Subject: [Diffserv] RFC 2597

From: "John Dullaert" <[email-masked]>
Date: Thu, 18 May 2000 10:11:23 -0400

Hello, I am a Masters student commencing a thesis in the end-to-end IP QoS
realm, focusing on Diffserv.  I am currently in the literature search
stage, trying to narrow my focus to a realistic problem definition.

The point of this e-mail is that I am looking for clarification to better
my understanding of subject RFC, which defines the AF PHB.  The recommended
codepoints follow the direction laid out in RFC 2474, in that the
requirement for improved service for increasing Class Selector Codepoints
(bit values of the first three bits) is met.  On the other hand, and this
is where I am seeking the clarification, it appears that service decreases
(more packets are dropped) the higher the bit value for the drop precedence
level (last three bits).  At first glance, this appears counter-intuitive.
At second glance, it makes sense if the definition was done from a
perspective of drop probability and matching the higher value to the higher
probability, which is unfortunately not consistent with the perspective
used for the first three bits.  Hence, my need for clarification.

Thank you for your time,

John C. Dullaert
Captain
Royal Military College of Canada
ECE Grad Student
548-8053
fax 548-8408

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From: Brian E Carpenter <[email-masked]>
Date: Fri, 19 May 2000 09:01:22 -0500

John,

First of all, the four AF PHBs are identically defined - they are
not, repeat not, in any type of priority order. This is different
from the class selectors, which are tightly defined to be backwards
compatible with IP Precedence.

Second, you are correct that the three code points for each AF PHB happen
to be defined such that a numerically larger value in three particular bits
corresponds to a higher probability of the packet being dropped. However,
this an arbitrary choice with no particular significance.

The point is that *all* the recommended diffserv code points are just 
arbitrary bit patterns; we could have chosen them at random except for the
desire to be backwards compatible with IP Precedence. Also as described
in RFC 2474, implementations must allow for an arbitrary mapping of
any code point to any PHB; the standard code points are only recommendations.

  Brian Carpenter