wdiff draft-ietf-rtgwg-mrt-frr-architecture-02.txt draft-ietf-rtgwg-mrt-frr-architecture-03.txt

Routing Area Working Group A. Atlas, Ed.
Internet-Draft R. Kebler
Intended status: Standards Track Juniper Networks
Expires: August 28, 2013 January 13, 2014 G. Enyedi
A. Csaszar
J. Tantsura
Ericsson
M. Konstantynowicz
Cisco Systems
R. White
Verisign
M. Shand
February 24,
VCE
July 12, 2013

An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees
draft-ietf-rtgwg-mrt-frr-architecture-02
draft-ietf-rtgwg-mrt-frr-architecture-03

Abstract

As IP and LDP Fast-Reroute are increasingly deployed, the coverage
limitations

With increasing deployment of Loop-Free Alternates are seen as a problem (LFA) [RFC5286],
it is clear that
requires a straightforward and consistent complete solution for IP and LDP,
for unicast and multicast. LDP Fast-Reroute is
required. This draft describes an architecture
based on redundant backup trees where a single failure can cut specification provides that solution. IP/LDP Fast-
Reroute with Maximally Redundant Trees (MRT-FRR) is a
point-of-local-repair from the destination only on one of technology that
gives link-protection and node-protection with 100% coverage in any
network topology that is still connected after the pair of
redundant trees.

One innovative algorithm failure.

MRT removes all need to compute such topologies engineer for coverage. MRT is maximally
disjoint backup trees. Each also extremely
computationally efficient. For any router can compute its next-hops for
each pair of maximally disjoint trees rooted at each node in the IGP
area with computational complexity similar to that required by
Dijkstra.

The additional state, address and network, the MRT
computation requirements are
believed to be significantly is less than the Not-Via architecture
requires. LFA computation for a node with three or
more neighbors.

Status of this This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on August 28, 2013. January 13, 2014.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 3
1.1. Goals for Extending IP Fast-Reroute coverage beyond LFA Importance of 100% Coverage . . . . . . . . . . . . . . . 4
1.2. Partial Deployment and Backwards Compatibility . . . . . 5
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 6
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. 6
4. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 6
4. 7
5. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . . 8
5. 9
6. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . . . 9
5.1. 10
6.1. LDP Unicast Forwarding - Avoid Tunneling . . . . . . . . . 10
5.2.
6.2. IP Unicast Traffic . . . . . . . . . . . . . . . . . . . . 10
6. 11
7. Protocol Extensions and Considerations: OSPF and ISIS . . . . 12
7.
8. Protocol Extensions and considerations: LDP . . . . . . . . . 14
8. Multi-homed Prefixes . . . . . . . . . . . . . . . . . . . . . 15
9. Inter-Area and ABR Forwarding Behavior . . . . . . . . . . . . 16 15
10. Issues with Area Abstraction Prefixes Multiply Attached to the MRT Island . . . . . . . . 18
10.1. Endpoint Selection . . . . . . . . . . 19
11. Partial Deployment and Islands of Compatible MRT FRR
routers . . . . . . . . . 19
10.2. Named Proxy-Nodes . . . . . . . . . . . . . . . . . . 20
12. . 21
10.2.1. Computing if an Island Neighbor (IN) is loop-free . 22
10.3. MRT Alternates for Destinations Outside the MRT Island . 23
11. Network Convergence and Preparing for the Next Failure . . . . 22
12.1. 24
11.1. Micro-forwarding loop prevention and MRTs . . . . . . . . 22
12.2. 24
11.2. MRT Recalculation . . . . . . . . . . . . . . . . . . . . 23
13. 24
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
14. 25
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
15. 25
14. Security Considerations . . . . . . . . . . . . . . . . . . . 24
16. 25
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
16.1. 25
15.1. Normative References . . . . . . . . . . . . . . . . . . . 24
16.2. 25
15.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. General Issues with Area Abstraction . . . 24 . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25 28

1. Introduction

There

This document gives a complete solution for IP/LDP fast-reroute
[RFC5714]. MRT-FRR creates two alternate trees separate from the
primary next-hop forwarding used during stable operation. These two
trees are maximally diverse from each other, providing link and node
protection for 100% of paths and failures as long as the failure does
not cut the network into multiple pieces. This document defines the
architecture for IP/LDP fast-reroute with MRT. The associated
protocol extensions are defined in [I-D.atlas-ospf-mrt] and
[I-D.atlas-mpls-ldp-mrt]. The exact MRT algorithm is still work required defined in
[I-D.enyedi-rtgwg-mrt-frr-algorithm].

IP/LDP Fast-Reroute with MRT (MRT-FRR) uses two maximally diverse
forwarding topologies to provide alternates. A primary next-hop
should be on only one of the diverse forwarding topologies; thus, the
other can be used to completely provide an alternate. Once traffic has been
moved to one of MRTs, it is not subject to further repair actions.
Thus, the traffic will not loop even if a worse failure (e.g. node)
occurs when protection was only available for a simpler failure (e.g.
link).

In addition to supporting IP and LDP Fast-
Reroute[RFC5714] for unicast fast-reroute, the
diverse forwarding topologies and multicast traffic. This draft
proposes an architecture to provide guarantee of 100% coverage for unicast
traffic. The associated permit
fast-reroute technology to be applied to multicast architecture is traffic as
described in [I-D.atlas-rtgwg-mrt-mc-arch].

Loop-free alternates (LFAs)[RFC5286]

Other existing or proposed solutions are partial solutions or have
significant issues, as described below.

Summary Comparison of IP/LDP FRR Methods

Table 1

Loop-Free Alternates (LFA): LFAs [RFC5286] provide a useful mechanism limited
topology-dependent coverage for link and node protection but getting complete coverage is quite hard.
[LFARevisited] defines sufficient conditions protection.
Restrictions on choice of alternates can be relaxed to determine improve
coverage, but this can cause forwarding loops if a
network provides link-protecting LFAs and also proves that augmenting worse failure
is experienced than protected against. Augmenting a network to
provide better coverage is NP-hard.
[I-D.ietf-rtgwg-lfa-applicability] NP-hard [LFARevisited]. [RFC6571]
discusses the applicability of LFA to different topologies with a
focus on common PoP architectures.

While

Remote LFA: Remote LFAs [I-D.ietf-rtgwg-remote-lfa] improve
coverage over LFAs for link protection but still cannot guarantee
complete coverage. The trade-off of looping traffic to improve
coverage is still made. Remote LFAs can provide node-protection
[I-D.litkowski-rtgwg-node-protect-remote-lfa] but not guaranteed
coverage and the computation required is quite high (an SPF per
neighbor's neighbor). [I-D.bryant-ipfrr-tunnels] describes
additional mechanisms to further improve coverage, at the cost of
added complexity.

Not-Via: Not-Via [I-D.ietf-rtgwg-ipfrr-notvia-addresses] is defined as
an architecture, in practice, it has proved too complicated the
only other solution that provides 100% coverage for link and
stateful to spark substantial interest in implementation or
deployment. Academic node
failures and does not have potential looping. However, the
computation is very high (an SPF per failure point) and academic
implementations [LightweightNotVia] exist and have found the address
management complexity high (but no
standardization has been done to reduce this).

A different approach is needed and that is what is described here.
It be high.

1.1. Importance of 100% Coverage
Fast-reroute is based on upon the idea of using disjoint backup topologies as
realized by Maximally Redundant Trees (described in
[LightweightNotVia]); single failure assumption - that the general architecture can also apply to
future improved redundant tree algorithms.

1.1. Goals for Extending IP Fast-Reroute coverage beyond LFA

Any scheme proposed
time between single failures is long enough for extending IPFRR a network topology coverage
beyond LFA, apart from attaining basic IPFRR properties, should also
aim to achieve the following usability goals:

o ensure maximum physically feasible link
reconverge and node disjointness
regardless of topology,

o automatically compute backup next-hops based start forwarding on the topology
information distributed by link-state IGP,

o do new shortest paths. That does
not require any signaling in imply that the case of network will only experience one failure and use pre-
programmed backup next-hops or
change.

It is straightforward to analyze a particular network topology for forwarding,

o introduce minimal amount
coverage. However, a real network does not always have the same
topology. For instance, maintenance events will take links or nodes
out of additional addressing and state use. Simply costing out a link can have a significant effect
on
routers,

o enable gradual introduction of what LFAs are available. Similarly, after a single failure has
happened, the new scheme topology is changed and backward
compatibility,

o its associated coverage.
Finally, many networks have new routers or links added and do not impose requirements removed;
each of those changes can have an effect on the coverage for external computation.

2. Terminology

2-connected: A graph that has no cut-vertices. This
topology-sensitive methods such as LFA and Remote LFA. If fast-
reroute is important for the network services provided, then a graph method
that requires two nodes guarantees 100% coverage is important to be removed before the accomodate natural
network is
partitioned.

2-connected cluster: A maximal set topology changes.

Asymmetric link costs are also a common aspect of nodes that networks. There
are 2-connected.

2-edge-connected: A network graph where at least two links must be
removed three common causes for them. First, any broadcast
interface is represented by a pseudo-node and has asymmetric link
costs to partition the network.

ADAG: Almost Directed Acyclic Graph - and from that pseudo-node. Second, when routers come up or
a graph that, if all links
incoming link with LDP comes up, it is recommended in [RFC5443] and
[RFC3137] that the link metric be raised to the root were removed, would maximum cost; this
may not be symmetric and for [RFC3137] is not expected to be. Third,
techniques such as IGP metric tuning for traffic-engineering can
result in asymmetric link costs. A fast-reroute solution needs to
handle network topologies with asymmetric link costs.

When a DAG.

block: Either a 2-connected cluster, network needs to use a cut-edge, micro-loop prevention mechanism
[RFC5715] such as Ordered FIB[I-D.ietf-rtgwg-ordered-fib] or an isolated
vertex.

cut-link: A link whose removal partitions Farside
Tunneling[RFC5715], then the network. A cut-link
by definition must whole IGP area needs to have alternates
available so that the micro-loop prevention mechanism, which requires
slower network convergence, can take the necessary time without
impacting traffic badly. Without complete coverage, traffic to the
unprotected destinations will be dropped for significantly longer
than with current convergence - where routers individually converge
as fast as possible.

1.2. Partial Deployment and Backwards Compatibility

MRT-FRR supports partial deployment. As with many new features, the
protocols (OSPF, LDP, ISIS) indicate their capability to support MRT.
Inside the MRT-capable connected between two cut-vertices. If
there are multiple parallel links, then they are referred group of routers (referred to as
cut-links in this document if removing an
MRT Island), the set of parallel links
would partition MRTs are computed. Alternates to destinations
outside the network.

cut-vertex: A vertex whose removal partitions MRT Island are computed and depend upon the network.

DAG: Directed Acyclic Graph - existence of
a graph where all links are directed loop-free neighbor of the MRT Island for that destination.

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and there "OPTIONAL" in this
document are no cycles to be interpreted as described in it.

GADAG: Generalized ADAG - a [RFC2119]

3. Terminology

network graph: A graph that is reflects the combination of network topology where all
links connect exactly two nodes and broadcast links have been
transformed into the
ADAGs standard pseudo-node representation.

Redundant Trees (RT): A pair of all blocks. trees where the path from any node
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.
These can be computed in 2-connected graphs.

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 the same node X to the root along the second tree share the
minimum number of nodes and the minimum number of links. Each
such shared node is a cut-vertex. Any shared links are cut-links.
Any RT is an MRT but many MRTs are not RTs.

network graph: A graph that reflects

MRT-Red: MRT-Red is used to describe one of the network two MRTs; it is
used to described the associated forwarding topology and MT-ID.
Specifically, MRT-Red is the decreasing MRT where all links connect exactly in the
GADAG are taken in the direction from a higher topologically
ordered node to a lower one.

MRT-Blue: MRT-Blue is used to describe one of the two nodes MRTs; it is
used to described the associated forwarding topology and broadcast links have been
transformed into MT-ID.
Specifically, MRT-Blue is the standard pseudo-node representation.

Redundant Trees (RT): A pair of trees increasing MRT where links in the path
GADAG are taken in the direction from any a lower topologically
ordered node
X to a higher one.

Rainbow MRT: It is useful to have an MT-ID that refers to the root R along
multiple MRT topologies and to the first tree default topology. This is node-disjoint with
referred to as the
path from Rainbow MRT MT-ID and is used by LDP to reduce
signaling and permit the same node X label to always be advertised to all
peers for the root along same (MT-ID, Prefix).

MRT Island: From the second tree.
These can be computed computing router, the set of routers that
support a particular MRT profile and are connected.

Island Border Router (IBR): A router in 2-connected graphs.

3. Maximally Redundant Trees (MRT)

In the last few years, there's been substantial research on how MRT Island that is
connected to
compute a router not in the MRT Island and use redundant trees. Redundant trees both routers are directed
spanning trees that provide disjoint paths towards their
in a common root.
These redundant trees only exist area or level.

Island Neighbor (IN): A router that is not in the MRT Island but is
adjacent to an IBR and provide in the same area/level as the IBR.

cut-link: A link protection 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 2-edge-connected and node protection if 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. Such connectiveness may not

2-edge-connected: A network graph where at least two links must be the case in real
networks, either due
removed to architecture partition the network.

block: Either a 2-connected cluster, a cut-edge, or due 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 previous failure.
The work on maximally redundant trees has added 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
that can be attached to the MRT Island via at least two useful pieces routers.
It is named if there is a way that make them ready traffic can be encapsulated to
reach specifically that proxy node; this could be because there is
an LDP FEC for use the associated prefix or because MRT-Red and MRT-
Blue IP addresses are advertised in a real network.

o Computable regardless an undefined fashion for that
proxy-node.

4. Maximally Redundant Trees (MRT)

A pair of network topology: The maximally redundant
trees Maximally Redundant Trees are computed so directed spanning trees that only the cut-edges
provide maximally disjoint paths towards their common root. Only
links or cut-vertices nodes whose failure would partition the network (i.e. cut-
links and cut-vertices) are shared between the multiple trees.

o Computationally practical The algorithm
to compute MRTs is based on a common network
topology database. Algorithm variants given in [I-D.enyedi-rtgwg-mrt-frr-algorithm].
This algorithm can compute be computed in O( e) or O(e + n log n), as given in [I-D.enyedi-rtgwg-mrt-frr-algorithm].

There is, of course, significantly more in the literature related to
redundant trees and even fast-reroute, but the formulation of the
Maximally Redundant Trees (MRT) algorithm makes n); it very well suited
to use in routers.

A known disadvantage of MRT, and redundant trees in general, is that
the trees do not necessarily provide shortest detour paths. The use
of the shortest-path-first algorithm in tree-building and including
all links in the network as possibilities for one path or another
should improve this. less than
three SPFs. Modeling is underway to investigate and compare
the results comparing MRT alternates to the optimal
[I-D.enyedi-rtgwg-mrt-frr-algorithm]. Providing shortest detour
paths would require failure-specific detour paths to the
destinations, but the state-reduction advantage of MRT lies in the
detour being established per destination (root) instead of per
destination AND per failure.

The specific algorithms to compute MRTs as well as the logic behind
that algorithm and alternative computational approaches
are given in
detail described in [I-D.enyedi-rtgwg-mrt-frr-algorithm]. Those interested are
highly recommended to read that document. This document
describes how the MRTs can be used and not how to compute them.

MRT provides destination-based trees for each destination. Each
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
alternate will be selected between the MRT-Blue and MRT-Red.

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
the destination D on the Blue MRT that is as disjoint as possible
from the path on the Red MRT. The two paths along the two MRTs to a
given destination-root of a 2-connected graph are node-disjoint and
link-disjoint, while in any non-2-connected graph, only the cut-
vertices and cut-edges can be contained by both of the paths.

For example, in Figure 1, there is a network graph that is
2-connected in (a) and associated MRTs in (b) and (c). One can
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.
These are clearly link and node-disjoint. These MRTs are redundant
trees because the paths are disjoint.

[E]---[D]---| [E]<--[D]<--| [E]-->[D]---|
| | | | ^ | | |
| | | V | | V V
[R] [F] [C] [R] [F] [C] [R] [F] [C]
| | | ^ ^ ^ | |
| | | | | | V |
[A]---[B]---| [A]-->[B]---| [A]---[B]<--| [A]<--[B]<--|

(a) (b) (c)
a 2-connected graph Blue MRT towards R Red MRT towards R

Figure 1: A 2-connected Network

By contrast, in Figure 2, the network in (a) is not 2-connected. If
F, G or the link F<->G failed, then the network would be partitioned.
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
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
MRT is G->F->D->E->R and on the Red MRT is G->F->B->A->R.

[E]---[D]---|
| | | |----[I]
| | | | |
[R]---[C] [F]---[G] |
| | | | |
| | | |----[J]
[A]---[B]---|

(a)
a non-2-connected graph

[E]<--[D]<--| [E]-->[D]---| [E]-->[D]
| ^ | [I] | | [I] |----[I]
V | | ^ | V V |
[R]<--[C] ^
[R] [C] [F]<--[G] | [R]---[C] [R]<--[C] [F]<--[G] |
^ ^ | | ^ | | ^ V ^ | | |--->[J]
| V | |----[J] | | [J]
[A]-->[B]---| [A]<--[B]<--|

(b) (c)
Blue MRT towards R Red MRT towards R

Figure 2: A non-2-connected network

5. Maximally Redundant Trees (MRT) and Fast-Reroute

In normal IGP routing, each router has its shortest-path-tree to all
destinations. From the perspective of a particular destination, D,
this looks like a reverse SPT (rSPT). To use maximally redundant
trees, in addition, each destination D has two MRTs associated with
it; by convention these will be called the blue MRT-Blue and red MRTs. MRT-Red.
MRT-FRR is realized by using multi-topology forwarding. There is a
MRT-Blue forwarding topology and a MRT-Red forwarding topology.

Any IP/LDP fast-reroute technique beyond LFA requires an additional
dataplane procedure, such as an additional forwarding mechanism. The
well-known options are multi-topology forwarding (used by MRT-FRR),
tunneling (e.g. [I-D.ietf-rtgwg-ipfrr-notvia-addresses] or
[I-D.ietf-rtgwg-remote-lfa]), and per-interface forwarding (e.g. Loop-
Free
Loop-Free Failure Insensitive Routing in [EnyediThesis]), and multi-
topology forwarding. MRT is realized by using multi-topology
forwarding. There is a Blue MRT forwarding topology and a Red MRT
forwarding topology.

MRTs are practical to maintain redundancy even after a single link or
node failure. If a pair of MRTs is computed rooted at each
destination, all the destinations remain reachable along one of the
MRTs in the case of a single link or node failure. [EnyediThesis]).

When there is a link or node failure affecting affecting, but not partitioning,
the rSPT, 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
would normally forward traffic to R across the C<->R link. If that
C<->R link fails, then C could use either the Blue MRT path C->D->E->R or the Red MRT path
C->B->A->R. C->D->E->R.

As is always the case with fast-reroute technologies, forwarding does
not change until a local failure is detected. Packets are forwarded
along the shortest path. The appropriate alternate to use is pre-
computed. [I-D.enyedi-rtgwg-mrt-frr-algorithm] describes exactly how
to determine whether the Blue MRT MRT-Blue next-hops or the Red MRT MRT-Red next-hops
should be the MRT alternate next-hops for a particular primary next-
hop N to a particular destination D.

MRT alternates are always available to use, unless the network has
been partitioned. use. It is a local decision
whether to use an MRT alternate, a Loop-Free Alternate or some other
type of alternate.
When a network needs to use a micro-loop prevention mechanism
[RFC5715] such as Ordered FIB[I-D.ietf-rtgwg-ordered-fib] or Farside
Tunneling[RFC5715], then the whole IGP area needs to have alternates
available so that the micro-loop prevention mechanism, which requires
slower network convergence, can take the necessary time without
impacting traffic badly.

As described in [RFC5286], when a worse failure than is anticipated
happens, using LFAs that are not downstream neighbors can cause
micro-looping. An Section 1.1 of [RFC5286] gives an example is given of link-protecting link-
protecting alternates causing a loop on node failure. Even if a
worse failure than anticipated happened, happens, the use of MRT alternates
will not cause looping. Therefore, while node-protecting LFAs may be prefered, an
preferred, the certainty that no alternate-induced looping will occur
is an advantage of using MRT alternates when the available node-protecting node-
protecting LFA is not a downstream path.

6. Unicast Forwarding with MRT Fast-Reroute

With LFA, there is no need to tunnel unicast traffic, whether IP or
LDP. The traffic is simply sent to an alternate. As mentioned
earlier in Section 4, 5, MRT needs multi-topology forwarding.
Unfortunately, neither IP nor LDP provide provides extra bits for a packet to
indicate its topology.

Once the MRTs are computed, the two sets of MRTs are seen by the
forwarding plane as essentially two additional topologies. The same
considerations apply for forwarding along the MRTs as for handling
multiple topologies.

5.1.

6.1. LDP Unicast Forwarding - Avoid Tunneling

For LDP, it is very desirable to avoid tunneling because, for at
least node protection, tunneling requires knowledge of remote LDP
label mappings and thus requires targeted LDP sessions and the
associated management complexity. There are two different mechanisms
that can be used. used; Option A MUST be supported.

1. Option A - Encode MT-ID in Labels: In addition to sending a
single label for a FEC, a router would provide two additional
labels with the MT-IDs associated with the Blue MRT or Red MRT
forwarding topologies. This is very simple for hardware support.
It does reduce the label space for other uses. It also increases
the memory to store the labels and the communication required by
LDP.

2. Option B - Create Topology-Identification Labels: Use the label-
stacking ability of MPLS and specify only two additional labels -
one for each associated MRT color - by a new FEC type. When
sending a packet onto an MRT, first swap the LDP label and then
push the topology-identification label for that MRT color. When
receiving a packet with a topology-identification label, pop it
and use it to guide the next-hop selection in combination with
the next label in the stack; then swap the remaining label, if
appropriate, and push the topology-identification label for the
next-hop. This has minimal usage of additional labels, memory
and LDP communication. It does increase the size of packets and
the complexity of the required label operations and look-ups.
This can use the same mechanisms as are needed for context-aware
label spaces.

Note that with LDP unicast forwarding, regardless of whether
topology-identification label or encoding topology in label is used,
no additional loopbacks per router are required. This is because LDP
labels are used on a hop-by-hop basis to identify MRT-blue and MRT-
red forwading topologies.

For greatest hardware compatibility, routers implementing MRT LDP
fast-reroute MUST support Option A of encoding the MT-ID in the
labels. The extensions to indicate an MT-ID for a FEC are described
in Section 3.2.1 of [I-D.ietf-mpls-ldp-multi-topology]

5.2. [I-D.ietf-mpls-ldp-multi-topology].

6.2. IP Unicast Traffic

For IP, there is no currently practical alternative except tunneling. tunneling
to gain the bits needed to indicate the MRT-Blue or MRT-Red
forwarding topology. The choice of tunnel egress could MAY be flexible
since any router closer to the destination than the next-hop can
work. This architecture assumes that the original destination in the area,
area is selected (see Section 10 for handling of multi-homed
prefixes); another possible choice is the next-next-hop towards the
destination. For LDP traffic, using the original destination
simplifies MRT-FRR by avoiding the need for targeted LDP sessions to
the
next-next-hop, etc.. next-next-hop. For IP, that consideration doesn't apply but
consistency with LDP is RECOMMENDED. If the tunnel egress is the
original destination router, then the traffic remains on the
redundant tree with sub-optimal routing. If the tunnel egress is the next-next-hop,
then protection of multi-homed prefixes and node-failure for ABRs is
not available. Selection of the tunnel
egress is a router-local decision.

There are three options available for marking IP packets with which
MRT it should be forwarded in. For greatest hardware compatibility
and ease in removing the MRT-topology marking at area/level
boundaries, routers that support MPLS and implement IP MRT fast-
reroute MUST support Option A - using an LDP label that indicates the
destination and MT-ID.

1. Tunnel IP packets via an LDP LSP. This has the advantage that
more installed routers can do line-rate encapsulation and
decapsulation. Also, no additional IP addresses would need to be
allocated or signaled.

a. Option A - LDP Destination-Topology Label: Use a label that
indicates both destination and MRT. This method allows easy
tunneling to the next-next-hop as well as to the IGP-area
destination. For a proxy-node, the destination to use is the
non-proxy-node immediately before the proxy-node on that
particular color MRT.

b. Option B - LDP Topology Label: Use a Topology-Identifier
label on top of the IP packet. This is very simple. If
tunneling to a next-next-hop is desired, then a two-deep
label stack can be used with [ Topology-ID label, Next-Next-
Hop Label ].

2. Tunnel IP packets in IP. Each router supporting this option
would announce two additional loopback addresses and their
associated MRT color. Those addresses are used as destination
addresses for MRT-blue and MRT-red IP tunnels respectively. They
allow the transit nodes to identify the traffic as being
forwarded along either MRT-blue or MRT-red tree topology to reach
the tunnel destination. Announcements of these two additional
loopback addresses per router with their MRT color requires IGP
extensions.

For greatest hardware compatibility

7. Protocol Extensions and ease in removing Considerations: OSPF and ISIS

For simplicity, the MRT-
topology marking at area/level boundaries, routers that approach of defining a well-known profile is
taken in [I-D.atlas-ospf-mrt]. The purpose of communicating support MPLS
and implement IP
for MRT fast-reroute SHOULD support Option A - using an
LDP label that indicates in the destination and MT-ID.

For proxy-nodes associated with one or more multi-homed prefixes,
there IGP is no router associated with the proxy-node, so its loopbacks
can't be known or used. Instead, the loopback addresses of to indicate thatqq the
routers that MRT-Blue and MRT-Red
forwarding topologies are attached to created for transit traffic. This section
describes the proxy-node can be used. One of
those routers will various options to be on the Red selected. The default MRT
profile is described here and the other on signaling extensions for OSPF are
given in [I-D.atlas-ospf-mrt].

For any MRT profile, the Blue MRT.
The MRT-red loopback of MRT Island is created by starting from the first router would be used to reach
computing router. If the computing router on supports the Red default MRT and similarly the MRT-blue loopback of
profile, add it to the
second MRT Island. Add a router would be used. The routers connected to the proxy-node
are the end of the area/level and can decapsulate the traffic and
properly forward it into MRT Island if
the next area.

6. Protocol Extensions and Considerations: OSPF and ISIS

There are two possible approaches to what additional information to
distribute in router supports the IGP. The first is to allow full flexibility in all
information and distribute whichever values default MRT profile and combinations are
desired. The second is connected to simply distribute flags indicating a
particular well-known profile is supported. Thus the
MRT Island
Creation process is trivial. The profile approach is recommended,
with via bidirectional links eligible for MRT.

If a router advertises support for multiple MRT profiles, then it
MUST create the added flexibility transit forwarding topologies for each of being able to specify more specific
information if necessary and supported.

For example, those,
unless the profile specifies No Forwarding Mechanism (e.g. as might
be done for a simple profile "metric-insensitive used only for multicast global protection). A
router MUST NOT advertise multiple MRT unicast fast-
reroute via LDP" could specify: profiles that overlap in their
MRT-Red MT-ID or MRT-Blue MT-ID.

The MRT Island Creation: Only include other routers advertising this
profile. Profile also defines different behaviors such as how MRT Algorithm ID: The
recomputation is handled and how area/level boundaries are dealt
with.

MRT Algorithm: MRT Lowpoint algorithm defined in
[I-D.enyedi-rtgwg-mrt-frr-algorithm].

Red MRT

MRT-Red MT-ID: The Red MRT MT-ID is the single well-known experimental 3997, final value
allocated assigned by IANA
allocated from the OSPF, ISIS, LDP and PIM MT-ID spaces.

Blue MRT space

MRT-Blue MT-ID: The Blue MRT MT-ID is the single well-known experimental 3998, final value
allocated assigned by IANA
allocated from the OSPF, ISIS, LDP and PIM MT-ID spaces. space

GADAG Root Election Selection Priority: Pick Among the router routers in the MRT Island
and with the lowest highest priority advertised, an implementation MUST
pick the router with the highest Router ID to be the GADAG root.

Forwarding Mechanisms for IP: Use IP-in-LDP.

MRT Capabilities: Computes MRTs, IP Fast-Reroute, Mechanisms: LDP Fast-Reroute

Recalculation: Recalculation of MRTs SHOULD occur as described in
Section 11.2. This allows the MRT forwarding topologies to
support IP/LDP fast-reroute traffic.

Area/Level Border Behavior: As described in Section 9, ABRs/LBRs
SHOULD ensure that traffic leaving the area also exits the MRT-Red
or MRT-Blue forwarding topology.

The following captures an initial understanding of describes the aspects that
must to be considered to fully form define a
profile to advertise. For some profiles, associated information may
need to be distributed, such as GADAG Root Election Selection Priority, Red
MRT Loopback Address, Blue MRT Loopback Address, or MRT Algorithm ID. Address.

MRT Island Creation ID: This identifies the process that the router
uses to form an MRT Island. By advertising an ID for the process,
it is possible to have different processes in the future. It may
be desirable to advertise a list ordered by preference to allow
transitions.

MRT Algorithm ID: Algorithm: This identifies the particular MRT algorithm used by
the router. By having an router for this profile. Algorithm ID, it is possible to
change the algorithm used or use different ones in different
networks. It may transitions can be desirable to advertise a list ordered managed
by
preference to allow transitions.

Red advertising multiple MRT profiles.

MRT-Red MT-ID: This specifies the MT-ID to be associated with the
Red MRT
MRT-Red forwarding topology. It is needed for use in LDP
signaling. All routers in the MRT Island MUST agree on a value.

Blue MRT

MRT-Blue MT-ID: This specifies the MT-ID to be associated with the
Blue MRT
MRT-Blue forwarding topology. It is needed for use in LDP
signaling. All routers in the MRT Island MUST agree on a value.

GADAG Root Election Selection Priority: This specifies the priority of the
router for being used as the GADAG root of its island. A GADAG
root is elected from the set of routers with the highest priority;
ties are broken based upon highest Router ID. The sensitivity of
the MRT Algorithms profile might specify this to GADAG root selection is still being
evaluated. This provides
provide the network operator with a knob to force a particular
GADAG root selection. If not specified in the MRT profile, the
highest Router ID in the profile's MRT Island will be elected the
GADAG Root. If a GADAG Root Selection Priority is specified, then
the MRT profile must also specify how the GADAG Root is elected.

Forwarding Mechanism for IP: Mechanism: This specifies which forwarding mechanisms
the router supports for IP transit traffic. An MRT island must
support a common set of forwarding mechanisms, which may be less
than
program appropriate next-hops into the full set advertised. Multiple forwarding mechanisms may plane. The
known options are IPv4, IPv6, LDP, and None. If IPv4 is
supported, then both MRT-Red and MRT-Blue IPv4 Loopback Addresses
SHOULD be specified, such as IP-in-IPv4, IP-in-IPv6 or IP-in-LDP Label.
None specified. If IPv6 is also an option.

Red MRT supported, both MRT-Red and MRT-
Blue IPv6 Loopback Addresses SHOULD be specified. If LDP is
supported, then LDP support and signaling extensions MUST be
supported.

MRT-Red Loopback Address: This provides the router's loopback
address to reach the router via the Red MRT MRT-Red forwarding topology.
It can, of course, be specified for both IPv4 and IPv6.

Blue MRT

MRT-Blue Loopback Address: This provides the router's loopback
address to reach the router via the Blue MRT MRT-Blue forwarding topology.
It can, of course, be specified for both IPv4 and IPv6.

MRT Capabilities Available: This is the set

Recalculation: As part of capabilities that
the router is configured to support.

MRT Capabilities Required: This is what process and timing should the set of capabilities that
other routers must have available to new
MRTs be added into the MRT island.

MRT Capability: Computes MRTs: The router can compute MRTs.

MRT Capability: IP Fast-Reroute: The router can use the computed
MRTs for IP fast-reroute.

MRT Capability: LDP Fast-Reroute: The router can use on a modified topology? Section 11.2 describes
the computed
MRTs for LDP minimum behavior required to support fast-reroute.

MRT Capability: PIM Fast-Reroute: The router can use

Area/Level Border Behavior: Should inter-area traffic on the computed
MRTs for PIM fast-reroute.

MRT Capability: mLDP Fast-Reroute: The router can use MRT-
Blue or MRT-Red be put back onto the computed
MRTs for mLDP fast-reroute.

MRT Capability: PIM Global Protection: The router can use shortest path tree? Should
it be swapped from MRT-Blue or MRT-Red in one area/level to MRT-
Red or MRT-Blue in the
computed MRTs for PIM Global Protection 1+1.

MRT Capability: mLDP Global Protection: The router can use next area/level to avoid the
computed MRTs for mLDP Global Protection 1+1.

The assumption is that a router will form potential
failure of an MRT island, compute MRTs
within that island, and then use those MRTs ABR? (See [I-D.atlas-rtgwg-mrt-mc-arch] for use-
case details.

Other Profile-Specific Behavior: Depending upon the purposes
specified in the profile. If multiple profiles are supported with
different purposes (e.g. mLDP Global Protection), then the router may
use a different profile and associated MRT island to be used use-case for
the
purposes in that different profile. If a router wanted to form
multiple MRT islands for different application purposes, that could profile, there may be done by specifying different Red MRT MT-ID and Blue MRT MT-IDs. additional profile-specific behavior.

As with LFA, it is expected that OSPF Virtual Links will not be
supported.

8. Protocol Extensions and considerations: LDP

Capability negotiation in
The protocol extensions for LDP is needed to are defined in
[I-D.atlas-mpls-ldp-mrt]. A router must indicate that it has the
ability to support for MRT; having this explicit allows the use of MRT-specific signaling
extensions. MRT-
specific processing, such as special handling of FECs sent with the
Rainbow MRT MT-ID.

A router also needs to indicate, via FEC advertisement,
whether it supports LDP Destination-Topology Labels, LDP Topology
Labels, or both. Since the label or labels are swapped at each LSR,
consistency across sent with the network is not required.

If both mechanisms are supported, then if a Destination-Topology
label is provided for a FEC, that should be used so Rainbow MRT MT-ID indicates that an ABR/LBR
can indicate the appropriate labels, as discussed in Section
Section 9.

8. Multi-homed Prefixes

One advantage of LFAs that is necessary FEC applies
to preserve is all the ability MRT-Blue and MRT-Red MT-IDs in supported MRT profiles as
well as to
protect multi-homed prefixes against ABR failure. For instance, if a
prefix from the backbone default shortest-path based MT-ID 0. The Rainbow MRT
MT-ID is available via both ABR A and ABR B, if A
fails, then the traffic should be redirected to B. This can also be
done for backups via MRT.

This generalizes defined to any multi-homed prefix. A multi-homed prefix
could be:

o An out-of-area prefix announced by more than one ABR,

o An AS-External route announced by 2 or more ASBRs,

o A prefix with iBGP multipath provide an easy way to different ASBRs,

o etc.

For each prefix, handle the attached ABRs are selected and a proxy-node special
signaling that is
created connected needed at ABRs or LBRs. It avoids the problem of
needing to those ABRs. If there exist multiple multi-homed
prefixes that share signal different MPLS labels for the same connectivity and costs to each of those
ABRs, then a single proxy-node can be FEC. Because
the Rainbow MRT MT-ID is used to represent only by ABRs/LBRs or the set. An
example of this LDP egress, it
is shown in Figure 3.

2 2 2 2
A----B----C A----B----C
2 | | 2 2 | | 2
| | | |
[ABR1] [ABR2] [ABR1] [ABR2]
| | | |
p,10 p,15 10 |---[P]---| 15

(a) Initial topology (b)with proxy-node

A<---B<---C A--->B--->C
| ^ ^ |
V | | V
[ABR1] [ABR2] [ABR1] [ABR2]
| |
|-->[P] [P]<--|

Figure 3: Prefixes Advertised by Multiple ABRs profile specific. The proxy-nodes and associated links are added to the network
topology after all real links have been assigned to a direction proposed experimental value is 3999
and
before the actual MRTs are computed. Proxy-nodes cannot be transited
when computing the MRTs. In addition to computing the pair of MRTs
associated with each router destination D in the area, a pair of MRTs
can final value will be computed for each such proxy-node to fully protect against ABR
failure.

Each ABR or attaching router must remove the MRT marking[see
Section 5] assigned by IANA and then forward the traffic outside of the area (or
island of MRT-fast-reroute-supporting routers).

If ASBR protection is desired, this has additonal complexities if allocated from the
ASBRs
LDP MT-ID space. The authoritative values are given in different areas. Similarly, protecting labeled BGP
traffic in the event of an ASBR failure has additional complexities
due to the per-ASBR label spaces involved.
[I-D.atlas-mpls-ldp-mrt].

9. Inter-Area and ABR Forwarding Behavior

In regular forwarding, packets destined outside the area arrive at
the ABR and the ABR

An ABR/LBR has two forwarding roles. First, it forwards them traffic
inside its area. Second, it forwards traffic from one area into
another. These same two roles apply for MRT transit traffic.
Traffic on MRT-Red or MRT-Blue destined inside the other area because the
next-hops from needs to stay
on MRT-Red or MRT-Blue in that area. However, it is desirable for
traffic leaving the area with the best route (according to tie-
breaking rules) are used by the ABR. The question is then what also exit MRT-Red or MRT-Blue back to do
with packets marked with an MRT that are received by the ABR.
shortest-path forwarding.

For unicast fast-reroute, MRT-FRR, the need to stay on an MRT forwarding topology
terminates at the ABR/LBR whose best route is via a different area/level. area/
level. It is highly desirable to go back to the default forwarding
topology when leaving an area/level. There are three basic reasons
for this. First, the default topology uses shortest paths; the
packet will thus take the shortest possible route to the destination.
Second, this allows failures that might appear in multiple areas
(e.g. ABR/LBR failures) to be separately identified and repaired
around. Third, the packet can be fast-
rerouted fast-rerouted again, if necessary,
due to a failure in a different area.

An ABR/LBR that receives a packet marked with an MRT on MRT-Red or MRT-Blue towards a
destination in another area/level should forward the MRT marked packet in the
area/level with the best route along its associated
MRT. MRT-Red or MRT-Blue. If the
packet came from that area/level, this correctly avoids the failure.

How does an
However, if the traffic came from a different area/level, the packet
should be removed from MRT-Red or MRT-Blue and forwarded on the
shortest-path default forwarding topology.

To avoid per-interface forwarding state for MRT-Red and MRT-Blue, the
ABR/LBR ensure needs to arrange that MRT-marked packets do not destined to a different area
arrive at the ABR/LBR? There are two different mechanisms depending upon the ABR/LBR already not marked as MRT-Red or MRT-Blue.

For LDP forwarding mechanism being used.

If where the LDP MPLS label encodes the MT-ID as well as the destination, then specifies (MT-ID, FEC), the
ABR/LBR is responsible for advertising a particular the proper label to each
neighbor. Additionally, an LDP label is associated with an MT-ID due
to the MT FEC that was used and not due to any intrisic particular
value for the label. Assume that an ABR/LBR has allocated three labels for a
particular destination; those labels are L_primary, L_blue, and
L_red. When the ABR/LBR advertises label bindings to routers in the
area with the best route to the destination, the ABR/
LBR ABR/LBR provides
L_primary for the default topology, L_blue for the Blue
MRT MRT-Blue MT-ID and
L_red for the Red MRT MRT-Red MT-ID, exactly as expected. However, when the
ABR/LBR advertises label bindings to routers in other areas, the ABR/LBR ABR/
LBR advertises L_primary for the Rainbow MRT MT-ID, which is then
used for the default topology, for the Blue MRT MT-ID, MRT-Blue MT-ID and for the Red MRT
MRT-Red MT-ID.

The ABR/LBR installs all next-hops from the best area area: primary next-
hops for L_primary based on
the default topology, L_primary, MRT-Blue next-hops for L_blue based on the Blue MRT forwarding
topology, L_blue, and MRT-Red next-
hops for L_red based on L_red. Because the Red ABR/LBR advertised (Rainbow MRT forwarding topology.
Therefore, MT-ID,
FEC) with L_primary to neighbors not in the best area, packets from the non-best area
those neighbors will arrive at the ABR/LBR with a label L_primary and
will be forwarded into the best area along the default topology. By
controlling what labels are advertised, the ABR/LBR can thus enforce
that packets exiting the area do so on the shortest-path default
topology.

If IP-in-IP IP forwarding is used, then the ABR/LBR behavior is dependent upon
the outermost IP address. If the outermost IP address is an MRT
loopback address of the ABR/LBR, then the packet is decapsulated and
forwarded based upon the inner IP address, which should go on the
default SPT topology. If the outermost IP address is not an MRT
loopback address of the ABR/LBR, then the packet is simply forwarded
along the associated forwarding topology. A PLR sending traffic to a
destination outside its local area/level will pick the MRT and use
the associated MRT loopback address of the ABR/
LBR immediately before selected ABR/LBR connected
to the proxy-node on that MRT. external destination.

Thus, regardless of which of these two forwarding mechanisms are
used, there is no need for additional computation or per-area
forwarding state.

+----[C]---- --[D]--[E] --[D]--[E]
| \ / \ / \
p--[A] Area 10 [ABR1] Area 0 [H]--p +-[ABR1] Area 0 [H]-+
| / \ / | \ / |
+----[B]---- --[F]--[G] | --[F]--[G] |
| |
| other |
+----------[p]-------+
area

(a) Example topology (b) Proxy node view in Area 0 nodes

+----[C]<--- [D]->[E]
V \ \
+-[A] Area 10 [ABR1] Area 0 [H]-+
| ^ / / |
| +----[B]<--- [F]->[G] V
| |
+------------->[p]<--------------+

(d) Blue MRT in Area 0 (e) Red MRT in Area 0

Figure 4: 3: ABR Forwarding Behavior and MRTs

The other potential forwarding mechanisms mechanism described in Section 6 is using
Topology-Identification Labels. This mechanism would require additional
computation by the penultimate that
any router along the in-local-area MRT
immediately before the ABR/LBR whose MRT-Red or MRT-Blue next-hop is reached. The penultimate router
can an ABR/LBR would
need to determine that whether the ABR/LBR will would forward the packet out of area/
level and, in that case,
the penultimate area/level. If so, then that router can remove should pop off the MRT
marking but still forward topology-
identification label before forwarding the packet along the MRT next-hop to reach the ABR. ABR/LBR.

For instance, example, in Figure 4, 3, if node H fails, node E has to put traffic
towards prefix p onto the red MRT. MRT-Red. But since node D knows that ABR1 will
use a best from another area, it is safe for D to remove pop the MRT marking Topology-
Identification Label and just send forward the packet to ABR1 still on along the red MRT but unmarked.
MRT-Red next-hop. ABR1 will use the shortest path in Area 10.

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
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
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-
state advertisement (LSA). In this one case, suboptimal forwarding
along the MRT in the other area would happen. If this is that becomes a
realistic deployment scenario, OSPF extensions could be considered.
This is not covered in [I-D.atlas-ospf-mrt].

10. Issues with Area Abstraction Prefixes Multiply Attached to the MRT fast-reroute provides complete coverage in Island

How a area that computing router S determines its local MRT Island for each
supported MRT profile is
2-connected. Where a failure would partition the network, already discussed in Section 7.

There are two types of course,
no alternate can protect against prefixes or FECs that failure. Similarly, there may be multiply attached
to an MRT Island. The first type are
ways of connecting multi-homed prefixes that make it impractical to
protect them without excessive complexity.

Figure 5: AS external prefixes in different areas

Consider domain or protocol boundary. The second type
represent routers that do not support the network profile for the MRT Island.
The key difference is whether the traffic, once out of the MRT
Island, remains in Figure 5 the same area/level and assume there is might reenter the MRT
Island if a richer
connective topology loop-free exit point is not selected.

One property of LFAs that isn't shown, where is necessary to preserve is the same ability to
protect multi-homed prefixes against ABR failure. For instance, if a
prefix from the backbone is
announced by ASBR X available via both ABR A and ABR B, if A
fails, then the traffic should be redirected to B. This can also be
done for backups via MRT.

If ASBR Y which protection is desired, this has additonal complexities if the
ASBRs are in different non-backbone areas. If Similarly, protecting labeled BGP
traffic in the link from A to ASBR X fails, then event of an MRT alternate
could forward the packet to ABR 1 and ABR 1 could forward it ASBR failure has additional complexities
due to D,
but then D would find the shortest per-ASBR label spaces involved.

As discussed in [RFC5286], a multi-homed prefix could be:

o An out-of-area prefix announced by more than one ABR,

o An AS-External route is back via ABR 1 announced by 2 or more ASBRs,

o A prefix with iBGP multipath to Area
20. different ASBRs,

o etc.

There are also two different approaches to protection. The only real way first is
to get it from A do endpoint selection to ASBR Y is pick a router to explicitly tunnel it to ASBR Y.

Tunnelling to the backup ASBR where that
router is for future consideration. The
previously proposed PHP approach needs to have an exception if BGP
policies (e.g. BGP local preference) determines which ASBR loop-free with respect to use.
Consider the case in Figure 6. If failure-point. Conceptually,
the link between A and ASBR X (the
preferred border router) fails, A can put the packets set of candidate routers to p onto provide LFAs expands to all routers,
with an MRT alternate, even tunnel it towards ASBR Y. Node B, however, must
not remove attached to the MRT marking in this case, as nodes in Area 0,
including ASBR Y itself would not know that their preferred ASBR is
down.

Area 20 BB Area 0
p ---[ASBR X]-X-[A]---[B]---[ABR 1]---[D]---[ASBR Y]--- p

BGP prefers ASBR X for prefix p

Figure 6: Failure of path towards ASBR preferred by BGP prefix.

The fine details of how second is to solve multi-area external prefix cases, use a proxy-node, that can be named via MPLS label
or
identifying certain cases as too unlikely IP address, and too complex pick the appropriate label or IP address to protect
is for further consideration.

11. Partial Deployment and Islands of Compatible MRT FRR routers reach
it on either MRT-Blue or MRT-Red as appropriate to avoid the failure
point. A natural concern with new functionality proxy-node can represent a destination prefix that can be
attached to the MRT Island via at least two routers. It is how termed a
named proxy-node if there is a way that traffic can be encapsulated
to have it reach specifically that proxy-node; this could be useful
when it because there is not deployed across
an entire IGP area. In LDP FEC for the case of
MRT FRR, where it provides alternates when appropriate LFAs aren't
available, there associated prefix or because MRT-Red and MRT-Blue
IP addresses are also deployment scenarios where it may make
sense to only enable some routers advertised in an area with MRT FRR. A simple
example of such as-yet undefined fashion for that
proxy-node. Traffic to a scenario would be named proxy-node may take a ring of 6 or more routers different path
than traffic to the attaching router; traffic is also explicitly
forwarded from the attaching router along a predetermined interface
towards the relevant prefixes.

For IP traffic, multi-homed prefixes can use endpoint selection. For
IP traffic that is connected via two routers destined to a router outside the rest of MRT Island, if
that router is the area.

First, egress for a FEC advertised into the MRT Island,
then the named proxy-node approach can be used.

For LDP traffic, there is always a FEC advertised into the MRT
Island. The named proxy-node approach should be used, unless the
computing router S must determine its local island of
compatible MRT fast-reroute routers. A router that has knows the label for the FEC at the selected
endpoint.

If a common FEC is advertised from outside the MRT Island into the MRT
Island and the forwarding mechanism specified in the profile flag includes
LDP, then the routers learning that FEC MUST also advertise labels
for (MRT-Red, FEC) and is connected either (MRT-Blue, FEC) to S or neighbors inside the MRT
Island. If the forwarding mechanism includes LDP, any router
receiving a FEC corresponding to another a router
already determined outside the MRT Island or
to a multi-homed prefix MUST compute and install the transit MRT-Blue
and MRT-Red next-hops for that FEC; the associated FECs ( (MT-ID 0,
FEC), (MRT-Red, FEC), and (MRT-Blue, FEC)) MUST also be in S's local island can be added provided via
LDP to S's
local island.

Destinations neighbors inside the local island can obviously use MRT
alternates. Destinations outside the Island.

10.1. Endpoint Selection

Endpoint Selection is a local island can be treated
like matter for a multi-homed prefix with caveats router in the MRT Island
since it pertains to avoid looping. For LDP
labels including both destination selecting and topology, using an alternate and does not
affect the routers at transit MRT-Red and MRT-Blue forwarding topologies.

Let the
borders of computing router be S and the local island need next-hop F be the node whose
failure is to originate labels be avoided. Let the destination be prefix p. Have A
be the router to which the prefix p is attached for S's shortest path
to p.

The candidates for endpoint selection are those to which the original
FEC and
destination prefix is attached in the associated MRT-specific labels. Packets sent area/level. For a particular
candidate B, it is necessary to an LDP
label marked as blue determine if B is loop-free to reach
p with respect to S and F for node-protection or red MRT at least with
respect to a destination outside S and the local
island link (S, F) for link-protection. If B will have
always prefer to send traffic to p via a different area/level, then
this is definitional. Otherwise, distance-based computations are
necessary and an SPF from B's perspective may be necessary. The
following equations give the last router checks needed; the rationale is similar
to that given in [RFC5286].

Loop-Free for S: D_opt(B, p) < D_opt(B, S) + D_opt(S, p)

Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(F, p)

The latter is equivalent to the local island swap following, which avoids the label need to one
compute the shortest path from F to p.

Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(S, p) - D_opt(S,
F)

Finally, the destination rules for Endpoint selection are given below. The basic
idea is to repair to the prefix-advertising router selected for the
shortest-path and forward only to select and tunnel to a different endpoint
if necessary (e.g. A=F or F is a cut-vertex or the link (S,F) is a
cut-link).

1. Does S have a node-protecting alternate to A? If so, select
that. Tunnel the packet to A along that alternate. For example,
if LDP is the outgoing
interface on forwarding mechanism, then push the MRT towards label (MRT-Red,
A) or (MRT-Blue, A) onto the packet.

2. If not, then is there a router outside the local island B that
was represented by the proxy-node.

For IP in IP encapsulations, remote destinations' loopback addresses
for is loop-free to reach p
while avoiding both F and S? If so, select B as the MRTs cannot be used, even if they were available. Instead, end-point.
Determine the MRT loopback address of alternate to reach B while avoiding F. Tunnel
the router attached packet to B along that alternate. For example, with LDP,
push the label (MRT-Red, B) or (MRT-Blue, B) onto the packet.

3. If not, then does S have a link-protecting alternate to A? If
so, select that.

4. If not, then is there a proxy-node,
which represents destinations outside router B that is loop-free to reach p
while avoiding S and the local island, can be used.
Packets sent link from S to F? If so, select B as
the endpoint and the router's MRT loopback address alternate that for reaching B from S
avoiding the link (S,F).

The endpoint selected will receive a packet destined to itself and,
being the egress, will pop that MPLS label (or have their
outer IP header removed signaled Implicit
Null) and will need forward based on what is underneath. This suffices for IP
traffic where the MPLS labels understood by the endpoint router are
not needed.

10.2. Named Proxy-Nodes

A clear advantage to using a named proxy-node is that it is possible
to be explicitly forwarded
along the outgoing interface on forward from the MRT towards Island along an interface to a router outside the
local
loop-free island neighbor (LFIN) when that was represented by interface may not be a
primary next-hop. For LDP traffic where the proxy-node. This behavior
requires essentially remembering label indicates both the MT-ID indicated by
topology and the outer IP
address. An alternate option FEC, it is necessary to either use a named proxy-
node or deal with learning remote MPLS labels.

A named proxy-node represents one or more destinations and, for LDP
forwarding, has a FEC associated with it that is signaled into the
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
Island, the label will swap to meaning (MT-ID 0, FEC). It would be
possible to advertise different
loopback have named proxy-nodes for IP forwarding, but this would
require extensions to signal two IP addresses to be associated with
MRT-Red and MRT-Blue for the proxy-node; proxy-node. A named proxy-node can be
uniquely represented by the outer two routers in the MRT Island to which it
is connected. The extensions to signal such IP
address would still addresses are not
defined in [I-D.atlas-ospf-mrt]. The details of what label-bindings
must be removed but it would indicate originated are described in [I-D.atlas-mpls-ldp-mrt].

Computing the outgoing
interface MRT next-hops to use a named proxy-node and no lookup would be necessary on the internal IP
address while maintaining MT-ID context. MRT
alternate for the computing router S to avoid a particular failure
node F is extremely straightforward. The details of the simple
constant-time functions, Select_Proxy_Node_NHs() and
Select_Alternates_Proxy_Node(), are given in
[I-D.enyedi-rtgwg-mrt-frr-algorithm]. A key point is that computing
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
impacting other existing MRT paths. This maps very well to, for
example, how OSPFv2 [[RFC2328] Section 16.5] does incremental updates
for new summary-LSAs.

The key question is which how to attach the named proxy-node to the MRT
Island; all the routers outside in the MRT island Island MUST do this consistently.
No more than 2 routers in the MRT Island can packets be
forwarded to so that they selected; one should
only be selected if there are not forwarded back into no others that meet the MRT island.
An example necessary
criteria. The named proxy-node is logically part of the necessary network graph transformations are given
in Figure 7. area/level.

There are two parts sources for candidate routers in the MRT Island to
connect to the computation. First, named proxy-node. The first set are those routers
that are advertising the MRT
island prefix; the cost assigned to each such
router is collapsed into a single node; this assumes that the announced cost of
transiting to the prefix. The second set are those
routers in the MRT island is nothing and is pessimistic but allows
for simpler computation. Then, for each destination (other than Island that are connected to routers not in the
MRT island), Island but in the same area/level; such routers adjacent will be defined
as Island Border Routers (IBRs). The routers connected to the IBRs
that are not in the MRT island Island and are checked to
see if they in the same area/level are
Island Neighbors (INs).

Since packets sent to the named proxy-node along MRT-Red or MRT-Blue
may come from any router inside the MRT Island, it is necessary that
whatever router to which an IBR forwards the packet be loop-free with respect
regard to the whole MRT island and Island for the destination. Thus, an IBR is
a candidate router only if it possesses at least one IN whose path to
the prefix does not enter the MRT Island. The loop-free neighbors cost assigned to each
(IBR, IN) pair is the D_opt(IN, prefix) plus Cost(IBR, IN).

From the set of prefix-advertising routers and the MRT island that IBRs, the two
lowest cost routers are
closest to selected and ties are broken based upon the destination
lowest Router ID. For ease of discussion, such selected routers are selected. Then,
proxy-node attachment routers and the two selected will be named A
and B.

A proxy-node attachment router has a graph of just special forwarding role. When a
packet is received destined to (MRT-Red, prefix) or (MRT-Blue,
prefix), if the
MRT island proxy-node attachment router is augmented with proxy-nodes that are attached via an IBR, it MUST swap
to the
outgoing interfaces default topology (e.g. swap to the selected loop-free neighbors. Finally, label for (MT-ID 0, prefix)
or remove the outer IP encapsulation) and forward the packet to the
IN whose cost was used in the selection. If the MRTs rooted at each proxy-node are computed on that augmented MRT
island graph. Essentially,
attachment router is not an IBR, then the packet MUST be removed from
the MRT island must have a forwarding topology and sent along the interface that caused
the router to advertise the prefix; this interface might be out of
the area/level/AS.

10.2.1. Computing if an Island Neighbor (IN) is loop-free
neighbor

As discussed, the Island Neighbor needs to be able loop-free with regard
to have an alternate.

[G]---[E]---(B)---(C)---(D) the whole MRT Island for the destination. Conceptually, the cost
of transiting the MRT Island should be regarded as 0. This can be
done by collapsing the MRT Island into a single node, as seen in
Figure 4, and then computing SPFs from each Island Neighbor and from
the MRT Island itself.

[G]---[E]---(V)---(U)---(T)
| \ | | |
| \ | | |
| \ | | |
[H]---[F]---(A)---(S)----|
[H]---[F]---(R)---(S)----|

(1) Network Graph with Partial Deployment

[E],[F],[G],[H] : No support for MRT-FRR
(A),(B),(C),(D),(S): MRT
(R),(S),(T),(U),(V): MRT Island - supports MRT-FRR MRT

[G]---[E]----| |---(B)---(C)---(D) |---(V)---(U)---(T)
| \ | | | | |
| \ | ( MRT Island ) [ proxy ] | |
| \ | | | | |
[H]---[F]----| |---(A)---(S)----| |---(R)---(S)----|

(2) Graph for determining (3) Graph for MRT computation
loop-free neighbors

Figure 7: 4: Computing alternates to destinations outside the MRT Island

The simple way to do this without manipulating the topology is to
compute the SPFs from each IN and a node in the MRT Island (e.g. the
GADAG root), but use a link metric of 0 for all links between routers
in the MRT Island. The distances computed via SPF this way will be
refered to as Dist_mrt0.

An IN is loop-free with respect to a destination D if: Dist_mrt0(IN,
D) < Dist_mrt0(IN, MRT Island Router) + Dist_mrt0(MRT Island Router,
D). Any router in the MRT Island can be used since the cost of
transiting between MRT Island routers is 0. The GADAG Root is
recommended for consistency.

10.3. MRT Alternates for Destinations Outside the MRT Island

A natural concern with new functionality is how to have it be useful
when it is not deployed across an entire IGP area. In the case of
MRT FRR, where it provides alternates when appropriate LFAs aren't
available, there are also deployment scenarios where it may make
sense to only enable some routers in an area with MRT FRR. A simple
example of such a scenario would be a ring of 6 or more routers that
is connected via two routers to the rest of the area.

Destinations inside the local island can obviously use MRT
alternates. Destinations outside the local island can be treated
like a multi-homed prefix and either Endpoint Selection or Named
Proxy-Nodes can be used. Named Proxy-Nodes MUST be supported when
LDP forwarding is supported and a label-binding for the destination
is sent to an IBR.

Naturally, there are more complicated options to improve coverage,
such as connecting multiple MRT islands across tunnels, but it is not
clear that the need
for the additional complexity is necessary.

12. has not been justified.

11. Network Convergence and Preparing for the Next Failure

After a failure, MRT detours ensure that packets reach their intended
destination while the IGP has not reconverged onto the new topology.
As link-state updates reach the routers, the IGP process calculates
the new shortest paths. Two things need attention: micro-loop
prevention and MRT re-calculation.

12.1.

11.1. Micro-forwarding loop prevention and MRTs

As is well known[RFC5715], micro-loops can occur during IGP
convergence; such loops can be local to the failure or remote from
the failure. Managing micro-loops is an orthogonal issue to having
alternates for local repair, such as MRT fast-reroute provides.

There are two possible micro-loop prevention mechanism mechanisms discussed in
[RFC5715]. The first is Ordered FIB [I-D.ietf-rtgwg-ordered-fib].
The second is Farside Tunneling which requires tunnels or an
alternate topology to reach routers on the farside of the failure.

Since MRTs provide an alternate topology through which traffic can be
sent and which can be manipulated separately from the SPT, it is
possible that MRTs could be used to support Farside Tunneling.
Details of how to do so are outside the scope of this document.

12.2.

Micro-loop mitigation mechanisms can also work when combined with
MRT.

11.2. MRT Recalculation

When a failure event happens, traffic is put by the PLRs onto the MRT
topologies. After that, each router recomputes its shortest path
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
MRT topologies should each router install the recomputed MRTs into
the FIBs.

At each router, therefore, the sequence is as follows:

1. Receive failure notification

2. Recompute SPT

3. Install new SPT

4. Recompute MRTs

5. Wait If the network was stable before the failure occured, wait a
configured (or advertised) period for all routers to be using
their SPTs and traffic to drain from the MRTs.

5. Recompute MRTs

6. Install new MRTs.

While the recomputed MRTs are not installed in the FIB, protection
coverage is lowered. Therefore, it is important to recalculate the
MRTs and install them quickly.

13.

12. Acknowledgements

The authors would like to thank Mike Shand for his valuable review
and contributions.

The authors would like to thank Joel Halpern, Hannes Gredler, Jeff Tantsura, Ted
Qian, Kishore Tiruveedhula, Shraddha Hegde, Santosh Esale, Nitin
Bahadur, Harish
Sitaraman and Sitaraman, Raveendra Torvi and Chris Bowers for their
suggestions and review.

14.

13. IANA Considerations

This doument includes no request to IANA.

15.

14. Security Considerations

This architecture is not currently believed to introduce new security
concerns.

16.

15. References

16.1.

15.1. Normative References

[I-D.enyedi-rtgwg-mrt-frr-algorithm]
Atlas, A., Envedi, G., Csaszar, A., and A. Gopalan, A., and C.
Bowers, "Algorithms for computing Maximally Redundant
Trees for IP/LDP Fast- Reroute",
draft-enyedi-rtgwg-mrt-frr-algorithm-02 draft-enyedi-rtgwg-mrt-
frr-algorithm-03 (work in progress), October 2012. July 2013.

[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008.

[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", RFC
5714, January 2010.

16.2.

15.2. Informative References

[EnyediThesis]
Enyedi, G., "Novel Algorithms for IP Fast Reroute",
Department of Telecommunications and Media Informatics,
Budapest University of Technology and Economics Ph.D.
Thesis, February 2011,
<http://timon.tmit.bme.hu/theses/thesis_book.pdf>.

[I-D.atlas-mpls-ldp-mrt]
Atlas, A., Tiruveedhula, K., Tantsura, J., and IJ.
Wijnands, "LDP Extensions to Support Maximally Redundant
Trees", draft-atlas-mpls-ldp-mrt-00 (work in progress),
July 2013.

[I-D.atlas-ospf-mrt]
Atlas, A., Hegde, S., Chris, C., and J. Tantsura, "OSPF
Extensions to Support Maximally Redundant Trees", draft-
atlas-ospf-mrt-00 (work in progress), July 2013.

[I-D.atlas-rtgwg-mrt-mc-arch]
Atlas, A., Kebler, R., Wijnands, I., Csaszar, A., and G.
Envedi, "An Architecture for Multicast Protection Using
Maximally Redundant Trees",
draft-atlas-rtgwg-mrt-mc-arch-00 draft-atlas-rtgwg-mrt-mc-
arch-02 (work in progress),
March 2012. 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.ietf-mpls-ldp-multi-topology]
Zhao, Q., Fang, L., Zhou, C., Li, L., and K. Raza, "LDP
Extensions for Multi Topology Routing",
draft-ietf-mpls-ldp-multi-topology-06 draft-ietf-mpls-
ldp-multi-topology-08 (work in progress),
December 2012. May 2013.

[I-D.ietf-rtgwg-ipfrr-notvia-addresses]
Bryant, S., Previdi, S., and M. Shand, "A Framework for IP
and MPLS Fast Reroute Using Not-via Addresses",
draft-ietf-rtgwg-ipfrr-notvia-addresses-10 draft-
ietf-rtgwg-ipfrr-notvia-addresses-11 (work in progress), December 2012.

[I-D.ietf-rtgwg-lfa-applicability]
Filsfils, C. and P. Francois, "LFA applicability in SP
networks", draft-ietf-rtgwg-lfa-applicability-06 (work in
progress), January 2012.
May 2013.

[I-D.ietf-rtgwg-ordered-fib]
Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
Francois, P., and O. Bonaventure, "Framework for Loop-free
convergence using oFIB", draft-ietf-rtgwg-ordered-fib-09 draft-ietf-rtgwg-ordered-fib-12
(work in progress), January May 2013.

[I-D.ietf-rtgwg-remote-lfa]
Bryant, S., Filsfils, C., Previdi, S., Shand, M., and S.
Ning, "Remote LFA FRR", draft-ietf-rtgwg-remote-lfa-01 draft-ietf-rtgwg-remote-lfa-02
(work in progress), December 2012. May 2013.

[I-D.litkowski-rtgwg-node-protect-remote-lfa]
Litkowski, S., "Node protecting remote LFA", draft-
litkowski-rtgwg-node-protect-remote-lfa-00 (work in
progress), April 2013.

[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>. <http://opti.tmit.bme.hu/~tapolcai
/papers/retvari2011lfa_infocom.pdf>.

[LightweightNotVia]
Enyedi, G., Retvari, G., Szilagyi, P., and A. Csaszar, "IP
Fast ReRoute: Lightweight Not-Via without Additional
Addresses", Proceedings of IEEE INFOCOM , 2009,
<http://mycite.omikk.bme.hu/doc/71691.pdf>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

[RFC3137] Retana, A., Nguyen, L., White, R., Zinin, A., and D.
McPherson, "OSPF Stub Router Advertisement", RFC 3137,
June 2001.

[RFC5443] Jork, M., Atlas, A., and L. Fang, "LDP IGP
Synchronization", RFC 5443, March 2009.

[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, January 2010.

[RFC6571] Filsfils, C., Francois, P., Shand, M., Decraene, B.,
Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
Alternate (LFA) Applicability in Service Provider (SP)
Networks", RFC 6571, June 2012.

Appendix A. General Issues with Area Abstraction

When a multi-homed prefix is connected in two different areas, it may
be impractical to protect them without adding the complexity of
explicit tunneling. This is also a problem for LFA and Remote-LFA.

Figure 5: AS external prefixes in different areas

Consider the network in Figure 5 and assume there is a richer
connective topology that isn't shown, where the same prefix is
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
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
20. This problem occurs because the routers, including the ABR, in
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
ASBR Y. If the traffic is unlabeled or the appropriate MPLS labels
are known, then explicit tunneling MAY be used as long as the
shortest-path of the tunnel avoids the failure point. In that case,
A must determine that it should use an explicit tunnel instead of an
MRT alternate.

Authors' Addresses

Alia Atlas (editor)
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA

Email: akatlas@juniper.net

Robert Kebler
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA

Email: rkebler@juniper.net
Gabor Sandor Enyedi
Ericsson
Konyves Kalman krt 11.
Budapest 1097
Hungary

Email: Gabor.Sandor.Enyedi@ericsson.com

Andras Csaszar
Ericsson
Konyves Kalman krt 11
Budapest 1097
Hungary

Email: Andras.Csaszar@ericsson.com

Jeff Tantsura
Ericsson
300 Holger Way
San Jose, CA 95134
USA

Email: jeff.tantsura@ericsson.com

Maciek Konstantynowicz
Cisco Systems

Email: maciek@bgp.nu

Russ White
Verisign
12061 Bluemont Way
Reston, VA 20190
USA

Email: riwhite@verisign.com

Mike Shand
VCE

Email: mike@mshand.org.uk russw@riw.us