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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.  

Frank E

-----Original Message-----
From: Harry Hvostov [mailto:HHvostov@xxxxxxxxxxxx]
Sent: Thursday, June 07, 2001 1:13 PM
To: 'FEffenberger@xxxxxxxxxxxxxxxxx'
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

Harry Hvostov
R&D Director
CTO Office

Luminous Networks, Inc
10460 Bubb Road
Cupertino, CA 95014

Tel: (408) 342-2512
Fax: (408) 863-1107

-----Original Message-----
From: FEffenberger@xxxxxxxxxxxxxxxxx
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
unique physical 
address associated with this station,"  

So, it seems that the standard allows for the reception of unicast pause,
but not 
the transmission of same.  Could any of the IEEE alumni explain why this is

Dr. Frank J. Effenberger
Director, System Engineering
Quantum Bridge Communications
(978) 983-2532