UDP Tracker Protocol for BitTorrent

Introduction

To discover other peers in a swarm a client announces it's existance to a tracker. The HTTP protocol is used and a typical request contains the following parameters: info_hash, key, peer_id, port, downloaded, left, uploaded and compact. A response contains a list of peers (host and port) and some other information. The request and response are both quite short. Since TCP is used, a connection has to be opened and closed, introducing additional overhead.

Overhead

Using HTTP introduces significant overhead. There's overhead at the ethernet layer (14 bytes per packet), at the IP layer (20 bytes per packet), at the TCP layer (20 bytes per packet) and at the HTTP layer. About 10 packets are used for a request plus response containing 50 peers and the total number of bytes used is about 1206 [1]. This overhead can be reduced significantly by using a UDP based protocol. The protocol proposed here uses 4 packets and about 618 bytes, reducing traffic by 50%. For a client, saving 1 kbyte every hour isn't significant, but for a tracker serving a million peers, reducing traffic by 50% matters a lot. An additional advantage is that a UDP based binary protocol doesn't require a complex parser and no connection handling, reducing the complexity of tracker code and increasing it's performance.

UDP connections / spoofing

==

In the ideal case, only 2 packets would be necessary. However, it is possible to spoof the source address of a UDP packet. The tracker has to ensure this doesn't occur, so it calculates a value (connection_id) and sends it to the client. If the client spoofed it's source address, it won't receive this value (unless it's sniffing the network). The connection_id will then be send to the tracker again in packet 3. The tracker verifies the connection_id and ignores the request if it doesn't match. Connection IDs should not be guessable by the client. This is comparable to a TCP handshake and a syn cookie like approach can be used to storing the connection IDs on the tracker side. A connection ID can be used for multiple requests. A client can use a connection ID until one minute after it has received it. Trackers should accept the connection ID until two minutes after it has been send.

Time outs

=

UDP is an 'unreliable' protocol. This means it doesn't retransmit lost packets itself. The application is responsible for this. If a response is not received after 15 * 2 ^ n seconds, the client should retransmit the request, where n starts at 0 and is increased up to 8 (3840 seconds) after every retransmission. Note that it is necessary to rerequest a connection ID when it has expired.

Examples

Normal announce::

t = 0: connect request t = 1: connect response t = 2: announce request t = 3: announce response

Connect times out::

t = 0: connect request t = 15: connect request t = 45: connect request t = 105: connect request etc

Announce times out::

t = 0: t = 0: connect request t = 1: connect response t = 2: announce request t = 17: announce request t = 47: announce request t = 107: connect request (because connection ID expired) t = 227: connect request etc

Multiple requests::

t = 0: connect request t = 1: connect response t = 2: announce request t = 3: announce response t = 4: announce request t = 5: announce response t = 60: announce request t = 61: announce response t = 62: connect request t = 63: connect response t = 64: announce request t = 64: scrape request t = 64: scrape request t = 64: announce request t = 65: announce response t = 66: announce response t = 67: scrape response t = 68: scrape response

UDP tracker 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.

Connect --------

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

1. Choose a random transaction ID. 2. Fill the connect request structure. 3. Send the packet.

connect request::

Offset Size Name Value 0 64-bit integer protocol_id 0x41727101980 // magic constant 8 32-bit integer action 0 // connect 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 response::

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

Announce --------

1. Choose a random transaction ID. 2. Fill the announce request structure. 3. Send the packet.

IPv4 announce request::

Offset Size Name Value 0 64-bit integer connection_id 8 32-bit integer action 1 // announce 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 0 // 0: none; 1: completed; 2: started; 3: stopped 84 32-bit integer IP address 0 // default 88 32-bit integer key 92 32-bit integer num_want -1 // default 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 occurred.

Do note that most trackers will only honor the ``IP address`` field under limited circumstances.

IPv4 announce response::

Offset Size Name Value 0 32-bit integer action 1 // announce 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

IPv6

IPv6 announces have the same structure as v4 ones, including the used ``action`` number except that the stride size of ```` pairs in the response is 18 bytes instead of 6.

That means the ``IP address`` field in the request remains 32bits wide which makes this field not usable under IPv6 and thus should always be set to 0.

Which format is used is determined by the address family of the underlying UDP packet. I.e. packets from a v4 address use the v4 format, those from a v6 address use the v6 format.

Clients that resolve hostnames to v4 and v6 and then announce to both should use the same ``key`` for both so that trackers that care about accurate statistics-keeping can match the two announces.

Scrape ------

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 request structure. 3. Send the packet.

scrape request::

Offset Size Name Value 0 64-bit integer connection_id 8 32-bit integer action 2 // scrape 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 response::

Offset Size Name Value 0 32-bit integer action 2 // scrape 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.

Errors ------

error response::

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

Existing implementations

Azureus, libtorrent [2], opentracker [3], XBT Client and XBT Tracker support this protocol.

Extensions

==

Extension bits or a version field are not included. Clients and trackers should not assume packets to be of a certain size. This way, additional fields can be added without breaking compatibility.

See BEP 41 [4] for an extension negotiation protocol.

References and Footnotes

.. [1] http://xbtt.sourceforge.net/udp_tracker_protocol.html .. [2] http://www.rasterbar.com/products/libtorrent/udp_tracker_protocol.html .. [3] http://opentracker.blog.h3q.com/ .. [4] http://bittorrent.org/beps/bep_0041.html

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