[bittorrent] BT protocol Q

Simon Spero ses at unc.edu
Fri Apr 29 12:49:21 EDT 2005

On Apr 29, 2005, at 12:30 PM, Justin Cormack wrote:

> Does it really happen other than when the peer has actually gone down
> but tcp hasnt timed out yet? There is very little overhead in having
> quite large numbers of requests outstanding at a time.

It happens quite a bit;  I haven't quantified it over  a large  
population yet.   I expect that the primary culprit are highly  
asymmetric links, a la DSL.

  If a DSL peer begins a transfer, it must have queued up requests  
for data in proportion to the ratios of expected throughput in each  
direction.   Since bittorrent clients only have weak ability to  
predict this  throughput,  this is a hard measure to calculate, and  
can waste bandwidth if cancel requests need to be sent but can't.

  Since transfer block sizes really need to be exactly the  same size  
as the entire piece, this problem can be significant. 

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