1. Inter-area MPLS TE Architecture and Protocol Extensions
draft-venkatachalam-interarea-mpls-te-01.txt draft-dharanikota-interarea-mpls-te-ext-01.txt
Senthil Venkatachalam - Alcatel USA
Sudheer Dharanikota - Nayna Networks Inc.
© Senthil Venkatachalam and Sudheer Dharanikota, December 2000.
All rights reserved.

2. Senthil, Sudheer, Thomas
Outline
Introduction to the problem
Our solution
Routing component
Signaling component
Configuration component
Conclusions
What next?
December 12, 2000
Senthil, Sudheer, Thomas

3. Introduction to the Problem
Why is inter-area LSP setup not attempted till now?
Routing:
TE metric values are defined and propagated for intra-area only
Interface-level TE metrics need to be extended and propagated to networks.
Signaling:
No effective way to signal routing constraints across areas
Also no specific mechanisms available for requesting a group of constraints.
What is needed in a solution?
Known TE LSP setup methods with backward compatibility
Known routing mechanisms for backward compatibility
Scalability
Possible solutions for inter-area LSP setup
LSP stitching => manual config. (not suitable for on-demand services)
Offline computation (Single point-of-failure and NRT)
On-line TE compute server (Additional HW and new paradigm)
Manual - Tedious?
Random+Crankback - Expensive?
Offline - Slow/n-RT
TE Compute Servers - not scalable, additional hardware
December 12, 2000
Senthil, Sudheer, Thomas

4. Introduction to the Problem
What is the problem?
Past: Complete topology information is not propagated across the area.
Intention: Reduce the IGP routing table size.
Problem: Sub-optimal paths are taken by the SPF based routers.
Solution: Summary routes across areas.
Assumption: Number of such routes would be less.
Current: Path attributes are not propagated across an area.
Problems:
Path computation: Sub-optimal paths chosen by CSPF based routers across areas.
Crankback: Crankback to the source or to an arbitrary ABR!
Solution: Limited summarization of TE information across areas.
Current: Single constraint problem.
The need for more than one constraint to calculate an end-to-end path.
Alternative constraints can be specified to reduce the crankback and hence the set up time.
due to unavailability of resources and unavailability of the most recent information.
Solution: Group the constraints (called criteria) and give an alternative.
Manual - Tedious?
Random+Crankback - Expensive?
Offline - Slow/n-RT
TE Compute Servers - not scalable, additional hardware
December 12, 2000
Senthil, Sudheer, Thomas

5. Senthil, Sudheer, Thomas
Our solution
Criteria
~ is a “group of constraints to be applied for a path computation”
A primary and secondary criteria are defined for a path
Routing protocols
Extended to advertise TE information across areas (Opaque TE summary LSA)
Extended CSPF for inter area
Signaling protocols
Extended to carry primary and secondary criteria
Extended to carry new error conditions
Area 0
Area 1
Area 2
LSP Section 1
LSP Section 2
LSP Section 3
December 12, 2000
Senthil, Sudheer, Thomas

6. Senthil, Sudheer, Thomas
Routing Component
Extension of IGPs for Inter-area TE attribute summarization
~ is the TE equivalent of IGP metric summarization
Summarized TE metrics
For OSPF:
[draft-venkatachalam-ospf-traffic-00.txt]
Opaque (Area scope) LSA - advertised into neighboring areas
For ISIS
[draft-dharanikota-interarea-mpls-te-ext-01.txt]
New TLVs defined for summarized TE metrics and advertised into other areas
Extensions to CSPF:
At head node of each LSP section (LER or ABR):
Use the TE-summary-LSAs (if available) to determine, a priori:
Whether the constraints on the inter-area LSP can be met, and if so,
The ABR that is “closest” to the destination in terms of the given constraints
Determine an intra-area route (CSPF) to the chosen ABR
December 12, 2000
Senthil, Sudheer, Thomas

7. Routing: Additive TE Metric Summarization
Additive metric
Use simple Dijkstra algorithm to compute summary
E.g.: ospf-metric, TE-metric, delay
Area 2 R1
Area 1 R3 10 10 R4 R2 N4 5
Summary For N4
Delay = 15 ms R5 5 R6 5
December 12, 2000
Senthil, Sudheer, Thomas

8. Routing: Max-Min TE Metric Summarization
Max-Min metric
Use a modified Dijkstra algorithm to summarize
Summarized metric value = Max over all path costs, where
Path cost = Min of component link costs
E.g.: Unreserved bandwidth in CT1, CT2, max. reservable bandwidth 3 5 4
Area 1 R2 R3 R6 R4 R5
Area 2 R1 7
Summary For N4
Unreserved BW
in CT1 = 4 Mbps N4
December 12, 2000
Senthil, Sudheer, Thomas

9. Routing: Other TE Metric Summarization
Other metrics: use a modified Dijkstra algorithm to summarize
Summarized metric value = F (over all path costs), where
Path cost = G (over all component links of that path)
Functions F,G: configurable!
E.g.: resource class/colors, SRLG. Example for bit-mapped colors can be: F = OR, G = AND
Summary For N4
Color Supported = 0x93
0xFF
0x13
0xb3
0x95
Area 1 R2 R3 R6 R4 R5
Area 2 R1
0x07 N4
December 12, 2000
Senthil, Sudheer, Thomas

10. Senthil, Sudheer, Thomas
Signaling Component
Extension of the signaling protocols to carry the routing criteria
Effective crankback using:
Crankback at the preceding head node
Primary and secondary criteria
Secondary criteria is inferior to primary in terms of requirements
Crankback topology feedback
Signaling protocols are extended for failure condition notifications
Signaling triggers CSPF under two conditions:
Loose path specification
Crankback occurrence (to prune the failed nodes) [draft-ietf-mpls-te-feed-01.txt]
When backup LSP is required:
RRO of the primary path is used
CSPF during the backup calculation prunes the nodes in RRO
December 12, 2000
Senthil, Sudheer, Thomas

11. Signaling: Inter-Area LSP Setup
(ABR) R5
(ABR) R7 R3
(ABR) R1 R6
(ABR) R4
(ABR)
TE-Summary-LSA
PATH
PATH ERROR
RESV
December 12, 2000
Senthil, Sudheer, Thomas

12. Signaling: Crankback to Preceding ABR
Area 0
Area 1
Area 2 R2
(ABR) R5
(ABR) R7 R3
(ABR) R1 R6
(ABR) R4
(ABR)
TE-Summary-LSA
PATH
PATH ERROR
RESV
December 12, 2000
Senthil, Sudheer, Thomas

13. Signaling: Use of Secondary Criteria
Use of the secondary criteria to minimize crankback
Area 0
Area 1
Area 2 R2
(ABR) R5
(ABR) R7 R3
(ABR) R1 R6
(ABR) R4
(ABR)
Modified (Secondary)
RESV
PATH
TE-Summary-LSA
PATH
RESV
PATH ERROR
December 12, 2000
Senthil, Sudheer, Thomas

14. Configuration component
Routing configuration
TE attributes to be summarized
TE routes to be summarized
LSP setup configuration
Primary and secondary criteria of the LSP
Signaling configuration
Report of notifications
December 12, 2000
Senthil, Sudheer, Thomas

15. Senthil, Sudheer, Thomas
Conclusions
Defined
Concept of criteria = Set of constraints
Routing extensions for inter area TE information
Signaling extensions for carrying constraints and effective crankback
Highlights
Finds an “optimal” path to the inter-area destination
If TE attribute summary available => most optimal route, less crankback
If no TE attribute summary NOT available => sub-optimal route, more crankback
For loose and strict source route specifications
Faster connection setup time
Uses secondary criteria to further reduce the possibility of crankback
Uses TE feedback when crankback does need to occur
Use of existing routing & signaling models (backward compatible)
No new hardware/compute servers with connections into multiple areas
December 12, 2000
Senthil, Sudheer, Thomas

16. Senthil, Sudheer, Thomas
Conclusions
Scenarios covered
TE summary LSA propagated
Loose source route: Can be computed with our modified CSPF
Strict source route:
Can determine apriori, if the resources are available for primary or secondary for the entire inter-area path
TE summary LSA not propagated
Loose source route: Reduce the crankback with the help of primary and secondary criteria
Can check at the head-end of every LSP section if the resources are available for primary or secondary
Scalability
Routing tradeoffs between the
“Selection of the optimal path” Vs. ”Granularity of the route aggregation”
“Accuracy of the optimal path” Vs. “Cost of frequent routing updates/TE database”
“Distributed dynamic setup” Vs. “Centralized computation”
Signaling tradeoffs between the
“Providing alternate criteria” Vs. “Simple crankback”
December 12, 2000
Senthil, Sudheer, Thomas

17. Senthil, Sudheer, Thomas
What next?
Any suggestions, questions?
Accept the following as working group documents:
draft-venkatachalam-interarea-mpls-te-01.txt
draft-dharanikota-interarea-mpls-te-ext-01.txt
December 12, 2000
Senthil, Sudheer, Thomas