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Versions: 00 01 02 03 draft-ietf-rtgwg-ipfrr-notvia-addresses

INTERNET DRAFT IP Fast Reroute Using Not-via Addresses     March 2006




Network Working Group                                         S. Bryant
Internet Draft                                                 M. Shand
Expiration Date: September 2006                              S. Previdi
                                                          Cisco Systems

                                                             March 2006



              IP Fast Reroute Using Not-via Addresses
         <draft-bryant-shand-ipfrr-notvia-addresses-02.txt>


Status of this Memo

   By submitting this Internet-Draft, each author represents that
   any applicable patent or other IPR claims of which he or she is
   aware have been or will be disclosed, and any of which he or she
   becomes aware will be disclosed, in accordance with Section 6 of
   BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   http://www.ietf.org/1id-abstracts.html

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Abstract
   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.




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Conventions used in this document

   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].

Table of Contents
1. Introduction........................................................3

2. Overview of Not-via Repairs.........................................3

3. Repair Computation..................................................5

4. How a repairing node repairs........................................6
 4.1 Node Failure.................................................. ...6
 4.2 Link Failure.................................................. ...6
 4.3 Multi-homed Prefix............................................. ..7
 4.4 Shared Risk Link Groups........................................ ..8
 4.5 Multicast...................................................... .12
 4.6 Fast Reroute in an MPLS LDP Network............................ .12
5. Local Area Networks................................................12
 5.1 Simple LAN Repair.............................................. .13
 5.2 LAN Component Repair........................................... .14
 5.3 LAN Repair Using Diagnostics................................... .15
6. Loop Free Alternates...............................................15
 6.1 Optimizing not-via computations using LFAs..................... .16
 6.2 Use of LFAs with SRLGs.......................................... 17
7. Equal Cost Multi-Path..............................................17

8. Multiple Simultaneous Failures.....................................17

9. Encapsulation......................................................17

10. Routing Extensions................................................17

11. Incremental Deployment............................................18

12. IANA considerations...............................................18

13. Security Considerations...........................................18



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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 (IPFRR), 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
   steer the repaired packet round an identified 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 in not partitioned by the
   failure, will always achieve a repair.

2.  Overview of Not-via Repairs

   The purpose of a repair is to deliver packets to their
   destination without traversing a known failure in the network,
   i.e. to deliver the packet not via the failure. A special address
   is assigned to each protected component. This address is called
   the not-via address. The semantics of a not-via address are that
   a packet addressed to a not-via address must be delivered to the
   router advertising that address, not via the protected component
   (link, node, SRLG etc.) with which that address is associated.

   A simple example would be node repair in which an additional
   address is assigned to each interface in the network. To repair a
   failure, the repairing router encapsulates the packet to the not-
   via address of the router interface on the far side of the
   failure. The routers on the repair path then know to which router
   they must deliver the packet, and which network component they
   must avoid. The network fragment shown in Figure 1 illustrates a
   not-via repair for the case of a router failure.











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              A
              |                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 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.

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

      Figure 2: The set of Not-via P Addresses


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3.  Repair 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 an 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 2, 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 on the modified topology. This
        incremental calculation is terminated when all of the not-
        via P addresses are attached to the SPT.

     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.

   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 6.1





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4.  How a repairing node repairs

   This section explains the operation of each type of repair
   necessary in the network.

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.

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 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 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.

   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. 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

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   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 loop.

4.3 Multi-homed Prefix

   A multi-homed Prefix (MHP) is reachable via more than one router
   in the network. When IPFRR router S (Figure 3) discovers that P
   has failed, it needs to send MHP packets addressed to X, which
   are normally reachable through P, to an alternate router, which
   is still able to reach X.

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

                Figure 3: Multi-home 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 continuing to run the incremental SPF with P
   failed until all of P's not-via addresses and its MHPs (X) are
   attached.

   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


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   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 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.

   Notice that using the not-via approach, only one level of
   encapsulation was needed to repair MHPs to the alternate router.

4.4 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 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

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   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

   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 de-capsulated 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.

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


           Figure 6: Shared Risk Link Group members in series


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   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.

   It is often the case that a single link may be a member of
   multiple SRLGs, and those SRLG 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 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 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


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   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. It practice it is likely that either SRLG "a" or
   SRLG "b" has failed, but these were indistinguishable from the
   point of view of S. However, each link repair is considered
   independently. So, when the packet arrives at G, if only SRLG "b"
   has failed it will be delivered across the link GD, while if only
   SRLG "a" has failed it will be repaired around the path G-F-H-D.
   This is illustrated in Figure 9 below.

                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


   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
   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,

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        aside from the complexity of performing the diagnostics,
        this requires multiple not-via addresses per interface,
        which has poor scaling properties.


4.5 Multicast

   Multicast traffic is repaired in a similar way to unicast,
   however 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.

   A more complete description of multicast operation will be
   provided in a future version of this draft.

4.6 Fast Reroute in an MPLS LDP Network.

   Not-via addresses are IP addresses and LDP 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 [NNHL]. Where
   an extended SRLG is being repaired, S must determine which
   routers its traffic would traverse on egress from the SRLG, and
   then establish directed LDP sessions with each of those routers.

5.  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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                        +--------------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 must run a connectivity check to each
   of its protected LAN adjacencies P, Q, and R, using, for example
   BFD [BFD].

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

     . its own interface to the LAN,

     . the LAN itself,

     . the LAN interface of P,

     . the node P.

5.1 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
                        +-------------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, 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 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

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   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 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


5.3 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.

6.  Loop Free Alternates

   The use of loop free alternates (LFA) as a repair mechanism has
   been studied [LFA]. Where an LFA exists, S may use this in place
   of the not-via repair mechanism for unicast packets (including
   MHP alternate routers). Multicast traffic requires the use of a
   repair encapsulation so that the packets are delivered to the
   router at the repair endpoint in order to correctly re-join the
   multicast tree and so that the necessary RPF check can be made.

   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

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   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.

   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 failure.

6.1  Optimizing not-via computations using LFAs

   The above observation permits an optimization to the not-via
   computations. 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 set.

   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
   not-via computation is performed on the latest available
   information.

   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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6.2  Use of LFAs with SRLGs

   Section 4.4 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 [LFA].

7.  Equal Cost Multi-Path

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

   A router computing a not-via repair path MAY subject the repair
   to ECMP.

8.  Multiple Simultaneous Failures

   The failure of a node or an SRLG can result in multiple
   correlated failures, which may be repaired using the mechanisms
   described in this design. This design will not correctly repair a
   set of unanticipated multiple failures. Such failures are out of
   scope of this design.

   It is important that the routers in the network are able to
   discriminate between these two classes of failure, and take
   appropriate action.

9.  Encapsulation

   Any IETF specified IP in IP encapsulation may be used to carry a
   not-via repair. IP in IP [IPIP], GRE [RFC1701] and L2TPv3
   [L2TPv3], 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 chose 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.

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.

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   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 [RFC1701], L2TPv3
   etc). However, the deployment of mixed IP encapsulation types
   within a network is deprecated.

11.  Incremental Deployment

   Incremental deployment is supported by excluding routers that are
   not calculating not-via routes from the base topology. In that
   way repairs may be steered around island 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 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]
   [L2TPv3]. 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 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

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   vulnerable to such an attack than a network that undertook a full
   convergence in response to a link up/down event.

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   described in this document or the extent to which any license
   under such rights might or might not be available; nor does it
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   rights in RFC documents can be found in BCP 78 and BCP 79.

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   The IETF invites any interested party to bring to its attention
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   proprietary rights that may cover technology that may be required
   to implement this standard.  Please address the information to
   the IETF at       ietf-ipr@ietf.org.


 Full copyright statement

   Copyright (C) The Internet Society (2006). This document is
   subject to the rights, licenses and restrictions contained in BCP
   78, and except as set forth therein, the authors retain all their
   rights.

   This document and the information contained herein are provided
   on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY
   THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY
   RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS
   FOR A PARTICULAR PURPOSE.

Normative References

   There are no normative references.




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Informative References

   Internet-drafts are works in progress available from
   <http://www.ietf.org/internet-drafts/>

   [BFD]         Katz, D., Ward, D., "Bidirectional Forwarding
                 Detection", < draft-ietf-bfd-base-04.txt>, July
                 2005, (work in progress).


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

   [IPFRR]       Shand, M., Bryant, S., "IP Fast-reroute
                 Framework",
                 <draft-ietf-rtgwg-ipfrr-framework-05.txt>,
                 October 2005, (work in progress).

   [l2TPV3]      J. Lau, Ed., et al., "Layer Two Tunneling
                 Protocol - Version 3 (L2TPv3)", RFC 3931, March
                 2005.

   [LFA]         A. Atlas, Ed, A. Zinin, Ed, "Basic
                 Specification for IP Fast-Reroute: Loop-free
                 Alternates", <draft-ietf-rtgwg-ipfrr-spec-base-
                 04.txt>, July 2005, (work in progress).

   [LDP]         Andersson, L., Doolan, P., Feldman, N.,
                 Fredette, A. and B. Thomas, "LDP
                 Specification", RFC 3036,
                 January 2001.

   [NNHL]        Shen, N., et al "Discovering LDP Next-Nexthop
                 Labels", <draft-shen-mpls-ldp-nnhop-label-
                 02.txt>, May 2005, (work in progress)

   [MPLS-TE]     Ping Pan, et al, "Fast Reroute Extensions to
                 RSVP-TE for LSP Tunnels", RFC 4090, May 2005.

   [TUNNEL]      Bryant, S., et al, "IP Fast Reroute using
                 tunnels", <draft-bryant-ipfrr-tunnels-02.txt>,
                 April 2005 (work in progress).








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Authors' Addresses

   Stewart Bryant
   Cisco Systems,
   250, Longwater Avenue,
   Green Park,
   Reading, RG2 6GB,
   United Kingdom.             Email: stbryant@cisco.com



   Stefano Previdi
   Cisco Systems
   Via Del Serafico, 200
   00142 Rome,
   Italy                      Email: sprevidi@cisco.com


   Mike Shand
   Cisco Systems,
   250, Longwater Avenue,
   Green Park,
   Reading, RG2 6GB,
   United Kingdom.             Email: mshand@cisco.com



























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