[CLOSED] Implement interested/choke semantics from BEP 3?

[feross]

Issue by feross
Sunday May 18, 2014 at 01:33 GMT
Originally opened as https://github.com/feross/bittorrent-client/issues/15

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Choking is done for several reasons. TCP congestion control behaves very poorly when sending over many connections at once. Also, choking lets each peer use a tit-for-tat-ish algorithm to ensure that they get a consistent download rate.

The choking algorithm described below is the currently deployed one. It is very important that all new algorithms work well both in a network consisting entirely of themselves and in a network consisting mostly of this one.

There are several criteria a good choking algorithm should meet. It should cap the number of simultaneous uploads for good TCP performance. It should avoid choking and unchoking quickly, known as 'fibrillation'. It should reciprocate to peers who let it download. Finally, it should try out unused connections once in a while to find out if they might be better than the currently used ones, known as optimistic unchoking.

The currently deployed choking algorithm avoids fibrillation by only changing who's choked once every ten seconds. It does reciprocation and number of uploads capping by unchoking the four peers which it has the best download rates from and are interested. Peers which have a better upload rate but aren't interested get unchoked and if they become interested the worst uploader gets choked. If a downloader has a complete file, it uses its upload rate rather than its download rate to decide who to unchoke.

For optimistic unchoking, at any one time there is a single peer which is unchoked regardless of its upload rate (if interested, it counts as one of the four allowed downloaders.) Which peer is optimistically unchoked rotates every 30 seconds. To give them a decent chance of getting a complete piece to upload, new connections are three times as likely to start as the current optimistic unchoke as anywhere else in the rotation.

http://www.bittorrent.org/beps/bep_0003.html

[feross]

Comment by fisch0920
Sunday May 18, 2014 at 09:47 GMT

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Definitely need to add optimistic unchoking.

Though this paper is a bit dated, it has the clearest description of the unchoking scenarios I've found.

[feross]

Comment by feross
Monday May 19, 2014 at 02:13 GMT

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Here's another paper to look at: http://www.bittorrent.org/bittorrentecon.pdf

[feross]

Comment by fisch0920
Friday May 30, 2014 at 15:31 GMT

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This issue should be fully addressed in my latest PR.

[feross]

Comment by feross
Tuesday Jun 03, 2014 at 17:23 GMT

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Fixed in #20 - thanks @fisch0920 !