Date: Wed, 28 Nov 2001 06:38:23 +0100 From: Cliff Sarginson <cliff@raggedclown.net> To: freebsd-questions@freebsd.org Subject: Re: RFC 1122 and the Urgent flag Message-ID: <20011128063823.A31258@raggedclown.net> In-Reply-To: <20011128130241.G579@k7.mavetju.org> References: <20011127202507.A960@starpower.net> <20011128130241.G579@k7.mavetju.org>
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On Wed, Nov 28, 2001 at 01:02:41PM +1100, Edwin Groothuis wrote:
> On Tue, Nov 27, 2001 at 08:28:08PM -0500, Bob Hall wrote:
> > I've been studying the OReilly Internet protocols book and I
> > found a paragraph that says that many BSD systems are not
> > compliant with RFC 1122 in how they deal with the Urgent flag.
> > The book is at least two years old now, so I'm wondering if
> > this is still true, and specifically, if it is true for
> > FreeBSD. This isn't an urgent question, but I've done a
> > search in the list archives and on google and haven't found
> > an answer. No apps will die if I don't get an answer, it
> > just relates to something I'm studying.
>
> According to Richard Stevens TCP/IP Illustrated Volume 1 section
> 20.8 on page 292 and following (which I had accidently laying open
> on the TCP setup):
>
> There is continuing debate about whether the urgent pointer
> points to the last byte of urgent data, or to the byte
> following the last byte of urgent data. The original TCP
> specification gave both interpretations but the Host
> Requirements RFC identifies which is correct: the urgent
> pointer points to the last byte of urgent data.
>
> The problem, however, is that most implementations (i.e.,
> the Berkeley-derived implementations) continue to use the
> wrong interpretation. An implementation that follows the
> specification in the Host Requirements RFC might be compliant,
> but might not communicate correctly with most other hosts.
>
According to "Building Internet Firewalls" 2nd Edition, page 87,
(which I had accidentally laying open on the Protocols above
IP Section)
URG and PSH are supposed to be used to identify critical
data .... URG more generally marks data that the sender
thinks is particularly important (sometimes incorrectly
called "out of bounds" data). In practise, neither of
these is reliably implemented and for most parts firewalls
do not need to take special action on them....
--
Regards
Cliff
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