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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?

Zephyr
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3 Answers3

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Well, according to this book(http://prntscr.com/guxx03), in go back N if any ACK is lost in the window(1-2-3-4-5-6), Go back N re-transmitt ALL the frames in the current window

Mendo
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in your example, in GBN if ack3 is lost since the receiver get the pckt4 so it sends cumulative ack which is ack4 so the sender doesn't retransmit any ack

niusha sbr
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ACK means something like "I've received everything up to this package (included)"

If an ACK=4 package is lost, the ACK=5 package would mean that "I've received everything up to package 5" so it means that package 4 did arrive as well.

If the original package 4 is lost instead of it's ACK, then the server would send ACK=3 for arriving packages 5, 6 ... That means something like "Hey, this package is not the one I was expecting, the last package i've got from you is the 3 (ACK=3)"

Alejandro Bello Iglesias
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