BEP:21
Title:Extension for partial seeds
Version: 596058cfa19fe07f178437e97f418e64cd1a7f1a
Last-Modified:Tue May 13 23:49:33 2008 +0000
Author: Arvid Norberg <arvid@bittorrent.com>, Greg Hazel <greg@bittorrent.com>, Aaron Grunthal
Status: Draft
Type:Standards Track
Created:8-May-2008
Post-History:

The purpose of this extension is to allow further optimizations of bittorrent swarms when peers are partial seeds. A partial seed is a peer that is incomplete without downloading anything more. This happens for multi file torrents where users only download some of the files.

Extension Header

A peer that is a partial seed SHOULD include an extra header in the extension handshake (specified in BEP 0010), 'upload_only'. Setting the value of this key to 1 indicates that this peer is not interested in downloading anything.

Example extension handshake message:

{'m': {'ut_metadata', 3}, 'upload_only': 1}

Tracker Scrapes

The tracker scrape conventions defines three values per torrent, 'complete', 'incomplete' and 'downloaded'. The purpose of this extensions is to let clients distinguish between partial seeds and downloaders, both of which currently would be classified as incomplete.

If the tracker supports this extension, it MUST add a fourth field, 'downloaders'. This field is the number of active downloaders in the swarm, it does not include partial seeds. The number of partial seeds can be calculated by: incomplete - downloaders.

Tracker Announce

In order to tell the tracker that a peer is a partial seed, it MUST send an event=paused parameter in every announce while it is a partial seed.

Rationale

Allowing peers to scrape a tracker and distinguish between active downloaders and partial seeds makes it more efficient to determine what to seed based on the downloader/seed ratio.

The reason why every announce should contain event=paused is to avoid relying on the state being stored in the tracker. In case there's a failure and a backup tracker is used, it can recover all of the swarm state because the clients are announcing that they are partial seeds.

References