Subject: [Pce] Comments on draft-ietf-pce-path-key

From: Dhruv Dhody <[email-masked]>
Date: Mon, 08 Feb 2010 10:56:08 +0530

Dear Authors, 

Kindly find following comments with respect to Path-Key draft. 

PathKey and Hop-Count 

Hop count is needed during the inter-domain best path computation when HOP
LIMIT constraint is given. 

Using the ERO Object length one can easily find the hop-count. 

But if PCRep message has the 'Path Key Subobject' present in the PCEP ERO
then there is no way to find the actual hop-count at PCE (1) because the
actual hops are replaced with a path key. 

Suggestion: If path key exists in the ERO Object, it should be followed by
TWO metric objects.

The first metric object carries the metric value of the CPS .Metric type of
this object is TE/IGP which was received in the path request. So in case of
path key exists the second metric object carries Hopcount of the CPS. Metric
type of this object is Type 3 - Hop Counts

Error Handling

In RP Object, if the path-key flag is set and there is no Path-key Object
included in PCReq message, then the receiving PCEP Peer MUST trigger the
Error message with Error Type 6 , Error Value 4 to set PATH-KEY object
missing. 

Thanks & Regards,

Dhruv Dhody

Huawei Technologies India Pvt. Ltd,

Bangalore, India

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From: "Adrian Farrel" <[email-masked]>
Date: Mon, 8 Feb 2010 10:52:45 -0000

Hi Dhruv,

> Kindly find following comments with respect to Path-Key draft.

Heads-up that this has been RFC 5520 since April 2009.

> type of this object is Type 3 - Hop Counts

The example you give of hop-count being deduced from an explicit ERO is an 
example of why hop count is a special metric. But when we do 
constraint-based computations, hop-count is *usually* less important than 
other metrics. And you should notice that even without and PKS, we cannot 
know the per-hop values of the other metrics for a returned ERO.

But that is OK, because we do not need to know this!

We are only interested in the net metrics for the path fragment returned in 
the PCRep. That is, we need to know the cumulative effect of including this 
fragment in the whole path.

PCEP already provides this by allowing the METRIC object on the PCRep 
message with the semntic that it applies to the whole path fragment.

> Error message with Error Type 6 , Error Value 4 to set PATH-KEY object

Yes, this would be correct behaviour.

Cheers,
Adrian