The R5N Distributed Hash Table

The R5N Distributed Hash Table Fraunhofer AISEC

Lichtenbergstrasse 11 Garching 85748 DE martin.schanzenbach@aisec.fraunhofer.de

Berner Fachhochschule

Hoeheweg 80 Biel/Bienne 2501 CH grothoff@gnunet.org

GNUnet e.V.

Boltzmannstrasse 3 Garching 85748 DE fix@gnunet.org

General Independent Stream distributed hash tables This document contains the R⁵N DHT technical specification. R⁵N 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. This document defines the normative wire format of protocol messages, routing algorithms, cryptographic routines and security considerations for use by implementers. This specification was developed outside the IETF and does not have IETF consensus. It is published here to guide implementation of R⁵N and to ensure interoperability among implementations including the pre-existing GNUnet implementation.

Introduction This specification describes the protocol of R⁵N. R⁵N is a Distributed Hash Table (DHT) is an acronym for "randomized recursive routing for restricted-route networks" and its first academic description can be found in . DHTs are a key data structure for the construction of decentralized applications. and they generally provide a robust and efficient means to distribute the storage and retrieval of key-value pairs. The core idea behind R⁵N is to combine an initial randomized routing algorithm with an efficient, classical 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 O(log n) routing complexity. R⁵N also includes advanced features like tracing paths messages take through the network, response filters and on-path application-specific data validation. This document defines the normative wire format of peer-to-peer messages, routing algorithms, cryptographic routines and security considerations for use by implementors.

Requirements Notation 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 when, and only when, they appear in all capitals, as shown here.

Terminology

Address: is a UTF-8 URI which can be used as address to contact a peer. An example of an addressing scheme used in this document is "r5n+ip+tcp", which refers to a standard TCP/IP socket connection. The "hier"-part of the URI must provide a suitable address for the given addressing scheme. The following is a non-normative example of address strings:
Applications: Applications are components which directly use the DHT overlay interfaces. Possible applications include the GNU Name System and the CADET transport system .
Application API: The application API exposes the core operations of the DHT overlay to applications. This includes storing blocks in the DHT and retrieving blocks from the DHT.
Block: Variable-size unit of payload stored in the DHT under a Key. Commonly also called a "value" when talking about a DHT as a "key-value store".
Block Storage: The Block Storage component is used to persist and manage Block data by peers. It includes logic for enforcing storage quotas, caching strategies and data validation.
Block-Type: A unique 32-bit value identifying the data format of a Block. Block-Types are either private or registered in the GANA block type registry (see ).
Initiator: The peer that initially creates and sends a message (, , , ).
HELLO block: A HELLO block is a block with a dedicated block type and is specified in this document. The HELLO block is used to store and retrieve Peer addresses. In this document, HELLO blocks are used by the peer discovery mechanism.
HELLO URL: HELLO URLs are URL-formatted HELLO blocks. They can used for out-of-band exchanges of peer information and are used for address update signalling messages to neighbours.
Key: 512-bit identifier of a location in the DHT. Multiple Blocks can be stored under the same key. Peer Addresses are valid keys.
Message Processing: The Message Processing component processes requests from and generates responses to applications and the underlay network.
Neighbor: A neighbor is a peer which is directly able to communicate with our peer via the Underlay Interface.
Peer: A host that is participating in the overlay. Peers are 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 hosts as needed by the Routing Algorithm.
Peer Address: The Peer Address is the identifier used on the Overlay to address a peer. It is a SHA-512 hash of the Peer ID.
Peer ID: The Peer ID is the public key which is used to authenticate a peer in the underlay. The Peer ID is the public key of the corresponding Ed25519 peer private key.
Routing: The Routing component includes the routing table as well as routing and peer selection logic. It facilitates the R⁵N routing algorithm with required data structures and algorithms.
Responsible Peer: The peer N that is responsible for a specific key K, as defined by the SelectClosestPeer(K, P) algorithm (see .
Underlay Interface: The Underlay Interface is an abstraction layer on top of the supported links of a peer. Peers may be linked by a variety of different transports, including "classical" protocols such as TCP, UDP and TLS or advanced protocols such as GNUnet, I2P or Tor.

Overview In R⁵N peers communicate with each other in order to realize and maintain two basic operations of a distributed hash table:

PUT: This operation stores a block payload on one or more peers with the goal of making the block availiable for queries using the GET operation. In the classical definition of a dictionary interface, this operation would be called "insert".
GET: This operation queries the network of peers for blocks previously stored under or near the key. In the classical definition of a dictionary interface, this operation would be called "find".

A peer or its implementation does not necessarily need to expose the above operations to applications but it commonly will. For example, the peer could be a server purely used for bootstrapping, routing or supporting the overlay network with resources. An example for possible semantics of the above operations provided as an API to applications by an implementation are outlined in . In a trivial scenario where there is only one peer (the local host), R⁵N operates in a very similar fashion 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. R⁵N is agnostic to the underlying transport protocol which is why this document defines a common addressing and messaging interface in . The interface provided by this underlay is used across the specification of the R⁵N protocol. It also serves as a set of requirements of possible transport mechanisms that can be used to implement R⁵N with. That being said, common transport protocols such as TCP/IP or UDP/IP and their interfaces are suitable R⁵N underlays used by existing implementations. Specifics about the protocols of the underlays providing connectivity or the applications using the DHT are out of the scope of this document. However, 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. In order to establish an initial connection to a network of R⁵N peers, an initial, addressable bootstrap peer is required. Further peers, including neighbors, are then learned via a peer discovery process as defined in . Across this document, the functional components of an R⁵N implementation are divided into routing (), message processing () and block processing (). Applications that require application-specific block payloads are expected to register a block type in the GANA block type registry () and provide a specification of the associated block operations (). to implementors of R⁵N. illustrates the architectural overview of R⁵N.

Underlay In the network underlay, a peer is addressable by traditional means out of scope of this document. For example, the peer may have a TCP/IP address, or a HTTPS endpoint. 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 connectivity information to other peers in the DHT overlay. This format is the "HELLO" Block (described in ), which contains URIs. The scheme of each URI indicates which underlay understands the respective address given in the rest of the URI. It is expected that the underlay provides basic mechanisms to manage peer connectivity and addressing. The required functionalities can be represented by the following API:

TRY_CONNECT(N, A): A function which allows the local peer to attempt the establishment of a connection to another peer N using an address A. When the connection attempt is successful, information on the new peer is offered through the PEER_CONNECTED signal.
HOLD(P): A function which tells the underlay to keep a hold on to a connection to a peer P. 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.
DROP(P): A function which tells the underlay to drop the connection to a peer P. This function is only there for symmetry and used during the peer's shutdown to release all of the remaining HOLDs. As R⁵N always prefers the longest-lived connections, it would never drop an active connection that it has called HOLD() on before. Nevertheless, underlay implementations should not rely on this always being true. A call to DROP() also does not imply that the underlay must close the connection: it merely removes the preference to preserve the connection that was established by HOLD().
SEND(P, M): A function that allows the local peer to send a protocol message M to a peer P.
L2NSE = ESTIMATE_NETWORK_SIZE(): A procedure that provides an estimate of the network size. The result, L2NSE, must be the base-2 logarithm of the estimated number of peers in the network. It is used by the routing algorithm. If the underlay does not support a protocol for network size estimation (such as cite paper NSE) the value is assumed to be provided as a configuration parameter to the implementation.

The above procedures are meant to be actively executed by the implementation as part of the peer-to-peer protocol. In addition, the underlay is expected to emit the following signals (usually implemented as callbacks) based on network events observed by the underlay implementation:

PEER_CONNECTED -> P: is a signal that allows the DHT to react to a newly connected peer P. Such an event triggers, for example, updates in the routing table and gossiping of HELLOs to that peer.
PEER_DISCONNECTED -> P: is a signal that allows the DHT to react to a recently disconnected peer. Such an event triggers, for example, updates in the routing table.
ADDRESS_ADDED -> A: The underlay signals indicates that an address A 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. A must be a URI suitable for inclusion in a HELLO payload .
ADDRESS_DELETED -> A: This underlay signals indicates that an address A was removed from the set of addresses the local peer is possibly reachable under. Addresses must have been added before they may be deleted. This information is used to no longer advertise this address to other peers.
RECEIVE -> (P, M): This signal informs the local peer that a protocol message M was received from a peer P.

These signals then drive updates of the routing table, local storage and message transmission.

Routing In order to select peers which are suitable destinations for routing messages, R⁵N uses a hybrid approach: Given an estimated network size N, the peer selection for the first N hops is random. After the initial N hops, peer selection follows an XOR-based peer distance calculation. To enable routing, any R⁵N implementation must keep information about its current set of neighbors. Upon receiving a connection notification from the Underlay through PEER_CONNECTED, information on the new neighbor MUST be added to the routing table. Peers added to the routing table SHOULD be signalled to the Underlay as important connections using HOLD. Similarly when a disconnect is indicated by the Underlay through PEER_DISCONNECTED messages for all addresses of the peer it MUST be removed from the routing table. In order to achieve O(log n) routing performance, the data structure for managing neighbors and their metadata MUST be implemented using the k-buckets concept of as defined in . Maintenance of the routing table (after bootstrapping) is described in . Unlike , routing decisions in R⁵N are also influenced by a Bloom filter in the message that prevents routing loops. This data structure is discussed in . describes the key functions provided on top of these data structures.

Routing Table Whenever a PEER_CONNECTED 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 k-buckets. Each k-bucket contains a list of neighbors. The i-th k-bucket stores neighbors whose peer IDs are between distance 2^i and 2^(i+1) from the local peer. System constraints will typically force an implementation to impose some upper limit on the number of neighbors kept per k-bucket. Upon insertion, the implementation MUST call HOLD on the respective connection. Implementations SHOULD try to keep at least 5 entries per k-bucket. Embedded systems that cannot manage this number of connections MAY 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 MUST NOT include connections to such restricted systems in their k-buckets, thereby effectively excluding them when making routing decisions. If a system hits constraints with respect to the number of active connections, an implementation MUST evict peers from those k-buckets with the largest number of neighbors. The eviction strategy MUST be to drop the shortest-lived connections first. The implementation MAY cache valid HELLOs of disconnected peers outside of the routing table and sporadically or periodically try to (re-)establish connection to the peer by issuing TRY_CONNECT requests on the Underlay.

Peer Discovery Initially, the implementation depends upon either the Underlay providing at least one initial connection to a peer (signalled through PEER_CONNECTED), or the application/end-user providing at least one working HELLO which is then in turn used to call TRY_CONNECT on the Underlay in order to trigger a subsequent PEER_CONNECTED signal from the Underlay. This is commonly achieved through the configuration of hardcoded bootstrap peers or bootstrap servers either for the Underlay or the R⁵N implementation. While details on how the first connection is established MAY depend on the specific implementation, this SHOULD usually be done by an out-of-band exchange of the information from a HELLO block. Section specifies a URL format for encoding HELLO blocks as text strings which allow portable, human-readable, text-based serialization format that can, for example, be encoded into a QR for dissemination. HELLO URLs SHOULD be supported by implementations for both import and export of HELLOs. To discover peers for its routing table, a peer will initiate GetMessage requests asking for blocks of type HELLO using its own peer address as QUERY_HASH. The PEER_BF is initialized and set using the peers own peer address as well as the addresses of all currently connected peers. These requests MUST use the FindApproximate and DemultiplexEverywhere flags. FindApproximate will ensure that other peers will reply with keys they merely consider close-enough, while DemultiplexEverywhere will cause each peer on the path to respond, which is likely to yield HELLO s of peers that are useful somewhere in the routing table. The RECOMMENDED replication level set in the REPL_LVL field is 4. The size and format of the result filter is specified in . The XQUERY is empty. In order to facilitate the above, the Underlay is expected to provide the implementation with one or more addresses signalled through ADDRESS_ADDED. Zero addresses MAY be provided if a peer can only establish outgoing connections and is otherwise unreachable. An implementation MUST advertise its addresses periodically to its neighbors through HelloMessages. The advertisement interval and expiration should be configurable or chosen at the discretion of the implementation based on external factors such as DHCP leases. The specific frequency of advertisements MAY 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, but SHOULD be a fraction of the expiration period. Whenever a peer receives such a HELLO message from another peer that is already in the routing table, it must cache it as long as that peer is in its routing table (or until the HELLO expires) and serve it in response to GET requests for HELLO blocks (see ). This behaviour makes it unnecessary to initiate dedicated PutMessages containing HELLO blocks by the implementation.

Peer Bloom Filter As DHT GetMessages and PutMessages traverse a random path through the network for the first N hops, it is essential that routing loops are avoided. This peer Bloom filter is constant in size at L=1024 buckets (128 bytes) and k=16 buckets per element. The peer Bloom filter is part of the routing metadata in messages in order to prevent circular routes and is updated at each hop with the hops peer identity. For the next hop selection in both the random and the deterministic case, any peer which is in the Bloom filter for the respective message is not included in the peer selection process. Any peer which is forwarding GetMessages or PutMessages () adds its own peer ID 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. The peer Bloom filter follows the definition in . The set of elements E consists of of all possible 256-bit peer IDs. The mapping function M is defined as follows: M(e) -> SHA-512 (e) as uint32[] The element e is hashed using SHA-512. The resulting byte string is interpreted as a string of k=16 32-bit integers in network byte order which are used to set and check the bucket bits in B using BF-SET and BF-TEST. 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.

Routing Functions Using the data structures described so far, the R⁵N routing component provides the following functions for message processing ():

GetDistance(A, B) -> Distance as Integer: 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.
SelectClosestpeer(K, B) -> N: This function selects the neighbor N from our routing table with the shortest XOR-distance to the key K. This means that for all other peers N' in the routing table GetDistance(N, K) < GetDistance(N',K). Peers with a positive test against the peer Bloom filter B are not considered.
SelectRandompeer(B) -> N: This function selects a random peer N from all neighbors. Peers with a positive test in the peer Bloom filter B are not considered.
Selectpeer(K, H, B) -> N: This function selects a neighbor N depending on the number of hops H parameter. If H < NETWORK_SIZE_ESTIMATE this function MUST return SelectRandompeer(B) and SelectClosestpeer(K, B) otherwise.
IsClosestPeer(N, K, B) -> true | false: This function checks if N is the closest peer for K (cf. SelectClosestpeer(K)). Peers with a positive test in the Bloom filter B are not considered.
ComputeOutDegree(REPL_LVL, HOPCOUNT, L2NSE) -> Number: This function computes the number of neighbors that a message should be forwarded to. The arguments are the desired replication level (REPL_LVL), the HOPCOUNT of the message so far and and the current network size estimate (L2NSE) 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.
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 return 1 + (RM1 / (L2NSE + RM1 * HOPCOUNT)) ]]>
The above calculation may yield values that are not discrete. Hence, the result MUST be rounded probabilistically to the nearest discrete value, using the fraction as the probability for rounding up. This probabillistic rounding is necessary to achieve the statistically expected value of the replication level and average number of peers a message is forwarded to.

Pending Table R⁵N 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 doing 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 associates a query hash with the associated originator of the request. The pending table MUST store entries for the last MAX_RECENT requests the peer has encountered. To ensure that the peer does not run out of memory, information about older requests is discarded. The value of MAX_RECENT MAY be configurable and SHOULD be at least 128k. For each entry in the pending table, the DHT MUST track not only the query key and the origin, but also the extended query, requested block type and flags, and the result filter. If the query did not provide a result filter, a fresh result filter MUST still be created to filter duplicate replies. Details of how a result filter works depend on the type, as described in . When a second query from the same origin for the same query hash is received, the DHT MUST attempt to merge the new request with the state for the old request. If this is not possible, the existing result filter MUST be discarded and replaced with the result filter of the incoming message. We note that for local applications, a fixed limit on the number of concurrent requests may be problematic. Hence, it is RECOMMENDED that implementations track requests from local applications separately and preserve the information until the application explicitly stops the request.

Message Processing Further, the implementation MAY act as an initiator of messages. If instructed through an application-facing API such as the one outlined in , the peer may acts as an initiator of GetMessages or PutMessages. The status of initiator is relevant for peers when processing ResultMessages and the potential handover of results to the application. The implementation MUST listen for RECEIVE(P, M) signals from the Underlay and respond to the respective messages sent by the peer P. Wheather initiated locally or received from a neighbour, the implementation processes the messages according to the wire formats and the required validations detailed in the following. Where required, the local peer's ID is referred to as SELF.

Message components This section describes some data structures and fields shared by various message types.

Header A message header that identifies the message length and type is shared across all messages used in the R⁵N protocol.

where:

MSIZE: denotes the size of this message in network byte order.
MTYPE: is the 16-bit message type. Message types are registered in the GANA "GNUnet Message Type" registry .

Flags Flags is a 16-bit vector representing binary options. Each flag is represented by a bit in the field starting from 0 as the rightmost bit to 15 as the leftmost bit.

0: DemultiplexEverywhere: 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 look for answers in their local storage for GetMessages and cache the block in their local storage for PutMessages and ResultMessages.
1: RecordRoute: This bit indicates to keep track of the path that the message takes in the P2P network.
2: FindApproximate: This bit allows results where the key does not match exactly.
3: Truncated: This is a special flag which is set if a peer truncated the path and thus the first hop on the path is given without a signature to enable checking of the next signature. MUST never be set in a query.
4-15: Reserved: The remaining bits are reserved for future use and MUST be set to 0 when initiating an operation. If non-zero bits are received, implementations MUST preserve these bits when forwarding messages.

Path Element A Path Element represents a hop in the path a message has taken through the network. The wire format of a Path Element is illustrated in .

where:

SIGNATURE: is a 64 byte EdDSA signature using the current hop's private key affirming the previous and next hops.
PEER ID: is the EdDSA public key of the peer on the path.

An ordered list of Path Elements may be appended to any routed PutMessages or ResultMessages. The signature of a Path Element is created by the current hop after it made its routing decision identifiying the successor peer. shows the wire format of an example path from Peers A over B and C as it would be received by D in the PUTPATH of a PutMessage or the combined PUTPATH and GETPATH of a ResultMessage. The wire format of the Path Elements allows a natural extension of the PUTPATH along the route of the ResultMessage to the destination forming the GETPATH. The PutMessage would indicate in the PATH_LEN field a length of 3. The ResultMessage would indicate a path length of 3 as the sum of the field values in PUTPATH_L and GETPATH_L.

A path may be truncated in which case the signature of the truncated Path Element is omitted leaving only the Peer ID required for the verification of the subsequent Path Element signature. Such a truncated path is indicated with the respective flag (). The Peer ID of the last Path Element is omitted as it must be that of the sender of the PutMesssage or ResultMessage. The wire format of a truncated example path from Peers B over C to D is illustrated in . The wire format of an example path from Peers B over C as it would be received by D in a PutMessage or ResultMessage is illustrated in . A ResultMessage would indicate in the PATH_LEN field a length of 1. A PutMessage would indicate a length of 1 as the sum of PUTPATH_L and GETPATH_L fields.

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 ID and the peer ID 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 .

SIZE: A 32-bit value containing the length of the signed data in bytes in network byte order. The length of the signed data MUST be 144 bytes.
PURPOSE: A 32-bit signature purpose flag. This field MUST be 6 (in network byte order).
EXPIRATION: denotes the absolute 64-bit expiration date of the block. In microseconds since midnight (0 hour), January 1, 1970 UTC in network byte order.
BLOCK HASH: a SHA-512 hash over the block payload.
PEER PREDECESSOR: the Peer ID of the previous hop. If the signing peer initiated the PUT, this field is set to all zeroes.
PEER SUCCESSOR: the Peer ID of the next hop (not of the signer).

HelloMessage When the Underlay notifies the implementation of added or removed addresses through ADDRESS_ADDED and ADDRESS_DELETED it MAY disseminate those changes to neighbors using HelloMessages. Initiation of HelloMessages by the implementation itself is RECOMMENDED. HelloMessages 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 HELLO blocks (see ) to answer peer discovery queries from other peers.

Wire Format

where:

HEADER: the common message header. Its MTYPE field must be set to the value 157 in network byte order.
RESERVED: is a 16-bit field that must be zero.
URL_CTR: is a 16-bit number that gives the total number of addresses encoded in the ADDRESSES field. In network byte order.
SIGNATURE: is a 64 byte EdDSA signature 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 HELLO block as described in .
EXPIRATION: 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 HELLO URL). Stored in network byte order.
ADDRESSES: A sequence of exactly URL_CTR addresses () which can be used to contact the peer. Each address MUST be 0-terminated. The set of addresses MAY be empty.

Processing If the initiator of a HelloMessage is SELF, the message is simply sent to all neighbors P currently in the routing table using SEND. Otherwise, upon receiving a HelloMessage from a peer P an implementation MUST process it step by step as follows:

If P is not in its routing table, the message is discarded.
The signature is verified, including a check that the expiration time is in the future. If the signature is invalid, the message is discarded.
The information contained in the HelloMessage can be used to synthesize a block of type HELLO (). The block is cached in the routing table until it expires, the peer is removed from the routing table, or the information is replaced by another message from the peer. The implementation SHOULD instruct the Underlay to connect to all now available addresses using TRY_CONNECT in order to make the underlay aware of alternative addresses for this connection and to maintain optimal connectivity.
Received HelloMessages MUST NOT be forwarded.

PutMessage PutMessages are used to store information at other peers in the DHT. Any API which allows applications to initiate PutMessages needs to provide sufficient, implementation-specific information needed to construct the initial PutMessage. For example, implementations supporting multiple applications and blocks will have block type and message flag parameters in addition to the actual data payload and key.

Wire Format

where:

HEADER: is the common message header. Its MTYPE field is set by the initiator to the value 146 in network byte order. Read-only.
BTYPE: is a 32-bit block type. The block type indicates the content type of the payload. Set by the initiator. Read-only. In network byte order.
FLAGS: is a 16-bit vector with binary options (see ). Set by the initiator. Read-only.
HOPCOUNT: is a 16-bit number indicating how many hops this message has traversed to far. Set by the initiator to 0. Incremented by processing peers. In network byte order.
REPL_LVL: is a 16-bit number indicating the desired replication level of the data. Set by the initiator. Read-only. In network byte order.
PATH_LEN: is a 16-bit number indicating the number of Path Elements recorded in PUTPATH. As PUTPATH is optional, this value may be zero. If the PUTPATH is enabled, set initially to 0 by the initiator. Incremented by processing peers. In network byte order.
EXPIRATION: denotes the absolute 64-bit expiration date of the content. Set by the initiator. Read-only. In microseconds since midnight (0 hour), January 1, 1970 in network byte order.
PEER_BF: A peer Bloom filter to stop circular routes (see ). 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 ID using BF-SET.
BLOCK_KEY: The key under which the PutMessage wants to store content under. Set by the initiator. Read-only.
TRUNCATED ORIGIN: is only provided if the TRUNCATED flag is set in FLAGS. If present, this is the public key of the peer just before the first entry on the PUTPATH and the first peer on the PUTPATH is not the actual origin of the message. Thus, to verify the first signature on the PUTPATH, 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.
PUTPATH: the variable-length PUT path. The path consists of a list of PATH_LEN Path Elements. Set by the initiator to 0. Incremented by processing peers.
LAST HOP SIGNATURE: is only provided if the RECORD ROUTE flag is set in FLAGS. If present, this is an EdDSA signature of 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 PATH_LEN is 0, otherwise the last peer in PUTPATH) to the target peer. Modified by processing peers (if flag is set).
BLOCK: the variable-length block payload. The contents are determined by the BTYPE field. The length is determined by MSIZE minus the size of all of the other fields. Set by the initiator. Read-only.

Processing Upon receiving a PutMessage from a peer P , or created through initiation by an overlay API, an implementation MUST process it step by step as follows:

The EXPIRATION field is evaluated. If the message is expired, it MUST be discarded.
If the BTYPE is not supported by the implementation, no validation of the block payload is performed and processing continues at (5). If the BTYPE is ANY, then the message MUST be discarded. Else, the block MUST be validated as defined in (3) and (4).
The message is evaluated using the block validation functions matching the BTYPE. First, the client attempts to derive the key using the respective DeriveBlockKey procedure as described in . If a key can be derived and does not match, the message MUST be discarded.
Next, the ValidateBlockStoreRequest procedure for the BTYPE as described in is used to validate the block payload. If the block payload is invalid, the message MUST be discarded.
The peer address of the sender peer P SHOULD be in PEER_BF. If not, the implementation MAY log an error, but MUST continue.
If the RecordRoute flag is not set, the PATH_LEN MUST be set to zero. If the flag is set and PATH_LEN is non-zero, the local peer SHOULD verify the signatures from the PUTPATH. Verification MAY involve checking all signatures or any random subset of the signatures. It is RECOMMENDED 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 PUTPATH MUST be truncated to only include the elements following the invalid signature.
If the local peer is the closest peer (cf. IsClosestPeer(SELF, BLOCK_KEY, PeerFilter)) or the DemultiplexEverywhere flag ist set, the message SHOULD be stored locally in the block storage if possible. The implementation MAY choose not store the block if external factors or configurations prevent this, such as limited (alottted) disk space.
If the BTYPE of the message indicates a HELLO block, the peer MUST be considered for the local routing table by using the peer address in BLOCK_KEY. If the peer is not either already connected or the respective k-bucket is not already full the peer MUST try to establish a connection to the peer indicated in the HELLO block using the address information from the HELLO block and the Underlay function TRY_CONNECT. The implementation MUST instruct the Underlay to try to connect to all provided addresses using TRY_CONNECT in order to make the underlay aware of multiple addresses for this connection. When a connection is established, the signal PEER_CONNECTED will cause the peer to be added to the respective k-bucket of the routing table ().
Given the value in REPL_LVL, HOPCOUNT and the result of IsClosestPeer(SELF, BLOCK_KEY, PeerFilter) the number of peers to forward to MUST be calculated using ComputeOutDegree(). The implementation SHOULD select up to this number of peers to forward the message to. The implementation MAY forward to fewer or no peers in order to handle resource constraints such as limited bandwidth. For each selected peer with peer address P a dedicated PutMessage_P is created containing the original (and where applicable already updated) fields of the received PutMessage. In each message the all selected addresses and the local peer MUST be added to the PEER_BF and the HOPCOUNT is incremented by 1. If the RecordRoute flag is set, a new Path Element is created using the predecessor peer ID and the signature of the current peer. The Path Element is added to the PUTPATH fields and the PATH_LEN field is incremented by 1. When creating the Path Element signature, the successor must be set to the recipient peer P of the PutMessageP. The successor in the new Path Element is the recipient peer P of Finally, the messages are sent using SEND(P, PutMessageP) each recipient.

GetMessage GetMessages are used to request information from other peers in the DHT. Any overlay API which allows applications to initiate GetMessages needs to provide sufficient, implementation-specific information needed to construct the initial GetMessage. For example, implementations supporting multiple applications and blocks will have block type and message flag parameters.

Wire Format

where:

HEADER: is the common message header. Its MTYPE field is set by the initiator to the value 147 in network byte order. Read-only.
BTYPE: is a 32-bit block type field. The block type indicates the content type of the payload. Set by the initiator. Read-only. In network byte order.
FLAGS: is a 16-bit vector with binary options (see ). Set by the initiator. Read-only.
HOPCOUNT: is a 16-bit number indicating how many hops this message has traversed to far. Set by the initiator to 0. Incremented by processing peers. In network byte order.
REPL_LVL: is a 16-bit number indicating the desired replication level of the data. Set by the initiator. Read-only. In network byte order.
RF_SIZE: is a 16-bit number indicating the length of the result filter RESULT_FILTER. Set by the initiator. Read-only. In network byte order.
PEER_BF: A peer Bloom filter to stop circular routes (see ). Set by the initiator to include itself and all connected neighbors in the routing table. Modified by processing peers to include their own peer address.
QUERY_HASH: 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.
RESULT_FILTER: the variable-length result filter, described in . Set by the initiator. Modified by processing peers.
XQUERY: the variable-length extended query. Optional. Set by the initiator. Read-only.

Result Filter The result filter is used to indicate to other peers which results are not of interest when processing a GetMessage (). Any peer which is processing GetMessages and has a result which matches the query key MUST check the result filter and only send a reply message if the result does not test positive under the result filter. Before forwarding the GetMessage, the result filter MUST be updated using the result of the BTYPE-specific FilterResult (see ) function to filter out all results already returned by the local peer. How a result filter is implemented depends on the block type as described in . 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. How exactly a block result is added to a result filter is specified as part of the definition of a block type (cf. ).

Processing Upon receiving a GetMessage from a peer P, or created through initiation by the overlay API, an implementation MUST process it step by step as follows:

The QUERY_HASH and XQUERY fields are validated against the requested BTYPE as defined by its respective ValidateBlockQuery procedure. If validation function yields REQUEST_INVALID, the message MUST be discarded. If the BTYPE is not supported, the message MUST be forwarded (Skip to step 4). If the BTYPE is ANY, the message is processed without validation.
The peer address of the sender peer P SHOULD be in the PEER_BF Bloom filter. If not, the implementation MAY log an error, but MUST continue.
The local peer MUST try to produce a reply in any of the following cases: (1) If the local peer is the closest peer (cf. IsClosestPeer (SELF, QueryHash, PeerFilter), or (2) if the DemultiplexEverywhere flag is set, or (3) if the local peer is not the closest and the GetRequest was answered previously resulting in a cached reply (). The reply is produced (if one is available) using the following steps:
1. If the BTYPE does not indicate a request for a HELLO block or ANY, the implementation MUST only consider blocks in the local block storage and previously cached ResultMessages. Otherwise, the implementation MUST only consider its own addresses and the addresses it has cached for the peers in its routing table.
2. If FLAGS indicate a FindApproximate request, the peer SHOULD try to respond with the closest block (smallest value of GetDistance(QUERY_HASH, BLOCK_KEY)) it can find that is not filtered by the RESULT_BF. Otherwise, the peer MUST respond if it has a valid block with a BLOCK_KEY that matches the QUERY_HASH exactly and that is not filtered by the RESULT_BF.
3. Any resulting (synthesized) block MUST be encapsulated in a ResultMessage. The cached or created ResultMessage SHOULD be transmitted to the neighbor from which the request was received.
Implementations MAY not to reply if they are resource-constrained. However, ResultMessages MUST be given the highest priority among competing transmissions. If the BTYPE is supported and ValidateBlockReply for the given query has yielded a status of FILTER_LAST, processing MUST end and not continue with forwarding of the request to other peers.
Using the value in REPL_LVL, the number of peers to forward to MUST be calculated using ComputeOutDegree(). If there is at least one peer to forward to, the implementation SHOULD select up to this number of peers to forward the message to. The implementation MAY forward to fewer or no peers in order to handle resource constraints such as bandwidth. The peer Bloom filter PEER_BF MUST be updated with the local peer address SELF for any forwarded message. For all peers with peer address P chosen to forward the message to, SEND(P, GetMessageP) is called. Here, GetMessageP is the original message with the updated fields for HOPCOUNT (incremented by 1), PEER_BF and RESULT_FILTER. The implementation MUST cache the originator peer address and the GetMessage in order to correctly process any ResultMessages.

ResultMessage ResultMessages are used to return information to other peers in the DHT or to applications using the overlay API that previously initiated a GetMessage. The initiator of a ResultMessage is a peer triggered through the processing of a GetMessage.

Wire Format

where:

HEADER: is the common message header. Its MTYPE field must be set to the value 148 in network byte order. Set by the initiator. Read-only.
BTYPE: is a 32-bit block type field. The block type indicates the content type of the payload. Set by the initiator. Read-only. In network byte order.
RESERVED: is a 16-bit value. Implementations MUST set this value to zero when originating a result message. Implementations MUST forward this value unchanged even if it is non-zero.
FLAGS: is a 16-bit vector with binary options (see ). Set by the initiator.
PUTPATH_L: is a 16-bit number indicating the number of Path Elements recorded in PUTPATH. As PUTPATH is optional, this value may be zero even if the message has traversed several peers. Set by the initiator to the PATH_LEN of the PutMessage from which the block originated. Modified by processing peers in case of path truncation. In network byte order.
GETPATH_L: is a 16-bit number indicating the number of Path Elements recorded in GETPATH. As GETPATH is optional, this value may be zero even if the message has traversed several peers. Set by the initiator to 0. Modified by processing peers. In network byte order.
EXPIRATION: 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 PutMessage from which the block originated. Read-only.
QUERY_HASH: the query hash corresponding to the GetMessage which caused this reply message to be sent. Set by the initiator using the value of the GetMessage. Read-only.
TRUNCATED ORIGIN: is only provided if the TRUNCATED flag is set in FLAGS. If present, this is the public key of the peer just before the first entry on the PUTPATH and the first peer on the PUTPATH is not the actual origin of the message. Thus, to verify the first signature on the PUTPATH, this public key must be used. Note that due to the truncation, this last hop cannot be verified to exist. Set by processing peers.
PUTPATH: the variable-length PUT path. The path consists of a list of PUTPATH_L Path Elements. Set by the initiator to the the PUTPATH of the PutMessage from which the block originated. Modified by processing peers in case of path truncation.
GETPATH: the variable-length PUT path. The path consists of a list of GETPATH_L Path Elements. Set by processing peers.
LAST HOP SIGNATURE: is only provided if the RecordRoute flag is set in FLAGS. If present, this is an EdDSA signature of 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 PATH_LEN is 0, otherwise the last peer in PUTPATH) to the target peer.
BLOCK: 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.

Processing Upon receiving a ResultMessage from a connected peer or triggered by the processing of a GetMessage, an implementation MUST process it step by step as follows:

First, the EXPIRATION field is evaluated. If the message is expired, it MUST be discarded.
If the BTYPE is supported, then the BLOCK MUST be validated against the requested BTYPE. To do this, the peer checks that the block is valid using ValidateBlockStoreRequest. If the result is BLOCK_INVALID, the message MUST be discarded.
If the PUTPATH_L or the GETPATH_L are non-zero, the local peer SHOULD verify the signatures from the PUTPATH and the GETPATH. Verification MAY involve checking all signatures or any random subset of the signatures. It is RECOMMENDED 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 MUST be truncated to only include the elements following the invalid signature. In particular, any invalid signature on the GETPATH will cause PUTPATH_L to be set to 0.
The peer also attempts to compute the key using DeriveBlockKey. This may result in NONE. The result is used later. Note that even if a key was computed, it does not have to match the QUERY_HASH.
If the BTYPE of the message indicates a HELLO block, the peer SHOULD be considered for the local routing table by using the peer address computed from the block usingDeriveBlockKey. An implementation MAY choose to ignore the HELLO, 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 MUST try to establish a connection to the peer indicated in the HELLO block using the address information from the HELLO block and the Underlay function TRY_CONNECT. The implementation MUST instruct the Underlay to connect to all provided addresses using TRY_CONNECT in order to make the underlay aware of multiple addresses for this connection. When a connection is established, the signal PEER_CONNECTED will cause the peer to be added to the respective k-bucket of the routing table ().
If the QUERY_HASH of this ResultMessage is found in the list of pending local or remote queries, then for each matching pending query:
1. If the FindApproximate flag was not set in the query and the BTYPE allowed the implementation to compute the key from the block, the computed key must exactly match the QUERY_HASH, otherwise the result does not match the pending query and processing continues with the next pending query.
2. If the BTYPE is supported, result block MUST be validated against the specific query using the respective FilterBlockResult function. This function MUST update the result filter if a result is returned to the originator of the query.
3. If the BTYPE is not supported, filtering of exact duplicate replies MUST still be performed before forwarding the reply. Such duplicate filtering MAY be implemented probabilistically, for example using a Bloom filter. The result of this duplicate filtering is always either FILTER_MORE or FILTER_DUPLICATE.
4. If the RecordRoute flag is set in FLAGS, the local peer address MUST be appended to the GETPATH of the message and the respective signature MUST be set using the query origin as the PEER SUCCESSOR and the response origin as the PEER PREDECESSOR. If the flag is not set, the GETPATH_L and PUTPATH_L MUST be set to zero when forwarding the result.
If the result is either FILTER_MORE or FILTER_LAST, the message is forwarded to the origin of the query through either the overlay API or using SEND(P, ResultMessage') where ResultMessage' is the now modified message. If the result was FILTER_LAST, the query is removed from the list of pending queries.
Finally, the implementation SHOULD cache ResultMessages in order to provide already seen replies to future GetMessages. The implementation MAY choose not no cache any or a limited number of ResultMessages for reasons such as resource limitations.

Blocks This section describes various considerations R⁵N implementations must consider with respect to blocks. Specifically, implementations SHOULD be able to validate and persist blocks. Implementations MAY not support validation for all types of blocks. On some devices, storing blocks MAY also be impossible due to lack of storage capacity. Applications can and should define their own block types. The block type determines the format and handling of the block payload by peers in PutMessages and ResultMessages. Block types MUST be registered with GANA (see ).

Block Operations Block validation may be necessary for all types of DHT messages. To enable these validations, any block type specification MUST define the following functions:

ValidateBlockQuery(Key, XQuery) -> RequestEvaluationResult

is used to evaluate the request for a block as part of GetMessage processing. Here, the block payload is unkown, but if possible the XQuery and Key SHOULD be verified. Possible values for the RequestEvaluationResult are:

REQUEST_VALID: Query is valid.
REQUEST_INVALID: Query format does not match block type. For example, a mandatory XQuery was not provided, or of the size of the XQuery is not appropriate for the block type.

DeriveBlockKey(Block) -> Key | NONE

is used to synthesize the block key from the block payload as part of PutMessage and ResultMessage processing. The special return value of NONE implies that this block type does not permit deriving the key from the block. A Key may be returned for a block that is ill-formed.

ValidateBlockStoreRequest(Block) -> BlockEvaluationResult

is used to evaluate a block payload as part of PutMessage and ResultMessage processing. Possible values for the BlockEvaluationResult are:

BLOCK_VALID: Block is valid.
BLOCK_INVALID: Block payload does not match the block type.

SetupResultFilter(FilterSize, Mutator) -> RF

is used to setup an empty result filter. The arguments are the set of results that must be filtered at the initiator, and a MUTATOR value which MAY be used to deterministically re-randomize probabilistic data structures. The specification MUST also include the wire format for BF.

FilterResult(Block, Key, RF, XQuery) -> (FilterEvaluationResult, RF')

is used to filter results against specific queries. This function does not check the validity of Block itself or that it matches the given key, as this must have been checked earlier. Thus, locally stored blocks from previously observed ResultMessages and PutMessages use this function to perform filtering based on the request parameters of a particular GET operation. Possible values for the FilterEvaluationResult are:

FILTER_MORE: Valid result, and there may be more.
FILTER_LAST: Last possible valid result.
FILTER_DUPLICATE: Valid result, but duplicate (was filtered by the result filter).
FILTER_IRRELEVANT: Block does not satisfy the constraints imposed by the XQuery.

If the main evaluation result is FILTER_MORE, the function also returns an updated result filter where the block is added to the set of filtered replies. An implementation is not expected to actually differenciate between the FILTER_DUPLICATE and FILTER_IRRELEVANT return values: in both cases the block is ignored for this query.

HELLO Blocks For bootstrapping and peer discovery, the DHT implementation uses its own block type called "HELLO". HELLO blocks are the only type of block that MUST be supported by every R⁵N implementation. A block with this block type contains the peer ID of the peer that published the HELLO together with a set of addresses of this peer. The key of a HELLO block is the SHA-512 of the peer ID and thus the peer's address in the DHT. The HELLO block type wire format is illustrated in . A query for block of type HELLO MUST NOT include extended query data (XQuery). Any implementation encountering a request for a HELLO with non-empty XQuery data MUST consider the request invalid and ignore it.

PEER-ID

is the Peer-ID of the peer which has generated this HELLO.

EXPIRATION

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 HELLO URL). Stored in network byte order.

ADDRESSES

is a list of UTF-8 addresses () which can be used to contact the peer. Each address MUST be 0-terminated. The set of addresses MAY be empty.

SIGNATURE

is the signature of the HELLO. It covers a 64-bit pseudo header derived from the information in the HELLO block. The pseudo header includes the expiration time, a constant that uniquely identifies the purpose of the signature, and a hash over the addresses. The wire format is illustrated in .

SIZE: A 32-bit value containing the length of the signed data in bytes in network byte order. The length of the signed data MUST be 80 bytes.
PURPOSE: A 32-bit signature purpose flag. This field MUST be 7 (in network byte order).
EXPIRATION: denotes the absolute 64-bit expiration date of the HELLO. In microseconds since midnight (0 hour), January 1, 1970 in network byte order.
H_ADDRS: a SHA-512 hash over the addresses in the HELLO. H_ADDRS is generated over the ADDRESSES field as provided in the HELLO block using SHA-512 .

The HELLO block functions MUST be implemented as follows:

ValidateBlockQuery(Key, XQuery) -> RequestEvaluationResult

To validate a block query for a HELLO is to simply check that the XQuery is empty. If it is empty, REQUEST_VALID ist returned. Otherwise, REQUEST_INVALID.

DeriveBlockKey(Block) -> Key | NONE

To derive a block key for a HELLO is to simply hash the peer ID from the HELLO. The result of this function is always the SHA-512 hash over the PEER-ID.

ValidateBlockStoreRequest(Block) -> BlockEvaluationResult

To validate a block store request is to verify the EdDSA SIGNATURE over the hashed ADDRESSES against the public key from the peer ID field. If the signature is valid BLOCK_VALID is returned. Otherwise BLOCK_INVALID.

SetupResultFilter(FilterSize, Mutator) -> RF

The RESULT_FILTER for HELLO blocks is implemented using a Bloom filter following the definition from and consists of a variable number of buckets L. L depends on the number of connected peers |E| known to the peer creating a HELLO block from its own addresses: L is set to the minimum of 2¹⁸ bits (2¹⁵ bytes) and the lowest power of 2 that is strictly larger than 2*K*|E| bits (K*|E|/4 bytes). The k-value for the Bloom filter is 16. The elements used in the Bloom filter consist of an XOR between the H_ADDRS field (as computed using SHA-512 over the ADDRESSES) and the SHA-512 hash of the MUTATOR field from a given HELLO block. The mapping function M(H_ADDRS XOR MUTATOR) is defined as follows: M(e = H_ADDR XOR MUTATOR) -> e as uint32[] M 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 bucket bits in B using BF-SET and BF-TEST. The 32-bit Mutator is prepended to the L-bit Bloom filter bucket field HELLO_BF containing B to create the result filter for a HELLO block:

where:

MUTATOR: The 32-bit mutator for the result filter.
HELLO_BF: The L-bit Bloom filter buckets byte array.

The MUTATOR 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 MUST not change the mutator value included in the RESULT_FILTER as they might not be able to recalculate the result filter with a different MUTATOR value. 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. HELLO result filters can be merged if the Bloom filters have the same size and MUTATOR 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 MUTATOR, predecessor and Bloom filter size.

FilterResult(Block, Key, RF, XQuery) -> (FilterEvaluationResult, RF')

The H_ADDRS field is XORed with the SHA-512 hash of the MUTATOR field from the HELLO block and the resulting value is checked against the Bloom filter in RF. Consequently, HELLOs with completely identical sets of addresses will be filtered and FILTER_DUPLICATE is returned. Any small variation in the set of addresses will cause the block to no longer be filtered (with high probability) and FILTER_MORE is returned.

Persistence An implementation SHOULD provide a local persistence mechanism for blocks. Embedded systems that lack storage capability MAY 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 MUST provide the following functionality:

Store(Key, Block): 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 PUTPATH (and if applicable TRUNCATED ORIGIN) of that version of the block.
Lookup(Key) -> List of Blocks: Retrieves blocks stored under the specified key.
LookupApproximate(Key) -> List of Blocks: Retrieves the blocks stored under the specified key and any blocks under keys close to the specified key, in order of decreasing proximity.

Approximate Search Considerations 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 LookupApproximate API must be implemented with care with respect to efficiency. It is RECOMMENDED to limit the number of results from the LookupApproximate procedure to a result size which is easily manageable by the local system. In order to efficiently find a suitable result set, the implementation SHOULD follow the following procedure:

Sort all blocks by the block key in ascending (decending) order. The block keys are interpreted as integer.
Alternatingly select a block with a key larger and smaller from the sortings. The resulting set is sorted by XOR distance. The selection process continues until the upper bound for the result set is reached and both sortings do not yield any closer blocks.

An implementation MAY 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, the procedure may become ineffective for large storages.

Caching Strategy Considerations An implementation MUST implement an eviction strategy for blocks stored in the block storage layer. In order to ensure the freshness of blocks, an implementation MUST evict expired blocks in favor of new blocks. An implementation MAY 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. An implementation MAY preserve blocks which are close to the local peer ID. An implementation MAY provide configurable storage quotas and adapt its eviction strategy based on the current storage size or other constrained resources.

Security Considerations If an upper bound to the maximum number of neighbors in a k-bucket is reached, the implementation MUST prefer to preserve the oldest working connections instead of new connections. This makes Sybil attacks less effective as an adversary would have to invest more resources over time to mount an effective attack. The ComputeOutDegree function limits the REPL_LVL to a maximum of 16. This imposes an upper limit on bandwidth amplification an attacker may achieve for a given network size and topology.

Approximate Result Filtering When a FindApproximate request is encountered, a peer will try to respond with the closest block it has that is not filtered by the result bloom filter. Implementations MUST ensure that the cost of evaluating any such query is reasonably small. For example, implementations MAY consider 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.

Access control By design R⁵N 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 which, like R⁵N, provides a peer-to-peer (P2P) signaling protocol with extensible routing and topology mechanisms. Some decentralized applications such as the GNU Name System () require a more open system that enables ad-hoc participation and other means to prevent common attacks on P2P overlays. GNS, for example, would be in conflict with its goals of providing a solution to the issues of a "Single Hierarchy with a Centrally Controlled Root" and "Distribution and Management of Root Servers" in DNS as raised in .

IANA Considerations IANA maintains a registry called the "Uniform Resource Identifier (URI) Schemes" registry.

IANA maintains 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.

IANA maintains the "Uniform Resource Identifier (URI) Schemes" registry. The registry should be updated to include an entry for the 'r5n+udp+ip' URI scheme. IANA is requested to update that entry to reference this document when published as an RFC.

GANA Considerations

Block Type Registry GANA is requested to create a "DHT Block Types" registry. The registry shall record for each entry:

Name: The name of the block type (case-insensitive ASCII string, restricted to alphanumeric characters
Number: 32-bit
Comment: Optionally, a brief English text describing the purpose of the block type (in UTF-8)
Contact: Optionally, the contact information of a person to contact for further information
References: Required, references (such as an RFC) specifying the block type and its block functions

The registration policy for this sub-registry is "First Come First Served", as described in . GANA created the registry as follows:

GNUnet URI schema Subregistry GANA is requested to create a "gnunet://" sub-registry. The registry shall record for each entry:

Name: The name of the subsystem (case-insensitive ASCII string, restricted to alphanumeric characters
Comment: Optionally, a brief English text describing the purpose of the subsystem (in UTF-8)
Contact: Optionally, the contact information of a person to contact for further information
References: Optionally, references describing the syntax of the URL (such as an RFC or LSD)

The registration policy for this sub-registry is "First Come First Served", as described in . GANA created this registry as follows:

GNUnet Signature Purpose Registry GANA amended the "GNUnet Signature Purpose" registry as follows:

GNUnet Message Type Registry GANA is requested to amend the "GNUnet Message Type" registry as follows:

Test Vectors

Normative References &RFC2119; &RFC3629; &RFC3986; &RFC4634; &RFC5234; &RFC6940; &RFC8126; &RFC8174; &RFC8324; &I-D.schanzen-gns; High-Speed High-Security SignaturesUniversity of Illinois at ChicagoTechnische Universiteit EindhovenTechnische Universiteit EindhovenNational Taiwan UniversityAcademia Sinica GNUnet Assigned Numbers Authority (GANA)GNUnet e.V. Informative References R5N: Randomized recursive routing for restricted-route networks Technische Universität München Technische Universität München Kademlia: A peer-to-peer information system based on the xor metric. CADET: Confidential ad-hoc decentralized end-to-end transport Technische Universität München Technische Universität München

Bloom filters in R⁵N R⁵N 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 R⁵N. 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". Bloom filters are defined as a string of L bits called "buckets". The buckets 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. A mapping function M is used to map each ID of each element from the set to a subset of k buckets. In the original proposal by Bloom, M is non-injective and can thus map the same element multiple times to the same bucket. The type of the mapping function can thus be described by the following mathematical notation:

B^k ------------------------------------ # L = Number of buckets # B = 0,1,2,3,4,...L-1 (the buckets) # k = Number of buckets per element # E = Set of elements ------------------------------------ Example: L=256, k=3 M('element-data') = {4,6,255} ]]> When adding an element to the Bloom filter bf using BF-SET(bf,e), each integer n of the mapping M(e) is interpreted as a bit offset n mod L within bf and set to 1. When testing if an element may be in the Bloom filter bf using BF-TEST(bf,e), each bit offset n mod L within bf MUST have been set to 1. Otherwise, the element is not considered to be in the Bloom filter.

Overlay Operations 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.

GET operation A basic GET operation interface may be exposed as: GET(Query-Key, Block-Type) -> Results as List The procedure typically takes at least two arguments to initiate a lookup:

QueryKey:: is the 512-bit key to look for in the DHT.
Block-Type:: is the type of block to look for, possibly "any".

The GET procedure may allow a set of optional parameters in order to control or modify the query:

Replication-Level:: is an integer which controls how many nearest peers the request should reach.
Flags:: is a 16-bit vector which indicates certain processing requirements for messages. Any combination of flags as defined in may be specified.
eXtended-Query (XQuery):: is medatadata which may be required depending on the respective Block-Type. A Block-Type must define if the XQuery 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 .
Result-Filter:: is data for a Block-type-specific filter which allows applications to indicate results which are not relevant anymore to the caller (see ).

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:

Block-Type:: is the desired type of block in the result.
Block-Data:: is the application-specific block payload. Contents are specific to the Block-Type.
Block-Expiration:: is the expiration time of the block. After this time, the result should no longer be used.
Key:: is the key under which the block was stored. This may be different from the key that was queried if the flag FindApproximate was set.
GET-Path:: is a signed path of the IDs of peers which the query traversed through the network. The DHT will try to make the path available if the RecordRoute flag was set by the application calling the PUT procedure. The reported path may have been silently truncated from the beginning.
PUT-Path:: is a signed path of the IDs of peers which the result message traversed. The DHT will try to make the path available if the RecordRoute flag was set for the GET procedure. The reported path may have been silently truncated from the beginning. 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 GET-Path.

PUT operation A PUT operation interface may be exposed as: PUT(Key, Block-Type, Block-Expiration, Block-Data) The procedure typically takes at least four parameters:

Key:: is the key under which to store the block.
Block-Type:: is the type of the block to store.
Block-Expiration:: specifies when the block should expire.
Block-Data:: is the application-specific payload of the block to store.

The PUT procedure may allow a set of optional parameters in order to control or modify the query:

Replication-Level:: is an integer which controls how many nearest peers the request should reach.
Flags:: is a bit-vector which indicates certain processing requirements for messages. Any combination of flags as defined in may be specified.

The PUT procedure does not necessarily yield any information.

HELLO URLs The general format of a HELLO 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 () of the Peer ID, a Base32-encoded EdDSA signature, 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 "://". For example, consider the following URL:

It specifies that the peer with the ID "RH1M...6Y3G" is reachable via "udp" at 127.0.0.1 on port 2086 until 1647134480 seconds after the Epoch. Note that "udp" here is underspecified and just used as a simple example. In practice, the key (addr-name) refers to a scheme supported by a DHT Underlay. The general syntax of HELLO URLs specified using Augmented Backus-Naur Form (ABNF) of is:

'scheme' is defined in in Section 3.1. 'pchar' is defined in , Appendix A.