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Challenges

PCE-based Computation Procedure To Compute Shortest Constrained P2MP Inter-domain TE LSP draft-zhao-pce-pcep-inter-domain-p2mp-procedures-03.txt. Challenges. 2. Complex problem (the intra-domain Steiner tree MCT computation for P2MP LSP is NP-complete).

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Challenges

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  1. PCE-based Computation Procedure To Compute Shortest Constrained P2MP Inter-domain TE LSPdraft-zhao-pce-pcep-inter-domain-p2mp-procedures-03.txt

  2. Challenges 2 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010 Complex problem (the intra-domain Steiner tree MCT computation for P2MP LSP is NP-complete). End-to-end Path has to be remerged free (may also need to be cross-over free). Topology information is not shared across the domains. Signaling crankbacks due to remerges should be avoided.

  3. Issues with P2MP Tree computation using P2P BRPC PCE2 Remerge happens at (C) in domain 3 PCE3 PCE4 B18 D2 G B16 B19 B24 C B23 J D1 D B17 B20 B26 B15 H B22 B21 B1 PCE1 S B2 Result of BRPC for D1: (S,D1)=S->B1->B15->J->B17->B23->C->D->B19->B24->D1 = 10 Result of BRPC for D2: (S,D2)= S->B1->B15->G->B16->B18->C->D->D2 = 8 We see the same issue when two different BNs tryto expend EROs for two sibling S2Ls 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

  4. Option: P2MP-BRPC using Incremental In-progress Tree • Selects a destination D1 from destination set (e.g. has the longest AS/domain-hop path). • Recall this is a heuristics for an NP-complete problem. • Compute CSPF to destination D1 using P2P BRPC. • Save Path to D1 in “in-progress sub-tree” which is composed of known paths of destinations attempted so far. • Carry “in-progress sub-tree” during path computation for other destination. • When a PCE receives P2MP path computation request, it inspects the presence of the in-progress sub-tree, and colors all links belonging to the in-progress tree and present in its TED with zero link costs. • Such a cost assignment is reasonable since once traffic is delivered over a link to one destination, there is no extra cost (e.g., no extra resources required) to deliver the data over the same link to another destination. The idea is to try to find shortest paths to join other destinations to the “in-progress sub-tree”. 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

  5. VSPT(3, D2) = B23, C, D, D2 Append (Path(D1), Path(D2)) P2MP-BRPC using Incremental In-progress Tree Example PCE2 PCE3 PCE4 B18 BRPC Path(D1) in PCReq msg D2 G B16 B19 B24 C B23 J D1 D B17 B20 B26 B15 H B22 B21 B1 B13 PCE5 B5 PCE1 PCE6 E S B2 B6 B12 B4 F B D3 D6 B11 Tree of SPs: (S,D1)=S->B1->B15->J->B17->B23->C->D->B19->B24->D1 = 10 (S,D2)= S->B1->B15->G->B16->B18->C->D->D2 = 8 (S,D3)=S->B2->B4->B->B6->B12->F->D3 = 7 (S,D4)= S->B2->B4->B->B7->B9->I->D4 = 7 B7 Tree of SPs (ctd): (S,D5)= S->B2->B4->B->B7->B9->I->D5 = 6 (S,D6)= S->B2->B4->B->D6 = 4 Tree cost = (D1..D6) COST(SP(Di))) B9 B10 PCE7 I D4 D5 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

  6. Attributes of P2MP-BRPC using Incremental In-progress Tree • Makes use of existing BRPC algorithm with minimal changes to it. • Only need to carry “in progress tree” in PCReq message of PCEP. • With the exception of need to modify procedure at a BN which hosts IP addresses found in the “in-progress tree”, all other machinery of BRPC has been reused. • Using path keys, the procedure can work well with the cases when a BN does not provide hop-by-hop details in the VSPT. • Computationally efficient: • Worst case complexity: Number of destination * complexity of BRPC. • The tree computation does not require the knowledge of all destinations during the computation, i.e. additional destinations can be incrementally added to the in-progress sub-tree after it is originally computed. • Additions of new destinations do not alter the paths to existing destinations and hence do not cause re-configuration of the entire LSP every time a new destination is added. • The resultant P2MP tree is always remerge free. • The tree computation is sensitive to the order in which the component paths are computed and may potentially lead to a suboptimal tree. However selection of a destination which has longest AS/domain-hop path can be used as another heuristic to improve cost of the tree. • The algorithm has to be run sequentially (i.e., destination by destination). 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

  7. Attributes of P2MP-BRPC using XVSPT • Solution is documented in the Appendix slides. • Can run computation for all destinations in parallel. • Information carried in XVSPT is more than its VSPT counter part and hence use of XVSPT may lead to more optimal paths. • Requires encoding and computation of XVSPT. • Requires changes at Ingress PCE to append and perform path computation on the appended XVSPT. • The resultant P2MP tree is always remerge free. 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

  8. Next Steps • Looking for comments/ feedback on the document. • Would like the document to be accepted as a WG document. 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

  9. Thank You. 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

  10. Backup Slides 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

  11. Option2: Extended VSPT for P2MP tree computation PCE2 PCE4 PCE3 B18 D2 G B16 B19 B24 C B23 J D1 K D B17 B20 B26 B15 H B22 B21 B1 B13 PCE5 B5 PCE1 PCE6 E S B2 B6 B12 B4 F B D3 D6 B11 • - Actual BRPC procedure is extended to compute and carry XVSPT. • - 2 Phased approach: • Phase 1: PCE (1) collects XVSPTs for each of the P2MP destinations • Phase 2: using the appended tree, PCE(1), computes the P2MP tree B7 B9 B10 PCE7 I D4 11 D5 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

  12. 1 B24 D1 2 B26 2 B29 XVSPT(4, D1) XVSPT Computation in destination domain Example: Phase I at PCE4 PCE4 B19 B24 D1 B20 B26 H B29 PCE(4) computes path XVSPT(4) • VSPT is extended such that it includes Shortest Paths (SPs) from: • Destination (leaf) to all exit BNs in the destination domain. 12 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

  13. XVSPT Computation in transit domain Example: Phase I at PCE3 B18 4 4 1 B24 B23 D1 3 2 B22 B26 3 2 B29 B21 XVSPT(3, D1) PCE3 PCE4 B18 D2 B16 B19 B24 C B23 D1 D B17 B20 B26 H B22 B21 B29 B13 B5 B20, B26 not considered PCE(3) computes path XVSPT(3) using XVSPT(4) • VSPT is extended such that it includes Shortest Paths (SPs) from: • From all entry-BNs to all exit-BNs at every transit domain. • Can optimize by excluding entry-BNs— and sub-trees off of them— that are not along the E2E P2P SP to destination. 13 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

  14. XVSPT Computation at PCE2 PCE2 PCE3 PCE4 B18 D2 G B16 B16 B19 J B24 C K B23 D1 D B17 B20 B26 B15 H B22 B21 B1 B13 B5 PCE1 B20, B26 not considered PCE(2) computes path XVSPT(2) using XVSPT(3) 14 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

  15. XVSPT Computation at PCE1 PCE2 PCE3 PCE4 B18 D2 G B16 B16 B19 B24 C D1 D B15 B22 B21 B1 B13 B5 PCE1 S PCE(1) composes the append P2MP tree from information collected. 15 15 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

  16. Extended VSPT Phase 2 B18 D2 G B16 B19 B24 C D1 D B15 B22 B21 B1 B13 B5 PCE1 E S B2 B6 B12 B4 F B D3 • PCE(1) completes the appended P2MP tree from information collected in XVSPTs for destinations D1,.. D5 • When VSPTs for all destinations are back, PCE(1) runs a suitable spanning tree heuristic to compute a P2MP Tree computation based on the appended tree. • PCE(1) replies with P2MP tree to PCC(S) D6 B11 B7 B9 B10 I D4 D5 16 16 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

  17. Attributes of P2MP-BRPC using XVSPT • Can run computation for all destinations in parallel. • Information carried in XVSPT is more than its VSPT counter part and hence use of XVSPT may lead to more optimal paths. • Requires encoding and computation of XVSPT. • Requires changes at Ingress PCE to append and perform path computation on the appended XVSPT. • The resultant P2MP tree is always remerge free. 77th IETF, PCE WG, Anaheim, CA, USAMarch 2010

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