RE: WG Adoption Call for draft-psarkar-rtgwg-rlfa-node-protection

[Uma Chunduri <[e-mail]>]

Chris,

In-line [Uma]:
--
Uma C.

From: Chris Bowers [mailto:[e-mail]]
Sent: Wednesday, March 19, 2014 8:07 PM
To: Uma Chunduri; Pushpasis Sarkar; [e-mail]; Alia Atlas
Cc: [e-mail]; [e-mail]
Subject: RE: WG Adoption Call for draft-psarkar-rtgwg-rlfa-node-protection

Uma,

At the London meeting we presented a sample of results from modeling different PQ-selection heuristics in real network topologies.  They are in slides 7-14 of this link.

http://www.ietf.org/proceedings/89/slides/slides-89-rtgwg-4.pdf

[Uma]:What is Topology-A and Topology-B represent? Are these 2 from the 14 SP topologies?
Else we don't know what are the main characteristics of the topology.
X-axis is not marked in the graph to get an idea of the total number of nodes in each topology; however I see anywhere between 500 - 1100 PQ nodes for
most PLRs (I am sure this is also function of number of  no of links per PLR apart from other factors).


The comparison of per-PLR coverage for different heuristics for selecting a subset of PQ-nodes is shown for two topologies on p.11-14.  Not shown in these slides are the results when one considers all possible PQ-nodes, but this was also computed.  The exhaustive PQ-node results show that for some topologies, there is potential to improve coverage with more PQ-nodes or a better choice of PQ-nodes, especially with respect to node-protecting coverage.  However, for these two topologies, even with a simple heuristic for selecting 16 PQ nodes, RLFA offers a significant improvement in coverage compared to local LFA (as shown on p.9-10 ), so some service providers may find it useful.

[Uma]: I also see the coverage varies on PLT to PLR.  Though this result is giving some idea, probably it's still difficult to  conceive default heuristic is indeed is the best bet for all/majority the topologies. On the other hand, if the results are varying for each type of heuristic viz., AVOID_THEN_DIST/ DIST_THEN_AVOID/ RR_ AVOID_THEN_DIST/ SOME _BEST_PROP _XX_HEURISTIC  and we strive for best coverage, then how can we assure you get the same pre-computed PQ node irrespective of the vendor ? And also this predictability seems to be the premise of the standardization of the solution...

As has been pointed out by others, having to deal with the constraint imposed by limiting possible repair tunnels to those that can be created with a single LDP label (extended P-space) is generally going to be problematic.
[Uma]: Though it's problematic, probably this is operationally simple.
Using explicitly routed repair tunnels (created using RSVP or segment routing) or using an alternate topology (as with MRT) may be more productive approaches to improving coverage and backup path quality.
[Uma]: Agree.

Chris