I was learning about the process of closing TCP connection and the process differs in many sources I've come across. In official Cisco cert guide (100-105) the process is as follows:
1) ACK, FIN ----> 2) <---- ACK 3) <---- ACK, FIN 4) ACK ---->
I just don't see the purpose of ACK in 3) as there is already an acknowledgement in 2). Also, in older Cisco study materials, the process goes without 2) as well.
On top of that, I have this WireShark capture that made me even more confused:
Basically the same thing, what is the extra ACK in packet #38 for? Acknowledgement was already sent in #37. And both also have the same Ack number.
Could someone give me a little insight if all those are possible in certain situations or which one is correct?