Subject: [Diffserv] EF Issue comments

From: [email-masked]
Date: Fri, 28 Jul 2000 20:59:31 -0400

After reading the drafts and lengthy discussion, it
seems to me that RFC 2598 and draft-charny-ef-definition
may be approaching the concept of EF from two different
and potentially complementary viewpoints.

-- RFC 2598 --

Consider the following excerpts from RFC 2598 Sections 1 and 2:

   Thus a service that ensures no queues for some aggregate is
   equivalent to bounding rates such that, at every transit node, the
   aggregate's maximum arrival rate is less than that aggregate's
   minimum departure rate.

   The EF PHB is defined as a forwarding treatment for a particular
   diffserv aggregate where the departure rate of the aggregate's
   packets from any diffserv node must equal or exceed a configurable
   rate.  The EF traffic SHOULD receive this rate independent of the
   intensity of any other traffic attempting to transit the node.

This appears to be a contradiction, because the first excerpt indicates
that under the intended use, traffic will never be offered at the
configured minimum departure rate, hence can't "receive" that
configured rate over any suitably long time period if the definition
of "receive" involves measuring the exiting EF traffic.  I think
that what was intended was a meaning of "receive" along the lines
of "have the opportunity to receive" (e.g., the output scheduler
is configured to send traffic at that rate), with the intention that
the higher departure rate drains queues caused by bursts from
multiplexing/aggregation/etc.

The "average" sentence that is at the root of the measurement
controversy immediately follows the second excerpt above:

   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.

This makes more sense when "average" is understood to modify
"have the opportunity to receive" the configured rate, but
still has fencepost problems with intervals that aren't multiples
of the "time to send an output link MTU sized packet at the
configured rate".  For example, assume that all packets are
MTU-sized and the configured EF rate is 1/3 of the line rate.
Then every 3 packet output window has to contain an opportunity
to send one EF packet, and considering multiple overlapping 3
packet windows results in a limit of sending at most two non-EF
packets between opportunities to send EF packets.  Nonetheless,
there are 4 packet windows that contain only one opportunity to
send an EF packet and hence have an average that's too low.
Limiting the intervals to multiples of the time to send an
output link MTU-sized packet is a cleaner approach than requiring
that only intervals aligned to an opportunity to send an MTU-size
packet be considered.  Interesting things happen when the configured
rate fraction is not expressible as 1/n -- for example, 2/5 results
in a limit of sending 1.5 non-EF packets between EF MTU sized packets;
if all packets are MTU sized, one possible result is that the
2/5 rate fraction is realized as 1/2.

This approach looks like it should generalize to packets of arbitrary
sizes (e.g., via suitable use of fixed capacity token buckets to model
the behavior), but I don't have the time to do that at the moment.

-- draft-charny-ef-definition --

The Charny draft takes a different approach to the issue.  Rather
than considering the opportunities to send packets, it considers
what should happen when packets arrive in a fashion that cannot
utilize all of the configured opportunities to send them.  It
appears to me that the formal definitions in the draft are correct,
although their accessibility is greatly improved by both Anna Charny's
second attempt to explain what the intuition behind the definition
(mid-day Thursday) and Bill Courtney's case by case dissection and
explanation of the formula that defines F(j) in the draft (early
Friday morning).  Both of these pieces of text would be valuable
editions to the next version of this draft.

-- Upshot --

(1) Something is wrong with the "SHOULD average at least the
	configured rate" sentence in RFC 2598.
(2) RFC 2598 and draft-charny take different approaches to
	defining the EF PHB, but (at least to me) this does
	appear to be the same PHB.
(3) This difference in approaches to defining the PHB
	would be useful to discuss as part of Brian's
	"Issue #0".

--David

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David L. Black, Senior Technologist
EMC Corporation, 42 South St., Hopkinton, MA  01748
+1 (508) 435-1000 x75140, FAX: +1 (508) 497-6909
[email-masked]  Cellular: +1 (978) 394-7754
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