[8023-CMSG] Server/NIC analogy
All,
During a private discussion this afternoon regarding the results of
last week's meeting, the concept of feedback came up - whether
it was necessary or not. There was some level of discussion about
this during the meeting but no one seemed to be able to provide an
adequate justification for providing congestion feedback and why
the more common approach of packet drop wasn't adequate.
During this afternoon's discussion, I came up with something that
I think might be justification. I'm probably just painting a big target
on my chest but let's see how this goes.
Consider a stand-alone server with a 1G Ethernet NIC. Today's
CPUs could easily generate enough traffic to swamp the 1G
Ethernet link (okay this is a bit of an assumption on my part
but if they can't today they will be able to tomorrow). I don't
build these things, nor have I looked at their architecture all
that closely in a number of years, but I'll step out on a limb and
state that there's a (most likely proprietary) mechanism for the NIC
to tell the CPU that the link is too slow to handle all the packets
that it is trying to transmit. I'll step even farther out on that same
limb and state that the mechanism is not packet drop.
Now, let's use this analogy to consider a server card in a backplane
that communicates to the world via a port interface line card. The
server card communicates to the port interface line card using a
link compliant with the newly emerging Backplane Ethernet standard.
(Okay, so I'm looking a little into the future.) If you consider the
entire chassis analogous to the server/NIC in my initial example
then it would seem plausible that you would want to communicate
buffer congestion on the port interface line card back to the
server card using a mechanism other than packet drop.
I'll just close my eyes now. Fire at will.
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
(Will this cut down on my spam???)
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