Subject: Re: RE: [802.1] 802.1ad Customer FCS Retention

From: Stewart Bryant <[email-masked]>
Date: Mon, 24 Nov 2003 00:25:29 +0000

>>If you assume a PW to be a sort of bridge,
> 
> NH=> A PW is for sure not a bridge!  A PW is an adaptation funtion 
> that lies between a client layer network and a server layer network 
> of MPLS or IP.  

Neil

Surely that is just one way to model the PW? Can't a PW also be
modelled as a repeater with certain internal delay and error
characteristics.

Stewart

---

From: mark seery <[email-masked]>
Date: Mon, 24 Nov 2003 05:51:41 -0800

Stewart Bryant wrote:

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Stewart,

I have worked on equipment that transported Ethernet over a tunnel, 
without doing bridging, and the Ethernet testers have commented to me 
"this is a repeater". So I think the analogy you make is intuitive for 
Ethernet people. It is not technically correct because of the formal 
definition of repeaters (IMO), but in spirit it is. The more difficult 
issue is that repeaters have 802 on ingress and egress, so if we are to 
extend this analogy, then a PW is an internal connection within the 
"virtual repeater" that is not part of a formal 802 model, other than it 
should not introduce errors greater than some prescription.

All Neil is suggesting is that the internal connection is "transporting" 
the ethernet layer, and he is further arguing for a principle of layer 
encapsulation (similar to object encapsulation in software) which is 
very similar in spirit to the principle of minimum intervention sited in 
the PW architecture document (3.3.5). All that is left is to argue about 
whether the FCS belongs to the layer being transported.

I would assume a real 802 repeater would not strip the FCS, so if we 
were to take the analogy to the extreme, we would not strip the FCS here 
either, but of course as this is not a real repeater function, so we are 
left with balancing the models and requirements of at multiple views: 
IEEE, IP, IP-MPLS, ITU, carrier operations/engineering.

In a later email you say:

"For the PW to be an adaptation function, one would have to assert hat 
the contents of the PW had some meaning within the PSN.  I don't beleive 
it has, because we never communicate any context information to any 
object in the PSN (either by signalling or  self description) which it 
can use to interpret the contents of the PW."

If we were to assert that the contents of the PW had some meaning within 
the PSN, I would suggest that would be an interworking function, not an 
adaptation function.

mark