X+Y+Z (was Re: [PWE3] FCS retention in PWE3)

[mark seery <mark@interflect.com>]

neil.2.harrison@bt.com wrote:

> Hi Stewart,
>
> With all due respect I really don't see what all the fuss is about here.  If the PW adaptation function did not mess about with the clients there would be no problems at all.  There should be exceedingly good reasons why any server layer technology should modify a client layer network's CI (characteristic information)....I am afraid BW squeezing is not a good reason IMO.  So the default behaviour in all client->server adaptation functions (because that is what a PW is doing) should be *do not change the client layer traffic unit at all*.
>
> This is not the only area where problems will arise.  Like-mode technology service I/W is another area where I can see that problems will arise due to the very fact PWs even exist....as, in principle, they are not serving any 'must-have' architectural technical purpose but (IMO) simply a political one.  However, if folks steadfastly refuse to see that MPLS is not the same as IP and both create layer networks (one belonging to the co-ps mode and one the cnls mode) then I am afraid we are never going to get this sorted properly.
>
> regards, Neil
>
>  
Hi Neil, understand where you are coming from. I think the key issue is 
we need to have a common language in which to discuss the differences. 
Language is a problem here because it effects the preparedness of some 
listeners.

So let me take a crack at this and see if it makes a difference.

In much of our history, both datacom and telecom, we have thought in 
terms of layers. Whether we say physical/data link/network, or we say 
client/server we are thinking in terms of layers; and often we think in 
terms of X over Y.

In the case of VLANs and IP-MPLS, we are dealing with X+Y. It could be 
argued that X+Y works when the full semantics of a layer are contained 
within the aggregate. This could be argued for both the VLAN and IP-MPLS 
case. When we then take MPLS and use it for PW, we have to make the 
decision about whether X+Y still works, or whether we need to move to X 
over Y, and make MPLS a full layer, because the X has different 
semantics now than it did for the IP-MPLS case. Another potential way of 
looking at it is that when we use PW-MPLS for Ethernet, we have created 
X+Y+Z.

The key observation is that the role of MPLS may be different in IP-MPLS 
than it is in PW-MPLS, which is of course why the PID probably ended up 
in PW rather than MPLS.

In saying this I am not trying to suggest that either X+Y or X+Y+Z meets 
yours, or anyone else's requirements. I am just trying to suggest a 
culture-neutral language that might be helpful in thinking through the 
issues you have raised.

best,
mark


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