TCP TIME WAIT – Purpose of TIME WAIT in TCP Connection Tear Down

Does that make any sense?

No.

The FIN is send because the sender decided that it wants to close the connection. Even if you would change the recipient that it will ignore the FIN the sender side of the application will still consider the connection closed and not send or receive any more data on it.

Just compare it to a phone call. If the other end decided to hang up the phone and no longer talk to you it does not matter if you are trying to ignore this, the other end still will not talk to you any more.

What are my options?

This must be fixed inside the application. For example the application could reuse the existing TCP connection like done with HTTP keep-alive.

A -----FIN-----> B
FIN_WAIT_1       CLOSE_WAIT
A <----ACK------ B
FIN_WAIT_2

(B can send more data here, this is half-close state)

A <----FIN------ B
TIME_WAIT        LAST_ACK
A -----ACK-----> B
|                CLOSED
2MSL Timer
|
CLOSED

The ACK to a FIN is required because the end sending the FIN will retransmit it until it receives an ACK. So, the question is, why TCP keeps sending the FIN if not ACK is received? My understanding is that given that a connection may be in a half-close state, the side that is receiving the last FIN (A in your diagram) may be waiting for data "indefinitely" wasting resources in that end when no more data will be received. B needs to be sure that A received the FIN (and closes the connection), hence it requires an ACK.

Edit

To be more precise about half-close. In your example, A can close its side of the connection sending the first FIN and receiving the first ACK, but B can keep sending more data at will for any period of time before closing the connection and sending the last FIN. So, the time between the first FIN/ACK sequence and the second one can't be determined or timed-out. A needs the last FIN to be sure B has closed its side of the connection.

Edit 2

What happen if the last FIN from B to A is lost? Then, B will not receive the ACK and will retransmit the FIN until it receives an ACK. So, A will eventually get the FIN and transition to the TIME_WAIT state.

A -----FIN-----> B
FIN_WAIT_1       CLOSE_WAIT
A <----ACK------ B
FIN_WAIT_2

(B can send more data here, this is half-close state)

A  (Lost)X<--FIN------ B
FIN_WAIT_2       LAST_ACK
                 (timeout waiting for ACK)
A <----FIN------ B
TIME_WAIT
A -----ACK-----> B
|                CLOSED
2MSL Timer
|
CLOSED

What happen if the last ACK is lost? Then, B will think that A did not receive the FIN and will retransmit the FIN. From B point of view this is the same as if the FIN was lost, from A point of view this different since A now is in the TIME_WAIT or CLOSED state. When A receives the new FIN from A if it is in the TIME_WAIT state it will send the ACK again.

A -----FIN-----> B
FIN_WAIT_1       CLOSE_WAIT
A <----ACK------ B
FIN_WAIT_2

(B can send more data here, this is half-close state)

A <----FIN------ B
TIME_WAIT        LAST_ACK
A -----ACK-->X(Lost)   B
TIME_WAIT        LAST_ACK
                 (timeout waiting for ACK)
A <----FIN------ B
A -----ACK-----> B
|                CLOSED
2MSL Timer
|
CLOSED

If A is in the CLOSED state it will send a RESET, in either case B will be able to close its side of the connection.

A -----FIN-----> B
FIN_WAIT_1       CLOSE_WAIT
A <----ACK------ B
FIN_WAIT_2

(B can send more data here, this is half-close state)

A <----FIN------ B
TIME_WAIT        LAST_ACK
A -----ACK-->X(Lost)   B
TIME_WAIT        LAST_ACK
|
2MSL Timer
|
CLOSED
                 (timeout waiting for ACK)
A <----FIN------ B
A -----RST-----> B
                 CLOSED