COMP3250 Computer Networks

Lecturer In Charge of Assignment Two:

• Total Marks: 15
• Weighting (Value): 15%
• Due: Week 12 Friday, 27 th of October 2023, 11:55 pm. (Via Turnitin)

Objectives

This assignment will test your knowledge in the following areas:

• Transmission Control Protocol (TCP)
• Internet Protocol Security (IPsec)

Assignment Type: Problem Solving (Individual Assignment)

The purpose of the problem-solving assignment is to help the students to get accus

tomed to dealing with real world problem situations/issues. It is designed to help

students analyse a particular problem and find its best solution. There is no word

limit for answering questions.

A Cautionary Note

These questions were fed into ChatGPT and some other common AI tools that are

out there prior to being published. Students are advised to exercise caution since

answers provided by ChatGPT are not always correct and can even be downright

misleading. Students who do not have a strong understanding of the subject matter

or do not cross-check with other resources might fall victim to ChatGPTs false con

fidence. We strongly advise you to NOT use answers generated by ChatGPT as is.

Use your own words when you answerquestions and not ChatGPT.

Questions

Question 1: (IPsec) [5 marks] In the figure shown below, router RTA is the

headquarter gateway router and RTB is the branch office gateway router.

Routers RTA and RTB use VPN technology to guarantee privacy, security

and anonymity for intra-organizational information sharing over the public

internet. To this effect, the two routers have established an IPSec Security

Association (SA) and use the IPSec ESP protocol in tunnel mode for bi-di

rectional information exchange. Suppose the router RTA receives an ordi

nary IPv4 datagram from a local host with an IP address of 10.28.1.17 (in

the headquarters network) which is destined to a host with an IP address of

10.28.2.48 (in the branch-office network)

1. [1mark] Draw the IPsec datagram format.
2. [1mark] Identify the source and destination IP address in the original

IP datagram.

3. [1mark] Identify the source and destination IP addresses in the new

IP header.

4. [1mark] Identify the protocol number in the protocol field of the new

IP header.

5. [1mark] Bob, who is a man-in-the-middle, is sitting somewhere be

tween the two gateway routers RTA and RTB. Assume that Bob does

not have any knowledge of shared authentication and encryption keys

used by the SA. What information Bob can and cannot extract from

this IPSec datagram traversingthe public Internet?

Question 2a: (TCP) [4 marks] Suppose a host H initializes a TCP connection

with a server S. S allocates a buffer size of 20KB (1KB=1024B) and sets its

window to cover the whole buffer (rwnd = 20). When a TCP connection is

initially established between H and S, the value of congestion window size

(cwnd) is initialized to 32KB. Assuming H is sending data to S with a MSS

(maximum segment size) = 1KB. The initial sequence number is set to 100,

with average RTT = 200ms. The server S acknowledges each segment from

1. Assume that no segment is lost, duplicated, or delayed along the path to

the destination.

1. [1mark] Determine the SYN and ACK numbers in the SYN+ACK packet

sent from the server to the client in response to the clients SYN message

during the initial TCP 3-Way handshake.

2. [1mark] What are the congestion and receive window sizes (cwnd and

rwnd) specified in the 8th ACK segment which the client receives from

the server?

3. [1mark] What is the sequence number of the 9th segment sent from host

H to server S.

4. [1mark] During what transmission round, the server announces its win-

dow size as rwnd=0 and how many segments have been successfully re-

ceived by the server Sprior to this announcement?

Question 2b: (TCP) [6 marks] Considering a scenario in TCP Tahoe involving

congestion control: Assume the slow start threshold is initially set to 16 segments

(half of the maximum window size). A time-out occurs after the 3rd RTT, and a

three-ACKs event occurs after the 13th RTT, connection is terminated after the

20th RTT.

1. [1mark] Draw a figure showing the TCP congestion window size as a func

tion of RTT based on TCP Tahoe protocol.

2. [1mark] Identify the intervals of time when TCP Tahoe slow start is operat

ing.

3. [1mark] Identify the intervals of time when TCP congestion avoidance is op

erating.

4. [1mark] What are the values of the congestion window size and ssthresh

when the three duplicate ACKs event occurs?

5. [1mark] What is the congestion window size when the connection is termi

nated?

6. [1mark] What will be the congestion window size when the connection is

terminated if TCP Reno is used instead of TCP Tahoe?

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