[e2e] High Packet Loss and TCP

[Durga Prasad Pandey]

I am looking into the possibility of congestion in a delay tolerant 
network(Characterized by high latency, intermittent connectivity, and high 
error rates). Since I am new to this area, could I ask for suggestions on 
how to approach the task of checking congestion in the network as a whole? 
In other words, if we define congestion to occur when the net amount of 
data flowing into a network through various nodes is greater than the net 
amount of data flowing out, what parametsrs would one use to identify 
congestion?
Another question is: Are there some free network simulators available on 
the web?

I will appreciate any help.

Durga

On Thu, 1 May 2003, David G. Andersen wrote:

> On Thu, May 01, 2003 at 03:40:42PM -0400, Ross Callon quacked:
> > My understanding is that there is some level of packet loss 
> > which causes TCP to back off to the point of stopping. My 
> > impression is that this is a sufficiently high loss rate that it 
> > shouldn't happen in a network which is behaving properly, 
> > and if it happens this should be considered a network failure 
> > rather than a TCP problem. (I am pretty sure that I saw this 
> > sort of behavior a few years ago when trying to access large 
> > files over a very bad link). 
> > 
> > Is there a paper which would describe what the appropriate 
> > loss rate is that would cause this problem? Is there any 
> > general understanding of what level of packet loss will cause
> > serious problems?
> 
> My favorite source for this is from Padhye et al., 
> which shows a pretty drastic knee in the TCP performance curve
> when the loss rate starts to exceed about 30%.  It's not a
> total "outage", but it does represent about a two order of
> magnitude reduction in throughput, and renders the network
> pretty unusable.
> 
> J. Padhye, V. Firoiu, D. Towsley, and J. Kurose
> Modeling TCP Throughput:  A Simple Model and its Empirical Validation.
> In Proc. ACM SIGCOMM, September 1998.
> 
> ftp://gaia.cs.umass.edu/pub/Padhye-Firoiu98-TCP-throughput.ps.Z
> 
>    -Dave
> 
> 

-- 
Durga Prasad Pandey
Assistant Online Editor, ACM Crossroads
dpsmiles at acm.org