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SEHS3321 1920s1 Assignment 1 1
SEHS3321 Computer Network Administration
Individual Assignment 1
Due Date: 8
th October 2019, 18:00
Submission Instruction
Submit your assignment in softcopy using MS Word or PDF file type. Upload the file via the
assignment submission link on Moodle. Late submission will NOT be marked.
Question 1
By using the browser “Internet Explorer” or “Google Chrome”, you can enter the developer
mode by pressing the function key F12. Go to the “Networking” tab, and press F5 to start
recording/capturing the network traffic. Clear the cached (temporary saved) images and files
first. Then load the SPEED homepage (http://www.speed-polyu.edu.hk). Answer the following
questions by analyzing the HTTP request and response messages. (Note: you should view source
to read the message header in chrome.)
a) How many HTTP requests were sent for loading the homepage?
b) Look at the HTTP request and response of the homepage,
i. What is the user agent of the browser?
ii. What is the connection type? Persistent or non-persistent?
iii. What are the server and PHP version used?
iv. In the response header, the cache-control field is “no-cache”. What does it mean?
v. In the response header, the keep-alive field is “timeout=5, max=100”. What does it
vi. In the response header, the transfer encoding field is “chunked”. What does it mean?
c) Look at the HTTP request and response of the file “logo_speed.gif”,
i. What is/are the accepted format(s) by the browser?
ii. What is the last modified date of the file?
iii. What is the size of the file?
iv. In the request header, there is a “referrer” field. What does it mean?
d) In the “Program Search” of the webpage, enter “84048” and select “Bachelor’s Degree”, and
click the icon to submit the search request.
i. What is the HTTP request method to submit the search request?
ii. What is the URL of the webpage that handles the request?
iii. What is the content-type of the HTTP request message?
iv. What is the content of the HTTP request body?
SEHS3321 1920s1 Assignment 1 2
Question 2
a) The screen below shows the DNS queries of the domains www.speed-polyu.edu.hk and
www.polyu.edu.hk using the command “nslookup”.
i. What is the meaning of “non-authoritative answer”?
ii. How do you interpret the difference of the two query results?
b) By using the command “nslookup” with your own default DNS server, provide the result
screen capture of the followings:
i. Find the name and IP address of mail server for your SPEED email.
ii. List the DNS servers of The Polytechnic University of Hong Kong.
iii. List the alias and CNAME records of “www.cisco.com”.
iv. List the mail servers of Gmail by Google. Which one is used with the highest priority?
v. List all root DNS servers.
c) Is it possible for an organization’s Web server and mail server to have exactly the same alias
for a hostname? Why?
SEHS3321 1920s1 Assignment 1 3
Question 3
a) UDP is unreliable while TCP provides reliable transfer service. Explain why an application
developer might still choose to run an application over UDP rather than TCP.
b) Encode each digit of your student ID number by 4-bit binary digits. Excluding the last letter
‘S’, the result should contain 32 bits. Calculate the 1s complement of the sum of all 8-bit
words in the bit stream encoded from your student ID number. Show your steps.
c) Suppose 8 bits are used for the sequence number field. What should be the appropriate
window size (in terms of number of packets) for sender and receiver if we are using:
i. Go-Back-N?
ii. Selective Repeat?
Justify your answers.
Question 4 (Let d be the last digit of your student ID number. If it is 0, take d = 10.)
Alice and Bob are communicating over a TCP connection, and Bob has already received from
Alice all bytes up through byte 358. Suppose Alice then sends two segments to Bob back-to-back.
The first and second segments contain (d x 50) and 80 bytes of data, respectively. In the first
segment, the sequence number is 359, the source port number is 1028, and the destination port
number is 80. Bob sends an acknowledgement whenever he receives a segment from Alice.
a) In the second segment sent from Alice to Bob, what are the sequence number, source port
number and destination port number?
b) If the first segment arrives before the second segment, in the acknowledgement of the first
arriving segment, what are the acknowledgement number, the source port number and the
destination port number?
c) If the second segment arrives before the first segment, in the acknowledgement of the first
arriving segment, what is the acknowledgement number?
d) Suppose the two segments sent by Alice arrive in order at Bob. The first acknowledgement is
lost and the second acknowledgement arrives after the first timeout interval. Draw a timing
diagram, showing these segments and all other segments and acknowledgements sent.
(Assume there is no additional packet loss.) For each segment in your figure, provide the
sequence number and the number of bytes of data; for each acknowledgement that you
add, provide the acknowledgement number.
- End of Assignment -

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