Overview

Each node has a globally unique identifier known as the "node ID." Node IDs are chosen at random from the same 160-bit space as BitTorrent infohashes [2]. A "distance metric" is used to compare two node IDs or a node ID and an infohash for "closeness." Nodes must maintain a routing table containing the contact information for a small number of other nodes. The routing table becomes more detailed as IDs get closer to the node's own ID. Nodes know about many other nodes in the DHT that have IDs that are "close" to their own but have only a handful of contacts with IDs that are very far away from their own.

In Kademlia, the distance metric is XOR and the result is interpreted as an unsigned integer. distance(A,B) = |A xor B| Smaller values are closer.

When a node wants to find peers for a torrent, it uses the distance metric to compare the infohash of the torrent with the IDs of the nodes in its own routing table. It then contacts the nodes it knows about with IDs closest to the infohash and asks them for the contact information of peers currently downloading the torrent. If a contacted node knows about peers for the torrent, the peer contact information is returned with the response. Otherwise, the contacted node must respond with the contact information of the nodes in its routing table that are closest to the infohash of the torrent. The original node iteratively queries nodes that are closer to the target infohash until it cannot find any closer nodes. After the search is exhausted, the client then inserts the peer contact information for itself onto the responding nodes with IDs closest to the infohash of the torrent.

The return value for a query for peers includes an opaque value known as the "token." For a node to announce that its controlling peer is downloading a torrent, it must present the token received from the same queried node in a recent query for peers. When a node attempts to "announce" a torrent, the queried node checks the token against the querying node's IP address. This is to prevent malicious hosts from signing up other hosts for torrents. Since the token is merely returned by the querying node to the same node it received the token from, the implementation is not defined. Tokens must be accepted for a reasonable amount of time after they have been distributed. The BitTorrent implementation uses the SHA1 hash of the IP address concatenated onto a secret that changes every five minutes and tokens up to ten minutes old are accepted.

Routing Table

Every node maintains a routing table of known good nodes. The nodes in the routing table are used as starting points for queries in the DHT. Nodes from the routing table are returned in response to queries from other nodes.

Not all nodes that we learn about are equal. Some are "good" and some are not. Many nodes using the DHT are able to send queries and receive responses, but are not able to respond to queries from other nodes. It is important that each node's routing table must contain only known good nodes. A good node is a node has responded to one of our queries within the last 15 minutes. A node is also good if it has ever responded to one of our queries and has sent us a query within the last 15 minutes. After 15 minutes of inactivity, a node becomes questionable. Nodes become bad when they fail to respond to multiple queries in a row. Nodes that we know are good are given priority over nodes with unknown status.

The routing table covers the entire node ID space from 0 to 2¹⁶⁰. The routing table is subdivided into "buckets" that each cover a portion of the space. An empty table has one bucket with an ID space range of min=0, max=2¹⁶⁰. When a node with ID "N" is inserted into the table, it is placed within the bucket that has min <= N < max. An empty table has only one bucket so any node must fit within it. Each bucket can only hold K nodes, currently eight, before becoming "full." When a bucket is full of known good nodes, no more nodes may be added unless our own node ID falls within the range of the bucket. In that case, the bucket is replaced by two new buckets each with half the range of the old bucket and the nodes from the old bucket are distributed among the two new ones. For a new table with only one bucket, the full bucket is always split into two new buckets covering the ranges 0..2¹⁵⁹ and 2¹⁵⁹..2¹⁶⁰.

When the bucket is full of good nodes, the new node is simply discarded. If any nodes in the bucket are known to have become bad, then one is replaced by the new node. If there are any questionable nodes in the bucket have not been seen in the last 15 minutes, the least recently seen node is pinged. If the pinged node responds then the next least recently seen questionable node is pinged until one fails to respond or all of the nodes in the bucket are known to be good. If a node in the bucket fails to respond to a ping, it is suggested to try once more before discarding the node and replacing it with a new good node. In this way, the table fills with stable long running nodes.

Each bucket should maintain a "last changed" property to indicate how "fresh" the contents are. When a node in a bucket is pinged and it responds, or a node is added to a bucket, or a node in a bucket is replaced with another node, the bucket's last changed property should be updated. Buckets that have not been changed in 15 minutes should be "refreshed." This is done by picking a random ID in the range of the bucket and performing a find_nodes search on it. Nodes that are able to receive queries from other nodes usually do not need to refresh buckets often. Nodes that are not able to receive queries from other nodes usually will need to refresh all buckets periodically to ensure there are good nodes in their table when the DHT is needed.

Upon inserting the first node into its routing table and when starting up thereafter, the node should attempt to find the closest nodes in the DHT to itself. It does this by issuing find_node messages to closer and closer nodes until it cannot find any closer. The routing table should be saved between invocations of the client software.

BitTorrent Protocol Extension

The BitTorrent protocol has been extended to exchange node UDP port numbers between peers that are introduced by a tracker. In this way, clients can get their routing tables seeded automatically through the download of regular torrents. Newly installed clients who attempt to download a trackerless torrent on the first try will not have any nodes in their routing table and will need the contacts included in the torrent file.

Peers supporting the DHT set the last bit of the 8-byte reserved flags exchanged in the BitTorrent protocol handshake. Peer receiving a handshake indicating the remote peer supports the DHT should send a PORT message. It begins with byte 0x09 and has a two byte payload containing the UDP port of the DHT node in network byte order. Peers that receive this message should attempt to ping the node on the received port and IP address of the remote peer. If a response to the ping is recieved, the node should attempt to insert the new contact information into their routing table according to the usual rules.

Torrent File Extensions

A trackerless torrent dictionary does not have an "announce" key. Instead, a trackerless torrent has a "nodes" key. This key should be set to the K closest nodes in the torrent generating client's routing table. Alternatively, the key could be set to a known good node such as one operated by the person generating the torrent. Please do not automatically add "router.bittorrent.com" to torrent files or automatically add this node to clients routing tables.

nodes = [["<host>", <port>], ["<host>", <port>], ...]
nodes = [["127.0.0.1", 6881], ["your.router.node", 4804], ["2001:db8:100:0:d5c8:db3f:995e:c0f7", 1941]]

KRPC Protocol

The KRPC protocol is a simple RPC mechanism consisting of bencoded dictionaries sent over UDP. A single query packet is sent out and a single packet is sent in response. There is no retry. There are three message types: query, response, and error. For the DHT protocol, there are four queries: ping, find_node, get_peers, and announce_peer.

A KRPC message is a single dictionary with three keys common to every message and additional keys depending on the type of message. Every message has a key "t" with a string value representing a transaction ID. This transaction ID is generated by the querying node and is echoed in the response, so responses may be correlated with multiple queries to the same node. The transaction ID should be encoded as a short string of binary numbers, typically 2 characters are enough as they cover 2^16 outstanding queries. Every message also has a key "y" with a single character value describing the type of message. The value of the "y" key is one of "q" for query, "r" for response, or "e" for error. A key "v" should be included in every message with a client version string. The string should be a two character client identifier registered in BEP 20 [3] followed by a two character version identifier. Not all implementations include a "v" key so clients should not assume its presence.

generic error = {"t":"aa", "y":"e", "e":[201, "A Generic Error Ocurred"]}
bencoded = d1:eli201e23:A Generic Error Ocurrede1:t2:aa1:y1:ee

DHT Queries

All queries have an "id" key and value containing the node ID of the querying node. All responses have an "id" key and value containing the node ID of the responding node.

arguments:  {"id" : "<querying nodes id>"}

response: {"id" : "<queried nodes id>"}

ping Query = {"t":"aa", "y":"q", "q":"ping", "a":{"id":"abcdefghij0123456789"}}
bencoded = d1:ad2:id20:abcdefghij0123456789e1:q4:ping1:t2:aa1:y1:qe

Response = {"t":"aa", "y":"r", "r": {"id":"mnopqrstuvwxyz123456"}}
bencoded = d1:rd2:id20:mnopqrstuvwxyz123456e1:t2:aa1:y1:re

arguments:  {"id" : "<querying nodes id>", "target" : "<id of target node>"}

response: {"id" : "<queried nodes id>", "nodes" : "<compact node info>"}

find_node Query = {"t":"aa", "y":"q", "q":"find_node", "a": {"id":"abcdefghij0123456789", "target":"mnopqrstuvwxyz123456"}}
bencoded = d1:ad2:id20:abcdefghij01234567896:target20:mnopqrstuvwxyz123456e1:q9:find_node1:t2:aa1:y1:qe

Response = {"t":"aa", "y":"r", "r": {"id":"0123456789abcdefghij", "nodes": "def456..."}}
bencoded = d1:rd2:id20:0123456789abcdefghij5:nodes9:def456...e1:t2:aa1:y1:re

arguments:  {"id" : "<querying nodes id>", "info_hash" : "<20-byte infohash of target torrent>"}

response: {"id" : "<queried nodes id>", "token" :"<opaque write token>", "values" : ["<peer 1 info string>", "<peer 2 info string>"]}

or: {"id" : "<queried nodes id>", "token" :"<opaque write token>", "nodes" : "<compact node info>"}

get_peers Query = {"t":"aa", "y":"q", "q":"get_peers", "a": {"id":"abcdefghij0123456789", "info_hash":"mnopqrstuvwxyz123456"}}
bencoded = d1:ad2:id20:abcdefghij01234567899:info_hash20:mnopqrstuvwxyz123456e1:q9:get_peers1:t2:aa1:y1:qe

Response with peers = {"t":"aa", "y":"r", "r": {"id":"abcdefghij0123456789", "token":"aoeusnth", "values": ["axje.u", "idhtnm"]}}
bencoded = d1:rd2:id20:abcdefghij01234567895:token8:aoeusnth6:valuesl6:axje.u6:idhtnmee1:t2:aa1:y1:re

Response with closest nodes = {"t":"aa", "y":"r", "r": {"id":"abcdefghij0123456789", "token":"aoeusnth", "nodes": "def456..."}}
bencoded = d1:rd2:id20:abcdefghij01234567895:nodes9:def456...5:token8:aoeusnthe1:t2:aa1:y1:re

arguments:  {"id" : "<querying nodes id>",
  "implied_port": <0 or 1>,
  "info_hash" : "<20-byte infohash of target torrent>",
  "port" : <port number>,
  "token" : "<opaque token>"}

response: {"id" : "<queried nodes id>"}

announce_peers Query = {"t":"aa", "y":"q", "q":"announce_peer", "a": {"id":"abcdefghij0123456789", "implied_port": 1, "info_hash":"mnopqrstuvwxyz123456", "port": 6881, "token": "aoeusnth"}}
bencoded = d1:ad2:id20:abcdefghij012345678912:implied_porti1e9:info_hash20:mnopqrstuvwxyz1234564:porti6881e5:token8:aoeusnthe1:q13:announce_peer1:t2:aa1:y1:qe

Response = {"t":"aa", "y":"r", "r": {"id":"mnopqrstuvwxyz123456"}}
bencoded = d1:rd2:id20:mnopqrstuvwxyz123456e1:t2:aa1:y1:re

References

Copyright

This document has been placed in the public domain.