netdev - Re: listen(2) backlog changes in or around Linux 3.1?

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Date:	Mon, 22 Oct 2012 15:11:23 -0700
From:	Vijay Subramanian <subramanian.vijay@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	enh <enh@...gle.com>,
	Venkat Venkatsubra <venkat.x.venkatsubra@...cle.com>,
	netdev@...r.kernel.org
Subject: Re: listen(2) backlog changes in or around Linux 3.1?

>
> I wonder then if we dont need to retransmit the synack when req moves
> into accept_queue then ?

If I understood the code correctly, the socket moves into accept_queue
only when the
third ack (with or without data) comes in. So, there should be no need
to resend syn-ack. The issue is that there is no mechanism to promote
req sockets which have finished TWHS to accept_queue currently.
Socket can move into accept_queue only when third ack is processed.
If we stop resending synacks, then socket will move into accept_queue
when client sends data.

>
> Or else how the client can 'knows' it can send data to server ?

>From client's point of view, TWHS is finished.  Client is already in
established state and
can even now send data. Currently, such packets with data will be
dropped if accept_queue is full.
If accept_queue is not full, socket moves into accept_queue and
established state and processes the data.

I think the only thing my patch does is reorder the tests so that
needless syn-ack retransmissions are stopped.

>
> All these facilities sound very complex and not really usable by clients
> (ie users not willing to wait more than few seconds anyway)
>

Fair enough. We can drop this if it is not worth the trouble or if I
have missed any other scenario.

Thanks for your review and time!
Vijay
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