Extension for partial seeds

: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

Copyright

This document has been placed in the public domain.

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