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(COMP361)mid-term-fall-2006.pdf
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COMP 361 Computer Communications Networks I
Fall Semester 2006
Midterm Examination
Date: October 25, 2006 Time: 6:00 pm C 8:00 pm


Name:
_______________
Student ID:
_______________
Email:
_______________




Instructions
1. Please write your name, student ID and email address on this page.

2. Write your student number on each of the following pages.

3. Please answer all questions within the space provided on the examination paper. Please be concise.

4. This paper consists of 5 questions and 10 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
20


3
20


4
22


5
28


TOTAL
100










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

a) FTP uses a single connection for all operations. T F


b) TCP is a pure GBN protocol. T F


c) For a result of Traceroute, the round-trip delay of packet n sent to a router n can be longer than the round-trip delay of packet n+1 sent to router n+1. T F


d) Datagram networks maintain some connection-state information in the switches. T F


e) GBN does not need receiver buffer management. T F


f) Applications built upon UDP can not have reliable data transmission. T F


g) UDP provides an unreliable connection oriented service T F


h) Rdt 3.0 assumes underlying channel can lose packets and bit error in packets. T F



i) Two distinct web pages from the same server can be sent over the same persistent connection T F


j) Suppose Host A send over a TCP connection to Host B one segment with sequence number 36 and 4 bytes of data. In the same segment, the acknowledgment number is necessarily 40. T F





2. (20 points) Please briefly answer the following questions:

a) What are the layers in the Internet protocol stack? What are the pros and cons of such a protocol layering? (5 points)



Answer: They are physical, link, network, transport, application layers. [1]
Pros: separate a complex system to simpler components, which makes it easier to develop and update system components. [2]
Cons: one layer may duplicate lower-layer functionality; one layer may need information in another layer [2]












b) What is the function of caching? Give two examples of caching used in the Internet. (5 points)



Answer: Caching mechanism in the Internet stores recently requested contents in local, which can accelerate future requests and reserve network resources. [3]
Web caching and DNS caching. [2]