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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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
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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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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
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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 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
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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 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.
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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).
+------------+----------+----------+----------+---------+-----------+
| 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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+-------------+---------------------------+------------+------------+
| 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
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qualify the condition for a given destination does not guarantee node
protection for the path segment from the PQ-node 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 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.
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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.
+-------------+---------------------------+------------+------------+
| 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
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required, the rest of the primary next hops may not guarantee node
protection. Figure 7 below shows one such example topology.
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 }
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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.
+--------+----------+-------+--------+--------+---------+-----------+
| 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.
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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.
+----------+----------+-------+--------+--------+--------+----------+
| 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.
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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.
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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