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(COMP361)[2011](s)final~=2_k^_86546.pdf
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COMP 361 Computer Communication Networks I
Spring Semester 2011
Final Examination
Date: May 24, 2011 Time: 4:30 pm C 6:30 pm
Name:_____________________ Lecture: Student ID: __________Email:__ ________
Instructions
1. Please write your name, student ID and email address on this page.

2. Please answer all questions within the space provided on the examination paper. You may use back of the pages for your rough work.

3. Please be concise, and this is NOT an essay contest.

4. This paper consists of 7 questions and 7 pages.

5. Please read each question very carefully and answer the questions clearly to the point. Make sure your answers are neatly written, legible, and readable.

6. Show all the steps used in deriving your answer, wherever appropriate.




Question
Points
Score

1
10


2
15


3
16


4
10


5
10


6
15


7
24


Total
100






1. (10 points) Answer the following true/false questions by circling either T or F.



a) The Ethernet achieves better utilization than Token Ring under heavy load T F

b) Only a client can close a TCP connection T F

c) DHCP can assign IP addresses to hosts T F

d) MAC address has 4 bytes T F

e) The maximum throughput of ALOHA is twice than that of slotted ALOHA T F

f) NAT allows one IP address to be shared by multiple hosts T F

g) BGP is a Distance-Vector (DV) based protocol using the shortest-path routing. T F

h) OSPF is a link-state based routing protocol. T F

i) TCP does both flow control and congestion control T F

j) ARP maps a MAC address to an IP address T F


2. (15 points) TCP Tahoe works as follows, when congestion window size (cwnd in number of MSS) is smaller than the value of ssthread, a TCP sender is in slow-start phase in which cwnd grows exponentially. When cwnd >= ssthread, a TCP sender is in congestion avoidance phase, in which cwnd grows linearly. When congestion occurs (either timeout or three duplicated ACKs), ssthread is reduced to cwnd/2, and cwnd = 1 MSS).


Now suppose that initially, ssthread = 8, at timeslot 1, cwnd = 1, the 1st congestion occurs when cwnd = 12 and the 2nd congestion occurs when cwnd = 10. Please plot cwnd vs. time in the following figure.
0246810121416024681012timecwnd

3. (16 points) Link-state routing


a) What is the major overhead in a link-state routing protocol? What is the benefit of that? (4 points)
Answer: The major overhead is to broadcast the link state of every link to all routers. It makes link-state routing to be a global routing algorithm, which is robust and converges fast.
b) Consider a network given below with the indicated link costs, please fill out the routing table at node v using link-state routing algorithm. (12 points)


Answer:
Dijkstra algor