CNT 4704: Computer Communication Networks

Fall 2009

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Homework 2

(assigned 10/06; due: 10/13, submitted via webcourse)

1. Chapter 3, problem 1.
    Suppose Client A initiates a Telnet session with Server S. At about the same time, Client B also initiates a Telnet session with Server S. Provide possible source and destination port numbers for
    a). The segments sent from A to B.
    b). The segments sent from B to S.
    c). The segments sent from S to A.
    d). The segments sent from S to B.
    e). If  A and B are different hosts, is it possible that the source port number in the segments from A to S is the same as that from B to S?
    f).  How about if they are the same host?

2. Chapter 3, problem 4.
    a). Suppose you have the following 2 bytes: 00110100 and 01101001. What is the 1s complement of these 2 bytes?
    b). Suppose you have the following 2 bytes: 11110101 and 00101001. What is the 1s complement of these 2 bytes?
    c). For the bytes in part (a), give an example where one bit is flipped in each of the 2 bytes and yet the 1s complement doesn't change.

3. Chapter 3, problem 12.
    Consider the rdt 3.0 protocol. Draw a diagram showing that if the network connection between the sender and receiver cna reorder messages (that is, that two messages propagating in the medium between the sender and receiver can be reordered), then the alternating-bit protocol will not work correctly (make sure you clearly identify the sense in which it will not work correctly). Your diagram should have the sender on the left and the receiver on the right, with the time axis running down the page, showing data (D) and acknowledgement (A) message exchange. Make sure you indicate the sequence number associated with any data or acknowledgement segment.

4. Chapter 3, problem 22 (Problem 21 in 4th edition). Justify your answer even if the answer is true. For statement (a), you should justify it by drawing a protocol running example.
    Answer true or false to the following statements and briefly justify your answer:
     a). With the SR protocol, it is possible fo rthe sender to receive an ACK for a packet that falls outside of its current window.
     b). With GBN, it is possible fo rthe sender to receive an ACK for a packet that falls outside of its current window.
     c). The alternating-bit protocol is the same as the SR protocol with a sender and receiver window size of 1.
     d). The alternating-bit protocol is the same as the GBN protocol with a sender and receiver window size of 1.

5. Chapter 3, problem 24 (Problem 23 in 4th edition). For qustion (b), assume that the network is perfect and there is no error or retransmissions.
    Consider transferring an enormous file of L bytes from Host A to Host B. Assume an MSS of 1,460 bytes.
    a). What is the maximum value of L such that TCP sequence numbers are not exhausted? Recall that the TCP sequence number field has 4 bytes.
    b). For the L you obtain in (a), find how long it takes to transmit the file. Assume that a total of 66 bytes of transport, netowrk and data-link header are added to each segment before the resulting packet is sent out over a 10Mbps link. Ignore flow control and congestion control so A can pump out the segments back to back and continuously.

6. Chapter 3, problem 37 (Probelm 33 in 4th edition).

7. Consider the cross-country example shown in lecture notes Chapter3-part2.ppt (page 10-12). How big would the window size have to be for the channel utilization to be greater than 90 percent?