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From:   <sjpark@amazon.com>
To:     David Laight <David.Laight@ACULAB.COM>
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        SeongJae Park <sjpark@amazon.de>
Subject: Re: RE: [PATCH 0/3] Fix reconnection latency caused by FIN/ACK handling race
Date:   Fri, 31 Jan 2020 16:05:44 +0100
Message-ID: <20200131150544.26333-1-sjpark@amazon.com>
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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_2
> 
> Why 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
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
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