UDP Tracker Connection Request Binary Structure

This article provides a technical overview of the binary structure of a connection request packet in the BitTorrent UDP Tracker Protocol (BEP 15). Before a BitTorrent client can announce or scrape torrent data via UDP, it must establish a temporary connection with the tracker. Below is the detailed breakdown of the exact byte layout, field definitions, data types, and endianness required to construct a valid connection request.

Packet Overview and Endianness

A UDP tracker connection request is a fixed-size packet consisting of 16 bytes (128 bits).

All values in the BitTorrent UDP tracker protocol are transmitted in network byte order (Big-Endian).

Binary Layout

The 16-byte packet is partitioned into three distinct fields:

Byte Offset	Field Name	Data Type	Size (Bytes)	Description / Value
`0 - 7`	`protocol_id`	64-bit unsigned integer	8	Magic constant: `0x41727101980`
`8 - 11`	`action`	32-bit unsigned integer	4	Action identifier: `0` (Connect)
`12 - 15`	`transaction_id`	32-bit unsigned integer	4	Random identifier generated by the client

Field Details

1. Protocol ID (Bytes 0–7)

Value: 0x0000041727101980 (Decimal: 4497486125440)
Purpose: Acts as a magic number to identify the protocol and prevent the tracker from processing accidental or malformed UDP packets.

2. Action (Bytes 8–11)

Value: 0 (0x00000000)
Purpose: Specifies the requested action. In the UDP tracker protocol, 0 is strictly designated for establishing a connection.

3. Transaction ID (Bytes 12–15)

Value: Randomized 32-bit integer (e.g., 0x7A3F1C2B)
Purpose: Generated by the client to correlate the tracker’s asynchronous response with this specific request. The tracker must return this exact transaction_id in its response packet.

Example Packet Representation

A raw hexadecimal representation of a valid connection request appears as follows:

00 00 04 17 27 10 19 80 00 00 00 00 7a 3f 1c 2b
|---------------------| |---------| |---------|
      protocol_id         action   transaction_id
       (8 bytes)         (4 bytes)    (4 bytes)

Upon receiving this request, the UDP tracker validates the protocol_id and action, generates a 64-bit connection_id, and echoes back the transaction_id in a 16-byte response packet.