Dr. M. Moh,        CS 258              Homework 5                             Spring 2008

(Revised 2/25/08)

 

1.       (Problem 4-40) An organization has a class C networks 200.1.1 and wants to form subnets for four departments, which hosts as follows: A – 72 hosts, B – 35 hosts, C – 20 hosts, and D – 18 hosts. There are 145 hosts in all.

a.       Give a possible arrangement of subnet masks to make this possible.

b.       Suggest what the organization might do if department D grows to 34 hosts.

 

2.       Corrected

(Problem 4-45) Table 4.16 (shown below) is a routing table using CIDR. Address bytes are in hexadecimal. The notation “/12” denotes a netmask with 12 leading 1 bits, that is, FF.F0.0.0. Note that the last three entries cover every address and thus serve in lieu of a default router. State to what next hop the following will be delivered.

a.       C4.5E.13.87.

b.       C4.5E.22.09.

c.       C3.41.80.02.

d.       5E.43.91.12.

e.       C4.6D.31.2E.

f.        C4.6B.31.2E.

 

(Table corrected)

Net/Mask-Length

Next Hop

C4.50.0.0/12

A

C4.5E.10.0/20

B

C4.60.0.0/12

C

C4.68.0.0/14

D

80.0.0.0/1

E

40.0.0.0/2

F

00.0.0.0/2

G

 

 

3.       (Problem 5-9) You are hired to design a reliable byte-stream protocol that uses a sliding window (like TCP). This protocol will run over a 100-Mbps network. The RTT of the network is 100ms, and the maximum segment lifetime is 60 sec.

 

a.       How many bits would you include in the AdvertisedWindow and SequenceNum fields of your protocol header?

b.       How would you determine the numbers given above, and which values might be less certain?

 

4.       (Problem 6-5) Is it possible for TCP Reno to reach a state with the congestion window size much larger than (e.g., twice as large as) RTT × bandwidth? If so, explain how, and if it is likely.

 

5.       (Problem 6-16) Assume that TCP implements an extension that allows window sizes much larger than 64 KB. Suppose that you are suing this extended TCP over a 1-Gbps link with a latency of 100 ms to transfer a 10-MB file, and the TCP receive window is 1 MB. If TCP sends a 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 1MB?

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?