Homework 2 - Grading Notes

Question 1

For the reliable stop n' wait protocol, some of the common mistakes observed were:

The sequence number on the acknowledgement was not checked before declaring that the transmission was successful.
Duplicate packets were not dropped i.e. the sequence number on the incoming packet did not match the expected sequence number
Acknowledgements were not sent for duplicate packets. Duplicate packets arrive because the sender did not get the acknowledgement. If you do not acknowledge the duplicate packets, the sender will keep resending it, till it reaches the retry limit, and then report a lost packet.

A common trend observed was that though a single-bit sequence number is sufficient for this protocol (hence the name 'Alternating Bit Protocol'), most of the solutions used larger sequence numbers.

Another point to note: the timeout specified is the duration you need to wait to receive the ACK. The way it was implemented in most answers was to wait till some data was available on the wire, and retransmit if this data was not the ACK. While points were not deducted for this, observe that this approach will not strictly be correct all the time.

Question 2

There were mainly two mistakes made in this question. Several of you forgot about the (1-p)^(n-i) term, which is the probability that (n-i) nodes are NOT transmitting. The other point was that some of you used only the value of (n choose 10), instead of (n choose i), where i >= 10. This is because the net is saturated if any 'i' nodes out of n are transmitting simultaneously, and not only if exactly 10 are doing so.

Question 3

A lot of people forgot to factor in the propagation delay (RTT/2) into the equations for the first 2 questions. There were not many mistakes in questions 3 and 4. However, many of you did not take the one's complement of the checksum, before operating on it.