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Enhanced Path Computation for Multi-Area Multi-AS Networks

This document explores solutions and enhancements for path computation in multi-area multi-AS networks, focusing on optimality, scalability, and simplicity. It introduces CSPF seed, heap, and relay concepts with mathematical foundations and PCE extensions. It addresses existing issues with global TED, crankback, and RBPC, presenting a Divide and Conquer approach using Path Trees. The text details CSPF seed properties, simplifies complex cases, and emphasizes Multi-AS path computation with information hiding and loose EROs for effective network management.

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Enhanced Path Computation for Multi-Area Multi-AS Networks

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  1. Relayed CSPF for Multi-Area Multi-AS PCEdraft-lu-relayed-cspf-00.txt IETF 79 - Beijing, China November 7-12, 2010 Wenhu Lu, SriganeshKini, Srikanth Narayanan

  2. Agenda • Solutions analysis • CSPF seed, heap, and relay • Mathematical base • PCE Elect • PCEP Extension • Relay Timer • Multi-AS

  3. Existing Solutions • Global TED • Too big, negating the purpose of having multiple areas or ASes; • violates the information hiding and confidentiality requirement and is unacceptable by ISPs • Crankback • Not scale • Not optimal • Labor intensive • RBPC – RFC5441 • Assume tailend is location-known, not always true • Can’t handle multi-homed cases • Complexity - Virtual Shortest Path Tree (VSPT)

  4. We look for • Solutions with • optimality • Scalability • simplicity

  5. Divide & Conquer • Path Tree Path Trees Headend Headend Area-1 Global TED Area-2 Area-3 Tailend Tailend

  6. CSPF Seed • The initial node for CSPF, or SPF • Seed to the heap • SPF is a seed based algorithm • The entire Shortest Path Tree (SPT) is built upon this seed.

  7. Multiple Seeds • A heap can have >1 seeds • In SPF expansion, the heap contains many nodes • can be perceived as seeds for further expansion.

  8. Heap Equivalence • A heap with one initial seed • is equivalent to that with multiple intermediate seeds in any SPF stages for the destinations that have not yet been reached. • The deposit time of seeds • is insensitive to destinations that have not yet been reached, provided that the seeds carry correct attributes values such as cost and nexthop. • The multiple seeds in property 1 • can further be reduced to those that constitute a set of nodes besides which the destinations are not viable.

  9. Simple Case • For convenience, assume every router is also a PCE • Start from Area “North” where Headend “H” resides • Compute paths to “A” and “B” • Note that “C” is not used • PCE-North sends “relay” request to PCE-South, say “B” • PCE-South uses “A” and “B” as seeds for CSPF in area “South” • Note that “A” and “B” have non-zero initial cost, hop-count, etc. • These two seeds will be the base to reach tailend “T” • PCE-South replys to PCE-North which stitch the paths H C North A B T South

  10. PCE Elect • For an exit area • Only one BN is necessary • Either “A” or “B” will work • Can be achieved through IGP-TE extension • draft-lu-ospf-area-tlv-00.txt

  11. Multi-Exit-Areas • Two paths avaible • Race condition B A D PCRep D B H PCReq T PCReq E A PCRep C C F

  12. Relay Timer • To collect multiple PCRep • Timeout to avoid infinite wait

  13. PCEP Seed TLV 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Len | Node-ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Node-ID (Cont) | Sub-Type | Sub-Len | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Seg-ID | Cost | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cost (Cont) | Hops | Sub-Type | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sub-Type #1 | | | // // | Sub-Type #M | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  14. Multi-AS Path Computation • Information Hiding • Loose EROs • Loose ERO recovery method • Pre-computed EROs • Re-Query • Transit Link • Included in the seed attributes

  15. Thank You

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