EFMWG: RE: MAC control Pause clarification
Yes, the standard prohibits the transmission of unicast pause frames.
It is quite clear on that. However, since the reception of same is
allowed, it appears as if the authors had future extensions in mind.
I also got a possible suggestion for the reason why this might be,
in that bridges must filter out pause frames, so that they don't
propagate through the network. This is easier to do when they all
have the same (multicast) address.
Also, use of pause is explicitly limited to point to point systems.
(See note in section 31B.1.) Hence use of Pause over PONs is
prohibited de jure.
I was hoping that the actual authors of the language could explain
this themselves. At the last meeting, some people were saying
"Don't change the pause Opcode, get a new Opcode." But given
the existing standard, it looks like some future augmentation was
contemplated when clause 31B was drafted.
From: Harry Hvostov [mailto:HHvostov@xxxxxxxxxxxx]
Sent: Thursday, June 07, 2001 1:13 PM
Subject: RE: MAC control Pause clarification
Do you believe the standard precludes transmissions of the unicast pause
frames? The frame format appears to be quite capable of supporting unicast
Luminous Networks, Inc
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Sent: Wednesday, June 06, 2001 8:03 PM
Subject: MAC control Pause clarification
I got the impression from the discussion at the last meeting that the pause
command was not defined for use with unicast MAC addresses.
I checked in Annex 31B, and I found conflicting information:
In section 31B.3.1, Transmit operation, it says in the first line item (a)
that "The destinationParam ... is currently restricted to the value
specified in 31B.1"
That value is the globally assigned multicast address for pause.
In section 31B.3.3, Receive operation, it says in the second paragraph that
"Upon receipt of a valid MAC Control frame with ... the destination address
either: (1) the reserved multicast address specified in 31B.1 or (2) the
address associated with this station,"
So, it seems that the standard allows for the reception of unicast pause,
the transmission of same. Could any of the IEEE alumni explain why this is
Dr. Frank J. Effenberger
Director, System Engineering
Quantum Bridge Communications