root / dotorg / v5 / html / beps / bep_0005.rst

BEP: 5
Title: DHT Protocol
Version: $Revision$
Last-Modified: $Date$
Author:  Andrew Loewenstern <drue@bittorrent.com>
Status:  Draft
Type:    Standards Track
Created: 31-Jan-2008
Post-History:

BitTorrent uses a "distributed sloppy hash table" (DHT) for storing
peer contact information for "trackerless" torrents. In effect, each
peer becomes a tracker. The protocol is based on Kademila [#Kademlia]_ and is
implemented over UDP.

Please note the terminology used in this document to avoid
confusion. A "peer" is a client/server listening on a TCP port that
implements the BitTorrent protocol. A "node" is a client/server
listening on a UDP port implementing the distributed hash table
protocol. The DHT is composed of nodes and stores the location of
peers. BitTorrent clients include a DHT node, which is used to contact
other nodes in the DHT to get the location of peers to download from
using the BitTorrent protocol.


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 [#entropy]_.  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
2\ :sup:`160`\ .  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=2\ :sup:`160`\ . When a node with ID
"N" is inserted into the table, it is placed within the bucket that
has min &lt;= N &lt; 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..2\ :sup:`159`\  and
2\ :sup:`159`\ ..2\ :sup:`160`\ .

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]]

  

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 two 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. The other key contained in every KRPC message is "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.

Contact Encoding
----------------

Contact information for peers is encoded as a 6-byte string. Also
known as "Compact IP-address/port info" the 4-byte IP address is in
network byte order with the 2 byte port in network byte order
concatenated onto the end.
  
Contact information for nodes is encoded as a 26-byte string.
Also known as "Compact node info" the 20-byte Node ID in network byte
order has the compact IP-address/port info concatenated to the end.

Queries
-------

Queries, or KRPC message dictionaries with a "y" value of "q",
contain two additional keys; "q" and "a". Key "q" has a string value
containing the method name of the query. Key "a" has a dictionary value
containing named arguments to the query.

Responses
---------

Responses, or KRPC message dictionaries with a "y" value of "r",
contain one additional key "r". The value of "r" is a dictionary
containing named return values. Response messages are sent upon
successful completion of a query.

Errors
------

Errors, or KRPC message dictionaries with a "y" value of "e",
contain one additional key "e". The value of "e" is a list. The first
element is an integer representing the error code. The second element
is a string containing the error message. Errors are sent when a query
cannot be fulfilled. The following table describes the possible error
codes:

+----------+------------------------------------------+
|  Code    | Description                              |
+----------+------------------------------------------+
|  201     |   Generic Error                          |
+----------+------------------------------------------+
|  202     |   Server Error                           |
+----------+------------------------------------------+
|  203     | Protocol Error, such as a malformed      |
|          | packet, invalid arguments, or bad token  |
+----------+------------------------------------------+
|  204     |   Method Unknown                         |
+----------+------------------------------------------+

Example Error Packets:

::

  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.

ping
----

The most basic query is a ping. "q" = "ping" A ping query has a
single argument, "id" the value is a 20-byte string containing the
senders node ID in network byte order. The appropriate response to a
ping has a single key "id" containing the node ID of the responding
node.

::

  arguments:  {"id"&nbsp;: "<querying nodes id>"}
  
  response: {"id"&nbsp;: "<queried nodes id>"}


Example Packets
::

  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


find_node
---------

Find node is used to find the contact information for a node given
its ID. "q" == "find_node" A find_node query has two arguments, "id"
containing the node ID of the querying node, and "target" containing
the ID of the node sought by the queryer. When a node receives a
find_node query, it should respond with a key "nodes" and value of a
string containing the compact node info for the target node or the K
(8) closest good nodes in its own routing table.

::

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

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


Example Packets
::

  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


get_peers
---------

Get peers associated with a torrent infohash. "q" = "get_peers" A
get_peers query has two arguments, "id" containing the node ID of the
querying node, and "info_hash" containing the infohash of the torrent.
If the queried node has peers for the infohash, they are returned in a
key "values" as a list of strings. Each string containing "compact" format
peer information for a single peer. If the queried node has no
peers for the infohash, a key "nodes" is returned containing the K
nodes in the queried nodes routing table closest to the infohash
supplied in the query. In either case a "token" key is also included in
the return value. The token value is a required argument for a future
announce_peer query. The token value should be a short binary string.

::

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

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

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


Example Packets:
::

  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


announce_peer
-------------

Announce that the peer, controlling the querying node, is downloading
a torrent on a port. announce_peer has four arguments: "id" containing the node ID of the
querying node, "info_hash" containing the infohash of the torrent,
"port" containing the port as an integer, and the "token" received in
response to a previous get_peers query. The queried node must verify
that the token was previously sent to the same IP address as the
querying node. Then the queried node should store the IP address of the
querying node and the supplied port number under the infohash in its
store of peer contact information.

::

  arguments:  {"id" : "<querying nodes id>", "info_hash" : "<20-byte infohash of target torrent>", "port" : <port number>, "token" : "<opaque token>"}
  
  response: {"id" : "<queried nodes id>"}
  

Example Packets:
::

  announce_peers Query = {"t":"aa", "y":"q", "q":"announce_peer", "a": {"id":"abcdefghij0123456789", "info_hash":"mnopqrstuvwxyz123456", "port": 6881, "token": "aoeusnth"}}
  bencoded = d1:ad2:id20:abcdefghij01234567899:info_hash20:<br />
  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
==========

.. [#Kademlia] Peter Maymounkov, David Mazieres, "Kademlia: A Peer-to-peer Information System Based on the XOR Metric", *IPTPS 2002*. http://www.cs.rice.edu/Conferences/IPTPS02/109.pdf

.. [#entropy] Use SHA1 and plenty of entropy to ensure a unique ID.



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