Differences between TCP and Go Back N

Question

I was reading Computer Networking from Kurose, and while reading in the TCP chapter about the differences between TCP and Go Back N I found something that I don't fully understand. The book says the following about some of the differences between the two protocols:

"many TCP implementations buffer correctly received but out-of-order segs rather than discard.

also, suppose a seqof segs 1, 2, …N, are received correctively in-order,ACK(n), n < N, gets lost, and remaining N-1 acks arrive at sender before their respective timeouts TCP retransmit most one seg, i.e., seg n, instead of pkts, n, n+1, …, N TCP wouldn’t even retransmit seg n if ACK(n+1) arrived before timeout for seg n"

I understand the buffering of out-of-order segments, but I don't understand the other behavior, and I think it is because I don't fully understand Go Back N. Following that example, if ACK(n+t) arrives before Go Back N timeout, the protocol would continue as if seg n was in fact received, which is the case, because of the accumulative ACKS... so, Go Back N wouldn't retransmit that segment either.... or am I missing something?

Answer 1

The quote says that the ACK(n) got lost, not the nth segment got lost. In such case, nothing needs to be re-transmitted, because ACK(n + x) means that everything upto n + x was successfully received.

Answer 2

I was confused by the statement from the book too, but I think I have found the answer:

Consider also what happens when the sender sends a sequence of segments 1, 2, . . . , N, and all of the segments arrive in order without error at the receiver. Further suppose that the acknowledgment for packet n < N gets lost, but the remaining N – 1 acknowledgments arrive at the sender before their respective timeouts. In this example, GBN would retransmit not only packet n, but also all of the subsequent packets n + 1, n + 2, . . . , N. TCP, on the other hand, would retransmit at most one segment, namely, segment n. Moreover, TCP would not even retransmit segment n if the acknowledgment for segment n + 1 arrived before the timeout for segment n.

Actually, in the above example, even though the ACK for packet n+1 arrives at the sender before its timeout, one has to be aware that the timer for packet n could have timed-out before that arrival. So, because packet n timeout and the GBN has not seen ACK(n+1) or ACK(n+2)... so far, it will trigger the re-transmission of all packets after n .

However, for TCP, the sender would only send packet n again at this specific moment.

P.S. this question has been very old. But, anyway, hopefully that might help anyone.