draft-ietf-rtgwg-mrt-frr-architecture-09.txt   draft-ietf-rtgwg-mrt-frr-architecture-10.txt 
Routing Area Working Group A. Atlas Routing Area Working Group A. Atlas
Internet-Draft C. Bowers Internet-Draft C. Bowers
Intended status: Standards Track Juniper Networks Intended status: Standards Track Juniper Networks
Expires: July 13, 2016 G. Enyedi Expires: August 8, 2016 G. Enyedi
Ericsson Ericsson
January 10, 2016 February 5, 2016
An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees
draft-ietf-rtgwg-mrt-frr-architecture-09 draft-ietf-rtgwg-mrt-frr-architecture-10
Abstract Abstract
This document defines the architecture for IP/LDP Fast-Reroute using This document defines the architecture for IP and LDP Fast-Reroute
Maximally Redundant Trees (MRT-FRR). MRT-FRR is a technology that using Maximally Redundant Trees (MRT-FRR). MRT-FRR is a technology
gives link-protection and node-protection with 100% coverage in any that gives link-protection and node-protection with 100% coverage in
network topology that is still connected after the failure. any network topology that is still connected after the failure.
Status of This Memo Status of This Memo
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Importance of 100% Coverage . . . . . . . . . . . . . . . 8 1.1. Importance of 100% Coverage . . . . . . . . . . . . . . . 4
1.2. Partial Deployment and Backwards Compatibility . . . . . 9 1.2. Partial Deployment and Backwards Compatibility . . . . . 5
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 9 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 9 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 11 4. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 7
5. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . 12 5. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . 9
6. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . . 13 6. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . . 9
6.1. MRT Forwarding Mechanisms . . . . . . . . . . . . . . . . 14 6.1. Introduction to MRT Forwarding Options . . . . . . . . . 10
6.1.1. MRT LDP labels . . . . . . . . . . . . . . . . . . . 14 6.1.1. MRT LDP labels . . . . . . . . . . . . . . . . . . . 10
6.1.1.1. Topology-scoped FEC encoded using a single label 6.1.1.1. Topology-scoped FEC encoded using a single label
(Option 1A) . . . . . . . . . . . . . . . . . . . 14 (Option 1A) . . . . . . . . . . . . . . . . . . . 10
6.1.1.2. Topology and FEC encoded using a two label stack 6.1.1.2. Topology and FEC encoded using a two label stack
(Option 1B) . . . . . . . . . . . . . . . . . . . 15 (Option 1B) . . . . . . . . . . . . . . . . . . . 11
6.1.1.3. Compatibility of Option 1A and 1B . . . . . . . . 15 6.1.1.3. Compatibility of MRT LDP Label Options 1A and 1B 12
6.1.1.4. Mandatory support for MRT LDP Label option 1A . . 15 6.1.1.4. Required support for MRT LDP Label options . . . 12
6.1.2. MRT IP tunnels (Options 2A and 2B) . . . . . . . . . 16 6.1.2. MRT IP tunnels (Options 2A and 2B) . . . . . . . . . 12
6.2. Forwarding LDP Unicast Traffic over MRT Paths . . . . . . 16 6.2. Forwarding LDP Unicast Traffic over MRT Paths . . . . . . 13
6.2.1. Forwarding LDP traffic using MRT LDP Labels (Option 6.2.1. Forwarding LDP traffic using MRT LDP Label Option 1A 13
1A) . . . . . . . . . . . . . . . . . . . . . . . . . 17 6.2.2. Forwarding LDP traffic using MRT LDP Label Option 1B 14
6.2.2. Forwarding LDP traffic using MRT LDP Labels (Option
1B) . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.2.3. Other considerations for forwarding LDP traffic using 6.2.3. Other considerations for forwarding LDP traffic using
MRT LDP Labels . . . . . . . . . . . . . . . . . . . 17 MRT LDP Labels . . . . . . . . . . . . . . . . . . . 14
6.3. Forwarding IP Unicast Traffic over MRT Paths . . . . . . 18 6.2.4. Required support for LDP traffic . . . . . . . . . . 14
6.3.1. Tunneling IP traffic using MRT LDP Labels . . . . . . 18 6.3. Forwarding IP Unicast Traffic over MRT Paths . . . . . . 14
6.3.1.1. Tunneling IP traffic using MRT LDP Labels (Option 6.3.1. Tunneling IP traffic using MRT LDP Labels . . . . . . 15
1A) . . . . . . . . . . . . . . . . . . . . . . . 18 6.3.1.1. Tunneling IP traffic using MRT LDP Label Option
6.3.1.2. Tunneling IP traffic using MRT LDP Labels (Option 1A . . . . . . . . . . . . . . . . . . . . . . . 15
1B) . . . . . . . . . . . . . . . . . . . . . . . 19 6.3.1.2. Tunneling IP traffic using MRT LDP Label Option
6.3.2. Tunneling IP traffic using MRT IP Tunnels . . . . . . 19 1B . . . . . . . . . . . . . . . . . . . . . . . 15
6.3.3. Required support . . . . . . . . . . . . . . . . . . 19 6.3.2. Tunneling IP traffic using MRT IP Tunnels . . . . . . 16
7. MRT Island Formation . . . . . . . . . . . . . . . . . . . . 19 6.3.3. Required support for IP traffic . . . . . . . . . . . 16
7.1. IGP Area or Level . . . . . . . . . . . . . . . . . . . . 20 7. MRT Island Formation . . . . . . . . . . . . . . . . . . . . 16
7.2. Support for a specific MRT profile . . . . . . . . . . . 20 7.1. IGP Area or Level . . . . . . . . . . . . . . . . . . . . 17
7.2. Support for a specific MRT profile . . . . . . . . . . . 17
7.3. Excluding additional routers and interfaces from the MRT 7.3. Excluding additional routers and interfaces from the MRT
Island . . . . . . . . . . . . . . . . . . . . . . . . . 21 Island . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.3.1. Existing IGP exclusion mechanisms . . . . . . . . . . 21 7.3.1. Existing IGP exclusion mechanisms . . . . . . . . . . 18
7.3.2. MRT-specific exclusion mechanism . . . . . . . . . . 22 7.3.2. MRT-specific exclusion mechanism . . . . . . . . . . 19
7.4. Connectivity . . . . . . . . . . . . . . . . . . . . . . 22 7.4. Connectivity . . . . . . . . . . . . . . . . . . . . . . 19
7.5. Algorithm for MRT Island Identification . . . . . . . . . 22 7.5. Algorithm for MRT Island Identification . . . . . . . . . 19
8. MRT Profile . . . . . . . . . . . . . . . . . . . . . . . . . 22 8. MRT Profile . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. MRT Profile Options . . . . . . . . . . . . . . . . . . . 22 8.1. MRT Profile Options . . . . . . . . . . . . . . . . . . . 19
8.2. Router-specific MRT paramaters . . . . . . . . . . . . . 24 8.2. Router-specific MRT paramaters . . . . . . . . . . . . . 21
8.3. Default MRT profile . . . . . . . . . . . . . . . . . . . 24 8.3. Default MRT profile . . . . . . . . . . . . . . . . . . . 21
9. LDP signaling extensions and considerations . . . . . . . . . 25 9. LDP signaling extensions and considerations . . . . . . . . . 22
10. Inter-area Forwarding Behavior . . . . . . . . . . . . . . . 25 10. Inter-area Forwarding Behavior . . . . . . . . . . . . . . . 22
10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A . . 26 10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A . . 23
10.1.1. Motivation for Creating the Rainbow-FEC . . . . . . 27 10.1.1. Motivation for Creating the Rainbow-FEC . . . . . . 24
10.2. ABR Forwarding Behavior with IP Tunneling (option 2) . . 27 10.2. ABR Forwarding Behavior with IP Tunneling (option 2) . . 24
10.3. ABR Forwarding Behavior with LDP Label option 1B . . . . 28 10.3. ABR Forwarding Behavior with MRT LDP Label option 1B . . 25
11. Prefixes Multiply Attached to the MRT Island . . . . . . . . 29 11. Prefixes Multiply Attached to the MRT Island . . . . . . . . 26
11.1. Protecting Multi-Homed Prefixes using Tunnel Endpoint 11.1. Protecting Multi-Homed Prefixes using Tunnel Endpoint
Selection . . . . . . . . . . . . . . . . . . . . . . . 31 Selection . . . . . . . . . . . . . . . . . . . . . . . 28
11.2. Protecting Multi-Homed Prefixes using Named Proxy-Nodes 32 11.2. Protecting Multi-Homed Prefixes using Named Proxy-Nodes 29
11.3. MRT Alternates for Destinations Outside the MRT Island . 34 11.3. MRT Alternates for Destinations Outside the MRT Island . 31
12. Network Convergence and Preparing for the Next Failure . . . 34 12. Network Convergence and Preparing for the Next Failure . . . 31
12.1. Micro-loop prevention and MRTs . . . . . . . . . . . . . 35 12.1. Micro-loop prevention and MRTs . . . . . . . . . . . . . 32
12.2. MRT Recalculation for the Default MRT Profile . . . . . 36 12.2. MRT Recalculation for the Default MRT Profile . . . . . 33
13. Implementation Status . . . . . . . . . . . . . . . . . . . . 37 13. Implementation Status . . . . . . . . . . . . . . . . . . . . 34
14. Operational Considerations . . . . . . . . . . . . . . . . . 38 14. Operational Considerations . . . . . . . . . . . . . . . . . 35
14.1. Verifying Forwarding on MRT Paths . . . . . . . . . . . 38 14.1. Verifying Forwarding on MRT Paths . . . . . . . . . . . 35
14.2. Traffic Capacity on Backup Paths . . . . . . . . . . . . 39 14.2. Traffic Capacity on Backup Paths . . . . . . . . . . . . 36
14.3. MRT IP Tunnel Loopback Address Management . . . . . . . 40 14.3. MRT IP Tunnel Loopback Address Management . . . . . . . 38
14.4. MRT-FRR in a Network with Degraded Connectivity . . . . 41 14.4. MRT-FRR in a Network with Degraded Connectivity . . . . 38
14.5. Partial Deployment of MRT-FRR in a Network . . . . . . . 41 14.5. Partial Deployment of MRT-FRR in a Network . . . . . . . 38
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 41 15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42 16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39
17. Security Considerations . . . . . . . . . . . . . . . . . . . 42 17. Security Considerations . . . . . . . . . . . . . . . . . . . 40
18. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 42 18. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 40
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 43 19. References . . . . . . . . . . . . . . . . . . . . . . . . . 41
19.1. Normative References . . . . . . . . . . . . . . . . . . 43 19.1. Normative References . . . . . . . . . . . . . . . . . . 41
19.2. Informative References . . . . . . . . . . . . . . . . . 44 19.2. Informative References . . . . . . . . . . . . . . . . . 42
Appendix A. General Issues with Area Abstraction . . . . . . . . 46 Appendix A. Inter-level Forwarding Behavior for IS-IS . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 47 Appendix B. General Issues with Area Abstraction . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45
1. Introduction 1. Introduction
This document describes a solution for IP/LDP fast-reroute [RFC5714]. This document describes a solution for IP/LDP fast-reroute [RFC5714].
MRT-FRR creates two alternate trees separate from the primary next- MRT-FRR creates two alternate forwarding trees which are distinct
hop forwarding used during stable operation. These two trees are from the primary next-hop forwarding used during stable operation.
maximally diverse from each other, providing link and node protection These two trees are maximally diverse from each other, providing link
for 100% of paths and failures as long as the failure does not cut and node protection for 100% of paths and failures as long as the
the network into multiple pieces. This document defines the failure does not cut the network into multiple pieces. This document
architecture for IP/LDP fast-reroute with MRT. defines the architecture for IP/LDP fast-reroute with MRT.
[I-D.ietf-rtgwg-mrt-frr-algorithm] describes how to compute maximally [I-D.ietf-rtgwg-mrt-frr-algorithm] describes how to compute maximally
redundant trees using a specific algorithm, the MRT Lowpoint redundant trees using a specific algorithm, the MRT Lowpoint
algorithm. The MRT Lowpoint algorithm is used by a router that algorithm. The MRT Lowpoint algorithm is used by a router that
supports the Default MRT Profile, as specified in this document. supports the Default MRT Profile, as specified in this document.
IP/LDP Fast-Reroute with MRT (MRT-FRR) uses two maximally diverse IP/LDP Fast-Reroute with MRT (MRT-FRR) uses two maximally diverse
forwarding topologies to provide alternates. A primary next-hop forwarding topologies to provide alternates. A primary next-hop
should be on only one of the diverse forwarding topologies; thus, the should be on only one of the diverse forwarding topologies; thus, the
other can be used to provide an alternate. Once traffic has been other can be used to provide an alternate. Once traffic has been
skipping to change at page 4, line 24 skipping to change at page 4, line 24
While MRT provides 100% protection for a single link or node failure, While MRT provides 100% protection for a single link or node failure,
it may not protect traffic in the event of multiple simultaneous it may not protect traffic in the event of multiple simultaneous
failures, nor does take into account Shared Risk Link Groups (SRLGs). failures, nor does take into account Shared Risk Link Groups (SRLGs).
Also, while the MRT Lowpoint algorithm is computationally efficient, Also, while the MRT Lowpoint algorithm is computationally efficient,
it is also new. In order for MRT-FRR to function properly, all of it is also new. In order for MRT-FRR to function properly, all of
the other nodes in the network that support MRT must correctly the other nodes in the network that support MRT must correctly
compute next-hops based on the same algorithm, and install the compute next-hops based on the same algorithm, and install the
corresponding forwarding state. This is in contrast to other FRR corresponding forwarding state. This is in contrast to other FRR
methods where the calculation of backup paths generally involves methods where the calculation of backup paths generally involves
repeated application of the simpler and widely-deployed SPF repeated application of the simpler and widely-deployed shortest path
algorithm, and backup paths themselves re-use the forwarding state first (SPF) algorithm, and backup paths themselves re-use the
used for shortest path forwarding of normal traffic. Section 14 forwarding state used for shortest path forwarding of normal traffic.
provides operational guidance related to verification of MRT Section 14 provides operational guidance related to verification of
forwarding paths. MRT forwarding paths.
In addition to supporting IP and LDP unicast fast-reroute, the In addition to supporting IP and LDP unicast fast-reroute, the
diverse forwarding topologies and guarantee of 100% coverage permit diverse forwarding topologies and guarantee of 100% coverage permit
fast-reroute technology to be applied to multicast traffic as fast-reroute technology to be applied to multicast traffic as
described in [I-D.atlas-rtgwg-mrt-mc-arch]. However, the current described in [I-D.atlas-rtgwg-mrt-mc-arch]. However, the current
document does not address the multicast applications of MRTs. document does not address the multicast applications of MRTs.
Figure 1 compares different methods of providing FRR for IP and LDP
traffic, illustrating some of the trade-offs between the different
approaches. For several methods, the methods are separated into
link-protecting (LP) and node-protecting (NP) variants. The first
column indicates whether the method provides 100% protection coverage
(when topologically feasible). The second column indicates whether
or not traffic traversing alternate path can loop when multiple
failures occur.
The third column gives an estimate of the amount of computation
required at each node to support the FRR method, in addition to the
usual SPF computation rooted at the computing node itself. This
metric of comparison is important for implementations that seek to
quickly recompute repair paths following their initial use after a
topology change, without reliance on an external system, in order to
minimize the risk of a new failure occurring before the new repair
paths are in place.
The fourth column indicates the maximum number of additional labels
that need to be applied to packets to support the FRR method, while
the fifth column gives the size of the MPLS label table needed to
support the FRR method. These two metrics may be useful for
evaluating requirements placed on hardware to support the different
FRR methods.
The last column indicates the additional requirements placed on the
control plane by the FRR method, beyond what is required for IGP
shortest path forwarding with LDP.
+---------+-----+------+-------------+-------+-------------+----------+
| Method |100% | Alt. | Additional | Max. | Label table | Control |
| |prot.| can | computation | addit.| size | plane |
| |cov. | loop?| at each | labels|(relative to | reqs. |
| | | | node | | SPF labels) | |
+---------+-----+------+-------------+-------+-------------+----------+
| MRT-FRR | Yes | No | equivalent | 0(LDP)| 3x | MRT- |
| LP and | | | of less | 1(IP) | | specific |
| NP | | | than | | | protocol |
| | | | 3 SPFs | | | extens. |
+---------+-----+------+-------------+-------+-------------+----------+
| LFA LP | No | Yes | SPF per | 0 | 1x | None |
| and NP | | | neighbor | | | |
+---------+-----+------+-------------+-------+-------------+----------+
| Remote | No | Yes | SPF per | 1 | 1x | T-LDP |
| LFA LP | | | neighbor | | | session |
| | | | | | | for each |
| | | | | | | selected |
| | | | | | | PQ node |
+---------+-----+------+-------------+-------+-------------+----------+
| Remote | No | Yes | SPF per nbr | 1 | 1x | T-LDP |
| LFA NP | | | and SPF per | | | session |
| | | | per PQ node | | | for each |
| | | | evaluated | | | selected |
| | | | | | | PQ node |
+---------+-----+------+-------------+-------+-------------+----------+
| Not-Via | Yes | No | SPF per | 1 | (average | T-LDP |
| LP and | | | link and | | number of | session |
| NP | | | per node | | neighbors) | for each |
| | | | in topology | | x | nbr's nbr|
+---------+-----+------+-------------+-------+-------------+----------+
| TI-LFA | Yes | Yes | SPF per | 2 | 1x | uses |
| LP with | | | neighbor | | | SPRING |
| symm. | | | | | | for |
| metrics | | | | | | label |
| | | | | | | dist. |
+---------+-----+------+-------------+-------+-------------+----------+
| TI-LFA | Yes | Yes | # of SPFs |depth | 1x | uses |
| NP or | | | dependent |depend-| | SPRING |
| LP with | | | on topology |ent on | | for |
| asymm. | | | |topo. | | label |
| metrics | | | | | | dist. |
+---------+-----+------+-------------+-------+-------------+----------+
Figure 1: Comparison of IP/LDP FRR Methods
MRT: MRT provides 100% coverage for link and node protection, and
traffic on the alternate paths will not loop. The computation
required on each node is equivalent to less than 3 additional SPFs
in terms of computational complexity. For IP traffic, MRT
requires one additional label, while for LDP traffic, no
additional labels are needed. However, the size of the MPLS label
table is three times what would normally be required for shortest
path LDP forwarding, due to the presence of additional red and
blue labels for each FEC. MRT requires protocol extensions in
order to allow a node to advertise support for MRT as well as a
value for the GADAG Root Selection Priority. It also requires
support for multi-topology LDP extensions.
Loop-Free Alternates (LFA): LFAs [RFC5286] provide limited
topology-dependent coverage for link and node protection.
Restrictions on choice of alternates can be relaxed to improve
coverage, but this can cause forwarding loops if a worse failure
is experienced than protected against. [RFC6571] discusses the
applicability of LFA to different topologies with a focus on
common PoP architectures. The computation required is one SPF per
neighbor. LFA imposes no additional labels imposed, has no impact
on the label table size, and has no additional control plane
requirements.
Remote LFA: Remote LFA [RFC7490] improves coverage over LFA for
both link and node protection, but it does not guarantee 100%
coverage. The alternates can also loop with worse than expected
failures. Computation for link protection is one SPF per
neighbor, while computation for node protection requires an
additional SPF per PQ node [I-D.ietf-rtgwg-rlfa-node-protection].
Remote LFA can impose up to one additional label on the packet,
but does not increase the size of the label table. It requires a
T-LDP session for each selected PQ node.
Not-Via: Not-Via [RFC6981] provides 100% coverage for link and node
failures and does not have potential looping among alternates.
The computation is high with an SPF per potential failure point
(all links and nodes in the topology). When implemented with LDP,
Not-Via adds one additional label to a tunnelled packet. It
significantly increases the size of the label table, multiplying
it by roughly the average number of neighbors. Not-Via also
requires targeted LDP sessions to each neighbor's neighbor.
TI-LFA: Topology Independent Loop-free Alternate Fast Re-route (TI-
LFA) [I-D.francois-rtgwg-segment-routing-ti-lfa] aims to provide
link and node protection of node and adjacency segments within the
Segment Routing (SR) framework. It guarantees complete coverage.
The TI-LFA computation for link-protection is fairly
straightforward, while the computation for node-protection is more
complex. For link-protection with symmetric link costs, TI-LFA
can provide complete coverage by pushing up to two additional
labels. For node protection on arbitrary topologies, the label
stack size can grow significantly based on repair path. Note that
TI-LFA requires shortest path forwarding based on SR Node-SIDs, as
opposed to LDP labels, in order to construct label stacks for
backups paths without relying on a large number of targeted LDP
sessions to learn remote FEC-label bindings. It also requires the
use of Adj-SIDs to achieve 100% coverage.
1.1. Importance of 100% Coverage 1.1. Importance of 100% Coverage
Fast-reroute is based upon the single failure assumption - that the Fast-reroute is based upon the single failure assumption - that the
time between single failures is long enough for a network to time between single failures is long enough for a network to
reconverge and start forwarding on the new shortest paths. That does reconverge and start forwarding on the new shortest paths. That does
not imply that the network will only experience one failure or not imply that the network will only experience one failure or
change. change.
It is straightforward to analyze a particular network topology for It is straightforward to analyze a particular network topology for
coverage. However, a real network does not always have the same coverage. However, a real network does not always have the same
topology. For instance, maintenance events will take links or nodes topology. For instance, maintenance events will take links or nodes
out of use. Simply costing out a link can have a significant effect out of use. Simply costing out a link can have a significant effect
on what LFAs are available. Similarly, after a single failure has on what loop-free alternates (LFAs) are available. Similarly, after
happened, the topology is changed and its associated coverage. a single failure has happened, the topology is changed and its
Finally, many networks have new routers or links added and removed; associated coverage. Finally, many networks have new routers or
each of those changes can have an effect on the coverage for links added and removed; each of those changes can have an effect on
topology-sensitive methods such as LFA and Remote LFA. If fast- the coverage for topology-sensitive methods such as LFA and Remote
reroute is important for the network services provided, then a method LFA. If fast-reroute is important for the network services provided,
that guarantees 100% coverage is important to accomodate natural then a method that guarantees 100% coverage is important to
network topology changes. accommodate natural network topology changes.
Asymmetric link costs are also a common aspect of networks. There
are at least three common causes for them. First, any broadcast
interface is represented by a pseudo-node and has asymmetric link
costs to and from that pseudo-node. Second, when routers come up or
a link with LDP comes up, it is recommended in [RFC5443] and
[RFC6987] that the link metric be raised to the maximum cost; this
may not be symmetric and for [RFC6987] is not expected to be. Third,
techniques such as IGP metric tuning for traffic-engineering can
result in asymmetric link costs. A fast-reroute solution needs to
handle network topologies with asymmetric link costs.
When a network needs to use a micro-loop prevention mechanism When a network needs to use Ordered FIB[RFC6976] or Nearside
[RFC5715] such as Ordered FIB[RFC6976] or Nearside Tunneling[RFC5715] as a micro-loop prevention mechanism [RFC5715],
Tunneling[RFC5715], then the whole IGP area needs to have alternates then the whole IGP area needs to have alternates available. This
available so that the micro-loop prevention mechanism, which requires allows the micro-loop prevention mechanism, which requires slower
slower network convergence, can take the necessary time without network convergence, to take the necessary time without adversely
adversely impacting traffic. Without complete coverage, traffic to impacting traffic. Without complete coverage, traffic to the
the unprotected destinations will be dropped for significantly longer unprotected destinations will be dropped for significantly longer
than with current convergence - where routers individually converge than with current convergence - where routers individually converge
as fast as possible. See Section 12.1 for more discussion of micro- as fast as possible. See Section 12.1 for more discussion of micro-
loop prevention and MRTs. loop prevention and MRTs.
1.2. Partial Deployment and Backwards Compatibility 1.2. Partial Deployment and Backwards Compatibility
MRT-FRR supports partial deployment. Routers advertise their ability MRT-FRR supports partial deployment. Routers advertise their ability
to support MRT. Inside the MRT-capable connected group of routers to support MRT. Inside the MRT-capable connected group of routers
(referred to as an MRT Island), the MRTs are computed. Alternates to (referred to as an MRT Island), the MRTs are computed. Alternates to
destinations outside the MRT Island are computed and depend upon the destinations outside the MRT Island are computed and depend upon the
skipping to change at page 10, line 15 skipping to change at page 6, line 25
Maximally Redundant Trees (MRT): A pair of trees where the path Maximally Redundant Trees (MRT): A pair of trees where the path
from any node X to the root R along the first tree and the path from any node X to the root R along the first tree and the path
from the same node X to the root along the second tree share the from the same node X to the root along the second tree share the
minimum number of nodes and the minimum number of links. Each minimum number of nodes and the minimum number of links. Each
such shared node is a cut-vertex. Any shared links are cut-links. such shared node is a cut-vertex. Any shared links are cut-links.
In graphs that are not 2-connected, it is not possible to compute In graphs that are not 2-connected, it is not possible to compute
RTs. However, it is possible to compute MRTs. MRTs are maximally RTs. However, it is possible to compute MRTs. MRTs are maximally
redundant in the sense that they are as redundant as possible redundant in the sense that they are as redundant as possible
given the constraints of the network graph. given the constraints of the network graph.
DAG: Directed Acyclic Graph - a graph where all links are directed Directed Acyclic Graph (DAG): A graph where all links are directed
and there are no cycles in it. and there are no cycles in it.
ADAG: Almost Directed Acyclic Graph - a graph that, if all links Almost Directed Acyclic Graph (ADAG): A graph with one node
incoming to the root were removed, would be a DAG. designated as the root. The graph has the property that if all
links incoming to the root were removed, then resulting graph
would be a DAG.
GADAG: Generalized ADAG - a graph that is the combination of the Generalized ADAG (GADAG): A graph that is the combination of the
ADAGs of all blocks. ADAGs of all blocks.
MRT-Red: MRT-Red is used to describe one of the two MRTs; it is MRT-Red: MRT-Red is used to describe one of the two MRTs; it is
used to describe the associated forwarding topology and MPLS used to describe the associated forwarding topology and MPLS
multi-topology identifier (MT-ID). Specifically, MRT-Red is the multi-topology identifier (MT-ID). Specifically, MRT-Red is the
decreasing MRT where links in the GADAG are taken in the direction decreasing MRT where links in the GADAG are taken in the direction
from a higher topologically ordered node to a lower one. from a higher topologically ordered node to a lower one.
MRT-Blue: MRT-Blue is used to describe one of the two MRTs; it is MRT-Blue: MRT-Blue is used to describe one of the two MRTs; it is
used to described the associated forwarding topology and MPLS MT- used to described the associated forwarding topology and MPLS MT-
skipping to change at page 11, line 9 skipping to change at page 7, line 19
connected to a router not in the MRT Island and both routers are connected to a router not in the MRT Island and both routers are
in a common area or level. in a common area or level.
Island Neighbor (IN): A router that is not in the MRT Island but is Island Neighbor (IN): A router that is not in the MRT Island but is
adjacent to an IBR and in the same area/level as the IBR. adjacent to an IBR and in the same area/level as the IBR.
named proxy-node: A proxy-node can represent a destination prefix named proxy-node: A proxy-node can represent a destination prefix
that can be attached to the MRT Island via at least two routers. that can be attached to the MRT Island via at least two routers.
It is named if there is a way that traffic can be encapsulated to It is named if there is a way that traffic can be encapsulated to
reach specifically that proxy node; this could be because there is reach specifically that proxy node; this could be because there is
an LDP FEC for the associated prefix or because MRT-Red and MRT- an LDP FEC (Forwarding Equivalence Class) for the associated
Blue IP addresses are advertised in an undefined fashion for that prefix or because MRT-Red and MRT-Blue IP addresses are advertised
proxy-node. in an undefined fashion for that proxy-node.
4. Maximally Redundant Trees (MRT) 4. Maximally Redundant Trees (MRT)
A pair of Maximally Redundant Trees is a pair of directed spanning A pair of Maximally Redundant Trees is a pair of directed spanning
trees that provides maximally disjoint paths towards their common trees that provides maximally disjoint paths towards their common
root. Only links or nodes whose failure would partition the network root. Only links or nodes whose failure would partition the network
(i.e. cut-links and cut-vertices) are shared between the trees. The (i.e. cut-links and cut-vertices) are shared between the trees. The
MRT Lowpoint algorithm is given in MRT Lowpoint algorithm is given in
[I-D.ietf-rtgwg-mrt-frr-algorithm]. This algorithm can be computed [I-D.ietf-rtgwg-mrt-frr-algorithm]. This algorithm can be computed
in O(e + n log n); it is less than three SPFs. This document in O(e + n log n); it is less than three SPFs. This document
skipping to change at page 11, line 34 skipping to change at page 7, line 44
MRT provides destination-based trees for each destination. Each MRT provides destination-based trees for each destination. Each
router stores its normal primary next-hop(s) as well as MRT-Blue router stores its normal primary next-hop(s) as well as MRT-Blue
next-hop(s) and MRT-Red next-hop(s) toward each destination. The next-hop(s) and MRT-Red next-hop(s) toward each destination. The
alternate will be selected between the MRT-Blue and MRT-Red. alternate will be selected between the MRT-Blue and MRT-Red.
The most important thing to understand about MRTs is that for each The most important thing to understand about MRTs is that for each
pair of destination-routed MRTs, there is a path from every node X to pair of destination-routed MRTs, there is a path from every node X to
the destination D on the Blue MRT that is as disjoint as possible the destination D on the Blue MRT that is as disjoint as possible
from the path on the Red MRT. from the path on the Red MRT.
For example, in Figure 2, there is a network graph that is For example, in Figure 1, there is a network graph that is
2-connected in (a) and associated MRTs in (b) and (c). One can 2-connected in (a) and associated MRTs in (b) and (c). One can
consider the paths from B to R; on the Blue MRT, the paths are consider the paths from B to R; on the Blue MRT, the paths are
B->F->D->E->R or B->C->D->E->R. On the Red MRT, the path is B->A->R. B->F->D->E->R or B->C->D->E->R. On the Red MRT, the path is B->A->R.
These are clearly link and node-disjoint. These MRTs are redundant These are clearly link and node-disjoint. These MRTs are redundant
trees because the paths are disjoint. trees because the paths are disjoint.
[E]---[D]---| [E]<--[D]<--| [E]-->[D]---| [E]---[D]---| [E]<--[D]<--| [E]-->[D]---|
| | | | ^ | | | | | | | ^ | | |
| | | V | | V V | | | V | | V V
[R] [F] [C] [R] [F] [C] [R] [F] [C] [R] [F] [C] [R] [F] [C] [R] [F] [C]
| | | ^ ^ ^ | | | | | ^ ^ ^ | |
| | | | | | V | | | | | | | V |
[A]---[B]---| [A]-->[B]---| [A]<--[B]<--| [A]---[B]---| [A]-->[B]---| [A]<--[B]<--|
(a) (b) (c) (a) (b) (c)
a 2-connected graph Blue MRT towards R Red MRT towards R a 2-connected graph Blue MRT towards R Red MRT towards R
Figure 2: A 2-connected Network Figure 1: A 2-connected Network
By contrast, in Figure 3, the network in (a) is not 2-connected. If By contrast, in Figure 2, the network in (a) is not 2-connected. If
F, G or the link F<->G failed, then the network would be partitioned. F, G or the link F<->G failed, then the network would be partitioned.
It is clearly impossible to have two link-disjoint or node-disjoint It is clearly impossible to have two link-disjoint or node-disjoint
paths from G, I or J to R. The MRTs given in (b) and (c) offer paths paths from G, I or J to R. The MRTs given in (b) and (c) offer paths
that are as disjoint as possible. For instance, the paths from B to that are as disjoint as possible. For instance, the paths from B to
R are the same as in Figure 2 and the path from G to R on the Blue R are the same as in Figure 1 and the path from G to R on the Blue
MRT is G->F->D->E->R and on the Red MRT is G->F->B->A->R. MRT is G->F->D->E->R and on the Red MRT is G->F->B->A->R.
[E]---[D]---| [E]---[D]---|
| | | |----[I] | | | |----[I]
| | | | | | | | | |
[R]---[C] [F]---[G] | [R]---[C] [F]---[G] |
| | | | | | | | | |
| | | |----[J] | | | |----[J]
[A]---[B]---| [A]---[B]---|
skipping to change at page 12, line 35 skipping to change at page 8, line 48
| ^ | [I] | |----[I] | ^ | [I] | |----[I]
V | | | V V ^ V | | | V V ^
[R] [C] [F]<--[G] | [R]<--[C] [F]<--[G] | [R] [C] [F]<--[G] | [R]<--[C] [F]<--[G] |
^ ^ ^ V ^ | | ^ ^ ^ V ^ | |
| | |----[J] | | [J] | | |----[J] | | [J]
[A]-->[B]---| [A]<--[B]<--| [A]-->[B]---| [A]<--[B]<--|
(b) (c) (b) (c)
Blue MRT towards R Red MRT towards R Blue MRT towards R Red MRT towards R
Figure 3: A non-2-connected network Figure 2: A non-2-connected network
5. Maximally Redundant Trees (MRT) and Fast-Reroute 5. Maximally Redundant Trees (MRT) and Fast-Reroute
In normal IGP routing, each router has its shortest path tree (SPT)to In normal IGP routing, each router has its shortest path tree (SPT)
all destinations. From the perspective of a particular destination, to all destinations. From the perspective of a particular
D, this looks like a reverse SPT. To use maximally redundant trees, destination, D, this looks like a reverse SPT. To use maximally
in addition, each destination D has two MRTs associated with it; by redundant trees, in addition, each destination D has two MRTs
convention these will be called the MRT-Blue and MRT-Red. MRT-FRR is associated with it; by convention these will be called the MRT-Blue
realized by using multi-topology forwarding. There is a MRT-Blue and MRT-Red. MRT-FRR is realized by using multi-topology forwarding.
forwarding topology and a MRT-Red forwarding topology. There is a MRT-Blue forwarding topology and a MRT-Red forwarding
topology.
Any IP/LDP fast-reroute technique beyond LFA requires an additional Any IP/LDP fast-reroute technique beyond LFA requires an additional
dataplane procedure, such as an additional forwarding mechanism. The dataplane procedure, such as an additional forwarding mechanism. The
well-known options are multi-topology forwarding (used by MRT-FRR), well-known options are multi-topology forwarding (used by MRT-FRR),
tunneling (e.g. [RFC6981] or [RFC7490]), and per-interface tunneling (e.g. [RFC6981] or [RFC7490]), and per-interface
forwarding (e.g. Loop-Free Failure Insensitive Routing in forwarding (e.g. Loop-Free Failure Insensitive Routing in
[EnyediThesis]). [EnyediThesis]).
When there is a link or node failure affecting, but not partitioning, When there is a link or node failure affecting, but not partitioning,
the network, each node will still have at least one path via one of the network, each node will still have at least one path via one of
the MRTs to reach the destination D. For example, in Figure 3, C the MRTs to reach the destination D. For example, in Figure 2, C
would normally forward traffic to R across the C<->R link. If that would normally forward traffic to R across the C<->R link. If that
C<->R link fails, then C could use the Blue MRT path C->D->E->R. C<->R link fails, then C could use the Blue MRT path C->D->E->R.
As is always the case with fast-reroute technologies, forwarding does As is always the case with fast-reroute technologies, forwarding does
not change until a local failure is detected. Packets are forwarded not change until a local failure is detected. Packets are forwarded
along the shortest path. The appropriate alternate to use is pre- along the shortest path. The appropriate alternate to use is pre-
computed. [I-D.ietf-rtgwg-mrt-frr-algorithm] describes exactly how computed. [I-D.ietf-rtgwg-mrt-frr-algorithm] describes exactly how
to determine whether the MRT-Blue next-hops or the MRT-Red next-hops to determine whether the MRT-Blue next-hops or the MRT-Red next-hops
should be the MRT alternate next-hops for a particular primary next- should be the MRT alternate next-hops for a particular primary next-
hop to a particular destination. hop to a particular destination.
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looping among alternates. Section 1.1 of [RFC5286] gives an example looping among alternates. Section 1.1 of [RFC5286] gives an example
of link-protecting alternates causing a loop on node failure. Even of link-protecting alternates causing a loop on node failure. Even
if a worse failure than anticipated happens, the use of MRT if a worse failure than anticipated happens, the use of MRT
alternates will not cause looping. alternates will not cause looping.
6. Unicast Forwarding with MRT Fast-Reroute 6. Unicast Forwarding with MRT Fast-Reroute
There are three possible types of routers involved in forwarding a There are three possible types of routers involved in forwarding a
packet along an MRT path. At the MRT ingress router, the packet packet along an MRT path. At the MRT ingress router, the packet
leaves the shortest path to the destination and follows an MRT path leaves the shortest path to the destination and follows an MRT path
to the destination. In a FRR application, the MRT ingress router is to the destination. In an FRR application, the MRT ingress router is
the PLR. An MRT transit router takes a packet that arrives already the PLR. An MRT transit router takes a packet that arrives already
associated with the particular MRT, and forwards it on that same MRT. associated with the particular MRT, and forwards it on that same MRT.
In some situations (to be discussed later), the packet will need to In some situations (to be discussed later), the packet will need to
leave the MRT path and return to the shortest path. This takes place leave the MRT path and return to the shortest path. This takes place
at the MRT egress router. The MRT ingress and egress functionality at the MRT egress router. The MRT ingress and egress functionality
may depend on the underlying type of packet being forwarded (LDP or may depend on the underlying type of packet being forwarded (LDP or
IP). The MRT transit functionality is independent of the type of IP). The MRT transit functionality is independent of the type of
packet being forwarded. We first consider several MRT transit packet being forwarded. We first consider several MRT transit
forwarding mechanisms. Then we look at how these forwarding forwarding mechanisms. Then we look at how these forwarding
mechanisms can be applied to carrying LDP and IP traffic. mechanisms can be applied to carrying LDP and IP traffic.
6.1. MRT Forwarding Mechanisms 6.1. Introduction to MRT Forwarding Options
The following options for MRT forwarding mechanisms are considered. The following options for MRT forwarding mechanisms are considered.
1. MRT LDP Labels 1. MRT LDP Labels
A. Topology-scoped FEC encoded using a single label A. Topology-scoped FEC encoded using a single label
B. Topology and FEC encoded using a two label stack B. Topology and FEC encoded using a two label stack
2. MRT IP Tunnels 2. MRT IP Tunnels
skipping to change at page 15, line 8 skipping to change at page 11, line 20
This forwarding mechanism has the useful property that the FEC This forwarding mechanism has the useful property that the FEC
associated with the packet is maintained in the labels at each hop associated with the packet is maintained in the labels at each hop
along the MRT. We will take advantage of this property when along the MRT. We will take advantage of this property when
specifying how to carry LDP traffic on MRT paths using multi-topology specifying how to carry LDP traffic on MRT paths using multi-topology
LDP labels. LDP labels.
This approach is very simple for hardware to support. However, it This approach is very simple for hardware to support. However, it
reduces the label space for other uses, and it increases the memory reduces the label space for other uses, and it increases the memory
needed to store the labels and the communication required by LDP to needed to store the labels and the communication required by LDP to
distribute FEC-label bindings. distribute FEC-label bindings. In general, this approach will also
increase the time needed to install the FRR entries in the Forwarding
Information Base (FIB) and hence the time needed before the next
failure can be protected.
This forwarding option uses the LDP signaling extensions described in This forwarding option uses the LDP signaling extensions described in
[RFC7307]. The MRT-specific LDP extensions required to support this [RFC7307]. The MRT-specific LDP extensions required to support this
option will be described elsewhere. option will be described elsewhere.
6.1.1.2. Topology and FEC encoded using a two label stack (Option 1B) 6.1.1.2. Topology and FEC encoded using a two label stack (Option 1B)
With this forwarding mechanism, a two label stack is used to encode With this forwarding mechanism, a two label stack is used to encode
the topology and the FEC of the packet. The top label (topology-id the topology and the FEC of the packet. The top label (topology-id
label) identifies the MRT forwarding topology, while the second label label) identifies the MRT forwarding topology, while the second label
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labels at each hop along the MRT. labels at each hop along the MRT.
This forwarding mechanism has minimal usage of additional labels, This forwarding mechanism has minimal usage of additional labels,
memory and LDP communication. It does increase the size of packets memory and LDP communication. It does increase the size of packets
and the complexity of the required label operations and look-ups. and the complexity of the required label operations and look-ups.
This forwarding option is consistent with context-specific label This forwarding option is consistent with context-specific label
spaces, as described in [RFC5331]. However, the precise LDP behavior spaces, as described in [RFC5331]. However, the precise LDP behavior
required to support this option for MRT has not been specified. required to support this option for MRT has not been specified.
6.1.1.3. Compatibility of Option 1A and 1B 6.1.1.3. Compatibility of MRT LDP Label Options 1A and 1B
In principle, MRT transit forwarding mechanisms 1A and 1B can coexist MRT transit forwarding based on MRT LDP Label options 1A and 1B can
in the same network, with a packet being forwarding along a single coexist in the same network, with a packet being forwarded along a
MRT path using the single label of option 1A for some hops and the single MRT path using the single label of option 1A for some hops and
two label stack of option 1B for other hops. the two label stack of option 1B for other hops. However, to
simplify the process of MRT Island formation we require that all
routers in the MRT Island support at least one common forwarding
mechanism. As an example, the Default MRT Profile requires support
for the MRT LDP Label Option 1A forwarding mechanism. This ensures
that the routers in an MRT island supporting the Default MRT Profile
will be able to establish MRT forwarding paths based on MRT LDP Label
Option 1A. However, an implementation supporting Option 1A may also
support Option 1B. If the scaling or performance characteristics for
the two options differ in this implementation, then it may be
desirable for a pair of adjacent routers to use Option 1B labels
instead of the Option 1A labels. If those routers successfully
negotiate the use of Option 1B labels, they are free to use them.
This can occur without any of the other routers in the MRT Island
being made aware of it.
6.1.1.4. Mandatory support for MRT LDP Label option 1A Note that this document only defines the Default MRT Profile which
requires support for the MRT LDP Label Option 1A forwarding
mechanism.
If a router supports a profile that includes the MRT LDP Label option 6.1.1.4. Required support for MRT LDP Label options
for MRT transit forwarding mechanism, then it MUST support option 1A,
which encodes topology-scoped FECs using a single label. If a router supports a profile that includes the MRT LDP Label Option
1A for the MRT transit forwarding mechanism, then it MUST support
option 1A, which encodes topology-scoped FECs using a single label.
The router MAY also support option 1B.
If a router supports a profile that includes the MRT LDP Label Option
1B for the MRT transit forwarding mechanism, then it MUST support
option 1B, which encodes the topology and FEC using a two label
stack. The router MAY also support option 1A.
6.1.2. MRT IP tunnels (Options 2A and 2B) 6.1.2. MRT IP tunnels (Options 2A and 2B)
IP tunneling can also be used as an MRT transit forwarding mechanism. IP tunneling can also be used as an MRT transit forwarding mechanism.
Each router supporting this MRT transit forwarding mechanism Each router supporting this MRT transit forwarding mechanism
announces two additional loopback addresses and their associated MRT announces two additional loopback addresses and their associated MRT
color. Those addresses are used as destination addresses for MRT- color. Those addresses are used as destination addresses for MRT-
blue and MRT-red IP tunnels respectively. The special loopback blue and MRT-red IP tunnels respectively. The special loopback
addresses allow the transit nodes to identify the traffic as being addresses allow the transit nodes to identify the traffic as being
forwarded along either the MRT-blue or MRT-red topology to reach the forwarded along either the MRT-blue or MRT-red topology to reach the
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In principle, it is possible to carry LDP traffic in MRT IP tunnels. In principle, it is possible to carry LDP traffic in MRT IP tunnels.
However, for LDP traffic, it is desirable to avoid tunneling. However, for LDP traffic, it is desirable to avoid tunneling.
Tunneling LDP traffic to a remote node requires knowledge of remote Tunneling LDP traffic to a remote node requires knowledge of remote
FEC-label bindings so that the LDP traffic can continue to be FEC-label bindings so that the LDP traffic can continue to be
forwarded properly when it leaves the tunnel. This requires targeted forwarded properly when it leaves the tunnel. This requires targeted
LDP sessions which can add management complexity. As described LDP sessions which can add management complexity. As described
below, the two MRT forwarding mechanisms that use LDP labels do not below, the two MRT forwarding mechanisms that use LDP labels do not
require targeted LDP sessions. require targeted LDP sessions.
6.2.1. Forwarding LDP traffic using MRT LDP Labels (Option 1A) 6.2.1. Forwarding LDP traffic using MRT LDP Label Option 1A
The MRT LDP Label option 1A forwarding mechanism uses topology-scoped The MRT LDP Label option 1A forwarding mechanism uses topology-scoped
FECs encoded using a single label as described in section FECs encoded using a single label as described in section
Section 6.1.1.1. When a PLR receives an LDP packet that needs to be Section 6.1.1.1. When a PLR receives an LDP packet that needs to be
forwarded on the Red MRT (for example), it does a label swap forwarded on the Red MRT (for example), it does a label swap
operation, replacing the usual LDP label for the FEC with the Red MRT operation, replacing the usual LDP label for the FEC with the Red MRT
label for that FEC received from the next-hop router in the Red MRT label for that FEC received from the next-hop router in the Red MRT
computed by the PLR. When the next-hop router in the Red MRT computed by the PLR. When the next-hop router in the Red MRT
receives the packet with the Red MRT label for the FEC, the MRT receives the packet with the Red MRT label for the FEC, the MRT
transit forwarding functionality continues as described in transit forwarding functionality continues as described in
Section 6.1.1.1. In this way the original FEC associated with the Section 6.1.1.1. In this way the original FEC associated with the
packet is maintained at each hop along the MRT. packet is maintained at each hop along the MRT.
6.2.2. Forwarding LDP traffic using MRT LDP Labels (Option 1B) 6.2.2. Forwarding LDP traffic using MRT LDP Label Option 1B
The MRT LDP Label option 1B forwarding mechanism encodes the topology The MRT LDP Label option 1B forwarding mechanism encodes the topology
and the FEC using a two label stack as described in Section 6.1.1.2. and the FEC using a two label stack as described in Section 6.1.1.2.
When a PLR receives an LDP packet that needs to be forwarded on the When a PLR receives an LDP packet that needs to be forwarded on the
Red MRT, it first does a normal LDP label swap operation, replacing Red MRT, it first does a normal LDP label swap operation, replacing
the incoming normal LDP label associated with a given FEC with the the incoming normal LDP label associated with a given FEC with the
outgoing normal LDP label for that FEC learned from the next-hop on outgoing normal LDP label for that FEC learned from the next-hop on
the Red MRT. In addition, the PLR pushes the topology-identification the Red MRT. In addition, the PLR pushes the topology-identification
label associated with the Red MRT, and forward the packet to the label associated with the Red MRT, and forward the packet to the
appropriate next-hop on the Red MRT. When the next-hop router in the appropriate next-hop on the Red MRT. When the next-hop router in the
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destination FEC is used. The alternates selected in destination FEC is used. The alternates selected in
[I-D.ietf-rtgwg-mrt-frr-algorithm] use the MRT path to the [I-D.ietf-rtgwg-mrt-frr-algorithm] use the MRT path to the
destination FEC, so targeted LDP sessions are not needed. If instead destination FEC, so targeted LDP sessions are not needed. If instead
one found it desirable to have the PLR use an MRT to reach the one found it desirable to have the PLR use an MRT to reach the
primary next-next-hop for the FEC, and then continue forwarding the primary next-next-hop for the FEC, and then continue forwarding the
LDP packet along the shortest path tree from the primary next-next- LDP packet along the shortest path tree from the primary next-next-
hop, this would require tunneling to the primary next-next-hop and a hop, this would require tunneling to the primary next-next-hop and a
targeted LDP session for the PLR to learn the FEC-label binding for targeted LDP session for the PLR to learn the FEC-label binding for
primary next-next-hop to correctly forward the packet. primary next-next-hop to correctly forward the packet.
6.2.4. Required support for LDP traffic
For greatest hardware compatibility, routers implementing MRT fast- For greatest hardware compatibility, routers implementing MRT fast-
reroute of LDP traffic MUST support Option 1A of encoding the MT-ID reroute of LDP traffic MUST support Option 1A of encoding the MT-ID
in the labels (See Section 9). in the labels (See Section 9).
6.3. Forwarding IP Unicast Traffic over MRT Paths 6.3. Forwarding IP Unicast Traffic over MRT Paths
For IPv4 traffic, there is no currently practical alternative except For IPv4 traffic, there is no currently practical alternative except
tunneling to gain the bits needed to indicate the MRT-Blue or MRT-Red tunneling to gain the bits needed to indicate the MRT-Blue or MRT-Red
forwarding topology. For IPv6 traffic, in principle one could define forwarding topology. For IPv6 traffic, in principle one could define
bits in the IPv6 options header to indicate the MRT-Blue or MRT-Red bits in the IPv6 options header to indicate the MRT-Blue or MRT-Red
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choice is the next-next-hop towards the destination. As discussed in choice is the next-next-hop towards the destination. As discussed in
the previous section, for LDP traffic, using the MRT to the original the previous section, for LDP traffic, using the MRT to the original
destination simplifies MRT-FRR by avoiding the need for targeted LDP destination simplifies MRT-FRR by avoiding the need for targeted LDP
sessions to the next-next-hop. For IP, that consideration doesn't sessions to the next-next-hop. For IP, that consideration doesn't
apply. apply.
Some situations require tunneling IP traffic along an MRT to a tunnel Some situations require tunneling IP traffic along an MRT to a tunnel
endpoint that is not the destination of the IP traffic. These endpoint that is not the destination of the IP traffic. These
situations will be discussed in detail later. We note here that an situations will be discussed in detail later. We note here that an
IP packet with a destination in a different IGP area/level from the IP packet with a destination in a different IGP area/level from the
PLR should be tunneled on the MRT to the ABR/LBR on the shortest path PLR should be tunneled on the MRT to the Area Border Router (ABR) or
to the destination. For a destination outside of the PLR's MRT Level Border Router (LBR) on the shortest path to the destination.
Island, the packet should be tunneled on the MRT to a non-proxy-node For a destination outside of the PLR's MRT Island, the packet should
immediately before the named proxy-node on that particular color MRT. be tunneled on the MRT to a non-proxy-node immediately before the
named proxy-node on that particular color MRT.
6.3.1. Tunneling IP traffic using MRT LDP Labels 6.3.1. Tunneling IP traffic using MRT LDP Labels
An IP packet can be tunneled along an MRT path by pushing the An IP packet can be tunneled along an MRT path by pushing the
appropriate MRT LDP label(s). Tunneling using LDP labels, as opposed appropriate MRT LDP label(s). Tunneling using LDP labels, as opposed
to IP headers, has the the advantage that more installed routers can to IP headers, has the the advantage that more installed routers can
do line-rate encapsulation and decapsulation using LDP than using IP. do line-rate encapsulation and decapsulation using LDP than using IP.
Also, no additional IP addresses would need to be allocated or Also, no additional IP addresses would need to be allocated or
signaled. signaled.
6.3.1.1. Tunneling IP traffic using MRT LDP Labels (Option 1A) 6.3.1.1. Tunneling IP traffic using MRT LDP Label Option 1A
The MRT LDP Label option 1A forwarding mechanism uses topology-scoped The MRT LDP Label option 1A forwarding mechanism uses topology-scoped
FECs encoded using a single label as described in section FECs encoded using a single label as described in section
Section 6.1.1.1. When a PLR receives an IP packet that needs to be Section 6.1.1.1. When a PLR receives an IP packet that needs to be
forwarded on the Red MRT to a particular tunnel endpoint, it does a forwarded on the Red MRT to a particular tunnel endpoint, it does a
label push operation. The label pushed is the Red MRT label for a label push operation. The label pushed is the Red MRT label for a
FEC originated by the tunnel endpoint, learned from the next-hop on FEC originated by the tunnel endpoint, learned from the next-hop on
the Red MRT. the Red MRT.
6.3.1.2. Tunneling IP traffic using MRT LDP Labels (Option 1B) 6.3.1.2. Tunneling IP traffic using MRT LDP Label Option 1B
The MRT LDP Label option 1B forwarding mechanism encodes the topology The MRT LDP Label option 1B forwarding mechanism encodes the topology
and the FEC using a two label stack as described in Section 6.1.1.2. and the FEC using a two label stack as described in Section 6.1.1.2.
When a PLR receives an IP packet that needs to be forwarded on the When a PLR receives an IP packet that needs to be forwarded on the
Red MRT to a particular tunnel endpoint, the PLR pushes two labels on Red MRT to a particular tunnel endpoint, the PLR pushes two labels on
the IP packet. The first (inner) label is the normal LDP label the IP packet. The first (inner) label is the normal LDP label
learned from the next-hop on the Red MRT, associated with a FEC learned from the next-hop on the Red MRT, associated with a FEC
originated by the tunnel endpoint. The second (outer) label is the originated by the tunnel endpoint. The second (outer) label is the
topology-identification label associated with the Red MRT. topology-identification label associated with the Red MRT.
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identification label directly onto the packet. An MRT transit router identification label directly onto the packet. An MRT transit router
would need to pop the topology-id label, do an IP route lookup in the would need to pop the topology-id label, do an IP route lookup in the
context of that topology-id , and push the topology-id label. context of that topology-id , and push the topology-id label.
6.3.2. Tunneling IP traffic using MRT IP Tunnels 6.3.2. Tunneling IP traffic using MRT IP Tunnels
In order to tunnel over the MRT to a particular tunnel endpoint, the In order to tunnel over the MRT to a particular tunnel endpoint, the
PLR encapsulates the original IP packet with an additional IP header PLR encapsulates the original IP packet with an additional IP header
using the MRT-Blue or MRT-Red loopack address of the tunnel endpoint. using the MRT-Blue or MRT-Red loopack address of the tunnel endpoint.
6.3.3. Required support 6.3.3. Required support for IP traffic
For greatest hardware compatibility and ease in removing the MRT- For greatest hardware compatibility and ease in removing the MRT-
topology marking at area/level boundaries, routers that support MPLS topology marking at area/level boundaries, routers that support MPLS
and implement IP MRT fast-reroute MUST support tunneling of IP and implement IP MRT fast-reroute MUST support tunneling of IP
traffic using MRT LDP Labels Option 1A (topology-scoped FEC encoded traffic using MRT LDP Label Option 1A (topology-scoped FEC encoded
using a single label). using a single label).
7. MRT Island Formation 7. MRT Island Formation
The purpose of communicating support for MRT is to indicate that the The purpose of communicating support for MRT is to indicate that the
MRT-Blue and MRT-Red forwarding topologies are created for transit MRT-Blue and MRT-Red forwarding topologies are created for transit
traffic. The MRT architecture allows for different, potentially traffic. The MRT architecture allows for different, potentially
incompatible options. In order to create consistent MRT forwarding incompatible options. In order to create consistent MRT forwarding
topologies, the routers participating in a particular MRT Island need topologies, the routers participating in a particular MRT Island need
to use the same set of options. These options are grouped into MRT to use the same set of options. These options are grouped into MRT
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and can be used by MRT in Island formation, subject to the and can be used by MRT in Island formation, subject to the
interpretation defined here. interpretation defined here.
Other information needs to be communicated between routers for which Other information needs to be communicated between routers for which
there do not currently exist protocol extensions. This new there do not currently exist protocol extensions. This new
information needs to be shared among all routers in an IGP area, so information needs to be shared among all routers in an IGP area, so
defining extensions to existing IGPs to carry this information makes defining extensions to existing IGPs to carry this information makes
sense. These new protocol extensions will be defined elsewhere. sense. These new protocol extensions will be defined elsewhere.
Deployment scenarios using multi-topology OSPF or IS-IS, or running Deployment scenarios using multi-topology OSPF or IS-IS, or running
both ISIS and OSPF on the same routers is out of scope for this both IS-IS and OSPF on the same routers is out of scope for this
specification. As with LFA, it is expected that OSPF Virtual Links specification. As with LFA, it is expected that OSPF Virtual Links
will not be supported. will not be supported.
At a high level, an MRT Island is defined as the set of routers
supporting the same MRT profile, in the same IGP area/level and the
bi-directional links interconnecting those routers. More detailed
descriptions of these criteria are given below.
7.1. IGP Area or Level 7.1. IGP Area or Level
All links in an MRT Island MUST be bidirectional and belong to the All links in an MRT Island are bidirectional and belong to the same
same IGP area or level. For ISIS, a link belonging to both level 1 IGP area or level. For IS-IS, a link belonging to both level 1 and
and level 2 would qualify to be in multiple MRT Islands. A given ABR level 2 would qualify to be in multiple MRT Islands. A given ABR or
or LBR can belong to multiple MRT Islands, corresponding to the areas LBR can belong to multiple MRT Islands, corresponding to the areas or
or levels in which it participates. Inter-area forwarding behavior levels in which it participates. Inter-area forwarding behavior is
is discussed in Section 10. discussed in Section 10.
7.2. Support for a specific MRT profile 7.2. Support for a specific MRT profile
All routers in an MRT Island MUST support the same MRT profile. A All routers in an MRT Island support the same MRT profile. A router
router advertises support for a given MRT profile using an 8-bit MRT advertises support for a given MRT profile using an 8-bit MRT Profile
Profile ID value. The registry for the MRT Profile ID is defined in ID value. The registry for the MRT Profile ID is defined in this
this document. The protocol extensions for advertising the MRT document. The protocol extensions for advertising the MRT Profile ID
Profile ID value will be defined elsewhere. A given router can value will be defined elsewhere. A given router can support multiple
support multiple MRT profiles and participate in multiple MRT MRT profiles and participate in multiple MRT Islands. The options
Islands. The options that make up an MRT profile, as well as the that make up an MRT profile, as well as the default MRT profile, are
default MRT profile, are defined in Section 8. defined in Section 8.
Note that a router may advertise support for multiple different MRT The process of MRT Island formation takes place independently for
profiles. The process of MRT Island formation takes place each MRT profile advertised by a given router. For example, consider
independently for each MRT profile advertised by a given router. For a network with 40 connected routers in the same area advertising
example, consider a network with 40 connected routers in the same support for MRT Profile A and MRT Profile B. Two distinct MRT
area advertising support for MRT Profile A and MRT Profile B. Two Islands will be formed corresponding to Profile A and Profile B, with
distinct MRT Islands will be formed corresponding to Profile A and each island containing all 40 routers. A complete set of maximally
Profile B, with each island containing all 40 routers. A complete redundant trees will be computed for each island following the rules
set of maximally redundant trees will be computed for each island defined for each profile. If we add a third MRT Profile to this
following the rules defined for each profile. If we add a third MRT example, with Profile C being advertised by a connected subset of 30
Profile to this example, with Profile C being advertised by a routers, there will be a third MRT Island formed corresponding to
connected subset of 30 routers, there will be a third MRT Island those 30 routers, and a third set of maximally redundant trees will
formed corresponding to those 30 routers, and a third set of be computed. In this example, 40 routers would compute and install
maximally redundant trees will be computed. In this example, 40 two sets of MRT transit forwarding entries corresponding to Profiles
routers would compute and install two sets of MRT transit forwarding A and B, while 30 routers would compute and install three sets of MRT
entries corresponding to Profiles A and B, while 30 routers would transit forwarding entries corresponding to Profiles A, B, and C.
compute and install three sets of MRT transit forwarding entries
corresponding to Profiles A, B, and C.
7.3. Excluding additional routers and interfaces from the MRT Island 7.3. Excluding additional routers and interfaces from the MRT Island
MRT takes into account existing IGP mechanisms for discouraging MRT takes into account existing IGP mechanisms for discouraging
traffic from using particular links and routers, and it introduces an traffic from using particular links and routers, and it introduces an
MRT-specific exclusion mechanism for links. MRT-specific exclusion mechanism for links.
7.3.1. Existing IGP exclusion mechanisms 7.3.1. Existing IGP exclusion mechanisms
Mechanisms for discouraging traffic from using particular links Mechanisms for discouraging traffic from using particular links
already exist in ISIS and OSPF. In ISIS, an interface configured already exist in IS-IS and OSPF. In IS-IS, an interface configured
with a metric of 2^24-2 (0xFFFFFE) will only be used as a last with a metric of 2^24-2 (0xFFFFFE) will only be used as a last
resort. (An interface configured with a metric of 2^24-1 (0xFFFFFF) resort. (An interface configured with a metric of 2^24-1 (0xFFFFFF)
will not be advertised into the topology.) In OSPF, an interface will not be advertised into the topology.) In OSPF, an interface
configured with a metric of 2^16-1 (0xFFFF) will only be used as a configured with a metric of 2^16-1 (0xFFFF) will only be used as a
last resort. These metrics can be configured manually to enforce last resort. These metrics can be configured manually to enforce
administrative policy, or they can be set in an automated manner as administrative policy, or they can be set in an automated manner as
with LDP IGP synchronization [RFC5443]. with LDP IGP synchronization [RFC5443].
Mechanisms also already exist in ISIS and OSPF to discourage or Mechanisms also already exist in IS-IS and OSPF to discourage or
prevent transit traffic from using a particular router. In ISIS, the prevent transit traffic from using a particular router. In IS-IS,
overload bit is prevents transit traffic from using a router. the overload bit is prevents transit traffic from using a router.
For OSPFv2 and OSPFv3, [RFC6987] specifies setting all outgoing For OSPFv2 and OSPFv3, [RFC6987] specifies setting all outgoing
interface metrics to 0xFFFF to discourage transit traffic from using interface metrics to 0xFFFF to discourage transit traffic from using
a router.( [RFC6987] defines the metric value 0xFFFF as a router.( [RFC6987] defines the metric value 0xFFFF as
MaxLinkMetric, a fixed architectural value for OSPF.) For OSPFv3, MaxLinkMetric, a fixed architectural value for OSPF.) For OSPFv3,
[RFC5340] specifies that a router be excluded from the intra-area [RFC5340] specifies that a router be excluded from the intra-area
shortest path tree computation if the V6-bit or R-bit of the LSA shortest path tree computation if the V6-bit or R-bit of the LSA
options is not set in the Router LSA. options is not set in the Router LSA.
The following rules for MRT Island formation ensure that MRT FRR The following rules for MRT Island formation ensure that MRT FRR
protection traffic does not use a link or router that is discouraged protection traffic does not use a link or router that is discouraged
or prevented from carrying traffic by existing IGP mechanisms. or prevented from carrying traffic by existing IGP mechanisms.
1. A bidirectional link MUST be excluded from an MRT Island if 1. A bidirectional link MUST be excluded from an MRT Island if
either the forward or reverse cost on the link is 0xFFFFFE (for either the forward or reverse cost on the link is 0xFFFFFE (for
ISIS) or 0xFFFF for OSPF. IS-IS) or 0xFFFF for OSPF.
2. A router MUST be excluded from an MRT Island if it is advertised 2. A router MUST be excluded from an MRT Island if it is advertised
with the overload bit set (for ISIS), or it is advertised with with the overload bit set (for IS-IS), or it is advertised with
metric values of 0xFFFF on all of its outgoing interfaces (for metric values of 0xFFFF on all of its outgoing interfaces (for
OSPFv2 and OSPFv3). OSPFv2 and OSPFv3).
3. A router MUST be excluded from an MRT Island if it is advertised 3. A router MUST be excluded from an MRT Island if it is advertised
with either the V6-bit or R-bit of the LSA options not set in the with either the V6-bit or R-bit of the LSA options not set in the
Router LSA. Router LSA.
7.3.2. MRT-specific exclusion mechanism 7.3.2. MRT-specific exclusion mechanism
This architecture also defines a means of excluding an otherwise This architecture also defines a means of excluding an otherwise
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A valid GADAG Root Selection Policy MUST be such that each router A valid GADAG Root Selection Policy MUST be such that each router
in the MRT island chooses the same GADAG root based on information in the MRT island chooses the same GADAG root based on information
available to all routers in the MRT island. GADAG Root Selection available to all routers in the MRT island. GADAG Root Selection
Priority values, advertised as router-specific MRT parameters, MAY Priority values, advertised as router-specific MRT parameters, MAY
be used in a GADAG Root Selection Policy. be used in a GADAG Root Selection Policy.
MRT Forwarding Mechanism: This specifies which forwarding mechanism MRT Forwarding Mechanism: This specifies which forwarding mechanism
the router uses to carry transit traffic along MRT paths. A the router uses to carry transit traffic along MRT paths. A
router which supports a specific MRT forwarding mechanism must router which supports a specific MRT forwarding mechanism must
program appropriate next-hops into the forwarding plane. The program appropriate next-hops into the forwarding plane. The
current options are MRT LDP Labels, IPv4 Tunneling, IPv6 current options are MRT LDP Label Option 1A, MRT LDP Label Option
Tunneling, and None. If the MRT LDP Labels option is supported, 1B, IPv4 Tunneling, IPv6 Tunneling, and None. If IPv4 is
then option 1A and the appropriate signaling extensions MUST be supported, then both MRT-Red and MRT-Blue IPv4 Loopback Addresses
supported. If IPv4 is supported, then both MRT-Red and MRT-Blue SHOULD be specified. If IPv6 is supported, both MRT-Red and MRT-
IPv4 Loopback Addresses SHOULD be specified. If IPv6 is Blue IPv6 Loopback Addresses SHOULD be specified.
supported, both MRT-Red and MRT-Blue IPv6 Loopback Addresses
SHOULD be specified.
Recalculation: Recalculation specifies the process and timing by Recalculation: Recalculation specifies the process and timing by
which new MRTs are computed after the topology has been modified. which new MRTs are computed after the topology has been modified.
Area/Level Border Behavior: This specifies how traffic traveling on Area/Level Border Behavior: This specifies how traffic traveling on
the MRT-Blue or MRT-Red in one area should be treated when it the MRT-Blue or MRT-Red in one area should be treated when it
passes into another area. passes into another area.
Other Profile-Specific Behavior: Depending upon the use-case for Other Profile-Specific Behavior: Depending upon the use-case for
the profile, there may be additional profile-specific behavior. the profile, there may be additional profile-specific behavior.
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MRT-Red MPLS MT-ID: This value will be allocated from the MPLS MRT-Red MPLS MT-ID: This value will be allocated from the MPLS
Multi-Topology Identifiers Registry. The IANA request for this Multi-Topology Identifiers Registry. The IANA request for this
allocation will be in another document. allocation will be in another document.
MRT-Blue MPLS MT-ID: This value will be allocated from the MPLS MRT-Blue MPLS MT-ID: This value will be allocated from the MPLS
Multi-Topology Identifiers Registry. The IANA request for this Multi-Topology Identifiers Registry. The IANA request for this
allocation will be in another document. allocation will be in another document.
GADAG Root Selection Policy: Among the routers in the MRT Island GADAG Root Selection Policy: Among the routers in the MRT Island
and with the most preferred GADAG Root Selection Priority with the lowest numerical value advertised for GADAG Root
advertised (corresponding to the lowest numerical value of GADAG Selection Priority, an implementation MUST pick the router with
Root Selection Priority), an implementation MUST pick the router the highest Router ID to be the GADAG root. Note that a lower
with the highest Router ID to be the GADAG root. numerical value for GADAG Root Selection Priority indicates a
higher preference for selection.
Forwarding Mechanisms: MRT LDP Labels Forwarding Mechanisms: MRT LDP Label Option 1A
Recalculation: Recalculation of MRTs SHOULD occur as described in Recalculation: Recalculation of MRTs SHOULD occur as described in
Section 12.2. This allows the MRT forwarding topologies to Section 12.2. This allows the MRT forwarding topologies to
support IP/LDP fast-reroute traffic. support IP/LDP fast-reroute traffic.
Area/Level Border Behavior: As described in Section 10, ABRs/LBRs Area/Level Border Behavior: As described in Section 10, ABRs/LBRs
SHOULD ensure that traffic leaving the area also exits the MRT-Red SHOULD ensure that traffic leaving the area also exits the MRT-Red
or MRT-Blue forwarding topology. or MRT-Blue forwarding topology.
9. LDP signaling extensions and considerations 9. LDP signaling extensions and considerations
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on MRT-Red or MRT-Blue in that area. However, it is desirable for on MRT-Red or MRT-Blue in that area. However, it is desirable for
traffic leaving the area to also exit MRT-Red or MRT-Blue and return traffic leaving the area to also exit MRT-Red or MRT-Blue and return
to shortest path forwarding. to shortest path forwarding.
For unicast MRT-FRR, the need to stay on an MRT forwarding topology For unicast MRT-FRR, the need to stay on an MRT forwarding topology
terminates at the ABR/LBR whose best route is via a different area/ terminates at the ABR/LBR whose best route is via a different area/
level. It is highly desirable to go back to the default forwarding level. It is highly desirable to go back to the default forwarding
topology when leaving an area/level. There are three basic reasons topology when leaving an area/level. There are three basic reasons
for this. First, the default topology uses shortest paths; the for this. First, the default topology uses shortest paths; the
packet will thus take the shortest possible route to the destination. packet will thus take the shortest possible route to the destination.
Second, this allows failures that might appear in multiple areas Second, this allows a single router failure that manifests itself in
(e.g. ABR/LBR failures) to be separately identified and repaired multiple areas (as would be the case with an ABR/LBR failure) to be
around. Third, the packet can be fast-rerouted again, if necessary, separately identified and repaired around. Third, the packet can be
due to a failure in a different area. fast-rerouted again, if necessary, due to a second distinct failure
in a different area.
An ABR/LBR that receives a packet on MRT-Red or MRT-Blue towards In OSPF, an ABR that receives a packet on MRT-Red or MRT-Blue towards
destination Z should continue to forward the packet along MRT-Red or destination Z should continue to forward the packet along MRT-Red or
MRT-Blue only if the best route to Z is in the same area as the MRT-Blue only if the best route to Z is in the same OSPF area as the
interface that the packet was received on. Otherwise, the packet interface that the packet was received on. Otherwise, the packet
should be removed from MRT-Red or MRT-Blue and forwarded on the should be removed from MRT-Red or MRT-Blue and forwarded on the
shortest-path default forwarding topology. shortest-path default forwarding topology.
To avoid per-interface forwarding state for MRT-Red and MRT-Blue, the The above description applies to OSPF. The same essential behavior
ABR/LBR needs to arrange that packets destined to a different area also applies to IS-IS if one substitutes IS-IS level for OSPF area.
arrive at the ABR/LBR already not marked as MRT-Red or MRT-Blue. However, the analogy with OSPF is not exact. An interface in OSPF
can only be in one area, whereas an interface in IS-IS can be in both
Level-1 and Level-2. Therefore, to avoid confusion and address this
difference, we explicitly describe the behavior for IS-IS in
Appendix A. In the following sections only the OSPF terminology is
used.
10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A 10.1. ABR Forwarding Behavior with MRT LDP Label Option 1A
For LDP forwarding where a single label specifies (MT-ID, FEC), the For LDP forwarding where a single label specifies (MT-ID, FEC), the
ABR/LBR is responsible for advertising the proper label to each ABR is responsible for advertising the proper label to each neighbor.
neighbor. Assume that an ABR/LBR has allocated three labels for a Assume that an ABR has allocated three labels for a particular
particular destination; those labels are L_primary, L_blue, and destination; those labels are L_primary, L_blue, and L_red. To those
L_red. To those routers in the same area as the best route to the routers in the same area as the best route to the destination, the
destination, the ABR/LBR advertises the following FEC-label bindings: ABR advertises the following FEC-label bindings: L_primary for the
L_primary for the default topology, L_blue for the MRT-Blue MT-ID and default topology, L_blue for the MRT-Blue MT-ID and L_red for the
L_red for the MRT-Red MT-ID, as expected. However, to routers in MRT-Red MT-ID, as expected. However, to routers in other areas, the
other areas, the ABR/LBR advertises the following FEC-label bindings: ABR advertises the following FEC-label bindings: L_primary for the
L_primary for the default topology, and L_primary for the Rainbow MRT default topology, and L_primary for the Rainbow MRT MT-ID.
MT-ID. Associating L_primary with the Rainbow MRT MT-ID causes the Associating L_primary with the Rainbow MRT MT-ID causes the receiving
receiving routers to use L_primary for the MRT-Blue MT-ID and for the routers to use L_primary for the MRT-Blue MT-ID and for the MRT-Red
MRT-Red MT-ID. MT-ID.
The ABR/LBR installs all next-hops for the best area: primary next- The ABR installs all next-hops for the best area: primary next-hops
hops for L_primary, MRT-Blue next-hops for L_blue, and MRT-Red next- for L_primary, MRT-Blue next-hops for L_blue, and MRT-Red next-hops
hops for L_red. Because the ABR/LBR advertised (Rainbow MRT MT-ID, for L_red. Because the ABR advertised (Rainbow MRT MT-ID, FEC) with
FEC) with L_primary to neighbors not in the best area, packets from L_primary to neighbors not in the best area, packets from those
those neighbors will arrive at the ABR/LBR with a label L_primary and neighbors will arrive at the ABR with a label L_primary and will be
will be forwarded into the best area along the default topology. By forwarded into the best area along the default topology. By
controlling what labels are advertised, the ABR/LBR can thus enforce controlling what labels are advertised, the ABR can thus enforce that
that packets exiting the area do so on the shortest-path default packets exiting the area do so on the shortest-path default topology.
topology.
10.1.1. Motivation for Creating the Rainbow-FEC 10.1.1. Motivation for Creating the Rainbow-FEC
The desired forwarding behavior could be achieved in the above The desired forwarding behavior could be achieved in the above
example without using the Rainbow-FEC. This could be done by having example without using the Rainbow-FEC. This could be done by having
the ABR/LBR advertise the following FEC-label bindings to neighbors the ABR advertise the following FEC-label bindings to neighbors not
not in the best area: L1_primary for the default topology, L1_primary in the best area: L1_primary for the default topology, L1_primary for
for the MRT-Blue MT-ID, and L1_primary for the MRT-Red MT-ID. Doing the MRT-Blue MT-ID, and L1_primary for the MRT-Red MT-ID. Doing this
this would require machinery to spoof the labels used in FEC-label would require machinery to spoof the labels used in FEC-label binding
binding advertisements on a per-neighbor basis. Such label-spoofing advertisements on a per-neighbor basis. Such label-spoofing
machinery does not currently exist in most LDP implmentations and machinery does not currently exist in most LDP implementations and
doesn't have other obvious uses. doesn't have other obvious uses.
Many existing LDP implmentations do however have the ability to Many existing LDP implementations do however have the ability to
filter FEC-label binding advertisements on a per-neighbor basis. The filter FEC-label binding advertisements on a per-neighbor basis. The
Rainbow-FEC allows us to re-use the existing per-neighbor FEC Rainbow-FEC allows us to re-use the existing per-neighbor FEC
filtering machinery to achieve the desired result. By introducing filtering machinery to achieve the desired result. By introducing
the Rainbow FEC, we can use per-neighbor FEC-filtering machinery to the Rainbow FEC, we can use per-neighbor FEC-filtering machinery to
advertise the FEC-label binding for the Rainbow-FEC (and filter those advertise the FEC-label binding for the Rainbow-FEC (and filter those
for MRT-Blue and MRT-Red) to non-best-area neighbors of the ABR. for MRT-Blue and MRT-Red) to non-best-area neighbors of the ABR.
The use of the Rainbow-FEC by the ABR for non-best-area An ABR may choose to either advertise the Rainbow-FEC or advertise
advertisements is RECOMMENDED. An ABR MAY advertise the label for separate MRT-Blue and MRT-Red advertisements. This is a local
the default topology in separate MRT-Blue and MRT-Red advertisements. choice. A router that supports the MRT LDP Label Option 1A
However, a router that supports the LDP Label MRT Forwarding Forwarding Mechanism MUST be able to receive and correctly interpret
Mechanism MUST be able to receive and correctly interpret the the Rainbow-FEC.
Rainbow-FEC.
10.2. ABR Forwarding Behavior with IP Tunneling (option 2) 10.2. ABR Forwarding Behavior with IP Tunneling (option 2)
If IP tunneling is used, then the ABR/LBR behavior is dependent upon If IP tunneling is used, then the ABR behavior is dependent upon the
the outermost IP address. If the outermost IP address is an MRT outermost IP address. If the outermost IP address is an MRT loopback
loopback address of the ABR/LBR, then the packet is decapsulated and address of the ABR, then the packet is decapsulated and forwarded
forwarded based upon the inner IP address, which should go on the based upon the inner IP address, which should go on the default SPT
default SPT topology. If the outermost IP address is not an MRT topology. If the outermost IP address is not an MRT loopback address
loopback address of the ABR/LBR, then the packet is simply forwarded of the ABR, then the packet is simply forwarded along the associated
along the associated forwarding topology. A PLR sending traffic to a forwarding topology. A PLR sending traffic to a destination outside
destination outside its local area/level will pick the MRT and use its local area/level will pick the MRT and use the associated MRT
the associated MRT loopback address of the selected ABR/LBR loopback address of the selected ABR advertising the lowest cost to
advertising the lowest cost to the external destination. the external destination.
Thus, for these two MRT Forwarding Mechanisms (MRT LDP Label option Thus, for these two MRT Forwarding Mechanisms (MRT LDP Label option
1A and IP tunneling option 2), there is no need for additional 1A and IP tunneling option 2), there is no need for additional
computation or per-area forwarding state. computation or per-area forwarding state.
10.3. ABR Forwarding Behavior with LDP Label option 1B 10.3. ABR Forwarding Behavior with MRT LDP Label option 1B
The other MRT forwarding mechanism described in Section 6 uses two The other MRT forwarding mechanism described in Section 6 uses two
labels, a topology-id label, and a FEC-label. This mechanism would labels, a topology-id label, and a FEC-label. This mechanism would
require that any router whose MRT-Red or MRT-Blue next-hop is an ABR/ require that any router whose MRT-Red or MRT-Blue next-hop is an ABR
LBR would need to determine whether the ABR/LBR would forward the would need to determine whether the ABR would forward the packet out
packet out of the area/level. If so, then that router should pop off of the area/level. If so, then that router should pop off the
the topology-identification label before forwarding the packet to the topology-identification label before forwarding the packet to the
ABR/LBR. ABR.
For example, in Figure 4, if node H fails, node E has to put traffic For example, in Figure 3, if node H fails, node E has to put traffic
towards prefix p onto MRT-Red. But since node D knows that ABR1 will towards prefix p onto MRT-Red. But since node D knows that ABR1 will
use a best route from another area, it is safe for D to pop the use a best route from another area, it is safe for D to pop the
Topology-Identification Label and just forward the packet to ABR1 Topology-Identification Label and just forward the packet to ABR1
along the MRT-Red next-hop. ABR1 will use the shortest path in Area along the MRT-Red next-hop. ABR1 will use the shortest path in Area
10. 10.
In all cases for ISIS and most cases for OSPF, the penultimate router In all cases for IS-IS and most cases for OSPF, the penultimate
can determine what decision the adjacent ABR will make. The one case router can determine what decision the adjacent ABR will make. The
where it can't be determined is when two ASBRs are in different non- one case where it can't be determined is when two ASBRs are in
backbone areas attached to the same ABR, then the ASBR's Area ID may different non-backbone areas attached to the same ABR, then the
be needed for tie-breaking (prefer the route with the largest OPSF ASBR's Area ID may be needed for tie-breaking (prefer the route with
area ID) and the Area ID isn't announced as part of the ASBR link- the largest OPSF area ID) and the Area ID isn't announced as part of
state advertisement (LSA). In this one case, suboptimal forwarding the ASBR link-state advertisement (LSA). In this one case,
along the MRT in the other area would happen. If that becomes a suboptimal forwarding along the MRT in the other area would happen.
realistic deployment scenario, protocol extensions could be developed If that becomes a realistic deployment scenario, protocol extensions
to address this issue. could be developed to address this issue.
+----[C]---- --[D]--[E] --[D]--[E] +----[C]---- --[D]--[E] --[D]--[E]
| \ / \ / \ | \ / \ / \
p--[A] Area 10 [ABR1] Area 0 [H]--p +-[ABR1] Area 0 [H]-+ p--[A] Area 10 [ABR1] Area 0 [H]--p +-[ABR1] Area 0 [H]-+
| / \ / | \ / | | / \ / | \ / |
+----[B]---- --[F]--[G] | --[F]--[G] | +----[B]---- --[F]--[G] | --[F]--[G] |
| | | |
| other | | other |
+----------[p]-------+ +----------[p]-------+
area area
skipping to change at page 29, line 38 skipping to change at page 26, line 38
/ \ / \ / \ / \
[ABR1] Area 0 [H]-+ +-[ABR1] [H] [ABR1] Area 0 [H]-+ +-[ABR1] [H]
/ | | \ / | | \
[F]->[G] V V -<[F]<-[G] [F]->[G] V V -<[F]<-[G]
| | | |
| | | |
[p]<------+ +--------->[p] [p]<------+ +--------->[p]
(d) Blue MRT in Area 0 (e) Red MRT in Area 0 (d) Blue MRT in Area 0 (e) Red MRT in Area 0
Figure 4: ABR Forwarding Behavior and MRTs Figure 3: ABR Forwarding Behavior and MRTs
11. Prefixes Multiply Attached to the MRT Island 11. Prefixes Multiply Attached to the MRT Island
How a computing router S determines its local MRT Island for each How a computing router S determines its local MRT Island for each
supported MRT profile is already discussed in Section 7. supported MRT profile is already discussed in Section 7.
There are two types of prefixes or FECs that may be multiply attached There are two types of prefixes or FECs that may be multiply attached
to an MRT Island. The first type are multi-homed prefixes that to an MRT Island. The first type are multi-homed prefixes that
usually connect at a domain or protocol boundary. The second type usually connect at a domain or protocol boundary. The second type
represent routers that do not support the profile for the MRT Island. represent routers that do not support the profile for the MRT Island.
skipping to change at page 30, line 30 skipping to change at page 27, line 30
As discussed in [RFC5286], a multi-homed prefix could be: As discussed in [RFC5286], a multi-homed prefix could be:
o An out-of-area prefix announced by more than one ABR, o An out-of-area prefix announced by more than one ABR,
o An AS-External route announced by 2 or more ASBRs, o An AS-External route announced by 2 or more ASBRs,
o A prefix with iBGP multipath to different ASBRs, o A prefix with iBGP multipath to different ASBRs,
o etc. o etc.
See Appendix A for a discussion of a general issue with multi-homed See Appendix B for a discussion of a general issue with multi-homed
prefixes connected in two different areas. prefixes connected in two different areas.
There are also two different approaches to protection. The first is There are also two different approaches to protection. The first is
tunnel endpoint selection where the PLR picks a router to tunnel to tunnel endpoint selection where the PLR picks a router to tunnel to
where that router is loop-free with respect to the failure-point. where that router is loop-free with respect to the failure-point.
Conceptually, the set of candidate routers to provide LFAs expands to Conceptually, the set of candidate routers to provide LFAs expands to
all routers that can be reached via an MRT alternate, attached to the all routers that can be reached via an MRT alternate, attached to the
prefix. prefix.
The second is to use a proxy-node, that can be named via MPLS label The second is to use a proxy-node, that can be named via MPLS label
skipping to change at page 31, line 46 skipping to change at page 28, line 46
The candidates for tunnel endpoint selection are those to which the The candidates for tunnel endpoint selection are those to which the
destination prefix is attached in the area/level. For a particular destination prefix is attached in the area/level. For a particular
candidate B, it is necessary to determine if B is loop-free to reach candidate B, it is necessary to determine if B is loop-free to reach
p with respect to S and F for node-protection or at least with p with respect to S and F for node-protection or at least with
respect to S and the link (S, F) for link-protection. If B will respect to S and the link (S, F) for link-protection. If B will
always prefer to send traffic to p via a different area/level, then always prefer to send traffic to p via a different area/level, then
this is definitional. Otherwise, distance-based computations are this is definitional. Otherwise, distance-based computations are
necessary and an SPF from B's perspective may be necessary. The necessary and an SPF from B's perspective may be necessary. The
following equations give the checks needed; the rationale is similar following equations give the checks needed; the rationale is similar
to that given in [RFC5286]. to that given in [RFC5286]. In the inequalities below, D_opt(X,Y)
means the shortest distance from node X to node Y, and D_opt(X,p)
means the shortest distance from node X to prefix p.
Loop-Free for S: D_opt(B, p) < D_opt(B, S) + D_opt(S, p) Loop-Free for S: D_opt(B, p) < D_opt(B, S) + D_opt(S, p)
Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(F, p) Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(F, p)
The latter is equivalent to the following, which avoids the need to The latter is equivalent to the following, which avoids the need to
compute the shortest path from F to p. compute the shortest path from F to p.
Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(S, p) - D_opt(S, Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(S, p) - D_opt(S,
F) F)
skipping to change at page 38, line 35 skipping to change at page 35, line 35
algorithm, augmentation of GADAG with additional links, and algorithm, augmentation of GADAG with additional links, and
calculation of MRT transit next-hops alternate next-hops based on calculation of MRT transit next-hops alternate next-hops based on
draft "draft-enyedi-rtgwg-mrt-frr-algorithm-03". This draft "draft-enyedi-rtgwg-mrt-frr-algorithm-03". This
implementation also includes IS-IS extension for MRT based on implementation also includes IS-IS extension for MRT based on
"draft-li-mrt-00". "draft-li-mrt-00".
o Licensing: proprietary o Licensing: proprietary
o Implementation experience: It is important produce a second o Implementation experience: It is important produce a second
implementation to verify the algorithm is implemented correctly implementation to verify the algorithm is implemented correctly
without looping. It is important to verify the ISIS extensions without looping. It is important to verify the IS-IS extensions
work for MRT-FRR. work for MRT-FRR.
o Contact information: lizhenbin@huawei.com, eric.wu@huawei.com o Contact information: lizhenbin@huawei.com, eric.wu@huawei.com
14. Operational Considerations 14. Operational Considerations
The following aspects of MRT-FRR are useful to consider when The following aspects of MRT-FRR are useful to consider when
deploying the technology in different operational environments and deploying the technology in different operational environments and
network topologies. network topologies.
14.1. Verifying Forwarding on MRT Paths 14.1. Verifying Forwarding on MRT Paths
The forwarding paths created by MRT-FRR are not used by normal (non- The forwarding paths created by MRT-FRR are not used by normal (non-
FRR) traffic. They are only used to carry FRR traffic for a short FRR) traffic. They are only used to carry FRR traffic for a short
period of time after a failure has been detected. It is RECOMMENDED period of time after a failure has been detected. It is RECOMMENDED
that an operator proactively monitor the MRT forwarding paths in that an operator proactively monitor the MRT forwarding paths in
order to be certain that the paths will be able to carry FRR traffic order to be certain that the paths will be able to carry FRR traffic
when needed. Therefore, an implementation SHOULD provide an operator when needed. Therefore, an implementation SHOULD provide an operator
with the ability to test MRT paths with OAM traffic. For example, with the ability to test MRT paths with Operations, Administration,
when MRT paths are realized using LDP labels distributed for and Maintenance (OAM) traffic. For example, when MRT paths are
topology-scoped FECs, an implementation can use the MPLS ping and realized using LDP labels distributed for topology-scoped FECs, an
traceroute as defined in [RFC4379] and extended in [RFC7307] for implementation can use the MPLS ping and traceroute as defined in
topology-scoped FECs. [RFC4379] and extended in [RFC7307] for topology-scoped FECs.
14.2. Traffic Capacity on Backup Paths 14.2. Traffic Capacity on Backup Paths
During a fast-reroute event initiated by a PLR in response to a During a fast-reroute event initiated by a PLR in response to a
network failure, the flow of traffic in the network will generally network failure, the flow of traffic in the network will generally
not be identical to the flow of traffic after the IGP forwarding not be identical to the flow of traffic after the IGP forwarding
state has converged, taking the failure into account. Therefore, state has converged, taking the failure into account. Therefore,
even if a network has been engineered to have enough capacity on the even if a network has been engineered to have enough capacity on the
appropriate links to carry all traffic after the IGP has converged appropriate links to carry all traffic after the IGP has converged
after the failure, the network may still not have enough capacity on after the failure, the network may still not have enough capacity on
skipping to change at page 40, line 7 skipping to change at page 37, line 7
In order to reduce or eliminate the potential for transient traffic In order to reduce or eliminate the potential for transient traffic
loss due to inadequate capacity during fast-reroute events, an loss due to inadequate capacity during fast-reroute events, an
operator can model the amount of traffic taking different paths operator can model the amount of traffic taking different paths
during a fast-reroute event. If it is determined that there is not during a fast-reroute event. If it is determined that there is not
enough capacity to support a given fast-reroute event, the operator enough capacity to support a given fast-reroute event, the operator
can address the issue either by augmenting capacity on certain links can address the issue either by augmenting capacity on certain links
or modifying the backup paths themselves. or modifying the backup paths themselves.
The MRT Lowpoint algorithm produces a pair of diverse paths to each The MRT Lowpoint algorithm produces a pair of diverse paths to each
destination. These paths are generated by following the directed destination. These paths are generated by following the directed
links on a common GADAG. MRT-FRR allows an operator to exclude a links on a common GADAG. The decision process for constructing the
link from the MRT Island, and thus the GADAG, by advertising it as GADAG in the MRT Lowpoint algorithm takes into account individual IGP
MRT-Ineligible. Such a link will not be used on the MRT forwarding link metrics. At any given node, links are explored in order from
path for any destination. Advertising links as MRT-Ineligible is the lowest IGP metric to highest IGP metric. Additionally, the process
main tool provided by MRT-FRR for keeping backup traffic off of lower for constructing the MRT-Red and Blue trees uses SPF traversals of
bandwidth links during fast-reroute events. the GADAG. Therefore, the IGP link metric values affect the computed
backup paths. However, adjusting the IGP link metrics is not a
generally applicable tool for modifying the MRT backup paths.
Achieving a desired set of MRT backup paths by adjusting IGP metrics
while at the same time maintaining the desired flow of traffic along
the shortest paths is not possible in general.
MRT-FRR allows an operator to exclude a link from the MRT Island, and
thus the GADAG, by advertising it as MRT-Ineligible. Such a link
will not be used on the MRT forwarding path for any destination.
Advertising links as MRT-Ineligible is the main tool provided by MRT-
FRR for keeping backup traffic off of lower bandwidth links during
fast-reroute events.
Note that all of the backup paths produced by the MRT Lowpoint Note that all of the backup paths produced by the MRT Lowpoint
algorithm are closely tied to the common GADAG computed as part of algorithm are closely tied to the common GADAG computed as part of
that algorithm. Therefore, it is generally not possible to modify a that algorithm. Therefore, it is generally not possible to modify a
subset of paths without affecting other paths. This precludes more subset of paths without affecting other paths. This precludes more
fine-grained modification of individual backup paths when using only fine-grained modification of individual backup paths when using only
paths computed by the MRT Lowpoint algorithm. paths computed by the MRT Lowpoint algorithm.
However, it may be desirable to allow an operator to use MRT-FRR However, it may be desirable to allow an operator to use MRT-FRR
alternates together with alternates provided by other FRR alternates together with alternates provided by other FRR
skipping to change at page 42, line 8 skipping to change at page 39, line 17
diverse. diverse.
15. Acknowledgements 15. Acknowledgements
The authors would like to thank Mike Shand for his valuable review The authors would like to thank Mike Shand for his valuable review
and contributions. and contributions.
The authors would like to thank Joel Halpern, Hannes Gredler, Ted The authors would like to thank Joel Halpern, Hannes Gredler, Ted
Qian, Kishore Tiruveedhula, Shraddha Hegde, Santosh Esale, Nitin Qian, Kishore Tiruveedhula, Shraddha Hegde, Santosh Esale, Nitin
Bahadur, Harish Sitaraman, Raveendra Torvi, Anil Kumar SN, Bruno Bahadur, Harish Sitaraman, Raveendra Torvi, Anil Kumar SN, Bruno
Decraene, Eric Wu, Janos Farkas, Rob Shakir, and Stewart Bryant for Decraene, Eric Wu, Janos Farkas, Rob Shakir, Stewart Bryant, and
their suggestions and review. Alvaro Retana for their suggestions and review.
16. IANA Considerations 16. IANA Considerations
IANA is requested to create a registry entitled "MRT Profile IANA is requested to create a registry entitled "MRT Profile
Identifier Registry". The range is 0 to 255. The Default MRT Identifier Registry". The range is 0 to 255. The Default MRT
Profile defined in this document has value 0. Values 1-200 are Profile defined in this document has value 0. Values 1-200 are
allocated by Standards Action. Values 201-220 are for allocated by Standards Action. Values 201-220 are for Experimental
experimentation. Values 221-255 are for vendor private use. Use. Values 221-254 are for Private Use. Value 255 is reserved for
future registry extension. (The allocation and use policies are
described in [RFC5226].)
The initial registry is shown below.
Value Description Reference
------- ---------------------------------------- ------------
0 Default MRT Profile [This draft]
1-200 Unassigned
201-220 Experimental Use
221-254 Private Use
255 Reserved (for future registry extension)
The MRT Profile Identifier Registry is a new registry in the IANA
Matrix. Following existing conventions, http://www.iana.org/
protocols should display a new header entitled "Maximally Redundant
Tree (MRT) Parameters". Under that header, there should be an entry
for "MRT Profile Identifier Registry" with a link to the registry
itself at http://www.iana.org/assignments/mrt-parameters/mrt-
parameters.xhtml#mrt-profile-registry.
17. Security Considerations 17. Security Considerations
In general, MRT forwarding paths do not follow shortest paths. The In general, MRT forwarding paths do not follow shortest paths. The
transit forwarding state corresponding to the MRT paths is created transit forwarding state corresponding to the MRT paths is created
during normal operations (before a failure occurs). Therefore, a during normal operations (before a failure occurs). Therefore, a
malicious packet with an appropriate header injected into the network malicious packet with an appropriate header injected into the network
from a compromised location would be forwarded to a destination along from a compromised location would be forwarded to a destination along
a non-shortest path. When this technology is deployed, a network a non-shortest path. When this technology is deployed, a network
security design should not rely on assumptions about potentially security design should not rely on assumptions about potentially
skipping to change at page 43, line 44 skipping to change at page 41, line 44
Envedi, G., Csaszar, A., Atlas, A., Bowers, C., and A. Envedi, G., Csaszar, A., Atlas, A., Bowers, C., and A.
Gopalan, "Algorithms for computing Maximally Redundant Gopalan, "Algorithms for computing Maximally Redundant
Trees for IP/LDP Fast- Reroute", draft-ietf-rtgwg-mrt-frr- Trees for IP/LDP Fast- Reroute", draft-ietf-rtgwg-mrt-frr-
algorithm-06 (work in progress), October 2015. algorithm-06 (work in progress), October 2015.
[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, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IP Fast Reroute: Loop-Free Alternates", RFC 5286, IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5286, September 2008, DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5286>. <http://www.rfc-editor.org/info/rfc5226>.
[RFC7307] Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D.
King, "LDP Extensions for Multi-Topology", RFC 7307,
DOI 10.17487/RFC7307, July 2014,
<http://www.rfc-editor.org/info/rfc7307>.
19.2. Informative References 19.2. Informative References
[EnyediThesis] [EnyediThesis]
Enyedi, G., "Novel Algorithms for IP Fast Reroute", Enyedi, G., "Novel Algorithms for IP Fast Reroute",
Department of Telecommunications and Media Informatics, Department of Telecommunications and Media Informatics,
Budapest University of Technology and Economics Ph.D. Budapest University of Technology and Economics Ph.D.
Thesis, February 2011, Thesis, February 2011,
<http://timon.tmit.bme.hu/theses/thesis_book.pdf>. <http://timon.tmit.bme.hu/theses/thesis_book.pdf>.
[I-D.atlas-rtgwg-mrt-mc-arch] [I-D.atlas-rtgwg-mrt-mc-arch]
Atlas, A., Kebler, R., Wijnands, I., Csaszar, A., and G. Atlas, A., Kebler, R., Wijnands, I., Csaszar, A., and G.
Envedi, "An Architecture for Multicast Protection Using Envedi, "An Architecture for Multicast Protection Using
Maximally Redundant Trees", draft-atlas-rtgwg-mrt-mc- Maximally Redundant Trees", draft-atlas-rtgwg-mrt-mc-
arch-02 (work in progress), July 2013. arch-02 (work in progress), July 2013.
[I-D.francois-rtgwg-segment-routing-ti-lfa]
Francois, P., Filsfils, C., Bashandy, A., and B. Decraene,
"Topology Independent Fast Reroute using Segment Routing",
draft-francois-rtgwg-segment-routing-ti-lfa-00 (work in
progress), August 2015.
[I-D.ietf-rtgwg-lfa-manageability]
Litkowski, S., Decraene, B., Filsfils, C., Raza, K.,
Horneffer, M., and P. Sarkar, "Operational management of
Loop Free Alternates", draft-ietf-rtgwg-lfa-
manageability-11 (work in progress), June 2015.
[I-D.ietf-rtgwg-rlfa-node-protection]
Sarkar, P., Hegde, S., Bowers, C., Gredler, H., and S.
Litkowski, "Remote-LFA Node Protection and Manageability",
draft-ietf-rtgwg-rlfa-node-protection-05 (work in
progress), December 2015.
[LightweightNotVia]
Enyedi, G., Retvari, G., Szilagyi, P., and A. Csaszar, "IP
Fast ReRoute: Lightweight Not-Via without Additional
Addresses", Proceedings of IEEE INFOCOM , 2009,
<http://mycite.omikk.bme.hu/doc/71691.pdf>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998, DOI 10.17487/RFC2328, April 1998,
<http://www.rfc-editor.org/info/rfc2328>. <http://www.rfc-editor.org/info/rfc2328>.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol [RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379, Label Switched (MPLS) Data Plane Failures", RFC 4379,
DOI 10.17487/RFC4379, February 2006, DOI 10.17487/RFC4379, February 2006,
<http://www.rfc-editor.org/info/rfc4379>. <http://www.rfc-editor.org/info/rfc4379>.
[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>.
[RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream [RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
Label Assignment and Context-Specific Label Space", Label Assignment and Context-Specific Label Space",
RFC 5331, DOI 10.17487/RFC5331, August 2008, RFC 5331, DOI 10.17487/RFC5331, August 2008,
<http://www.rfc-editor.org/info/rfc5331>. <http://www.rfc-editor.org/info/rfc5331>.
[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, DOI 10.17487/RFC5340, July 2008, for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<http://www.rfc-editor.org/info/rfc5340>. <http://www.rfc-editor.org/info/rfc5340>.
[RFC5443] Jork, M., Atlas, A., and L. Fang, "LDP IGP [RFC5443] Jork, M., Atlas, A., and L. Fang, "LDP IGP
skipping to change at page 45, line 26 skipping to change at page 43, line 13
2009, <http://www.rfc-editor.org/info/rfc5443>. 2009, <http://www.rfc-editor.org/info/rfc5443>.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", [RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, DOI 10.17487/RFC5714, January 2010, RFC 5714, DOI 10.17487/RFC5714, January 2010,
<http://www.rfc-editor.org/info/rfc5714>. <http://www.rfc-editor.org/info/rfc5714>.
[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, DOI 10.17487/RFC5715, January Convergence", RFC 5715, DOI 10.17487/RFC5715, January
2010, <http://www.rfc-editor.org/info/rfc5715>. 2010, <http://www.rfc-editor.org/info/rfc5715>.
[RFC6571] Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene,
B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
Alternate (LFA) Applicability in Service Provider (SP)
Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012,
<http://www.rfc-editor.org/info/rfc6571>.
[RFC6976] Shand, M., Bryant, S., Previdi, S., Filsfils, C., [RFC6976] Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
Francois, P., and O. Bonaventure, "Framework for Loop-Free Francois, P., and O. Bonaventure, "Framework for Loop-Free
Convergence Using the Ordered Forwarding Information Base Convergence Using the Ordered Forwarding Information Base
(oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July (oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July
2013, <http://www.rfc-editor.org/info/rfc6976>. 2013, <http://www.rfc-editor.org/info/rfc6976>.
[RFC6981] Bryant, S., Previdi, S., and M. Shand, "A Framework for IP [RFC6981] Bryant, S., Previdi, S., and M. Shand, "A Framework for IP
and MPLS Fast Reroute Using Not-Via Addresses", RFC 6981, and MPLS Fast Reroute Using Not-Via Addresses", RFC 6981,
DOI 10.17487/RFC6981, August 2013, DOI 10.17487/RFC6981, August 2013,
<http://www.rfc-editor.org/info/rfc6981>. <http://www.rfc-editor.org/info/rfc6981>.
skipping to change at page 46, line 5 skipping to change at page 43, line 34
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running [RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982, Code: The Implementation Status Section", RFC 6982,
DOI 10.17487/RFC6982, July 2013, DOI 10.17487/RFC6982, July 2013,
<http://www.rfc-editor.org/info/rfc6982>. <http://www.rfc-editor.org/info/rfc6982>.
[RFC6987] Retana, A., Nguyen, L., Zinin, A., White, R., and D. [RFC6987] Retana, A., Nguyen, L., Zinin, A., White, R., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 6987, McPherson, "OSPF Stub Router Advertisement", RFC 6987,
DOI 10.17487/RFC6987, September 2013, DOI 10.17487/RFC6987, September 2013,
<http://www.rfc-editor.org/info/rfc6987>. <http://www.rfc-editor.org/info/rfc6987>.
[RFC7307] Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D.
King, "LDP Extensions for Multi-Topology", RFC 7307,
DOI 10.17487/RFC7307, July 2014,
<http://www.rfc-editor.org/info/rfc7307>.
[RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N. [RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)", So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
RFC 7490, DOI 10.17487/RFC7490, April 2015, RFC 7490, DOI 10.17487/RFC7490, April 2015,
<http://www.rfc-editor.org/info/rfc7490>. <http://www.rfc-editor.org/info/rfc7490>.
Appendix A. General Issues with Area Abstraction Appendix A. Inter-level Forwarding Behavior for IS-IS
In the description below, we use the terms "Level-1-only interface",
"Level-2-only interface", and "Level-1-and-Level-2 interface" to mean
in interface which has formed only a Level-1 adjacency, only a
Level-2 adjacency, or both Level-1 and Level-2 adjacencies. Note
that IS-IS also defines the concept of areas. A router is configured
with an IS-IS area identifier, and a given router may be configured
with multiple IS-IS area identifiers. For an IS-IS Level-1 adjacency
to form between two routers, at least one IS-IS area identifier must
match. IS-IS Level-2 adjacencies to not require any area identifiers
to match. The behavior described below does not explicitly refer to
IS-IS area identifiers. However, IS-IS area identifiers will
indirectly affect the behavior by affecting the formation of Level-1
adjacencies.
First consider a packet destined to Z on MRT-Red or MRT-Blue received
on a Level-1-only interface. If the best shortest path route to Z
was learned from a Level-1 advertisement, then the packet should
continue to be forwarded along MRT-Red or MRT-Blue. If instead the
best route was learned from a Level-2 advertisement, then the packet
should be removed from MRT-Red or MRT-Blue and forwarded on the
shortest-path default forwarding topology.
Now consider a packet destined to Z on MRT-Red or MRT-Blue received
on a Level-2-only interface. If the best route to Z was learned from
a Level-2 advertisement, then the packet should continue to be
forwarded along MRT-Red or MRT-Blue. If instead the best route was
learned from a Level-1 advertisement, then the packet should be
removed from MRT-Red or MRT-Blue and forwarded on the shortest-path
default forwarding topology.
Finally, consider a packet destined to Z on MRT-Red or MRT-Blue
received on a Level-1-and-Level-2 interface. This packet should
continue to be forwarded along MRT-Red or MRT-Blue, regardless of
which level the route was learned from.
An implementation may simplify the decision-making process above by
using the interface of the next-hop for the route to Z to determine
the level that the best route to Z was learned from. If the next-hop
points out a Level-1-only interface, then the route was learned from
a Level-1 advertisement. If the next-hop points out a Level-2-only
interface, then the route was learned from a Level-2 advertisement.
A next-hop that points out a Level-1-and-Level-2 interface does not
provide enough information to determine the source of the best route.
With this simplification, an implementation would need to continue
forwarding along MRT-Red or MRT-Blue when the next-hop points out a
Level-1-and-Level-2 interface. Therefore, a packet on MRT-Red or
MRT-Blue going from Level-1 to Level-2 (or vice versa) that traverses
a Level-1-and-Level-2 interface in the process will remain on MRT-Red
or MRT-Blue. This simplification may not always produce the optimal
forwarding behavior, but it does not introduce interoperability
problems. The packet will stay on an MRT backup path longer than
necessary, but it will still reach its destination.
Appendix B. General Issues with Area Abstraction
When a multi-homed prefix is connected in two different areas, it may When a multi-homed prefix is connected in two different areas, it may
be impractical to protect them without adding the complexity of be impractical to protect them without adding the complexity of
explicit tunneling. This is also a problem for LFA and Remote-LFA. explicit tunneling. This is also a problem for LFA and Remote-LFA.
50 50
|----[ASBR Y]---[B]---[ABR 2]---[C] Backbone Area 0: |----[ASBR Y]---[B]---[ABR 2]---[C] Backbone Area 0:
| | ABR 1, ABR 2, C, D | | ABR 1, ABR 2, C, D
| | | |
| | Area 20: A, ASBR X | | Area 20: A, ASBR X
| | | |
p ---[ASBR X]---[A]---[ABR 1]---[D] Area 10: B, ASBR Y p ---[ASBR X]---[A]---[ABR 1]---[D] Area 10: B, ASBR Y
5 p is a Type 1 AS-external 5 p is a Type 1 AS-external
Figure 5: AS external prefixes in different areas Figure 4: AS external prefixes in different areas
Consider the network in Figure 5 and assume there is a richer Consider the network in Figure 4 and assume there is a richer
connective topology that isn't shown, where the same prefix is connective topology that isn't shown, where the same prefix is
announced by ASBR X and ASBR Y which are in different non-backbone announced by ASBR X and ASBR Y which are in different non-backbone
areas. If the link from A to ASBR X fails, then an MRT alternate areas. If the link from A to ASBR X fails, then an MRT alternate
could forward the packet to ABR 1 and ABR 1 could forward it to D, could forward the packet to ABR 1 and ABR 1 could forward it to D,
but then D would find the shortest route is back via ABR 1 to Area but then D would find the shortest route is back via ABR 1 to Area
20. This problem occurs because the routers, including the ABR, in 20. This problem occurs because the routers, including the ABR, in
one area are not yet aware of the failure in a different area. one area are not yet aware of the failure in a different area.
The only way to get it from A to ASBR Y is to explicitly tunnel it to The only way to get it from A to ASBR Y is to explicitly tunnel it to
ASBR Y. If the traffic is unlabeled or the appropriate MPLS labels ASBR Y. If the traffic is unlabeled or the appropriate MPLS labels
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