CMPSCI 653: Computer Networks
Homework 1: Network Layer and Transport Layer
Web posted: Mar 23, 19:00 Due: March 30, 17:00

VIP/NTU Students: Due one week from when you receive the assignment or view Lecture 15 whichever is later.
  1. Distance Vector Routing
    Chapter 4, Problem 6 from the Ross Kurose text book (page 317).
    Online version is here (look at the Problems section, not the review questions).

  2. Multicast routing
    Consider the two basic approaches identified toward achieving multicast: unicast emulation and network-layer-multicast. Consider a sender and 32 receivers. Suppose that the sender is connected to the receiver through a binary tree of routers. What is the cost of sending a multicast packet, in the case of unicast emulation and network-layer multicast, for this topology? Assume that every time a packet (or a copy of the packet) traverses a link, it incurs a cost C. What topology for interconnecting the senders, receivers, and routers will bring the cost of unicast emulation and network-layer multicast as far apart as possible? You can choose as many routers as you like.

  3. Congestion Control
    As a possible congestion control mechanism in a subnet using virtual circuits internally, a router could refrain from acknowledging a packet until (i) it knows its last transmission along the virtual circuit was received successfully, and (ii) it has a free buffer. For simplicity, assume that the routers use a stop-and-wait protocol and a that each virtual circuit has one buffer dedicated to it for each direction of traffic. If it takes T seconds to transmit a packet (data or acks) and there are n routers on the path, what is the rate at which packets are delivered to the destination host? Assume that transmission errors are rare, and the host router connection is infinitely fast.

  4. UDP
    Why does UDP exist? Would it not have been sufficient to let applications send raw IP packets?

  5. TCP
    Consider the idealized model for the steady-state dynamics of TCP: the average throughput of a connection in the period where the congestion window varies from (W*MSS)/2 to W*MSS is (0.75 * W * MSS)/RTT. In this scenario, assume that only one packet is lost at the very end of each period. Show that the loss rate is equal to L = 1/[ 3/8 * W*W + 3W/4]
Prashant Shenoy
Last modified: Wed Mar 29 10:07:26 EST 2000