Duplicating ACKs and Window Sizes

https://serverfault.com/questions/503345/three-duplicate-acknowledgementss

I am confused. Do ACK values repeat when a packet is lost or when an out of sequence packet is received? For example in the diagram, the ACK values are same at the 12th and 13th segment. However, there's a drastic change in the window size. What exactly is the link?

I also read that selective acknowledgments could be used in segement 8. I know what they do by definition but I don't understand why we could use them here in segment 8 particularly. Any ideas? If it's because of the same SEQ & ACK numbers in the corresponding segments, it would be the same for segment 10, right?

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– D.W.
Commented Apr 6, 2020 at 16:49

In simple words: ack is the way of the receiver to tell you "i got data until some num now send me from this num",now if a packet is lost it means the reciver will not get its data.

If i understood your question right then we can see the acks in the 12th and 13th packets are identical,the reason is because not data was sent between them to the reciver

Now why the window size is bigger?its simply because the receiver reads the data in a certain speed and the buffer now has more space in it

So th reason they have same acks its because no data was sent to the receiver And the reason why the window size is bigger is because the receiver reads from the buffer

• I also read that selective acknowledgments could be used in segement 8. I know what they do by definition but I don't understand why we could use them here in segment 8 particularly. Any ideas? If it's because of the same SEQ & ACK numbers in the corresponding segments, it would be the same for segment 10, right?
– x89
Commented Apr 9, 2020 at 2:56
• Notice that the 6th packet is lost the 8th packet is an ack that says "i got data until 2230" andin the 9th packet seems like the sender resends the lost packet,how do i know that?its because you can see the sequence number in 2230 just like the 6th one who was lost the 10th packet is an ack that says "i got data until 3848" Commented Apr 9, 2020 at 7:44
• Makes sense. In this case, SACK can also be used in segement 14th right? with a left edge of 5830 and right edge of 6631.
– x89
Commented Apr 10, 2020 at 2:17
• oh no wait, 14th is from client to server
– x89
Commented Apr 10, 2020 at 2:29