Routing Area Working Group A. Atlas, Ed.
Internet-Draft R. Kebler
Intended status: Standards Track Juniper Networks
Expires: September 13, 2012 G. Enyedi
A. Csaszar
Ericsson
M. Konstantynowicz
R. White
Cisco Systems
M. Shand
March 12, 2012
An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees
draft-ietf-rtgwg-mrt-frr-architecture-01
Abstract
As IP and LDP Fast-Reroute are increasingly deployed, the coverage
limitations of Loop-Free Alternates are seen as a problem that
requires a straightforward and consistent solution for IP and LDP,
for unicast and multicast. This draft describes an architecture
based on redundant backup trees where a single failure can cut a
point-of-local-repair from the destination only on one of the pair of
redundant trees.
One innovative algorithm to compute such topologies is maximally
disjoint backup trees. Each 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 computation requirements are
believed to be significantly less than the Not-Via architecture
requires.
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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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 September 13, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Goals for Extending IP Fast-Reroute coverage beyond LFA . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 6
4. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . . 8
5. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . . . 9
5.1. LDP Unicast Forwarding - Avoid Tunneling . . . . . . . . . 10
5.2. IP Unicast Traffic . . . . . . . . . . . . . . . . . . . . 10
6. Protocol Extensions and Considerations: OSPF and ISIS . . . . 12
7. Multi-homed Prefixes . . . . . . . . . . . . . . . . . . . . . 14
8. Inter-Area and ABR Forwarding Behavior . . . . . . . . . . . . 15
9. Issues with Area Abstraction . . . . . . . . . . . . . . . . . 18
10. Partial Deployment and Islands of Compatible MRT FRR
routers . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
11. Network Convergence and Preparing for the Next Failure . . . . 21
11.1. Micro-forwarding loop prevention and MRTs . . . . . . . . 21
11.2. MRT Recalculation . . . . . . . . . . . . . . . . . . . . 22
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
14. Security Considerations . . . . . . . . . . . . . . . . . . . 23
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
15.1. Normative References . . . . . . . . . . . . . . . . . . . 23
15.2. Informative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Introduction
There is still work required to completely provide IP and LDP Fast-
Reroute[RFC5714] for unicast and multicast traffic. This draft
proposes an architecture to provide 100% coverage for unicast
traffic. The associated multicast architecture is described in
[I-D.atlas-rtgwg-mrt-mc-arch].
Loop-free alternates (LFAs)[RFC5286] provide a useful mechanism for
link and node protection but getting complete coverage is quite hard.
[LFARevisited] defines sufficient conditions to determine if a
network provides link-protecting LFAs and also proves that augmenting
a network to provide better coverage is NP-hard.
[I-D.ietf-rtgwg-lfa-applicability] discusses the applicability of LFA
to different topologies with a focus on common PoP architectures.
While Not-Via [I-D.ietf-rtgwg-ipfrr-notvia-addresses] is defined as
an architecture, in practice, it has proved too complicated and
stateful to spark substantial interest in implementation or
deployment. 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 is based on the idea of using disjoint backup topologies as
realized by Maximally Redundant Trees (described in
[LightweightNotVia]); 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 for extending IPFRR 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 and node disjointness
regardless of topology,
o automatically compute backup next-hops based on the topology
information distributed by link-state IGP,
o do not require any signaling in the case of failure and use pre-
programmed backup next-hops for forwarding,
o introduce minimal amount of additional addressing and state on
routers,
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o enable gradual introduction of the new scheme and backward
compatibility,
o and do not impose requirements for external computation.
2. Terminology
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.
ADAG: Almost Directed Acyclic Graph - a graph that, if all links
incoming to the root were removed, would be a DAG.
block: Either a 2-connected cluster, a cut-edge, or an isolated
vertex.
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.
cut-vertex: A vertex whose removal partitions the network.
DAG: Directed Acyclic Graph - a graph where all links are directed
and there are no cycles in it.
GADAG: Generalized ADAG - a graph that is the combination of the
ADAGs of all blocks.
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 the network topology where all
links connect exactly two nodes and broadcast links have been
transformed into the standard pseudo-node representation.
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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.
3. Maximally Redundant Trees (MRT)
In the last few years, there's been substantial research on how to
compute and use redundant trees. Redundant trees are directed
spanning trees that provide disjoint paths towards their common root.
These redundant trees only exist and provide link protection if the
network is 2-edge-connected and node protection if the network is
2-connected. Such connectiveness may not be the case in real
networks, either due to architecture or due to a previous failure.
The work on maximally redundant trees has added two useful pieces
that make them ready for use in a real network.
o Computable regardless of network topology: The maximally redundant
trees are computed so that only the cut-edges or cut-vertices are
shared between the multiple trees.
o Computationally practical algorithm is based on a common network
topology database. Algorithm variants can compute 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 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. Modeling is underway to investigate and compare
the 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 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.
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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) (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.
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[E]---[D]---|
| | | |----[I]
| | | | |
[R]---[C] [F]---[G] |
| | | | |
| | | |----[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] | V | |----[J]
[A]-->[B]---| [A]<--[B]<--|
(b) (c)
Blue MRT towards R Red MRT towards R
Figure 2: A non-2-connected network
4. 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 and red MRTs.
Any IP/LDP fast-reroute technique beyond LFA requires an additional
dataplane procedure, such as an additional forwarding mechanism. The
well-known options are tunneling (e.g.
[I-D.ietf-rtgwg-ipfrr-notvia-addresses]), per-interface forwarding
(e.g. 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.
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When there is a link or node failure affecting the rSPT, 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.
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 next-hops or the Red MRT 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. 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 example is given of link-protecting alternates
causing a loop on node failure. Even if a worse failure than
anticipated happened, the use of MRT alternates will not cause
looping. Therefore, while node-protecting LFAs may be prefered, an
advantage to using MRT alternates when such a node-protecting LFA is
not a downstream path is the certainty that no alternate-induced
looping will occur.
5. 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, MRT needs multi-topology forwarding.
Unfortunately, neither IP nor LDP provide 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.
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5.1. 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.
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. IP Unicast Traffic
For IP, there is no currently practical alternative except tunneling.
The tunnel egress could be the original destination in the area, the
next-next-hop, etc.. If the tunnel egress is the original
destination router, then the traffic remains on the redundant tree
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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.
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 multi-homed prefixes, this requires that
additional labels be advertised for each proxy-node.
B. Option B - LDP Topology Label: Use a Topology-Identifier
label on top of the IP packet. This is very simple and
doesn't require additional labels for proxy-nodes. 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 and ease in removing the MRT-
topology marking at area/level boundaries, routers that support MPLS
and implement IP MRT fast-reroute SHOULD support Option A - using an
LDP label that indicates the destination and MT-ID.
For proxy-nodes associated with one or more multi-homed prefixes,
there is no router associated with the proxy-node, so its loopbacks
can't be known or used. Instead, the loopback addresses of the two
routers that are attached to the proxy-node can be used. One of
those routers will be on the Red MRT and the other on the Blue MRT.
The MRT-red loopback of the first router would be used to reach the
router on the Red MRT and similarly the MRT-blue loopback of the
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second 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 the next area.
6. Protocol Extensions and Considerations: OSPF and ISIS
This captures an initial understanding of what may need to be
specified. In cases of partial deployment, it is necessary for a
router to determine a consistent set of routers to include in the
island of MRT support. To facilitate this, each router can announce
both what its capabilities are and what it requires from other
routers to add them to the MRT island. Generally, there will be a
set of information advertised about the MRT support. This
information has only area/level-wide scope.
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: This identifies the particular MRT algorithm used
by the router. By having an Algorithm ID, it is possible to
change the algorithm used or use different ones in different
networks. It may be desirable to advertise a list ordered by
preference to allow transitions.
Red MRT MT-ID: This specifies the MT-ID to be associated with the
Red MRT forwarding topology. It is needed for use in signaling.
All routers in the MRT Island MUST agree on a value.
Blue MRT MT-ID: This specifies the MT-ID to be associated with the
Blue MRT forwarding topology. It is needed for use in signaling.
All routers in the MRT Island MUST agree on a value.
GADAG Root Election 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 to GADAG root selection is still being
evaluated. This provides the network operator with a knob to
force particular GADAG root selection.
Forwarding Mechanism for IP: This specifies which forwarding
mechanisms the router supports for IP traffic. An MRT island must
support a common set of forwarding mechanisms, which may be less
than the full set advertised. Multiple forwarding mechanisms may
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be specified, such as IP-in-IPv4, IP-in-IPv6 or IP-in-LDP-
Destination-Topology Label. None is also an option.
Forwarding Mechanism for LDP: This specifies which forwarding
mechanisms the router supports for LDP traffic. An MRT island
must support a common set of forwarding mechanisms, which may be
less than the full set advertised. The expected mechanisms are
"Encode MT-ID in Labels" or None.
Red MRT Loopback Address: This provides the router's loopback
address to reach the router via the Red MRT forwarding topology.
It can, of course, be specified for both IPv4 and IPv6.
Blue MRT Loopback Address: This provides the router's loopback
address to reach the router via the Blue MRT forwarding topology.
It can, of course, be specified for both IPv4 and IPv6.
MRT Capabilities Available: This is the set of capabilities that
the router is configured to support.
MRT Capabilities Required: This is the set of capabilities that
other routers must have available to 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 the computed
MRTs for LDP fast-reroute.
MRT Capability: PIM Fast-Reroute: The router can use the computed
MRTs for PIM fast-reroute.
MRT Capability: mLDP Fast-Reroute: The router can use the computed
MRTs for mLDP fast-reroute.
MRT Capability: PIM Global Protection: The router can use the
computed MRTs for PIM Global Protection 1+1.
MRT Capability: mLDP Global Protection: The router can use the
computed MRTs for mLDP Global Protection 1+1.
The assumption is that a router will form 1 MRT island, compute MRTs
within that island, and then use those MRTs for the different
purposes. Including a router that, for instance, doesn't support
mLDP Global Protection would mean that the whole MRT island could not
support it. In a fully deployed case, of course, the whole area/
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level would support MRT and the complexities of MRT island formation
would be minimal.
If a router wanted to form multiple MRT islands for different
application purposes, that could be done by specifying different Red
MRT MT-ID and Blue MRT MT-IDs.
As with LFA, it is expected that OSPF Virtual Links will not be
supported.
7. Multi-homed Prefixes
One advantage 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.
This generalizes 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 to different ASBRs,
o etc.
For each prefix, the two lowest total cost ABRs are selected and a
proxy-node is created connected to those two ABRs. If there exist
multiple multi-homed prefixes that share the same two best
connectivity, then a single proxy-node can be used to represent the
set. An example of this is shown in Figure 3.
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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]<--|
(c) Blue MRT (d) Red MRT
Figure 3: Prefixes Advertised by Multiple ABRs
The proxy-nodes and associated links are added to the network
topology after all real links have been assigned to a direction 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 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] 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 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.
8. Inter-Area and ABR Forwarding Behavior
In regular forwarding, packets destined outside the area arrive at
the ABR and the ABR forwards them into the other area because the
next-hops from the area with the best route (according to tie-
breaking rules) are used by the ABR. The question is then what to do
with packets marked with an MRT that are received by the ABR.
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For unicast fast-reroute, 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 marked with an MRT towards a
destination in another area should forward the MRT marked packet in
the area with the best route along its associated MRT. If the packet
came from that area, this correctly avoids the failure.
How does an ABR/LBR ensure that MRT-marked packets do not arrive at
the ABR/LBR? There are two different mechanisms depending upon the
forwarding mechanism being used.
If the LDP label encodes the MT-ID as well as the destination, then
the ABR/LBR is responsible for advertising a particular 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 provides L_primary for the default topology, L_blue for the Blue
MRT MT-ID and L_red for the Red MRT 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 default
topology, for the Blue MRT MT-ID, and for the Red MRT MT-ID. The
ABR/LBR installs next-hops from the best area for L_primary based on
the default topology, for L_blue based on the Blue MRT forwarding
topology, and for L_red based on the Red MRT forwarding topology.
Therefore, packets from the non-best area 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 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
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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 the proxy-node on that MRT.
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]<--------------+
(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 4: ABR Forwarding Behavior and MRTs
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The other potential forwarding mechanisms require additional
computation by the penultimate router along the in-local-area MRT
immediately before the ABR/LBR is reached. The penultimate router
can determine that the ABR/LBR will forward the packet out of area/
level and, in that case, the penultimate router can remove the MRT
marking but still forward the packet along the MRT next-hop to reach
the ABR. For instance, in Figure 4, if node H fails, node E has to
put traffic towards prefix p onto the red MRT. But since node D
knows that ABR1 will use a best from another area, it is safe for D
to remove the MRT marking and just send the packet to ABR1 still on
the red MRT but unmarked. 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 a realistic
deployment scenario, OSPF extensions could be considered.
9. Issues with Area Abstraction
MRT fast-reroute provides complete coverage in a area that is
2-connected. Where a failure would partition the network, of course,
no alternate can protect against that failure. Similarly, there are
ways of connecting multi-homed prefixes that make it impractical to
protect them without excessive complexity.
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
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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. The only real way to get it from A to ASBR Y is to explicitly
tunnel it to ASBR Y.
Tunnelling to the backup ASBR 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 to use.
Consider the case in Figure 6. If the link between A and ASBR X (the
preferred border router) fails, A can put the packets to p onto an
MRT alternate, even tunnel it towards ASBR Y. Node B, however, must
not remove 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
The fine details of how to solve multi-area external prefix cases, or
identifying certain cases as too unlikely and too complex to protect
is for further consideration.
10. Partial Deployment and Islands of Compatible MRT FRR routers
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.
First, a computing router S must determine its local island of
compatible MRT fast-reroute routers. A router that has common
forwarding mechanisms and common algorithm and is connected to either
to S or to another router already determined to be in S's local
island can be added to S's local island.
Destinations inside the local island can obviously use MRT
alternates. Destinations outside the local island can be treated
like a multi-homed prefix with caveats to avoid looping. For LDP
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labels including both destination and topology, the routers at the
borders of the local island need to originate labels for the original
FEC and the associated MRT-specific labels. Packets sent to an LDP
label marked as blue or red MRT to a destination outside the local
island will have the last router in the local island swap the label
to one for the destination and forward the packet along the outgoing
interface on the MRT towards a router outside the local island that
was represented by the proxy-node.
For IP in IP encapsulations, remote destinations' loopback addresses
for the MRTs cannot be used, even if they were available. Instead,
the MRT loopback address of the router attached to a proxy-node,
which represents destinations outside the local island, can be used.
Packets sent to the router's MRT loopback address will have their
outer IP header removed and will need to be explicitly forwarded
along the outgoing interface on the MRT towards a router outside the
local island that was represented by the proxy-node. This behavior
requires essentially remembering the MT-ID indicated by the outer IP
address. An alternate option would be to advertise different
loopback addresses to be associated with the proxy-node; the outer IP
address would still be removed but it would indicate the outgoing
interface to use and no lookup would be necessary on the internal IP
address while maintaining MT-ID context.
A key question is which routers outside the MRT island can packets be
forwarded to so that they are not forwarded back into the MRT island.
An example of the necessary network graph transformations are given
in Figure 7. There are two parts to the computation. First, the MRT
island is collapsed into a single node; this assumes that the cost of
transiting the MRT island is nothing and is pessimistic but allows
for simpler computation. Then, for each destination (other than the
MRT island), the routers adjacent to the MRT island are checked to
see if they are loop-free with respect to the MRT island and the
destination. The two loop-free neighbors of the MRT island that are
closest to the destination are selected. Then, a graph of just the
MRT island is augmented with proxy-nodes that are attached via the
outgoing interfaces to the selected loop-free neighbors. Finally,
the MRTs rooted at each proxy-node are computed on that augmented MRT
island graph. Essentially, the MRT island must have a loop-free
neighbor to be able to have an alternate.
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[G]---[E]---(B)---(C)---(D)
| \ | | |
| \ | | |
| \ | | |
[H]---[F]---(A)---(S)----|
(1) Network Graph with Partial Deployment
[E],[F],[G],[H] : No support for MRT-FRR
(A),(B),(C),(D),(S): MRT Island - supports MRT-FRR
[G]---[E]----| |---(B)---(C)---(D)
| \ | | | | |
| \ | ( MRT Island ) [ proxy ] | |
| \ | | | | |
[H]---[F]----| |---(A)---(S)----|
(2) Graph for determining (3) Graph for MRT computation
loop-free neighbors
Figure 7: Computing alternates to destinations outside the MRT Island
Naturally, there are more complicated options to improve coverage,
such as connecting multiple MRT islands across tunnels, but it is not
clear that the additional complexity is necessary.
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 mechanism 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.
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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 of this document.
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 configured period for all routers to be using their SPTs and
traffic to drain from the 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 as quickly as possible.
The installation of the MRTs can be staged such that the affected or
broken MRTs are updated first and then the unbroken.
12. Acknowledgements
The authors would like to thank Hannes Gredler, Jeff Tantsura, Ted
Qian, Kishore Tiruveedhula, Santosh Esale, Nitin Bahadur, Harish
Sitaraman and Raveendra Torvi for their suggestions and review.
13. IANA Considerations
This doument includes no request to IANA.
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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]
Enyedi, G., Atlas, A. and A. Csaszar, "Algorithms for
computing Maximally Redundant Trees for IP/LDP Fast-
Reroute", draft-enyedi-rtgwg-mrt-frr-algorithm-01 (work
in progress), March 2012.
[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.
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-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 (work in progress),
March 2012.
[I-D.ietf-mpls-ldp-multi-topology]
Zhao, Q., Fang, L., Zhou, C., Li, L., and N. So, "LDP
Extensions for Multi Topology Routing",
draft-ietf-mpls-ldp-multi-topology-03 (work in progress),
March 2012.
[I-D.ietf-rtgwg-ipfrr-notvia-addresses]
Bryant, S., Previdi, S., and M. Shand, "IP Fast Reroute
Using Not-via Addresses",
draft-ietf-rtgwg-ipfrr-notvia-addresses-08 (work in
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progress), December 2011.
[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.
[I-D.ietf-rtgwg-ordered-fib]
Shand, M., Bryant, S., Previdi, S., and C. Filsfils,
"Loop-free convergence using oFIB",
draft-ietf-rtgwg-ordered-fib-05 (work in progress),
April 2011.
[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>.
[RFC5715] Shand, M. and S. Bryant, "A Framework for Loop-Free
Convergence", RFC 5715, January 2010.
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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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
Maciek Konstantynowicz
Cisco Systems
Email: maciek@bgp.nu
Russ White
Cisco Systems
Email: russwh@cisco.com
Mike Shand
Email: mike@mshand.org.uk
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