Subject: [tcpm] About the urgent pointer...

From: David Borman <[email-masked]>
Date: Wed, 11 Mar 2009 15:56:11 -0500

Hello,

One of the items on the TCPM agenda for IETF 74 is the TCP Urgent  
pointer.  I want this to be a very focused discussion around one  
question:  Should the urgent pointer point to the last byte of urgent  
data, or the first byte of non-urgent data?

RFC 793 has one place (p. 17) where it says that it is the first byte  
of non-urgent data, and at least two places (p. 41 and p. 56) where it  
says that it is the last byte of urgent data.  Though RFC 961 and RFC  
1122 resolve this ambiguity and are clear that the urgent pointer is  
defined to point to the last byte of urgent data, as draft-gont-tcpm- 
urgent-data-01 points out most systems actually implement it as the  
first byte of non-urgent data.  This goes back to the original BSD  
code from CSRG.

So, the question is *not* what do the RFCs say, because they are  
clear.  The question is, what do we do about the mismatch between what  
the RFCs say, and what is generally implemented?

			-David Borman

---

From: Fernando Gont <[email-masked]>
Date: Thu, 12 Mar 2009 15:38:30 -0200

Hello, David,

> So, the question is *not* what do the RFCs say, because they are clear. 
> The question is, what do we do about the mismatch between what the RFCs
> say, and what is generally implemented?

One important thing to note is that it is impossible to tell (without
any additional mechanism) what semantics for the UP a TCP implements.
This means that if were were to try to get real stacks implement the
semantics for the UP as specified in RFC 1122, we would basically break
the urgent mechanism.

My understanding from the talks that I had with a few folks at the last
IETF is that the only sensible thing to do is to change the spec
(considering that whether the UP points to the last byte of urgent data
vs. the byte following that chunk, does not really matter -- as long as
all stacks do the same... which is already the case).

Nevertheles, my personal feeling is that regardless of which specific
outcome, we agree that tcpm wg should do something on this issue (and
probably the last posts on this subject are an indication that at least
is clarification is needed).

That said, I believe it would be a good thing to poll the wg to adopt
this I-D -- if anything, to have a starting point to work on this issue.
(FWIW, this last revision has been substantially revised to address the
feedback I got from you and Joe at the last IETF, and feedback from
Alfred that I got while he was helping (a lot) with a related document).

Thanks!

Kind regards,
-- 
Fernando Gont
e-mail: [email-masked] || [email-masked]
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