Computer Networking

Test 3, 2 May 2013

There are six questions; answer at least five of them. You have one hour, from 1 to 2.

Unless a question specifically tells you not to, make sure you justify your answer, explain your reasoning, show your work.

Up to the first 75 words appearing after a question are taken to be the answer to the question. Words following the 75th word are not part of the answer and will be ignored.

Do not use the word “it” in your answers. Each occurrence of “it” in an answer takes one point off the answer’s grade.

  1. Suppose a TCP connection negotiates a window size of W packets for packets of size S bytes. If the sender halves the packet size, can it double the window size? Justify your answer.

    Yes. The window size represents the receiver's buffer capacity, and halving the packet size while doubling the window size leaves the overall window capacity unchanged.

  2. Which of the three peer-to-peer network architectures does a network of bridged LANs most closely resemble? Justify your answer.

    A hybrid peer-to-peer architecture. The bridges in a bridged LAN resemble a structured layer in a hybrid architecture, and the LANs resemble the unstructured layer. The bridges and hosts have distinct orientation and function in a LAN, which resembles the organization of an hybrid architecture.

  3. When a TCP connection encounters congestion, the sender resets its threshold value. Could it ever be possible that the new threshold value is larger than the old threshold value? Explain your answer.

    Yes. The threshold is set to half the congestion window size when congestion is detected. Before congestion, the congestion window grows, either exponentially during slow start or linearly during congestion avoidance. The threshold constrains how fast the congestion window grows, not whether or not it grows at all. During a long uncongested period the congestion window size could grow to be more than twice the threshold.

  4. How many links would you expect there to be in a Chord network with n nodes? Show your work.

    For n nodes in the ring, around n log n links. A Chord ring has around n = 2m nodes maximum. Each node has a finger table of size m = log n for a total of nm = n log n links.

  5. Describe three kinds of reliability that could be added to a datagram service, and the mechanisms required to provide the reliability.

    Using sequence numbers provide duplicate-datagram protection, Using checksums provides datagram-corruption protection. Using acknowledgments, time-outs, and retransmissions provides protection against lost datagrams.

  6. Which kind of network routing algorithm would be more appropriate to adopt a model for a peer-to-peer network routing algorithm? Justify your answer.

    Distance-vector algorithms provide a more appropriate model. A distance-vector algorithm works directly with local information, and only indirectly with global information. This matches well with unstructured and the more simply structured peer-to-peer networks, where global information is hard if not impossible to come by.

This page last modified on 2013 April 3.