Test series question(Aceacademy)

Question

Can someone explain it to me as to why in selective repeat it's not possible for an acknowledgment to come for a packet that falls outside the current window? Because it may be possible that there is some delayed acknowledgment. It's possible in all sliding window protocols then why is statement 2 only true?

Moreover, in the solution, they mentioned that statement 2 is true because GBN has cumulative ack because of which if we receive ack 2 then the sender will assume that both packets 1 and 2 have been received successfully and so it slides the window to remove 1 and 2 from it but later we might get ack 1 which I feel is not possible because here we are talking about cumulative ack not independent. So how is this reason valid?

Comments

Both are TRUE

In general case, SR protocol slides it's window only when an ACK for a packet is received. Depending on the way duplicate packets are handled at the receiver side the answer can be YES.

GBN, this definitely holds true due to the usage of cumulative ACKs.

a) True. Suppose the sender has a window size of 3 and sends packets 1, 2, 3 at t 0 . At t1 (t1 > t 0) the receiver ACKS 1, 2, 3. At t 2 (t 2 > t1) the sender times out and resends 1, 2, 3. At t 3 the receiver receives the duplicates and re-acknowledges 1, 2, 3. At t 4 the sender receives the ACKs that the receiver sent at t1 and advances its window to 4, 5, 6. At t 5 the sender receives the ACKs 1, 2, 3 the receiver sent at t 2 . These ACKs are outside its window.

 b) True. By essentially the same scenario as in (a). 

Referlink: https://www.ics.uci.edu/~keldefra/teaching/spring2013/uci_cs132/problemsets/CS132_EECS148_ProblemSet3_Solution.pdf

Both a and b are true.