Date: Tue, 07 Jan 2014 14:40:06 -0800 From: Peter Wemm <peter@wemm.org> To: freebsd-net@freebsd.org Subject: Re: TCP question: Is this simultaneous close handling broken? Message-ID: <52CC8246.7080609@wemm.org> In-Reply-To: <52CC5F2E.5030201@wemm.org> References: <52CB3AE9.3030107@wemm.org> <52CC5F2E.5030201@wemm.org>
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[-- Attachment #1 --] On 1/7/14, 12:10 PM, Peter Wemm wrote: > On 1/6/14, 3:23 PM, Peter Wemm wrote: >> We've hit a weird problem at work when dealing with simultaneous closes. >> In this particular case, it's a FreeBSD-7.4 machine talking some random >> Linux host. >> >> There is a client/server protocol in use, and both ends are doing a close >> at the same time. It might be a shutdown, I haven't seen all the code yet. > [..] >> A packet capture, with relative timestamps: >> >> 000050 freebsd.28411 > linux.14001: F 6486:6486(0) ack 232 >> 000031 linux.14001 > freebsd.28411: F 232:232(0) ack 6486 >> 000333 linux.14001 > freebsd.28411: . ack 6487 >> [200ms retransmit timer fires on linux] >> 200490 linux.14001 > freebsd.28411: F 232:232(0) ack 6487 >> 000011 freebsd.28411 > linux.14001: . ack 233 > [..] >> What am I looking at? Who's at fault? It looks like we're failing to >> recognize the ack for our fin. > > It definitely looks like FreeBSD at fault. We've simply not acked their FIN > until they retransmitted it. > > I've looked at the commit logs and I don't see anything obvious that stands > out to me for a fix for this. Most of the changes seem to be lock structure > changes than protocol fixes. I see things like ECN and other protocol > features being added as well. > > Where should I look in the code? It turns out it's fixed in HEAD. ------------------------------------------------------------------------ r258821 | eadler | 2013-12-01 19:11:25 -0800 (Sun, 01 Dec 2013) | 14 lines In a situation where: - The remote host sends a FIN - in an ACK for a sequence number for which an ACK has already been received - There is still unacked data on route to the remote host - The packet does not contain a window update The packet may be dropped without processing the FIN flag. PR: kern/99188 Submitted by: Staffan Ulfberg <staffan@ulfberg.se> Discussed with: andre MFC after: never ------------------------------------------------------------------------ The bug in question is from Date: Mon, 19 Jun 2006 23:45:05 +0200 (CEST) This is the exact situation we hit. -- Peter Wemm - peter@wemm.org; peter@FreeBSD.org; peter@yahoo-inc.com; KI6FJV UTF-8: for when a ' just won\342\200\231t do. [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/ iEYEARECAAYFAlLMgkYACgkQFRKuUnJ3cX/KuQCfV6gHtCN58vr6A+fpUsVPJ7YL jWIAn0itVXvDRhEGCH+zrsX5XO9rJNq/ =aWiN -----END PGP SIGNATURE-----
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