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Douglas E. Comer
Computer Science Department
Purdue University
West Lafayette, IN 47907
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HON: Faculty Notes to Chapter 13
Home > Faculty > Chapter Notes > Chapter 13
Chapter 13
This chapter is in some sense a continuation of Chapter 12.
Experiment 13.1 deals with the tricky task of reassembling IP fragments.
There is a great deal to be learned from the the second
Optional Extension, but
there is a major problem involved in obtaining the data needed
to test solutions. It is absolutely essential for the students
to wrestle with
RFC 815. Simply put, the simplistic idea that I have had
for years as to how fragmented datagrams are reassembled turns
out to be simplistic. You have to deal with the possibility
that datagrams have been fragmented in different ways. For
example, imagine a 90 byte datagram that has been fragmented.
I had always assumed that the worst you would have to deal
with would be a standard length, say 30, and fragments
arriving as [60,89], [0,29], and [30,59]. Wrong! You have to
deal with the possibility of [30,59], [22,31], etc. You cannot
make assumptions about the fragments as they arrive.
So RFC815 turns out to be extremely interesting. If you do not
teach the algorithms course, you might mention it to the
person who does. It contains an elegant and practical example.
The problem is simply where to find such totally "messed up"
fragments. I am sure that they occur somewhere in the world,
but I am equally sure that they are not going to cooperate by
coming through my computer just when I am sniffing packets.
I chose to implement the algorithm by randomly writing chunks
of a file to a second file. This represents a worst case
scenario because you cannot make any assumptions about
the resulting "fragments". Fascinating exercise!
You will find two programs under the solutions to Experiment
13.1 called scramble.c and
unscramble.c. See
Experiment 13.1 for a warning about trying this with very
large files.
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