Subject: [Diffserv] Yet another EF definition

From: Steve Blake <[email-masked]>
Date: Fri, 28 Jul 2000 10:36:08 -0400

I have only skimmed draft-charny-ef-definition-00.txt (although everything
I read in it looked fine), and I have not been able to follow every message
on the list this week, so my apologies if the following restates something
already obvious to everyone else, but the most concise verbal definition
of EF I could come up with that captures the *operationally significant*
characteristics of the behavior is as follows:

  A node exhibits behavior EF(T(), D(), MTU, C, R) for output link O with
  rate C, MTU, and configured EF rate R <= C if, for any EF-feasible traffic
  pattern for O (satisfying the constraint that the amount of received EF
  traffic for O over any interval T(R, MTU) is strictly less than 
  R * T(R, MTU)), the peak packet forwarding delay variation (the maximum
  difference in the packet forwarding delay (the difference between the 
  receipt and transmission time of the last byte of a packet) across all
  packets) for the EF traffic for O is bounded by D(MTU, C, R) and the
  packet loss is negligible (i.e., zero), in the presence of arbitrary
  amounts of non-EF traffic for O.

The appropriate requirements for T() and D() would be PDB-dependent.  A
precise definition would have to define the measured traffic over an 
interval to account for variances in L2 header size, packet fragmentation,
etc.  Any particular implementation would only be able to support certain
bounds D() given T(), MTU, C, and R, and the node's number of ports N.

Regards,

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Steven L. Blake                  <[email-masked]>
Ericsson IP Infrastructure                  (919)472-9913

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From: Brian E Carpenter <[email-masked]>
Date: Fri, 28 Jul 2000 11:14:59 -0500

Steve:

For clarification, are you assuming that R/C is not bounded at 0.5 or any
other value less than unity?

   Brian

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From: Steve Blake <[email-masked]>
Date: Fri, 28 Jul 2000 12:54:11 -0400

> other value less than unity?

Correct.  D() is a function of MTU, C, R, so obviously any particular
implementation has its own feasible space for (MTU, R) such that D is
finite (and a smaller space such that D satisfies some desired constraint).
I am pretty sure that in the case of a hypothetical output-queued node
with a strict priority scheduler, R = C results in a finite D, given
an EF-feasible input pattern as I have defined it.

This is probably just a simplistic restatement of what is already proven in
draft-charny-ef-definition-00.txt.

Regards,

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From: Martin Westhead <[email-masked]>
Date: Sun, 30 Jul 2000 16:00:56 +0100

Hi,

I'd like to say that I think Steve's approach deserves some
consideration.

It seems to me that the property of EF behaviour that we really want to
capture with a definition is a bound on the jitter through a router?

We have two proposals:
	- RFC2598 which is intuitive but appears to capture the wrong behaviour
and 	
	- drafy-charny... which despite proofs of correctness feels
unsatisfying, to me at least, because its formulation does not follow
easily from intuition and it feels more complicated than it should be.

Is the problem with both proposals that they are actually trying to
define a jitter bound in terms of packet rates?

It should be straightforward to agree a definition of jitter through a
router. You could then discuss different bounds that could be applied to
it (e.g. fixed, percentile, as a function of MTU, C, R etc.) Such a
definition should be robust against both the rate problem and the router
delay problem identified in drafy-charny... but could be much more
intuitive in its formulation.
Or have I missed something that makes this approach infeasible?

Cheers,

Martin

-- 
--------|epcc|------------------------------------------------ 
Dr Martin D. Westhead | <[email-masked]> 
                      | http://www.epcc.ed.ac.uk/~martinwe 
Principal Consultant  | phone : +44 (0)131 650 5958       
    Research          | mobile: +44 (0)7050 193530
-------------------------------------------------|epcc|-------

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From: Anna Charny <[email-masked]>
Date: Sun, 30 Jul 2000 14:10:25 -0400

Martin,

I too feel that Steve's  approach deserves a serious consideration.  I do 
have some preliminary concerns with Steve's definition, which unfortunately 
I do not have time to discuss now, since I have to catch a flight to 
Pittsburgh.  But I certainly think  it needs to be understood carefully. 
One of  its attractions is that unlike RFC 2598 itself, which can delay a 
packet indefinitely without violating the Ef definition,  and unlike our 
draft which preserves this property of the RFC, Steve's definition bounds 
individual packet jitter explicitly, by the definition itself.

In fact, we  are planning to meet with Steve in Pittsburgh and see if we 
can reach consensus on the properties, both absolute and in relation to the 
RFC and our draft..

I suggest that we delay this very worthwhile discussion until after Pittsburgh.

Regards,
Anna

> > bounds D() given T(), MTU, C, and R, and the node's number of ports N.

---------------------------------------
Anna Charny
Cisco Systems, Inc
300 Apollo Drive
Chelmsford, MA  01824
Tel. (978)-244-8172
Fax (978)-244-8126

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From: Steve Blake <[email-masked]>
Date: Sun, 30 Jul 2000 14:25:53 -0400

> individual packet jitter explicitly, by the definition itself.

To clarify, my proposed EF definition says nothing explicitly about
maximum forwarding latency, but does say that packet loss must be
neglible.  I think this is equivalent to saying that the forwarding
latency must be finite.  :)

Regards,