[Pce] P2MP PCE

Hi JL,

In Vancouver you said you had some thoughts and issues with the P2MP PCEP 
drafts draft-yasukawa-pce-p2mp-app-01.txt and 
draft-yasukawa-pce-p2mp-req-04.txt.

The main issue that you raised at the microphone was that you felt that the 
I-Ds were too strong in their assertion that P2MP computation would never be 
done at a head-end LSR.

Looking back at the I-Ds, I see that the requirements draft makes no claims 
about the applicability of the PCE architecture to P2MP LSP computation: it 
devolves this responsibility to the applicability I-D.

The applicability I-D does discuss the potentially greater computational 
load for P2MP LSPs compared to P2P LSPs. It states:
   Roughly speaking, the load to compute a least-cost-to-leaf tree is
   the same as the cost to compute a single optimal path to each leaf in
   turn. The load to compute a Steiner tree is approximately an order of
   magnitude greater.

>From what you said in Vancouver, you are suggesting that approximations to 
Steiner may be achieved in roughly the same order of magnitude as a 
least-cost-to-leaf tree. Would be enough to you if we added to this 
paragraph to say...
   ...although powerful heuristics exist that may make it possible
   to approximate a Steiner tree in the same computational time
   as a least-cost-to-leaf tree.

Were there any other issues you wanted to raise?

Cheers,
Adrian

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