[ NNSquad ] Re: [OT?] NN definition(s?)

[John Pettitt <jpp@cloudview.com>]

Richard Bennett wrote:
> So I have two questions: 
>
> 1) Does anybody else think Comcast interferes with BitTorrent
> *downloads*?
>
>   
The behavior of BitTorrent swarms under different network conditions is 
PhD thesis fodder.    However in the general case if a swarm has 
relatively few pure seeds (clients that have completed the download) 
then download speed will tend to be related to upload speed for two 
reasons.  First in the big picture the swarm of clients can be looked at 
as a bucket of bits, with each client pouring bits into the bucket and 
taking them out at the same time - average download will equal average 
upload.   The number of bit's available to download is going to be 
impacted by upload blocking and so the overall swarm download speed will 
suffer.   Second individual clients practice tit for tat - the faster 
you send data to me the faster I'll send it to you.  If the connection 
to you keeps getting RST that is not going to work.   Detecting when a 
client is just uploading requires deep packet inspection to look at the 
bitmap of which blocks the client has.  Doing it with an encrypted 
connection requires traffic analysis that is easily thrown off when 
there are multiple torrents running on the client at different stages of 
completion.

I haven't had a chance to study exactly where in the conversation 
Comcast is sending RST.  Depending on how good the traffic analysis 
engine sandvine uses is it's possible to impact uploads and not download 
but that would be quite complex to do, particularly with encrypted 
connections, and require a lot of state to be kept for each connection 
to understand what is going on.  It's quite possible that people are 
downloading torrents with many seeds where they will see good download 
speeds even if they do no uploading themselves, however if upload 
blocking becomes pervasive this will change.

John
(Disclaimer: I used to be VP Engineering at BitTorrent - I don't work 
for them any more and this post may not reflect their views)