| libtorrent

# Bittorrent UDP-tracker protocol extension

Author: | Arvid Norberg, arvid@libtorrent.org

Table of contents

- introduction
- connecting
- announcing
- scraping
- errors
- actions
- extensions
  - authentication
- request string
- credits

# introduction

A tracker with the protocol "udp://" in its URI is supposed to be contacted using this protocol.

This protocol is supported by xbt-tracker.

For additional information and descriptions of the terminology used in this document, see the protocol specification

All values are sent in network byte order (big-endian). The sizes are specified with ANSI-C standard types.

If no response to a request is received within 15 seconds, resend the request. If no reply has been received after 60 seconds, stop retrying.

# connecting

Client sends packet:

size | name | description
--- | --- | ---
int64_t | connection_id | Must be initialized to 0x41727101980 in network byte order. This will identify the protocol.
int32_t | action | 0 for a connection request
int32_t | transaction_id | Randomized by client.

Server replies with packet:

size | name | description
--- | --- | ---
int32_t | action | Describes the type of packet, in this case it should be 0, for connect. If 3 (for error) see errors.
int32_t | transaction_id | Must match the transaction_id sent from the client.
int64_t | connection_id | A connection id, this is used when further information is exchanged with the tracker, to identify you. This connection id can be reused for multiple requests, but if it's cached for too long, it will not be valid anymore.

# announcing

Client sends packet:

size | name | description
--- | --- | ---
int64_t | connection_id | The connection id acquired from establishing the connection.
int32_t | action | Action. in this case, 1 for announce. See actions.
int32_t | transaction_id | Randomized by client.
int8_t[20] | info_hash | The info-hash of the torrent you want announce yourself in.
int8_t[20] | peer_id | Your peer id.
int64_t | downloaded | The number of byte you've downloaded in this session.
int64_t | left | The number of bytes you have left to download until you're finished.
int64_t | uploaded | The number of bytes you have uploaded in this session.
int32_t | event | The event, one of none = 0 completed = 1 started = 2 stopped = 3
uint32_t | ip | Your ip address. Set to 0 if you want the tracker to use the `sender` of this UDP packet.
uint32_t | key | A unique key that is randomized by the client.
int32_t | num_want | The maximum number of peers you want in the reply. Use -1 for default.
uint16_t | port | The port you're listening on.
uint16_t | extensions | See extensions

Server replies with packet:

size | name | description
--- | --- | ---
int32_t | action | The action this is a reply to. Should in this case be 1 for announce. If 3 (for error) see errors. See actions.
int32_t | transaction_id | Must match the transaction_id sent in the announce request.
int32_t | interval | the number of seconds you should wait until re-announcing yourself.
int32_t | leechers | The number of peers in the swarm that has not finished downloading.
int32_t | seeders | The number of peers in the swarm that has finished downloading and are seeding.

The rest of the server reply is a variable number of the following structure:

size | name | description
--- | --- | ---
int32_t | ip | The ip of a peer in the swarm.
uint16_t | port | The peer's listen port.

# scraping

Client sends packet:

size | name | description
--- | --- | ---
int64_t | connection_id | The connection id retrieved from the establishing of the connection.
int32_t | action | The action, in this case, 2 for scrape. See actions.
int32_t | transaction_id | Randomized by client.

The following structure is repeated for each info-hash to scrape, but limited by the MTU.

size | name | description
--- | --- | ---
int8_t[20] | info_hash | The info hash that is to be scraped.

Server replies with packet:

size | name | description
--- | --- | ---
int32_t | action | The action, should in this case be 2 for scrape. If 3 (for error) see errors.
int32_t | transaction_id | Must match the sent transaction id.

The rest of the packet contains the following structures once for each info-hash you asked in the scrape request.

size | name | description
--- | --- | ---
int32_t | complete | The current number of connected seeds.
int32_t | downloaded | The number of times this torrent has been downloaded.
int32_t | incomplete | The current number of connected leechers.

# errors

In case of a tracker error,

server replies packet:

size | name | description
--- | --- | ---
int32_t | action | The action, in this case 3, for error. See actions.
int32_t | transaction_id | Must match the transaction_id sent from the client.
int8_t[] | error_string | The rest of the packet is a string describing the error.

# actions

The action fields has the following encoding:

> - connect = 0
> - announce = 1
> - scrape = 2
> - error = 3 (only in server replies)

# extensions

The extensions field is a bitmask. The following bits are assigned:

> - 1 = authentication.
> - 2 = request string.

If multiple bits are present in the extension field, the extension bodies are appended to the packet in the order of least significant bit first. For instance, if both bit 1 and 2 are set, the extension represented by bit 1 comes first, followed by the extension represented by bit 2.

## authentication

The packet will have an authentication part appended to it. It has the following format:

size | name | description
--- | --- | ---
int8_t | username_length | The number of characters in the username.
int8_t[] | username | The username, the number of characters as specified in the previous field.
uint8_t[8] | passwd_hash | sha1(packet + sha1(password)) The packet in this case means the entire packet except these 8 bytes that are the password hash. These are the 8 first bytes (most significant) from the 20 bytes hash calculated.

# request string

The request string extension is meant to allow torrent creators pass along cookies back to the tracker. This can be useful for authenticating that a torrent is allowed to be tracked by a tracker for instance. It could also be used to authenticate users by generating torrents with unique tokens in the tracker URL for each user. The extension body has the following format:

size | name | description
--- | --- | ---
int8_t | request length | The number of bytes in the request string.
int8_t[] | request string | The string that comes after the host- name and port in the UDP tracker URL. Typically this starts with "/announce" The bittorrent client is not expected to append query string arguments for stats reporting, like "uploaded" and "downloaded" since this is already reported in the UDP tracker protocol. However, the client is free to add arguments as extensions.

# credits

Protocol designed by Olaf van der Spek and extended by Arvid Norberg