Lesser comments on Updated Draft Liaiosn to Q6/15

"Adrian Farrel" <adrian@olddog.co.uk> Tue, 10 March 2009 17:20 UTC

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From: "Adrian Farrel" <adrian@olddog.co.uk>
To: <ccamp@ops.ietf.org>
Cc: "Malcolm Betts" <betts01@nortel.com>, "Greg Bernstein" <gregb@grotto-networking.com>
Subject: Lesser comments on Updated Draft Liaiosn to Q6/15
Date: Tue, 10 Mar 2009 17:20:47 -0000
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Thanks Malcolm and Greg,

> Deployment scenario 3 "Concern for "basic" impairments" my
> recollection is that we were considering network scenarios in
> which we could allocate sufficient margin so that a "simple"
> computation of the accumulation could be used and still have
> a high probability that the optical path would be viable and
> would not perturb any existing paths.

Thanks. Yes. "Basic" probably conveys that I was struggling for the right 
Greg suggested "Networks in which approximate impairment estimation is 
sufficient", and I will merge this with what you suggested (as in this case 
more may be more!).

> Deployment scenario 4 "Concern for "advanced" impairments"
> again my recollection is that in this scenario a full computation
> of the accumulation of impairments including the impact on
> existing paths is required.  This significantly increases the scope
> of information required and the compute time.

Well, let's not get into the compute time :-)

But you're right, again I was fishing for words, so I will merge what you 
said with what Greg supplied.

> One final concern:
> "With this in mind, CCAMP is looking to Q6/15 to work as a partner in
> establishing:
> - ....
> - the rules by which such impairments are accumulated along a path
>  ...."
> This implies that we will standardize an aspect of path computation i.e.
> the method of computing the accumulation of impairments.  If the path
> computation is performed in a single PCE then it is only necessary to
> standardize the collection of the impairments, if it is distributed
> across multiple PCEs it may be preferable to only expose a "figure of
> merit" for the portion of the path that has been computed vs.
> standardization of the method of computation.  This should be discussed
> with the experts from Q6.

I think you are inferring a little more than was intended. In the PCE 
working group they talk of "objective functions". For example, they may say 
"minimize cost" and mean that the cost of a path is computed by accumulating 
cost in a particular way. But that does not specify a mechanism for 
computing the least cost path - only the way that the cost of a path is 

So, it was my intent here to make the same statement. That is, that CCAMP is 
looking for the rules by which the end-to-end impairments of a path may be 
determined from a knowledge of parameters of the path and impairments on the 
path segments.

I will add this clarifying text.

And note that yes, this should be discussed with Q6. That's why we are 
meeting them :-)