[8023-CMSG] To Pause or not to Pause, That is the Question
All,
This note is in response to a private thread that began before the
reflector was in place. I've moved it here without reproducing all
the previous material. Those who generated that material are
encouraged to raise their arguments again or to simply continue
this thread.
So, most agree that PAUSE doesn't work except in very special
cases or for some handpicked tests and, even then, it can have a big
impact on throughput. It sounds like PAUSE exacerbates the problem
of congestion, spreading it backwards through the network.
Why do we think that flow- or class-based PAUSE will work? What
problem does this fix that link-based PAUSE can't? Don't answer too
quickly.
Each flow/class on a particular link is likely to have multiple sources
so by PAUSEing one of them, doesn't that have the same effect that
link-based PAUSE has, though perhaps only for one flow/class?
A flow can be defined many different ways and, based on your
definition, there can be many, many flows over a single link. When
we use PAUSE, either link-based or flow/class-based, we're acting
on a buffer. We can't act on each flow individually. That's not practical
for a real implementation because it is not practical for a device to have
a buffer for each flow. Therefore, numerous flows are combined into
a "class" (and I use this term generically) and each class is assigned
a buffer. All we can do is flow-control that buffer.
So, now I'll ask the question again. What does flow/class-based PAUSE
have over link-based PAUSE? An answer to this is critical is we're to
stand up in WG 802.3 on Thursday, July 15 and ask to become a Task
Force.
Regards,
Ben
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Benjamin Brown
178 Bear Hill Road
Chichester, NH 03258
603-491-0296 - Cell
603-798-4115 - Office
benjamin-dot-brown-at-ieee-dot-org
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