I am trying to understand the working of Go-Back N using this website:- http://www.ccs-labs.org/teaching/rn/animations/gbn_sr/

Here, suppose we are sending frames 1,2,3,4,5,6 (window size 4) and in the current window we have frames 1,2,3,4. Now suppose ACK 4 is lost then according to the website none of frames in the current window are re-transmitted.

According to my understanding, frames 3,4,5,6 (of current window) must be re-transmitted.

Where am I going wrong?

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    Aug 9, 2017 at 14:14

2 Answers 2


If ACK 4 is lost, packets 1,2,3 are probably already ACk'ed, so the windows has advanced to 4,5,6,... Packets 5,6(,7) are on their way, and when the timeout runs out 4 is retransmitted. ACKs do not have to be in order but the window won't advance beyond a non-ACKed packet.

  • ACK 4 means that server needs 4th packet and it is sent to 3rd packet of the sender. If ACK 4 is lost then 3rd packet of the sender won't receive any ACK so it remains in the window. So in window 3,4,5,6 are there . Now are you saying that all 3,4,5,6(whole window ) is retransmitted? Because the animation in the website doesn't retransmit
    – Zephyr
    Jun 10, 2017 at 9:59
  • If ACK 4 is lost, packet 4 is retransmitted after timeout - where's the connection to packet 3? ACKs are (obviously) numbered, so the sender knows whch packets have been received and which haven't.
    – Zac67
    Jun 10, 2017 at 10:02

If ACK 4 is lost, and ACK for all previous or at least number 3 has been received, current window will be [4,5,6,(7)] where 7 is not yet available to send. Go back N goes back to packet which was not acked yet and starts from there.

Otherwise, it will start window at the number after latest received ACK.

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