From owner-freebsd-net@FreeBSD.ORG  Wed Sep 13 15:19:06 2006
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Date: Wed, 13 Sep 2006 19:19:04 +0400 (MSD)
From: Igor Sysoev <is@rambler-co.ru>
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To: Andre Oppermann <andre@freebsd.org>
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Cc: freebsd-net@freebsd.org, silby@freebsd.org
Subject: Re: Improved TCP syncookie implementation
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On Sun, 3 Sep 2006, Andre Oppermann wrote:

> I've pretty much rewritten our implementation of TCP syncookies to get
> rid of some locking in TCP syncache and to improve their functionality.
>
> The RFC1323 timestamp option is used to carry the full TCP SYN+SYN/ACK
> optional feature information.  This means that a FreeBSD host may run
> with syncookies only and not degrade TCP connections made through it.
> All important TCP connection setup negotiated options are preserved
> (send/receive window scaling, SACK, MSS) without storing any state on
> the host during the SYN-SYN/ACK phase.  As a nice side effect the
> timestamps we respond with are randomized instead of directly using
> ticks (which reveals out uptime).

As I understand syncache is used to retransmit SYN/ACK.
What would be if

1) a client sent SYN,
2) we sent SYN/ACK with cookie,
3) the client sent ACK, but the ACK was lost

?

I suppose the client will see timed out error.


Igor Sysoev
http://sysoev.ru/en/