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 CSE 461: Computer Communication and Networks (Autumn 2005)
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Homework 4

Textbook problems:

5.22, 5.25, 5.32, 6.16, 6.20, 6.22, 6.28

tcpdump puzzler (hard):

I have gathered a tcpdump trace gathered between my home machine (on a wireless network in my house, connected to the Internet via a cable modem connection) the NYU CS department web server. Your job is to:

  1. Use the /usr/sbin/tcpdump tool on attu.cs.washington.edu to extract from the tcpdump trace file the sequence of packets that were exchanged.

  2. Using this, generate a time-sequence plot for the data flowing from the Web server to my home machine. A time-sequence plot has time on the x-axis, and sequence numbers on the y-axis. Your graph should contain two lines:

    • sequence numbers of data packets flowing from the server to the client. In other words, each dot plotted corresponds to a tcp data segment sent by the server. The dot's x-axis value is the time the data segment was seen, and its y-axis value is the sequence number of the last byte in that data segment.

    • sequence numbers of acknowledgement packets flowing from the client to the server

  3. Using this plot, your job is to deduce as much information as you can about this tcp transfer. For example, you might be able to deduce things such as the approximate round-trip time between the machines, the number of bytes transferred to my home machine, whether packets were dropped or reordered, the MSS mandated by my home machine, and perhaps much more.

Here's what you have to work with:

    smalltransfer.dump: the output of running the following command on my home computer: 
            /usr/sbin/tcpdump -w smalltransfer.dump -s 0 -n -i en1 \
	port 80 and host www.cs.nyu.edu
    You can play back this tcpdump file using the tcpdump program itself, e.g.: 
            /usr/sbin/tcpdump -r smalltransfer.dump -s 0 -XX
    (Note that you can read the man page for tcpdump on attu, if you want to find out more about tcpdump.)

    Here's what you should turn in for this problem:

    • your time-sequence plot
    • a list of facts you deduced, and for each, a description of your supporting evidence and the steps you used to deduce that fact.

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