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To: Mike Silbersack <silby@silby.com>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: Your message of "Wed, 21 Apr 2004 01:50:28 CDT."
             <20040421014736.H1228@odysseus.silby.com> 
Date: Wed, 21 Apr 2004 08:50:07 +0200
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cc: freebsd-security@freebsd.org
cc: Don Lewis <truckman@freebsd.org>
cc: avalon@caligula.anu.edu.au
Subject: Re: [Full-Disclosure] IETF Draft - Fix for TCP vulnerability (fwd) 
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In message <20040421014736.H1228@odysseus.silby.com>, Mike Silbersack writes:
>
>On Tue, 20 Apr 2004, Don Lewis wrote:
>
>> I am concerned that step C will not solve the compatibility problem. The
>> FreeBSD host is sending a FIN to close an established connection, and
>> the peer host adding the window size advertised in the FIN packet to the
>> sequence number acknowledged in the FIN packet, and using the sum as the
>> sequence number for the RST packet, which puts the sequence number at
>> the end of the receive window.
>
>Would it be feasible for us to create a four to five element array to
>track "resettable" sequence numbers?  This could hold the sequence numbers
>of the last few packets transmitted, and account for that edge case as
>well.

Sounds like an interesting idea.

Technically you will have to hold the sequence numbers for all
non-ACK'ed packets, which may be up to the window divided by the
MTU.  In the conventional case, worst case is 237 sequence numbers
(65535/276).   This sounds like a lot until one realizes that at
the same time we hold 64k of un-ACK'ed data.

A prototype would be a good thing...

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.