Subject: [Pce] Domain Sequence

From: dhruv dhody <[email-masked]>
Date: Thu, 17 Dec 2009 11:39:40 +0530

Dear Authors,

In "draft-king-pce-hierarchy-fwk-02", section 5.6.2 [Hierarchical PCE
End-to-End Path Computation Procedure]: 

Domain paths are denoted as (S-BN11-BN21-D2-BN23-BN31-D,
S-BN11-BN21-D2-BN24-BN32-D, and S-BN13-BN41-D4-BN42-BN33-D). 

i.e. Parent PCE must know about the BNs and there interconnections. 

What is the need for this? 

Why can't the domain sequence be represented just by a sequence of domains
i.e. [S-D2-D and S-D4-D]? 

BRPC procedure can take domain sequence as input and figure out the
appropriate BNs and there interconnection while applying the VSPT algorithm.

Also note that, Child PCE can use the PCED-discovery [NEIG-PCE-DOMAIN
Sub-TLV] to specify the neighbor PCE-Domain(s) to Parent PCE, without
mentioning any BNs link-interconnection information.  

Background:

P2P inter-domain as in RFC 5441 [Section 4.1]

The PCC MAY indicate the sequence of domains to be traversed using the
Include Route Object (IRO) defined in [RFC5440] so that it is available to
all PCEs. 
 
Here domains sequence can be carried as SUB-OBJECTS [defined in RFC 3209],
Type = 32 for AS; a new Type = 5 for Area-id can be defined. There is no
need to carry BNs. That decision should be taken while applying the BRPC
procedures based on all the constraints. 
This works when domain sequence is administratively predetermined or
discovered. 
 
P2MP inter-domain as in
"draft-zhao-pce-pcep-inter-domain-p2mp-procedures-02" [Section 8.2.2] 

The PCE Sequence Object is added to the existing PCE protocol.

Instead of PCE sequence, domain sequence works much better. 

-         Existing IRO and sub-objects can be used, similar to P2P BRPC
(backward compatible)

-         PCE(i) for a particular domain (i) is selected dynamically by
PCE(i-1) based on the domain information in the domain-sequence. For PCE
Sequence, PCC/PCE in domain (1), should know about all the PCEs before the
start of procedure, and cannot be changed on the fly.

In summary:

1) Domain sequence should carry only the domain-information and not the BNs.
[H-PCE]

2) Domain sequence can be used instead of PCE-Sequence [P2MP-BRPC]

Kindly give any comments/suggestions. 

Regards,

Dhruv Dhody

Huawei Technologies Co.,Ltd.

Bangalore, India

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From: "Adrian Farrel" <[email-masked]>
Date: Wed, 30 Dec 2009 19:18:45 -0000

Hi Dhruv,

> What is the need for this?

I think you are refering to a specific example of the potential paths 
determined in a parent PCE that will give rise to computation requests sent 
to child PCEs to compute specific segments of the potential paths.

Nothing here says that there is a specific representation of domain paths.
But note that there is no need to represent the domain path to anyone by 
anyone in this model unless one of
- the ingress PCC has some restriction it wants to apply
- the path computation will be hybrid using some elements of BRPC
- the resulting path is a hybrid that will require some elements of PDPC

Thus, the question simply does not arrise.

> appropriate BNs and there interconnection while applying the VSPT 

Thanks, yes, that is how BRPC works.

> mentioning any BNs link-interconnection information.

That is true.
However, in a complex mesh of domains, knowledge of the inter-domain links 
can help the parent PCE to reduce the potential domain paths and so reduce 
the processing that the child PCEs have to do.

Cheers,
Adrian