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Versions: (draft-litkowski-rtgwg-lfa-manageability) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 7916

Routing Area Working Group                             S. Litkowski, Ed.
Internet-Draft                                               B. Decraene
Intended status: Standards Track                                  Orange
Expires: December 27, 2015                                   C. Filsfils
                                                                 K. Raza
                                                           Cisco Systems
                                                            M. Horneffer
                                                        Deutsche Telekom
                                                               P. Sarkar
                                                        Juniper Networks
                                                           June 25, 2015


             Operational management of Loop Free Alternates
                 draft-ietf-rtgwg-lfa-manageability-11

Abstract

   Loop Free Alternates (LFA), as defined in RFC 5286 is an IP Fast
   ReRoute (IP FRR) mechanism enabling traffic protection for IP traffic
   (and MPLS LDP traffic by extension).  Following first deployment
   experiences, this document provides operational feedback on LFA,
   highlights some limitations, and proposes a set of refinements to
   address those limitations.  It also proposes required management
   specifications.

   This proposal is also applicable to remote LFA solution.

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 [RFC2119].

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."



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   This Internet-Draft will expire on December 27, 2015.

Copyright Notice

   Copyright (c) 2015 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Operational issues with default LFA tie breakers  . . . . . .   4
     3.1.  Case 1: PE router protecting failures within core network   4
     3.2.  Case 2: PE router choosen to protect core failures while
           P router LFA exists . . . . . . . . . . . . . . . . . . .   5
     3.3.  Case 3: suboptimal P router alternate choice  . . . . . .   6
     3.4.  Case 4: No-transit LFA computing node . . . . . . . . . .   7
   4.  Need for coverage monitoring  . . . . . . . . . . . . . . . .   8
   5.  Need for LFA activation granularity . . . . . . . . . . . . .   9
   6.  Configuration requirements  . . . . . . . . . . . . . . . . .   9
     6.1.  LFA enabling/disabling scope  . . . . . . . . . . . . . .  10
     6.2.  Policy based LFA selection  . . . . . . . . . . . . . . .  10
       6.2.1.  Connected vs remote alternates  . . . . . . . . . . .  11
       6.2.2.  Mandatory criteria  . . . . . . . . . . . . . . . . .  12
       6.2.3.  Additional criteria . . . . . . . . . . . . . . . . .  12
       6.2.4.  Criteria evaluation . . . . . . . . . . . . . . . . .  12
       6.2.5.  Retrieving alternate path attributes  . . . . . . . .  16
       6.2.6.  ECMP LFAs . . . . . . . . . . . . . . . . . . . . . .  22
   7.  Operational aspects . . . . . . . . . . . . . . . . . . . . .  23
     7.1.  No-transit condition on LFA computing node  . . . . . . .  23
     7.2.  Manual triggering of FRR  . . . . . . . . . . . . . . . .  24
     7.3.  Required local information  . . . . . . . . . . . . . . .  25
     7.4.  Coverage monitoring . . . . . . . . . . . . . . . . . . .  25
     7.5.  LFA and network planning  . . . . . . . . . . . . . . . .  26
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  27
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  27



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     11.1.  Normative References . . . . . . . . . . . . . . . . . .  27
     11.2.  Informative References . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

1.  Introduction

   Following the first deployments of Loop Free Alternates (LFA), this
   document provides feedback to the community about the management of
   LFA.

      Section 3 provides real uses cases illustrating some limitations
      and suboptimal behavior.

      Section 4 provides requirements for LFA simulations.

      Section 5 proposes requirements for activation granularity and
      policy based selection of the alternate.

      Section 6 express requirements for the operational management of
      LFA and especially a policy framework to manage alternates.

      Section 7 details some operational considerations of LFA like IS-
      IS overload bit management or troubleshooting informations.

2.  Definitions

   o  Per-prefix LFA : LFA computation, and best alternate evaluation is
      done for each destination prefix, as opposed to "Per-next hop"
      simplification also proposed in [RFC5286] Section 3.8.

   o  PE router : Provider Edge router.  These routers are connecting
      customers

   o  P router : Provider router.  These routers are core routers,
      without customer connections.  They provide transit between PE
      routers and they form the core network.

   o  Core network : subset of the network composed by P routers and
      links between them.

   o  Core link : network link part of the core network i.e. a P router
      to P router link.

   o  Link-protecting LFA : alternate providing protection against link
      failure.

   o  Node-protecting LFA : alternate providing protection against node
      failure.



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   o  Connected alternate : alternate adjacent (at IGP level) to the
      point of local repair (i.e. an IGP neighbor).

   o  Remote alternate : alternate which is does not share an IGP
      adjacency with the point of local repair.

3.  Operational issues with default LFA tie breakers

   [RFC5286] introduces the notion of tie breakers when selecting the
   LFA among multiple candidate alternate next-hops.  When multiple LFA
   exist, RFC 5286 has favored the selection of the LFA providing the
   best coverage of the failure cases.  While this is indeed a goal,
   this is one among multiple and in some deployment this lead to the
   selection of a suboptimal LFA.  The following sections details real
   use cases of such limitations.

   Note that the use case of LFA computation per destination (per-prefix
   LFA) is assumed throughout this analysis.  We also assume in the
   network figures that all IP prefixes are advertised with zero cost.

3.1.  Case 1: PE router protecting failures within core network

       P1 --------- P2 ---------- P3 --------- P4
       |      1           100           1       |
       |                                        |
       | 100                                    | 100
       |                                        |
       |      1           100           1       |  1     5k
       P5 --------- P6 ---------- P7 --------- P8 --- P9 -- PE1
       | |         | |          |             |
     5k| |5k     5k| |5k        | 5k          | 5k
       | |         | |          |             |
       | +-- PE4 --+ |          +---- PE2 ----+
       |             |                 |
       +---- PE5 ----+                 | 5k
                                       |
                                      PE3

                                                   Figure 1

   Px routers are P routers using n*10G links.  PEs are connected using
   links with lower bandwidth.

   In figure 1, let us consider the traffic flowing from PE1 to PE4.
   The nominal path is P9-P8-P7-P6-PE4.  Let us consider the failure of
   link P7-P8.  As P4 primary path to PE4 is P8-P7-P6-PE4, P4 is not an
   LFA for P8 (because P4 will loop back traffic to P8) and the only
   available LFA is PE2.



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   When the core link P8-P7 fails, P8 switches all traffic destined to
   PE4/PE5 towards the node PE2.  Hence a PE node and PE links are used
   to protect the failure of a core link.  Typically, PE links have less
   capacity than core links and congestion may occur on PE2 links.  Note
   that although PE2 was not directly affected by the failure, its links
   become congested and its traffic will suffer from the congestion.

   In summary, in case of P8-P7 link failure, the impact on customer
   traffic is:

   o  From PE2 point of view :

      *  without LFA: no impact

      *  with LFA: traffic is partially dropped (but possibly
         prioritized by a QoS mechanism).  It must be highlighted that
         in such situation, traffic not affected by the failure may be
         affected by the congestion.

   o  From P8 point of view:

      *  without LFA: traffic is totally dropped until convergence
         occurs.

      *  with LFA: traffic is partially dropped (but possibly
         prioritized by a QoS mechanism).

   Besides the congestion aspects of using an Edge router as an
   alternate to protect a core failure, a service provider may consider
   this as a bad routing design and would like to prevent it.

3.2.  Case 2: PE router choosen to protect core failures while P router
      LFA exists


















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       P1 --------- P2 ------------ P3 -------- P4
       |      1           100       |     1     |
       |                            |           |
       | 100                        | 30        | 30
       |                            |           |
       |     1         50       50  |    10     |   1    5k
       P5 --------- P6 --- P10 ---- P7 -------- P8 --- P9 -- PE1
       | |         | |        \                |
     5k| |5k     5k| |5k       \ 5k            | 5k
       | |         | |          \              |
       | +-- PE4 --+ |           +---- PE2 ----+
       |             |                 |
       +---- PE5 ----+                 | 5k
                                       |
                                      PE3

                             Figure 2

   Px routers are P routers meshed with n*10G links.  PEs are meshed
   using links with lower bandwidth.

   In the figure 2, let us consider the traffic coming from PE1 to PE4.
   Nominal path is P9-P8-P7-P10-P6-PE4.  Let us consider the failure of
   the link P7-P8.  For P8, P4 is a link-protecting LFA and PE2 is a
   node-protecting LFA.  PE2 is chosen as best LFA due to its better
   protection type.  Just like in case 1, this may lead to congestion on
   PE2 links upon LFA activation.

3.3.  Case 3: suboptimal P router alternate choice






















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               +--- PE3 --+
              /            \
        1000 /              \ 1000
            /                \
    +----- P1 ---------------- P2 ----+
    |      |       500         |      |
    | 10   |                   |      | 10
    |      |                   |      |
    R5     | 10                | 10   R7
    |      |                   |      |
    | 10   |                   |      | 10
    |      |       500         |      |
    +---- P3 ---------------- P4 -----+
            \                 /
        1000 \               / 1000
              \             /
               +--- PE1 ---+

               Figure 3

   Px routers are P routers.  P1-P2 and P3-P4 links are 1G links.  All
   others inter Px links are 10G links.

   In the figure above, let us consider the failure of link P1-P3.  For
   destination PE3, P3 has two possible alternates:

   o  P4, which is node-protecting

   o  R5, which is link-protecting

   P4 is chosen as best LFA due to its better protection type.  However,
   it may not be desirable to use P4 for bandwidth capacity reason.  A
   service provider may prefer to use high bandwidth links as prefered
   LFA.  In this example, prefering shortest path over protection type
   may achieve the expected behavior, but in cases where metric are not
   reflecting bandwidth, it would not work and some other criteria would
   need to be involved when selecting the best LFA.

3.4.  Case 4: No-transit LFA computing node












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       P1       P2
       |   \  /   |
    50 | 50 \/ 50 | 50
       |    /\    |
       PE1-+  +-- PE2
        \        /
      45 \      / 45
          -PE3-
      (No-transit condition set)

               Figure 4

   IS-IS and OSPF protocols define some way to prevent a router to be
   used as transit.

   IS-IS overload bit is defined in [ISO10589] and OSPF R-bit is defined
   in [RFC5340].  OSPF Stub Router is also defined in [RFC6987] as a
   method to prevent transit on a node by advertising MaxLinkMetric on
   all non stub links.

   In the figure above, PE3 has its no-transit condition set
   (permanently, for design reason) and wants to protect traffic using
   LFA for destination PE2.

   On PE3, the loop-free condition is not satisfied : 100 !< 45 + 45.
   PE1 is thus not considered as an LFA.  However thanks to the no-
   transit condition on PE3, we know that PE1 will not loop the traffic
   back to PE3.  So PE1 is an LFA to reach PE2.

   In case of no-transit condition set on a node, LFA behavior must be
   clarified.

4.  Need for coverage monitoring

   As per [RFC6571], LFA coverage highly depends on the used network
   topology.  Even if remote LFA ([RFC7490]) extends significantly the
   coverage of the basic LFA specification, there is still some cases
   where protection would not be available.  As network topologies are
   constantly evolving (network extension, capacity addings, latency
   optimization etc.), the protection coverage may change.  Fast reroute
   functionality may be critical for some services supported by the
   network, a service provider must constantly know what protection
   coverage is currently available on the network.  Moreover, predicting
   the protection coverage in case of network topology change is
   mandatory.

   Today network simulation tool associated with whatif scenarios
   functionality are often used by service providers for the overall



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   network design (capacity, path optimization etc.).  Section 7.5,
   Section 7.4 and Section 7.3 of this document propose to add LFA
   informations into such tool and within routers, so a service provider
   may be able :

   o  to evaluate protection coverage after a topology change.

   o  to adjust the topology change to cover the primary need (e.g.
      latency optimization or bandwidth increase) as well as LFA
      protection.

   o  to monitor constantly the LFA coverage in the live network and
      being alerted.

   Documentation of LFA selection algorithms by implementers (default
   and tuning options) is important in order to leave possibility for
   3rd party modules to model these policy-LFA expressions.

5.  Need for LFA activation granularity

   As in all FRR mechanism, LFA installs backup paths in Forwarding
   Information Base (FIB).  Depending on the hardware used by a service
   provider, FIB resource may be critical.  Activating LFA, by default,
   on all available components (IGP topologies, interface, address
   families etc.) may lead to waste of FIB resource as generally in a
   network only few destinations should be protected (e.g. loopback
   addresses supporting MPLS services) compared to the number of
   destinations in the RIB.

   Moreover a service provider may implement multiple different FRR
   mechanism in its networks for different usages (MRT, TE FRR).  In
   this scenario, an implementation MAY allow to compute alternates for
   a specific destination even if the destination is already protected
   by another mechanism.  This will bring redundancy and let the ability
   for the operator to select the best option for FRR using a policy
   language.

   Section 6 of this document propose some implementation guidelines.

6.  Configuration requirements

   Controlling best alternate and LFA activation granularity is a
   requirement for Service Providers.  This section defines
   configuration requirements for LFA.







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6.1.  LFA enabling/disabling scope

   The granularity of LFA activation SHOULD be controlled (as alternate
   next hop consume memory in forwarding plane).

   An implementation of LFA SHOULD allow its activation with the
   following granularities:

   o  Per routing context: VRF, virtual/logical router, global routing
      table, etc.

   o  Per interface

   o  Per protocol instance, topology, area

   o  Per prefixes: prefix protection SHOULD have a higher priority
      compared to interface protection.  This means that if a specific
      prefix must be protected due to a configuration request, LFA MUST
      be computed and installed for this prefix even if the primary
      outgoing interface is not configured for protection.

   An implementation of LFA MAY allow its activation with the following
   criteria:

   o  Per address-family: ipv4 unicast, ipv6 unicast

   o  Per MPLS control plane: for MPLS control planes that inherit
      routing decision from the IGP routing protocol, MPLS dataplane may
      be protected by LFA.  The implementation may allow operator to
      control this inheritance of protection from the IP prefix to the
      MPLS label bound to this prefix.  The protection inheritance will
      concern : IP to MPLS, MPLS to MPLS, and MPLS to IP entries.  As
      example, LDP and segment-routing extensions for ISIS and OSPF are
      control plane eligible to this inheritance of protection.

6.2.  Policy based LFA selection

   When multiple alternates exist, LFA selection algorithm is based on
   tie breakers.  Current tie breakers do not provide sufficient control
   on how the best alternate is chosen.  This document proposes an
   enhanced tie breaker allowing service providers to manage all
   specific cases:

   1.  An implementation of LFA SHOULD support policy-based decision for
       determining the best LFA.

   2.  Policy based decision SHOULD be based on multiple criterions,
       with each criteria having a level of preference.



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   3.  If the defined policy does not allow the determination of a
       unique best LFA, an implementation SHOULD pick only one based on
       its own decision.  An implementation SHOULD also support election
       of multiple LFAs, for loadbalancing purposes.

   4.  Policy SHOULD be applicable to a protected interface or to a
       specific set of destinations.  In case of application on the
       protected interface, all destinations primarily routed on this
       interface SHOULD use the interface policy.

   5.  It is an implementation choice to reevaluate policy dynamically
       or not (in case of policy change).  If a dynamic approach is
       chosen, the implementation SHOULD recompute the best LFAs and
       reinstall them in FIB, without service disruption.  If a non-
       dynamic approach is chosen, the policy would be taken into
       account upon the next IGP event.  In this case, the
       implementation SHOULD support a command to manually force the
       recomputation/reinstallation of LFAs.

6.2.1.  Connected vs remote alternates

   In addition to connected LFAs, tunnels (e.g.  IP, LDP, RSVP-TE or
   Segment Routing) to distant routers may be used to complement LFA
   coverage (tunnel tail used as virtual neighbor).  When a router has
   multiple alternate candidates for a specific destination, it may have
   connected alternates and remote alternates (reachable via a tunnel).
   Connected alternates may not always provide an optimal routing path
   and it may be preferable to select a remote alternate over a
   connected alternate.  Some usage of tunnels to extend LFA ([RFC5286])
   coverage is described in either [RFC7490] or
   [I-D.francois-segment-routing-ti-lfa].  These documents present some
   use cases of LDP tunnels ([RFC7490]) or Segment Routing tunnels
   ([I-D.francois-segment-routing-ti-lfa]).  This document considers any
   type of tunneling techniques to reach remote alternates (IP, GRE,
   LDP, RSVP-TE, L2TP, Segment Routing etc.) and does not restrict the
   remote alternates to the usage presented in the referenced document.

   In figure 1, there is no P router alternate for P8 to reach PE4 or
   PE5 , so P8 is using PE2 as alternate, which may generate congestion
   when FRR is activated.  Instead, we could have a remote alternate for
   P8 to protect traffic to PE4 and PE5.  For example, a tunnel from P8
   to P3 (following shortest path) can be setup and P8 would be able to
   use P3 as remote alternate to protect traffic to PE4 and PE5.  In
   this scenario, traffic will not use a PE link during FRR activation.

   When selecting the best alternate, the selection algorithm MUST
   consider all available alternates (connected or tunnel).  For example




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   with Remote LFA, computation of PQ set ([RFC7490]) SHOULD be
   performed before best alternate selection.

6.2.2.  Mandatory criteria

   An implementation of LFA MUST support the following criteria:

   o  Non candidate link: A link marked as "non candidate" will never be
      used as LFA.

   o  A primary next hop being protected by another primary next hop of
      the same prefix (ECMP case).

   o  Type of protection provided by the alternate: link protection,
      node protection.  In case of node protection preference, an
      implementation SHOULD support fall back to link protection if node
      protection is not available.

   o  Shortest path: lowest IGP metric used to reach the destination.

   o  SRLG (as defined in [RFC5286] Section 3, see also Section 6.2.4.1
      for more details).

6.2.3.  Additional criteria

   An implementation of LFA SHOULD support the following criteria:

   o  Downstreamness of an alternate : preference of a downstream path
      over a non downstream path SHOULD be configurable.

   o  Link coloring with : include, exclude and preference based system
      (see Section 6.2.4.2).

   o  Link Bandwidth (see Section 6.2.4.3).

   o  Alternate preference/Node coloring (see Section 6.2.4.4).

6.2.4.  Criteria evaluation

6.2.4.1.  SRLG

   [RFC5286] Section 3. proposes to reuse GMPLS IGP extensions to encode
   Shared Risk Link Groups ([RFC4205] and [RFC4203]).  The section is
   also describing the algorithm to compute SRLG protection.

   When SRLG protection is computed, an implementation SHOULD allow the
   following :




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   o  Exclusion alternates violating SRLG.

   o  Maintenance of a preference system between alternates based on
      SRLG violations.  How the preference system is implemented is out
      of scope of this document but here are few examples :

      *  Preference based on number of violations.  In this case : the
         more violations = the less preferred.

      *  Preference based on violation cost.  In this case, each SRLG
         violation has an associated cost.  The lower violation cost sum
         is preferred.

   When applying SRLG criteria, the SRLG violation check SHOULD be
   performed on source to alternate as well as alternate to destination
   paths based on the SRLG set of the primary path.  In the case of
   remote LFA, PQ to destination path attributes would be retrieved from
   SPT rooted at PQ.

6.2.4.2.  Link coloring

   Link coloring is a powerful system to control the choice of
   alternates.  Link colors are markers that will allow to encode
   properties of a particular link.  Protecting interfaces are tagged
   with colors.  Protected interfaces are configured to include some
   colors with a preference level, and exclude others.

   Link color information SHOULD be signalled in the IGP and admin-
   groups IGP extensions ([RFC5305] and [RFC3630]) that are already
   standardized, implemented and widely-used, SHOULD be used for
   encoding and signalling link colors.

                  PE2
                  |   +---- P4
                  |  /
         PE1 ---- P1 --------- P2
                  |      10Gb
              1Gb |
                  |
                  P3

                        Figure 8

   Example : P1 router is connected to three P routers and two PEs.

   P1 is configured to protect the P1-P4 link.  We assume that given the
   topology, all neighbors are candidate LFA.  We would like to enforce
   a policy in the network where only a core router may protect against



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   the failure of a core link, and where high capacity links are
   prefered.

   In this example, we can use the proposed link coloring by:

   o  Marking PEs links with color RED

   o  Marking 10Gb CORE link with color BLUE

   o  Marking 1Gb CORE link with color YELLOW

   o  Configured the protected interface P1->P4 with :

      *  Include BLUE, preference 200

      *  Include YELLOW, preference 100

      *  Exclude RED

   Using this, PE links will never be used to protect against P1-P4 link
   failure and 10Gb link will be be preferred.

   The main advantage of this solution is that it can easily be
   duplicated on other interfaces and other nodes without change.  A
   Service Provider has only to define the color system (associate color
   with a significance), as it is done already for TE affinities or BGP
   communities.

   An implementation of link coloring:

   o  SHOULD support multiple include and exclude colors on a single
      protected interface.

   o  SHOULD provide a level of preference between included colors.

   o  SHOULD support multiple colors configuration on a single
      protecting interface.

6.2.4.3.  Bandwidth

   As mentioned in previous sections, not taking into account bandwidth
   of an alternate could lead to congestion during FRR activation.  We
   propose to base the bandwidth criteria on the link speed information
   for the following reason :

   o  if a router S has a set of X destinations primarly forwarded to N,
      using per prefix LFA may lead to have a subset of X protected by a
      neighbor N1, another subset by N2, another subset by Nx etc.



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   o  S is not aware about traffic flows to each destination and is not
      able to evaluate how much traffic will be sent to N1,N2, etc.  Nx
      in case of FRR activation.

   Based on this, it is not useful to gather available bandwidth on
   alternate paths, as the router does not know how much bandwidth it
   requires for protection.  The proposed link speed approach provides a
   good approximation with a small cost as information is easily
   available.

   The bandwidth criteria of the policy framework SHOULD work in at
   least two ways :

   o  PRUNE : exclude a LFA if link speed to reach it is lower than the
      link speed of the primary next hop interface.

   o  PREFER : prefer a LFA based on its bandwidth to reach it compared
      to the link speed of the primary next hop interface.

6.2.4.4.  Alternate preference/Node coloring

   Rather than tagging interface on each node (using link color) to
   identify alternate node type (as example), it would be helpful if
   routers could be identified in the IGP.  This would allow a grouped
   processing on multiple nodes.  As an implementation need to exclude
   some specific alternates (see Section 6.2.3), an implementation :

   o  SHOULD be able to give a preference to specific alternate.

   o  SHOULD be able to give a preference to a group of alternate.

   o  SHOULD be able to exclude a specific alternate.

   o  SHOULD be able to exclude a group of alternate.

   A specific alternate may be identified by its interface, IP address
   or router ID and group of alternates may be identified by a marker
   (tag) advertised in IGP.  The IGP encoding and signalling for marking
   group of alternates SHOULD be done using
   [I-D.ietf-isis-node-admin-tag], [I-D.ietf-ospf-node-admin-tag].
   Using a tag/marker is referred as Node coloring in comparison to link
   coloring option presented in Section 6.2.4.2.

   Consider the following network:







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                  PE3
                  |
                  |
                  PE2
                  |   +---- P4
                  |  /
         PE1 ---- P1 -------- P2
                  |      10Gb
              1Gb |
                  |
                  P3

             Figure 9



   In the example above, each node is configured with a specific tag
   flooded through the IGP.

   o  PE1,PE3: 200 (non candidate).

   o  PE2: 100 (edge/core).

   o  P1,P2,P3: 50 (core).

   A simple policy could be configured on P1 to choose the best
   alternate for P1->P4 based on router function/role as follows :

   o  criteria 1 -> alternate preference: exclude tag 100 and 200.

   o  criteria 2 -> bandwidth.

6.2.5.  Retrieving alternate path attributes

6.2.5.1.  Alternate path

   The alternate path is composed of two distinct parts : PLR to
   alternate and alternate to destination.













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      N1 -- R1 ---- R2
     /50     \       \
    /         R3 --- R4
   /                   \
   S -------- E ------- D
   \\                  //
    \\                //
     N2 ---- PQ ---- R5

          Figure 5

   In the figure above, we consider a primary path from S to D, S using
   E as primary nexthop.  All metrics are 1 except {S,N1}=50.  Two
   alternate paths are available:

   o  {S,N1,R1,R2|R3,R4,D} where N1 is a connected alternate.  This
      consists of two sub-paths:

      *  {S,N1}: path from PLR to the alternate.

      *  {N1,R1,R2|R3,R4,D}: path from alternate to destination.

   o  {S,N2,PQ,R5,D} where PQ is a remote alternate.  Again the path
      consists of two sub-paths:

      *  {S,N2,PQ}: path from PLR to the alternate.

      *  {PQ,R5,D}: path from alternate to destination.

   As displayed in the figure, some part of the alternate path may
   fanout in multipath due to ECMP.

6.2.5.2.  Alternate path attributes

   Some criterions listed in the previous sections are requiring to
   retrieve some characteristic of the alternate path (SRLG, bandwidth,
   color, tag etc.).  We call these characteristics "path attributes".
   A path attribute can record a list of node properties (e.g. node tag)
   or link properties (e.g. link color).

   This document defines two types of path attributes:

   o  Cumulative attribute: when a path attribute is cumulative, the
      implementation SHOULD record the value of the attribute on each
      element (link and node) along the alternate path.  SRLG, link
      color, and node color are cumulative attributes.





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   o  Unitary attribute: when a path attribute is unitary, the
      implementation SHOULD record the value of the attribute only on
      the first element along the alternate path (first node, or first
      link).  Bandwidth is a unitary attribute.

      N1 -- R1 ---- R2
     /               \
    / 50              R4
   /                   \
   S -------- E ------- D

   In the figure above, N1 is a connected alternate to each D from S.
   We consider that all links have a RED color except {R1,R2} which is
   BLUE.  We consider all links to be 10Gbps, except {N1,R1} which is
   2.5Gbps.  The bandwidth attribute collected for the alternate path
   will be 10Gbps.  As the attribute is unitary, only the link speed of
   the first link {S,N1} is recorded.  The link color attribute
   collected for the alternate path will be {RED,RED,BLUE,RED,RED}. As
   the attribute is cumulative, the value of the attribute on each link
   along the path is recorded.

6.2.5.3.  Connected alternate

   For alternate path using a connected alternate:

   o  attributes from PLR to alternate are retrieved from the interface
      connected to the alternate.  In case the alternate is connected
      through multiple interfaces, the evaluation of attributes SHOULD
      be done once per interface (each interface is considered as a
      separate alternate) and once per ECMP group of interfaces (Layer 3
      bundle).

   o  path attributes from alternate to destination are retrieved from
      SPF rooted at the alternate.  As the alternate is a connected
      alternate, the SPF has already been computed to find the
      alternate, so there is no need of additional computation.

        N1 -- R1 ---- R2
     50//50             \
      //                 \
   i1//i2                 \
    S -------- E -------- D

           Figure 6

   In the figure above, we consider a primary path from S to D, S using
   E as primary nexthop.  All metrics are considered as 1 expect {S,N1}




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   links which are using metric of 50.  We consider the following SRLG
   groups on links:

   o  {S,N1} using i1 : SRLG1,SRLG10

   o  {S,N1} using i2 : SRLG2,SRLG20

   o  {N1,R1} : SRLG3

   o  {R1,R2} : SRLG4

   o  {R2,D} : SRLG5

   o  {S,E} : SRLG10

   o  {E,D} : SRLG6

   S is connected to the alternate using two interfaces i1 and i2.

   If i1 and i2 are not part of an ECMP group, the evaluation of
   attributes is done once per interface, and each interface is
   considered as a separate alternate path.  Two alternate paths will be
   available with the associated SRLG attributes :

   o  Alternate path #1 : {S,N1 using if1,R1,R2,D}:
      SRLG1,SRLG10,SRLG3,SRLG4,SRLG5.

   o  Alternate path #2 : {S,N1 using if2,R1,R2,D}:
      SRLG2,SRLG20,SRLG3,SRLG4,SRLG5.

   Alternate path #1 is sharing risks with primary path and may be
   depreferred or pruned by user defined policy.

   If i1 and i2 are part of an ECMP group, the evaluation of attributes
   is done once per ECMP group, and the implementation considers a
   single alternate path {S,N1 using if1|if2,R1,R2,D} with the following
   SRLG attributes: SRLG1,SRLG10,SRLG2,SRLG20,SRLG3,SRLG4,SRLG5.
   Alternate path is sharing risks with primary path and may be
   depreferred or pruned by user defined policy.

6.2.5.4.  Remote alternate

   For alternate path using a remote alternate (tunnel) :

   o  Attributes on the path from the PLR to alternate are retrieved
      using the PLR's primary SPF (when using a PQ-node from P-Space) or
      the immediate neighbor's SPF (when using a PQ from extended
      P-Space).  These are then combined with the attributes of the



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      link(s) to reach the immediate neighbor.  In both cases, no
      additional SPF is required.

   o  Attributes from remote alternate to destination path may be
      retrieved from SPF rooted at the remote alternate.  An additional
      forward SPF is required for each remote alternate (PQ-node) as
      indicated in [I-D.ietf-rtgwg-rlfa-node-protection] section 3.2 .
      In some remote alternate scenarios, like [I-D.francois-segment-
      routing-ti-lfa], alternate to destination path attributes may be
      obtained using a different technique.

   The number of remote alternates may be very high. .  In case of
   remote LFA, simulations of real-world network topologies have shown
   that order of hundreths of PQ may be possible.  The computational
   overhead to collect all path attributes of all PQ to destination
   paths may grow beyond practical reason.

   To handle this situation, implementations need to limit the number of
   remote alternates to be evaluated to a finite number before
   collecting alternate path attributes and running the policy
   evaluation.  [I-D.ietf-rtgwg-rlfa-node-protection] Section 2.3.3
   provides a way to reduce the number of PQ to be evaluated.

   Some other remote alternate techniques using static or dynamic
   tunnels may not require this pruning.


























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                  Link            Remote              Remote
                  alternate       alternate           alternate
                 -------------  ------------------   -------------
   Alternates    |  LFA      |  |   rLFA (PQs)   |   |  Static/  |
                 |           |  |                |   |  Dynamic  |
   sources       |           |  |                |   |  tunnels  |
                 -------------  ------------------   -------------
                      |                   |                  |
                      |                   |                  |
                      |        --------------------------    |
                      |        |  Prune some alternates |    |
                      |        | (sorting strategy)     |    |
                      |        --------------------------    |
                      |                   |                  |
                      |                   |                  |
                  ------------------------------------------------
                  |          Collect alternate attributes        |
                  ------------------------------------------------
                                          |
                                          |
                               -------------------------
                               |    Evaluate policy    |
                               -------------------------
                                          |
                                          |
                                   Best alternates

6.2.5.5.  Collecting attributes in case of multipath

   As described in Section 6.2.5, there may be some situation where an
   alternate path or part of an alternate path fans out to multiple
   paths (e.g.  ECMP).  When collecting path attributes in such case, an
   implementation SHOULD consider the union of attributes of each sub-
   path.

   In the figure 5 (in Section 6.2.5), S has two alternates paths to
   reach D.  Each alternate path fans out into multipath due to ECMP.
   Considering the following link color attributes : all links are RED
   except {R1,R3} which is BLUE.  The user wants to use an alternate
   path with only RED links.  The first alternate path
   {S,N1,R1,R2|R3,R4,D} does not fit the constraint, as {R1,R3} is BLUE.
   The second alternate path {S,N2,PQ,R5,D} fits the constraint and will
   be preferred as it uses only RED links.








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6.2.6.  ECMP LFAs

           10
      PE2 - PE3
       |     |
    50 |  5  | 50
       P1----P2
       \\    //
    50  \\  // 50
         PE1

           Figure 7

   Links between P1 and PE1 are L1 and L2, links between P2 and PE1 are
   L3 and L4

   In the figure above, primary path from PE1 to PE2 is through P1 using
   ECMP on two parallel links L1 and L2.  In case of standard ECMP
   behavior, if L1 is failing, postconvergence next hop would become L2
   and there would be no longer ECMP.  If LFA is activated, as stated in
   [RFC5286] Section 3.4., "alternate next-hops may themselves also be
   primary next-hops, but need not be" and "alternate next-hops should
   maximize the coverage of the failure cases".  In this scenario there
   is no alternate providing node protection, LFA will so prefer L2 as
   alternate to protect L1 which makes sense compared to postconvergence
   behavior.

   Considering a different scenario using figure 7, where L1 and L2 are
   configured as a layer 3 bundle using a local feature, as well as L3/
   L4 being a second layer 3 bundle.  Layer 3 bundles are configured as
   if a link in the bundle is failing, the traffic must be rerouted out
   of the bundle.  Layer 3 bundles are generally introduced to increase
   bandwidth between nodes.  In nominal situation, ECMP is still
   available from PE1 to PE2, but if L1 is failing, postconvergence next
   hop would become ECMP on L3 and L4.  In this case, LFA behavior
   SHOULD be adapted in order to reflect the bandwidth requirement.















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   We would expect the following FIB entry on PE1 :


       On PE1 : PE2 +--> ECMP -> L1
                    |     |
                    |     +----> L2
                    |
                    +--> LFA(ECMP) -> L3
                          |
                          +---------> L4


   If L1 or L2 is failing, traffic must be switched on the LFA ECMP
   bundle rather than using the other primary next hop.

   As mentioned in [RFC5286] Section 3.4., protecting a link within an
   ECMP by another primary next hop is not a MUST.  Moreover, we already
   presented in this document, that maximizing the coverage of the
   failure case may not be the right approach and policy based choice of
   alternate may be preferred.

   An implementation SHOULD allow to prefer to protect a primary next
   hop by another primary next hop.  An implementation SHOULD allow to
   prefer to protect a primary next hop by a NON primary next hop.  An
   implementation SHOULD allow to use an ECMP bundle as a LFA.

7.  Operational aspects

7.1.  No-transit condition on LFA computing node

   In [RFC5286], Section 3.5, the setting of the no-transit condition
   (through IS-IS overload or OSPF R-bit) in LFA computation is only
   taken into account for the case where a neighbor has the no-transit
   condition set.

   In addition to RFC 5286 inequality 1 Loop-Free Criterion
   (Distance_opt(N, D) < Distance_opt(N, S) + Distance_opt(S, D)), the
   IS-IS overload bit or OSPF R-bit of the LFA calculating neighbor (S)
   SHOULD be taken into account.  Indeed, if it has the IS-IS overload
   bit set or OSPF R-bit clear, no neighbor will loop back to traffic to
   itself.

   An OSPF router acting as a stub router [RFC 6987] SHOULD behave as if
   R-bit was clear regarding LFA computation.







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7.2.  Manual triggering of FRR

   Service providers often perform manual link shutdown (using router
   CLI) to perform some network changes/tests.  A manual link shutdown
   may be done at multiple level : physical interface, logical
   interface, IGP interface, BFD session etc.  Especially testing or
   troubleshooting FRR requires to perform the manual shutdown on the
   remote end of the link as generally a local shutdown would not
   trigger FRR.

   To enhance such situation, an implementation SHOULD support
   triggering/activating LFA Fast Reroute for a given link when a manual
   shutdown is done on a component that currently supports FRR
   activation.

   An implementation MAY also support FRR activation for a specific
   interface or a specific prefix on a primary next-hop interface and
   revert without any action on any running component of the node (links
   or protocols).  In this use case, the FRR activation time need to be
   controlled by a timer in case the operator forgot to revert traffic
   on primary path.  When the timer expires, the traffic is
   automatically reverted to the primary path.  This will make easier
   tests of fast-reroute path and then revert back to the primary path
   without causing a global network convergence.

   For example :

   o  if an implementation supports FRR activation upon BFD session down
      event, this implementation SHOULD support FRR activation when a
      manual shutdown is done on the BFD session.  But if an
      implementation does not support FRR activation on BFD session
      down, there is no need for this implementation to support FRR
      activation on manual shutdown of BFD session.

   o  if an implementation supports FRR activation on physical link down
      event (e.g.  Rx laser Off detection, or error threshold raised
      etc.), this implementation SHOULD support FRR activation when a
      manual shutdown at physical interface is done.  But if an
      implementation does not support FRR activation on physical link
      down event, there is no need for this implementation to support
      FRR activation on manual physical link shutdown.

   o  A CLI command may allow to switch from primary path to FRR path
      for testing FRR path for a specific.  There is no impact on
      controlplane, only dataplane of the local node could be changed.
      A similar command may allow to switch back traffic from FRR path
      to primary path.




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7.3.  Required local information

   LFA introduction requires some enhancement in standard routing
   information provided by implementations.  Moreover, due to the non
   100% coverage, coverage informations is also required.

   Hence an implementation :

   o  MUST be able to display, for every prefix, the primary next hop as
      well as the alternate next hop information.

   o  MUST provide coverage information per activation domain of LFA
      (area, level, topology, instance, virtual router, address family
      etc.).

   o  MUST provide number of protected prefixes as well as non protected
      prefixes globally.

   o  SHOULD provide number of protected prefixes as well as non
      protected prefixes per link.

   o  MAY provide number of protected prefixes as well as non protected
      prefixes per priority if implementation supports prefix-priority
      insertion in RIB/FIB.

   o  SHOULD provide a reason for choosing an alternate (policy and
      criteria) and for excluding an alternate.

   o  SHOULD provide the list of non protected prefixes and the reason
      why they are not protected (no protection required or no alternate
      available).

7.4.  Coverage monitoring

   It is pretty easy to evaluate the coverage of a network in a nominal
   situation, but topology changes may change the coverage.  In some
   situations, the network may no longer be able to provide the required
   level of protection.  Hence, it becomes very important for service
   providers to get alerted about changes of coverage.

   An implementation SHOULD :

   o  provide an alert system if total coverage (for a node) is below a
      defined threshold or comes back to a normal situation.

   o  provide an alert system if coverage of a specific link is below a
      defined threshold or comes back to a normal situation.




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   An implementation MAY :

   o  trigger an alert if a specific destination is not protected
      anymore or when protection comes back up for this destination

   Although the procedures for providing alerts are beyond the scope of
   this document, we recommend that implementations consider standard
   and well used mechanisms like syslog or SNMP traps.

7.5.  LFA and network planning

   The operator may choose to run simulations in order to ensure full
   coverage of a certain type for the whole network or a given subset of
   the network.  This is particularly likely if he operates the network
   in the sense of the third backbone profiles described in [RFC6571],
   that is, he seeks to design and engineer the network topology in a
   way that a certain coverage is always achieved.  Obviously a complete
   and exact simulation of the IP FRR coverage can only be achieved, if
   the behavior is deterministic and if the algorithm used is available
   to the simulation tool.  Thus, an implementation SHOULD:

   o  Behave deterministic in its selection LFA process.  I.e. in the
      same topology and with the same policy configuration, the
      implementation MUST always choose the same alternate for a given
      prefix.

   o  Document its behavior.  The implementation SHOULD provide enough
      documentation of its behavior that allows an implementer of a
      simulation tool, to foresee the exact choice of the LFA
      implementation for every prefix in a given topology.  This SHOULD
      take into account all possible policy configuration options.  One
      possible way to document this behavior is to disclose the
      algorithm used to choose alternates.

8.  Security Considerations

   The policy mechanism introduced in this document allows to tune the
   selection of the alternate.  This is not seen as a security threat
   as:

   o  all candidates are already eligible as per [RFC5286] and
      considered useable.

   o  the policy is based on information from the router's own
      configuration and from the IGP which are both considered trusted.

   Hence this document does not introduce new security considerations
   compared to [RFC5286].



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   This document does not introduce any change in security consideration
   compared to [RFC5286].  The policy mechanism introduced in this
   document allow to tune the best alternate choice but does not change
   the list of alternates that are eligible.  As defined in [RFC5286]
   Section 7., this best alternate "can be used anyway when a different
   topological change occurs, and hence this can't be viewed as a new
   security threat.".

9.  IANA Considerations

   This document has no action for IANA.

10.  Contributors

   Significant contributions were made by Pierre Francois, Hannes
   Gredler, Chris Bowers, Jeff Tantsura, Uma Chunduri, Acee Lindem and
   Mustapha Aissaoui which the authors would like to acknowledge.

11.  References

11.1.  Normative References

   [I-D.ietf-isis-node-admin-tag]
              Sarkar, P., Gredler, H., Hegde, S., Litkowski, S.,
              Decraene, B., Li, Z., Aries, E., Rodriguez, R., and H.
              Raghuveer, "Advertising Per-node Admin Tags in IS-IS",
              draft-ietf-isis-node-admin-tag-02 (work in progress), June
              2015.

   [I-D.ietf-ospf-node-admin-tag]
              Hegde, S., Raghuveer, H., Gredler, H., Shakir, R.,
              Smirnov, A., Li, Z., and B. Decraene, "Advertising per-
              node administrative tags in OSPF", draft-ietf-ospf-node-
              admin-tag-02 (work in progress), June 2015.

   [ISO10589]
              "Intermediate system to Intermediate system intra-domain
              routing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473), ISO/IEC
              10589:2002, Second Edition.", Nov 2002.

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

   [RFC3137]  Retana, A., Nguyen, L., White, R., Zinin, A., and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 3137,
              June 2001.



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   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630, September
              2003.

   [RFC4203]  Kompella, K. and Y. Rekhter, "OSPF Extensions in Support
              of Generalized Multi-Protocol Label Switching (GMPLS)",
              RFC 4203, October 2005.

   [RFC4205]  Kompella, K. and Y. Rekhter, "Intermediate System to
              Intermediate System (IS-IS) Extensions in Support of
              Generalized Multi-Protocol Label Switching (GMPLS)", RFC
              4205, October 2005.

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

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, October 2008.

   [RFC5307]  Kompella, K. and Y. Rekhter, "IS-IS Extensions in Support
              of Generalized Multi-Protocol Label Switching (GMPLS)",
              RFC 5307, October 2008.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, July 2008.

   [RFC6571]  Filsfils, C., Francois, P., Shand, M., Decraene, B.,
              Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
              Alternate (LFA) Applicability in Service Provider (SP)
              Networks", RFC 6571, June 2012.

   [RFC6987]  Retana, A., Nguyen, L., Zinin, A., White, R., and D.
              McPherson, "OSPF Stub Router Advertisement", RFC 6987,
              September 2013.

   [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, April 2015.

11.2.  Informative References

   [I-D.francois-segment-routing-ti-lfa]
              Francois, P., Filsfils, C., Bashandy, A., and B. Decraene,
              "Topology Independent Fast Reroute using Segment Routing",
              draft-francois-segment-routing-ti-lfa-00 (work in
              progress), November 2013.





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   [I-D.ietf-rtgwg-rlfa-node-protection]
              Sarkar, P., Gredler, H., Hegde, S., Bowers, C., Litkowski,
              S., and H. Raghuveer, "Remote-LFA Node Protection and
              Manageability", draft-ietf-rtgwg-rlfa-node-protection-02
              (work in progress), June 2015.

Authors' Addresses

   Stephane Litkowski (editor)
   Orange

   Email: stephane.litkowski@orange.com


   Bruno Decraene
   Orange

   Email: bruno.decraene@orange.com


   Clarence Filsfils
   Cisco Systems

   Email: cfilsfil@cisco.com


   Kamran Raza
   Cisco Systems

   Email: skraza@cisco.com


   Martin Horneffer
   Deutsche Telekom

   Email: Martin.Horneffer@telekom.de


   Pushpasis Sarkar
   Juniper Networks

   Email: psarkar@juniper.net









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