Subject: RE: [Diffserv] Possible compromise between EFRESOLVE and draft-ch arny

From: ROBERTS James FTRD/DAC/ISS <[email-masked]>
Date: Mon, 11 Dec 2000 10:58:24 +0100

I still don't understand how EFRESOLVE as it stands can be accepted as a
satisfactory PHB definition. Its usefulness depends on it being possible to
calculate a sufficiently small value of S for some candidate schedulers
under specified conditions of burstiness. It is surely necessary to
demonstrate that the definition is meaningful in at least one plausible
scenario. No one has yet responded to the request for such an example made
initially be Jon Bennett.

If, to simplify, we assume there is only EF traffic in the network and the
routers provide perfect FIFO output queuing, is it then possible to come up
with the "network calculus" necessary to quantify how burstiness evolves
from node to node and how it impacts the necessary bound S? Current research
(the previously cited paper by Charny and Le Boudec and references therein)
would suggest this is extremely difficult or even impossible.

The modified rate guarantee given by Bruce Davie avoids the requirement to  
specify burstiness. The bounds E_a and E_p measure the divergence of a real
router from an ideal represented by the perfect FIFO output queuing
scheduler referred to above. This obviously doesn't solve the problem of
evaluating burstiness and its impact on packet delay variation in the node
or through the network. The advantage I see is that it leaves this question
in the research domain where it still belongs. The per hop behaviour
definition does not depend on a hypothetical solution to a difficult
problem.

I happen to believe that perfect FIFO output queuing, and slight divergences
from it measured by small values of E_a and E_p, would provide a
satisfactory basis for building useful end to end services over EF with
statistical bounds on delay and delay variation. The necessary network
engineering depends significantly on the values of E_a and E_p. On the other
hand, I really doubt that it is possible to find a deterministic delay bound
S*MTU/R that is sufficiently small to be of any practical use in building
services.

I therefore add my support to the modified definition of the EF PHB coming
from the 'group of twelve'.    

Jim Roberts

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From: demir <[email-masked]>
Date: Mon, 11 Dec 2000 03:32:52 -0800 (PST)

Very nicely said. I have made below points before. However, I haven't
received any response. May be, I didn't say that clear(?)

> from the 'group of twelve'.    

It didn't happen to me to accept such a solution to be accepted for a PHB
where there are "toplogy-dependent" parameters. However, if this is
acceptable by Diffserv, I think B. Davie's (and the group's
solution) seems a better choice.

Alper K. Demir

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From: demir <[email-masked]>
Date: Mon, 11 Dec 2000 07:43:49 -0800 (PST)

I think that if B. Davie and his group's definition is the one that will
be accepted, it would be better to define B and E_p as:

B=SIGMA Bi, where Bi is "bound on the burstiness of the input traffic from
i-th interface

E_p=SIGMA E_pi, where E_pi is packet error allowed for i-th interface

Alper K. Demir

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From: Brian E Carpenter <[email-masked]>
Date: Mon, 11 Dec 2000 10:08:06 -0600

ROBERTS James FTRD/DAC/ISS wrote:
...
> would suggest this is extremely difficult or even impossible.

We are not attempting to solve research problems, and we all hopefully
agree that this "network calculus" is a research problem. Our job is
very limited: specify a well-defined behaviour with some measureable
properties. Requiring that we understand all the implications of a
non-existent network calculus for compositions of this behaviour
is by definition impossible. So we are going to re-specify EF without
attempting the impossible. We're engineers.

  Brian Carpenter
  diffserv co-chair