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If Go back N protocol is used with a window size of 4 and the ACK for packet 2 gets lost.

Now how to show that the events untill packet 2 is acknowledged at the sender side ?

2 Answers

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Go Back N uses cumulative ack. Now remember that in Go back N the receiver side has window size = 1.

Now if suppose packet 2 gets lost then receiver window will expect packet no 2. In that case even if sender sends packet 3, as receiver window is looking for packet no 2, it will discard packet 3 and resend ack of packet 1 (which it last received). Sender window then will understand that packet 2 is not received by receiver so next it will send this window: [2,3,4,5].

Now in this question they are saying that the ack of packet 2 is lost. Meaning receiver window has received packet 2. Now it is expecting packet 3. As packet 3 is the next packet for the sender, sender will send it and receiver will receive it (receiver will not discard packet 3 because packet 2 is already received). Now ack of packet 3 will be sent by the receiver. When sender receives ack of packet 3 it will know that if receiver would not have received packet 2 it would have discarded packet 3 and sent ack of packet 1. But as it sent ack of packet 3 that means packet 2 is already received. So sender will not retransmit from packet 2.

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For this question also read-

http://www.myreadingroom.co.in/notes-and-studymaterial/68-dcn/813-go-back-n-arq-protocol.html

Resending the frame heading

let me know if you have any doubt

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