draft-ietf-rtgwg-mrt-frr-architecture-08.txt   draft-ietf-rtgwg-mrt-frr-architecture-09.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: June 23, 2016 G. Enyedi Expires: July 13, 2016 G. Enyedi
Ericsson Ericsson
December 21, 2015 January 10, 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-08 draft-ietf-rtgwg-mrt-frr-architecture-09
Abstract Abstract
This document defines the architecture for IP/LDP Fast-Reroute using This document defines the architecture for IP/LDP Fast-Reroute using
Maximally Redundant Trees (MRT-FRR). MRT-FRR is a technology that Maximally Redundant Trees (MRT-FRR). MRT-FRR is a technology that
gives link-protection and node-protection with 100% coverage in any gives link-protection and node-protection with 100% coverage in any
network topology that is still connected after the failure. network topology that is still connected after the failure.
MRT removes the need to engineer for coverage. MRT is also extremely
computationally efficient. For any router in the network, the MRT
computation is less than the LFA computation for a node with three or
more neighbors.
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 . . . . . . . . . . . . . . . 5 1.1. Importance of 100% Coverage . . . . . . . . . . . . . . . 8
1.2. Partial Deployment and Backwards Compatibility . . . . . 6 1.2. Partial Deployment and Backwards Compatibility . . . . . 9
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 6 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 9
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 8 4. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 11
5. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . 10 5. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . 12
6. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . . 11 6. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . . 13
6.1. MRT Forwarding Mechanisms . . . . . . . . . . . . . . . . 11 6.1. MRT Forwarding Mechanisms . . . . . . . . . . . . . . . . 14
6.1.1. MRT LDP labels . . . . . . . . . . . . . . . . . . . 12 6.1.1. MRT LDP labels . . . . . . . . . . . . . . . . . . . 14
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) . . . . . . . . . . . . . . . . . . . 12 (Option 1A) . . . . . . . . . . . . . . . . . . . 14
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) . . . . . . . . . . . . . . . . . . . 13 (Option 1B) . . . . . . . . . . . . . . . . . . . 15
6.1.1.3. Compatibility of Option 1A and 1B . . . . . . . . 13 6.1.1.3. Compatibility of Option 1A and 1B . . . . . . . . 15
6.1.1.4. Mandatory support for MRT LDP Label option 1A . . 13 6.1.1.4. Mandatory support for MRT LDP Label option 1A . . 15
6.1.2. MRT IP tunnels (Options 2A and 2B) . . . . . . . . . 13 6.1.2. MRT IP tunnels (Options 2A and 2B) . . . . . . . . . 16
6.2. Forwarding LDP Unicast Traffic over MRT Paths . . . . . . 14 6.2. Forwarding LDP Unicast Traffic over MRT Paths . . . . . . 16
6.2.1. Forwarding LDP traffic using MRT LDP Labels (Option 6.2.1. Forwarding LDP traffic using MRT LDP Labels (Option
1A) . . . . . . . . . . . . . . . . . . . . . . . . . 14 1A) . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.2.2. Forwarding LDP traffic using MRT LDP Labels (Option 6.2.2. Forwarding LDP traffic using MRT LDP Labels (Option
1B) . . . . . . . . . . . . . . . . . . . . . . . . . 15 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 . . . . . . . . . . . . . . . . . . . 15 MRT LDP Labels . . . . . . . . . . . . . . . . . . . 17
6.3. Forwarding IP Unicast Traffic over MRT Paths . . . . . . 15 6.3. Forwarding IP Unicast Traffic over MRT Paths . . . . . . 18
6.3.1. Tunneling IP traffic using MRT LDP Labels . . . . . . 16 6.3.1. Tunneling IP traffic using MRT LDP Labels . . . . . . 18
6.3.1.1. Tunneling IP traffic using MRT LDP Labels (Option 6.3.1.1. Tunneling IP traffic using MRT LDP Labels (Option
1A) . . . . . . . . . . . . . . . . . . . . . . . 16 1A) . . . . . . . . . . . . . . . . . . . . . . . 18
6.3.1.2. Tunneling IP traffic using MRT LDP Labels (Option 6.3.1.2. Tunneling IP traffic using MRT LDP Labels (Option
1B) . . . . . . . . . . . . . . . . . . . . . . . 16 1B) . . . . . . . . . . . . . . . . . . . . . . . 19
6.3.2. Tunneling IP traffic using MRT IP Tunnels . . . . . . 17 6.3.2. Tunneling IP traffic using MRT IP Tunnels . . . . . . 19
6.3.3. Required support . . . . . . . . . . . . . . . . . . 17 6.3.3. Required support . . . . . . . . . . . . . . . . . . 19
7. MRT Island Formation . . . . . . . . . . . . . . . . . . . . 17 7. MRT Island Formation . . . . . . . . . . . . . . . . . . . . 19
7.1. IGP Area or Level . . . . . . . . . . . . . . . . . . . . 18 7.1. IGP Area or Level . . . . . . . . . . . . . . . . . . . . 20
7.2. Support for a specific MRT profile . . . . . . . . . . . 18 7.2. Support for a specific MRT profile . . . . . . . . . . . 20
7.3. Excluding additional routers and interfaces from the MRT 7.3. Excluding additional routers and interfaces from the MRT
Island . . . . . . . . . . . . . . . . . . . . . . . . . 18 Island . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.3.1. Existing IGP exclusion mechanisms . . . . . . . . . . 18 7.3.1. Existing IGP exclusion mechanisms . . . . . . . . . . 21
7.3.2. MRT-specific exclusion mechanism . . . . . . . . . . 19 7.3.2. MRT-specific exclusion mechanism . . . . . . . . . . 22
7.4. Connectivity . . . . . . . . . . . . . . . . . . . . . . 22
7.4. Connectivity . . . . . . . . . . . . . . . . . . . . . . 19 7.5. Algorithm for MRT Island Identification . . . . . . . . . 22
7.5. Example algorithm . . . . . . . . . . . . . . . . . . . . 19 8. MRT Profile . . . . . . . . . . . . . . . . . . . . . . . . . 22
8. MRT Profile . . . . . . . . . . . . . . . . . . . . . . . . . 19 8.1. MRT Profile Options . . . . . . . . . . . . . . . . . . . 22
8.1. MRT Profile Options . . . . . . . . . . . . . . . . . . . 20 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 . . . . . . . . . . . . . . . 23 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 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 . . . . . . . . . . . . . . . . . . . . . . . 28 Selection . . . . . . . . . . . . . . . . . . . . . . . 31
11.2. Protecting Multi-Homed Prefixes using Named Proxy-Nodes 29 11.2. Protecting Multi-Homed Prefixes using Named Proxy-Nodes 32
11.3. MRT Alternates for Destinations Outside the MRT Island . 31 11.3. MRT Alternates for Destinations Outside the MRT Island . 34
12. Network Convergence and Preparing for the Next Failure . . . 31 12. Network Convergence and Preparing for the Next Failure . . . 34
12.1. Micro-forwarding loop prevention and MRTs . . . . . . . 32 12.1. Micro-loop prevention and MRTs . . . . . . . . . . . . . 35
12.2. MRT Recalculation for the Default MRT Profile . . . . . 32 12.2. MRT Recalculation for the Default MRT Profile . . . . . 36
13. Implementation Status . . . . . . . . . . . . . . . . . . . . 33 13. Implementation Status . . . . . . . . . . . . . . . . . . . . 37
14. Operations and Management Considerations . . . . . . . . . . 34 14. Operational Considerations . . . . . . . . . . . . . . . . . 38
15. Applying Policy to Select from Multiple Possible Alternates 14.1. Verifying Forwarding on MRT Paths . . . . . . . . . . . 38
for FRR . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 14.2. Traffic Capacity on Backup Paths . . . . . . . . . . . . 39
16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 35 14.3. MRT IP Tunnel Loopback Address Management . . . . . . . 40
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 14.4. MRT-FRR in a Network with Degraded Connectivity . . . . 41
18. Security Considerations . . . . . . . . . . . . . . . . . . . 36 14.5. Partial Deployment of MRT-FRR in a Network . . . . . . . 41
19. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 36 15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 41
20. References . . . . . . . . . . . . . . . . . . . . . . . . . 36 16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42
20.1. Normative References . . . . . . . . . . . . . . . . . . 36 17. Security Considerations . . . . . . . . . . . . . . . . . . . 42
20.2. Informative References . . . . . . . . . . . . . . . . . 37 18. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 42
Appendix A. General Issues with Area Abstraction . . . . . . . . 40 19. References . . . . . . . . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40 19.1. Normative References . . . . . . . . . . . . . . . . . . 43
19.2. Informative References . . . . . . . . . . . . . . . . . 44
Appendix A. General Issues with Area Abstraction . . . . . . . . 46
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 47
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 trees separate from the primary next-
hop forwarding used during stable operation. These two trees are hop forwarding used during stable operation. These two trees are
maximally diverse from each other, providing link and node protection maximally diverse from each other, providing link and node protection
for 100% of paths and failures as long as the failure does not cut for 100% of paths and failures as long as the failure does not cut
the network into multiple pieces. This document defines the the network into multiple pieces. This document defines the
architecture for IP/LDP fast-reroute with MRT. The associated architecture for IP/LDP fast-reroute with MRT.
protocol extensions are defined in [I-D.ietf-ospf-mrt] and
[I-D.ietf-mpls-ldp-mrt]. The exact MRT algorithm is defined in [I-D.ietf-rtgwg-mrt-frr-algorithm] describes how to compute maximally
[I-D.ietf-rtgwg-mrt-frr-algorithm]. redundant trees using a specific algorithm, the MRT Lowpoint
algorithm. The MRT Lowpoint algorithm is used by a router that
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
moved to one of MRTs, it is not subject to further repair actions. moved to one of the MRTs by one point of local repair (PLR), that
Thus, the traffic will not loop even if a worse failure (e.g. node) traffic is not subject to further repair actions by another PLR, even
occurs when protection was only available for a simpler failure (e.g. in the event of multiple simultaneous failures. Therefore, traffic
link). repaired by MRT-FRR will not loop between different PLRs responding
to different simultaneous failures.
While MRT provides 100% protection for a single link or node failure,
it may not protect traffic in the event of multiple simultaneous
failures, nor does take into account Shared Risk Link Groups (SRLGs).
Also, while the MRT Lowpoint algorithm is computationally efficient,
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
compute next-hops based on the same algorithm, and install the
corresponding forwarding state. This is in contrast to other FRR
methods where the calculation of backup paths generally involves
repeated application of the simpler and widely-deployed SPF
algorithm, and backup paths themselves re-use the forwarding state
used for shortest path forwarding of normal traffic. Section 14
provides operational guidance related to verification of 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]. described in [I-D.atlas-rtgwg-mrt-mc-arch]. However, the current
document does not address the multicast applications of MRTs.
Other existing or proposed solutions are partial solutions or have Figure 1 compares different methods of providing FRR for IP and LDP
significant issues, as described below. 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.
Summary Comparison of IP/LDP FRR Methods 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
| Method | Coverage | Alternate | Computation (in SPFs) | that need to be applied to packets to support the FRR method, while
| | | Looping? | | the fifth column gives the size of the MPLS label table needed to
+---------+-------------+-------------+-----------------------------+ support the FRR method. These two metrics may be useful for
| MRT-FRR | 100% | None | less than 3 | evaluating requirements placed on hardware to support the different
| | Link/Node | | | FRR methods.
| | | | |
| LFA | Partial | Possible | per neighbor |
| | Link/Node | | |
| | | | |
| Remote | Partial | Possible | per neighbor (link) or |
| LFA | Link/Node | | neighbor's neighbor (node) |
| | | | |
| Not-Via | 100% | None | per link and node |
| | Link/Node | | |
| | | | |
| TI-LFA | 100% | Possible | per neighbor (link) or |
| | Link/Node | | neighbor's neighbor (node) |
+---------+-------------+-------------+-----------------------------+
Table 1 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 Loop-Free Alternates (LFA): LFAs [RFC5286] provide limited
topology-dependent coverage for link and node protection. topology-dependent coverage for link and node protection.
Restrictions on choice of alternates can be relaxed to improve Restrictions on choice of alternates can be relaxed to improve
coverage, but this can cause forwarding loops if a worse failure coverage, but this can cause forwarding loops if a worse failure
is experienced than protected against. Augmenting a network to is experienced than protected against. [RFC6571] discusses the
provide better coverage is NP-hard [LFARevisited]. [RFC6571] applicability of LFA to different topologies with a focus on
discusses the applicability of LFA to different topologies with a common PoP architectures. The computation required is one SPF per
focus on common PoP architectures. 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 LFAs [RFC7490] improve coverage over LFAs for Remote LFA: Remote LFA [RFC7490] improves coverage over LFA for
link protection but still cannot guarantee complete coverage. The both link and node protection, but it does not guarantee 100%
trade-off of looping traffic to improve coverage is still made. coverage. The alternates can also loop with worse than expected
Remote LFAs can provide node-protection failures. Computation for link protection is one SPF per
[I-D.ietf-rtgwg-rlfa-node-protection] but not guaranteed coverage neighbor, while computation for node protection requires an
and the computation required is quite high (an SPF for each PQ- additional SPF per PQ node [I-D.ietf-rtgwg-rlfa-node-protection].
node evaluated). [I-D.bryant-ipfrr-tunnels] describes additional Remote LFA can impose up to one additional label on the packet,
mechanisms to further improve coverage, at the cost of added but does not increase the size of the label table. It requires a
complexity. T-LDP session for each selected PQ node.
Not-Via: Not-Via [RFC6981] is the only other solution that provides Not-Via: Not-Via [RFC6981] provides 100% coverage for link and node
100% coverage for link and node failures and does not have failures and does not have potential looping among alternates.
potential looping. However, the computation is very high (an SPF The computation is high with an SPF per potential failure point
per failure point) and academic implementations (all links and nodes in the topology). When implemented with LDP,
[LightweightNotVia] have found the address management complexity Not-Via adds one additional label to a tunnelled packet. It
to be high. 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- TI-LFA: Topology Independent Loop-free Alternate Fast Re-route (TI-
LFA) [I-D.francois-rtgwg-segment-routing-ti-lfa] aims to provide LFA) [I-D.francois-rtgwg-segment-routing-ti-lfa] aims to provide
link and node protection of node and adjacency segments within the link and node protection of node and adjacency segments within the
Segment Routing (SR) framework. It guarantees complete coverage. Segment Routing (SR) framework. It guarantees complete coverage.
The TI-LFA computation for link-protection is fairly The TI-LFA computation for link-protection is fairly
straightforward, while the computation for node-protection is more straightforward, while the computation for node-protection is more
complex. For link-protection with symmetric link costs, TI-LFA complex. For link-protection with symmetric link costs, TI-LFA
can provide complete coverage by pushing up to two additional can provide complete coverage by pushing up to two additional
labels. For node protection on arbitrary topologies, the label labels. For node protection on arbitrary topologies, the label
skipping to change at page 6, line 25 skipping to change at page 8, line 46
interface is represented by a pseudo-node and has asymmetric link interface is represented by a pseudo-node and has asymmetric link
costs to and from that pseudo-node. Second, when routers come up or 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 a link with LDP comes up, it is recommended in [RFC5443] and
[RFC6987] that the link metric be raised to the maximum cost; this [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, may not be symmetric and for [RFC6987] is not expected to be. Third,
techniques such as IGP metric tuning for traffic-engineering can techniques such as IGP metric tuning for traffic-engineering can
result in asymmetric link costs. A fast-reroute solution needs to result in asymmetric link costs. A fast-reroute solution needs to
handle network topologies with asymmetric link costs. handle network topologies with asymmetric link costs.
When a network needs to use a micro-loop prevention mechanism When a network needs to use a micro-loop prevention mechanism
[RFC5715] such as Ordered FIB[RFC6976] or Farside Tunneling[RFC5715], [RFC5715] such as Ordered FIB[RFC6976] or Nearside
then the whole IGP area needs to have alternates available so that Tunneling[RFC5715], then the whole IGP area needs to have alternates
the micro-loop prevention mechanism, which requires slower network available so that the micro-loop prevention mechanism, which requires
convergence, can take the necessary time without adversely impacting slower network convergence, can take the necessary time without
traffic. Without complete coverage, traffic to the unprotected adversely impacting traffic. Without complete coverage, traffic to
destinations will be dropped for significantly longer than with the unprotected destinations will be dropped for significantly longer
current convergence - where routers individually converge as fast as than with current convergence - where routers individually converge
possible. as fast as possible. See Section 12.1 for more discussion of micro-
loop prevention and MRTs.
1.2. Partial Deployment and Backwards Compatibility 1.2. Partial Deployment and Backwards Compatibility
MRT-FRR supports partial deployment. As with many new features, the MRT-FRR supports partial deployment. Routers advertise their ability
protocols (OSPF, LDP, ISIS) indicate their capability to support MRT. to support MRT. Inside the MRT-capable connected group of routers
Inside the MRT-capable connected group of routers (referred to as an (referred to as an MRT Island), the MRTs are computed. Alternates to
MRT Island), the MRTs are computed. Alternates to destinations destinations outside the MRT Island are computed and depend upon the
outside the MRT Island are computed and depend upon the existence of existence of a loop-free neighbor of the MRT Island for that
a loop-free neighbor of the MRT Island for that destination. destination. MRT Islands are discussed in detail in Section 7, and
partial deployment is discussed in more detail in Section 14.5.
2. Requirements Language 2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Terminology 3. Terminology
network graph: A graph that reflects the network topology where all network graph: A graph that reflects the network topology where all
links connect exactly two nodes and broadcast links have been links connect exactly two nodes and broadcast links have been
transformed into the standard pseudo-node representation. transformed into the standard pseudo-node representation.
cut-link: A link whose removal partitions the network. A cut-link
by definition must be connected between two cut-vertices. If
there are multiple parallel links, then they are referred to as
cut-links in this document if removing the set of parallel links
would partition the network graph.
cut-vertex: A vertex whose removal partitions the network graph.
2-connected: A graph that has no cut-vertices. This is a graph
that requires two nodes to be removed before the network is
partitioned.
2-connected cluster: A maximal set of nodes that are 2-connected.
block: Either a 2-connected cluster, a cut-edge, or an isolated
vertex.
Redundant Trees (RT): A pair of trees where the path from any node Redundant Trees (RT): A pair of trees where the path from any node
X to the root R along the first tree is node-disjoint with the X to the root R along the first tree is node-disjoint with the
path from the same node X to the root along the second tree. path from the same node X to the root along the second tree.
These can be computed in 2-connected graphs. Redundant trees can always be computed in 2-connected graphs.
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.
Any RT is an MRT but many MRTs are not RTs. In graphs that are not 2-connected, it is not possible to compute
RTs. However, it is possible to compute MRTs. MRTs are maximally
redundant in the sense that they are as redundant as possible
given the constraints of the network graph.
DAG: Directed Acyclic Graph - a graph where all links are directed
and there are no cycles in it.
ADAG: Almost Directed Acyclic Graph - a graph that, if all links
incoming to the root were removed, would be a DAG.
GADAG: Generalized ADAG - a graph that is the combination of the
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 described the associated forwarding topology and MPLS MT- used to describe the associated forwarding topology and MPLS
ID. Specifically, MRT-Red is the decreasing MRT where links in multi-topology identifier (MT-ID). Specifically, MRT-Red is the
the GADAG are taken in the direction from a higher topologically decreasing MRT where links in the GADAG are taken in the direction
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-
ID. Specifically, MRT-Blue is the increasing MRT where links in ID. Specifically, MRT-Blue is the increasing MRT where links in
the GADAG are taken in the direction from a lower topologically the GADAG are taken in the direction from a lower topologically
ordered node to a higher one. ordered node to a higher one.
Rainbow MRT: It is useful to have an MPLS MT-ID that refers to the Rainbow MRT: It is useful to have an MPLS MT-ID that refers to the
multiple MRT topologies and to the default topology. This is multiple MRT forwarding topologies and to the default forwarding
referred to as the Rainbow MRT MPLS MT-ID and is used by LDP to topology. This is referred to as the Rainbow MRT MPLS MT-ID and
reduce signaling and permit the same label to always be advertised is used by LDP to reduce signaling and permit the same label to
to all peers for the same (MT-ID, Prefix). always be advertised to all peers for the same (MT-ID, Prefix).
MRT Island: The set of routers that support a particular MRT MRT Island: The set of routers that support a particular MRT
profile and the links connecting them that support MRT. profile and the links connecting them that support MRT.
Island Border Router (IBR): A router in the MRT Island that is Island Border Router (IBR): A router in the MRT Island that is
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.
cut-link: A link whose removal partitions the network. A cut-link
by definition must be connected between two cut-vertices. If
there are multiple parallel links, then they are referred to as
cut-links in this document if removing the set of parallel links
would partition the network graph.
cut-vertex: A vertex whose removal partitions the network graph.
2-connected: A graph that has no cut-vertices. This is a graph
that requires two nodes to be removed before the network is
partitioned.
2-connected cluster: A maximal set of nodes that are 2-connected.
2-edge-connected: A network graph where at least two links must be
removed to partition the network.
block: Either a 2-connected cluster, a cut-edge, or an isolated
vertex.
DAG: Directed Acyclic Graph - a graph where all links are directed
and there are no cycles in it.
ADAG: Almost Directed Acyclic Graph - a graph that, if all links
incoming to the root were removed, would be a DAG.
GADAG: Generalized ADAG - a graph that is the combination of the
ADAGs of all blocks.
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 for the associated prefix or because MRT-Red and MRT-
Blue IP addresses are advertised in an undefined fashion for that Blue IP addresses are advertised in an undefined fashion for that
proxy-node. 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
algorithm to compute MRTs 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. Modeling results in O(e + n log n); it is less than three SPFs. This document
comparing the alternate path lengths obtained with MRT to other describes how the MRTs can be used and not how to compute them.
approaches are described in [I-D.ietf-rtgwg-mrt-frr-algorithm]. This
document describes how the MRTs can be used and not how to compute
them.
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 1, there is a network graph that is For example, in Figure 2, 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 1: A 2-connected Network Figure 2: A 2-connected Network
By contrast, in Figure 2, the network in (a) is not 2-connected. If By contrast, in Figure 3, 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 1 and the path from G to R on the Blue R are the same as in Figure 2 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 10, line 27 skipping to change at page 12, line 35
| ^ | [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 2: A non-2-connected network Figure 3: 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 to all In normal IGP routing, each router has its shortest path tree (SPT)to
destinations. From the perspective of a particular destination, D, all destinations. From the perspective of a particular destination,
this looks like a reverse SPT (rSPT). To use maximally redundant D, this looks like a reverse SPT. To use maximally redundant trees,
trees, in addition, each destination D has two MRTs associated with in addition, each destination D has two MRTs associated with it; by
it; by convention these will be called the MRT-Blue and MRT-Red. convention these will be called the MRT-Blue and MRT-Red. MRT-FRR is
MRT-FRR is realized by using multi-topology forwarding. There is a realized by using multi-topology forwarding. There is a MRT-Blue
MRT-Blue forwarding topology and a MRT-Red forwarding topology. 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 2, C the MRTs to reach the destination D. For example, in Figure 3, 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.
MRT alternates are always available to use. It is a local decision MRT alternates are always available to use. It is a local decision
whether to use an MRT alternate, a Loop-Free Alternate or some other whether to use an MRT alternate, a Loop-Free Alternate or some other
type of alternate. type of alternate.
As described in [RFC5286], when a worse failure than is anticipated As described in [RFC5286], when a worse failure than is anticipated
happens, using LFAs that are not downstream neighbors can cause happens, using LFAs that are not downstream neighbors can cause
micro-looping. Section 1.1 of [RFC5286] gives an example of link- looping among alternates. Section 1.1 of [RFC5286] gives an example
protecting alternates causing a loop on node failure. Even if a of link-protecting alternates causing a loop on node failure. Even
worse failure than anticipated happens, the use of MRT alternates if a worse failure than anticipated happens, the use of MRT
will not cause looping. Therefore, while node-protecting LFAs may be alternates will not cause looping.
preferred, the certainty that no alternate-induced looping will occur
is an advantage of using MRT alternates when the available node-
protecting LFA is not a downstream path.
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 a 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
skipping to change at page 12, line 51 skipping to change at page 15, line 12
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.
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 are described in [I-D.ietf-mpls-ldp-mrt]. 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
(FEC label) identifies the FEC. The top label would be a new FEC (FEC label) identifies the FEC. The top label would be a new FEC
type with two values corresponding to MRT Red and Blue topologies. type with two values corresponding to MRT Red and Blue topologies.
When an MRT transit router receives a packet with a topology-id When an MRT transit router receives a packet with a topology-id
skipping to change at page 14, line 12 skipping to change at page 16, line 21
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
tunnel destination. For example, an MRT ingress router can cause a tunnel destination. For example, an MRT ingress router can cause a
packet to be tunneled along the MRT-red path to router X by packet to be tunneled along the MRT-red path to router X by
encapsulating the packet using the MRT-red loopback address encapsulating the packet using the MRT-red loopback address
advertised by router X. Upon receiving the packet, router X would advertised by router X. Upon receiving the packet, router X would
remove the encapsulation header and forward the packet based on the remove the encapsulation header and forward the packet based on the
original destination address. original destination address.
Announcements of these two additional loopback addresses per router
with their MRT color requires IGP extensions, which have not been
defined.
Either IPv4 (option 2A) or IPv6 (option 2B) can be used as the Either IPv4 (option 2A) or IPv6 (option 2B) can be used as the
tunneling mechanism. tunneling mechanism.
Note that the two forwarding mechanisms using LDP Label options do Note that the two forwarding mechanisms using LDP Label options do
not require additional loopbacks per router, as is required by the IP not require additional loopbacks per router, as is required by the IP
tunneling mechanism. This is because LDP labels are used on a hop- tunneling mechanism. This is because LDP labels are used on a hop-
by-hop basis to identify MRT-blue and MRT-red forwarding topologies. by-hop basis to identify MRT-blue and MRT-red forwarding topologies.
6.2. Forwarding LDP Unicast Traffic over MRT Paths 6.2. Forwarding LDP Unicast Traffic over MRT Paths
skipping to change at page 16, line 9 skipping to change at page 18, line 15
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
forwarding topology. However, in this document, we have chosen not forwarding topology. However, in this document, we have chosen not
to define a solution that would work for IPv6 traffic but not for to define a solution that would work for IPv6 traffic but not for
IPv4 traffic. IPv4 traffic.
The choice of tunnel egress MAY be flexible since any router closer The choice of tunnel egress is flexible since any router closer to
to the destination than the next-hop can work. This architecture the destination than the next-hop can work. This architecture
assumes that the original destination in the area is selected (see assumes that the original destination in the area is selected (see
Section 11 for handling of multi-homed prefixes); another possible Section 11 for handling of multi-homed prefixes); another possible
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. However, consistency with LDP is RECOMMENDED. 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 ABR/LBR on the shortest path
to the destination. For a destination outside of the PLR's MRT to the destination. For a destination outside of the PLR's MRT
Island, the packet should be tunneled on the MRT to a non-proxy-node Island, the packet should be tunneled on the MRT to a non-proxy-node
immediately before the named proxy-node on that particular color MRT. immediately before the named proxy-node on that particular color MRT.
skipping to change at page 17, line 34 skipping to change at page 19, line 40
6.3.3. Required support 6.3.3. Required support
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 Labels 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 in the IGP is to The purpose of communicating support for MRT is to indicate that the
indicate that the MRT-Blue and MRT-Red forwarding topologies are MRT-Blue and MRT-Red forwarding topologies are created for transit
created for transit traffic. The MRT architecture allows for traffic. The MRT architecture allows for different, potentially
different, potentially incompatible options. In order to create incompatible options. In order to create consistent MRT forwarding
constistent MRT forwarding topologies, the routers participating in a topologies, the routers participating in a particular MRT Island need
particular MRT Island need to use the same set of options. These to use the same set of options. These options are grouped into MRT
options are grouped into MRT profiles. In addition, the routers in profiles. In addition, the routers in an MRT Island all need to use
an MRT Island all need to use the same set of nodes and links within the same set of nodes and links within the Island when computing the
the Island when computing the MRT forwarding topologies. This MRT forwarding topologies. This section describes the information
section describes the information used by a router to determine the used by a router to determine the nodes and links to include in a
nodes and links to include in a particular MRT Island. Some of this particular MRT Island. Some information already exists in the IGPs
information is shared among routers using the newly-defined IGP
signaling extensions for MRT described in [I-D.ietf-ospf-mrt] and
[I-D.ietf-isis-mrt]. Other information already exists in the IGPs
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
there do not currently exist protocol extensions. This new
information needs to be shared among all routers in an IGP area, so
defining extensions to existing IGPs to carry this information makes
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 ISIS 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.
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 MUST be bidirectional and belong to the
same IGP area or level. For ISIS, a link belonging to both level 1 same IGP area or level. For ISIS, a link belonging to both level 1
and level 2 would qualify to be in multiple MRT Islands. A given ABR and level 2 would qualify to be in multiple MRT Islands. A given ABR
or LBR can belong to multiple MRT Islands, corresponding to the areas or LBR can belong to multiple MRT Islands, corresponding to the areas
or levels in which it participates. Inter-area forwarding behavior or levels in which it participates. Inter-area forwarding behavior
is discussed in Section 10. is 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 MUST support the same MRT profile. A
router advertises support for a given MRT profile using the IGP router advertises support for a given MRT profile using an 8-bit MRT
extensions defined in [I-D.ietf-ospf-mrt] and [I-D.ietf-isis-mrt] Profile ID value. The registry for the MRT Profile ID is defined in
using an 8-bit Profile ID value. A given router can support multiple this document. The protocol extensions for advertising the MRT
MRT profiles and participate in multiple MRT Islands. The options Profile ID value will be defined elsewhere. A given router can
that make up an MRT profile, as well as the default MRT profile, are support multiple MRT profiles and participate in multiple MRT
defined in Section 8. Islands. The options that make up an MRT profile, as well as the
default MRT profile, are defined in Section 8.
Note that a router may advertise support for multiple different MRT
profiles. The process of MRT Island formation takes place
independently for each MRT profile advertised by a given router. For
example, consider a network with 40 connected routers in the same
area advertising support for MRT Profile A and MRT Profile B. Two
distinct MRT Islands will be formed corresponding to Profile A and
Profile B, with each island containing all 40 routers. A complete
set of maximally redundant trees will be computed for each island
following the rules defined for each profile. If we add a third MRT
Profile to this example, with Profile C being advertised by a
connected subset of 30 routers, there will be a third MRT Island
formed corresponding to those 30 routers, and a third set of
maximally redundant trees will be computed. In this example, 40
routers would compute and install two sets of MRT transit forwarding
entries corresponding to Profiles A and B, while 30 routers would
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 ISIS and OSPF. In ISIS, 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 exist in ISIS and OSPF to prevent transit traffic Mechanisms also already exist in ISIS and OSPF to discourage or
from using a particular router. In ISIS, the overload bit is used prevent transit traffic from using a particular router. In ISIS, the
for this purpose. In OSPF, [RFC6987] specifies setting all outgoing overload bit is prevents transit traffic from using a router.
interface metrics to 0xFFFF to accomplish this.
For OSPFv2 and OSPFv3, [RFC6987] specifies setting all outgoing
interface metrics to 0xFFFF to discourage transit traffic from using
a router.( [RFC6987] defines the metric value 0xFFFF as
MaxLinkMetric, a fixed architectural value for OSPF.) For OSPFv3,
[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
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
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. ISIS) 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 ISIS), 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
OSPF). OSPFv2 and OSPFv3).
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
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
usable link from MRT Islands. [I-D.ietf-ospf-mrt] and usable link from MRT Islands. The protocol extensions for
[I-D.ietf-isis-mrt] define the IGP extensions for OSPF and ISIS used advertising that a link is MRT-Ineligible will be defined elsewhere.
to advertise that a link is MRT-Ineligible. A link with either A link with either interface advertised as MRT-Ineligible MUST be
interface advertised as MRT-Ineligible MUST be excluded from an MRT excluded from an MRT Island. Note that an interface advertised as
Island. Note that an interface advertised as MRT-Ineligigle by a MRT-Ineligible by a router is ineligible with respect to all profiles
router is ineligible with respect to all profiles advertised by that advertised by that router.
router.
7.4. Connectivity 7.4. Connectivity
All of the routers in an MRT Island MUST be connected by All of the routers in an MRT Island MUST be connected by
bidirectional links with other routers in the MRT Island. bidirectional links with other routers in the MRT Island.
Disconnected MRT Islands will operate independently of one another. Disconnected MRT Islands will operate independently of one another.
7.5. Example algorithm 7.5. Algorithm for MRT Island Identification
An algorithm that allows a computing router to identify the routers An algorithm that allows a computing router to identify the routers
and links in the local MRT Island satisfying the above rules is given and links in the local MRT Island satisfying the above rules is given
in section 5.2 of [I-D.ietf-rtgwg-mrt-frr-algorithm]. in section 5.2 of [I-D.ietf-rtgwg-mrt-frr-algorithm].
8. MRT Profile 8. MRT Profile
An MRT Profile is a set of values and options related to MRT An MRT Profile is a set of values and options related to MRT
behavior. The complete set of options is designated by the behavior. The complete set of options is designated by the
corresponding 8-bit Profile ID value. corresponding 8-bit Profile ID value.
This document specifies the values and options that correspond to the This document specifies the values and options that correspond to the
Default MRT Profile (Profile ID = 0). Future documents may define Default MRT Profile (Profile ID = 0). Future documents may define
other MRT Profiles by specifying the MRT Profile Options below. other MRT Profiles by specifying the MRT Profile Options below.
8.1. MRT Profile Options 8.1. MRT Profile Options
Below is a description of the values and options that define an MRT Below is a description of the values and options that define an MRT
Profile. Profile.
MRT Algorithm: This identifies the particular MRT algorithm used by MRT Algorithm: This identifies the particular algorithm for
the router for this profile. Algorithm transitions can be managed computing maximally redundant trees used by the router for this
by advertising multiple MRT profiles. profile.
MRT-Red MT-ID: This specifies the MT-ID to be associated with the MRT-Red MT-ID: This specifies the MPLS MT-ID to be associated with
MRT-Red forwarding topology. It is needed for use in LDP the MRT-Red forwarding topology. It is allocated from the MPLS
signaling. All routers in the MRT Island MUST agree on a value. Multi-Topology Identifiers Registry.
MRT-Blue MT-ID: This specifies the MT-ID to be associated with the MRT-Blue MT-ID: This specifies the MPLS MT-ID to be associated with
MRT-Blue forwarding topology. It is needed for use in LDP the MRT-Blue forwarding topology. It is allocated from the MPLS
signaling. All routers in the MRT Island MUST agree on a value. Multi-Topology Identifiers Registry.
GADAG Root Selection Policy: This specifes the manner in which the GADAG Root Selection Policy: This specifies the manner in which the
GADAG root is selected. All routers in the MRT island need to use GADAG root is selected. All routers in the MRT island need to use
the same GADAG root in the calculations used construct the MRTs. the same GADAG root in the calculations used construct the MRTs.
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 in the IGP as router-specific MRT Priority values, advertised as router-specific MRT parameters, MAY
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 Labels, IPv4 Tunneling, IPv6
Tunneling, and None. If the MRT LDP Labels option is supported, Tunneling, and None. If the MRT LDP Labels option is supported,
then option 1A and the appropriate signaling extensions MUST be then option 1A and the appropriate signaling extensions MUST be
supported. If IPv4 is supported, then both MRT-Red and MRT-Blue supported. If IPv4 is supported, then both MRT-Red and MRT-Blue
IPv4 Loopback Addresses SHOULD be specified. If IPv6 is IPv4 Loopback Addresses SHOULD be specified. If IPv6 is
supported, both MRT-Red and MRT-Blue IPv6 Loopback Addresses supported, both MRT-Red and MRT-Blue IPv6 Loopback Addresses
SHOULD be specified. The None option in may be useful for SHOULD be specified.
multicast global protection.
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: Should inter-area traffic on the MRT- Area/Level Border Behavior: This specifies how traffic traveling on
Blue or MRT-Red be put back onto the shortest path tree? Should the MRT-Blue or MRT-Red in one area should be treated when it
it be swapped from MRT-Blue or MRT-Red in one area/level to MRT- passes into another area.
Red or MRT-Blue in the next area/level to avoid the potential
failure of an ABR? (See [I-D.atlas-rtgwg-mrt-mc-arch] for use-
case details.
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.
When a new MRT Profile is defined, new and unique values should be
allocated from the MPLS Multi-Topology Identifiers Registry,
corresponding to the MRT-Red and MRT-Blue MT-ID values for the new
MRT Profile .
If a router advertises support for multiple MRT profiles, then it If a router advertises support for multiple MRT profiles, then it
MUST create the transit forwarding topologies for each of those, MUST create the transit forwarding topologies for each of those,
unless the profile specifies the None option for MRT Forwarding unless the profile specifies the None option for MRT Forwarding
Mechanism. A router MUST NOT advertise multiple MRT profiles that Mechanism.
overlap in their MRT-Red MT-ID or MRT-Blue MT-ID.
The ability of MRT-FRR to support transit forwarding entries for
multiple profiles can be used to facilitate a smooth transition from
an existing deployed MRT Profile to a new MRT Profile. The new
profile can be activated in parallel with the existing profile,
installing the transit forwarding entries for the new profile without
affecting the transit forwarding entries for the existing profile.
Once the new transit forwarding state has been verified, the router
can be configured to use the alternates computed by the new profile
in the event of a failure.
8.2. Router-specific MRT paramaters 8.2. Router-specific MRT paramaters
For some profiles, additional router-specific MRT parameters may need For some profiles, additional router-specific MRT parameters may need
to be distributed via the IGP. While the set of options indicated by to be advertised. While the set of options indicated by the MRT
the MRT Profile ID must be identical for all routers in an MRT Profile ID must be identical for all routers in an MRT Island, these
Island, these router-specific MRT parameters may differ between router-specific MRT parameters may differ between routers in the same
routers in the same MRT island. Several such parameters are MRT island. Several such parameters are described below.
described below.
GADAG Root Selection Priority: A GADAG Root Selection Policy MAY GADAG Root Selection Priority: A GADAG Root Selection Policy MAY
rely on the GADAG Root Selection Priority values advertised by rely on the GADAG Root Selection Priority values advertised by
each router in the MRT island. A GADAG Root Selection Policy may each router in the MRT island. A GADAG Root Selection Policy may
use the GADAG Root Selection Priority to allow network operators use the GADAG Root Selection Priority to allow network operators
to configure a parameter to ensure that the GADAG root is selected to configure a parameter to ensure that the GADAG root is selected
from a particular subset of routers. An example of this use of from a particular subset of routers. An example of this use of
the GADAG Root Selection Priority value by the GADAG Root the GADAG Root Selection Priority value by the GADAG Root
Selection Policy is given in the Default MRT profile below. Selection Policy is given in the Default MRT profile below.
MRT-Red Loopback Address: This provides the router's loopback MRT-Red Loopback Address: This provides the router's loopback
address to reach the router via the MRT-Red forwarding topology. address to reach the router via the MRT-Red forwarding topology.
It can be specified for either IPv4 and IPv6. It can be specified for either IPv4 or IPv6. Note that this
parameter is not needed to support the Default MRT profile.
MRT-Blue Loopback Address: This provides the router's loopback MRT-Blue Loopback Address: This provides the router's loopback
address to reach the router via the MRT-Blue forwarding topology. address to reach the router via the MRT-Blue forwarding topology.
It can be specified for either IPv4 and IPv6. It can be specified for either IPv4 and IPv6. Note that this
parameter is not needed to support the Default MRT profile.
The extensions to OSPF and ISIS for advertising a router's GADAG Root Protocol extensions for advertising a router's GADAG Root Selection
Selection Priority value are defined in [I-D.ietf-ospf-mrt] and Priority value will be defined in other documents. Protocol
[I-D.ietf-isis-mrt]. IGP extensions for the advertising a router's extensions for the advertising a router's MRT-Red and MRT-Blue
MRT-Red and MRT-Blue Loopback Addresses have not been defined. Loopback Addresses will be defined elsewhere.
8.3. Default MRT profile 8.3. Default MRT profile
The following set of options defines the default MRT Profile. The The following set of options defines the default MRT Profile. The
default MRT profile is indicated by the MRT Profile ID value of 0. default MRT profile is indicated by the MRT Profile ID value of 0.
MRT Algorithm: MRT Lowpoint algorithm defined in MRT Algorithm: MRT Lowpoint algorithm defined in
[I-D.ietf-rtgwg-mrt-frr-algorithm]. [I-D.ietf-rtgwg-mrt-frr-algorithm].
MRT-Red MPLS MT-ID: [I-D.ietf-mpls-ldp-mrt] contains the IANA MRT-Red MPLS MT-ID: This value will be allocated from the MPLS
request for allocation of this value from the MPLS Multi-Topology Multi-Topology Identifiers Registry. The IANA request for this
Identifiers Registry. Prototype experiments have used a value of allocation will be in another document.
3997.
MRT-Blue MPLS MT-ID: [I-D.ietf-mpls-ldp-mrt] contains the IANA MRT-Blue MPLS MT-ID: This value will be allocated from the MPLS
request for allocation of this value from the MPLS Multi-Topology Multi-Topology Identifiers Registry. The IANA request for this
Identifiers Registry. Prototype experiments have used a value of allocation will be in another document.
3998.
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 highest priority advertised, an implementation MUST and with the most preferred GADAG Root Selection Priority
pick the router with the highest Router ID to be the GADAG root. advertised (corresponding to the lowest numerical value of GADAG
Root Selection Priority), an implementation MUST pick the router
with the highest Router ID to be the GADAG root.
Forwarding Mechanisms: MRT LDP Labels Forwarding Mechanisms: MRT LDP Labels
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
The protocol extensions for LDP are defined in The protocol extensions for LDP will be defined in another document.
[I-D.ietf-mpls-ldp-mrt]. A router must indicate that it has the A router must indicate that it has the ability to support MRT; having
ability to support MRT; having this explicit allows the use of MRT- this explicit allows the use of MRT-specific processing, such as
specific processing, such as special handling of FECs sent with the special handling of FECs sent with the Rainbow MRT MT-ID.
Rainbow MRT MT-ID.
A FEC sent with the Rainbow MRT MT-ID indicates that the FEC applies A FEC sent with the Rainbow MRT MT-ID indicates that the FEC applies
to all the MRT-Blue and MRT-Red MT-IDs in supported MRT profiles. to all the MRT-Blue and MRT-Red MT-IDs in supported MRT profiles.
The FEC-label bindings for the default shortest-path based MT-ID 0 The FEC-label bindings for the default shortest-path based MT-ID 0
MUST still be sent (even though it could be inferred from the Rainbow MUST still be sent (even though it could be inferred from the Rainbow
FEC-label bindings) to ensure continuous operation of normal LDP FEC-label bindings) to ensure continuous operation of normal LDP
forwarding. The Rainbow MRT MT-ID is defined to provide an easy way forwarding. The Rainbow MRT MT-ID is defined to provide an easy way
to handle the special signaling that is needed at ABRs or LBRs. It to handle the special signaling that is needed at ABRs or LBRs. It
avoids the problem of needing to signal different MPLS labels to avoids the problem of needing to signal different MPLS labels to
different LDP neighbors for the same FEC. Because the Rainbow MRT different LDP neighbors for the same FEC. Because the Rainbow MRT
MT-ID is used only by ABRs/LBRs or an LDP egress router, it is not MT-ID is used only by ABRs/LBRs or an LDP egress router, it is not
MRT profile specific. MRT profile specific.
[I-D.ietf-mpls-ldp-mrt] contains the IANA request for the Rainbow MRT The value of the Rainbow MRT MPLS MT-ID will be allocated from the
MPLS MT-ID. MPLS Multi-Topology Identifiers Registry. The IANA request for this
allocation will be in another document.
10. Inter-area Forwarding Behavior 10. Inter-area Forwarding Behavior
Unless otherwise specified, in this section we will use the terms
area and ABR to indicate either an OSPF area and OSPF ABR or ISIS
level and ISIS LBR.
An ABR/LBR has two forwarding roles. First, it forwards traffic An ABR/LBR has two forwarding roles. First, it forwards traffic
within areas. Second, it forwards traffic from one area into within areas. Second, it forwards traffic from one area into
another. These same two roles apply for MRT transit traffic. another. These same two roles apply for MRT transit traffic.
Traffic on MRT-Red or MRT-Blue destined inside the area needs to stay Traffic on MRT-Red or MRT-Blue destined inside the area needs to stay
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
skipping to change at page 25, line 10 skipping to change at page 27, line 49
the outermost IP address. If the outermost IP address is an MRT the outermost IP address. If the outermost IP address is an MRT
loopback address of the ABR/LBR, then the packet is decapsulated and loopback address of the ABR/LBR, then the packet is decapsulated and
forwarded based upon the inner IP address, which should go on the forwarded based upon the inner IP address, which should go on the
default SPT topology. If the outermost IP address is not an MRT default SPT topology. If the outermost IP address is not an MRT
loopback address of the ABR/LBR, then the packet is simply forwarded loopback address of the ABR/LBR, then the packet is simply forwarded
along the associated forwarding topology. A PLR sending traffic to a along the associated forwarding topology. A PLR sending traffic to a
destination outside its local area/level will pick the MRT and use destination outside its local area/level will pick the MRT and use
the associated MRT loopback address of the selected ABR/LBR the associated MRT loopback address of the selected ABR/LBR
advertising the lowest cost to the external destination. advertising the lowest cost to 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 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 LBR would need to determine whether the ABR/LBR would forward the
packet out of the area/level. If so, then that router should pop off packet out of the area/level. If so, then that router should pop off
the topology-identification label before forwarding the packet to the the topology-identification label before forwarding the packet to the
ABR/LBR. ABR/LBR.
For example, in Figure 3, if node H fails, node E has to put traffic For example, in Figure 4, 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 ISIS and most cases for OSPF, the penultimate router
can determine what decision the adjacent ABR will make. The one case can determine what decision the adjacent ABR will make. The one case
where it can't be determined is when two ASBRs are in different non- where it can't be determined is when two ASBRs are in different non-
backbone areas attached to the same ABR, then the ASBR's Area ID may backbone areas attached to the same ABR, then the ASBR's Area ID may
be needed for tie-breaking (prefer the route with the largest OPSF be needed for tie-breaking (prefer the route with the largest OPSF
area ID) and the Area ID isn't announced as part of the ASBR link- area ID) and the Area ID isn't announced as part of the ASBR link-
state advertisement (LSA). In this one case, suboptimal forwarding state advertisement (LSA). In this one case, suboptimal forwarding
along the MRT in the other area would happen. If that becomes a along the MRT in the other area would happen. If that becomes a
realistic deployment scenario, OSPF extensions could be considered. realistic deployment scenario, protocol extensions could be developed
This is not covered in [I-D.ietf-ospf-mrt]. 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 26, line 38 skipping to change at page 29, 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 3: ABR Forwarding Behavior and MRTs Figure 4: 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.
The key difference is whether the traffic, once out of the MRT The key difference is whether the traffic, once out of the MRT
Island, remains in the same area/level and might re-enter the MRT Island, might re-enter the MRT Island if a loop-free exit point is
Island if a loop-free exit point is not selected. not selected.
FRR using LFA has the useful property that it is able to protect FRR using LFA has the useful property that it is able to protect
multi-homed prefixes against ABR failure. For instance, if a prefix multi-homed prefixes against ABR failure. For instance, if a prefix
from the backbone is available via both ABR A and ABR B, if A fails, from the backbone is available via both ABR A and ABR B, if A fails,
then the traffic should be redirected to B. This can be accomplished then the traffic should be redirected to B. This can be accomplished
with MRT FRR as well. with MRT FRR as well.
If ASBR protection is desired, this has additional complexities if If ASBR protection is desired, this has additional complexities if
the ASBRs are in different areas. Similarly, protecting labeled BGP the ASBRs are in different areas. Similarly, protecting labeled BGP
traffic in the event of an ASBR failure has additional complexities traffic in the event of an ASBR failure has additional complexities
skipping to change at page 27, line 30 skipping to change at page 30, 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
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
or IP address, and pick the appropriate label or IP address to reach or IP address, and pick the appropriate label or IP address to reach
it on either MRT-Blue or MRT-Red as appropriate to avoid the failure it on either MRT-Blue or MRT-Red as appropriate to avoid the failure
skipping to change at page 30, line 9 skipping to change at page 33, line 14
A named proxy-node represents one or more destinations and, for LDP A named proxy-node represents one or more destinations and, for LDP
forwarding, has a FEC associated with it that is signalled into the forwarding, has a FEC associated with it that is signalled into the
MRT Island. Therefore, it is possible to explicitly label packets to MRT Island. Therefore, it is possible to explicitly label packets to
go to (MRT-Red, FEC) or (MRT-Blue, FEC); at the border of the MRT go to (MRT-Red, FEC) or (MRT-Blue, FEC); at the border of the MRT
Island, the label will swap to meaning (MT-ID 0, FEC). It would be Island, the label will swap to meaning (MT-ID 0, FEC). It would be
possible to have named proxy-nodes for IP forwarding, but this would possible to have named proxy-nodes for IP forwarding, but this would
require extensions to signal two IP addresses to be associated with require extensions to signal two IP addresses to be associated with
MRT-Red and MRT-Blue for the proxy-node. A named proxy-node can be MRT-Red and MRT-Blue for the proxy-node. A named proxy-node can be
uniquely represented by the two routers in the MRT Island to which it uniquely represented by the two routers in the MRT Island to which it
is connected. The extensions to signal such IP addresses are not is connected. The extensions to signal such IP addresses will be
defined in [I-D.ietf-ospf-mrt]. The details of what label-bindings defined elsewhere. The details of what label-bindings must be
must be originated are described in [I-D.ietf-mpls-ldp-mrt]. originated will be described in another document.
Computing the MRT next-hops to a named proxy-node and the MRT Computing the MRT next-hops to a named proxy-node and the MRT
alternate for the computing router S to avoid a particular failure alternate for the computing router S to avoid a particular failure
node F is straightforward. The details of the simple constant-time node F is straightforward. The details of the simple constant-time
functions, Select_Proxy_Node_NHs() and functions, Select_Proxy_Node_NHs() and
Select_Alternates_Proxy_Node(), are given in Select_Alternates_Proxy_Node(), are given in
[I-D.ietf-rtgwg-mrt-frr-algorithm]. A key point is that computing [I-D.ietf-rtgwg-mrt-frr-algorithm]. A key point is that computing
these MRT next-hops and alternates can be done as new named proxy- these MRT next-hops and alternates can be done as new named proxy-
nodes are added or removed without requiring a new MRT computation or nodes are added or removed without requiring a new MRT computation or
impacting other existing MRT paths. This maps very well to, for impacting other existing MRT paths. This maps very well to, for
skipping to change at page 32, line 5 skipping to change at page 35, line 7
for the additional complexity has not been justified. for the additional complexity has not been justified.
12. Network Convergence and Preparing for the Next Failure 12. Network Convergence and Preparing for the Next Failure
After a failure, MRT detours ensure that packets reach their intended After a failure, MRT detours ensure that packets reach their intended
destination while the IGP has not reconverged onto the new topology. destination while the IGP has not reconverged onto the new topology.
As link-state updates reach the routers, the IGP process calculates As link-state updates reach the routers, the IGP process calculates
the new shortest paths. Two things need attention: micro-loop the new shortest paths. Two things need attention: micro-loop
prevention and MRT re-calculation. prevention and MRT re-calculation.
12.1. Micro-forwarding loop prevention and MRTs 12.1. Micro-loop prevention and MRTs
As is well known[RFC5715], micro-loops can occur during IGP A micro-loop is a transient packet forwarding loop among two or more
convergence; such loops can be local to the failure or remote from routers that can occur during convergence of IGP forwarding state.
the failure. Managing micro-loops is an orthogonal issue to having [RFC5715] discusses several techniques for preventing micro-loops.
alternates for local repair, such as MRT fast-reroute provides. This section discusses how MRT-FRR relates to two of the micro-loop
prevention techniques discussed in [RFC5715], Nearside Tunneling and
Farside Tunneling.
There are two possible micro-loop prevention mechanisms discussed in In Nearside Tunneling, a router (PLR) adjacent to a failure perform
[RFC5715]. The first is Ordered FIB [RFC6976]. The second is local repair and inform remote routers of the failure. The remote
Farside Tunneling which requires tunnels or an alternate topology to routers initially tunnel affected traffic to the nearest PLR, using
reach routers on the farside of the failure. tunnels which are unaffected by the failure. Once the forwarding
state for normal shortest path routing has converged, the remote
routers return the traffic to shortest path forwarding. MRT-FRR is
relevant for Nearside Tunneling for the following reason. The
process of tunneling traffic to the PLRs and waiting a sufficient
amount of time for IGP forwarding state convergence with Nearside
Tunneling means that traffic will generally be relying on the local
repair at the PLR for longer than it would in the absence of Nearside
Tunneling. Since MRT-FRR provides 100% coverage for single link and
node failure, it may be an attractive option to provide the local
repair paths when Nearside Tunneling is deployed.
Since MRTs provide an alternate topology through which traffic can be MRT-FRR is also relevant for the Farside Tunneling micro-loop
sent and which can be manipulated separately from the SPT, it is prevention technique. In Farside Tunneling, remote routers tunnel
possible that MRTs could be used to support Farside Tunneling. traffic affected by a failure to a node downstream of the failure
Details of how to do so are outside the scope of this document. with respect to traffic destination. This node can be viewed as
being on the farside of the failure with respect to the node
initiating the tunnel. Note that the discussion of Farside Tunneling
in [RFC5715] focuses on the case where the farside node is
immediately adjacent to a failed link or node. However, the farside
node may be any node downstream of the failure with respect to
traffic destination, including the destination itself. The tunneling
mechanism used to reach the farside node must be unaffected by the
failure. The alternative forwarding paths created by MRT-FRR have
the potential to be used to forward traffic from the remote routers
upstream of the failure all the way to the destination. In the event
of failure, either the MRT-Red or MRT-Blue path from the remote
upstream router to the destination is guaranteed to avoid a link
failure or inferred node failure. The MRT forwarding paths are also
guaranteed to not be subject to micro-loops because they are locked
to the topology before the failure.
Micro-loop mitigation mechanisms can also work when combined with We note that the computations in [I-D.ietf-rtgwg-mrt-frr-algorithm]
MRT. address the case of a PLR adjacent to a failure determining which
choice of MRT-Red or MRT-Blue will avoid a failed link or node. More
computation may be required for an arbitrary remote upstream router
to determine whether to choose MRT-Red or MRT-Blue for a given
destination and failure.
12.2. MRT Recalculation for the Default MRT Profile 12.2. MRT Recalculation for the Default MRT Profile
This section describes how the MRT recalculation SHOULD be performed This section describes how the MRT recalculation SHOULD be performed
for the Default MRT Profile. This is intended to support FRR for the Default MRT Profile. This is intended to support FRR
applications. Other approaches are possible, but they are not applications. Other approaches are possible, but they are not
specified in this document. specified in this document.
When a failure event happens, traffic is put by the PLRs onto the MRT When a failure event happens, traffic is put by the PLRs onto the MRT
topologies. After that, each router recomputes its shortest path topologies. After that, each router recomputes its shortest path
tree (SPT) and moves traffic over to that. Only after all the PLRs tree (SPT) and moves traffic over to that. Only after all the PLRs
have switched to using their SPTs and traffic has drained from the have switched to using their SPTs and traffic has drained from the
MRT topologies should each router install the recomputed MRTs into MRT topologies should each router install the recomputed MRTs into
the FIBs. the FIBs.
At each router, therefore, the sequence is as follows: At each router, therefore, the sequence is as follows:
1. Receive failure notification 1. Receive failure notification
2. Recompute SPT 2. Recompute SPT.
3. Install new SPT 3. Install the new SPT in the FIB.
4. If the network was stable before the failure occured, wait a 4. If the network was stable before the failure occured, wait a
configured (or advertised) period for all routers to be using configured (or advertised) period for all routers to be using
their SPTs and traffic to drain from the MRTs. their SPTs and traffic to drain from the MRTs.
5. Recompute MRTs 5. Recompute MRTs.
6. Install new MRTs.
6. Install new MRTs in the FIB.
While the recomputed MRTs are not installed in the FIB, protection While the recomputed MRTs are not installed in the FIB, protection
coverage is lowered. Therefore, it is important to recalculate the coverage is lowered. Therefore, it is important to recalculate the
MRTs and install them quickly. MRTs and install them quickly.
New protocol extensions for advertising the time needed to recompute
shortest path routes and install them in the FIB will be defined
elsewhere.
13. Implementation Status 13. Implementation Status
[RFC Editor: please remove this section prior to publication.] [RFC Editor: please remove this section prior to publication.]
This section records the status of known implementations of the This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC6982]. Internet-Draft, and is based on a proposal described in [RFC6982].
The description of implementations in this section is intended to The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation RFCs. Please note that the listing of any individual implementation
skipping to change at page 33, line 38 skipping to change at page 37, line 33
to assign due consideration to documents that have the benefit of to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature. and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as It is up to the individual working groups to use this information as
they see fit". they see fit".
Juniper Networks Implementation Juniper Networks Implementation
o Organization responsible for the implementation: Juniper Networks o Organization responsible for the implementation: Juniper Networks
o Implementation name: MRT-FRR algorithm o Implementation name: MRT-FRR
o Implementation description: The MRT-FRR algorithm using OSPF as o Implementation description: MRT-FRR using OSPF as the IGP has been
the IGP has been implemented and verified. implemented and verified.
o The implementation's level of maturity: prototype o The implementation's level of maturity: prototype
o Protocol coverage: This implementation of the MRT algorithm o Protocol coverage: This implementation of the MRT-FRR includes
includes Island identification, GADAG root selection, Lowpoint Island identification, GADAG root selection, MRT Lowpoint
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-ietf-rtgwg-mrt-frr-algorithm-00". This draft "draft-ietf-rtgwg-mrt-frr-algorithm-00". This
implementation also includes the M-bit in OSPF based on "draft- implementation also includes the M-bit in OSPF based on "draft-
atlas-ospf-mrt-01" as well as LDP MRT Capability based on "draft- atlas-ospf-mrt-01" as well as LDP MRT Capability based on "draft-
atlas-mpls-ldp-mrt-00". atlas-mpls-ldp-mrt-00".
o Licensing: proprietary o Licensing: proprietary
o Implementation experience: Implementation was useful for verifying o Implementation experience: Implementation was useful for verifying
functionality and lack of gaps. It has also been useful for functionality and lack of gaps. It has also been useful for
improving aspects of the algorithm. improving aspects of the algorithm.
o Contact information: akatlas@juniper.net, shraddha@juniper.net, o Contact information: akatlas@juniper.net, shraddha@juniper.net,
kishoret@juniper.net kishoret@juniper.net
Huawei Technology Implementation Huawei Technology Implementation
o Organization responsible for the implementation: Huawei Technology o Organization responsible for the implementation: Huawei Technology
skipping to change at page 34, line 21 skipping to change at page 38, line 16
improving aspects of the algorithm. improving aspects of the algorithm.
o Contact information: akatlas@juniper.net, shraddha@juniper.net, o Contact information: akatlas@juniper.net, shraddha@juniper.net,
kishoret@juniper.net kishoret@juniper.net
Huawei Technology Implementation Huawei Technology Implementation
o Organization responsible for the implementation: Huawei Technology o Organization responsible for the implementation: Huawei Technology
Co., Ltd. Co., Ltd.
o Implementation name: MRT-FRR algorithm and IS-IS extensions for o Implementation name: MRT-FRR and IS-IS extensions for MRT.
MRT.
o Implementation description: The MRT-FRR algorithm, IS-IS o Implementation description: The MRT-FRR using IS-IS extensions for
extensions for MRT and LDP multi-topology have been implemented MRT and LDP multi-topology have been implemented and verified.
and verified.
o The implementation's level of maturity: prototype o The implementation's level of maturity: prototype
o Protocol coverage: This implementation of the MRT algorithm o Protocol coverage: This implementation of the MRT algorithm
includes Island identification, GADAG root selection, Lowpoint includes Island identification, GADAG root selection, MRT Lowpoint
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 ISIS 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. Operations and Management Considerations 14. Operational Considerations
An implementation SHOULD provide an operator with the ability to test The following aspects of MRT-FRR are useful to consider when
MRT paths with OAM traffic. For example, when MRT paths are realized deploying the technology in different operational environments and
using LDP labels distributed for topology-scoped FECs, an network topologies.
implementation can use the MPLS ping and traceroute as defined in
[RFC4379] and extended in [RFC7307] for topology-scoped FECs.
15. Applying Policy to Select from Multiple Possible Alternates for FRR 14.1. Verifying Forwarding on MRT Paths
For a given topology, GADAG root, and profile, MRT will provide a The forwarding paths created by MRT-FRR are not used by normal (non-
node-protecting alternate path from each PLR to each destination for FRR) traffic. They are only used to carry FRR traffic for a short
any single link or node failure, if such a path exists. Therefore, period of time after a failure has been detected. It is RECOMMENDED
an implementation may choose to only use the alternates determined by that an operator proactively monitor the MRT forwarding paths in
MRT to provide 100% FRR coverage. order to be certain that the paths will be able to carry FRR traffic
when needed. Therefore, an implementation SHOULD provide an operator
with the ability to test MRT paths with OAM traffic. For example,
when MRT paths are realized using LDP labels distributed for
topology-scoped FECs, an implementation can use the MPLS ping and
traceroute as defined in [RFC4379] and extended in [RFC7307] for
topology-scoped FECs.
However, it may be desirable to allow an operator to use MRT 14.2. Traffic Capacity on Backup Paths
During a fast-reroute event initiated by a PLR in response to a
network failure, the flow of traffic in the network will generally
not be identical to the flow of traffic after the IGP forwarding
state has converged, taking the failure into account. Therefore,
even if a network has been engineered to have enough capacity on the
appropriate links to carry all traffic after the IGP has converged
after the failure, the network may still not have enough capacity on
the appropriate links to carry the flow of traffic during a fast-
reroute event. This can result in more traffic loss during the fast-
reroute event than might otherwise be expected.
Note that there are two somewhat distinct aspects to this phenomenon.
The first is that the path from the PLR to the destination during the
fast-reroute event may be different from the path after the IGP
converges. In this case, any traffic for the destination that
reaches the PLR during the fast-reroute event will follow a different
path from the PLR to the destination than will be followed after IGP
convergence.
The second aspect is that the amount of traffic arriving at the PLR
for affected destinations during the fast-reroute event may be larger
than the amount of traffic arriving at the PLR for affected
destinations after IGP convergence. Immediately after a failure, any
non-PLR routers that were sending traffic to the PLR before the
failure will continue sending traffic to the PLR, and that traffic
will be carried over backup paths from the PLR to the destinations.
After IGP convergence, upstream non-PLR routers may direct some
traffic away from the PLR.
In order to reduce or eliminate the potential for transient traffic
loss due to inadequate capacity during fast-reroute events, an
operator can model the amount of traffic taking different paths
during a fast-reroute event. If it is determined that there is not
enough capacity to support a given fast-reroute event, the operator
can address the issue either by augmenting capacity on certain links
or modifying the backup paths themselves.
The MRT Lowpoint algorithm produces a pair of diverse paths to each
destination. These paths are generated by following the directed
links on a common GADAG. 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
algorithm are closely tied to the common GADAG computed as part of
that algorithm. Therefore, it is generally not possible to modify a
subset of paths without affecting other paths. This precludes more
fine-grained modification of individual backup paths when using only
paths computed by the MRT Lowpoint algorithm.
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
technologies. A policy-based alternate selection process can allow technologies. A policy-based alternate selection process can allow
an operator to select the best alternate from those provided by MRT an operator to select the best alternate from those provided by MRT
and other FRR technologies. As an example, it may be desirable to and other FRR technologies. As a concrete example, it may be
implement a policy where a node-protecting LFA (if it exists for a desirable to implement a policy where a downstream LFA (if it exists
given failure mode and destination) is preferred over a given MRT for a given failure mode and destination) is preferred over a given
alternate. [I-D.ietf-rtgwg-lfa-manageability] discusses many of the MRT alternate. This combination gives the operator the ability to
potential criteria that one might take into account when evaluating affect where traffic flows during a fast-reroute event, while still
different alternates for selection. producing backup paths that use no additional labels for LDP traffic
and will not loop under multiple failures. This and other choices of
alternate selection policy can be evaluated in the context of their
effect on fast-reroute traffic flow and available capacity, as well
as other deployment considerations.
Note that future documents may define MRT profiles in addition to the Note that future documents may define MRT profiles in addition to the
default profile defined here. Different MRT profiles will generally default profile defined here. Different MRT profiles will generally
produce alternate paths with different properties. An implementation produce alternate paths with different properties. An implementation
may allow an operator to use different MRT profiles instead of or in may allow an operator to use different MRT profiles instead of or in
addition to the default profile. addition to the default profile.
16. Acknowledgements 14.3. MRT IP Tunnel Loopback Address Management
As described in Section 6.1.2, if an implementation uses IP tunneling
as the mechanism to realize MRT forwarding paths, each node must
advertise an MRT-Red and an MRT-Blue loopback address. These IP
addresses must be unique within the routing domain to the extent that
they do not overlap with each other or with any other routing table
entries. It is expected that operators will use existing tools and
processes for managing infrastructure IP addresses to manage these
additional MRT-related loopback addresses.
14.4. MRT-FRR in a Network with Degraded Connectivity
Ideally, routers is a service provider network using MRT-FRR will be
initially deployed in a 2-connected topology, allowing MRT-FRR to
find completely diverse paths to all destinations. However, a
network can differ from an ideal 2-connected topology for many
possible reasons, including network failures and planned maintenance
events.
MRT-FRR is designed to continue to function properly when network
connectivity is degraded. When a network contains cut-vertices or
cut-links dividing the network into different 2-connected blocks,
MRT-FRR will continue to provide completely diverse paths for
destinations within the same block as the PLR. For a destination in
a different block from the PLR, the redundant paths created by MRT-
FRR will be link and node diverse within each block, and the paths
will only share links and nodes that are cut-links or cut-vertices in
the topology.
If a network becomes partitioned with one set of routers having no
connectivity to another set of routers, MRT-FRR will function
independently in each set of connected routers, providing redundant
paths to destinations in same set of connected routers as a given
PLR.
14.5. Partial Deployment of MRT-FRR in a Network
A network operator may choose to deploy MRT-FRR only on a subset of
routers in an IGP area. MRT-FRR is designed to accommodate this
partial deployment scenario. Only routers that advertise support for
a given MRT profile will be included in a given MRT Island. For a
PLR within the MRT Island, MRT-FRR will create redundant forwarding
paths to all destinations with the MRT Island using maximally
redundant trees all the way to those destinations. For destinations
outside of the MRT Island, MRT-FRR creates paths to the destination
which use forwarding state created by MRT-FRR within the MRT Island
and shortest path forwarding state outside of the MRT Island. The
paths created by MRT-FRR to non-Island destinations are guaranteed to
be diverse within the MRT Island (if topologically possible).
However, the part of the paths outside of the MRT Island may not be
diverse.
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, and Janos Farkas for their suggestions and review. Decraene, Eric Wu, Janos Farkas, Rob Shakir, and Stewart Bryant for
their suggestions and review.
17. IANA Considerations 16. IANA Considerations
Please create an MRT Profile registry for the MRT Profile TLV. The IANA is requested to create a registry entitled "MRT Profile
range is 0 to 255. The default MRT Profile has value 0. Values Identifier Registry". The range is 0 to 255. The Default MRT
1-200 are by Standards Action. Values 201-220 are for Profile defined in this document has value 0. Values 1-200 are
allocated by Standards Action. Values 201-220 are for
experimentation. Values 221-255 are for vendor private use. experimentation. Values 221-255 are for vendor private use.
18. 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
malicious traffic only following shortest paths. malicious traffic only following shortest paths.
It should be noted that the creation of non-shortest forwarding paths It should be noted that the creation of non-shortest forwarding paths
is not unique to MRT. is not unique to MRT.
19. Contributors MRT-FRR requires that routers advertise information used in the
formation of MRT backup paths. While this document does not specify
the protocol extensions used to advertise this information, we
discuss security considerations related to the information itself.
Injecting false MRT-related information could be used to direct some
MRT backup paths over compromised transmission links. Combined with
the ability to generate network failures, this could be used to send
traffic over compromised transmission links during a fast-reroute
event. In order to prevent this potential exploit, a receiving
router needs to be able to authenticate MRT-related information that
claims to have been advertised by another router.
18. Contributors
Robert Kebler Robert Kebler
Juniper Networks Juniper Networks
10 Technology Park Drive 10 Technology Park Drive
Westford, MA 01886 Westford, MA 01886
USA USA
Email: rkebler@juniper.net Email: rkebler@juniper.net
Andras Csaszar Andras Csaszar
Ericsson Ericsson
Konyves Kalman krt 11 Konyves Kalman krt 11
skipping to change at page 36, line 46 skipping to change at page 43, line 29
Ericsson Ericsson
300 Holger Way 300 Holger Way
San Jose, CA 95134 San Jose, CA 95134
USA USA
Email: jeff.tantsura@ericsson.com Email: jeff.tantsura@ericsson.com
Russ White Russ White
VCE VCE
Email: russw@riw.us Email: russw@riw.us
20. References 19. References
20.1. Normative References 19.1. Normative References
[I-D.ietf-rtgwg-mrt-frr-algorithm] [I-D.ietf-rtgwg-mrt-frr-algorithm]
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
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for [RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286, IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008, DOI 10.17487/RFC5286, September 2008,
<http://www.rfc-editor.org/info/rfc5286>. <http://www.rfc-editor.org/info/rfc5286>.
20.2. 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.bryant-ipfrr-tunnels]
Bryant, S., Filsfils, C., Previdi, S., and M. Shand, "IP
Fast Reroute using tunnels", draft-bryant-ipfrr-tunnels-03
(work in progress), November 2007.
[I-D.francois-rtgwg-segment-routing-ti-lfa] [I-D.francois-rtgwg-segment-routing-ti-lfa]
Francois, P., Filsfils, C., Bashandy, A., and B. Decraene, Francois, P., Filsfils, C., Bashandy, A., and B. Decraene,
"Topology Independent Fast Reroute using Segment Routing", "Topology Independent Fast Reroute using Segment Routing",
draft-francois-rtgwg-segment-routing-ti-lfa-00 (work in draft-francois-rtgwg-segment-routing-ti-lfa-00 (work in
progress), August 2015. progress), August 2015.
[I-D.ietf-isis-mrt]
Li, Z., Wu, N., <>, Q., Atlas, A., Bowers, C., and J.
Tantsura, "Intermediate System to Intermediate System (IS-
IS) Extensions for Maximally Redundant Trees (MRT)",
draft-ietf-isis-mrt-01 (work in progress), October 2015.
[I-D.ietf-mpls-ldp-mrt]
Atlas, A., Tiruveedhula, K., Bowers, C., Tantsura, J., and
I. Wijnands, "LDP Extensions to Support Maximally
Redundant Trees", draft-ietf-mpls-ldp-mrt-02 (work in
progress), September 2015.
[I-D.ietf-ospf-mrt]
Atlas, A., Hegde, S., Bowers, C., Tantsura, J., and Z. Li,
"OSPF Extensions to Support Maximally Redundant Trees",
draft-ietf-ospf-mrt-01 (work in progress), October 2015.
[I-D.ietf-rtgwg-lfa-manageability] [I-D.ietf-rtgwg-lfa-manageability]
Litkowski, S., Decraene, B., Filsfils, C., Raza, K., Litkowski, S., Decraene, B., Filsfils, C., Raza, K.,
Horneffer, M., and P. Sarkar, "Operational management of Horneffer, M., and P. Sarkar, "Operational management of
Loop Free Alternates", draft-ietf-rtgwg-lfa- Loop Free Alternates", draft-ietf-rtgwg-lfa-
manageability-11 (work in progress), June 2015. manageability-11 (work in progress), June 2015.
[I-D.ietf-rtgwg-rlfa-node-protection] [I-D.ietf-rtgwg-rlfa-node-protection]
Sarkar, P., Hegde, S., Bowers, C., Gredler, H., and S. Sarkar, P., Hegde, S., Bowers, C., Gredler, H., and S.
Litkowski, "Remote-LFA Node Protection and Manageability", Litkowski, "Remote-LFA Node Protection and Manageability",
draft-ietf-rtgwg-rlfa-node-protection-05 (work in draft-ietf-rtgwg-rlfa-node-protection-05 (work in
progress), December 2015. progress), December 2015.
[LFARevisited]
Retvari, G., Tapolcai, J., Enyedi, G., and A. Csaszar, "IP
Fast ReRoute: Loop Free Alternates Revisited", Proceedings
of IEEE INFOCOM , 2011,
<http://opti.tmit.bme.hu/~tapolcai/papers/
retvari2011lfa_infocom.pdf>.
[LightweightNotVia] [LightweightNotVia]
Enyedi, G., Retvari, G., Szilagyi, P., and A. Csaszar, "IP Enyedi, G., Retvari, G., Szilagyi, P., and A. Csaszar, "IP
Fast ReRoute: Lightweight Not-Via without Additional Fast ReRoute: Lightweight Not-Via without Additional
Addresses", Proceedings of IEEE INFOCOM , 2009, Addresses", Proceedings of IEEE INFOCOM , 2009,
<http://mycite.omikk.bme.hu/doc/71691.pdf>. <http://mycite.omikk.bme.hu/doc/71691.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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>.
[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
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<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
Synchronization", RFC 5443, DOI 10.17487/RFC5443, March Synchronization", RFC 5443, DOI 10.17487/RFC5443, March
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
skipping to change at page 40, line 20 skipping to change at page 46, line 30
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 4: AS external prefixes in different areas Figure 5: AS external prefixes in different areas
Consider the network in Figure 4 and assume there is a richer Consider the network in Figure 5 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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