Tuesday, September 14, 2010

Making Sense of Packet Dumps

I recently read a paper called End-to-End Internet Packet Dynamics by Varn Paxson. This paper tried to make sense of packet dumps from 20,000 100 Kb TCP connections. It's a little old (1999), but I think it does a great job. The task is extremely difficult because of the complexity of the system being measured. Any particular effect could be caused by the packet sniffer, the TCP implementation on the sender or the receiver, or any of the network links or routers in between. And as a passive observer, the analysis program can only guess at the internal state of each component of the system.

I was particularly interested by some of the unexpected effects described in the paper, such as non-FIFO queuing, non-independent loss events, and route fluttering. In the face of such idiosyncratic behavior, I wonder what other bizarre effects have continued unnoticed for years. Occasionally, I'm just amazed that such a complex system as the Internet works at all.

I'm impressed both with the insightful observations from the author, and also with his acknowledgement that some of the conclusions might be wrong. Unfortunately, as the author acknowledges, many of the measured quantities exhibit extremely high variance, and some of the observations only apply to particular links or operating systems. This analysis is difficult to perform, but it really needs to happen again and again as the Internet continues to evolve as a system.

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