[Pce] Terminology for PCE-Initiated LSP (Was I-D Action: draft-ietf-pce-questions-02.txt)

[Dhruv Dhody <[e-mail]>]

WG,

Regarding PCE-Initiated LSP...

(1) http://tools.ietf.org/html/draft-ietf-pce-questions-04#section-20
use the term "suggest" or "recommend" LSP and considers "LSP Instantiation" as out of scope.

(2) http://tools.ietf.org/html/draft-ietf-pce-pce-initiated-lsp-00
http://tools.ietf.org/html/draft-ali-pce-remote-initiated-gmpls-lsp-03 (WG -00 yet to be posted)
http://tools.ietf.org/html/draft-palle-pce-stateful-pce-initiated-p2mp-lsp-01
all use the term "LSP instantiation" for PCE-Initiated LSP.

We need to come to some consensus regarding this and use the same terminology across WG documents.

Thoughts?

Dhruv


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Dhruv Dhody
System Architect,
Huawei Technologies India Pvt. Ltd.,
Banagalore
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From: Adrian Farrel [mailto:[e-mail]]
Sent: 17 March 2014 17:11
To: Dhruv Dhody
Cc: [e-mail]; 'Dhruv Dhody'
Subject: RE: [Pce] I-D Action: draft-ietf-pce-questions-02.txt

Sorry for the HTML, but I am hoping this thread can be closed soon...

You're killing me :-)
Hoping you are not red/green colourblind.

Look in line for [DD].

[Snipped to only the points for discussion]


* Sec 5.  How Do I Select Between PCEs?

Along with capability, you can also mention the PCE's preference for each

computation scope as carried in the PATH-SCOPE subtlv.



You are going to have to remind me, I'm afraid. Where is the PATH-SCOPE subtlv defined?

I suppose I was considering that a PCC is only going to choose a PCE in its own domain, but I can add a note.

[DD] Its in RFC5088, 5089, the idea being PCC should select the PCE in its own domain which matches with the path computation scope (inter-area, inter-AS, inter-layer) along with the capability.



Ah, ah, ah! I was busy thinking this was a PCEP sub-tlv.



OLD:

   When more then one PCE is discovered or configured, a PCC will need

   to select which PCE to use.  It may make this decision on any

   arbitrary algorithm (for example, first-listed, or round-robin), but

   it may also be the case that different PCEs have different

   capabilities, in which case the PCC will want to select the PCE most

   likely to be able to satisfy any one request.  The first requirement,

     of course, is that the PCE can compute paths for the relevant domain.

NEW:

   When more than one PCE is discovered or configured, a PCC will need

   to select which PCE to use.  It may make this decision on any

   arbitrary algorithm (for example, first-listed, or round-robin), but

   it may also be the case that different PCEs have different

   capabilities and path computation scope, in which case the PCC will

   want to select the PCE most likely to be able to satisfy any one request.

     The first requirement, of course, is that the PCE can compute paths for

     the relevant domain.



Yeah, OK for that. It duplicates the final sentence I added to this paragraph, but it does no harm.



* Sec 20.  Comparison of Stateless and Stateful PCE

Can you have a re look at this wrt draft-ietf-pce-pce-initiated-lsp-00 is now a WG draft.



Mutter!

Which document is right?

What specific issue are you raising?

[DD] http://tools.ietf.org/html/draft-ietf-pce-questions-03#section-20 says

                              | Stateless |  Stateful |

      ------------------------+-----------+-----------+

      Passive                 |     1     |     2     |

      Active delegated LSPs   |     3     |     4     |

      Active suggest new LSPs |     5     |     6     |

        Active instantiate LSPs |     7     |     7     |



      7. These modes are out of scope for PCE as currently described.



Where does draft-ietf-pce-pce-initiated-lsp fits in, in my reading this was 7, and thus I suggested it should not be out of scope.

If you agree, ignore below text...



Otherwise we have some confusion over the terms...

IMO 'Suggest new LSP' was same as PCE sends an update message triggering setup of a new LSP in MBB fashion.

'Instantiate LSP' was PCE send an LSP initiate message to instantiate an LSP. (7)

Maybe you can clarify what is your understanding of these terms...



OK, the difference I am trying to arrive at is whether the Active PCE can *require* the establishment of an LSP, or whether it is *suggesting* it.



The difference is (IMHO) between an NMS or CLI that dictates what the LSR does, and an Active PCE that requests the LSR to act.

More precisely, absent access controls, an LSR obeys the NMS, but an LSR can implement policy to filter or modify requests from a PCE.



I realise that this view might not be popular with implementers of Active PCEs, but I think that they are failing to distinguish between the blob of code they are writing and naming "Company Foo's most Excellent Active PCE", and the architectural components.  This, at least, is part of what I am trying to convey in Section 19 of this document and in the ABNO document.



The bottom line, I think, is that a PCE is not a provisioning tool, it is a path computation element. The fact that PCEP is used as a provisioning protocol does not make the thing that does the provisioning into a PCE.



Of course, I don't want to go against consensus with this, but I do think it is an important architectural principle. Thus, I have been looking for words that suit both viewpoints...



For an Active PCE to "suggest new LSPs" or to "recommend new LSPs" as described in Sections 19 and 20 allows, IMHO, an LSR to have a policy that says "always do what is recommended" and so achieving PCE-initiated LSPs. Yet, these words also allow a policy of "look both ways before crossing the road" which fits more closely with my world-view.



>From my perspective "sending an update message to trigger setup of a new LSP in MBB fashion" is no different from setting up any other LSP. First there was no LSP, then there is one. So I think the distinction you are drawing doesn't work.



I hope that explains my motivation, and I believe the words in the draft fit this explanation.



Cheers,

Adrian