CODEWITHSUNDEEP
network-programmingflow-controldata-link-layergo-back-n
Zephyr
Zephyr

Reputation: 1621

Go-Back N protocol ACK

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?

Upvotes: 1

Views: 1981

Answers (3)

Alejandro Bello Iglesias
Alejandro Bello Iglesias

Reputation: 109

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)"

Upvotes: 0

niusha sbr
niusha sbr

Reputation: 11

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

Upvotes: 1

Mendo
Mendo

Reputation: 11

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

Upvotes: 1

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