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|
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<rfc xmlns:xi="http://www.w3.org/2001/XInclude" category="info" docName="draft-schanzen-r5n-04" ipr="trust200902" obsoletes="" updates="" submissionType="IETF" xml:lang="en" version="3">
<front>
<title abbrev="The R5N Distributed Hash Table">
The R5N Distributed Hash Table
</title>
<seriesInfo name="Internet-Draft" value="draft-schanzen-r5n-04"/>
<author fullname="Martin Schanzenbach" initials="M." surname="Schanzenbach">
<organization>Fraunhofer AISEC</organization>
<address>
<postal>
<street>Lichtenbergstrasse 11</street>
<city>Garching</city>
<code>85748</code>
<country>DE</country>
</postal>
<email>martin.schanzenbach@aisec.fraunhofer.de</email>
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</author>
<author fullname="Christian Grothoff" initials="C." surname="Grothoff">
<organization>Berner Fachhochschule</organization>
<address>
<postal>
<street>Hoeheweg 80</street>
<city>Biel/Bienne</city>
<code>2501</code>
<country>CH</country>
</postal>
<email>grothoff@gnunet.org</email>
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<author fullname="Bernd Fix" initials="B." surname="Fix">
<organization>GNUnet e.V.</organization>
<address>
<postal>
<street>Boltzmannstrasse 3</street>
<city>Garching</city>
<code>85748</code>
<country>DE</country>
</postal>
<email>fix@gnunet.org</email>
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</author>
<!-- Meta-data Declarations -->
<area>General</area>
<workgroup>Independent Stream</workgroup>
<keyword>distributed hash tables</keyword>
<abstract>
<t>
This document contains the R<sup>5</sup>N DHT technical specification.
R<sup>5</sup>N is a secure distributed hash table (DHT) routing algorithm
and data structure for decentralized applications.
It features an open peer-to-peer overlay routing mechanism which supports ad-hoc
permissionless participation and support for topologies in restricted-route
environments. Optionally, the paths data takes through the overlay can be
recorded, allowing decentralized applications to use the DHT to discover routes.
</t>
<t>
This document defines the normative wire format of protocol messages,
routing algorithms, cryptographic routines and security considerations for
use by implementers.
</t>
<t>
This specification was developed outside the IETF and does not have IETF
consensus. It is published here to guide implementation of R<sup>5</sup>N and to
ensure interoperability among implementations including the pre-existing
GNUnet implementation.
</t>
</abstract>
</front>
<middle>
<section anchor="introduction" numbered="true" toc="default">
<name>Introduction</name>
<t>
This specification describes the protocol of R<sup>5</sup>N.
R<sup>5</sup>N is a Distributed Hash Table (DHT). The name is an acronym for
"randomized recursive routing for restricted-route
networks" and its first academic description can be found in
<xref target="R5N"/>.
</t>
<t>
DHTs are a key data structure for the construction of decentralized applications
and generally provide a robust and efficient means to distribute the
storage and retrieval of key-value pairs.
</t>
<t>
The core idea behind R<sup>5</sup>N is to combine a randomized routing
algorithm with an efficient, deterministic closest-peer algorithm.
This allows us to construct an algorithm that is able to escape and circumvent
restricted route environments while at the same time allow for a logarithmically bounded
routing complexity.
</t>
<t>
R<sup>5</sup>N also includes advanced features like recording the path a
key-value pair took
through the network, response filters and on-path application-specific data
validation.
</t>
<t>
This document defines the normative wire format of peer-to-peer
messages, routing algorithms, cryptographic routines and security
considerations for use by implementors.
</t>
<section numbered="true" toc="default">
<name>Requirements Notation</name>
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only
when, they appear in all capitals, as shown here.
</t>
</section>
<section numbered="true" toc="default">
<name>System Model</name>
<t>
DHTs usually operate as overlay networks consisting of peers
communicating over the existing Internet. Hence canonical
DHT designs often assume that the IP protocol provides the
peers of the overlay with unrestricted end-to-end pairwise
connectivity. However, in practice firewalls and network
address translation (NAT) <xref target="RFC2663"/> make it
difficult for peers operating on consumer end-devices to
directly communicate, especially in the absence of core
network infrastructure enabling NAT traversal via protocols
such as interactive connectivity establishment (ICE) <xref target="RFC5245"/>.
</t>
<t>
Furthermore, not all peer-to-peer networks consistently
operate over the Internet, such as mobile ad-hoc networks
(MANETs). While routing protocols have been designed for
such networks (<xref target="RFC3561"/>) these generally
have issues with security in the presence of malicious
participants, as they vulnerable to impersonation attacks.
The usual solution to these issues is to assert that the
entire MANET is a closed network and to require
authentication on all control messages. In contrast, the
system model for R<sup>5</sup>N is that of an open network
without any kind of authorities that could restrict access
only to trusted participants.
</t>
</section>
<section numbered="true" toc="default">
<name>Security Model</name>
<t>
We assume that the network is open and thus a fraction of
the participating peers is malicious. Malicious peers may
create, alter, delay or drop messages. We also assume that
an adversary can control (or fake) many peers <xref target="Sybil"/>, thus any kind
of voting or punishment of malicious peers would be rather
pointless.
</t>
<t>
Honest peers are expected to establish and maintain many
connections. We assume that as a result the adversary is
generally unable to prevent honest peers from maintaining a
sufficient number of direct connections with other honest
peers to achieve acceptable performance. As the number of
malicious peers and their connections increases, performance
of the system should gracefully degrade, and
only collapse for peers that an adversary has fully isolated
from the benign network.
</t>
<t>
The main security objectives are to provide honest nodes
correct results and to limit the propagation of invalid
data. Invalid data includes both invalid key-value pairs as
well as invalid routing path data if such routing meta-data
is present. While malicious nodes may make up arbitrary
key-value pairs and paths within the adversary's domain,
invalid key-value pairs are ideally
discarded at the first honest node, and path data
honestly state entry- and exit-points
from the honest network into the subset of malicious nodes.
</t>
<t>
Malicious nodes may attempt to exhaust the
storage capacity of honest nodes by distributing well-formed
(but possibly otherwise useless) application data. We assume
that storage space is relatively cheap compared to bandwidth
and that honest nodes also frequently re-publish the useful
data that they publish. As a result, an adversary
may reduce the effectiveness and longevity of
data cached in the DHT, but is assumed to not be able to
effectively prevent publication and retrieval of application
data by honest nodes.
</t>
</section>
</section>
<section anchor="terminology">
<name>Terminology</name>
<dl>
<dt>Address</dt>
<dd>
<t>
An <em>address</em> is a UTF-8 <xref target="RFC3629"/> string which can be
used to address a <em>peer</em> through the Underlay (<xref target="underlay"/>).
The format of an address is not enforced by this specification,
but it is expected that in most cases the address is a URI <xref target="RFC3986"/>.
</t>
</dd>
<dt>Applications</dt>
<dd>
<em>Applications</em> are higher-layer components which directly use the
<em>Core Operations</em>.
Possible <em>applications</em> include the GNU Name System
<xref target="RFC9498"/> and the GNUnet
Confidential Ad-hoc Decentralized End-to-End Transport (CADET)
<xref target="cadet"/>.
</dd>
<dt>Core Operations</dt>
<dd>
The <em>Core Operations</em> provide an interface to the
core operations of the DHT overlay to <em>applications</em>.
This includes storing <em>blocks</em> in the DHT and retrieving
<em>blocks</em> from the DHT.
</dd>
<dt>Block</dt>
<dd>
Variable-size unit of payload stored in the DHT
under a <em>key</em>.
In the context of "key-value stores" this
refers to "value" stored under a <em>key</em>.
</dd>
<dt>Block Storage</dt>
<dd>
The <em>block storage</em> component is used to persist and manage
<em>blocks</em> stored by <em>peers</em>.
It includes logic for enforcing storage quotas, caching strategies and
block validation.
</dd>
<dt>Block Type</dt>
<dd>
A unique 32-bit value identifying the data format of a <em>block</em>.
<em>Block types</em> are public and applications that require
application-specific
block payloads are expected to register one or more
block types in the GANA Block-Type registry
(<xref target="gana_block_type"/>) and provide a specification
of the associated block operations (<xref
target="block_functions"/>) to implementors of
R<sup>5</sup>N.
</dd>
<dt>Bootstrapping</dt>
<dd>
<em>Bootstrapping</em> is the process of establishing a connection
to the peer-to-peer network.
It requires an initial, non-empty set of reachable <em>peers</em> and corresponding
<em>addresses</em> supported by the implementation to connect to.
</dd>
<dt>Initiator</dt>
<dd>
The <em>peer</em> that initially creates and sends a DHT protocol message (<xref target="p2p_hello"/>,
<xref target="p2p_put"/>, <xref target="p2p_get"/>, <xref target="p2p_result"/>).
</dd>
<dt>HELLO block</dt>
<dd>
A <tt>HELLO block</tt> is a type of <em>block</em> that is used to store and retrieve <em>addresses</em> of a <em>peer</em>.
It are used by the peer discovery mechanism in <xref target="find_peer"/>.
</dd>
<dt>HELLO URL</dt>
<dd>
<tt>HELLO</tt> URLs are <tt>HELLO</tt> blocks represented as URLs.
They are used for out-of-band exchanges of <em>peer</em> <em>addresses</em>
and for signalling address updates to <em>neighbours</em>.
Implementation details of HELLO URLs and examples are found in <xref target="hello_url"/>.
</dd>
<dt>Key</dt>
<dd>
512-bit identifier of a location in the DHT. Multiple <tt>blocks</tt> can be
stored under the same <em>key</em>. A <em>peer identity</em> is also a <tt>key</tt>.
In the context of "key-value stores" this
refers to "key" under which <em>blocks</em> are stored.
</dd>
<dt>Message Processing</dt>
<dd>
The <em>message processing</em> component of the DHT implementation processes
requests from and generates responses to <em>applications</em>
and the <em>underlay interface</em>.
</dd>
<dt>Neighbor</dt>
<dd>
A neighbor is a <em>peer</em> which is directly able to communicate
with our <em>peer</em> via the <em>underlay interface</em>.
</dd>
<dt>Peer</dt>
<dd>
A host that is participating in the overlay by running an implementation
of the DHT protocol. Each participating host is
responsible for holding some portion of the data that has been
stored in the overlay, and they are responsible for routing
messages on behalf of other <em>peers</em> as needed by the <em>routing
algorithm</em>.
</dd>
<dt>Peer Identity</dt>
<dd>
The <em>peer identity</em> is the identifier used on the overlay
to identify a <em>peer</em>. It is a SHA-512 hash of the <em>peer public key</em>.
</dd>
<dt>Peer Public Key</dt>
<dd>
The <em>peer public key</em> is the key used to authenticate
a <em>peer</em> in the underlay.
</dd>
<dt>Routing</dt>
<dd>
The <em>routing</em> component includes the routing table as well as
routing and <em>peer</em> selection logic. It facilitates the R<sup>5</sup>N routing
algorithm with required data structures and algorithms.
</dd>
<dt>Underlay Interface</dt>
<dd>
The <em>underlay interface</em> is an abstraction layer on top of the
supported links of a <em>peer</em>. Peers may be linked by a variety of
different transports, including "classical" protocols such as
TCP, UDP and TLS or higher-layer protocols such as GNUnet, I2P or Tor.
<!-- FIXME: add references to GNUnet/I2P/Tor here! -->
</dd>
</dl>
</section>
<section numbered="true" toc="default">
<name>Motivation</name>
<section numbered="true" toc="default">
<name>Restricted-route topologies</name>
<t>
Restricted-route topologies emerge when a connected underlay
topology prevents (or restricts) direct connections between
some of the nodes. This commonly occurs through the use of
NAT (<xref target="RFC2663"/>). Nodes operated behind a NAT
cause common DHT routing algorithms such as Kademlia <xref
target="Kademlia"/> to exhibit degraded performance or even
to fail. While excluding such nodes is an option, this
limits load distribution and is ineffective for some
networks, such as MANETs.
</t>
<t>
In general, nodes may not be mutually reachable (for example
due to a firewall or NAT) despite being "neighbours"
according to the routing table construction algorithm of a
particular DHT. For example, Kademlia uses the XOR metric
and would generally connect nodes that have peer identities
with a small XOR distance. However, the XOR distance between
(basically randomly assigned) peer identities is completely
divorced from the ability of the nodes to directly
communicate. DHTs usually use greedy routing to store data
at the peer(s) closest to the key. In cases where a DHT
cannot connect peers according to the construction rules of
its routing algorithm, the topology may ends up with
multiple (local) minima for a given key. Using canonical
greedy routing from a particular fixed location in the
network, a node may then only be able to publish and
retrieve data in the proximity of its local minima.
</t>
<t>
R<sup>5</sup>N addresses this problem by prepending a random
walk before a classical, deterministic XOR-based routing
algorithm is employed. The optimal number of random hops
taken is equal to the mixing time of the graph. The mixing
time for various graphs is well known; for small-world
networks <xref target="Smallworld"/>, the mixing time has
been shown to be around <tt>O(log n)</tt> where <tt>n</tt>
is the number of nodes in the network
<xref target="Smallworldmix"/>.
</t>
<t>
Thus, if the network exhibits the properties of a small
world topology <xref target="Smallworld"/>, a random walk of
length <tt>O(log n)</tt> will cause the algorithm to land on
a random node in the network. Consequently, the
deterministic part of the algorithm will encounter a random
local minimum. It is then possible to repeat this process
in order to store or retrieve data in the context of all or
at least multiple local minima. The ideal length of the
random walk and the number of repetitions expected to cover
all local minima depends on the network topology. Our
design assumes that the benign subset of the network forms a
small-world topology <xref target="Smallworld"/> and then
obtains an estimate of the current number of nodes
<tt>n</tt> in the network and then uses <tt>log n</tt> for
the actual length of the random walk.
</t>
</section>
<section numbered="true" toc="default">
<name>Key differences to RELOAD</name>
<t>
<xref target="RFC6940"/> specifies the RELOAD DHT. The R<sup>5</sup>N DHT
described in this document differs from RELOAD in its objectives
and thus in its design.
The authors of RELOAD make the case that P2P networks are often established
among a set of peers that do not trust each other.
It addresses this issue by requiring that node identifiers
are either assigned by a central authority, or self-issued in the case of closed networks.
In other words, by enforcing the P2P network to be established among a set
of <em>trusted</em> peers.
This misses the point that this openness is a core requirement of efficient and
useful DHTs as they serve a fundamental part in a decentralized network
infrastructure.
R<sup>5</sup>N, by contrast, is intended for open
overlay networks, and thus does not include a central enrollment server to
certify participants and does not limit participation in another way.
As participants could be malicious, R<sup>5</sup>N
includes on-path customizable key-value validation to delete malformed
data and path randomiziation
to help evade malicious peers. R<sup>5</sup>N also expects to perform
over a network where not every peer can communicate with every other peer,
and where thus its route discovery feature provides utility to higher-level
applications. As a result, both the features and the security properties
of RELOAD and R<sup>5</sup>N are different, except in that both allow
storing and retrieving key-value pairs.
<!--
2023/08/20 CG: I believe the above text addresses the comments from MSC below ...
2022/12/23 MSC: I moved references to rfc6940 to security considerations.
I think we should talk about R5N in the positive here only, not about
RELOAD in the negative.
- Lean. Can be implemented. Not overengineered.
- Path tracking (more difficult) -> Not built in
- Certificates central server ?
- "self-signed certificates can be used in closed networks."
- "Security Framework: A P2P network will often be established among a
set of peers that do not trust each other. RELOAD leverages a
central enrollment server to provide credentials for each peer,
which can then be used to authenticate each operation. This
greatly reduces the possible attack surface." bizarre statement.
- For a PUT, reload requires that
"Each element is signed by a credential which is authorized to
write this Kind at this Resource-ID. If this check fails, the
request <bcp14>MUST</bcp14> be rejected with an Error_Forbidden error."
-->
<!--FIXME: Here we should also cite and discuss RELOAD (https://datatracker.ietf.org/doc/html/rfc6940)
and establish why we need this spec and are not a "Topology plugin"
in RELOAD. The argumentation revolves around the trust model (openness) and
security aspects (path signatures).-->
</t>
</section>
</section>
<section>
<name>Overview</name>
<t>
In R<sup>5</sup>N peers provide to their applications
the two fundamental core operations of any DHT:
</t>
<ul>
<li>
<tt>PUT</tt>: This operation stores a block
under a key on one or more peers with
the goal of making the block availiable for queries using the <tt>GET</tt> operation.
In the classical definition of a dictionary interface, this operation would be
called "insert".
</li>
<li>
<tt>GET</tt>: This operation queries the network of peers for any number of blocks
previously stored under or near a key.
In the classical definition of a dictionary interface, this operation would be
called "find".
</li>
</ul>
<t>
An example for possible semantics of the above operations
provided as an API to applications by an implementation are
outlined in <xref target="overlay"/>.
</t>
<t>
A peer does not necessarily need to expose the above
operations to applications, but it commonly will. A
peer that does not expose the above operations could
be a host purely used for bootstrapping,
routing or supporting
the overlay network with resources.
</t>
<t>
Similarly, there could be hosts on the network that
participate in the DHT but do not route traffic or store
data. Examples for such hosts would be mobile devices with
limited bandwidth, battery and storage capacity. Such hosts
may be used to run applications that use the DHT. However, we
will not refer to such hosts as peers.
</t>
<t>
In a trivial scenario where there is only one peer (on the local host),
R<sup>5</sup>N operates similarly to a dictionary data structure.
However, the default use case is one where nodes communicate directly and
indirectly in order to realize a distributed storage mechanism.
This communication requires a lower-level peer addressing and message transport
mechanism such as TCP/IP.
R<sup>5</sup>N is agnostic to the underlying transport protocol which is why
this document defines a common addressing and messaging interface in
<xref target="underlay"/>.
The interface provided by this underlay is used across the specification of the
R<sup>5</sup>N protocol.
It also serves as a set of requirements of possible transport mechanisms that
can be used to implement R<sup>5</sup>N with.
That being said, common transport protocols such as TCP/IP or UDP/IP and their
interfaces are suitable R<sup>5</sup>N underlays and are used as such by existing
implementations.
</t>
<!-- consider moving some of this back into sec considerations -->
<t>
Specifics about the protocols of the underlays implementing
the underlay interface or the applications
using the DHT are out of the scope of this document.
</t>
<t>
To establish an initial connection to a network of
R<sup>5</sup>N peers, at least one initial, addressable
peer is required as part of the
bootstrapping process. Further peers,
including neighbors, are then learned via a peer
discovery process as defined in <xref target="find_peer"/>.
</t>
<t>
Across this document, the functional components of an
R<sup>5</sup>N implementation are divided into
routing (<xref target="routing"/>), message
processing (<xref target="p2p_messages"/>) and
block storage (<xref target="blockstorage"/>).
<xref target="figure_r5n_arch"/> illustrates
the architectural overview of R<sup>5</sup>N.
</t>
<figure anchor="figure_r5n_arch" title="The R5N architecture.">
<artwork><![CDATA[
| +-----------------+ +-------+
Applications | | GNU Name System | | CADET | ...
| +-----------------+ +-------+
-------------+------------------------------------ Core Operations
| ^
| | +---------------+
| | | Block Storage |
| | +---------------+
| | ^
R5N | v v
| +--------------------+ +---------+
| | Message Processing |<-->| Routing |
| +--------------------+ +---------+
| ^ ^
| v v
-------------+------------------------------------ Underlay Interface
| +--------+ +--------+ +----------+
| |GNUnet | |IP | | QUIC |
Connectivity | |Underlay| |Underlay| | Underlay | ...
| |Link | |Link | | Link |
| +--------+ +--------+ +----------+
]]>
</artwork>
</figure>
</section>
<section anchor="underlay" numbered="true" toc="default">
<name>Underlay</name>
<t>
A peer <bcp14>MUST</bcp14> support one or more underlay
protocols.
Peers supporting multiple underlays effectively
create a bridge between different networks. How peers are
addressed in a specific underlay is out of scope of this
document. For example, a peer may have a TCP/IP address, or
expose a QUIC endpoint, or both. While the specific addressing options
and mechanisms are out of scope for this document,
it is necessary to define a
universal addressing format in order to facilitate the
distribution of address information to other
peers in the DHT overlay.
This standardized format
is the HELLO block (described in <xref
target="hello_block"/>), which contains sets of addresses.
If the address is a URI, it may indicate which
underlay understands the respective address.
</t>
<!--
1) The current API is always fire+forget, it doesn't allow for flow
control. I think we need to add that, possibly for sending and receiving.
IDK.
CG: I think we should not have flow control for the DHT; overkill,
should instead simply define transmission as unreliable.
2) I'm not sure what to do with the crypto: mandate EdDSA or allow the
underlay to do whatever public keys it likes.
We need keys in the overlay. (Path signatures). Do they need to
be the same keys???
CG: I'd mandate EdDSA. CONG will have mitigation to establish
EdDSA keys over libp2p, even if libp2p does not use EdDSA. But,
that said, I'm not sure if we should even mandate AE on the
underlay.
3) I think we may want to mandate that the lower layer at least
authenticate the other peer (i.e. every UDP message could be in
cleartext, but would need to come with an EdDSA signature, alas 92 byte
overhead and a signature verification _required_). Otherwise, I don't
see how we can offer even the most minimal protections against peer
impersonation attacks. WDYT?
CG: Yes, I think authentication should be mandatory, but not
any _specific_ type of encryption.
Security considerations? Prerequisites?
-->
<t>
It is expected that the underlay provides basic mechanisms to
manage peer connectivity and addressing.
The essence of the underlay interface is
captured by the following set of API calls:
</t>
<dl>
<dt>
<tt>TRY_CONNECT(P, A)</tt>
</dt>
<dd>
This call allows the DHT implementation to signal to the
underlay that the DHT wants to establish a connection to the
target peer <tt>P</tt> using the given address <tt>A</tt>.
If the connection attempt is successful, information on the
new peer connection will be offered through the
<tt>PEER_CONNECTED</tt> signal.
<!--
Underlay implementations
can ignore calls with addresses they do not support.
-->
</dd>
<dt>
<tt>HOLD(P)</tt>
</dt>
<dd>
This call tells the underlay to hold on to a connection
to a peer <tt>P</tt>. Underlays are usually limited in the number
of active connections. With this function the DHT can indicate to the
underlay which connections should preferably be preserved.
</dd>
<dt>
<tt>DROP(P)</tt>
</dt>
<dd>
This call tells the underlay to drop the connection to a
peer <tt>P</tt>. This call is only there for symmetry and
used during the peer's shutdown to release all of the remaining
<tt>HOLDs</tt>.
<!-- FIXME: are we supposed to call DROP if a peer disconnects?
NOTE: That would seem to be an implementation detail beyond what needs
to be in the RFC. An API may mandate DROP on disconnect, or
it may simply state that when the underlay signals a
disconnect, all holds are automatically dropped.
-->
As R<sup>5</sup>N always prefers the longest-lived
connections, it would never drop an active connection that it
has called <tt>HOLD()</tt> on before. Nevertheless, underlay implementations
should not rely on this always being true. A call to <tt>DROP()</tt> also
does not imply that the underlay must close the connection: it merely
removes the preference to preserve the connection that was established
by <tt>HOLD()</tt>.
</dd>
<dt>
<tt>SEND(P, M)</tt>
</dt>
<dd>
This call allows the local peer to send a protocol message
<tt>M</tt> to a peer <tt>P</tt>. Sending messages is expected
to be done on a best-effort basis, thus the underlay does not
have to guarantee delivery or message ordering. If the underlay
implements flow- or congestion-control, it may
discard messages to limit its queue size.
</dd>
<dt>
<tt>ESTIMATE_NETWORK_SIZE() -> L2NSE</tt>
</dt>
<dd>
This call must return an estimate of the network size. The
resulting <tt>L2NSE</tt> value must be the base-2 logarithm
of the <em>estimated</em> number of peers in the network.
This estimate is used by the routing algorithm. If the underlay does
not support a protocol for network size estimation (such as
<xref target="NSE"/>) the value is assumed to be provided as
a configuration parameter to the underlay implementation.
</dd>
</dl>
<t>
The above calls are meant to be actively executed by the
implementation as part of the peer-to-peer protocol. In
addition, the underlay creates <em>signals</em> to drive
updates of the routing table, local storage and message
processing (<xref target="p2p_messages"/>). Specifically,
the underlay is expected to emit the following
signals (usually implemented as callbacks) based on network
events observed by the underlay implementation:
</t>
<dl>
<dt>
<tt>PEER_CONNECTED -> P</tt>
</dt>
<dd>
This signal allows the DHT to react to a newly connected
peer <tt>P</tt>. Such an event triggers, for example,
updates in the routing table and gossiping of HELLOs to that
peer. Underlays may include meta-data about the connection,
for example to indicate that the connection is from a
resource-constrained host that does not intend to function
as a full peer and thus should not be considered
for routing.
</dd>
<dt>
<tt>PEER_DISCONNECTED -> P</tt>
</dt>
<dd>
This signal allows the DHT to react to a recently
disconnected peer. Such an event primarily triggers updates
in the routing table.
</dd>
<dt>
<tt>ADDRESS_ADDED -> A</tt>
</dt>
<dd>
The underlay signals indicates that an address <tt>A</tt>
was added for our local peer and that henceforth the peer
may be reachable under this address. This information is
used to advertise connectivity information about the local
peer to other peers. <tt>A</tt> is an
address suitable for inclusion in a <tt>HELLO</tt> payload
<xref target="hello_block"/>.
</dd>
<dt>
<tt>ADDRESS_DELETED -> A</tt>
</dt>
<dd>
This underlay signal indicates that an address <tt>A</tt>
was removed from the set of addresses the local peer is
possibly reachable under. The signal is used
to stop advertising this address to other peers.
</dd>
<dt>
<tt>RECEIVE -> (P, M)</tt>
</dt>
<dd>
This signal informs the local peer that a protocol
message <tt>M</tt> was received from a peer <tt>P</tt>.
</dd>
</dl>
</section>
<section anchor="routing" numbered="true" toc="default">
<name>Routing</name>
<t>
To enable routing, any R<sup>5</sup>N implementation must keep
information about its current set of neighbors.
Upon receiving a connection notification from the
underlay interface through a
<tt>PEER_CONNECTED</tt> signal, information on the new neighbor
<bcp14>MUST</bcp14> be added to the routing table, except if the
respective <tt>k</tt>-bucket in the routing table is full or if meta-data
is present that indicates that the peer does not wish to participate
in routing.
Peers added to the routing table <bcp14>SHOULD</bcp14> be signalled to the
underlay as important connections using a <tt>HOLD()</tt> call.
Similarly when a disconnect is indicated by the underlay through
a <tt>PEER_DISCONNECTED</tt> signal, the peer
<bcp14>MUST</bcp14> be removed from the routing table.
<!-- FIXME: are we supposed to call DROP if we called HOLD if a peer disconnects!?? -->
</t>
<t>
To achieve logarithmically bounded routing performance,
the data structure for managing neighbors and their
metadata <bcp14>MUST</bcp14> be implemented using the k-buckets concept of
<xref target="Kademlia"/> as defined in <xref target="routing_table"/>.
Maintenance of the routing table (after bootstrapping) is
described in <xref target="find_peer"/>.
</t>
<t>
Unlike <xref target="Kademlia"/>, routing decisions in
R<sup>5</sup>N are also influenced by a Bloom filter in the message
that prevents routing loops. This data structure is discussed in
<xref target="routing_bloomfilter"/>.
</t>
<t>
In order to select peers which are suitable destinations for
routing messages, R<sup>5</sup>N uses a hybrid approach: Given
an estimated network size <tt>L2NSE</tt> retrieved using
<tt>ESTIMATE_NETWORK_SIZE()</tt>, the peer selection for the
first <tt>L2NSE</tt> hops is random. After the initial
<tt>L2NSE</tt> hops, peer selection follows an XOR-based peer
distance calculation.
<xref target="routing_functions"/>
describes the corresponding routing functions.
</t>
<t>
Finally, each <tt>ResultMessage</tt> is routed back along the
path that the corresponding <tt>GetMessage</tt> took
previously. This is enabled by tracking state per
<tt>GetMessage</tt> in the pending table described in
<xref target="pending_table"/>.
</t>
<section anchor="routing_table">
<name>Routing Table</name>
<t>
Whenever a <tt>PEER_CONNECTED</tt> signal is received from
the underlay, the respective peer is considered for
insertion into the routing table. The routing table
consists of an array of <tt>k</tt>-buckets. Each
<tt>k</tt>-bucket contains a list of neighbors.
The i-th <tt>k</tt>-bucket stores neighbors whose peer
public keys are between XOR-distance 2<sup>i</sup> and
2<sup>i+1</sup> from the local peer; <tt>i</tt> can be
directly computed from the two peer identities using the
<tt>GetDistance()</tt> function. System constraints will
typically force an implementation to impose some upper limit
on the number of neighbors kept per
<tt>k</tt>-bucket. Upon insertion, the implementation
<bcp14>MUST</bcp14> call <tt>HOLD()</tt> on the respective
neighor.
</t>
<t>
Implementations <bcp14>SHOULD</bcp14> try to keep at least
5 entries per <tt>k</tt>-bucket. Embedded systems that cannot manage
this number of connections <bcp14>MAY</bcp14> use connection-level
signalling to indicate that they are merely a client utilizing a
DHT and not able to participate in routing. DHT peers receiving
such connections <bcp14>MUST NOT</bcp14> include connections to
such restricted systems in their <tt>k</tt>-buckets, thereby effectively
excluding them when making routing decisions.
</t>
<t>
If a system hits constraints with respect to
the number of active connections, an implementation
<bcp14>MUST</bcp14> evict neighbours from those <tt>k</tt>-buckets with the
largest number of neighbors. The eviction strategy <bcp14>MUST</bcp14> be
to drop the shortest-lived connection per <tt>k</tt>-bucket first.
</t>
<t>
Implementations <bcp14>MAY</bcp14> cache valid addresses of disconnected
peers outside of the routing table and sporadically or periodically try to (re-)establish connection
to the peer by making <tt>TRY_CONNECT()</tt> calls to the underlay interface
if the respective <tt>k</tt>-bucket has empty slots.
</t>
</section>
<section anchor="find_peer">
<name>Peer Discovery</name>
<t>
Initially, implementations require at least one initial connection to a
neighbor (signalled through
<tt>PEER_CONNECTED</tt>).
The first connection <bcp14>SHOULD</bcp14> be established
by an out-of-band exchange of the information from a
<tt>HELLO</tt> block.
This is commonly achieved through the
configuration of hardcoded bootstrap peers or bootstrap
servers either for the underlay or the R<sup>5</sup>N
implementation.
</t>
<t>
Implementations <bcp14>MAY</bcp14> have other means to achieve this
initial connection.
For example, implementations could allow the application or
even end-user to provide a working <tt>HELLO</tt>
which is then in turn used to call <tt>TRY_CONNECT()</tt> on
the underlay in order to trigger a subsequent
<tt>PEER_CONNECTED</tt> signal from the underlay
interface.
<xref target="hello_url"/> specifies
a URL format for encoding HELLO blocks as text strings. The
URL format thus provides a portable, human-readable,
text-based serialization format that can, for example, be
encoded into a QR code for dissemination.
HELLO URLs <bcp14>SHOULD</bcp14> be supported by implementations for
both import and export of <tt>HELLOs</tt>.
</t>
<t>
To discover additional peers for its routing table, a peer
<bcp14>MUST</bcp14> initiate <tt>GetMessage</tt> requests
(see <xref target="p2p_get"/>) asking for blocks of type
<tt>HELLO</tt> using its own peer identity in the
<tt>QUERY_HASH</tt> field of the message. The
<tt>PEER_BF</tt> field of the <tt>GetMessage</tt>
<bcp14>MUST</bcp14> be
initialized to filter the peer's own peer identity as well as the peer
identities of all currently connected
neighbors. These requests <bcp14>MUST</bcp14> use
the <tt>FindApproximate</tt> and
<tt>DemultiplexEverywhere</tt>
flags. <tt>FindApproximate</tt> will ensure that other peers
will reply with results where the keys are merely considered
close-enough, while <tt>DemultiplexEverywhere</tt> will
cause each peer on the path to respond if it has relevant
information. The combination of these flags is thus likely
to yield <tt>HELLOs</tt> of peers that are useful somewhere
in the initiator's routing table. The <bcp14>RECOMMENDED</bcp14>
replication level to be set in the <tt>REPL_LVL</tt> field
is 4. The size and format of the result filter is specified
in <xref target="hello_block"/>. The <tt>XQUERY</tt>
<bcp14>MUST</bcp14> be empty.
</t>
<t>
In order to facilitate peers answering requests for
<tt>HELLOs</tt>, the underlay is expected to provide the
implementation with addresses signalled through
<tt>ADDRESS_ADDED</tt>. It is possible that no addresses are
provided if a peer can only establish outgoing connections
and is otherwise unreachable. An implementation
<bcp14>MUST</bcp14> advertise its addresses periodically to
its neighbors through <tt>HelloMessages</tt>. The
advertisement interval and expiration <bcp14>SHOULD</bcp14>
be configurable.
If the values are chosen at the discretion of the
implementation, it is <bcp14>RECOMMENDED</bcp14> to choose external
factors such as expiration of DHCP leases to determine the values.
The specific frequency of advertisements
<bcp14>SHOULD</bcp14> be smaller than the expiration
period.
It <bcp14>MAY</bcp14> additionally depend on available bandwidth,
the set of already connected neighbors, the workload of the system and
other factors which are at the discretion of the developer.
If <tt>HelloMessages</tt> are not updated before they expire,
peers might be unable to discover and connect to the respective
peer, and thus miss out on quality routing table entries. This
would degrade the performance of the DHT and <bcp14>SHOULD</bcp14>
thus be avoided by advertising updated <tt>HELLOs</tt> before the
previous one expires. When using unreliable underlays, an implementation
<bcp14>MAY</bcp14> use higher frequencies and transmit
more <tt>HelloMessages</tt> within an expiration interval
to ensure that neighbours almost always have non-expired
<tt>HelloMessages</tt> at their disposal even if some messages
are lost.
</t>
<t>
Whenever a peer receives such a <tt>HelloMessage</tt>
from another peer that is already in the routing
table, it must cache it as long as that peer remains in its
routing table (or until the <tt>HELLO</tt> expires) and
serve it in response to <tt>GET</tt> requests for
<tt>HELLO</tt> blocks (see <xref
target="p2p_get_processing"/>). This behaviour makes it
unnecessary for peers to initiate dedicated
<tt>PutMessages</tt> containing <tt>HELLO</tt> blocks.
</t>
</section>
<section anchor="routing_bloomfilter">
<name>Peer Bloom Filter</name>
<t>
As DHT <tt>GetMessages</tt> and <tt>PutMessages</tt>
traverse a random path through the network for the first
<tt>L2NSE</tt> hops, a key design objective of
R<sup>5</sup>N is to avoid routing loops. The peer Bloom
filter is part of the routing metadata in messages to
prevent circular routes. It is updated at each hop where the
hop's peer identity derived from the peer's public key is
added to it.
The peer Bloom filter follows the definition in <xref target="bloom_filters"/>.
It <bcp14>MUST</bcp14> be <tt>L=1024</tt> bits
(128 bytes) in size and <bcp14>MUST</bcp14> set <tt>k=16</tt> bits per
element.
The set of elements <tt>E</tt> consists of of all possible 256-bit peer
public keys and the mapping function <tt>M</tt> is defined as follows:
</t>
<t>
<tt>M(e) -> SHA-512 (e) as uint32[]</tt>
</t>
<t>
The element <tt>e</tt> is the peer public key which is hashed using SHA-512.
The resulting 512-bit peer identity is interpreted as an array of k=16
32-bit integers in network byte order which are used to set and check the bits
in <tt>B</tt> using <tt>BF-SET()</tt> and <tt>BF-TEST()</tt>.
</t>
<t>
At this size, the Bloom filter reaches a false-positive rate of
approximately fifty percent at about 200 entries. For peer
discovery where the Bloom filter is initially populated with
peer identities from the local routing table, the 200
entries would still be enough for 40 buckets assuming 5
peers per bucket, which corresponds to an overlay network
size of approximately 1 trillion peers. Thus,
<tt>L=1024</tt> bits should suffice for all conceivable
use-cases.
</t>
<t>
For the next hop selection in both the random
and the deterministic case, any peer which is in the peer
Bloom filter for the respective message is excluded from the
peer selection process.
Any peer which is forwarding <tt>GetMessages</tt> or <tt>PutMessages</tt>
(<xref target="p2p_messages"/>) thus adds its own peer public key to the
peer Bloom filter.
This allows other peers to (probabilistically) exclude already
traversed peers when searching for the next hops in the routing table.
</t>
<t>
We note that the peer Bloom filter may exclude peers due to false-postive
matches. This is acceptable as routing should nevertheless
terminate (with high probability) in close vicinity of the key. Furthermore,
due to the randomization of the first L2NSE hops, it is possible that
false-positives will be different when a request is repeated.
</t>
</section>
<section anchor="routing_functions">
<name>Routing Functions</name>
<t>
Using the data structures described so far,
the R<sup>5</sup>N routing component provides
the following functions for
message processing (<xref target="p2p_messages"/>):
</t>
<dl>
<dt>
<tt>GetDistance(A, B) -> Distance</tt>
</dt>
<dd>
This function calculates the binary XOR between A and B.
The resulting distance is interpreted as an integer where
the leftmost bit is the most significant bit.
</dd>
<dt>
<tt>SelectClosestPeer(K, B) -> N</tt>
</dt>
<dd>
This function selects the neighbor <tt>N</tt> from our
routing table with the shortest XOR-distance to the key <tt>K</tt>.
This means that for all other peers <tt>N'</tt> in the routing table
<tt>GetDistance(N, K) < GetDistance(N',K)</tt>.
Peers with a positive test against the peer Bloom
filter <tt>B</tt> are not considered.
</dd>
<dt>
<tt>SelectRandomPeer(B) -> N</tt>
</dt>
<dd>
This function selects a random peer <tt>N</tt> from
all neighbors.
Peers with a positive test in the peer Bloom
filter <tt>B</tt> are not considered.
</dd>
<dt>
<tt>SelectPeer(K, H, B) -> N</tt>
</dt>
<dd>
This function selects a neighbor <tt>N</tt> depending on the
number of hops <tt>H</tt> parameter.
If <tt>H < NETWORK_SIZE_ESTIMATE</tt>
returns <tt>SelectRandomPeer(B)</tt>, and otherwise
returns <tt>SelectClosestPeer(K, B)</tt>.
</dd>
<dt>
<tt>IsClosestPeer(N, K, B) -> true | false</tt>
</dt>
<dd>
This function checks if <tt>N</tt> is the closest peer for <tt>K</tt>
(cf. <tt>SelectClosestPeer(K, B)</tt>).
Peers with a positive test in the Bloom filter <tt>B</tt> are not considered.
</dd>
<dt>
<tt>ComputeOutDegree(REPL_LVL, HOPCOUNT, L2NSE) -> Number</tt>
</dt>
<dd>
<t>
This function computes the number of neighbors
that a message should be forwarded to. The arguments
are the desired replication level (<tt>REPL_LVL</tt>),
the <tt>HOPCOUNT</tt> of the message so far and
and the current network size estimate (<tt>L2NSE</tt>)
as provided by the underlay.
The result is the non-negative number of next hops to
select. The following figure gives the
pseudocode for computing the number of neighbors
the peer should attempt to forward the message to.
</t>
<figure anchor="compute_out_degree" title="Computing the number of next hops.">
<artwork name="" type="" align="left" alt=""><![CDATA[
ComputeOutDegree(REPL_LVL, HOPCOUNT, L2NSE):
if (HOPCOUNT > L2NSE * 4)
return 0;
if (HOPCOUNT > L2NSE * 2)
return 1;
if (0 = REPL_LEVL)
REPL_LEVL = 1
if (REPL_LEVEL > 16)
REPL_LEVEL = 16
RM1 = REPL_LEVEL - 1
FRAC = 1 + (RM1 / (L2NSE + RM1 * HOPCOUNT))
return PROUND(FRAC)
]]></artwork>
</figure>
<t>
The above calculation of <tt>FRAC</tt> may yield values that are
not discrete.
The result is <tt>FRAC</tt> rounded probabilistically
(<tt>PROUND</tt>) to the nearest
discrete value, using the fraction
as the probability for rounding up.
For example, a value of <tt>3.14</tt> is rounded up to <tt>4</tt> with
a probability of 14% and rounded down to <tt>3</tt> with a probability
of 86%.
This probabilistic rounding is necessary to achieve
the statistically expected value of the replication
level and average number of peers a message is forwarded to.
</t>
</dd>
</dl>
</section>
<section anchor="pending_table">
<name>Pending Table</name>
<t>
R<sup>5</sup>N performs stateful routing where the messages
only carry the query hash and do not encode the ultimate
source or destination of the request. Routing a request
towards the key is done hop-by-hop using the routing table and the
query hash. The pending table is used to route responses
back to the originator. In the pending table each peer
primarily maps a query hash to the associated
originator of the request. The pending table <bcp14>MUST</bcp14>
store entries for the last <tt>MAX_RECENT</tt> requests
the peer has encountered.
To ensure that the peer does
not run out of memory, information about older requests
<bcp14>MAY</bcp14> be discarded.
The value of <tt>MAX_RECENT</tt> <bcp14>MAY</bcp14> be configurable
at the host level to use available memory resources without
conflicting with other system requirements and limitations.
<tt>MAX_RECENT</tt> <bcp14>SHOULD</bcp14>
be at least 128 * 10<sup>3</sup>.
If the pending table is smaller, the likelihood grows that a peer
receives a response to a query but is unable to forward it to the
initiator because it forgot the predecessor. Eventually, the
initiator would likely succeed by repeating the query.
However, this would be much more expensive than peers having an adequately
sized pending table.
</t>
<t>
For each entry in the pending table, the DHT
<bcp14>MUST</bcp14> track the query key, the peer identity
of the previous hop, the extended query, requested block type,
flags, and the result filter. If the query did not provide
a result filter, a fresh result filter <bcp14>MUST</bcp14>
still be created to filter duplicate replies. Details of
how a result filter works depend on the type, as described
in <xref target="block_functions"/>.
</t>
<t>
When a second query from the same origin for the
same query hash is received, the DHT <bcp14>MUST</bcp14>
attempt to merge the new request with the state for
the old request. If this is not possible (say because
the MUTATOR differs), the
existing result filter <bcp14>MUST</bcp14> be
discarded and replaced with the result
filter of the incoming message.
</t>
<t>
We note that for local applications, a fixed limit on
the number of concurrent requests may be problematic.
Hence, it is <bcp14>RECOMMENDED</bcp14> that implementations
track requests from local applications separately and
preserve the information about requests from local
applications until the local application explicitly
stops the request.
</t>
</section>
</section>
<section anchor="p2p_messages" numbered="true" toc="default">
<name>Message Processing</name>
<t>
An implementation will process
messages either because it needs to transmit messages as part of routing
table maintenance, or due to requests from local applications, or
because it received a message from a neighbor.
If instructed through an application-facing API such as the one outlined
in <xref target="overlay"/>, a peer acts as an initiator
of <tt>GetMessages</tt>
or <tt>PutMessages</tt>.
The status of initiator is relevant for peers when processing <tt>ResultMessages</tt>
due to the required handover of results to the application that requested
the respective result.
</t>
<t>
The implementation <bcp14>MUST</bcp14> listen for <tt>RECEIVE(P, M)</tt> signals
from the underlay and react to the respective messages sent by
the peer <tt>P</tt>.
</t>
<t>
Whether initiated locally or received from a neighbor, an implementation
processes messages according to the wire formats and the required
validations detailed in the following sections.
Where required, the local peer public key is referred to as <tt>SELF</tt>.
</t>
<section anchor="message_components">
<name>Message components</name>
<t>
This section describes some data structures and fields shared
by various types of messages.
</t>
<section anchor="route_flags">
<name>Flags</name>
<t>
Flags is an 8-bit vector representing binary options.
Each flag is represented by a bit in the field starting from 0 as
the rightmost bit to 7 as the leftmost bit.
</t>
<dl>
<dt>0: DemultiplexEverywhere</dt>
<dd>
This bit indicates that each peer along the way should process the request.
If the bit is not set, intermediate peers only route the message and only
peers which consider themselves closest to the key (based on their
routing table) look for answers
in their local storage for <tt>GetMessages</tt>, or respectively cache the
block in their local storage for <tt>PutMessages</tt> and <tt>ResultMessages</tt>.
</dd>
<dt>1: RecordRoute</dt>
<dd>
This bit indicates to keep track of the path that the message takes
in the P2P network.
</dd>
<dt>2: FindApproximate</dt>
<dd>
This bit asks peers to return results even if the key
does not exactly match the query hash.
</dd>
<dt>3: Truncated</dt>
<dd>
This is a special flag which is set if a peer truncated the
recorded path.
This results in the first hop on the path to be given without a signature
to enable checking of the next signature.
This flag <bcp14>MUST NOT</bcp14> be set in
a <tt>GetMessage</tt>.
</dd>
<dt>4-7: Reserved</dt>
<dd>
The remaining bits are reserved for future use and
<bcp14>MUST</bcp14> be set to zero when initiating an operation.
If non-zero bits are received, implementations <bcp14>MUST</bcp14>
preserve these bits when forwarding messages.
</dd>
</dl>
</section>
<section anchor="p2p_pathelement">
<!-- TODO-GROTHOFF: Discuss this change again. The text is currently not correct
it is very difficult to understand. Is it worth 32 byte;
CG: I've fixed the figures, tried to clarify the text. Is it OK now? -->
<name>Path Element</name>
<t>
A path element represents a hop in the path a message has taken
through the overlay network.
The wire format of a path element is illustrated in
<xref target="figure_pathelement"/>.
</t>
<figure anchor="figure_pathelement" title="The Wire Format of a path element.">
<artwork name="" type="" align="left" alt=""><![CDATA[
0 8 16 24 32 40 48 56
+-----+-----+-----+-----+-----+-----+-----+-----+
| SIGNATURE |
| (64 bytes) |
| |
| |
| |
| |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
| PRED PEER PUBLIC KEY |
| (32 bytes) |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
]]></artwork>
</figure>
<t>where:</t>
<dl>
<dt>SIGNATURE</dt>
<dd>
is a 64 byte EdDSA signature <xref target="ed25519"/> created
using the current hop's private
key which affirms the public keys of the peers from the
previous and next hops.
</dd>
<dt>PRED PEER PUBLIC KEY</dt>
<dd>
is the EdDSA public key <xref target="ed25519"/> of the previous peer on the path.
</dd>
</dl>
<t>
An ordered list of path elements may be appended to any routed
<tt>PutMessages</tt> or <tt>ResultMessages</tt>.
The last signature (after which the peer public key is omitted)
is created by the current hop only after the peer made its routing
decision identifiying the successor peer. The peer public key is not
included after the last signature as it must be that of the sender of
the message and including it would thus be redundant.
Similarly, the predecessor of the first element of
an untruncated path is not stated explicitly, as it must be <tt>ZERO</tt>
(32 NULL-bytes).
</t>
<t>
<xref target="figure_path_ex"/> shows the wire format of an example
path from peer A over peers B and C and D as it would be received by peer E in the
<tt>PUTPATH</tt> of a <tt>PutMessage</tt>, or as the combined
<tt>PUTPATH</tt> and <tt>GETPATH</tt> of a <tt>ResultMessage</tt>.
The wire format of the path elements allows a natural
extension of the <tt>PUTPATH</tt> along the route of the <tt>ResultMessage</tt>
to the destination forming the <tt>GETPATH</tt>.
The <tt>PutMessage</tt> would indicate in the <tt>PATH_LEN</tt> field
a length of 3.
The <tt>ResultMessage</tt> would indicate a path length of 3 as the
sum of the field values in <tt>PUTPATH_L</tt> and <tt>GETPATH_L</tt>.
Basically, the last signature does not count for the path length.
</t>
<figure anchor="figure_path_ex" title="Example of a path as found in PutMessages or ResultMessages from Peer A to Peer D as transmitted by Peer D.">
<artwork name="" type="" align="left" alt=""><![CDATA[
0 8 16 24 32 40 48 56
+-----+-----+-----+-----+-----+-----+-----+-----+
| SIGNATURE A |
| (64 bytes) |
| |
| (Over ZERO and PEER B signed by PEER A) |
| |
| |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
| PEER A |
| (32 bytes) |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
| SIGNATURE B |
| (64 bytes) |
| |
| (Over PEER A and PEER C signed by PEER B) |
| |
| |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
| PEER B |
| (32 bytes) |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
| SIGNATURE C |
| (64 bytes) |
| |
| (Over PEER B and PEER D signed by PEER C) |
| |
| |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
| PEER C |
| (32 bytes) |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
| SIGNATURE D (last sig) |
| (64 bytes) |
| |
| (Over PEER C and receiver signed by PEER D) |
| |
| |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
]]></artwork>
</figure>
<t>
A path may be truncated in which case the signature of the truncated
path element is omitted leaving only the public key of the peer preceeding
the truncation which is required for the
verification of the subsequent path element signature.
Such a truncated path is indicated with the respective
truncated flag (<xref target="route_flags"/>).
For truncated paths, the peer public key of the signer of the last path element is
again omitted as it must be that of
the sender of the <tt>PutMesssage</tt> or <tt>ResultMessage</tt>. Similarly,
the public key of the receiving peer used in the last path element is omitted as
it must be SELF.
The wire format of a truncated example path from peers B over C and D to E
(possibly still originating at A, but the origin is unknowable to E due to truncation)
is illustrated in <xref target="figure_path_ex_trunc"/>.
Here, a <tt>ResultMessage</tt> would indicate in the <tt>PATH_LEN</tt> field
a length of 1 while
a <tt>PutMessage</tt> would indicate a length of 1 as the sum of
<tt>PUTPATH_L</tt> and <tt>GETPATH_L</tt> fields.
Basically, the truncated peer and the last signature do not count
for the path length.
</t>
<figure anchor="figure_path_ex_trunc" title="Example of a truncated path from Peer B to Peer D as transmitted by Peer D.">
<artwork name="" type="" align="left" alt=""><![CDATA[
0 8 16 24 32 40 48 56
+-----+-----+-----+-----+-----+-----+-----+-----+
| PEER B (truncated) |
| (32 byte) |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
| SIGNATURE C |
| (64 bytes) |
| |
| (Over PEER B and PEER D signed by PEER C) |
| |
| |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
| PEER C |
| (32 bytes) |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
| SIGNATURE D (last sig) |
| (64 byte) |
| |
| (Over PEER C and receiver signed by PEER D) |
| |
| |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
]]></artwork>
</figure>
<t>
The SIGNATURE field in a path element covers a 64-bit contextualization header, the
the block expiration, a hash of the block
payload, as well as the predecessor peer public key and the peer public key of the
successor that the peer making the signature is routing the
message to. Thus, the signature made by SELF basically says that
SELF received the block payload from PEER PREDECESSOR and has forwarded
it to PEER SUCCESSOR. The wire format is illustrated
in <xref target="figure_pathelewithpseudo"/>.
</t>
<figure anchor="figure_pathelewithpseudo" title="The Wire Format of the path element for Signing.">
<artwork name="" type="" align="left" alt=""><![CDATA[
0 8 16 24 32 40 48 56
+-----+-----+-----+-----+-----+-----+-----+-----+
| SIZE | PURPOSE |
+-----+-----+-----+-----+-----+-----+-----+-----+
| EXPIRATION |
+-----+-----+-----+-----+-----+-----+-----+-----+
| BLOCK HASH |
| (64 byte) |
| |
| |
| |
| |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
| PEER PREDECESSOR |
| (32 byte) |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
| PEER SUCCESSOR |
| (32 byte) |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
]]></artwork>
</figure>
<dl>
<dt>SIZE</dt>
<dd>
A 32-bit value containing the length of the signed data in bytes
in network byte order.
The length of the signed data <bcp14>MUST</bcp14> be 144 bytes.
</dd>
<dt>PURPOSE</dt>
<dd>
A 32-bit signature purpose flag. This field <bcp14>MUST</bcp14> be 6 (in network
byte order).
</dd>
<dt>EXPIRATION</dt>
<dd>
denotes the absolute 64-bit expiration date of the block
in microseconds since midnight (0 hour), January 1, 1970
UTC in network byte order.
</dd>
<dt>BLOCK HASH</dt>
<dd>
a SHA-512 hash over the block payload.
</dd>
<dt>PEER PREDECESSOR</dt>
<dd>
the peer public key of the previous hop. If the signing peer initiated
the PUT, this field is set to all zeroes.
</dd>
<dt>PEER SUCCESSOR</dt>
<dd>
the peer public key of the next hop (not of the signer).
</dd>
</dl>
</section>
</section>
<section anchor="p2p_hello" numbered="true" toc="default">
<name>HelloMessage</name>
<t>
When the underlay signals the implementation of added or
removed addresses through <tt>ADDRESS_ADDED</tt> and
<tt>ADDRESS_DELETED</tt> an implementation
<bcp14>MUST</bcp14> disseminate those changes to neighbors
using <tt>HelloMessages</tt> (as already discussed in
section <xref target="find_peer"/>). <tt>HelloMessages</tt>
are used to inform neighbors of a peer about the sender's
available addresses. The recipients use these messages to
inform their respective underlays about ways to sustain the
connections and to generate <tt>HELLO</tt> blocks (see <xref
target="hello_block"/>) to answer peer discovery queries
from other peers.
</t>
<section anchor="p2p_hello_wire">
<name>Wire Format</name>
<figure anchor="figure_hellomsg" title="The HelloMessage Wire Format.">
<artwork name="" type="" align="left" alt=""><![CDATA[
0 8 16 24 32 40 48 56
+-----+-----+-----+-----+-----+-----+-----+-----+
| MSIZE | MTYPE | VERSION | NUM_ADDRS |
+-----+-----+-----+-----+-----+-----+-----+-----+
| SIGNATURE /
/ (64 bytes) |
+-----+-----+-----+-----+-----+-----+-----+-----+
| EXPIRATION |
+-----+-----+-----+-----+-----+-----+-----+-----+
/ ADDRESSES (variable length) /
+-----+-----+-----+-----+-----+-----+-----+-----+
]]></artwork>
</figure>
<t>where:</t>
<dl>
<dt>MSIZE</dt>
<dd>
denotes the size of this message in network byte order.
</dd>
<dt>MTYPE</dt>
<dd>
is the 16-bit message type.
It must be set to
the value 157 in network byte order as defined in the GANA "GNUnet Message Type" registry
(see <xref target="gana_message_type"/>).
</dd>
<dt>VERSION</dt>
<dd>
is a 16-bit field that indicates the version of the <tt>HelloMessage</tt>. Must be zero.
In the future, this may be used to extend or update the <tt>HelloMessage</tt> format.
</dd>
<dt>NUM_ADDRS</dt>
<dd>
is a 16-bit number in network byte order that gives the
total number of addresses encoded in the
<tt>ADDRESSES</tt> field.
</dd>
<dt>SIGNATURE</dt>
<dd>
is a 64 byte EdDSA signature <xref target="ed25519"/> using the sender's private
key affirming the information contained in the message.
The signature is signing exactly the same data that is being
signed in a <tt>HELLO</tt> block as described in <xref target="hello_block"/>.
</dd>
<dt>EXPIRATION</dt>
<dd>
denotes the absolute 64-bit expiration date of the content.
The value specified is microseconds since midnight (0 hour),
January 1, 1970, but must be a multiple of one million
(so that it can be represented in seconds in a <tt>HELLO</tt> URL).
Stored in network byte order.
</dd>
<dt>ADDRESSES</dt>
<dd>
A sequence of exactly <tt>NUM_ADDRS</tt>
addresses which can be used to contact the peer.
Each address <bcp14>MUST</bcp14> be 0-terminated.
If <tt>NUM_ADDRS = 0</tt> then this field is omitted (0 bytes).
</dd>
</dl>
</section>
<section anchor="p2p_hello_processing">
<name>Processing</name>
<t>
If the initiator of a <tt>HelloMessage</tt> is <tt>SELF</tt>, the message
is simply sent to all neighbors <tt>P</tt> currently in the routing table
using the <tt>SEND()</tt> function of the underlay as defined in
<xref target="underlay"/>.
</t>
<t>
Upon receiving a <tt>HelloMessage</tt> from a peer <tt>P</tt>
an implementation <bcp14>MUST</bcp14> process it step by step as follows:
</t>
<ol>
<li>
If <tt>P</tt> is not in its routing table, the message is discarded.
</li>
<li>
The signature is verified, including a check that the expiration time
is in the future. If the signature is invalid, the message is discarded.
</li>
<li>
The information contained in the <tt>HelloMessage</tt>
is used to synthesize a block of type <tt>HELLO</tt>
(<xref target="hello_block"/>). The block is cached in
the routing table until it expires, or the peer is
removed from the routing table, or the information is
replaced by another message from the peer. The
implementation <bcp14>SHOULD</bcp14> instruct the
underlay to connect to all now available addresses using
<tt>TRY_CONNECT()</tt> in order to make the underlay
aware of alternative addresses for this connection and
to maintain optimal connectivity.
</li>
<li>
Received <tt>HelloMessages</tt> <bcp14>MUST NOT</bcp14>
be forwarded.
</li>
</ol>
</section>
</section>
<section anchor="p2p_put" numbered="true" toc="default">
<name>PutMessage</name>
<t>
<tt>PutMessages</tt> are used to store information at other
peers in the DHT. Any application-facing API which allows
applications to initiate <tt>PutMessages</tt> to store data
in the DHT needs to receive sufficient, possibly
implementation-specific information to construct the initial
<tt>PutMessage</tt>. In general, application-facing APIs
supporting multiple applications and block types need to be
given the block type (<tt>BTYPE</tt>) and message
<tt>FLAGS</tt> in addition to the actual <tt>BLOCK</tt>
payload. The <tt>BLOCK_KEY</tt> could be provided
explicitly, or in some cases might be derived using the
<tt>DeriveBlockKey()</tt> function from the block type
specific operations defined in <xref
target="block_functions"/>.
</t>
<section anchor="p2p_put_wire">
<name>Wire Format</name>
<figure anchor="figure_putmsg" title="The PutMessage Wire Format.">
<artwork name="" type="" align="left" alt=""><![CDATA[
0 8 16 24 32 40 48 56
+-----+-----+-----+-----+-----+-----+-----+-----+
| MSIZE | MTYPE | BTYPE |
+-----+-----+-----+-----+-----+-----+-----+-----+
| VER |FLAGS| HOPCOUNT | REPL_LVL | PATH_LEN |
+-----+-----+-----+-----+-----+-----+-----+-----+
| EXPIRATION |
+-----+-----+-----+-----+-----+-----+-----+-----+
| PEER_BF /
/ (128 byte) |
+-----+-----+-----+-----+-----+-----+-----+-----+
| BLOCK_KEY /
/ (64 byte) |
+-----+-----+-----+-----+-----+-----+-----+-----+
/ TRUNCATED ORIGIN (0 or 32 bytes) /
+-----+-----+-----+-----+-----+-----+-----+-----+
/ PUTPATH (variable length) /
+-----+-----+-----+-----+-----+-----+-----+-----+
/ LAST HOP SIGNATURE (0 or 64 bytes) /
+-----+-----+-----+-----+-----+-----+-----+-----+
/ BLOCK (variable length) /
+-----+-----+-----+-----+-----+-----+-----+-----+
]]></artwork>
</figure>
<t>where:</t>
<dl>
<dt>MSIZE</dt>
<dd>
denotes the size of this message in network byte order.
</dd>
<dt>MTYPE</dt>
<dd>
is the 16-bit message type. Read-only. It must be set
to the value 146 in network byte order as defined in the
GANA "GNUnet Message Type" registry <xref
target="gana_message_type"/>.
</dd>
<dt>BTYPE</dt>
<dd>
is a 32-bit block type in network byte order. The block
type indicates the content type of the payload. Set by
the initiator. Read-only.
</dd>
<dt>VER</dt>
<dd>
is a 8-bit protocol version.
Set to zero. May be used in future protocol versions.
</dd>
<dt>FLAGS</dt>
<dd>
is a 8-bit vector with binary options (see <xref target="route_flags"/>).
Set by the initiator. Read-only.
</dd>
<dt>HOPCOUNT</dt>
<dd>
is a 16-bit number in network byte order indicating how
many hops this message has traversed to far. Set by the
initiator to zero. <bcp14>MUST</bcp14> be incremented by one
by each peer before forwarding the request.
</dd>
<dt>REPL_LVL</dt>
<dd>
is a 16-bit number in network byte order indicating the
desired replication level of the data. Set by the
initiator. Read-only.
</dd>
<dt>PATH_LEN</dt>
<dd>
is a 16-bit number in network byte order indicating the
number of path elements recorded in <tt>PUTPATH</tt>. As <tt>PUTPATH</tt>
is optional, this value may be zero. If the <tt>PUTPATH</tt> is
enabled, set initially to zero by the initiator. Updated
by processing peers to match the path length in the
message.
</dd>
<dt>EXPIRATION</dt>
<dd>
denotes the absolute 64-bit expiration date of the
content in microseconds since midnight (0 hour), January
1, 1970 in network byte order. Set by the
initiator. Read-only.
</dd>
<dt>PEER_BF</dt>
<dd>
A peer Bloom filter to stop circular routes (see <xref
target="routing_bloomfilter"/>). Set by the initiator
to contain the local peer and all neighbors it is
forwarded to. Modified by processing peers to include
their own peer public key using <tt>BF-SET()</tt>.
</dd>
<dt>BLOCK_KEY</dt>
<dd>
The key under which the <tt>PutMessage</tt> wants to store content
under.
Set by the initiator. Read-only.
</dd>
<dt>TRUNCATED ORIGIN</dt>
<dd>
is only provided if the <tt>Truncated</tt> flag is set
in <tt>FLAGS</tt>. If present, this is the public key of
the peer just before the first entry on the
<tt>PUTPATH</tt> and the first peer on the
<tt>PUTPATH</tt> is not the actual origin of the
message. Thus, to verify the first signature on the
<tt>PUTPATH</tt>, this public key must be used. Note
that due to the truncation, this last hop cannot be
verified to exist. Value is modified by processing
peers.
</dd>
<dt>PUTPATH</dt>
<dd>
the variable-length <tt>PUT</tt> path.
The path consists of a list of <tt>PATH_LEN</tt> path elements.
Set by the initiator to zero.
Incremented by processing peers.
</dd>
<dt>LAST HOP SIGNATURE</dt>
<dd>
is only provided if the <tt>RecordRoute</tt> flag
is set in <tt>FLAGS</tt>. If present, this is
an EdDSA signature <xref target="ed25519"/> by the sender of this message
(using the same format as the signatures in PUTPATH)
affirming that the sender forwarded the message from
the predecessor (all zeros if <tt>PATH_LEN</tt> is zero,
otherwise the last peer in <tt>PUTPATH</tt>) to
the target peer.
Modified by processing peers (if flag is set).
</dd>
<dt>BLOCK</dt>
<dd>
the variable-length block payload. The contents are
determined by the <tt>BTYPE</tt> field. The length is
determined by <tt>MSIZE</tt> minus the size of all of
the other fields. Set by the initiator. Read-only.
</dd>
</dl>
</section>
<section anchor="p2p_put_processing">
<name>Processing</name>
<t>
Upon receiving a <tt>PutMessage</tt> from a peer <tt>P</tt>,
or created through initiation by an overlay API,
an implementation <bcp14>MUST</bcp14> process it step by step as follows:
</t>
<ol>
<li>
The <tt>EXPIRATION</tt> field is evaluated.
If the message is expired, it <bcp14>MUST</bcp14> be discarded.
</li>
<li>
If the <tt>BTYPE</tt> is <tt>ANY</tt>, then the message
<bcp14>MUST</bcp14> be discarded. If the <tt>BTYPE</tt>
is not supported by the implementation, no validation of
the block payload is performed and processing continues
at (5). Else, the block <bcp14>MUST</bcp14> be
validated as defined in (3) and (4).
</li>
<li>
The message is evaluated using the block validation
functions matching the <tt>BTYPE</tt>. First, the client
attempts to derive the key using the respective
<tt>DeriveBlockKey</tt> procedure as described in <xref
target="block_functions"/>. If a key can be derived and
does not match, the message <bcp14>MUST</bcp14> be
discarded.
</li>
<li>
Next, the <tt>ValidateBlockStoreRequest</tt> procedure
for the <tt>BTYPE</tt> as described in <xref
target="block_functions"/> is used to validate the block
payload. If the block payload is invalid, the message
<bcp14>MUST</bcp14> be discarded.
</li>
<li>
The peer identity of the sender peer <tt>P</tt>
<bcp14>SHOULD</bcp14> be in <tt>PEER_BF</tt>. If not,
the implementation <bcp14>MAY</bcp14> log an error, but
<bcp14>MUST</bcp14> continue.
</li>
<li>
If the <tt>RecordRoute</tt> flag is not set, the
<tt>PATH_LEN</tt> <bcp14>MUST</bcp14> be set to zero.
If the flag is set and <tt>PATH_LEN</tt> is non-zero,
the local peer <bcp14>SHOULD</bcp14> verify the
signatures from the <tt>PUTPATH</tt>. Verification
<bcp14>MAY</bcp14> involve checking all signatures or
any random subset of the signatures. It is
<bcp14>RECOMMENDED</bcp14> that peers adapt their
behavior to available computational resources so as to
not make signature verification a bottleneck. If an
invalid signature is found, the <tt>PUTPATH</tt>
<bcp14>MUST</bcp14> be truncated to only include the
elements following the invalid signature.
</li>
<li>
If the local peer is the closest peer
(cf. <tt>IsClosestPeer(SELF, BLOCK_KEY,
PeerFilter)</tt>) or the <tt>DemultiplexEverywhere</tt>
flag ist set, the message <bcp14>SHOULD</bcp14> be
stored locally in the block storage if possible. The
implementation <tt>MAY</tt> choose not store the block
if external factors or configurations prevent this, such
as limited (allotted) disk space.
</li>
<li>
If the <tt>BTYPE</tt> of the message indicates a
<tt>HELLO</tt> block, the peer <bcp14>MUST</bcp14> be
considered for the local routing table by using the peer
identity in <tt>BLOCK_KEY</tt>. If neither the peer is
already connected nor the respective k-bucket is already
full, then the peer <bcp14>MUST</bcp14> try to establish
a connection to the peer indicated in the <tt>HELLO</tt>
block using the address information from the
<tt>HELLO</tt> block and the underlay's
<tt>TRY_CONNECT()</tt> function. The implementation
<bcp14>MUST</bcp14> instruct the underlay to try to
connect to all provided addresses using
<tt>TRY_CONNECT()</tt> in order to make the underlay
aware of multiple addresses for this connection. When a
connection can be established, the underlay's
<tt>PEER_CONNECTED</tt> signal will cause the peer to be
added to the respective k-bucket of the routing table
(<xref target="routing"/>).
</li>
<li>
Given the value in <tt>REPL_LVL</tt>, <tt>HOPCOUNT</tt>
and <tt>FALSE = IsClosestPeer(SELF, BLOCK_KEY,
PeerFilter)</tt> the number of peers to forward to
<bcp14>MUST</bcp14> be calculated using
<tt>ComputeOutDegree()</tt>. The implementation
<bcp14>SHOULD</bcp14> select this number of peers
to forward the message to using the function
<tt>SelectPeer()</tt> (<xref
target="routing_functions"/>) using the
<tt>BLOCK_KEY</tt>, <tt>HOPCOUNT</tt>, and utilizing
<tt>PEER_BF</tt> as peer Bloom filter. For each
selected peer <tt>PEER_BF</tt> is updated with that peer
in between calls to <tt>SelectPeer()</tt>. The
implementation <bcp14>MAY</bcp14> forward to fewer or no
peers in order to handle resource constraints such as
limited bandwidth or simply because there are not enough
suitable connected peers. For each selected peer with
peer identity <tt>P</tt> a dedicated
<tt>PutMessage_P</tt> is created containing the original
(and where applicable already updated) fields of the
received <tt>PutMessage</tt>. In each message
<em>all</em> selected peer identities and the local peer
identity <bcp14>MUST</bcp14> be added to the
<tt>PEER_BF</tt> and the <tt>HOPCOUNT</tt>
<bcp14>MUST</bcp14> be incremented by one. If the
<tt>RecordRoute</tt> flag is set, a new path element is
created using the predecessor peer public key and the
signature of the current peer. The path element is
added to the <tt>PUTPATH</tt> fields and the
<tt>PATH_LEN</tt> field is incremented by one. When
creating the path element signature, the successor must
be set to the recipient peer <tt>P</tt> of the
<tt>PutMessage_P</tt>. The successor in the new path
element is the recipient peer <tt>P</tt>. If the path
becomes too long for the resulting message to be
transmitted by the underlay, it <bcp14>MUST</bcp14> be
truncated. Finally, the messages are sent using
<tt>SEND(P, PutMessage_P)</tt> to each recipient.
</li>
</ol>
</section>
</section>
<section anchor="p2p_get" numbered="true" toc="default">
<name>GetMessage</name>
<t>
<tt>GetMessages</tt> are used to request information from
other peers in the DHT. An application-level API which
allows applications to initiate <tt>GetMessages</tt> needs
to provide sufficient, implementation-specific information
needed to construct the initial <tt>GetMessage</tt>. For
example, implementations supporting multiple applications
and blocks will need to be given the block type, message
<tt>FLAG</tt> parameters and possibly an <tt>XQUERY</tt> in
addition to just the <tt>QUERY_HASH</tt>. In some cases, it
might also be useful to enable the application to assist in
the construction of the <tt>RESULT_FILTER</tt> such that it
can filter already known results. Note that the
<tt>RESULT_FILTER</tt> may need to be re-constructed every
time the query is retransmitted by the initiator (details
depending on the <tt>BTYPE</tt>) and thus a
<tt>RESULT_FILTER</tt> can often not be passed directly as
an argument by the application to an application API.
Instead, applications would typically provide the set of
results to be filtered, allowing the DHT to construct the
<tt>RESULT_FILTER</tt> whenever it retransmits a
<tt>GetMessage</tt> request as initiator.
</t>
<section anchor="p2p_get_wire">
<name>Wire Format</name>
<figure anchor="figure_getmsg" title="The GetMessage Wire Format.">
<artwork name="" type="" align="left" alt=""><![CDATA[
0 8 16 24 32 40 48 56
+-----+-----+-----+-----+-----+-----+-----+-----+
| MSIZE | MTYPE | BTYPE |
+-----+-----+-----+-----+-----+-----+-----+-----+
| VER |FLAGS| HOPCOUNT | REPL_LVL | RF_SIZE |
+-----+-----+-----+-----+-----+-----+-----+-----+
| PEER_BF /
/ (128 byte) |
+-----+-----+-----+-----+-----+-----+-----+-----+
| QUERY_HASH /
/ (64 byte) |
+-----+-----+-----+-----+-----+-----+-----+-----+
| RESULT_FILTER /
/ (variable length) /
+-----+-----+-----+-----+-----+-----+-----+-----+
/ XQUERY (variable length) /
+-----+-----+-----+-----+-----+-----+-----+-----+
]]></artwork>
</figure>
<t>where:</t>
<dl>
<dt>MSIZE</dt>
<dd>
denotes the size of this message in network byte order.
</dd>
<dt>MTYPE</dt>
<dd>
is the 16-bit message type. Read-only. It must be set
to the value 147 in network byte order as defined in the
GANA "GNUnet Message Type" registry <xref
target="gana_message_type"/>.
</dd>
<dt>BTYPE</dt>
<dd>
is a 32-bit block type field in network byte order. The
block type indicates the content type of the
payload. Set by the initiator. Read-only.
</dd>
<dt>VER</dt>
<dd>
is a 8-bit protocol version.
Set to zero. May be used in future protocol versions.
</dd>
<dt>FLAGS</dt>
<dd>
is a 8-bit vector with binary options (see <xref target="route_flags"/>).
Set by the initiator. Read-only.
</dd>
<dt>HOPCOUNT</dt>
<dd>
is a 16-bit number in network byte order indicating how
many hops this message has traversed to far. Set by the
initiator to zero. Incremented by each peer by one
per hop.
</dd>
<dt>REPL_LVL</dt>
<dd>
is a 16-bit number in network byte order indicating the
desired replication level of the data. Set by the
initiator. Read-only.
</dd>
<dt>RF_SIZE</dt>
<dd>
is a 16-bit number in network byte order indicating the
length of the <tt>RESULT_FILTER</tt>. Set by the
initiator. Read-only.
</dd>
<dt>PEER_BF</dt>
<dd>
A peer Bloom filter to stop circular routes (see <xref target="routing_bloomfilter"/>).
Set by the initiator to include itself and all connected neighbors in the routing table.
Modified by processing peers to include their own peer identity.
</dd>
<dt>QUERY_HASH</dt>
<dd>
The query used to indicate what the key is under which
the initiator is looking for blocks with this request.
The block type may use a different evaluation logic to
determine applicable result blocks. Set by the
initiator. Read-only.
</dd>
<dt>RESULT_FILTER</dt>
<dd>
the variable-length result filter with <tt>RF_SIZE</tt>
bytes as described in <xref target="result_filter"/>.
Set by the initiator. Modified by processing peers
based on results returned.
</dd>
<dt>XQUERY</dt>
<dd>
the variable-length extended query. Optional.
Set by the initiator. Read-only. The length is
determined by subtracting the length of all
other fields from <tt>MSIZE</tt>.
</dd>
</dl>
</section>
<section anchor="result_filter">
<name>Result Filter</name>
<t>
A result filter is used to indicate to other peers which
results are not of interest when processing a
<tt>GetMessage</tt> (<xref target="p2p_get"/>). Any peer
which is processing <tt>GetMessages</tt> and has a result
which matches the query key <bcp14>MUST</bcp14> check the
result filter and only send a reply message if the result
does not test positive under the result filter. Before
forwarding the <tt>GetMessage</tt>, the result filter
<bcp14>MUST</bcp14> be updated using the result of the
<tt>BTYPE</tt>-specific <tt>FilterResult</tt> (see <xref
target="block_functions"/>) function to filter out all
results already returned by the local peer.
</t>
<t>
How a result filter is implemented depends on the block type
as described in <xref target="block_functions"/>.
Result filters may be probabilistic data structures. Thus,
it is possible that a desireable result is filtered by a result
filter because of a false-positive test.
</t>
<t>
How exactly a block result is added to a result filter is
specified as part of the definition of a block type
(cf. <xref target="hello_block"/>).
</t>
</section>
<section anchor="p2p_get_processing">
<name>Processing</name>
<t>
Upon receiving a <tt>GetMessage</tt> from a peer <tt>P</tt>, or
created through initiation by the overlay API, an
implementation <bcp14>MUST</bcp14> process it step by step as follows:
</t>
<ol>
<li>
If the <tt>BTYPE</tt> is supported, the
<tt>QUERY_HASH</tt> and <tt>XQUERY</tt> fields are
validated as defined by the respective
<tt>ValidateBlockQuery()</tt> procedure for this type. If
the result yields <tt>REQUEST_INVALID</tt>, the message
<bcp14>MUST</bcp14> be discarded and processing ends.
If the <tt>BTYPE</tt> is not supported, the message
<bcp14>MUST</bcp14> be forwarded (Skip to step 4). If
the <tt>BTYPE</tt> is <tt>ANY</tt>, the message is
processed further without validation.
</li>
<li>
The peer identity of the sender peer <tt>P</tt>
<bcp14>SHOULD</bcp14> be in the <tt>PEER_BF</tt> peer
Bloom filter. If not, the implementation
<bcp14>MAY</bcp14> log an error, but <bcp14>MUST</bcp14>
continue.
</li>
<li>
<t>
The local peer <bcp14>SHOULD</bcp14> try to produce a
reply in any of the following cases: (1) If the local
peer is the closest peer (cf. <tt>IsClosestPeer(SELF,
QueryHash, PeerFilter)</tt>, or (2) if the
<tt>DemultiplexEverywhere</tt> flag is set, or (3) if
the local peer is not the closest and a previously
cached <tt>ResultMessage</tt> also matches this
request (<xref target="p2p_result_processing"/>).
</t>
<t>
The reply is produced (if one is available) using the following
steps:
</t>
<ol type="%c)">
<li>
If the <tt>BTYPE</tt> is <tt>HELLO</tt>, the
implementation <bcp14>MUST</bcp14> only consider
synthesizing its own addresses and the addresses it
has cached for the peers in its routing table as
<tt>HELLO</tt> block replies. Otherwise, if the
<tt>BTYPE</tt> does not indicate a request for a
<tt>HELLO</tt> block or <tt>ANY</tt>, the
implementation <bcp14>MUST</bcp14> only consider
blocks in the local block storage and previously
cached <tt>ResultMessages</tt>.
</li>
<li>
If the <tt>FLAGS</tt> field includes the flag
<tt>FindApproximate</tt>, the peer
<bcp14>SHOULD</bcp14> respond with the closest block
(smallest value of <tt>GetDistance(QUERY_HASH,
BLOCK_KEY)</tt>) it can find that is not filtered by
the <tt>RESULT_BF</tt>. Otherwise, the peer
<bcp14>MUST</bcp14> respond with the block with a
<tt>BLOCK_KEY</tt> that matches the
<tt>QUERY_HASH</tt> exactly and that is not filtered
by the <tt>RESULT_BF</tt>.
</li>
<li>
Any resulting (synthesized) block is encapsulated in
a <tt>ResultMessage</tt>. The
<tt>ResultMessage</tt> <bcp14>SHOULD</bcp14> be
transmitted to the neighbor from which the request
was received.
</li>
</ol>
<t>
Implementations <bcp14>MAY</bcp14> not reply if they
are resource-constrained. However,
<tt>ResultMessages</tt> <bcp14>MUST</bcp14> be given
the highest priority among competing transmissions.
</t>
<t>
If the <tt>BTYPE</tt> is supported and
<tt>ValidateBlockReply</tt> for the given query has
yielded a status of <tt>FILTER_LAST</tt>, processing
<bcp14>MUST</bcp14> end and not continue with
forwarding of the request to other peers.
</t>
</li>
<li>
The implementation <tt>MUST</tt> create (or merge) an
entry in the pending table (see <xref
target="pending_table"/>) for the query represented by
this <tt>GetMessage</tt>. The pending table
<bcp14>MUST</bcp14> store the last <tt>MAX_RECENT</tt>
requests, and peers thus <bcp14>MUST</bcp14> discard the
oldest existing request if memory constraints on the
pending table are encountered. Note that peers
<bcp14>MUST</bcp14> clean up state for queries that had
response with a status of <tt>FILTER_LAST</tt> even if
they are not the oldest query in the pending table.
</li>
<li>
Using the value in <tt>REPL_LVL</tt>, the number of
peers to forward to <bcp14>MUST</bcp14> be calculated
using <tt>ComputeOutDegree()</tt>. If there is at least
one peer to forward to, the implementation
<bcp14>SHOULD</bcp14> select up to this number of peers
to forward the message to. Furthermore, the
implementation <bcp14>SHOULD</bcp14> select up to this
number of peers to using the function
<tt>SelectPeer()</tt> (from <xref
target="routing_functions"/>) using the
<tt>QUERY_HASH</tt>, <tt>HOPCOUNT</tt>, and the
<tt>PEER_BF</tt>. The implementation <bcp14>MAY</bcp14>
forward to fewer or no peers in order to handle resource
constraints such as bandwidth. Before forwarding, the
peer Bloom filter <tt>PEER_BF</tt> <bcp14>MUST</bcp14>
be updated to filter all selected peers and the local
peer identity <tt>SELF</tt>. For all peers with peer
identity <tt>P</tt> chosen to forward the message to,
<tt>SEND(P, GetMessage_P)</tt> is called. Here,
<tt>GetMessage_P</tt> is the original message with the
updated fields for <tt>HOPCOUNT</tt> (incremented by
one), updated <tt>PEER_BF</tt> and updated
<tt>RESULT_FILTER</tt> (based on results already
returned).
</li>
</ol>
</section>
</section>
<section anchor="p2p_result" numbered="true" toc="default">
<name>ResultMessage</name>
<t>
<tt>ResultMessages</tt> are used to return information to
other peers in the DHT or to applications using the overlay
API that previously initiated a <tt>GetMessage</tt>. The
initiator of a <tt>ResultMessage</tt> is a peer triggered
through the processing of a <tt>GetMessage</tt>.
</t>
<section anchor="p2p_result_wire">
<name>Wire Format</name>
<figure anchor="figure_resmsg" title="The ResultMessage Wire Format">
<artwork name="" type="" align="left" alt=""><![CDATA[
0 8 16 24 32 40 48 56
+-----+-----+-----+-----+-----+-----+-----+-----+
| MSIZE | MTYPE | BTYPE |
+-----+-----+-----+-----+-----+-----+-----+-----+
| RESERVED | VER |FLAGS| PUTPATH_L | GETPATH_L |
+-----+-----+-----+-----+-----+-----+-----+-----+
| EXPIRATION |
+-----+-----+-----+-----+-----+-----+-----+-----+
| QUERY_HASH /
/ (64 byte) |
+-----+-----+-----+-----+-----+-----+-----+-----+
/ TRUNCATED ORIGIN (0 or 32 bytes) /
+-----+-----+-----+-----+-----+-----+-----+-----+
/ PUTPATH /
/ (variable length) /
+-----+-----+-----+-----+-----+-----+-----+-----+
/ GETPATH /
/ (variable length) /
+-----+-----+-----+-----+-----+-----+-----+-----+
/ LAST HOP SIGNATURE (0 or 64 bytes) /
+-----+-----+-----+-----+-----+-----+-----+-----+
/ BLOCK /
/ (variable length) /
+-----+-----+-----+-----+-----+-----+-----+-----+
]]></artwork>
</figure>
<t>where:</t>
<dl>
<dt>MSIZE</dt>
<dd>
denotes the size of this message in network byte order.
</dd>
<dt>MTYPE</dt>
<dd>
is the 16-bit message type. Set by the
initiator. Read-only. It must be set to the value 148
in network byte order as defined in the GANA "GNUnet
Message Type" registry (see <xref
target="gana_message_type"/>).
</dd>
<dt>BTYPE</dt>
<dd>
is a 32-bit block type field in network byte order. The
block type indicates the content type of the payload.
Set by the initiator. Read-only.
</dd>
<dt>RESERVED</dt>
<dd>
is a 16-bit value. Implementations <bcp14>MUST</bcp14>
set this value to zero when originating a result message.
Implementations <bcp14>MUST</bcp14> forward
this value unchanged even if it is non-zero.
</dd>
<dt>VER</dt>
<dd>
is a 8-bit protocol version in network byte order.
Set to zero. May be used in future protocol versions.
</dd>
<dt>FLAGS</dt>
<dd>
is a 8-bit vector with binary options (see <xref
target="route_flags"/>). Set by the initiator of the
response based on the flags retained from the original
<tt>PutMessage</tt>, possibly setting the
<tt>Truncated</tt> bit if the initiator is forced to
truncate the path. For <tt>HELLO</tt> blocks, the
<tt>FLAGS</tt> should simply be cleared.
</dd>
<dt>PUTPATH_L</dt>
<dd>
is a 16-bit number in network byte order indicating the
number of path elements recorded in <tt>PUTPATH</tt>. As
<tt>PUTPATH</tt> is optional, this value may be zero
even if the message has traversed several peers. Set by
the initiator to the <tt>PATH_LEN</tt> of the
<tt>PutMessage</tt> from which the block originated.
Modified by processing peers in case of path truncation.
</dd>
<dt>GETPATH_L</dt>
<dd>
is a 16-bit number in network byte order indicating the
number of path elements recorded in <tt>GETPATH</tt>. As
<tt>GETPATH</tt> is optional, this value may be zero
even if the message has traversed several peers.
<bcp14>MUST</bcp14> be set to zero by the initiator.
Modified by processing peers to match the path length in
the message.
</dd>
<dt>EXPIRATION</dt>
<dd>
denotes the absolute 64-bit expiration date of the
content in microseconds since midnight (0 hour), January
1, 1970 in network byte order. Set by the initiator to
the expiration value as recorded from the
<tt>PutMessage</tt> from which the block originated.
Read-only.
</dd>
<dt>QUERY_HASH</dt>
<dd>
the query hash corresponding to the <tt>GetMessage</tt>
which caused this reply message to be sent. Set by the
initiator using the value of the <tt>GetMessage</tt>.
Read-only.
</dd>
<dt>TRUNCATED ORIGIN</dt>
<dd>
is only provided if the <tt>Truncated</tt> flag is set
in <tt>FLAGS</tt>. If present, this is the public key of
the peer just before the first entry on the
<tt>PUTPATH</tt> and the first peer on the
<tt>PUTPATH</tt> is not the actual origin of the
message. Thus, to verify the first signature on the
<tt>PUTPATH</tt>, this public key must be used. Note
that due to the truncation, this last hop cannot be
verified to exist. Set by processing peers.
</dd>
<dt>PUTPATH</dt>
<dd>
the variable-length PUT path. The path consists of a
list of <tt>PUTPATH_L</tt> path elements. Set by the
initiator to the the <tt>PUTPATH</tt> of the
<tt>PutMessage</tt> from which the block originated.
Modified by processing peers in case of path truncation.
</dd>
<dt>GETPATH</dt>
<dd>
the variable-length PUT path. The path consists of a
list of <tt>GETPATH_L</tt> path elements. Set by
processing peers.
</dd>
<dt>LAST HOP SIGNATURE</dt>
<dd>
is only provided if the <tt>RecordRoute</tt> flag is set
in <tt>FLAGS</tt>. If present, this is an EdDSA
signature <xref target="ed25519"/> by the sender of this
message (using the same format as the signatures in
<tt>PUTPATH</tt>) affirming that the sender forwarded
the message from the predecessor (all zeros if
<tt>PATH_LEN</tt> is zero, otherwise the last peer in
<tt>PUTPATH</tt>) to the target peer.
</dd>
<dt>BLOCK</dt>
<dd>
the variable-length resource record data payload. The
contents are defined by the respective type of the
resource record. Set by the initiator. Read-only.
</dd>
</dl>
</section>
<section anchor="p2p_result_processing">
<name>Processing</name>
<t>
Upon receiving a <tt>ResultMessage</tt> from a connected
peer or triggered by the processing of a
<tt>GetMessage</tt>, an implementation <bcp14>MUST</bcp14>
process it step by step as follows:
</t>
<ol>
<li>
First, the <tt>EXPIRATION</tt> field is evaluated. If
the message is expired, it <bcp14>MUST</bcp14> be
discarded.
</li>
<li>
If the <tt>BTYPE</tt> is supported, then the
<tt>BLOCK</tt> <bcp14>MUST</bcp14> be validated against
the requested <tt>BTYPE</tt>. To do this, the peer
checks that the block is valid using
<tt>ValidateBlockStoreRequest</tt>. If the result is
<tt>BLOCK_INVALID</tt>, the message <bcp14>MUST</bcp14>
be discarded.
</li>
<li>
If the <tt>PUTPATH_L</tt> or the <tt>GETPATH_L</tt> are
non-zero, the local peer <bcp14>SHOULD</bcp14> verify
the signatures from the <tt>PUTPATH</tt> and the
<tt>GETPATH</tt>. Verification <bcp14>MAY</bcp14>
involve checking all signatures or any random subset of
the signatures. It is <bcp14>RECOMMENDED</bcp14> that
peers adapt their behavior to available computational
resources so as to not make signature verification a
bottleneck. If an invalid signature is found, the path
<bcp14>MUST</bcp14> be truncated to only include the
elements following the invalid signature. In
particular, any invalid signature on the
<tt>GETPATH</tt> will cause <tt>PUTPATH_L</tt> to be set
to zero.
</li>
<li>
The peer also attempts to compute the key using
<tt>DeriveBlockKey</tt>. This may result in
<tt>NONE</tt>. The result is used later. Note that
even if a key was computed, it does not have to match
the <tt>QUERY_HASH</tt>.
</li>
<li>
If the <tt>BTYPE</tt> of the message indicates a
<tt>HELLO</tt> block, the peer <bcp14>SHOULD</bcp14> be
considered for the local routing table by using the peer
identity computed from the block using
<tt>DeriveBlockKey</tt>. An implementation
<bcp14>MAY</bcp14> choose to ignore the <tt>HELLO</tt>,
for example because the routing table or the respective
k-bucket is already full. If the peer is a suitable
candidate for insertion, the local peer
<bcp14>MUST</bcp14> try to establish a connection to the
peer indicated in the <tt>HELLO</tt> block using the
address information from the <tt>HELLO</tt> block and
the underlay's <tt>TRY_CONNECT()</tt> function. The
implementation <bcp14>MUST</bcp14> instruct the underlay
to connect to all provided addresses using
<tt>TRY_CONNECT()</tt> in order to make the underlay
aware of multiple addresses for this connection. When a
connection is established, the signal
<tt>PEER_CONNECTED</tt> will cause the peer to be added
to the respective k-bucket of the routing table (see
<xref target="routing"/>).
</li>
<li>
If the <tt>QUERY_HASH</tt> of this
<tt>ResultMessage</tt> does not match an entry in the
pending table (<xref target="pending_table"/>), then the
message is discarded and processing ends. Otherwise,
processing continues for each entry in the table as
follows.
</li>
<li>
<ol type="%c)">
<li>
If the <tt>FindApproximate</tt> flag was not set in
the query and the <tt>BTYPE</tt> enabled the
implementation to compute the key from the block,
the computed key must exactly match the
<tt>QUERY_HASH</tt>, otherwise the result does not
match the pending query and processing continues
with the next pending table entry.
</li>
<li>
If the <tt>BTYPE</tt> is supported, result block
<bcp14>MUST</bcp14> be validated against the
specific query using the respective
<tt>FilterBlockResult</tt> function. This function
<bcp14>MUST</bcp14> update the result filter if a
result is returned to the originator of the query.
</li>
<li>
If the <tt>BTYPE</tt> is not supported, filtering of
exact duplicate replies <bcp14>MUST</bcp14> still be
performed before forwarding the reply. Such
duplicate filtering <bcp14>MAY</bcp14> be
implemented probabilistically, for example using a
Bloom filter. The result of this duplicate
filtering is always either <tt>FILTER_MORE</tt> or
<tt>FILTER_DUPLICATE</tt>.
</li>
<li>
If the <tt>RecordRoute</tt> flag is set in
<tt>FLAGS</tt>, the local peer identity
<bcp14>MUST</bcp14> be appended to the
<tt>GETPATH</tt> of the message and the respective
signature <bcp14>MUST</bcp14> be set using the query
origin as the <tt>PEER SUCCESSOR</tt> and the
response origin as the <tt>PEER PREDECESSOR</tt>.
If the flag is not set, the <tt>GETPATH_L</tt> and
<tt>PUTPATH_L</tt> <bcp14>MUST</bcp14> be set to
zero when forwarding the result.
</li>
<li>
If the result filter result is either
<tt>FILTER_MORE</tt> or <tt>FILTER_LAST</tt>, the
message is forwarded to the origin of the query as
defined in the entry which may either be the local
peer or a remote peer. In case this is a query of
the local peer the result may have to be provided to
applications through the overlay API. Otherwise,
the result is forwarded using <tt>SEND(P,
ResultMessage')</tt> where <tt>ResultMessage'</tt>
is the now modified message. If the result was
<tt>FILTER_LAST</tt>, the query <bcp14>MUST</bcp14>
be removed from the pending table.
</li>
</ol>
</li>
<li>
Finally, the implementation <bcp14>SHOULD</bcp14> cache
<tt>ResultMessages</tt> in order to provide already seen
replies to future <tt>GetMessages</tt>. The
implementation <bcp14>MAY</bcp14> choose not no cache
any or a limited number of <tt>ResultMessages</tt> for
reasons such as resource limitations.
</li>
</ol>
</section>
</section>
</section>
<section anchor="blockstorage" numbered="true" toc="default">
<name>Blocks</name>
<t>
This section describes various considerations R<sup>5</sup>N
implementations must consider with respect to blocks.
Specifically, implementations <bcp14>SHOULD</bcp14> be validate and persist
blocks.
Implementations
<bcp14>MAY</bcp14> not support validation for all types
of blocks.
For example, on some devices, storing blocks is impossible due to lack of
storage capacity.
Block storage improves lookup performance for local applications and also
other peers. Not storing blocks results in degraded performance.
</t>
<t>
The block type determines the format and handling of the block
payload by peers in <tt>PutMessages</tt> and
<tt>ResultMessages</tt>. Applications can and should define
their own block types. Block types <bcp14>MUST</bcp14> be
registered with GANA (see <xref target="gana_block_type"/>).
Especially when new block types are introduced, some peers
<bcp14>MAY</bcp14> lack support for the respective block
operations.
</t>
<t>
</t>
<section anchor="block_functions">
<name>Block Operations</name>
<t>
Block validation operations are used as part of message
processing (see <xref target="p2p_messages"/>) for all
types of DHT messages. To enable these validation
operations, any block type specification
<bcp14>MUST</bcp14> define the following functions:
</t>
<dl>
<dt>ValidateBlockQuery(Key, XQuery)
-> RequestEvaluationResult</dt>
<dd>
<t>
is used to evaluate the request for a block as part of
<tt>GetMessage</tt> processing. Here, the block payload is unkown,
but if possible the <tt>XQuery</tt> and <tt>Key</tt>
<bcp14>SHOULD</bcp14> be verified. Possible values for
the <tt>RequestEvaluationResult</tt> are:
</t>
<dl>
<dt>REQUEST_VALID</dt>
<dd>
The query is valid.
</dd>
<dt>REQUEST_INVALID</dt>
<dd>
The query format does not match the block type. For example,
a mandatory <tt>XQuery</tt> was not provided, or of
the size of the <tt>XQuery</tt> is not appropriate
for the block type.
</dd>
</dl>
</dd>
<dt>DeriveBlockKey(Block) -> Key | NONE</dt>
<dd>
is used to synthesize the block key from the block
payload as part of <tt>PutMessage</tt> and
<tt>ResultMessage</tt> processing. The special return
value of <tt>NONE</tt> implies that this block type does
not permit deriving the key from the block. A
<tt>Key</tt> may be returned for a block that is
ill-formed.
</dd>
<dt>ValidateBlockStoreRequest(Block)
-> BlockEvaluationResult</dt>
<dd>
<t>
is used to evaluate a block payload as part of
<tt>PutMessage</tt> and <tt>ResultMessage</tt>
processing. Possible values for the
<tt>BlockEvaluationResult</tt> are:
</t>
<dl>
<dt>BLOCK_VALID</dt>
<dd>
The block is valid.
</dd>
<dt>BLOCK_INVALID</dt>
<dd>
The block payload does not match the block type.
</dd>
</dl>
</dd>
<dt>SetupResultFilter(FilterSize, Mutator) -> RF</dt>
<dd>
is used to setup an empty result filter. The arguments
are typically the size of the set of results that must
be filtered at the initiator, and a <tt>Mutator</tt>
value which <bcp14>MAY</bcp14> be used to
deterministically re-randomize probabilistic data
structures. <tt>RF</tt> <bcp14>MUST</bcp14> be a byte
sequence suitable for transmission over the network.
</dd>
<dt>FilterResult(Block, Key, RF, XQuery) -> (FilterEvaluationResult, RF')</dt>
<dd>
<t>
is used to filter results against specific queries.
This function does not check the validity of
<tt>Block</tt> itself or that it matches the given key,
as this must have been checked earlier. Locally
stored blocks from previously observed
<tt>ResultMessages</tt> and <tt>PutMessages</tt> use
this function to perform filtering based on the request
parameters of a particular GET operation. Possible
values for the <tt>FilterEvaluationResult</tt> are:
</t>
<dl>
<dt>FILTER_MORE</dt>
<dd>
<tt>Block</tt> is a valid result, and there may be more.
</dd>
<dt>FILTER_LAST</dt>
<dd>
The given <tt>Block</tt> is the last possible valid result.
</dd>
<dt>FILTER_DUPLICATE</dt>
<dd>
<tt>Block</tt> is a valid result, but considered to be
a duplicate (was filtered by the <tt>RF</tt>) and
<bcp14>SHOULD NOT</bcp14> be returned to the previous
hop. Peers that do not understand the block type
<bcp14>MAY</bcp14> return such duplicate results
anyway and implementations must take this into account.
</dd>
<dt>FILTER_IRRELEVANT</dt>
<dd>
<tt>Block</tt> does not satisfy the constraints
imposed by the <tt>XQuery</tt>. The result
<bcp14>SHOULD NOT</bcp14> be returned to the previous
hop. Peers that do not understand the block type
<bcp14>MAY</bcp14> return such irrelevant results
anyway and implementations must take this into account.
</dd>
</dl>
<t>
If the main evaluation result is <tt>FILTER_MORE</tt>,
the function also returns an updated result filter
where the <tt>Block</tt> is added to the set of
filtered replies. An implementation is not expected
to actually differenciate between the
<tt>FILTER_DUPLICATE</tt> and
<tt>FILTER_IRRELEVANT</tt> return values: in both
cases the <tt>Block</tt> is ignored for this query.
</t>
</dd>
</dl>
</section>
<section anchor="hello_block">
<name>HELLO Blocks</name>
<t>
For bootstrapping and peer discovery, the DHT
implementation uses its own block type called "HELLO".
<tt>HELLO</tt> blocks are the only type of block that
<bcp14>MUST</bcp14> be supported by every R<sup>5</sup>N
implementation. A block with this block type contains the
peer public key of the peer that published the
<tt>HELLO</tt> together with a set of addresses of this
peer. The key of a <tt>HELLO</tt> block is the SHA-512
hash <xref target="RFC4634"/> of the peer public key and
thus the peer's identity in the DHT.
</t>
<t>
The <tt>HELLO</tt> block type wire format is illustrated
in <xref target="figure_hello"/>. A query for block of
type <tt>HELLO</tt> <bcp14>MUST NOT</bcp14> include
extended query data (XQuery). Any implementation
encountering a request for a <tt>HELLO</tt> with non-empty
XQuery data <bcp14>MUST</bcp14> consider the request
invalid and ignore it.
</t>
<figure anchor="figure_hello" title="The HELLO Block Format.">
<artwork name="" type="" align="left" alt=""><![CDATA[
0 8 16 24 32 40 48 56
+-----+-----+-----+-----+-----+-----+-----+-----+
| PEER PUBLIC KEY |
| (32 byte) |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
| SIGNATURE |
| (64 byte) |
| |
| |
| |
| |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
| EXPIRATION |
+-----+-----+-----+-----+-----+-----+-----+-----+
/ ADDRESSES /
/ (variable length) /
+-----+-----+-----+-----+-----+-----+-----+-----+
]]></artwork>
</figure>
<dl>
<dt>PEER PUBLIC KEY</dt>
<dd>
is the public key of the peer to which the
<tt>ADDRESSES</tt> belong. This is also the public key
needed to verify the <tt>SIGNATURE</tt>.
</dd>
<dt>EXPIRATION</dt>
<dd>
denotes the absolute 64-bit expiration date of the
content. The value specified is microseconds since
midnight (0 hour), January 1, 1970 in network byte
order, but must be a multiple of one million (so that it
can be represented in seconds in a <tt>HELLO</tt> URL).
</dd>
<dt>ADDRESSES</dt>
<dd>
is a list of UTF-8 addresses (<xref target="terminology"/>)
which can be used to contact the peer.
Each address <bcp14>MUST</bcp14> be 0-terminated.
The set of addresses <bcp14>MAY</bcp14> be empty, for example
if the peer knows that it cannot be reached from the outside (i.e. NAT).
</dd>
<dt>SIGNATURE</dt>
<dd>
<t>
is the EdDSA signature <xref target="ed25519"/> of the
<tt>HELLO</tt> block. The signature covers various
information derived from the actual data in the
<tt>HELLO</tt> block. The data signed over includes the
block expiration time, a constant that uniquely
identifies the purpose of the signature, and a hash of
the addresses with 0-terminators in the same order as
they are present in the <tt>HELLO</tt> block. The
format is illustrated in <xref
target="figure_hellowithpseudo"/>.
</t>
<figure anchor="figure_hellowithpseudo" title="The Wire Format of the HELLO for Signing.">
<artwork name="" type="" align="left" alt=""><![CDATA[
0 8 16 24 32 40 48 56
+-----+-----+-----+-----+-----+-----+-----+-----+
| SIZE | PURPOSE |
+-----+-----+-----+-----+-----+-----+-----+-----+
| EXPIRATION |
+-----+-----+-----+-----+-----+-----+-----+-----+
| H_ADDRS |
| (64 byte) |
| |
| |
| |
| |
| |
| |
+-----+-----+-----+-----+-----+-----+-----+-----+
]]></artwork>
</figure>
<dl>
<dt>SIZE</dt>
<dd>
A 32-bit value containing the length of the signed data
in bytes in network byte order. The length of the
signed data <bcp14>MUST</bcp14> be 80 bytes.
</dd>
<dt>PURPOSE</dt>
<dd>
A 32-bit signature purpose flag. This field
<bcp14>MUST</bcp14> be 7 in network byte order.
</dd>
<dt>EXPIRATION</dt>
<dd>
denotes the absolute 64-bit expiration date of the
<tt>HELLO</tt> in microseconds since midnight (0 hour),
January 1, 1970 in network byte order.
</dd>
<dt>H_ADDRS</dt>
<dd>
a SHA-512 hash over the addresses in the <tt>HELLO</tt>.
<tt>H_ADDRS</tt> is generated over the
<tt>ADDRESSES</tt> field as provided in the
<tt>HELLO</tt> block using SHA-512 <xref
target="RFC4634"/>.
</dd>
</dl>
</dd>
</dl>
<t>
The <tt>HELLO</tt> block operations <bcp14>MUST</bcp14> be
implemented as follows:
</t>
<dl>
<dt>ValidateBlockQuery(Key, XQuery)
-> RequestEvaluationResult</dt>
<dd>
To validate a block query for a <tt>HELLO</tt> is to
simply check that the <tt>XQuery</tt> is empty. If it is empty,
<tt>REQUEST_VALID</tt> ist returned. Otherwise,
<tt>REQUEST_INVALID</tt> is returned.
</dd>
<dt>DeriveBlockKey(Block) -> Key | NONE</dt>
<dd>
To derive a block key for a <tt>HELLO</tt> is to simply
hash the <tt>PEER PUBLIC KEY</tt> from the <tt>HELLO</tt>. The
result of this function is thus always the SHA-512 hash over
the <tt>PEER PUBLIC KEY</tt>.
</dd>
<dt>ValidateBlockStoreRequest(Block)
-> BlockEvaluationResult</dt>
<dd>
To validate a block store request is to verify the EdDSA
<tt>SIGNATURE</tt> over the hashed <tt>ADDRESSES</tt>
against the public key from the <tt>PEER PUBLIC KEY</tt>
field. If the signature is valid <tt>BLOCK_VALID</tt> is
returned. Otherwise <tt>BLOCK_INVALID</tt> is returned.
</dd>
<dt>SetupResultFilter(FilterSize, Mutator) -> RF</dt>
<dd>
<t>
The <tt>RF</tt> for <tt>HELLO</tt> blocks is implemented
using a Bloom filter following the definition from <xref
target="bloom_filters"/> and consists of a variable number
of bits <tt>L</tt>. <tt>L</tt> depends on the
<tt>FilterSize</tt> which will be the number of
connected peers <tt>|E|</tt> known to the peer creating a
<tt>HELLO</tt> block from its own addresses: <tt>L</tt> is
set to the minimum of 2<sup>18</sup> bits (2<sup>15</sup>
bytes) and the lowest power of 2 that is strictly larger
than <tt>2*K*|E|</tt> bits (<tt>K*|E|/4</tt> bytes).
</t>
<t>
The <tt>k</tt>-value for the Bloom filter
<bcp14>MUST</bcp14> be 16. The elements used in the Bloom
filter consist of an XOR between the <tt>H_ADDRS</tt>
field (as computed using SHA-512 over the
<tt>ADDRESSES</tt>) and the SHA-512 hash of the
<tt>MUTATOR</tt> field from a given <tt>HELLO</tt> block.
The mapping function M(<tt>H_ADDRS XOR MUTATOR</tt>) is
defined as follows:
</t>
<t>
<tt>M(e = H_ADDR XOR MUTATOR) -> e as uint32[]</tt>
</t>
<t>
<tt>M</tt> is an identity function and returns the 512-bit
XOR result unmodified. This resulting byte string is
interpreted as k=16 32-bit integers in network byte order
which are used to set and check the bits in <tt>B</tt>
using <tt>BF-SET()</tt> and <tt>BF-TEST()</tt>. The
32-bit <tt>MUTATOR</tt> is prepended to the L-bit Bloom filter
field <tt>HELLO_BF</tt> containing <tt>B</tt> to create
the result filter for a <tt>HELLO</tt> block:
</t>
<figure anchor="hello_rf" title="The HELLO Block Result Filter.">
<artwork name="" type="" align="left" alt=""><![CDATA[
0 8 16 24 32 40 48 56
+-----+-----+-----+-----+-----+-----+-----+-----+
| MUTATOR | HELLO_BF /
+-----+-----+-----+-----+ (variable length) /
/ /
+-----+-----+-----+-----+-----+-----+-----+-----+
]]></artwork>
</figure>
<t>where:</t>
<dl>
<dt>MUTATOR</dt>
<dd>
The 32-bit mutator for the result filter.
</dd>
<dt>HELLO_BF</dt>
<dd>
The L-bit Bloom filter array.
</dd>
</dl>
<t>
The <tt>MUTATOR</tt> value is used to additionally
"randomize" the computation of the Bloom filter while
remaining deterministic across peers. It is only ever set
by the peer initiating the GET request, and changed every
time the GET request is repeated. Peers forwarding GET
requests <bcp14>MUST</bcp14> not change the mutator value
included in the <tt>RESULT_FILTER</tt> as they might not
be able to recalculate the result filter with a different
<tt>MUTATOR</tt> value.
</t>
<t>
Consequently, repeated requests have statistically
independent probabilities of creating false-positives in a
result filter. Thus, even if for one request a result
filter may exclude a result as a false-positive match,
subsequent requests are likely to not have the same
false-positives.
</t>
<t>
<tt>HELLO</tt> result filters can be merged if the Bloom
filters have the same size and <tt>MUTATOR</tt> by setting
all bits to 1 that are set in either Bloom filter. This
is done whenever a peer receives a query with the same
<tt>MUTATOR</tt>, predecessor and Bloom filter size.
</t>
</dd>
<dt>FilterResult(Block, Key, RF, XQuery) -> (FilterEvaluationResult, RF')</dt>
<dd>
The <tt>H_ADDRS</tt> field is XORed with the SHA-512 hash
of the <tt>MUTATOR</tt> field from the <tt>HELLO</tt>
block and the resulting value is checked against the
Bloom filter in <tt>RF</tt>. Consequently,
<tt>HELLOs</tt> with completely identical sets of
addresses will be filtered and <tt>FILTER_DUPLICATE</tt>
is returned. Any small variation in the set of addresses
will cause the block to no longer be filtered (with high
probability) and <tt>FILTER_MORE</tt> is returned.
</dd>
</dl>
</section>
<section>
<name>Persistence</name>
<t>
An implementation <bcp14>SHOULD</bcp14> provide a local
persistence mechanism for blocks. Embedded systems that
lack storage capability <bcp14>MAY</bcp14> use
connection-level signalling to indicate that they are merely
a client utilizing a DHT and are not able to participate
with storage. The local storage <bcp14>MUST</bcp14> provide
the following functionality:
</t>
<dl>
<dt>Store(Key, Block)</dt>
<dd>
Stores a block under the specified key. If an block with identical
payload exists already under the same key, the meta data should
be set to the maximum expiration time of both blocks and use the
corresponding <tt>PUTPATH</tt> (and if applicable
<tt>TRUNCATED ORIGIN</tt>) of that version of the block.
</dd>
<dt>Lookup(Key) -> Set of Blocks</dt>
<dd>
Retrieves blocks stored under the specified key.
</dd>
<dt>LookupApproximate(Key) -> List of Blocks</dt>
<dd>
Retrieves the blocks stored under the specified key and
any blocks under keys close to the specified key, in order
of decreasing proximity.
</dd>
</dl>
<section anchor="approx_search">
<name>Approximate Search Considerations</name>
<t>
Over time a peer may accumulate a significant number of blocks
which are stored locally in the persistence layer.
Due to the expected high number of blocks, the method to
retrieve blocks close to the specified lookup key in the
<tt>LookupApproximate</tt> API must be implemented with care
with respect to efficiency.
</t>
<t>
It is <bcp14>RECOMMENDED</bcp14> to limit the number of
results from the <tt>LookupApproximate</tt> procedure to a
result size which is easily manageable by the local system.
The <bcp14>RECOMMENDED</bcp14> default is to return blocks
with the four closest keys. Note that filtering by the
<tt>RF</tt> will be done by the DHT afterwards and it is
<bcp14>NOT RECOMMENDED</bcp14> to fetch additional records
even if all four closest keys are filtered by the
<tt>RF</tt>. The main reason for this is to ensure peers do
not spend extensive resources to process approximate
lookups. In particular, implementations <bcp14>MUST</bcp14>
limit the worst-case effort they spent on approximate
lookups.
</t>
<t>
In order to efficiently find a suitable result set, the
implementation <bcp14>SHOULD</bcp14> follow the following
procedure:
</t>
<ol>
<li>
Sort all blocks by the block key in ascending (decending) order.
The block keys are interpreted as integers.
</li>
<li>
Alternatingly select a block with a key larger and smaller
from the sortings. The resulting set is then sorted by
the XOR distance to the query. The selection process
continues until the upper bound for the result set is
reached and both sortings do not yield any closer blocks.
</li>
</ol>
<t>
An implementation <bcp14>MAY</bcp14> decide to use a custom
algorithm in order to find the closest blocks in the local
storage. But, especially for more primitive approaches
(such as only comparing XOR distances for all blocks in the
storage), more simplistic procedures may become ineffective
for large data sets and care <bcp14>MUST</bcp14> be taken
to strictly bound the maximum effort expended per query.
</t>
</section>
<section>
<name>Caching Strategy Considerations</name>
<t>
An implementation <bcp14>MUST</bcp14> implement an
eviction strategy for blocks stored in the block storage
layer.
</t>
<t>
In order to ensure the freshness of blocks, an implementation
<bcp14>MUST</bcp14> evict expired blocks in favor of
new blocks.
</t>
<t>
An implementation <bcp14>MAY</bcp14> preserve blocks which
are often requested. This approach can be expensive as it
requires the implementation to keep track of how often a
block is requested.
</t>
<t>
An implementation <bcp14>MAY</bcp14> preserve blocks which
are close to the local peer public key.
</t>
<t>
An implementation <bcp14>MAY</bcp14> provide configurable
storage quotas and adapt its eviction strategy based on
the current storage size or other constrained resources.
</t>
</section>
</section>
</section>
<section anchor="security" numbered="true" toc="default">
<name>Security Considerations</name>
<!-- FIXME: Here we should (again) discuss how the system is open and
does not have/require a trust anchor a priori. This is (again) in contrast
to RELOAD -->
<t>
If an upper bound to the maximum number of neighbors in a
k-bucket is reached, the implementation <bcp14>MUST</bcp14>
prefer to preserve the oldest working connections instead of
new connections. This makes Sybil attacks <xref
target="Sybil"/> less effective as an adversary would have to
invest more resources over time to mount an effective attack.
</t>
<t>
The <tt>ComputeOutDegree</tt> function limits the
<tt>REPL_LVL</tt> to a maximum of 16. This imposes
an upper limit on bandwidth amplification an attacker
may achieve for a given network size and topology.
</t>
<section>
<name>Disjoint Underlay or Application Protocol Support</name>
<t>
We note that peers
implementing disjoint sets of underlay protocols may
experience difficulties communicating (unless other peers
bridge the respective underlays). Similarly, peers that
do not support a particular application will not be able
to validate application-specific payloads and may thus be
tricked into storing or forwarding corrupt blocks.
</t>
</section>
<section>
<name>Approximate Result Filtering</name>
<t>
When a <tt>FindApproximate</tt> flag is encountered in a
query, a peer will try to respond with the closest block it
has that is not filtered by the result Bloom filter
(<tt>RF</tt>). Implementations <bcp14>MUST</bcp14> ensure
that the cost of evaluating any such query is reasonably
small. For example, implementations <bcp14>SHOULD</bcp14>
consider ways to avoid an exhaustive search of their
database. Not doing so can lead to denial of service
attacks as there could be cases where too many local results
are filtered by the result filter.
</t>
</section>
<section>
<name>Access Control</name>
<t>
By design R<sup>5</sup>N does not rely on strict admission
control through the use of either centralized enrollment
servers or pre-shared keys. This is a key distintion over
protocols that do rely on this kind of access control such
as <xref target="RFC6940"/> which, like R<sup>5</sup>N,
provides a peer-to-peer (P2P) signaling protocol with
extensible routing and topology mechanisms. Some
decentralized applications, such as the GNU Name System
(<xref target="RFC9498"/>), require an open system that
enables ad-hoc participation.
</t>
</section>
</section>
<section anchor="iana" numbered="true" toc="default">
<name>IANA Considerations</name>
<t>
IANA maintains a registry called the "Uniform Resource Identifier
(URI) Schemes" registry.
The registry should be updated to include
an entry for the 'gnunet' URI scheme. IANA is requested to
update that entry to reference this document when published
as an RFC.
</t>
</section>
<section anchor="gana" numbered="true" toc="default">
<name>GANA Considerations</name>
<section anchor="gana_block_type" numbered="true" toc="default">
<name>Block Type Registry</name>
<t>
GANA <xref target="GANA"/>
is requested to create a "DHT Block Types" registry.
The registry shall record for each entry:
</t>
<ul>
<li>Name: The name of the block type (case-insensitive ASCII
string, restricted to alphanumeric characters</li>
<li>Number: 32-bit</li>
<li>Comment: Optionally, a brief English text describing the purpose of
the block type (in UTF-8)</li>
<li>Contact: Optionally, the contact information of a person to contact for
further information</li>
<li>References: Required, references (such as an RFC) specifying the block type and its block functions</li>
</ul>
<t>
The registration policy for this sub-registry is "First Come First
Served", as described in <xref target="RFC8126"/>.
GANA created the registry as follows:
</t>
<!-- NOTE: changed GNS Reference to This.I-D because we either need to define it here
or in the GNS RFC. And I think here is better or in a separate document
=> not in here. Use separate document for NAMERECORD draft.
-MSC -->
<figure anchor="figure_btypenums" title="The Block Type Registry.">
<artwork name="" type="" align="left" alt=""><![CDATA[
Number| Name | References | Description
------+----------------+------------+-------------------------
0 ANY [This.I-D] Reserved
13 DHT_HELLO [This.I-D] Address data for a peer
Contact: r5n-registry@gnunet.org
]]></artwork>
</figure>
</section>
<section anchor="gana_gnunet_url" numbered="true" toc="default">
<name>GNUnet URI Schema Subregistry</name>
<t>
GANA <xref target="GANA"/>
is requested to create a "gnunet://" sub-registry.
The registry shall record for each entry:
</t>
<ul>
<li>Name: The name of the subsystem (case-insensitive ASCII
string, restricted to alphanumeric characters)</li>
<li>Comment: Optionally, a brief English text describing the purpose of
the subsystem (in UTF-8)</li>
<li>Contact: Optionally, the contact information of a person to contact for
further information</li>
<li>References: Optionally, references describing the syntax of the URL
(such as an RFC or LSD)</li>
</ul>
<t>
<!-- FIXME: See GNS wording for this which is already improved / ISE compliant -->
The registration policy for this sub-registry is "First Come First
Served", as described in <xref target="RFC8126"/>.
GANA created this registry as follows:
</t>
<figure anchor="figure_gnunetscheme" title="GNUnet scheme Subregistry.">
<artwork name="" type="" align="left" alt=""><![CDATA[
Name | References | Description
---------------+------------+-------------------------
HELLO [This.I-D] How to contact a peer.
ADDRESS N/A Network address.
Contact: gnunet-registry@gnunet.org
]]></artwork>
</figure>
</section>
<section anchor="gana_signature_purpose" numbered="true" toc="default">
<name>GNUnet Signature Purpose Registry</name>
<t>
GANA amended the "GNUnet Signature Purpose" registry
as follows:
</t>
<figure anchor="figure_purposenums" title="The Signature Purpose Registry Entries.">
<artwork name="" type="" align="left" alt=""><![CDATA[
Purpose | Name | References | Description
--------+-----------------+------------+---------------
6 DHT PATH ELEMENT [This.I-D] DHT message routing data
7 HELLO PAYLOAD [This.I-D] Peer contact information
]]></artwork>
</figure>
</section>
<section anchor="gana_message_type" numbered="true" toc="default">
<name>GNUnet Message Type Registry</name>
<t>
GANA is requested to amend the "GNUnet Message Type" registry
as follows:
</t>
<figure anchor="figure_messagetypeenums" title="The Message Type Registry Entries.">
<artwork name="" type="" align="left" alt=""><![CDATA[
Type | Name | References | Description
--------+-----------------+------------+---------------
146 DHT PUT [This.I-D] Store information in DHT
147 DHT GET [This.I-D] Request information from DHT
148 DHT RESULT [This.I-D] Return information from DHT
157 HELLO Message [This.I-D] Peer contact information
]]></artwork>
</figure>
</section>
</section>
<!-- gana -->
<!--<section anchor="testvectors">
<name>Test Vectors</name>
</section>-->
</middle>
<back>
<references>
<name>Normative References</name>
&RFC2119;
&RFC2663;
&RFC3561;
&RFC3629;
&RFC3986;
&RFC4634;
&RFC5234;
&RFC5245;
&RFC6940;
&RFC8126;
&RFC8174;
&RFC9498;
<reference anchor="ed25519" target="http://link.springer.com/chapter/10.1007/978-3-642-23951-9_9"><front><title>High-Speed High-Security Signatures</title><author initials="D." surname="Bernstein" fullname="Daniel Bernstein"><organization>University of Illinois at Chicago</organization></author><author initials="N." surname="Duif" fullname="Niels Duif"><organization>Technische Universiteit Eindhoven</organization></author><author initials="T." surname="Lange" fullname="Tanja Lange"><organization>Technische Universiteit Eindhoven</organization></author><author initials="P." surname="Schwabe" fullname="Peter Schwabe"><organization>National Taiwan University</organization></author><author initials="B." surname="Yang" fullname="Bo-Yin Yang"><organization>Academia Sinica</organization></author><date year="2011"/></front></reference>
<reference anchor="GANA" target="https://gana.gnunet.org/"><front><title>GNUnet Assigned Numbers Authority (GANA)</title><author><organization>GNUnet e.V.</organization></author><date month="April" year="2020"/></front></reference>
</references>
<references>
<name>Informative References</name>
<reference anchor="R5N" target="https://doi.org/10.1109/ICNSS.2011.6060022">
<front>
<title>R5N: Randomized recursive routing for restricted-route networks</title>
<author initials="N. S." surname="Evans" fullname="Nathan S. Evans">
<organization>Technische Universität München</organization>
</author>
<author initials="C." surname="Grothoff" fullname="Christian Grothoff">
<organization>Technische Universität München</organization>
</author>
<date year="2011"/>
</front>
</reference>
<reference anchor="NSE" target="https://doi.org/10.1007/978-3-642-30045-5_23">
<front>
<title>Efficient and Secure Decentralized Network Size Estimation</title>
<author initials="N. S." surname="Evans" fullname="Nathan S. Evans">
<organization>Technische Universität München</organization>
</author>
<author initials="B." surname="Polot" fullname="Bartlomiej Polot">
<organization>Technische Universität München</organization>
</author>
<author initials="C." surname="Grothoff" fullname="Christian Grothoff">
<organization>Technische Universität München</organization>
</author>
<date year="2012"/>
</front>
</reference>
<reference anchor="Kademlia" target="http://css.csail.mit.edu/6.824/2014/papers/kademlia.pdf">
<front>
<title>Kademlia: A peer-to-peer information system based on the xor metric.</title>
<author initials="P." surname="Maymounkov" fullname="Petar Maymounkov">
</author>
<author initials="D." surname="Mazieres" fullname="David Mazieres">
</author>
<date year="2002"/>
</front>
</reference>
<reference anchor="Sybil" target="https://link.springer.com/chapter/10.1007/3-540-45748-8_24">
<front>
<title>The Sybil Attack</title>
<author initials="J." surname="Douceur" fullname="John R. Douceur">
</author>
<date year="2002"/>
</front>
</reference>
<reference anchor="Smallworldmix" target="https://api.semanticscholar.org/CorpusID:44240877">
<front>
<title>A Measurement of Mixing Time in Social Networks</title>
<author initials="M." surname="Dell'Amico" fullname="Matteo Dell'Amico">
</author>
<date year="2009"/>
</front>
</reference>
<reference anchor="Smallworld" target="https://www.nature.com/articles/35022643">
<front>
<title>Navigation in a small world</title>
<author initials="J." surname="Kleinberg" fullname="Jon M. Kleinberg">
</author>
<date year="2000"/>
</front>
</reference>
<reference anchor="cadet" target="https://doi.org/10.1109/MedHocNet.2014.6849107">
<front>
<title>CADET: Confidential ad-hoc decentralized end-to-end transport</title>
<author initials="B." surname="Polot" fullname="Bartlomiej Polot">
<organization>Technische Universität München</organization>
</author>
<author initials="C." surname="Grothoff" fullname="Christian Grothoff">
<organization>Technische Universität München</organization>
</author>
<date year="2014"/>
</front>
</reference>
</references>
<section anchor="bloom_filters" numbered="true" toc="default">
<name>Bloom filters in R<sup>5</sup>N</name>
<t>
R<sup>5</sup>N uses Bloom filters in several places. This section
gives some general background on Bloom filters and defines functions
on this data structure shared by the various use-cases in R<sup>5</sup>N.
</t>
<t>
A Bloom filter (BF) is a space-efficient probabilistic datastructure
to test if an element is part of a set of elements.
Elements are identified by an element ID.
Since a BF is a probabilistic datastructure, it is possible to have
false-positives: when asked if an element is in the set, the answer from
a BF is either "no" or "maybe".
</t>
<t>
Bloom filters are defined as a string of <tt>L</tt> bits.
The bits are initially always empty, meaning that the bits are set to
zero.
There are two functions which can be invoked on the Bloom filter "bf":
BF-SET(bf, e) and BF-TEST(bf, e) where "e" is an element that is to
be added to the Bloom filter or queried against the set.
</t>
<t>
A mapping function M is used to map each ID of each element from the set to a
subset of k bits.
In the original proposal by Bloom, M is non-injective and can thus map the same
element multiple times to the same bit.
The type of the mapping function can thus be described by the following
mathematical notation:
</t>
<figure anchor="figure_bf_func" title="Bloom filter mapping function.">
<artwork><![CDATA[
------------------------------------
# M: E->B^k
------------------------------------
# L = Number of bits
# B = 0,1,2,3,4,...L-1 (the bits)
# k = Number of bits per element
# E = Set of elements
------------------------------------
Example: L=256, k=3
M('element-data') = {4,6,255}
]]>
</artwork>
</figure>
<t>
When adding an element to the Bloom filter <tt>bf</tt> using
<tt>BF-SET(bf, e)</tt>, each integer <tt>n</tt> of the mapping
<tt>M(e)</tt> is interpreted as a bit offset <tt>n mod L</tt> within
<tt>bf</tt> and set to 1.
</t>
<t>
When testing if an element may be in the Bloom filter <tt>bf</tt> using
<tt>BF-TEST(bf,e)</tt>, each bit offset <tt>n mod L</tt> within
<tt>bf</tt> <bcp14>MUST</bcp14> have been set to 1.
Otherwise, the element is not considered to be in the Bloom filter.
</t>
</section>
<section anchor="overlay" numbered="true" toc="default">
<name>Overlay Operations</name>
<t>
An implementation of this specification commonly exposes the two overlay
operations "GET" and "PUT".
The following are non-normative examples of APIs for those operations.
Their behaviour is described prosaically in order to give implementers a fuller
picture of the protocol.
</t>
<section>
<name>GET operation</name>
<t>
A basic GET operation interface may be exposed as:
</t>
<t>
<tt>GET(Query-Key, Block-Type) -> Results as List</tt>
</t>
<t>
The procedure typically takes at least two arguments to initiate a lookup:
</t>
<dl>
<dt><tt>QueryKey</tt>:</dt>
<dd>
is the 512-bit key to look for in the DHT.
</dd>
<dt>Block-Type:</dt>
<dd>
is the type of block to look for, possibly "any".
</dd>
</dl>
<t>
The GET procedure may allow additional optional parameters in order to
control or modify the query:
</t>
<dl>
<dt>Replication-Level:</dt>
<dd>
is an integer which controls how many nearest peers the request
should reach.
</dd>
<dt>Flags:</dt>
<dd>
is a 16-bit vector which indicates certain
processing requirements for messages.
Any combination of flags as defined in <xref target="route_flags"/>
may be specified.
</dd>
<dt>eXtended-Query (XQuery):</dt>
<dd>
is medatadata which may be
required depending on the respective <tt>Block-Type</tt>.
A <tt>Block-Type</tt> must define if the <tt>XQuery</tt> can or must
be used and what the specific format of its contents should be.
Extended queries are in general used to implement domain-specific filters.
These might be particularly useful in combination with FindApproximate
to add a well-defined filter by an application-specific distance.
Regardless, the DHT does not define any particular semantics for an XQuery.
See also <xref target="blockstorage"/>.
</dd>
<dt>Result-Filter:</dt>
<dd>
is data for a <tt>Block-type</tt>-specific filter
which allows applications to
indicate results which are
not relevant anymore to the
caller (see <xref target="result_filter"/>).
</dd>
</dl>
<t>
The GET procedure should be implemented as an asynchronous
operation that returns individual results as they are found
in the DHT. It should terminate only once the application
explicitly cancels the operation. A single result commonly
consists of:</t>
<dl>
<dt>Block-Type:</dt>
<dd>
is the desired type of block in the result.
</dd>
<dt>Block-Data:</dt>
<dd>
is the application-specific block payload. Contents are
specific to the <tt>Block-Type</tt>.
</dd>
<dt>Block-Expiration:</dt>
<dd>
is the expiration time of the block. After this time, the result should no
longer be used.
</dd>
<dt>Key:</dt>
<dd>
is the key under which the block was stored. This may be different from the
key that was queried if the flag <tt>FindApproximate</tt> was set.
</dd>
<dt>GET-Path:</dt>
<dd>
is a signed path of the public keys of peers which the
query traversed through the network. The DHT will try to
make the path available if the <tt>RecordRoute</tt> flag
was set by the application calling the PUT procedure. The
reported path may have been truncated from the beginning.
The API <bcp14>SHOULD</bcp14> signal truncation by exposing
the <tt>Truncated</tt> flag.
</dd>
<dt>PUT-Path:</dt>
<dd>
is a signed path of the public keys of peers which the
result message traversed. The DHT will try to make the
path available if the <tt>RecordRoute</tt> flag was set
for the GET procedure. The reported path may have been
truncated from the beginning. The API
<bcp14>SHOULD</bcp14> signal truncation by exposing the
<tt>Truncated</tt> flag. As the block was cached by the
node at the end of this path, this path is more likely to
be stale compared to the <tt>GET-Path</tt>.
</dd>
<dt>Truncated:</dt>
<dd>
is true if the <tt>GET-Path</tt> or <tt>PUT-Path</tt>
was truncated, otherwise false.
</dd>
</dl>
</section>
<section>
<name>PUT operation</name>
<t>
A PUT operation interface may be exposed as:
</t>
<t>
<tt>PUT(Key, Block-Type, Block-Expiration, Block-Data)</tt>
</t>
<t>
The procedure typically takes at least four parameters:
</t>
<dl>
<dt>Key:</dt>
<dd>is the key under which to store the block.</dd>
<dt>Block-Type:</dt>
<dd>is the type of the block to store.</dd>
<dt>Block-Expiration:</dt>
<dd>specifies when the block should expire.</dd>
<dt>Block-Data:</dt>
<dd>is the application-specific payload of the block to store.</dd>
</dl>
<t>
The PUT procedure may accept additional optional parameters that
control or modify the operation:
</t>
<dl>
<dt>Replication-Level:</dt>
<dd>
is an integer which controls how many nearest peers the request
should reach.
</dd>
<dt>Flags:</dt>
<dd>
is a bit-vector which indicates certain processing
requirements for messages. Any combination of flags as
defined in <xref target="route_flags"/> may be specified.
</dd>
</dl>
<t>
The PUT procedure does not necessarily yield any information.
</t>
</section>
</section>
<section anchor="hello_url">
<name>HELLO URLs</name>
<t>
The general format of a <tt>HELLO</tt> URL uses "gnunet://"
as the scheme, followed by "hello/" for the name
of the GNUnet subsystem, followed by "/"-separated values
with the GNS Base32 encoding <xref target="RFC9498"/> of
the peer public key, a Base32-encoded EdDSA signature
<xref target="ed25519"/>, and an expiration
time in seconds since the UNIX Epoch in decimal format.
After this a "?" begins a list of key-value pairs where the key
is the URI scheme of one of the peer's addresses and the value
is the URL-escaped payload of the address URI without the "://".
</t>
<t>
The general syntax of <tt>HELLO</tt> URLs specified using
Augmented Backus-Naur Form (ABNF) of <xref target="RFC5234"/> is:
</t>
<figure>
<artwork type="abnf"><![CDATA[
hello-URL = "gnunet://hello[:version]/" meta [ "?" addrs ]
version = *(DIGIT)
meta = pid "/" sig "/" exp
pid = *bchar
sig = *bchar
exp = *DIGIT
addrs = addr *( "&" addr )
addr = addr-name "=" addr-value
addr-name = scheme
addr-value = *pchar
bchar = *(ALPHA / DIGIT)
]]>
</artwork>
</figure>
<t>
'scheme' is defined in <xref target="RFC3986" /> in Section 3.1.
'pchar' is defined in <xref target="RFC3986" />, Appendix A.
</t>
<t>
For example, consider the following URL:
</t>
<figure>
<artwork type="abnf"><![CDATA[
gnunet://hello/1MVZC83SFHXMADVJ5F4
S7BSM7CCGFNVJ1SMQPGW9Z7ZQBZ689ECG/
CFJD9SY1NY5VM9X8RC5G2X2TAA7BCVCE16
726H4JEGTAEB26JNCZKDHBPSN5JD3D60J5
GJMHFJ5YGRGY4EYBP0E2FJJ3KFEYN6HYM0G/
1708333757?foo=example.com&bar+baz=1.2.3.4%3A5678%2Ffoo
]]>
</artwork>
</figure>
<t>
It specifies that the peer with the <tt>pid</tt> "1MVZ..."
is reachable via "foo" at "example.com" and "bar+baz" at
"1.2.3.4" on port 5678 until
1708333757 seconds after the Epoch. Note that "foo"
and "bar+baz" here are underspecified and just used as a simple example.
In practice, the <tt>addr-name</tt> refers to a scheme supported by a
DHT underlay.
</t>
</section>
</back>
</rfc>
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