Request for Comments: 8102
Category: Standards Track
ISSN: 20701721
Arrcus, Inc.
S. Hegde
C. Bowers
Juniper Networks, Inc.
H. Gredler
RtBrick, Inc.
S. Litkowski
Orange
March 2017
RemoteLFA Node Protection and Manageability
Abstract

The loopfree alternates (LFAs) computed following the current remoteLFA specification guarantees only link protection. The resulting remoteLFA next hops (also called "PQnodes") may not guarantee node protection for all destinations being protected by it.
This document describes an extension to the remoteloopfreebased IP fast reroute mechanisms that specifies procedures for determining whether or not a given PQnode provides node protection for a specific destination. The document also shows how the same procedure can be utilized for the collection of complete characteristics for alternate paths. Knowledge about the characteristics of all alternate paths is a precursor to applying the operatordefined policy for eliminating paths not fitting the constraints.
Status of This Memo

This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfceditor.org/info/rfc8102.
Copyright Notice

Copyright © 2017 IETF Trust and the persons identified as the document authors. All rights reserved.
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Requirements Language . . . . . . . . . . . . . . . . . . 5 2. Node Protection with RemoteLFA . . . . . . . . . . . . . . . 5 2.1. The Problem . . . . . . . . . . . . . . . . . . . . . . . 5 2.2. Additional Definitions . . . . . . . . . . . . . . . . . 7 2.2.1. LinkProtecting Extended PSpace . . . . . . . . . . 7 2.2.2. NodeProtecting Extended PSpace . . . . . . . . . . 7 2.2.3. QSpace . . . . . . . . . . . . . . . . . . . . . . . 8 2.2.4. LinkProtecting PQSpace . . . . . . . . . . . . . . 8 2.2.5. Candidate NodeProtecting PQSpace . . . . . . . . . 8 2.2.6. CostBased Definitions . . . . . . . . . . . . . . . 8 2.2.6.1. LinkProtecting Extended PSpace . . . . . . . . 9 2.2.6.2. NodeProtecting Extended PSpace . . . . . . . . 9 2.2.6.3. QSpace . . . . . . . . . . . . . . . . . . . . . 10 2.3. Computing NodeProtecting RLFA Path . . . . . . . . . . 10 2.3.1. Computing Candidate NodeProtecting PQNodes for Primary Next Hops . . . . . . . . . . . . . . . . . . 10 2.3.2. Computing NodeProtecting Paths from PQNodes to Destinations . . . . . . . . . . . . . . . . . . . . 12 2.3.3. Computing NodeProtecting RLFA Paths for Destinations with Multiple Primary NextHop Nodes . . 14 2.3.4. Limiting Extra Computational Overhead . . . . . . . . 18 3. Manageability of RemoteLFA Alternate Paths . . . . . . . . . 19 3.1. The Problem . . . . . . . . . . . . . . . . . . . . . . . 19 3.2. The Solution . . . . . . . . . . . . . . . . . . . . . . 20 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 5. Security Considerations . . . . . . . . . . . . . . . . . . . 20 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 21 6.1. Normative References . . . . . . . . . . . . . . . . . . 21 6.2. Informative References . . . . . . . . . . . . . . . . . 21 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
1. Introduction

The RemoteLFA specification [RFC7490] provides loopfree alternates that guarantee only link protection. The resulting remoteLFA alternate next hops (also referred to as the "PQnodes") may not provide node protection for all destinations covered by the same remoteLFA alternate, in case of failure of the primary nexthop node, and it does not provide a means to determine the same.
Also, the LFA Manageability document [RFC7916] requires a computing router to find all possible alternate next hops (including all possible remoteLFA), collect the complete set of path characteristics for each alternate path, run an alternateselection policy (configured by the operator), and find the best alternate path. This will require that the remoteLFA implementation gathers all the required path characteristics along each link on the entire remoteLFA alternate path.
With current LFA [RFC5286] and remoteLFA implementations, the forward SPF (and reverse SPF) is run with the computing router and its immediate onehop routers as the roots. While that enables computation of path attributes (e.g., Shared Risk Link Group (SRLG) and Admingroups) for the first alternate path segment from the computing router to the PQnode, there is no means for the computing router to gather any path attributes for the path segment from the PQnode to the destination. Consequently, any policybased selection of alternate paths will consider only the path attributes from the computing router up until the PQnode.
This document describes a procedure for determining node protection with remoteLFA. The same procedure is also extended for the collection of a complete set of path attributes, enabling more accurate policybased selection for alternate paths obtained with remoteLFA.
1.1. Abbreviations

This document uses the following list of abbreviations:

LFA:
LoopFree Alternates RLFA or RLFA: Remote LoopFree Alternates

ECMP:
EqualCost Multiple Path

SPF:

Shortest Path First graph computations
NH:
NextHop node

1.2. Requirements Language

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

Node protection is required to provide protection of traffic on a given forwarding node against the failure of the firsthop node on the primary forwarding path. Such protection becomes more critical in the absence of mechanisms like nonstop routing in the network. Certain operators refrain from deploying nonstoprouting in their network, due to the required complex state synchronization between redundant control plane hardwares it requires, and the significant additional computation and performance overheads it comes along with. In such cases, node protection is essential to guarantee uninterrupted flow of traffic, even in the case of an entire forwarding node going down.
The following sections discuss the nodeprotection problem in the context of remoteLFA 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 remoteLFA document [RFC7490] is being reused with slight modification.
D1 / SxE / \ N R3D2 \ / R1R2
Figure 1: Topology 1

In the above topology, for all (nonECMP) destinations reachable via the SE link, there is no standard LFA alternate. As per the remote LFA [RFC7490] alternate specifications, node R2 being the only PQ node for the SE link provides the next hop for all of the above destinations. Table 1 shows all possible primary and remoteLFA alternate paths for each destination.
+++++  Destination  Primary Path  PQnode  RemoteLFA 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: RemoteLFA Backup Paths via PQNode R2

A closer look at Table 1 shows that, while the PQnode 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 next hop E, the alternate path from the remoteLFA next hop R2 to E and D1 also becomes unavailable. So, for a remote LFA next hop to provide node protection for a given destination, the shortest path from the given PQnode to the given destination MUST NOT traverse the primary next hop.
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 / SxE / / \ N+ R3D2 \ / R1R2
Figure 2: Topology 2

In the above topology, the SE link is no longer 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 Pspace and the Qspace of E (with respect to the SE link). Table 2 shows all possible primary and RLFA alternate paths via PQnode R3 for each destination reachable through the SE link in the above topology. The RLFA alternate paths via PQnode R2 remain the same as in Table 1.
+++++  Destination  Primary Path  PQnode  RemoteLFA 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: RemoteLFA Backup Paths via PQNode R3

Again, a closer look at Table 2 shows that, unlike Table 1 where the single PQnode R2 provided node protection for destinations R3 and D2, if we choose R3 as the RLFA next hop, it no longer provides node protection for R3 and D2. If S chooses R3 as the RLFA next hop and if there is a nodefailure on primary next hop E, then one of the parallel ECMP paths between N and R3 also becomes unavailable on the alternate path from S to RLFA next hop R3. So, for a remoteLFA next hop to provide node protection for a given destination, the shortest paths from S to the chosen PQnode MUST NOT traverse the primary nexthop node.
2.2. Additional Definitions

This document adds and enhances the following definitions, extending the ones mentioned in the RemoteLFA specification [RFC7490].
2.2.1. LinkProtecting Extended PSpace

The RemoteLFA specification [RFC7490] already defines this. The linkprotecting extended Pspace for a link SE 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 SE link on any of the shortest paths 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 (SE) being protected.
For a costbased definition for linkprotecting extended Pspace, refer to Section 2.2.6.1.
2.2.2. NodeProtecting Extended PSpace

The nodeprotecting extended Pspace 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 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 costbased definition for nodeprotecting extended Pspace, refer to Section 2.2.6.2.
2.2.3. QSpace

The RemoteLFA document [RFC7490] already defines this. The Qspace for a link SE being protected is the set of nodes that can reach primary node E, without traversing the SE link on any of the shortest paths from the node itself to primary next hop E. This MUST exclude any node for which there is at least one ECMP path from the node to the primary next hop E traversing the link (SE) being protected.
For a costbased definition for QSpace, refer to Section 2.2.6.3.
2.2.4. LinkProtecting PQSpace

A node Y is in a linkprotecting PQspace with respect to the link (SE) being protected if and only if Y is present in both link protecting extended Pspace and the Qspace for the link being protected.
2.2.5. Candidate NodeProtecting PQSpace

A node Y is in a candidate nodeprotecting PQspace with respect to the node (E) being protected if and only if Y is present in both the nodeprotecting extended Pspace and the Qspace for the link being protected.
Please note that a node Y being in a candidate nodeprotecting PQ space does not guarantee that the RLFA 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 PQnode Y is unaffected by the same failure event. The PQnodes in the candidate nodeprotecting PQspace may provide node protection for only a subset of destinations that are reachable through the corresponding primary link.
2.2.6. CostBased Definitions

This section provides costbased definitions for some of the terms introduced in Section 2.2 of this document.
2.2.6.1. LinkProtecting Extended PSpace

Please refer to Section 2.2.1 for a formal definition of link protecting extended Pspace.
A node Y is in a linkprotecting extended Pspace with respect to the link (SE) being protected if and only if there exists at least one direct neighbor of S (Ni) other than primary next hop 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 the most optimum path from A to B.
Ni : A direct neighbor of S other than primary next hop E. Y : The node being evaluated for linkprotecting extended PSpace.
Figure 3: LinkProtecting ExtPSpace Condition
2.2.6.2. NodeProtecting Extended PSpace

Please refer to Section 2.2.2 for a formal definition of node protecting extended Pspace.
A node Y is in a nodeprotecting extended Pspace with respect to the node E being protected if and only if there exists at least one direct neighbor of S (Ni) other than primary next hop 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 the most optimum path from A to B.
E : The primary next hop on the shortest path from S to destination. Ni : A direct neighbor of S other than primary next hop E. Y : The node being evaluated for nodeprotecting extended PSpace.
Figure 4: NodeProtecting ExtPSpace Condition

Please note that a node Y satisfying the condition in Figure 4 above only guarantees that the RLFA 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. QSpace

Please refer to Section 2.2.3 for a formal definition of QSpace.
A node Y is in Qspace with respect to the link (SE) 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 the most optimum path from A to B.
E : The primary next hop on the shortest path from S to destination. Y : The node being evaluated for QSpace.
Figure 5: QSpace Condition
2.3. Computing NodeProtecting RLFA Path

The RLFA alternate path through a given PQnode to a given destination is comprised of two path segments as follows:
 Path segment from the computing router to the PQnode (RemoteLFA alternate next hop), and
 Path segment from the PQnode to the destination being protected.
So, to ensure that an RLFA 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 2.3.1 and 2.3.2 show how this can be ensured.
2.3.1. Computing Candidate NodeProtecting PQNodes for Primary Next
Hops

To choose a nodeprotecting RLFA next hop for a destination R3, router S needs to consider a PQnode from the candidate node protecting PQspace for the primary next hop E on the shortest path from S to R3. As mentioned in Section 2.2.2, to consider a PQnode as a candidate nodeprotecting PQnode, there must be at least one direct neighbor Ni of S, such that all shortest paths from Ni to the PQnode do not traverse primary nexthop node E.
Implementations SHOULD run the inequality in Section 2.2.6.2, Figure 4 for all direct neighbors, other than primary nexthop node E, to determine whether a node Y is a candidate nodeprotecting 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 shows how the above condition can be used to determine the candidate nodeprotecting PQspace for SE link (primary next hop E).
+++++++  Candidate  Direct  D_opt  D_opt  D_opt  Condition   PQnode  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: NodeProtection Evaluation for RLFA Repair Tunnel to PQ
Node

As seen in the above Table 3, R3 does not meet the nodeprotecting extended pspace inequality; so, while R2 is in candidate node protecting PQspace, 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 computations [RFC5286]. So, S may reuse the list of links and nodes collected from the same SPF computations to decide whether or not a node Y is a candidate nodeprotecting PQnode. A node Y shall be considered as a nodeprotecting PQnode if and only if there is at least one direct neighbor of S, other than the primary next hop E for which the primary nexthop node E does not exist on the list of nodes traversed on any of the shortest paths from the direct neighbor to the PQnode. Table 4 is an illustration of the mechanism with the topology in Figure 2.
+++++  Candidate  Repair Tunnel Path  Link  Node   PQnode  (Repairing router to PQ  Protection  Protection    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 RemoteLFA Tunnel to the PQNode

As seen in the above Table 4, while R2 is a candidate nodeprotecting remoteLFA next hop for R3 and D2, it is not so for E and D1, since the primary next hop E is on the shortest path from R2 to E and D1.
2.3.2. Computing NodeProtecting Paths from PQNodes to Destinations

Once a computing router finds all the candidate nodeprotecting 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 RLFA paths for destinations reachable through the same primary link in the primary SPF graph.
To find a nodeprotecting RLFA path for a given destination, the computing router needs to pick a subset of PQnodes from the candidate nodeprotecting PQspace for the corresponding primary next hop, such that all the path(s) from the PQnode(s) to the given destination remain unaffected in the event of a node failure of the primary nexthop node. To determine whether a given PQnode belongs to such a subset of PQnodes, the computing router MUST ensure that none of the primary nexthop nodes are found on any of the shortest paths from the PQnode to the given destination.
This document proposes an additional forward SPF computation for each of the PQnodes to discover all shortest paths from the PQnodes to the destination. This will help determine whether or not a given primary nexthop node is on the shortest paths from the PQnode to the given destination. To determine whether or not a given candidate nodeprotecting PQnode provides nodeprotecting alternate for a given destination, all the shortest paths from the PQnode to the given destination have to be inspected to check if the primary next hop node is found on any of these shortest paths. To compute all the shortest paths from a candidate nodeprotecting PQnode to one or more destinations, the computing router MUST run the forward SPF on the candidate nodeprotecting PQnode. Soon after running the forward SPF, the computer router SHOULD run the inequality in Figure 6 below, once for each destination. A PQnode that does not qualify the condition for a given destination does not guarantee node protection for the path segment from the PQnode to the specific destination.
D_opt(Y,D) < D_opt(Y,E) + Distance_opt(E,D)
Where,

D_opt(A,B) : Distance on the most optimum path from A to B.
D : The destination node. E : The primary next hop on the shortest path from S to destination. Y : The nodeprotecting PQnode being evaluated

Figure 6: NodeProtecting Condition for PQNode 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 next hop) as the root, run as part of the regular LFA and remoteLFA implementation. The Distance_opt(Y, D) metric can only be determined by the additional forward SPF run with PQnode Y as the root. With reference to the topology in Figure 2, Table 5 shows that the above condition can be used to determine node protection with a node protecting PQnode R2.
+++++++  Destination  PrimaryNH  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: NodeProtection Evaluation for RLFA Path Segment between
PQNode and Destination

As seen in the example above, R2 does not meet the nodeprotecting inequality for destination E and D1. And so, once again, while R2 is a nodeprotecting remoteLFA next hop 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, the inequality in Figure 6 above need not be evaluated. Instead, to determine whether or not a PQnode provides node protection for a given destination, the list of nodes computed from forward SPF that run on the PQnode for the given destination SHOULD be inspected. In case the list contains the primary nexthop node, the PQnode does not provide node protection. Else, the PQnode guarantees the node protecting alternate for the given destination. Below is an illustration of the mechanism with candidate nodeprotecting PQnode R2 in the topology in Figure 2.
+++++  Destination  Shortest Path (Repairing  Link  Node    router to PQnode)  Protection  Protection  +++++  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 RemoteLFA Path between PQnode and
Destination

As seen in the above example, while R2 is a candidate nodeprotecting RLFA next hop for R3 and D2, it is not so for E and D1, since the primary next hop E is on the shortest path from R2 to E and D1.
The procedure described in this document helps no more than to determine whether or not a given remoteLFA alternate provides node protection for a given destination. It does not find out any new remoteLFA alternate next hops, outside the ones already computed by the standard remoteLFA procedure. However, in the case of availability of more than one PQnode (remoteLFA alternates) for a destination where node protection is required for the given primary next hop, this procedure will eliminate the PQnodes that do not provide node protection and choose only the ones that do.
2.3.3. Computing NodeProtecting RLFA Paths for Destinations with
Multiple Primary NextHop Nodes

In certain scenarios, when one or more destinations may be reachable via multiple ECMP (equalcostmultipath) nexthop nodes and only link protection is required, there is no need to compute any alternate paths for such destinations. In the event of failure of one of the nexthop links, the remaining primary next hops shall always provide link protection. However, if node protection is required, the rest of the primary next hops may not guarantee node protection. Figure 7 below shows one such example topology.
D1 2 / SxE1 / \ / \ / x / \ / \ / \ NE2 R3D2 \ 2 / \ / \ / R1R2 2
Primary Next hops:
Destination D1 = [{ SE1, E1}, {SE2, E2}] Destination D2 = [{ SE1, E1}, {SE2, E2}]
Figure 7: Topology with Multiple ECMP Primary Next Hops

In the above example topology, costs of all links are 1, except the following links:
Link: SE1, Cost: 2 Link: NE2: Cost: 2 Link: R1R2: Cost: 2
In the above topology, on computing router S, destinations D1 and D2 are reachable via two ECMP nexthop nodes E1 and E2. However, the primary paths via nexthop node E2 also traverse via the nexthop node E1. So, in the event of node failure of nexthop node E1, both primary paths (via E1 and E2) become unavailable. Hence, if node protection is desired for destinations D1 and D2, alternate paths that do not traverse any of the primary nexthop nodes E1 and E2 need to be computed. In the above topology, the only alternate neighbor N does not provide such an LFA alternate path. Hence, one or more RLFA nodeprotecting alternate paths for destinations D1 and D2, needs to be computed.
In the above topology, the linkprotecting PQnodes are as follows:
Primary Next Hop: E1, LinkProtecting PQNode: { R2 } Primary Next Hop: E2, LinkProtecting PQNode: { R2 }
To find one (or more) nodeprotecting RLFA paths for destinations D1 and D2, one (or more) nodeprotecting PQnode(s) need to be determined first. Inequalities specified in Sections 2.2.6.2 and 2.2.6.3 can be evaluated to compute the nodeprotecting PQspace for each of the nexthop nodes E1 and E2, as shown in Table 7 below. To select a PQnode as a nodeprotecting PQnode for a destination with multiple primary nexthop nodes, the PQnode MUST satisfy the inequality for all primary nexthop nodes. Any PQnode that is NOT a nodeprotecting PQnode for all the primary nexthop nodes MUST NOT be chosen as the nodeprotecting PQnode for the destination.
++++++++  Primary Candidate Direct D_opt  D_opt  D_opt  Condition   Next  PQ  Nbr  (Ni,Y)  (Ni,E)  (E,Y)  Met   Hop  node (Y)  (Ni)       (E)        ++++++++  E1  R2  N  3  3  2  Yes      (N,R2)  (N,E1)  (E1,R2)    E2  R2  N  3  2  3  Yes      (N,R2)  (N,E2)  (E2,R2)   ++++++++

Table 7: Computing NodeProtected PQNodes for Next Hop E1 and E2
In SPF implementations that also produce a list of links and nodes traversed on the shortest path(s) from a given root to others, the tunnelrepair paths from the computing router to candidate PQnode can be examined to ensure that none of the primary nexthop nodes are traversed. PQnodes that provide one or more Tunnelrepair paths that do not traverse any of the primary nexthop nodes are to be considered as nodeprotecting PQnodes. Table 8 below shows the possible tunnelrepair paths to PQnode R2.
+++++  PrimaryNH  PQNode  TunnelRepair  Exclude All   (E)  (Y)  Paths  PrimaryNH  +++++  E1, E2  R2  S==>N==>R1==>R2  Yes  +++++

Table 8: TunnelRepair Paths to PQNode R2

From Tables 7 and 8 in the example above, R2 is a nodeprotecting PQ node for both primary next hops E1 and E2 and should be chosen as the nodeprotecting PQnode for destinations D1 and D2 that are both reachable via the primary nexthop nodes E1 and E2.
Next, to find a nodeprotecting RLFA path from a nodeprotecting PQ node to destinations D1 and D2, inequalities specified in Figure 6 should be evaluated to ensure that R2 provides a nodeprotecting RLFA path for each of these destinations, as shown below in Table 9. For an RLFA path to qualify as a nodeprotecting RLFA path for a destination with multiple ECMP primary nexthop nodes, the RLFA path from the PQnode to the destination MUST satisfy the inequality for all primary nexthop nodes.
++++++++  Destinat  Primary  PQ  D_opt  D_opt  D_opt  Condition  ion (D)  NH (E)  Node  (Y, D)  (Y, E)  (E, D)  Met     (Y)      ++++++++  D1  E1  R2  3 (R2,  2 (R2,  1 (E1,  No      D1)  E1)  D1)    D1  E2  R2  3 (R2,  3 (R2,  2 (E2,  Yes      D1)  E2)  D1)    D2  E1  R2  2 (R2,  2 (R2,  2 (E1,  Yes      D2)  E1)  D2)    D2  E2  R2  2 (R2,  2 (R2,  3 (E2,  Yes      D2)  E2)  D2)   ++++++++
Table 9: Finding NodeProtecting RLFA Path for
Destinations D1 and D2

In SPF implementations that also produce a list of links and nodes traversed on the shortest path(s) from a given root to others, the RLFA paths via a nodeprotecting PQnode to the final destination can be examined to ensure that none of the primary nexthop nodes are traversed. One or more RLFA paths that do not traverse any of the primary nexthop nodes guarantees node protection in the event of failure of any of the primary nexthop nodes. Table 10 shows the possible RLFApaths for destinations D1 and D2 via the node protecting PQnode R2.
++++++  Destination  PrimaryNH  PQNode  RLFA Paths  Exclude   (D)  (E)  (Y)   All       PrimaryNH  ++++++  D1  E1, E2  R2  S==>N==>R1==>R2  No      >R3>E1>D1          D2  E1, E2  R2  S==>N==>R1==>R2  Yes      >R3>D2   ++++++
Table 10: RLFA Paths for Destinations D1 and D2

From Tables 9 and 10 in the example above, the RLFA path from R2 does not meet the nodeprotecting inequality for destination D1, while it does meet the same inequality for destination D2. So, while R2 provides a nodeprotecting RLFA alternate for D2, it fails to provide node protection for destination D1. Finally, while it is possible to get a nodeprotecting RLFA path for D2, no such node protecting RLFA path can be found for D1.
2.3.4. Limiting Extra Computational Overhead

In addition to the extra reverse SPF computations suggested by the RemoteLFA document [RFC7490] (one reverse SPF for each of the directly connected neighbors), this document proposes a forward SPF computation for each PQnode discovered in the network. Since the average number of PQnodes 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 PQnode may add considerably to the overall SPF computation time.
To limit the computational overhead of the approach proposed, this document specifies that implementations MUST choose a subset from the entire set of PQnodes computed in the network, with a finite limit on the number of PQnodes in the subset. Implementations MUST choose a default value for this limit and may provide the user with a configuration knob to override the default limit. This document suggests 16 as a default value for this limit. Implementations MUST also evaluate some default preference criteria while considering a PQnode in this subset. The exact default preference criteria to be used is outside the scope of this document and is a matter of implementation. Finally, implementations MAY also allow the 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 PQnode selection that will put a score on each PQnode, proportional to the number of primary interfaces for which it provides coverage, its distance from the computing router, and its routerid (or systemid in case of ISIS). PQnodes that cover more primary interfaces SHOULD be preferred over PQnodes that cover fewer primary interfaces. When two or more PQnodes cover the same number of primary interfaces, PQnodes that are closer (based on metric) to the computing router SHOULD be preferred over PQnodes farther away from it. For PQnodes that cover the same number of primary interfaces and are the same distance from the computing router, the PQnode with smaller routerid (or systemid in case of ISIS) SHOULD be preferred.
Once a subset of PQnodes is found, a computing router shall run a forward SPF on each of the PQnodes in the subset to continue with procedures proposed in Section 2.3.2.
3. Manageability of RemoteLFA Alternate Paths
3.1. The Problem

With the regular remoteLFA [RFC7490] functionality, the computing router may compute more than one PQnode as usable remoteLFA alternate next hops. Additionally, [RFC7916] specifies an LFA (and a remoteLFA) manageability framework, in which an alternate selection policy may be configured to let the network operator choose one of them as the most appropriate remoteLFA alternates. For such a policybased alternate selection to run, the computing router needs to collect all the relevant path characteristics (as specified in Section 6.2.4 of [RFC7916]) for each of the alternate paths (one through each of the PQnodes). As mentioned before in Section 2.3, the RLFA alternate path through a given PQnode to a given destination is comprised of two path segments. Section 6.2.4 of [RFC7916] specifies that any kind of alternate selection policy must consider path characteristics for both path segments while evaluating one or more RLFA alternate paths.
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 2.3.2 of this document, there is no way to determine the path characteristics for the second path segment (i.e., from the PQnode to the destination). In the absence of the path characteristics for the second path segment, two remoteLFA alternate paths may be equally preferred based on the first path segment characteristics only, although the second path segment attributes may be different.
3.2. The Solution

The additional forward SPF computation proposed in Section 2.3.2 shall also collect links, nodes, and path characteristics along the second path segment. This shall enable the collection of complete path characteristics for a given remoteLFA 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.
As already specified in Section 2.3.4, to limit the computational overhead of the proposed approach, forward SPF computations must be run on a selected subset from the entire set of PQnodes computed in the network, with a finite limit on the number of PQnodes 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 alternateselection policy, this is needed to keep the computational complexity within affordable limits. However, if the alternateselection policy is very restrictive, this may leave few destinations in the entire topology without protection. Yet this limitation provides a necessary tradeoff between extensive coverage and immense computational overhead.
The mechanism proposed in this section does not modify or invalidate any part of [RFC7916]. This document specifies a mechanism to meet the requirements specified in Section 6.2.5.4 of [RFC7916].
4. IANA Considerations

This document does not require any IANA actions.
5. 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 PQnode discovered in the whole network.
6. References
6.1. 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.rfceditor.org/info/rfc2119>. [RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for IP Fast Reroute: LoopFree Alternates", RFC 5286, DOI 10.17487/RFC5286, September 2008, <http://www.rfceditor.org/info/rfc5286>. [RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N. So, "Remote LoopFree Alternate (LFA) Fast Reroute (FRR)", RFC 7490, DOI 10.17487/RFC7490, April 2015, <http://www.rfceditor.org/info/rfc7490>.
6.2. Informative References

[RFC7916] Litkowski, S., Ed., Decraene, B., Filsfils, C., Raza, K., Horneffer, M., and P. Sarkar, "Operational Management of LoopFree Alternates", RFC 7916, DOI 10.17487/RFC7916, July 2016, <http://www.rfceditor.org/info/rfc7916>.
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 draft versions of this document.
Authors' Addresses

Pushpasis Sarkar (editor)
Arrcus, Inc.Email: 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 United States of America
Email:
cbowers@juniper.net
Hannes Gredler
RtBrick, Inc.Email:
hannes@rtbrick.com
Stephane Litkowski
OrangeEmail: stephane.litkowski@orange.com