Subject: [CCAMP] draft-ietf-ccamp-lsp-hierarchy-bis

From: Benoit C Tremblay <[email-masked]>
Date: Fri, 11 Jun 2010 00:24:16 -0400

Hi Adrian and Shiomoto-san,

We evaluated the implementation of  the extensions proposed in draft-ietf-ccamp-lsp-hierarchy-bis.
The proposed extensions are pretty easy to implement and fulfills the lacks that were present in the existing HLSP RFC related to how to advertise the resulting TE-link. However, we believe that a feature is still missing and would like to share our observation with you.

When advertising the (virtual) TE-link corresponding to an HLSP, RFC4206 recommends to set its TE-metric to max(1, (TE-metric of the FA-LSP path) - 1).  However, in many cases none of the ingress or egress node knows the resulting TE-metric of the signaled HLSP.

Here are a few examples of scenarios where the nodes do not know the TE-metric of the HLSP:
- On the ingress node, the path is only partially computed and the further path computation is delegated to node down the signaling path.
- The Record Route Object (RRO) that collects the path (either from the ingress to the egress node or from the egress node to the ingress node) is optional and might not be received by the remote node.
- The RRO object may contain node IDs instead of links.  If multiple links with different costs exist between two neighbor nodes, the ingress or egress node might not be able to determine which link has been used.
- In some cases, the RRO contains path keys to identify segments of the LSP instead of the node or links.

When the end nodes can not accurately determine the TE-metric of the HLSP, a default or approximate value is used.  The default value does not usually correspond to the actual metric of the HLSP and can introduce distortion in further path computations.

Have you considered how the metric of the TE-link corresponding to the HLSP would be computed by the ingress and the egress nodes?

Regards,
Benoit

BENOIT TREMBLAY
Researcher

Ericsson Canada inc.
Broadband and Systems Research
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