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In the Figure 24.13 of Data Communications and Networking, 5/e the first ACK message from the server holds y-1 in it's Sequence no. field. Note that the first FIN message from the client message had y in it's Acknowledgement no. field. So shouldn't the first ACK message from the server be holding y in it's Sequence no. field instead of holding y-1? Is the diagram faulty or is there a reason behind it?

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The FIN segment consumes 1 sequence number as it does not carry data. ACK does not consume a sequence number. The FIN segment is sent by the client process. The server process receives a FIN segment but does not send a FIN segment as the connection is not closed, since it is a half-close process. Server process send only an ACK segment to acknowledge receipt of FIN segment from the client. Hence, the sequence number decreases by 1 and becomes $y-1$.

Contrast this with the Three way handshaking, where the sequence number is $y$ because in that process, the server sends a ACK segment as it acknowledges receipt of FIN segment from the client and sends a FIN segment as it terminates the connection.

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  • $\begingroup$ Thanks for your edit to the title. "Doubt about" is only valid in some regional English dialects (not that it makes it wrong, but potentially confusing). However, more generally, putting "question", "doubt", "issue" etc. in titles is generally considered noise, which doesn't help the question be clearer, so it is better to just remove such phrases. Welcome to the site. $\endgroup$ – 6005 Jun 23 at 14:34

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