UDP tracker protocol

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原文：http://xbtt.sourceforge.net/udp_tracker_protocol.html


UDP tracker protocol

Introduction

The UDP tracker protocol is a high-performance low-overhead BitTorrent tracker protocol. URLs for this protocol look like udp://tracker:port.

Azureus, XBT Client and XBT Tracker support this protocol.

All values are send in network byte order (big endian). Do not expect packets to be exactly of a certain size. Future extensions could increase the size of packets.

Set n to 0.
If no response is received after 60 * 2 ^ n seconds, resend the connect request and increase n.
If a response is received, reset n to 0.
HTTP vs UDP

    * Ethernet: 14 bytes
    * IP: 20 bytes
    * TCP: 20 bytes
    * UDP: 8 bytes

Protocol Packets Non-user User Total
HTTP 10 540 247 + 119 + 6 * N = 366 + 6 * N 906 + 6 * N
UDP 4 168 16 + 16 + 98 + 20 + 6 * N = 150 + 6 * N 318 + 6 * N
HTTP - UDP 6 372 216 588
HTTP / UDP 2.5 3.2 1.5 2.0

The UDP tracker protocol uses (less than) 50% of the bandwidth the HTTP tracker protocol uses. And because UDP is stateless, limits to the number of (open) TCP connections a router or server can handle, do not apply.
Structures

Before announcing or scraping, you have to obtain a connection ID.

   1. Choose a (random) transaction ID.
   2. Fill the connect input structure.
   3. Send the packet.

connect input Offset Size Name Value
0 64-bit integer connection_id 0x41727101980
8 32-bit integer action 0
12 32-bit integer transaction_id
16

   1. Receive the packet.
   2. Check whether the packet is at least 16 bytes.
   3. Check whether the transaction ID is equal to the one you chose.
   4. Check whether the action is connect.
   5. Store the connection ID for future use.

connect output Offset Size Name Value
0 32-bit integer action 0
4 32-bit integer transaction_id
8 64-bit integer connection_id
16

   1. Choose a (random) transaction ID.
   2. Fill the announce input structure.
   3. Send the packet.

announce input Offset Size Name Value
0 64-bit integer connection_id
8 32-bit integer action 1
12 32-bit integer transaction_id
16 20-byte string info_hash
36 20-byte string peer_id
56 64-bit integer downloaded
64 64-bit integer left
72 64-bit integer uploaded
80 32-bit integer event
84 32-bit integer IP address 0
88 32-bit integer key
92 32-bit integer num_want -1
96 16-bit integer port
98

   1. Receive the packet.
   2. Check whether the packet is at least 20 bytes.
   3. Check whether the transaction ID is equal to the one you chose.
   4. Check whether the action is announce.
   5. Do not announce again until interval seconds have passed or an event has happened.

announce output Offset Size Name Value
0 32-bit integer action 1
4 32-bit integer transaction_id
8 32-bit integer interval
12 32-bit integer leechers
16 32-bit integer seeders
20 + 6 * n 32-bit integer IP address
24 + 6 * n 16-bit integer TCP port
20 + 6 * N

Up to about 74 torrents can be scraped at once. A full scrape can't be done with this protocol.

   1. Choose a (random) transaction ID.
   2. Fill the scrape input structure.
   3. Send the packet.

scrape input Offset Size Name Value
0 64-bit integer connection_id
8 32-bit integer action 2
12 32-bit integer transaction_id
16 + 20 * n 20-byte string info_hash
16 + 20 * N

   1. Receive the packet.
   2. Check whether the packet is at least 8 bytes.
   3. Check whether the transaction ID is equal to the one you chose.
   4. Check whether the action is scrape.

scrape output Offset Size Name Value
0 32-bit integer action 2
4 32-bit integer transaction_id
8 + 12 * n 32-bit integer seeders
12 + 12 * n 32-bit integer completed
16 + 12 * n 32-bit integer leechers
8 + 12 * N

If the tracker encounters an error, it might send an error packet.

   1. Receive the packet.
   2. Check whether the packet is at least 8 bytes.
   3. Check whether the transaction ID is equal to the one you chose.

error output Offset Size Name Value
0 32-bit integer action 3
4 32-bit integer transaction_id
8 string message

If the tracker requires authentication, an authentication structure has to be appended to every packet you send to the tracker. The hash is the first 8 bytes of sha1(input + username + sha1(password)).
authenticate input Offset Size Name
0 8-byte zero-padded string username
8 8-byte string hash
16
Actions

    * 0: connect
    * 1: announce
    * 2: scrape
    * 3: error

Events

    * 0: none
    * 1: completed
    * 2: started
    * 3: stopped