06-piece-exchange.md

Piece Exchange

Overview

After metadata is received and verified, the downloader requests pieces from peers.

Flow

1. Seeder sends bitfield (or have_all) showing which pieces it has
2. Downloader sends interested (message ID 2)
3. Seeder sends unchoke (message ID 1)
4. Downloader sends request messages (message ID 6)
5. Seeder responds with piece messages (message ID 7)
6. After all pieces: downloader sends not_interested (message ID 3)

Request Format

[length=13][id=6][index:uint32][begin:uint32][length:uint32]

• index: piece index (0-based)
• begin: byte offset within the piece
• length: number of bytes requested (typically 16384 = 16KB)

Piece Format

[length=9+N][id=7][index:uint32][begin:uint32][data:N bytes]

Piece Verification

After receiving all blocks for a piece, the client computes SHA-1 (or SHA-256) of the assembled piece and compares against the hash in the info dict.

Captured Piece Exchange Events

[+3586ms] have_all (downloader)

{
  "event": "have_all",
  "role": "downloader",
  "direction": "received"
}

[+3591ms] have_all (seeder)

{
  "event": "have_all",
  "role": "seeder",
  "direction": "received"
}

[+3590ms] downloader_done

{
  "event": "downloader_done",
  "downloaded": 49152,
  "uploaded": 157,
  "peers": 1,
  "ratio": 0.5627240143369175
}