CS 655: Introduction to Computer Networks Fall 2022

Homework 3

To be completed individually. Please review the academic conduct rules mentioned in the syllabus. Answer all questions. Submit on Gradescope.

This assignment is part of BU CS 655 material and is provided for educational purposes. Please do NOT share or post this assignment handout or solution, on any public site, e.g. github. Of course, you are not allowed to share your solution with classmates.

1. Suppose that you are using an extended version of TCP Reno that allows window sizes much larger than 64K bytes.1 Suppose you are using it over a 1Gbps link with a round-trip time (RTT) of 200ms to transfer 16M-byte file, and the TCP receiver’s advertised window is 2M bytes. If TCP sends 1K-byte segments, and assuming no congestion and no lost segments:

   1. How many RTTs does it take until the sender’s congestion window reaches 2M bytes? Recall that the congestion window is initialized to the size of a single segment, and assume that the slow-start threshold is initialized to a value higher than the receiver’s advertised window.

   2. How many RTTs does it take to send the file?

   3. If the time to send the file is given by the number of required RTTs times the RTT value, what is the effective throughput for the transfer? What percentage of the link capacity is utilized?

2. Consider the network given in Figure below with link costs indicated. Give the datagram-forwarding (next-hop routing) table for routers (nodes) A, B, and C. Assume that a shortest-path-first algorithm is used to select the (least cost) route to each destination.

   

   1 A 16-bit receiver’s advertised window in the TCP segment means that 216 = 64K bytes is traditionally a maximum limit on the send window.

3. Consider a campus-area network that runs the distance-vector routing protocol RIP (Routing Information Protocol), where router K has the following routing table.

   Destination	Distance	Next-Hop
   Net 1	0	direct
   Net 2	0	direct
   Net 5	8	Router L
   Net 17	6	Router M
   Net 24	6	Router J
   Net 30	2	Router Q
   Net 42	2	Router J

   Suppose router K receives the following routing update from router J.

   Destination	Distance
   Net 1	2
   Net 5	4
   Net 17	7
   Net 22	8
   Net 24	5
   Net 30	10
   Net 42	3

   Give router K’s routing table after it incorporates this update from router J. Note that RIP assumes that the distance over each network (i.e., between two neighbor routers) is 1.

4. Assume a distance-vector routing algorithm is used in a WAN of 60 switches (nodes). If costs are recorded as 8-bit numbers and cost vectors are exchanged twice a second, how much capacity per (full-duplex) link is chewed up by the distributed routing algorithm? Assume that each node has three links to other nodes.

5. Solve the exercise given in the GENI lab on “Designing subnets”.

6. What are the CIDR addresses for a network if all its addresses start with 145.98? And if this network has exactly two subnets, what are the CIDR addresses for each of its subnets?