CS 457 - Fall 2012

Homework 2

1. Problem 1 (10pts): Why is it important for protocols configured on top of the Ethernet to have a length field in their header indicating how long the message is ?
   
   
2. Problem 2 (10pts): What kind of problem can arise when two hosts on the same Ethernet share the same hardware address? Describe what happens and why that behavior is a problem.
   
   
3. Problem 3 (15pts): The 1982 Ethernet specification allowed between any two stations up to 1,500m of coaxial cable, 1,000 m of other point to point link cable, and two repeaters. Each station or repeater connects to the coaxial cable via up to 50 , of drop cable. Typical delays associated with each device are given in following table:

   Item	Delay
   Coaxial cable	Propagation speed .77c
   Link drop cable	Propagation speed .65c
   Repeaters	Approximately 0.6 µs each
   Transceivers	Approximately 0.2 µs each

   Here c=speed of light in vaccum = 3 x 10 ⁸ m/s. What is the worst case round trip propagation delay, measured in bits, due to the sources listed?
   
   
4. Problem 4 (15pts): Suppose the roud trip propagation delay for Ethernet is 46.4 µs this yields a minimum packet size of 512 bits (464 bits corresponding to propagation delay + 48 bits of jam signal).
   1. What happens to the minimum packet size if the delay time is held constant, and the signaling rate rises to 100 Mbps
   2. What are the drawbacks to so large a minimum packet size?
   3. If compatibility were not an issue, how might the specification be written so as to permit a smaller minimum packet size?


5. Problem 5 (25pts): Let A and B be two stations attempting to transmit on an Ethernet, Each has a steady queue of frames ready to send; A's frames will be numbered A₁, A₂, and so on, and B's similarly. Let T= 51.2 µs be the exponential backoff base unit.
   Suppose A and B simultaneously attempt to send frame 1, collide, and happen to choose backoff times of 0 x T and 1 x T respectively, meaning A wins the race and transmits A₁ while B waits. At the end of this transmission, B will attempt to retransmit B₁ while A will attempt to transmit A₂. These first attempts will collide, but now A backs off for either 0 x T or 1 x T, while B backs off for time equal to one of 0 x T, .... , 3 x T.
   
   
   1. Give the probability that A wins this second backoff race immediately after this first collision, that is, A's first chioce of backoff time k x 51.2 is less than B's.
   2. Suppose A wins this second backoff race, A transmits A₃, and when it is finished, A and B collide again as A tries to tranmit A₄ and B tries once more to transmit B₁. Give the probability that A wins this third backoff race immediately after the first collision.
   3. Give a reasonable lower bound for the probability that A wins all the remaining backoff races.
   4. Then what happens to the frame B₁.
   [Note: This scenario is known as the Ethernet capture effect.]
   


6. Problem 6 (10pts): For a 100-Mbps token ring network with a token rotation time of 200 µs and that allows each station to transmit one 1-KB packet each time it possesses the token, calculate the maximum effective throughout rate that any one host can achieve. Do this assuming (a) immediate release and (b) delayed release.
   Note: Read the description of early vs. delayed release from the textbook.



7. Problem 7 (15pts): ARP (Address resolution protocol): Read the manual page for  the command "arp" to understand the different options. Make sure you explain at least the following options: -v, -n, -a -d -s, -i. Run the command on the machines in the lab. Explain what you see. Then, use the "ping" command to ping another machine in the lab. Make sure that there is no arp entry for that machine. Run the arp command again and explain what happened. Log on the machine you just pinged and use the ifconfig command to justify what you saw using arp. Finally, ping a machine outside the lab (such as cnn.com) and explain the output of the arp command. You must produce printouts of the results of each command you run. Use either the script commnd or redirection to save the output to a file and theninclude it in your report.

FIRSTNAME_LASTNAME_HW2.txt/.pdf