CS 4514 Computer Networks WPI, C Term 1995
Craig E. Wills Final Exam (100 pts)
Given: Tuesday, Feb 28, 1995

NAME: BOX:

1.5em 1.5em

This is a closed book (and notes) examination. Answer all questions on the exam itself. Take the number of points assigned to each problem and the amount of space provided for your answer as a measure of the length and difficulty of the expected solution. The exam totals 100 points.

Potentially useful formulas:

• Nyquist rule for a noiseless channel:
  max data rate = bps where H is the bandwidth and V is the number of states encoded.

• Shannon's theorem for channels with noise:
  max data rate = where noise is measured in decibels (db) and
  db =

SCORE:

1. (32 points) This entire question (parts a-e) refer to the network of IMP's shown in the following figure.

   1. (7 points) Compute and show the sink tree for IMP C in the space given in the figure. The numbers shown for each link indicate the delay (bidirectional) for that line.

   2. (7 points) If IMP C wants to send a packet to IMP A by which path will the packet travel? How does each IMP along the path know the next IMP to which to send the packet?

   3. (6 points) A packet is sent by IMP B and received by IMP C. If flooding is being used to route packets then which IMP(s) is the packet sent to by IMP C? Give the algorithm an IMP uses when it processes an arriving packet.

   4. (6 points) A packet is sent by IMP B and received by IMP C. If reverse path forwarding is being used to route packets then which IMP(s) is the packet sent to by IMP C? Give the algorithm an IMP uses when it processes an arriving packet.

   5. (6 points) Assume that IMP C sends an echo request to IMP G, who replies with an echo reply packet. How long will IMP C have to wait for the response from IMP G? What protocol would be used in the Internet to make such a request/reply?

2. (8 points) Assume that a packet has to traverse various networks in its path from the source to the destination host and that each network has a different size for the maximum transmission unit. This situation will likely cause fragmentation of the packet to be needed. What is an advantage of doing reassembly of fragments at the destination host versus doing it as the packet travels across each network?

3. (8 points) One approach for handling network congestion is to simply discard incoming packets until congestion is relieved at a node. What is the problem with this simple approach? What modification to this approach can be done to make it better?

4. (8 points) Assume that there are never any transmission errors over a particular physical channel with a large propagation delay. Which protocol would you expect to have better performance across the channel: selective repeat or go back n? Assume that each protocol uses a 3-bit sequence number.

5. (8 points) This question pertains to project 2, which used the oracle server. What transport level protocol is used for communication with the oracle server? Specify the messages exchanged between a client and the oracle server for the client to request and receive the available list of services.

6. (10 points) This question refers to the following figure showing five Ethernet networks (LAN A-E) connected to a backbone Ethernet network.

   Assume that each of the bridges shown are spanning tree bridges that initially have no knowledge about hosts on the network. Indicate on which of the network(s) (the five LANs plus the backbone) the following frames will be transmitted. The frames are sent in the order shown with the source and destination corresponding to hosts in the figure.

   1. Src Host: 1, Dest Host: 3

   2. Src Host: 3, Dest Host: 1

   3. Src Host: 4, Dest Host: 3

   4. Src Host: 6, Dest Host: 7

   5. Src Host: 1, Dest Host: 4

7. (8 points) An important problem in the TCP protocol is determining the retransmission time out. How is this determination done in TCP?

8. (10 points) A user invokes the Unix command ``ping machine-name'' to determine if the given machine is alive. Indicate the network traffic generated (directly and indirectly) to perform this command in terms of the protocols we have discussed.

9. (8 points) This question relates to your last project on a reliable, in-order delivery mechanism. What if the data link layer was expected to provide reliable, but not necessarily in-order delivery of packets. Briefly indicate what changes this change in requirements would have for the stop and wait protocol, which allows only one outstanding unACKed frame. Answer the same question for a protocol such as go back n or selective repeat, which allow multiple outstanding frames.