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hw3 by huanghengdong


									                                      CS2520 Wide Area Networks

                                              Assignment #3

                                            Due date: 2/3/2009

1. The offset field in the IPv4 header measures the offset in 8-byte units. Why? (10 pt)

2. An Ethernet switch is simply a bridge that has the ability to forward some number of packets in parallel,
assuming the input and output ports are all distinct. Suppose two such N-port switches, for a large value
of N, are each able to forward individually up to three packets in parallel. They are then connected to one
another in series by joining a pair of ports, one from each switch; the joining link is the bottleneck as it
can, of course, carry only one packet at a time.

        (a) Suppose we choose two connections through this combined switch at random. What is the
            probability that both connections can be forwarded in parallel? (10 pt)

        (b) What if three connections are chosen at random? (10 pt)

3. Path-MTU discovery is a way to find the minimum MTU along the path so that the sender can use it to
prevent packet fragmentation in that path. Describe how Path-MTU discovery works. (20 pt)
4. IPv4 currently uses 32-bit addresses. If we could redesign IP to use the 6-byte MAC address instead of
the 32-bit address, would we be able to eliminate the need for ARP? Explain why or why not. (20 pt)

5. Give one reason why a large queue at a router is desirable? Give two reasons why a larger queue is
undesirable? (20 pt)

6. When you click “reload” at your browser when the data transfer is slow, sometimes you see faster
download of the data. Give one reason why? (10 pt)

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