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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-00.txt


1. Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   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 that do not go through the nexthop
   router that is being protected.

   This document also provide a way to maximize the use of loose
   segments in order to simplify the implementation of repair paths.









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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.

   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



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   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:

   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.



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      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.


7.2. Last 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 last loose segment. From there on, the packets can
   be forwarded towards destination based on normal routing, and the
   packets will not come back to the router being protected.

   It can be proven that this optimization is also valid for repair



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   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 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
   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>.




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   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 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
   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.




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   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])) {
               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 {



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               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

   LSP source route and RSVP-TE (possibly with loose segment
   optimization), can be used to construct arbiturary repair paths.

   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. Link metrics are show next to the links.

                   10
                 A-----B
             10 / \   / \ 10
               /  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 LSP source route and RSVP-
   TE today.


8. Security Considerations

   This document does not introduce any new security issues.













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9. Full Copyright Statement

   Copyright (C) The Internet Society (2002). All Rights Reserved.
   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the  purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS 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.


10. 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.

    [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-00.txt, May 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.



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    [LOOPFREE] Atlas, Torvi, Choudhury, Martin, Imhoff, Fedyk,
       "IP/LDP Local Protection", draft-atlas-ip-local-protect-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
       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.


11. 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


12. Appendix A: Classification of Repair Path Mechanisms


12.1. Two Hops: Strict - Loose

    Downstream Path (Loop-Free Alternative) [LOOPFREE]
    ECMP









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12.2. Two Hops: Loose - Loose

    Tunnel Approach without directed forwarding [TUNNEL].


12.3. Three Hops: Loose - Strict - Loose

    Tunnel Approach with directed forwarding [TUNNEL].


12.4. Three Hops: Strict - Strict - Loose

    U-Turn: only support a subset of the cases where the first hop node
    must be an upstream node. [UTURN]


12.5. Three Hops: Strict - Loose - Loose

    Tunnel Approach with first strict hop optimization and without
    directed forwarding [TUNNEL].


12.6. Four Hops - Strict, Loose, then Strict, Loose

    Tunnel Approach with first strict hop optimization and directed
    forwarding [TUNNEL].


12.7. Arbitrary Mix of Loose and Strict

    LSP Source Route [LSP-SRC-RT]
    RSVP-TE [RSVPTE]
    RSVP-TE with Loose Hop Optimization [LOOSE-OPT].


















Tian                                                           [Page 11]


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