Hello I am trying to understand the GBN protocol and I encountered the following question during my homework.

Assume that we have GBN protocol with a window size of 20. The sender sends 15 packets one after the other with seq numbers from 1-15. After that, the sender is waiting to get acks from the server (the sender doesn't have any more info yet).

Now Firstly ack #3 arrives then ack #7 arrives and then ack #11 arrives. after that, we get a TIME-OUT.

the question is: in response to that T.O mentioned above the the sender will send ----- packets

As I understand after we get the 11th ack we know that everything before has been working just fine.

But I'm getting confused with the window size and the sequence number of the messages.

Please anyone can explain and help.


  • $\begingroup$ since timeout has occurred it will follow slow start phase till n/2 window size, i.e. 5 in this case and after that it will increment by 1 mss $\endgroup$ – Turing101 Jun 14 '19 at 3:36
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    $\begingroup$ Moreover the question is a bit ambiguous, as u are saying that the window size is 20, but here we can see that an acknowlegement is sent after every 4th packet and in GBN we sent cumulative acknowledgement after each window, so I think the window size should be 4 rather than 20, and 20 is the total number fo packets that we are trying too send $\endgroup$ – Turing101 Jun 14 '19 at 3:39
  • $\begingroup$ Sorry but I dont know why you say it will follow slow start phase till n/2 any reference? $\endgroup$ – Amitay Tsinis Jun 14 '19 at 8:43
  • $\begingroup$ geeksforgeeks.org/computer-network-tcp-congestion-control $\endgroup$ – Turing101 Jun 14 '19 at 10:15

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