Subject: Re: [Pce] Please comment draft-zhang-pce-comm-app-model-00

From: "Adrian Farrel" <[email-masked]>
Date: Fri, 1 Jul 2005 17:57:37 +0100

Hi,

Thanks for your draft.

I think it is important to contrast the situation where all paths are
computed by a central entity, and the case where there are multiple points
within the network that could compute a path. The former case is seen in
traditional transport networks where a central planning office is
responsible for determining paths. It also applies, as you state, where a
single PCE is responsible for a whole domain.

The latter case occurs, as you say, when there is more than one PCE in a
domain. We should note very clearly that this second case arises in
pre-PCE MPLS-TE or GMPLS networks where the ingress LSR is often given the
responsibility of computing the path of the LSP it is requested to
establish.

The case you are making is one for universal knowledge, and the you need
to make it clear that the trade-off is between the resource contention
that you describe, and the problems associated with distributing and
collecting a full knowledge set.

You should also contrast your requirements with the "stateful" PCE
described in the PCE Architecture draft.

It is important to observe that the PCE computes paths but does not cause
them to be established. A path computation might result in an LSP that
fails for some reason (perhaps a new fault in the network), or might never
be used to set up an LSP (it might be a path computed for information, or
to be kept in readiness for rapid restoration signaling). Thus, knowledge
of path computations is not the same as knowledge of current or planned
resource status.

Note also that not all paths are computed by a PCE. Some may be computed
by an LSR, and some may be supplied by an operator or application.

I would like your draft to focus more on the problem. What is the
likelihood of contention, and what is the consequence. How significant is
the consequence?

Then you can discuss the tradeoff. Race conditions are annoying, but not
significantly harmful. Avoiding race conditions requires centralization of
function, or centralization of information.

Thanks,
Adrian

----- Original Message ----- 
From: "Zhang Renhai" <[email-masked]>
To: <[email-masked]>
Sent: Tuesday, June 14, 2005 2:40 AM
Subject: [Pce] Please comment draft-zhang-pce-comm-app-model-00

> Dear WG,
>
> A new ID has just been finished and posted
>
(http://www.ietf.org/internet-drafts/draft-zhang-pce-comm-app-model-00.txt
),
> which proposed a application model. Thinking of the satuation where the
> network resources are reserved and released frequently, for instance,
VoIP
> environment,this draft give a mechanisms on this respect, mostly on
resolving
> race conditions and on computation load balancing.
> It's 10 pages long. Comments are very welcome.
>
> Thanks and Regards.
>
> Rh Zhang
>
>

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