CS 457 - Fall 2012

Homework 5

Date assigned: Oct 30, 2012
Date due: Nov 7, 2012
Submission tag: HW5

  1. (10pts): Exercise 6.16 (p.532)
    Assume that TCP implements an extension that allows window sizes much larger than 64KB. Suppose that you are using this extended TCP over a 1-Gbps link with a latency of 100ms to transfer a 10-MB file, and the TCP receive window is 1 MB. If TCP sends 1-KB packets (assuming no congestion and no lost packets)
    a)How many RTTs does it take until slow start opens the send window to 1 MB
    b)How many RTTs does it take to send the file?
    c)If the time to send the file is given by the number of required RTTs multiplied by the link latency, what is the effective throughput for the transfer? What percentage of the link bandwidth is utilized ?

  2. (10pts): Exercise 6.19 (p.533)
    During linear increase, TCP computes an increment to the congestion window as

    Increment = MSS x (MSS/CongestionWindow)

    Explain why computing this increment each time an ACK arrives may not result in the correct increment. Give a more precise definition for this increment.

  3. (10pts): Exercise 6.20 (p.533)
    Under what circumstances may coarse-grained timeouts still occur in TCP even when the fast retransmit mechanism is being used.

  4. (70pts) Use a tool called tcpdump on the lab machines and this trace file to answer the following questions. You will need to read the man page for tcpdump to understand how it works. The file is a packet capture one large file transfer.
    1. When did the transfer start? When did it end?
    2. What was the size of the data transferred?
    3. What was the average roundtrip time?
    4. How many packets were transferred?
    5. What was the data packet size for most packets?
    6. What was the window size at each endpoint?
    7. What was the average transfer rate?

All exercise numbers and page numbers are according to 4th Edition of the Textbook.

Submission Guidelines:
Submit your homework via email to the TA. The homework solutions must be typed, no hand written homeworks. Format of the file must be .txt or .pdf. Name your submission file as follows:

FIRSTNAME_LASTNAME_HW5.txt/.pdf

For example, John_Smith_HW5.pdf. The subject line of your email must be CS457 Submission: John_Smith_HW5.pdf
For questions regarding this homework email the TA with subject line CS457 Query: Regarding HW5

Note: All homework assignments are individual assignments. Please do your own work.