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This is a modified version of the Internet RFC suitable for machine-translating. Original version is available here: RFC8102


PROPOSED STANDARD

Internet Engineering Task Force (IETF)                    P. Sarkar, Ed.
Request for Comments: 8102                                  Arrcus, Inc.
Category: Standards Track                                       S. Hegde
ISSN: 2070-1721                                                C. Bowers
                                                  Juniper Networks, Inc.
                                                              H. Gredler
                                                           RtBrick, Inc.
                                                            S. Litkowski
                                                                  Orange
                                                              March 2017


              Remote-LFA Node Protection and Manageability

Abstract

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

This document describes an extension to the remote-loop-free-based IP fast reroute mechanisms that specifies procedures for determining whether or not a given PQ-node 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 operator-defined 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.rfc-editor.org/info/rfc8102.

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RFC 8102         R-LFA Node Protection and Manageability      March 2017
Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

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RFC 8102         R-LFA Node Protection and Manageability      March 2017
Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Node Protection with Remote-LFA . . . . . . . . . . . . . . . 5
2.1. The Problem . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Additional Definitions . . . . . . . . . . . . . . . . . 7
2.2.1. Link-Protecting Extended P-Space . . . . . . . . . . 7
2.2.2. Node-Protecting Extended P-Space . . . . . . . . . . 7
2.2.3. Q-Space . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.4. Link-Protecting PQ-Space . . . . . . . . . . . . . . 8
2.2.5. Candidate Node-Protecting PQ-Space . . . . . . . . . 8
2.2.6. Cost-Based Definitions . . . . . . . . . . . . . . . 8
2.2.6.1. Link-Protecting Extended P-Space . . . . . . . . 9
2.2.6.2. Node-Protecting Extended P-Space . . . . . . . . 9
2.2.6.3. Q-Space . . . . . . . . . . . . . . . . . . . . . 10
2.3. Computing Node-Protecting R-LFA Path . . . . . . . . . . 10
2.3.1. Computing Candidate Node-Protecting PQ-Nodes for
Primary Next Hops . . . . . . . . . . . . . . . . . . 10
2.3.2. Computing Node-Protecting Paths from PQ-Nodes to
Destinations . . . . . . . . . . . . . . . . . . . . 12
2.3.3. Computing Node-Protecting R-LFA Paths for
Destinations with Multiple Primary Next-Hop Nodes . . 14
2.3.4. Limiting Extra Computational Overhead . . . . . . . . 18
3. Manageability of Remote-LFA 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

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1. Introduction

The Remote-LFA specification [RFC7490] provides loop-free alternates that guarantee only link protection. The resulting remote-LFA alternate next hops (also referred to as the "PQ-nodes") may not provide node protection for all destinations covered by the same remote-LFA alternate, in case of failure of the primary next-hop 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 remote-LFA), collect the complete set of path characteristics for each alternate path, run an alternate-selection policy (configured by the operator), and find the best alternate path. This will require that the remote-LFA implementation gathers 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 with the computing router and its immediate one-hop routers as the roots. While that enables computation of path attributes (e.g., Shared Risk Link Group (SRLG) and Admin-groups) for the 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 the 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 the collection of a complete set of path attributes, enabling more accurate policy-based selection for alternate paths obtained with remote-LFA.
1.1.  Abbreviations

This document uses the following list of abbreviations:

LFA: Loop-Free Alternates

RLFA or R-LFA: Remote Loop-Free Alternates

ECMP: Equal-Cost Multiple Path

SPF: Shortest Path First graph computations

NH: Next-Hop node

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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 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 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 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 document [RFC7490] 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 the next hop for all of the above destinations. Table 1 shows all possible primary and remote-LFA alternate paths for each destination.

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    +-------------+--------------+---------+-------------------------+
    | 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 next hop E, the alternate path from the remote-LFA 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 PQ-node 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
/
S-x-E
                                      /   / \
                                     N---+   R3--D2
                                      \     /
                                      R1---R2

Figure 2: Topology 2

In the above topology, the S-E 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 P-space and the Q-space of E (with respect to the S-E link). Table 2 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 remain the same as in Table 1.

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     +-------------+--------------+---------+------------------------+
     | 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 next hop, it no longer provides node protection for R3 and D2. If S chooses R3 as the R-LFA next hop and if there is a node-failure 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 R-LFA next hop R3. So, for a remote-LFA next hop to provide node protection for a given destination, the shortest paths from S to the chosen PQ-node MUST NOT traverse the primary next-hop node.
2.2.  Additional Definitions

This document adds and enhances the following definitions, extending the ones mentioned in the Remote-LFA specification [RFC7490].
2.2.1.  Link-Protecting Extended P-Space

The Remote-LFA specification [RFC7490] 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 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 (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 next-hop 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.



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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 document [RFC7490] already defines this. The Q-space for a link S-E being protected is the set of nodes that can reach primary node E, without traversing the S-E 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 (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 a link-protecting PQ-space with respect 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 a candidate node-protecting PQ-space with respect to the node (E) being protected if and only if Y is present in both the node-protecting extended P-space and the Q-space for the link being protected.

Please note that a node Y being in a 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 next-hop 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.

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2.2.6.1.  Link-Protecting Extended P-Space

Please refer to Section 2.2.1 for a formal definition of link- protecting extended P-space.

A node Y is in a link-protecting extended P-space with respect to the link (S-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,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 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 of node- protecting extended P-space.

A node Y is in a node-protecting extended P-space 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 node-protecting
extended P-Space.

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

Please note 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.


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2.2.6.3.  Q-Space

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

A node Y is in Q-space with respect 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 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 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 next hop), and

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

So, to ensure that an 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 next-hop node. Sections 2.3.1 and 2.3.2 show how this can be ensured.

2.3.1. Computing Candidate Node-Protecting PQ-Nodes for Primary Next Hops

To choose a node-protecting R-LFA next hop for a destination R3, router S needs to consider a PQ-node from the candidate node- protecting PQ-space for the primary next hop E on the shortest path from S to R3. As mentioned in Section 2.2.2, to consider a PQ-node as a 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 do not traverse primary next-hop node E. Implementations SHOULD run the inequality in Section 2.2.6.2, Figure 4 for all direct neighbors, other than primary next-hop 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 shows how the above condition can be used to determine the candidate node-protecting PQ-space for S-E link (primary next hop E).




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   +------------+----------+----------+----------+---------+-----------+
   | 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; 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 computations [RFC5286]. So, S may re-use the list of links and nodes collected from the same SPF computations to decide whether or not a node Y is a candidate node-protecting PQ-node. 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 next hop E for which the primary next-hop node E does not exist on the list of nodes traversed on any of the shortest paths from the direct neighbor to the PQ-node. Table 4 is an illustration of the mechanism with the topology in Figure 2.

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RFC 8102         R-LFA Node Protection and Manageability      March 2017
   +-------------+---------------------------+------------+------------+
   | Candidate   | Repair Tunnel Path        | Link       | Node       |
   | PQ-node     | (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 Remote-LFA Tunnel to the PQ-Node

As seen in the above Table 4, while R2 is a candidate node-protecting remote-LFA 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 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 next hop, 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 next-hop node. To determine whether a given PQ-node belongs to such a subset of PQ-nodes, the computing router MUST ensure that none of the primary next-hop nodes are found 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. This will help determine whether or not a given primary next-hop node is on the shortest paths from the PQ-node to the given destination. To determine whether or not a given candidate node-protecting PQ-node provides node-protecting alternate for a given destination, all the shortest paths from the PQ-node 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 node-protecting PQ-node to one or more destinations, the computing router MUST run the forward SPF on the candidate node-protecting PQ-node. Soon after running the forward SPF, the computer router SHOULD run the inequality in Figure 6 below, once for each destination. 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 specific destination.


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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 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 next hop) 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 shows that the above condition can be used to determine node protection with a 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 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 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 PQ-node provides node protection for a given destination, the list of nodes computed from forward SPF that run on the PQ-node for the given destination SHOULD be inspected. In case the list contains the primary next-hop node, the PQ-node does not provide node protection. Else, the PQ-node guarantees the 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.

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   +-------------+---------------------------+------------+------------+
   | Destination | Shortest Path (Repairing  | Link       | Node       |
   |             | router to PQ-node)        | 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 Remote-LFA Path between PQ-node and
Destination

As seen in the above example, while R2 is a candidate node-protecting R-LFA 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 remote-LFA alternate provides node protection for a given destination. It does not find out any new remote-LFA alternate next hops, outside the ones already computed by the standard remote-LFA procedure. However, in the case of availability of more than one PQ-node (remote-LFA alternates) for a destination where node protection is required for the given primary next hop, this procedure will eliminate the PQ-nodes that do not provide node protection and choose only the ones that do.

2.3.3. Computing Node-Protecting R-LFA Paths for Destinations with Multiple Primary Next-Hop Nodes

In certain scenarios, when one or more destinations may be reachable via multiple ECMP (equal-cost-multi-path) next-hop 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 next-hop 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.


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D1
                              2    /
                          S---x---E1
                         / \     / \
/ x / \
                       /     \ /     \
                      N-------E2      R3--D2
                       \  2          /
                        \           /
                         \         /
                         R1-------R2
                              2

Primary Next hops:
Destination D1 = [{ S-E1, E1}, {S-E2, E2}] Destination D2 = [{ S-E1, E1}, {S-E2, 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: S-E1, Cost: 2

Link: N-E2: Cost: 2

Link: R1-R2: Cost: 2

In the above topology, on computing router S, destinations D1 and D2 are reachable via two ECMP next-hop nodes E1 and E2. However, the primary paths via next-hop node E2 also traverse via the next-hop node E1. So, in the event of node failure of next-hop 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 next-hop 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 R-LFA node-protecting alternate paths for destinations D1 and D2, needs to be computed.

In the above topology, the link-protecting PQ-nodes are as follows:

Primary Next Hop: E1, Link-Protecting PQ-Node: { R2 }

Primary Next Hop: E2, Link-Protecting PQ-Node: { R2 } To find one (or more) node-protecting R-LFA paths for destinations D1 and D2, one (or more) node-protecting PQ-node(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 node-protecting PQ-space for each of the next-hop nodes E1 and E2, as shown in Table 7 below. To select a PQ-node as a node-protecting PQ-node for a destination with multiple primary next-hop nodes, the PQ-node MUST satisfy the inequality for all primary next-hop nodes. Any PQ-node that is NOT a node-protecting PQ-node for all the primary next-hop nodes MUST NOT be chosen as the node-protecting PQ-node for the destination.


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   +--------+----------+-------+--------+--------+---------+-----------+
   | 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 Node-Protected PQ-Nodes 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 tunnel-repair paths from the computing router to candidate PQ-node can be examined to ensure that none of the primary next-hop nodes are traversed. PQ-nodes that provide one or more Tunnel-repair paths that do not traverse any of the primary next-hop nodes are to be considered as node-protecting PQ-nodes. Table 8 below shows the possible tunnel-repair paths to PQ-node R2.
   +--------------+------------+-------------------+-------------------+
   |  Primary-NH  |  PQ-Node   |   Tunnel-Repair   |    Exclude All    |
   |     (E)      |    (Y)     |       Paths       |     Primary-NH    |
   +--------------+------------+-------------------+-------------------+
   |    E1, E2    |     R2     |  S==>N==>R1==>R2  |        Yes        |
   +--------------+------------+-------------------+-------------------+

Table 8: Tunnel-Repair Paths to PQ-Node R2

From Tables 7 and 8 in the example above, R2 is a node-protecting PQ- node for both primary next hops E1 and E2 and should be chosen as the node-protecting PQ-node for destinations D1 and D2 that are both reachable via the primary next-hop nodes E1 and E2. Next, to find a node-protecting R-LFA path from a node-protecting PQ- node to destinations D1 and D2, inequalities specified in Figure 6 should be evaluated to ensure that R2 provides a node-protecting R-LFA path for each of these destinations, as shown below in Table 9. For an R-LFA path to qualify as a node-protecting R-LFA path for a destination with multiple ECMP primary next-hop nodes, the R-LFA path from the PQ-node to the destination MUST satisfy the inequality for all primary next-hop nodes.



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   +----------+----------+-------+--------+--------+--------+----------+
   | 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 Node-Protecting R-LFA 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 R-LFA paths via a node-protecting PQ-node to the final destination can be examined to ensure that none of the primary next-hop nodes are traversed. One or more R-LFA paths that do not traverse any of the primary next-hop nodes guarantees node protection in the event of failure of any of the primary next-hop nodes. Table 10 shows the possible R-LFA-paths for destinations D1 and D2 via the node- protecting PQ-node R2.

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   +-------------+------------+---------+-----------------+------------+
   | Destination | Primary-NH | PQ-Node |   R-LFA Paths   |  Exclude   |
   |     (D)     |    (E)     |   (Y)   |                 |    All     |
   |             |            |         |                 | Primary-NH |
   +-------------+------------+---------+-----------------+------------+
   |      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: R-LFA Paths for Destinations D1 and D2

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

In addition to the extra reverse SPF computations suggested by the Remote-LFA document [RFC7490] (one reverse SPF for each of the directly connected neighbors), this document proposes a forward SPF computation for each 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 specifies 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 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 PQ-node 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 PQ-node selection that 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 that 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 computing router, the PQ-node with smaller router-id (or system-id in case of IS-IS) SHOULD be preferred.




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


3. Manageability 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 next hops. Additionally, [RFC7916] specifies an LFA (and a remote-LFA) manageability framework, in which an alternate selection policy may be configured to let the network operator choose one of them as the most appropriate remote-LFA alternates. For such a policy-based 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 PQ-nodes). As mentioned before in Section 2.3, the R-LFA alternate path through a given PQ-node 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 PQ-node to the destination). In the absence of the path characteristics for the second path segment, two remote-LFA alternate paths may be equally preferred based on the first path segment characteristics only, although the second path segment attributes may be different.


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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 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.

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 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 to keep the computational complexity within affordable limits. However, if the alternate-selection 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 PQ-node discovered in the whole network.

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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.rfc-editor.org/info/rfc2119>.

[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>.
6.2.  Informative References

[RFC7916] Litkowski, S., Ed., Decraene, B., Filsfils, C., Raza, K.,
Horneffer, M., and P. Sarkar, "Operational Management of Loop-Free Alternates", RFC 7916, DOI 10.17487/RFC7916, July 2016, <http://www.rfc-editor.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.

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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 Orange

Email: stephane.litkowski@orange.com















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