Subject: [Diffserv] Expedited Forwarding & Virtual wire

From: Guven Mercankosk <[email-masked]>
Date: Thu, 27 Jul 2000 21:00:17 +0800

In relation to EF definition, it might be a good idea to review what is
the thing that the authors are trying to achieve.

From both RFC 2478 and the Virtual Wire draft, it is clear to me beyond
any doubt that the authors are trying define an end-to-end service
through a DS-domain that appears like an unshared point-to-point
connection. Towards that objective they identify and state two
requirements.

Requirement 1

RFC 2478 clearly states that the nodes are required to be configured so
that the aggregate has a well defined minimum departure rate. It further
qualifies 'well defined' to mean being independent of the dynamic state
of the node.

In the case of priority queueing, the link rate always provides a well
defined minimum departure rate. I claim that the non preemptive nature
of priority queueing does not alter the well defined nature of the
minimum departure rate (see draft-mercankosk-diffserv-pdb-vw-00.txt,
Section 4.4, Appendix E & Section 5.1) So, at least one scheduling
algorithm exists that satisfies the first requirement.

Requirement 2

The Virtual Wire draft identifies the necessity of conditioning the
aggregate so that its arrival rate at any node is always less than that
node's configured minimum departure rate. Based on the conceptual model,
this condition is satisfied by limiting the packets from a given micro
flows to arrive at the ingress no closer than a packet time at the VW
configured rate.

Since the aggregate at any hop is limited to a fraction of the link
capacity, with the use of priority queueing the second requirement is
also satisfied.

At this stage, I would like to point out that the requirements are
clear, unambiguous and that there are mechanisms to satisfy them.

I have to admit that when I read the documents, I had problems in the
way EF had been defined and with its application to various delay
components. However, my gut feeling was telling me that the requirements
as stated above were right. Using the requirements and the end-to-end
service characteristics given above, I have made an attempt to rewrite
the document as a way of being constructive. The result is the
'draft-mercankosk-diffserv-pdb-vw-00.txt.'

The study points out to one additional requirement and provides a set of
tools to engineer a network using virtual wires. The analytical tools
provided uses only one additional assumption other than those implied by
the requirements. The additional assumption relates to the independence
of the timing of each flow's arrival.

In the process, I managed to avoid any reference to fluid models. IMHO,
the confusions are resulting due to the use of fluid models. Probably,
this statement is controversial enough to warrant stopping right here.

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From: PANDARAM SUJAN REDDY <[email-masked]>
Date: Thu, 27 Jul 2000 13:56:17 -0500 (CDT)

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From: Brian E Carpenter <[email-masked]>
Date: Thu, 27 Jul 2000 12:45:59 -0500

For the record, I think you mean 2598 not 2478.

  Brian

-- 
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Brian E Carpenter 
Program Director, Internet Standards & Technology, IBM 
On assignment for IBM at http://www.iCAIR.org 
Internet Society: http://www.isoc.org
Non-IBM email: [email-masked]