netdev - Re: [PATCH][RFC] tcp: fix ambiguity in the `before' relation

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Date:	Thu, 21 Dec 2006 14:42:38 +0000
From:	Gerrit Renker <gerrit@....abdn.ac.uk>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH][RFC] tcp: fix ambiguity in the `before' relation

Hi David,

many thanks for taking the matter seriously and investigating
it further. 

|  I went over this patch and analysis a dozen times, because I
|  couldn't believe something like this has been broken for
|  so long :-)
It gave me some grief too, when I looked at DCCP sequence numbers %-)
RFC 1982 provides some definitions, but leaves the case a = (b + 2^31) % 2^32
open to the implementation (suggests `undefined').

I think the new definition is more conformant with RFC 1982 than the old one,
since the ambiguity is now removed with regard to a = (b + 2^31) % 32, and it
is not "unnecessarily burdensome to implement" (section 3.2 of RFC 1982).

|  Even BSD suffers of this issue, since the beginning.  See
|  SEQ_LT() in tcp_seq.h, and it seems that BSD's timestamp
|  sequence checking has the issue too (see TSTMP_LT() macro
|  in OpenBSD's tcp_input.c)
I didn't know about OpenBSD, but in Stevens vol 2 (sec. 24.7) it is
already defined in this way. 

Best regards & merry Christmas
Gerrit
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