Subject: [tsvwg] Delay vs. Loss draft: Terminology

From: "Black, David" <[email-masked]>
Date: Wed, 17 Jul 2013 17:37:21 -0400

<WG chair hat off>

This note concerns draft-polk-tsvwg-delay-vs-loss-ds-service-classes

I believe I see some sloppy use of terminology in this draft and the
related discussion.

CS4 was originally defined as a DSCP value in RFC 2474 (Diffserv field
in IP headers) and is used only as a DSCP value in RFC 4594 (Diffserv
Service classes).

That said, RFC 2474 can be read to allow use of CS4 as a PHB name - it's
effectively shorthand for "the PHB assigned to the CS4 DSCP - as is done
in draft-polk-tsvwg-rfc4594-update does.  While that may not be the best
usage, I think it's ok.

OTOH, draft-polk-tsvwg-delay-vs-loss-ds-service-classes appears to be
using strings such as "CS4", "CS4-Discardable" and "CS4+" as service
class names (see Sections 5 and 6).  That inclusion of the DSCP name
in the service class name is (IMHO), a stunningly bad idea that is
going to lead to confusion, and moreover is at odds with one of the basic
principles of the Diffserv architecture, namely separation of traffic
behavior (PHB and subsequently, service class) from the DSCP used in
the IP header to request suitable treatment in a particular network.

As an author of both RFC 2474 and 2475, I really would like to
see draft-polk-tsvwg-delay-vs-loss-ds-service-classes revised to be
clear on DSCP vs. PHB vs. service class and in particular to cease
and desist from using CS4 (or any other DSCP name) as the name or part
of the name of a service class - that will inevitably lead to the bad
and wrong assumption that such a service class can only ever be
deployed with the CS4 DSCP - while that sort of relationship is
RECOMMENDED by RFC 4594, it is not REQUIRED (where "RECOMMENNDED"
and "REQUIRED" have their RFC 2119 meanings).

An example of the possible confusion that is that there will be
networks in which the CS4 DSCP will be deployed with only one
of the service classes currently called "CS4" and "CS4-Discardable" in
draft-polk-tsvwg-delay-vs-loss-ds-service-classes.  Is anyone prepared
to try to explain with a straight face why the "CS4" service class
isn't using the "CS4" DSCP?

Thanks,
--David
----------------------------------------------------
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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From: Brian E Carpenter <[email-masked]>
Date: Thu, 18 Jul 2013 11:35:51 +1200

> the IP header to request suitable treatment in a particular network.

Hear, hear! It's especially true because CSn was included explicitly
for backwards-compatibility with RFC 791, so attempting to name new
behaviours as derivatives of CSn is particularly misleading. If you're
looking for a PHB that's like CS4 but "more discardable" that would be
CS3, anyway.

The CSn, including CS0, are unique in that the assigned code
points are not *recommended* mappings - section 4.2.2.1 of
RFC 2474 reserves them unconditionally. No wiggle room here;
there's a 1:1 correspondence between DSCP CSn and PHB CSn.
But that applies nowhere else in diffserv.

    Brian