path: root/doc/rfc/rfc6981.txt

Internet Engineering Task Force (IETF)                         S. Bryant
Request for Comments: 6981                                    S. Previdi
Category: Informational                                    Cisco Systems
ISSN: 2070-1721                                                 M. Shand
                                                  Individual Contributor
                                                             August 2013


    A Framework for IP and MPLS Fast Reroute Using Not-Via Addresses

Abstract

   This document presents an illustrative framework for providing fast
   reroute in an IP or MPLS network through encapsulation and forwarding
   to "not-via" addresses.  The general approach described here uses a
   single level of encapsulation and could be used to protect unicast,
   multicast, and LDP traffic against link, router, and shared risk
   group failure, regardless of network topology and metrics.

   The mechanisms presented in this document are purely illustrative of
   the general approach and do not constitute a protocol specification.
   The document represents a snapshot of the work of the Routing Area
   Working Group at the time of publication and is published as a
   document of record.  Further work is needed before implementation or
   deployment.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6981.










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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) 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.





































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Table of Contents

   1. Introduction ....................................................4
      1.1. The Purpose of This Document ...............................4
      1.2. Overview ...................................................4
   2. Requirements Language ...........................................5
   3. Overview of Not-Via Repairs .....................................5
      3.1. Use of Equal-Cost Multi-Path ...............................6
      3.2. Use of LFA Repairs .........................................6
   4. Not-Via Repair Path Computation .................................7
      4.1. Computing Not-Via Repairs in Distance and Path
           Vector Routing Protocols ...................................8
   5. Operation of Repairs ............................................8
      5.1. Node Failure ...............................................8
      5.2. Link Failure ...............................................9
           5.2.1. Loop Prevention under Node Failure ..................9
      5.3. Multi-Homed Prefixes .......................................9
      5.4. Installation of Repair Paths ..............................11
   6. Compound Failures ..............................................12
      6.1. Shared Risk Link Groups ...................................12
      6.2. Local Area Networks .......................................17
           6.2.1. Simple LAN Repair ..................................18
           6.2.2. LAN Component Repair ...............................19
           6.2.3. LAN Repair Using Diagnostics .......................19
      6.3. Multiple Independent Failures .............................20
           6.3.1. Looping Repairs ....................................20
           6.3.2. Outline Solution ...................................21
           6.3.3. Mutually Looping Repairs ...........................22
                  6.3.3.1. Dropping Looping Packets ..................23
                  6.3.3.2. Computing Non-looping Repairs of Repairs ..23
           6.3.4. Mixing LFAs and Not-Via ............................25
   7. Optimizing Not-Via Computations Using LFAs .....................26
   8. Multicast ......................................................27
   9. Fast Reroute in an MPLS LDP Network ............................27
   10. Encapsulation .................................................28
   11. Routing Extensions ............................................28
   12. Incremental Deployment ........................................28
   13. Manageability Considerations ..................................29
      13.1. Pre-failure Configuration ................................29
      13.2. Pre-failure Monitoring and Operational Support ...........29
      13.3. Failure Action Monitoring ................................30
   14. Security Considerations .......................................30
   15. Acknowledgements ..............................................31
   16. References ....................................................31
      16.1. Normative References .....................................31
      16.2. Informative References ...................................31
   Appendix A. Q-Space ...............................................33




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1.  Introduction

1.1.  The Purpose of This Document

   This document presents an illustrative framework for providing fast
   reroute around a failure in an IP or MPLS network based on the
   concept of tunneling or encapsulating packets via an IP address that
   is known to avoid the failure.  The general approach described here
   uses a single level of encapsulation and could be used to protect
   unicast, multicast, and LDP traffic against link, router, and shared
   risk group failure, regardless of network topology and metrics.

   At the time of publication, there is no demand to deploy this
   technology; however, in view of the subtleties involved in the design
   of routing protocol extensions to provide IP Fast Reroute (IPFRR)
   [RFC5714], the Routing Area Working Group considered it desirable to
   publish this document to place on record the design considerations of
   the not-via address approach.

   The mechanisms presented in this document are purely illustrative of
   the general approach and do not constitute a protocol specification.
   The document represents a snapshot of the work of the working group
   at the time of publication and is published as a document of record.
   Additional work is needed to specify the necessary routing protocol
   extensions necessary to support this IPFRR method before
   implementation or deployment.

1.2.  Overview

   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 IPFRR [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 explicitly steer the
   repaired packet around 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 that, provided the network
   is not partitioned by the failure, will always achieve a repair.






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2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Overview of Not-Via Repairs

   This section provides a brief overview of the not-via method of
   IPFRR.  Consider the network fragment shown in Figure 1 below, in
   which 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.

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

                Figure 1: Not-Via Repair of Router Failure

   In the not-via IPFRR method, S encapsulates the packet to Bp, where
   Bp is an address on node B that has the property of not being
   reachable from node P, i.e., the notation Bp means "an address of
   node B that is only reachable not via node P".  We later show how to
   install the path from S to Bp such that it 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 and the path from S to Bp has been installed,
   then the packet will be successfully delivered to B.  In the example
   in Figure 1, this is the path S-X-Y-Z-B.  When the packet addressed
   to Bp arrives at B, B removes the encapsulation and forwards the
   repaired packet towards 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
   backtrack 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.





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   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.  Similarly, P will require a set of
   not-via addresses (one for each neighbor) allowing traffic to be
   directed to P without traversing each of those neighbors.

   The not-via addresses are advertised in the routing protocol in a way
   that clearly identifies them as not-via addresses and not 'ordinary'
   addresses.

                                       A
                                       |Ap
                                       |
                             Sp      Pa|Pb
                            S----------P----------B
                                     Ps|Pc      Bp
                                       |
                                     Cp|
                                       C

                 Figure 2: The Set of Not-Via P Addresses

3.1.  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 ECMP.

3.2.  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
   (LFAs) and/or downstream paths as documented in [RFC5286].  In
   particular, LFAs do not require the assignment and management of
   additional IP addresses to nodes, they do not require nodes in the
   network to be upgraded in order to calculate not-via repair paths,
   and they do not require the use of encapsulation.

   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 that are repairable by LFAs
   are so repaired, and the remainder of the destinations are repaired
   using the not-via 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



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   the failure, but it may be more optimal for some other traffic.  This
   document assumes that LFAs will be used where available, but the
   distribution of repairs between the two mechanisms is a local
   implementation choice.

4.  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 (SPF) algorithm, 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, routers A, B, C, X, Y, Z, and P will consider each
   router in turn, assume that the router has failed, and then calculate
   its own route to each of the not-via addresses advertised by the
   neighbors of that router.  In other words, in the case of a presumed
   failure of P, ALL routers (S, A, B, C, X, Y, and Z in this case)
   calculate their routes 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 reattaching 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 reattached to the Shortest Path Tree (SPT), and then
       terminate it.

   3.  Reverts to the base topology.




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   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.

   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.

4.1.  Computing Not-Via Repairs in Distance and Path Vector Routing
      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 that 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 needs to 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.

5.  Operation of Repairs

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

5.1.  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.  The next-next hop is the router that the next-hop
   router regards as its own next hop to the destination.  In Figure 1,
   B is S's next-next hop to D.





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   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.

5.2.  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
   occurred.

   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 send the encapsulated
   packet on the shortest available path to P.  This path is calculated
   by running an SPF with the link S-P removed.  Note that this may
   again be an incremental calculation, which can terminate when address
   Ps has been reattached.

5.2.1.  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 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 6.3.  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 A-P 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 a loop.

5.3.  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.





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   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 that is still able to reach X.

            X                          X                          X
            |                          |                          |
            |                          |                          |
            |                Sp        |Pb                        |
            Z...............S----------P----------B...............Y
                                     Ps|Pc      Bp
                                       |
                                     Cp|
                                       C

                      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 removed.  This is
   accomplished by the normal process of reattaching 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 removed 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.



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   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.

5.4.  Installation of Repair Paths

   The following algorithm is used by node S (Figure 3) to pre-calculate
   and install repair paths in the Forwarding Information Base (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 that 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) that can still be reached
       when node P fails.  These will be multi-homed prefixes that are
       not repairable by LFA, and for which the normal attachment node
       is P (or a router for which P is a single point of failure), and
       that 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:

           o  Y, where the next hop towards Y is P, or

           o  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).



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

       For each destination (DS) that remains, install a not-via repair
       to Ps (P not via S).  Note that these are destinations for which
       node P is a single point of failure, and they 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 of the loop.

   Note that where multiple next-next hops are available to reach DR,
   any or several of them may be chosen from a routing correctness point
   of view.  Unless other factors require consideration, the closest
   next-next hop to the repairing router would be the normal choice.

6.  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.

6.1.  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
   needs to be computed to avoid not only the adjacent link but also all
   the links that are members of the same SRLG.






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   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, these are 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
   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
   case).

                                  a   Ps
                             S----------P---------D
                             |          |
                             |    a     |
                             A----------B
                             |          |
                             |          |
                             C----------E

                     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
                            S----------P---------D
                            |          |         |
                            |    a     |         |
                            A----------B         |
                            |          |         |
                            |          |         |
                            C----------E---------F

           Figure 5: Shared Risk Link Group Members in Series -
              Decapsulation and Re-encapsulation by One Node







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   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
   occur.

   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 that
   is a member of the same SRLG.  In some cases, the decapsulation and
   re-encapsulation take 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.

                             a   Ps              a  Dg
                        S----------P---------G--------D
                        |          |         |        |
                        |    a     |         |        |
                        A----------B         |        |
                        |          |         |        |
                        |          |         |        |
                        C----------E---------F--------H

           Figure 6: Shared Risk Link Group Members in Series -
           Decapsulation and Re-encapsulation by Different Nodes

   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
   destination.


















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   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
                          S----------P---------G--------D
                          |          |         |        |
                          |    a     |         |        |
                          A----------B         |        |
                          |          |         |        |
                          |    b     |         |   b    |
                          C----------E---------F--------H
                          |          |
                          |          |
                          J----------K

                Figure 7: Multiple Shared Risk Link Groups

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

                              ab  Ps              a  Dg
                         S----/-----P---------G---/----D
                         |          |         |        |
                         |    a     |         |        |
                         A----/-----B         |        |
                         |          |         |        |
                         |    b     |         |   b    |
                         C----/-----E---------F---/----H
                         |          |
                         |          |
                         J----------K

        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 G-D is a member of SRLG "a"
   and F-H is a member of SRLG "b".  This is true if BOTH SRLGs "a" and
   "b" have in fact failed, which would be an instance of multiple
   independent failures.  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 needs to repair on the



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   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 G-D will still be available.  In this case, the
   packet will be delivered as normal across the link G-D.

                              ab  Ps              a  Dg
                         S----/-----P---------G---/----D
                         |          |         |        |
                         |    a     |         |        |
                         A----/-----B         |        |
                         |          |         |        |
                         |    b     |         |   b    |
                         C----------E---------F--------H
                         |          |
                         |          |
                         J----------K

        Figure 9: Topology Used for Repair Computation for Link G-D

   If both SRLG "a" and SRLG "b" had failed, the packet would be
   repaired as far as P by S and would be forwarded by P to G.  G would
   encapsulate the packet to D using the not-via address Dg and forward
   it to F.  F would recognize that its next hop to Dg (H) was
   unreachable due to the failure of link F-H (part of SRLG "b") and
   would drop the packet, because packets addressed to a not-via address
   are not repaired in basic not-via IPFRR.

   The repair of multiple independent failures is not provided by the
   basic not-via IPFRR method described so far in this memo.

   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
   strategy may occasionally fail to identify a repair even though a
   viable repair path exists in the network.  The use of suboptimal
   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.




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   3.  Using some form of SRLG diagnostic (for example, by running
       Bidirectional Forwarding Detection (BFD) [RFC5880] over alternate
       repair paths) to determine which SRLG member(s) have 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 properties.

   4.  Using the mechanism described in Section 6.3.

6.2.  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.

                                     +--------------Q------C
                                     |
                                     |
                                     |
                   A--------S-------(N)-------------P------B
                                     |
                                     |
                                     |
                                     +--------------R------D

                      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 needs to run a connectivity check to each
   of its protected LAN adjacencies P, Q, and R, using, for example, BFD
   [RFC5880].













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   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

6.2.1.  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.

                                                 Ql       Cl
                                     +-------------Q--------C
                                     |              Qc
                                     |
                    As       Sl      |           Pl       Bl
                   A--------S-------(N)------------P--------B
                          Sa         |              Pb
                                     |
                                     |           Rl       Dl
                                     +-------------R--------D
                                                    Rd

                 Figure 11: Local Area Networks - LAN SRLG

   In this case, if 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).

   While 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 suboptimal repair paths and, in some cases, the inability
   to identify a viable repair.









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6.2.2.  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 needs to repair traffic sent through P and B, to an
   address Bpn (B not-via P,N, i.e., B 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.  A similar process would also
   apply 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 (P).

   Notice also that each neighbor of a router connected to a LAN needs
   to 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,
   are advertising exactly one address more than they would otherwise
   have advertised if this degree of connectivity had been achieved
   using point-to-point links.

                                                Qs Qp Qc    Cqn
                                      +--------------Q---------C
                                      |         Qr Qn        Cq
                                      |
                     Asn   Sa Sp Sq   |         Ps Pq Pb    Bpn
                    A--------S-------(N)-------------P---------B
                     As       Sr Sn   |         Pr Pn        Bp
                                      |
                                      |         Rs Rp Pd    Drn
                                      +--------------R---------D
                                                Rq Rn        Dr

             Figure 12: Local Area Networks - Component Repair

6.2.3.  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



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   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.

6.3.  Multiple Independent Failures

   IPFRR repair of multiple simultaneous failures that 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 5.2.1, 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 that 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 where
   multiple repairs do not induce loops.

   This section describes the properties of multiple unrelated failures
   and proposes some methods that may be used to address this problem.

6.3.1.  Looping Repairs

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

                           A------//------B------------D
                          /                \
                         /                  \
                        F                    G
                         \                  /
                          \                /
                           X------//------Y

             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 other
   paths present in Figure 13, 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.




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   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 in 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.

                 A------//------B------------X------//------Y------D
                 |              |            |              |
                 |              |            |              |
                 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.

6.3.2.  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 that 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



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   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
   preventing the loop that 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
   contemplated.

   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 maximum
   transmission unit (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.

6.3.3.  Mutually 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 apply the "Abandoning All Hope" (AAH) mechanism ([RFC6976],
   Appendix A) and immediately invoke normal reconvergence.

   Note that it is not sufficient to expedite the issuance of a Link
   State Packet (LSP) reporting the failure, since this may be treated
   as a permitted simultaneous failure by the ordered FIB (oFIB)
   algorithm [RFC6976].  It is therefore necessary to explicitly trigger
   an oFIB AAH.





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6.3.3.1.  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 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 signaled oFIB this should be sub-second).

   While 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
   predetermination that has been applied to existing IPFRR solutions.

6.3.3.2.  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 pre-compute 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
   packets.

   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 removed.  Note that the assumed simultaneous failure
   and resulting repair path only apply to the repair path computed for
   the conflicting not-via addresses and are 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.  Node 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,




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   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 neighbors of a router).
   When A is computing its set of repair paths, it does so for all its k
   neighbors.  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 that
   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 X-Y 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,
   that 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 X-Y 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
   (Appendix A) 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 that are only
   bi-connected.  In these cases, repair is clearly impossible -- the
   failure of both links partitions the network.  It would be



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   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.

6.3.4.  Mixing LFAs and Not-Via

   So far in this section, we have assumed that all repairs use not-via
   tunnels.  However, in practice we may wish to use LFAs or downstream
   routes where available.  This complicates the issue, because their
   use results in packets that 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 that 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.  However, if the path information is
   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 that
   has failed and was not accounted for on the computed path.  If the
   A-B repair (Figure 13) 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 that 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



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   addressed to Yx can be sent to the same neighbor.  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 removed.
   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 that traverses A-B, we
   must perform the downstream calculation for that not-via address in
   the topology with link Q-P removed), then the downstream repair for
   Yx can safely be used.  These packets cannot revisit 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).

7.  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
   S-P 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 Link
   State Advertisement (LSA) associated with each link protected by an
   LFA.  Routers computing not-via routes can then omit the running of
   the iSPF for links with this bit set.

   When running the iSPF for a particular link A-B, the calculating
   router first checks whether the link A-B 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 A-B
   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.




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   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 S-P for which S-P is in the path to P, there is a significant
   reduction in the computation required.

8.  Multicast

   Multicast traffic can be repaired in a way similar 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 Reverse
   Path Forwarding (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: by
   definition, their normal path to the repair endpoint is not via the
   failure.

   A more complete description of multicast operation is left for
   further study.

9.  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 reroute in an MPLS network by first
   pushing the label that 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 a method similar to the
   next-next-hop label distribution mechanism proposed in [NNHL].








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10.  Encapsulation

   Any IETF-specified IP-in-IP encapsulation may be used to carry a
   not-via repair.  IP in IP [RFC2003], Generic Routing Encapsulation
   (GRE) [RFC1701], and the Layer 2 Tunneling Protocol (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 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.

   Care needs to be taken to ensure that the encapsulation used to
   provide a repair tunnel does not result in the packet exceeding the
   MTU of the links traversed by that repair.

11.  Routing Extensions

   IPFRR requires routing protocol 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 routers capable of supporting not-via routes
   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.

   If the optimization proposed in Section 7 is to be used, then the use
   of the LFA in place of the not-via repair MUST also be signaled in
   the routing protocol.

12.  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




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   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.

13.  Manageability Considerations

   [RFC5714] outlines the general set of manageability considerations
   that apply to the general case of IPFRR.  We slightly expand this and
   add details that are not-via specific.  There are three classes of
   manageability considerations:

   1.  Pre-failure configuration

   2.  Pre-failure monitoring and operational support

   3.  Failure action monitoring

13.1.  Pre-failure Configuration

   Pre-failure configuration for not-via includes:

   o  Enabling/disabling not-via IPFRR support.

   o  Enabling/disabling protection on a per-link or per-node basis.

   o  Expressing preferences regarding the links/nodes used for repair
      paths.

   o  Configuration of failure detection mechanisms.

   o  Setting a preference concerning the use of LFAs.

   o  Configuring a not-via address (per interface) or not-via address
      set (per node).

   o  Configuring any SRLG rules or preferences.

   Any standard configuration method may be used.  The selection of the
   method to be used is outside the scope of this document.

13.2.  Pre-failure Monitoring and Operational Support

   Pre-failure monitoring and operational support for not-via include:

   o  Notification of links/nodes/destinations that cannot be protected.

   o  Notification of pre-computed repair paths.




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   o  Notification of repair type to be used (LFA or not-via).

   o  Notification of not-via address assignment.

   o  Notification of path or address optimizations used.

   o  Testing repair paths.  Note that not-via addresses look identical
      to "ordinary" addresses as far as tools such as traceroute and
      ping are concerned, and thus it is anticipated that these will be
      used to verify the established repair path.

   Any standard IETF method may be used for the above.  The selection of
   the method to be used is outside the scope of this document.

13.3.  Failure Action Monitoring

   Failure action monitoring for not-via includes:

   o  Counts of failure detections, protection invocations, and packets
      forwarded over repair paths.

   o  Logging of the events, using a sufficiently accurate and precise
      timestamp.

   o  Validation that the packet loss was within specification, using a
      suitable loss verification tool.

   o  Capture of the in-flight repair packet flows, using a tool such as
      IP Flow Information Export (IPFIX) [RFC5101].

   Note that monitoring the repair in action requires the capture of the
   signatures of a short, possibly sub-second network transient; this
   technique is not a well-developed IETF technology.

14.  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 [RFC6169].  The primary method of protection SHOULD be
   through the use of a private address space for the not-via addresses
   [RFC1918] [RFC4193].  Repair endpoint addresses MUST NOT be
   advertised outside the routing domain over which not-via is deployed
   and MUST be filtered at the network entry points.  In addition, a
   mechanism might be developed that allows 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



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   addressing an excessive traffic load to the decapsulation 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.

15.  Acknowledgements

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

16.  References

16.1.  Normative References

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

16.2.  Informative References

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

   [NNHL]        Shen, N., Chen, E., and A. Tian, "Discovering LDP Next-
                 Nexthop Labels", Work in Progress, May 2005.

   [REMOTE-LFA]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and
                 N. So, "Remote LFA FRR", Work in Progress, May 2013.

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

   [RFC1918]     Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G.,
                 and E. Lear, "Address Allocation for Private
                 Internets", BCP 5, RFC 1918, February 1996.

   [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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   [RFC4193]     Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
                 Addresses", RFC 4193, October 2005.

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

   [RFC5101]     Claise, B., "Specification of the IP Flow Information
                 Export (IPFIX) Protocol for the Exchange of IP Traffic
                 Flow Information", RFC 5101, January 2008.

   [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.

   [RFC6169]     Krishnan, S., Thaler, D., and J. Hoagland, "Security
                 Concerns with IP Tunneling", RFC 6169, April 2011.

   [RFC6976]     Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
                 Francois, P., and O. Bonaventure, "Framework for Loop-
                 Free Convergence Using the Ordered Forwarding
                 Information Base (oFIB) Approach", RFC 6976, July 2013.
























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Appendix A.  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 (or node).  It is described fully in
   [REMOTE-LFA].

                                   S---Eq
                                  /     \
                                 A       Dq
                                  \     /
                                   B---Cq

         Figure 15: The Q Space of E with Respect to the Link S-E

   Consider a repair of link S-E (Figure 15).  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 that
   traverses the failed link excised (including those that 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 15, the Q-space comprises
   nodes E, D, and C only.


























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Authors' Addresses

   Stewart Bryant
   Cisco Systems
   10 New Square, Bedfont Lakes
   Feltham, Middlesex  TW18 8HA
   UK

   EMail: stbryant@cisco.com


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

   EMail: sprevidi@cisco.com


   Mike Shand
   Individual Contributor

   EMail: imc.shand@googlemail.com



























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