Network Working Group Albert J. Tian Internet Draft Naiming Shen Expiration Date: Jan 2005 Redback Networks July 2004 Fast Reroute using Alternative Shortest Paths draft-tian-frr-alt-shortest-path-01.txt 1. Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 2. Abstract Repair path mechanism is an important element in IP/LDP fast reroute. In this document we propose a way to calculate local repair paths using alternative shortest paths. The solution can provide complete coverage for all destinations within an IGP area that can possibly be protected. In this document we also suggested an "Explicit Path with Loose Segment" model that can be used to characterize, classify, and analyse repair paths. We proved some of the common properties of loose segments, and showed that the model can be used to characterize all existing fast reroute solutions. Tian [Page 1]

Internet Draft draft-tian-frr-alt-shortest-path-01.txt July 2004 Based on the properties of loose segments, we also provided a way to maximize the use of loose segments in a repair path based on alternative shortest path, in order to simplify the repair path implementation. 3. Introduction To construct a repair path, the termination point of the repair path must be determined first. Then we can calculate the repair path from the router at point of local repair (PLR) to the termination point without going through the nexthop router being protected. The resulting explicit path from the calculation is usually a strict path that lists all nodes on the path. The path can then be simplified by maximizing the use of loose hops. The resulting path can then be implemented using mechanisms such as LSP source route or RSVP-TE. 4. Select the Termination Point of Repair Path Since node protection can also cover link failure and its in general difficult to distinguish between link and node failure, node failure is always assumed, unless the nexthop is a single point of failure. On a PLR router P, to protect a destination D from the failure of nexthop N, the termination point T of a repair path can be one of the following: If the nexthop N is not the primary egress point E for D, then either a) terminate at the primary egress point E for D, or b) terminate at the next-nexthop node from P to E [NHFRR]. If the nexthop N is the primary egress point E for D, then c) if there exists an alternative egress point E' for D, terminate at E'; d) if there are no alternative egress points, terminate at E and attempt link protection. Terminating repair path at next-nexthop has several advantages over terminating at egress point: 1) since there are usually much less next-nexthops than egress points, next-nexthop based solution requires much less repair paths to be calculated and maintained. 2) next-nexthop based repair path can protect multicast traffic [NHFRR-MCAST], while egress based repair path can not. Tian [Page 2]

Internet Draft draft-tian-frr-alt-shortest-path-01.txt July 2004 In some cases, next-nexthop based repair paths may be less optimal for some destinations, but this usually is not a concern. If the nexthop is the only egress, then it is a single point of failure. In this case, link protection is attempted. Repair paths can be calculated easily by disqualifying the link between P and N. The rest of the discussions in this document will focus on the node protection cases (i.e. cases a, b and c above). 5. Use Alternative Shortest Paths as Repair Paths Once the termination point T is decided, we can move on to the calculation of repair paths from P to T. Repair paths are used by the PLR router P to quickly recover from the failure of a nexthop N, therefore the repair path can not go through the nexthop N that is being protected. For a destination D, one way to find the repair path on PLR router P to protect nexthop N's failure is to calculate shortest alternative paths from P to termination point T that do not go through N. For networks that are running link state IGP such as OSPF or ISIS, a simple way to calculate alternative shortest paths is to remove N from the link state database and re-run SPF from PLR P's point of view. This SPF will find all the alternative shortest paths from P to all possible T not going through N, therefore it will find out all the repair paths needed to cover N's failure, except for cases where N is a single point of failure. To protect all P's nexthops, the same calculation needs to be done for each nexthop. We use the notion ASP-N<A,B> to represent the set of alternative shortest paths between A and B that do not go through N. 6. Construct Repair Paths using Explicit Paths Since repair paths can not follow normal IP routing, therefore they have to be explicitly paths. Even in ECMP cases, when one of the ECMP nexthop fails, traffic has to be explicitly directed to the other ECMP nexthops. Therefore the ECMP based repair paths are still explicit paths. An explicit path can be expressed as a list of nexthops that the path must traverse. Each hop can be either strict or loose. In general there are two ways to implement an explicit path: Tian [Page 3]

Internet Draft draft-tian-frr-alt-shortest-path-01.txt July 2004 a) Stateful Explicit Path: this method installs special forwarding state on each router that is specified in the explicit path. An example of this method is RSVP-TE. There is little or no per packet overhead, but states need to be maintained in the network. This method can handle arbitrary explicit paths. Also RSVP-TE can support QoS along the path. b) Source Routed Explicit Path: this method use some form of source routing to encode the path in the packet itself. An example of this method is LSP source route [LSP-SRC-RT]. The benefit of this method is that no state need to be maintained in the network, therefore this method can scale to a large number of explicit paths. The limitation is that due to the per packet overhead, this method is only suitable for simple paths with small number of routers specified. Please note that a simple path is not necessarily short. One loose hop specified in the path can traverse a large number of routers. There are other solutions for fast reroute. They can all be viewed as some form of limited source routing. We will discuss this later in appendix A. 7. Loose Hops in Explicit Repair Paths There are several benefits of using loose segments in repair paths. 7.1. Reduce Number of Hops Specified In any case, there is incentive to reduce the number of hops that need to be specified in an explicit path. The simplest way to reduce the number of routers that need to be specified in an explicit path is through the use of loose hops. For stateful explicit paths, if loose segment optimization using tunnels is enabled [LOOSE-OPT], then the use of loose segments can reduce the amount of state installed in the network. For source routed explicit paths, the use of loose segments can reduce the per packet overhead. Tian [Page 4]

Internet Draft draft-tian-frr-alt-shortest-path-01.txt July 2004 7.2. Trailing Loose Hop Optimization If the last segment of an explicit repair path is a loose segment, then as an optimization the explicit path can terminate early at the beginning of the trailing loose segment. From there on, the packets can be forwarded towards destination based on normal routing, and the actual path traversed by the packets will not be affected by the failure of the router being protected. It can be proven that this optimization is also valid for repair paths terminating on next-nexthop router. Consider the following topology where P is the PLR, N is the nexthop being protected, H is the next-nexthop, E is the egress. Path P-A-B-H is the next-nexthop based repair path. All links shown below except link <P,N> are loose segments that may traverse multiple routers. A-----B--------+ / \ / \ | / \ / \ | P-----N-----H-----E Figure 2 It can be proven that if segment <B,H> can be a loose hop for repair path P-A-B-H, then repair path P-A-B is sufficient to protect all traffic from P that passes through next-nexthop H. 7.3. Characters of Loose Hops One requirement for a repair path is that it can not pass through the router being protected regardless of its status. Therefore any loose hop in an explicit repair path must not pass through the router being protected. 7.3.1. Theorem 1 The following theorem can help identifying the loose segments in an explicit path. Theorem 1: Let SP<A,B> be the set of shortest paths between A and B. If paths in SP<A,B> do not pass through N when N is available, then the set Tian [Page 5]

Internet Draft draft-tian-frr-alt-shortest-path-01.txt July 2004 SP<A,B> will not change when N and only N becomes unavailable. Proof: When node N becomes unavailable, the cost of the links between N and its neighbors increase from some finite value to infinity. If the cost of any path between A and B is changed after N's failure, it can only become higher. Therefore N's failure will not add any new paths to SP<A,B>. If N is the only node that becomes unavailable, then the costs of links not connected to N will not be changed. Therefore the cost of the paths not passing through N will not be changed. That means the costs of the paths in SP<A,B> will not be changed. Therefore N's failure will not remove any paths from SP<A,B>. So N's failure will not change SP<A,B>. END. Basically theorem 1 means that if the shortest paths between A and B do not pass through the router N being protected, then segment <A,B> can be safely used as a loose segment in a repair path protecting N, because the actual path for the loose segment will not be affected by N's failure. 7.3.2. Theorem 2 The following theorem can further improve theorem 1. Theorem 2: Let ASP_N<A,B> be the set of alternative shortest paths between A and B that do not go through N. Let l(ASP_N<A,B>) be the path length for ASP_N<A,B>. It is the shortest distance between A and B for paths that do not go through N. Let d(A,N) be the shortest distance between A and N. Let d(N,B) be the shortest distance between N and B. If the following condition holds, then SP<A,B> do not go through N. l(ASP_N<A,B>) < d(A,N) + d(N,B) ....... Condition 1 Proof All the paths between A and B can be divided into two sets, those that pass through N, and those that do not pass through N. For paths Tian [Page 6]

Internet Draft draft-tian-frr-alt-shortest-path-01.txt July 2004 that pass through N, the shortest distance between A and B is d(A,N) + d(N,B). For paths that do not pass through N, by definition the shortest path is ASP_N<A,B>, and its length is l(ASP_N<A,B>). Because condition 1 is true, ASP_N<A,B> is shorter than any path that goes through N. Therefore ASP_N<A,B> is the shortest among all paths between A and B. Therefore ASP_N<A,B> is SP<A,B>. Since ASP_N<A,B> do not go through N, SP<A,B> do not go through N. END. Essentially Theorem 2 means that if the length of the alternative shortest path between A and B that do not go through N is shorter than the distance between A and N plus the distance between N and B, then the segment <A,B> can be used as a loose segment in a repair path protecting N. 7.4. Finding Loose Segments in Alternative Shortest Path Based on theorem 1 and theorem 2, an algorithm can be devised to find loose hops in alternative shortest paths. Given an alternative shortest path ASP_N<P,T> = <R1, R2, ..., Rm> from PLR P to termination point T not going through nexthop N, it can be used to protect N. Run SPF from N's point of view to find out d(N,X), for all X. If all link metrics are symmetrical, then d(X,N) = d(N,X), for all X. If some link metrics are asymmetrical, then run an additional reverse metric SPF from N's point of view to find out d(X,N), for all X. Let c(Ri, Rj) be the link metric from Ri to Rj. The following algorithm maximizes the length of loose hops in the alternative shortest path. It evaluates a segment on the path against theorem 2, if it can be a loose hop, then extend the segment by one hop and re-evaluate again, till a point that the segment is no longer a loose segment. In this way the algorithm finds the longest loose segment on the path for a given starting point. Algorithm Find_Loose_Hops { len = 0; start = 1; end = start; while (end < m) { if (len + c(R[end],R[end+1]) >= d(R[start],N) + d(N,R[end])) { Tian [Page 7]

Internet Draft draft-tian-frr-alt-shortest-path-01.txt July 2004 if (len == 0) { output segment <R[end], R[end+1]> as a strict hop; end = end+1; start = end; if (start > m) break; } else { output segment <R[start],R[end]> as a loose hop; start = end; } } else { len = len + c(R[end],R[end+1]); end = end+1; if (end == m) { output segment <R[start],R[end]> as a loose hop; } } } } 7.5. Implementing Repair Paths IP TE Route Switched Path [IP-TE-RSP] can be used to implement arbitrary repair paths in a pure IP network. If MPLS is enabled, then LSP source route [LSP-SRC-RT] and RSVP-TE (possibly with loose segment optimization [LOOSE-OPT]), can be used to construct arbiturary repair paths using MPLS. All of the above mechanisms can benefit from the use of loose segments in repair paths. In most cases, a repair path is expected to conatin some loose segments, in perticular a long loose segment at the end. For RSVP based solutions, if optimization of loose segments is enabled, then the actual amount of state information maintained in a network can be greatly reduced. For LSP source route, the use of loose segments can reduce the label stack size. Please note that under some topologies the repair paths may require multiple strict hops to route around the router being protected. For example, in the following topology as shown in Figure 1, P-N-H-E was the primary path. In order for PLR P to implement repair path P-A-B- H-E to protect failure in N, the first three segments must all be strict. The repair path is of type SSSL. Link metrics are show next to the links. 2 A-----B 2 / \ / \ 2 Tian [Page 8]

Internet Draft draft-tian-frr-alt-shortest-path-01.txt July 2004 / 1\ /1 \ P-----N-----H-----E 1 1 10 Figure 2 A sample topology that requires complex repair path This repair path can only be supported by IP TE RSP, LSP source route, and RSVP-TE today. 8. Security Considerations This document does not introduce any new security issues. 9. References [IPFRR] M. Shand, "IP Fast Reroute Framework", draft-ietf-rtgwg-ipfrr-framework-01.txt, Work in progress. [NHFRR] N. Shen, P. Pang, "Nexthop Fast ReRoute for IP and MPLS", draft-shen-nhop-fastreroute-00.txt, Work in progress. [IP-TE-RSP] N. Shen, A. Tian, J, Zhuang, "IP Traffic Engineering With Route Switched Paths (RSPs)", Work in progress. [NHFRR-MCAST] N. Shen, L. Wei, D. Farinacci, "Discovering PIM-SM Next-Nexthop Downstream Nodes", draft-shen-pim-nnhop-nodes-00.txt, Work in progress. [LSP-SRC-RT] A. Tian, G. Apostolopoulos, "Source Routed MPLS LSP using Domain Wide Label", draft-tian-mpls-lsp-source-route-01.txt, July 2004, Work in progress. [LOOSE-OPT] A. Tian, N. Shen, "Loose Segment Optimization in Explicit Paths", draft-tian-rsvp-loose-seg-opt-00.txt, Work in progress. [LOOPFREE] Atlas, Torvi, Choudhury, Martin, Imhoff, Fedyk, "IP/LDP Local Protection", draft-atlas-ip-local-protect-loopfree-00.txt, Work in progress. [UTURN] Atlas, et al., "U-turn Alternates for IP/LDP Local Protection", draft-atlas-ip-local-protect-uturn-00.txt, Work in progress. [TUNNEL] Bryant, Filsfils, Previdi, Shand, "IP Fast Reroute using Tian [Page 9]

Internet Draft draft-tian-frr-alt-shortest-path-01.txt July 2004 tunnels", draft-bryant-ipfrr-tunnels-00.txt, May 2004, Work In Progress. [RSVPTE] Awduche, et al., "Extensions to RSVP for LSP Tunnels", RFC 3209, December 2001. [LDP] L. Andersson, P. Doolan, N. Feldman, A. Fredette, and B. Thomas, "LDP Specification", RFC 3036, January 2001. 10. Author Information Albert Jining Tian Redback Networks, Inc. 300 Holger Way San Jose, CA 95134 Email: tian@redback.com Naiming Shen Redback Networks, Inc. 300 Holger Way San Jose, CA 95134 Email: naiming@redback.com 11. Appendix A: Classification of Repair Path Mechanisms 11.1. Two Hops: Strict - Loose (Type SL) Downstream Path (Loop-Free Alternative) [LOOPFREE] ECMP 11.2. Two Hops: Loose - Loose (Type LL) Tunnel Approach without directed forwarding [TUNNEL]. 11.3. Three Hops: Loose - Strict - Loose (Type LSL) Tunnel Approach with directed forwarding [TUNNEL]. Tian [Page 10]

Internet Draft draft-tian-frr-alt-shortest-path-01.txt July 2004 11.4. Three Hops: Strict - Strict - Loose (Type SSL) U-Turn: only support a subset of the cases where the first hop node must be an upstream node. [UTURN] 11.5. Three Hops: Strict - Loose - Loose (Type SLL) Tunnel Approach with first strict hop optimization and without directed forwarding [TUNNEL]. 11.6. Four Hops - Strict, Loose, then Strict, Loose (Type SLSL) Tunnel Approach with first strict hop optimization and directed forwarding [TUNNEL]. 11.7. Arbitrary Mix of Loose and Strict Hops IP TE RSP [IP-TE-RSP] LSP Source Route [LSP-SRC-RT] RSVP-TE [RSVPTE] RSVP-TE with Loose Hop Optimization [LOOSE-OPT]. 12. Full Copyright Statement Copyright (C) The Internet Society (year). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Tian [Page 11]