[bittorrent] BT protocol Q

Elliott Mitchell ehem at m5p.com
Mon May 2 15:35:59 EDT 2005

>From: David P. Mott <dpmott at sep.com>
> So, I don't see any reason (except for very large meta files) why you 
> couldn't make the piece length 2^17 or smaller, such that you could 
> request entire pieces at once.

There are others, and in fact they may be much more important than the
above, though they're not so immediately obvious. With smaller (and
therefore more) pieces you end up flooding the swam with HAVE messages.
Even with 256KB pieces the HAVE messages are 50% of the protocol
overhead. Larger pieces also strongly encourage clients to grab nearby
blocks, increasing locality of reference and therefore performance.

> The large size of the meta files, however, is the other nasty part of BT, 
> and has historically hindered scalability across large trackers because 
> of the bandwidth required to download them.

That burden is not on the tracker. The metafile can be anywhere, though
it may often be on the tracker.

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