Subject: RE: Address learning / was more comments on draft-ietf-bmwg-mswit ch-01.txt

From: Jim McQuaid <[email-masked]>
Date: Fri, 11 Jun 1999 13:55:09 -0400

Perfect Address Learning

There are two paths (at least) to go down here.

ONE: similar to 1944/2544 is to progressively increase the offered load of
addresses to be learned until ONE is dropped.  That might termed "address
learning throughput" which is the perfect performance limit.

TWO: given than switches may NOT learn it the first time they see it (due to
congestion, laziness etc.) they probably DO learn that address in question
the second time they see it.  So the test method becomes a measure of the
time to learn N addresses, such as 1000 addresses.  Some switches will do it
in one pass of 1000 frames, others will require multiple passes.

If the real world behavior is to "eventually" learn all offered addresses,
either test approach is probably overkill.

Jim McQuaid

Ganymede Software
[email-masked]

-----Original Message-----
From: David Newman [mailto:[email-masked]]
Sent: Monday, June 07, 1999 5:41 PM
To: [email-masked]
Subject: RE: more comments on draft-ietf-bmwg-mswitch-01.txt

> The address learning methodology tries to determine the loss-less address
> learning rate, rather than the typical address learning rate.  

Although I'm sympathetic to the point you raise (the current test is a
pain to run), I'm not so sure about the solution proposed. Any metric
other than a perfect learning rate implies that some amount of loss is
permissible. 

This is an approach that is inconsistent with the bmwg's previous work.
For example, RFC 1242, section 3.12, states that loss is what happens in
overload situations, which this isn't. And the goal of RFC 2544's
description of frame loss tests (section 26.3) is to find the highest
offered rate at which *no* loss occurs. 

What is an acceptable amount of loss? This is a slippery slope.

> For instance if a switch is offered 1000 packets in 1 second and learns
900
> of these addresses, then the learning rate is 900 packets per second (not
> test inconclusive invoke a binary search).

900 out of what? This approach requires the reporting of two metrics
(addresses offered, addresses learned) while a perfect-learning approach
requires reporting of just one number (addresses learned). 

And if I'm one of the 100 that got dropped I'm going to be a pretty
unhappy camper. . .

dn