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~1 min readPeer Wire Protocol

Peer Messages

After handshake, peers exchange typed messages with this frame:

4 bytes: length prefix (big-endian)
1 byte:  message ID (only present if length > 0)
N bytes: payload

A length of 0 is a keep-alive (no ID, no payload). Sent every 2 minutes.

Message types:

ID	Name	Payload
0	choke	-
1	unchoke	-
2	interested	-
3	not interested	-
4	have	4 bytes: piece index
5	bitfield	bytes: 1 bit per piece (have=1)
6	request	12 bytes: index + offset + length (each 4 bytes)
7	piece	4 byte index + 4 byte offset + N bytes block
8	cancel	same as request
9	port	2 bytes: DHT port

Pieces are typically 256 KB to 4 MB. Requested in 16 KB blocks (per BEP 3 convention). So a 1 MB piece = 64 sequential request messages, and we accumulate 64 piece responses.

The request pipeline: send multiple requests before waiting for responses to keep the TCP pipe full. Typical: 5-20 outstanding requests at a time.

For this lesson: encode/decode message frames.

next: Practice

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