Network Working Group                                          S. Bryant
Internet-Draft                                                S. Previdi
Intended status: Informational                             Cisco Systems
Expires: June 23, 2012                                          M. Shand
                                                  Individual Contributor
                                                       December 21, 2011

                IP Fast Reroute Using Not-via Addresses


   This draft describes a mechanism that provides fast reroute in an IP
   network through encapsulation to "not-via" addresses.  A single level
   of encapsulation is used.  The mechanism protects unicast, multicast
   and LDP traffic against link, router and shared risk group failure,
   regardless of network topology and metrics.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on June 23, 2012.

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   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Overview of Not-via Repairs  . . . . . . . . . . . . . . . . .  3
     2.1.  Use of Equal Cost Multi-Path . . . . . . . . . . . . . . .  4
     2.2.  Use of LFA repairs . . . . . . . . . . . . . . . . . . . .  4
   3.  Not-via Repair Path Computation  . . . . . . . . . . . . . . .  5
     3.1.  Computing not-via repairs in routing vector protocols  . .  6
   4.  Operation of Repairs . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Node Failure . . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Link Failure . . . . . . . . . . . . . . . . . . . . . . .  7
       4.2.1.  Loop Prevention Under Node Failure . . . . . . . . . .  7
     4.3.  Multi-homed Prefixes . . . . . . . . . . . . . . . . . . .  7
     4.4.  Installation of Repair Paths . . . . . . . . . . . . . . .  9
   5.  Compound Failures  . . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  Shared Risk Link Groups  . . . . . . . . . . . . . . . . . 10
       5.1.1.  Use of LFAs with SRLGs . . . . . . . . . . . . . . . . 14
     5.2.  Local Area Networks  . . . . . . . . . . . . . . . . . . . 14
       5.2.1.  Simple LAN Repair  . . . . . . . . . . . . . . . . . . 15
       5.2.2.  LAN Component Repair . . . . . . . . . . . . . . . . . 16
       5.2.3.  LAN Repair Using Diagnostics . . . . . . . . . . . . . 17
     5.3.  Multiple Independent Failures  . . . . . . . . . . . . . . 17
       5.3.1.  Looping Repairs  . . . . . . . . . . . . . . . . . . . 18
       5.3.2.  Outline Solution . . . . . . . . . . . . . . . . . . . 19
       5.3.3.  Looping Repairs  . . . . . . . . . . . . . . . . . . . 20  Dropping Looping Packets . . . . . . . . . . . . . 20  Computing non-looping Repairs of Repairs . . . . . 21
       5.3.4.  Mixing LFAs and Not-via  . . . . . . . . . . . . . . . 23
   6.  Optimizing not-via computations using LFAs . . . . . . . . . . 24
   7.  Multicast  . . . . . . . . . . . . . . . . . . . . . . . . . . 25
   8.  Fast Reroute in an MPLS LDP Network. . . . . . . . . . . . . . 25
   9.  Encapsulation  . . . . . . . . . . . . . . . . . . . . . . . . 25
   10. Routing Extensions . . . . . . . . . . . . . . . . . . . . . . 26
   11. Incremental Deployment . . . . . . . . . . . . . . . . . . . . 26
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 26
   14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
   15. Informative References . . . . . . . . . . . . . . . . . . . . 27
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28

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1.  Introduction

   When a link or a router fails, only the neighbors of the failure are
   initially aware that the failure has occurred.  In a network
   operating IP fast reroute [RFC5714], the routers that are the
   neighbors of the failure repair the failure.  These repairing routers
   have to steer packets to their destinations despite the fact that
   most other routers in the network are unaware of the nature and
   location of the failure.

   A common limitation in most IPFRR mechanisms is an inability to
   indicate the identity of the failure and to explicitly steer the
   repaired packet round the failure.  The extent to which this
   limitation affects the repair coverage is topology dependent.  The
   mechanism proposed here is to encapsulate the packet to an address
   that explicitly identifies the network component that the repair must
   avoid.  This produces a repair mechanism, which, provided the network
   is not partitioned by the failure, will always achieve a repair.

2.  Overview of Not-via Repairs

   This section provides a brief overview of the not-via method of

                 |                Bp is the address to use to get
                 |                  a packet to B not-via P
      S----------P----------B. . . . . . . . . .D
       \         |        Bp^
        \        |          |
         \       |          |
          \      C          |
           \                |
              Repair to Bp

                Figure 1: Not-via repair of router failure

   Assume that S has a packet for some destination D that it would
   normally send via P and B, and that S suspects that P has failed.  S
   encapsulates the packet to Bp.  The path from S to Bp is the shortest
   path from S to B not going via P. If the network contains a path from
   S to B that does not transit router P, i.e. the network is not
   partitioned by the failure of P, then the packet will be successfully
   delivered to B. When the packet addressed to Bp arrives at B, B
   removes the encapsulation and forwards the repaired packet towards

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   its final destination.

   Note that if the path from B to the final destination includes one or
   more nodes that are included in the repair path, a packet may back
   track after the encapsulation is removed.  However, because the
   decapsulating router is always closer to the packet destination than
   the encapsulating router, the packet will not loop.

   For complete protection, all of P's neighbors will require a not-via
   address that allows traffic to be directed to them without traversing
   P. This is shown in Figure 2.

       Sp      Pa|Pb
               Ps|Pc      Bp

                 Figure 2: The set of Not-via P Addresses

2.1.  Use of Equal Cost Multi-Path

   A router can use an equal cost multi-path (ECMP) repair in place of a
   not-via repair.

   A router computing a not-via repair path may subject the repair to

2.2.  Use of LFA repairs

   The not-via approach provides complete repair coverage and therefore
   may be used as the sole repair mechanism.  There are, however,
   advantages in using not-via in combination with loop free alternates
   (LFA) and or downstream paths as documented in [RFC5286].

   LFAs are computed on a per destination basis and in general, only a
   subset of the destinations requiring repair will have a suitable LFA
   repair.  In this case, those destinations which are repairable by
   LFAs are so repaired and the remainder of the destinations are
   repaired using the not-via encapsulation.  This has the advantage of
   reducing the volume of traffic that requires encapsulation.  On the
   other hand, the path taken by an LFA repair may be less optimal than
   that of the equivalent not-via repair for traffic destined to nodes
   close to the far end of the failure, but may be more optimal for some

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   other traffic.  The description in this document assumes that LFAs
   will be used where available, but the distribution of repairs between
   the two mechanisms is a local implementation choice.

3.  Not-via Repair Path Computation

   The not-via repair mechanism requires that all routers on the path
   from S to B (Figure 1) have a route to Bp.  They can calculate this
   by failing node P, running a Shortest Path First Algorithm (SPF), and
   finding the shortest route to B.

   A router has no simple way of knowing whether it is on the shortest
   path for any particular repair.  It is therefore necessary for every
   router to calculate the path it would use in the event of any
   possible router failure.  Each router therefore "fails" every router
   in the network, one at a time, and calculates its own best route to
   each of the neighbors of that router.  In other words, with reference
   to Figure 1, some router X will consider each router in turn to be P,
   fail P, and then calculate its own route to each of the not-via P
   addresses advertised by the neighbors of P. i.e.  X calculates its
   route to Sp, Ap, Bp, and Cp, in each case, not via P.

   To calculate the repair paths a router has to calculate n-1 SPFs
   where n is the number of routers in the network.  This is expensive
   to compute.  However, the problem is amenable to a solution in which
   each router (X) proceeds as follows.  X first calculates the base
   topology with all routers functional and determines its normal path
   to all not-via addresses.  This can be performed as part of the
   normal SPF computation.  For each router P in the topology, X then
   performs the following actions:-

   1.  Removes router P from the topology.

   2.  Performs an incremental SPF (iSPF) [ISPF] on the modified
       topology.  The iSPF process involves detaching the sub-tree
       affected by the removal of router P, and then re-attaching the
       detached nodes.  However, it is not necessary to run the iSPF to
       completion.  It is sufficient to run the iSPF up to the point
       where all of the nodes advertising not-via P addresses have been
       re-attached to the SPT, and then terminate it.

   3.  Reverts to the base topology.

   This algorithm is significantly less expensive than a set of full
   SPFs.  Thus, although a router has to calculate the repair paths for
   n-1 failures, the computational effort is much less than n-1 SPFs.

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   Experiments on a selection of real world network topologies with
   between 40 and 400 nodes suggest that the worst-case computational
   complexity using the above optimizations is equivalent to performing
   between 5 and 13 full SPFs.  Further optimizations are described in
   section 6.

3.1.  Computing not-via repairs in routing vector protocols

   While this document focuses on link state routing protocols, it is
   equally possible to compute not-via repairs in distance vector (e.g.
   RIP) or path vector (e.g.  BGP) routing protocols.  This can be
   achieved with very little protocol modification by advertising the
   not-via address in the normal way, but ensuring that the information
   about a not-via address Ps is not propagated through the node S. In
   the case of link protection this simply means that the advertisement
   from P to S is suppressed, with the result that S and all other nodes
   compute a route to Ps which doesn't traverse S, as required.

   In the case of node protection, where P is the protected node, and N
   is some neighbor, the advertisement of Np must be suppressed not only
   across the link N->P, but also across any link to P. The simplest way
   of achieving this is for P itself to perform the suppression of any
   address of the form Xp.

4.  Operation of Repairs

   This section explains the basic operation of the not-via repair of
   node and link failure.

4.1.  Node Failure

   When router P fails (Figure 2) S encapsulates any packet that it
   would send to B via P to Bp, and then sends the encapsulated packet
   on the shortest path to Bp.  S follows the same procedure for routers
   A and C in Figure 2.  The packet is decapsulated at the repair target
   (A, B or C) and then forwarded normally to its destination.  The
   repair target can be determined as part of the normal SPF by
   recording the "next-next-hop" for each destination in addition to the
   normal next-hop.

   Notice that with this technique only one level of encapsulation is
   needed, and that it is possible to repair ANY failure regardless of
   link metrics and any asymmetry that may be present in the network.
   The only exception to this is where the failure was a single point of
   failure that partitioned the network, in which case ANY repair is
   clearly impossible.

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4.2.  Link Failure

   The normal mode of operation of the network would be to assume router
   failure.  However, where some destinations are only reachable through
   the failed router, it is desirable that an attempt be made to repair
   to those destinations by assuming that only a link failure has

   To perform a link repair, S encapsulates to Ps (i.e. it instructs the
   network to deliver the packet to P not-via S).  All of the neighbors
   of S will have calculated a path to Ps in case S itself had failed.
   S could therefore give the packet to any of its neighbors (except, of
   course, P).  However, S should preferably send the encapsulated
   packet on the shortest available path to P. This path is calculated
   by running an SPF with the link SP failed.  Note that this may again
   be an incremental calculation, which can terminate when address Ps
   has been reattached.

4.2.1.  Loop Prevention Under Node Failure

   It is necessary to consider the behavior of IPFRR solutions when a
   link repair is attempted in the presence of node failure.  In its
   simplest form the not-via IPFRR solution prevents the formation of
   loops forming as a result of mutual repair, by never providing a
   repair path for a not-via address.  The repair of packets with not-
   via addresses is considered in more detail in Section 5.3.  Referring
   to Figure 2, if A was the neighbor of P that was on the link repair
   path from S to P, and P itself had failed, the repaired packet from S
   would arrive at A encapsulated to Ps.  A would have detected that the
   AP link had failed and would normally attempt to repair the packet.
   However, no repair path is provided for any not-via address, and so A
   would be forced to drop the packet, thus preventing the formation of

4.3.  Multi-homed Prefixes

   A multi-homed Prefix (MHP) is a prefix that is reachable via more
   than one router in the network.  Some of these may be repairable
   using LFAs as described in [RFC5286].  Only those without such a
   repair need be considered here.

   When IPFRR router S (Figure 3) discovers that P has failed, it needs
   to send packets addressed to the MHP X, which is normally reachable
   through P, to an alternate router, which is still able to reach X.

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      X                          X                          X
      |                          |                          |
      |                          |                          |
      |                Sp        |Pb                        |
                               Ps|Pc      Bp

                      Figure 3: Multi-homed Prefixes

   S should choose the closest router that can reach X during the
   failure as the alternate router.  S determines which router to use as
   the alternate while running the SPF with P failed.  This is
   accomplished by the normal process of re-attaching a leaf node to the
   core topology (this is sometimes known as a "partial SPF").

   First, consider the case where the shortest alternate path to X is
   via Z. S can reach Z without using the failed router P. However, S
   cannot just send the packet towards Z, because the other routers in
   the network will not be aware of the failure of P, and may loop the
   packet back to S. S therefore encapsulates the packet to Z (using a
   normal address for Z).  When Z receives the encapsulated packet it
   removes the encapsulation and forwards the packet to X.

   Now consider the case where the shortest alternate path to X is via
   Y, which S reaches via P and B. To reach Y, S must first repair the
   packet to B using the normal not-via repair mechanism.  To do this S
   encapsulates the packet for X to Bp.  When B receives the packet it
   removes the encapsulation and discovers that the packet is intended
   for MHP X. The situation now reverts to the previous case, in which
   the shortest alternate path does not require traversal of the
   failure.  B therefore follows the algorithm above and encapsulates
   the packet to Y (using a normal address for Y).  Y removes the
   encapsulation and forwards the packet to X.

   It may be that the cost of reaching X using local delivery from the
   alternate router (i.e.  Z or Y) is greater than the cost of reaching
   X via P. Under those circumstances, the alternate router would
   normally forward to X via P, which would cause the IPFRR repair to
   loop.  To prevent the repair from looping the alternate router must
   locally deliver a packet received via a repair encapsulation.  This
   may be specified by using a special address with the above semantics.
   Note that only one such address is required per node.  Notice that
   using the not-via approach, only one level of encapsulation was
   needed to repair MHPs to the alternate router.

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4.4.  Installation of Repair Paths

   The following algorithm is used by node S (Figure 3) to pre-
   calculate and install repair paths in the FIB, ready for immediate
   use in the event of a failure.  It is assumed that the not-via repair
   paths have already been calculated as described above.

   For each neighbor P, consider all destinations which are reachable
   via P in the current topology:-

   1.  For all destinations with an ECMP or LFA repair (as described in
       [RFC5286]) install that repair.

   2.  For each destination (DR) that remains, identify in the current
       topology the next-next-hop (H) (i.e. the neighbor of P that P
       will use to send the packet to DR).  This can be determined
       during the normal SPF run by recording the additional
       information.  If S has a path to the not-via address Hp (H not
       via P), install a not-via repair to Hp for the destination DR.

   3.  Identify all remaining destinations (M) which can still be
       reached when node P fails.  These will be multi-homed prefixes
       that are not repairable by LFA, for which the normal attachment
       node is P, or a router for which P is a single point of failure,
       and have an alternative attachment point that is reachable after
       P has failed.  One way of determining these destinations would be
       to run an SPF rooted at S with node P removed, but an
       implementation may record alternative attachment points during
       the normal SPF run.  In either case, the next best point of
       attachment can also be determined for use in step (4) below.

   4.  For each multi-homed prefix (M) identified in step (3):-

       A.  Identify the new attachment node (as shown in Figure 3).
           This may be:-

           a.  Y, where the next hop towards Y is P, or

           b.  Z, where the next hop towards Z is not P.

           If the attachment node is Z, install the repair for M as a
           tunnel to Z' (where Z' is the address of Z that is used to
           force local forwarding).

       B.  For the subset of prefixes (M) that remain (having attachment
           point Y), install the repair path previously installed for
           destination Y.

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       For each destination (DS) that remains, install a not-via repair
       to Ps (P not via S).  Note, these are destinations for which node
       P is a single point of failure, and can only be repaired by
       assuming that the apparent failure of node P was simply a failure
       of the S-P link.  Note that, if available, a downstream path to P
       may be used for such a repair.  This cannot generate a persistent
       loop in the event of the failure of node P, but if one neighbor
       of P uses a not-via repair and another uses a downstream path, it
       is possible for a packet sent on the downstream path to be
       returned to the sending node inside a not-via encapsulation.
       Since packets destined to not-via addresses are not repaired, the
       packet will be dropped after executing a single turn loop.

5.  Compound Failures

   The following types of failures involve more than one component:

   1.  Shared Risk Link Groups

   2.  Local Area Networks

   3.  Multiple Independent Failures

   The considerations that apply in each of the above situations are
   described in the following sections.

5.1.  Shared Risk Link Groups

   A Shared Risk Link Group (SRLG) is a set of links whose failure can
   be caused by a single action such as a conduit cut or line card
   failure.  When repairing the failure of a link that is a member of an
   SRLG, it must be assumed that all the other links that are also
   members of the SRLG have also failed.  Consequently, any repair path
   must be computed to avoid not just the adjacent link, but also all
   the links which are members of the same SRLG.

   In Figure 4 below, the links S-P and A-B are both members of SRLG
   "a".  The semantics of the not-via address Ps changes from simply "P
   not-via the link S-P" to be "P not-via the link S-P or any other link
   with which S-P shares an SRLG" In Figure 4 this is the links that are
   members of SRLG "a".  I.e. links S-P and A-B.  Since the information
   about SRLG membership of all links is available in the Link State
   Database, all nodes computing routes to the not-via address Ps can
   infer these semantics, and perform the computation by failing all the
   links in the SRLG when running the iSPF.

   Note that it is not necessary for S to consider repairs to any other

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   nodes attached to members of the SRLG (such as B).  It is sufficient
   for S to repair to the other end of the adjacent link (P in this

                   a   Ps
              |          |
              |    a     |
              |          |
              |          |

                     Figure 4: Shared Risk Link Group

   In some cases, it may be that the links comprising the SRLG occur in
   series on the path from S to the destination D, as shown in Figure 5.
   In this case, multiple consecutive repairs may be necessary.  S will
   first repair to Ps, then P will repair to Dp.  In both cases, because
   the links concerned are members of SRLG "a" the paths are computed to
   avoid all members of SRLG "a".

                   a   Ps    a   Dp
              |          |         |
              |    a     |         |
              A----------B         |
              |          |         |
              |          |         |

            Figure 5: Shared Risk Link Group members in series

   While the use of multiple repairs in series introduces some
   additional overhead, these semantics avoid the potential
   combinatorial explosion of not-via addresses that could otherwise

   Note that although multiple repairs are used, only a single level of
   encapsulation is required.  This is because the first repair packet
   is decapsulated before the packet is re-encapsulated using the not-
   via address corresponding to the far side of the next link which is a
   member of the same SRLG.  In some cases the de-capsulation and re-
   encapsulation takes place (at least notionally) at a single node,
   while in other cases, these functions may be performed by different
   nodes.  This scenario is illustrated in Figure 6 below.

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                   a   Ps              a  Dg
              |          |         |        |
              |    a     |         |        |
              A----------B         |        |
              |          |         |        |
              |          |         |        |

            Figure 6: Shared Risk Link Group members in series

   In this case, S first encapsulates to Ps, and node P decapsulates the
   packet and forwards it "native" to G using its normal FIB entry for
   destination D. G then repairs the packet to Dg.

   It can be shown that such multiple repairs can never form a loop
   because each repair causes the packet to move closer to its

   It is often the case that a single link may be a member of multiple
   SRLGs, and those SRLGs may not be isomorphic.  This is illustrated in
   Figure 7 below.

                   ab  Ps              a  Dg
              |          |         |        |
              |    a     |         |        |
              A----------B         |        |
              |          |         |        |
              |    b     |         |   b    |
              |          |
              |          |

                Figure 7: Multiple Shared Risk Link Groups

   The link SP is a member of SRLGs "a" and "b".  When a failure of the
   link SP is detected, it must be assumed that BOTH SRLGs have failed.
   Therefore the not-via path to Ps must be computed by failing all
   links which are members of SRLG "a" or SRLG "b".  I.e. the semantics
   of Ps is now "P not-via any links which are members of any of the
   SRLGs of which link SP is a member".  This is illustrated in Figure 8

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                   ab  Ps              a  Dg
              |          |         |        |
              |    a     |         |        |
              A----/-----B         |        |
              |          |         |        |
              |    b     |         |   b    |
              |          |
              |          |

        Figure 8: Topology used for repair computation for link S-P

   In this case, the repair path to Ps will be S-A-C-J-K-E-B-P.  It may
   appear that there is no path to D because GD is a member of SRLG "a"
   and FH is a member of SRLG "b".  This is true if BOTH SRLGs "a" and
   "b" have in fact failed.  But that would be an instance of multiple
   uncorrelated failures which are out of scope for this design.  In
   practice it is likely that there is only a single failure, i.e.
   either SRLG "a" or SRLG "b" has failed, but not both.  These two
   possibilities are indistinguishable from the point of view of the
   repairing router S and so it must repair on the assumption that both
   are unavailable.  However, each link repair is considered
   independently.  The repair to Ps delivers the packet to P which then
   forwards the packet to G. When the packet arrives at G, if SRLG "a"
   has failed it will be repaired around the path G-F-H-D.  This is
   illustrated in Figure 9 below.  If, on the other hand, SRLG "b" has
   failed, link GD will still be available.  In this case the packet
   will be delivered as normal across the link GD.

                   ab  Ps              a  Dg
              |          |         |        |
              |    a     |         |        |
              A----/-----B         |        |
              |          |         |        |
              |    b     |         |   b    |
              |          |
              |          |

        Figure 9: Topology used for repair computation for link G-D

   A repair strategy that assumes the worst-case failure for each link
   can often result in longer repair paths than necessary.  In cases
   where only a single link fails, rather than the full SRLG, this

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   strategy may occasionally fail to identify a repair even though a
   viable repair path exists in the network.  The use of sub-optimal
   repair paths is an inevitable consequence of this compromise
   approach.  The failure to identify any repair is a serious
   deficiency, but is a rare occurrence in a robustly designed network.
   This problem can be addressed by:-

   1.  Reporting that the link in question is irreparable, so that the
       network designer can take appropriate action.

   2.  Modifying the design of the network to avoid this possibility.

   3.  Using some form of SRLG diagnostic (for example, by running BFD
       over alternate repair paths) to determine which SRLG member(s)
       has actually failed and using this information to select an
       appropriate pre-computed repair path.  However, aside from the
       complexity of performing the diagnostics, this requires multiple
       not-via addresses per interface, which has poor scaling

   4.  Using the mechanism described in Section 5.3

5.1.1.  Use of LFAs with SRLGs

   Section 4.1 above describes the repair of links which are members of
   one or more SRLGs.  LFAs can be used for the repair of such links
   provided that any other link with which S-P shares an SRLG is avoided
   when computing the LFA.  This is described for the simple case of
   "local-SRLGs" in [RFC5286].

5.2.  Local Area Networks

   LANs are a special type of SRLG and are solved using the SRLG
   mechanisms outlined above.  With all SRLGs there is a trade-off
   between the sophistication of the fault detection and the size of the
   SRLG.  Protecting against link failure of the LAN link(s) is
   relatively straightforward, but as with all fast reroute mechanisms,
   the problem becomes more complex when it is desired to protect
   against the possibility of failure of the nodes attached to the LAN
   as well as the LAN itself.

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                      Figure 10: Local Area Networks

   Consider the LAN shown in Figure 10.  For connectivity purposes, we
   consider that the LAN is represented by the pseudonode (N).  To
   provide IPFRR protection, S must run a connectivity check to each of
   its protected LAN adjacencies P, Q, and R, using, for example BFD

   When S discovers that it has lost connectivity to P, it is unsure
   whether the failure is:

   o  its own interface to the LAN,

   o  the LAN itself,

   o  the LAN interface of P,

   o  the node P.

5.2.1.  Simple LAN Repair

   A simple approach to LAN repair is to consider the LAN and all of its
   connected routers as a single SRLG.  Thus, the address P not via the
   LAN (Pl) would require P to be reached not-via any router connected
   to the LAN.  This is shown in Figure 11.

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                                       Ql       Cl
                           |              Qc
          As       Sl      |           Pl       Bl
                Sa         |              Pb
                           |           Rl       Dl

                 Figure 11: Local Area Networks - LAN SRLG

   In this case, when S detected that P had failed it would send traffic
   reached via P and B to B not-via the LAN or any router attached to
   the LAN (i.e. to Bl).  Any destination only reachable through P would
   be addressed to P not-via the LAN or any router attached to the LAN
   (except of course P).

   Whilst this approach is simple, it assumes that a large portion of
   the network adjacent to the failure has also failed.  This will
   result in the use of sub-optimal repair paths and in some cases the
   inability to identify a viable repair.

5.2.2.  LAN Component Repair

   In this approach, possible failures are considered at a finer
   granularity, but without the use of diagnostics to identify the
   specific component that has failed.  Because S is unable to diagnose
   the failure it must repair traffic sent through P and B, to B not-
   via P,N (i.e. not via P and not via N), on the conservative
   assumption that both the entire LAN and P have failed.  Destinations
   for which P is a single point of failure must as usual be sent to P
   using an address that avoids the interface by which P is reached from
   S, i.e. to P not-via N. Similarly for routers Q and R.

   Notice that each router that is connected to a LAN must, as usual,
   advertise one not-via address for each neighbor.  In addition, each
   router on the LAN must advertise an extra address not via the
   pseudonode (N).

   Notice also that each neighbor of a router connected to a LAN must
   advertise two not-via addresses, the usual one not via the neighbor
   and an additional one, not via either the neighbor or the pseudonode.
   The required set of LAN address assignments is shown in Figure 12
   below.  Each router on the LAN, and each of its neighbors, is
   advertising exactly one address more than it would otherwise have

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   advertised if this degree of connectivity had been achieved using
   point-to-point links.

                                     Qs Qp Qc    Cqn
                           |         Qr Qn        Cq
          Asn   Sa Sp Sq   |         Ps Pq Pb    Bpn
          As       Sr Sn   |         Pr Pn        Bp
                           |         Rs Rp Pd    Drn
                                     Rq Rn        Dr

                      Figure 12: Local Area Networks

5.2.3.  LAN Repair Using Diagnostics

   A more specific LAN repair can be undertaken by using diagnostics.
   In order to explicitly diagnose the failed network component, S
   correlates the connectivity reports from P and one or more of the
   other routers on the LAN, in this case, Q and R. If it lost
   connectivity to P alone, it could deduce that the LAN was still
   functioning and that the fault lay with either P, or the interface
   connecting P to the LAN.  It would then repair to B not via P (and P
   not-via N for destinations for which P is a single point of failure)
   in the usual way.  If S lost connectivity to more than one router on
   the LAN, it could conclude that the fault lay only with the LAN, and
   could repair to P, Q and R not-via N, again in the usual way.

5.3.  Multiple Independent Failures

   IPFRR repair of multiple simultaneous failures which are not members
   of a known SRLG is complicated by the problem that the use of
   multiple concurrent repairs may result in looping repair paths.  As
   described in Section 4.2.1, the simplest method of preventing such
   loops, is to ensure that packets addressed to a not-via address are
   not repaired but instead are dropped.  It is possible that a network
   may experience multiple simultaneous failures.  This may be due to
   simple statistical effects, but the more likely cause is
   unanticipated SRLGs.  When multiple failures which are not part of an
   anticipated group are detected, repairs are abandoned and the network
   reverts to normal convergence.  Although safe, this approach is
   somewhat draconian, since there are many circumstances were multiple
   repairs do not induce loops.

   This section describes the properties of multiple unrelated failures

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   and proposes some methods that may be used to address this problem.

5.3.1.  Looping Repairs

   Let us assume that the repair mechanism is based on solely on not-via
   repairs.  LFA or downstream routes may be incorporated, and will be
   dealt with later.

             /                \
            /                  \
           F                    G
            \                  /
             \                /

             Figure 13: The General Case of Multiple Failures

   The essential case is as illustrated in Figure 13.  Note that
   depending on the repair case under consideration, there may be paths
   present in Figure 13, that are in addition to those shown in the
   figure.  For example there may be paths between A and B, and/or
   between X and Y. These paths are omitted for graphical clarity.

   There are three cases to consider:

      1) Consider the general case of a pair of protected links A-B and
      X-Y as shown in the network fragment shown Figure 13.  If the
      repair path for A-B does not traverse X-Y and the repair path for
      X-Y does not traverse A-B, this case is completely safe and will
      not cause looping or packet loss.

      A more common variation of this case is shown in Figure 14, which
      shows two failures in different parts of the network in which a
      packet from A to D traverses two concatenated repairs.

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       |              |            |              |
       |              |            |              |
       M--------------+            N--------------+

                       Figure 14: Concatenated Repairs

      2) In Figure 13, the repair for A-B traverses X-Y, but the repair
      for X-Y does not traverse A-B.  This case occurs when the not-via
      path from A to B traverses link X-Y, but the not-via path from X
      to Y traverses some path not shown in Figure 13.  Without the
      multi-failure mechanism described in this section the repaired
      packet for A-B would be dropped when it reached X-Y, since the
      repair of repaired packets would be forbidden.  However, if this
      packet were allowed to be repaired, the path to D would be
      complete and no harm would be done, although two levels of
      encapsulation would be required.

      3) The repair for A-B traverses X-Y AND the repair for X-Y
      traverses A-B.  In this case unrestricted repair would result in
      looping packets and increasing levels of encapsulation.

   The challenge in applying IPFRR to a network that is undergoing
   multiple failures is, therefore, to identify which of these cases
   exist in the network and react accordingly.

5.3.2.  Outline Solution

   When A is computing the not-via repair path for A-B (i.e. the path
   for packets addressed to Ba, read as "B not-via A") it is aware of
   the list of nodes which this path traverses.  This can be recorded by
   a simple addition to the SPF process, and the not-via addresses
   associated with each forward link can be determined.  If the path
   were A, F, X, Y, G, B, (Figure 13) the list of not-via addresses
   would be: Fa, Xf, Yx, Gy, Bg.  Under standard not-via operation, A
   would populate its FIB such that all normal addresses normally
   reachable via A-B would be encapsulated to Ba when A-B fails, but
   traffic addressed to any not-via address arriving at A would be
   dropped.  The new procedure modifies this such that any traffic for a
   not-via address normally reachable over A-B is also encapsulated to
   Ba unless the not-via address is one of those previously identified
   as being on the path to Ba, for example Yx, in which case the packet
   is dropped.

   The above procedure allows cases 1 and 2 above to be repaired, while

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   preventing the loop which would result from case 3.

   Note that this is accomplished by pre-computing the required FIB
   entries, and does not require any detailed packet inspection.  The
   same result could be achieved by checking for multiple levels of
   encapsulation and dropping any attempt to triple encapsulate.
   However, this would require more detailed inspection of the packet,
   and causes difficulties when more than 2 "simultaneous" failures are

   So far we have permitted benign repairs to coexist, albeit sometimes
   requiring multiple encapsulation.  Note that in many cases there will
   be no performance impact since unless both failures are on the same
   node, the two encapsulations or two decapsulations will be performed
   at different nodes.  There is however the issue of the MTU impact of
   multiple encapsulations.

   In the following sub-section we consider the various strategies that
   may be applied to case 3 - mutual repairs that would loop.

5.3.3.  Looping Repairs

   In case 3, the simplest approach is to simply not install repairs for
   repair paths that might loop.  In this case, although the potentially
   looping traffic is dropped, the traffic is not repaired.  If we
   assume that a hold-down is applied before reconvergence in case the
   link failure was just a short glitch, and if a loop free convergence
   mechanism further delays convergence, then the traffic will be
   dropped for an extended period.  In these circumstances it would be
   better to "abandon all hope" (AAH) [I-D.ietf-rtgwg-ordered-fib]
   (Appendix A) and immediately invoke normal re-convergence.

   Note that it is not sufficient to expedite the issuance of an LSP
   reporting the failure, since this may be treated as a permitted
   simultaneous failure by the ordered FIB (oFIB) algorithm
   [I-D.ietf-rtgwg-ordered-fib].  It is therefore necessary to
   explicitly trigger an oFIB AAH.  Dropping Looping Packets

   One approach to case 3 is to allow the repair, and to experimentally
   discover the incompatibility of the repairs if and when they occur.
   With this method we permit the repair in case 3 and trigger AAH when
   a packet drop count on the not-via address has been incremented.
   Alternatively, it is possible to wait until the LSP describing the
   change is issued normally (i.e. when X announces the failure of X-Y).
   When the repairing node A, which has precomputed that X-Y failures
   are mutually incompatible with its own repairs receives this LSP it

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   can then issue the AAH.  This has the disadvantage that it does not
   overcome the hold-down delay, but it requires no "data-driven"
   operation, and it still has the required effect of abandoning the
   oFIB which is probably the longer of the delays (although with
   signalled oFIB this should be sub-second).

   Whilst both of the experimental approaches described above are
   feasible, they tend to induce AAH in the presence of otherwise
   feasible repairs, and they are contrary to the philosophy of repair
   pre-determination that has been applied to existing IPFRR solutions.  Computing non-looping Repairs of Repairs

   An alternative approach to simply dropping the looping packets, or to
   detecting the loop after it has occurred, is to use secondary SRLGs.
   With a link state routing protocol it is possible to precompute the
   incompatibility of the repairs in advance and to compute an
   alternative SRLG repair path.  Although this does considerably
   increase the computational complexity it may be possible to compute
   repair paths that avoid the need to simply drop the offending

   This approach requires us to identify the mutually incompatible
   failures, and advertise them as "secondary SRLGs".  When computing
   the repair paths for the affected not-via addresses these links are
   simultaneously failed.  Note that the assumed simultaneous failure
   and resulting repair path only applies to the repair path computed
   for the conflicting not-via addresses, and is not used for normal
   addresses.  This implies that although there will be a longer repair
   path when there is more than one failure, if there is a single
   failure the repair path length will be "normal".

   Ideally we would wish to only invoke secondary SRLG computation when
   we are sure that the repair paths are mutually incompatible.
   Consider the case of node A in Figure 13.  A first identifies that
   the repair path for A-B is via F-X-Y-G-B.  It then explores this path
   determining the repair path for each link in the path.  Thus, for
   example, it performs a check at X by running an SPF rooted at X with
   the X-Y link removed to determine whether A-B is indeed on X's repair
   path for packets addressed to Yx.

   Some optimizations are possible in this calculation, which appears at
   first sight to be order hk (where h is the average hop length of
   repair paths and k is the average number of neighbours of a router).
   When A is computing its set of repair paths, it does so for all its k
   neighbours.  In each case it identifies a list of node pairs
   traversed by each repair.  These lists may often have one or more
   node pairs in common, so the actual number of link failures which

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   require investigation is the union of these sets.  It is then
   necessary to run an SPF rooted at the first node of each pair (the
   first node because the pairings are ordered representing the
   direction of the path), with the link to the second node removed.
   This SPF, while not an incremental, can be terminated as soon as the
   not-via address is reached.  For example, when running the SPF rooted
   at X, with the link X-Y removed, the SPF can be terminated when Yx is
   reached.  Once the path has been found, the path is checked to
   determine if it traverses any of A's links in the direction away from
   A. Note that, because the node pair XY may exist in the list for more
   than one of A's links (i.e. it lies on more than one repair path), it
   is necessary to identify the correct list, and hence link which has a
   mutually looping repair path.  That link of A is then advertised by A
   as a secondary SRLG paired with the link X-Y.  Also note that X will
   be running this algorithm as well, and will identify that XY is
   paired with A-B and so advertise it.  This could perhaps be used as a
   further check.

   The ordering of the pairs in the lists is important. i.e.  X-Y and
   Y-X are dealt with separately.  If and only if the repairs are
   mutually incompatible, we need to advertise the pair of links as a
   secondary SRLG, and then ALL nodes compute repair paths around both
   failures using an additional not-via address with the semantics not-
   via A-B AND not-via X-Y.

   A further possibility is that because we are going to the trouble of
   advertising these SRLG sets, we could also advertise the new repair
   path and only get the nodes on that path to perform the necessary
   computation.  Note also that once we have reached Q space with
   respect to the two failures we need no longer continue the
   computation, so we only need to notify the nodes on the path that are
   not in Q-space.

   One cause of mutually looping repair paths is the existence of nodes
   with only two links, or sections of the network which are only bi-
   connected.  In these cases, repair is clearly impossible - the
   failure of both links partitions the network.  It would be
   advantageous to be able to identify these cases, and inhibit the
   fruitless advertisement of the secondary SRLG information.  This
   could be achieved by the node detecting the requirement for a
   secondary SRLG, first running the not-via computation with both links
   removed.  If this does not result in a path, it is clear that the
   network would be partitioned by such a failure, and so no
   advertisement is required.

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5.3.4.  Mixing LFAs and Not-via

   So far in this section we have assumed that all repairs use not-via
   tunnels.  However, in practise we may wish to use LFAs or downstream
   routes where available.  This complicates the issue, because their
   use results in packets which are being repaired, but NOT addressed to
   not-via addresses.  If BOTH links are using downstream routes there
   is no possibility of looping, since it is impossible to have a pair
   of nodes which are both downstream of each other [RFC5286].

   Loops can however occur when LFAs are used.  An obvious example is
   the well known node repair problem with LFAs [RFC5286].  If one link
   is using a downstream route, while the other is using a not-via
   tunnel, the potential mechanism described above would work provided
   it were possible to determine the nodes on the path of the downstream
   route.  Some methods of computing downstream routes do not provide
   this path information.  If the path information is however available,
   the link using a downstream route will have a discard FIB entry for
   the not-via address of the other link.  The consequence is that
   potentially looping packets will be discarded when they attempt to
   cross this link.

   In the case where the mutual repairs are both using not-via repairs,
   the loop will be broken when the packet arrives at the second
   failure.  However packets are unconditionally repaired by means of a
   downstream routes, and thus when the mutual pair consists of a
   downstream route and a not-via repair, the looping packet will only
   be dropped when it gets back to the first failure. i.e. it will
   execute a single turn of the loop before being dropped.

   There is a further complication with downstream routes, since
   although the path may be computed to the far side of the failure, the
   packet may "peel off" to its destination before reaching the far side
   of the failure.  In this case it may traverse some other link which
   has failed and was not accounted for on the computed path.  If the
   A-B repair (Figure 1) is a downstream route and the X-Y repair is a
   not-via repair, we can have the situation where the X-Y repair
   packets encapsulated to Yx follow a path which attempts to traverse
   A-B.  If the A-B repair path for "normal" addresses is a downstream
   route, it cannot be assumed that the repair path for packets
   addressed to Yx can be sent to the same neighbour.  This is because
   the validity of a downstream route must be ascertained in the
   topology represented by Yx, i.e. that with the link X-Y failed.  This
   is not the same topology that was used for the normal downstream
   calculation, and use of the normal downstream route for the
   encapsulated packets may result in an undetected loop.  If it is
   computationally feasible to check the downstream route in this
   topology (i.e. for any not-via address Qp which traverses A-B we must

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   perform the downstream calculation for that not-via address in the
   topology with link Q-P failed.), then the downstream repair for Yx
   can safely be used.  These packets cannot re-visit X-Y, since by
   definition they will avoid that link.  Alternatively, the packet
   could be always repaired in a not-via tunnel. i.e. even though the
   normal repair for traffic traversing A-B would be to use a downstream
   route, we could insist that such traffic addressed to a not-via
   address must use a tunnel to Ba.  Such a tunnel would only be
   installed for an address Qp if it were established that it did not
   traverse Q-P (using the rules described above).

6.  Optimizing not-via computations using LFAs

   If repairing node S has an LFA to the repair endpoint it is not
   necessary for any router to perform the incremental SPF with the link
   SP removed in order to compute the route to the not-via address Ps.
   This is because the correct routes will already have been computed as
   a result of the SPF on the base topology.  Node S can signal this
   condition to all other routers by including a bit in its LSP or LSA
   associated with each LFA protected link.  Routers computing not-via
   routes can then omit the running of the iSPF for links with this bit

   When running the iSPF for a particular link AB, the calculating
   router first checks whether the link AB is present in the existing
   SPT.  If the link is not present in the SPT, no further work is
   required.  This check is a normal part of the iSPF computation.

   If the link is present in the SPT, this optimization introduces a
   further check to determine whether the link is marked as protected by
   an LFA in the direction in which the link appears in the SPT.  If so
   the iSPF need not be performed.  For example, if the link appears in
   the SPT in the direction A->B and A has indicated that the link AB is
   protected by an LFA no further action is required for this link.

   If the receipt of this information is delayed, the correct operation
   of the protocol is not compromised provided that the necessity to
   perform a not-via computation is re-evaluated whenever new
   information arrives.

   This optimization is not particularly beneficial to nodes close to
   the repair since, as has been observed above, the computation for
   nodes on the LFA path is trivial.  However, for nodes upstream of the
   link SP for which S-P is in the path to P, there is a significant
   reduction in the computation required.

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7.  Multicast

   Multicast traffic can be repaired in a similar way to unicast.  The
   multicast forwarder is able to use the not-via address to which the
   multicast packet was addressed as an indication of the expected
   receive interface and hence to correctly run the required RPF check.

   In some cases, all the destinations, including the repair endpoint,
   are repairable by an LFA.  In this case, all unicast traffic may be
   repaired without encapsulation.  Multicast traffic still requires
   encapsulation, but for the nodes on the LFA repair path the
   computation of the not-via forwarding entry is unnecessary since, by
   definition, their normal path to the repair endpoint is not via the

   A more complete description of multicast operation is for further

8.  Fast Reroute in an MPLS LDP Network.

   Not-via addresses are IP addresses and LDP [RFC5036] will distribute
   labels for them in the usual way.  The not-via repair mechanism may
   therefore be used to provide fast re-route in an MPLS network by
   first pushing the label which the repair endpoint uses to forward the
   packet, and then pushing the label corresponding to the not-via
   address needed to effect the repair.  Referring once again to
   Figure 1, if S has a packet destined for D that it must reach via P
   and B, S first pushes B's label for D. S then pushes the label that
   its next hop to Bp needs to reach Bp.

   Note that in an MPLS LDP network it is necessary for S to have the
   repair endpoint's label for the destination.  When S is effecting a
   link repair it already has this.  In the case of a node repair, S
   either needs to set up a directed LDP session with each of its
   neighbor's neighbors, or it needs to use the next-next hop label
   distribution mechanism proposed in [I-D.shen-mpls-ldp-nnhop-label].

9.  Encapsulation

   Any IETF specified IP in IP encapsulation may be used to carry a not-
   via repair.  IP in IP [RFC2003], GRE [RFC1701] and L2TPv3 [RFC3931],
   all have the necessary and sufficient properties.  The requirement is
   that both the encapsulating router and the router to which the
   encapsulated packet is addressed have a common ability to process the
   chosen encapsulation type.  When an MPLS LDP network is being
   protected, the encapsulation would normally be an additional MPLS

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   label.  In an MPLS enabled IP network an MPLS label may be used in
   place of an IP in IP encapsulation in the case above.

10.  Routing Extensions

   IPFRR requires IGP extensions.  Each IPFRR router that is directly
   connected to a protected network component must advertise a not-via
   address for that component.  This must be advertised in such a way
   that the association between the protected component (link, router or
   SRLG) and the not-via address can be determined by the other routers
   in the network.

   It is necessary that not-via capable routers advertise in the IGP
   that they will calculate not-via routes.

   It is necessary for routers to advertise the type of encapsulation
   that they support (MPLS, GRE, L2TPv3 etc).  However, the deployment
   of mixed IP encapsulation types within a network is discouraged.

11.  Incremental Deployment

   Incremental deployment is supported by excluding routers that are not
   calculating not-via routes (as indicated by their capability
   information flooded with their link state information) from the base
   topology used for the computation of repair paths.  In that way
   repairs may be steered around islands of routers that are not IPFRR
   capable.  Routers that are protecting a network component need to
   have the capability to encapsulate and decapsulate packets.  However,
   routers that are on the repair path only need to be capable of
   calculating not-via paths and including the not-via addresses in
   their FIB i.e. these routers do not need any changes to their
   forwarding mechanism.

12.  IANA Considerations

   There are no IANA considerations that arise from this draft.

13.  Security Considerations

   The repair endpoints present vulnerability in that they might be used
   as a method of disguising the delivery of a packet to a point in the
   network.  The primary method of protection should be through the use
   of a private address space for the not-via addresses.  These
   addresses must not be advertised outside the area, and should be

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   filtered at the network entry points.  In addition, a mechanism might
   be developed that allowed the use of the mild security available
   through the use of a key [RFC1701] [RFC3931].  With the deployment of
   such mechanisms, the repair endpoints would not increase the security
   risk beyond that of existing IP tunnel mechanisms.  An attacker may
   attempt to overload a router by addressing an excessive traffic load
   to the de-capsulation endpoint.  Typically, routers take a 50%
   performance penalty in decapsulating a packet.  The attacker could
   not be certain that the router would be impacted, and the extremely
   high volume of traffic needed, would easily be detected as an
   anomaly.  If an attacker were able to influence the availability of a
   link, they could cause the network to invoke the not-via repair
   mechanism.  A network protected by not-via IPFRR is less vulnerable
   to such an attack than a network that undertook a full convergence in
   response to a link up/down event.

14.  Acknowledgements

   The authors would like to acknowledge contributions made by Alia
   Atlas and John Harper.

15.  Informative References

              Shand, M., Bryant, S., Previdi, S., and C. Filsfils,
              "Loop-free convergence using oFIB",
              draft-ietf-rtgwg-ordered-fib-05 (work in progress),
              April 2011.

              Shen, N., "Discovering LDP Next-Nexthop Labels",
              draft-shen-mpls-ldp-nnhop-label-02 (work in progress),
              May 2005.

   [ISPF]     McQuillan, J., Richer, I., and E. Rosen, "ARPANET Routing
              Algorithm Improvements"", BBN Technical Report 3803, 1978.

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

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.

   [RFC3931]  Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
              Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.

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   [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.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

Authors' Addresses

   Stewart Bryant
   Cisco Systems
   250, Longwater Avenue.
   Reading, Berks  RG2 6GB


   Stefano Previdi
   Cisco Systems
   Via Del Serafico, 200
   00142 Rome,


   Mike Shand
   Individual Contributor


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