Merge pull request #61 from bittorrent/dht-version-string

Document client version field in DHT messages

beps/bep_0005.rst

@@ -186,17 +186,20 @@ 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
+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. The other key contained in every KRPC message is "y"
-with a single character value describing the type of message. The value
+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.
+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 [#BEP-20]_ followed by a two character version identifier. Not all
+implementations include a "v" key so clients should not assume its presence.
 
 Contact Encoding
 ----------------
@@ -418,6 +421,9 @@ References
 
 .. [#entropy] Use SHA1 and plenty of entropy to ensure a unique ID.
 
+.. [#BEP-20] BEP_0020. Peer ID Conventions.
+   (http://www.bittorrent.org/beps/bep_0020.html)
+
 Copyright
 =========