Routing Area Working Group A. Atlas, Ed.
Internet-Draft R. Kebler
Intended status: Standards Track Juniper Networks
Expires: January 13, 2014 G. Enyedi
A. Csaszar
J. Tantsura
Ericsson
M. Konstantynowicz
Cisco Systems
R. White
VCE
July 12, 2013
An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees
draft-ietf-rtgwg-mrt-frr-architecture-03
Abstract
With increasing deployment of Loop-Free Alternates (LFA) [RFC5286],
it is clear that a complete solution for IP and LDP Fast-Reroute is
required. This specification provides that solution. IP/LDP Fast-
Reroute with Maximally Redundant Trees (MRT-FRR) is a technology that
gives link-protection and node-protection with 100% coverage in any
network topology that is still connected after the failure.
MRT removes all 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
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
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 13, 2014.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Importance of 100% Coverage . . . . . . . . . . . . . . . 4
1.2. Partial Deployment and Backwards Compatibility . . . . . 5
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 6
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 7
5. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . 9
6. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . . 10
6.1. LDP Unicast Forwarding - Avoid Tunneling . . . . . . . . 10
6.2. IP Unicast Traffic . . . . . . . . . . . . . . . . . . . 11
7. Protocol Extensions and Considerations: OSPF and ISIS . . . . 12
8. Protocol Extensions and considerations: LDP . . . . . . . . . 14
9. Inter-Area and ABR Forwarding Behavior . . . . . . . . . . . 15
10. Prefixes Multiply Attached to the MRT Island . . . . . . . . 18
10.1. Endpoint Selection . . . . . . . . . . . . . . . . . . . 19
10.2. Named Proxy-Nodes . . . . . . . . . . . . . . . . . . . 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 . . . 24
11.1. Micro-forwarding loop prevention and MRTs . . . . . . . 24
11.2. MRT Recalculation . . . . . . . . . . . . . . . . . . . 24
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
14. Security Considerations . . . . . . . . . . . . . . . . . . . 25
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
15.1. Normative References . . . . . . . . . . . . . . . . . . 25
15.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. General Issues with Area Abstraction . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
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1. Introduction
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 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 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 unicast fast-reroute, the
diverse forwarding topologies and guarantee of 100% coverage permit
fast-reroute technology to be applied to multicast traffic as
described in [I-D.atlas-rtgwg-mrt-mc-arch].
Other existing or proposed solutions are partial solutions or have
significant issues, as described below.
Summary Comparison of IP/LDP FRR Methods
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+-----------+---------------+---------------+-----------------------+
| Method | Coverage | Alternate | Computation (in SPFs) |
| | | Looping? | |
+-----------+---------------+---------------+-----------------------+
| MRT-FRR | 100% | None | less than 3 |
| | Link/Node | | |
| | | | |
| LFA | Partial | Possible | per neighbor |
| | Link/Node | | |
| | | | |
| Remote | Partial | Possible | per neighbor (link) |
| LFA | Link/Node | | or neighbor's |
| | | | neighbor (node) |
| | | | |
| Not-Via | 100% | None | per link and node |
| | Link/Node | | |
+-----------+---------------+---------------+-----------------------+
Table 1
Loop-Free Alternates (LFA): LFAs [RFC5286] provide limited
topology-dependent coverage for link and node protection.
Restrictions on choice of alternates can be relaxed to improve
coverage, but this can cause forwarding loops if a worse failure
is experienced than protected against. Augmenting a network to
provide better coverage is NP-hard [LFARevisited]. [RFC6571]
discusses the applicability of LFA to different topologies with a
focus on common PoP architectures.
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 the
only other solution that provides 100% coverage for link and node
failures and does not have potential looping. However, the
computation is very high (an SPF per failure point) and academic
implementations [LightweightNotVia] have found the address
management complexity to be high.
1.1. Importance of 100% Coverage
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Fast-reroute is based upon the single failure assumption - that the
time between single failures is long enough for a network to
reconverge and start forwarding on the new shortest paths. That does
not imply that the network will only experience one failure or
change.
It is straightforward to analyze a particular network topology for
coverage. However, a real network does not always have the same
topology. For instance, maintenance events will take links or nodes
out of use. Simply costing out a link can have a significant effect
on what LFAs are available. Similarly, after a single failure has
happened, the topology is changed and its associated coverage.
Finally, many networks have new routers or links added and removed;
each of those changes can have an effect on the coverage for
topology-sensitive methods such as LFA and Remote LFA. If fast-
reroute is important for the network services provided, then a method
that guarantees 100% coverage is important to accomodate natural
network topology changes.
Asymmetric link costs are also a common aspect of networks. There
are at least three common causes for them. First, any broadcast
interface is represented by a pseudo-node and has asymmetric link
costs to and from that pseudo-node. Second, when routers come up or
a link with LDP comes up, it is recommended in [RFC5443] and
[RFC3137] that the link metric be raised to the 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 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. 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 group of routers (referred to as an
MRT Island), the MRTs are computed. Alternates to destinations
outside the MRT Island are computed and depend upon the existence of
a loop-free neighbor of the MRT Island for that destination.
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2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]
3. Terminology
network graph: A graph that reflects the network topology where all
links connect exactly two nodes and broadcast links have been
transformed into the standard pseudo-node representation.
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
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.
MRT-Red: MRT-Red is used to describe one of the two MRTs; it is
used to described the associated forwarding topology and MT-ID.
Specifically, MRT-Red is the decreasing MRT where links 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 MRTs; it is
used to described the associated forwarding topology and MT-ID.
Specifically, MRT-Blue is the increasing MRT where links in the
GADAG are taken in the direction from a lower topologically
ordered node to a higher one.
Rainbow MRT: It is useful to have an MT-ID that refers to the
multiple MRT topologies and to the default topology. This is
referred to as the Rainbow MRT MT-ID and is used by LDP to reduce
signaling and permit the same label to always be advertised to all
peers for the same (MT-ID, Prefix).
MRT Island: From the computing router, the set of routers that
support a particular MRT profile and are connected.
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
in a common area or level.
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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.
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
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
reach specifically that proxy node; this could be because there is
an LDP FEC for the associated prefix or because MRT-Red and MRT-
Blue IP addresses are advertised in an undefined fashion for that
proxy-node.
4. Maximally Redundant Trees (MRT)
A pair of Maximally Redundant Trees are directed spanning trees that
provide maximally disjoint paths towards their common root. Only
links or nodes whose failure would partition the network (i.e. cut-
links and cut-vertices) are shared between the trees. The algorithm
to compute MRTs is given in [I-D.enyedi-rtgwg-mrt-frr-algorithm].
This algorithm can be computed in O(e + n log n); it is less than
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three SPFs. Modeling results comparing MRT alternates to the optimal
are described in [I-D.enyedi-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
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.
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) (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] |
| | | | |
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| | | |----[J]
[A]---[B]---|
(a)
a non-2-connected graph
[E]<--[D]<--| [E]-->[D]
| ^ | [I] | |----[I]
V | | | V V ^
[R] [C] [F]<--[G] | [R]<--[C] [F]<--[G] |
^ ^ ^ 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 MRT-Blue and 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 Failure Insensitive Routing in [EnyediThesis]).
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 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 the Blue MRT path 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 MRT-Blue next-hops or the MRT-Red next-hops
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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. It is a local decision
whether to use an MRT alternate, a Loop-Free Alternate or some other
type of alternate.
As described in [RFC5286], when a worse failure than is anticipated
happens, using LFAs that are not downstream neighbors can cause
micro-looping. Section 1.1 of [RFC5286] gives an example of link-
protecting alternates causing a loop on node failure. Even if a
worse failure than anticipated happens, the use of MRT alternates
will not cause looping. Therefore, while node-protecting LFAs may be
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
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 5, MRT needs multi-topology forwarding.
Unfortunately, neither IP nor LDP 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.
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; 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 -
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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].
6.2. IP Unicast Traffic
For IP, there is no currently practical alternative except tunneling
to gain the bits needed to indicate the MRT-Blue or MRT-Red
forwarding topology. The choice of tunnel egress 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 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. 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. 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.
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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.
7. Protocol Extensions and Considerations: OSPF and ISIS
For simplicity, the approach of defining a well-known profile is
taken in [I-D.atlas-ospf-mrt]. The purpose of communicating support
for MRT in the IGP is to indicate thatqq the MRT-Blue and MRT-Red
forwarding topologies are created for transit traffic. This section
describes the various options to be selected. The default MRT
profile is described here and the signaling extensions for OSPF are
given in [I-D.atlas-ospf-mrt].
For any MRT profile, the MRT Island is created by starting from the
computing router. If the computing router supports the default MRT
profile, add it to the MRT Island. Add a router to the MRT Island if
the router supports the default MRT profile and is connected to the
MRT Island via bidirectional links eligible for MRT.
If a router advertises support for multiple MRT profiles, then it
MUST create the transit forwarding topologies for each of those,
unless the profile specifies No Forwarding Mechanism (e.g. as might
be done for a profile used only for multicast global protection). A
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router MUST NOT advertise multiple MRT profiles that overlap in their
MRT-Red MT-ID or MRT-Blue MT-ID.
The MRT Profile also defines different behaviors such as how MRT
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].
MRT-Red MT-ID: experimental 3997, final value assigned by IANA
allocated from the LDP MT-ID space
MRT-Blue MT-ID: experimental 3998, final value assigned by IANA
allocated from the LDP MT-ID space
GADAG Root Selection Priority: Among the routers in the MRT Island
and with the highest priority advertised, an implementation MUST
pick the router with the highest Router ID to be the GADAG root.
Forwarding Mechanisms: LDP
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 describes the aspects to be considered to define a
profile to advertise. For some profiles, associated information may
need to be distributed, such as GADAG Root Selection Priority, Red
MRT Loopback Address, Blue MRT Loopback Address.
MRT Algorithm: This identifies the particular MRT algorithm used by
the router for this profile. Algorithm transitions can be managed
by advertising multiple MRT profiles.
MRT-Red MT-ID: This specifies the MT-ID to be associated with the
MRT-Red forwarding topology. It is needed for use in LDP
signaling. All routers in the MRT Island MUST agree on a value.
MRT-Blue MT-ID: This specifies the MT-ID to be associated with the
MRT-Blue forwarding topology. It is needed for use in LDP
signaling. All routers in the MRT Island MUST agree on a value.
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GADAG Root Selection Priority: A MRT profile might specify this to
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: This specifies which forwarding mechanisms
the router supports for transit traffic. An MRT island must
program appropriate next-hops into the forwarding 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. If IPv6 is 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 MRT-Red forwarding topology.
It can, of course, be specified for both IPv4 and IPv6.
MRT-Blue Loopback Address: This provides the router's loopback
address to reach the router via the MRT-Blue forwarding topology.
It can, of course, be specified for both IPv4 and IPv6.
Recalculation: As part of what process and timing should the new
MRTs be computed on a modified topology? Section 11.2 describes
the minimum behavior required to support fast-reroute.
Area/Level Border Behavior: Should inter-area traffic on the MRT-
Blue or MRT-Red be put back onto the 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 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
the profile, there may be additional profile-specific behavior.
As with LFA, it is expected that OSPF Virtual Links will not be
supported.
8. Protocol Extensions and considerations: LDP
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The protocol extensions for LDP are defined in
[I-D.atlas-mpls-ldp-mrt]. A router must indicate that it has the
ability to support MRT; having this explicit allows the use of MRT-
specific processing, such as special handling of FECs sent with the
Rainbow MRT MT-ID.
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 as
well as to the default shortest-path based MT-ID 0. 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 avoids the problem of
needing to signal different MPLS labels for the same FEC. Because
the Rainbow MRT MT-ID is used only by ABRs/LBRs or the LDP egress, it
is not MRT profile specific. The proposed experimental value is 3999
and the final value will be assigned by IANA and allocated from the
LDP MT-ID space. The authoritative values are given in
[I-D.atlas-mpls-ldp-mrt].
9. Inter-Area and ABR Forwarding Behavior
An ABR/LBR has two forwarding roles. First, it forwards 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 area needs to stay
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 back to the
shortest-path forwarding.
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/
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 again, if necessary,
due to a failure in a different area.
An ABR/LBR that receives a packet on MRT-Red or MRT-Blue towards a
destination in another area/level should forward the packet in the
area/level with the best route along MRT-Red or MRT-Blue. If the
packet came from that area/level, this correctly avoids the failure.
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.
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To avoid per-interface forwarding state for MRT-Red and MRT-Blue, the
ABR/LBR needs to arrange that packets destined to a different area
arrive at the ABR/LBR already not marked as MRT-Red or MRT-Blue.
For LDP forwarding where the MPLS label specifies (MT-ID, FEC), the
ABR/LBR is responsible for advertising the proper label to each
neighbor. 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 provides
L_primary for the default topology, L_blue for the MRT-Blue MT-ID and
L_red for the MRT-Red MT-ID, exactly as expected. However, when the
ABR/LBR advertises label bindings to routers in other areas, the ABR/
LBR advertises L_primary for the Rainbow MRT MT-ID, which is then
used for the default topology, for the MRT-Blue MT-ID and for the
MRT-Red MT-ID.
The ABR/LBR installs all next-hops from the best area: primary next-
hops for L_primary, MRT-Blue next-hops for L_blue, and MRT-Red next-
hops for L_red. Because the ABR/LBR advertised (Rainbow MRT MT-ID,
FEC) with L_primary to neighbors not in the best area, packets from
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 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 selected ABR/LBR connected
to the 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] |
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| |
| 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]<--------------+
(c) rSPT towards destination p
->[D]->[E] -<[D]<-[E]
/ \ / \
[ABR1] Area 0 [H]-+ +-[ABR1] [H]
/ | | \
[F]->[G] V V -<[F]<-[G]
| |
| |
[p]<------+ +--------->[p]
(d) Blue MRT in Area 0 (e) Red MRT in Area 0
Figure 3: ABR Forwarding Behavior and MRTs
The other forwarding mechanism described in Section 6 is using
Topology-Identification Labels. This mechanism would 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 packet out of
the area/level. If so, then that router should pop off the topology-
identification label before forwarding the packet to the ABR/LBR.
For example, in Figure 3, if node H fails, node E has to put traffic
towards prefix p onto MRT-Red. But since node D knows that ABR1 will
use a best from another area, it is safe for D to pop the Topology-
Identification Label and just forward the packet to ABR1 along the
MRT-Red next-hop. ABR1 will use the shortest path in Area 10.
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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 that becomes a
realistic deployment scenario, OSPF extensions could be considered.
This is not covered in [I-D.atlas-ospf-mrt].
10. Prefixes Multiply Attached to the MRT Island
How a computing router S determines its local MRT Island for each
supported MRT profile is already discussed in Section 7.
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
usually connect at a domain or protocol boundary. The second type
represent routers that do not support the profile for the MRT Island.
The key difference is whether the traffic, once out of the MRT
Island, remains in the same area/level and might reenter the MRT
Island if a loop-free exit point is not selected.
One property of LFAs that is necessary to preserve is the ability to
protect 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, then the traffic should be redirected to B. This can also be
done for backups via MRT.
If ASBR protection is desired, this has additonal complexities if the
ASBRs are 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.
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 announced by 2 or more ASBRs,
o A prefix with iBGP multipath to different ASBRs,
o etc.
There are also two different approaches to protection. The first is
to do endpoint selection to pick a router to tunnel to where that
router is loop-free with respect to the failure-point. Conceptually,
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the set of candidate routers to provide LFAs expands to all routers,
with an MRT alternate, attached to the prefix.
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
it on either MRT-Blue or MRT-Red as appropriate to avoid the failure
point. A proxy-node can represent a destination prefix that can be
attached to the MRT Island via at least two routers. It is termed a
named proxy-node if there is a way that traffic can be encapsulated
to reach specifically that proxy-node; this could be because there is
an LDP FEC for the associated prefix or because MRT-Red and MRT-Blue
IP addresses are advertised in an as-yet undefined fashion for that
proxy-node. Traffic to a named proxy-node may take a 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 destined to a router outside the MRT Island, if
that router is the 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 knows the label for the FEC at the selected
endpoint.
If a FEC is advertised from outside the MRT Island into the MRT
Island and the forwarding mechanism specified in the profile includes
LDP, then the routers learning that FEC MUST also advertise labels
for (MRT-Red, FEC) and (MRT-Blue, FEC) to neighbors inside the MRT
Island. If the forwarding mechanism includes LDP, any router
receiving a FEC corresponding to a router 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 provided via
LDP to neighbors inside the MRT Island.
10.1. Endpoint Selection
Endpoint Selection is a local matter for a router in the MRT Island
since it pertains to selecting and using an alternate and does not
affect the transit MRT-Red and MRT-Blue forwarding topologies.
Let the computing router be S and the next-hop F be the node whose
failure is to 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.
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The candidates for endpoint selection are those to which the
destination prefix is attached in the area/level. For a particular
candidate B, it is necessary to determine if B is loop-free to reach
p with respect to S and F for node-protection or at least with
respect to S and the link (S, F) for link-protection. If B will
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 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 following, which avoids the need to
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 rules for Endpoint selection are given below. The basic
idea is to repair to the prefix-advertising router selected for the
shortest-path and 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 forwarding mechanism, then push the label (MRT-Red,
A) or (MRT-Blue, A) onto the packet.
2. If not, then is there a router B that is loop-free to reach p
while avoiding both F and S? If so, select B as the end-point.
Determine the MRT alternate to reach B while avoiding F. Tunnel
the 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 router B that is loop-free to reach p
while avoiding S and the link from S to F? If so, select B as
the endpoint and the MRT 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 signaled Implicit
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Null) and 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 explicitly forward from the MRT Island along an interface to a
loop-free island neighbor (LFIN) when that interface may not be a
primary next-hop. For LDP traffic where the label indicates both the
topology and the 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 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. A named proxy-node can be
uniquely represented by the two routers in the MRT Island to which it
is connected. The extensions to signal such IP addresses are not
defined in [I-D.atlas-ospf-mrt]. The details of what label-bindings
must be originated are described in [I-D.atlas-mpls-ldp-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
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 how to attach the named proxy-node to the MRT
Island; all the routers in the MRT Island MUST do this consistently.
No more than 2 routers in the MRT Island can be selected; one should
only be selected if there are no others that meet the necessary
criteria. The named proxy-node is logically part of the area/level.
There are two sources for candidate routers in the MRT Island to
connect to the named proxy-node. The first set are those routers
that are advertising the prefix; the cost assigned to each such
router is the announced cost to the prefix. The second set are those
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routers in the MRT Island that are connected to routers not in the
MRT Island but in the same area/level; such routers will be defined
as Island Border Routers (IBRs). The routers connected to the IBRs
that are not in the MRT Island and are 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
regard to the whole MRT 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 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 IBRs, the two
lowest cost routers are selected and ties are broken based upon the
lowest Router ID. For ease of discussion, such selected routers are
proxy-node attachment routers and the two selected will be named A
and B.
A proxy-node attachment router has a special forwarding role. When a
packet is received destined to (MRT-Red, prefix) or (MRT-Blue,
prefix), if the proxy-node attachment router is an IBR, it MUST swap
to the default topology (e.g. swap to the 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 proxy-node
attachment router is not an IBR, then the packet MUST be removed from
the MRT 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
As discussed, the Island Neighbor needs to be loop-free with regard
to 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]---(R)---(S)----|
(1) Network Graph with Partial Deployment
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[E],[F],[G],[H] : No support for MRT
(R),(S),(T),(U),(V): MRT Island - supports MRT
[G]---[E]----| |---(V)---(U)---(T)
| \ | | | | |
| \ | ( MRT Island ) [ proxy ] | |
| \ | | | | |
[H]---[F]----| |---(R)---(S)----|
(2) Graph for determining (3) Graph for MRT computation
loop-free neighbors
Figure 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 the need
for the additional complexity has not been justified.
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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.
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 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.
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. 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.
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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.
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, Ted
Qian, Kishore Tiruveedhula, Shraddha Hegde, Santosh Esale, Nitin
Bahadur, Harish Sitaraman, Raveendra Torvi and Chris Bowers for their
suggestions and review.
13. IANA Considerations
This doument includes no request to IANA.
14. Security Considerations
This architecture is not currently believed to introduce new security
concerns.
15. References
15.1. Normative References
[I-D.enyedi-rtgwg-mrt-frr-algorithm]
Atlas, A., Envedi, G., Csaszar, A., Gopalan, A., and C.
Bowers, "Algorithms for computing Maximally Redundant
Trees for IP/LDP Fast- Reroute", draft-enyedi-rtgwg-mrt-
frr-algorithm-03 (work in progress), 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.
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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-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.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-08 (work in progress), 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-11 (work in progress),
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-12
(work in progress), May 2013.
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Internet-Draft MRT Unicast FRR Architecture July 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-02
(work in progress), 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>.
[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.
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50
|----[ASBR Y]---[B]---[ABR 2]---[C] Backbone Area 0:
| | ABR 1, ABR 2, C, D
| |
| | Area 20: A, ASBR X
| |
p ---[ASBR X]---[A]---[ABR 1]---[D] Area 10: B, ASBR Y
5 p is a Type 1 AS-external
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
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Internet-Draft MRT Unicast FRR Architecture July 2013
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
VCE
Email: russw@riw.us
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