Graphing packet loss

Michael Halcrow mike at halcrow.us
Thu Dec 29 19:04:16 MST 2005


On Thu, Dec 29, 2005 at 05:11:18PM -0700, Hans Fugal wrote:
> My dad is having serious come-and-go packet loss issues with his ISP
> (a Vernal local wireless setup). I'd like to give him some leverage
> with some nice cacti graphs of packet loss, but I'm having a hard
> time pinning down precisely what to graph.

I would pay attention to TCP packets sent out wherein the RTT is
greater than 1.5 seconds. Just grep the tcpdump output, keeping a hash
table of what is sent out vs. what you get back.

Keep in mind that TCP has an additive-increase multiplicative-decrease
strategy, and, unfortunately, packet loss is the only congestion
indicator that TCP offers (which is why such schemes incorporating
explicit congestion feedback, like XCP, are a really good idea). So
packet loss is very frequent for TCP connections; if you want to be
able to differentiate from packet loss due to congestion vs. packet
loss due to other reasons, then you are up for a challenge, but it has
been done (see papers referenced below). Chances are, your dad's ISP
is sending packets through a congested bottleneck, and he has a long
latency to get to that bottleneck, and so his TCP traffic is being
treated unfairly due to weaknesses in TCP.

If you are interested in some interesting reading material on the
subject, see:

http://www.cs.utexas.edu/users/lam/Vita/IEEE/YangLam00.pdf
http://www.acm.org/sigcomm/sigcomm2002/papers/xcp.pdf
http://www.acm.org/sigs/sigcomm/sigcomm2004/papers/p581-kim1.pdf

Mike
.___________________________________________________________________.
                         Michael A. Halcrow                          
       Security Software Engineer, IBM Linux Technology Center       
GnuPG Fingerprint: 419C 5B1E 948A FA73 A54C  20F5 DB40 8531 6DCA 8769

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