CS 176A -- Introduction to Computer Communication Networks
Homework Assignment #4
Due in Class, Thursday, December 5, 2002

  1. Suppose I decide to configure my router to filter all ICMP traffic since ICMP messages can pose a security threat. What would be a negative consequence of this decision? [HINT: http://www.networkmagazine.com/article/NMG20000829S0003 might help some people answer this question.]

  2. A reason for using a dynamic approach to address resolution is that a static approach can be error-prone and time consuming. What would be a benefit to using a static approach?

  3. The state stored in the ARP cache periodically times out. Discuss one benefit and one drawback to having a short timeout value.

  4. Suppose that the TCP congestion window is set to 18 KB and a timeout occurs. How big will the window be if the next four transmission bursts are all successful? Assume that the maximum segment size is 1 KB.

  5. To get around the problem of sequence numbers wrapping around while old packets still exist, one could use 64-bit sequence numbers. However, theoretically, an optical fiber can run at 75 Tbps (what is Tbps?). What maximum packet lifetime is required to make sure that future 75 Tbps networks do not have wraparound problems even with 64-bit sequence numbers? Assume that each byte has its own sequence number as TCP does.

  6. Why does UDP exist? Would it not have been enough to just let user processes send raw IP packets?

  7. Unlike TCP, UDP does not perform congestion control. What are some of the potential effects of this fact? Discuss at least one positive and one negative effect.

  8. Suppose that I notice that a large number of connection requests are being sent to port 1214. What might be happening (i.e., what application uses port 1214)?

  9. Explain how a SYN flooding attack works. [HINT: http://www.alco.dk/virus/190996.htm might help some people answer this question.]

  10. DNS uses UDP instead of TCP. If a DNS packet is lost, there is no automatic recover. How is this problem handled?

  11. Can a machine with a single DNS name have multiple IP addresses? How could this occur? Consider a hypothetical situation where it does happen, when a lookup occurs, what is the result?