Network Working Group                                          S. Bryant
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                              S. Previdi
Expires: November 24, 2013                                 Cisco Systems
                                                                M. Shand
                                                 Independent Contributor
                                                                   N. So
                                                     Tata Communications
                                                            May 23, 2013

                             Remote LFA FRR


   This draft describes an extension to the basic IP fast re-route
   mechanism described in RFC5286 that provides additional backup
   connectivity for link failures when none can be provided by the basic

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC2119 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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."

   This Internet-Draft will expire on November 24, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Terminology

   This draft uses the terms defined in [RFC5714].  This section defines
   additional terms used in this draft.

   Extended P-space

                  The union of the P-space of the neighbours of a
                  specific router with respect to the protected link.

   P-space        P-space is the set of routers reachable from a
                  specific router without any path (including equal cost
                  path splits) transiting the protected link.

                  For example, the P-space of S, is the set of routers
                  that S can reach without using the protected link S-E.

   PQ node        A node which is a member of both the extended P-space
                  and the Q-space.

   Q-space        Q-space is the set of routers from which a specific
                  router can be reached without any path (including
                  equal cost path splits) transiting the protected link.

   Repair tunnel  A tunnel established for the purpose of providing a
                  virtual neighbor which is a Loop Free Alternate.

   Remote LFA     The tail-end of a repair tunnel.  This tail-end is a
                  member of both the extended-P space the Q space.  It
                  is also termed a "PQ" node.

   In this document we use the notation X-Y to mean the path from X to Y
   over the link directly connecting X and Y, whilst the notation X->Y
   refers to the shortest path from X to Y via some set of unspecified
   nodes including the null set (i.e.  including over a link directly
   connecting X and Y).

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2.  Introduction

   RFC 5714 [RFC5714] describes a framework for IP Fast Re-route and
   provides a summary of various proposed IPFRR solutions.  A basic
   mechanism using loop-free alternates (LFAs) is described in [RFC5286]
   that provides good repair coverage in many
   topologies[I-D.filsfils-rtgwg-lfa-applicability], especially those
   that are highly meshed.  However, some topologies, notably ring based
   topologies are not well protected by LFAs alone.  This is illustrated
   in Figure 1 below.

            /     \
           A       D
            \     /

                     Figure 1: A simple ring topology

   If all link costs are equal, the link S-E cannot be fully protected
   by LFAs.  The destination C is an ECMP from S, and so can be
   protected when S-E fails, but D and E are not protectable using LFAs

   This draft describes extensions to the basic repair mechanism in
   which tunnels are used to provide additional logical links which can
   then be used as loop free alternates where none exist in the original
   topology.  For example if a tunnel is provided between S and C as
   shown in Figure 2 then C, now being a direct neighbor of S would
   become an LFA for D and E.  The non-failure traffic distribution is
   not disrupted by the provision of such a tunnel since it is only used
   for repair traffic and MUST NOT be used for normal traffic.

         / \   \
        A   \   D
         \   \ /

                    Figure 2: The addition of a tunnel

   The use of this technique is not restricted to ring based topologies,
   but is a general mechanism which can be used to enhance the
   protection provided by LFAs.

   This technique describes in this document is directed at providing
   repairs in the case of link failures.  Considerations regarding node
   failures are discussed in Section 6.

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3.  Repair Paths

   As with LFA FRR, when a router detects an adjacent link failure, it
   uses one or more repair paths in place of the failed link.  Repair
   paths are pre-computed in anticipation of later failures so they can
   be promptly activated when a failure is detected.

   A tunneled repair path tunnels traffic to some staging point in the
   network from which it is assumed that, in the absence of multiple
   failures, it will travel to its destination using normal forwarding
   without looping back.  This is equivalent to providing a virtual
   loop-free alternate to supplement the physical loop-free alternates.
   Hence the name "Remote LFA FRR".  When a link cannot be entirely
   protected with local LFA neighbors, the protecting router seeks the
   help of a remote LFA staging point.

3.1.  Tunnels as Repair Paths

   Consider an arbitrary protected link S-E.  In LFA FRR, if a path to
   the destination from a neighbor N of S does not cause a packet to
   loop back over the link S-E (i.e.  N is a loop-free alternate), then
   S can send the packet to N and the packet will be delivered to the
   destination using the pre-failure forwarding information.  If there
   is no such LFA neighbor, then S may be able to create a virtual LFA
   by using a tunnel to carry the packet to a point in the network which
   is not a direct neighbor of S from which the packet will be delivered
   to the destination without looping back to S.  In this document such
   a tunnel is termed a repair tunnel.  The tail-end of this tunnel is
   called a "remote LFA" or a "PQ node".

   Note that the repair tunnel terminates at some intermediate router
   between S and E, and not E itself.  This is clearly the case, since
   if it were possible to construct a tunnel from S to E then a
   conventional LFA would have been sufficient to effect the repair.

3.2.  Tunnel Requirements

   There are a number of IP in IP tunnel mechanisms that may be used to
   fulfil the requirements of this design, such as IP-in-IP [RFC1853]
   and GRE[RFC1701] .

   In an MPLS enabled network using LDP[RFC5036], a simple label
   stack[RFC3032] may be used to provide the required repair tunnel.  In
   this case the outer label is S's neighbor's label for the repair
   tunnel end point, and the inner label is the repair tunnel end
   point's label for the packet destination.  In order for S to obtain
   the correct inner label it is necessary to establish a directed LDP
   session[RFC5036] to the tunnel end point.

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   The selection of the specific tunnelling mechanism (and any necessary
   enhancements) used to provide a repair path is outside the scope of
   this document.  The authors simply note that deployment in an MPLS/
   LDP environment is extremely simple and straight-forward as an LDP
   LSP from S to the PQ node is readily available, and hence does not
   require any new protocol extension or design change.  This LSP is
   automatically established as a basic property of LDP behavior.  The
   performance of the encapsulation and decapsulation is also excellent
   as encapsulation is just a push of one label (like conventional MPLS
   TE FRR) and the decapsulation occurs naturally at the penultimate hop
   before the PQ node.

   When a failure is detected, it is necessary to immediately redirect
   traffic to the repair path.  Consequently, the repair tunnel used
   must be provisioned beforehand in anticipation of the failure.  Since
   the location of the repair tunnels is dynamically determined it is
   necessary to establish the repair tunnels without management action.
   Multiple repairs may share a tunnel end point.

4.  Construction of Repair Paths

4.1.  Identifying Required Tunneled Repair Paths

   Not all links will require protection using a tunneled repair path.
   Referring to Figure 1, if E can already be protected via an LFA, S-E
   does not need to be protected using a repair tunnel, since all
   destinations normally reachable through E must therefore also be
   protectable by an LFA.  Such an LFA is frequently termed a "link
   LFA".  Tunneled repair paths are only required for links which do not
   have a link LFA.

4.2.  Determining Tunnel End Points

   The repair tunnel endpoint needs to be a node in the network
   reachable from S without traversing S-E.  In addition, the repair
   tunnel end point needs to be a node from which packets will normally
   flow towards their destination without being attracted back to the
   failed link S-E.

   Note that once released from the tunnel, the packet will be
   forwarded, as normal, on the shortest path from the release point to
   its destination.  This may result in the packet traversing the router
   E at the far end of the protected link S-E., but this is obviously
   not required.

   The properties that are required of repair tunnel end points are

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   o  The repair tunneled point MUST be reachable from the tunnel source
      without traversing the failed link; and

   o  When released, tunneled packets MUST proceed towards their
      destination without being attracted back over the failed link.

   Provided both these requirements are met, packets forwarded over the
   repair tunnel will reach their destination and will not loop.

   In some topologies it will not be possible to find a repair tunnel
   endpoint that exhibits both the required properties.  For example if
   the ring topology illustrated in Figure 1 had a cost of 4 for the
   link B-C, while the remaining links were cost 1, then it would not be
   possible to establish a tunnel from S to C (without resorting to some
   form of source routing).

4.2.1.  Computing Repair Paths

   The set of routers which can be reached from S without traversing S-E
   is termed the P-space of S with respect to the link S-E.  The P-space
   can be obtained by computing a shortest path tree (SPT) rooted at S
   and excising the sub-tree reached via the link S-E (including those
   which are members of an ECMP).  In the case of Figure 1 the P-space
   comprises nodes A and B only.  Expressed in cost terms the set of
   routers {P} are those for which the shortest path cost S->P is
   strictly less than the shortest path cost S->E->P.

   The set of routers from which the node E can be reached, by normal
   forwarding, without traversing the link S-E is termed the Q-space of
   E with respect to the link S-E.  The Q-space can be obtained by
   computing a reverse shortest path tree (rSPT) rooted at E, with the
   sub-tree which traverses the failed link excised (including those
   which are members of an ECMP).  The rSPT uses the cost towards the
   root rather than from it and yields the best paths towards the root
   from other nodes in the network.  In the case of Figure 1 the Q-space
   comprises nodes C and D only.  Expressed in cost terms the set of
   routers {Q} are those for which the shortest path cost E->Q is
   strictly less than the shortest path cost E->S->Q.  In Figure 1 the
   intersection of the E's Q-space with S's P-space defines the set of
   viable repair tunnel end-points, known as "PQ nodes".  As can be
   seen, for the case of Figure 1 there is no common node and hence no
   viable repair tunnel end-point.

   Note that the Q-space calculation could be conducted for each
   individual destination and a per-destination repair tunnel end point
   determined.  However this would, in the worst case, require an SPF
   computation per destination which is not currently considered to be
   scalable.  We therefore use the Q-space of E as a proxy for the

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   Q-space of each destination.  This approximation is obviously correct
   since the repair is only used for the set of destinations which were,
   prior to the failure, routed through node E.  This is analogous to
   the use of link-LFAs rather than per-prefix LFAs.

4.2.2.  Extended P-space

   The description in Section 4.2.1 calculated router S's P-space rooted
   at S itself.  However, since router S will only use a repair path
   when it has detected the failure of the link S-E, the initial hop of
   the repair path need not be subject to S's normal forwarding decision
   process.  Thus we introduce the concept of extended P-space.  Router
   S's extended P-space is the union of the P-spaces of each of S's
   neighbours.  This may be calculated by computing the an SPT at each
   of S's neighbors (N) (excluding E) and excising the subtree reached
   via the path N->S->E.  The use of extended P-space may allow router S
   to reach potential repair tunnel end points that were otherwise
   unreachable.  In cost terms a router is in extended P-space if the
   shortest path cost S-N->P is strictly less than the shortest path
   cost S-E->P.

   Another way to describe extended P-space is that it is the union of (
   un-extended ) P-space and the set of destinations for which S has a
   per-prefix LFA protecting the link S-E.  i.e.  the repair tunnel end
   point can be reached either directly or using a per-prefix LFA.

   Since in the case of Figure 1 node A is a per-prefix LFA for the
   destination node C, the set of extended P-space nodes comprises nodes
   A, B and C.  Since node C is also in E's Q-space, there is now a node
   common to both extended P-space and Q-space which can be used as a
   repair tunnel end-point to protect the link S-E.

4.2.3.  Selecting Repair Paths

   The mechanisms described above will identify all the possible repair
   tunnel end points that can be used to protect a particular link.  In
   a well-connected network there are likely to be multiple possible
   release points for each protected link.  All will deliver the packets
   correctly so, arguably, it does not matter which is chosen.  However,
   one repair tunnel end point may be preferred over the others on the
   basis of path cost or some other selection criteria.

   There is no technical requirement for the selection criteria to be
   consistent across all routers, but such consistency may be desirable
   from an operational point of view.  In general there are advantages
   in choosing the repair tunnel end point closest (shortest metric) to
   S.  Choosing the closest maximises the opportunity for the traffic to
   be load balanced once it has been released from the tunnel.  For

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   consistency in behavior is RECOMMENDED that member of the set of
   routers {P} with the lowest cost S->P be the default choice for P.
   In the event of a tie the router with the lowest node identifier
   SHOULD be selected.

5.  Example Application of Remote LFAs

   An example of a commonly deployed topology which is not fully
   protected by LFAs alone is shown in Figure 3.  PE1 and PE2 are
   connected in the same site.  P1 and P2 may be geographically
   separated (inter-site).  In order to guarantee the lowest latency
   path from/to all other remote PEs, normally the shortest path follows
   the geographical distance of the site locations.  Therefore, to
   ensure this, a lower IGP metric (5) is assigned between PE1 and PE2.
   A high metric (1000) is set on the P-PE links to prevent the PEs
   being used for transit traffic.  The PEs are not individually dual-
   homed in order to reduce costs.

   This is a common topology in SP networks.

   When a failure occurs on the link between PE1 and P2, PE1 does not
   have an LFA for traffic reachable via P1.  Similarly, by symmetry, if
   the link between PE2 and P1 fails, PE2 does not have an LFA for
   traffic reachable via P2.

   Increasing the metric between PE1 and PE2 to allow the LFA would
   impact the normal traffic performance by potentially increasing the

          |    100    |
           \         /
       1000 \       / 1000

                       Figure 3: Example SP topology

   Clearly, full protection can be provided, using the techniques
   described in this draft, by PE1 choosing P2 as a PQ node, and PE2
   choosing P1 as a PQ node.

6.  Node Failures

   When the failure is a node failure rather than a link failure there
   is a danger that the RLFA repair will loop.  This is discussed in
   detail in [I-D.bryant-ipfrr-tunnels].  In summary problem is that two
   of more of E's neighbors each with E as the next hop to some

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   destination D may attempt to repair a packet addressed to destination
   D via the other neighbor and then E, thus causing a loop to form.  As
   will be noted from [I-D.bryant-ipfrr-tunnels], this can rapidly
   become a complex problem to address.

   There are a number of ways to minimize the probability of a loop
   forming when a node failure occurs and there exists the possibility
   that two of E's neighbors may form a mutual repair.

   1.  Detect when a packet has arrived on some interface I that is also
       the interface used to reach the first hop on the RLFA path to PQ,
       and drop the packet.  This is useful in the case of a ring

   2.  Require that the path from PQ to destination D never passes
       through E (including in the ECMP case), i.e.  only use node
       protecting paths in which the cost PQ to D is strictly less than
       the cost PQ to E plus the cost E to D.

   3.  Require that where the packet may pass through another neighbor
       of E, that node is down stream (i.e.  strictly closer to D than
       the repairing node).  This means that some neighbor of E (X) can
       repair via some other neighbor of E (Y), but Y cannot repair via

   Case 1 accepts that loops may form and suppresses them by dropping
   packets.  Dropping packets may be considered less detrimental than
   looping packets.  Cases 2 and 3 above prevent the formation of a
   loop, but at the expense of a reduced repair coverage and at the cost
   of additional complexity in the algorithm to compute the repair path.

   The probability of a node failure and the consequences of node
   failure in any particular topology will depend on the node design,
   the particular topology in use, and node failure strategy (including
   the null strategy).  It is recommended that a network operator
   perform an analysis of the consequences and probability of node
   failure in their network, and determine whether the incidence and
   consequence of occurrence are acceptable.

7.  Operation in an LDP environment

   Where this technique is used in an MPLS network using LDP [RFC5036],
   S will need to push two labels onto the repair packet.  First it
   needs to push PQ's label to the destination, and then it needs to
   push its own label for PQ.  In the example Section 3.1 S already has
   the first hop (B) label for the PQ node (C) as a result of the
   ordinary operation of LDP.  To get the PQ node (C) label for the
   destination (D), S needs to establish a targeted LDP session with C.

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   The label stack for normal operation and RLFA operation is shown
   below in Figure 4.

   +-----------------+     +-----------------+     +-----------------+
   |    datalink     |     |    datalink     |     |    datalink     |
   +-----------------+     +-----------------+     +-----------------+
   | S's label for D |     | E's label for D |     | C's label for D |
   +-----------------+     +-----------------+     +-----------------+
   |    Payload      |     |    Payload      |     | B's label for C |
   +-----------------+     +-----------------+     +-----------------+
           X                       Y               |    Payload      |

   X = Normal label stack packet arriving at S
   Y = Normal label stack packet leaving S
   Z = RLFA label stack to D via C as PQ node

                                 Figure 4

   To establish an targeted LDP session with a candidate PQ node the
   repairing node (S) needs to know what IP address PQ is willing to use
   for targeted LDP sessions.  This in turn requires PQ to advertise
   this address in the IGP in use.  What address is used, how this is
   advertised in the IGP, and whether this is a special IP address or an
   IP address also used for some other purpose is out of scope for this
   document and must be specified in an IGP specific RFC.

8.  Historical Note

   The basic concepts behind Remote LFA were invented in 2002 and were
   later included in [I-D.bryant-ipfrr-tunnels], submitted in 2004.

   [I-D.bryant-ipfrr-tunnels], targeted a 100% protection coverage and
   hence included additional mechanisms on top of the Remote LFA
   concept.  The addition of these mechanisms made the proposal very
   complex and computationally intensive and it was therefore not
   pursued as a working group item.

   As explained in [I-D.filsfils-rtgwg-lfa-applicability], the purpose
   of the LFA FRR technology is not to provide coverage at any cost.  A
   solution for this already exists with MPLS TE FRR.  MPLS TE FRR is a
   mature technology which is able to provide protection in any topology
   thanks to the explicit routing capability of MPLS TE.

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   The purpose of LFA FRR technology is to provide for a simple FRR
   solution when such a solution is possible.  The first step along this
   simplicity approach was "local" LFA [RFC5286].  We propose "Remote
   LFA" as a natural second step.  The following section motivates its
   benefits in terms of simplicity, incremental deployment and
   significant coverage increase.

9.  Benefits

   Remote LFAs preserve the benefits of RFC5286: simplicity, incremental
   deployment and good protection coverage.

9.1.  Simplicity

   The remote LFA algorithm is simple to compute.

   o  The extended P space does not require any new computation (it is
      known once per-prefix LFA computation is completed).

   o  The Q-space is a single reverse SPF rooted at the neighbor.

   o  The directed LDP session is automatically computed and

   In edge topologies (square, ring), the directed LDP session position
   and number is deterministic and hence troubleshooting is simple.

   In core topologies, our simulation indicates that the 90th percentile
   number of LDP sessions per node to achieve the significant Remote LFA
   coverage observed in section 7.3 is <= 6.  This is insignificant
   compared to the number of LDP sessions commonly deployed per router
   which is frequently is in the several hundreds.

9.2.  Incremental Deployment

   The establishment of the directed LDP session to the PQ node does not
   require any new technology on the PQ node.  Indeed, routers commonly
   support the ability to accept a remote request to open a directed LDP
   session.  The new capability is restricted to the Remote-LFA
   computing node (the originator of the LDP session).

9.3.  Significant Coverage Extension

   The previous sections have already explained how Remote LFAs provide
   protection for frequently occurring edge topologies: square and
   rings.  In the core, we extend the analysis framework in section 4.3
   of [I-D.filsfils-rtgwg-lfa-applicability]and provide hereafter the
   Remote LFA coverage results for the 11 topologies:

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   | Topology | Per-link LFA | Per-prefix LFA | Remote LFA |
   |    T1    |      45%     |       77%      |    78%     |
   |    T2    |      49%     |       99%      |   100%     |
   |    T3    |      88%     |       99%      |    99%     |
   |    T4    |      68%     |       84%      |    92%     |
   |    T5    |      75%     |       94%      |    99%     |
   |    T6    |      87%     |       99%      |   100%     |
   |    T7    |      16%     |       67%      |    96%     |
   |    T8    |      87%     |      100%      |   100%     |
   |    T9    |      67%     |       80%      |    98%     |
   |    T10   |      98%     |      100%      |   100%     |
   |    T11   |      59%     |       77%      |    95%     |
   |  Average |      67%     |       89%      |    96%     |
   |  Median  |      68%     |       94%      |    99%     |

   Another study[ISOCORE2010] confirms the significant coverage increase
   provided by Remote LFAs.

10.  Complete Protection

   As shown in the previous table, Remote LFA provides for 96% average
   (99% median) protection in the 11 analyzed SP topologies.

   In an MPLS network, this is achieved without any scalability impact
   as the tunnels to the PQ nodes are always present as a property of an
   LDP-based deployment.

   In the very few cases where P and Q spaces have an empty
   intersection, one could select the closest node in the Q space and
   signal an explicitely-routed RSVP TE LSP to that Q node.  A directed
   LDP session is then established with the selected Q node and the rest
   of the solution is identical to that described elsewhere in this

   The drawbacks of this solution are:

   1.  only available for MPLS network;

   2.  the addition of LSPs in the SP infrastructure.

   This extension is described for exhaustivity.  In practice, the
   "Remote LFA" solution should be preferred for three reasons: its
   simplicity, its excellent coverage in the analyzed backbones and its

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   complete coverage in the most frequent access/aggregation topologies
   (box or ring).

11.  IANA Considerations

   There are no IANA considerations that arise from this architectural
   description of IPFRR.  The RFC Editor may remove this section on

12.  Security Considerations

   The security considerations of RFC 5286 also apply.

   To prevent their use as an attack vector the repair tunnel endpoints
   SHOULD be assigned from a set of addresses that are not reachable
   from outside the routing domain.

13.  Acknowledgments

   The authors acknowledge the technical contributions made to this work
   by Stefano Previdi.

14.  Informative References

              Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP
              Fast Reroute using tunnels", draft-bryant-ipfrr-tunnels-03
              (work in progress), November 2007.

              Filsfils, C., Francois, P., Shand, M., Decraene, B.,
              Uttaro, J., Leymann, N., and M. Horneffer, "LFA
              applicability in SP networks", draft-filsfils-rtgwg-lfa-
              applicability-00 (work in progress), March 2010.

              So, N., Lin, T., and C. Chen, "LFA (Loop Free Alternates)
              Case Studies in Verizon's LDP Network", 2010.

   [RFC1701]  Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
              Routing Encapsulation (GRE)", RFC 1701, October 1994.

   [RFC1853]  Simpson, W., "IP in IP Tunneling", RFC 1853, October 1995.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.

   [RFC5036]  Andersson, L., Minei, I., and B. Thomas, "LDP
              Specification", RFC 5036, October 2007.

   [RFC5286]  Atlas, A. and A. Zinin, "Basic Specification for IP Fast
              Reroute: Loop-Free Alternates", RFC 5286, September 2008.

   [RFC5714]  Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
              5714, January 2010.

Authors' Addresses

   Stewart Bryant
   Cisco Systems
   250, Longwater, Green Park,
   Reading  RG2 6GB, UK


   Clarence Filsfils
   Cisco Systems
   De Kleetlaan 6a
   1831 Diegem


   Stefano Previdi
   Cisco Systems


   Mike Shand
   Independent Contributor


Bryant, et al.         Expires November 24, 2013               [Page 14]

Internet-Draft               Remote LFA FRR                     May 2013

   Ning So
   Tata Communications
   Mobile Broadband Services


Bryant, et al.         Expires November 24, 2013               [Page 15]