Subject: [tsvwg] David Black's comments on draft-geib-tsvwg-diffserv-intercon-

From: "Black, David" <[email-masked]>
Date: Thu, 28 Feb 2013 18:09:09 -0500

These comments are as an individual - <WG co-chair hat off>

In general, this is a valuable contribution, including the specific
proposal of the 4 diffserv interconnect classes.  It may be a good
idea to add a mapping from the current RFC 4594 classes into
these classes as a complement to the current PHB-oriented material.

Abstract and Introduction: A possible tsvwg direction forward is to
revise RFC 5127 to focus on network internals and produce a separate RFC for
interconnection of network edges - that separation is consistent with the
discussion in Section 3 of this draft about mapping between internal and
interconnection schemes.

That distinction between schemes is crucial, IMHO - it's in the second
paragraph of this draft's introduction, but could use some more emphasis.
OTOH, I'd remove the text that focuses this draft on revising RFC 5127, as
that may not be the most effective path forward, although it'd probably
be ok to retain some of the language in the introduction about concepts
in this draft being relevant to a revision of RFC 5127.

The introduction should explain more about copying of the "IP Precedence Bits"
to the MPLS TC or Ethernet P-Bits - the crucial thing that needs to be stated
is that the bits involved are the first 3 bits in the DSCP, and the values
for the recommended DSCPs for AF and EF were chosen in part based on this
consideration, so that EF has a higher priority than AF (and most other
traffic) when this is done.  This is described in Section 5, but should
also be explained in the introduction. 

The last paragraph on p.3 indicates that internal PHBs and codepoints may
be added later - I don't think that would be a good idea.  Rather, it'd be
better (IMHO) to focus this draft on the interconnect problem to keep it
cleanly separated from the RFC 4594 update discussion that also affects RFC 5127.
That paragraph could also use some minor rephrasing to avoid any implication
that this draft has priority on use of MPLS TC space - talking about other
uses as peer users of that space would be better than the current "leave some
MPLS TC codespace" language.

It's recently been discovered that draft-ietf-idr-sla-exchange-00.txt is
related to this work, so that draft should be mentioned.  The
idr-sla-exchange draft is rather descriptive, in contrast to this diffserv
interconnect draft being prescriptive.

The middle bullet in the list at the end of section 3 isn't quite right.
There are only 8 codepoints available for DSCP encoding in MPLS and Ethernet,
but some PHBs (e.g., AF) can consume more than one codepoint.  The last bullet
at the end of section 3 is unclear on whether this is about DSCPs, MPLS (and
Ethernet) codepoints, or both.

The first 3 paragraphs of section 4 feel like they belong in a related work
section, possibly an appendix, which would also be a good place for a longer
discussion of Y.1566, which is the basis for a lot of this material.

I'm not sure I understand the point of Section 6, and would suggest removing
it.  I would also combine sections 5 and 7 into a single section.

Thanks,
--David
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David L. Black, Distinguished Engineer
EMC Corporation, 176 South St., Hopkinton, MA  01748
+1 (508) 293-7953             FAX: +1 (508) 293-7786
[email-masked]        Mobile: +1 (978) 394-7754
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