Charles Swiger wrote:The TCP stack wants the remote end to acknowledge the last FIN it sends and close the connection cleanly, and there is a timer (2 * MSL?) which gets started when a connection moves into the closing stages (FIN_WAIT_1, FIN_WAIT_2, LAST_ACK).
So FIN_WAIT_2 is the time the server waits to receive the FIN from the client, but although it doesn't get this last FIN message yet, it sends ACK's for the expected FIN message?
FIN_WAIT_2 is a name describing the state of a TCP connection. It's defined in a state diagram in RFC-793. But otherwise, your description is pretty good:
[ ... ]
| CLOSE +---------+
| ------- | ESTAB |
| snd FIN +---------+
| CLOSE | | rcv FIN
V ------- | | -------
+---------+ snd FIN / \ snd ACK +---------+
| FIN |<----------------- ------------------>| CLOSE |
| WAIT-1 |------------------ | WAIT |
+---------+ rcv FIN \ +---------+
| rcv ACK of FIN ------- | CLOSE |
| -------------- snd ACK | ------- |
V x V snd FIN V
+---------+ +---------+ +---------+
|FINWAIT-2| | CLOSING | | LAST-ACK|
+---------+ +---------+ +---------+
| rcv ACK of FIN | rcv ACK of FIN |
| rcv FIN -------------- | Timeout=2MSL -------------- |
| ------- x V ------------ x V
\ snd ACK +---------+delete TCB +---------+
------------------------>|TIME WAIT|------------------>| CLOSED |
+---------+ +---------+ TCP Connection State Diagram
Figure 6.It's not uncommon for some clients (think dialup users running Win98), to drop the connection rather rather than close it properly.So the client has time to ACK the server's FIN, but the user closes the dialup connection, so the application hasn't sent the FIN?
Your http server answers the HTTP request, and tries to close the connection. It sends a the data, followed by a FIN. It is waiting to receive an ACK of the FIN, and then receive a FIN from the client indicating that the client has no more data to send and is willing to close the connection.
-- -Chuck
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