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(COMP361)midterm-key-fall2005.pdf
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THE HONGKONG UNIVERSITY OF SCIENCE & TECHNOLOGY
COMP361: Computer Communication Networks I

Fall Semester, 2005
Thursday, October 27, 2005

MIDTERM EXAMINATION WITH KEY
STUDENT NAME: __________________________________
STUDENT NUMBER: __________________________________
LAB NUMBER: __________________________________

Instructions to Students
1.
There are 5 problems and 6 pages. Check that you have all 6 pages.

2.
Write your name, student number and Section number on this page.

3.
Write your student number on each of the following pages.

4.
Answer all questions in the space provided.

5.
Rough work should be done only on the back pages and afterwards draw a diagonal line through it to show that it is not part of your answer.

6.
Leave all pages stapled together.

7.
The examination period will last for 1 hour and 30 minutes.

8.
Stop writing immediately when the time is up.



For Grading Purpose Only
Problem 1 _______________ / 10 Problem 2 _______________ / 25 Problem 3 _______________ / 15 Problem 4 _______________ / 20 Problem 5 _______________ / 18
TOTAL: ______________ / 88 COMP 361 Mid-term Exam Fall 2004 Student Id: _________________
Problem 1 (10 points) (Justify all your answers)
a) Explain the difference between an iterative DNS query and a recursive DNS query. [5 points]

In the iterative query, the local DNS server queries the root and obtains the DNS server address of the TLD server, then queries the TLD server to obtain the address of an Authoritative DNS server then queries the authoritative DNS server to obtain the address of the target host.
In the recursive query, the local DNS server queries the root DNS, who will query the TLD. Then the TLD queries the Authoritative DNS. Once this reply is obtained it is forwarded back to the local DNS server via the TLD, and the root.
Any answer that captures this essential difference should get 5 marks.
b) Suppose we are retrieving a web page from web server S. Suppose the IP address for S is not cached in your local host nor in your local DNS server, so that you have to run a DNS lookup to obtain the IP address. Three DNS servers -- namely, Root DNS server, TLD DNS server and Authoritative DNS server -- are traversed before your host receives the IP address in return. Assume the query from your local DNS server to the Root DNS server is a recursive request and that the subsequent requests are iterative. Indicate and label the steps of message delivery in the following figure. [5 points]


Problem 2 (25 points) (Justify all your answers)
Consider sending a file of F bits from A to C over the network below, consisting of one intermediate node and two links. L1 transmits at R1 bps and L2 transmits at R2 bps (R2>R1). The network is lightly loaded so that there are no queuing delays (no waiting time). If packet
COMP 361 Mid-term Exam Fall 2004 Student Id: _________________
switching is used, eac