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Douglas E. Comer
Computer Science Department
Purdue University
West Lafayette, IN 47907
webmaster: W. David Laverell
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Hands-On Networking: Experiment 12.2
Home > Student > Experiments > 12.2
Experiment 12.2
Everything said in the notes on Experiment 12.1 applies
here. If you have finished 12.1, this experiment should
not be too difficult and could be a lot of fun.
On my SunBlade 100 running Solaris 8 I used
/usr/include/inet/ip.h. One fascinating issue
comes up in the computation of the payload. If you
consult Professor Comer's description of The IP Datagram
Header Format on page 324 of Computer Networks and
Internets you will see that the header length is a
half-octet. By the way, using the sizeof
your ip header structure is cheating! So you have to figure
out how to get the correct 4 bits from it. You have the
advantage of knowing that it is 5, the unit being 32-bit
words. The other 4 bits are 4 (at least at the time of this
writing). So try this experiment: guess ahead of time whether
you and the byte with 0xf0 or with 0xf.
(Either way you have to shift the get the count in bytes.)
In a survey of bitheads in our department two out of three
said 0xf. That's correct. Please let me know if it is the
opposite on your platform!
The Optional Extensions are well worth doing, especially
Number 11 which asks you to look at values from the options
field.
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