Routing Area Working Group                                P. Sarkar, Ed.
Internet-Draft                                                  S. Hegde                                    Individual Contributor
Intended status: Standards Track                                S. Hegde
Expires: April 11, 2017                                        C. Bowers
Expires: June 12, 2016
                                                  Juniper Networks, Inc.
                                                              H. Gredler
                                                            Unaffiliated
                                                            RtBrick Inc.
                                                            S. Litkowski
                                                                  Orange
                                                       December 10, 2015
                                                         October 8, 2016

              Remote-LFA Node Protection and Manageability
                draft-ietf-rtgwg-rlfa-node-protection-05
                draft-ietf-rtgwg-rlfa-node-protection-06

Abstract

   The loop-free alternates computed following the current Remote-LFA
   specification guarantees only link-protection.  The resulting Remote-
   LFA nexthops (also called PQ-nodes), may not guarantee node-
   protection for all destinations being protected by it.

   This document describes procedures for determining if a given PQ-node
   provides node-protection for a specific destination or not.  The
   document also shows how the same procedure can be utilised for
   collection of complete characteristics for alternate paths.
   Knowledge about the characteristics of all alternate path is
   precursory to apply operator defined policy for eliminating paths not
   fitting constraints.

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 [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."

   This Internet-Draft will expire on June 12, 2016. April 11, 2017.

Copyright Notice

   Copyright (c) 2015 2016 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.  Node Protection with Remote-LFA . . . . . . . . . . . . . . .   3
     2.1.  The Problem . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Additional Definitions  . . . . . . . . . . . . . . . . .   6
       2.2.1.  Link-Protecting Extended P-Space  . . . . . . . . . .   6
       2.2.2.  Node-Protecting Extended P-Space  . . . . . . . . . .   6
       2.2.3.  Q-Space . . . . . . . . . . . . . . . . . . . . . . .   6
       2.2.4.  Link-Protecting PQ Space  . . . . . . . . . . . . . .   6
       2.2.5.  Candidate Node-Protecting PQ Space  . . . . . . . . .   7
       2.2.6.  Cost-Based Definitions  . . . . . . . . . . . . . . .   7
         2.2.6.1.  Link-Protecting Extended P-Space  . . . . . . . .   7
         2.2.6.2.  Node-Protecting Extended P-Space  . . . . . . . .   7
         2.2.6.3.  Q-Space . . . . . . . . . . . . . . . . . . . . .   8
     2.3.  Computing Node-protecting R-LFA Path  . . . . . . . . . .   9
       2.3.1.  Computing Candidate Node-protecting PQ-Nodes for
               Primary nexthops  . . . . . . . . . . . . . . . . . .   9
       2.3.2.  Computing node-protecting paths from PQ-nodes to
               destinations  . . . . . . . . . . . . . . . . . . . .  11
       2.3.3.  Limiting extra computational overhead . . . . . . . .  13
   3.  Manageabilty of Remote-LFA Alternate Paths  . . . . . . . . .  14
     3.1.  The Problem . . . . . . . . . . . . . . . . . . . . . . .  14
     3.2.  The Solution  . . . . . . . . . . . . . . . . . . . . . .  15
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   The Remote-LFA [RFC7490] specification provides loop-free alternates
   that guarantee only link-protection.  The resulting Remote-LFA
   alternate nexthops (also referred to as the PQ-nodes) may not provide
   node-protection for all destinations covered by the same, in case of
   failure of the primary nexthop node.  Neither does the specification
   provide a means to determine the same.

   Also, the LFA Manageability [I-D.ietf-rtgwg-lfa-manageability]
   document, requires a computing router to find all possible (including
   all possible Remote-LFA) alternate nexthops, collect the complete set
   of path characteristics for each alternate path, run a alternate-
   selection policy (configured by the operator), and find the best
   alternate path.  This will require the Remote-LFA implementation to
   gather all the required path characteristics along each link on the
   entire Remote-LFA alternate path.

   With current LFA [RFC5286] and Remote-LFA implementations, the
   forward SPF (and reverse SPF) is run on the computing router and its
   immediate 1-hop routers as the roots.  While that enables computation
   of path attributes (e.g.  SRLG, Admin-groups) for first alternate
   path segment from the computing router to the PQ-node, there is no
   means for the computing router to gather any path attributes for the
   path segment from the PQ-node to destination.  Consequently any
   policy-based selection of alternate paths will consider only the path
   attributes from the computing router up until the PQ-node.

   This document describes a procedure for determining node-protection
   with Remote-LFA.  The same procedure is also extended for collection
   of a complete set of path attributes, enabling more accurate policy-
   based selection for alternate paths obtained with Remote-LFA.

2.  Node Protection with Remote-LFA

   Node-protection is required to provide protection of traffic on a
   given forwarding node, against the failure of the first-hop node on
   the primary forwarding path.  Such protection becomes more critical
   in the absence of mechanisms like non-stop-routing in the network.
   Certain operators refrain from deploying non-stop-routing in their
   network, due to the significant additional performance complexities
   it introduces.  In such cases node-protection is essential to
   guarantee un-interrupted flow of traffic, even in the case of an
   entire forwarding node going down.

   The following sections discuss the node-protection problem in the
   context of Remote-LFA and propose a solution.

2.1.  The Problem

   To better illustrate the problem and the solution proposed in this
   document the following topology diagram from the Remote-LFA [RFC7490]
   draft is being re-used with slight modification.

                                             D1
                                            /
                                       S-x-E
                                      /     \
                                     N       R3--D2
                                      \     /
                                      R1---R2

                           Figure 1: Topology 1

   In the above topology, for all (non-ECMP) destinations reachable via
   the S-E link there is no standard LFA alternate.  As per the Remote-
   LFA [RFC7490] alternate specifications node R2 being the only PQ-node
   for the S-E link provides nexthop for all the above destinations.
   Table 1 below, shows all possible primary and Remote-LFA alternate
   paths for each destination.

    +-------------+--------------+---------+-------------------------+
    | Destination | Primary Path | PQ-node | Remote-LFA Backup Path  |
    +-------------+--------------+---------+-------------------------+
    | R3          | S->E->R3     | R2      | S=>N=>R1=>R2->R3        |
    | E           | S->E         | R2      | S=>N=>R1=>R2->R3->E     |
    | D1          | S->E->D1     | R2      | S=>N=>R1=>R2->R3->E->D1 |
    | D2          | S->E->R3->D2 | R2      | S=>N=>R1=>R2->R3->D2    |
    +-------------+--------------+---------+-------------------------+

              Table 1: Remote-LFA backup paths via PQ-node R2

   A closer look at Table 1 shows that, while the PQ-node R2 provides
   link-protection for all the destinations, it does not provide node-
   protection for destinations E and D1.  In the event of the node-
   failure on primary nexthop E, the alternate path from Remote-LFA
   nexthop R2 to E and D1 also becomes unavailable.  So for a Remote-LFA
   nexthop to provide node-protection for a given destination, it is
   mandatory that, the shortest path from the given PQ-node to the given
   destination MUST not traverse the primary nexthop.

   In another extension of the topology in Figure 1 let us consider an
   additional link between N and E with the same cost as the other
   links.

                                             D1
                                            /
                                       S-x-E
                                      /   / \
                                     N---+   R3--D2
                                      \     /
                                      R1---R2

                           Figure 2: Topology 2

   In the above topology, the S-E link is no more on any of the shortest
   paths from N to R3, E and D1.  Hence R3, E and D1 are also included
   in both the Extended-P space and Q space of E (w.r.t S-E link).
   Table 2 below, shows all possible primary and R-LFA alternate paths
   via PQ-node R3, for each destination reachable through the S-E link
   in the above topology.  The R-LFA alternate paths via PQ-node R2
   remains same as in Table 1.

     +-------------+--------------+---------+------------------------+
     | Destination | Primary Path | PQ-node | Remote-LFA Backup Path |
     +-------------+--------------+---------+------------------------+
     | R3          | S->E->R3     | R3      | S=>N=>E=>R3            |
     | E           | S->E         | R3      | S=>N=>E=>R3->E         |
     | D1          | S->E->D1     | R3      | S=>N=>E=>R3->E->D1     |
     | D2          | S->E->R3->D2 | R3      | S=>N=>E=>R3->D2        |
     +-------------+--------------+---------+------------------------+

              Table 2: Remote-LFA backup paths via PQ-node R3

   Again a closer look at Table 2 shows that, unlike Table 1, where the
   single PQ-node R2 provided node-protection for destinations R3 and
   D2, if we choose R3 as the R-LFA nexthop, it does not provide node-
   protection for R3 and D2 anymore.  If S chooses R3 as the R-LFA
   nexthop, in the event of the node-failure on primary nexthop E, on
   the alternate path from S to R-LFA nexthop R3, one of parallel ECMP
   path between N and R3 also becomes unavailable.  So for a Remote-LFA
   nexthop to provide node-protection for a given destination, it is
   also mandatory that, the shortest path from S to the chosen PQ-node
   MUST not traverse the primary nexthop node.

2.2.  Additional Definitions

   This document adds and enhances the following definitions extending
   the ones mentioned in Remote-LFA [RFC7490] specification.

2.2.1.  Link-Protecting Extended P-Space

   The Remote-LFA [RFC7490] specification already defines this.  The
   link-protecting extended P-space for a link S-E being protected is
   the set of routers that are reachable from one or more direct
   neighbors of S, except primary node E, without traversing the S-E
   link on any of the shortest path from the direct neighbor to the
   router.  This MUST exclude any direct neighbor for which there is at
   least one ECMP path from the direct neighbor traversing the link(S-E)
   being protected.

   For a cost-based definition for Link-protecting Extended P-Space
   refer to Section 2.2.6.1.

2.2.2.  Node-Protecting Extended P-Space

   The node-protecting extended P-space for a primary nexthop node E
   being protected, is the set of routers that are reachable from one or
   more direct neighbors of S, except primary node E, without traversing
   the node E.  This MUST exclude any direct neighbors for which there
   is at least one ECMP path from the direct neighbor traversing the
   node E being protected.

   For a cost-based definition for Node-protecting Extended P-Space
   refer to Section 2.2.6.2.

2.2.3.  Q-Space

   The Remote-LFA [RFC7490] draft already defines this.  The Q-space for
   a link S-E being protected is the set of routers that can reach
   primary node E, without traversing the S-E link on any of the
   shortest path from the node Y to primary nexthop E.  This MUST
   exclude any destination for which there is at least one ECMP path
   from the node Y to the primary nexthop E traversing the link(S-E)
   being protected.

   For a cost-based definition for Q-Space refer to Section 2.2.6.3.

2.2.4.  Link-Protecting PQ Space

   A node Y is in link-protecting PQ space w.r.t to the link (S-E) being
   protected, if and only if, Y is present in both link-protecting
   extended P-space and the Q-space for the link being protected.

2.2.5.  Candidate Node-Protecting PQ Space

   A node Y is in candidate node-protecting PQ space w.r.t to the node
   (E) being protected, if and only if, Y is present in both node-
   protecting extended P-space and the Q-space for the link being
   protected.

   It must be noted, that a node Y being in candidate node-protecting
   PQ-space, does not guarantee that the R-LFA alternate path via the
   same, in entirety, is unaffected in the event of a node failure of
   primary nexthop node E.  It only guarantees that the path segment
   from S to PQ-node Y is unaffected by the same failure event.  The PQ-
   nodes in the candidate node-protecting PQ space may provide node
   protection for only a subset of destinations that are reachable
   through the corresponding primary link.

2.2.6.  Cost-Based Definitions

   This section provides cost-based definitions for some of the terms
   introduced in Section 2.2 of this document.

2.2.6.1.  Link-Protecting Extended P-Space

   Please refer to Section 2.2.1 for a formal definition for Link-
   protecting Extended P-Space.

   A node Y is in link-protecting extended P-space w.r.t to the link
   (S-E) being protected, if and only if, there exists at least one
   direct neighbor of S, Ni, other than primary nexthop E, that
   satisfies the following condition.

   D_opt(Ni,Y) < D_opt(Ni,S) + D_opt(S,Y)

   Where,
     D_opt(A,B) : Distance on most optimum path from A to B.
            Ni  : A direct neighbor of S other than primary
                  nexthop E.
             Y  : The node being evaluated for link-protecting
                  extended P-Space.

              Figure 3: Link-Protecting Ext-P-Space Condition

2.2.6.2.  Node-Protecting Extended P-Space

   Please refer to Section 2.2.2 for a formal definition for Node-
   protecting Extended P-Space.

   A node Y is in node-protecting extended P-space w.r.t to the node E
   being protected, if and only if, there exists at least one direct
   neighbor of S, Ni, other than primary nexthop E, that satisfies the
   following condition.

   D_opt(Ni,Y) < D_opt(Ni,E) + D_opt(E,Y)

   Where,
     D_opt(A,B) : Distance on most optimum path from A to B.
             E  : The primary nexthop on shortest path from S
                  to destination.
             Ni : A direct neighbor of S other than primary
                  nexthop E.
              Y : The node being evaluated for node-protecting
                  extended P-Space.

              Figure 4: Node-Protecting Ext-P-Space Condition

   It must be noted that a node Y satisfying the condition in Figure 4
   above only guarantees that the R-LFA alternate path segment from S
   via direct neighbor Ni to the node Y is not affected in the event of
   a node failure of E.  It does not yet guarantee that the path segment
   from node Y to the destination is also unaffected by the same failure
   event.

2.2.6.3.  Q-Space

   Please refer to Section 2.2.3 for a formal definition for Q-Space.

   A node Y is in Q-space w.r.t to the link (S-E) being protected, if
   and only if, the following condition is satisfied.

   D_opt(Y,E) < D_opt(S,E) + D_opt(Y,S)

   Where,
     D_opt(A,B) : Distance on most optimum path from A to B.
             E  : The primary nexthop on shortest path from S
                  to destination.
             Y  : The node being evaluated for Q-Space.

                        Figure 5: Q-Space Condition

2.3.  Computing Node-protecting R-LFA Path

   The R-LFA alternate path through a given PQ-node to a given
   destination is comprised of two path segments as follows.

   1.  Path segment from the computing router to the PQ-node (Remote-LFA
       alternate nexthop), and

   2.  Path segment from the PQ-node to the destination being protected.

   So to ensure a R-LFA alternate path for a given destination provides
   node-protection we need to ensure that none of the above path
   segments are affected in the event of failure of the primary nexthop
   node.  Sections Section 2.3.1 and Section 2.3.2 shows how this can be
   ensured.

2.3.1.  Computing Candidate Node-protecting PQ-Nodes for Primary
        nexthops

   To choose a node-protecting R-LFA nexthop for a destination R3,
   router S needs to consider a PQ-node from the candidate node-
   protecting PQ-space for the primary nexthop E on shortest path from S
   to R3.  As mentioned in Section 2.2.2, to consider a PQ-node as
   candidate node-protecting PQ-node, there must be at least one direct
   neighbor Ni of S, such that all shortest paths from Ni to the PQ-node
   does not traverse primary nexthop node E.

   Implementations should run the inequality in Section 2.2.2 Figure 4
   for all direct neighbor, other than primary nexthop node E, to
   determine whether a node Y is a candidate node-protecting PQ-node.
   All of the metrics needed by this inequality would have been already
   collected from the forward SPFs rooted at each of direct neighbor S,
   computed as part of standard LFA [RFC5286] implementation.  With
   reference to the topology in Figure 2, Table 3 below shows how the
   above condition can be used to determine the candidate node-
   protecting PQ-space for S-E link (primary nexthop E)
   +------------+----------+----------+----------+---------+-----------+
   | Candidate  |  Direct  |  D_opt   |  D_opt   |  D_opt  | Condition |
   |  PQ-node   | Nbr (Ni) |  (Ni,Y)  |  (Ni,E)  |  (E,Y)  |    Met    |
   |    (Y)     |          |          |          |         |           |
   +------------+----------+----------+----------+---------+-----------+
   |     R2     |    N     | 2 (N,R2) | 1 (N,E)  |    2    |    Yes    |
   |            |          |          |          |  (E,R2) |           |
   |     R3     |    N     | 2 (N,R3) | 1 (N,E)  |    1    |     No    |
   |            |          |          |          |  (E,R3) |           |
   +------------+----------+----------+----------+---------+-----------+

    Table 3: Node-protection evaluation for R-LFA repair tunnel to PQ-
                                   node

   As seen in the above Table 3 , R3 does not meet the node-protecting
   extended-p-space inequality and so, while R2 is in candidate node-
   protecting PQ space, R3 is not.

   Some SPF implementations may also produce a list of links and nodes
   traversed on the shortest path(s) from a given root to others.  In
   such implementations, router S may have executed a forward SPF with
   each of its direct neighbors as the SPF root, executed as part of the
   standard LFA [RFC5286] computations.  So S may re-use the list of
   links and nodes collected from the same SPF computations, to decide
   whether a node Y is a candidate node-protecting PQ-node or not.  A
   node Y shall be considered as a node-protecting PQ-node, if and only
   if, there is at least one direct neighbor of S, other than the
   primary nexthop E, for which, the primary nexthop node E does not
   exist on the list of nodes traversed on any of the shortest path(s)
   from the direct neighbor to the PQ-node.  Table 4 below is an
   illustration of the mechanism with the topology in Figure 2.

   +-----------+-------------------+-----------------+-----------------+
   | Candidate | Repair Tunnel     | Link-Protection | Node-Protection |
   | PQ-node   | Path(Repairing    |                 |                 |
   |           | router to PQ-     |                 |                 |
   |           | node)             |                 |                 |
   +-----------+-------------------+-----------------+-----------------+
   | R2        | S->N->R1->R2      | Yes             | Yes             |
   | R2        | S->E->R3->R2      | No              | No              |
   | R3        | S->N->E->R3       | Yes             | No              |
   +-----------+-------------------+-----------------+-----------------+

          Table 4: Protection of Remote-LFA tunnel to the PQ-node

   As seen in the above Table 4 while R2 is candidate node-protecting
   Remote-LFA nexthop for R3 and D2, it is not so for E and D1, since
   the primary nexthop E is in the shortest path from R2 to E and D1.

2.3.2.  Computing node-protecting paths from PQ-nodes to destinations

   Once a computing router finds all the candidate node-protecting PQ-
   nodes for a given directly attached primary link, it shall follow the
   procedure as proposed in this section, to choose one or more node-
   protecting R-LFA paths, for destinations reachable through the same
   primary link in the primary SPF graph.

   To find a node-protecting R-LFA path for a given destination, the
   computing router needs to pick a subset of PQ-nodes from the
   candidate node-protecting PQ-space for the corresponding primary
   nexthop, such that all the path(s) from the PQ-node(s) to the given
   destination remain unaffected in the event of a node failure of the
   primary nexthop node.  To ensure this, the computing router will need
   to ensure that, the primary nexthop node should not be on any of the
   shortest paths from the PQ-node to the given destination.

   This document proposes an additional forward SPF computation for each
   of the PQ-nodes, to discover all shortest paths from the PQ-nodes to
   the destination.  The additional forward SPF computation for each PQ-
   node, shall help determine, if a given primary nexthop node is on the
   shortest paths from the PQ-node to the given destination or not.  To
   determine if a given candidate node-protecting PQ-node provides node-
   protecting alternate for a given destination, the primary nexthop
   node should not be on any of the shortest paths from the PQ-node to
   the given destination.  On running the forward SPF on a candidate
   node-protecting PQ-node the computing router shall run the inequality
   in Figure 6 below.  A PQ-node that does not qualify the condition for
   a given destination, does not guarantee node-protection for the path
   segment from the PQ-node to the given destination.

   D_opt(Y,D) < D_opt(Y,E) + Distance_opt(E,D)

   Where,
     D_opt(A,B) : Distance on most optimum path from A to B.
             D  : The destination node.
             E  : The primary nexthop on shortest path from S
                  to destination.
             Y  : The node-protecting PQ-node being evaluated

      Figure 6: Node-Protecting Condition for PQ-node to Destination

   All of the above metric costs except D_opt(Y, D), can be obtained
   with forward and reverse SPFs with E(the primary nexthop) as the
   root, run as part of the regular LFA and Remote-LFA implementation.
   The Distance_opt(Y, D) metric can only be determined by the
   additional forward SPF run with PQ-node Y as the root.  With
   reference to the topology in Figure 2, Table 5 below shows how the
   above condition can be used to determine node-protection with node-
   protecting PQ-node R2.

   +-------------+------------+---------+--------+---------+-----------+
   | Destination | Primary-NH |  D_opt  | D_opt  |  D_opt  | Condition |
   |     (D)     |    (E)     |  (Y, D) | (Y, E) |  (E, D) |    Met    |
   +-------------+------------+---------+--------+---------+-----------+
   |      R3     |     E      |    1    |   2    |    1    |    Yes    |
   |             |            | (R2,R3) | (R2,E) |  (E,R3) |           |
   |      E      |     E      |    2    |   2    | 0 (E,E) |     No    |
   |             |            |  (R2,E) | (R2,E) |         |           |
   |      D1     |     E      |    3    |   2    |    1    |     No    |
   |             |            | (R2,D1) | (R2,E) |  (E,D1) |           |
   |      D2     |     E      |    2    |   2    |    1    |    Yes    |
   |             |            | (R2,D2) | (R2,E) |  (E,D2) |           |
   +-------------+------------+---------+--------+---------+-----------+

    Table 5: Node-protection evaluation for R-LFA path segment between
                          PQ-node and destination

   As seen in the above example above, R2 does not meet the node-
   protecting inequality for destination E, and D1.  And so, once again,
   while R2 is a node-protecting Remote-LFA nexthop for R3 and D2, it is
   not so for E and D1.

   In SPF implementations that also produce a list of links and nodes
   traversed on the shortest path(s) from a given root to others, to
   determine whether a PQ-node provides node-protection for a given
   destination or not, the list of nodes computed from forward SPF run
   on the PQ-node, for the given destination, should be inspected.  In
   case the list contains the primary nexthop node, the PQ-node does not
   provide node-protection.  Else, the PQ-node guarantees node-
   protecting alternate for the given destination.  Below is an
   illustration of the mechanism with candidate node-protecting PQ-node
   R2 in the topology in Figure 2.

   +-------------+-----------------+-----------------+-----------------+
   | Destination | Shortest Path   | Link-Protection | Node-Protection |
   |             | (Repairing      |                 |                 |
   |             | router to PQ-   |                 |                 |
   |             | node)           |                 |                 |
   +-------------+-----------------+-----------------+-----------------+
   | R3          | R2->R3          | Yes             | Yes             |
   | E           | R2->R3->E       | Yes             | No              |
   | D1          | R2->R3->E->D1   | Yes             | No              |
   | D2          | R2->R3->D2      | Yes             | Yes             |
   +-------------+-----------------+-----------------+-----------------+

        Table 6: Protection of Remote-LFA path between PQ-node and
                                destination

   As seen in the above example while R2 is candidate node-protecting
   R-LFA nexthop for R3 and D2, it is not so for E and D1, since the
   primary nexthop E is in the shortest path from R2 to E and D1.

   The procedure described in this document helps no more than to
   determine whether a given Remote-LFA alternate provides node-
   protection for a given destination or not.  It does not find out any
   new Remote-LFA alternate nexthops, outside the ones already computed
   by standard Remote-LFA procedure.  However, in case of availability
   of more than one PQ-node (Remote-LFA alternates) for a destination,
   and node-protection is required for the given primary nexthop, this
   procedure will eliminate the PQ-nodes that do not provide node-
   protection and choose only the ones that does.

2.3.3.  Limiting extra computational overhead

   In addition to the extra reverse SPF computation, one per directly
   connected neighbor, suggested by the Remote-LFA [RFC7490] draft, this
   document proposes a forward SPF per PQ-node discovered in the
   network.  Since the average number of PQ-nodes found in any network
   is considerably more than the number of direct neighbors of the
   computing router, the proposal of running one forward SPF per PQ-node
   may add considerably to the overall SPF computation time.

   To limit the computational overhead of the approach proposed, this
   document proposes that implementations MUST choose a subset from the
   entire set of PQ-nodes computed in the network, with a finite limit
   on the number of PQ-nodes in the subset.  Implementations MUST choose
   a default value for this limit and may provide user with a
   configuration knob to override the default limit.  Implementations
   MUST also evaluate some default preference criteria while considering
   a PQ-node in this subset.  Finally, implementations MAY also allow
   user to override the default preference criteria, by providing a
   policy configuration for the same.

   This document proposes that implementations SHOULD use a default
   preference criteria for PQ-node selection which will put a score on
   each PQ-node, proportional to the number of primary interfaces for
   which it provides coverage, its distance from the computing router,
   and its router-id (or system-id in case of IS-IS).  PQ-nodes that
   cover more primary interfaces SHOULD be preferred over PQ-nodes that
   cover fewer primary interfaces.  When two or more PQ-nodes cover the
   same number of primary interfaces, PQ-nodes which are closer (based
   on metric) to the computing router SHOULD be preferred over PQ-nodes
   farther away from it.  For PQ-nodes that cover the same number of
   primary interfaces and are the same distance from the the computing
   router, the PQ-node with smaller router-id (or system-id in case of
   IS-IS) SHOULD be preferred.

   Once a subset of PQ-nodes is found, computing router shall run a
   forward SPF on each of the PQ-nodes in the subset to continue with
   procedures proposed in section Section 2.3.2.

3.  Manageabilty of Remote-LFA Alternate Paths

3.1.  The Problem

   With the regular Remote-LFA [RFC7490] functionality the computing
   router may compute more than one PQ-node as usable Remote-LFA
   alternate nexthops.  Additionally an alternate selection policy may
   be configured to enable the network operator to choose one of them as
   the most appropriate Remote-LFA alternate.  For such policy-based
   alternate selection to run, all the relevant path characteristics for
   each the alternate paths (one through each of the PQ-nodes), needs to
   be collected.  As mentioned before in section Section 2.3 the R-LFA
   alternate path through a given PQ-node to a given destination is
   comprised of two path segments.

   The first path segment (i.e. from the computing router to the PQ-
   node) can be calculated from the regular forward SPF done as part of
   standard and remote LFA computations.  However without the mechanism
   proposed in section Section 2.3.2 of this document, there is no way
   to determine the path characteristics for the second path segment
   (i.e from the PQ-node to the destination).  In the absence of the
   path characteristics for the second path segment, two Remote-LFA
   alternate path may be equally preferred based on the first path
   segments characteristics only, although the second path segment
   attributes may be different.

3.2.  The Solution

   The additional forward SPF computation proposed in section
   Section 2.3.2 document shall also collect links, nodes and path
   characteristics along the second path segment.  This shall enable
   collection of complete path characteristics for a given Remote-LFA
   alternate path to a given destination.  The complete alternate path
   characteristics shall then facilitate more accurate alternate path
   selection while running the alternate selection policy.

   Like specified in Section 2.3.3 to limit the computational overhead
   of the approach proposed, forward SPF computations MUST be run on a
   selected subset from the entire set of PQ-nodes computed in the
   network, with a finite limit on the number of PQ-nodes in the subset.
   The detailed suggestion on how to select this subset is specified in
   the same section.  While this limits the number of possible alternate
   paths provided to the alternate-selection policy, this is needed keep
   the computational complexity within affordable limits.  However if
   the alternate-selection policy is very restrictive this may leave few
   destinations in the entire toplogy without protection.  Yet this
   limitation provides a necessary tradeoff between extensive coverage
   and immense computational overhead.

4.  Acknowledgements

   Many thanks to Bruno Decraene for providing his useful comments.  We
   would also like to thank Uma Chunduri for reviewing this document and
   providing valuable feedback.  Also, many thanks to Harish Raghuveer
   for his review and comments on the initial versions of this document.

5.  IANA Considerations

   N/A. - No protocol changes are proposed in this document.

6.  Security Considerations

   This document does not introduce any change in any of the protocol
   specifications.  It simply proposes to run an extra SPF rooted on
   each PQ-node discovered in the whole network.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

7.2.  Informative References

   [I-D.ietf-rtgwg-lfa-manageability]
              Litkowski, S., Decraene, B., Filsfils, C., Raza, K.,
              Horneffer, M., and P. Sarkar,
              M. Horneffer, "Operational management of Loop Free
              Alternates", draft-ietf-rtgwg-lfa-
              manageability-11 draft-ietf-rtgwg-lfa-manageability-11 (work
              in progress), June 2015.

   [RFC5286]  Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
              IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,
              <http://www.rfc-editor.org/info/rfc5286>.

   [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, DOI 10.17487/RFC7490, April 2015,
              <http://www.rfc-editor.org/info/rfc7490>.

Authors' Addresses

   Pushpasis Sarkar (editor)
   Juniper Networks, Inc.
   Electra, Exora Business Park
   Bangalore, KA  560103
   India
   Individual Contributor

   Email: pushpasis.ietf@gmail.com; psarkar@juniper.net pushpasis.ietf@gmail.com

   Shraddha Hegde
   Juniper Networks, Inc.
   Electra, Exora Business Park
   Bangalore, KA  560103
   India

   Email: shraddha@juniper.net

   Chris Bowers
   Juniper Networks, Inc.
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   US

   Email: cbowers@juniper.net

   Hannes Gredler
   Unaffiliated
   RtBrick Inc.

   Email: hannes@gredler.at hannes@rtbrick.com
   Stephane Litkowski
   Orange

   Email: stephane.litkowski@orange.com