draft-ietf-rtgwg-remote-lfa-10.txt   draft-ietf-rtgwg-remote-lfa-11.txt 
Network Working Group S. Bryant Network Working Group S. Bryant
Internet-Draft C. Filsfils Internet-Draft C. Filsfils
Intended status: Standards Track S. Previdi Intended status: Standards Track S. Previdi
Expires: July 10, 2015 Cisco Systems Expires: August 3, 2015 Cisco Systems
M. Shand M. Shand
Independent Contributor Independent Contributor
N. So N. So
Vinci Systems Vinci Systems
January 6, 2015 January 30, 2015
Remote Loop-Free Alternate Fast Re-Route Remote Loop-Free Alternate (LFA) Fast Re-Route (FRR)
draft-ietf-rtgwg-remote-lfa-10 draft-ietf-rtgwg-remote-lfa-11
Abstract Abstract
This document describes an extension to the basic IP fast re-route This document describes an extension to the basic IP fast re-route
mechanism described in RFC5286, that provides additional backup mechanism described in RFC5286, that provides additional backup
connectivity for point to point link failures when none can be connectivity for point to point link failures when none can be
provided by the basic mechanisms. provided by the basic mechanisms.
Requirements Language Requirements Language
skipping to change at page 1, line 44 skipping to change at page 1, line 44
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 10, 2015. This Internet-Draft will expire on August 3, 2015.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Repair Paths . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Overview of Solution . . . . . . . . . . . . . . . . . . . . 4
3.1. Tunnels as Repair Paths . . . . . . . . . . . . . . . . . 6 4. Repair Paths . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Tunnel Requirements . . . . . . . . . . . . . . . . . . . 6 4.1. Tunnels as Repair Paths . . . . . . . . . . . . . . . . . 6
4. Construction of Repair Paths . . . . . . . . . . . . . . . . 7 4.2. Tunnel Requirements . . . . . . . . . . . . . . . . . . . 7
4.1. Identifying Required Tunneled Repair Paths . . . . . . . 7 5. Construction of Repair Paths . . . . . . . . . . . . . . . . 8
4.2. Determining Tunnel End Points . . . . . . . . . . . . . . 8 5.1. Identifying Required Tunneled Repair Paths . . . . . . . 8
4.2.1. Computing Repair Paths . . . . . . . . . . . . . . . 8 5.2. Determining Tunnel End Points . . . . . . . . . . . . . . 8
4.2.2. Selecting Repair Paths . . . . . . . . . . . . . . . 11 5.2.1. Computing Repair Paths . . . . . . . . . . . . . . . 9
4.3. A Cost Based RLFA Algorithm . . . . . . . . . . . . . . . 11 5.2.2. Selecting Repair Paths . . . . . . . . . . . . . . . 11
4.4. Interactions with IS-IS Overload, RFC6987, and Costed 5.3. A Cost Based RLFA Algorithm . . . . . . . . . . . . . . . 12
Out Links . . . . . . . . . . . . . . . . . . . . . . . . 16 5.4. Interactions with IS-IS Overload, RFC6987, and Costed
5. Example Application of Remote LFAs . . . . . . . . . . . . . 17 Out Links . . . . . . . . . . . . . . . . . . . . . . . . 17
6. Node Failures . . . . . . . . . . . . . . . . . . . . . . . . 18 6. Example Application of Remote LFAs . . . . . . . . . . . . . 18
7. Operation in an LDP environment . . . . . . . . . . . . . . . 19 7. Node Failures . . . . . . . . . . . . . . . . . . . . . . . . 18
8. Analysis of Real World Topologies . . . . . . . . . . . . . . 20 8. Operation in an LDP environment . . . . . . . . . . . . . . . 20
8.1. Topology Details . . . . . . . . . . . . . . . . . . . . 21 9. Analysis of Real World Topologies . . . . . . . . . . . . . . 21
8.2. LFA only . . . . . . . . . . . . . . . . . . . . . . . . 21 9.1. Topology Details . . . . . . . . . . . . . . . . . . . . 21
8.3. RLFA . . . . . . . . . . . . . . . . . . . . . . . . . . 22 9.2. LFA only . . . . . . . . . . . . . . . . . . . . . . . . 22
8.4. Comparison of LFA an RLFA results . . . . . . . . . . . . 23 9.3. RLFA . . . . . . . . . . . . . . . . . . . . . . . . . . 23
9. Management and Operational Considerations . . . . . . . . . . 24 9.4. Comparison of LFA an RLFA results . . . . . . . . . . . . 24
10. Historical Note . . . . . . . . . . . . . . . . . . . . . . . 25 10. Management and Operational Considerations . . . . . . . . . . 25
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 11. Historical Note . . . . . . . . . . . . . . . . . . . . . . . 26
12. Security Considerations . . . . . . . . . . . . . . . . . . . 25 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 25 13. Security Considerations . . . . . . . . . . . . . . . . . . . 26
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 26 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
14.1. Normative References . . . . . . . . . . . . . . . . . . 26 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
14.2. Informative References . . . . . . . . . . . . . . . . . 26 15.1. Normative References . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28 15.2. Informative References . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Terminology 1. Introduction
RFC 5714 [RFC5714] describes a framework for IP Fast Re-route (IPFRR)
and provides a summary of various proposed IPFRR solutions. A basic
mechanism using loop-free alternates (LFAs) is described in [RFC5286]
that provides good repair coverage in many topologies [RFC6571],
especially those that are highly meshed. However, some topologies,
notably ring based topologies are not well protected by LFAs alone
because there is no neighbor of the point of local repair (PLR) that
has a cost to the destination without traversing the failure that is
cheaper than the cost to the destination via the failure.
The method described in this document extends LFA approach described
in [RFC5286] to cover many of these cases by tunneling the packets
that require IPFRR to a node that is both reachable from the PLR and
can reach the destination.
2. Terminology
This document uses the terms defined in [RFC5714]. This section This document uses the terms defined in [RFC5714]. This section
defines additional terms that are used in this document. defines additional terms that are used in this document.
FIB Forwarding Information (data)Base. The database used
by a packet forwarder to determine the actions it
should take on a packet it is processing.
Repair tunnel A tunnel established for the purpose of providing a Repair tunnel A tunnel established for the purpose of providing a
virtual neighbor which is a Loop Free Alternate. virtual neighbor which is a Loop Free Alternate.
P-space P-space of a router with respect to a protected link P-space The P-space of a router with respect to a protected
is the set of routers reachable from that specific link is the set of routers reachable from that
router using the pre-convergence FIB, without any path specific router using the pre-convergence shortest
(including equal cost path splits) transiting that paths, without any of those paths (including equal
protected link. cost path splits) transiting that protected link.
For example, the P-space of S with respect to link For example, the P-space of S with respect to link
S-E, is the set of routers that S can reach without S-E, is the set of routers that S can reach without
using the protected link S-E. using the protected link S-E.
Extended P-space Extended P-space
Consider the set of neighbours of a router protecting Consider the set of neighbours of a router protecting
a link. Exclude from that set of routers the router a link. Exclude from that set of routers the router
reachable over the protected link. The extended reachable over the protected link. The extended
P-space of the protecting router with respect to the P-space of the protecting router with respect to the
protected link is the union of the P-spaces of the protected link is the union of the P-spaces of the
neighbours in that set of neighbours with respect to neighbours in that set of neighbours with respect to
the protected link (see Section 4.2.1.2). the protected link (see Section 5.2.1.2).
Q-space Q-space of a router with respect to a protected link Q-space Q-space of a router with respect to a protected link
is the set of routers from which that specific router is the set of routers from which that specific router
can be reached without any path (including equal cost can be reached without any path (including equal cost
path splits) transiting that protected link. path splits) transiting that protected link.
PQ node A PQ node of a node S with respect to a protected link PQ node A PQ node of a node S with respect to a protected link
S-E is a node which is a member of both the P-space S-E is a node which is a member of both the P-space
(or the extended P-space) of S with respect to that (or the extended P-space) of S with respect to that
protected link S-E and the Q-space of E with respect protected link S-E and the Q-space of E with respect
to that protected link S-E. A repair tunnel endpoint to that protected link S-E. A repair tunnel endpoint
is chosen from the set of PQ-nodes. is chosen from the set of PQ-nodes.
Remote LFA (RLFA) The use of a PQ node rather than a neighbour of Remote LFA (RLFA) The use of a PQ node rather than a neighbour of
the repairing node as the next hop in an LFA repair the repairing node as the next hop in an LFA repair
[RFC5286]. [RFC5286].
In this document we use the notation X-Y to mean the path from X to Y In this document the notation X-Y is used to mean the path from X to
over the link directly connecting X and Y, whilst the notation X->Y Y over the link directly connecting X and Y, whilst the notation X->Y
refers to the shortest path from X to Y via some set of unspecified refers to the shortest path from X to Y via some set of unspecified
nodes including the null set (i.e. Including over a link directly nodes including the null set (i.e. Including over a link directly
connecting X and Y). connecting X and Y).
2. Introduction 3. Overview of Solution
RFC 5714 [RFC5714] describes a framework for IP Fast Re-route (IPFRR) The problem of LFA IPFRR reachability in some networks is illustrated
and provides a summary of various proposed IPFRR solutions. A basic by the network fragment shown in Figure 1 below.
mechanism using loop-free alternates (LFAs) is described in [RFC5286]
that provides good repair coverage in many topologies [RFC6571],
especially those that are highly meshed. However, some topologies,
notably ring based topologies are not well protected by LFAs alone.
This is illustrated in Figure 1 below.
S---E S---E
/ \ / \
A D A D
\ / \ /
B---C B---C
Figure 1: A simple ring topology Figure 1: A simple ring topology
If all link costs are equal, traffic transiting link S-E cannot be If all link costs are equal, traffic transiting link S-E cannot be
fully protected by LFAs. The destination C is an ECMP from S, and so fully protected by LFAs. The destination C is an ECMP from S, and so
traffic to C can be protected when S-E fails, but traffic to D and E traffic to C can be protected when S-E fails, but traffic to D and E
are not protectable using LFAs. are not protectable using LFAs.
This document describes extensions to the basic repair mechanism in This document describes extensions to the basic repair mechanism in
which tunnels are used to provide additional logical links which can which tunnels are used to provide additional logical links which can
then be used as loop free alternates where none exist in the original then be used as loop free alternates where none exist in the original
topology. In Figure 1 S can reach A, B, and C without going via S-E; topology. In Figure 1 S can reach A, B, and C without going via S-E;
these form S's extended P-space with respect to S-E. The routers these form S's extended P-space with respect to S-E. The routers
that can reach E without going through S-E will be E's Q-space; these that can reach E without going through S-E will be in E's Q-space
are D and C. B has equal-cost paths via B-A-S-E and B-C-D-E and so with respect to link S-E; these are D and C. B has equal-cost paths
may go through S-E. The single node in both S's extended P-space and to E via B-A-S-E and B-C-D-E and so the forwarder at S might choose
E's Q-space is C; thus node C is selected as the repair tunnel's end- to send a packet to E via link S-E. Hence B is not in the Q-space of
point. Thus, if a tunnel is provided between S and C as shown in E with respect to link S-E. The single node in both S's extended
Figure 2 then C, now being a direct neighbor of S would become an LFA P-space and E's Q-space is C; thus node C is selected as the repair
for D and E. The definition of (extended-)P space and Q space are tunnel's end-point. Thus, if a tunnel is provided between S and C as
provided in Section 1 and details of the calculation of the tunnel shown in Figure 2 then C, now being a direct neighbor of S would
end points is provided in Section 4.2. become an LFA for D and E. The definition of (extended-)P space and
Q space are provided in Section 2 and details of the calculation of
the tunnel end points is provided in Section 5.2.
The non-failure traffic distribution is not disrupted by the The non-failure traffic distribution is not disrupted by the
provision of such a tunnel since it is only used for repair traffic provision of such a tunnel since it is only used for repair traffic
and MUST NOT be used for normal traffic. Note that OAM traffic and MUST NOT be used for normal traffic. Note that Operations and
specifically to verify the viability of the repair MAY traverse the Maintenance (OAM) traffic specifically to verify the viability of the
tunnel prior to a failure. repair MAY traverse the tunnel prior to a failure.
S---E S---E
/ \ \ / \ \
A \ D A \ D
\ \ / \ \ /
B---C B---C
Figure 2: The addition of a tunnel Figure 2: The addition of a tunnel
The use of this technique is not restricted to ring based topologies, The use of this technique is not restricted to ring based topologies,
but is a general mechanism which can be used to enhance the but is a general mechanism which can be used to enhance the
protection provided by LFAs. A study of the protection achieved protection provided by LFAs. A study of the protection achieved
using remote LFA in typical service provider core networks is using remote LFA in typical service provider core networks is
provided in Section 8, and a side by side comparison between LFA and provided in Section 9, and a side by side comparison between LFA and
remote LFA is provided in Section 8.4. remote LFA is provided in Section 9.4.
Remote LFA is suitable for incremental deployment within a network, Remote LFA is suitable for incremental deployment within a network,
including a network that is already deploying LFA. Computation of including a network that is already deploying LFA. Computation of
the repair path requires acceptable CPU resources, and takes place the repair path requires acceptable CPU resources, and takes place
exclusively on the repairing node. In MPLS networks the targeted LDP exclusively on the repairing node. In MPLS networks the targeted LDP
protocol needed to learn the label binding at the repair tunnel protocol needed to learn the label binding at the repair tunnel
endpoint is a well understood and widely deployed technology. endpoint Section 8 is a well understood and widely deployed
technology.
The technique described in this document is directed at providing The technique described in this document is directed at providing
repairs in the case of link failures. Considerations regarding node repairs in the case of link failures. Considerations regarding node
failures are discussed in Section 6. This memo describes a solution failures are discussed in Section 7. This memo describes a solution
to the case where the failure occurs on a point to point link. It to the case where the failure occurs on a point to point link. It
covers the case where the repair first hop is reached via a broadcast covers the case where the repair first hop is reached via a broadcast
or non-broadcast multi-access (NBMA) link such as a LAN, and the case or non-broadcast multi-access (NBMA) link such as a LAN, and the case
where the P or Q node is attached via such a link. It does not where the P or Q node is attached via such a link. It does not
however cover the more complicated case where the failed interface is however cover the more complicated case where the failed interface is
a broadcast or non-broadcast multi-access (NBMA) link. a broadcast or non-broadcast multi-access (NBMA) link.
This document considers the case when the repair path is confined to This document considers the case when the repair path is confined to
either a single area or to the level two routing domain. In all either a single area or to the level two routing domain. In all
other cases, the chosen PQ node should be regarded as a tunnel other cases, the chosen PQ node should be regarded as a tunnel
adjacency of the repairing node, and the considerations described in adjacency of the repairing node, and the considerations described in
Section 6 of [RFC5286] taken into account. Section 6 of [RFC5286] taken into account.
3. Repair Paths 4. Repair Paths
As with LFA FRR, when a router detects an adjacent link failure, it As with LFA FRR, when a router detects an adjacent link failure, it
uses one or more repair paths in place of the failed link. Repair uses one or more repair paths in place of the failed link. Repair
paths are pre-computed in anticipation of later failures so they can paths are pre-computed in anticipation of later failures so they can
be promptly activated when a failure is detected. be promptly activated when a failure is detected.
A tunneled repair path tunnels traffic to some staging point in the A tunneled repair path tunnels traffic to some staging point in the
network from which it is known that, in the absence of a worse than network from which it is known that, in the absence of a worse than
anticipated failure, the traffic will travel to its destination using anticipated failure, the traffic will travel to its destination using
normal forwarding without looping back. This is equivalent to normal forwarding without looping back. This is equivalent to
providing a virtual loop-free alternate to supplement the physical providing a virtual loop-free alternate to supplement the physical
loop-free alternates. Hence the name "Remote LFA FRR". In its loop-free alternates. Hence the name "Remote LFA FRR". In its
simplest form, when a link cannot be entirely protected with local simplest form, when a link cannot be entirely protected with local
LFA neighbors, the protecting router seeks the help of a remote LFA LFA neighbors, the protecting router seeks the help of a remote LFA
staging point. Network manageability considerations may lead to a staging point. Network manageability considerations may lead to a
repair strategy that uses a remote LFA more frequently repair strategy that uses a remote LFA more frequently
[I-D.ietf-rtgwg-lfa-manageability]. [I-D.ietf-rtgwg-lfa-manageability].
Examples of worse failures are node failures (see Section 6 ), the Examples of worse failures are node failures (see Section 7 ), the
failure of a shared risk link group (SRLG), the independent failure of a shared risk link group (SRLG), the independent
concurrent failures of multiple links, broadcast or non-broadcast concurrent failures of multiple links, broadcast or non-broadcast
multi-access (NBMA) links Section 2 ; protecting against such worse multi-access (NBMA) links Section 3 ; protecting against such worse
failures is out of scope for this specification. failures is out of scope for this specification.
3.1. Tunnels as Repair Paths 4.1. Tunnels as Repair Paths
Consider an arbitrary protected link S-E. In LFA FRR, if a path to Consider an arbitrary protected link S-E. In LFA FRR, if a path to
the destination from a neighbor N of S does not cause a packet to the destination from a neighbor N of S does not cause a packet to
loop back over the link S-E (i.e. N is a loop-free alternate), then loop back over the link S-E (i.e. N is a loop-free alternate), then
S can send the packet to N and the packet will be delivered to the S can send the packet to N and the packet will be delivered to the
destination using the pre-failure forwarding information. If there destination using the pre-failure forwarding information. If there
is no such LFA neighbor, then S may be able to create a virtual LFA is no such LFA neighbor, then S may be able to create a virtual LFA
by using a tunnel to carry the packet to a point in the network which by using a tunnel to carry the packet to a point in the network which
is not a direct neighbor of S from which the packet will be delivered is not a direct neighbor of S from which the packet will be delivered
to the destination without looping back to S. In this document such to the destination without looping back to S. In this document such
a tunnel is termed a repair tunnel. The tail-end of this tunnel (the a tunnel is termed a repair tunnel. The tail-end of this tunnel (the
repair tunnel endpoint) is a "PQ node" and the repair mechanism is a repair tunnel endpoint) is a "PQ node" and the repair mechanism is a
"remote LFA". This tunnel MUST NOT traverse the link S-E. "remote LFA". This tunnel MUST NOT traverse the link S-E.
Note that the repair tunnel terminates at some intermediate router Note that the repair tunnel terminates at some intermediate router
between S and E, and not E itself. This is clearly the case, since between S and E, and not E itself. This is clearly the case, since
if it were possible to construct a tunnel from S to E then a if it were possible to construct a tunnel from S to E then a
conventional LFA would have been sufficient to effect the repair. conventional LFA would have been sufficient to effect the repair.
3.2. Tunnel Requirements 4.2. Tunnel Requirements
There are a number of IP in IP tunnel mechanisms that may be used to There are a number of IP in IP tunnel mechanisms that may be used to
fulfil the requirements of this design, such as IP-in-IP [RFC1853] fulfil the requirements of this design, such as IP-in-IP [RFC1853]
and GRE[RFC1701] . and GRE[RFC1701] .
In an MPLS enabled network using LDP[RFC5036], a simple label In an MPLS enabled network using LDP[RFC5036], a simple label
stack[RFC3032] may be used to provide the required repair tunnel. In stack[RFC3032] may be used to provide the required repair tunnel. In
this case the outer label is S's neighbor's label for the repair this case the outer label is S's neighbor's label for the repair
tunnel end point, and the inner label is the repair tunnel end tunnel end point, and the inner label is the repair tunnel end
point's label for the packet destination. In order for S to obtain point's label for the packet destination. In order for S to obtain
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into service. This is not anticipated to be a problem in normal into service. This is not anticipated to be a problem in normal
operation since the managed introduction and removal of links is operation since the managed introduction and removal of links is
relatively rare as is the incidence of failure in a well managed relatively rare as is the incidence of failure in a well managed
network. network.
When a failure is detected, it is necessary to immediately redirect When a failure is detected, it is necessary to immediately redirect
traffic to the repair path. Consequently, the repair tunnel used traffic to the repair path. Consequently, the repair tunnel used
MUST be provisioned beforehand in anticipation of the failure. Since MUST be provisioned beforehand in anticipation of the failure. Since
the location of the repair tunnels is dynamically determined it is the location of the repair tunnels is dynamically determined it is
necessary to automatically establish the repair tunnels. Multiple necessary to automatically establish the repair tunnels. Multiple
repairs MAY share a tunnel end point. repair tunnels may share a tunnel end point.
4. Construction of Repair Paths 5. Construction of Repair Paths
4.1. Identifying Required Tunneled Repair Paths 5.1. Identifying Required Tunneled Repair Paths
Not all links will require protection using a tunneled repair path. Not all links will require protection using a tunneled repair path.
Referring to Figure 1, if E can already be protected via an LFA, S-E Referring to Figure 1, if E can already be protected via an LFA, S-E
does not need to be protected using a repair tunnel, since all does not need to be protected using a repair tunnel, since all
destinations normally reachable through E must therefore also be destinations normally reachable through E must therefore also be
protectable by an LFA. Such an LFA is frequently termed a "link protectable by an LFA. Such an LFA is frequently termed a "link
LFA". Tunneled repair paths (which may be calculated per-prefix) are LFA". Tunneled repair paths (which may be calculated per-prefix) are
only required for links which do not have a link or per-prefix LFA. only required for links which do not have a link or per-prefix LFA.
It should be noted that using the Q-space of E as a proxy for the It should be noted that using the Q-space of E as a proxy for the
Q-space of each destination can result in failing to identify valid Q-space of each destination can result in failing to identify valid
remote LFAs. The extent to which this reduces the effective remote LFAs. The extent to which this reduces the effective
protection coverage is topology dependent. protection coverage is topology dependent.
4.2. Determining Tunnel End Points 5.2. Determining Tunnel End Points
The repair tunnel endpoint needs to be a node in the network The repair tunnel endpoint needs to be a node in the network
reachable from S without traversing S-E. In addition, the repair reachable from S without traversing S-E. In addition, the repair
tunnel end point needs to be a node from which packets will normally tunnel end point needs to be a node from which packets will normally
flow towards their destination without being attracted back to the flow towards their destination without being attracted back to the
failed link S-E. failed link S-E.
Note that once released from the tunnel, the packet will be Note that once released from the tunnel, the packet will be
forwarded, as normal, on the shortest path from the release point to forwarded, as normal, on the shortest path from the release point to
its destination. This may result in the packet traversing the router its destination. This may result in the packet traversing the router
skipping to change at page 8, line 39 skipping to change at page 9, line 7
repair tunnel will reach their destination, and will not loop after a repair tunnel will reach their destination, and will not loop after a
single link failure. single link failure.
In some topologies it will not be possible to find a repair tunnel In some topologies it will not be possible to find a repair tunnel
endpoint that exhibits both the required properties. For example if endpoint that exhibits both the required properties. For example if
the ring topology illustrated in Figure 1 had a cost of 4 for the the ring topology illustrated in Figure 1 had a cost of 4 for the
link B-C, while the remaining links were cost 1, then it would not be link B-C, while the remaining links were cost 1, then it would not be
possible to establish a tunnel from S to C (without resorting to some possible to establish a tunnel from S to C (without resorting to some
form of source routing). form of source routing).
4.2.1. Computing Repair Paths 5.2.1. Computing Repair Paths
To compute the repair path for link S-E we need to determine the set To compute the repair path for link S-E it is necessary to determine
of routers which can be reached from S without traversing S-E, and the set of routers which can be reached from S without traversing
match this with the set of routers from which the node E can be S-E, and match this with the set of routers from which the node E can
reached, by normal forwarding, without traversing the link S-E. be reached, by normal forwarding, without traversing the link S-E.
The approach described in this memo is as follows: The approach used in this memo is as follows:
o We describe how to compute the set of routers which can be reached o The method of computing the set of routers which can be reached
from S on the shortest path tree without traversing S-E. We call from S on the shortest path tree without traversing S-E is
this the S's P-space with respect to the failure of link S-E. described. This is called the S's P-space with respect to the
failure of link S-E.
o We show how to extend the distance of the tunnel endpoint from the o The distance of the tunnel endpoint from the point of local repair
point of local repair (PLR) by noting that S is able to use the (PLR) is increased by noting that S is able to use the P-Space of
P-Space of its neighbours since S can determine which neighbour it its neighbours with respect to the failure of link S-E, since S
will use as the next hop for the repair. We call this the S's can determine which neighbour it will use as the next hop for the
Extended P-Space with respect to the failure of link S-E. The use repair. This is called the S's Extended P-space with respect to
of extended P-Space allows greater repair coverage and is the the failure of link S-E. The use of extended P-space allows
preferred approach. greater repair coverage and is the preferred approach.
o Finally we show how to compute the set of routers from which the o Finally two methods of computing the set of routers from which the
node E can be reached, by normal forwarding, without traversing node E can be reached, by normal forwarding, without traversing
the link S-E. This is called the Q-space of E with respect to the the link S-E. This is called the Q-space of E with respect to the
link S-E. link S-E.
The selection of the preferred node from the set of nodes that an in The selection of the preferred node from the set of nodes that an in
both Extended P-Space and Q-Space is described in Section 4.2.2. both Extended P-Space and Q-Space with respect to the S-E is
described in Section 5.2.2.
A suitable cost based algorithm to compute the set of nodes common to A suitable cost based algorithm to compute the set of nodes common to
both extended P-space and Q-space is provided in Section 4.3. both extended P-space and Q-space with respect to the S-E is provided
in Section 5.3.
4.2.1.1. P-space 5.2.1.1. P-space
The set of routers which can be reached from S on the shortest path The set of routers which can be reached from S on the shortest path
tree without traversing S-E is termed the P-space of S with respect tree without traversing S-E is termed the P-space of S with respect
to the link S-E. The P-space can be obtained by computing a shortest to the link S-E. This P-space can be obtained by computing a
path tree (SPT) rooted at S and excising the sub-tree reached via the shortest path tree (SPT) rooted at S and excising the sub-tree
link S-E (including those routers which are members of an ECMP that reached via the link S-E (including those routers which are members
includes link S-E). The exclusion of routers reachable via an ECMP of an ECMP that includes link S-E). The exclusion of routers
that includes S-E prevents the forwarding subsystem from attempting reachable via an ECMP that includes S-E prevents the forwarding
to a repair endpoint via the failed link S-E. Thus for example, if subsystem from attempting to execute a repair via the failed link
the SPF computation stores at each node the next-hops to be used to S-E. Thus for example, if the SPF computation stores at each node
reach that node from S, then the node can be added to P-space if none the next-hops to be used to reach that node from S, then the node can
of its next-hops are S-E. In the case of Figure 1 the P-space be added to P-space if none of its next-hops are link S-E. In the
comprises nodes A and B only. Expressed in cost terms the set of case of Figure 1 this P-space comprises nodes A and B only.
routers {P} are those for which the shortest path cost S->P is Expressed in cost terms the set of routers {P} are those for which
strictly less than the shortest path cost S->E->P. the shortest path cost S->P is strictly less than the shortest path
cost S->E->P.
4.2.1.2. Extended P-space 5.2.1.2. Extended P-space
The description in Section 4.2.1.1 calculated router S's P-space The description in Section 5.2.1.1 calculated router S's P-space
rooted at S itself. However, since router S will only use a repair rooted at S itself. However, since router S will only use a repair
path when it has detected the failure of the link S-E, the initial path when it has detected the failure of the link S-E, the initial
hop of the repair path need not be subject to S's normal forwarding hop of the repair path need not be subject to S's normal forwarding
decision process. Thus we introduce the concept of extended P-space. decision process. Thus the concept of extended P-space is
Router S's extended P-space is the union of the P-spaces of each of introduced. Router S's extended P-space is the union of the P-spaces
S's neighbours (N). This may be calculated by computing an SPT at of each of S's neighbours (N). This may be calculated by computing a
each of S's neighbors (excluding E) and excising the subtree reached shortest path tree (SPT) at each of S's neighbors (excluding E) and
via the path N->S->E. Note this will excise those routers which are excising the subtree reached via the path N->S->E. Note this will
reachable through all ECMPs that includes link S-E. The use of excise those routers which are reachable through all ECMPs that
extended P-space may allow router S to reach potential repair tunnel includes link S-E. The use of extended P-space may allow router S to
end points that were otherwise unreachable. In cost terms a router reach potential repair tunnel end points that were otherwise
(P) is in extended P-space if the shortest path cost N->P is strictly unreachable. In cost terms a router (P) is in extended P-space if
less than the shortest path cost N->S->E->P. In other words, once the shortest path cost N->P is strictly less than the shortest path
the packet it forced to N by S, it is lower cost for it to continue cost N->S->E->P. In other words, once the packet it forced to N by
on to P by any path except one that takes it back to S and then S, it is a lower cost for it to continue on to P by any path except
across the S->E link. one that takes it back to S and then across the S->E link.
Since in the case of Figure 1 node A is a per-prefix LFA for the Since in the case of Figure 1 node A is a per-prefix LFA for the
destination node C, the set of extended P-space nodes comprises nodes destination node C, the set of extended P-space nodes with respect to
A, B and C. Since node C is also in E's Q-space, there is now a node link S-E comprises nodes A, B and C. Since node C is also in E's
common to both extended P-space and Q-space which can be used as a Q-space with respect to link S-E, there is now a node common to both
repair tunnel end-point to protect the link S-E. extended P-space and Q-space which can be used as a repair tunnel
end-point to protect the link S-E.
4.2.1.3. Q-space 5.2.1.3. Q-space
The set of routers from which the node E can be reached, by normal The set of routers from which the node E can be reached, by normal
forwarding, without traversing the link S-E is termed the Q-space of forwarding, without traversing the link S-E is termed the Q-space of
E with respect to the link S-E. The Q-space can be obtained by E with respect to the link S-E. The Q-space can be obtained by
computing a reverse shortest path tree (rSPT) rooted at E, with the computing a reverse shortest path tree (rSPT) rooted at E, with the
sub-tree which traverses the failed link excised (including those sub-tree which might traverse the protected link S-E excised (i.e.
which are members of an ECMP). The rSPT uses the cost towards the those nodes that would send the packet via S-E plus those nodes which
root rather than from it and yields the best paths towards the root have an ECMP set to E with one or more members of that ECMP set
from other nodes in the network. In the case of Figure 1 the Q-space traversing the protected link S-E). The rSPT uses the cost towards
comprises nodes C and D only. Expressed in cost terms the set of the root rather than from it and yields the best paths towards the
routers {Q} are those for which the shortest path cost Q<-E is root from other nodes in the network. In the case of Figure 1 the
strictly less than the shortest path cost Q<-S<-E. In Figure 1 the Q-space of E with respect to S-E comprises nodes C and D only.
intersection of the E's Q-space with S's P-space defines the set of
viable repair tunnel end-points, known as "PQ nodes". As can be Expressed in cost terms the set of routers {Q} are those for which
the shortest path cost Q<-E is strictly less than the shortest path
cost Q<-S<-E. In Figure 1 the intersection of the E's Q-space with
respect to S-E with S's P-space with respect to S-E defines the set
of viable repair tunnel end-points, known as "PQ nodes". As can be
seen, for the case of Figure 1 there is no common node and hence no seen, for the case of Figure 1 there is no common node and hence no
viable repair tunnel end-point. However when the extended the viable repair tunnel end-point. However when the extended the
extended P-space Section 4.2.1.2 at S is considered a suitable extended P-space Section 5.2.1.2 at S with respect to S-E is
intersection is found at C. considered, a suitable intersection is found at C.
Note that the Q-space calculation could be conducted for each Note that the Q-space calculation could be conducted for each
individual destination and a per-destination repair tunnel end point individual destination and a per-destination repair tunnel end point
determined. However this would, in the worst case, require an SPF determined. However this would, in the worst case, require an SPF
computation per destination which is not currently considered to be computation per destination which is not currently considered to be
scalable. We therefore use the Q-space of E as a proxy for the scalable. Therefore the Q-space of E with respect to link S-E is
Q-space of each destination. This approximation is obviously correct used as a proxy for the Q-space of each destination. This
since the repair is only used for the set of destinations which were, approximation is obviously correct since the repair is only used for
prior to the failure, routed through node E. This is analogous to the set of destinations which were, prior to the failure, routed
the use of link-LFAs rather than per-prefix LFAs. through node E. This is analogous to the use of link-LFAs rather
than per-prefix LFAs.
4.2.2. Selecting Repair Paths 5.2.2. Selecting Repair Paths
The mechanisms described above will identify all the possible repair The mechanisms described above will identify all the possible repair
tunnel end points that can be used to protect a particular link. In tunnel end points that can be used to protect a particular link. In
a well-connected network there are likely to be multiple possible a well-connected network there are likely to be multiple possible
release points for each protected link. All will deliver the packets release points for each protected link. All will deliver the packets
correctly so, arguably, it does not matter which is chosen. However, correctly so, arguably, it does not matter which is chosen. However,
one repair tunnel end point may be preferred over the others on the one repair tunnel end point may be preferred over the others on the
basis of path cost or some other selection criteria. basis of path cost or some other selection criteria.
There is no technical requirement for the selection criteria to be There is no technical requirement for the selection criteria to be
skipping to change at page 11, line 39 skipping to change at page 12, line 12
manageability considerations may lead to a repair strategy that uses manageability considerations may lead to a repair strategy that uses
a remote LFA more frequently [I-D.ietf-rtgwg-lfa-manageability]. a remote LFA more frequently [I-D.ietf-rtgwg-lfa-manageability].
As described in [RFC5286], always selecting a PQ node that is As described in [RFC5286], always selecting a PQ node that is
downstream to the destination with respect to the repairing node, downstream to the destination with respect to the repairing node,
prevents the formation of loops when the failure is worse than prevents the formation of loops when the failure is worse than
expected. The use of downstream nodes reduces the repair coverage, expected. The use of downstream nodes reduces the repair coverage,
and operators are advised to determine whether adequate coverage is and operators are advised to determine whether adequate coverage is
achieved before enabling this selection feature. achieved before enabling this selection feature.
4.3. A Cost Based RLFA Algorithm 5.3. A Cost Based RLFA Algorithm
The preceding text has mostly described the computation of the remote The preceding text has described the computation of the remote LFA
LFA repair target (PQ) in terms of the intersection of two repair target (PQ) in terms of the intersection of two reachability
reachability graphs computed using SPFs. This section describes a graphs computed using a shortest path first (SPF) algorithm. This
method of computing the remote LFA repair target for a specific section describes a method of computing the remote LFA repair target
failed link using a cost based algorithm. The pseudo-code provided for a specific failed link using a cost based algorithm. The pseudo-
in this section avoids unnecessary SPF computations, but for the sake code provided in this section avoids unnecessary SPF computations,
of readability, it does not otherwise try to optimize the code. The but for the sake of readability, it does not otherwise try to
algorithm covers the case where the repair first hop is reached via a optimize the code. The algorithm covers the case where the repair
broadcast or non-broadcast multi-access (NBMA) link such as a LAN. first hop is reached via a broadcast or non-broadcast multi-access
It also covers the case where the P or Q node is attached via such a (NBMA) link such as a LAN. It also covers the case where the P or Q
link. It does not cover the case where the failed interface is a node is attached via such a link. It does not cover the case where
broadcast or non-broadcast multi-access (NBMA) link. To address that the failed interface is a broadcast or non-broadcast multi-access
case it is necessary to compute the Q space of each neighbor of the (NBMA) link. To address that case it is necessary to compute the Q
repairing router reachable though the LAN, i.e. to treat the space of each neighbor of the repairing router reachable though the
pseudonode as a node failure. This is because the Q spaces of the LAN, i.e. to treat the pseudonode [RFC1195] as a node failure. This
neighbors of the pseudonode may be disjoint requiring use of a is because the Q spaces of the neighbors of the pseudonode may be
neighbor specific PQ node. The reader is referred to disjoint requiring use of a neighbor specific PQ node. The reader is
[I-D.ietf-rtgwg-rlfa-node-protection] for further information on the referred to [I-D.ietf-rtgwg-rlfa-node-protection] for further
use of RLFA for node repairs. information on the use of RLFA for node repairs.
The following notation is used: The following notation is used:
o D_opt(a,b) is the shortest distance from node a to node b as o D_opt(a,b) is the shortest distance from node a to node b as
computed by the SPF. computed by the SPF.
o dest is the packet destination o dest is the packet destination
o fail_intf is the failed interface (S-E in the example) o fail_intf is the failed interface (S-E in the example)
skipping to change at page 14, line 24 skipping to change at page 14, line 24
Compute_and_Store_Forward_SPF(root) Compute_and_Store_Forward_SPF(root)
Compute_Forward_SPF(root) Compute_Forward_SPF(root)
foreach node y in network foreach node y in network
store D_opt(root,y) store D_opt(root,y)
///////////////////////////////////////////////////////////////// /////////////////////////////////////////////////////////////////
// //
// This computes the optimum distance from each neighbour (other // This computes the optimum distance from each neighbour (other
// than the neighbour reachable through the failed link) and // than the neighbour reachable through the failed link) and
// every other node in the network // every other node in the network
//
// Note that we compute this for all neighbours including the
// neighbour on the far side the failure. This is done on the
// expectation that more than on link will be protected, and
// that the results are stored for later use.
//
Compute_Neighbor_SPFs() Compute_Neighbor_SPFs()
foreach interface intf in self foreach interface intf in self
Compute_and_Store_Forward_SPF(intf.remote_node) Compute_and_Store_Forward_SPF(intf.remote_node)
///////////////////////////////////////////////////////////////// /////////////////////////////////////////////////////////////////
// //
// The reverse SPF computes the cost from each remote node to // The reverse SPF computes the cost from each remote node to
// root. This is achieved by running the normal SPF algorithm, // root. This is achieved by running the normal SPF algorithm,
// but using the link cost in the direction from the next hop // but using the link cost in the direction from the next hop
skipping to change at page 16, line 38 skipping to change at page 17, line 26
if (y.valid_tunnel_endpoint) if (y.valid_tunnel_endpoint)
Compute_and_Store_Forward_SPF(y) Compute_and_Store_Forward_SPF(y)
if ((D_opt(y,dest) < D_opt(self,dest)) if ((D_opt(y,dest) < D_opt(self,dest))
y.valid_tunnel_endpoint = true y.valid_tunnel_endpoint = true
else else
y.valid_tunnel_endpoint = false y.valid_tunnel_endpoint = false
// //
///////////////////////////////////////////////////////////////// /////////////////////////////////////////////////////////////////
4.4. Interactions with IS-IS Overload, RFC6987, and Costed Out Links 5.4. Interactions with IS-IS Overload, RFC6987, and Costed Out Links
Since normal link state routing takes into account the IS-IS overload Since normal link state routing takes into account the IS-IS overload
bit, [RFC6987], and costing out of links as described in Section 3.5 bit, [RFC6987], and costing out of links as described in Section 3.5
of [RFC5286], the forward SPFs performed by the PLR rooted at the of [RFC5286], the forward SPFs performed by the PLR rooted at the
neighbors of the PLR also need to take this into account. A repair neighbors of the PLR also need to take this into account. A repair
tunnel path from a neighbor of the PLR to a repair tunnel endpoint tunnel path from a neighbor of the PLR to a repair tunnel endpoint
will generally avoid the nodes and links excluded by the IGP will generally avoid the nodes and links excluded by the IGP
overload/costing out rules. However, there are two situations where overload/costing out rules. However, there are two situations where
this behavior may result in a repair path traversing a link or router this behavior may result in a repair path traversing a link or router
that should be excluded: that should be excluded:
skipping to change at page 17, line 24 skipping to change at page 18, line 9
2. The IS-IS overload bit and the mechanism of [RFC6987] only 2. The IS-IS overload bit and the mechanism of [RFC6987] only
prevent transit traffic from traversing a node. They do not prevent transit traffic from traversing a node. They do not
prevent traffic destined to a node. The per-neighbor forward prevent traffic destined to a node. The per-neighbor forward
SPFs using the standard IGP overload rules will not prevent a PLR SPFs using the standard IGP overload rules will not prevent a PLR
from choosing a repair tunnel endpoint that is advertising a from choosing a repair tunnel endpoint that is advertising a
desire to not carry transit traffic. Therefore, the PLR MUST NOT desire to not carry transit traffic. Therefore, the PLR MUST NOT
use a repair tunnel endpoint with the IS-IS overload bit set, or use a repair tunnel endpoint with the IS-IS overload bit set, or
where all outgoing interfaces have the cost set to MaxLinkMetric where all outgoing interfaces have the cost set to MaxLinkMetric
for OSPF. for OSPF.
5. Example Application of Remote LFAs 6. Example Application of Remote LFAs
An example of a commonly deployed topology which is not fully An example of a commonly deployed topology which is not fully
protected by LFAs alone is shown in Figure 3. PE1 and PE2 are protected by LFAs alone is shown in Figure 3. PE1 and PE2 are
connected in the same site. P1 and P2 may be geographically connected in the same site. P1 and P2 may be geographically
separated (inter-site). In order to guarantee the lowest latency separated (inter-site). In order to guarantee the lowest latency
path from/to all other remote PEs, normally the shortest path follows path from/to all other remote PEs, normally the shortest path follows
the geographical distance of the site locations. Therefore, to the geographical distance of the site locations. Therefore, to
ensure this, a lower IGP metric (5) is assigned between PE1 and PE2. ensure this, a lower IGP metric (5) is assigned between PE1 and PE2.
A high metric (1000) is set on the P-PE links to prevent the PEs A high metric (1000) is set on the P-PE links to prevent the PEs
being used for transit traffic. The PEs are not individually dual- being used for transit traffic. The PEs are not individually dual-
skipping to change at page 18, line 19 skipping to change at page 18, line 47
PE1---PE2 PE1---PE2
5 5
Figure 3: Example SP topology Figure 3: Example SP topology
Clearly, full protection can be provided, using the techniques Clearly, full protection can be provided, using the techniques
described in this document, by PE1 choosing P2 as the remote LFA described in this document, by PE1 choosing P2 as the remote LFA
repair target node, and PE2 choosing P1 as the remote LFA repair repair target node, and PE2 choosing P1 as the remote LFA repair
target. target.
6. Node Failures 7. Node Failures
When the failure is a node failure rather than a point-to-point link When the failure is a node failure rather than a point-to-point link
failure there is a danger that the RLFA repair will loop. This is failure there is a danger that the RLFA repair will loop. This is
discussed in detail in [I-D.bryant-ipfrr-tunnels]. In summary the discussed in detail in [I-D.bryant-ipfrr-tunnels]. In summary the
problem is that two of more of E's neighbors each with E as the next problem is that two of more of E's neighbors each with E as the next
hop to some destination D may attempt to repair a packet addressed to hop to some destination D may attempt to repair a packet addressed to
destination D via the other neighbor and then E, thus causing a loop destination D via the other neighbor and then E, thus causing a loop
to form. A similar problem exists in the case of a shared risk link to form. A similar problem exists in the case of a shared risk link
group failure where the PLR for each failure attempts to repair via group failure where the PLR for each failure attempts to repair via
the other failure. As will be noted from [I-D.bryant-ipfrr-tunnels], the other failure. As will be noted from [I-D.bryant-ipfrr-tunnels],
skipping to change at page 19, line 26 skipping to change at page 20, line 8
failure in any particular topology will depend on the node design, failure in any particular topology will depend on the node design,
the particular topology in use, and the strategy adopted under node the particular topology in use, and the strategy adopted under node
failure. It is recommended that a network operator perform an failure. It is recommended that a network operator perform an
analysis of the consequences and probability of node failure in their analysis of the consequences and probability of node failure in their
network, and determine whether the incidence and consequence of network, and determine whether the incidence and consequence of
occurrence are acceptable. occurrence are acceptable.
This topic is further discussed in This topic is further discussed in
[I-D.ietf-rtgwg-rlfa-node-protection]. [I-D.ietf-rtgwg-rlfa-node-protection].
7. Operation in an LDP environment 8. Operation in an LDP environment
Where this technique is used in an MPLS network using LDP [RFC5036], Where this technique is used in an MPLS network using LDP [RFC5036],
and S is a transit node, S will need to swap the top label in the and S is a transit node, S will need to swap the top label in the
stack for the remote LFA repair target's (PQ's) label to the stack for the remote LFA repair target's (PQ's) label to the
destination, and to then push its own label for the remote LFA repair destination, and to then push its own label for the remote LFA repair
target. target.
In the example Figure 2 S already has the first hop (A) label for the In the example Figure 2 S already has the first hop (A) label for the
remote LFA repair target (C) as a result of the ordinary operation of remote LFA repair target (C) as a result of the ordinary operation of
LDP. To get the remote LFA repair target's label (C's label) for the LDP. To get the remote LFA repair target's label (C's label) for the
skipping to change at page 20, line 41 skipping to change at page 21, line 14
In the absence of a protocol to learn the preferred IP address for In the absence of a protocol to learn the preferred IP address for
targeted LDP, an LSR should attempt a targeted LDP session with the targeted LDP, an LSR should attempt a targeted LDP session with the
Router ID [RFC2328] [RFC5305] [RFC5340] [RFC6119] Router ID [RFC2328] [RFC5305] [RFC5340] [RFC6119]
[I-D.ietf-ospf-routable-ip-address], unless it is configured [I-D.ietf-ospf-routable-ip-address], unless it is configured
otherwise. otherwise.
No protection is available until the TLDP session has been No protection is available until the TLDP session has been
established and a label for the destination has been learned from the established and a label for the destination has been learned from the
remote LFA repair target. If for any reason the TLDP session cannot remote LFA repair target. If for any reason the TLDP session cannot
not be established, an implementation SHOULD advise the operator be established, an implementation SHOULD advise the operator about
about the protection setup issue using any well known mechanism such the protection setup issue through the network management system.
as Syslog [RFC5424] or SNMP [RFC3411].
8. Analysis of Real World Topologies 9. Analysis of Real World Topologies
This section gives the results of analysing a number of real world This section gives the results of analysing a number of real world
service provider topologies collected between the end of 2012 and service provider topologies collected between the end of 2012 and
early 2013 early 2013
8.1. Topology Details 9.1. Topology Details
The figure below characterises each topology (topo) studied in terms The figure below characterises each topology (topo) studied in terms
of : of :
o The number of nodes (# nodes) excluding pseudonodes. o The number of nodes (# nodes) excluding pseudonodes.
o The number of bidirectional links ( # links) including parallel o The number of bidirectional links ( # links) including parallel
links and links to and from pseudonodes. links and links to and from pseudonodes.
o The number of node pairs that are connected by one or more links o The number of node pairs that are connected by one or more links
skipping to change at page 21, line 43 skipping to change at page 22, line 24
| 7 | 55 | 237 | 159 | 67 | 2 | | 7 | 55 | 237 | 159 | 67 | 2 |
| 8 | 779 | 1848 | 1441 | 199 | 437 | | 8 | 779 | 1848 | 1441 | 199 | 437 |
| 9 | 263 | 482 | 413 | 41 | 12 | | 9 | 263 | 482 | 413 | 41 | 12 |
| 10 | 86 | 375 | 145 | 64 | 22 | | 10 | 86 | 375 | 145 | 64 | 22 |
| 11 | 162 | 1083 | 351 | 201 | 49 | | 11 | 162 | 1083 | 351 | 201 | 49 |
| 12 | 380 | 1174 | 763 | 231 | 0 | | 12 | 380 | 1174 | 763 | 231 | 0 |
| 13 | 1051 | 2087 | 2037 | 48 | 64 | | 13 | 1051 | 2087 | 2037 | 48 | 64 |
| 14 | 92 | 291 | 204 | 64 | 2 | | 14 | 92 | 291 | 204 | 64 | 2 |
+------+---------+---------+---------+--------+--------+ +------+---------+---------+---------+--------+--------+
8.2. LFA only 9.2. LFA only
The figure below shows the percentage of protected destinations (% The figure below shows the percentage of protected destinations (%
prot) and percentage of guaranteed node protected destinations ( % prot) and percentage of guaranteed node protected destinations ( %
gtd N) for the set of topologies characterized in Section 8.1 gtd N) for the set of topologies characterized in Section 9.1
achieved using only LFA repairs. achieved using only LFA repairs.
These statistics were generated by considering each node and then These statistics were generated by considering each node and then
considering each link to each next hop to each destination. The considering each link to each next hop to each destination. The
percentage of such links across the entire network that are protected percentage of such links across the entire network that are protected
against link failure was determined. This is the percentage of against link failure was determined. This is the percentage of
protected destinations. If a link is protected against the failure protected destinations. If a link is protected against the failure
of the next hop node, this is considered guaranteed node protecting of the next hop node, this is considered guaranteed node protecting
(GNP) and percentage of guaranteed node protected destinations is (GNP) and percentage of guaranteed node protected destinations is
calculated using the same method used for calculating the link calculated using the same method used for calculating the link
skipping to change at page 22, line 34 skipping to change at page 23, line 24
| 7 | 93.9 | 35.4 | | 7 | 93.9 | 35.4 |
| 8 | 95.3 | 48.1 | | 8 | 95.3 | 48.1 |
| 9 | 82.2 | 49.5 | | 9 | 82.2 | 49.5 |
| 10 | 98.5 | 14.9 | | 10 | 98.5 | 14.9 |
| 11 | 99.6 | 24.8 | | 11 | 99.6 | 24.8 |
| 12 | 99.5 | 62.4 | | 12 | 99.5 | 62.4 |
| 13 | 92.4 | 51.6 | | 13 | 92.4 | 51.6 |
| 14 | 99.3 | 48.6 | | 14 | 99.3 | 48.6 |
+------+--------+---------+ +------+--------+---------+
8.3. RLFA 9.3. RLFA
The figure below shows the percentage of protected destinations (% The figure below shows the percentage of protected destinations (%
prot) and % guaranteed node protected destinations ( % gtd N) for prot) and % guaranteed node protected destinations ( % gtd N) for
RLFA protection in the topologies studies. In addition, it show the RLFA protection in the topologies studies. In addition, it show the
percentage of destinations using an RLFA repair (% PQ) together with percentage of destinations using an RLFA repair (% PQ) together with
the total number of unidirectional RLFA targeted LDP session the total number of unidirectional RLFA targeted LDP session
established (# PQ), the number of PQ sessions which would be required established (# PQ), the number of PQ sessions which would be required
for complete protection, but which could not be established because for complete protection, but which could not be established because
there was no PQ node, i.e. the number of cases whether neither LFA or there was no PQ node, i.e. the number of cases whether neither LFA or
RLFA protection was possible (no PQ). It also shows the 50 (p50), 90 RLFA protection was possible (no PQ). It also shows the 50 (p50), 90
skipping to change at page 23, line 42 skipping to change at page 24, line 32
| 10 | 99.6 | 14.1 | 1 | 49 | 7 | 1 | 2 | 5 | | 10 | 99.6 | 14.1 | 1 | 49 | 7 | 1 | 2 | 5 |
| 11 | 99.9 | 24.9 | 0.3 | 85 | 1 | 0 | 2 | 8 | | 11 | 99.9 | 24.9 | 0.3 | 85 | 1 | 0 | 2 | 8 |
| 12 | 99.999 | 62.8 | 0.5 | 512 | 4 | 0 | 0 | 3 | | 12 | 99.999 | 62.8 | 0.5 | 512 | 4 | 0 | 0 | 3 |
| 13 | 97.5 | 54.6 | 5.1 | 1188 | 95 | 0 | 2 | 27 | | 13 | 97.5 | 54.6 | 5.1 | 1188 | 95 | 0 | 2 | 27 |
| 14 | 100 | 48.6 | 0.7 | 79 | 0 | 0 | 2 | 4 | | 14 | 100 | 48.6 | 0.7 | 79 | 0 | 0 | 2 | 4 |
+------+--------+--------+------+------+-------+-----+-----+------+ +------+--------+--------+------+------+-------+-----+-----+------+
Another study[ISOCORE2010] confirms the significant coverage increase Another study[ISOCORE2010] confirms the significant coverage increase
provided by Remote LFAs. provided by Remote LFAs.
8.4. Comparison of LFA an RLFA results 9.4. Comparison of LFA an RLFA results
The table below provides a side by side comparison the LFA and the The table below provides a side by side comparison the LFA and the
remote LFA results. This shows a significant improvement in the remote LFA results. This shows a significant improvement in the
percentage of protected destinations and normally a modest percentage of protected destinations and normally a modest
improvement in the percentage of guaranteed node protected improvement in the percentage of guaranteed node protected
destinations. destinations.
+------+--------+--------+---------+---------+ +------+--------+--------+---------+---------+
| topo | LFA | RLFA | LFA | RLFA | | topo | LFA | RLFA | LFA | RLFA |
| | % prot | %prot | % gtd N | % gtd N | | | % prot | %prot | % gtd N | % gtd N |
skipping to change at page 24, line 39 skipping to change at page 25, line 39
always present as a property of an LDP-based deployment. always present as a property of an LDP-based deployment.
In the small number of cases where there is no intersection between In the small number of cases where there is no intersection between
the (extended)P-space and the Q-space, a number of solutions to the (extended)P-space and the Q-space, a number of solutions to
providing a suitable path between such disjoint regions in the providing a suitable path between such disjoint regions in the
network have been discussed in the working group. For example an network have been discussed in the working group. For example an
explicitly routed LSP between P and Q might be set up using RSVP-TE explicitly routed LSP between P and Q might be set up using RSVP-TE
or using Segment Routing [I-D.filsfils-spring-segment-routing]. Such or using Segment Routing [I-D.filsfils-spring-segment-routing]. Such
extended repair methods are outside the scope of this document. extended repair methods are outside the scope of this document.
9. Management and Operational Considerations 10. Management and Operational Considerations
The management of LFA and remote LFA is the subject of ongoing work The management of LFA and remote LFA is the subject of ongoing work
withing the IETF [I-D.ietf-rtgwg-lfa-manageability] to which the withing the IETF [I-D.ietf-rtgwg-lfa-manageability] to which the
reader is referred. Management considerations may lead to a reader is referred. Management considerations may lead to a
preference for the use of a remote LFA over an available LFA. This preference for the use of a remote LFA over an available LFA. This
preference is a matter for the network operator, and not a matter of preference is a matter for the network operator, and not a matter of
protocol correctness. protocol correctness.
When the network re-converges, microloops [RFC5715] may form due to When the network re-converges, microloops [RFC5715] can form due to
transient inconsistencies in the router FIBs. If it is determined transient inconsistencies in the forwarding tables of different
that microloops are a significant issue in the deployment, then a routers. If it is determined that microloops are a significant issue
suitable loop free convergence methods such as one of those described in the deployment, then a suitable loop free convergence methods such
in [RFC5715], [RFC6976], or [I-D.litkowski-rtgwg-uloop-delay] should as one of those described in [RFC5715], [RFC6976], or
be implemented. [I-D.litkowski-rtgwg-uloop-delay] should be implemented.
10. Historical Note 11. Historical Note
The basic concepts behind Remote LFA were invented in 2002 and were The basic concepts behind Remote LFA were invented in 2002 and were
later included in [I-D.bryant-ipfrr-tunnels], submitted in 2004. later included in [I-D.bryant-ipfrr-tunnels], submitted in 2004.
[I-D.bryant-ipfrr-tunnels], targeted a 100% protection coverage and [I-D.bryant-ipfrr-tunnels], targeted a 100% protection coverage and
hence included additional mechanisms on top of the Remote LFA hence included additional mechanisms on top of the Remote LFA
concept. The addition of these mechanisms made the proposal very concept. The addition of these mechanisms made the proposal very
complex and computationally intensive and it was therefore not complex and computationally intensive and it was therefore not
pursued as a working group item. pursued as a working group item.
skipping to change at page 25, line 29 skipping to change at page 26, line 29
not to provide coverage at any cost. A solution for this already not to provide coverage at any cost. A solution for this already
exists with MPLS TE FRR. MPLS TE FRR is a mature technology which is exists with MPLS TE FRR. MPLS TE FRR is a mature technology which is
able to provide protection in any topology thanks to the explicit able to provide protection in any topology thanks to the explicit
routing capability of MPLS TE. routing capability of MPLS TE.
The purpose of LFA FRR technology is to provide for a simple FRR The purpose of LFA FRR technology is to provide for a simple FRR
solution when such a solution is possible. The first step along this solution when such a solution is possible. The first step along this
simplicity approach was "local" LFA [RFC5286]. This specification of simplicity approach was "local" LFA [RFC5286]. This specification of
"Remote LFA" is a natural second step. "Remote LFA" is a natural second step.
11. IANA Considerations 12. IANA Considerations
There are no IANA considerations that arise from this architectural There are no IANA considerations that arise from this architectural
description of IPFRR. The RFC Editor may remove this section on description of IPFRR. The RFC Editor may remove this section on
publication. publication.
12. Security Considerations 13. Security Considerations
The security considerations of [RFC5286] also apply. The security considerations of [RFC5286] also apply.
Targeted LDP sessions and MPLS tunnels are normal features of an MPLS Targeted LDP sessions and MPLS tunnels are normal features of an MPLS
network and their use in this application raises no additional network and their use in this application raises no additional
security concerns. security concerns.
To prevent their use as an attack vector IP repair tunnel endpoints IP repair tunnel endpoints (where used) SHOULD be assigned from a set
(where used) SHOULD be assigned from a set of addresses that are not of addresses that are not reachable from outside the routing domain.
reachable from outside the routing domain. This would prevent their use as an attack vector.
13. Acknowledgments Other than OAM traffic, used to verify the correct operation of a
repair tunnel, only traffic that is being protected as a result of a
link failure is placed a repair tunnel. The repair tunnel MUST NOT
be advertised by the routing protocol as a link that may be used to
carry normal user traffic, or routing protocol traffic.
14. Acknowledgments
The authors wish to thank Levente Csikor and Chris Bowers for their The authors wish to thank Levente Csikor and Chris Bowers for their
contribution to the cost based algorithm text. We thank Alia Atlas, contribution to the cost based algorithm text. The authors thank
Ross Callon, Stephane Litkowski, Bharath R, Pushpasis Sarkar and Alia Atlas, Ross Callon, Stephane Litkowski, Bharath R, Pushpasis
Adrian Farrel for their review of this document. Sarkar and Adrian Farrel for their review of this document.
14. References 15. References
14.1. Normative References 15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast [RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008. Reroute: Loop-Free Alternates", RFC 5286, September 2008.
14.2. Informative References [RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
5714, January 2010.
15.2. Informative References
[I-D.bryant-ipfrr-tunnels] [I-D.bryant-ipfrr-tunnels]
Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP
Fast Reroute using tunnels", draft-bryant-ipfrr-tunnels-03 Fast Reroute using tunnels", draft-bryant-ipfrr-tunnels-03
(work in progress), November 2007. (work in progress), November 2007.
[I-D.filsfils-spring-segment-routing] [I-D.filsfils-spring-segment-routing]
Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R., Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe, Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
"Segment Routing Architecture", draft-filsfils-spring- "Segment Routing Architecture", draft-filsfils-spring-
segment-routing-04 (work in progress), July 2014. segment-routing-04 (work in progress), July 2014.
[I-D.ietf-ospf-routable-ip-address] [I-D.ietf-ospf-routable-ip-address]
Xu, X., Chunduri, U., and M. Bhatia, "Carrying Routable IP Xu, X., Chunduri, U., and M. Bhatia, "Carrying Routable IP
Addresses in OSPF RI LSA", draft-ietf-ospf-routable-ip- Addresses in OSPF RI LSA", draft-ietf-ospf-routable-ip-
address-01 (work in progress), October 2014. address-01 (work in progress), October 2014.
[I-D.ietf-rtgwg-lfa-manageability] [I-D.ietf-rtgwg-lfa-manageability]
Litkowski, S., Decraene, B., Filsfils, C., Raza, K., Litkowski, S., Decraene, B., Filsfils, C., Raza, K.,
Horneffer, M., and p. psarkar@juniper.net, "Operational Horneffer, M., and P. Sarkar, "Operational management of
management of Loop Free Alternates", draft-ietf-rtgwg-lfa- Loop Free Alternates", draft-ietf-rtgwg-lfa-
manageability-06 (work in progress), January 2015. manageability-07 (work in progress), January 2015.
[I-D.ietf-rtgwg-rlfa-node-protection] [I-D.ietf-rtgwg-rlfa-node-protection]
psarkar@juniper.net, p., Gredler, H., Hegde, S., Bowers, Sarkar, P., Gredler, H., Hegde, S., Bowers, C., Litkowski,
C., Litkowski, S., and H. Raghuveer, "Remote-LFA Node S., and H. Raghuveer, "Remote-LFA Node Protection and
Protection and Manageability", draft-ietf-rtgwg-rlfa-node- Manageability", draft-ietf-rtgwg-rlfa-node-protection-01
protection-01 (work in progress), December 2014. (work in progress), December 2014.
[I-D.litkowski-rtgwg-uloop-delay] [I-D.litkowski-rtgwg-uloop-delay]
Litkowski, S., Decraene, B., Filsfils, C., and P. Litkowski, S., Decraene, B., Filsfils, C., and P.
Francois, "Microloop prevention by introducing a local Francois, "Microloop prevention by introducing a local
convergence delay", draft-litkowski-rtgwg-uloop-delay-03 convergence delay", draft-litkowski-rtgwg-uloop-delay-03
(work in progress), February 2014. (work in progress), February 2014.
[ISOCORE2010] [ISOCORE2010]
So, N., Lin, T., and C. Chen, "LFA (Loop Free Alternates) So, N., Lin, T., and C. Chen, "LFA (Loop Free Alternates)
Case Studies in Verizon's LDP Network", 2010. Case Studies in Verizon's LDP Network", 2010.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic [RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC 1701, October 1994. Routing Encapsulation (GRE)", RFC 1701, October 1994.
[RFC1853] Simpson, W., "IP in IP Tunneling", RFC 1853, October 1995. [RFC1853] Simpson, W., "IP in IP Tunneling", RFC 1853, October 1995.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., [RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001. Encoding", RFC 3032, January 2001.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP [RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007. Specification", RFC 5036, October 2007.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008. Engineering", RFC 5305, October 2008.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, July 2008. for IPv6", RFC 5340, July 2008.
[RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
5714, January 2010.
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free [RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, January 2010. Convergence", RFC 5715, January 2010.
[RFC6119] Harrison, J., Berger, J., and M. Bartlett, "IPv6 Traffic [RFC6119] Harrison, J., Berger, J., and M. Bartlett, "IPv6 Traffic
Engineering in IS-IS", RFC 6119, February 2011. Engineering in IS-IS", RFC 6119, February 2011.
[RFC6571] Filsfils, C., Francois, P., Shand, M., Decraene, B., [RFC6571] Filsfils, C., Francois, P., Shand, M., Decraene, B.,
Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
Alternate (LFA) Applicability in Service Provider (SP) Alternate (LFA) Applicability in Service Provider (SP)
Networks", RFC 6571, June 2012. Networks", RFC 6571, June 2012.
 End of changes. 75 change blocks. 
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