How ACKs work

up2thetime

Trying to understand windowing and ACKs..

If PC1 is trying to send data to PC2 and PC2 has given a window of 3000

PC1 sends:
1000
2000
3000

PC2 replies with:
ACK 4000

PC1 will then send:
4000
5000
6000

Let us say an error occurs upon sending the 5000 (packet dropped or bad cabling)... This means that PC2 will reply with ACK 5000 which indicates it has not gotten 5000. When PC1 receives this, will it resend:
(a) just 5000
(b) 5000 and 6000
or
(c) 5000 6000 7000 since the window size is still 3000

(Assume that even though there was an error in the transmission of 5000, 6000 was still received successfully).

Or will PC2 indicate to PC1 that it JUST needs 5000 and the window size becomes irrelevant for this retransmission..

Much thanks!

fieldmonkey

I was studying this a couple of days ago and this is how I understand that TCP will react to lost segments.

In this case, the receiver will send an ACK 5000 back to the sender requesting that lost segment only. The receiver will continue to send this ACK = 5000 until received.

Now if the receiver also never received SEQ 6000, it would send an ACK for that sequence also. Whether that is simultaneous or not, I have no idea.

Once the sender receives the ACK = 5000 from the receiver, it will send the SEQ 5000 again (ONLY) and then wait for a reply ACK from the receiver, which would be ACK = X000, where X represents the next sequence needed. At that point the transmission would begin again restablishing the initial window size at the next needed sequence number.

For example, If the ACK came back at 5000, it would then begin retransmission of the same window size sending 6000, 7000 and 8000. If there was no problem with the SEQ 6000 the receiver would send an ACK = 7000, requesting 7000, 8000 and 9000.