Subject: RE: DiffServ field

From: "Christian Huitema" <[email-masked]>
Date: Wed, 30 Jul 2003 08:35:38 -0700

> >Does anyone know is use of DiffServ field supported in some equipment
and
> >networks?
> 
> It is supported in some equipment. I wouldn't be able to tell you how
> networks configure their equipment; they generally view this as NDA
> information.

We have found some issues when trying to use the diffserv field in
practical deployments. There are a number of routers in the network that
were built and deployed before diffserv was widely accepted as a
standard. Understandably, these routers implement the previous standard,
i.e. the RFC 791 definition of the TOS bit. Specifically, the old
routers look at the three precedence bits and treat it as a priority
level. In practice, this restricts the number of diffserv code points
that can be safely used. You don't want for example to define a code
point for "less than best effort" in which the three precedence bits
happen to mean "high priority" in the old routers...

-- Christian Huitema

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From: Brian E Carpenter <[email-masked]>
Date: Thu, 31 Jul 2003 08:36:52 +0200

That's why some care was used in choosing the recommended
code points in the various RFCs. If you use the default values,
a router set up for precedence will behave reasonably.

But indeed, an IPv4 network where some routers have legacy TOS
behaviour and others have diffserv behaviour will not deliver
perfect diffserv. We shouldn't have this problem for IPv6, where
TOS behaviour was never defined.

   Brian

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Brian E Carpenter 
Distinguished Engineer, Internet Standards & Technology, IBM 

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