1 files changed, 11802 insertions, 0 deletions

diff --git a/src/service/transport/gnunet-service-transport.c b/src/service/transport/gnunet-service-transport.c
new file mode 100644
index 000000000..ed45462cb
--- /dev/null
+++ b/src/service/transport/gnunet-service-transport.c
@@ -0,0 +1,11802 @@
+/*
+   This file is part of GNUnet.
+   Copyright (C) 2010-2016, 2018, 2019 GNUnet e.V.
+
+   GNUnet is free software: you can redistribute it and/or modify it
+   under the terms of the GNU Affero General Public License as published
+   by the Free Software Foundation, either version 3 of the License,
+   or (at your option) any later version.
+
+   GNUnet is distributed in the hope that it will be useful, but
+   WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+     SPDX-License-Identifier: AGPL3.0-or-later
+ */
+/**
+ * @file transport/gnunet-service-transport.c
+ * @brief main for gnunet-service-transport
+ * @author Christian Grothoff
+ *
+ * TODO:
+ * Implement next:
+ * - review retransmission logic, right now there is no smartness there!
+ *   => congestion control, etc [PERFORMANCE-BASICS]
+ *
+ * Optimizations-Statistics:
+ * - Track ACK losses based on ACK-counter [ROUTING]
+ * - Need to track total bandwidth per VirtualLink and adjust how frequently
+ *   we send FC messages based on bandwidth-delay-product (and relation
+ *   to the window size!). See OPTIMIZE-FC-BDP.
+ * - Consider more statistics in #check_connection_quality() [FIXME-CONQ-STATISTICS]
+ * - Adapt available_fc_window_size, using larger values for high-bandwidth
+ *   and high-latency links *if* we have the RAM [GOODPUT / utilization / stalls]
+ * - Set last_window_consum_limit promise properly based on
+ *   latency and bandwidth of the respective connection [GOODPUT / utilization / stalls]
+ *
+ * Optimizations-DV:
+ * - When forwarding DV learn messages, if a peer is reached that
+ *   has a *bidirectional* link to the origin beyond 1st hop,
+ *   do NOT forward it to peers _other_ than the origin, as
+ *   there is clearly a better path directly from the origin to
+ *   whatever else we could reach.
+ * - When we passively learned DV (with unconfirmed freshness), we
+ *   right now add the path to our list but with a zero path_valid_until
+ *   time and only use it for unconfirmed routes.  However, we could consider
+ *   triggering an explicit validation mechanism ourselves, specifically routing
+ *   a challenge-response message over the path [ROUTING]
+ * = if available, try to confirm unconfirmed DV paths when trying to establish
+ *   virtual link for a `struct IncomingRequest`. (i.e. if DVH is
+ *   unconfirmed, incoming requests cause us to try to validate a passively
+ *   learned path (requires new message type!))
+ *
+ * Optimizations-Fragmentation:
+ * - Fragments send over a reliable channel could do without the
+ *   AcknowledgementUUIDP altogether, as they won't be acked! [BANDWIDTH]
+ *   (-> have 2nd type of acknowledgment message; low priority, as we
+ *       do not have an MTU-limited *reliable* communicator) [FIXME-FRAG-REL-UUID]
+ * - if messages are below MTU, consider adding ACKs and other stuff
+ *   to the same transmission to avoid tiny messages (requires planning at
+ *   receiver, and additional MST-style demultiplex at receiver!) [PACKET COUNT]
+ *
+ * Optimizations-internals:
+ * - queue_send_msg by API design has to make a copy
+ *   of the payload, and route_message on top of that requires a malloc/free.
+ *   Change design to approximate "zero" copy better... [CPU]
+ * - could avoid copying body of message into each fragment and keep
+ *   fragments as just pointers into the original message and only
+ *   fully build fragments just before transmission (optimization, should
+ *   reduce CPU and memory use) [CPU, MEMORY]
+ */
+#include "platform.h"
+#include "gnunet_util_lib.h"
+#include "gnunet_statistics_service.h"
+#include "gnunet_transport_monitor_service.h"
+#include "gnunet_peerstore_service.h"
+#include "gnunet_hello_uri_lib.h"
+#include "gnunet_signatures.h"
+#include "transport.h"
+
+/**
+ * Size of ring buffer to cache CORE and forwarded DVBox messages.
+ */
+#define RING_BUFFER_SIZE 16
+
+/**
+ * Maximum number of FC retransmissions for a running retransmission task.
+ */
+#define MAX_FC_RETRANSMIT_COUNT 1000
+
+/**
+ * Maximum number of messages we acknowledge together in one
+ * cumulative ACK.  Larger values may save a bit of bandwidth.
+ */
+#define MAX_CUMMULATIVE_ACKS 64
+
+/**
+ * What is the 1:n chance that we send a Flow control response when
+ * receiving a flow control message that did not change anything for
+ * us? Basically, this is used in the case where both peers are stuck
+ * on flow control (no window changes), but one might continue sending
+ * flow control messages to the other peer as the first FC message
+ * when things stalled got lost, and then subsequently the other peer
+ * does *usually* not respond as nothing changed.  So to ensure that
+ * eventually the FC messages stop, we do send with 1/8th probability
+ * an FC message even if nothing changed.  That prevents one peer
+ * being stuck in sending (useless) FC messages "forever".
+ */
+#define FC_NO_CHANGE_REPLY_PROBABILITY 8
+
+/**
+ * What is the size we assume for a read operation in the
+ * absence of an MTU for the purpose of flow control?
+ */
+#define IN_PACKET_SIZE_WITHOUT_MTU 128
+
+/**
+ * Number of slots we keep of historic data for computation of
+ * goodput / message loss ratio.
+ */
+#define GOODPUT_AGING_SLOTS 4
+
+/**
+ * How big is the flow control window size by default;
+ * limits per-neighbour RAM utilization.
+ */
+#define DEFAULT_WINDOW_SIZE (128 * 1024)
+
+/**
+ * For how many incoming connections do we try to create a
+ * virtual link for (at the same time!).  This does NOT
+ * limit the number of incoming connections, just the number
+ * for which we are actively trying to find working addresses
+ * in the absence (!) of our own applications wanting the
+ * link to go up.
+ */
+#define MAX_INCOMING_REQUEST 16
+
+/**
+ * Maximum number of peers we select for forwarding DVInit
+ * messages at the same time (excluding initiator).
+ */
+#define MAX_DV_DISCOVERY_SELECTION 16
+
+/**
+ * Window size. How many messages to the same target do we pass
+ * to CORE without a RECV_OK in between? Small values limit
+ * thoughput, large values will increase latency.
+ *
+ * FIXME-OPTIMIZE: find out what good values are experimentally,
+ * maybe set adaptively (i.e. to observed available bandwidth).
+ */
+#define RECV_WINDOW_SIZE 4
+
+/**
+ * Minimum number of hops we should forward DV learn messages
+ * even if they are NOT useful for us in hope of looping
+ * back to the initiator?
+ *
+ * FIXME: allow initiator some control here instead?
+ */
+#define MIN_DV_PATH_LENGTH_FOR_INITIATOR 3
+
+/**
+ * Maximum DV distance allowed ever.
+ */
+#define MAX_DV_HOPS_ALLOWED 16
+
+/**
+ * Maximum number of DV learning activities we may
+ * have pending at the same time.
+ */
+#define MAX_DV_LEARN_PENDING 64
+
+/**
+ * Maximum number of DV paths we keep simultaneously to the same target.
+ */
+#define MAX_DV_PATHS_TO_TARGET 3
+
+/**
+ * If a queue delays the next message by more than this number
+ * of seconds we log a warning. Note: this is for testing,
+ * the value chosen here might be too aggressively low!
+ */
+#define DELAY_WARN_THRESHOLD \
+        GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_SECONDS, 5)
+
+/**
+ * If a DVBox could not be forwarded after this number of
+ * seconds we drop it.
+ */
+#define DV_FORWARD_TIMEOUT \
+        GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_SECONDS, 60)
+
+/**
+ * Default value for how long we wait for reliability ack.
+ */
+#define DEFAULT_ACK_WAIT_DURATION \
+        GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_SECONDS, 1)
+
+/**
+ * We only consider queues as "quality" connections when
+ * suppressing the generation of DV initiation messages if
+ * the latency of the queue is below this threshold.
+ */
+#define DV_QUALITY_RTT_THRESHOLD \
+        GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_SECONDS, 1)
+
+/**
+ * How long do we consider a DV path valid if we see no
+ * further updates on it? Note: the value chosen here might be too low!
+ */
+#define DV_PATH_VALIDITY_TIMEOUT \
+        GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_MINUTES, 5)
+
+/**
+ * How long do we cache backchannel (struct Backtalker) information
+ * after a backchannel goes inactive?
+ */
+#define BACKCHANNEL_INACTIVITY_TIMEOUT \
+        GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_MINUTES, 5)
+
+/**
+ * How long before paths expire would we like to (re)discover DV paths? Should
+ * be below #DV_PATH_VALIDITY_TIMEOUT.
+ */
+#define DV_PATH_DISCOVERY_FREQUENCY \
+        GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_MINUTES, 4)
+
+/**
+ * How long are ephemeral keys valid?
+ */
+#define EPHEMERAL_VALIDITY \
+        GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_HOURS, 4)
+
+/**
+ * How long do we keep partially reassembled messages around before giving up?
+ */
+#define REASSEMBLY_EXPIRATION \
+        GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_MINUTES, 4)
+
+/**
+ * What is the fastest rate at which we send challenges *if* we keep learning
+ * an address (gossip, DHT, etc.)?
+ */
+#define FAST_VALIDATION_CHALLENGE_FREQ \
+        GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_MINUTES, 1)
+
+/**
+ * What is the slowest rate at which we send challenges?
+ */
+#define MAX_VALIDATION_CHALLENGE_FREQ \
+        GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_DAYS, 1)
+
+/**
+ * How long until we forget about historic accumulators and thus
+ * reset the ACK counter? Should exceed the maximum time an
+ * active connection experiences without an ACK.
+ */
+#define ACK_CUMMULATOR_TIMEOUT \
+        GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_HOURS, 4)
+
+/**
+ * What is the non-randomized base frequency at which we
+ * would initiate DV learn messages?
+ */
+#define DV_LEARN_BASE_FREQUENCY GNUNET_TIME_UNIT_MINUTES
+
+/**
+ * How many good connections (confirmed, bi-directional, not DV)
+ * do we need to have to suppress initiating DV learn messages?
+ */
+#define DV_LEARN_QUALITY_THRESHOLD 100
+
+/**
+ * When do we forget an invalid address for sure?
+ */
+#define MAX_ADDRESS_VALID_UNTIL \
+        GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_MONTHS, 1)
+
+/**
+ * How long do we consider an address valid if we just checked?
+ */
+#define ADDRESS_VALIDATION_LIFETIME \
+        GNUNET_TIME_relative_multiply (GNUNET_TIME_UNIT_HOURS, 4)
+
+/**
+ * What is the maximum frequency at which we do address validation?
+ * A random value between 0 and this value is added when scheduling
+ * the #validation_task (both to ensure we do not validate too often,
+ * and to randomize a bit).
+ */
+#define MIN_DELAY_ADDRESS_VALIDATION GNUNET_TIME_UNIT_MILLISECONDS
+
+/**
+ * How many network RTTs before an address validation expires should we begin
+ * trying to revalidate? (Note that the RTT used here is the one that we
+ * experienced during the last validation, not necessarily the latest RTT
+ * observed).
+ */
+#define VALIDATION_RTT_BUFFER_FACTOR 3
+
+/**
+ * How many messages can we have pending for a given communicator
+ * process before we start to throttle that communicator?
+ *
+ * Used if a communicator might be CPU-bound and cannot handle the traffic.
+ */
+#define COMMUNICATOR_TOTAL_QUEUE_LIMIT 512
+
+/**
+ * How many messages can we have pending for a given queue (queue to
+ * a particular peer via a communicator) process before we start to
+ * throttle that queue?
+ */
+#define QUEUE_LENGTH_LIMIT 32
+
+
+GNUNET_NETWORK_STRUCT_BEGIN
+
+/**
+ * Unique identifier we attach to a message.
+ */
+struct MessageUUIDP
+{
+  /**
+   * Unique value, generated by incrementing the
+   * `message_uuid_ctr` of `struct Neighbour`.
+   */
+  uint64_t uuid GNUNET_PACKED;
+};
+
+
+/**
+ * Unique identifier to map an acknowledgement to a transmission.
+ */
+struct AcknowledgementUUIDP
+{
+  /**
+   * The UUID value.
+   */
+  struct GNUNET_Uuid value;
+};
+
+/**
+ * Outer layer of an encapsulated backchannel message.
+ */
+struct TransportBackchannelEncapsulationMessage
+{
+  /**
+   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_BACKCHANNEL_ENCAPSULATION.
+   */
+  struct GNUNET_MessageHeader header;
+
+  /* Followed by *another* message header which is the message to
+     the communicator */
+
+  /* Followed by a 0-terminated name of the communicator */
+};
+
+
+/**
+ * Body by which a peer confirms that it is using an ephemeral key.
+ */
+struct EphemeralConfirmationPS
+{
+  /**
+   * Purpose is #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_EPHEMERAL
+   */
+  struct GNUNET_CRYPTO_EccSignaturePurpose purpose;
+
+  /**
+   * How long is this signature over the ephemeral key valid?
+   *
+   * Note that the receiver MUST IGNORE the absolute time, and only interpret
+   * the value as a mononic time and reject "older" values than the last one
+   * observed.  This is necessary as we do not want to require synchronized
+   * clocks and may not have a bidirectional communication channel.
+   *
+   * Even with this, there is no real guarantee against replay achieved here,
+   * unless the latest timestamp is persisted.  While persistence should be
+   * provided via PEERSTORE, we do not consider the mechanism reliable!  Thus,
+   * communicators must protect against replay attacks when using backchannel
+   * communication!
+   */
+  struct GNUNET_TIME_AbsoluteNBO sender_monotonic_time;
+
+  /**
+   * Target's peer identity.
+   */
+  struct GNUNET_PeerIdentity target;
+
+  /**
+   * Ephemeral key setup by the sender for @e target, used
+   * to encrypt the payload.
+   */
+  struct GNUNET_CRYPTO_EcdhePublicKey ephemeral_key;
+};
+
+
+/**
+ * Plaintext of the variable-size payload that is encrypted
+ * within a `struct TransportBackchannelEncapsulationMessage`
+ */
+struct TransportDVBoxPayloadP
+{
+  /**
+   * Sender's peer identity.
+   */
+  struct GNUNET_PeerIdentity sender;
+
+  /**
+   * Signature of the sender over an
+   * #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_EPHEMERAL.
+   */
+  struct GNUNET_CRYPTO_EddsaSignature sender_sig;
+
+  /**
+   * Current monotonic time of the sending transport service.  Used to
+   * detect replayed messages.  Note that the receiver should remember
+   * a list of the recently seen timestamps and only reject messages
+   * if the timestamp is in the list, or the list is "full" and the
+   * timestamp is smaller than the lowest in the list.
+   *
+   * Like the @e ephemeral_validity, the list of timestamps per peer should be
+   * persisted to guard against replays after restarts.
+   */
+  struct GNUNET_TIME_AbsoluteNBO monotonic_time;
+
+  /* Followed by a `struct GNUNET_MessageHeader` with a message
+     for the target peer */
+};
+
+
+/**
+ * Outer layer of an encapsulated unfragmented application message sent
+ * over an unreliable channel.
+ */
+struct TransportReliabilityBoxMessage
+{
+  /**
+   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_BOX
+   */
+  struct GNUNET_MessageHeader header;
+
+  /**
+   * Number of messages still to be sent before a commulative
+   * ACK is requested.  Zero if an ACK is requested immediately.
+   * In NBO.  Note that the receiver may send the ACK faster
+   * if it believes that is reasonable.
+   */
+  uint32_t ack_countdown GNUNET_PACKED;
+
+  /**
+   * Unique ID of the message used for signalling receipt of
+   * messages sent over possibly unreliable channels.  Should
+   * be a random.
+   */
+  struct AcknowledgementUUIDP ack_uuid;
+};
+
+
+/**
+ * Acknowledgement payload.
+ */
+struct TransportCummulativeAckPayloadP
+{
+  /**
+   * How long was the ACK delayed for generating cumulative ACKs?
+   * Used to calculate the correct network RTT by taking the receipt
+   * time of the ack minus the transmission time of the sender minus
+   * this value.
+   */
+  struct GNUNET_TIME_RelativeNBO ack_delay;
+
+  /**
+   * UUID of a message being acknowledged.
+   */
+  struct AcknowledgementUUIDP ack_uuid;
+};
+
+
+/**
+ * Confirmation that the receiver got a
+ * #GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_BOX. Note that the
+ * confirmation may be transmitted over a completely different queue,
+ * so ACKs are identified by a combination of PID of sender and
+ * message UUID, without the queue playing any role!
+ */
+struct TransportReliabilityAckMessage
+{
+  /**
+   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_ACK
+   */
+  struct GNUNET_MessageHeader header;
+
+  /**
+   * Counter of ACKs transmitted by the sender to us. Incremented
+   * by one for each ACK, used to detect how many ACKs were lost.
+   */
+  uint32_t ack_counter GNUNET_PACKED;
+
+  /* followed by any number of `struct TransportCummulativeAckPayloadP`
+     messages providing ACKs */
+};
+
+
+/**
+ * Outer layer of an encapsulated fragmented application message.
+ */
+struct TransportFragmentBoxMessage
+{
+  /**
+   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_FRAGMENT
+   */
+  struct GNUNET_MessageHeader header;
+
+  /**
+   * Offset of this fragment in the overall message.
+   */
+  uint16_t frag_off GNUNET_PACKED;
+
+  /**
+   * Total size of the message that is being fragmented.
+   */
+  uint16_t msg_size GNUNET_PACKED;
+
+  /**
+   * Unique ID of this fragment (and fragment transmission!). Will
+   * change even if a fragment is retransmitted to make each
+   * transmission attempt unique! If a client receives a duplicate
+   * fragment (same @e frag_off for same @a msg_uuid, it must send
+   * #GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_ACK immediately.
+   */
+  struct AcknowledgementUUIDP ack_uuid;
+
+  /**
+   * Original message ID for of the message that all the fragments
+   * belong to.  Must be the same for all fragments.
+   */
+  struct MessageUUIDP msg_uuid;
+};
+
+
+/**
+ * Content signed by the initator during DV learning.
+ *
+ * The signature is required to prevent DDoS attacks. A peer sending out this
+ * message is potentially generating a lot of traffic that will go back to the
+ * initator, as peers receiving this message will try to let the initiator
+ * know that they got the message.
+ *
+ * Without this signature, an attacker could abuse this mechanism for traffic
+ * amplification, sending a lot of traffic to a peer by putting out this type
+ * of message with the victim's peer identity.
+ *
+ * Even with just a signature, traffic amplification would be possible via
+ * replay attacks. The @e monotonic_time limits such replay attacks, as every
+ * potential amplificator will check the @e monotonic_time and only respond
+ * (at most) once per message.
+ */
+struct DvInitPS
+{
+  /**
+   * Purpose is #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_INITIATOR
+   */
+  struct GNUNET_CRYPTO_EccSignaturePurpose purpose;
+
+  /**
+   * Time at the initiator when generating the signature.
+   *
+   * Note that the receiver MUST IGNORE the absolute time, and only interpret
+   * the value as a mononic time and reject "older" values than the last one
+   * observed.  This is necessary as we do not want to require synchronized
+   * clocks and may not have a bidirectional communication channel.
+   *
+   * Even with this, there is no real guarantee against replay achieved here,
+   * unless the latest timestamp is persisted.  Persistence should be
+   * provided via PEERSTORE if possible.
+   */
+  struct GNUNET_TIME_AbsoluteNBO monotonic_time;
+
+  /**
+   * Challenge value used by the initiator to re-identify the path.
+   */
+  struct GNUNET_CRYPTO_ChallengeNonceP challenge;
+};
+
+
+/**
+ * Content signed by each peer during DV learning.
+ *
+ * This assues the initiator of the DV learning operation that the hop from @e
+ * pred via the signing peer to @e succ actually exists.  This makes it
+ * impossible for an adversary to supply the network with bogus routes.
+ *
+ * The @e challenge is included to provide replay protection for the
+ * initiator. This way, the initiator knows that the hop existed after the
+ * original @e challenge was first transmitted, providing a freshness metric.
+ *
+ * Peers other than the initiator that passively learn paths by observing
+ * these messages do NOT benefit from this. Here, an adversary may indeed
+ * replay old messages.  Thus, passively learned paths should always be
+ * immediately marked as "potentially stale".
+ */
+struct DvHopPS
+{
+  /**
+   * Purpose is #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_HOP
+   */
+  struct GNUNET_CRYPTO_EccSignaturePurpose purpose;
+
+  /**
+   * Identity of the previous peer on the path.
+   */
+  struct GNUNET_PeerIdentity pred;
+
+  /**
+   * Identity of the next peer on the path.
+   */
+  struct GNUNET_PeerIdentity succ;
+
+  /**
+   * Challenge value used by the initiator to re-identify the path.
+   */
+  struct GNUNET_CRYPTO_ChallengeNonceP challenge;
+};
+
+
+/**
+ * An entry describing a peer on a path in a
+ * `struct TransportDVLearnMessage` message.
+ */
+struct DVPathEntryP
+{
+  /**
+   * Identity of a peer on the path.
+   */
+  struct GNUNET_PeerIdentity hop;
+
+  /**
+   * Signature of this hop over the path, of purpose
+   * #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_HOP
+   */
+  struct GNUNET_CRYPTO_EddsaSignature hop_sig;
+};
+
+
+/**
+ * Internal message used by transport for distance vector learning.
+ * If @e num_hops does not exceed the threshold, peers should append
+ * themselves to the peer list and flood the message (possibly only
+ * to a subset of their neighbours to limit discoverability of the
+ * network topology).  To the extend that the @e bidirectional bits
+ * are set, peers may learn the inverse paths even if they did not
+ * initiate.
+ *
+ * Unless received on a bidirectional queue and @e num_hops just
+ * zero, peers that can forward to the initator should always try to
+ * forward to the initiator.
+ */
+struct TransportDVLearnMessage
+{
+  /**
+   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_DV_LEARN
+   */
+  struct GNUNET_MessageHeader header;
+
+  /**
+   * Number of hops this messages has travelled, in NBO. Zero if
+   * sent by initiator.
+   */
+  uint16_t num_hops GNUNET_PACKED;
+
+  /**
+   * Bitmask of the last 16 hops indicating whether they are confirmed
+   * available (without DV) in both directions or not, in NBO.  Used
+   * to possibly instantly learn a path in both directions.  Each peer
+   * should shift this value by one to the left, and then set the
+   * lowest bit IF the current sender can be reached from it (without
+   * DV routing).
+   */
+  uint16_t bidirectional GNUNET_PACKED;
+
+  /**
+   * Peers receiving this message and delaying forwarding to other
+   * peers for any reason should increment this value by the non-network
+   * delay created by the peer.
+   */
+  struct GNUNET_TIME_RelativeNBO non_network_delay;
+
+  /**
+   * Time at the initiator when generating the signature.
+   *
+   * Note that the receiver MUST IGNORE the absolute time, and only interpret
+   * the value as a mononic time and reject "older" values than the last one
+   * observed.  This is necessary as we do not want to require synchronized
+   * clocks and may not have a bidirectional communication channel.
+   *
+   * Even with this, there is no real guarantee against replay achieved here,
+   * unless the latest timestamp is persisted.  Persistence should be
+   * provided via PEERSTORE if possible.
+   */
+  struct GNUNET_TIME_AbsoluteNBO monotonic_time;
+
+  /**
+   * Signature of this hop over the path, of purpose
+   * #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_INITIATOR
+   */
+  struct GNUNET_CRYPTO_EddsaSignature init_sig;
+
+  /**
+   * Identity of the peer that started this learning activity.
+   */
+  struct GNUNET_PeerIdentity initiator;
+
+  /**
+   * Challenge value used by the initiator to re-identify the path.
+   */
+  struct GNUNET_CRYPTO_ChallengeNonceP challenge;
+
+  /* Followed by @e num_hops `struct DVPathEntryP` values,
+     excluding the initiator of the DV trace; the last entry is the
+     current sender; the current peer must not be included. */
+};
+
+
+/**
+ * Outer layer of an encapsulated message send over multiple hops.
+ * The path given only includes the identities of the subsequent
+ * peers, i.e. it will be empty if we are the receiver. Each
+ * forwarding peer should scan the list from the end, and if it can,
+ * forward to the respective peer. The list should then be shortened
+ * by all the entries up to and including that peer.  Each hop should
+ * also increment @e total_hops to allow the receiver to get a precise
+ * estimate on the number of hops the message travelled.  Senders must
+ * provide a learned path that thus should work, but intermediaries
+ * know of a shortcut, they are allowed to send the message via that
+ * shortcut.
+ *
+ * If a peer finds itself still on the list, it must drop the message.
+ *
+ * The payload of the box can only be decrypted and verified by the
+ * ultimate receiver. Intermediaries do not learn the sender's
+ * identity and the path the message has taken.  However, the first
+ * hop does learn the sender as @e total_hops would be zero and thus
+ * the predecessor must be the origin (so this is not really useful
+ * for anonymization).
+ */
+struct TransportDVBoxMessage
+{
+  /**
+   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_DV_BOX
+   */
+  struct GNUNET_MessageHeader header;
+
+  /**
+   * Flag if the payload is a control message. In NBO.
+   */
+  unsigned int without_fc;
+
+  /**
+   * Number of total hops this messages travelled. In NBO.
+   * @e origin sets this to zero, to be incremented at
+   * each hop.  Peers should limit the @e total_hops value
+   * they accept from other peers.
+   */
+  uint16_t total_hops GNUNET_PACKED;
+
+  /**
+   * Number of hops this messages includes. In NBO.  Reduced by one
+   * or more at each hop.  Peers should limit the @e num_hops value
+   * they accept from other peers.
+   */
+  uint16_t num_hops GNUNET_PACKED;
+
+  /**
+   * Ephemeral key setup by the sender for target, used to encrypt the
+   * payload.  Intermediaries must not change this value.
+   */
+  struct GNUNET_CRYPTO_EcdhePublicKey ephemeral_key;
+
+  /**
+   * We use an IV here as the @e ephemeral_key is re-used for
+   * #EPHEMERAL_VALIDITY time to avoid re-signing it all the time.
+   * Intermediaries must not change this value.
+   */
+  struct GNUNET_ShortHashCode iv;
+
+  /**
+   * HMAC over the ciphertext of the encrypted, variable-size body
+   * that follows.  Verified via DH of target and @e ephemeral_key.
+   * Intermediaries must not change this value.
+   */
+  struct GNUNET_HashCode hmac;
+
+  /**
+   * Size this msg had initially. This is needed to calculate the hmac at the target.
+   * The header size can not be used for that, because the box size is getting smaller at each hop.
+   *
+   */
+  uint16_t orig_size GNUNET_PACKED;
+
+  /* Followed by @e num_hops `struct GNUNET_PeerIdentity` values;
+     excluding the @e origin and the current peer, the last must be
+     the ultimate target; if @e num_hops is zero, the receiver of this
+     message is the ultimate target. */
+
+  /* Followed by encrypted, variable-size payload, which
+     must begin with a `struct TransportDVBoxPayloadP` */
+
+  /* Followed by the actual message, which itself must not be a
+     a DV_LEARN or DV_BOX message! */
+};
+
+
+/**
+ * Message send to another peer to validate that it can indeed
+ * receive messages at a particular address.
+ */
+struct TransportValidationChallengeMessage
+{
+  /**
+   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_ADDRESS_VALIDATION_CHALLENGE
+   */
+  struct GNUNET_MessageHeader header;
+
+  /**
+   * Always zero.
+   */
+  uint32_t reserved GNUNET_PACKED;
+
+  /**
+   * Challenge to be signed by the receiving peer.
+   */
+  struct GNUNET_CRYPTO_ChallengeNonceP challenge;
+
+  /**
+   * Timestamp of the sender, to be copied into the reply to allow
+   * sender to calculate RTT.  Must be monotonically increasing!
+   */
+  struct GNUNET_TIME_AbsoluteNBO sender_time;
+};
+
+
+/**
+ * Message signed by a peer to confirm that it can indeed
+ * receive messages at a particular address.
+ */
+struct TransportValidationPS
+{
+  /**
+   * Purpose is #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_CHALLENGE
+   */
+  struct GNUNET_CRYPTO_EccSignaturePurpose purpose;
+
+  /**
+   * How long does the sender believe the address on
+   * which the challenge was received to remain valid?
+   */
+  struct GNUNET_TIME_RelativeNBO validity_duration;
+
+  /**
+   * Challenge signed by the receiving peer.
+   */
+  struct GNUNET_CRYPTO_ChallengeNonceP challenge;
+};
+
+
+/**
+ * Message  send to a peer to respond to a
+ * #GNUNET_MESSAGE_TYPE_ADDRESS_VALIDATION_CHALLENGE
+ */
+struct TransportValidationResponseMessage
+{
+  /**
+   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_ADDRESS_VALIDATION_RESPONSE
+   */
+  struct GNUNET_MessageHeader header;
+
+  /**
+   * Always zero.
+   */
+  uint32_t reserved GNUNET_PACKED;
+
+  /**
+   * The peer's signature matching the
+   * #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_CHALLENGE purpose.
+   */
+  struct GNUNET_CRYPTO_EddsaSignature signature;
+
+  /**
+   * The challenge that was signed by the receiving peer.
+   */
+  struct GNUNET_CRYPTO_ChallengeNonceP challenge;
+
+  /**
+   * Original timestamp of the sender (was @code{sender_time}),
+   * copied into the reply to allow sender to calculate RTT.
+   */
+  struct GNUNET_TIME_AbsoluteNBO origin_time;
+
+  /**
+   * How long does the sender believe this address to remain
+   * valid?
+   */
+  struct GNUNET_TIME_RelativeNBO validity_duration;
+};
+
+
+/**
+ * Message for Transport-to-Transport Flow control. Specifies the size
+ * of the flow control window, including how much we believe to have
+ * consumed (at transmission time), how much we believe to be allowed
+ * (at transmission time), and how much the other peer is allowed to
+ * send to us, and how much data we already received from the other
+ * peer.
+ */
+struct TransportFlowControlMessage
+{
+  /**
+   * Type is #GNUNET_MESSAGE_TYPE_TRANSPORT_FLOW_CONTROL
+   */
+  struct GNUNET_MessageHeader header;
+
+  /**
+   * Sequence number of the flow control message. Incremented by one
+   * for each message.  Starts at zero when a virtual link goes up.
+   * Used to detect one-sided connection drops. On wrap-around, the
+   * flow control counters will be reset as if the connection had
+   * dropped.
+   */
+  uint32_t seq GNUNET_PACKED;
+
+  /**
+   * Flow control window size in bytes, in NBO.
+   * The receiver can send this many bytes at most.
+   */
+  uint64_t inbound_window_size GNUNET_PACKED;
+
+  /**
+   * How many bytes has the sender sent that count for flow control at
+   * this time.  Used to allow the receiver to estimate the packet
+   * loss rate.
+   */
+  uint64_t outbound_sent GNUNET_PACKED;
+
+  /**
+   * Latest flow control window size we learned from the other peer,
+   * in bytes, in NBO.  We are limited to sending at most this many
+   * bytes to the other peer.  May help the other peer detect when
+   * flow control messages were lost and should thus be retransmitted.
+   * In particular, if the delta to @e outbound_sent is too small,
+   * this signals that we are stalled.
+   */
+  uint64_t outbound_window_size GNUNET_PACKED;
+
+  /**
+   * Timestamp of the sender.  Must be monotonically increasing!
+   * Used to enable receiver to ignore out-of-order packets in
+   * combination with the @e seq. Note that @e seq will go down
+   * (back to zero) whenever either side believes the connection
+   * was dropped, allowing the peers to detect that they need to
+   * reset the counters for the number of bytes sent!
+   */
+  struct GNUNET_TIME_AbsoluteNBO sender_time;
+};
+
+
+GNUNET_NETWORK_STRUCT_END
+
+
+/**
+ * What type of client is the `struct TransportClient` about?
+ */
+enum ClientType
+{
+  /**
+   * We do not know yet (client is fresh).
+   */
+  CT_NONE = 0,
+
+  /**
+   * Is the CORE service, we need to forward traffic to it.
+   */
+  CT_CORE = 1,
+
+  /**
+   * It is a monitor, forward monitor data.
+   */
+  CT_MONITOR = 2,
+
+  /**
+   * It is a communicator, use for communication.
+   */
+  CT_COMMUNICATOR = 3,
+
+  /**
+   * "Application" telling us where to connect (i.e. TOPOLOGY, DHT or CADET).
+   */
+  CT_APPLICATION = 4
+};
+
+
+/**
+ * Which transmission options are allowable for transmission?
+ * Interpreted bit-wise!
+ */
+enum RouteMessageOptions
+{
+  /**
+   * Only confirmed, non-DV direct neighbours.
+   */
+  RMO_NONE = 0,
+
+  /**
+   * We are allowed to use DV routing for this @a hdr
+   */
+  RMO_DV_ALLOWED = 1,
+
+  /**
+   * We are allowed to use unconfirmed queues or DV routes for this message
+   */
+  RMO_UNCONFIRMED_ALLOWED = 2,
+
+  /**
+   * Reliable and unreliable, DV and non-DV are all acceptable.
+   */
+  RMO_ANYTHING_GOES = (RMO_DV_ALLOWED | RMO_UNCONFIRMED_ALLOWED),
+
+  /**
+   * If we have multiple choices, it is OK to send this message
+   * over multiple channels at the same time to improve loss tolerance.
+   * (We do at most 2 transmissions.)
+   */
+  RMO_REDUNDANT = 4
+};
+
+
+/**
+ * When did we launch this DV learning activity?
+ */
+struct LearnLaunchEntry
+{
+  /**
+   * Kept (also) in a DLL sorted by launch time.
+   */
+  struct LearnLaunchEntry *prev;
+
+  /**
+   * Kept (also) in a DLL sorted by launch time.
+   */
+  struct LearnLaunchEntry *next;
+
+  /**
+   * Challenge that uniquely identifies this activity.
+   */
+  struct GNUNET_CRYPTO_ChallengeNonceP challenge;
+
+  /**
+   * When did we transmit the DV learn message (used to calculate RTT) and
+   * determine freshness of paths learned via this operation.
+   */
+  struct GNUNET_TIME_Absolute launch_time;
+};
+
+
+/**
+ * Information we keep per #GOODPUT_AGING_SLOTS about historic
+ * (or current) transmission performance.
+ */
+struct TransmissionHistoryEntry
+{
+  /**
+   * Number of bytes actually sent in the interval.
+   */
+  uint64_t bytes_sent;
+
+  /**
+   * Number of bytes received and acknowledged by the other peer in
+   * the interval.
+   */
+  uint64_t bytes_received;
+};
+
+
+/**
+ * Performance data for a transmission possibility.
+ */
+struct PerformanceData
+{
+  /**
+   * Weighted average for the RTT.
+   */
+  struct GNUNET_TIME_Relative aged_rtt;
+
+  /**
+   * Historic performance data, using a ring buffer of#GOODPUT_AGING_SLOTS
+   * entries.
+   */
+  struct TransmissionHistoryEntry the[GOODPUT_AGING_SLOTS];
+
+  /**
+   * What was the last age when we wrote to @e the? Used to clear
+   * old entries when the age advances.
+   */
+  unsigned int last_age;
+};
+
+
+/**
+ * Client connected to the transport service.
+ */
+struct TransportClient;
+
+/**
+ * A neighbour that at least one communicator is connected to.
+ */
+struct Neighbour;
+
+/**
+ * Entry in our #dv_routes table, representing a (set of) distance
+ * vector routes to a particular peer.
+ */
+struct DistanceVector;
+
+/**
+ * A queue is a message queue provided by a communicator
+ * via which we can reach a particular neighbour.
+ */
+struct Queue;
+
+/**
+ * Message awaiting transmission. See detailed comments below.
+ */
+struct PendingMessage;
+
+/**
+ * One possible hop towards a DV target.
+ */
+struct DistanceVectorHop;
+
+/**
+ * A virtual link is another reachable peer that is known to CORE.  It
+ * can be either a `struct Neighbour` with at least one confirmed
+ * `struct Queue`, or a `struct DistanceVector` with at least one
+ * confirmed `struct DistanceVectorHop`.  With a virtual link we track
+ * data that is per neighbour that is not specific to how the
+ * connectivity is established.
+ */
+struct VirtualLink;
+
+
+/**
+ * Context from #handle_incoming_msg().  Closure for many
+ * message handlers below.
+ */
+struct CommunicatorMessageContext
+{
+  /**
+   * Kept in a DLL of `struct VirtualLink` if waiting for CORE
+   * flow control to unchoke.
+   */
+  struct CommunicatorMessageContext *next;
+
+  /**
+   * Kept in a DLL of `struct VirtualLink` if waiting for CORE
+   * flow control to unchoke.
+   */
+  struct CommunicatorMessageContext *prev;
+
+  /**
+   * Which communicator provided us with the message.
+   */
+  struct TransportClient *tc;
+
+  /**
+   * Additional information for flow control and about the sender.
+   */
+  struct GNUNET_TRANSPORT_IncomingMessage im;
+
+  /**
+   * The message to demultiplex.
+   */
+  const struct GNUNET_MessageHeader *mh;
+
+  /**
+   * Number of hops the message has travelled (if DV-routed).
+   * FIXME: make use of this in ACK handling!
+   */
+  uint16_t total_hops;
+};
+
+
+/**
+ * Entry for the ring buffer caching messages send to core, when virtual link is avaliable.
+ **/
+struct RingBufferEntry
+{
+  /**
+   * Communicator context for this ring buffer entry.
+   **/
+  struct CommunicatorMessageContext *cmc;
+
+  /**
+   * The message in this entry.
+   **/
+  struct GNUNET_MessageHeader *mh;
+};
+
+
+/**
+ * Closure for #core_env_sent_cb.
+ */
+struct CoreSentContext
+{
+  /**
+   * Kept in a DLL to clear @e vl in case @e vl is lost.
+   */
+  struct CoreSentContext *next;
+
+  /**
+   * Kept in a DLL to clear @e vl in case @e vl is lost.
+   */
+  struct CoreSentContext *prev;
+
+  /**
+   * Virtual link this is about.
+   */
+  struct VirtualLink *vl;
+
+  /**
+   * How big was the message.
+   */
+  uint16_t size;
+
+  /**
+   * By how much should we increment @e vl's
+   * incoming_fc_window_size_used once we are done sending to CORE?
+   * Use to ensure we do not increment twice if there is more than one
+   * CORE client.
+   */
+  uint16_t isize;
+};
+
+
+/**
+ * Information we keep for a message that we are reassembling.
+ */
+struct ReassemblyContext
+{
+  /**
+   * Original message ID for of the message that all the fragments
+   * belong to.
+   */
+  struct MessageUUIDP msg_uuid;
+
+  /**
+   * Which neighbour is this context for?
+   */
+  struct VirtualLink *virtual_link;
+
+  /**
+   * Entry in the reassembly heap (sorted by expiration).
+   */
+  struct GNUNET_CONTAINER_HeapNode *hn;
+
+  /**
+   * Bitfield with @e msg_size bits representing the positions
+   * where we have received fragments.  When we receive a fragment,
+   * we check the bits in @e bitfield before incrementing @e msg_missing.
+   *
+   * Allocated after the reassembled message.
+   */
+  uint8_t *bitfield;
+
+  /**
+   * At what time will we give up reassembly of this message?
+   */
+  struct GNUNET_TIME_Absolute reassembly_timeout;
+
+  /**
+   * Time we received the last fragment.  @e avg_ack_delay must be
+   * incremented by now - @e last_frag multiplied by @e num_acks.
+   */
+  struct GNUNET_TIME_Absolute last_frag;
+
+  /**
+   * How big is the message we are reassembling in total?
+   */
+  uint16_t msg_size;
+
+  /**
+   * How many bytes of the message are still missing?  Defragmentation
+   * is complete when @e msg_missing == 0.
+   */
+  uint16_t msg_missing;
+
+  /* Followed by @e msg_size bytes of the (partially) defragmented original
+   * message */
+
+  /* Followed by @e bitfield data */
+};
+
+
+/**
+ * A virtual link is another reachable peer that is known to CORE.  It
+ * can be either a `struct Neighbour` with at least one confirmed
+ * `struct Queue`, or a `struct DistanceVector` with at least one
+ * confirmed `struct DistanceVectorHop`.  With a virtual link we track
+ * data that is per neighbour that is not specific to how the
+ * connectivity is established.
+ */
+struct VirtualLink
+{
+  /**
+   * Identity of the peer at the other end of the link.
+   */
+  struct GNUNET_PeerIdentity target;
+
+  /**
+   * Map with `struct ReassemblyContext` structs for fragments under
+   * reassembly. May be NULL if we currently have no fragments from
+   * this @e pid (lazy initialization).
+   */
+  struct GNUNET_CONTAINER_MultiHashMap32 *reassembly_map;
+
+  /**
+   * Heap with `struct ReassemblyContext` structs for fragments under
+   * reassembly. May be NULL if we currently have no fragments from
+   * this @e pid (lazy initialization).
+   */
+  struct GNUNET_CONTAINER_Heap *reassembly_heap;
+
+  /**
+   * Task to free old entries from the @e reassembly_heap and @e reassembly_map.
+   */
+  struct GNUNET_SCHEDULER_Task *reassembly_timeout_task;
+
+  /**
+   * Communicators blocked for receiving on @e target as we are waiting
+   * on the @e core_recv_window to increase.
+   */
+  struct CommunicatorMessageContext *cmc_head;
+
+  /**
+   * Communicators blocked for receiving on @e target as we are waiting
+   * on the @e core_recv_window to increase.
+   */
+  struct CommunicatorMessageContext *cmc_tail;
+
+  /**
+   * Head of list of messages pending for this VL.
+   */
+  struct PendingMessage *pending_msg_head;
+
+  /**
+   * Tail of list of messages pending for this VL.
+   */
+  struct PendingMessage *pending_msg_tail;
+
+  /**
+   * Kept in a DLL to clear @e vl in case @e vl is lost.
+   */
+  struct CoreSentContext *csc_tail;
+
+  /**
+   * Kept in a DLL to clear @e vl in case @e vl is lost.
+   */
+  struct CoreSentContext *csc_head;
+
+  /**
+   * Task scheduled to possibly notfiy core that this peer is no
+   * longer counting as confirmed.  Runs the #core_visibility_check(),
+   * which checks that some DV-path or a queue exists that is still
+   * considered confirmed.
+   */
+  struct GNUNET_SCHEDULER_Task *visibility_task;
+
+  /**
+   * Task scheduled to periodically retransmit FC messages (in
+   * case one got lost).
+   */
+  struct GNUNET_SCHEDULER_Task *fc_retransmit_task;
+
+  /**
+   * Number of FC retransmissions for this running task.
+   */
+  unsigned int fc_retransmit_count;
+
+  /**
+   * Is this VirtualLink confirmed.
+   * A unconfirmed VirtualLink might exist, if we got a FC from that target.
+   */
+  unsigned int confirmed;
+
+  /**
+   * Neighbour used by this virtual link, NULL if @e dv is used.
+   */
+  struct Neighbour *n;
+
+  /**
+   * Distance vector used by this virtual link, NULL if @e n is used.
+   */
+  struct DistanceVector *dv;
+
+  /**
+   * Sender timestamp of @e n_challenge, used to generate out-of-order
+   * challenges (as sender's timestamps must be monotonically
+   * increasing).  FIXME: where do we need this?
+   */
+  struct GNUNET_TIME_Absolute n_challenge_time;
+
+  /**
+   * When did we last send a
+   * #GNUNET_MESSAGE_TYPE_TRANSPORT_FLOW_CONTROL message?
+   * Used to determine whether it is time to re-transmit the message.
+   */
+  struct GNUNET_TIME_Absolute last_fc_transmission;
+
+  /**
+   * Sender timestamp of the last
+   * #GNUNET_MESSAGE_TYPE_TRANSPORT_FLOW_CONTROL message we have
+   * received.  Note that we do not persist this monotonic time as we
+   * do not really have to worry about ancient flow control window
+   * sizes after restarts.
+   */
+  struct GNUNET_TIME_Absolute last_fc_timestamp;
+
+  /**
+   * Expected RTT from the last FC transmission. (Zero if the last
+   * attempt failed, but could theoretically be zero even on success.)
+   */
+  struct GNUNET_TIME_Relative last_fc_rtt;
+
+  /**
+   * Used to generate unique UUIDs for messages that are being
+   * fragmented.
+   */
+  uint64_t message_uuid_ctr;
+
+  /**
+   * Memory allocated for this virtual link.  Expresses how much RAM
+   * we are willing to allocate to this virtual link.  OPTIMIZE-ME:
+   * Can be adapted to dedicate more RAM to links that need it, while
+   * sticking to some overall RAM limit.  For now, set to
+   * #DEFAULT_WINDOW_SIZE.
+   */
+  uint64_t available_fc_window_size;
+
+  /**
+   * Memory actually used to buffer packets on this virtual link.
+   * Expresses how much RAM we are currently using for virtual link.
+   * Note that once CORE is done with a packet, we decrement the value
+   * here.
+   */
+  uint64_t incoming_fc_window_size_ram;
+
+  /**
+   * Last flow control window size we provided to the other peer, in
+   * bytes.  We are allowing the other peer to send this
+   * many bytes.
+   */
+  uint64_t incoming_fc_window_size;
+
+  /**
+   * How much of the window did the other peer successfully use (and
+   * we already passed it on to CORE)? Must be below @e
+   * incoming_fc_window_size.   We should effectively signal the
+   * other peer that the window is this much bigger at the next
+   * opportunity / challenge.
+   */
+  uint64_t incoming_fc_window_size_used;
+
+  /**
+   * What is our current estimate on the message loss rate for the sender?
+   * Based on the difference between how much the sender sent according
+   * to the last #GNUNET_MESSAGE_TYPE_TRANSPORT_FLOW_CONTROL message
+   * (@e outbound_sent field) and how much we actually received at that
+   * time (@e incoming_fc_window_size_used).  This delta is then
+   * added onto the @e incoming_fc_window_size when determining the
+   * @e outbound_window_size we send to the other peer.  Initially zero.
+   * May be negative if we (due to out-of-order delivery) actually received
+   * more than the sender claims to have sent in its last FC message.
+   */
+  int64_t incoming_fc_window_size_loss;
+
+  /**
+   * Our current flow control window size in bytes.  We
+   * are allowed to transmit this many bytes to @a n.
+   */
+  uint64_t outbound_fc_window_size;
+
+  /**
+   * How much of our current flow control window size have we
+   * used (in bytes).  Must be below
+   * @e outbound_fc_window_size.
+   */
+  uint64_t outbound_fc_window_size_used;
+
+  /**
+   * What is the most recent FC window the other peer sent us
+   * in `outbound_window_size`? This is basically the window
+   * size value the other peer has definitively received from
+   * us. If it matches @e incoming_fc_window_size, we should
+   * not send a FC message to increase the FC window. However,
+   * we may still send an FC message to notify the other peer
+   * that we received the other peer's FC message.
+   */
+  uint64_t last_outbound_window_size_received;
+
+  /**
+   * Generator for the sequence numbers of
+   * #GNUNET_MESSAGE_TYPE_TRANSPORT_FLOW_CONTROL messages we send.
+   */
+  uint32_t fc_seq_gen;
+
+  /**
+   * Last sequence number of a
+   * #GNUNET_MESSAGE_TYPE_TRANSPORT_FLOW_CONTROL message we have
+   * received.
+   */
+  uint32_t last_fc_seq;
+
+  /**
+   * How many more messages can we send to CORE before we exhaust
+   * the receive window of CORE for this peer? If this hits zero,
+   * we must tell communicators to stop providing us more messages
+   * for this peer.  In fact, the window can go negative as we
+   * have multiple communicators, so per communicator we can go
+   * down by one into the negative range. Furthermore, we count
+   * delivery per CORE client, so if we had multiple cores, that
+   * might also cause a negative window size here (as one message
+   * would decrement the window by one per CORE client).
+   */
+  int core_recv_window;
+};
+
+
+/**
+ * Data structure kept when we are waiting for an acknowledgement.
+ */
+struct PendingAcknowledgement
+{
+  /**
+   * If @e pm is non-NULL, this is the DLL in which this acknowledgement
+   * is kept in relation to its pending message.
+   */
+  struct PendingAcknowledgement *next_pm;
+
+  /**
+   * If @e pm is non-NULL, this is the DLL in which this acknowledgement
+   * is kept in relation to its pending message.
+   */
+  struct PendingAcknowledgement *prev_pm;
+
+  /**
+   * If @e queue is non-NULL, this is the DLL in which this acknowledgement
+   * is kept in relation to the queue that was used to transmit the
+   * @a pm.
+   */
+  struct PendingAcknowledgement *next_queue;
+
+  /**
+   * If @e queue is non-NULL, this is the DLL in which this acknowledgement
+   * is kept in relation to the queue that was used to transmit the
+   * @a pm.
+   */
+  struct PendingAcknowledgement *prev_queue;
+
+  /**
+   * If @e dvh is non-NULL, this is the DLL in which this acknowledgement
+   * is kept in relation to the DVH that was used to transmit the
+   * @a pm.
+   */
+  struct PendingAcknowledgement *next_dvh;
+
+  /**
+   * If @e dvh is non-NULL, this is the DLL in which this acknowledgement
+   * is kept in relation to the DVH that was used to transmit the
+   * @a pm.
+   */
+  struct PendingAcknowledgement *prev_dvh;
+
+  /**
+   * Pointers for the DLL of all pending acknowledgements.
+   * This list is sorted by @e transmission time.  If the list gets too
+   * long, the oldest entries are discarded.
+   */
+  struct PendingAcknowledgement *next_pa;
+
+  /**
+   * Pointers for the DLL of all pending acknowledgements.
+   * This list is sorted by @e transmission time.  If the list gets too
+   * long, the oldest entries are discarded.
+   */
+  struct PendingAcknowledgement *prev_pa;
+
+  /**
+   * Unique identifier for this transmission operation.
+   */
+  struct AcknowledgementUUIDP ack_uuid;
+
+  /**
+   * Message that was transmitted, may be NULL if the message was ACKed
+   * via another channel.
+   */
+  struct PendingMessage *pm;
+
+  /**
+   * Distance vector path chosen for this transmission, NULL if transmission
+   * was to a direct neighbour OR if the path was forgotten in the meantime.
+   */
+  struct DistanceVectorHop *dvh;
+
+  /**
+   * Queue used for transmission, NULL if the queue has been destroyed
+   * (which may happen before we get an acknowledgement).
+   */
+  struct Queue *queue;
+
+  /**
+   * Time of the transmission, for RTT calculation.
+   */
+  struct GNUNET_TIME_Absolute transmission_time;
+
+  /**
+   * Number of bytes of the original message (to calculate bandwidth).
+   */
+  uint16_t message_size;
+
+  /**
+   * How often the PendingMessage was send via the Queue of this PendingAcknowledgement.
+   */
+  unsigned int num_send;
+};
+
+
+/**
+ * One possible hop towards a DV target.
+ */
+struct DistanceVectorHop
+{
+  /**
+   * Kept in a MDLL, sorted by @e timeout.
+   */
+  struct DistanceVectorHop *next_dv;
+
+  /**
+   * Kept in a MDLL, sorted by @e timeout.
+   */
+  struct DistanceVectorHop *prev_dv;
+
+  /**
+   * Kept in a MDLL.
+   */
+  struct DistanceVectorHop *next_neighbour;
+
+  /**
+   * Kept in a MDLL.
+   */
+  struct DistanceVectorHop *prev_neighbour;
+
+  /**
+   * Head of DLL of PAs that used our @a path.
+   */
+  struct PendingAcknowledgement *pa_head;
+
+  /**
+   * Tail of DLL of PAs that used our @a path.
+   */
+  struct PendingAcknowledgement *pa_tail;
+
+  /**
+   * What would be the next hop to @e target?
+   */
+  struct Neighbour *next_hop;
+
+  /**
+   * Distance vector entry this hop belongs with.
+   */
+  struct DistanceVector *dv;
+
+  /**
+   * Array of @e distance hops to the target, excluding @e next_hop.
+   * NULL if the entire path is us to @e next_hop to `target`. Allocated
+   * at the end of this struct. Excludes the target itself!
+   */
+  const struct GNUNET_PeerIdentity *path;
+
+  /**
+   * At what time do we forget about this path unless we see it again
+   * while learning?
+   */
+  struct GNUNET_TIME_Absolute timeout;
+
+  /**
+   * For how long is the validation of this path considered
+   * valid?
+   * Set to ZERO if the path is learned by snooping on DV learn messages
+   * initiated by other peers, and to the time at which we generated the
+   * challenge for DV learn operations this peer initiated.
+   */
+  struct GNUNET_TIME_Absolute path_valid_until;
+
+  /**
+   * Performance data for this transmission possibility.
+   */
+  struct PerformanceData pd;
+
+  /**
+   * Number of hops in total to the `target` (excluding @e next_hop and `target`
+   * itself). Thus 0 still means a distance of 2 hops (to @e next_hop and then
+   * to `target`).
+   */
+  unsigned int distance;
+};
+
+
+/**
+ * Entry in our #dv_routes table, representing a (set of) distance
+ * vector routes to a particular peer.
+ */
+struct DistanceVector
+{
+  /**
+   * To which peer is this a route?
+   */
+  struct GNUNET_PeerIdentity target;
+
+  /**
+   * Known paths to @e target.
+   */
+  struct DistanceVectorHop *dv_head;
+
+  /**
+   * Known paths to @e target.
+   */
+  struct DistanceVectorHop *dv_tail;
+
+  /**
+   * Task scheduled to purge expired paths from @e dv_head MDLL.
+   */
+  struct GNUNET_SCHEDULER_Task *timeout_task;
+
+  /**
+   * Do we have a confirmed working queue and are thus visible to
+   * CORE?  If so, this is the virtual link, otherwise NULL.
+   */
+  struct VirtualLink *vl;
+
+  /**
+   * Signature affirming @e ephemeral_key of type
+   * #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_EPHEMERAL
+   */
+  struct GNUNET_CRYPTO_EddsaSignature sender_sig;
+
+  /**
+   * How long is @e sender_sig valid
+   */
+  struct GNUNET_TIME_Absolute ephemeral_validity;
+
+  /**
+   * What time was @e sender_sig created
+   */
+  struct GNUNET_TIME_Absolute monotime;
+
+  /**
+   * Our ephemeral key.
+   */
+  struct GNUNET_CRYPTO_EcdhePublicKey ephemeral_key;
+
+  /**
+   * Master secret for the setup of the Key material for the backchannel.
+   */
+  struct GNUNET_HashCode *km;
+};
+
+
+/**
+ * Entry identifying transmission in one of our `struct
+ * Queue` which still awaits an ACK.  This is used to
+ * ensure we do not overwhelm a communicator and limit the number of
+ * messages outstanding per communicator (say in case communicator is
+ * CPU bound) and per queue (in case bandwidth allocation exceeds
+ * what the communicator can actually provide towards a particular
+ * peer/target).
+ */
+struct QueueEntry
+{
+  /**
+   * Kept as a DLL.
+   */
+  struct QueueEntry *next;
+
+  /**
+   * Kept as a DLL.
+   */
+  struct QueueEntry *prev;
+
+  /**
+   * Queue this entry is queued with.
+   */
+  struct Queue *queue;
+
+  /**
+   * Pending message this entry is for, or NULL for none.
+   */
+  struct PendingMessage *pm;
+
+  /**
+   * Message ID used for this message with the queue used for transmission.
+   */
+  uint64_t mid;
+};
+
+
+/**
+ * A queue is a message queue provided by a communicator
+ * via which we can reach a particular neighbour.
+ */
+struct Queue
+{
+  /**
+   * Kept in a MDLL.
+   */
+  struct Queue *next_neighbour;
+
+  /**
+   * Kept in a MDLL.
+   */
+  struct Queue *prev_neighbour;
+
+  /**
+   * Kept in a MDLL.
+   */
+  struct Queue *prev_client;
+
+  /**
+   * Kept in a MDLL.
+   */
+  struct Queue *next_client;
+
+  /**
+   * Head of DLL of PAs that used this queue.
+   */
+  struct PendingAcknowledgement *pa_head;
+
+  /**
+   * Tail of DLL of PAs that used this queue.
+   */
+  struct PendingAcknowledgement *pa_tail;
+
+  /**
+   * Head of DLL of unacked transmission requests.
+   */
+  struct QueueEntry *queue_head;
+
+  /**
+   * End of DLL of unacked transmission requests.
+   */
+  struct QueueEntry *queue_tail;
+
+  /**
+   * Which neighbour is this queue for?
+   */
+  struct Neighbour *neighbour;
+
+  /**
+   * Which communicator offers this queue?
+   */
+  struct TransportClient *tc;
+
+  /**
+   * Address served by the queue.
+   */
+  const char *address;
+
+  /**
+   * Is this queue of unlimited length.
+   */
+  unsigned int unlimited_length;
+
+  /**
+   * Task scheduled for the time when this queue can (likely) transmit the
+   * next message.
+   */
+  struct GNUNET_SCHEDULER_Task *transmit_task;
+
+  /**
+   * How long do *we* consider this @e address to be valid?  In the past or
+   * zero if we have not yet validated it.  Can be updated based on
+   * challenge-response validations (via address validation logic), or when we
+   * receive ACKs that we can definitively map to transmissions via this
+   * queue.
+   */
+  struct GNUNET_TIME_Absolute validated_until;
+
+  /**
+   * Performance data for this queue.
+   */
+  struct PerformanceData pd;
+
+  /**
+   * Message ID generator for transmissions on this queue to the
+   * communicator.
+   */
+  uint64_t mid_gen;
+
+  /**
+   * Unique identifier of this queue with the communicator.
+   */
+  uint32_t qid;
+
+  /**
+   * Maximum transmission unit supported by this queue.
+   */
+  uint32_t mtu;
+
+  /**
+   * Messages pending.
+   */
+  uint32_t num_msg_pending;
+
+  /**
+   * Bytes pending.
+   */
+  uint32_t num_bytes_pending;
+
+  /**
+   * Length of the DLL starting at @e queue_head.
+   */
+  unsigned int queue_length;
+
+  /**
+   * Capacity of the queue.
+   */
+  uint64_t q_capacity;
+
+  /**
+   * Queue priority
+   */
+  uint32_t priority;
+
+  /**
+   * Network type offered by this queue.
+   */
+  enum GNUNET_NetworkType nt;
+
+  /**
+   * Connection status for this queue.
+   */
+  enum GNUNET_TRANSPORT_ConnectionStatus cs;
+
+  /**
+   * Set to #GNUNET_YES if this queue is idle waiting for some
+   * virtual link to give it a pending message.
+   */
+  int idle;
+};
+
+
+/**
+ * A neighbour that at least one communicator is connected to.
+ */
+struct Neighbour
+{
+  /**
+   * Which peer is this about?
+   */
+  struct GNUNET_PeerIdentity pid;
+
+  /**
+   * Head of MDLL of DV hops that have this neighbour as next hop. Must be
+   * purged if this neighbour goes down.
+   */
+  struct DistanceVectorHop *dv_head;
+
+  /**
+   * Tail of MDLL of DV hops that have this neighbour as next hop. Must be
+   * purged if this neighbour goes down.
+   */
+  struct DistanceVectorHop *dv_tail;
+
+  /**
+   * Head of DLL of queues to this peer.
+   */
+  struct Queue *queue_head;
+
+  /**
+   * Tail of DLL of queues to this peer.
+   */
+  struct Queue *queue_tail;
+
+  /**
+   * Handle for an operation to fetch @e last_dv_learn_monotime information from
+   * the PEERSTORE, or NULL.
+   */
+  struct GNUNET_PEERSTORE_IterateContext *get;
+
+  /**
+   * Handle to a PEERSTORE store operation to store this @e pid's @e
+   * @e last_dv_learn_monotime.  NULL if no PEERSTORE operation is pending.
+   */
+  struct GNUNET_PEERSTORE_StoreContext *sc;
+
+  /**
+   * Do we have a confirmed working queue and are thus visible to
+   * CORE?  If so, this is the virtual link, otherwise NULL.
+   */
+  struct VirtualLink *vl;
+
+  /**
+   * Latest DVLearn monotonic time seen from this peer.  Initialized only
+   * if @e dl_monotime_available is #GNUNET_YES.
+   */
+  struct GNUNET_TIME_Absolute last_dv_learn_monotime;
+
+  /**
+   * Do we have the latest value for @e last_dv_learn_monotime from
+   * PEERSTORE yet, or are we still waiting for a reply of PEERSTORE?
+   */
+  int dv_monotime_available;
+};
+
+
+/**
+ * Another peer attempted to talk to us, we should try to establish
+ * a connection in the other direction.
+ */
+struct IncomingRequest
+{
+  /**
+   * Kept in a DLL.
+   */
+  struct IncomingRequest *next;
+
+  /**
+   * Kept in a DLL.
+   */
+  struct IncomingRequest *prev;
+
+  /**
+   * Notify context for new HELLOs.
+   */
+  struct GNUNET_PEERSTORE_NotifyContext *nc;
+
+  /**
+   * Which peer is this about?
+   */
+  struct GNUNET_PeerIdentity pid;
+};
+
+
+/**
+ * A peer that an application (client) would like us to talk to directly.
+ */
+struct PeerRequest
+{
+  /**
+   * Which peer is this about?
+   */
+  struct GNUNET_PeerIdentity pid;
+
+  /**
+   * Client responsible for the request.
+   */
+  struct TransportClient *tc;
+
+  /**
+   * Notify context for new HELLOs.
+   */
+  struct GNUNET_PEERSTORE_NotifyContext *nc;
+
+  /**
+   * What kind of performance preference does this @e tc have?
+   *
+   * TODO: use this!
+   */
+  enum GNUNET_MQ_PriorityPreferences pk;
+
+  /**
+   * How much bandwidth would this @e tc like to see?
+   */
+  struct GNUNET_BANDWIDTH_Value32NBO bw;
+};
+
+
+/**
+ * Types of different pending messages.
+ */
+enum PendingMessageType
+{
+  /**
+   * Ordinary message received from the CORE service.
+   */
+  PMT_CORE = 0,
+
+  /**
+   * Fragment box.
+   */
+  PMT_FRAGMENT_BOX = 1,
+
+  /**
+   * Reliability box.
+   */
+  PMT_RELIABILITY_BOX = 2,
+
+  /**
+   * Pending message created during #forward_dv_box().
+   */
+  PMT_DV_BOX = 3
+};
+
+
+/**
+ * Transmission request that is awaiting delivery.  The original
+ * transmission requests from CORE may be too big for some queues.
+ * In this case, a *tree* of fragments is created.  At each
+ * level of the tree, fragments are kept in a DLL ordered by which
+ * fragment should be sent next (at the head).  The tree is searched
+ * top-down, with the original message at the root.
+ *
+ * To select a node for transmission, first it is checked if the
+ * current node's message fits with the MTU.  If it does not, we
+ * either calculate the next fragment (based on @e frag_off) from the
+ * current node, or, if all fragments have already been created,
+ * descend to the @e head_frag.  Even though the node was already
+ * fragmented, the fragment may be too big if the fragment was
+ * generated for a queue with a larger MTU. In this case, the node
+ * may be fragmented again, thus creating a tree.
+ *
+ * When acknowledgements for fragments are received, the tree
+ * must be pruned, removing those parts that were already
+ * acknowledged.  When fragments are sent over a reliable
+ * channel, they can be immediately removed.
+ *
+ * If a message is ever fragmented, then the original "full" message
+ * is never again transmitted (even if it fits below the MTU), and
+ * only (remaining) fragments are sent.
+ */
+struct PendingMessage
+{
+  /**
+   * Kept in a MDLL of messages for this @a vl.
+   */
+  struct PendingMessage *next_vl;
+
+  /**
+   * Kept in a MDLL of messages for this @a vl.
+   */
+  struct PendingMessage *prev_vl;
+
+  /**
+   * Kept in a MDLL of messages from this @a client (if @e pmt is #PMT_CORE)
+   */
+  struct PendingMessage *next_client;
+
+  /**
+   * Kept in a MDLL of messages from this @a client  (if @e pmt is #PMT_CORE)
+   */
+  struct PendingMessage *prev_client;
+
+  /**
+   * Kept in a MDLL of messages from this @a cpm (if @e pmt is
+   * #PMT_FRAGMENT_BOx)
+   */
+  struct PendingMessage *next_frag;
+
+  /**
+   * Kept in a MDLL of messages from this @a cpm  (if @e pmt is
+   * #PMT_FRAGMENT_BOX)
+   */
+  struct PendingMessage *prev_frag;
+
+  /**
+   * Head of DLL of PAs for this pending message.
+   */
+  struct PendingAcknowledgement *pa_head;
+
+  /**
+   * Tail of DLL of PAs for this pending message.
+   */
+  struct PendingAcknowledgement *pa_tail;
+
+  /**
+   * This message, reliability *or* DV-boxed. Only possibly available
+   * if @e pmt is #PMT_CORE.
+   */
+  struct PendingMessage *bpm;
+
+  /**
+   * Target of the request (always the ultimate destination!).
+   * Might be NULL in case of a forwarded DVBox we have no validated neighbour.
+   */
+  struct VirtualLink *vl;
+
+  /**
+   * In case of a not validated neighbour, we store the target peer.
+   **/
+  struct GNUNET_PeerIdentity target;
+
+  /**
+   * Set to non-NULL value if this message is currently being given to a
+   * communicator and we are awaiting that communicator's acknowledgement.
+   * Note that we must not retransmit a pending message while we're still
+   * in the process of giving it to a communicator. If a pending message
+   * is free'd while this entry is non-NULL, the @e qe reference to us
+   * should simply be set to NULL.
+   */
+  struct QueueEntry *qe;
+
+  /**
+   * Client that issued the transmission request, if @e pmt is #PMT_CORE.
+   */
+  struct TransportClient *client;
+
+  /**
+   * Head of a MDLL of fragments created for this core message.
+   */
+  struct PendingMessage *head_frag;
+
+  /**
+   * Tail of a MDLL of fragments created for this core message.
+   */
+  struct PendingMessage *tail_frag;
+
+  /**
+   * Our parent in the fragmentation tree.
+   */
+  struct PendingMessage *frag_parent;
+
+  /**
+   * At what time should we give up on the transmission (and no longer retry)?
+   */
+  struct GNUNET_TIME_Absolute timeout;
+
+  /**
+   * What is the earliest time for us to retry transmission of this message?
+   */
+  struct GNUNET_TIME_Absolute next_attempt;
+
+  /**
+   * UUID to use for this message (used for reassembly of fragments, only
+   * initialized if @e msg_uuid_set is #GNUNET_YES).
+   */
+  struct MessageUUIDP msg_uuid;
+
+  /**
+   * UUID we use to identify this message in our logs.
+   * Generated by incrementing the "logging_uuid_gen".
+   */
+  uint64_t logging_uuid;
+
+  /**
+   * Type of the pending message.
+   */
+  enum PendingMessageType pmt;
+
+  /**
+   * Preferences for this message.
+   * TODO: actually use this!
+   */
+  enum GNUNET_MQ_PriorityPreferences prefs;
+
+  /**
+   * If pmt is of type PMT_DV_BOX we store the used path here.
+   */
+  struct DistanceVectorHop *used_dvh;
+
+  /**
+   * Size of the original message.
+   */
+  uint16_t bytes_msg;
+
+  /**
+   * Offset at which we should generate the next fragment.
+   */
+  uint16_t frag_off;
+
+  /**
+   * Are we sending fragments at the moment?
+   */
+  uint32_t frags_in_flight;
+
+  /**
+   * How many fragments do we have?
+   **/
+  uint16_t frag_count;
+
+  /**
+   * #GNUNET_YES once @e msg_uuid was initialized
+   */
+  int16_t msg_uuid_set;
+
+  /* Followed by @e bytes_msg to transmit */
+};
+
+
+/**
+ * Acknowledgement payload.
+ */
+struct TransportCummulativeAckPayload
+{
+  /**
+   * When did we receive the message we are ACKing?  Used to calculate
+   * the delay we introduced by cummulating ACKs.
+   */
+  struct GNUNET_TIME_Absolute receive_time;
+
+  /**
+   * UUID of a message being acknowledged.
+   */
+  struct AcknowledgementUUIDP ack_uuid;
+};
+
+
+/**
+ * Data structure in which we track acknowledgements still to
+ * be sent to the
+ */
+struct AcknowledgementCummulator
+{
+  /**
+   * Target peer for which we are accumulating ACKs here.
+   */
+  struct GNUNET_PeerIdentity target;
+
+  /**
+   * ACK data being accumulated.  Only @e num_acks slots are valid.
+   */
+  struct TransportCummulativeAckPayload ack_uuids[MAX_CUMMULATIVE_ACKS];
+
+  /**
+   * Task scheduled either to transmit the cumulative ACK message,
+   * or to clean up this data structure after extended periods of
+   * inactivity (if @e num_acks is zero).
+   */
+  struct GNUNET_SCHEDULER_Task *task;
+
+  /**
+   * When is @e task run (only used if @e num_acks is non-zero)?
+   */
+  struct GNUNET_TIME_Absolute min_transmission_time;
+
+  /**
+   * Counter to produce the `ack_counter` in the `struct
+   * TransportReliabilityAckMessage`.  Allows the receiver to detect
+   * lost ACK messages.  Incremented by @e num_acks upon transmission.
+   */
+  uint32_t ack_counter;
+
+  /**
+   * Number of entries used in @e ack_uuids.  Reset to 0 upon transmission.
+   */
+  unsigned int num_acks;
+};
+
+
+/**
+ * One of the addresses of this peer.
+ */
+struct AddressListEntry
+{
+  /**
+   * Kept in a DLL.
+   */
+  struct AddressListEntry *next;
+
+  /**
+   * Kept in a DLL.
+   */
+  struct AddressListEntry *prev;
+
+  /**
+   * Which communicator provides this address?
+   */
+  struct TransportClient *tc;
+
+  /**
+   * Store hello handle
+   */
+  struct GNUNET_PEERSTORE_StoreHelloContext *shc;
+
+  /**
+   * The actual address.
+   */
+  const char *address;
+
+  /**
+   * Signed address
+   */
+  void *signed_address;
+
+  /**
+   * Signed address length
+   */
+  size_t signed_address_len;
+
+  /**
+   * Current context for storing this address in the peerstore.
+   */
+  struct GNUNET_PEERSTORE_StoreContext *sc;
+
+  /**
+   * Task to periodically do @e st operation.
+   */
+  struct GNUNET_SCHEDULER_Task *st;
+
+  /**
+   * What is a typical lifetime the communicator expects this
+   * address to have? (Always from now.)
+   */
+  struct GNUNET_TIME_Relative expiration;
+
+  /**
+   * Address identifier used by the communicator.
+   */
+  uint32_t aid;
+
+  /**
+   * Network type offered by this address.
+   */
+  enum GNUNET_NetworkType nt;
+};
+
+
+/**
+ * Client connected to the transport service.
+ */
+struct TransportClient
+{
+  /**
+   * Kept in a DLL.
+   */
+  struct TransportClient *next;
+
+  /**
+   * Kept in a DLL.
+   */
+  struct TransportClient *prev;
+
+  /**
+   * Handle to the client.
+   */
+  struct GNUNET_SERVICE_Client *client;
+
+  /**
+   * Message queue to the client.
+   */
+  struct GNUNET_MQ_Handle *mq;
+
+  /**
+   * What type of client is this?
+   */
+  enum ClientType type;
+
+  union
+  {
+    /**
+     * Information for @e type #CT_CORE.
+     */
+    struct
+    {
+      /**
+       * Head of list of messages pending for this client, sorted by
+       * transmission time ("next_attempt" + possibly internal prioritization).
+       */
+      struct PendingMessage *pending_msg_head;
+
+      /**
+       * Tail of list of messages pending for this client.
+       */
+      struct PendingMessage *pending_msg_tail;
+    } core;
+
+    /**
+     * Information for @e type #CT_MONITOR.
+     */
+    struct
+    {
+      /**
+       * Peer identity to monitor the addresses of.
+       * Zero to monitor all neighbours.  Valid if
+       * @e type is #CT_MONITOR.
+       */
+      struct GNUNET_PeerIdentity peer;
+
+      /**
+       * Is this a one-shot monitor?
+       */
+      int one_shot;
+    } monitor;
+
+
+    /**
+     * Information for @e type #CT_COMMUNICATOR.
+     */
+    struct
+    {
+      /**
+       * If @e type is #CT_COMMUNICATOR, this communicator
+       * supports communicating using these addresses.
+       */
+      char *address_prefix;
+
+      /**
+       * Head of DLL of queues offered by this communicator.
+       */
+      struct Queue *queue_head;
+
+      /**
+       * Tail of DLL of queues offered by this communicator.
+       */
+      struct Queue *queue_tail;
+
+      /**
+       * Head of list of the addresses of this peer offered by this
+       * communicator.
+       */
+      struct AddressListEntry *addr_head;
+
+      /**
+       * Tail of list of the addresses of this peer offered by this
+       * communicator.
+       */
+      struct AddressListEntry *addr_tail;
+
+      /**
+       * Number of queue entries in all queues to this communicator. Used
+       * throttle sending to a communicator if we see that the communicator
+       * is globally unable to keep up.
+       */
+      unsigned int total_queue_length;
+
+      /**
+       * Characteristics of this communicator.
+       */
+      enum GNUNET_TRANSPORT_CommunicatorCharacteristics cc;
+    } communicator;
+
+    /**
+     * Information for @e type #CT_APPLICATION
+     */
+    struct
+    {
+      /**
+       * Map of requests for peers the given client application would like to
+       * see connections for.  Maps from PIDs to `struct PeerRequest`.
+       */
+      struct GNUNET_CONTAINER_MultiPeerMap *requests;
+    } application;
+  } details;
+};
+
+
+/**
+ * State we keep for validation activities.  Each of these
+ * is both in the #validation_heap and the #validation_map.
+ */
+struct ValidationState
+{
+  /**
+   * For which peer is @a address to be validated (or possibly valid)?
+   * Serves as key in the #validation_map.
+   */
+  struct GNUNET_PeerIdentity pid;
+
+  /**
+   * How long did the peer claim this @e address to be valid? Capped at
+   * minimum of #MAX_ADDRESS_VALID_UNTIL relative to the time where we last
+   * were told about the address and the value claimed by the other peer at
+   * that time.  May be updated similarly when validation succeeds.
+   */
+  struct GNUNET_TIME_Absolute valid_until;
+
+  /**
+   * How long do *we* consider this @e address to be valid?
+   * In the past or zero if we have not yet validated it.
+   */
+  struct GNUNET_TIME_Absolute validated_until;
+
+  /**
+   * When did we FIRST use the current @e challenge in a message?
+   * Used to sanity-check @code{origin_time} in the response when
+   * calculating the RTT. If the @code{origin_time} is not in
+   * the expected range, the response is discarded as malicious.
+   */
+  struct GNUNET_TIME_Absolute first_challenge_use;
+
+  /**
+   * When did we LAST use the current @e challenge in a message?
+   * Used to sanity-check @code{origin_time} in the response when
+   * calculating the RTT.  If the @code{origin_time} is not in
+   * the expected range, the response is discarded as malicious.
+   */
+  struct GNUNET_TIME_Absolute last_challenge_use;
+
+  /**
+   * Next time we will send the @e challenge to the peer, if this time is past
+   * @e valid_until, this validation state is released at this time.  If the
+   * address is valid, @e next_challenge is set to @e validated_until MINUS @e
+   * validation_delay * #VALIDATION_RTT_BUFFER_FACTOR, such that we will try
+   * to re-validate before the validity actually expires.
+   */
+  struct GNUNET_TIME_Absolute next_challenge;
+
+  /**
+   * Current backoff factor we're applying for sending the @a challenge.
+   * Reset to 0 if the @a challenge is confirmed upon validation.
+   * Reduced to minimum of #FAST_VALIDATION_CHALLENGE_FREQ and half of the
+   * existing value if we receive an unvalidated address again over
+   * another channel (and thus should consider the information "fresh").
+   * Maximum is #MAX_VALIDATION_CHALLENGE_FREQ.
+   */
+  struct GNUNET_TIME_Relative challenge_backoff;
+
+  /**
+   * Initially set to "forever". Once @e validated_until is set, this value is
+   * set to the RTT that tells us how long it took to receive the validation.
+   */
+  struct GNUNET_TIME_Relative validation_rtt;
+
+  /**
+   * The challenge we sent to the peer to get it to validate the address. Note
+   * that we rotate the challenge whenever we update @e validated_until to
+   * avoid attacks where a peer simply replays an old challenge in the future.
+   * (We must not rotate more often as otherwise we may discard valid answers
+   * due to packet losses, latency and reorderings on the network).
+   */
+  struct GNUNET_CRYPTO_ChallengeNonceP challenge;
+
+  /**
+   * Claimed address of the peer.
+   */
+  char *address;
+
+  /**
+   * Entry in the #validation_heap, which is sorted by @e next_challenge. The
+   * heap is used to figure out when the next validation activity should be
+   * run.
+   */
+  struct GNUNET_CONTAINER_HeapNode *hn;
+
+  /**
+   * Handle to a PEERSTORE store operation for this @e address.  NULL if
+   * no PEERSTORE operation is pending.
+   */
+  struct GNUNET_PEERSTORE_StoreContext *sc;
+
+  /**
+   * Self-imposed limit on the previous flow control window. (May be zero,
+   * if we never used data from the previous window or are establishing the
+   * connection for the first time).
+   */
+  uint32_t last_window_consum_limit;
+
+  /**
+   * We are technically ready to send the challenge, but we are waiting for
+   * the respective queue to become available for transmission.
+   */
+  int awaiting_queue;
+};
+
+
+/**
+ * A Backtalker is a peer sending us backchannel messages. We use this
+ * struct to detect monotonic time violations, cache ephemeral key
+ * material (to avoid repeatedly checking signatures), and to synchronize
+ * monotonic time with the PEERSTORE.
+ */
+struct Backtalker
+{
+  /**
+   * Peer this is about.
+   */
+  struct GNUNET_PeerIdentity pid;
+
+  /**
+   * Last (valid) monotonic time received from this sender.
+   */
+  struct GNUNET_TIME_Absolute monotonic_time;
+
+  /**
+   * When will this entry time out?
+   */
+  struct GNUNET_TIME_Absolute timeout;
+
+  /**
+   * Last (valid) ephemeral key received from this sender.
+   */
+  struct GNUNET_CRYPTO_EcdhePublicKey last_ephemeral;
+
+  /**
+   * Task associated with this backtalker. Can be for timeout,
+   * or other asynchronous operations.
+   */
+  struct GNUNET_SCHEDULER_Task *task;
+
+  /**
+   * Communicator context waiting on this backchannel's @e get, or NULL.
+   */
+  struct CommunicatorMessageContext *cmc;
+
+  /**
+   * Handle for an operation to fetch @e monotonic_time information from the
+   * PEERSTORE, or NULL.
+   */
+  struct GNUNET_PEERSTORE_IterateContext *get;
+
+  /**
+   * Handle to a PEERSTORE store operation for this @e pid's @e
+   * monotonic_time.  NULL if no PEERSTORE operation is pending.
+   */
+  struct GNUNET_PEERSTORE_StoreContext *sc;
+
+  /**
+   * Number of bytes of the original message body that follows after this
+   * struct.
+   */
+  size_t body_size;
+};
+
+
+/**
+ * Ring buffer for a CORE message we did not deliver to CORE, because of missing virtual link to sender.
+ */
+static struct RingBufferEntry *ring_buffer[RING_BUFFER_SIZE];
+
+/**
+ * Head of the ring buffer.
+ */
+static unsigned int ring_buffer_head;
+
+/**
+ * Is the ring buffer filled up to RING_BUFFER_SIZE.
+ */
+static unsigned int is_ring_buffer_full;
+
+/**
+ * Ring buffer for a forwarded DVBox message we did not deliver to the next hop, because of missing virtual link that hop.
+ */
+static struct PendingMessage *ring_buffer_dv[RING_BUFFER_SIZE];
+
+/**
+ * Head of the ring buffer.
+ */
+static unsigned int ring_buffer_dv_head;
+
+/**
+ * Is the ring buffer filled up to RING_BUFFER_SIZE.
+ */
+static unsigned int is_ring_buffer_dv_full;
+
+/**
+ * Head of linked list of all clients to this service.
+ */
+static struct TransportClient *clients_head;
+
+/**
+ * Tail of linked list of all clients to this service.
+ */
+static struct TransportClient *clients_tail;
+
+/**
+ * Statistics handle.
+ */
+static struct GNUNET_STATISTICS_Handle *GST_stats;
+
+/**
+ * Configuration handle.
+ */
+static const struct GNUNET_CONFIGURATION_Handle *GST_cfg;
+
+/**
+ * Our public key.
+ */
+static struct GNUNET_PeerIdentity GST_my_identity;
+
+/**
+ * Our HELLO
+ */
+struct GNUNET_HELLO_Builder *GST_my_hello;
+
+/**
+ * Our private key.
+ */
+static struct GNUNET_CRYPTO_EddsaPrivateKey *GST_my_private_key;
+
+/**
+ * Map from PIDs to `struct Neighbour` entries.  A peer is
+ * a neighbour if we have an MQ to it from some communicator.
+ */
+static struct GNUNET_CONTAINER_MultiPeerMap *neighbours;
+
+/**
+ * Map from PIDs to `struct Backtalker` entries.  A peer is
+ * a backtalker if it recently send us backchannel messages.
+ */
+static struct GNUNET_CONTAINER_MultiPeerMap *backtalkers;
+
+/**
+ * Map from PIDs to `struct AcknowledgementCummulator`s.
+ * Here we track the cumulative ACKs for transmission.
+ */
+static struct GNUNET_CONTAINER_MultiPeerMap *ack_cummulators;
+
+/**
+ * Map of pending acknowledgements, mapping `struct AcknowledgementUUID` to
+ * a `struct PendingAcknowledgement`.
+ */
+static struct GNUNET_CONTAINER_MultiUuidmap *pending_acks;
+
+/**
+ * Map from PIDs to `struct DistanceVector` entries describing
+ * known paths to the peer.
+ */
+static struct GNUNET_CONTAINER_MultiPeerMap *dv_routes;
+
+/**
+ * Map from PIDs to `struct ValidationState` entries describing
+ * addresses we are aware of and their validity state.
+ */
+static struct GNUNET_CONTAINER_MultiPeerMap *validation_map;
+
+/**
+ * Map from PIDs to `struct VirtualLink` entries describing
+ * links CORE knows to exist.
+ */
+static struct GNUNET_CONTAINER_MultiPeerMap *links;
+
+/**
+ * Map from challenges to `struct LearnLaunchEntry` values.
+ */
+static struct GNUNET_CONTAINER_MultiShortmap *dvlearn_map;
+
+/**
+ * Head of a DLL sorted by launch time.
+ */
+static struct LearnLaunchEntry *lle_head = NULL;
+
+/**
+ * Tail of a DLL sorted by launch time.
+ */
+static struct LearnLaunchEntry *lle_tail = NULL;
+
+/**
+ * MIN Heap sorted by "next_challenge" to `struct ValidationState` entries
+ * sorting addresses we are aware of by when we should next try to (re)validate
+ * (or expire) them.
+ */
+static struct GNUNET_CONTAINER_Heap *validation_heap;
+
+/**
+ * Database for peer's HELLOs.
+ */
+static struct GNUNET_PEERSTORE_Handle *peerstore;
+
+/**
+ * Task run to initiate DV learning.
+ */
+static struct GNUNET_SCHEDULER_Task *dvlearn_task;
+
+/**
+ * Task to run address validation.
+ */
+static struct GNUNET_SCHEDULER_Task *validation_task;
+
+/**
+ * List of incoming connections where we are trying
+ * to get a connection back established. Length
+ * kept in #ir_total.
+ */
+static struct IncomingRequest *ir_head;
+
+/**
+ * Tail of DLL starting at #ir_head.
+ */
+static struct IncomingRequest *ir_tail;
+
+/**
+ * Length of the DLL starting at #ir_head.
+ */
+static unsigned int ir_total;
+
+/**
+ * Generator of `logging_uuid` in `struct PendingMessage`.
+ */
+static unsigned long long logging_uuid_gen;
+
+/**
+ * Monotonic time we use for HELLOs generated at this time.  TODO: we
+ * should increase this value from time to time (i.e. whenever a
+ * `struct AddressListEntry` actually expires), but IF we do this, we
+ * must also update *all* (remaining) addresses in the PEERSTORE at
+ * that time! (So for now only increased when the peer is restarted,
+ * which hopefully roughly matches whenever our addresses change.)
+ */
+static struct GNUNET_TIME_Absolute hello_mono_time;
+
+/**
+ * Indication if we have received a shutdown signal
+ * and are in the process of cleaning up.
+ */
+static int in_shutdown;
+
+/**
+ * Get an offset into the transmission history buffer for `struct
+ * PerformanceData`.  Note that the caller must perform the required
+ * modulo #GOODPUT_AGING_SLOTS operation before indexing into the
+ * array!
+ *
+ * An 'age' lasts 15 minute slots.
+ *
+ * @return current age of the world
+ */
+static unsigned int
+get_age ()
+{
+  struct GNUNET_TIME_Absolute now;
+
+  now = GNUNET_TIME_absolute_get ();
+  return now.abs_value_us / GNUNET_TIME_UNIT_MINUTES.rel_value_us / 15;
+}
+
+
+/**
+ * Release @a ir data structure.
+ *
+ * @param ir data structure to release
+ */
+static void
+free_incoming_request (struct IncomingRequest *ir)
+{
+  GNUNET_CONTAINER_DLL_remove (ir_head, ir_tail, ir);
+  GNUNET_assert (ir_total > 0);
+  ir_total--;
+  GNUNET_PEERSTORE_hello_changed_notify_cancel (ir->nc);
+  ir->nc = NULL;
+  GNUNET_free (ir);
+}
+
+
+/**
+ * Release @a pa data structure.
+ *
+ * @param pa data structure to release
+ */
+static void
+free_pending_acknowledgement (struct PendingAcknowledgement *pa)
+{
+  struct Queue *q = pa->queue;
+  struct PendingMessage *pm = pa->pm;
+  struct DistanceVectorHop *dvh = pa->dvh;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "free_pending_acknowledgement\n");
+  if (NULL != q)
+  {
+    GNUNET_CONTAINER_MDLL_remove (queue, q->pa_head, q->pa_tail, pa);
+    pa->queue = NULL;
+  }
+  if (NULL != pm)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "remove pa from message\n");
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "remove pa from message %" PRIu64 "\n",
+                pm->logging_uuid);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "remove pa from message %u\n",
+                pm->pmt);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "remove pa from message %s\n",
+                GNUNET_uuid2s (&pa->ack_uuid.value));
+    GNUNET_CONTAINER_MDLL_remove (pm, pm->pa_head, pm->pa_tail, pa);
+    pa->pm = NULL;
+  }
+  if (NULL != dvh)
+  {
+    GNUNET_CONTAINER_MDLL_remove (dvh, dvh->pa_head, dvh->pa_tail, pa);
+    pa->queue = NULL;
+  }
+  GNUNET_assert (GNUNET_YES ==
+                 GNUNET_CONTAINER_multiuuidmap_remove (pending_acks,
+                                                       &pa->ack_uuid.value,
+                                                       pa));
+  GNUNET_free (pa);
+}
+
+
+/**
+ * Free fragment tree below @e root, excluding @e root itself.
+ * FIXME: this does NOT seem to have the intended semantics
+ * based on how this is called. Seems we generally DO expect
+ * @a root to be free'ed itself as well!
+ *
+ * @param root root of the tree to free
+ */
+static void
+free_fragment_tree (struct PendingMessage *root)
+{
+  struct PendingMessage *frag;
+
+  while (NULL != (frag = root->head_frag))
+  {
+    struct PendingAcknowledgement *pa;
+
+    free_fragment_tree (frag);
+    while (NULL != (pa = frag->pa_head))
+    {
+      GNUNET_CONTAINER_MDLL_remove (pm, frag->pa_head, frag->pa_tail, pa);
+      pa->pm = NULL;
+    }
+    GNUNET_CONTAINER_MDLL_remove (frag, root->head_frag, root->tail_frag, frag);
+    if (NULL != frag->qe)
+    {
+      GNUNET_assert (frag == frag->qe->pm);
+      frag->qe->pm = NULL;
+    }
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Free frag %p\n",
+                frag);
+    GNUNET_free (frag);
+  }
+}
+
+
+/**
+ * Release memory associated with @a pm and remove @a pm from associated
+ * data structures.  @a pm must be a top-level pending message and not
+ * a fragment in the tree.  The entire tree is freed (if applicable).
+ *
+ * @param pm the pending message to free
+ */
+static void
+free_pending_message (struct PendingMessage *pm)
+{
+  struct TransportClient *tc = pm->client;
+  struct VirtualLink *vl = pm->vl;
+  struct PendingAcknowledgement *pa;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Freeing pm %p\n",
+              pm);
+  if (NULL != tc)
+  {
+    GNUNET_CONTAINER_MDLL_remove (client,
+                                  tc->details.core.pending_msg_head,
+                                  tc->details.core.pending_msg_tail,
+                                  pm);
+  }
+  if ((NULL != vl) && (NULL == pm->frag_parent))
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Removing pm %" PRIu64 "\n",
+                pm->logging_uuid);
+    GNUNET_CONTAINER_MDLL_remove (vl,
+                                  vl->pending_msg_head,
+                                  vl->pending_msg_tail,
+                                  pm);
+  }
+  while (NULL != (pa = pm->pa_head))
+  {
+    if (NULL == pa)
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "free pending pa  null\n");
+    if (NULL == pm->pa_tail)
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "free pending pa_tail null\n");
+    if (NULL == pa->prev_pa)
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "free pending pa prev null\n");
+    if (NULL == pa->next_pa)
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "free pending pa next null\n");
+    GNUNET_CONTAINER_MDLL_remove (pm, pm->pa_head, pm->pa_tail, pa);
+    pa->pm = NULL;
+  }
+
+  free_fragment_tree (pm);
+  if (NULL != pm->qe)
+  {
+    struct QueueEntry *qe = pm->qe;
+
+    GNUNET_assert (pm == pm->qe->pm);
+    pm->qe->pm = NULL;
+    GNUNET_CONTAINER_DLL_remove (qe->queue->queue_head,
+                                 qe->queue->queue_tail,
+                                 qe);
+    qe->queue->queue_length--;
+    GNUNET_free (qe);
+  }
+  if (NULL != pm->bpm)
+  {
+    free_fragment_tree (pm->bpm);
+    if (NULL != pm->bpm->qe)
+    {
+      struct QueueEntry *qe = pm->bpm->qe;
+
+      qe->pm = NULL;
+      GNUNET_CONTAINER_DLL_remove (qe->queue->queue_head,
+                                   qe->queue->queue_tail,
+                                   qe);
+      qe->queue->queue_length--;
+      GNUNET_free (qe);
+    }
+    GNUNET_free (pm->bpm);
+  }
+
+  GNUNET_free (pm);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Freeing pm done\n");
+}
+
+
+/**
+ * Free @a rc
+ *
+ * @param rc data structure to free
+ */
+static void
+free_reassembly_context (struct ReassemblyContext *rc)
+{
+  struct VirtualLink *vl = rc->virtual_link;
+
+  GNUNET_assert (rc == GNUNET_CONTAINER_heap_remove_node (rc->hn));
+  GNUNET_assert (GNUNET_OK ==
+                 GNUNET_CONTAINER_multihashmap32_remove (vl->reassembly_map,
+                                                         rc->msg_uuid.uuid,
+                                                         rc));
+  GNUNET_free (rc);
+}
+
+
+/**
+ * Task run to clean up reassembly context of a neighbour that have expired.
+ *
+ * @param cls a `struct Neighbour`
+ */
+static void
+reassembly_cleanup_task (void *cls)
+{
+  struct VirtualLink *vl = cls;
+  struct ReassemblyContext *rc;
+
+  vl->reassembly_timeout_task = NULL;
+  while (NULL != (rc = GNUNET_CONTAINER_heap_peek (vl->reassembly_heap)))
+  {
+    if (0 == GNUNET_TIME_absolute_get_remaining (rc->reassembly_timeout)
+        .rel_value_us)
+    {
+      free_reassembly_context (rc);
+      continue;
+    }
+    GNUNET_assert (NULL == vl->reassembly_timeout_task);
+    vl->reassembly_timeout_task =
+      GNUNET_SCHEDULER_add_at (rc->reassembly_timeout,
+                               &reassembly_cleanup_task,
+                               vl);
+    return;
+  }
+}
+
+
+/**
+ * function called to #free_reassembly_context().
+ *
+ * @param cls NULL
+ * @param key unused
+ * @param value a `struct ReassemblyContext` to free
+ * @return #GNUNET_OK (continue iteration)
+ */
+static int
+free_reassembly_cb (void *cls, uint32_t key, void *value)
+{
+  struct ReassemblyContext *rc = value;
+
+  (void) cls;
+  (void) key;
+  free_reassembly_context (rc);
+  return GNUNET_OK;
+}
+
+
+/**
+ * Free virtual link.
+ *
+ * @param vl link data to free
+ */
+static void
+free_virtual_link (struct VirtualLink *vl)
+{
+  struct PendingMessage *pm;
+  struct CoreSentContext *csc;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "free virtual link %p\n",
+              vl);
+
+  if (NULL != vl->reassembly_map)
+  {
+    GNUNET_CONTAINER_multihashmap32_iterate (vl->reassembly_map,
+                                             &free_reassembly_cb,
+                                             NULL);
+    GNUNET_CONTAINER_multihashmap32_destroy (vl->reassembly_map);
+    vl->reassembly_map = NULL;
+    GNUNET_CONTAINER_heap_destroy (vl->reassembly_heap);
+    vl->reassembly_heap = NULL;
+  }
+  if (NULL != vl->reassembly_timeout_task)
+  {
+    GNUNET_SCHEDULER_cancel (vl->reassembly_timeout_task);
+    vl->reassembly_timeout_task = NULL;
+  }
+  while (NULL != (pm = vl->pending_msg_head))
+    free_pending_message (pm);
+  GNUNET_assert (GNUNET_YES ==
+                 GNUNET_CONTAINER_multipeermap_remove (links, &vl->target, vl));
+  if (NULL != vl->visibility_task)
+  {
+    GNUNET_SCHEDULER_cancel (vl->visibility_task);
+    vl->visibility_task = NULL;
+  }
+  if (NULL != vl->fc_retransmit_task)
+  {
+    GNUNET_SCHEDULER_cancel (vl->fc_retransmit_task);
+    vl->fc_retransmit_task = NULL;
+  }
+  while (NULL != (csc = vl->csc_head))
+  {
+    GNUNET_CONTAINER_DLL_remove (vl->csc_head, vl->csc_tail, csc);
+    GNUNET_assert (vl == csc->vl);
+    csc->vl = NULL;
+  }
+  GNUNET_break (NULL == vl->n);
+  GNUNET_break (NULL == vl->dv);
+  GNUNET_free (vl);
+}
+
+
+/**
+ * Free validation state.
+ *
+ * @param vs validation state to free
+ */
+static void
+free_validation_state (struct ValidationState *vs)
+{
+  GNUNET_assert (
+    GNUNET_YES ==
+    GNUNET_CONTAINER_multipeermap_remove (validation_map, &vs->pid, vs));
+  GNUNET_CONTAINER_heap_remove_node (vs->hn);
+  vs->hn = NULL;
+  if (NULL != vs->sc)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "store cancel\n");
+    GNUNET_PEERSTORE_store_cancel (vs->sc);
+    vs->sc = NULL;
+  }
+  GNUNET_free (vs->address);
+  GNUNET_free (vs);
+}
+
+
+/**
+ * Lookup neighbour for peer @a pid.
+ *
+ * @param pid neighbour to look for
+ * @return NULL if we do not have this peer as a neighbour
+ */
+static struct Neighbour *
+lookup_neighbour (const struct GNUNET_PeerIdentity *pid)
+{
+  return GNUNET_CONTAINER_multipeermap_get (neighbours, pid);
+}
+
+
+/**
+ * Lookup virtual link for peer @a pid.
+ *
+ * @param pid virtual link to look for
+ * @return NULL if we do not have this peer as a virtual link
+ */
+static struct VirtualLink *
+lookup_virtual_link (const struct GNUNET_PeerIdentity *pid)
+{
+  return GNUNET_CONTAINER_multipeermap_get (links, pid);
+}
+
+
+/**
+ * Details about what to notify monitors about.
+ */
+struct MonitorEvent
+{
+  /**
+   * @deprecated To be discussed if we keep these...
+   */
+  struct GNUNET_TIME_Absolute last_validation;
+  struct GNUNET_TIME_Absolute valid_until;
+  struct GNUNET_TIME_Absolute next_validation;
+
+  /**
+   * Current round-trip time estimate.
+   */
+  struct GNUNET_TIME_Relative rtt;
+
+  /**
+   * Connection status.
+   */
+  enum GNUNET_TRANSPORT_ConnectionStatus cs;
+
+  /**
+   * Messages pending.
+   */
+  uint32_t num_msg_pending;
+
+  /**
+   * Bytes pending.
+   */
+  uint32_t num_bytes_pending;
+};
+
+
+/**
+ * Free a @a dvh. Callers MAY want to check if this was the last path to the
+ * `target`, and if so call #free_dv_route to also free the associated DV
+ * entry in #dv_routes (if not, the associated scheduler job should eventually
+ * take care of it).
+ *
+ * @param dvh hop to free
+ */
+static void
+free_distance_vector_hop (struct DistanceVectorHop *dvh)
+{
+  struct Neighbour *n = dvh->next_hop;
+  struct DistanceVector *dv = dvh->dv;
+  struct PendingAcknowledgement *pa;
+
+  while (NULL != (pa = dvh->pa_head))
+  {
+    GNUNET_CONTAINER_MDLL_remove (dvh, dvh->pa_head, dvh->pa_tail, pa);
+    pa->dvh = NULL;
+  }
+  GNUNET_CONTAINER_MDLL_remove (neighbour, n->dv_head, n->dv_tail, dvh);
+  GNUNET_CONTAINER_MDLL_remove (dv, dv->dv_head, dv->dv_tail, dvh);
+  GNUNET_free (dvh);
+}
+
+
+/**
+ * Task run to check whether the hops of the @a cls still
+ * are validated, or if we need to core about disconnection.
+ *
+ * @param cls a `struct VirtualLink`
+ */
+static void
+check_link_down (void *cls);
+
+
+/**
+ * Send message to CORE clients that we lost a connection.
+ *
+ * @param pid peer the connection was for
+ */
+static void
+cores_send_disconnect_info (const struct GNUNET_PeerIdentity *pid)
+{
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Informing CORE clients about disconnect from %s\n",
+              GNUNET_i2s (pid));
+  for (struct TransportClient *tc = clients_head; NULL != tc; tc = tc->next)
+  {
+    struct GNUNET_MQ_Envelope *env;
+    struct DisconnectInfoMessage *dim;
+
+    if (CT_CORE != tc->type)
+      continue;
+    env = GNUNET_MQ_msg (dim, GNUNET_MESSAGE_TYPE_TRANSPORT_DISCONNECT);
+    dim->peer = *pid;
+    GNUNET_MQ_send (tc->mq, env);
+  }
+}
+
+
+/**
+ * Free entry in #dv_routes.  First frees all hops to the target, and
+ * if there are no entries left, frees @a dv as well.
+ *
+ * @param dv route to free
+ */
+static void
+free_dv_route (struct DistanceVector *dv)
+{
+  struct DistanceVectorHop *dvh;
+
+  while (NULL != (dvh = dv->dv_head))
+    free_distance_vector_hop (dvh);
+  if (NULL == dv->dv_head)
+  {
+    struct VirtualLink *vl;
+
+    GNUNET_assert (
+      GNUNET_YES ==
+      GNUNET_CONTAINER_multipeermap_remove (dv_routes, &dv->target, dv));
+    if (NULL != (vl = dv->vl))
+    {
+      GNUNET_assert (dv == vl->dv);
+      vl->dv = NULL;
+      if (NULL == vl->n)
+      {
+        cores_send_disconnect_info (&dv->target);
+        free_virtual_link (vl);
+      }
+      else
+      {
+        GNUNET_SCHEDULER_cancel (vl->visibility_task);
+        vl->visibility_task = GNUNET_SCHEDULER_add_now (&check_link_down, vl);
+      }
+      dv->vl = NULL;
+    }
+
+    if (NULL != dv->timeout_task)
+    {
+      GNUNET_SCHEDULER_cancel (dv->timeout_task);
+      dv->timeout_task = NULL;
+    }
+    GNUNET_free (dv->km);
+    GNUNET_free (dv);
+  }
+}
+
+
+/**
+ * Notify monitor @a tc about an event.  That @a tc
+ * cares about the event has already been checked.
+ *
+ * Send @a tc information in @a me about a @a peer's status with
+ * respect to some @a address to all monitors that care.
+ *
+ * @param tc monitor to inform
+ * @param peer peer the information is about
+ * @param address address the information is about
+ * @param nt network type associated with @a address
+ * @param me detailed information to transmit
+ */
+static void
+notify_monitor (struct TransportClient *tc,
+                const struct GNUNET_PeerIdentity *peer,
+                const char *address,
+                enum GNUNET_NetworkType nt,
+                const struct MonitorEvent *me)
+{
+  struct GNUNET_MQ_Envelope *env;
+  struct GNUNET_TRANSPORT_MonitorData *md;
+  size_t addr_len = strlen (address) + 1;
+
+  env = GNUNET_MQ_msg_extra (md,
+                             addr_len,
+                             GNUNET_MESSAGE_TYPE_TRANSPORT_MONITOR_DATA);
+  md->nt = htonl ((uint32_t) nt);
+  md->peer = *peer;
+  md->last_validation = GNUNET_TIME_absolute_hton (me->last_validation);
+  md->valid_until = GNUNET_TIME_absolute_hton (me->valid_until);
+  md->next_validation = GNUNET_TIME_absolute_hton (me->next_validation);
+  md->rtt = GNUNET_TIME_relative_hton (me->rtt);
+  md->cs = htonl ((uint32_t) me->cs);
+  md->num_msg_pending = htonl (me->num_msg_pending);
+  md->num_bytes_pending = htonl (me->num_bytes_pending);
+  memcpy (&md[1], address, addr_len);
+  GNUNET_MQ_send (tc->mq, env);
+}
+
+
+/**
+ * Send information in @a me about a @a peer's status with respect
+ * to some @a address to all monitors that care.
+ *
+ * @param peer peer the information is about
+ * @param address address the information is about
+ * @param nt network type associated with @a address
+ * @param me detailed information to transmit
+ */
+static void
+notify_monitors (const struct GNUNET_PeerIdentity *peer,
+                 const char *address,
+                 enum GNUNET_NetworkType nt,
+                 const struct MonitorEvent *me)
+{
+  for (struct TransportClient *tc = clients_head; NULL != tc; tc = tc->next)
+  {
+    if (CT_MONITOR != tc->type)
+      continue;
+    if (tc->details.monitor.one_shot)
+      continue;
+    if ((GNUNET_NO == GNUNET_is_zero (&tc->details.monitor.peer)) &&
+        (0 != GNUNET_memcmp (&tc->details.monitor.peer, peer)))
+      continue;
+    notify_monitor (tc, peer, address, nt, me);
+  }
+}
+
+
+/**
+ * Called whenever a client connects.  Allocates our
+ * data structures associated with that client.
+ *
+ * @param cls closure, NULL
+ * @param client identification of the client
+ * @param mq message queue for the client
+ * @return our `struct TransportClient`
+ */
+static void *
+client_connect_cb (void *cls,
+                   struct GNUNET_SERVICE_Client *client,
+                   struct GNUNET_MQ_Handle *mq)
+{
+  struct TransportClient *tc;
+
+  (void) cls;
+  tc = GNUNET_new (struct TransportClient);
+  tc->client = client;
+  tc->mq = mq;
+  GNUNET_CONTAINER_DLL_insert (clients_head, clients_tail, tc);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Client %p of type %u connected\n",
+              tc,
+              tc->type);
+  return tc;
+}
+
+
+/**
+ * Release memory used by @a neighbour.
+ *
+ * @param neighbour neighbour entry to free
+ */
+static void
+free_neighbour (struct Neighbour *neighbour)
+{
+  struct DistanceVectorHop *dvh;
+  struct VirtualLink *vl;
+
+  GNUNET_assert (NULL == neighbour->queue_head);
+  GNUNET_assert (GNUNET_YES ==
+                 GNUNET_CONTAINER_multipeermap_remove (neighbours,
+                                                       &neighbour->pid,
+                                                       neighbour));
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Freeing neighbour\n");
+  while (NULL != (dvh = neighbour->dv_head))
+  {
+    struct DistanceVector *dv = dvh->dv;
+
+    free_distance_vector_hop (dvh);
+    if (NULL == dv->dv_head)
+      free_dv_route (dv);
+  }
+  if (NULL != neighbour->get)
+  {
+    GNUNET_PEERSTORE_iterate_cancel (neighbour->get);
+    neighbour->get = NULL;
+  }
+  if (NULL != neighbour->sc)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "store cancel\n");
+    GNUNET_PEERSTORE_store_cancel (neighbour->sc);
+    neighbour->sc = NULL;
+  }
+  if (NULL != (vl = neighbour->vl))
+  {
+    GNUNET_assert (neighbour == vl->n);
+    vl->n = NULL;
+    if (NULL == vl->dv)
+    {
+      cores_send_disconnect_info (&vl->target);
+      free_virtual_link (vl);
+    }
+    else
+    {
+      GNUNET_SCHEDULER_cancel (vl->visibility_task);
+      vl->visibility_task = GNUNET_SCHEDULER_add_now (&check_link_down, vl);
+    }
+    neighbour->vl = NULL;
+  }
+  GNUNET_free (neighbour);
+}
+
+
+/**
+ * Send message to CORE clients that we lost a connection.
+ *
+ * @param tc client to inform (must be CORE client)
+ * @param pid peer the connection is for
+ */
+static void
+core_send_connect_info (struct TransportClient *tc,
+                        const struct GNUNET_PeerIdentity *pid)
+{
+  struct GNUNET_MQ_Envelope *env;
+  struct ConnectInfoMessage *cim;
+
+  GNUNET_assert (CT_CORE == tc->type);
+  env = GNUNET_MQ_msg (cim, GNUNET_MESSAGE_TYPE_TRANSPORT_CONNECT);
+  cim->id = *pid;
+  GNUNET_MQ_send (tc->mq, env);
+}
+
+
+/**
+ * Send message to CORE clients that we gained a connection
+ *
+ * @param pid peer the queue was for
+ */
+static void
+cores_send_connect_info (const struct GNUNET_PeerIdentity *pid)
+{
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Informing CORE clients about connection to %s\n",
+              GNUNET_i2s (pid));
+  for (struct TransportClient *tc = clients_head; NULL != tc; tc = tc->next)
+  {
+    if (CT_CORE != tc->type)
+      continue;
+    core_send_connect_info (tc, pid);
+  }
+}
+
+
+/**
+ * We believe we are ready to transmit a message on a queue. Gives the
+ * message to the communicator for transmission (updating the tracker,
+ * and re-scheduling itself if applicable).
+ *
+ * @param cls the `struct Queue` to process transmissions for
+ */
+static void
+transmit_on_queue (void *cls);
+
+
+/**
+ * Check if the communicator has another queue with higher prio ready for sending.
+ */
+static unsigned int
+check_for_queue_with_higher_prio (struct Queue *queue, struct Queue *queue_head)
+{
+  for (struct Queue *s = queue_head; NULL != s;
+       s = s->next_client)
+  {
+    if (s->tc->details.communicator.address_prefix !=
+        queue->tc->details.communicator.address_prefix)
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "queue address %s qid %u compare with queue: address %s qid %u\n",
+                  queue->address,
+                  queue->qid,
+                  s->address,
+                  s->qid);
+      if ((s->priority > queue->priority) && (0 < s->q_capacity) &&
+          (QUEUE_LENGTH_LIMIT > s->queue_length) )
+        return GNUNET_YES;
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "Lower prio\n");
+    }
+  }
+  return GNUNET_NO;
+}
+
+
+/**
+ * Called whenever something changed that might effect when we
+ * try to do the next transmission on @a queue using #transmit_on_queue().
+ *
+ * @param queue the queue to do scheduling for
+ * @param p task priority to use, if @a queue is scheduled
+ */
+static void
+schedule_transmit_on_queue (struct GNUNET_TIME_Relative delay,
+                            struct Queue *queue,
+                            enum GNUNET_SCHEDULER_Priority p)
+{
+  if (check_for_queue_with_higher_prio (queue,
+                                        queue->tc->details.communicator.
+                                        queue_head))
+    return;
+
+  if (queue->tc->details.communicator.total_queue_length >=
+      COMMUNICATOR_TOTAL_QUEUE_LIMIT)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Transmission throttled due to communicator queue limit\n");
+    GNUNET_STATISTICS_update (
+      GST_stats,
+      "# Transmission throttled due to communicator queue limit",
+      1,
+      GNUNET_NO);
+    queue->idle = GNUNET_NO;
+    return;
+  }
+  if (queue->queue_length >= QUEUE_LENGTH_LIMIT)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Transmission throttled due to communicator queue length limit\n");
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# Transmission throttled due to queue queue limit",
+                              1,
+                              GNUNET_NO);
+    queue->idle = GNUNET_NO;
+    return;
+  }
+  if (0 == queue->q_capacity)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Transmission throttled due to communicator message queue qid %u has capacity %"
+                PRIu64 ".\n",
+                queue->qid,
+                queue->q_capacity);
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# Transmission throttled due to message queue capacity",
+                              1,
+                              GNUNET_NO);
+    queue->idle = GNUNET_NO;
+    return;
+  }
+  /* queue might indeed be ready, schedule it */
+  if (NULL != queue->transmit_task)
+    GNUNET_SCHEDULER_cancel (queue->transmit_task);
+  queue->transmit_task =
+    GNUNET_SCHEDULER_add_delayed_with_priority (delay, p, &transmit_on_queue,
+                                                queue);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Considering transmission on queue `%s' QID %llu to %s\n",
+              queue->address,
+              (unsigned long long) queue->qid,
+              GNUNET_i2s (&queue->neighbour->pid));
+}
+
+
+/**
+ * Task run to check whether the hops of the @a cls still
+ * are validated, or if we need to core about disconnection.
+ *
+ * @param cls a `struct VirtualLink`
+ */
+static void
+check_link_down (void *cls)
+{
+  struct VirtualLink *vl = cls;
+  struct DistanceVector *dv = vl->dv;
+  struct Neighbour *n = vl->n;
+  struct GNUNET_TIME_Absolute dvh_timeout;
+  struct GNUNET_TIME_Absolute q_timeout;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Checking if link is down\n");
+  vl->visibility_task = NULL;
+  dvh_timeout = GNUNET_TIME_UNIT_ZERO_ABS;
+  if (NULL != dv)
+  {
+    for (struct DistanceVectorHop *pos = dv->dv_head; NULL != pos;
+         pos = pos->next_dv)
+      dvh_timeout = GNUNET_TIME_absolute_max (dvh_timeout,
+                                              pos->path_valid_until);
+    if (0 == GNUNET_TIME_absolute_get_remaining (dvh_timeout).rel_value_us)
+    {
+      vl->dv->vl = NULL;
+      vl->dv = NULL;
+    }
+  }
+  q_timeout = GNUNET_TIME_UNIT_ZERO_ABS;
+  for (struct Queue *q = n->queue_head; NULL != q; q = q->next_neighbour)
+    q_timeout = GNUNET_TIME_absolute_max (q_timeout, q->validated_until);
+  if (0 == GNUNET_TIME_absolute_get_remaining (q_timeout).rel_value_us)
+  {
+    vl->n->vl = NULL;
+    vl->n = NULL;
+  }
+  if ((NULL == vl->n) && (NULL == vl->dv))
+  {
+    cores_send_disconnect_info (&vl->target);
+    free_virtual_link (vl);
+    return;
+  }
+  vl->visibility_task =
+    GNUNET_SCHEDULER_add_at (GNUNET_TIME_absolute_max (q_timeout, dvh_timeout),
+                             &check_link_down,
+                             vl);
+}
+
+
+/**
+ * Free @a queue.
+ *
+ * @param queue the queue to free
+ */
+static void
+free_queue (struct Queue *queue)
+{
+  struct Neighbour *neighbour = queue->neighbour;
+  struct TransportClient *tc = queue->tc;
+  struct MonitorEvent me = { .cs = GNUNET_TRANSPORT_CS_DOWN,
+                             .rtt = GNUNET_TIME_UNIT_FOREVER_REL };
+  struct QueueEntry *qe;
+  int maxxed;
+  struct PendingAcknowledgement *pa;
+  struct VirtualLink *vl;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Cleaning up queue %u\n", queue->qid);
+  if (NULL != queue->transmit_task)
+  {
+    GNUNET_SCHEDULER_cancel (queue->transmit_task);
+    queue->transmit_task = NULL;
+  }
+  while (NULL != (pa = queue->pa_head))
+  {
+    GNUNET_CONTAINER_MDLL_remove (queue, queue->pa_head, queue->pa_tail, pa);
+    pa->queue = NULL;
+  }
+
+  GNUNET_CONTAINER_MDLL_remove (neighbour,
+                                neighbour->queue_head,
+                                neighbour->queue_tail,
+                                queue);
+  GNUNET_CONTAINER_MDLL_remove (client,
+                                tc->details.communicator.queue_head,
+                                tc->details.communicator.queue_tail,
+                                queue);
+  maxxed = (COMMUNICATOR_TOTAL_QUEUE_LIMIT <=
+            tc->details.communicator.
+            total_queue_length);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Cleaning up queue with length %u\n",
+              queue->queue_length);
+  while (NULL != (qe = queue->queue_head))
+  {
+    GNUNET_CONTAINER_DLL_remove (queue->queue_head, queue->queue_tail, qe);
+    queue->queue_length--;
+    tc->details.communicator.total_queue_length--;
+    if (NULL != qe->pm)
+    {
+      GNUNET_assert (qe == qe->pm->qe);
+      qe->pm->qe = NULL;
+    }
+    GNUNET_free (qe);
+  }
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Cleaning up queue with length %u\n",
+              queue->queue_length);
+  GNUNET_assert (0 == queue->queue_length);
+  if ((maxxed) && (COMMUNICATOR_TOTAL_QUEUE_LIMIT >
+                   tc->details.communicator.total_queue_length))
+  {
+    /* Communicator dropped below threshold, resume all _other_ queues */
+    GNUNET_STATISTICS_update (
+      GST_stats,
+      "# Transmission throttled due to communicator queue limit",
+      -1,
+      GNUNET_NO);
+    for (struct Queue *s = tc->details.communicator.queue_head; NULL != s;
+         s = s->next_client)
+      schedule_transmit_on_queue (GNUNET_TIME_UNIT_ZERO,
+                                  s,
+                                  GNUNET_SCHEDULER_PRIORITY_DEFAULT);
+  }
+  notify_monitors (&neighbour->pid, queue->address, queue->nt, &me);
+  GNUNET_free (queue);
+
+  vl = lookup_virtual_link (&neighbour->pid);
+  if ((NULL != vl) && (neighbour == vl->n))
+  {
+    GNUNET_SCHEDULER_cancel (vl->visibility_task);
+    check_link_down (vl);
+  }
+  if (NULL == neighbour->queue_head)
+  {
+    free_neighbour (neighbour);
+  }
+}
+
+
+/**
+ * Free @a ale
+ *
+ * @param ale address list entry to free
+ */
+static void
+free_address_list_entry (struct AddressListEntry *ale)
+{
+  struct TransportClient *tc = ale->tc;
+
+  GNUNET_CONTAINER_DLL_remove (tc->details.communicator.addr_head,
+                               tc->details.communicator.addr_tail,
+                               ale);
+  if (NULL != ale->sc)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "store cancel\n");
+    GNUNET_PEERSTORE_store_cancel (ale->sc);
+    ale->sc = NULL;
+  }
+  if (NULL != ale->st)
+  {
+    GNUNET_SCHEDULER_cancel (ale->st);
+    ale->st = NULL;
+  }
+  if (NULL != ale->signed_address)
+    GNUNET_free (ale->signed_address);
+  GNUNET_free (ale);
+}
+
+
+/**
+ * Stop the peer request in @a value.
+ *
+ * @param cls a `struct TransportClient` that no longer makes the request
+ * @param pid the peer's identity
+ * @param value a `struct PeerRequest`
+ * @return #GNUNET_YES (always)
+ */
+static int
+stop_peer_request (void *cls,
+                   const struct GNUNET_PeerIdentity *pid,
+                   void *value)
+{
+  struct TransportClient *tc = cls;
+  struct PeerRequest *pr = value;
+
+  GNUNET_PEERSTORE_hello_changed_notify_cancel (pr->nc);
+  pr->nc = NULL;
+  GNUNET_assert (
+    GNUNET_YES ==
+    GNUNET_CONTAINER_multipeermap_remove (tc->details.application.requests,
+                                          pid,
+                                          pr));
+  GNUNET_free (pr);
+
+  return GNUNET_OK;
+}
+
+
+static void
+do_shutdown (void *cls);
+
+/**
+ * Called whenever a client is disconnected.  Frees our
+ * resources associated with that client.
+ *
+ * @param cls closure, NULL
+ * @param client identification of the client
+ * @param app_ctx our `struct TransportClient`
+ */
+static void
+client_disconnect_cb (void *cls,
+                      struct GNUNET_SERVICE_Client *client,
+                      void *app_ctx)
+{
+  struct TransportClient *tc = app_ctx;
+
+  (void) cls;
+  (void) client;
+  GNUNET_CONTAINER_DLL_remove (clients_head, clients_tail, tc);
+  switch (tc->type)
+  {
+  case CT_NONE:
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Unknown Client %p disconnected, cleaning up.\n",
+                tc);
+    break;
+
+  case CT_CORE: {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "CORE Client %p disconnected, cleaning up.\n",
+                  tc);
+
+      struct PendingMessage *pm;
+
+      while (NULL != (pm = tc->details.core.pending_msg_head))
+      {
+        GNUNET_CONTAINER_MDLL_remove (client,
+                                      tc->details.core.pending_msg_head,
+                                      tc->details.core.pending_msg_tail,
+                                      pm);
+        pm->client = NULL;
+      }
+    }
+    break;
+
+  case CT_MONITOR:
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "MONITOR Client %p disconnected, cleaning up.\n",
+                tc);
+
+    break;
+
+  case CT_COMMUNICATOR: {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "COMMUNICATOR Client %p disconnected, cleaning up.\n",
+                  tc);
+
+      struct Queue *q;
+      struct AddressListEntry *ale;
+
+      while (NULL != (q = tc->details.communicator.queue_head))
+        free_queue (q);
+      while (NULL != (ale = tc->details.communicator.addr_head))
+        free_address_list_entry (ale);
+      GNUNET_free (tc->details.communicator.address_prefix);
+    }
+    break;
+
+  case CT_APPLICATION:
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "APPLICATION Client %p disconnected, cleaning up.\n",
+                tc);
+
+    GNUNET_CONTAINER_multipeermap_iterate (tc->details.application.requests,
+                                           &stop_peer_request,
+                                           tc);
+    GNUNET_CONTAINER_multipeermap_destroy (tc->details.application.requests);
+    break;
+  }
+  GNUNET_free (tc);
+  if ((GNUNET_YES == in_shutdown) && (NULL == clients_head))
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                "Our last client disconnected\n");
+    do_shutdown (cls);
+  }
+}
+
+
+/**
+ * Iterator telling new CORE client about all existing
+ * connections to peers.
+ *
+ * @param cls the new `struct TransportClient`
+ * @param pid a connected peer
+ * @param value the `struct Neighbour` with more information
+ * @return #GNUNET_OK (continue to iterate)
+ */
+static int
+notify_client_connect_info (void *cls,
+                            const struct GNUNET_PeerIdentity *pid,
+                            void *value)
+{
+  struct TransportClient *tc = cls;
+  struct Neighbour *n = value;
+  struct VirtualLink *vl = n->vl;
+
+  if ((NULL == vl) || (GNUNET_NO == vl->confirmed))
+    return GNUNET_OK;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Telling new CORE client about existing connection to %s\n",
+              GNUNET_i2s (pid));
+  core_send_connect_info (tc, pid);
+  return GNUNET_OK;
+}
+
+
+/**
+ * Initialize a "CORE" client.  We got a start message from this
+ * client, so add it to the list of clients for broadcasting of
+ * inbound messages.
+ *
+ * @param cls the client
+ * @param start the start message that was sent
+ */
+static void
+handle_client_start (void *cls, const struct StartMessage *start)
+{
+  struct TransportClient *tc = cls;
+  uint32_t options;
+
+  options = ntohl (start->options);
+  if ((0 != (1 & options)) &&
+      (0 != GNUNET_memcmp (&start->self, &GST_my_identity)))
+  {
+    /* client thinks this is a different peer, reject */
+    GNUNET_break (0);
+    GNUNET_SERVICE_client_drop (tc->client);
+    return;
+  }
+  if (CT_NONE != tc->type)
+  {
+    GNUNET_break (0);
+    GNUNET_SERVICE_client_drop (tc->client);
+    return;
+  }
+  tc->type = CT_CORE;
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "New CORE client with PID %s registered\n",
+              GNUNET_i2s (&start->self));
+  GNUNET_CONTAINER_multipeermap_iterate (neighbours,
+                                         &notify_client_connect_info,
+                                         tc);
+  GNUNET_SERVICE_client_continue (tc->client);
+}
+
+
+/**
+ * Client asked for transmission to a peer.  Process the request.
+ *
+ * @param cls the client
+ * @param obm the send message that was sent
+ */
+static int
+check_client_send (void *cls, const struct OutboundMessage *obm)
+{
+  struct TransportClient *tc = cls;
+  uint16_t size;
+  const struct GNUNET_MessageHeader *obmm;
+
+  if (CT_CORE != tc->type)
+  {
+    GNUNET_break (0);
+    return GNUNET_SYSERR;
+  }
+  size = ntohs (obm->header.size) - sizeof(struct OutboundMessage);
+  if (size < sizeof(struct GNUNET_MessageHeader))
+  {
+    GNUNET_break (0);
+    return GNUNET_SYSERR;
+  }
+  obmm = (const struct GNUNET_MessageHeader *) &obm[1];
+  if (size != ntohs (obmm->size))
+  {
+    GNUNET_break (0);
+    return GNUNET_SYSERR;
+  }
+  return GNUNET_OK;
+}
+
+
+/**
+ * Send a response to the @a pm that we have processed a "send"
+ * request.  Sends a confirmation to the "core" client responsible for
+ * the original request and free's @a pm.
+ *
+ * @param pm handle to the original pending message
+ */
+static void
+client_send_response (struct PendingMessage *pm)
+{
+  struct TransportClient *tc = pm->client;
+  struct VirtualLink *vl = pm->vl;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "client send response\n");
+  if (NULL != tc)
+  {
+    struct GNUNET_MQ_Envelope *env;
+    struct SendOkMessage *so_msg;
+
+    env = GNUNET_MQ_msg (so_msg, GNUNET_MESSAGE_TYPE_TRANSPORT_SEND_OK);
+    so_msg->peer = vl->target;
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Confirming transmission of <%" PRIu64 "> to %s\n",
+                pm->logging_uuid,
+                GNUNET_i2s (&vl->target));
+    GNUNET_MQ_send (tc->mq, env);
+  }
+  free_pending_message (pm);
+}
+
+
+/**
+ * Pick @a hops_array_length random DV paths satisfying @a options
+ *
+ * @param dv data structure to pick paths from
+ * @param options constraints to satisfy
+ * @param[out] hops_array set to the result
+ * @param hops_array_length length of the @a hops_array
+ * @return number of entries set in @a hops_array
+ */
+static unsigned int
+pick_random_dv_hops (const struct DistanceVector *dv,
+                     enum RouteMessageOptions options,
+                     struct DistanceVectorHop **hops_array,
+                     unsigned int hops_array_length)
+{
+  uint64_t choices[hops_array_length];
+  uint64_t num_dv;
+  unsigned int dv_count;
+
+  /* Pick random vectors, but weighted by distance, giving more weight
+     to shorter vectors */
+  num_dv = 0;
+  dv_count = 0;
+  for (struct DistanceVectorHop *pos = dv->dv_head; NULL != pos;
+       pos = pos->next_dv)
+  {
+    if ((0 == (options & RMO_UNCONFIRMED_ALLOWED)) &&
+        (GNUNET_TIME_absolute_get_remaining (pos->path_valid_until)
+         .rel_value_us == 0))
+      continue;   /* pos unconfirmed and confirmed required */
+    num_dv += MAX_DV_HOPS_ALLOWED - pos->distance;
+    dv_count++;
+  }
+  if (0 == dv_count)
+    return 0;
+  if (dv_count <= hops_array_length)
+  {
+    dv_count = 0;
+    for (struct DistanceVectorHop *pos = dv->dv_head; NULL != pos;
+         pos = pos->next_dv)
+      hops_array[dv_count++] = pos;
+    return dv_count;
+  }
+  for (unsigned int i = 0; i < hops_array_length; i++)
+  {
+    int ok = GNUNET_NO;
+    while (GNUNET_NO == ok)
+    {
+      choices[i] =
+        GNUNET_CRYPTO_random_u64 (GNUNET_CRYPTO_QUALITY_WEAK, num_dv);
+      ok = GNUNET_YES;
+      for (unsigned int j = 0; j < i; j++)
+        if (choices[i] == choices[j])
+        {
+          ok = GNUNET_NO;
+          break;
+        }
+    }
+  }
+  dv_count = 0;
+  num_dv = 0;
+  for (struct DistanceVectorHop *pos = dv->dv_head; NULL != pos;
+       pos = pos->next_dv)
+  {
+    uint32_t delta = MAX_DV_HOPS_ALLOWED - pos->distance;
+
+    if ((0 == (options & RMO_UNCONFIRMED_ALLOWED)) &&
+        (GNUNET_TIME_absolute_get_remaining (pos->path_valid_until)
+         .rel_value_us == 0))
+      continue;   /* pos unconfirmed and confirmed required */
+    for (unsigned int i = 0; i < hops_array_length; i++)
+      if ((num_dv <= choices[i]) && (num_dv + delta > choices[i]))
+        hops_array[dv_count++] = pos;
+    num_dv += delta;
+  }
+  return dv_count;
+}
+
+
+/**
+ * Communicator started.  Test message is well-formed.
+ *
+ * @param cls the client
+ * @param cam the send message that was sent
+ */
+static int
+check_communicator_available (
+  void *cls,
+  const struct GNUNET_TRANSPORT_CommunicatorAvailableMessage *cam)
+{
+  struct TransportClient *tc = cls;
+  uint16_t size;
+
+  if (CT_NONE != tc->type)
+  {
+    GNUNET_break (0);
+    return GNUNET_SYSERR;
+  }
+  tc->type = CT_COMMUNICATOR;
+  size = ntohs (cam->header.size) - sizeof(*cam);
+  if (0 == size)
+    return GNUNET_OK; /* receive-only communicator */
+  GNUNET_MQ_check_zero_termination (cam);
+  return GNUNET_OK;
+}
+
+
+/**
+ * Send ACK to communicator (if requested) and free @a cmc.
+ *
+ * @param cmc context for which we are done handling the message
+ */
+static void
+finish_cmc_handling_with_continue (struct CommunicatorMessageContext *cmc,
+                                   unsigned
+                                   int continue_client)
+{
+  if (0 != ntohl (cmc->im.fc_on))
+  {
+    /* send ACK when done to communicator for flow control! */
+    struct GNUNET_MQ_Envelope *env;
+    struct GNUNET_TRANSPORT_IncomingMessageAck *ack;
+
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Acknowledge message with flow control id %" PRIu64 "\n",
+                cmc->im.fc_id);
+    env = GNUNET_MQ_msg (ack, GNUNET_MESSAGE_TYPE_TRANSPORT_INCOMING_MSG_ACK);
+    ack->reserved = htonl (0);
+    ack->fc_id = cmc->im.fc_id;
+    ack->sender = cmc->im.neighbour_sender;
+    GNUNET_MQ_send (cmc->tc->mq, env);
+  }
+
+  if (GNUNET_YES == continue_client)
+  {
+    GNUNET_SERVICE_client_continue (cmc->tc->client);
+  }
+  GNUNET_free (cmc);
+}
+
+
+static void
+finish_cmc_handling (struct CommunicatorMessageContext *cmc)
+{
+  finish_cmc_handling_with_continue (cmc, GNUNET_YES);
+}
+
+
+/**
+ * Client confirms that it is done handling message(s) to a particular
+ * peer. We may now provide more messages to CORE for this peer.
+ *
+ * Notifies the respective queues that more messages can now be received.
+ *
+ * @param cls the client
+ * @param rom the message that was sent
+ */
+static void
+handle_client_recv_ok (void *cls, const struct RecvOkMessage *rom)
+{
+  struct TransportClient *tc = cls;
+  struct VirtualLink *vl;
+  uint32_t delta;
+  struct CommunicatorMessageContext *cmc;
+
+  if (CT_CORE != tc->type)
+  {
+    GNUNET_break (0);
+    GNUNET_SERVICE_client_drop (tc->client);
+    return;
+  }
+  vl = lookup_virtual_link (&rom->peer);
+  if ((NULL == vl) || (GNUNET_NO == vl->confirmed))
+  {
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# RECV_OK dropped: virtual link unknown",
+                              1,
+                              GNUNET_NO);
+    GNUNET_SERVICE_client_continue (tc->client);
+    return;
+  }
+  delta = ntohl (rom->increase_window_delta);
+  vl->core_recv_window += delta;
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "CORE ack receiving message, increased CORE recv window to %d\n",
+              vl->core_recv_window);
+  GNUNET_SERVICE_client_continue (tc->client);
+  if (vl->core_recv_window <= 0)
+    return;
+  /* resume communicators */
+  while (NULL != (cmc = vl->cmc_tail))
+  {
+    GNUNET_CONTAINER_DLL_remove (vl->cmc_head, vl->cmc_tail, cmc);
+    finish_cmc_handling (cmc);
+  }
+}
+
+
+/**
+ * Communicator started.  Process the request.
+ *
+ * @param cls the client
+ * @param cam the send message that was sent
+ */
+static void
+handle_communicator_available (
+  void *cls,
+  const struct GNUNET_TRANSPORT_CommunicatorAvailableMessage *cam)
+{
+  struct TransportClient *tc = cls;
+  uint16_t size;
+
+  size = ntohs (cam->header.size) - sizeof(*cam);
+  if (0 == size)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Receive-only communicator connected\n");
+    return;   /* receive-only communicator */
+  }
+  tc->details.communicator.address_prefix =
+    GNUNET_strdup ((const char *) &cam[1]);
+  tc->details.communicator.cc =
+    (enum GNUNET_TRANSPORT_CommunicatorCharacteristics) ntohl (cam->cc);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Communicator with prefix `%s' connected\n",
+              tc->details.communicator.address_prefix);
+  GNUNET_SERVICE_client_continue (tc->client);
+}
+
+
+/**
+ * Communicator requests backchannel transmission.  Check the request.
+ *
+ * @param cls the client
+ * @param cb the send message that was sent
+ * @return #GNUNET_OK if message is well-formed
+ */
+static int
+check_communicator_backchannel (
+  void *cls,
+  const struct GNUNET_TRANSPORT_CommunicatorBackchannel *cb)
+{
+  const struct GNUNET_MessageHeader *inbox;
+  const char *is;
+  uint16_t msize;
+  uint16_t isize;
+
+  (void) cls;
+  msize = ntohs (cb->header.size) - sizeof(*cb);
+  inbox = (const struct GNUNET_MessageHeader *) &cb[1];
+  isize = ntohs (inbox->size);
+  if (isize >= msize)
+  {
+    GNUNET_break (0);
+    return GNUNET_SYSERR;
+  }
+  is = (const char *) inbox;
+  is += isize;
+  msize -= isize;
+  GNUNET_assert (0 < msize);
+  if ('\0' != is[msize - 1])
+  {
+    GNUNET_break (0);
+    return GNUNET_SYSERR;
+  }
+  return GNUNET_OK;
+}
+
+
+/**
+ * Sign ephemeral keys in our @a dv are current.
+ *
+ * @param[in,out] dv virtual link to update ephemeral for
+ */
+static void
+sign_ephemeral (struct DistanceVector *dv)
+{
+  struct EphemeralConfirmationPS ec;
+
+  dv->monotime = GNUNET_TIME_absolute_get_monotonic (GST_cfg);
+  dv->ephemeral_validity =
+    GNUNET_TIME_absolute_add (dv->monotime, EPHEMERAL_VALIDITY);
+  ec.purpose.purpose = htonl (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_EPHEMERAL);
+  ec.target = dv->target;
+  ec.ephemeral_key = dv->ephemeral_key;
+  ec.sender_monotonic_time = GNUNET_TIME_absolute_hton (dv->monotime);
+  ec.purpose.size = htonl (sizeof(ec));
+  GNUNET_CRYPTO_eddsa_sign (GST_my_private_key,
+                            &ec,
+                            &dv->sender_sig);
+}
+
+
+/**
+ * Send the message @a payload on @a queue.
+ *
+ * @param queue the queue to use for transmission
+ * @param pm pending message to update once transmission is done, may be NULL!
+ * @param payload the payload to send (encapsulated in a
+ *        #GNUNET_MESSAGE_TYPE_TRANSPORT_SEND_MSG).
+ * @param payload_size number of bytes in @a payload
+ */
+static void
+queue_send_msg (struct Queue *queue,
+                struct PendingMessage *pm,
+                const void *payload,
+                size_t payload_size)
+{
+  struct Neighbour *n = queue->neighbour;
+  struct GNUNET_TRANSPORT_SendMessageTo *smt;
+  struct GNUNET_MQ_Envelope *env;
+  struct PendingAcknowledgement *pa;
+
+  GNUNET_log (
+    GNUNET_ERROR_TYPE_DEBUG,
+    "Queueing %u bytes of payload for transmission <%" PRIu64
+    "> on queue %llu to %s\n",
+    (unsigned int) payload_size,
+    (NULL == pm) ? 0 : pm->logging_uuid,
+    (unsigned long long) queue->qid,
+    GNUNET_i2s (&queue->neighbour->pid));
+  env = GNUNET_MQ_msg_extra (smt,
+                             payload_size,
+                             GNUNET_MESSAGE_TYPE_TRANSPORT_SEND_MSG);
+  smt->qid = htonl (queue->qid);
+  smt->mid = GNUNET_htonll (queue->mid_gen);
+  smt->receiver = n->pid;
+  memcpy (&smt[1], payload, payload_size);
+  {
+    /* Pass the env to the communicator of queue for transmission. */
+    struct QueueEntry *qe;
+
+    qe = GNUNET_new (struct QueueEntry);
+    qe->mid = queue->mid_gen;
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Create QueueEntry with MID %" PRIu64
+                " and QID %u and prefix %s\n",
+                qe->mid,
+                queue->qid,
+                queue->tc->details.communicator.address_prefix);
+    queue->mid_gen++;
+    qe->queue = queue;
+    if (NULL != pm)
+    {
+      qe->pm = pm;
+      // TODO Why do we have a retransmission. When we know, make decision if we still want this.
+      // GNUNET_assert (NULL == pm->qe);
+      if (NULL != pm->qe)
+      {
+        GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                    "Retransmitting message <%" PRIu64
+                    "> remove pm from qe with MID: %llu \n",
+                    pm->logging_uuid,
+                    (unsigned long long) pm->qe->mid);
+        pm->qe->pm = NULL;
+      }
+      pm->qe = qe;
+    }
+    GNUNET_CONTAINER_DLL_insert (queue->queue_head, queue->queue_tail, qe);
+    GNUNET_assert (CT_COMMUNICATOR == queue->tc->type);
+    if (0 == queue->q_capacity)
+    {
+      // Messages without FC or fragments can get here.
+      if (NULL != pm)
+        GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                    "Message %" PRIu64
+                    " (pm type %u) was not send because queue has no capacity.\n",
+                    pm->logging_uuid,
+                    pm->pmt);
+      GNUNET_free (env);
+      return;
+    }
+    queue->queue_length++;
+    queue->tc->details.communicator.total_queue_length++;
+    if (GNUNET_NO == queue->unlimited_length)
+      queue->q_capacity--;
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Queue %s with qid %u has capacity %" PRIu64 "\n",
+                queue->address,
+                queue->qid,
+                queue->q_capacity);
+    if (COMMUNICATOR_TOTAL_QUEUE_LIMIT ==
+        queue->tc->details.communicator.total_queue_length)
+      queue->idle = GNUNET_NO;
+    if (QUEUE_LENGTH_LIMIT == queue->queue_length)
+      queue->idle = GNUNET_NO;
+    if (0 == queue->q_capacity)
+      queue->idle = GNUNET_NO;
+
+    if (NULL != pm && NULL != (pa = pm->pa_head))
+    {
+      while (pm != pa->pm)
+        pa = pa->next_pa;
+      pa->num_send++;
+    }
+    // GNUNET_CONTAINER_multiuuidmap_get (pending_acks, &ack[i].ack_uuid.value);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Sending message MID %" PRIu64
+                " of type %u (%u) and size %lu with MQ %p QID %u\n",
+                GNUNET_ntohll (smt->mid),
+                ntohs (((const struct GNUNET_MessageHeader *) payload)->type),
+                ntohs (smt->header.size),
+                (unsigned long) payload_size,
+                queue->tc->mq,
+                queue->qid);
+    GNUNET_MQ_send (queue->tc->mq, env);
+  }
+}
+
+
+/**
+ * Pick a queue of @a n under constraints @a options and schedule
+ * transmission of @a hdr.
+ *
+ * @param n neighbour to send to
+ * @param hdr message to send as payload
+ * @param options whether queues must be confirmed or not,
+ *        and whether we may pick multiple (2) queues
+ * @return expected RTT for transmission, #GNUNET_TIME_UNIT_FOREVER_REL if sending failed
+ */
+static struct GNUNET_TIME_Relative
+route_via_neighbour (const struct Neighbour *n,
+                     const struct GNUNET_MessageHeader *hdr,
+                     enum RouteMessageOptions options)
+{
+  struct GNUNET_TIME_Absolute now;
+  unsigned int candidates;
+  unsigned int sel1;
+  unsigned int sel2;
+  struct GNUNET_TIME_Relative rtt;
+
+  /* Pick one or two 'random' queues from n (under constraints of options) */
+  now = GNUNET_TIME_absolute_get ();
+  /* FIXME-OPTIMIZE: give queues 'weights' and pick proportional to
+     weight in the future; weight could be assigned by observed
+     bandwidth (note: not sure if we should do this for this type
+     of control traffic though). */
+  candidates = 0;
+  for (struct Queue *pos = n->queue_head; NULL != pos;
+       pos = pos->next_neighbour)
+  {
+    if ((0 != (options & RMO_UNCONFIRMED_ALLOWED)) ||
+        (pos->validated_until.abs_value_us > now.abs_value_us))
+      candidates++;
+  }
+  if (0 == candidates)
+  {
+    /* This can happen rarely if the last confirmed queue timed
+       out just as we were beginning to process this message. */
+    GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+                "Could not route message of type %u to %s: no valid queue\n",
+                ntohs (hdr->type),
+                GNUNET_i2s (&n->pid));
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# route selection failed (all no valid queue)",
+                              1,
+                              GNUNET_NO);
+    return GNUNET_TIME_UNIT_FOREVER_REL;
+  }
+
+  rtt = GNUNET_TIME_UNIT_FOREVER_REL;
+  sel1 = GNUNET_CRYPTO_random_u32 (GNUNET_CRYPTO_QUALITY_WEAK, candidates);
+  if (0 == (options & RMO_REDUNDANT))
+    sel2 = candidates; /* picks none! */
+  else
+    sel2 = GNUNET_CRYPTO_random_u32 (GNUNET_CRYPTO_QUALITY_WEAK, candidates);
+  candidates = 0;
+  for (struct Queue *pos = n->queue_head; NULL != pos;
+       pos = pos->next_neighbour)
+  {
+    if ((0 != (options & RMO_UNCONFIRMED_ALLOWED)) ||
+        (pos->validated_until.abs_value_us > now.abs_value_us))
+    {
+      if ((sel1 == candidates) || (sel2 == candidates))
+      {
+        GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                    "Routing message of type %u to %s using %s (#%u)\n",
+                    ntohs (hdr->type),
+                    GNUNET_i2s (&n->pid),
+                    pos->address,
+                    (sel1 == candidates) ? 1 : 2);
+        rtt = GNUNET_TIME_relative_min (rtt, pos->pd.aged_rtt);
+        queue_send_msg (pos, NULL, hdr, ntohs (hdr->size));
+      }
+      candidates++;
+    }
+  }
+  return rtt;
+}
+
+
+/**
+ * Structure of the key material used to encrypt backchannel messages.
+ */
+struct DVKeyState
+{
+  /**
+   * State of our block cipher.
+   */
+  gcry_cipher_hd_t cipher;
+
+  /**
+   * Actual key material.
+   */
+  struct
+  {
+    /**
+     * Key used for HMAC calculations (via #GNUNET_CRYPTO_hmac()).
+     */
+    struct GNUNET_CRYPTO_AuthKey hmac_key;
+
+    /**
+     * Symmetric key to use for encryption.
+     */
+    char aes_key[256 / 8];
+
+    /**
+     * Counter value to use during setup.
+     */
+    char aes_ctr[128 / 8];
+  } material;
+};
+
+
+/**
+ * Given the key material in @a km and the initialization vector
+ * @a iv, setup the key material for the backchannel in @a key.
+ *
+ * @param km raw master secret
+ * @param iv initialization vector
+ * @param[out] key symmetric cipher and HMAC state to generate
+ */
+static void
+dv_setup_key_state_from_km (const struct GNUNET_HashCode *km,
+                            const struct GNUNET_ShortHashCode *iv,
+                            struct DVKeyState *key)
+{
+  /* must match what we defive from decapsulated key */
+  GNUNET_assert (GNUNET_YES ==
+                 GNUNET_CRYPTO_kdf (&key->material,
+                                    sizeof(key->material),
+                                    "transport-backchannel-key",
+                                    strlen ("transport-backchannel-key"),
+                                    km,
+                                    sizeof(*km),
+                                    iv,
+                                    sizeof(*iv),
+                                    NULL));
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Deriving backchannel key based on KM %s and IV %s\n",
+              GNUNET_h2s (km),
+              GNUNET_sh2s (iv));
+  GNUNET_assert (0 == gcry_cipher_open (&key->cipher,
+                                        GCRY_CIPHER_AES256 /* low level: go for speed */,
+                                        GCRY_CIPHER_MODE_CTR,
+                                        0 /* flags */));
+  GNUNET_assert (0 == gcry_cipher_setkey (key->cipher,
+                                          &key->material.aes_key,
+                                          sizeof(key->material.aes_key)));
+  gcry_cipher_setctr (key->cipher,
+                      &key->material.aes_ctr,
+                      sizeof(key->material.aes_ctr));
+}
+
+
+/**
+ * Do HMAC calculation for backchannel messages over @a data using key
+ * material from @a key.
+ *
+ * @param key key material (from DH)
+ * @param[out] hmac set to the HMAC
+ * @param data data to perform HMAC calculation over
+ * @param data_size number of bytes in @a data
+ */
+static void
+dv_hmac (const struct DVKeyState *key,
+         struct GNUNET_HashCode *hmac,
+         const void *data,
+         size_t data_size)
+{
+  GNUNET_CRYPTO_hmac (&key->material.hmac_key, data, data_size, hmac);
+}
+
+
+/**
+ * Perform backchannel encryption using symmetric secret in @a key
+ * to encrypt data from @a in to @a dst.
+ *
+ * @param[in,out] key key material to use
+ * @param dst where to write the result
+ * @param in input data to encrypt (plaintext)
+ * @param in_size number of bytes of input in @a in and available at @a dst
+ */
+static void
+dv_encrypt (struct DVKeyState *key, const void *in, void *dst, size_t in_size)
+{
+  GNUNET_assert (0 ==
+                 gcry_cipher_encrypt (key->cipher, dst, in_size, in, in_size));
+}
+
+
+/**
+ * Perform backchannel encryption using symmetric secret in @a key
+ * to encrypt data from @a in to @a dst.
+ *
+ * @param[in,out] key key material to use
+ * @param ciph cipher text to decrypt
+ * @param[out] out output data to generate (plaintext)
+ * @param out_size number of bytes of input in @a ciph and available in @a out
+ * @return GNUNET_OK on success
+ */
+static enum GNUNET_GenericReturnValue
+dv_decrypt (struct DVKeyState *key,
+            void *out,
+            const void *ciph,
+            size_t out_size)
+{
+  return (0 ==
+          gcry_cipher_decrypt (key->cipher,
+                               out, out_size,
+                               ciph, out_size)) ? GNUNET_OK : GNUNET_SYSERR;
+}
+
+
+/**
+ * Clean up key material in @a key.
+ *
+ * @param key key material to clean up (memory must not be free'd!)
+ */
+static void
+dv_key_clean (struct DVKeyState *key)
+{
+  gcry_cipher_close (key->cipher);
+  GNUNET_CRYPTO_zero_keys (&key->material, sizeof(key->material));
+}
+
+
+/**
+ * Function to call to further operate on the now DV encapsulated
+ * message @a hdr, forwarding it via @a next_hop under respect of
+ * @a options.
+ *
+ * @param cls closure
+ * @param next_hop next hop of the DV path
+ * @param hdr encapsulated message, technically a `struct TransportDFBoxMessage`
+ * @param options options of the original message
+ */
+typedef void (*DVMessageHandler) (void *cls,
+                                  struct Neighbour *next_hop,
+                                  const struct GNUNET_MessageHeader *hdr,
+                                  enum RouteMessageOptions options);
+
+/**
+ * Pick a path of @a dv under constraints @a options and schedule
+ * transmission of @a hdr.
+ *
+ * @param target neighbour to ultimately send to
+ * @param num_dvhs length of the @a dvhs array
+ * @param dvhs array of hops to send the message to
+ * @param hdr message to send as payload
+ * @param use function to call with the encapsulated message
+ * @param use_cls closure for @a use
+ * @param options whether path must be confirmed or not, to be passed to @a use
+ * @param without_fc shall this TransportDVBoxMessage be forwarded without flow control.
+ * @return expected RTT for transmission, #GNUNET_TIME_UNIT_FOREVER_REL if sending failed
+ */
+static struct GNUNET_TIME_Relative
+encapsulate_for_dv (struct DistanceVector *dv,
+                    unsigned int num_dvhs,
+                    struct DistanceVectorHop **dvhs,
+                    const struct GNUNET_MessageHeader *hdr,
+                    DVMessageHandler use,
+                    void *use_cls,
+                    enum RouteMessageOptions options,
+                    enum GNUNET_GenericReturnValue without_fc)
+{
+  struct TransportDVBoxMessage box_hdr;
+  struct TransportDVBoxPayloadP payload_hdr;
+  uint16_t enc_body_size = ntohs (hdr->size);
+  char enc[sizeof(struct TransportDVBoxPayloadP) + enc_body_size] GNUNET_ALIGN;
+  struct DVKeyState *key;
+  struct GNUNET_TIME_Relative rtt;
+  struct GNUNET_HashCode km;
+
+  key = GNUNET_new (struct DVKeyState);
+  /* Encrypt payload */
+  box_hdr.header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_DV_BOX);
+  box_hdr.total_hops = htons (0);
+  box_hdr.without_fc = htons (without_fc);
+  // update_ephemeral (dv);
+  if (0 ==
+      GNUNET_TIME_absolute_get_remaining (dv->ephemeral_validity).rel_value_us)
+  {
+    GNUNET_CRYPTO_eddsa_kem_encaps (&dv->target.public_key,
+                                    &dv->ephemeral_key,
+                                    &km);
+    dv->km = GNUNET_new (struct GNUNET_HashCode);
+    GNUNET_memcpy (dv->km, &km, sizeof(struct GNUNET_HashCode));
+    sign_ephemeral (dv);
+  }
+  box_hdr.ephemeral_key = dv->ephemeral_key;
+  payload_hdr.sender_sig = dv->sender_sig;
+
+  GNUNET_CRYPTO_random_block (GNUNET_CRYPTO_QUALITY_NONCE,
+                              &box_hdr.iv,
+                              sizeof(box_hdr.iv));
+  // We are creating this key, so this must work.
+  // FIXME: Possibly also add return values here. We are processing
+  // Input from other peers...
+  dv_setup_key_state_from_km (dv->km, &box_hdr.iv, key);
+  payload_hdr.sender = GST_my_identity;
+  payload_hdr.monotonic_time = GNUNET_TIME_absolute_hton (dv->monotime);
+  dv_encrypt (key, &payload_hdr, enc, sizeof(payload_hdr));
+  dv_encrypt (key,
+              hdr,
+              &enc[sizeof(struct TransportDVBoxPayloadP)],
+              enc_body_size);
+  dv_hmac (key, &box_hdr.hmac, enc, sizeof(enc));
+  dv_key_clean (key);
+  rtt = GNUNET_TIME_UNIT_FOREVER_REL;
+  /* For each selected path, take the pre-computed header and body
+     and add the path in the middle of the message; then send it. */
+  for (unsigned int i = 0; i < num_dvhs; i++)
+  {
+    struct DistanceVectorHop *dvh = dvhs[i];
+    unsigned int num_hops = dvh->distance + 1;
+    char buf[sizeof(struct TransportDVBoxMessage)
+             + sizeof(struct GNUNET_PeerIdentity) * num_hops
+             + sizeof(struct TransportDVBoxPayloadP)
+             + enc_body_size] GNUNET_ALIGN;
+    struct GNUNET_PeerIdentity *dhops;
+
+    box_hdr.header.size = htons (sizeof(buf));
+    box_hdr.orig_size = htons (sizeof(buf));
+    box_hdr.num_hops = htons (num_hops);
+    memcpy (buf, &box_hdr, sizeof(box_hdr));
+    dhops = (struct GNUNET_PeerIdentity *) &buf[sizeof(box_hdr)];
+    memcpy (dhops,
+            dvh->path,
+            dvh->distance * sizeof(struct GNUNET_PeerIdentity));
+    dhops[dvh->distance] = dv->target;
+    if (GNUNET_EXTRA_LOGGING > 0)
+    {
+      char *path;
+
+      path = GNUNET_strdup (GNUNET_i2s (&GST_my_identity));
+      for (unsigned int j = 0; j < num_hops; j++)
+      {
+        char *tmp;
+
+        GNUNET_asprintf (&tmp, "%s-%s", path, GNUNET_i2s (&dhops[j]));
+        GNUNET_free (path);
+        path = tmp;
+      }
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "Routing message of type %u to %s using DV (#%u/%u) via %s\n",
+                  ntohs (hdr->type),
+                  GNUNET_i2s (&dv->target),
+                  i + 1,
+                  num_dvhs,
+                  path);
+      GNUNET_free (path);
+    }
+    rtt = GNUNET_TIME_relative_min (rtt, dvh->pd.aged_rtt);
+    memcpy (&dhops[num_hops], enc, sizeof(enc));
+    use (use_cls,
+         dvh->next_hop,
+         (const struct GNUNET_MessageHeader *) buf,
+         options);
+    GNUNET_free (key);
+  }
+  return rtt;
+}
+
+
+/**
+ * Wrapper around #route_via_neighbour() that matches the
+ * #DVMessageHandler structure.
+ *
+ * @param cls unused
+ * @param next_hop where to send next
+ * @param hdr header of the message to send
+ * @param options message options for queue selection
+ */
+static void
+send_dv_to_neighbour (void *cls,
+                      struct Neighbour *next_hop,
+                      const struct GNUNET_MessageHeader *hdr,
+                      enum RouteMessageOptions options)
+{
+  (void) cls;
+  (void) route_via_neighbour (next_hop, hdr, RMO_UNCONFIRMED_ALLOWED);
+}
+
+
+/**
+ * We need to transmit @a hdr to @a target.  If necessary, this may
+ * involve DV routing.  This function routes without applying flow
+ * control or congestion control and should only be used for control
+ * traffic.
+ *
+ * @param target peer to receive @a hdr
+ * @param hdr header of the message to route and #GNUNET_free()
+ * @param options which transmission channels are allowed
+ * @return expected RTT for transmission, #GNUNET_TIME_UNIT_FOREVER_REL if sending failed
+ */
+static struct GNUNET_TIME_Relative
+route_control_message_without_fc (struct VirtualLink *vl,
+// route_control_message_without_fc (const struct GNUNET_PeerIdentity *target,
+                                  const struct GNUNET_MessageHeader *hdr,
+                                  enum RouteMessageOptions options)
+{
+  // struct VirtualLink *vl;
+  struct Neighbour *n;
+  struct DistanceVector *dv;
+  struct GNUNET_TIME_Relative rtt1;
+  struct GNUNET_TIME_Relative rtt2;
+  const struct GNUNET_PeerIdentity *target = &vl->target;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Trying to route message of type %u to %s without fc\n",
+              ntohs (hdr->type),
+              GNUNET_i2s (target));
+
+  // TODO Do this elsewhere. vl should be given as parameter to method.
+  // vl = lookup_virtual_link (target);
+  GNUNET_assert (NULL != vl && GNUNET_YES == vl->confirmed);
+  if (NULL == vl)
+    return GNUNET_TIME_UNIT_FOREVER_REL;
+  n = vl->n;
+  dv = (0 != (options & RMO_DV_ALLOWED)) ? vl->dv : NULL;
+  if (0 == (options & RMO_UNCONFIRMED_ALLOWED))
+  {
+    /* if confirmed is required, and we do not have anything
+       confirmed, drop respective options */
+    if (NULL == n)
+      n = lookup_neighbour (target);
+    if ((NULL == dv) && (0 != (options & RMO_DV_ALLOWED)))
+      dv = GNUNET_CONTAINER_multipeermap_get (dv_routes, target);
+  }
+  if ((NULL == n) && (NULL == dv))
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+                "Cannot route message of type %u to %s: no route\n",
+                ntohs (hdr->type),
+                GNUNET_i2s (target));
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# Messages dropped in routing: no acceptable method",
+                              1,
+                              GNUNET_NO);
+    return GNUNET_TIME_UNIT_FOREVER_REL;
+  }
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Routing message of type %u to %s with options %X\n",
+              ntohs (hdr->type),
+              GNUNET_i2s (target),
+              (unsigned int) options);
+  /* If both dv and n are possible and we must choose:
+     flip a coin for the choice between the two; for now 50/50 */
+  if ((NULL != n) && (NULL != dv) && (0 == (options & RMO_REDUNDANT)))
+  {
+    if (0 == GNUNET_CRYPTO_random_u32 (GNUNET_CRYPTO_QUALITY_WEAK, 2))
+      n = NULL;
+    else
+      dv = NULL;
+  }
+  if ((NULL != n) && (NULL != dv))
+    options &= ~RMO_REDUNDANT; /* We will do one DV and one direct, that's
+                                  enough for redundancy, so clear the flag. */
+  rtt1 = GNUNET_TIME_UNIT_FOREVER_REL;
+  rtt2 = GNUNET_TIME_UNIT_FOREVER_REL;
+  if (NULL != n)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Try to route message of type %u to %s without fc via neighbour\n",
+                ntohs (hdr->type),
+                GNUNET_i2s (target));
+    rtt1 = route_via_neighbour (n, hdr, options);
+  }
+  if (NULL != dv)
+  {
+    struct DistanceVectorHop *hops[2];
+    unsigned int res;
+
+    res = pick_random_dv_hops (dv,
+                               options,
+                               hops,
+                               (0 == (options & RMO_REDUNDANT)) ? 1 : 2);
+    if (0 == res)
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+                  "Failed to route message, could not determine DV path\n");
+      return rtt1;
+    }
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "encapsulate_for_dv 1\n");
+    rtt2 = encapsulate_for_dv (dv,
+                               res,
+                               hops,
+                               hdr,
+                               &send_dv_to_neighbour,
+                               NULL,
+                               options & (~RMO_REDUNDANT),
+                               GNUNET_YES);
+  }
+  return GNUNET_TIME_relative_min (rtt1, rtt2);
+}
+
+
+static void
+consider_sending_fc (void *cls);
+
+/**
+ * Something changed on the virtual link with respect to flow
+ * control. Consider retransmitting the FC window size.
+ *
+ * @param cls a `struct VirtualLink` to work with
+ */
+static void
+task_consider_sending_fc (void *cls)
+{
+  struct VirtualLink *vl = cls;
+  vl->fc_retransmit_task = NULL;
+  consider_sending_fc (cls);
+}
+
+
+/**
+ * Something changed on the virtual link with respect to flow
+ * control. Consider retransmitting the FC window size.
+ *
+ * @param cls a `struct VirtualLink` to work with
+ */
+static void
+consider_sending_fc (void *cls)
+{
+  struct VirtualLink *vl = cls;
+  struct GNUNET_TIME_Absolute monotime;
+  struct TransportFlowControlMessage fc;
+  struct GNUNET_TIME_Relative duration;
+  struct GNUNET_TIME_Relative rtt;
+
+  duration = GNUNET_TIME_absolute_get_duration (vl->last_fc_transmission);
+  /* OPTIMIZE-FC-BDP: decide sane criteria on when to do this, instead of doing
+     it always! */
+  /* For example, we should probably ONLY do this if a bit more than
+     an RTT has passed, or if the window changed "significantly" since
+     then. See vl->last_fc_rtt! NOTE: to do this properly, we also
+     need an estimate for the bandwidth-delay-product for the entire
+     VL, as that determines "significantly". We have the delay, but
+     the bandwidth statistics need to be added for the VL!*/(void) duration;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Sending FC seq %u to %s with new window %llu\n",
+              (unsigned int) vl->fc_seq_gen,
+              GNUNET_i2s (&vl->target),
+              (unsigned long long) vl->incoming_fc_window_size);
+  monotime = GNUNET_TIME_absolute_get_monotonic (GST_cfg);
+  vl->last_fc_transmission = monotime;
+  fc.header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_FLOW_CONTROL);
+  fc.header.size = htons (sizeof(fc));
+  fc.seq = htonl (vl->fc_seq_gen++);
+  fc.inbound_window_size = GNUNET_htonll (vl->incoming_fc_window_size
+                                          + vl->incoming_fc_window_size_used
+                                          + vl->incoming_fc_window_size_loss);
+  fc.outbound_sent = GNUNET_htonll (vl->outbound_fc_window_size_used);
+  fc.outbound_window_size = GNUNET_htonll (vl->outbound_fc_window_size);
+  fc.sender_time = GNUNET_TIME_absolute_hton (monotime);
+  rtt = route_control_message_without_fc (vl, &fc.header, RMO_DV_ALLOWED);
+  if (GNUNET_TIME_UNIT_FOREVER_REL.rel_value_us == rtt.rel_value_us)
+  {
+    rtt = GNUNET_TIME_UNIT_SECONDS;
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "FC retransmission to %s failed, will retry in %s\n",
+                GNUNET_i2s (&vl->target),
+                GNUNET_STRINGS_relative_time_to_string (rtt, GNUNET_YES));
+    vl->last_fc_rtt = GNUNET_TIME_UNIT_ZERO;
+  }
+  else
+  {
+    /* OPTIMIZE-FC-BDP: rtt is not ideal, we can do better! */
+    vl->last_fc_rtt = rtt;
+  }
+  if (NULL != vl->fc_retransmit_task)
+    GNUNET_SCHEDULER_cancel (vl->fc_retransmit_task);
+  if (MAX_FC_RETRANSMIT_COUNT == vl->fc_retransmit_count)
+  {
+    rtt = GNUNET_TIME_UNIT_MINUTES;
+    vl->fc_retransmit_count = 0;
+  }
+  vl->fc_retransmit_task =
+    GNUNET_SCHEDULER_add_delayed (rtt, &task_consider_sending_fc, vl);
+  vl->fc_retransmit_count++;
+}
+
+
+/**
+ * There is a message at the head of the pending messages for @a vl
+ * which may be ready for transmission. Check if a queue is ready to
+ * take it.
+ *
+ * This function must (1) check for flow control to ensure that we can
+ * right now send to @a vl, (2) check that the pending message in the
+ * queue is actually eligible, (3) determine if any applicable queue
+ * (direct neighbour or DVH path) is ready to accept messages, and
+ * (4) prioritize based on the preferences associated with the
+ * pending message.
+ *
+ * So yeah, easy.
+ *
+ * @param vl virtual link where we should check for transmission
+ */
+static void
+check_vl_transmission (struct VirtualLink *vl)
+{
+  struct Neighbour *n = vl->n;
+  struct DistanceVector *dv = vl->dv;
+  struct GNUNET_TIME_Absolute now;
+  struct VirtualLink *vl_next_hop;
+  int elig;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "check_vl_transmission to target %s\n",
+              GNUNET_i2s (&vl->target));
+  /* Check that we have an eligible pending message!
+     (cheaper than having #transmit_on_queue() find out!) */
+  elig = GNUNET_NO;
+  for (struct PendingMessage *pm = vl->pending_msg_head; NULL != pm;
+       pm = pm->next_vl)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "check_vl_transmission loop\n");
+    if (NULL != pm->qe)
+      continue;   /* not eligible, is in a queue! */
+    if (pm->bytes_msg + vl->outbound_fc_window_size_used >
+        vl->outbound_fc_window_size)
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "Stalled message %" PRIu64
+                  " transmission on VL %s due to flow control: %llu < %llu\n",
+                  pm->logging_uuid,
+                  GNUNET_i2s (&vl->target),
+                  (unsigned long long) vl->outbound_fc_window_size,
+                  (unsigned long long) (pm->bytes_msg
+                                        + vl->outbound_fc_window_size_used));
+      consider_sending_fc (vl);
+      return;     /* We have a message, but flow control says "nope" */
+    }
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Target window on VL %s not stalled. Scheduling transmission on queue\n",
+                GNUNET_i2s (&vl->target));
+    /* Notify queues at direct neighbours that we are interested */
+    now = GNUNET_TIME_absolute_get ();
+    if (NULL != n)
+    {
+      for (struct Queue *queue = n->queue_head; NULL != queue;
+           queue = queue->next_neighbour)
+      {
+        if ((GNUNET_YES == queue->idle) &&
+            (queue->validated_until.abs_value_us > now.abs_value_us))
+        {
+          GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                      "Direct neighbour %s not stalled\n",
+                      GNUNET_i2s (&n->pid));
+          schedule_transmit_on_queue (GNUNET_TIME_UNIT_ZERO,
+                                      queue,
+                                      GNUNET_SCHEDULER_PRIORITY_DEFAULT);
+          elig = GNUNET_YES;
+        }
+        else
+          GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                      "Neighbour Queue QID: %u (%u) busy or invalid\n",
+                      queue->qid,
+                      queue->idle);
+      }
+    }
+    /* Notify queues via DV that we are interested */
+    if (NULL != dv)
+    {
+      /* Do DV with lower scheduler priority, which effectively means that
+         IF a neighbour exists and is available, we prefer it. */
+      for (struct DistanceVectorHop *pos = dv->dv_head; NULL != pos;
+           pos = pos->next_dv)
+      {
+        struct Neighbour *nh = pos->next_hop;
+
+
+        if (pos->path_valid_until.abs_value_us <= now.abs_value_us)
+          continue;   /* skip this one: path not validated */
+        else
+        {
+          vl_next_hop = lookup_virtual_link (&nh->pid);
+          GNUNET_assert (NULL != vl_next_hop);
+          if (pm->bytes_msg + vl_next_hop->outbound_fc_window_size_used >
+              vl_next_hop->outbound_fc_window_size)
+          {
+            GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                        "Stalled message %" PRIu64
+                        " transmission on next hop %s due to flow control: %llu < %llu\n",
+                        pm->logging_uuid,
+                        GNUNET_i2s (&vl_next_hop->target),
+                        (unsigned long
+                         long) vl_next_hop->outbound_fc_window_size,
+                        (unsigned long long) (pm->bytes_msg
+                                              + vl_next_hop->
+                                              outbound_fc_window_size_used));
+            consider_sending_fc (vl_next_hop);
+            continue; /* We have a message, but flow control says "nope" for the first hop of this path */
+          }
+          for (struct Queue *queue = nh->queue_head; NULL != queue;
+               queue = queue->next_neighbour)
+            if ((GNUNET_YES == queue->idle) &&
+                (queue->validated_until.abs_value_us > now.abs_value_us))
+            {
+              GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                          "Next hop neighbour %s not stalled\n",
+                          GNUNET_i2s (&nh->pid));
+              schedule_transmit_on_queue (GNUNET_TIME_UNIT_ZERO,
+                                          queue,
+                                          GNUNET_SCHEDULER_PRIORITY_BACKGROUND);
+              elig = GNUNET_YES;
+            }
+            else
+              GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                          "DV Queue QID: %u (%u) busy or invalid\n",
+                          queue->qid,
+                          queue->idle);
+        }
+      }
+    }
+    if (GNUNET_YES == elig)
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "Eligible message %" PRIu64 " of size %u to %s: %llu/%llu\n",
+                  pm->logging_uuid,
+                  pm->bytes_msg,
+                  GNUNET_i2s (&vl->target),
+                  (unsigned long long) vl->outbound_fc_window_size,
+                  (unsigned long long) (pm->bytes_msg
+                                        + vl->outbound_fc_window_size_used));
+    break;
+  }
+}
+
+
+/**
+ * Client asked for transmission to a peer.  Process the request.
+ *
+ * @param cls the client
+ * @param obm the send message that was sent
+ */
+static void
+handle_client_send (void *cls, const struct OutboundMessage *obm)
+{
+  struct TransportClient *tc = cls;
+  struct PendingMessage *pm;
+  const struct GNUNET_MessageHeader *obmm;
+  uint32_t bytes_msg;
+  struct VirtualLink *vl;
+  enum GNUNET_MQ_PriorityPreferences pp;
+
+  GNUNET_assert (CT_CORE == tc->type);
+  obmm = (const struct GNUNET_MessageHeader *) &obm[1];
+  bytes_msg = ntohs (obmm->size);
+  pp = (enum GNUNET_MQ_PriorityPreferences) ntohl (obm->priority);
+  vl = lookup_virtual_link (&obm->peer);
+  if ((NULL == vl) || (GNUNET_NO == vl->confirmed))
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Don't have %s as a neighbour (anymore).\n",
+                GNUNET_i2s (&obm->peer));
+    /* Failure: don't have this peer as a neighbour (anymore).
+       Might have gone down asynchronously, so this is NOT
+       a protocol violation by CORE. Still count the event,
+       as this should be rare. */
+    GNUNET_SERVICE_client_continue (tc->client);
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# messages dropped (neighbour unknown)",
+                              1,
+                              GNUNET_NO);
+    return;
+  }
+
+  pm = GNUNET_malloc (sizeof(struct PendingMessage) + bytes_msg);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "1 created pm %p storing vl %p\n",
+              pm,
+              vl);
+  pm->logging_uuid = logging_uuid_gen++;
+  pm->prefs = pp;
+  pm->client = tc;
+  pm->vl = vl;
+  pm->bytes_msg = bytes_msg;
+  memcpy (&pm[1], obmm, bytes_msg);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Sending message of type %u  with %u bytes as <%" PRIu64
+              "> to %s\n",
+              ntohs (obmm->type),
+              bytes_msg,
+              pm->logging_uuid,
+              GNUNET_i2s (&obm->peer));
+  GNUNET_CONTAINER_MDLL_insert (client,
+                                tc->details.core.pending_msg_head,
+                                tc->details.core.pending_msg_tail,
+                                pm);
+  GNUNET_CONTAINER_MDLL_insert (vl,
+                                vl->pending_msg_head,
+                                vl->pending_msg_tail,
+                                pm);
+  check_vl_transmission (vl);
+  GNUNET_SERVICE_client_continue (tc->client);
+}
+
+
+/**
+ * Communicator requests backchannel transmission.  Process the request.
+ * Just repacks it into our `struct TransportBackchannelEncapsulationMessage *`
+ * (which for now has exactly the same format, only a different message type)
+ * and passes it on for routing.
+ *
+ * @param cls the client
+ * @param cb the send message that was sent
+ */
+static void
+handle_communicator_backchannel (
+  void *cls,
+  const struct GNUNET_TRANSPORT_CommunicatorBackchannel *cb)
+{
+  struct Neighbour *n;
+  struct VirtualLink *vl;
+  struct TransportClient *tc = cls;
+  const struct GNUNET_MessageHeader *inbox =
+    (const struct GNUNET_MessageHeader *) &cb[1];
+  uint16_t isize = ntohs (inbox->size);
+  const char *is = ((const char *) &cb[1]) + isize;
+  size_t slen = strlen (is) + 1;
+  char
+    mbuf[slen + isize
+         + sizeof(struct
+                  TransportBackchannelEncapsulationMessage)] GNUNET_ALIGN;
+  struct TransportBackchannelEncapsulationMessage *be =
+    (struct TransportBackchannelEncapsulationMessage *) mbuf;
+
+  /* 0-termination of 'is' was checked already in
+   #check_communicator_backchannel() */
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Preparing backchannel transmission to %s:%s of type %u and size %u\n",
+              GNUNET_i2s (&cb->pid),
+              is,
+              ntohs (inbox->type),
+              ntohs (inbox->size));
+  /* encapsulate and encrypt message */
+  be->header.type =
+    htons (GNUNET_MESSAGE_TYPE_TRANSPORT_BACKCHANNEL_ENCAPSULATION);
+  be->header.size = htons (sizeof(mbuf));
+  memcpy (&be[1], inbox, isize);
+  memcpy (&mbuf[sizeof(struct TransportBackchannelEncapsulationMessage)
+                + isize],
+          is,
+          strlen (is) + 1);
+  // route_control_message_without_fc (&cb->pid, &be->header, RMO_DV_ALLOWED);
+  vl = lookup_virtual_link (&cb->pid);
+  if ((NULL != vl) && (GNUNET_YES == vl->confirmed))
+  {
+    route_control_message_without_fc (vl, &be->header, RMO_DV_ALLOWED);
+  }
+  else
+  {
+    /* Use route via neighbour */
+    n = lookup_neighbour (&cb->pid);
+    if (NULL != n)
+      route_via_neighbour (
+        n,
+        &be->header,
+        RMO_NONE);
+  }
+  GNUNET_SERVICE_client_continue (tc->client);
+}
+
+
+/**
+ * Address of our peer added.  Test message is well-formed.
+ *
+ * @param cls the client
+ * @param aam the send message that was sent
+ * @return #GNUNET_OK if message is well-formed
+ */
+static int
+check_add_address (void *cls,
+                   const struct GNUNET_TRANSPORT_AddAddressMessage *aam)
+{
+  struct TransportClient *tc = cls;
+
+  if (CT_COMMUNICATOR != tc->type)
+  {
+    GNUNET_break (0);
+    return GNUNET_SYSERR;
+  }
+  GNUNET_MQ_check_zero_termination (aam);
+  return GNUNET_OK;
+}
+
+
+/**
+ * Ask peerstore to store our address.
+ *
+ * @param cls an `struct AddressListEntry *`
+ */
+static void
+store_pi (void *cls);
+
+
+/**
+ * Function called when peerstore is done storing our address.
+ *
+ * @param cls a `struct AddressListEntry`
+ * @param success #GNUNET_YES if peerstore was successful
+ */
+static void
+peerstore_store_own_cb (void *cls, int success)
+{
+  struct AddressListEntry *ale = cls;
+
+  ale->sc = NULL;
+  if (GNUNET_YES != success)
+    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                "Failed to store our own address `%s' in peerstore!\n",
+                ale->address);
+  else
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Successfully stored our own address `%s' in peerstore!\n",
+                ale->address);
+  /* refresh period is 1/4 of expiration time, that should be plenty
+     without being excessive. */
+  ale->st =
+    GNUNET_SCHEDULER_add_delayed (GNUNET_TIME_relative_divide (ale->expiration,
+                                                               4ULL),
+                                  &store_pi,
+                                  ale);
+}
+
+
+static void
+shc_cont (void *cls, int success)
+{
+  struct AddressListEntry *ale = cls;
+  struct GNUNET_TIME_Absolute expiration;
+  expiration = GNUNET_TIME_relative_to_absolute (ale->expiration);
+  ale->sc = GNUNET_PEERSTORE_store (peerstore,
+                                    "transport",
+                                    &GST_my_identity,
+                                    GNUNET_PEERSTORE_TRANSPORT_HELLO_KEY,
+                                    ale->signed_address,
+                                    ale->signed_address_len,
+                                    expiration,
+                                    GNUNET_PEERSTORE_STOREOPTION_MULTIPLE,
+                                    &peerstore_store_own_cb,
+                                    ale);
+  if (NULL == ale->sc)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+                "Failed to store our address `%s' with peerstore\n",
+                ale->address);
+    ale->st = GNUNET_SCHEDULER_add_delayed (GNUNET_TIME_UNIT_SECONDS,
+                                            &store_pi,
+                                            ale);
+   }
+}
+
+
+/**
+ * Ask peerstore to store our address.
+ *
+ * @param cls an `struct AddressListEntry *`
+ */
+static void
+store_pi (void *cls)
+{
+  struct AddressListEntry *ale = cls;
+  struct GNUNET_MQ_Envelope *env;
+  const struct GNUNET_MessageHeader *msg;
+  const char *dash;
+  char *address_uri;
+  char *prefix = GNUNET_HELLO_address_to_prefix (ale->address);
+  unsigned int add_success;
+
+  dash = strchr (ale->address, '-');
+  GNUNET_assert (NULL != dash);
+  dash++;
+  GNUNET_asprintf (&address_uri,
+                   "%s://%s",
+                   prefix,
+                   dash);
+  GNUNET_free (prefix);
+  ale->st = NULL;
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Storing our address `%s' in peerstore until %s!\n",
+              ale->address,
+              GNUNET_STRINGS_absolute_time_to_string (hello_mono_time));
+  add_success = GNUNET_HELLO_builder_add_address (GST_my_hello,
+                                                  address_uri);
+  if (GNUNET_OK != add_success)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+                "Storing our address `%s' %s\n",
+                address_uri,
+                GNUNET_NO == add_success ? "not done" : "failed");
+    GNUNET_free (address_uri);
+    return;
+  }
+  // FIXME hello_mono_time used here?? What about expiration in ale?
+  GNUNET_HELLO_sign_address (ale->address,
+                             ale->nt,
+                             hello_mono_time,
+                             GST_my_private_key,
+                             &ale->signed_address,
+                             &ale->signed_address_len);
+  GNUNET_free (address_uri);
+  env = GNUNET_HELLO_builder_to_env (GST_my_hello,
+                                     GST_my_private_key,
+                                     GNUNET_TIME_UNIT_ZERO);
+  msg = GNUNET_MQ_env_get_msg (env);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "store_pi 1\n");
+  ale->shc = GNUNET_PEERSTORE_hello_add (peerstore,
+                                         msg,
+                                         shc_cont,
+                                         ale);
+  GNUNET_free (env);
+}
+
+
+/**
+ * Address of our peer added.  Process the request.
+ *
+ * @param cls the client
+ * @param aam the send message that was sent
+ */
+static void
+handle_add_address (void *cls,
+                    const struct GNUNET_TRANSPORT_AddAddressMessage *aam)
+{
+  struct TransportClient *tc = cls;
+  struct AddressListEntry *ale;
+  size_t slen;
+
+  /* 0-termination of &aam[1] was checked in #check_add_address */
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Communicator added address `%s'!\n",
+              (const char *) &aam[1]);
+  slen = ntohs (aam->header.size) - sizeof(*aam);
+  ale = GNUNET_malloc (sizeof(struct AddressListEntry) + slen);
+  ale->tc = tc;
+  ale->address = (const char *) &ale[1];
+  ale->expiration = GNUNET_TIME_relative_ntoh (aam->expiration);
+  ale->aid = aam->aid;
+  ale->nt = (enum GNUNET_NetworkType) ntohl (aam->nt);
+  memcpy (&ale[1], &aam[1], slen);
+  GNUNET_CONTAINER_DLL_insert (tc->details.communicator.addr_head,
+                               tc->details.communicator.addr_tail,
+                               ale);
+  ale->st = GNUNET_SCHEDULER_add_now (&store_pi, ale);
+  GNUNET_SERVICE_client_continue (tc->client);
+}
+
+
+/**
+ * Address of our peer deleted.  Process the request.
+ *
+ * @param cls the client
+ * @param dam the send message that was sent
+ */
+static void
+handle_del_address (void *cls,
+                    const struct GNUNET_TRANSPORT_DelAddressMessage *dam)
+{
+  struct TransportClient *tc = cls;
+  struct AddressListEntry *alen;
+
+  if (CT_COMMUNICATOR != tc->type)
+  {
+    GNUNET_break (0);
+    GNUNET_SERVICE_client_drop (tc->client);
+    return;
+  }
+  for (struct AddressListEntry *ale = tc->details.communicator.addr_head;
+       NULL != ale;
+       ale = alen)
+  {
+    alen = ale->next;
+    if (dam->aid != ale->aid)
+      continue;
+    GNUNET_assert (ale->tc == tc);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Communicator deleted address `%s'!\n",
+                ale->address);
+    free_address_list_entry (ale);
+    GNUNET_SERVICE_client_continue (tc->client);
+    return;
+  }
+  GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+              "Communicator removed address we did not even have.\n");
+  GNUNET_SERVICE_client_continue (tc->client);
+  // GNUNET_SERVICE_client_drop (tc->client);
+}
+
+
+/**
+ * Given an inbound message @a msg from a communicator @a cmc,
+ * demultiplex it based on the type calling the right handler.
+ *
+ * @param cmc context for demultiplexing
+ * @param msg message to demultiplex
+ */
+static void
+demultiplex_with_cmc (struct CommunicatorMessageContext *cmc);
+
+
+/**
+ * Function called when we are done giving a message of a certain
+ * size to CORE and should thus decrement the number of bytes of
+ * RAM reserved for that peer's MQ.
+ *
+ * @param cls a `struct CoreSentContext`
+ */
+static void
+core_env_sent_cb (void *cls)
+{
+  struct CoreSentContext *ctx = cls;
+  struct VirtualLink *vl = ctx->vl;
+
+  if (NULL == vl)
+  {
+    /* lost the link in the meantime, ignore */
+    GNUNET_free (ctx);
+    return;
+  }
+  GNUNET_CONTAINER_DLL_remove (vl->csc_head, vl->csc_tail, ctx);
+  GNUNET_assert (vl->incoming_fc_window_size_ram >= ctx->size);
+  vl->incoming_fc_window_size_ram -= ctx->size;
+  vl->incoming_fc_window_size_used += ctx->isize;
+  consider_sending_fc (vl);
+  GNUNET_free (ctx);
+}
+
+
+static void
+finish_handling_raw_message (struct VirtualLink *vl,
+                             const struct GNUNET_MessageHeader *mh,
+                             struct CommunicatorMessageContext *cmc,
+                             unsigned int continue_client)
+{
+  uint16_t size = ntohs (mh->size);
+  int have_core;
+
+  if (vl->incoming_fc_window_size_ram > UINT_MAX - size)
+  {
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# CORE messages dropped (FC arithmetic overflow)",
+                              1,
+                              GNUNET_NO);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "CORE messages of type %u with %u bytes dropped (FC arithmetic overflow)\n",
+                (unsigned int) ntohs (mh->type),
+                (unsigned int) ntohs (mh->size));
+    finish_cmc_handling_with_continue (cmc, continue_client);
+    return;
+  }
+  if (vl->incoming_fc_window_size_ram + size > vl->available_fc_window_size)
+  {
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# CORE messages dropped (FC window overflow)",
+                              1,
+                              GNUNET_NO);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "CORE messages of type %u with %u bytes dropped (FC window overflow)\n",
+                (unsigned int) ntohs (mh->type),
+                (unsigned int) ntohs (mh->size));
+    finish_cmc_handling_with_continue (cmc, continue_client);
+    return;
+  }
+
+  /* Forward to all CORE clients */
+  have_core = GNUNET_NO;
+  for (struct TransportClient *tc = clients_head; NULL != tc; tc = tc->next)
+  {
+    struct GNUNET_MQ_Envelope *env;
+    struct InboundMessage *im;
+    struct CoreSentContext *ctx;
+
+    if (CT_CORE != tc->type)
+      continue;
+    vl->incoming_fc_window_size_ram += size;
+    env = GNUNET_MQ_msg_extra (im, size, GNUNET_MESSAGE_TYPE_TRANSPORT_RECV);
+    ctx = GNUNET_new (struct CoreSentContext);
+    ctx->vl = vl;
+    ctx->size = size;
+    ctx->isize = (GNUNET_NO == have_core) ? size : 0;
+    have_core = GNUNET_YES;
+    GNUNET_CONTAINER_DLL_insert (vl->csc_head, vl->csc_tail, ctx);
+    GNUNET_MQ_notify_sent (env, &core_env_sent_cb, ctx);
+    im->peer = cmc->im.sender;
+    memcpy (&im[1], mh, size);
+    GNUNET_MQ_send (tc->mq, env);
+    vl->core_recv_window--;
+  }
+  if (GNUNET_NO == have_core)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+                "Dropped message to CORE: no CORE client connected!\n");
+    /* Nevertheless, count window as used, as it is from the
+       perspective of the other peer! */
+    vl->incoming_fc_window_size_used += size;
+    /* TODO-M1 */
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Dropped message of type %u with %u bytes to CORE: no CORE client connected!\n",
+                (unsigned int) ntohs (mh->type),
+                (unsigned int) ntohs (mh->size));
+    finish_cmc_handling_with_continue (cmc, continue_client);
+    return;
+  }
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Delivered message from %s of type %u to CORE recv window %d\n",
+              GNUNET_i2s (&cmc->im.sender),
+              ntohs (mh->type),
+              vl->core_recv_window);
+  if (vl->core_recv_window > 0)
+  {
+    finish_cmc_handling_with_continue (cmc, continue_client);
+    return;
+  }
+  /* Wait with calling #finish_cmc_handling(cmc) until the message
+     was processed by CORE MQs (for CORE flow control)! */
+  GNUNET_CONTAINER_DLL_insert (vl->cmc_head, vl->cmc_tail, cmc);
+}
+
+
+/**
+ * Communicator gave us an unencapsulated message to pass as-is to
+ * CORE.  Process the request.
+ *
+ * @param cls a `struct CommunicatorMessageContext` (must call
+ * #finish_cmc_handling() when done)
+ * @param mh the message that was received
+ */
+static void
+handle_raw_message (void *cls, const struct GNUNET_MessageHeader *mh)
+{
+  struct CommunicatorMessageContext *cmc = cls;
+  // struct CommunicatorMessageContext *cmc_copy =
+  // GNUNET_new (struct CommunicatorMessageContext);
+  struct GNUNET_MessageHeader *mh_copy;
+  struct RingBufferEntry *rbe;
+  struct VirtualLink *vl;
+  uint16_t size = ntohs (mh->size);
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Handling raw message of type %u with %u bytes\n",
+              (unsigned int) ntohs (mh->type),
+              (unsigned int) ntohs (mh->size));
+
+  if ((size > UINT16_MAX - sizeof(struct InboundMessage)) ||
+      (size < sizeof(struct GNUNET_MessageHeader)))
+  {
+    struct GNUNET_SERVICE_Client *client = cmc->tc->client;
+
+    GNUNET_break (0);
+    finish_cmc_handling (cmc);
+    GNUNET_SERVICE_client_drop (client);
+    return;
+  }
+  vl = lookup_virtual_link (&cmc->im.sender);
+  if ((NULL == vl) || (GNUNET_NO == vl->confirmed))
+  {
+    /* FIXME: sender is giving us messages for CORE but we don't have
+       the link up yet! I *suspect* this can happen right now (i.e.
+       sender has verified us, but we didn't verify sender), but if
+       we pass this on, CORE would be confused (link down, messages
+       arrive).  We should investigate more if this happens often,
+       or in a persistent manner, and possibly do "something" about
+       it. Thus logging as error for now. */
+
+    mh_copy = GNUNET_malloc (size);
+    rbe = GNUNET_new (struct RingBufferEntry);
+    rbe->cmc = cmc;
+    /*cmc_copy->tc = cmc->tc;
+       cmc_copy->im = cmc->im;*/
+    GNUNET_memcpy (mh_copy, mh, size);
+
+    rbe->mh = mh_copy;
+
+    ring_buffer[ring_buffer_head] = rbe;// cmc_copy;
+    // cmc_copy->mh = (const struct GNUNET_MessageHeader *) mh_copy;
+    cmc->mh = (const struct GNUNET_MessageHeader *) mh_copy;
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Storing message for %s and type %u (%u) in ring buffer\n",
+                GNUNET_i2s (&cmc->im.sender),
+                (unsigned int) ntohs (mh->type),
+                (unsigned int) ntohs (mh_copy->type));
+    if (RING_BUFFER_SIZE - 1 == ring_buffer_head)
+    {
+      ring_buffer_head = 0;
+      is_ring_buffer_full = GNUNET_YES;
+    }
+    else
+      ring_buffer_head++;
+
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "%u items stored in ring buffer\n",
+                ring_buffer_head);
+
+    /*GNUNET_break_op (0);
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# CORE messages dropped (virtual link still down)",
+                              1,
+                              GNUNET_NO);
+
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "CORE messages of type %u with %u bytes dropped (virtual link still down)\n",
+                (unsigned int) ntohs (mh->type),
+                (unsigned int) ntohs (mh->size));
+    finish_cmc_handling (cmc);*/
+    GNUNET_SERVICE_client_continue (cmc->tc->client);
+    // GNUNET_free (cmc);
+    return;
+  }
+  finish_handling_raw_message (vl, mh, cmc, GNUNET_YES);
+}
+
+
+/**
+ * Communicator gave us a fragment box.  Check the message.
+ *
+ * @param cls a `struct CommunicatorMessageContext`
+ * @param fb the send message that was sent
+ * @return #GNUNET_YES if message is well-formed
+ */
+static int
+check_fragment_box (void *cls, const struct TransportFragmentBoxMessage *fb)
+{
+  uint16_t size = ntohs (fb->header.size);
+  uint16_t bsize = size - sizeof(*fb);
+
+  (void) cls;
+  if (0 == bsize)
+  {
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  if (bsize + ntohs (fb->frag_off) > ntohs (fb->msg_size))
+  {
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  if (ntohs (fb->frag_off) >= ntohs (fb->msg_size))
+  {
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  return GNUNET_YES;
+}
+
+
+/**
+ * Clean up an idle cumulative acknowledgement data structure.
+ *
+ * @param cls a `struct AcknowledgementCummulator *`
+ */
+static void
+destroy_ack_cummulator (void *cls)
+{
+  struct AcknowledgementCummulator *ac = cls;
+
+  ac->task = NULL;
+  GNUNET_assert (0 == ac->num_acks);
+  GNUNET_assert (
+    GNUNET_YES ==
+    GNUNET_CONTAINER_multipeermap_remove (ack_cummulators, &ac->target, ac));
+  GNUNET_free (ac);
+}
+
+
+/**
+ * Do the transmission of a cumulative acknowledgement now.
+ *
+ * @param cls a `struct AcknowledgementCummulator *`
+ */
+static void
+transmit_cummulative_ack_cb (void *cls)
+{
+  struct Neighbour *n;
+  struct VirtualLink *vl;
+  struct AcknowledgementCummulator *ac = cls;
+  char buf[sizeof(struct TransportReliabilityAckMessage)
+           + ac->num_acks
+           * sizeof(struct TransportCummulativeAckPayloadP)] GNUNET_ALIGN;
+  struct TransportReliabilityAckMessage *ack =
+    (struct TransportReliabilityAckMessage *) buf;
+  struct TransportCummulativeAckPayloadP *ap;
+
+  ac->task = NULL;
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Sending ACK with %u components to %s\n",
+              ac->num_acks,
+              GNUNET_i2s (&ac->target));
+  GNUNET_assert (0 < ac->num_acks);
+  ack->header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_ACK);
+  ack->header.size =
+    htons (sizeof(*ack)
+           + ac->num_acks * sizeof(struct TransportCummulativeAckPayloadP));
+  ack->ack_counter = htonl (ac->ack_counter += ac->num_acks);
+  ap = (struct TransportCummulativeAckPayloadP *) &ack[1];
+  for (unsigned int i = 0; i < ac->num_acks; i++)
+  {
+    ap[i].ack_uuid = ac->ack_uuids[i].ack_uuid;
+    ap[i].ack_delay = GNUNET_TIME_relative_hton (
+      GNUNET_TIME_absolute_get_duration (ac->ack_uuids[i].receive_time));
+  }
+  /*route_control_message_without_fc (
+    &ac->target,
+    &ack->header,
+    RMO_DV_ALLOWED);*/
+  vl = lookup_virtual_link (&ac->target);
+  if ((NULL != vl) && (GNUNET_YES == vl->confirmed))
+  {
+    route_control_message_without_fc (
+      vl,
+      &ack->header,
+      RMO_DV_ALLOWED);
+  }
+  else
+  {
+    /* Use route via neighbour */
+    n = lookup_neighbour (&ac->target);
+    if (NULL != n)
+      route_via_neighbour (
+        n,
+        &ack->header,
+        RMO_NONE);
+  }
+  ac->num_acks = 0;
+  ac->task = GNUNET_SCHEDULER_add_delayed (ACK_CUMMULATOR_TIMEOUT,
+                                           &destroy_ack_cummulator,
+                                           ac);
+}
+
+
+/**
+ * Transmit an acknowledgement for @a ack_uuid to @a pid delaying
+ * transmission by at most @a ack_delay.
+ *
+ * @param pid target peer
+ * @param ack_uuid UUID to ack
+ * @param max_delay how long can the ACK wait
+ */
+static void
+cummulative_ack (const struct GNUNET_PeerIdentity *pid,
+                 const struct AcknowledgementUUIDP *ack_uuid,
+                 struct GNUNET_TIME_Absolute max_delay)
+{
+  struct AcknowledgementCummulator *ac;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Scheduling ACK %s for transmission to %s\n",
+              GNUNET_uuid2s (&ack_uuid->value),
+              GNUNET_i2s (pid));
+  ac = GNUNET_CONTAINER_multipeermap_get (ack_cummulators, pid);
+  if (NULL == ac)
+  {
+    ac = GNUNET_new (struct AcknowledgementCummulator);
+    ac->target = *pid;
+    ac->min_transmission_time = max_delay;
+    GNUNET_assert (GNUNET_YES ==
+                   GNUNET_CONTAINER_multipeermap_put (
+                     ack_cummulators,
+                     &ac->target,
+                     ac,
+                     GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
+  }
+  else
+  {
+    if (MAX_CUMMULATIVE_ACKS == ac->num_acks)
+    {
+      /* must run immediately, ack buffer full! */
+      transmit_cummulative_ack_cb (ac);
+    }
+    GNUNET_SCHEDULER_cancel (ac->task);
+    ac->min_transmission_time =
+      GNUNET_TIME_absolute_min (ac->min_transmission_time, max_delay);
+  }
+  GNUNET_assert (ac->num_acks < MAX_CUMMULATIVE_ACKS);
+  ac->ack_uuids[ac->num_acks].receive_time = GNUNET_TIME_absolute_get ();
+  ac->ack_uuids[ac->num_acks].ack_uuid = *ack_uuid;
+  ac->num_acks++;
+  ac->task = GNUNET_SCHEDULER_add_at (ac->min_transmission_time,
+                                      &transmit_cummulative_ack_cb,
+                                      ac);
+}
+
+
+/**
+ * Closure for #find_by_message_uuid.
+ */
+struct FindByMessageUuidContext
+{
+  /**
+   * UUID to look for.
+   */
+  struct MessageUUIDP message_uuid;
+
+  /**
+   * Set to the reassembly context if found.
+   */
+  struct ReassemblyContext *rc;
+};
+
+
+/**
+ * Iterator called to find a reassembly context by the message UUID in the
+ * multihashmap32.
+ *
+ * @param cls a `struct FindByMessageUuidContext`
+ * @param key a key (unused)
+ * @param value a `struct ReassemblyContext`
+ * @return #GNUNET_YES if not found, #GNUNET_NO if found
+ */
+static int
+find_by_message_uuid (void *cls, uint32_t key, void *value)
+{
+  struct FindByMessageUuidContext *fc = cls;
+  struct ReassemblyContext *rc = value;
+
+  (void) key;
+  if (0 == GNUNET_memcmp (&fc->message_uuid, &rc->msg_uuid))
+  {
+    fc->rc = rc;
+    return GNUNET_NO;
+  }
+  return GNUNET_YES;
+}
+
+
+/**
+ * Communicator gave us a fragment.  Process the request.
+ *
+ * @param cls a `struct CommunicatorMessageContext` (must call
+ * #finish_cmc_handling() when done)
+ * @param fb the message that was received
+ */
+static void
+handle_fragment_box (void *cls, const struct TransportFragmentBoxMessage *fb)
+{
+  struct CommunicatorMessageContext *cmc = cls;
+  struct VirtualLink *vl;
+  struct ReassemblyContext *rc;
+  const struct GNUNET_MessageHeader *msg;
+  uint16_t msize;
+  uint16_t fsize;
+  uint16_t frag_off;
+  char *target;
+  struct GNUNET_TIME_Relative cdelay;
+  struct FindByMessageUuidContext fc;
+
+  vl = lookup_virtual_link (&cmc->im.sender);
+  if ((NULL == vl) || (GNUNET_NO == vl->confirmed))
+  {
+    struct GNUNET_SERVICE_Client *client = cmc->tc->client;
+
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "No virtual link for %s to handle fragment\n",
+                GNUNET_i2s (&cmc->im.sender));
+    GNUNET_break (0);
+    finish_cmc_handling (cmc);
+    GNUNET_SERVICE_client_drop (client);
+    return;
+  }
+  if (NULL == vl->reassembly_map)
+  {
+    vl->reassembly_map = GNUNET_CONTAINER_multihashmap32_create (8);
+    vl->reassembly_heap =
+      GNUNET_CONTAINER_heap_create (GNUNET_CONTAINER_HEAP_ORDER_MIN);
+    vl->reassembly_timeout_task =
+      GNUNET_SCHEDULER_add_delayed (REASSEMBLY_EXPIRATION,
+                                    &reassembly_cleanup_task,
+                                    vl);
+  }
+  msize = ntohs (fb->msg_size);
+  fc.message_uuid = fb->msg_uuid;
+  fc.rc = NULL;
+  (void) GNUNET_CONTAINER_multihashmap32_get_multiple (vl->reassembly_map,
+                                                       fb->msg_uuid.uuid,
+                                                       &find_by_message_uuid,
+                                                       &fc);
+  fsize = ntohs (fb->header.size) - sizeof(*fb);
+  if (NULL == (rc = fc.rc))
+  {
+    rc = GNUNET_malloc (sizeof(*rc) + msize    /* reassembly payload buffer */
+                        + (msize + 7) / 8 * sizeof(uint8_t) /* bitfield */);
+    rc->msg_uuid = fb->msg_uuid;
+    rc->virtual_link = vl;
+    rc->msg_size = msize;
+    rc->reassembly_timeout =
+      GNUNET_TIME_relative_to_absolute (REASSEMBLY_EXPIRATION);
+    rc->last_frag = GNUNET_TIME_absolute_get ();
+    rc->hn = GNUNET_CONTAINER_heap_insert (vl->reassembly_heap,
+                                           rc,
+                                           rc->reassembly_timeout.abs_value_us);
+    GNUNET_assert (GNUNET_OK ==
+                   GNUNET_CONTAINER_multihashmap32_put (
+                     vl->reassembly_map,
+                     rc->msg_uuid.uuid,
+                     rc,
+                     GNUNET_CONTAINER_MULTIHASHMAPOPTION_MULTIPLE));
+    target = (char *) &rc[1];
+    rc->bitfield = (uint8_t *) (target + rc->msg_size);
+    if (fsize != rc->msg_size)
+      rc->msg_missing = rc->msg_size;
+    else
+      rc->msg_missing = 0;
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Received fragment with size %u at offset %u/%u %u bytes missing from %s for NEW message %u\n",
+                fsize,
+                ntohs (fb->frag_off),
+                msize,
+                rc->msg_missing,
+                GNUNET_i2s (&cmc->im.sender),
+                (unsigned int) fb->msg_uuid.uuid);
+  }
+  else
+  {
+    target = (char *) &rc[1];
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Received fragment at offset %u/%u from %s for message %u\n",
+                ntohs (fb->frag_off),
+                msize,
+                GNUNET_i2s (&cmc->im.sender),
+                (unsigned int) fb->msg_uuid.uuid);
+  }
+  if (msize != rc->msg_size)
+  {
+    GNUNET_break (0);
+    finish_cmc_handling (cmc);
+    return;
+  }
+
+  /* reassemble */
+  if (0 == fsize)
+  {
+    GNUNET_break (0);
+    finish_cmc_handling (cmc);
+    return;
+  }
+  frag_off = ntohs (fb->frag_off);
+  if (frag_off + fsize > msize)
+  {
+    /* Fragment (plus fragment size) exceeds message size! */
+    GNUNET_break_op (0);
+    finish_cmc_handling (cmc);
+    return;
+  }
+  memcpy (&target[frag_off], &fb[1], fsize);
+  /* update bitfield and msg_missing */
+  for (unsigned int i = frag_off; i < frag_off + fsize; i++)
+  {
+    if (0 == (rc->bitfield[i / 8] & (1 << (i % 8))))
+    {
+      rc->bitfield[i / 8] |= (1 << (i % 8));
+      rc->msg_missing--;
+    }
+  }
+
+  /* Compute cumulative ACK */
+  cdelay = GNUNET_TIME_absolute_get_duration (rc->last_frag);
+  cdelay = GNUNET_TIME_relative_multiply (cdelay, rc->msg_missing / fsize);
+  if (0 == rc->msg_missing)
+    cdelay = GNUNET_TIME_UNIT_ZERO;
+  cummulative_ack (&cmc->im.sender,
+                   &fb->ack_uuid,
+                   GNUNET_TIME_relative_to_absolute (cdelay));
+  rc->last_frag = GNUNET_TIME_absolute_get ();
+  /* is reassembly complete? */
+  if (0 != rc->msg_missing)
+  {
+    finish_cmc_handling (cmc);
+    return;
+  }
+  /* reassembly is complete, verify result */
+  msg = (const struct GNUNET_MessageHeader *) &rc[1];
+  if (ntohs (msg->size) != rc->msg_size)
+  {
+    GNUNET_break (0);
+    free_reassembly_context (rc);
+    finish_cmc_handling (cmc);
+    return;
+  }
+  /* successful reassembly */
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Fragment reassembly complete for message %u\n",
+              (unsigned int) fb->msg_uuid.uuid);
+  /* FIXME: check that the resulting msg is NOT a
+     DV Box or Reliability Box, as that is NOT allowed! */
+  cmc->mh = msg;
+  demultiplex_with_cmc (cmc);
+  /* FIXME-OPTIMIZE: really free here? Might be bad if fragments are still
+     en-route and we forget that we finished this reassembly immediately!
+     -> keep around until timeout?
+     -> shorten timeout based on ACK? */
+  free_reassembly_context (rc);
+}
+
+
+/**
+ * Communicator gave us a reliability box.  Check the message.
+ *
+ * @param cls a `struct CommunicatorMessageContext`
+ * @param rb the send message that was sent
+ * @return #GNUNET_YES if message is well-formed
+ */
+static int
+check_reliability_box (void *cls,
+                       const struct TransportReliabilityBoxMessage *rb)
+{
+  (void) cls;
+  const struct GNUNET_MessageHeader *inbox =  (const struct
+                                               GNUNET_MessageHeader *) &rb[1];
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "check_send_msg with size %u: inner msg type %u and size %u (%lu %lu)\n",
+              ntohs (rb->header.size),
+              ntohs (inbox->type),
+              ntohs (inbox->size),
+              sizeof (struct TransportReliabilityBoxMessage),
+              sizeof (struct GNUNET_MessageHeader));
+  GNUNET_MQ_check_boxed_message (rb);
+  return GNUNET_YES;
+}
+
+
+/**
+ * Communicator gave us a reliability box.  Process the request.
+ *
+ * @param cls a `struct CommunicatorMessageContext` (must call
+ * #finish_cmc_handling() when done)
+ * @param rb the message that was received
+ */
+static void
+handle_reliability_box (void *cls,
+                        const struct TransportReliabilityBoxMessage *rb)
+{
+  struct CommunicatorMessageContext *cmc = cls;
+  const struct GNUNET_MessageHeader *inbox =
+    (const struct GNUNET_MessageHeader *) &rb[1];
+  struct GNUNET_TIME_Relative rtt;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Received reliability box from %s with UUID %s of type %u\n",
+              GNUNET_i2s (&cmc->im.sender),
+              GNUNET_uuid2s (&rb->ack_uuid.value),
+              (unsigned int) ntohs (inbox->type));
+  rtt = GNUNET_TIME_UNIT_SECONDS; /* FIXME: should base this on "RTT", but we
+                                     do not really have an RTT for the
+                                   * incoming* queue (should we have
+                                     the sender add it to the rb message?) */
+  cummulative_ack (
+    &cmc->im.sender,
+    &rb->ack_uuid,
+    (0 == ntohl (rb->ack_countdown))
+    ? GNUNET_TIME_UNIT_ZERO_ABS
+    : GNUNET_TIME_relative_to_absolute (
+      GNUNET_TIME_relative_divide (rtt, 8 /* FIXME: magic constant */)));
+  /* continue with inner message */
+  /* FIXME: check that inbox is NOT a DV Box, fragment or another
+     reliability box (not allowed!) */
+  cmc->mh = inbox;
+  demultiplex_with_cmc (cmc);
+}
+
+
+/**
+ * Check if we have advanced to another age since the last time.  If
+ * so, purge ancient statistics (more than GOODPUT_AGING_SLOTS before
+ * the current age)
+ *
+ * @param[in,out] pd data to update
+ * @param age current age
+ */
+static void
+update_pd_age (struct PerformanceData *pd, unsigned int age)
+{
+  unsigned int sage;
+
+  if (age == pd->last_age)
+    return; /* nothing to do */
+  sage = GNUNET_MAX (pd->last_age, age - 2 * GOODPUT_AGING_SLOTS);
+  for (unsigned int i = sage; i <= age - GOODPUT_AGING_SLOTS; i++)
+  {
+    struct TransmissionHistoryEntry *the = &pd->the[i % GOODPUT_AGING_SLOTS];
+
+    the->bytes_sent = 0;
+    the->bytes_received = 0;
+  }
+  pd->last_age = age;
+}
+
+
+/**
+ * Update @a pd based on the latest @a rtt and the number of bytes
+ * that were confirmed to be successfully transmitted.
+ *
+ * @param[in,out] pd data to update
+ * @param rtt latest round-trip time
+ * @param bytes_transmitted_ok number of bytes receiver confirmed as received
+ */
+static void
+update_performance_data (struct PerformanceData *pd,
+                         struct GNUNET_TIME_Relative rtt,
+                         uint16_t bytes_transmitted_ok)
+{
+  uint64_t nval = rtt.rel_value_us;
+  uint64_t oval = pd->aged_rtt.rel_value_us;
+  unsigned int age = get_age ();
+  struct TransmissionHistoryEntry *the = &pd->the[age % GOODPUT_AGING_SLOTS];
+
+  if (oval == GNUNET_TIME_UNIT_FOREVER_REL.rel_value_us)
+    pd->aged_rtt = rtt;
+  else
+    pd->aged_rtt.rel_value_us = (nval + 7 * oval) / 8;
+  update_pd_age (pd, age);
+  the->bytes_received += bytes_transmitted_ok;
+}
+
+
+/**
+ * We have successfully transmitted data via @a q, update metrics.
+ *
+ * @param q queue to update
+ * @param rtt round trip time observed
+ * @param bytes_transmitted_ok number of bytes successfully transmitted
+ */
+static void
+update_queue_performance (struct Queue *q,
+                          struct GNUNET_TIME_Relative rtt,
+                          uint16_t bytes_transmitted_ok)
+{
+  update_performance_data (&q->pd, rtt, bytes_transmitted_ok);
+}
+
+
+/**
+ * We have successfully transmitted data via @a dvh, update metrics.
+ *
+ * @param dvh distance vector path data to update
+ * @param rtt round trip time observed
+ * @param bytes_transmitted_ok number of bytes successfully transmitted
+ */
+static void
+update_dvh_performance (struct DistanceVectorHop *dvh,
+                        struct GNUNET_TIME_Relative rtt,
+                        uint16_t bytes_transmitted_ok)
+{
+  update_performance_data (&dvh->pd, rtt, bytes_transmitted_ok);
+}
+
+
+/**
+ * We have completed transmission of @a pm, remove it from
+ * the transmission queues (and if it is a fragment, continue
+ * up the tree as necessary).
+ *
+ * @param pm pending message that was transmitted
+ */
+static void
+completed_pending_message (struct PendingMessage *pm)
+{
+  struct PendingMessage *pos;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Complete transmission of message %" PRIu64 " %u\n",
+              pm->logging_uuid,
+              pm->pmt);
+  switch (pm->pmt)
+  {
+  case PMT_CORE:
+  case PMT_RELIABILITY_BOX:
+    /* Full message sent, we are done */
+    client_send_response (pm);
+    return;
+
+  case PMT_FRAGMENT_BOX:
+    /* Fragment sent over reliable channel */
+    pos = pm->frag_parent;
+    GNUNET_CONTAINER_MDLL_remove (frag, pos->head_frag, pos->tail_frag, pm);
+    free_pending_message (pm);
+    /* check if subtree is done */
+    while ((NULL == pos->head_frag) && (pos->frag_off == (pos->bytes_msg
+                                                          - sizeof(struct
+                                                                   TransportFragmentBoxMessage)))
+           &&
+           (NULL != pos->frag_parent))
+    {
+      pm = pos;
+      pos = pm->frag_parent;
+      if ((NULL == pos) && (PMT_DV_BOX == pm->pmt))
+      {
+        client_send_response (pm);
+        return;
+      }
+      else if (PMT_DV_BOX == pm->pmt)
+      {
+        client_send_response (pos);
+        return;
+      }
+      GNUNET_CONTAINER_MDLL_remove (frag, pos->head_frag, pos->tail_frag, pm);
+      free_pending_message (pm);
+    }
+
+    /* Was this the last applicable fragment? */
+    if ((NULL == pos->head_frag) && (NULL == pos->frag_parent) &&
+        (pos->frag_off == pos->bytes_msg))
+      client_send_response (pos);
+    return;
+
+  case PMT_DV_BOX:
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Completed transmission of message %" PRIu64 " (DV Box)\n",
+                pm->logging_uuid);
+    if (NULL != pm->frag_parent)
+    {
+      pos = pm->frag_parent;
+      free_pending_message (pm);
+      pos->bpm = NULL;
+      client_send_response (pos);
+    }
+    else
+      client_send_response (pm);
+    return;
+  }
+}
+
+
+/**
+ * The @a pa was acknowledged, process the acknowledgement.
+ *
+ * @param pa the pending acknowledgement that was satisfied
+ * @param ack_delay artificial delay from cumulative acks created by the
+ * other peer
+ */
+static void
+handle_acknowledged (struct PendingAcknowledgement *pa,
+                     struct GNUNET_TIME_Relative ack_delay)
+{
+  struct GNUNET_TIME_Relative delay;
+
+  delay = GNUNET_TIME_absolute_get_duration (pa->transmission_time);
+  delay = GNUNET_TIME_relative_subtract (delay, ack_delay);
+  if (NULL != pa->queue && 1 == pa->num_send)
+    update_queue_performance (pa->queue, delay, pa->message_size);
+  if (NULL != pa->dvh && 1 == pa->num_send)
+    update_dvh_performance (pa->dvh, delay, pa->message_size);
+  if (NULL != pa->pm)
+    completed_pending_message (pa->pm);
+  free_pending_acknowledgement (pa);
+}
+
+
+/**
+ * Communicator gave us a reliability ack.  Check it is well-formed.
+ *
+ * @param cls a `struct CommunicatorMessageContext` (unused)
+ * @param ra the message that was received
+ * @return #GNUNET_Ok if @a ra is well-formed
+ */
+static int
+check_reliability_ack (void *cls,
+                       const struct TransportReliabilityAckMessage *ra)
+{
+  unsigned int n_acks;
+
+  (void) cls;
+  n_acks = (ntohs (ra->header.size) - sizeof(*ra))
+           / sizeof(struct TransportCummulativeAckPayloadP);
+  if (0 == n_acks)
+  {
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  if ((ntohs (ra->header.size) - sizeof(*ra)) !=
+      n_acks * sizeof(struct TransportCummulativeAckPayloadP))
+  {
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  return GNUNET_OK;
+}
+
+
+/**
+ * Communicator gave us a reliability ack.  Process the request.
+ *
+ * @param cls a `struct CommunicatorMessageContext` (must call
+ * #finish_cmc_handling() when done)
+ * @param ra the message that was received
+ */
+static void
+handle_reliability_ack (void *cls,
+                        const struct TransportReliabilityAckMessage *ra)
+{
+  struct CommunicatorMessageContext *cmc = cls;
+  const struct TransportCummulativeAckPayloadP *ack;
+  unsigned int n_acks;
+  uint32_t ack_counter;
+
+  n_acks = (ntohs (ra->header.size) - sizeof(*ra))
+           / sizeof(struct TransportCummulativeAckPayloadP);
+  ack = (const struct TransportCummulativeAckPayloadP *) &ra[1];
+  for (unsigned int i = 0; i < n_acks; i++)
+  {
+    struct PendingAcknowledgement *pa =
+      GNUNET_CONTAINER_multiuuidmap_get (pending_acks, &ack[i].ack_uuid.value);
+    if (NULL == pa)
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+                  "Received ACK from %s with UUID %s which is unknown to us!\n",
+                  GNUNET_i2s (&cmc->im.sender),
+                  GNUNET_uuid2s (&ack[i].ack_uuid.value));
+      GNUNET_STATISTICS_update (
+        GST_stats,
+        "# FRAGMENT_ACKS dropped, no matching pending message",
+        1,
+        GNUNET_NO);
+      continue;
+    }
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Received ACK from %s with UUID %s\n",
+                GNUNET_i2s (&cmc->im.sender),
+                GNUNET_uuid2s (&ack[i].ack_uuid.value));
+    handle_acknowledged (pa, GNUNET_TIME_relative_ntoh (ack[i].ack_delay));
+  }
+
+  ack_counter = htonl (ra->ack_counter);
+  (void) ack_counter;  /* silence compiler warning for now */
+  // FIXME-OPTIMIZE: track ACK losses based on ack_counter somewhere!
+  // (DV and/or Neighbour?)
+  finish_cmc_handling (cmc);
+}
+
+
+/**
+ * Communicator gave us a backchannel encapsulation.  Check the message.
+ *
+ * @param cls a `struct CommunicatorMessageContext`
+ * @param be the send message that was sent
+ * @return #GNUNET_YES if message is well-formed
+ */
+static int
+check_backchannel_encapsulation (
+  void *cls,
+  const struct TransportBackchannelEncapsulationMessage *be)
+{
+  uint16_t size = ntohs (be->header.size) - sizeof(*be);
+  const struct GNUNET_MessageHeader *inbox =
+    (const struct GNUNET_MessageHeader *) &be[1];
+  const char *is;
+  uint16_t isize;
+
+  (void) cls;
+  if (ntohs (inbox->size) >= size)
+  {
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  isize = ntohs (inbox->size);
+  is = ((const char *) inbox) + isize;
+  size -= isize;
+  if ('\0' != is[size - 1])
+  {
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  return GNUNET_YES;
+}
+
+
+/**
+ * Communicator gave us a backchannel encapsulation.  Process the request.
+ * (We are the destination of the backchannel here.)
+ *
+ * @param cls a `struct CommunicatorMessageContext` (must call
+ * #finish_cmc_handling() when done)
+ * @param be the message that was received
+ */
+static void
+handle_backchannel_encapsulation (
+  void *cls,
+  const struct TransportBackchannelEncapsulationMessage *be)
+{
+  struct CommunicatorMessageContext *cmc = cls;
+  struct GNUNET_TRANSPORT_CommunicatorBackchannelIncoming *cbi;
+  struct GNUNET_MQ_Envelope *env;
+  struct TransportClient *tc;
+  const struct GNUNET_MessageHeader *inbox =
+    (const struct GNUNET_MessageHeader *) &be[1];
+  uint16_t isize = ntohs (inbox->size);
+  const char *target_communicator = ((const char *) inbox) + isize;
+  char *sender;
+  char *self;
+
+  GNUNET_asprintf (&sender,
+                   "%s",
+                   GNUNET_i2s (&cmc->im.sender));
+  GNUNET_asprintf (&self,
+                   "%s",
+                   GNUNET_i2s (&GST_my_identity));
+
+  /* Find client providing this communicator */
+  for (tc = clients_head; NULL != tc; tc = tc->next)
+    if ((CT_COMMUNICATOR == tc->type) &&
+        (0 ==
+         strcmp (tc->details.communicator.address_prefix, target_communicator)))
+      break;
+  if (NULL == tc)
+  {
+    char *stastr;
+
+    GNUNET_asprintf (
+      &stastr,
+      "# Backchannel message dropped: target communicator `%s' unknown",
+      target_communicator);
+    GNUNET_STATISTICS_update (GST_stats, stastr, 1, GNUNET_NO);
+    GNUNET_free (stastr);
+    finish_cmc_handling (cmc);
+    return;
+  }
+  /* Finally, deliver backchannel message to communicator */
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Delivering backchannel message from %s to %s of type %u to %s\n",
+              sender,
+              self,
+              ntohs (inbox->type),
+              target_communicator);
+  env = GNUNET_MQ_msg_extra (
+    cbi,
+    isize,
+    GNUNET_MESSAGE_TYPE_TRANSPORT_COMMUNICATOR_BACKCHANNEL_INCOMING);
+  cbi->pid = cmc->im.sender;
+  memcpy (&cbi[1], inbox, isize);
+  GNUNET_MQ_send (tc->mq, env);
+  finish_cmc_handling (cmc);
+}
+
+
+/**
+ * Task called when we should check if any of the DV paths
+ * we have learned to a target are due for garbage collection.
+ *
+ * Collects stale paths, and possibly frees the entire DV
+ * entry if no paths are left. Otherwise re-schedules itself.
+ *
+ * @param cls a `struct DistanceVector`
+ */
+static void
+path_cleanup_cb (void *cls)
+{
+  struct DistanceVector *dv = cls;
+  struct DistanceVectorHop *pos;
+
+  dv->timeout_task = NULL;
+  while (NULL != (pos = dv->dv_head))
+  {
+    GNUNET_assert (dv == pos->dv);
+    if (GNUNET_TIME_absolute_get_remaining (pos->timeout).rel_value_us > 0)
+      break;
+    free_distance_vector_hop (pos);
+  }
+  if (NULL == pos)
+  {
+    free_dv_route (dv);
+    return;
+  }
+  dv->timeout_task =
+    GNUNET_SCHEDULER_add_at (pos->timeout, &path_cleanup_cb, dv);
+}
+
+
+static void
+send_msg_from_cache (struct VirtualLink *vl)
+{
+
+  const struct GNUNET_PeerIdentity target = vl->target;
+
+
+  if ((GNUNET_YES == is_ring_buffer_full) || (0 < ring_buffer_head))
+  {
+    struct RingBufferEntry *ring_buffer_copy[RING_BUFFER_SIZE];
+    unsigned int tail = GNUNET_YES == is_ring_buffer_full ? ring_buffer_head :
+                        0;
+    unsigned int head = GNUNET_YES == is_ring_buffer_full ? RING_BUFFER_SIZE :
+                        ring_buffer_head;
+    struct GNUNET_TRANSPORT_IncomingMessage im;
+    struct CommunicatorMessageContext *cmc;
+    struct RingBufferEntry *rbe;
+    struct GNUNET_MessageHeader *mh;
+
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Sending from ring buffer, which has %u items\n",
+                ring_buffer_head);
+
+    ring_buffer_head = 0;
+    for (unsigned int i = 0; i < head; i++)
+    {
+      rbe = ring_buffer[(i + tail) % RING_BUFFER_SIZE];
+      cmc = rbe->cmc;
+      mh = rbe->mh;
+
+      im = cmc->im;
+      // mh = cmc->mh;
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "Sending to ring buffer target %s using vl target %s\n",
+                  GNUNET_i2s (&im.sender),
+                  GNUNET_i2s2 (&target));
+      if (0 == GNUNET_memcmp (&target, &im.sender))
+      {
+        GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                    "Finish handling message of type %u and size %u\n",
+                    (unsigned int) ntohs (mh->type),
+                    (unsigned int) ntohs (mh->size));
+        finish_handling_raw_message (vl, mh, cmc, GNUNET_NO);
+        GNUNET_free (mh);
+        GNUNET_free (rbe);
+      }
+      else
+      {
+        ring_buffer_copy[i] = rbe;
+        ring_buffer_head++;
+      }
+    }
+
+    if ((GNUNET_YES == is_ring_buffer_full) && (RING_BUFFER_SIZE - 1 >
+                                                ring_buffer_head))
+    {
+      is_ring_buffer_full = GNUNET_NO;
+    }
+
+    for (unsigned int i = 0; i < ring_buffer_head; i++)
+    {
+      ring_buffer[i] = ring_buffer_copy[i];
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "ring_buffer_copy[i]->mh->type for i %u %u\n",
+                  i,
+                  ring_buffer_copy[i]->mh->type);
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "ring_buffer[i]->mh->type for i %u %u\n",
+                  i,
+                  ring_buffer[i]->mh->type);
+    }
+
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "%u items still in ring buffer\n",
+                ring_buffer_head);
+  }
+
+  if ((GNUNET_YES == is_ring_buffer_full) || (0 < ring_buffer_dv_head))
+  {
+    struct PendingMessage *ring_buffer_dv_copy[RING_BUFFER_SIZE];
+    struct PendingMessage *pm;
+    unsigned int tail = GNUNET_YES == is_ring_buffer_dv_full ?
+                        ring_buffer_dv_head :
+                        0;
+    unsigned int head = GNUNET_YES == is_ring_buffer_dv_full ?
+                        RING_BUFFER_SIZE :
+                        ring_buffer_dv_head;
+
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Sending from ring buffer dv, which has %u items\n",
+                ring_buffer_dv_head);
+
+    ring_buffer_dv_head = 0;
+    for (unsigned int i = 0; i < head; i++)
+    {
+      pm = ring_buffer_dv[(i + tail) % RING_BUFFER_SIZE];
+
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "Sending to ring buffer target %s using vl target %s\n",
+                  GNUNET_i2s (&pm->target),
+                  GNUNET_i2s2 (&target));
+      if (0 == GNUNET_memcmp (&target, &pm->target))
+      {
+        GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                    "Adding PendingMessage to vl, checking transmission.\n");
+        pm->vl = vl;
+        GNUNET_CONTAINER_MDLL_insert (vl,
+                                      vl->pending_msg_head,
+                                      vl->pending_msg_tail,
+                                      pm);
+
+        check_vl_transmission (vl);
+      }
+      else
+      {
+        ring_buffer_dv_copy[i] = pm;
+        ring_buffer_dv_head++;
+      }
+    }
+
+    if (is_ring_buffer_dv_full && (RING_BUFFER_SIZE - 1 > ring_buffer_dv_head))
+    {
+      is_ring_buffer_dv_full = GNUNET_NO;
+    }
+
+    for (unsigned int i = 0; i < ring_buffer_dv_head; i++)
+      ring_buffer_dv[i] = ring_buffer_dv_copy[i];
+
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "%u items still in ring buffer dv.\n",
+                ring_buffer_dv_head);
+
+  }
+}
+
+
+/**
+ * The @a hop is a validated path to the respective target
+ * peer and we should tell core about it -- and schedule
+ * a job to revoke the state.
+ *
+ * @param hop a path to some peer that is the reason for activation
+ */
+static void
+activate_core_visible_dv_path (struct DistanceVectorHop *hop)
+{
+  struct DistanceVector *dv = hop->dv;
+  struct VirtualLink *vl;
+
+  vl = lookup_virtual_link (&dv->target);
+  if (NULL == vl)
+  {
+
+    vl = GNUNET_new (struct VirtualLink);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Creating new virtual link %p to %s using DV!\n",
+                vl,
+                GNUNET_i2s (&dv->target));
+    vl->confirmed = GNUNET_YES;
+    vl->message_uuid_ctr =
+      GNUNET_CRYPTO_random_u64 (GNUNET_CRYPTO_QUALITY_WEAK, UINT64_MAX);
+    vl->target = dv->target;
+    vl->core_recv_window = RECV_WINDOW_SIZE;
+    vl->available_fc_window_size = DEFAULT_WINDOW_SIZE;
+    vl->incoming_fc_window_size = DEFAULT_WINDOW_SIZE;
+    GNUNET_break (GNUNET_YES ==
+                  GNUNET_CONTAINER_multipeermap_put (
+                    links,
+                    &vl->target,
+                    vl,
+                    GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
+    vl->dv = dv;
+    dv->vl = vl;
+    vl->visibility_task =
+      GNUNET_SCHEDULER_add_at (hop->path_valid_until, &check_link_down, vl);
+    consider_sending_fc (vl);
+    /* We lacked a confirmed connection to the target
+       before, so tell CORE about it (finally!) */
+    cores_send_connect_info (&dv->target);
+    send_msg_from_cache (vl);
+  }
+  else
+  {
+    /* Link was already up, remember dv is also now available and we are done */
+    vl->dv = dv;
+    dv->vl = vl;
+    if (GNUNET_NO == vl->confirmed)
+    {
+      vl->confirmed = GNUNET_YES;
+      vl->visibility_task =
+        GNUNET_SCHEDULER_add_at (hop->path_valid_until, &check_link_down, vl);
+      consider_sending_fc (vl);
+      /* We lacked a confirmed connection to the target
+         before, so tell CORE about it (finally!) */
+      cores_send_connect_info (&dv->target);
+      send_msg_from_cache (vl);
+    }
+    else
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "Virtual link to %s could now also use DV!\n",
+                  GNUNET_i2s (&dv->target));
+  }
+}
+
+
+/**
+ * We have learned a @a path through the network to some other peer, add it to
+ * our DV data structure (returning #GNUNET_YES on success).
+ *
+ * We do not add paths if we have a sufficient number of shorter
+ * paths to this target already (returning #GNUNET_NO).
+ *
+ * We also do not add problematic paths, like those where we lack the first
+ * hop in our neighbour list (i.e. due to a topology change) or where some
+ * non-first hop is in our neighbour list (returning #GNUNET_SYSERR).
+ *
+ * @param path the path we learned, path[0] should be us,
+ *             and then path contains a valid path from us to
+ * `path[path_len-1]` path[1] should be a direct neighbour (we should check!)
+ * @param path_len number of entries on the @a path, at least three!
+ * @param network_latency how long does the message take from us to
+ * `path[path_len-1]`? set to "forever" if unknown
+ * @param path_valid_until how long is this path considered validated? Maybe
+ * be zero.
+ * @return #GNUNET_YES on success,
+ *         #GNUNET_NO if we have better path(s) to the target
+ *         #GNUNET_SYSERR if the path is useless and/or invalid
+ *                         (i.e. path[1] not a direct neighbour
+ *                        or path[i+1] is a direct neighbour for i>0)
+ */
+static int
+learn_dv_path (const struct GNUNET_PeerIdentity *path,
+               unsigned int path_len,
+               struct GNUNET_TIME_Relative network_latency,
+               struct GNUNET_TIME_Absolute path_valid_until)
+{
+  struct DistanceVectorHop *hop;
+  struct DistanceVector *dv;
+  struct Neighbour *next_hop;
+  unsigned int shorter_distance;
+
+  if (path_len < 3)
+  {
+    /* what a boring path! not allowed! */
+    GNUNET_break (0);
+    return GNUNET_SYSERR;
+  }
+  GNUNET_assert (0 == GNUNET_memcmp (&GST_my_identity, &path[0]));
+  next_hop = lookup_neighbour (&path[1]);
+  if (NULL == next_hop)
+  {
+    /* next hop must be a neighbour, otherwise this whole thing is useless! */
+    GNUNET_break (0);
+    return GNUNET_SYSERR;
+  }
+  for (unsigned int i = 2; i < path_len; i++)
+    if (NULL != lookup_neighbour (&path[i]))
+    {
+      /* Useless path: we have a direct connection to some hop
+         in the middle of the path, so this one is not even
+         terribly useful for redundancy */
+      GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+                  "Path of %u hops useless: directly link to hop %u (%s)\n",
+                  path_len,
+                  i,
+                  GNUNET_i2s (&path[i]));
+      GNUNET_STATISTICS_update (GST_stats,
+                                "# Useless DV path ignored: hop is neighbour",
+                                1,
+                                GNUNET_NO);
+      return GNUNET_SYSERR;
+    }
+  dv = GNUNET_CONTAINER_multipeermap_get (dv_routes, &path[path_len - 1]);
+  if (NULL == dv)
+  {
+    dv = GNUNET_new (struct DistanceVector);
+    dv->target = path[path_len - 1];
+    dv->timeout_task = GNUNET_SCHEDULER_add_delayed (DV_PATH_VALIDITY_TIMEOUT,
+                                                     &path_cleanup_cb,
+                                                     dv);
+    GNUNET_assert (GNUNET_OK ==
+                   GNUNET_CONTAINER_multipeermap_put (
+                     dv_routes,
+                     &dv->target,
+                     dv,
+                     GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
+  }
+  /* Check if we have this path already! */
+  shorter_distance = 0;
+  for (struct DistanceVectorHop *pos = dv->dv_head; NULL != pos;
+       pos = pos->next_dv)
+  {
+    if (pos->distance < path_len - 3)
+      shorter_distance++;
+    /* Note that the distances in 'pos' excludes us (path[0]),
+       the next_hop (path[1]) and the target so we need to subtract three
+       and check next_hop explicitly */
+    if ((pos->distance == path_len - 3) && (pos->next_hop == next_hop))
+    {
+      int match = GNUNET_YES;
+
+      for (unsigned int i = 0; i < pos->distance; i++)
+      {
+        if (0 != GNUNET_memcmp (&pos->path[i], &path[i + 2]))
+        {
+          match = GNUNET_NO;
+          break;
+        }
+      }
+      if (GNUNET_YES == match)
+      {
+        struct GNUNET_TIME_Relative last_timeout;
+
+        /* Re-discovered known path, update timeout */
+        GNUNET_STATISTICS_update (GST_stats,
+                                  "# Known DV path refreshed",
+                                  1,
+                                  GNUNET_NO);
+        last_timeout = GNUNET_TIME_absolute_get_remaining (pos->timeout);
+        pos->timeout =
+          GNUNET_TIME_relative_to_absolute (DV_PATH_VALIDITY_TIMEOUT);
+        pos->path_valid_until =
+          GNUNET_TIME_absolute_max (pos->path_valid_until, path_valid_until);
+        GNUNET_CONTAINER_MDLL_remove (dv, dv->dv_head, dv->dv_tail, pos);
+        GNUNET_CONTAINER_MDLL_insert (dv, dv->dv_head, dv->dv_tail, pos);
+        if (0 <
+            GNUNET_TIME_absolute_get_remaining (path_valid_until).rel_value_us)
+          activate_core_visible_dv_path (pos);
+        if (last_timeout.rel_value_us <
+            GNUNET_TIME_relative_subtract (DV_PATH_VALIDITY_TIMEOUT,
+                                           DV_PATH_DISCOVERY_FREQUENCY)
+            .rel_value_us)
+        {
+          /* Some peer send DV learn messages too often, we are learning
+             the same path faster than it would be useful; do not forward! */
+          GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+                      "Rediscovered path too quickly, not forwarding further\n");
+          return GNUNET_NO;
+        }
+        GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                    "Refreshed known path to %s valid until %s, forwarding further\n",
+                    GNUNET_i2s (&dv->target),
+                    GNUNET_STRINGS_absolute_time_to_string (
+                      pos->path_valid_until));
+        return GNUNET_YES;
+      }
+    }
+  }
+  /* Count how many shorter paths we have (incl. direct
+     neighbours) before simply giving up on this one! */
+  if (shorter_distance >= MAX_DV_PATHS_TO_TARGET)
+  {
+    /* We have a shorter path already! */
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Have many shorter DV paths %s, not forwarding further\n",
+                GNUNET_i2s (&dv->target));
+    return GNUNET_NO;
+  }
+  /* create new DV path entry */
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Discovered new DV path to %s valid until %s\n",
+              GNUNET_i2s (&dv->target),
+              GNUNET_STRINGS_absolute_time_to_string (path_valid_until));
+  hop = GNUNET_malloc (sizeof(struct DistanceVectorHop)
+                       + sizeof(struct GNUNET_PeerIdentity) * (path_len - 3));
+  hop->next_hop = next_hop;
+  hop->dv = dv;
+  hop->path = (const struct GNUNET_PeerIdentity *) &hop[1];
+  memcpy (&hop[1],
+          &path[2],
+          sizeof(struct GNUNET_PeerIdentity) * (path_len - 3));
+  hop->timeout = GNUNET_TIME_relative_to_absolute (DV_PATH_VALIDITY_TIMEOUT);
+  hop->path_valid_until = path_valid_until;
+  hop->distance = path_len - 3;
+  hop->pd.aged_rtt = network_latency;
+  GNUNET_CONTAINER_MDLL_insert (dv, dv->dv_head, dv->dv_tail, hop);
+  GNUNET_CONTAINER_MDLL_insert (neighbour,
+                                next_hop->dv_head,
+                                next_hop->dv_tail,
+                                hop);
+  if (0 < GNUNET_TIME_absolute_get_remaining (path_valid_until).rel_value_us)
+    activate_core_visible_dv_path (hop);
+  return GNUNET_YES;
+}
+
+
+/**
+ * Communicator gave us a DV learn message.  Check the message.
+ *
+ * @param cls a `struct CommunicatorMessageContext`
+ * @param dvl the send message that was sent
+ * @return #GNUNET_YES if message is well-formed
+ */
+static int
+check_dv_learn (void *cls, const struct TransportDVLearnMessage *dvl)
+{
+  uint16_t size = ntohs (dvl->header.size);
+  uint16_t num_hops = ntohs (dvl->num_hops);
+  const struct DVPathEntryP *hops = (const struct DVPathEntryP *) &dvl[1];
+
+  (void) cls;
+  if (size != sizeof(*dvl) + num_hops * sizeof(struct DVPathEntryP))
+  {
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  if (num_hops > MAX_DV_HOPS_ALLOWED)
+  {
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  for (unsigned int i = 0; i < num_hops; i++)
+  {
+    if (0 == GNUNET_memcmp (&dvl->initiator, &hops[i].hop))
+    {
+      GNUNET_break_op (0);
+      return GNUNET_SYSERR;
+    }
+    if (0 == GNUNET_memcmp (&GST_my_identity, &hops[i].hop))
+    {
+      GNUNET_break_op (0);
+      return GNUNET_SYSERR;
+    }
+  }
+  return GNUNET_YES;
+}
+
+
+/**
+ * Build and forward a DV learn message to @a next_hop.
+ *
+ * @param next_hop peer to send the message to
+ * @param msg message received
+ * @param bi_history bitmask specifying hops on path that were bidirectional
+ * @param nhops length of the @a hops array
+ * @param hops path the message traversed so far
+ * @param in_time when did we receive the message, used to calculate network
+ * delay
+ */
+static void
+forward_dv_learn (const struct GNUNET_PeerIdentity *next_hop,
+                  const struct TransportDVLearnMessage *msg,
+                  uint16_t bi_history,
+                  uint16_t nhops,
+                  const struct DVPathEntryP *hops,
+                  struct GNUNET_TIME_Absolute in_time)
+{
+  struct Neighbour *n;
+  struct VirtualLink *vl;
+  struct DVPathEntryP *dhops;
+  char buf[sizeof(struct TransportDVLearnMessage)
+           + (nhops + 1) * sizeof(struct DVPathEntryP)] GNUNET_ALIGN;
+  struct TransportDVLearnMessage *fwd = (struct TransportDVLearnMessage *) buf;
+  struct GNUNET_TIME_Relative nnd;
+
+  /* compute message for forwarding */
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Forwarding DV learn message originating from %s to %s\n",
+              GNUNET_i2s (&msg->initiator),
+              GNUNET_i2s2 (next_hop));
+  GNUNET_assert (nhops < MAX_DV_HOPS_ALLOWED);
+  fwd->header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_DV_LEARN);
+  fwd->header.size = htons (sizeof(struct TransportDVLearnMessage)
+                            + (nhops + 1) * sizeof(struct DVPathEntryP));
+  fwd->num_hops = htons (nhops + 1);
+  fwd->bidirectional = htons (bi_history);
+  nnd = GNUNET_TIME_relative_add (GNUNET_TIME_absolute_get_duration (in_time),
+                                  GNUNET_TIME_relative_ntoh (
+                                    msg->non_network_delay));
+  fwd->non_network_delay = GNUNET_TIME_relative_hton (nnd);
+  fwd->init_sig = msg->init_sig;
+  fwd->initiator = msg->initiator;
+  fwd->challenge = msg->challenge;
+  fwd->monotonic_time = msg->monotonic_time;
+  dhops = (struct DVPathEntryP *) &fwd[1];
+  GNUNET_memcpy (dhops, hops, sizeof(struct DVPathEntryP) * nhops);
+  dhops[nhops].hop = GST_my_identity;
+  {
+    struct DvHopPS dhp = {
+      .purpose.purpose = htonl (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_HOP),
+      .purpose.size = htonl (sizeof(dhp)),
+      .pred = (0 == nhops) ? msg->initiator : dhops[nhops - 1].hop,
+      .succ = *next_hop,
+      .challenge = msg->challenge
+    };
+    GNUNET_CRYPTO_eddsa_sign (GST_my_private_key,
+                              &dhp,
+                              &dhops[nhops].hop_sig);
+  }
+  /*route_control_message_without_fc (next_hop,
+                                    &fwd->header,
+                                    RMO_UNCONFIRMED_ALLOWED);*/
+  vl = lookup_virtual_link (next_hop);
+  if ((NULL != vl) && (GNUNET_YES == vl->confirmed))
+  {
+    route_control_message_without_fc (vl,
+                                      &fwd->header,
+                                      RMO_UNCONFIRMED_ALLOWED);
+  }
+  else
+  {
+    /* Use route via neighbour */
+    n = lookup_neighbour (next_hop);
+    if (NULL != n)
+      route_via_neighbour (
+        n,
+        &fwd->header,
+        RMO_UNCONFIRMED_ALLOWED);
+  }
+}
+
+
+/**
+ * Check signature of type #GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_INITIATOR
+ *
+ * @param sender_monotonic_time monotonic time of the initiator
+ * @param init the signer
+ * @param challenge the challenge that was signed
+ * @param init_sig signature presumably by @a init
+ * @return #GNUNET_OK if the signature is valid
+ */
+static int
+validate_dv_initiator_signature (
+  struct GNUNET_TIME_AbsoluteNBO sender_monotonic_time,
+  const struct GNUNET_PeerIdentity *init,
+  const struct GNUNET_CRYPTO_ChallengeNonceP *challenge,
+  const struct GNUNET_CRYPTO_EddsaSignature *init_sig)
+{
+  struct DvInitPS ip = { .purpose.purpose = htonl (
+                           GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_INITIATOR),
+                         .purpose.size = htonl (sizeof(ip)),
+                         .monotonic_time = sender_monotonic_time,
+                         .challenge = *challenge };
+
+  if (
+    GNUNET_OK !=
+    GNUNET_CRYPTO_eddsa_verify (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_INITIATOR,
+                                &ip,
+                                init_sig,
+                                &init->public_key))
+  {
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  return GNUNET_OK;
+}
+
+
+/**
+ * Closure for #dv_neighbour_selection and #dv_neighbour_transmission.
+ */
+struct NeighbourSelectionContext
+{
+  /**
+   * Original message we received.
+   */
+  const struct TransportDVLearnMessage *dvl;
+
+  /**
+   * The hops taken.
+   */
+  const struct DVPathEntryP *hops;
+
+  /**
+   * Time we received the message.
+   */
+  struct GNUNET_TIME_Absolute in_time;
+
+  /**
+   * Offsets of the selected peers.
+   */
+  uint32_t selections[MAX_DV_DISCOVERY_SELECTION];
+
+  /**
+   * Number of peers eligible for selection.
+   */
+  unsigned int num_eligible;
+
+  /**
+   * Number of peers that were selected for forwarding.
+   */
+  unsigned int num_selections;
+
+  /**
+   * Number of hops in @e hops
+   */
+  uint16_t nhops;
+
+  /**
+   * Bitmap of bidirectional connections encountered.
+   */
+  uint16_t bi_history;
+};
+
+
+/**
+ * Function called for each neighbour during #handle_dv_learn.
+ *
+ * @param cls a `struct NeighbourSelectionContext *`
+ * @param pid identity of the peer
+ * @param value a `struct Neighbour`
+ * @return #GNUNET_YES (always)
+ */
+static int
+dv_neighbour_selection (void *cls,
+                        const struct GNUNET_PeerIdentity *pid,
+                        void *value)
+{
+  struct NeighbourSelectionContext *nsc = cls;
+
+  (void) value;
+  if (0 == GNUNET_memcmp (pid, &nsc->dvl->initiator))
+    return GNUNET_YES; /* skip initiator */
+  for (unsigned int i = 0; i < nsc->nhops; i++)
+    if (0 == GNUNET_memcmp (pid, &nsc->hops[i].hop))
+      return GNUNET_YES;
+  /* skip peers on path */
+  nsc->num_eligible++;
+  return GNUNET_YES;
+}
+
+
+/**
+ * Function called for each neighbour during #handle_dv_learn.
+ * We call #forward_dv_learn() on the neighbour(s) selected
+ * during #dv_neighbour_selection().
+ *
+ * @param cls a `struct NeighbourSelectionContext *`
+ * @param pid identity of the peer
+ * @param value a `struct Neighbour`
+ * @return #GNUNET_YES (always)
+ */
+static int
+dv_neighbour_transmission (void *cls,
+                           const struct GNUNET_PeerIdentity *pid,
+                           void *value)
+{
+  struct NeighbourSelectionContext *nsc = cls;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "transmission %s\n",
+              GNUNET_i2s (pid));
+  (void) value;
+  if (0 == GNUNET_memcmp (pid, &nsc->dvl->initiator))
+    return GNUNET_YES; /* skip initiator */
+  for (unsigned int i = 0; i < nsc->nhops; i++)
+    if (0 == GNUNET_memcmp (pid, &nsc->hops[i].hop))
+      return GNUNET_YES;
+  /* skip peers on path */
+  for (unsigned int i = 0; i < nsc->num_selections; i++)
+  {
+    if (nsc->selections[i] == nsc->num_eligible)
+    {
+      forward_dv_learn (pid,
+                        nsc->dvl,
+                        nsc->bi_history,
+                        nsc->nhops,
+                        nsc->hops,
+                        nsc->in_time);
+      break;
+    }
+  }
+  nsc->num_eligible++;
+  return GNUNET_YES;
+}
+
+
+/**
+ * Computes the number of neighbours we should forward a DVInit
+ * message to given that it has so far taken @a hops_taken hops
+ * though the network and that the number of neighbours we have
+ * in total is @a neighbour_count, out of which @a eligible_count
+ * are not yet on the path.
+ *
+ * NOTE: technically we might want to include NSE in the formula to
+ * get a better grip on the overall network size. However, for now
+ * using NSE here would create a dependency issue in the build system.
+ * => Left for later, hardcoded to 50 for now.
+ *
+ * The goal of the fomula is that we want to reach a total of LOG(NSE)
+ * peers via DV (`target_total`).  We want the reach to be spread out
+ * over various distances to the origin, with a bias towards shorter
+ * distances.
+ *
+ * We make the strong assumption that the network topology looks
+ * "similar" at other hops, in particular the @a neighbour_count
+ * should be comparable at other hops.
+ *
+ * If the local neighbourhood is densely connected, we expect that @a
+ * eligible_count is close to @a neighbour_count minus @a hops_taken
+ * as a lot of the path is already known. In that case, we should
+ * forward to few(er) peers to try to find a path out of the
+ * neighbourhood. OTOH, if @a eligible_count is close to @a
+ * neighbour_count, we should forward to many peers as we are either
+ * still close to the origin (i.e.  @a hops_taken is small) or because
+ * we managed to get beyond a local cluster.  We express this as
+ * the `boost_factor` using the square of the fraction of eligible
+ * neighbours (so if only 50% are eligible, we boost by 1/4, but if
+ * 99% are eligible, the 'boost' will be almost 1).
+ *
+ * Second, the more hops we have taken, the larger the problem of an
+ * exponential traffic explosion gets.  So we take the `target_total`,
+ * and compute our degree such that at each distance d 2^{-d} peers
+ * are selected (corrected by the `boost_factor`).
+ *
+ * @param hops_taken number of hops DVInit has travelled so far
+ * @param neighbour_count number of neighbours we have in total
+ * @param eligible_count number of neighbours we could in
+ *        theory forward to
+ */
+static unsigned int
+calculate_fork_degree (unsigned int hops_taken,
+                       unsigned int neighbour_count,
+                       unsigned int eligible_count)
+{
+  double target_total = 50.0; /* FIXME: use LOG(NSE)? */
+  double eligible_ratio =
+    ((double) eligible_count) / ((double) neighbour_count);
+  double boost_factor = eligible_ratio * eligible_ratio;
+  unsigned int rnd;
+  double left;
+
+  if (hops_taken >= 64)
+  {
+    GNUNET_break (0);
+    return 0;   /* precaution given bitshift below */
+  }
+  for (unsigned int i = 1; i < hops_taken; i++)
+  {
+    /* For each hop, subtract the expected number of targets
+       reached at distance d (so what remains divided by 2^d) */
+    target_total -= (target_total * boost_factor / (1LLU << i));
+  }
+  rnd =
+    (unsigned int) floor (target_total * boost_factor / (1LLU << hops_taken));
+  /* round up or down probabilistically depending on how close we were
+     when floor()ing to rnd */
+  left = target_total - (double) rnd;
+  if (UINT32_MAX * left >
+      GNUNET_CRYPTO_random_u64 (GNUNET_CRYPTO_QUALITY_WEAK, UINT32_MAX))
+    rnd++; /* round up */
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Forwarding DV learn message of %u hops %u(/%u/%u) times\n",
+              hops_taken,
+              rnd,
+              eligible_count,
+              neighbour_count);
+  return rnd;
+}
+
+
+/**
+ * Function called when peerstore is done storing a DV monotonic time.
+ *
+ * @param cls a `struct Neighbour`
+ * @param success #GNUNET_YES if peerstore was successful
+ */
+static void
+neighbour_store_dvmono_cb (void *cls, int success)
+{
+  struct Neighbour *n = cls;
+
+  n->sc = NULL;
+  if (GNUNET_YES != success)
+    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                "Failed to store other peer's monotonic time in peerstore!\n");
+}
+
+
+/**
+ * Communicator gave us a DV learn message.  Process the request.
+ *
+ * @param cls a `struct CommunicatorMessageContext` (must call
+ * #finish_cmc_handling() when done)
+ * @param dvl the message that was received
+ */
+static void
+handle_dv_learn (void *cls, const struct TransportDVLearnMessage *dvl)
+{
+  struct CommunicatorMessageContext *cmc = cls;
+  enum GNUNET_TRANSPORT_CommunicatorCharacteristics cc;
+  int bi_hop;
+  uint16_t nhops;
+  uint16_t bi_history;
+  const struct DVPathEntryP *hops;
+  int do_fwd;
+  int did_initiator;
+  struct GNUNET_TIME_Absolute in_time;
+  struct Neighbour *n;
+
+  nhops = ntohs (dvl->num_hops);  /* 0 = sender is initiator */
+  bi_history = ntohs (dvl->bidirectional);
+  hops = (const struct DVPathEntryP *) &dvl[1];
+  if (0 == nhops)
+  {
+    /* sanity check */
+    if (0 != GNUNET_memcmp (&dvl->initiator, &cmc->im.sender))
+    {
+      GNUNET_break (0);
+      finish_cmc_handling (cmc);
+      return;
+    }
+  }
+  else
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "handle dv learn message last hop %s\n",
+                GNUNET_i2s (&hops[nhops - 1].hop));
+    /* sanity check */
+    if (0 != GNUNET_memcmp (&hops[nhops - 1].hop, &cmc->im.sender))
+    {
+      GNUNET_break (0);
+      finish_cmc_handling (cmc);
+      return;
+    }
+  }
+
+  GNUNET_assert (CT_COMMUNICATOR == cmc->tc->type);
+  cc = cmc->tc->details.communicator.cc;
+  bi_hop = (GNUNET_TRANSPORT_CC_RELIABLE ==
+            cc); // FIXME: add bi-directional flag to cc?
+  in_time = GNUNET_TIME_absolute_get ();
+
+  /* continue communicator here, everything else can happen asynchronous! */
+  finish_cmc_handling (cmc);
+
+  n = lookup_neighbour (&dvl->initiator);
+  if (NULL != n)
+  {
+    if ((n->dv_monotime_available == GNUNET_YES) &&
+        (GNUNET_TIME_absolute_ntoh (dvl->monotonic_time).abs_value_us <
+         n->last_dv_learn_monotime.abs_value_us))
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "DV learn from %s discarded due to time travel",
+                  GNUNET_i2s (&dvl->initiator));
+      GNUNET_STATISTICS_update (GST_stats,
+                                "# DV learn discarded due to time travel",
+                                1,
+                                GNUNET_NO);
+      return;
+    }
+    if (GNUNET_OK != validate_dv_initiator_signature (dvl->monotonic_time,
+                                                      &dvl->initiator,
+                                                      &dvl->challenge,
+                                                      &dvl->init_sig))
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "DV learn signature from %s invalid\n",
+                  GNUNET_i2s (&dvl->initiator));
+      GNUNET_break_op (0);
+      return;
+    }
+    n->last_dv_learn_monotime = GNUNET_TIME_absolute_ntoh (dvl->monotonic_time);
+    if (GNUNET_YES == n->dv_monotime_available)
+    {
+      if (NULL != n->sc)
+      {
+        GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                    "store cancel\n");
+        GNUNET_PEERSTORE_store_cancel (n->sc);
+      }
+      n->sc =
+        GNUNET_PEERSTORE_store (peerstore,
+                                "transport",
+                                &dvl->initiator,
+                                GNUNET_PEERSTORE_TRANSPORT_DVLEARN_MONOTIME,
+                                &dvl->monotonic_time,
+                                sizeof(dvl->monotonic_time),
+                                GNUNET_TIME_UNIT_FOREVER_ABS,
+                                GNUNET_PEERSTORE_STOREOPTION_REPLACE,
+                                &neighbour_store_dvmono_cb,
+                                n);
+    }
+  }
+  /* OPTIMIZE-FIXME: asynchronously (!) verify signatures!,
+     If signature verification load too high, implement random drop strategy */
+  for (unsigned int i = 0; i < nhops; i++)
+  {
+    struct DvHopPS dhp = { .purpose.purpose =
+                             htonl (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_HOP),
+                           .purpose.size = htonl (sizeof(dhp)),
+                           .pred = (0 == i) ? dvl->initiator : hops[i - 1].hop,
+                           .succ = (nhops == i + 1) ? GST_my_identity
+                                   : hops[i + 1].hop,
+                           .challenge = dvl->challenge };
+
+    if (GNUNET_OK !=
+        GNUNET_CRYPTO_eddsa_verify (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_HOP,
+                                    &dhp,
+                                    &hops[i].hop_sig,
+                                    &hops[i].hop.public_key))
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "DV learn from %s signature of hop %u invalid\n",
+                  GNUNET_i2s (&dvl->initiator),
+                  i);
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "signature of hop %s invalid\n",
+                  GNUNET_i2s (&hops[i].hop));
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "pred %s\n",
+                  GNUNET_i2s (&dhp.pred));
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "succ %s\n",
+                  GNUNET_i2s (&dhp.succ));
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "hash %s\n",
+                  GNUNET_sh2s (&dhp.challenge.value));
+      GNUNET_break_op (0);
+      return;
+    }
+  }
+  if (GNUNET_EXTRA_LOGGING > 0)
+  {
+    char *path;
+
+    path = GNUNET_strdup (GNUNET_i2s (&dvl->initiator));
+    for (unsigned int i = 0; i < nhops; i++)
+    {
+      char *tmp;
+
+      GNUNET_asprintf (&tmp,
+                       "%s%s%s",
+                       path,
+                       (bi_history & (1 << (nhops - i))) ? "<->" : "-->",
+                       GNUNET_i2s (&hops[i].hop));
+      GNUNET_free (path);
+      path = tmp;
+    }
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Received DVInit via %s%s%s\n",
+                path,
+                bi_hop ? "<->" : "-->",
+                GNUNET_i2s (&GST_my_identity));
+    GNUNET_free (path);
+  }
+  do_fwd = GNUNET_YES;
+  if (0 == GNUNET_memcmp (&GST_my_identity, &dvl->initiator))
+  {
+    struct GNUNET_PeerIdentity path[nhops + 1];
+    struct GNUNET_TIME_Relative host_latency_sum;
+    struct GNUNET_TIME_Relative latency;
+    struct GNUNET_TIME_Relative network_latency;
+
+    /* We initiated this, learn the forward path! */
+    path[0] = GST_my_identity;
+    path[1] = hops[0].hop;
+    host_latency_sum = GNUNET_TIME_relative_ntoh (dvl->non_network_delay);
+
+    // Need also something to lookup initiation time
+    // to compute RTT! -> add RTT argument here?
+    latency = GNUNET_TIME_absolute_get_duration (GNUNET_TIME_absolute_ntoh (
+                                                   dvl->monotonic_time));
+    GNUNET_assert (latency.rel_value_us >= host_latency_sum.rel_value_us);
+    // latency = GNUNET_TIME_UNIT_FOREVER_REL;   // FIXME: initialize properly
+    // (based on dvl->challenge, we can identify time of origin!)
+
+    network_latency = GNUNET_TIME_relative_subtract (latency, host_latency_sum);
+    /* assumption: latency on all links is the same */
+    network_latency = GNUNET_TIME_relative_divide (network_latency, nhops);
+
+    for (unsigned int i = 2; i <= nhops; i++)
+    {
+      struct GNUNET_TIME_Relative ilat;
+
+      /* assumption: linear latency increase per hop */
+      ilat = GNUNET_TIME_relative_multiply (network_latency, i);
+      path[i] = hops[i - 1].hop;
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "Learned path with %u hops to %s with latency %s\n",
+                  i,
+                  GNUNET_i2s (&path[i]),
+                  GNUNET_STRINGS_relative_time_to_string (ilat, GNUNET_YES));
+      learn_dv_path (path,
+                     i + 1,
+                     ilat,
+                     GNUNET_TIME_relative_to_absolute (
+                       ADDRESS_VALIDATION_LIFETIME));
+    }
+    /* as we initiated, do not forward again (would be circular!) */
+    do_fwd = GNUNET_NO;
+    return;
+  }
+  if (bi_hop)
+  {
+    /* last hop was bi-directional, we could learn something here! */
+    struct GNUNET_PeerIdentity path[nhops + 2];
+
+    path[0] = GST_my_identity;
+    path[1] = hops[nhops - 1].hop;   /* direct neighbour == predecessor! */
+    for (unsigned int i = 0; i < nhops; i++)
+    {
+      int iret;
+
+      if (0 == (bi_history & (1 << i)))
+        break;     /* i-th hop not bi-directional, stop learning! */
+      if (i == nhops - 1)
+      {
+        path[i + 2] = dvl->initiator;
+      }
+      else
+      {
+        path[i + 2] = hops[nhops - i - 2].hop;
+      }
+
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "Learned inverse path with %u hops to %s\n",
+                  i + 2,
+                  GNUNET_i2s (&path[i + 2]));
+      iret = learn_dv_path (path,
+                            i + 3,
+                            GNUNET_TIME_UNIT_FOREVER_REL,
+                            GNUNET_TIME_relative_to_absolute (
+                              ADDRESS_VALIDATION_LIFETIME));
+      if (GNUNET_SYSERR == iret)
+      {
+        /* path invalid or too long to be interesting for US, thus should also
+           not be interesting to our neighbours, cut path when forwarding to
+           'i' hops, except of course for the one that goes back to the
+           initiator */
+        GNUNET_STATISTICS_update (GST_stats,
+                                  "# DV learn not forwarded due invalidity of path",
+                                  1,
+                                  GNUNET_NO);
+        do_fwd = GNUNET_NO;
+        break;
+      }
+      if ((GNUNET_NO == iret) && (nhops == i + 1))
+      {
+        /* we have better paths, and this is the longest target,
+           so there cannot be anything interesting later */
+        GNUNET_STATISTICS_update (GST_stats,
+                                  "# DV learn not forwarded, got better paths",
+                                  1,
+                                  GNUNET_NO);
+        do_fwd = GNUNET_NO;
+        break;
+      }
+    }
+  }
+  if (MAX_DV_HOPS_ALLOWED == nhops)
+  {
+    /* At limit, we're out of here! */
+    return;
+  }
+
+  /* Forward to initiator, if path non-trivial and possible */
+  bi_history = (bi_history << 1) | (bi_hop ? 1 : 0);
+  did_initiator = GNUNET_NO;
+  if ((1 <= nhops) &&
+      (GNUNET_YES ==
+       GNUNET_CONTAINER_multipeermap_contains (neighbours, &dvl->initiator)))
+  {
+    /* send back to origin! */
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Sending DVL back to initiator %s\n",
+                GNUNET_i2s (&dvl->initiator));
+    forward_dv_learn (&dvl->initiator, dvl, bi_history, nhops, hops, in_time);
+    did_initiator = GNUNET_YES;
+  }
+  /* We forward under two conditions: either we still learned something
+     ourselves (do_fwd), or the path was darn short and thus the initiator is
+     likely to still be very interested in this (and we did NOT already
+     send it back to the initiator) */
+  if ((do_fwd) || ((nhops < MIN_DV_PATH_LENGTH_FOR_INITIATOR) &&
+                   (GNUNET_NO == did_initiator)))
+  {
+    /* Pick random neighbours that are not yet on the path */
+    struct NeighbourSelectionContext nsc;
+    unsigned int n_cnt;
+
+    n_cnt = GNUNET_CONTAINER_multipeermap_size (neighbours);
+    nsc.nhops = nhops;
+    nsc.dvl = dvl;
+    nsc.bi_history = bi_history;
+    nsc.hops = hops;
+    nsc.in_time = in_time;
+    nsc.num_eligible = 0;
+    GNUNET_CONTAINER_multipeermap_iterate (neighbours,
+                                           &dv_neighbour_selection,
+                                           &nsc);
+    if (0 == nsc.num_eligible)
+      return;   /* done here, cannot forward to anyone else */
+    nsc.num_selections = calculate_fork_degree (nhops, n_cnt, nsc.num_eligible);
+    nsc.num_selections =
+      GNUNET_MIN (MAX_DV_DISCOVERY_SELECTION, nsc.num_selections);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Forwarding DVL to %u other peers\n",
+                nsc.num_selections);
+    for (unsigned int i = 0; i < nsc.num_selections; i++)
+      nsc.selections[i] =
+        (nsc.num_selections == n_cnt)
+        ? i   /* all were selected, avoid collisions by chance */
+        : GNUNET_CRYPTO_random_u32 (GNUNET_CRYPTO_QUALITY_WEAK, n_cnt);
+    nsc.num_eligible = 0;
+    GNUNET_CONTAINER_multipeermap_iterate (neighbours,
+                                           &dv_neighbour_transmission,
+                                           &nsc);
+  }
+}
+
+
+/**
+ * Communicator gave us a DV box.  Check the message.
+ *
+ * @param cls a `struct CommunicatorMessageContext`
+ * @param dvb the send message that was sent
+ * @return #GNUNET_YES if message is well-formed
+ */
+static int
+check_dv_box (void *cls, const struct TransportDVBoxMessage *dvb)
+{
+  uint16_t size = ntohs (dvb->header.size);
+  uint16_t num_hops = ntohs (dvb->num_hops);
+  const struct GNUNET_PeerIdentity *hops =
+    (const struct GNUNET_PeerIdentity *) &dvb[1];
+
+  (void) cls;
+  if (size < sizeof(*dvb) + num_hops * sizeof(struct GNUNET_PeerIdentity)
+      + sizeof(struct GNUNET_MessageHeader))
+  {
+    GNUNET_break_op (0);
+    return GNUNET_SYSERR;
+  }
+  /* This peer must not be on the path */
+  for (unsigned int i = 0; i < num_hops; i++)
+    if (0 == GNUNET_memcmp (&hops[i], &GST_my_identity))
+    {
+      GNUNET_break_op (0);
+      return GNUNET_SYSERR;
+    }
+  return GNUNET_YES;
+}
+
+
+/**
+ * Create a DV Box message and queue it for transmission to
+ * @a next_hop.
+ *
+ * @param next_hop peer to receive the message next
+ * @param total_hops how many hops did the message take so far
+ * @param num_hops length of the @a hops array
+ * @param origin origin of the message
+ * @param hops next peer(s) to the destination, including destination
+ * @param payload payload of the box
+ * @param payload_size number of bytes in @a payload
+ */
+static void
+forward_dv_box (struct Neighbour *next_hop,
+                struct TransportDVBoxMessage *hdr,
+                uint16_t total_hops,
+                uint16_t num_hops,
+                const struct GNUNET_PeerIdentity *hops,
+                const void *enc_payload,
+                uint16_t enc_payload_size)
+{
+  struct VirtualLink *vl = next_hop->vl;
+  struct PendingMessage *pm;
+  size_t msg_size = sizeof(struct TransportDVBoxMessage)
+                    + num_hops * sizeof(struct GNUNET_PeerIdentity)
+                    + enc_payload_size;
+  char *buf;
+  char msg_buf[msg_size] GNUNET_ALIGN;
+  struct GNUNET_PeerIdentity *dhops;
+
+  hdr->num_hops = htons (num_hops);
+  hdr->total_hops = htons (total_hops);
+  hdr->header.size = htons (msg_size);
+  memcpy (msg_buf, hdr, sizeof(*hdr));
+  dhops = (struct GNUNET_PeerIdentity *) &msg_buf[sizeof(struct
+                                                         TransportDVBoxMessage)];
+  memcpy (dhops, hops, num_hops * sizeof(struct GNUNET_PeerIdentity));
+  memcpy (&dhops[num_hops], enc_payload, enc_payload_size);
+
+  if (GNUNET_YES == ntohs (hdr->without_fc))
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Forwarding control message (payload size %u) in DV Box to next hop %s (%u/%u) \n",
+                enc_payload_size,
+                GNUNET_i2s (&next_hop->pid),
+                (unsigned int) num_hops,
+                (unsigned int) total_hops);
+    route_via_neighbour (next_hop, (const struct
+                                    GNUNET_MessageHeader *) msg_buf,
+                         RMO_ANYTHING_GOES);
+  }
+  else
+  {
+    pm = GNUNET_malloc (sizeof(struct PendingMessage) + msg_size);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "2 created pm %p storing vl %p \n",
+                pm,
+                vl);
+    pm->pmt = PMT_DV_BOX;
+    pm->vl = vl;
+    pm->target = next_hop->pid;
+    pm->timeout = GNUNET_TIME_relative_to_absolute (DV_FORWARD_TIMEOUT);
+    pm->logging_uuid = logging_uuid_gen++;
+    pm->prefs = GNUNET_MQ_PRIO_BACKGROUND;
+    pm->bytes_msg = msg_size;
+    buf = (char *) &pm[1];
+    memcpy (buf, msg_buf, msg_size);
+
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Created pending message %" PRIu64
+                " for DV Box with next hop %s (%u/%u)\n",
+                pm->logging_uuid,
+                GNUNET_i2s (&next_hop->pid),
+                (unsigned int) num_hops,
+                (unsigned int) total_hops);
+
+    if ((NULL != vl) && (GNUNET_YES == vl->confirmed))
+    {
+      GNUNET_CONTAINER_MDLL_insert (vl,
+                                    vl->pending_msg_head,
+                                    vl->pending_msg_tail,
+                                    pm);
+
+      check_vl_transmission (vl);
+    }
+    else
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "The virtual link is not ready for forwarding a DV Box with payload, storing PendingMessage in ring buffer.\n");
+
+      ring_buffer_dv[ring_buffer_dv_head] = pm;
+      if (RING_BUFFER_SIZE - 1 == ring_buffer_dv_head)
+      {
+        ring_buffer_dv_head = 0;
+        is_ring_buffer_dv_full = GNUNET_YES;
+      }
+      else
+        ring_buffer_dv_head++;
+
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "%u items stored in DV ring buffer\n",
+                  ring_buffer_dv_head);
+    }
+  }
+}
+
+
+/**
+ * Free data structures associated with @a b.
+ *
+ * @param b data structure to release
+ */
+static void
+free_backtalker (struct Backtalker *b)
+{
+  if (NULL != b->get)
+  {
+    GNUNET_PEERSTORE_iterate_cancel (b->get);
+    b->get = NULL;
+    GNUNET_assert (NULL != b->cmc);
+    finish_cmc_handling (b->cmc);
+    b->cmc = NULL;
+  }
+  if (NULL != b->task)
+  {
+    GNUNET_SCHEDULER_cancel (b->task);
+    b->task = NULL;
+  }
+  if (NULL != b->sc)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "store cancel\n");
+    GNUNET_PEERSTORE_store_cancel (b->sc);
+    b->sc = NULL;
+  }
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Removing backtalker for %s\n",
+              GNUNET_i2s (&b->pid));
+  GNUNET_assert (
+    GNUNET_YES ==
+    GNUNET_CONTAINER_multipeermap_remove (backtalkers, &b->pid, b));
+  GNUNET_free (b);
+}
+
+
+/**
+ * Callback to free backtalker records.
+ *
+ * @param cls NULL
+ * @param pid unused
+ * @param value a `struct Backtalker`
+ * @return #GNUNET_OK (always)
+ */
+static int
+free_backtalker_cb (void *cls,
+                    const struct GNUNET_PeerIdentity *pid,
+                    void *value)
+{
+  struct Backtalker *b = value;
+
+  (void) cls;
+  (void) pid;
+  free_backtalker (b);
+  return GNUNET_OK;
+}
+
+
+/**
+ * Function called when it is time to clean up a backtalker.
+ *
+ * @param cls a `struct Backtalker`
+ */
+static void
+backtalker_timeout_cb (void *cls)
+{
+  struct Backtalker *b = cls;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "backtalker timeout.\n");
+  b->task = NULL;
+  if (0 != GNUNET_TIME_absolute_get_remaining (b->timeout).rel_value_us)
+  {
+    b->task = GNUNET_SCHEDULER_add_at (b->timeout, &backtalker_timeout_cb, b);
+    return;
+  }
+  GNUNET_assert (NULL == b->sc);
+  free_backtalker (b);
+}
+
+
+/**
+ * Function called with the monotonic time of a backtalker
+ * by PEERSTORE. Updates the time and continues processing.
+ *
+ * @param cls a `struct Backtalker`
+ * @param record the information found, NULL for the last call
+ * @param emsg error message
+ */
+static void
+backtalker_monotime_cb (void *cls,
+                        const struct GNUNET_PEERSTORE_Record *record,
+                        const char *emsg)
+{
+  struct Backtalker *b = cls;
+  struct GNUNET_TIME_AbsoluteNBO *mtbe;
+  struct GNUNET_TIME_Absolute mt;
+
+  (void) emsg;
+  if (NULL == record)
+  {
+    /* we're done with #backtalker_monotime_cb() invocations,
+       continue normal processing */
+    b->get = NULL;
+    GNUNET_assert (NULL != b->cmc);
+    b->cmc->mh = (const struct GNUNET_MessageHeader *) &b[1];
+    if (0 != b->body_size)
+      demultiplex_with_cmc (b->cmc);
+    else
+      finish_cmc_handling (b->cmc);
+    b->cmc = NULL;
+    return;
+  }
+  if (sizeof(*mtbe) != record->value_size)
+  {
+    GNUNET_break (0);
+    return;
+  }
+  mtbe = record->value;
+  mt = GNUNET_TIME_absolute_ntoh (*mtbe);
+  if (mt.abs_value_us > b->monotonic_time.abs_value_us)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Backtalker message from %s dropped, monotime in the past\n",
+                GNUNET_i2s (&b->pid));
+    GNUNET_STATISTICS_update (
+      GST_stats,
+      "# Backchannel messages dropped: monotonic time not increasing",
+      1,
+      GNUNET_NO);
+    b->monotonic_time = mt;
+    /* Setting body_size to 0 prevents call to #forward_backchannel_payload()
+     */
+    b->body_size = 0;
+    return;
+  }
+}
+
+
+/**
+ * Function called by PEERSTORE when the store operation of
+ * a backtalker's monotonic time is complete.
+ *
+ * @param cls the `struct Backtalker`
+ * @param success #GNUNET_OK on success
+ */
+static void
+backtalker_monotime_store_cb (void *cls, int success)
+{
+  struct Backtalker *b = cls;
+
+  if (GNUNET_OK != success)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                "Failed to store backtalker's monotonic time in PEERSTORE!\n");
+  }
+  b->sc = NULL;
+  if (NULL != b->task)
+  {
+    GNUNET_SCHEDULER_cancel (b->task);
+    b->task = NULL;
+  }
+  b->task = GNUNET_SCHEDULER_add_at (b->timeout, &backtalker_timeout_cb, b);
+}
+
+
+/**
+ * The backtalker @a b monotonic time changed. Update PEERSTORE.
+ *
+ * @param b a backtalker with updated monotonic time
+ */
+static void
+update_backtalker_monotime (struct Backtalker *b)
+{
+  struct GNUNET_TIME_AbsoluteNBO mtbe;
+
+  if (NULL != b->sc)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "store cancel before store with sc %p\n",
+                b->sc);
+    /*GNUNET_PEERSTORE_store_cancel (b->sc);
+      b->sc = NULL;*/
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "store cancel before store with sc %p is null\n",
+                b->sc);
+  }
+  else
+  {
+    GNUNET_SCHEDULER_cancel (b->task);
+    b->task = NULL;
+  }
+  mtbe = GNUNET_TIME_absolute_hton (b->monotonic_time);
+  b->sc =
+    GNUNET_PEERSTORE_store (peerstore,
+                            "transport",
+                            &b->pid,
+                            GNUNET_PEERSTORE_TRANSPORT_BACKCHANNEL_MONOTIME,
+                            &mtbe,
+                            sizeof(mtbe),
+                            GNUNET_TIME_UNIT_FOREVER_ABS,
+                            GNUNET_PEERSTORE_STOREOPTION_REPLACE,
+                            &backtalker_monotime_store_cb,
+                            b);
+}
+
+
+/**
+ * Communicator gave us a DV box.  Process the request.
+ *
+ * @param cls a `struct CommunicatorMessageContext` (must call
+ * #finish_cmc_handling() when done)
+ * @param dvb the message that was received
+ */
+static void
+handle_dv_box (void *cls, const struct TransportDVBoxMessage *dvb)
+{
+  struct CommunicatorMessageContext *cmc = cls;
+  uint16_t size = ntohs (dvb->header.size) - sizeof(*dvb);
+  uint16_t num_hops = ntohs (dvb->num_hops);
+  const struct GNUNET_PeerIdentity *hops =
+    (const struct GNUNET_PeerIdentity *) &dvb[1];
+  const char *enc_payload = (const char *) &hops[num_hops];
+  uint16_t enc_payload_size =
+    size - (num_hops * sizeof(struct GNUNET_PeerIdentity));
+  struct DVKeyState key;
+  struct GNUNET_HashCode hmac;
+  const char *hdr;
+  size_t hdr_len;
+
+  if (GNUNET_EXTRA_LOGGING > 0)
+  {
+    char *path;
+
+    path = GNUNET_strdup (GNUNET_i2s (&GST_my_identity));
+    for (unsigned int i = 0; i < num_hops; i++)
+    {
+      char *tmp;
+
+      GNUNET_asprintf (&tmp, "%s->%s", path, GNUNET_i2s (&hops[i]));
+      GNUNET_free (path);
+      path = tmp;
+    }
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Received DVBox with remaining path %s\n",
+                path);
+    GNUNET_free (path);
+  }
+
+  if (num_hops > 0)
+  {
+    /* We're trying from the end of the hops array, as we may be
+       able to find a shortcut unknown to the origin that way */
+    for (int i = num_hops - 1; i >= 0; i--)
+    {
+      struct Neighbour *n;
+
+      if (0 == GNUNET_memcmp (&hops[i], &GST_my_identity))
+      {
+        GNUNET_break_op (0);
+        finish_cmc_handling (cmc);
+        return;
+      }
+      n = lookup_neighbour (&hops[i]);
+      if (NULL == n)
+        continue;
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "Skipping %u/%u hops ahead while routing DV Box\n",
+                  i,
+                  num_hops);
+
+      forward_dv_box (n,
+                      (struct TransportDVBoxMessage *) dvb,
+                      ntohs (dvb->total_hops) + 1,
+                      num_hops - i - 1,    /* number of hops left */
+                      &hops[i + 1],    /* remaining hops */
+                      enc_payload,
+                      enc_payload_size);
+      GNUNET_STATISTICS_update (GST_stats,
+                                "# DV hops skipped routing boxes",
+                                i,
+                                GNUNET_NO);
+      GNUNET_STATISTICS_update (GST_stats,
+                                "# DV boxes routed (total)",
+                                1,
+                                GNUNET_NO);
+      finish_cmc_handling (cmc);
+      return;
+    }
+    /* Woopsie, next hop not in neighbours, drop! */
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# DV Boxes dropped: next hop unknown",
+                              1,
+                              GNUNET_NO);
+    finish_cmc_handling (cmc);
+    return;
+  }
+  /* We are the target. Unbox and handle message. */
+  GNUNET_STATISTICS_update (GST_stats,
+                            "# DV boxes opened (ultimate target)",
+                            1,
+                            GNUNET_NO);
+  cmc->total_hops = ntohs (dvb->total_hops);
+
+  // DH key derivation with received DV, could be garbage.
+  struct GNUNET_HashCode km;
+
+  if (GNUNET_YES != GNUNET_CRYPTO_eddsa_kem_decaps (GST_my_private_key,
+                                                    &dvb->ephemeral_key,
+                                                    &km))
+  {
+    GNUNET_break_op (0);
+    finish_cmc_handling (cmc);
+    return;
+  }
+  dv_setup_key_state_from_km (&km, &dvb->iv, &key);
+  hdr = (const char *) &dvb[1];
+  hdr_len = ntohs (dvb->orig_size) - sizeof(*dvb) - sizeof(struct
+                                                           GNUNET_PeerIdentity)
+            * ntohs (dvb->total_hops);
+
+  dv_hmac (&key, &hmac, hdr, hdr_len);
+  if (0 != GNUNET_memcmp (&hmac, &dvb->hmac))
+  {
+    /* HMAC mismatch, discard! */
+    GNUNET_break_op (0);
+    finish_cmc_handling (cmc);
+    return;
+  }
+  /* begin actual decryption */
+  {
+    struct Backtalker *b;
+    struct GNUNET_TIME_Absolute monotime;
+    struct TransportDVBoxPayloadP ppay;
+    char body[hdr_len - sizeof(ppay)] GNUNET_ALIGN;
+    const struct GNUNET_MessageHeader *mh;
+
+    GNUNET_assert (hdr_len >=
+                   sizeof(ppay) + sizeof(struct GNUNET_MessageHeader));
+    if (GNUNET_OK != dv_decrypt (&key, &ppay, hdr, sizeof(ppay)))
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                  "Error decrypting DV payload header\n");
+      GNUNET_break_op (0);
+      finish_cmc_handling (cmc);
+      return;
+    }
+    if (GNUNET_OK != dv_decrypt (&key, body,
+                                 &hdr[sizeof(ppay)], hdr_len - sizeof(ppay)))
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                  "Error decrypting DV payload\n");
+      GNUNET_break_op (0);
+      finish_cmc_handling (cmc);
+      return;
+    }
+    mh = (const struct GNUNET_MessageHeader *) body;
+    dv_key_clean (&key);
+    if (ntohs (mh->size) != sizeof(body))
+    {
+      GNUNET_break_op (0);
+      finish_cmc_handling (cmc);
+      return;
+    }
+    /* need to prevent box-in-a-box (and DV_LEARN) so check inbox type! */
+    switch (ntohs (mh->type))
+    {
+    case GNUNET_MESSAGE_TYPE_TRANSPORT_DV_BOX:
+      GNUNET_break_op (0);
+      finish_cmc_handling (cmc);
+      return;
+
+    case GNUNET_MESSAGE_TYPE_TRANSPORT_DV_LEARN:
+      GNUNET_break_op (0);
+      finish_cmc_handling (cmc);
+      return;
+
+    default:
+      /* permitted, continue */
+      break;
+    }
+    monotime = GNUNET_TIME_absolute_ntoh (ppay.monotonic_time);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Decrypted backtalk from %s\n",
+                GNUNET_i2s (&ppay.sender));
+    b = GNUNET_CONTAINER_multipeermap_get (backtalkers, &ppay.sender);
+    if ((NULL != b) && (monotime.abs_value_us < b->monotonic_time.abs_value_us))
+    {
+      GNUNET_STATISTICS_update (
+        GST_stats,
+        "# Backchannel messages dropped: monotonic time not increasing",
+        1,
+        GNUNET_NO);
+      finish_cmc_handling (cmc);
+      return;
+    }
+    if ((NULL == b) ||
+        (0 != GNUNET_memcmp (&b->last_ephemeral, &dvb->ephemeral_key)))
+    {
+      /* Check signature */
+      struct EphemeralConfirmationPS ec;
+
+      ec.purpose.purpose = htonl (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_EPHEMERAL);
+      ec.target = GST_my_identity;
+      ec.ephemeral_key = dvb->ephemeral_key;
+      ec.purpose.size =  htonl (sizeof(ec));
+      ec.sender_monotonic_time = ppay.monotonic_time;
+      if (
+        GNUNET_OK !=
+        GNUNET_CRYPTO_eddsa_verify (
+          GNUNET_SIGNATURE_PURPOSE_TRANSPORT_EPHEMERAL,
+          &ec,
+          &ppay.sender_sig,
+          &ppay.sender.public_key))
+      {
+        /* Signature invalid, discard! */
+        GNUNET_break_op (0);
+        finish_cmc_handling (cmc);
+        return;
+      }
+    }
+    /* Update sender, we now know the real origin! */
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "DVBox received for me from %s via %s\n",
+                GNUNET_i2s2 (&ppay.sender),
+                GNUNET_i2s (&cmc->im.sender));
+    cmc->im.sender = ppay.sender;
+
+    if (NULL != b)
+    {
+      /* update key cache and mono time */
+      b->last_ephemeral = dvb->ephemeral_key;
+      b->monotonic_time = monotime;
+      update_backtalker_monotime (b);
+      b->timeout =
+        GNUNET_TIME_relative_to_absolute (BACKCHANNEL_INACTIVITY_TIMEOUT);
+      cmc->mh = mh;
+      demultiplex_with_cmc (cmc);
+      return;
+    }
+    /* setup data structure to cache signature AND check
+       monotonic time with PEERSTORE before forwarding backchannel payload */
+    b = GNUNET_malloc (sizeof(struct Backtalker) + sizeof(body));
+    b->pid = ppay.sender;
+    b->body_size = sizeof(body);
+    memcpy (&b[1], body, sizeof(body));
+    GNUNET_assert (GNUNET_YES ==
+                   GNUNET_CONTAINER_multipeermap_put (
+                     backtalkers,
+                     &b->pid,
+                     b,
+                     GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
+    b->monotonic_time = monotime; /* NOTE: to be checked still! */
+    b->cmc = cmc;
+    b->timeout =
+      GNUNET_TIME_relative_to_absolute (BACKCHANNEL_INACTIVITY_TIMEOUT);
+    b->task = GNUNET_SCHEDULER_add_at (b->timeout, &backtalker_timeout_cb, b);
+    b->get =
+      GNUNET_PEERSTORE_iterate (peerstore,
+                                "transport",
+                                &b->pid,
+                                GNUNET_PEERSTORE_TRANSPORT_BACKCHANNEL_MONOTIME,
+                                &backtalker_monotime_cb,
+                                b);
+  } /* end actual decryption */
+}
+
+
+/**
+ * Client notified us about transmission from a peer.  Process the request.
+ *
+ * @param cls a `struct TransportClient` which sent us the message
+ * @param im the send message that was sent
+ * @return #GNUNET_YES if message is well-formed
+ */
+static int
+check_incoming_msg (void *cls,
+                    const struct GNUNET_TRANSPORT_IncomingMessage *im)
+{
+  struct TransportClient *tc = cls;
+
+  if (CT_COMMUNICATOR != tc->type)
+  {
+    GNUNET_break (0);
+    return GNUNET_SYSERR;
+  }
+  GNUNET_MQ_check_boxed_message (im);
+  return GNUNET_OK;
+}
+
+
+/**
+ * Closure for #check_known_address.
+ */
+struct CheckKnownAddressContext
+{
+  /**
+   * Set to the address we are looking for.
+   */
+  const char *address;
+
+  /**
+   * Set to a matching validation state, if one was found.
+   */
+  struct ValidationState *vs;
+};
+
+
+/**
+ * Test if the validation state in @a value matches the
+ * address from @a cls.
+ *
+ * @param cls a `struct CheckKnownAddressContext`
+ * @param pid unused (must match though)
+ * @param value a `struct ValidationState`
+ * @return #GNUNET_OK if not matching, #GNUNET_NO if match found
+ */
+static int
+check_known_address (void *cls,
+                     const struct GNUNET_PeerIdentity *pid,
+                     void *value)
+{
+  struct CheckKnownAddressContext *ckac = cls;
+  struct ValidationState *vs = value;
+
+  (void) pid;
+  if (0 != strcmp (vs->address, ckac->address))
+    return GNUNET_OK;
+  ckac->vs = vs;
+  return GNUNET_NO;
+}
+
+
+/**
+ * Task run periodically to validate some address based on #validation_heap.
+ *
+ * @param cls NULL
+ */
+static void
+validation_start_cb (void *cls);
+
+
+/**
+ * Set the time for next_challenge of @a vs to @a new_time.
+ * Updates the heap and if necessary reschedules the job.
+ *
+ * @param vs validation state to update
+ * @param new_time new time for revalidation
+ */
+static void
+update_next_challenge_time (struct ValidationState *vs,
+                            struct GNUNET_TIME_Absolute new_time)
+{
+  struct GNUNET_TIME_Relative delta;
+
+  if (new_time.abs_value_us == vs->next_challenge.abs_value_us)
+    return; /* be lazy */
+  vs->next_challenge = new_time;
+  if (NULL == vs->hn)
+    vs->hn =
+      GNUNET_CONTAINER_heap_insert (validation_heap, vs, new_time.abs_value_us);
+  else
+    GNUNET_CONTAINER_heap_update_cost (vs->hn, new_time.abs_value_us);
+  if ((vs != GNUNET_CONTAINER_heap_peek (validation_heap)) &&
+      (NULL != validation_task))
+    return;
+  if (NULL != validation_task)
+    GNUNET_SCHEDULER_cancel (validation_task);
+  /* randomize a bit */
+  delta.rel_value_us =
+    GNUNET_CRYPTO_random_u64 (GNUNET_CRYPTO_QUALITY_WEAK,
+                              MIN_DELAY_ADDRESS_VALIDATION.rel_value_us);
+  new_time = GNUNET_TIME_absolute_add (new_time, delta);
+  validation_task =
+    GNUNET_SCHEDULER_add_at (new_time, &validation_start_cb, NULL);
+}
+
+
+/**
+ * Start address validation.
+ *
+ * @param pid peer the @a address is for
+ * @param address an address to reach @a pid (presumably)
+ */
+static void
+start_address_validation (const struct GNUNET_PeerIdentity *pid,
+                          const char *address)
+{
+  struct GNUNET_TIME_Absolute now;
+  struct ValidationState *vs;
+  struct CheckKnownAddressContext ckac = { .address = address, .vs = NULL };
+
+  (void) GNUNET_CONTAINER_multipeermap_get_multiple (validation_map,
+                                                     pid,
+                                                     &check_known_address,
+                                                     &ckac);
+  if (NULL != (vs = ckac.vs))
+  {
+    /* if 'vs' is not currently valid, we need to speed up retrying the
+     * validation */
+    if (vs->validated_until.abs_value_us < vs->next_challenge.abs_value_us)
+    {
+      /* reduce backoff as we got a fresh advertisement */
+      vs->challenge_backoff =
+        GNUNET_TIME_relative_min (FAST_VALIDATION_CHALLENGE_FREQ,
+                                  GNUNET_TIME_relative_divide (
+                                    vs->challenge_backoff,
+                                    2));
+      update_next_challenge_time (vs,
+                                  GNUNET_TIME_relative_to_absolute (
+                                    vs->challenge_backoff));
+    }
+    return;
+  }
+  now = GNUNET_TIME_absolute_get_monotonic (GST_cfg);
+  vs = GNUNET_new (struct ValidationState);
+  vs->pid = *pid;
+  vs->valid_until =
+    GNUNET_TIME_relative_to_absolute (ADDRESS_VALIDATION_LIFETIME);
+  vs->first_challenge_use = now;
+  vs->validation_rtt = GNUNET_TIME_UNIT_FOREVER_REL;
+  GNUNET_CRYPTO_random_block (GNUNET_CRYPTO_QUALITY_NONCE,
+                              &vs->challenge,
+                              sizeof(vs->challenge));
+  vs->address = GNUNET_strdup (address);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Starting address validation `%s' of peer %s using challenge %s\n",
+              address,
+              GNUNET_i2s (pid),
+              GNUNET_sh2s (&vs->challenge.value));
+  GNUNET_assert (GNUNET_YES ==
+                 GNUNET_CONTAINER_multipeermap_put (
+                   validation_map,
+                   &vs->pid,
+                   vs,
+                   GNUNET_CONTAINER_MULTIHASHMAPOPTION_MULTIPLE));
+  update_next_challenge_time (vs, now);
+}
+
+
+static void
+hello_for_incoming_cb (void *cls,
+                       const struct GNUNET_PeerIdentity *pid,
+                       const char *uri)
+{
+  (void) cls;
+  int pfx_len;
+  const char *eou;
+  char *address;
+
+  eou = strstr (uri,
+                "://");
+  pfx_len = eou - uri;
+  eou += 3;
+  GNUNET_asprintf (&address,
+                   "%.*s-%s",
+                   pfx_len,
+                   uri,
+                   eou);
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "helo for client %s\n",
+              address);
+
+  start_address_validation (pid, address);
+  GNUNET_free (address);
+}
+
+
+/**
+ * Function called by PEERSTORE for each matching record.
+ *
+ * @param cls closure, a `struct IncomingRequest`
+ * @param record peerstore record information
+ * @param emsg error message, or NULL if no errors
+ */
+static void
+handle_hello_for_incoming (void *cls,
+                           const struct GNUNET_PeerIdentity *peer,
+                           const struct GNUNET_MessageHeader *hello,
+                           const char *emsg)
+{
+  struct GNUNET_HELLO_Builder *builder;
+
+  if (NULL != emsg)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+                "Got failure from PEERSTORE: %s\n",
+                emsg);
+    return;
+  }
+  if (0 == GNUNET_memcmp (peer, &GST_my_identity))
+    return;
+  builder = GNUNET_HELLO_builder_from_msg (hello);
+  GNUNET_HELLO_builder_iterate (builder,
+                                hello_for_incoming_cb,
+                                NULL);
+  GNUNET_HELLO_builder_free (builder);
+}
+
+
+/**
+ * Communicator gave us a transport address validation challenge.  Process the
+ * request.
+ *
+ * @param cls a `struct CommunicatorMessageContext` (must call
+ * #finish_cmc_handling() when done)
+ * @param tvc the message that was received
+ */
+static void
+handle_validation_challenge (
+  void *cls,
+  const struct TransportValidationChallengeMessage *tvc)
+{
+  struct CommunicatorMessageContext *cmc = cls;
+  struct TransportValidationResponseMessage tvr;
+  struct VirtualLink *vl;
+  struct GNUNET_TIME_RelativeNBO validity_duration;
+  struct IncomingRequest *ir;
+  struct Neighbour *n;
+  struct GNUNET_PeerIdentity sender;
+
+  /* DV-routed messages are not allowed for validation challenges */
+  if (cmc->total_hops > 0)
+  {
+    GNUNET_break_op (0);
+    finish_cmc_handling (cmc);
+    return;
+  }
+  validity_duration = cmc->im.expected_address_validity;
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Received address validation challenge %s\n",
+              GNUNET_sh2s (&tvc->challenge.value));
+  /* If we have a virtual link, we use this mechanism to signal the
+     size of the flow control window, and to allow the sender
+     to ask for increases. If for us the virtual link is still down,
+     we will always give a window size of zero. */
+  tvr.header.type =
+    htons (GNUNET_MESSAGE_TYPE_TRANSPORT_ADDRESS_VALIDATION_RESPONSE);
+  tvr.header.size = htons (sizeof(tvr));
+  tvr.reserved = htonl (0);
+  tvr.challenge = tvc->challenge;
+  tvr.origin_time = tvc->sender_time;
+  tvr.validity_duration = validity_duration;
+  {
+    /* create signature */
+    struct TransportValidationPS tvp = {
+      .purpose.purpose = htonl (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_CHALLENGE),
+      .purpose.size = htonl (sizeof(tvp)),
+      .validity_duration = validity_duration,
+      .challenge = tvc->challenge
+    };
+
+    GNUNET_CRYPTO_eddsa_sign (GST_my_private_key,
+                              &tvp,
+                              &tvr.signature);
+  }
+  sender = cmc->im.sender;
+  vl = lookup_virtual_link (&sender);
+  if ((NULL != vl) && (GNUNET_YES == vl->confirmed))
+  {
+    // route_control_message_without_fc (&cmc->im.sender,
+    route_control_message_without_fc (vl,
+                                      &tvr.header,
+                                      RMO_ANYTHING_GOES | RMO_REDUNDANT);
+  }
+  else
+  {
+    /* Use route via neighbour */
+    n = lookup_neighbour (&sender);
+    if (NULL != n)
+      route_via_neighbour (n, &tvr.header,
+                           RMO_ANYTHING_GOES | RMO_REDUNDANT
+                           | RMO_UNCONFIRMED_ALLOWED);
+  }
+
+  finish_cmc_handling (cmc);
+  if (NULL != vl)
+    return;
+
+  /* For us, the link is still down, but we need bi-directional
+     connections (for flow-control and for this to be useful for
+     CORE), so we must try to bring the link up! */
+
+  /* (1) Check existing queues, if any, we may be lucky! */
+  n = lookup_neighbour (&sender);
+  if (NULL != n)
+    for (struct Queue *q = n->queue_head; NULL != q; q = q->next_neighbour)
+      start_address_validation (&sender, q->address);
+  /* (2) Also try to see if we have addresses in PEERSTORE for this peer
+     we could use */
+  for (ir = ir_head; NULL != ir; ir = ir->next)
+    if (0 == GNUNET_memcmp (&ir->pid, &sender))
+      return;
+  /* we are already trying */
+  ir = GNUNET_new (struct IncomingRequest);
+  ir->pid = sender;
+  GNUNET_CONTAINER_DLL_insert (ir_head, ir_tail, ir);
+
+  ir->nc = GNUNET_PEERSTORE_hello_changed_notify (peerstore,
+                                                  GNUNET_NO,
+                                                  &handle_hello_for_incoming,
+                                                  NULL);
+  ir_total++;
+  /* Bound attempts we do in parallel here, might otherwise get excessive */
+  while (ir_total > MAX_INCOMING_REQUEST)
+    free_incoming_request (ir_head);
+}
+
+
+/**
+ * Closure for #check_known_challenge.
+ */
+struct CheckKnownChallengeContext
+{
+  /**
+   * Set to the challenge we are looking for.
+   */
+  const struct GNUNET_CRYPTO_ChallengeNonceP *challenge;
+
+  /**
+   * Set to a matching validation state, if one was found.
+   */
+  struct ValidationState *vs;
+};
+
+
+/**
+ * Test if the validation state in @a value matches the
+ * challenge from @a cls.
+ *
+ * @param cls a `struct CheckKnownChallengeContext`
+ * @param pid unused (must match though)
+ * @param value a `struct ValidationState`
+ * @return #GNUNET_OK if not matching, #GNUNET_NO if match found
+ */
+static int
+check_known_challenge (void *cls,
+                       const struct GNUNET_PeerIdentity *pid,
+                       void *value)
+{
+  struct CheckKnownChallengeContext *ckac = cls;
+  struct ValidationState *vs = value;
+
+  (void) pid;
+  if (0 != GNUNET_memcmp (&vs->challenge, ckac->challenge))
+    return GNUNET_OK;
+  ckac->vs = vs;
+  return GNUNET_NO;
+}
+
+
+/**
+ * Function called when peerstore is done storing a
+ * validated address.
+ *
+ * @param cls a `struct ValidationState`
+ * @param success #GNUNET_YES on success
+ */
+static void
+peerstore_store_validation_cb (void *cls, int success)
+{
+  struct ValidationState *vs = cls;
+
+  vs->sc = NULL;
+  if (GNUNET_YES == success)
+    return;
+  GNUNET_STATISTICS_update (GST_stats,
+                            "# Peerstore failed to store foreign address",
+                            1,
+                            GNUNET_NO);
+}
+
+
+/**
+ * Find the queue matching @a pid and @a address.
+ *
+ * @param pid peer the queue must go to
+ * @param address address the queue must use
+ * @return NULL if no such queue exists
+ */
+static struct Queue *
+find_queue (const struct GNUNET_PeerIdentity *pid, const char *address)
+{
+  struct Neighbour *n;
+
+  n = lookup_neighbour (pid);
+  if (NULL == n)
+    return NULL;
+  for (struct Queue *pos = n->queue_head; NULL != pos;
+       pos = pos->next_neighbour)
+  {
+    if (0 == strcmp (pos->address, address))
+      return pos;
+  }
+  return NULL;
+}
+
+
+/**
+ * Communicator gave us a transport address validation response.  Process the
+ * request.
+ *
+ * @param cls a `struct CommunicatorMessageContext` (must call
+ * #finish_cmc_handling() when done)
+ * @param tvr the message that was received
+ */
+static void
+handle_validation_response (
+  void *cls,
+  const struct TransportValidationResponseMessage *tvr)
+{
+  struct CommunicatorMessageContext *cmc = cls;
+  struct ValidationState *vs;
+  struct CheckKnownChallengeContext ckac = { .challenge = &tvr->challenge,
+                                             .vs = NULL};
+  struct GNUNET_TIME_Absolute origin_time;
+  struct Queue *q;
+  struct Neighbour *n;
+  struct VirtualLink *vl;
+  const struct GNUNET_TIME_Absolute now = GNUNET_TIME_absolute_get_monotonic (
+    GST_cfg);
+
+  /* check this is one of our challenges */
+  (void) GNUNET_CONTAINER_multipeermap_get_multiple (validation_map,
+                                                     &cmc->im.sender,
+                                                     &check_known_challenge,
+                                                     &ckac);
+  if (NULL == (vs = ckac.vs))
+  {
+    /* This can happen simply if we 'forgot' the challenge by now,
+       i.e. because we received the validation response twice */
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# Validations dropped, challenge unknown",
+                              1,
+                              GNUNET_NO);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Validation response %s dropped, challenge unknown\n",
+                GNUNET_sh2s (&tvr->challenge.value));
+    finish_cmc_handling (cmc);
+    return;
+  }
+
+  /* sanity check on origin time */
+  origin_time = GNUNET_TIME_absolute_ntoh (tvr->origin_time);
+  if ((origin_time.abs_value_us < vs->first_challenge_use.abs_value_us) ||
+      (origin_time.abs_value_us > vs->last_challenge_use.abs_value_us))
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Diff first use %" PRIu64 " and last use %" PRIu64 "\n",
+                vs->first_challenge_use.abs_value_us - origin_time.abs_value_us,
+                origin_time.abs_value_us - vs->last_challenge_use.abs_value_us);
+    GNUNET_break_op (0);
+    finish_cmc_handling (cmc);
+    return;
+  }
+
+  {
+    /* check signature */
+    struct TransportValidationPS tvp = {
+      .purpose.purpose = htonl (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_CHALLENGE),
+      .purpose.size = htonl (sizeof(tvp)),
+      .validity_duration = tvr->validity_duration,
+      .challenge = tvr->challenge
+    };
+
+    if (
+      GNUNET_OK !=
+      GNUNET_CRYPTO_eddsa_verify (GNUNET_SIGNATURE_PURPOSE_TRANSPORT_CHALLENGE,
+                                  &tvp,
+                                  &tvr->signature,
+                                  &cmc->im.sender.public_key))
+    {
+      GNUNET_break_op (0);
+      finish_cmc_handling (cmc);
+      return;
+    }
+  }
+
+  /* validity is capped by our willingness to keep track of the
+     validation entry and the maximum the other peer allows */
+  vs->valid_until = GNUNET_TIME_relative_to_absolute (
+    GNUNET_TIME_relative_min (GNUNET_TIME_relative_ntoh (
+                                tvr->validity_duration),
+                              MAX_ADDRESS_VALID_UNTIL));
+  vs->validated_until =
+    GNUNET_TIME_absolute_min (vs->valid_until,
+                              GNUNET_TIME_relative_to_absolute (
+                                ADDRESS_VALIDATION_LIFETIME));
+  vs->validation_rtt = GNUNET_TIME_absolute_get_duration (origin_time);
+  vs->challenge_backoff = GNUNET_TIME_UNIT_ZERO;
+  GNUNET_CRYPTO_random_block (GNUNET_CRYPTO_QUALITY_NONCE,
+                              &vs->challenge,
+                              sizeof(vs->challenge));
+  vs->first_challenge_use = GNUNET_TIME_absolute_subtract (
+    vs->validated_until,
+    GNUNET_TIME_relative_multiply (vs->validation_rtt,
+                                   VALIDATION_RTT_BUFFER_FACTOR));
+  if (GNUNET_TIME_absolute_cmp (vs->first_challenge_use, <, now))
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "First challenge use is now %" PRIu64 " %s \n",
+                vs->first_challenge_use.abs_value_us,
+                GNUNET_sh2s (&vs->challenge.value));
+    vs->first_challenge_use = now;
+  }
+  else
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "First challenge use is later %" PRIu64 " %s \n",
+                vs->first_challenge_use.abs_value_us,
+                GNUNET_sh2s (&vs->challenge.value));
+  vs->last_challenge_use =
+    GNUNET_TIME_UNIT_ZERO_ABS; /* challenge was not yet used */
+  update_next_challenge_time (vs, vs->first_challenge_use);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Validation response %s from %s accepted, address valid until %s\n",
+              GNUNET_sh2s (&tvr->challenge.value),
+              GNUNET_i2s (&cmc->im.sender),
+              GNUNET_STRINGS_absolute_time_to_string (vs->valid_until));
+  vs->sc = GNUNET_PEERSTORE_store (peerstore,
+                                   "transport",
+                                   &cmc->im.sender,
+                                   GNUNET_PEERSTORE_TRANSPORT_URLADDRESS_KEY,
+                                   vs->address,
+                                   strlen (vs->address) + 1,
+                                   vs->valid_until,
+                                   GNUNET_PEERSTORE_STOREOPTION_MULTIPLE,
+                                   &peerstore_store_validation_cb,
+                                   vs);
+  finish_cmc_handling (cmc);
+
+  /* Finally, we now possibly have a confirmed (!) working queue,
+     update queue status (if queue still is around) */
+  q = find_queue (&vs->pid, vs->address);
+  if (NULL == q)
+  {
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# Queues lost at time of successful validation",
+                              1,
+                              GNUNET_NO);
+    return;
+  }
+  q->validated_until = vs->validated_until;
+  q->pd.aged_rtt = vs->validation_rtt;
+  n = q->neighbour;
+  vl = lookup_virtual_link (&vs->pid);
+  if (NULL == vl)
+  {
+    vl = GNUNET_new (struct VirtualLink);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Creating new virtual link %p to %s using direct neighbour!\n",
+                vl,
+                GNUNET_i2s (&vs->pid));
+    vl->confirmed = GNUNET_YES;
+    vl->message_uuid_ctr =
+      GNUNET_CRYPTO_random_u64 (GNUNET_CRYPTO_QUALITY_WEAK, UINT64_MAX);
+    vl->target = n->pid;
+    vl->core_recv_window = RECV_WINDOW_SIZE;
+    vl->available_fc_window_size = DEFAULT_WINDOW_SIZE;
+    vl->incoming_fc_window_size = DEFAULT_WINDOW_SIZE;
+    GNUNET_break (GNUNET_YES ==
+                  GNUNET_CONTAINER_multipeermap_put (
+                    links,
+                    &vl->target,
+                    vl,
+                    GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
+    vl->n = n;
+    n->vl = vl;
+    q->idle = GNUNET_YES;
+    vl->visibility_task =
+      GNUNET_SCHEDULER_add_at (q->validated_until, &check_link_down, vl);
+    consider_sending_fc (vl);
+    /* We lacked a confirmed connection to the target
+       before, so tell CORE about it (finally!) */
+    cores_send_connect_info (&n->pid);
+    send_msg_from_cache (vl);
+  }
+  else
+  {
+    /* Link was already up, remember n is also now available and we are done */
+    if (NULL == vl->n)
+    {
+      vl->n = n;
+      n->vl = vl;
+      if (GNUNET_YES == vl->confirmed)
+        GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                    "Virtual link to %s could now also use direct neighbour!\n",
+                    GNUNET_i2s (&vs->pid));
+    }
+    else
+    {
+      GNUNET_assert (n == vl->n);
+    }
+    if (GNUNET_NO == vl->confirmed)
+    {
+      vl->confirmed = GNUNET_YES;
+      q->idle = GNUNET_YES;
+      vl->visibility_task =
+        GNUNET_SCHEDULER_add_at (q->validated_until, &check_link_down, vl);
+      consider_sending_fc (vl);
+      /* We lacked a confirmed connection to the target
+         before, so tell CORE about it (finally!) */
+      cores_send_connect_info (&n->pid);
+      send_msg_from_cache (vl);
+    }
+  }
+}
+
+
+/**
+ * Incoming meessage.  Process the request.
+ *
+ * @param im the send message that was received
+ */
+static void
+handle_incoming_msg (void *cls,
+                     const struct GNUNET_TRANSPORT_IncomingMessage *im)
+{
+  struct TransportClient *tc = cls;
+  struct CommunicatorMessageContext *cmc =
+    GNUNET_new (struct CommunicatorMessageContext);
+
+  cmc->tc = tc;
+  cmc->im = *im;
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Received message with size %u and flow control id %" PRIu64
+              " via communicator from peer %s\n",
+              ntohs (im->header.size),
+              im->fc_id,
+              GNUNET_i2s (&im->sender));
+  cmc->im.neighbour_sender = cmc->im.sender;
+  cmc->mh = (const struct GNUNET_MessageHeader *) &im[1];
+  demultiplex_with_cmc (cmc);
+}
+
+
+/**
+ * Communicator gave us a transport address validation response.  Process the
+ * request.
+ *
+ * @param cls a `struct CommunicatorMessageContext` (must call
+ * #finish_cmc_handling() when done)
+ * @param fc the message that was received
+ */
+static void
+handle_flow_control (void *cls, const struct TransportFlowControlMessage *fc)
+{
+  struct CommunicatorMessageContext *cmc = cls;
+  struct VirtualLink *vl;
+  uint32_t seq;
+  struct GNUNET_TIME_Absolute st;
+  uint64_t os;
+  uint64_t wnd;
+  uint32_t random;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Received FC from %s\n", GNUNET_i2s (&cmc->im.sender));
+  vl = lookup_virtual_link (&cmc->im.sender);
+  if (NULL == vl)
+  {
+    vl = GNUNET_new (struct VirtualLink);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "No virtual link for %p FC creating new unconfirmed virtual link to %s!\n",
+                vl,
+                GNUNET_i2s (&cmc->im.sender));
+    vl->confirmed = GNUNET_NO;
+    vl->message_uuid_ctr =
+      GNUNET_CRYPTO_random_u64 (GNUNET_CRYPTO_QUALITY_WEAK, UINT64_MAX);
+    vl->target = cmc->im.sender;
+    vl->core_recv_window = RECV_WINDOW_SIZE;
+    vl->available_fc_window_size = DEFAULT_WINDOW_SIZE;
+    vl->incoming_fc_window_size = DEFAULT_WINDOW_SIZE;
+    GNUNET_break (GNUNET_YES ==
+                  GNUNET_CONTAINER_multipeermap_put (
+                    links,
+                    &vl->target,
+                    vl,
+                    GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
+  }
+  st = GNUNET_TIME_absolute_ntoh (fc->sender_time);
+  if (st.abs_value_us < vl->last_fc_timestamp.abs_value_us)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "FC dropped: Message out of order\n");
+    /* out of order, drop */
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# FC dropped: message out of order",
+                              1,
+                              GNUNET_NO);
+    finish_cmc_handling (cmc);
+    return;
+  }
+  seq = ntohl (fc->seq);
+  if (seq < vl->last_fc_seq)
+  {
+    /* Wrap-around/reset of other peer; start all counters from zero */
+    vl->outbound_fc_window_size_used = 0;
+  }
+  vl->last_fc_seq = seq;
+  vl->last_fc_timestamp = st;
+  vl->outbound_fc_window_size = GNUNET_ntohll (fc->inbound_window_size);
+  os = GNUNET_ntohll (fc->outbound_sent);
+  vl->incoming_fc_window_size_loss =
+    (int64_t) (os - vl->incoming_fc_window_size_used);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Received FC from %s, seq %u, new window %llu (loss at %lld)\n",
+              GNUNET_i2s (&vl->target),
+              (unsigned int) seq,
+              (unsigned long long) vl->outbound_fc_window_size,
+              (long long) vl->incoming_fc_window_size_loss);
+  wnd = GNUNET_ntohll (fc->outbound_window_size);
+  random = GNUNET_CRYPTO_random_u32 (GNUNET_CRYPTO_QUALITY_WEAK,
+                                     UINT32_MAX);
+  if ((GNUNET_YES == vl->confirmed) && ((wnd < vl->incoming_fc_window_size
+                                         + vl->incoming_fc_window_size_used
+                                         + vl->incoming_fc_window_size_loss) ||
+                                        (vl->last_outbound_window_size_received
+                                         != wnd) ||
+                                        (0 == random
+                                         % FC_NO_CHANGE_REPLY_PROBABILITY)))
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Consider re-sending our FC message, as clearly the other peer's idea of the window is not up-to-date (%llu vs %llu) or %llu last received differs, or random reply %u\n",
+                (unsigned long long) wnd,
+                (unsigned long long) vl->incoming_fc_window_size,
+                (unsigned long long) vl->last_outbound_window_size_received,
+                random % FC_NO_CHANGE_REPLY_PROBABILITY);
+    consider_sending_fc (vl);
+  }
+  if ((wnd == vl->incoming_fc_window_size) &&
+      (vl->last_outbound_window_size_received == wnd) &&
+      (NULL != vl->fc_retransmit_task))
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Stopping FC retransmission to %s: peer is current at window %llu\n",
+                GNUNET_i2s (&vl->target),
+                (unsigned long long) wnd);
+    GNUNET_SCHEDULER_cancel (vl->fc_retransmit_task);
+    vl->fc_retransmit_task = NULL;
+    vl->fc_retransmit_count = 0;
+  }
+  vl->last_outbound_window_size_received = wnd;
+  /* FC window likely increased, check transmission possibilities! */
+  check_vl_transmission (vl);
+  finish_cmc_handling (cmc);
+}
+
+
+/**
+ * Given an inbound message @a msg from a communicator @a cmc,
+ * demultiplex it based on the type calling the right handler.
+ *
+ * @param cmc context for demultiplexing
+ * @param msg message to demultiplex
+ */
+static void
+demultiplex_with_cmc (struct CommunicatorMessageContext *cmc)
+{
+  struct GNUNET_MQ_MessageHandler handlers[] =
+  { GNUNET_MQ_hd_var_size (fragment_box,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_FRAGMENT,
+                           struct TransportFragmentBoxMessage,
+                           cmc),
+    GNUNET_MQ_hd_var_size (reliability_box,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_BOX,
+                           struct TransportReliabilityBoxMessage,
+                           cmc),
+    GNUNET_MQ_hd_var_size (reliability_ack,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_ACK,
+                           struct TransportReliabilityAckMessage,
+                           cmc),
+    GNUNET_MQ_hd_var_size (backchannel_encapsulation,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_BACKCHANNEL_ENCAPSULATION,
+                           struct TransportBackchannelEncapsulationMessage,
+                           cmc),
+    GNUNET_MQ_hd_var_size (dv_learn,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_DV_LEARN,
+                           struct TransportDVLearnMessage,
+                           cmc),
+    GNUNET_MQ_hd_var_size (dv_box,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_DV_BOX,
+                           struct TransportDVBoxMessage,
+                           cmc),
+    GNUNET_MQ_hd_fixed_size (
+      validation_challenge,
+      GNUNET_MESSAGE_TYPE_TRANSPORT_ADDRESS_VALIDATION_CHALLENGE,
+      struct TransportValidationChallengeMessage,
+      cmc),
+    GNUNET_MQ_hd_fixed_size (flow_control,
+                             GNUNET_MESSAGE_TYPE_TRANSPORT_FLOW_CONTROL,
+                             struct TransportFlowControlMessage,
+                             cmc),
+    GNUNET_MQ_hd_fixed_size (
+      validation_response,
+      GNUNET_MESSAGE_TYPE_TRANSPORT_ADDRESS_VALIDATION_RESPONSE,
+      struct TransportValidationResponseMessage,
+      cmc),
+    GNUNET_MQ_handler_end () };
+  int ret;
+  const struct GNUNET_MessageHeader *msg = cmc->mh;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Handling message of type %u with %u bytes\n",
+              (unsigned int) ntohs (msg->type),
+              (unsigned int) ntohs (msg->size));
+  ret = GNUNET_MQ_handle_message (handlers, msg);
+  if (GNUNET_SYSERR == ret)
+  {
+    GNUNET_break (0);
+    GNUNET_SERVICE_client_drop (cmc->tc->client);
+    GNUNET_free (cmc);
+    return;
+  }
+  if (GNUNET_NO == ret)
+  {
+    /* unencapsulated 'raw' message */
+    handle_raw_message (cmc, msg);
+  }
+}
+
+
+/**
+ * New queue became available.  Check message.
+ *
+ * @param cls the client
+ * @param aqm the send message that was sent
+ */
+static int
+check_add_queue_message (void *cls,
+                         const struct GNUNET_TRANSPORT_AddQueueMessage *aqm)
+{
+  struct TransportClient *tc = cls;
+
+  if (CT_COMMUNICATOR != tc->type)
+  {
+    GNUNET_break (0);
+    return GNUNET_SYSERR;
+  }
+  GNUNET_MQ_check_zero_termination (aqm);
+  return GNUNET_OK;
+}
+
+
+/**
+ * If necessary, generates the UUID for a @a pm
+ *
+ * @param pm pending message to generate UUID for.
+ */
+static void
+set_pending_message_uuid (struct PendingMessage *pm)
+{
+  if (pm->msg_uuid_set)
+    return;
+  pm->msg_uuid.uuid = pm->vl->message_uuid_ctr++;
+  pm->msg_uuid_set = GNUNET_YES;
+}
+
+
+/**
+ * Setup data structure waiting for acknowledgements.
+ *
+ * @param queue queue the @a pm will be sent over
+ * @param dvh path the message will take, may be NULL
+ * @param pm the pending message for transmission
+ * @return corresponding fresh pending acknowledgement
+ */
+static struct PendingAcknowledgement *
+prepare_pending_acknowledgement (struct Queue *queue,
+                                 struct DistanceVectorHop *dvh,
+                                 struct PendingMessage *pm)
+{
+  struct PendingAcknowledgement *pa;
+
+  pa = GNUNET_new (struct PendingAcknowledgement);
+  pa->queue = queue;
+  pa->dvh = dvh;
+  pa->pm = pm;
+  do
+  {
+    GNUNET_CRYPTO_random_block (GNUNET_CRYPTO_QUALITY_NONCE,
+                                &pa->ack_uuid,
+                                sizeof(pa->ack_uuid));
+  }
+  while (GNUNET_YES != GNUNET_CONTAINER_multiuuidmap_put (
+           pending_acks,
+           &pa->ack_uuid.value,
+           pa,
+           GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
+  GNUNET_CONTAINER_MDLL_insert (queue, queue->pa_head, queue->pa_tail, pa);
+  GNUNET_CONTAINER_MDLL_insert (pm, pm->pa_head, pm->pa_tail, pa);
+  if (NULL != dvh)
+    GNUNET_CONTAINER_MDLL_insert (dvh, dvh->pa_head, dvh->pa_tail, pa);
+  pa->transmission_time = GNUNET_TIME_absolute_get ();
+  pa->message_size = pm->bytes_msg;
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Waiting for ACKnowledgment `%s' for <%" PRIu64 ">\n",
+              GNUNET_uuid2s (&pa->ack_uuid.value),
+              pm->logging_uuid);
+  return pa;
+}
+
+
+/**
+ * Fragment the given @a pm to the given @a mtu.  Adds
+ * additional fragments to the neighbour as well. If the
+ * @a mtu is too small, generates and error for the @a pm
+ * and returns NULL.
+ *
+ * @param queue which queue to fragment for
+ * @param dvh path the message will take, or NULL
+ * @param pm pending message to fragment for transmission
+ * @return new message to transmit
+ */
+static struct PendingMessage *
+fragment_message (struct Queue *queue,
+                  struct DistanceVectorHop *dvh,
+                  struct PendingMessage *pm)
+{
+  struct PendingAcknowledgement *pa;
+  struct PendingMessage *ff;
+  uint16_t mtu;
+  uint16_t msize;
+
+  mtu = (UINT16_MAX == queue->mtu)
+        ? UINT16_MAX - sizeof(struct GNUNET_TRANSPORT_SendMessageTo)
+        : queue->mtu;
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Fragmenting message <%" PRIu64
+              "> with size %u to %s for MTU %u\n",
+              pm->logging_uuid,
+              pm->bytes_msg,
+              GNUNET_i2s (&pm->vl->target),
+              (unsigned int) mtu);
+  set_pending_message_uuid (pm);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Fragmenting message %" PRIu64 " <%" PRIu64
+              "> with size %u to %s for MTU %u\n",
+              pm->msg_uuid.uuid,
+              pm->logging_uuid,
+              pm->bytes_msg,
+              GNUNET_i2s (&pm->vl->target),
+              (unsigned int) mtu);
+
+  /* This invariant is established in #handle_add_queue_message() */
+  GNUNET_assert (mtu > sizeof(struct TransportFragmentBoxMessage));
+
+  /* select fragment for transmission, descending the tree if it has
+     been expanded until we are at a leaf or at a fragment that is small
+     enough
+   */
+  ff = pm;
+  msize = ff->bytes_msg;
+
+  while (((ff->bytes_msg > mtu) || (pm == ff)) &&
+         (ff->frag_off == msize) && (NULL != ff->head_frag))
+  {
+    ff = ff->head_frag;   /* descent into fragmented fragments */
+    msize = ff->bytes_msg - sizeof(struct TransportFragmentBoxMessage);
+  }
+
+  if (((ff->bytes_msg > mtu) || (pm == ff)) && (ff->frag_off < msize))
+  {
+    /* Did not yet calculate all fragments, calculate next fragment */
+    struct PendingMessage *frag;
+    struct TransportFragmentBoxMessage tfb;
+    const char *orig;
+    char *msg;
+    uint16_t fragmax;
+    uint16_t fragsize;
+    uint16_t msize;
+    uint16_t xoff = 0;
+
+    orig = (const char *) &ff[1];
+    msize = ff->bytes_msg;
+    if (pm != ff)
+    {
+      const struct TransportFragmentBoxMessage *tfbo;
+
+      tfbo = (const struct TransportFragmentBoxMessage *) orig;
+      orig += sizeof(struct TransportFragmentBoxMessage);
+      msize -= sizeof(struct TransportFragmentBoxMessage);
+      xoff = ntohs (tfbo->frag_off);
+    }
+    fragmax = mtu - sizeof(struct TransportFragmentBoxMessage);
+    fragsize = GNUNET_MIN (msize - ff->frag_off, fragmax);
+    frag =
+      GNUNET_malloc (sizeof(struct PendingMessage)
+                     + sizeof(struct TransportFragmentBoxMessage) + fragsize);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "3 created pm %p from pm %p storing vl %p from pm %p\n",
+                frag,
+                ff,
+                pm->vl,
+                pm);
+    frag->logging_uuid = logging_uuid_gen++;
+    frag->vl = pm->vl;
+    frag->frag_parent = ff;
+    frag->timeout = pm->timeout;
+    frag->bytes_msg = sizeof(struct TransportFragmentBoxMessage) + fragsize;
+    frag->pmt = PMT_FRAGMENT_BOX;
+    msg = (char *) &frag[1];
+    tfb.header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_FRAGMENT);
+    tfb.header.size =
+      htons (sizeof(struct TransportFragmentBoxMessage) + fragsize);
+    pa = prepare_pending_acknowledgement (queue, dvh, frag);
+    tfb.ack_uuid = pa->ack_uuid;
+    tfb.msg_uuid = pm->msg_uuid;
+    tfb.frag_off = htons (ff->frag_off + xoff);
+    tfb.msg_size = htons (pm->bytes_msg);
+    memcpy (msg, &tfb, sizeof(tfb));
+    memcpy (&msg[sizeof(tfb)], &orig[ff->frag_off], fragsize);
+    GNUNET_CONTAINER_MDLL_insert (frag, ff->head_frag,
+                                  ff->tail_frag, frag);
+    ff->frag_off += fragsize;
+    ff = frag;
+  }
+
+  /* Move head to the tail and return it */
+  GNUNET_CONTAINER_MDLL_remove (frag,
+                                ff->frag_parent->head_frag,
+                                ff->frag_parent->tail_frag,
+                                ff);
+  GNUNET_CONTAINER_MDLL_insert_tail (frag,
+                                     ff->frag_parent->head_frag,
+                                     ff->frag_parent->tail_frag,
+                                     ff);
+
+  return ff;
+}
+
+
+/**
+ * Reliability-box the given @a pm. On error (can there be any), NULL
+ * may be returned, otherwise the "replacement" for @a pm (which
+ * should then be added to the respective neighbour's queue instead of
+ * @a pm).  If the @a pm is already fragmented or reliability boxed,
+ * or itself an ACK, this function simply returns @a pm.
+ *
+ * @param queue which queue to prepare transmission for
+ * @param dvh path the message will take, or NULL
+ * @param pm pending message to box for transmission over unreliabile queue
+ * @return new message to transmit
+ */
+static struct PendingMessage *
+reliability_box_message (struct Queue *queue,
+                         struct DistanceVectorHop *dvh,
+                         struct PendingMessage *pm)
+{
+  struct TransportReliabilityBoxMessage rbox;
+  struct PendingAcknowledgement *pa;
+  struct PendingMessage *bpm;
+  char *msg;
+
+  if ((PMT_CORE != pm->pmt) && (PMT_DV_BOX != pm->pmt))
+    return pm; /* already fragmented or reliability boxed, or control message:
+                  do nothing */
+  if (NULL != pm->bpm)
+    return pm->bpm; /* already computed earlier: do nothing */
+  // TODO I guess we do not need this assertion. We might have a DLL with
+  // fragments, because the MTU changed, and we do not need to fragment anymore.
+  // But we should keep the fragments until message was completed, because
+  // the MTU might change again.
+  // GNUNET_assert (NULL == pm->head_frag);
+  if (pm->bytes_msg + sizeof(rbox) > UINT16_MAX)
+  {
+    /* failed hard */
+    GNUNET_break (0);
+    client_send_response (pm);
+    return NULL;
+  }
+
+  pa = prepare_pending_acknowledgement (queue, dvh, pm);
+
+  bpm = GNUNET_malloc (sizeof(struct PendingMessage) + sizeof(rbox)
+                       + pm->bytes_msg);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "4 created pm %p storing vl %p from pm %p\n",
+              bpm,
+              pm->vl,
+              pm);
+  bpm->logging_uuid = logging_uuid_gen++;
+  bpm->vl = pm->vl;
+  bpm->frag_parent = pm;
+  // Why was this needed?
+  // GNUNET_CONTAINER_MDLL_insert (frag, pm->head_frag, pm->tail_frag, bpm);
+  bpm->timeout = pm->timeout;
+  bpm->pmt = PMT_RELIABILITY_BOX;
+  bpm->bytes_msg = pm->bytes_msg + sizeof(rbox);
+  set_pending_message_uuid (bpm);
+  rbox.header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_RELIABILITY_BOX);
+  rbox.header.size = htons (sizeof(rbox) + pm->bytes_msg);
+  rbox.ack_countdown = htonl (0);  // FIXME: implement ACK countdown support
+
+  rbox.ack_uuid = pa->ack_uuid;
+  msg = (char *) &bpm[1];
+  memcpy (msg, &rbox, sizeof(rbox));
+  memcpy (&msg[sizeof(rbox)], &pm[1], pm->bytes_msg);
+  pm->bpm = bpm;
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Preparing reliability box for message <%" PRIu64
+              "> of size %d (%d) to %s on queue %s\n",
+              pm->logging_uuid,
+              pm->bytes_msg,
+              ntohs (((const struct GNUNET_MessageHeader *) &pm[1])->size),
+              GNUNET_i2s (&pm->vl->target),
+              queue->address);
+  return bpm;
+}
+
+
+static void
+reorder_root_pm (struct PendingMessage *pm,
+                 struct GNUNET_TIME_Absolute next_attempt)
+{
+  struct VirtualLink *vl = pm->vl;
+  struct PendingMessage *pos;
+
+  /* re-insert sort in neighbour list */
+  GNUNET_CONTAINER_MDLL_remove (vl,
+                                vl->pending_msg_head,
+                                vl->pending_msg_tail,
+                                pm);
+  pos = vl->pending_msg_tail;
+  while ((NULL != pos) &&
+         (next_attempt.abs_value_us > pos->next_attempt.abs_value_us))
+    pos = pos->prev_vl;
+  GNUNET_CONTAINER_MDLL_insert_after (vl,
+                                      vl->pending_msg_head,
+                                      vl->pending_msg_tail,
+                                      pos,
+                                      pm);
+}
+
+
+static unsigned int
+check_next_attempt_tree (struct PendingMessage *pm,
+                         struct GNUNET_TIME_Absolute next_attempt)
+{
+  struct PendingMessage *pos;
+
+  pos = pm->head_frag;
+  while (NULL != pos)
+  {
+    if (pos->next_attempt.abs_value_us != next_attempt.abs_value_us ||
+        GNUNET_YES == check_next_attempt_tree (pos, next_attempt))
+      return GNUNET_YES;
+    pos = pos->next_frag;
+  }
+
+  return GNUNET_NO;
+}
+
+
+/**
+ * Change the value of the `next_attempt` field of @a pm
+ * to @a next_attempt and re-order @a pm in the transmission
+ * list as required by the new timestamp.
+ *
+ * @param pm a pending message to update
+ * @param next_attempt timestamp to use
+ */
+static void
+update_pm_next_attempt (struct PendingMessage *pm,
+                        struct GNUNET_TIME_Absolute next_attempt)
+{
+  if (NULL == pm->frag_parent)
+  {
+    pm->next_attempt = next_attempt;
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Next attempt for message <%" PRIu64 "> set to %" PRIu64 "\n",
+                pm->logging_uuid,
+                next_attempt.abs_value_us);
+    reorder_root_pm (pm, next_attempt);
+  }
+  else if ((PMT_RELIABILITY_BOX == pm->pmt) || (PMT_DV_BOX == pm->pmt))// || (PMT_FRAGMENT_BOX == pm->pmt))
+  {
+    struct PendingMessage *root = pm->frag_parent;
+
+    while (NULL != root->frag_parent)
+      root = root->frag_parent;
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Next attempt for root message <%" PRIu64 "> set to %s\n",
+                root->logging_uuid,
+                GNUNET_STRINGS_absolute_time_to_string (next_attempt));
+    root->next_attempt = next_attempt;
+    reorder_root_pm (root, next_attempt);
+  }
+  else
+  {
+    struct PendingMessage *root = pm->frag_parent;
+
+    while (NULL != root->frag_parent)
+      root = root->frag_parent;
+
+    if (GNUNET_NO == root->frags_in_flight)
+    {
+      root->next_attempt = next_attempt;
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "Next attempt for fragmented message <%" PRIu64 "> (<%" PRIu64
+                  ">)set to %" PRIu64 "\n",
+                  pm->logging_uuid,
+                  root->logging_uuid,
+                  next_attempt.abs_value_us);
+    }
+
+    pm->next_attempt = root->next_attempt;
+
+    if (root->bytes_msg == root->frag_off)
+      root->frags_in_flight = check_next_attempt_tree (root,
+                                                       root->next_attempt);
+    else
+      root->frags_in_flight = GNUNET_YES;
+
+    if (GNUNET_NO == root->frags_in_flight)
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "We have no fragments in flight for message %" PRIu64
+                  ", reorder root! Next attempt is %" PRIu64 "\n",
+                  root->logging_uuid,
+                  root->next_attempt.abs_value_us);
+      reorder_root_pm (root, root->next_attempt);
+      root->frag_count = 0;
+      root->next_attempt = GNUNET_TIME_UNIT_ZERO_ABS;
+    }
+    else
+    {
+      double factor = ((double) root->frag_count - 1)
+                      / (double) root->frag_count;
+      struct GNUNET_TIME_Relative s1;
+      struct GNUNET_TIME_Relative s2;
+      struct GNUNET_TIME_Relative plus_mean =
+        GNUNET_TIME_absolute_get_duration (root->next_attempt);
+      struct GNUNET_TIME_Relative plus = GNUNET_TIME_absolute_get_duration (
+        next_attempt);
+
+      s1 = GNUNET_TIME_relative_multiply (plus_mean,
+                                          factor);
+      s2 = GNUNET_TIME_relative_divide (plus,
+                                        root->frag_count);
+      plus_mean = GNUNET_TIME_relative_add (s1, s2);
+      root->next_attempt = GNUNET_TIME_relative_to_absolute (plus_mean);
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "We have fragments in flight for message %" PRIu64
+                  ", do not reorder root! Actual next attempt %" PRIu64 "\n",
+                  root->logging_uuid,
+                  root->next_attempt.abs_value_us);
+    }
+  }
+}
+
+
+/**
+ * Context for #select_best_pending_from_link().
+ */
+struct PendingMessageScoreContext
+{
+  /**
+   * Set to the best message that was found, NULL for none.
+   */
+  struct PendingMessage *best;
+
+  /**
+   * DVH that @e best should take, or NULL for direct transmission.
+   */
+  struct DistanceVectorHop *dvh;
+
+  /**
+   * What is the estimated total overhead for this message?
+   */
+  size_t real_overhead;
+
+  /**
+   * Number of pending messages we seriously considered this time.
+   */
+  unsigned int consideration_counter;
+
+  /**
+   * Did we have to fragment?
+   */
+  int frag;
+
+  /**
+   * Did we have to reliability box?
+   */
+  int relb;
+
+  /**
+   * There are pending messages, but it was to early to send one of them.
+   */
+  int to_early;
+
+  /**
+   * There is a pending messages we are sending fragments at the moment.
+   */
+  unsigned int frags_in_flight;
+
+  /**
+   * When will we try to transmit the message again for which it was to early to retry.
+   */
+  struct GNUNET_TIME_Relative to_early_retry_delay;
+};
+
+
+/**
+ * Select the best pending message from @a vl for transmission
+ * via @a queue.
+ *
+ * @param[in,out] sc best message so far (NULL for none), plus scoring data
+ * @param queue the queue that will be used for transmission
+ * @param vl the virtual link providing the messages
+ * @param dvh path we are currently considering, or NULL for none
+ * @param overhead number of bytes of overhead to be expected
+ *        from DV encapsulation (0 for without DV)
+ */
+static void
+select_best_pending_from_link (struct PendingMessageScoreContext *sc,
+                               struct Queue *queue,
+                               struct VirtualLink *vl,
+                               struct DistanceVectorHop *dvh,
+                               size_t overhead)
+{
+  struct GNUNET_TIME_Absolute now;
+
+  now = GNUNET_TIME_absolute_get ();
+  sc->to_early = GNUNET_NO;
+  sc->frags_in_flight = GNUNET_NO;
+  for (struct PendingMessage *pos = vl->pending_msg_head; NULL != pos;
+       pos = pos->next_vl)
+  {
+    size_t real_overhead = overhead;
+    int frag;
+    int relb;
+
+    if ((NULL != dvh) && (PMT_DV_BOX == pos->pmt))
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "DV messages must not be DV-routed to next hop!\n");
+      continue;   /* DV messages must not be DV-routed to next hop! */
+    }
+    if (pos->next_attempt.abs_value_us > now.abs_value_us)
+    {
+      if (GNUNET_YES == pos->frags_in_flight)
+      {
+        sc->frags_in_flight = GNUNET_YES;
+        GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                    "Fragments in flight for message %" PRIu64 "\n",
+                    pos->logging_uuid);
+      }
+      else
+      {
+        GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                    "Maybe too early, because message are sorted by next_attempt, if there are no fragments in flight.Checked message %"
+                    PRIu64 "\n",
+                    pos->logging_uuid);
+        sc->to_early = GNUNET_YES;
+        sc->to_early_retry_delay = GNUNET_TIME_absolute_get_remaining (
+          pos->next_attempt);
+        continue;
+      }
+      // break;   /* too early for all messages, they are sorted by next_attempt */
+    }
+    if (NULL != pos->qe)
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "not eligible\n");
+      continue;   /* not eligible */
+    }
+    sc->consideration_counter++;
+    /* determine if we have to fragment, if so add fragmentation
+       overhead! */
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "check %" PRIu64 " for sc->best\n",
+                pos->logging_uuid);
+    frag = GNUNET_NO;
+    if (((0 != queue->mtu) &&
+         (pos->bytes_msg + real_overhead > queue->mtu)) ||
+        (pos->bytes_msg > UINT16_MAX - sizeof(struct
+                                              GNUNET_TRANSPORT_SendMessageTo))
+        ||
+        (NULL != pos->head_frag /* fragments already exist, should
+                                     respect that even if MTU is UINT16_MAX for
+                                     this queue */))
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "fragment msg with size %u, realoverhead is %lu\n",
+                  pos->bytes_msg,
+                  real_overhead);
+      frag = GNUNET_YES;
+      if (GNUNET_TRANSPORT_CC_RELIABLE == queue->tc->details.communicator.cc)
+      {
+        /* FIXME-FRAG-REL-UUID: we could use an optimized, shorter fragmentation
+           header without the ACK UUID when using a *reliable* channel! */
+      }
+      real_overhead = overhead + sizeof(struct TransportFragmentBoxMessage);
+    }
+    /* determine if we have to reliability-box, if so add reliability box
+       overhead */
+    relb = GNUNET_NO;
+    if ((GNUNET_NO == frag) &&
+        (0 == (pos->prefs & GNUNET_MQ_PREF_UNRELIABLE)) &&
+        (GNUNET_TRANSPORT_CC_RELIABLE != queue->tc->details.communicator.cc))
+    {
+      real_overhead += sizeof(struct TransportReliabilityBoxMessage);
+
+      if ((0 != queue->mtu) && (pos->bytes_msg + real_overhead > queue->mtu))
+      {
+        frag = GNUNET_YES;
+        real_overhead = overhead + sizeof(struct TransportFragmentBoxMessage);
+      }
+      else
+      {
+        relb = GNUNET_YES;
+      }
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "Create reliability box of msg with size %u, realoverhead is %lu %u %u %u\n",
+                  pos->bytes_msg,
+                  real_overhead,
+                  queue->mtu,
+                  frag,
+                  relb);
+    }
+
+    /* Finally, compare to existing 'best' in sc to see if this 'pos' pending
+       message would beat it! */
+    if (GNUNET_NO == sc->frags_in_flight && NULL != sc->best)
+    {
+      /* CHECK if pos fits queue BETTER (=smaller) than pm, if not: continue;
+         OPTIMIZE-ME: This is a heuristic, which so far has NOT been
+         experimentally validated. There may be some huge potential for
+         improvement here. Also, we right now only compare how well the
+         given message fits _this_ queue, and do not consider how well other
+         queues might suit the message. Taking other queues into consideration
+         may further improve the result, but could also be expensive
+         in terms of CPU time.  */
+      long long sc_score = sc->frag * 40 + sc->relb * 20 + sc->real_overhead;
+      long long pm_score = frag * 40 + relb * 20 + real_overhead;
+      long long time_delta =
+        (sc->best->next_attempt.abs_value_us - pos->next_attempt.abs_value_us)
+        / 1000LL;
+
+      /* "time_delta" considers which message has been 'ready' for transmission
+         for longer, if a message has a preference for low latency, increase
+         the weight of the time_delta by 10x if it is favorable for that message */
+      if ((0 != (pos->prefs & GNUNET_MQ_PREF_LOW_LATENCY)) &&
+          (0 != (sc->best->prefs & GNUNET_MQ_PREF_LOW_LATENCY)))
+        time_delta *= 10;     /* increase weight (always, both are low latency) */
+      else if ((0 != (pos->prefs & GNUNET_MQ_PREF_LOW_LATENCY)) &&
+               (time_delta > 0))
+        time_delta *= 10;     /* increase weight, favors 'pos', which is low latency */
+      else if ((0 != (sc->best->prefs & GNUNET_MQ_PREF_LOW_LATENCY)) &&
+               (time_delta < 0))
+        time_delta *= 10;     /* increase weight, favors 'sc->best', which is low latency */
+      if (0 != queue->mtu)
+      {
+        /* Grant bonus if we are below MTU, larger bonus the closer we will
+           be to the MTU */
+        if (queue->mtu > sc->real_overhead + sc->best->bytes_msg)
+          sc_score -= queue->mtu - (sc->real_overhead + sc->best->bytes_msg);
+        if (queue->mtu > real_overhead + pos->bytes_msg)
+          pm_score -= queue->mtu - (real_overhead + pos->bytes_msg);
+      }
+      if (sc_score + time_delta > pm_score)
+      {
+        GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                    "sc_score of %" PRIu64 " larger, keep sc->best %" PRIu64
+                    "\n",
+                    pos->logging_uuid,
+                    sc->best->logging_uuid);
+        continue;     /* sc_score larger, keep sc->best */
+      }
+    }
+    sc->best = pos;
+    sc->dvh = dvh;
+    sc->frag = frag;
+    sc->relb = relb;
+    sc->real_overhead = real_overhead;
+  }
+}
+
+
+/**
+ * Function to call to further operate on the now DV encapsulated
+ * message @a hdr, forwarding it via @a next_hop under respect of
+ * @a options.
+ *
+ * @param cls a `struct PendingMessageScoreContext`
+ * @param next_hop next hop of the DV path
+ * @param hdr encapsulated message, technically a `struct TransportDVBoxMessage`
+ * @param options options of the original message
+ */
+static void
+extract_box_cb (void *cls,
+                struct Neighbour *next_hop,
+                const struct GNUNET_MessageHeader *hdr,
+                enum RouteMessageOptions options)
+{
+  struct PendingMessageScoreContext *sc = cls;
+  struct PendingMessage *pm = sc->best;
+  struct PendingMessage *bpm;
+  uint16_t bsize = ntohs (hdr->size);
+
+  GNUNET_assert (NULL == pm->bpm);
+  bpm = GNUNET_malloc (sizeof(struct PendingMessage) + bsize);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "5 created pm %p storing vl %p from pm %p\n",
+              bpm,
+              pm->vl,
+              pm);
+  bpm->logging_uuid = logging_uuid_gen++;
+  bpm->pmt = PMT_DV_BOX;
+  bpm->vl = pm->vl;
+  bpm->timeout = pm->timeout;
+  bpm->bytes_msg = bsize;
+  bpm->frag_parent = pm;
+  set_pending_message_uuid (bpm);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Creating DV Box %" PRIu64 " for original message %" PRIu64
+              " (next hop is %s)\n",
+              bpm->logging_uuid,
+              pm->logging_uuid,
+              GNUNET_i2s (&next_hop->pid));
+  memcpy (&bpm[1], hdr, bsize);
+  pm->bpm = bpm;
+}
+
+
+/**
+ * We believe we are ready to transmit a `struct PendingMessage` on a
+ * queue, the big question is which one!  We need to see if there is
+ * one pending that is allowed by flow control and congestion control
+ * and (ideally) matches our queue's performance profile.
+ *
+ * If such a message is found, we give the message to the communicator
+ * for transmission (updating the tracker, and re-scheduling ourselves
+ * if applicable).
+ *
+ * If no such message is found, the queue's `idle` field must be set
+ * to #GNUNET_YES.
+ *
+ * @param cls the `struct Queue` to process transmissions for
+ */
+static void
+transmit_on_queue (void *cls)
+{
+  struct Queue *queue = cls;
+  struct Neighbour *n = queue->neighbour;
+  struct PendingMessageScoreContext sc;
+  struct PendingMessage *pm;
+
+  queue->transmit_task = NULL;
+  if (NULL == n->vl)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Virtual link `%s' is down, cannot have PM for queue `%s'\n",
+                GNUNET_i2s (&n->pid),
+                queue->address);
+    queue->idle = GNUNET_YES;
+    return;
+  }
+  memset (&sc, 0, sizeof(sc));
+  select_best_pending_from_link (&sc, queue, n->vl, NULL, 0);
+  if (NULL == sc.best)
+  {
+    /* Also look at DVH that have the n as first hop! */
+    for (struct DistanceVectorHop *dvh = n->dv_head; NULL != dvh;
+         dvh = dvh->next_neighbour)
+    {
+      select_best_pending_from_link (&sc,
+                                     queue,
+                                     dvh->dv->vl,
+                                     dvh,
+                                     sizeof(struct GNUNET_PeerIdentity)
+                                     * (1 + dvh->distance)
+                                     + sizeof(struct TransportDVBoxMessage)
+                                     + sizeof(struct TransportDVBoxPayloadP));
+    }
+  }
+  if (NULL == sc.best)
+  {
+    /* no message pending, nothing to do here! */
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "No pending messages, queue `%s' to %s now idle\n",
+                queue->address,
+                GNUNET_i2s (&n->pid));
+    if (GNUNET_YES == sc.to_early)
+      schedule_transmit_on_queue (sc.to_early_retry_delay,
+                                  queue,
+                                  GNUNET_SCHEDULER_PRIORITY_DEFAULT);
+    queue->idle = GNUNET_YES;
+    return;
+  }
+  /* There is a message pending, we are certainly not idle */
+  queue->idle = GNUNET_NO;
+
+  /* Given selection in `sc`, do transmission */
+  pm = sc.best;
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Selected message <%" PRIu64 ">\n",
+              pm->logging_uuid);
+  if (NULL != sc.dvh)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Is this %u a DV box?\n",
+                pm->pmt);
+    GNUNET_assert (PMT_DV_BOX != pm->pmt);
+    if ((NULL != sc.best->bpm) && (sc.best->bpm->used_dvh != sc.dvh))
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "Discard old box, because we have a new DV path.\n");
+      free_pending_message (sc.best->bpm);
+      sc.best->bpm = NULL;
+    }
+
+    if (NULL == sc.best->bpm)
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "encapsulate_for_dv 2\n");
+      encapsulate_for_dv (sc.dvh->dv,
+                          1,
+                          &sc.dvh,
+                          (const struct GNUNET_MessageHeader *) &sc.best[1],
+                          &extract_box_cb,
+                          &sc,
+                          RMO_NONE,
+                          GNUNET_NO);
+      GNUNET_assert (NULL != sc.best->bpm);
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "%lu %lu %lu %lu %u\n",
+                  sizeof(struct GNUNET_PeerIdentity),
+                  sizeof(struct TransportDVBoxMessage),
+                  sizeof(struct TransportDVBoxPayloadP),
+                  sizeof(struct TransportFragmentBoxMessage),
+                  ((const struct GNUNET_MessageHeader *) &sc.best[1])->size);
+      sc.best->bpm->used_dvh = sc.dvh;
+    }
+    pm = sc.best->bpm;
+  }
+  if (GNUNET_YES == sc.frag)
+  {
+    pm = fragment_message (queue, sc.dvh, pm);
+    if (NULL == pm)
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "Fragmentation failed queue %s to %s for <%" PRIu64
+                  ">, trying again\n",
+                  queue->address,
+                  GNUNET_i2s (&n->pid),
+                  sc.best->logging_uuid);
+      schedule_transmit_on_queue (GNUNET_TIME_UNIT_ZERO,
+                                  queue,
+                                  GNUNET_SCHEDULER_PRIORITY_DEFAULT);
+      return;
+    }
+  }
+  else if (GNUNET_YES == sc.relb)
+  {
+    pm = reliability_box_message (queue, sc.dvh, pm);
+    if (NULL == pm)
+    {
+      /* Reliability boxing failed, try next message... */
+      GNUNET_log (
+        GNUNET_ERROR_TYPE_DEBUG,
+        "Reliability boxing failed queue %s to %s for <%" PRIu64
+        ">, trying again\n",
+        queue->address,
+        GNUNET_i2s (&n->pid),
+        sc.best->logging_uuid);
+      schedule_transmit_on_queue (GNUNET_TIME_UNIT_ZERO,
+                                  queue,
+                                  GNUNET_SCHEDULER_PRIORITY_DEFAULT);
+      return;
+    }
+  }
+
+  /* Pass 'pm' for transission to the communicator */
+  GNUNET_log (
+    GNUNET_ERROR_TYPE_DEBUG,
+    "Passing message <%" PRIu64
+    "> to queue %s for peer %s (considered %u others)\n",
+    pm->logging_uuid,
+    queue->address,
+    GNUNET_i2s (&n->pid),
+    sc.consideration_counter);
+
+  /* Flow control: increment amount of traffic sent; if we are routing
+     via DV (and thus the ultimate target of the pending message is for
+     a different virtual link than the one of the queue), then we need
+     to use up not only the window of the direct link but also the
+     flow control window for the DV link! */
+  pm->vl->outbound_fc_window_size_used += pm->bytes_msg;
+
+  if (pm->vl != queue->neighbour->vl)
+  {
+    /* If the virtual link of the queue differs, this better be distance
+       vector routing! */
+    GNUNET_assert (NULL != sc.dvh);
+    /* If we do distance vector routing, we better not do this for a
+       message that was itself DV-routed */
+    GNUNET_assert (PMT_DV_BOX != sc.best->pmt);
+    /* We use the size of the unboxed message here, to avoid counting
+       the DV-Box header which is eaten up on the way by intermediaries */
+    queue->neighbour->vl->outbound_fc_window_size_used += sc.best->bytes_msg;
+  }
+  else
+  {
+    GNUNET_assert (NULL == sc.dvh);
+  }
+
+  queue_send_msg (queue, pm, &pm[1], pm->bytes_msg);
+
+  /* Check if this transmission somehow conclusively finished handing 'pm'
+     even without any explicit ACKs */
+  if ((PMT_CORE == pm->pmt) ||
+      (GNUNET_TRANSPORT_CC_RELIABLE == queue->tc->details.communicator.cc))
+  {
+    completed_pending_message (pm);
+  }
+  else
+  {
+    struct GNUNET_TIME_Relative wait_duration;
+    unsigned int wait_multiplier;
+
+    if (PMT_FRAGMENT_BOX == pm->pmt)
+    {
+      struct PendingMessage *root;
+
+      root = pm->frag_parent;
+      while (NULL != root->frag_parent)
+        root = root->frag_parent;
+
+      root->frag_count++;
+      wait_multiplier =  (unsigned int) ceil ((double) root->bytes_msg
+                                              / ((double) root->frag_off
+                                                 / (double) root->frag_count))
+                        * 4;
+    }
+    else
+    {
+      // No fragments, we use 4 RTT before retransmitting.
+      wait_multiplier = 4;
+    }
+
+    // Depending on how much pending message the VirtualLink is queueing, we wait longer.
+    // wait_multiplier = wait_multiplier * pm->vl->pending_msg_num;
+
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Wait multiplier %u\n",
+                wait_multiplier);
+
+    /* Message not finished, waiting for acknowledgement.
+       Update time by which we might retransmit 's' based on queue
+       characteristics (i.e. RTT); it takes one RTT for the message to
+       arrive and the ACK to come back in the best case; but the other
+       side is allowed to delay ACKs by 2 RTTs, so we use 4 RTT before
+       retransmitting.
+
+       OPTIMIZE: Note that in the future this heuristic should likely
+       be improved further (measure RTT stability, consider message
+       urgency and size when delaying ACKs, etc.) */
+
+    if (GNUNET_TIME_UNIT_FOREVER_REL.rel_value_us !=
+        queue->pd.aged_rtt.rel_value_us)
+      wait_duration = queue->pd.aged_rtt;
+    else
+    {
+      wait_duration = DEFAULT_ACK_WAIT_DURATION;
+      wait_multiplier = 4;
+    }
+    struct GNUNET_TIME_Absolute next = GNUNET_TIME_relative_to_absolute (
+      GNUNET_TIME_relative_multiply (
+        wait_duration, wait_multiplier));
+    struct GNUNET_TIME_Relative plus = GNUNET_TIME_relative_multiply (
+      wait_duration, wait_multiplier);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Waiting %s (%s) for ACK until %s\n",
+                GNUNET_STRINGS_relative_time_to_string (
+                  GNUNET_TIME_relative_multiply (
+                    queue->pd.aged_rtt, wait_multiplier), GNUNET_NO),
+                GNUNET_STRINGS_relative_time_to_string (plus, GNUNET_YES),
+                GNUNET_STRINGS_absolute_time_to_string (next));
+    update_pm_next_attempt (pm,
+                            GNUNET_TIME_relative_to_absolute (
+                              GNUNET_TIME_relative_multiply (wait_duration,
+                                                             wait_multiplier)));
+  }
+  /* finally, re-schedule queue transmission task itself */
+  schedule_transmit_on_queue (GNUNET_TIME_UNIT_ZERO,
+                              queue,
+                              GNUNET_SCHEDULER_PRIORITY_DEFAULT);
+}
+
+
+/**
+ * Queue to a peer went down.  Process the request.
+ *
+ * @param cls the client
+ * @param dqm the send message that was sent
+ */
+static void
+handle_del_queue_message (void *cls,
+                          const struct GNUNET_TRANSPORT_DelQueueMessage *dqm)
+{
+  struct TransportClient *tc = cls;
+
+  if (CT_COMMUNICATOR != tc->type)
+  {
+    GNUNET_break (0);
+    GNUNET_SERVICE_client_drop (tc->client);
+    return;
+  }
+  for (struct Queue *queue = tc->details.communicator.queue_head; NULL != queue;
+       queue = queue->next_client)
+  {
+    struct Neighbour *neighbour = queue->neighbour;
+
+    if ((ntohl (dqm->qid) != queue->qid) ||
+        (0 != GNUNET_memcmp (&dqm->receiver, &neighbour->pid)))
+      continue;
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Dropped queue %s to peer %s\n",
+                queue->address,
+                GNUNET_i2s (&neighbour->pid));
+    free_queue (queue);
+    GNUNET_SERVICE_client_continue (tc->client);
+    return;
+  }
+  GNUNET_break (0);
+  GNUNET_SERVICE_client_drop (tc->client);
+}
+
+
+/**
+ * Message was transmitted.  Process the request.
+ *
+ * @param cls the client
+ * @param sma the send message that was sent
+ */
+static void
+handle_send_message_ack (void *cls,
+                         const struct GNUNET_TRANSPORT_SendMessageToAck *sma)
+{
+  struct TransportClient *tc = cls;
+  struct QueueEntry *qe;
+  struct PendingMessage *pm;
+
+  if (CT_COMMUNICATOR != tc->type)
+  {
+    GNUNET_break (0);
+    GNUNET_SERVICE_client_drop (tc->client);
+    return;
+  }
+
+  /* find our queue entry matching the ACK */
+  qe = NULL;
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Looking for queue for PID %s\n",
+              GNUNET_i2s (&sma->receiver));
+  for (struct Queue *queue = tc->details.communicator.queue_head; NULL != queue;
+       queue = queue->next_client)
+  {
+    if (0 != GNUNET_memcmp (&queue->neighbour->pid, &sma->receiver))
+      continue;
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Found PID %s\n",
+                GNUNET_i2s (&queue->neighbour->pid));
+
+
+    for (struct QueueEntry *qep = queue->queue_head; NULL != qep;
+         qep = qep->next)
+    {
+      if (qep->mid != GNUNET_ntohll (sma->mid) || queue->qid != ntohl (
+            sma->qid))
+        continue;
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "QueueEntry MID: %" PRIu64 " on queue QID: %u, Ack MID: %"
+                  PRIu64 " Ack QID %u\n",
+                  qep->mid,
+                  queue->qid,
+                  GNUNET_ntohll (sma->mid),
+                  ntohl (sma->qid));
+      qe = qep;
+      if ((NULL != qe->pm) && (qe->pm->qe != qe))
+        GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                    "For pending message %" PRIu64 " we had retransmissions.\n",
+                    qe->pm->logging_uuid);
+      break;
+    }
+  }
+  if (NULL == qe)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "No QueueEntry found for Ack MID %" PRIu64 " QID: %u\n",
+                GNUNET_ntohll (sma->mid),
+                ntohl (sma->qid));
+    // TODO I guess this can happen, if the Ack from the peer comes before the Ack from the queue.
+    // Update: Maybe QueueEntry was accidentally freed during freeing PendingMessage.
+    /* this should never happen */
+    // GNUNET_break (0);
+    // GNUNET_SERVICE_client_drop (tc->client);
+    GNUNET_SERVICE_client_continue (tc->client);
+    return;
+  }
+  GNUNET_CONTAINER_DLL_remove (qe->queue->queue_head,
+                               qe->queue->queue_tail,
+                               qe);
+  qe->queue->queue_length--;
+  tc->details.communicator.total_queue_length--;
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Received ACK on queue %s to peer %s (new length: %u/%u)\n",
+              qe->queue->address,
+              GNUNET_i2s (&qe->queue->neighbour->pid),
+              qe->queue->queue_length,
+              tc->details.communicator.total_queue_length);
+  GNUNET_SERVICE_client_continue (tc->client);
+
+  /* if applicable, resume transmissions that waited on ACK */
+  if (COMMUNICATOR_TOTAL_QUEUE_LIMIT - 1 ==
+      tc->details.communicator.total_queue_length)
+  {
+    /* Communicator dropped below threshold, resume all queues
+       incident with this client! */
+    GNUNET_STATISTICS_update (
+      GST_stats,
+      "# Transmission throttled due to communicator queue limit",
+      -1,
+      GNUNET_NO);
+    for (struct Queue *queue = tc->details.communicator.queue_head;
+         NULL != queue;
+         queue = queue->next_client)
+    {
+      schedule_transmit_on_queue (GNUNET_TIME_UNIT_ZERO,
+                                  queue,
+                                  GNUNET_SCHEDULER_PRIORITY_DEFAULT);
+    }
+  }
+  else if (QUEUE_LENGTH_LIMIT - 1 == qe->queue->queue_length)
+  {
+    /* queue dropped below threshold; only resume this one queue */
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# Transmission throttled due to queue queue limit",
+                              -1,
+                              GNUNET_NO);
+    schedule_transmit_on_queue (GNUNET_TIME_UNIT_ZERO,
+                                qe->queue,
+                                GNUNET_SCHEDULER_PRIORITY_DEFAULT);
+  }
+  else if (1 == qe->queue->q_capacity)
+  {
+    // TODO I guess this will never happen, because the communicator triggers this by updating its queue length itself.
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Transmission rescheduled due to communicator message queue with qid %u has capacity %"
+                PRIu64 ".\n",
+                qe->queue->qid,
+                qe->queue->q_capacity);
+    /* message queue has capacity; only resume this one queue */
+    /* queue dropped below threshold; only resume this one queue */
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# Transmission throttled due to message queue capacity",
+                              -1,
+                              GNUNET_NO);
+    schedule_transmit_on_queue (GNUNET_TIME_UNIT_ZERO,
+                                qe->queue,
+                                GNUNET_SCHEDULER_PRIORITY_DEFAULT);
+  }
+
+  if (NULL != (pm = qe->pm))
+  {
+    struct VirtualLink *vl;
+
+    // GNUNET_assert (qe == pm->qe);
+    pm->qe = NULL;
+    /* If waiting for this communicator may have blocked transmission
+       of pm on other queues for this neighbour, force schedule
+       transmit on queue for queues of the neighbour */
+    if (NULL == pm->frag_parent)
+    {
+      vl = pm->vl;
+      if ((NULL != vl) &&
+          (NULL != vl->pending_msg_head) &&
+          (vl->pending_msg_head == pm))
+        check_vl_transmission (vl);
+    }
+  }
+  GNUNET_free (qe);
+}
+
+
+/**
+ * Iterator telling new MONITOR client about all existing
+ * queues to peers.
+ *
+ * @param cls the new `struct TransportClient`
+ * @param pid a connected peer
+ * @param value the `struct Neighbour` with more information
+ * @return #GNUNET_OK (continue to iterate)
+ */
+static int
+notify_client_queues (void *cls,
+                      const struct GNUNET_PeerIdentity *pid,
+                      void *value)
+{
+  struct TransportClient *tc = cls;
+  struct Neighbour *neighbour = value;
+
+  GNUNET_assert (CT_MONITOR == tc->type);
+  for (struct Queue *q = neighbour->queue_head; NULL != q;
+       q = q->next_neighbour)
+  {
+    struct MonitorEvent me = { .rtt = q->pd.aged_rtt,
+                               .cs = q->cs,
+                               .num_msg_pending = q->num_msg_pending,
+                               .num_bytes_pending = q->num_bytes_pending };
+
+    notify_monitor (tc, pid, q->address, q->nt, &me);
+  }
+  return GNUNET_OK;
+}
+
+
+/**
+ * Initialize a monitor client.
+ *
+ * @param cls the client
+ * @param start the start message that was sent
+ */
+static void
+handle_monitor_start (void *cls,
+                      const struct GNUNET_TRANSPORT_MonitorStart *start)
+{
+  struct TransportClient *tc = cls;
+
+  if (CT_NONE != tc->type)
+  {
+    GNUNET_break (0);
+    GNUNET_SERVICE_client_drop (tc->client);
+    return;
+  }
+  tc->type = CT_MONITOR;
+  tc->details.monitor.peer = start->peer;
+  tc->details.monitor.one_shot = ntohl (start->one_shot);
+  GNUNET_CONTAINER_multipeermap_iterate (neighbours, &notify_client_queues, tc);
+  GNUNET_SERVICE_client_mark_monitor (tc->client);
+  GNUNET_SERVICE_client_continue (tc->client);
+}
+
+
+/**
+ * Find transport client providing communication service
+ * for the protocol @a prefix.
+ *
+ * @param prefix communicator name
+ * @return NULL if no such transport client is available
+ */
+static struct TransportClient *
+lookup_communicator (const char *prefix)
+{
+  for (struct TransportClient *tc = clients_head; NULL != tc; tc = tc->next)
+  {
+    if (CT_COMMUNICATOR != tc->type)
+      continue;
+    if (0 == strcmp (prefix, tc->details.communicator.address_prefix))
+      return tc;
+  }
+  GNUNET_log (
+    GNUNET_ERROR_TYPE_WARNING,
+    "Somone suggested use of communicator for `%s', but we do not have such a communicator!\n",
+    prefix);
+  return NULL;
+}
+
+
+/**
+ * Signature of a function called with a communicator @a address of a peer
+ * @a pid that an application wants us to connect to.
+ *
+ * @param pid target peer
+ * @param address the address to try
+ */
+static void
+suggest_to_connect (const struct GNUNET_PeerIdentity *pid, const char *address)
+{
+  static uint32_t idgen;
+  struct TransportClient *tc;
+  char *prefix;
+  struct GNUNET_TRANSPORT_CreateQueue *cqm;
+  struct GNUNET_MQ_Envelope *env;
+  size_t alen;
+
+  prefix = GNUNET_HELLO_address_to_prefix (address);
+  if (NULL == prefix)
+  {
+    GNUNET_break (0);  /* We got an invalid address!? */
+    return;
+  }
+  tc = lookup_communicator (prefix);
+  if (NULL == tc)
+  {
+    GNUNET_STATISTICS_update (GST_stats,
+                              "# Suggestions ignored due to missing communicator",
+                              1,
+                              GNUNET_NO);
+    GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+                "Cannot connect to %s at `%s', no matching communicator present\n",
+                GNUNET_i2s (pid),
+                address);
+    GNUNET_free (prefix);
+    return;
+  }
+  /* forward suggestion for queue creation to communicator */
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Request #%u for `%s' communicator to create queue to `%s' at `%s'\n",
+              (unsigned int) idgen,
+              prefix,
+              GNUNET_i2s (pid),
+              address);
+  GNUNET_free (prefix);
+  alen = strlen (address) + 1;
+  env =
+    GNUNET_MQ_msg_extra (cqm, alen, GNUNET_MESSAGE_TYPE_TRANSPORT_QUEUE_CREATE);
+  cqm->request_id = htonl (idgen++);
+  cqm->receiver = *pid;
+  memcpy (&cqm[1], address, alen);
+  GNUNET_MQ_send (tc->mq, env);
+}
+
+
+/**
+ * The queue @a q (which matches the peer and address in @a vs) is
+ * ready for queueing. We should now queue the validation request.
+ *
+ * @param q queue to send on
+ * @param vs state to derive validation challenge from
+ */
+static void
+validation_transmit_on_queue (struct Queue *q, struct ValidationState *vs)
+{
+  struct TransportValidationChallengeMessage tvc;
+
+  vs->last_challenge_use = GNUNET_TIME_absolute_get_monotonic (GST_cfg);
+  tvc.header.type =
+    htons (GNUNET_MESSAGE_TYPE_TRANSPORT_ADDRESS_VALIDATION_CHALLENGE);
+  tvc.header.size = htons (sizeof(tvc));
+  tvc.reserved = htonl (0);
+  tvc.challenge = vs->challenge;
+  tvc.sender_time = GNUNET_TIME_absolute_hton (vs->last_challenge_use);
+  GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+              "Sending address validation challenge %s to %s\n",
+              GNUNET_sh2s (&tvc.challenge.value),
+              GNUNET_i2s (&q->neighbour->pid));
+  queue_send_msg (q, NULL, &tvc, sizeof(tvc));
+}
+
+
+/**
+ * Task run periodically to validate some address based on #validation_heap.
+ *
+ * @param cls NULL
+ */
+static void
+validation_start_cb (void *cls)
+{
+  struct ValidationState *vs;
+  struct Queue *q;
+  const struct GNUNET_TIME_Absolute now = GNUNET_TIME_absolute_get_monotonic (
+    GST_cfg);
+
+  (void) cls;
+  validation_task = NULL;
+  vs = GNUNET_CONTAINER_heap_peek (validation_heap);
+  /* drop validations past their expiration */
+  while (
+    (NULL != vs) &&
+    (0 == GNUNET_TIME_absolute_get_remaining (vs->valid_until).rel_value_us))
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Validation response %s cleaned up\n",
+                GNUNET_sh2s (&vs->challenge.value));
+    free_validation_state (vs);
+    vs = GNUNET_CONTAINER_heap_peek (validation_heap);
+  }
+  if (NULL == vs)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+                "Address validation task not scheduled anymore, nothing to do\n");
+    return;   /* woopsie, no more addresses known, should only
+                 happen if we're really a lonely peer */
+  }
+  q = find_queue (&vs->pid, vs->address);
+  if (GNUNET_TIME_absolute_cmp (vs->first_challenge_use, >, now))
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "To early to start next address validation for challenge %s\n",
+                GNUNET_sh2s (&vs->challenge.value));
+    return;
+  }
+  if (NULL == q)
+  {
+    vs->awaiting_queue = GNUNET_YES;
+    suggest_to_connect (&vs->pid, vs->address);
+  }
+  else
+    validation_transmit_on_queue (q, vs);
+  /* Finally, reschedule next attempt */
+  vs->challenge_backoff =
+    GNUNET_TIME_randomized_backoff (vs->challenge_backoff,
+                                    MAX_VALIDATION_CHALLENGE_FREQ);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Address validation task will run again in %s\n",
+              GNUNET_STRINGS_relative_time_to_string (vs->challenge_backoff,
+                                                      GNUNET_YES));
+  update_next_challenge_time (vs,
+                              GNUNET_TIME_relative_to_absolute (
+                                vs->challenge_backoff));
+}
+
+
+/**
+ * Closure for #check_connection_quality.
+ */
+struct QueueQualityContext
+{
+  /**
+   * Set to the @e k'th queue encountered.
+   */
+  struct Queue *q;
+
+  /**
+   * Set to the number of quality queues encountered.
+   */
+  unsigned int quality_count;
+
+  /**
+   * Set to the total number of queues encountered.
+   */
+  unsigned int num_queues;
+
+  /**
+   * Decremented for each queue, for selection of the
+   * k-th queue in @e q.
+   */
+  unsigned int k;
+};
+
+
+/**
+ * Check whether any queue to the given neighbour is
+ * of a good "quality" and if so, increment the counter.
+ * Also counts the total number of queues, and returns
+ * the k-th queue found.
+ *
+ * @param cls a `struct QueueQualityContext *` with counters
+ * @param pid peer this is about
+ * @param value a `struct Neighbour`
+ * @return #GNUNET_OK (continue to iterate)
+ */
+static int
+check_connection_quality (void *cls,
+                          const struct GNUNET_PeerIdentity *pid,
+                          void *value)
+{
+  struct QueueQualityContext *ctx = cls;
+  struct Neighbour *n = value;
+  int do_inc;
+
+  (void) pid;
+  do_inc = GNUNET_NO;
+  for (struct Queue *q = n->queue_head; NULL != q; q = q->next_neighbour)
+  {
+    ctx->num_queues++;
+    if (0 == ctx->k--)
+      ctx->q = q;
+    /* FIXME-CONQ-STATISTICS: in the future, add reliability / goodput
+       statistics and consider those as well here? */
+    if (q->pd.aged_rtt.rel_value_us < DV_QUALITY_RTT_THRESHOLD.rel_value_us)
+      do_inc = GNUNET_YES;
+  }
+  if (GNUNET_YES == do_inc)
+    ctx->quality_count++;
+  return GNUNET_OK;
+}
+
+
+/**
+ * Task run when we CONSIDER initiating a DV learn
+ * process. We first check that sending out a message is
+ * even possible (queues exist), then that it is desirable
+ * (if not, reschedule the task for later), and finally
+ * we may then begin the job.  If there are too many
+ * entries in the #dvlearn_map, we purge the oldest entry
+ * using #lle_tail.
+ *
+ * @param cls NULL
+ */
+static void
+start_dv_learn (void *cls)
+{
+  struct LearnLaunchEntry *lle;
+  struct QueueQualityContext qqc;
+  struct TransportDVLearnMessage dvl;
+
+  (void) cls;
+  dvlearn_task = NULL;
+  if (0 == GNUNET_CONTAINER_multipeermap_size (neighbours))
+    return; /* lost all connectivity, cannot do learning */
+  qqc.quality_count = 0;
+  qqc.num_queues = 0;
+  qqc.k = GNUNET_CONTAINER_multipeermap_size (neighbours);
+  GNUNET_CONTAINER_multipeermap_iterate (neighbours,
+                                         &check_connection_quality,
+                                         &qqc);
+  if (qqc.quality_count > DV_LEARN_QUALITY_THRESHOLD)
+  {
+    struct GNUNET_TIME_Relative delay;
+    unsigned int factor;
+
+    /* scale our retries by how far we are above the threshold */
+    factor = qqc.quality_count / DV_LEARN_QUALITY_THRESHOLD;
+    delay = GNUNET_TIME_relative_multiply (DV_LEARN_BASE_FREQUENCY, factor);
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "At connection quality %u, will launch DV learn in %s\n",
+                qqc.quality_count,
+                GNUNET_STRINGS_relative_time_to_string (delay, GNUNET_YES));
+    dvlearn_task = GNUNET_SCHEDULER_add_delayed (delay, &start_dv_learn, NULL);
+    return;
+  }
+  /* remove old entries in #dvlearn_map if it has grown too big */
+  while (MAX_DV_LEARN_PENDING <=
+         GNUNET_CONTAINER_multishortmap_size (dvlearn_map))
+  {
+    lle = lle_tail;
+    GNUNET_assert (GNUNET_YES ==
+                   GNUNET_CONTAINER_multishortmap_remove (dvlearn_map,
+                                                          &lle->challenge.value,
+                                                          lle));
+    GNUNET_CONTAINER_DLL_remove (lle_head, lle_tail, lle);
+    GNUNET_free (lle);
+  }
+  /* setup data structure for learning */
+  lle = GNUNET_new (struct LearnLaunchEntry);
+  GNUNET_CRYPTO_random_block (GNUNET_CRYPTO_QUALITY_NONCE,
+                              &lle->challenge,
+                              sizeof(lle->challenge));
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Starting launch DV learn with challenge %s\n",
+              GNUNET_sh2s (&lle->challenge.value));
+  GNUNET_CONTAINER_DLL_insert (lle_head, lle_tail, lle);
+  GNUNET_break (GNUNET_YES ==
+                GNUNET_CONTAINER_multishortmap_put (
+                  dvlearn_map,
+                  &lle->challenge.value,
+                  lle,
+                  GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
+  dvl.header.type = htons (GNUNET_MESSAGE_TYPE_TRANSPORT_DV_LEARN);
+  dvl.header.size = htons (sizeof(dvl));
+  dvl.num_hops = htons (0);
+  dvl.bidirectional = htons (0);
+  dvl.non_network_delay = GNUNET_TIME_relative_hton (GNUNET_TIME_UNIT_ZERO);
+  dvl.monotonic_time =
+    GNUNET_TIME_absolute_hton (GNUNET_TIME_absolute_get_monotonic (GST_cfg));
+  {
+    struct DvInitPS dvip = {
+      .purpose.purpose = htonl (
+        GNUNET_SIGNATURE_PURPOSE_TRANSPORT_DV_INITIATOR),
+      .purpose.size = htonl (sizeof(dvip)),
+      .monotonic_time = dvl.monotonic_time,
+      .challenge = lle->challenge
+    };
+
+    GNUNET_CRYPTO_eddsa_sign (GST_my_private_key,
+                              &dvip,
+                              &dvl.init_sig);
+  }
+  dvl.initiator = GST_my_identity;
+  dvl.challenge = lle->challenge;
+
+  qqc.quality_count = 0;
+  qqc.k = GNUNET_CRYPTO_random_u32 (GNUNET_CRYPTO_QUALITY_WEAK, qqc.num_queues);
+  qqc.num_queues = 0;
+  qqc.q = NULL;
+  GNUNET_CONTAINER_multipeermap_iterate (neighbours,
+                                         &check_connection_quality,
+                                         &qqc);
+  GNUNET_assert (NULL != qqc.q);
+
+  /* Do this as close to transmission time as possible! */
+  lle->launch_time = GNUNET_TIME_absolute_get ();
+
+  queue_send_msg (qqc.q, NULL, &dvl, sizeof(dvl));
+  /* reschedule this job, randomizing the time it runs (but no
+     actual backoff!) */
+  dvlearn_task = GNUNET_SCHEDULER_add_delayed (GNUNET_TIME_randomize (
+                                                 DV_LEARN_BASE_FREQUENCY),
+                                               &start_dv_learn,
+                                               NULL);
+}
+
+
+/**
+ * Get the IP address without the port number.
+ *
+ * @param address The string contains a communicator prefix, IP address and port
+ *        like this 'tcp-92.68.150.1:55452'.
+ * @return String with IP address only.
+ */
+static char *
+get_address_without_port (const char *address)
+{
+  const char *colon;
+  char *colon_rest;
+  size_t colon_rest_length;
+  char *address_without_port;
+
+  colon = strchr (address,':');
+  colon_rest = GNUNET_strndup (address, colon - address);
+  colon_rest_length = strlen (colon_rest);
+  address_without_port = GNUNET_strndup (&colon_rest[4], colon_rest_length - 4);
+  GNUNET_free (colon_rest);
+
+  return address_without_port;
+}
+
+
+/**
+ * A new queue has been created, check if any address validation
+ * requests have been waiting for it.
+ *
+ * @param cls a `struct Queue`
+ * @param pid peer concerned (unused)
+ * @param value a `struct ValidationState`
+ * @return #GNUNET_NO if a match was found and we can stop looking
+ */
+static int
+check_validation_request_pending (void *cls,
+                                  const struct GNUNET_PeerIdentity *pid,
+                                  void *value)
+{
+  struct Queue *q = cls;
+  struct ValidationState *vs = value;
+  char *address_without_port_vs;
+  char *address_without_port_q;
+  int success = GNUNET_YES;
+
+  address_without_port_vs = get_address_without_port (vs->address);
+  address_without_port_q = get_address_without_port (q->address);
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Check validation request pending for `%s' at `%s'/`%s' (vs)/(q)\n",
+              GNUNET_i2s (pid),
+              address_without_port_vs,
+              address_without_port_q);
+  (void) pid;
+  if ((GNUNET_YES == vs->awaiting_queue) &&
+      (0 == strcmp (address_without_port_vs, address_without_port_q)))
+  {
+
+    vs->awaiting_queue = GNUNET_NO;
+    validation_transmit_on_queue (q, vs);
+    success = GNUNET_NO;
+  }
+
+  GNUNET_free (address_without_port_vs);
+  GNUNET_free (address_without_port_q);
+  return success;
+}
+
+
+/**
+ * Function called with the monotonic time of a DV initiator
+ * by PEERSTORE. Updates the time.
+ *
+ * @param cls a `struct Neighbour`
+ * @param record the information found, NULL for the last call
+ * @param emsg error message
+ */
+static void
+neighbour_dv_monotime_cb (void *cls,
+                          const struct GNUNET_PEERSTORE_Record *record,
+                          const char *emsg)
+{
+  struct Neighbour *n = cls;
+  struct GNUNET_TIME_AbsoluteNBO *mtbe;
+
+  (void) emsg;
+  if (NULL == record)
+  {
+    /* we're done with #neighbour_dv_monotime_cb() invocations,
+       continue normal processing */
+    n->get = NULL;
+    n->dv_monotime_available = GNUNET_YES;
+    return;
+  }
+  if (sizeof(*mtbe) != record->value_size)
+  {
+    GNUNET_break (0);
+    return;
+  }
+  mtbe = record->value;
+  n->last_dv_learn_monotime =
+    GNUNET_TIME_absolute_max (n->last_dv_learn_monotime,
+                              GNUNET_TIME_absolute_ntoh (*mtbe));
+}
+
+
+/**
+ * New queue became available.  Process the request.
+ *
+ * @param cls the client
+ * @param aqm the send message that was sent
+ */
+static void
+handle_add_queue_message (void *cls,
+                          const struct GNUNET_TRANSPORT_AddQueueMessage *aqm)
+{
+  struct TransportClient *tc = cls;
+  struct Queue *queue;
+  struct Neighbour *neighbour;
+  struct GNUNET_TIME_Absolute validated_until = GNUNET_TIME_UNIT_ZERO_ABS;
+  const char *addr;
+  uint16_t addr_len;
+
+  if (ntohl (aqm->mtu) <= sizeof(struct TransportFragmentBoxMessage))
+  {
+    /* MTU so small as to be useless for transmissions,
+       required for #fragment_message()! */
+    GNUNET_break_op (0);
+    GNUNET_SERVICE_client_drop (tc->client);
+    return;
+  }
+  /* This may simply be a queue update */
+  for (queue = tc->details.communicator.queue_head;
+       NULL != queue;
+       queue = queue->next_client)
+  {
+    validated_until = queue->validated_until;
+    if (queue->qid != ntohl (aqm->qid))
+      continue;
+    break;
+  }
+
+  if (NULL != queue)
+  {
+    neighbour = queue->neighbour;
+  }
+  else
+  {
+    neighbour = lookup_neighbour (&aqm->receiver);
+    if (NULL == neighbour)
+    {
+      neighbour = GNUNET_new (struct Neighbour);
+      neighbour->pid = aqm->receiver;
+      GNUNET_assert (GNUNET_OK ==
+                     GNUNET_CONTAINER_multipeermap_put (
+                       neighbours,
+                       &neighbour->pid,
+                       neighbour,
+                       GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY));
+      neighbour->get =
+        GNUNET_PEERSTORE_iterate (peerstore,
+                                  "transport",
+                                  &neighbour->pid,
+                                  GNUNET_PEERSTORE_TRANSPORT_DVLEARN_MONOTIME,
+                                  &neighbour_dv_monotime_cb,
+                                  neighbour);
+    }
+    addr_len = ntohs (aqm->header.size) - sizeof(*aqm);
+    addr = (const char *) &aqm[1];
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "New queue %s to %s available with QID %u and q_len %" PRIu64
+                " and mtu %u\n",
+                addr,
+                GNUNET_i2s (&aqm->receiver),
+                ntohl (aqm->qid),
+                GNUNET_ntohll (aqm->q_len),
+                ntohl (aqm->mtu));
+    queue = GNUNET_malloc (sizeof(struct Queue) + addr_len);
+    queue->tc = tc;
+    if (GNUNET_TIME_UNIT_ZERO_ABS.abs_value_us != validated_until.abs_value_us)
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                  "New queue with QID %u inherit validated until\n",
+                  ntohl (aqm->qid));
+      queue->validated_until = validated_until;
+    }
+    queue->address = (const char *) &queue[1];
+    queue->pd.aged_rtt = GNUNET_TIME_UNIT_FOREVER_REL;
+    queue->qid = ntohl (aqm->qid);
+    queue->neighbour = neighbour;
+    if (GNUNET_TRANSPORT_QUEUE_LENGTH_UNLIMITED == GNUNET_ntohll (aqm->q_len))
+      queue->unlimited_length = GNUNET_YES;
+    queue->q_capacity = GNUNET_ntohll (aqm->q_len);
+    memcpy (&queue[1], addr, addr_len);
+    /* notify monitors about new queue */
+    {
+      struct MonitorEvent me = { .rtt = queue->pd.aged_rtt, .cs = queue->cs };
+
+      notify_monitors (&neighbour->pid, queue->address, queue->nt, &me);
+    }
+    GNUNET_CONTAINER_MDLL_insert (neighbour,
+                                  neighbour->queue_head,
+                                  neighbour->queue_tail,
+                                  queue);
+    GNUNET_CONTAINER_MDLL_insert (client,
+                                  tc->details.communicator.queue_head,
+                                  tc->details.communicator.queue_tail,
+                                  queue);
+
+  }
+  queue->mtu = ntohl (aqm->mtu);
+  queue->nt = (enum GNUNET_NetworkType) ntohl (aqm->nt);
+  queue->cs = (enum GNUNET_TRANSPORT_ConnectionStatus) ntohl (aqm->cs);
+  queue->idle = GNUNET_YES;
+  /* check if valdiations are waiting for the queue */
+  (void)
+  GNUNET_CONTAINER_multipeermap_get_multiple (validation_map,
+                                              &aqm->receiver,
+                                              &check_validation_request_pending,
+                                              queue);
+  /* look for traffic for this queue */
+  schedule_transmit_on_queue (GNUNET_TIME_UNIT_ZERO,
+                              queue, GNUNET_SCHEDULER_PRIORITY_DEFAULT);
+  /* might be our first queue, try launching DV learning */
+  if (NULL == dvlearn_task)
+    dvlearn_task = GNUNET_SCHEDULER_add_now (&start_dv_learn, NULL);
+  GNUNET_SERVICE_client_continue (tc->client);
+}
+
+
+/**
+ * @brief Handle updates to queues.
+ *
+ * @param cls the transport client.
+ * @param msg Message struct.
+ */
+static void
+handle_update_queue_message (void *cls,
+                             const struct
+                             GNUNET_TRANSPORT_UpdateQueueMessage *msg)
+{
+  struct TransportClient *tc = cls;
+  struct Queue *target_queue = NULL;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Received queue update message for %u with q_len %llu and mtu %u\n",
+              ntohl (msg->qid),
+              (unsigned long long) GNUNET_ntohll (msg->q_len),
+              ntohl (msg->mtu));
+  for (target_queue = tc->details.communicator.queue_head;
+       NULL != target_queue;
+       target_queue = target_queue->next_client)
+  {
+    if (ntohl (msg->qid) == target_queue->qid)
+      break;
+  }
+  if (NULL == target_queue)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+                "Queue to update no longer exists! Discarding update.\n");
+    return;
+  }
+
+  target_queue->nt = msg->nt;
+  target_queue->mtu = ntohl (msg->mtu);
+  target_queue->cs = msg->cs;
+  target_queue->priority = ntohl (msg->priority);
+  /* The update message indicates how many messages
+   * the queue should be able to handle.
+   */
+  if (GNUNET_TRANSPORT_QUEUE_LENGTH_UNLIMITED == GNUNET_ntohll (msg->q_len))
+    target_queue->unlimited_length = GNUNET_YES;
+  else
+    target_queue->unlimited_length = GNUNET_NO;
+  target_queue->q_capacity += GNUNET_ntohll (msg->q_len);
+  if (0 < target_queue->q_capacity)
+    schedule_transmit_on_queue (GNUNET_TIME_UNIT_ZERO,
+                                target_queue,
+                                GNUNET_SCHEDULER_PRIORITY_DEFAULT);
+  GNUNET_SERVICE_client_continue (tc->client);
+}
+
+
+/**
+ * Communicator tells us that our request to create a queue "worked", that
+ * is setting up the queue is now in process.
+ *
+ * @param cls the `struct TransportClient`
+ * @param cqr confirmation message
+ */
+static void
+handle_queue_create_ok (void *cls,
+                        const struct GNUNET_TRANSPORT_CreateQueueResponse *cqr)
+{
+  struct TransportClient *tc = cls;
+
+  if (CT_COMMUNICATOR != tc->type)
+  {
+    GNUNET_break (0);
+    GNUNET_SERVICE_client_drop (tc->client);
+    return;
+  }
+  GNUNET_STATISTICS_update (GST_stats,
+                            "# Suggestions succeeded at communicator",
+                            1,
+                            GNUNET_NO);
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Request #%u for communicator to create queue succeeded\n",
+              (unsigned int) ntohs (cqr->request_id));
+  GNUNET_SERVICE_client_continue (tc->client);
+}
+
+
+/**
+ * Communicator tells us that our request to create a queue failed. This
+ * usually indicates that the provided address is simply invalid or that the
+ * communicator's resources are exhausted.
+ *
+ * @param cls the `struct TransportClient`
+ * @param cqr failure message
+ */
+static void
+handle_queue_create_fail (
+  void *cls,
+  const struct GNUNET_TRANSPORT_CreateQueueResponse *cqr)
+{
+  struct TransportClient *tc = cls;
+
+  if (CT_COMMUNICATOR != tc->type)
+  {
+    GNUNET_break (0);
+    GNUNET_SERVICE_client_drop (tc->client);
+    return;
+  }
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Request #%u for communicator to create queue failed\n",
+              (unsigned int) ntohs (cqr->request_id));
+  GNUNET_STATISTICS_update (GST_stats,
+                            "# Suggestions failed in queue creation at communicator",
+                            1,
+                            GNUNET_NO);
+  GNUNET_SERVICE_client_continue (tc->client);
+}
+
+
+/**
+ * We have received a `struct ExpressPreferenceMessage` from an application
+ * client.
+ *
+ * @param cls handle to the client
+ * @param msg the start message
+ */
+static void
+handle_suggest_cancel (void *cls, const struct ExpressPreferenceMessage *msg)
+{
+  struct TransportClient *tc = cls;
+  struct PeerRequest *pr;
+
+  if (CT_APPLICATION != tc->type)
+  {
+    GNUNET_break (0);
+    GNUNET_SERVICE_client_drop (tc->client);
+    return;
+  }
+  pr = GNUNET_CONTAINER_multipeermap_get (tc->details.application.requests,
+                                          &msg->peer);
+  if (NULL == pr)
+  {
+    GNUNET_break (0);
+    GNUNET_SERVICE_client_drop (tc->client);
+    return;
+  }
+  (void) stop_peer_request (tc, &pr->pid, pr);
+  GNUNET_SERVICE_client_continue (tc->client);
+}
+
+
+static void
+hello_for_client_cb (void *cls,
+                     const struct GNUNET_PeerIdentity *pid,
+                     const char *uri)
+{
+  (void) cls;
+  int pfx_len;
+  const char *eou;
+  char *address;
+
+  eou = strstr (uri,
+                "://");
+  pfx_len = eou - uri;
+  eou += 3;
+  GNUNET_asprintf (&address,
+                   "%.*s-%s",
+                   pfx_len,
+                   uri,
+                   eou);
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "hello for client %s\n",
+              address);
+
+  start_address_validation (pid, address);
+  GNUNET_free (address);
+}
+
+
+/**
+ * Function called by PEERSTORE for each matching record.
+ *
+ * @param cls closure, a `struct PeerRequest`
+ * @param record peerstore record information
+ * @param emsg error message, or NULL if no errors
+ */
+static void
+handle_hello_for_client (void *cls,
+                         const struct GNUNET_PeerIdentity *peer,
+                         const struct GNUNET_MessageHeader *hello,
+                         const char *emsg)
+{
+  (void) cls;
+  struct GNUNET_HELLO_Builder *builder;
+
+  if (NULL != emsg)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
+                "Got failure from PEERSTORE: %s\n",
+                emsg);
+    return;
+  }
+  if (0 == GNUNET_memcmp (peer, &GST_my_identity))
+    return;
+  builder = GNUNET_HELLO_builder_from_msg (hello);
+  GNUNET_HELLO_builder_iterate (builder,
+                                hello_for_client_cb,
+                                NULL);
+  GNUNET_HELLO_builder_free (builder);
+}
+
+
+/**
+ * We have received a `struct ExpressPreferenceMessage` from an application
+ * client.
+ *
+ * @param cls handle to the client
+ * @param msg the start message
+ */
+static void
+handle_suggest (void *cls, const struct ExpressPreferenceMessage *msg)
+{
+  struct TransportClient *tc = cls;
+  struct PeerRequest *pr;
+
+  if (CT_NONE == tc->type)
+  {
+    tc->type = CT_APPLICATION;
+    tc->details.application.requests =
+      GNUNET_CONTAINER_multipeermap_create (16, GNUNET_YES);
+  }
+  if (CT_APPLICATION != tc->type)
+  {
+    GNUNET_break (0);
+    GNUNET_SERVICE_client_drop (tc->client);
+    return;
+  }
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Client suggested we talk to %s with preference %d at rate %u\n",
+              GNUNET_i2s (&msg->peer),
+              (int) ntohl (msg->pk),
+              (int) ntohl (msg->bw.value__));
+  pr = GNUNET_new (struct PeerRequest);
+  pr->tc = tc;
+  pr->pid = msg->peer;
+  pr->bw = msg->bw;
+  pr->pk = (enum GNUNET_MQ_PriorityPreferences) ntohl (msg->pk);
+  if (GNUNET_YES != GNUNET_CONTAINER_multipeermap_put (
+        tc->details.application.requests,
+        &pr->pid,
+        pr,
+        GNUNET_CONTAINER_MULTIHASHMAPOPTION_UNIQUE_ONLY))
+  {
+    GNUNET_break (0);
+    GNUNET_free (pr);
+    GNUNET_SERVICE_client_drop (tc->client);
+    return;
+  }
+  pr->nc = GNUNET_PEERSTORE_hello_changed_notify (peerstore,
+                                                  GNUNET_NO,
+                                                  &handle_hello_for_client,
+                                                  NULL);
+  GNUNET_SERVICE_client_continue (tc->client);
+}
+
+
+/**
+ * Check #GNUNET_MESSAGE_TYPE_TRANSPORT_REQUEST_HELLO_VALIDATION
+ * messages.
+ *
+ * @param cls a `struct TransportClient *`
+ * @param m message to verify
+ * @return #GNUNET_OK on success
+ */
+static int
+check_request_hello_validation (void *cls,
+                                const struct RequestHelloValidationMessage *m)
+{
+  (void) cls;
+  GNUNET_MQ_check_zero_termination (m);
+  return GNUNET_OK;
+}
+
+
+/**
+ * A client encountered an address of another peer. Consider validating it,
+ * and if validation succeeds, persist it to PEERSTORE.
+ *
+ * @param cls a `struct TransportClient *`
+ * @param m message to verify
+ */
+static void
+handle_request_hello_validation (void *cls,
+                                 const struct RequestHelloValidationMessage *m)
+{
+  struct TransportClient *tc = cls;
+
+  start_address_validation (&m->peer, (const char *) &m[1]);
+  GNUNET_SERVICE_client_continue (tc->client);
+}
+
+
+/**
+ * Free neighbour entry.
+ *
+ * @param cls NULL
+ * @param pid unused
+ * @param value a `struct Neighbour`
+ * @return #GNUNET_OK (always)
+ */
+static int
+free_neighbour_cb (void *cls,
+                   const struct GNUNET_PeerIdentity *pid,
+                   void *value)
+{
+  struct Neighbour *neighbour = value;
+
+  (void) cls;
+  (void) pid;
+  GNUNET_break (0);  // should this ever happen?
+  free_neighbour (neighbour);
+
+  return GNUNET_OK;
+}
+
+
+/**
+ * Free DV route entry.
+ *
+ * @param cls NULL
+ * @param pid unused
+ * @param value a `struct DistanceVector`
+ * @return #GNUNET_OK (always)
+ */
+static int
+free_dv_routes_cb (void *cls,
+                   const struct GNUNET_PeerIdentity *pid,
+                   void *value)
+{
+  struct DistanceVector *dv = value;
+
+  (void) cls;
+  (void) pid;
+  free_dv_route (dv);
+
+  return GNUNET_OK;
+}
+
+
+/**
+ * Free validation state.
+ *
+ * @param cls NULL
+ * @param pid unused
+ * @param value a `struct ValidationState`
+ * @return #GNUNET_OK (always)
+ */
+static int
+free_validation_state_cb (void *cls,
+                          const struct GNUNET_PeerIdentity *pid,
+                          void *value)
+{
+  struct ValidationState *vs = value;
+
+  (void) cls;
+  (void) pid;
+  free_validation_state (vs);
+  return GNUNET_OK;
+}
+
+
+/**
+ * Free pending acknowledgement.
+ *
+ * @param cls NULL
+ * @param key unused
+ * @param value a `struct PendingAcknowledgement`
+ * @return #GNUNET_OK (always)
+ */
+static int
+free_pending_ack_cb (void *cls, const struct GNUNET_Uuid *key, void *value)
+{
+  struct PendingAcknowledgement *pa = value;
+
+  (void) cls;
+  (void) key;
+  free_pending_acknowledgement (pa);
+  return GNUNET_OK;
+}
+
+
+/**
+ * Free acknowledgement cummulator.
+ *
+ * @param cls NULL
+ * @param pid unused
+ * @param value a `struct AcknowledgementCummulator`
+ * @return #GNUNET_OK (always)
+ */
+static int
+free_ack_cummulator_cb (void *cls,
+                        const struct GNUNET_PeerIdentity *pid,
+                        void *value)
+{
+  struct AcknowledgementCummulator *ac = value;
+
+  (void) cls;
+  (void) pid;
+  GNUNET_SCHEDULER_cancel (ac->task);
+  GNUNET_free (ac);
+  return GNUNET_OK;
+}
+
+
+/**
+ * Function called when the service shuts down.  Unloads our plugins
+ * and cancels pending validations.
+ *
+ * @param cls closure, unused
+ */
+static void
+do_shutdown (void *cls)
+{
+  struct LearnLaunchEntry *lle;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "shutdown logic\n");
+  (void) cls;
+  GNUNET_CONTAINER_multipeermap_iterate (neighbours,
+                                         &free_neighbour_cb, NULL);
+  if (NULL != validation_task)
+  {
+    GNUNET_SCHEDULER_cancel (validation_task);
+    validation_task = NULL;
+  }
+  if (NULL != dvlearn_task)
+  {
+    GNUNET_SCHEDULER_cancel (dvlearn_task);
+    dvlearn_task = NULL;
+  }
+  if (NULL != GST_stats)
+  {
+    GNUNET_STATISTICS_destroy (GST_stats, GNUNET_NO);
+    GST_stats = NULL;
+  }
+  if (NULL != GST_my_hello)
+  {
+    GNUNET_HELLO_builder_free (GST_my_hello);
+    GST_my_hello = NULL;
+  }
+  if (NULL != GST_my_private_key)
+  {
+    GNUNET_free (GST_my_private_key);
+    GST_my_private_key = NULL;
+  }
+  GNUNET_CONTAINER_multipeermap_iterate (ack_cummulators,
+                                         &free_ack_cummulator_cb,
+                                         NULL);
+  GNUNET_CONTAINER_multipeermap_destroy (ack_cummulators);
+  ack_cummulators = NULL;
+  GNUNET_CONTAINER_multiuuidmap_iterate (pending_acks,
+                                         &free_pending_ack_cb,
+                                         NULL);
+  GNUNET_CONTAINER_multiuuidmap_destroy (pending_acks);
+  pending_acks = NULL;
+  GNUNET_break (0 == GNUNET_CONTAINER_multipeermap_size (neighbours));
+  GNUNET_CONTAINER_multipeermap_destroy (neighbours);
+  neighbours = NULL;
+  GNUNET_break (0 == GNUNET_CONTAINER_multipeermap_size (links));
+  GNUNET_CONTAINER_multipeermap_destroy (links);
+  links = NULL;
+  GNUNET_CONTAINER_multipeermap_iterate (backtalkers,
+                                         &free_backtalker_cb,
+                                         NULL);
+  GNUNET_CONTAINER_multipeermap_destroy (backtalkers);
+  backtalkers = NULL;
+  GNUNET_CONTAINER_multipeermap_iterate (validation_map,
+                                         &free_validation_state_cb,
+                                         NULL);
+  GNUNET_CONTAINER_multipeermap_destroy (validation_map);
+  validation_map = NULL;
+  while (NULL != ir_head)
+    free_incoming_request (ir_head);
+  GNUNET_assert (0 == ir_total);
+  while (NULL != (lle = lle_head))
+  {
+    GNUNET_CONTAINER_DLL_remove (lle_head, lle_tail, lle);
+    GNUNET_free (lle);
+  }
+  if (NULL != peerstore)
+  {
+    GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+                "Disconnecting from PEERSTORE service\n");
+    GNUNET_PEERSTORE_disconnect (peerstore, GNUNET_NO);
+    peerstore = NULL;
+  }
+  GNUNET_CONTAINER_multishortmap_destroy (dvlearn_map);
+  dvlearn_map = NULL;
+  GNUNET_CONTAINER_heap_destroy (validation_heap);
+  validation_heap = NULL;
+  GNUNET_CONTAINER_multipeermap_iterate (dv_routes, &free_dv_routes_cb, NULL);
+  GNUNET_CONTAINER_multipeermap_destroy (dv_routes);
+  dv_routes = NULL;
+  GNUNET_SCHEDULER_shutdown ();
+}
+
+
+static void
+shutdown_task (void *cls)
+{
+  in_shutdown = GNUNET_YES;
+
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Shutdown task executed\n");
+  if (NULL != clients_head)
+  {
+    for (struct TransportClient *tc = clients_head; NULL != tc; tc = tc->next)
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                  "client still connected: %u\n",
+                  tc->type);
+    }
+  }
+  else
+    do_shutdown (cls);
+
+}
+
+
+/**
+ * Initiate transport service.
+ *
+ * @param cls closure
+ * @param c configuration to use
+ * @param service the initialized service
+ */
+static void
+run (void *cls,
+     const struct GNUNET_CONFIGURATION_Handle *c,
+     struct GNUNET_SERVICE_Handle *service)
+{
+  (void) cls;
+  (void) service;
+  /* setup globals */
+  hello_mono_time = GNUNET_TIME_absolute_get_monotonic (c);
+  in_shutdown = GNUNET_NO;
+  GST_cfg = c;
+  backtalkers = GNUNET_CONTAINER_multipeermap_create (16, GNUNET_YES);
+  pending_acks = GNUNET_CONTAINER_multiuuidmap_create (32768, GNUNET_YES);
+  ack_cummulators = GNUNET_CONTAINER_multipeermap_create (256, GNUNET_YES);
+  neighbours = GNUNET_CONTAINER_multipeermap_create (1024, GNUNET_YES);
+  links = GNUNET_CONTAINER_multipeermap_create (512, GNUNET_YES);
+  dv_routes = GNUNET_CONTAINER_multipeermap_create (1024, GNUNET_YES);
+  dvlearn_map = GNUNET_CONTAINER_multishortmap_create (2 * MAX_DV_LEARN_PENDING,
+                                                       GNUNET_YES);
+  validation_map = GNUNET_CONTAINER_multipeermap_create (1024, GNUNET_YES);
+  validation_heap =
+    GNUNET_CONTAINER_heap_create (GNUNET_CONTAINER_HEAP_ORDER_MIN);
+  GST_my_private_key =
+    GNUNET_CRYPTO_eddsa_key_create_from_configuration (GST_cfg);
+  if (NULL == GST_my_private_key)
+  {
+    GNUNET_log (
+      GNUNET_ERROR_TYPE_ERROR,
+      _ (
+        "Transport service is lacking key configuration settings. Exiting.\n"));
+    GNUNET_SCHEDULER_shutdown ();
+    return;
+  }
+  GNUNET_CRYPTO_eddsa_key_get_public (GST_my_private_key,
+                                      &GST_my_identity.public_key);
+  GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+              "My identity is `%s'\n",
+              GNUNET_i2s_full (&GST_my_identity));
+  GST_my_hello = GNUNET_HELLO_builder_new (&GST_my_identity);
+  GST_stats = GNUNET_STATISTICS_create ("transport", GST_cfg);
+  GNUNET_SCHEDULER_add_shutdown (&shutdown_task, NULL);
+  peerstore = GNUNET_PEERSTORE_connect (GST_cfg);
+  if (NULL == peerstore)
+  {
+    GNUNET_break (0);
+    GNUNET_SCHEDULER_shutdown ();
+    return;
+  }
+}
+
+
+/**
+ * Define "main" method using service macro.
+ */
+GNUNET_SERVICE_MAIN (
+  "transport",
+  GNUNET_SERVICE_OPTION_SOFT_SHUTDOWN,
+  &run,
+  &client_connect_cb,
+  &client_disconnect_cb,
+  NULL,
+  /* communication with applications */
+  GNUNET_MQ_hd_fixed_size (suggest,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_SUGGEST,
+                           struct ExpressPreferenceMessage,
+                           NULL),
+  GNUNET_MQ_hd_fixed_size (suggest_cancel,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_SUGGEST_CANCEL,
+                           struct ExpressPreferenceMessage,
+                           NULL),
+  GNUNET_MQ_hd_var_size (request_hello_validation,
+                         GNUNET_MESSAGE_TYPE_TRANSPORT_REQUEST_HELLO_VALIDATION,
+                         struct RequestHelloValidationMessage,
+                         NULL),
+  /* communication with core */
+  GNUNET_MQ_hd_fixed_size (client_start,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_START,
+                           struct StartMessage,
+                           NULL),
+  GNUNET_MQ_hd_var_size (client_send,
+                         GNUNET_MESSAGE_TYPE_TRANSPORT_SEND,
+                         struct OutboundMessage,
+                         NULL),
+  GNUNET_MQ_hd_fixed_size (client_recv_ok,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_RECV_OK,
+                           struct RecvOkMessage,
+                           NULL),
+  /* communication with communicators */
+  GNUNET_MQ_hd_var_size (communicator_available,
+                         GNUNET_MESSAGE_TYPE_TRANSPORT_NEW_COMMUNICATOR,
+                         struct GNUNET_TRANSPORT_CommunicatorAvailableMessage,
+                         NULL),
+  GNUNET_MQ_hd_var_size (communicator_backchannel,
+                         GNUNET_MESSAGE_TYPE_TRANSPORT_COMMUNICATOR_BACKCHANNEL,
+                         struct GNUNET_TRANSPORT_CommunicatorBackchannel,
+                         NULL),
+  GNUNET_MQ_hd_var_size (add_address,
+                         GNUNET_MESSAGE_TYPE_TRANSPORT_ADD_ADDRESS,
+                         struct GNUNET_TRANSPORT_AddAddressMessage,
+                         NULL),
+  GNUNET_MQ_hd_fixed_size (del_address,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_DEL_ADDRESS,
+                           struct GNUNET_TRANSPORT_DelAddressMessage,
+                           NULL),
+  GNUNET_MQ_hd_var_size (incoming_msg,
+                         GNUNET_MESSAGE_TYPE_TRANSPORT_INCOMING_MSG,
+                         struct GNUNET_TRANSPORT_IncomingMessage,
+                         NULL),
+  GNUNET_MQ_hd_fixed_size (queue_create_ok,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_QUEUE_CREATE_OK,
+                           struct GNUNET_TRANSPORT_CreateQueueResponse,
+                           NULL),
+  GNUNET_MQ_hd_fixed_size (queue_create_fail,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_QUEUE_CREATE_FAIL,
+                           struct GNUNET_TRANSPORT_CreateQueueResponse,
+                           NULL),
+  GNUNET_MQ_hd_var_size (add_queue_message,
+                         GNUNET_MESSAGE_TYPE_TRANSPORT_QUEUE_SETUP,
+                         struct GNUNET_TRANSPORT_AddQueueMessage,
+                         NULL),
+  GNUNET_MQ_hd_fixed_size (update_queue_message,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_QUEUE_UPDATE,
+                           struct GNUNET_TRANSPORT_UpdateQueueMessage,
+                           NULL),
+  GNUNET_MQ_hd_fixed_size (del_queue_message,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_QUEUE_TEARDOWN,
+                           struct GNUNET_TRANSPORT_DelQueueMessage,
+                           NULL),
+  GNUNET_MQ_hd_fixed_size (send_message_ack,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_SEND_MSG_ACK,
+                           struct GNUNET_TRANSPORT_SendMessageToAck,
+                           NULL),
+  /* communication with monitors */
+  GNUNET_MQ_hd_fixed_size (monitor_start,
+                           GNUNET_MESSAGE_TYPE_TRANSPORT_MONITOR_START,
+                           struct GNUNET_TRANSPORT_MonitorStart,
+                           NULL),
+  GNUNET_MQ_handler_end ());
+
+
+/* end of file gnunet-service-transport.c */