On Fri, Jan 31, 2020 at 7:25 AM sjpark@amazon.com wrote:
From: SeongJae Park sjpark@amazon.de
When closing a connection, the two acks that required to change closing socket's status to FIN_WAIT_2 and then TIME_WAIT could be processed in reverse order. This is possible in RSS disabled environments such as a connection inside a host.
For example, expected state transitions and required packets for the disconnection will be similar to below flow.
00 (Process A) (Process B) 01 ESTABLISHED ESTABLISHED 02 close() 03 FIN_WAIT_1 04 ---FIN--> 05 CLOSE_WAIT 06 <--ACK--- 07 FIN_WAIT_2 08 <--FIN/ACK--- 09 TIME_WAIT 10 ---ACK--> 11 LAST_ACK 12 CLOSED CLOSED
AFAICT this sequence is not quite what would happen, and that it would be different starting in line 8, and would unfold as follows:
08 close() 09 LAST_ACK 10 <--FIN/ACK--- 11 TIME_WAIT 12 ---ACK--> 13 CLOSED CLOSED
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,
AFAICT that is where the bug starts.
AFAICT, from first principles, when process A receives the FIN/ACK it should move to TIME_WAIT even if it has not received a preceding ACK. That's because ACKs are cumulative. So receiving a later cumulative ACK conveys all the information in the previous ACKs.
Also, consider the de facto standard state transition diagram from "TCP/IP Illustrated, Volume 2: The Implementation", by Wright and Stevens, e.g.:
https://courses.cs.washington.edu/courses/cse461/19sp/lectures/TCPIP_State_T...
This first-principles analysis agrees with the Wright/Stevens diagram, which says that a connection in FIN_WAIT_1 that receives a FIN/ACK should move to TIME_WAIT.
This seems like a faster and more robust solution than installing special timers.
Thoughts?
neal