# Bittorrent udp-tracker protocol extension

Author: | Arvid Norberg, arvid@rasterbar.com

Table of contents

- introduction
- connecting
- announcing
- scraping
- errors
- actions
- extensions
  - authentication
- 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 descritptions 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 reannouncing 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 retreived 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.
int16_t | num_info_hashes | The number of info-hashes that will follow.
uint16_t | extensions | Optional field. See extensions.

The following structure is repeated `num_info_hashes` times:

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 total number of completed downloads.
int32_t | downloaded | The current number of connected seeds.
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.

## 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.

# credits

Protocol designed by Olaf van der Spek