[bittorrent] packet loss

Andrew Brampton andrew at bramp.freeserve.co.uk
Tue Apr 19 05:35:48 EDT 2005


I thought I'll go off on a tangent again to provoke discussion :)

I don't know typical numbers for packet loss, but I can tell you some 
properties of it. Loss isn't constant over a length of time, its bursty [1]. 
So for 10 minutes you could have 0% then for the next 10 seconds you have 
100% (losing in your example 20 packets).

This behaviour is important if you decide to recover from loss... Two simple 
methods I know of solving loss is

Pre-emptive by using Forward Error Checking (FEC) [2] - This is where you 
add 10% for example to each UDP packet containing redundant data that allows 
you to re-construct ~10% of missing packets.
or
Re-transmission - You re-request any lost packets, this can be done by not 
acking (this is what TCP does), or by explicitly re-requesting.

Now because of the bursty nature of internet loss, FEC doesn't perform well 
in quickly recovering data, so in most cases its not very useful. However 
re-transmission takes time so that is also not very quick but might be the 
better option to guarantee complete delivery. However there are ways to 
improve FEC in bursty conditions [3].

Andrew

[1] PROMISE: Peer-to-Peer Media Streaming Using Collect Cast (2003), M. 
Hefeeda and A. Habib and B. Boyan and D. Xu and B. Bhargava
[2] Theory and Practice of Error Control Codes, R. E. Blahut, Jan 1983
[3] Distributed Video Streaming with Forward Error Correction (2002), T. 
Nguyen and A. Zakhor

----- Original Message ----- 
From: <larytet.8708132 at bloglines.com>
To: <bittorrent at lists.ibiblio.org>
Sent: Tuesday, April 19, 2005 3:57 AM
Subject: [bittorrent] packet loss


>i kept UDP session open for ~20 hours. 60 bytes of dummy payload was sent
> every 0.5s across two ISPs. Both ISPs are in the US. both connections are
> broadband. packet loss is under 0.5% (~600/130,000). probably real packet
> loss is ~1% because i measured packet loss according the number of missing
> acks.
>
> Is this packet loss ratio typical for the modern networks ? or is
> it relatively low/high ? 




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