Subject: [Diffserv] Intuitive EF ?

From: Grenville Armitage <[email-masked]>
Date: Tue, 25 Jul 2000 23:58:51 -0700

Okay, so I'm reading section 1 of draft-charny-ef-definition-00.txt
and the phrase "intuitive" arises many times. Here's another outside
observer's "intuition" about the salient text in RFC 2598:

 "It SHOULD average at least the configured rate when measured over
  any time interval equal to or longer than the time it takes to send
  an output link MTU sized packet at the configured rate."

Whenever I read this (and surrounding) text, I assumed the following:

	- It is descriptive of externally visible behavior
	- It is not descriptive of specific scheduler implementation
	- The "average" is measured from the first bit of an
	  EF packet belonging to the EF class under consideration.

This third point appears critical. I never saw the problems raised in
section 1.2.1 of draft-charny-ef-definition-00.txt since my
definition of "average" doesn't involve an interval with arbitrary
start/stop times. The start time is always fixed to be the moment
the first bit of an EF packet is transmitted, and the stop time
is arbitrary and greater than MTU/C beyond the start time. To me
this was always the intuitive interpretation of RFC2598's intent.

Re-analyzing sections 1.2.1 and 1.2.2 using my interpretation of
"average" results in everything working out fine (apparently).

I suspect the phrase "..over any time interval.." is a little
confusing. In draft-charny-ef-definition-00.txt it is apparently
interpreted to mean that the averaging can be done over an interval
with any start and any stop time (subject to the constraint of
stop time being at least MTU/C later than the start time). To me
it was more intuitive to assume that only the size of the interval
was "any" (subject to the same minimum of MTU/C), but that the
start time was synchronized to the transmission of packets in the
EF stream in some manner (since, afterall, it is the egressing
behavior as perceived by EF traffic that's being expressed here).

Now, maybe I'm just being mathematically challenged (perhaps that
makes me IETF-compliant ;)  But I like the thought that RFC2598 isn't
necessarily as internally contradictory as it might seem. What am I
missing here? (And why would this require "global clock
synchronization" as implied by the opening paragraph of
section 1.3 ?)

If I'm being an idiot, hopefully draft-charny-ef-definition-01.txt
will contain additional new text documenting and rebutting my version
of "average" :-)

cheers,
gja

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---

From: Brian E Carpenter <[email-masked]>
Date: Wed, 26 Jul 2000 09:07:16 -0500

Grenville,

I think the discussion is easier to understand if you accept that
there are two PHBs being discussed as if they were a single one.
I think of them as EF-Jacobson and EF-Charny. They both have
proponents that believe they are well defined and completely
implementable. 

The WG has a choice
 - clarify EF-Jacobson to remove the apparent misunderstandings
   by some would-be implementors
or
 - replace it by EF-Charny
or
 - clarify EF-Jacobson AND specify EF-Charny

Essentially this is the choice we will have to make in Pittsburgh,
since we certainly can't resolve the mathematical details in
a room of several hundred people.

    Brian