On Fri, 31 Jan 2020 14:00:27 +0000 David Laight David.Laight@ACULAB.COM wrote:
From: sjpark@amazon.com
Sent: 31 January 2020 12:24
...
The acks in lines 6 and 8 are the acks. If the line 8 packet is processed before the line 6 packet, it will be just ignored as it is not a expected packet, and the later process of the line 6 packet will change the status of Process A to FIN_WAIT_2, but as it has already handled line 8 packet, it will not go to TIME_WAIT and thus will not send the line 10 packet to Process B. Thus, Process B will left in CLOSE_WAIT status, as below.
00 (Process A) (Process B) 01 ESTABLISHED ESTABLISHED 02 close() 03 FIN_WAIT_1 04 ---FIN--> 05 CLOSE_WAIT 06 (<--ACK---) 07 (<--FIN/ACK---) 08 (fired in right order) 09 <--FIN/ACK--- 10 <--ACK--- 11 (processed in reverse order) 12 FIN_WAIT_2Why doesn't A treat the FIN/ACK (09) as valid (as if the ACK had got lost) and then ignore the ACK (10) because it refers to a closed socket?
Because the TCP protocol (RFC 793) doesn't have such speculation. TCP is stateful protocol. Thus, packets arrived in unexpected state are not required to be respected, AFAIU.
I presume that B sends two ACKs (06 and 07) because it can sit in an intermediate state and the first ACK stops the FIN being resent?
I think there is no such presume in the protocol, either.
I've implemented lots of protocols in my time, but not TCP.
If you find anything I'm misunderstanding, please don't hesitate to yell at me. Hope the previous discussion[1] regarding this issue to be helpful.
Thanks, SeongJae Park
[1] https://lore.kernel.org/bpf/20200129171403.3926-1-sjpark@amazon.com/
David
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