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Restoration by Path Concatenation: Fast Recovery of MPLS Paths

Restoration by Path Concatenation: Fast Recovery of MPLS Paths. Anat Bremler-Barr Yehuda Afek Haim Kaplan Tel-Aviv University Edith Cohen Michael Merritt AT&T Labs-Research. Agenda. MPLS - quick introduction A fast restoration scheme for MPLS. MPLS: Multi Protocol Label Switching.

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Restoration by Path Concatenation: Fast Recovery of MPLS Paths

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  1. Restoration by Path Concatenation:Fast Recovery of MPLS Paths Anat Bremler-Barr Yehuda Afek Haim Kaplan Tel-Aviv University Edith Cohen Michael Merritt AT&T Labs-Research

  2. Agenda • MPLS - quick introduction • A fast restoration scheme for MPLS

  3. MPLS: Multi Protocol Label Switching Two motivating forces: • Fast forwarding (eliminate IP-lookup) • Traffic Engineering & QoS

  4. IP Lookup forwarding Destination Address 1011001101011011001111110101 Prefix NxtHop • IP lookup - given an IP address, determine the next hop for reaching that destination • Fast Address lookup key component for high performance routers * 4 00* 12 011101110* 3 10000001* 3 10110* 3 101111* 5 10110011 * 2 10110011010* 4 Forwarding Table

  5. IP 8 (1, 2) (2, 17) (1, 6) (2, 21) (1, 8) (4, 7) IP 7 (2, 13) (3, 32) Multi Protocol Label Switching Label MPLS Header IP Packet IP Packet l – Short, fixed - length packet identifier – Label swapping (similar to forwarding algorithm used in Frame Relay and ATM) – Incoming Label Mapping (ILM) Incoming Label Mapping In Out Label (port, label) (port, label) Operation Swap Port 1 Port 2 Swap Swap Port 3 Port 4 Swap

  6. MPLS Header IP Packet 32-bits MPLS Forward Equivalence Class (FEC) • The same label to a stream/flow of IP packets: • Forwarded over the same path • Treated in the same manner • FEC/label binding mechanism • Currently based on destination IP address prefix • Future mappings based on TE-defined policy

  7. 134.5.1.5 134.5.1.5 84 3 134.5.1.5 134.5.1.5 ILM Table In Out (2, 84) (6, 3) FEC Table Destination Next Hop ILM Table ILM Table 134.5/16 (2, 84) In In Out Out 200.3.2/24 (3, 99) (3, 56) (1, 99) (2, 56) (5, 3) 1 2 2 3 2 6 3 5 MPLS Forwarding Example 134.5.6.1 134.5.1.5 200.3.2.1 200.3.2.7

  8. IP 21 IP 56 IP 21 12 IP 21 3 IP 21 MPLS Label Stack MPLS Label Stack – Stack of labels in the header IP Packet IP Packet MPLS Label MPLS Label – Each LSR processes the top label 2 1 2 2 6 1 5 4 5 3 3 ILM Table ILM Table ILM Table ILM Table In Out In Out In Out In Out (1, 21) (2, Push [12]) (2, 12) (6, 3 ) (1, 3) (5, Pop ) (2, 56) (4, 21) (3, 9) (2, Push [12]) (4, 9) (5, 7)

  9. Fault Teardown Calculate – loop free Establish In conclusion: • MPLS benefits: • No IP lookup • Traffic engineering • QoS • Restoration

  10. Part II Restoration by Path Concatenation:Fast Recovery of MPLS Paths

  11. Restoration By Path Concatenation (RBPC) Restore by concatenating existing paths m t s

  12. Main claim: • Unweighted case: Any shortest path after k edge failures is a concatenation of at most k+1 original surviving shortest paths. • Weighted case:k+1 paths and k edges • The basic set of Paths: Either All shortest paths or One shortest path for each pair of routers.

  13. One edge failure - concatenation of two paths Two edge failures - concatenation of three paths Example m s t o n

  14. 1 2 Ingress Routing Table (FEC) Destination Next Hop 134.5/16 (1, 30) (2, 27|87) (2, 87) 200.3.2/24 Path Concatenation with MPLS • Use the stack of labels mechanism: • source pushes two labels (one fault) 200.3.2/24 27 87 t s 30 134.5/16 No changes in ILM tables

  15. 2 1 V87 2 Concatenation mechanism in ATM or WDM • Need an IP-lookup at m !!! m V27 V87 VC Table of S t s V30 Dest label (vci/vpi) port t V30 1 m V87 2

  16. The restoration method requirements • Global knowledge at Ingress LSR • Store the global view locally (on a disk)

  17. v s t Limitations of RBPC • Bandwidth reservation: have not yet dealt with • Non shortest paths: Requires T.E. Algorithms at the source • Theory does not apply to node failure • Does not, in general work in directed graphs

  18. Main claim: • Unweighted case: Any shortest path after k edge failures is a concatenation of at most k+1 original surviving shortest paths. • Weighted case:k+1 paths and k edges • The basic set of Paths: Either All shortest paths or One shortest path for each pair of routers.

  19. e2 e1 e3 e1 e2 e1 e2 e3 s t Unweighted case: sketch of proof Let p be the shortest path after removing k edges. Let bypasses {bp1, bp2, bp3, bp4} be: p s t x w u v Claim: There are at most k bypasses ==> Main claim

  20. e1 e1 e2 e2 x y x y z w z w p s t • Proof by contradiction: • Assume there are more than k bypasses • Then exists p* (s->t), s.t., p* is shorter than p. e1 e1 e2 e3 e2 p s t constructing p*: claim: exists a subset of bypasses, s.t., each removed edge occurs in an even number of bypasses.

  21. Building blocks for the shortest path p*: e1 e1 e2 e2 x y x y z w z w p s t

  22. x z t P* must exist - Euler s y w e1 e2 e3 p* Building blocks for the shortest path p*: e1 e1 e2 e2 x y x y z w z w p s t

  23. Pre-provisioned method • For each link & LSP (label swapping path) going over it maintain (pre-provision) a restoration path • Similarly, for each two links in an LSP maintain a restoration path • Huge O/H: ILM tables • Not scalable

  24. The restoration method benefits • Fast restoration • Static set of paths • No messages for tearing down and setting up • Static & Small ILM tables • Only one router changes the FEC table. • Speed and simplicity of pre-provisioned restoration paths without the associated overhead.

  25. Empirical results Name Nodes Links Avg. degree ISP ~200 ~400 ~3.7 Internet 40,377 101,659 5.035 AS Graph 4,746 9,878 4.16 AS Graph AS

  26. After one link failure Network max ILM Avg ILM. Avg. Concate Length. savings savings s.factor ISP weighted 12.5% 25.6% 2.05 1.15 ISP unweighted 20.0% 32.3% 2 1.14 Internet 16.7% 22.8% 2 1.08 AS graph 25.0% 32.7% 2 1.19 RBPC ILM table size / pre-provisioned t.s.

  27. After two link failures Network max ILM Avg ILM. Avg. PC length Length. savings savings s.factor ISP weighted 2.3% 6.1% 2.38 1.77 ISP unweighted 3.6% 8.5% 2.20 1.34 Internet 3.0% 4.7% 2.06 1.15 AS graph 7.1% 16.4% 2.09 1.32 RBPC ILM table size / pre-provisioned t.s.

  28. v s t After one router failure Network max ILM Avg ILM. Avg. PC length Length. savings savings s.factor ISP weighted 25.0% 43.7% 2.10 1.38 ISP unweighted 20.0% 36.8% 2.03 1.18 Internet 12.5% 21.1% 2.02 1.08 AS graph 25.0% 38.5% 2.03 1.26 RBPC ILM table size / pre-provisioned t.s.

  29. After two router failures Network max ILM Avg ILM. Avg. PC length Length. savings savings s.f. ISP weighted 5.26% 11.1% 2.43 1.57 ISP unweighted 6.67% 13.3% 2.21 1.44 Internet 2.50% 4.1% 2.23 1.17 AS graph 8.33% 18.5% 2.17 1.31 RBPC ILM table size / pre-provisioned t.s.

  30. End

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