Description

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• This assignment has a total of 100 points.
• Please write your answer in the white space to the right of the corresponding problem.
1 Choose all that Apply – 3 × 3 points
Each question may have more than one correct answer. You will only get points if you identify all the correct answers.
1. A TCP socket is an end to end connection between two .
(a) processes
(b) threads
(c) hosts
(d) devices
2. Suppose Host A sends one segment with sequence number 40 and 8 bytes of data over a TCP connection to Host B. In this same segment the acknowledgement number is necessarily 48.
(a) True
(b) False
3. Consider Selective ACK protocol. Choose all that apply
(a) Receiver’s base sequence number can be smaller than transmitter’s base sequence number
(b) Receiver’s base sequence number can be smaller than transmitter’s tail sequence number
(c) Receiver’s base sequence number can be larger than transmitter’s tail sequence number.
(d) Receiver’s base sequence number can be larger than transmitter’s tail sequence number plus one.
2 TCP examples 1 – 2 × 3 + 3 points
Assume TCP is at slow start phase from CW = 1

1. CW at time t1 =
2. CW at time t2 =
3. CW at time t3 =
4. How should the TCP transmitter react after receiving A3? Please give CW head,
CW tail, and Send
NOTE: For questions that ask how TCP reacts, the following fields are defined as:
• CW head: Congestion Window Head (also called Base); An integer
• CW tail: Congestion Window Tail; An integer
• SSthresh: Slow Start Threshold; Numerical answers round to 1 decimal place.
• Send: The packets that need to be transmitted by the TCP transmitter.
A sequence of numbers. When the transmitter has no packets to send, write [].
Example: If CW=[4,5,6,7,8], then you should answer CW head as 4 and CW tail as 8.
5. How should the TCP transmitter react after receiving A2? Please give CW head,
CW tail, and Send
3 TCP example 2 – 2 x 3 + 4 x 3 points
Assume TCP is at slow start phase from CW = 1

1. CW at time t1 =
2. CW at time t2 =
3. CW at time t3 =
4. How should the TCP transmitter react after receiving packet P3’s timeout? Please giveCW head, CW tail, SSthresh and Send
5. How should the TCP transmitter react after receiving the penultimate ACK shown ingraph? Please give CW head, CW tail, SSthresh and Send
6. How should the TCP transmitter react after receiving the last shown ACK? Please give
CW head, CW tail, SSthresh and Send
4 TCP example 3 – 2 x 2 + 3 points
Assume packets before P10 have already been acknowledged in the past and TCP is in slow start.

1. CW at time t1 =
2. CW at time t2 =
3. How should the TCP transmitter react after receiving A13? Please give CW head,
CW tail, and Send
5 TCP example 4 – 2 x 2 + 3 + 4 points
Assume that the first ACK that is shown to arrive at the TCP transmitter is A5 and TCP is in slow start. Also assume packets before P5 have already been acknowledged in the past.

1. CW at time t1 =
2. CW at time t2 =
3. How should the TCP transmitter react after receiving A5? Please give CW head, CW tail, and Send
4. How should the TCP transmitter react after timeout? Please give CW head, CW tail,
SSthresh and Send
6 TCP example 5 – 4 × 6 points
Assume packets before P10 have already been acknowledged in the past

1. What should the values of CW be at times t1, t2, t3, and t4? (Round to 1 decimal place)
2. How should the TCP transmitter react upon receiving A10? Please give CW head, CW tail, SSthresh, and Send
3. How should the TCP transmitter react upon receiving A11? Please give CW head, CW tail, SSthresh, and Send
4. How should the TCP transmitter react upon receiving A13? Please give CW head, CW tail, SSthresh, and Send
5. How should the TCP transmitter react upon receiving A12? Please give CW head, CW tail, SSthresh, and Send
6. How should the TCP transmitter react upon receiving A14? Please give CW head,
CW tail, SSthresh, and Send
7 T/F Question – 5 + 5 points
Answer true or false of the following questions and briefly justify your answer:
1. With SR protocal, it is possible for the sender to receive an ACK for a packet that fallsoutside of its current window.
2. With GBN, it is possible for the sender to receive an ACK for a packet that falls outsideof its current window.
8 GBN Question – 6 x 2 points
Consider the Go-Back-N protocol with a send window size of N and a large sequence number range. Suppose that at time t, the next in-order packet that the receiver is expecting has a sequence number of k. Assume that, the medium may drop packets but does not reorder messages. Also, “window-base” as taught in class is also called the “window-head”.
1. What are the possible set of sequence number inside the sender’s window at time t?Justify your answer.
2. What are all possible values of ACK field in all possible message currently propagtingback to the sender at time t? Justify your answer.