Question

Question 1 (28\%) (This question is related to ILO2a - "be able to comprehend the challenges and explain the principles in providing reliable communication in an unreliable medium", and ILO3 - "be able to carry out reliable data transfer protocols in given networks".) a) Consider two hosts A and B that are quite far apart (i.e., d pg≫d tx). They are communicating using Go-Back-N protocol with a window size of 5 and a sequence number space of 4 bits. Assuming A is the source of the data and B is the sink, i.e., this is a one-way data flow, and packets may be lost or corrupted but without reordering due to network routing. At present, the receive window at B is at [13], i.e., the next expected sequence number is 13 . Consider the following (independent) events. i. B receives an uncorrupted data packet with sequence number 13 . What will B do? Given this scenario, estimate what are the possible sender's windows at A at this moment? ii. B receives an uncorrupted data packet with sequence number 11 . What will B do? Given this scenario, estimate what are the possible sender's windows at A at this moment? iii. B receives an uncorrupted data packet with sequence number 0 . What will B do? Given this scenario, estimate what are the possible sender's windows at A at this moment? b) Consider two hosts A and B that are quite far apart. They are communicating using Selective Repeat protocol with a window size of 5 and a sequence number space of 4 bits. Assuming A is the source of the data and B is the sink, i.e., this is a one-way data flow, and packets may be lost or corrupted but without reordering. Consider the following (independent) events. i. At present, the sender's window at A is at [5,6,7,8,9] and all labels are usable but not yet used. A receives an uncorrupted ACK packet with seq \# 4. Is it possible? If so, explain why A may receive an ACK 4 and estimate what are the possible receiver's ii. At present, the sender's window at A is at [5,6,7,8,9] and all labels are used, i.e., A ′s window is exhausted. A receives an uncorrupted ACK packet with seq \# 4 . Is it possible? If so, explain why A may receive an ACK 4 and estimate what are the possible receiver's window at B at the time B sent this ACK 4. If not, explain why it is not possible. iii. At present, the receive window at B is at [1,2,3,4,5]. B receives an uncorrupted data packet with seq \# 4. Is that possible? If so, what will B, do and estimate what are the possible sender's windows at A at this moment? if not, explain why it is not possible. iv. At present, the receive window at B is at [8,9,10,11,12]. B has successfully received packets with seq \# 10 and \# 11 and has sent ACK 10 and ACK 11 to A. Then B receives an uncorrupted data packet with seq \# 13. Is that possible? If so, what will B do, and estimate what are the possible sender's windows at A at this moment? If not, explain why it is not possible.

Answer 1

When B receives uncorrupted data packets with different sequence numbers, it will acknowledge the packets and adjust its window. The possible sender's windows at A are dependent on the previous packets sent.

Step-by-step explanation:

In the scenario where B receives an uncorrupted data packet with sequence number 13, B will acknowledge this packet and move its window to [14, 15, 0, 1, 2]. Since the next expected sequence number is 13, the possible sender's windows at A at this moment could be [9, 10, 11, 12, 13] or [10, 11, 12, 13, 14] depending on the previous packets sent.

In the scenario where B receives an uncorrupted data packet with sequence number 11, B will acknowledge this packet and move its window to [12, 13, 14, 15, 0]. Since the next expected sequence number is 13, the possible sender's windows at A at this moment could be [9, 10, 11, 12, 13] or [10, 11, 12, 13, 14] depending on the previous packets sent.

In the scenario where B receives an uncorrupted data packet with sequence number 0, B will acknowledge this packet and move its window to [1, 2, 3, 4, 5]. Since the next expected sequence number is 13, the possible sender's windows at A at this moment could be [9, 10, 11, 12, 13] or [10, 11, 12, 13, 14] depending on the previous packets sent.