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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

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Agenda l MPLS - quick introduction l A fast restoration scheme for MPLS

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

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

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Multi Protocol Label Switching l Label – Short, fixed-length packet identifier – Label swapping (similar to forwarding algorithm used in Frame Relay and ATM) IP Packet MPLS Header Incoming Label Mapping In (port, label) Out (port, label) (1, 2) (1, 6) (1, 8) (2, 13) (2, 17) (2, 21) (4, 7) (3, 32) Label Operation Swap 8IP7 – Incoming Label Mapping (ILM) Port 3 Port 1 Port 4 Port 2

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

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134.5.1.5 200.3.2.7 1 2 26 3 5 200.3.2.1 134.5.6.1 FEC Table DestinationNext Hop 134.5/16 200.3.2/24 (2, 84) (3, 99) ILM Table InOut (1, 99)(2, 56) ILM Table InOut (3, 56)(5, 3) ILM Table InOut (2, 84)(6, 3) 2 3 MPLS Forwarding Example 134.5.1.5 84 3

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3 MPLS Label Stack InOut ILM Table (2,Push [12])(1, 21) (3, 9)(2,Push [12]) InOut ILM Table (6, 3 )(2, 12) InOut ILM Table (5,Pop )(1, 3) InOut ILM Table (2, 56) (4, 21) (4, 9)(5, 7) 3 1 – Each LSR processes the top label – Stack of labels in the header IP Packet MPLS Label IP 21 2261545 2 12IP 213 IP 56

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In conclusion: l MPLS benefits: +No IP lookup +Traffic engineering +QoS -Restoration Fault Teardown Calculate – loop free Establish

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

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

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

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

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

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

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

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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 st v

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

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

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

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e1 e1 e2 e2 stst Building blocks for the shortest path p*: p x y x y z w z w

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

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

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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.

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Empirical results NameNodesLinksAvg. degree ISP ISP~200 ~400~3.7 Internet Internet40,377 101,659 5.035 AS Graph AS Graph 4,7469,878 4.16 AS AS Graph

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After one link failure Network max ILM Avg ILM. Avg. Concate Length. savings savings s.factor ISP weighted12.5% 25.6% 2.05 1.15 ISP unweighted 20.0% 32.3% 2 1.14 Internet16.7% 22.8% 2 1.08 AS graph 25.0% 32.7% 2 1.19 RBPC ILM table size / pre-provisioned t.s.

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After two link failures Network max ILM Avg ILM. Avg. PC length Length. savings savings s.factor ISP weighted2.3% 6.1% 2.38 1.77 ISP unweighted 3.6% 8.5% 2.20 1.34 Internet3.0% 4.7% 2.06 1.15 AS graph 7.1% 16.4% 2.09 1.32 RBPC ILM table size / pre-provisioned t.s.

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After one router failure Network max ILM Avg ILM. Avg. PC length Length. savings savings s.factor ISP weighted25.0% 43.7% 2.10 1.38 ISP unweighted 20.0% 36.8% 2.03 1.18 Internet12.5% 21.1% 2.02 1.08 AS graph 25.0% 38.5% 2.03 1.26 st v RBPC ILM table size / pre-provisioned t.s.

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After two router failures Network max ILM Avg ILM. Avg. PC length Length. savings savings s.f. ISP weighted5.26% 11.1% 2.43 1.57 ISP unweighted 6.67% 13.3% 2.21 1.44 Internet2.50% 4.1% 2.23 1.17 AS graph 8.33% 18.5% 2.17 1.31 RBPC ILM table size / pre-provisioned t.s.

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Edge bypass length Bypass ISP ISPASInternet Hopcountweighted UnweightedGraph 289.05% 90.11% 61.27% 54.96% 32.95% 2.99% 30.88% 37.68% 41.18% 1.79% 6.22% 2.37% 54.14% 5.08% 1.29% 1.72% 60.88% 0% 0.32% 2.05% 71.77% 0% 0% 0.64% 80% 0% 0% 0.95% 90% 0%0 % 0.23%

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End End

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Edge bypass: Weight S.F. ( Avg 1.15)

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Edge bypass: Hopcount S.F. (Avg 1.31)

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End-route: Weight S.F. (Avg. 1.07)

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End-route: Hopcount S.F. (Avg. 1.16)

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

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MPLS Multiple Protocol Label Switching 2003/2/19.

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