Re: [Diffserv-interest] Fw: RFC 3662 on A Lower Effort Per-Domain Behavior (PDB)

Roland Bless <bless@tm.uka.de> Sat, 20 December 2003 23:01 UTC

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Date: Sun, 21 Dec 2003 00:00:39 +0100
From: Roland Bless <bless@tm.uka.de>
To: "Jeff Norman" <jnorman@nortelnetworks.com>
Cc: diffserv interest <diffserv-interest@ietf.org>
Subject: Re: [Diffserv-interest] Fw: RFC 3662 on A Lower Effort Per-Domain Behavior (PDB)
Message-Id: <20031221000039.06b840ff.bless@tm.uka.de>
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References: <200312200004.hBK04ak24849@zcars04f.nortelnetworks.com> <20031220234842.7d0a4b0c.bless@tm.uka.de>
Organization: Institute of Telematics, University of Karlsruhe
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Hi Jeff, 

sorry, I hit the return key too early by accident. I had two additional
remarks (see inline).

On Sat, 20 Dec 2003 23:48:42 +0100 Roland Bless <bless@tm.uka.de>; wrote:

> On Fri, 19 Dec 2003 19:04:31 -0500 (EST) "Jeff Norman" <jnorman@nortelnetworks.com>; wrote:
> 
> > Hmmm.... I would have thought that a single DSCP value may have
> > been suggested for the LE PHB... perhaps a value of 000110 would
> 
> It's a _PDB_ definition actually, so the document describes which PHBs
> could be used (cf. RFC 3086). It doesn't need to make any suggestion 
> or recommendation for a DSCP value (which may be appropriate for a PHB 
> definition).
> 
> > suffice, although it's not compatible with precedence bits (which would
> > be CS0, although I think is better than CS1 which conflicts with AF1*).
>           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> No, that's not true: CS1 doesn't conflict with AF1x. 

See also RFC 2597:
7. Interactions with Other PHB Groups

   The AF codepoint mappings recommended above do not interfere with the
   local use spaces nor the Class Selector codepoints recommended in
   [Nichols].

> You'll ask: Why not? (It has been said so many times before:) 
> DSCPs have no structure, so CS1=001000 is not in AF1x={001010,001100,001110}
> (you should never apply any bit mask to the DSCP, because all six bits
> must be considered). RFC2474, sec 4.2.2.1:
>    The DS field values of 'xxx000|xx', or DSCP = 'xxx000' and CU
>    subfield unspecified, are reserved as a set of Class Selector
>    Codepoints.

Therefore, Class Selector Codepoints have always 000 in the lower three 
bits, so they do not interfere with any recommended AF DSCP values.

Regards,
 Roland

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