Computer Science
CS 460 Computer Communications and Networking

Homework #5

Review Questions (not graded, do not turn in)

  1. Kurose & Ross, Chapter 5, Review Question R2.

    If all the links in the Internet were to provide reliable delivery service, would the TCP reliable delivery service be redundant? Why or why not?

  2. At the end of Section 5.3, there is a list of four desirable characteristics of a broadcast channel. Which of these characteristics are exhibited by slotted ALOHA? By token passing? Explain each of your answers.
  3. Kurose & Ross, Chapter 5, Review Question R7.

    Why would the token-ring protocol be inefficient if a LAN had a very large perimeter?

  4. Kurose & Ross, Chapter 5, Review Question R9.

    Suppose nodes A, B, and C each attach to the same broadcast LAN (through their adapters). If A sends thousands of IP datagrams to B with each encapsulating frame addressed to the MAC address of B, will C's adapter process these frames? If so, will C's adapter pass the IP datagrams in these frames to the network layer C? How would your answers change if A sends frames with the MAC broadcast address?

  5. Kurose & Ross, Chapter 5, Review Question R14.

    In CSMA/CD, after the fifth collision, what is the probability that a node chooses K = 4? The result K = 4 corresponds to a delay of how many seconds on a 10 Mbps Ethernet?

Problems (10 points each)

  1. Kurose & Ross, Chapter 5, Problem P5.

    Consider the 5-bit generator, G=10011, and suppose that D has the value 1010101010. What is the value of R?

  2. Kurose & Ross, Chapter 5, Problem P14c, P14d.

    Consider three LANs interconnected by two routers, as shown below:


    Use these IP and MAC addresses:

    • A:, 00-00-00-00-00-00
    • B:, 11-11-11-11-11-11
    • Left router, interface on Subnet 1:, 22-22-22-22-22-22
    • Left router interface on Subnet 2:, 33-33-33-33-33-33
    • C:, 44-44-44-44-44-44
    • D:, 66-66-66-66-66-66
    • Right router, interface on Subnet 2:, 55-55-55-55-55-55
    • Right router, interface on Subnet 3: 133.333.333.002, 88-88-88-88-88-88
    • E: 133.333.333.001, 77-77-77-77-77-77
    • F: 133.333.333.003, 99-99-99-99-99-99
    1. Consider sending an IP datagram from Host E to Host B. Suppose all of the ARP tables are up to date. Enumerate all the steps, as done for the single-router example in Section 5.4.2.
    2. Repeat the above problem, now assuming that the ARP table in the sending host is empty (and the other tables are up to date).
  3. Consider the network from Problem P14, but now the router between subnets 2 and 3 is replaced by a switch. Explain how your answers to P14c and P14d will change.
  4. Kurose & Ross, Chapter 5, Problem P17.

    Recall that with the CSMA/ CD protocol, the adapter waits K*512 bit times after a collision, where K is drawn randomly. For K = 100, how long does the adapter wait until returning to Step 2 for a 10 Mbps Ethernet? For a 100 Mbps Ethernet?

  5. Kurose & Ross, Chapter 5, Problem P21.

    Suppose nodes A and B are on the same 10 Mbps Ethernet bus, and the propagation delay between the two nodes is 245 bit times. Suppose A and B send frames at the same time, the frames collide, and then A and B choose different values of K in the CSMA/CD algorithm. Assuming no other nodes are active, can the retransmissions from A and B collide? For our purposes, it suffices to work out the following example. Suppose A and B begin transmission at t = 0 bit times. They both detect collisions at t = 245 bit times. They finish transmitting a jam signal at t = 245 + 48 = 293 bit times. Suppose KA = 0 and KB = 1. At what time does B schedule its retransmission? At what time does A begin transmission? (Note: The nodes must wait for an idle channel after returning to Step 2 -- see protocol.) At what time does A's signal reach B? Does B refrain from transmitting at its scheduled time?

  6. Kurose & Ross, Chapter 5, Problem P33.

    In this problem, we explore the use of small packets for Voice-over-IP applications. One of the drawbacks of a small packet size is that a large fraction of link bandwidth is consumed by overhead bytes. To this end, suppose that the packet consists of P bytes and 5 bytes of header.

    1. Consider sending a digitally encoded voice source directly. Suppose the source is encoded at a constant rate of 128 kbps. Assume each packet is entirely filled before the source sends the packet into the network. The time required to fill a packet is the packetization delay. In terms of L, determine the packetization delay in milliseconds.
    2. Packetization delays greater than 20 msec can cause a noticeable and unpleasant echo. Determine the packetization delay for L = 1,500 bytes (roughly corresponding to a maximum-sized Ethernet packet) and for L = 50 (corresponding to an ATM packet).
    3. Calculate the store-and-forward delay at a single switch for a link rate of R = 622 Mbps for L = 1,500 bytes, and for L = 50 bytes.
    4. Comment on the advantages of using a small packet size.
  7. Kurose & Ross, Chapter 5, Problem P35.

    Consider the MPLS network shown below:


    Suppose that routers R5 and R6 are now MPLS enabled. We want to perform traffic engineering so that packets from R6 destined for A are switched to A via R6-R4-R3-R1, and packets from R5 destined for A are switched via R5-R4-R2-R1. Show the MPLS tables in R5 and R6, as well as the modified table in R4, that would make this possible.