draft-ietf-rtgwg-mrt-frr-architecture-00.txt   draft-ietf-rtgwg-mrt-frr-architecture-01.txt 
Routing Area Working Group A. Atlas, Ed. Routing Area Working Group A. Atlas, Ed.
Internet-Draft R. Kebler Internet-Draft R. Kebler
Intended status: Standards Track M. Konstantynowicz Intended status: Standards Track Juniper Networks
Expires: July 29, 2012 Juniper Networks Expires: September 13, 2012 G. Enyedi
G. Enyedi
A. Csaszar A. Csaszar
Ericsson Ericsson
M. Konstantynowicz
R. White R. White
Cisco Systems Cisco Systems
M. Shand M. Shand
January 26, 2012 March 12, 2012
An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees
draft-ietf-rtgwg-mrt-frr-architecture-00 draft-ietf-rtgwg-mrt-frr-architecture-01
Abstract Abstract
As IP and LDP Fast-Reroute are increasingly deployed, the coverage As IP and LDP Fast-Reroute are increasingly deployed, the coverage
limitations of Loop-Free Alternates are seen as a problem that limitations of Loop-Free Alternates are seen as a problem that
requires a straightforward and consistent solution for IP and LDP, requires a straightforward and consistent solution for IP and LDP,
for unicast and multicast. This draft describes an architecture for unicast and multicast. This draft describes an architecture
based on redundant backup trees where a single failure can cut a 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 point-of-local-repair from the destination only on one of the pair of
redundant trees. redundant trees.
skipping to change at page 2, line 7 skipping to change at page 2, line 7
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 29, 2012. This Internet-Draft will expire on September 13, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 3, line 12 skipping to change at page 3, line 12
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Goals for Extending IP Fast-Reroute coverage beyond LFA . 4 1.1. Goals for Extending IP Fast-Reroute coverage beyond LFA . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 6 3. Maximally Redundant Trees (MRT) . . . . . . . . . . . . . . . 6
4. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . . 8 4. Maximally Redundant Trees (MRT) and Fast-Reroute . . . . . . . 8
4.1. Multi-homed Prefixes . . . . . . . . . . . . . . . . . . . 9 5. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . . . 9
4.2. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . 10 5.1. LDP Unicast Forwarding - Avoid Tunneling . . . . . . . . . 10
4.2.1. LDP Unicast Forwarding - Avoid Tunneling . . . . . . . 11 5.2. IP Unicast Traffic . . . . . . . . . . . . . . . . . . . . 10
4.2.1.1. Protocol Extensions and Considerations: LDP . . . 12 6. Protocol Extensions and Considerations: OSPF and ISIS . . . . 12
4.2.2. IP Unicast Traffic . . . . . . . . . . . . . . . . . . 12 7. Multi-homed Prefixes . . . . . . . . . . . . . . . . . . . . . 14
4.2.2.1. Protocol Extensions and Considerations: OSPF 8. Inter-Area and ABR Forwarding Behavior . . . . . . . . . . . . 15
and ISIS . . . . . . . . . . . . . . . . . . . . . 13 9. Issues with Area Abstraction . . . . . . . . . . . . . . . . . 18
4.2.3. Inter-Area and ABR Forwarding Behavior . . . . . . . . 13 10. Partial Deployment and Islands of Compatible MRT FRR
4.2.4. Issues with Area Abstraction . . . . . . . . . . . . . 15 routers . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2.5. Partial Deployment and Islands of Compatible MRT 11. Network Convergence and Preparing for the Next Failure . . . . 21
FRR routers . . . . . . . . . . . . . . . . . . . . . 16 11.1. Micro-forwarding loop prevention and MRTs . . . . . . . . 21
4.2.6. Network Convergence and Preparing for the Next 11.2. MRT Recalculation . . . . . . . . . . . . . . . . . . . . 22
Failure . . . . . . . . . . . . . . . . . . . . . . . 17 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
4.2.6.1. Micro-forwarding loop prevention and MRTs . . . . 17 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
4.2.6.2. MRT Recalculation . . . . . . . . . . . . . . . . 17 14. Security Considerations . . . . . . . . . . . . . . . . . . . 23
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18 15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 15.1. Normative References . . . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18 15.2. Informative References . . . . . . . . . . . . . . . . . . 23
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
8.1. Normative References . . . . . . . . . . . . . . . . . . . 18
8.2. Informative References . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction 1. Introduction
There is still work required to completely provide IP and LDP Fast- There is still work required to completely provide IP and LDP Fast-
Reroute[RFC5714] for unicast and multicast traffic. This draft Reroute[RFC5714] for unicast and multicast traffic. This draft
proposes an architecture to provide 100% coverage. 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 Loop-free alternates (LFAs)[RFC5286] provide a useful mechanism for
link and node protection but getting complete coverage is quite hard. link and node protection but getting complete coverage is quite hard.
[LFARevisited] defines sufficient conditions to determine if a [LFARevisited] defines sufficient conditions to determine if a
network provides link-protecting LFAs and also proves that augmenting network provides link-protecting LFAs and also proves that augmenting
a network to provide better coverage is NP-hard. a network to provide better coverage is NP-hard.
[I-D.ietf-rtgwg-lfa-applicability] discusses the applicability of LFA [I-D.ietf-rtgwg-lfa-applicability] discusses the applicability of LFA
to different topologies with a focus on common PoP architectures. to different topologies with a focus on common PoP architectures.
While Not-Via [I-D.ietf-rtgwg-ipfrr-notvia-addresses] is defined as While Not-Via [I-D.ietf-rtgwg-ipfrr-notvia-addresses] is defined as
an architecture, in practice, it has proved too complicated and an architecture, in practice, it has proved too complicated and
stateful to spark substantial interest in implementation or stateful to spark substantial interest in implementation or
deployment. Academic implementations [LightweightNotVia] exist and deployment. Academic implementations [LightweightNotVia] exist and
have found the address management complexity high (but no have found the address management complexity high (but no
standardization has been done to reduce this). standardization has been done to reduce this).
A different approach is needed and that is what is described here. A different approach is needed and that is what is described here.
It is based on the idea of using disjoint backup topologies as It is based on the idea of using disjoint backup topologies as
realized by Maximally Redundant Trees (described in realized by Maximally Redundant Trees (described in
[LightweightNotVia]); the general architecture could also apply to [LightweightNotVia]); the general architecture can also apply to
future improved redundant tree algorithms. future improved redundant tree algorithms.
1.1. Goals for Extending IP Fast-Reroute coverage beyond LFA 1.1. Goals for Extending IP Fast-Reroute coverage beyond LFA
Any scheme proposed for extending IPFRR network topology coverage Any scheme proposed for extending IPFRR network topology coverage
beyond LFA, apart from attaining basic IPFRR properties, should also beyond LFA, apart from attaining basic IPFRR properties, should also
aim to achieve the following usability goals: aim to achieve the following usability goals:
o ensure maximum physically feasible link and node disjointness o ensure maximum physically feasible link and node disjointness
regardless of topology, regardless of topology,
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of the shortest-path-first algorithm in tree-building and including of the shortest-path-first algorithm in tree-building and including
all links in the network as possibilities for one path or another all links in the network as possibilities for one path or another
should improve this. Modeling is underway to investigate and compare should improve this. Modeling is underway to investigate and compare
the MRT alternates to the optimal the MRT alternates to the optimal
[I-D.enyedi-rtgwg-mrt-frr-algorithm]. Providing shortest detour [I-D.enyedi-rtgwg-mrt-frr-algorithm]. Providing shortest detour
paths would require failure-specific detour paths to the paths would require failure-specific detour paths to the
destinations, but the state-reduction advantage of MRT lies in the destinations, but the state-reduction advantage of MRT lies in the
detour being established per destination (root) instead of per detour being established per destination (root) instead of per
destination AND per failure. destination AND per failure.
The specific algorithm to compute MRTs as well as the logic behind The specific algorithms to compute MRTs as well as the logic behind
that algorithm and alternative computational approaches are given in that algorithm and alternative computational approaches are given in
detail in [I-D.enyedi-rtgwg-mrt-frr-algorithm]. Those interested are detail in [I-D.enyedi-rtgwg-mrt-frr-algorithm]. Those interested are
highly recommended to read that document. This document describes highly recommended to read that document. This document describes
how the MRTs can be used and not how to compute them. how the MRTs can be used and not how to compute them.
The most important thing to understand about MRTs is that for each The most important thing to understand about MRTs is that for each
pair of destination-routed MRTs, there is a path from every node X to pair of destination-routed MRTs, there is a path from every node X to
the destination D on the Blue MRT that is as disjoint as possible the destination D on the Blue MRT that is as disjoint as possible
from the path on the Red MRT. The two paths along the two MRTs to a 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, given destination-root of a 2-connected graph are node-disjoint and
while in any non-2-connected graph, only the cut-vertices and cut- link-disjoint, while in any non-2-connected graph, only the cut-
edges can be contained by both of the paths. vertices and cut-edges can be contained by both of the paths.
For example, in Figure 1, there is a network graph that is For example, in Figure 1, there is a network graph that is
2-connected in (a) and associated MRTs in (b) and (c). One can 2-connected in (a) and associated MRTs in (b) and (c). One can
consider the paths from B to R; on the Blue MRT, the paths are consider the paths from B to R; on the Blue MRT, the paths are
B->F->D->E->R or B->F->C->E->R. On the Red MRT, the path is B->A->R. B->F->D->E->R or B->C->D->E->R. On the Red MRT, the path is B->A->R.
These are clearly link and node-disjoint. These MRTs are redundant These are clearly link and node-disjoint. These MRTs are redundant
trees because the paths are disjoint. trees because the paths are disjoint.
[E]---[D]---| [E]<--[D]<--| [E]-->[D]---| [E]---[D]---| [E]<--[D]<--| [E]-->[D]---|
| | | | ^ | | | | | | | ^ | | |
| | | V | | V V | | | V | | V V
[R] [F] [C] [R] [F] [C] [R] [F] [C] [R] [F] [C] [R] [F] [C] [R] [F] [C]
| | | ^ ^ ^ | | | | | ^ ^ ^ | |
| | | | | | V | | | | | | | V |
[A]---[B]---| [A]-->[B]---| [A]---[B]<--| [A]---[B]---| [A]-->[B]---| [A]---[B]<--|
(a) (b) (c) (a) (b) (c)
a 2-connected graph Blue MRT towards R Red MRT towards R a 2-connected graph Blue MRT towards R Red MRT towards R
Figure 1: A 2-connected Network Figure 1: A 2-connected Network
By contrast, in Figure 2, the network in (a) is not 2-conneted. If By contrast, in Figure 2, the network in (a) is not 2-connected. If
F, G or the link F<->G failed, then the network would be partitioned. F, G or the link F<->G failed, then the network would be partitioned.
It is clearly impossible to have two link-disjoint or node-disjoint It is clearly impossible to have two link-disjoint or node-disjoint
paths from G, I or J to R. The MRTs given in (b) and (c) offer paths paths from G, I or J to R. The MRTs given in (b) and (c) offer paths
that are as disjoint as possible. For instance, the paths from B to that are as disjoint as possible. For instance, the paths from B to
R are the same as in Figure 1 and the path from G to R on the Blue R are the same as in Figure 1 and the path from G to R on the Blue
MRT is G->F->D->E->R and on the Red MRT is G->F->B->A->R. MRT is G->F->D->E->R and on the Red MRT is G->F->B->A->R.
[E]---[D]---| [E]---[D]---|
| | | |----[I] | | | |----[I]
| | | | | | | | | |
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Figure 2: A non-2-connected network Figure 2: A non-2-connected network
4. Maximally Redundant Trees (MRT) and Fast-Reroute 4. Maximally Redundant Trees (MRT) and Fast-Reroute
In normal IGP routing, each router has its shortest-path-tree to all In normal IGP routing, each router has its shortest-path-tree to all
destinations. From the perspective of a particular destination, D, destinations. From the perspective of a particular destination, D,
this looks like a reverse SPT (rSPT). To use maximally redundant this looks like a reverse SPT (rSPT). To use maximally redundant
trees, in addition, each destination D has two MRTs associated with trees, in addition, each destination D has two MRTs associated with
it; by convention these will be called the blue and red MRTs. 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 MRTs are practical to maintain redundancy even after a single link or
node failure. If a pair of MRTs is computed rooted at each node failure. If a pair of MRTs is computed rooted at each
destination, all the destinations remain reachable along one of the destination, all the destinations remain reachable along one of the
MRTs in the case of a single link or node failure. MRTs in the case of a single link or node failure.
When there is a link or node failure affecting the rSPT, each node 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 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 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 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 could use either the Blue MRT path C->D->E->R or the Red MRT path
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Tunneling[RFC5715], then the whole IGP area needs to have alternates Tunneling[RFC5715], then the whole IGP area needs to have alternates
available so that the micro-loop prevention mechanism, which requires available so that the micro-loop prevention mechanism, which requires
slower network convergence, can take the necessary time without slower network convergence, can take the necessary time without
impacting traffic badly. impacting traffic badly.
As described in [RFC5286], when a worse failure than is anticipated As described in [RFC5286], when a worse failure than is anticipated
happens, using LFAs that are not downstream neighbors can cause happens, using LFAs that are not downstream neighbors can cause
micro-looping. An example is given of link-protecting alternates micro-looping. An example is given of link-protecting alternates
causing a loop on node failure. Even if a worse failure than causing a loop on node failure. Even if a worse failure than
anticipated happened, the use of MRT alternates will not cause anticipated happened, the use of MRT alternates will not cause
looping. Therefore, while node-protecting LFAs may be prefered, looping. Therefore, while node-protecting LFAs may be prefered, an
there are advantages to using MRT alternates when such a node- advantage to using MRT alternates when such a node-protecting LFA is
protecting LFA is not a downstream path. not a downstream path is the certainty that no alternate-induced
looping will occur.
4.1. 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.
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 4.2] and then forward the traffic outside of the area (or
island of MRT-fast-reroute-supporting routers).
When directing traffic along an MRT towards a multi-homed prefix, if
a topology-identifier label[see Section 4.2.1] is not used, then the
proxy-node must be named and either additional LDP labels or IP
addresses associated with it.
4.2. Unicast Forwarding with MRT Fast-Reroute 5. Unicast Forwarding with MRT Fast-Reroute
With LFA, there is no need to tunnel unicast traffic, whether IP or With LFA, there is no need to tunnel unicast traffic, whether IP or
LDP. The traffic is simply sent to an alternate. The behavior with LDP. The traffic is simply sent to an alternate. As mentioned
MRT Fast-Reroute is different depending upon whether IP or LDP earlier in Section 4, MRT needs multi-topology forwarding.
unicast traffic is considered. Unfortunately, neither IP nor LDP provide extra bits for a packet to
indicate its topology.
Logically, one could use the same IP address or LDP FEC and then also
use 2 bits to express the topology to use. The topology options are
(00) IGP/SPT, (01) blue MRT, (10) red MRT. Unfortunately, there just
aren't 2 spare bits available in the IPv4 or IPv6 header. This has
different consequences for IP and LDP because LDP can just add a
topology label on top or take 2 spare bits from the label space.
Once the MRTs are computed, the two sets of MRTs are seen by the Once the MRTs are computed, the two sets of MRTs are seen by the
forwarding plane as essentially two additional topologies. The same forwarding plane as essentially two additional topologies. The same
considerations apply for forwarding along the MRTs as for handling considerations apply for forwarding along the MRTs as for handling
multiple topologies. multiple topologies.
4.2.1. LDP Unicast Forwarding - Avoid Tunneling 5.1. LDP Unicast Forwarding - Avoid Tunneling
For LDP, it is very desirable to avoid tunneling because, for at For LDP, it is very desirable to avoid tunneling because, for at
least node protection, tunneling requires knowledge of remote LDP least node protection, tunneling requires knowledge of remote LDP
label mappings and thus requires targeted LDP sessions and the label mappings and thus requires targeted LDP sessions and the
associated management complexity. There are two different mechanisms associated management complexity. There are two different mechanisms
that can be used. that can be used.
1. Option A - Encode Topology in Labels: In addition to sending a 1. Option A - Encode MT-ID in Labels: In addition to sending a
single label for a FEC, a router would provide two additional single label for a FEC, a router would provide two additional
labels with their associated MRT colors. This is simple, but labels with the MT-IDs associated with the Blue MRT or Red MRT
reduces the label space for other uses. It also increases the forwarding topologies. This is very simple for hardware support.
memory to store the labels and the communication required by LDP. 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- 2. Option B - Create Topology-Identification Labels: Use the label-
stacking ability of MPLS and specify only two additional labels - stacking ability of MPLS and specify only two additional labels -
one for each associated MRT color - by a new FEC type. When one for each associated MRT color - by a new FEC type. When
sending a packet onto an MTR, first swap the LDP label and then sending a packet onto an MRT, first swap the LDP label and then
push the topology-identification label for that MTR color. When push the topology-identification label for that MRT color. When
receiving a packet with a topology-identification label, pop it receiving a packet with a topology-identification label, pop it
and use it to guide the next-hop selection in combination with and use it to guide the next-hop selection in combination with
the next label in the stack; then swap the remaining label, if the next label in the stack; then swap the remaining label, if
appropriate, and push the topology-identification label for the appropriate, and push the topology-identification label for the
next-hop. This has minimal usage of additional labels, memory next-hop. This has minimal usage of additional labels, memory
and LDP communication. It does increase the size of packets and and LDP communication. It does increase the size of packets and
the complexity of the required label operations and look-ups. the complexity of the required label operations and look-ups.
This can use the same mechanisms as are needed for context-aware This can use the same mechanisms as are needed for context-aware
label spaces. label spaces.
Note that with LDP unicast forwarding, regardless of whether Note that with LDP unicast forwarding, regardless of whether
topology-identification label or encoding topology in label is used, topology-identification label or encoding topology in label is used,
no additional loopbacks per router are required as are required in no additional loopbacks per router are required. This is because LDP
the IP unicast forwarding case. This is because LDP labels are used labels are used on a hop-by-hop basis to identify MRT-blue and MRT-
on a hop-by-hop basis to identify MRT-blue and MRT-red forwarding red forwading topologies.
trees.
For greatest hardware compatibility, routers should support Option B
of encoding the topology in the labels.
4.2.1.1. Protocol Extensions and Considerations: LDP
This captures an initial understanding of what may need to be
specified.
1. Specify Topology in Label: When sending a Label Mapping, have the
ability to send a Label TLV and multiple Topology-Label TLVs.
The Topology-Label TLV would specify MRT and the associated MRT
color.
2. Topology-Identification Labels: Define a new FEC type that For greatest hardware compatibility, routers implementing MRT LDP
describes the topology for MRT and the associated MRT color. 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]
4.2.2. IP Unicast Traffic 5.2. IP Unicast Traffic
For IP, there is no currently practical alternative except tunneling. For IP, there is no currently practical alternative except tunneling.
The tunnel egress could be the original destination in the area, the The tunnel egress could be the original destination in the area, the
next-next-hop, etc.. If the tunnel egress is the original next-next-hop, etc.. If the tunnel egress is the original
destination router, then the traffic remains on the redundant tree destination router, then the traffic remains on the redundant tree
with sub-optimal routing. If the tunnel egress is the next-next-hop, 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 then protection of multi-homed prefixes and node-failure for ABRs is
not available. Selection of the tunnel egress is a router-local not available. Selection of the tunnel egress is a router-local
decision. decision.
skipping to change at page 13, line 15 skipping to change at page 11, line 40
2. Tunnel IP packets in IP. Each router supporting this option 2. Tunnel IP packets in IP. Each router supporting this option
would announce two additional loopback addresses and their would announce two additional loopback addresses and their
associated MRT color. Those addresses are used as destination associated MRT color. Those addresses are used as destination
addresses for MRT-blue and MRT-red IP tunnels respectively. They addresses for MRT-blue and MRT-red IP tunnels respectively. They
allow the transit nodes to identify the traffic as being allow the transit nodes to identify the traffic as being
forwarded along either MRT-blue or MRT-red tree topology to reach forwarded along either MRT-blue or MRT-red tree topology to reach
the tunnel destination. Announcements of these two additional the tunnel destination. Announcements of these two additional
loopback addresses per router with their MRT color requires IGP loopback addresses per router with their MRT color requires IGP
extensions. extensions.
For proxy-nodes associated with one or more multi-homed prefixes, the For greatest hardware compatibility and ease in removing the MRT-
problem is harder because there is no router associated with the topology marking at area/level boundaries, routers that support MPLS
proxy-node, so its loopbacks can't be known or used. In this case, and implement IP MRT fast-reroute SHOULD support Option A - using an
each router attached to the proxy-node could announce two common IP LDP label that indicates the destination and MT-ID.
addresses with their associated MRT colors. This would require
configuration as well as the previously mentioned IGP extensions.
Similarly, in the LDP case, two additional FEC bindings could be
announced.
4.2.2.1. Protocol Extensions and Considerations: OSPF and ISIS 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
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 This captures an initial understanding of what may need to be
specified. 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.
o Capabilities: Does a router support MRT? Does the router do MRT MRT Island Creation ID: This identifies the process that the router
tunneling with LDP or IP or GRE or...? 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.
o Topology Association: A router needs to advertise a loopback and MRT Algorithm ID: This identifies the particular MRT algorithm used
associate it with an MRT whether blue or red. Additional by the router. By having an Algorithm ID, it is possible to
flexibility for future uses would be good. 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.
o Proxy-nodes for Multi-homed Prefixes: We need a way to advertise Red MRT MT-ID: This specifies the MT-ID to be associated with the
common addresses with MRT for multi-homed prefixes' proxy-nodes. Red MRT forwarding topology. It is needed for use in signaling.
Currently, those proxy-nodes aren't named or considered. 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
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/
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 As with LFA, it is expected that OSPF Virtual Links will not be
supported. supported.
4.2.3. Inter-Area and ABR Forwarding Behavior 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.
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 In regular forwarding, packets destined outside the area arrive at
the ABR and the ABR forwards them into the other area because the 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- 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 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. with packets marked with an MRT that are received by the ABR.
The only option that doesn't require forwarding based upon incoming For unicast fast-reroute, the need to stay on an MRT forwarding
interface is to forward an MRT marked packet in the area with the topology terminates at the ABR/LBR whose best route is via a
best route along its associated MRT. If the packet came from that different area/level. It is highly desirable to go back to the
area, this correctly avoids the failure. If the packet came from a default forwarding topology when leaving an area/level. There are
different area, at least this gets the packet to the destination even three basic reasons for this. First, the default topology uses
though it is along an MRT rather than the shortest-path. 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
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] +----[C]---- --[D]--[E] --[D]--[E]
| \ / \ / \ | \ / \ / \
p--[A] Area 10 [ABR1] Area 0 [H]--p +-[ABR1] Area 0 [H]-+ p--[A] Area 10 [ABR1] Area 0 [H]--p +-[ABR1] Area 0 [H]-+
| / \ / | \ / | | / \ / | \ / |
+----[B]---- --[F]--[G] | --[F]--[G] | +----[B]---- --[F]--[G] | --[F]--[G] |
| | | |
| other | | other |
+----------[p]-------+ +----------[p]-------+
area area
skipping to change at page 14, line 44 skipping to change at page 18, line 5
/ | | \ / | | \
[F]->[G] V V -<[F]<-[G] [F]->[G] V V -<[F]<-[G]
| | | |
| | | |
[p]<------+ +--------->[p] [p]<------+ +--------->[p]
(d) Blue MRT in Area 0 (e) Red MRT in Area 0 (d) Blue MRT in Area 0 (e) Red MRT in Area 0
Figure 4: ABR Forwarding Behavior and MRTs Figure 4: ABR Forwarding Behavior and MRTs
To avoid using an out-of-area MRT, special action can be taken by the The other potential forwarding mechanisms require additional
penultimate router along the in-local-area MRT immediately before the computation by the penultimate router along the in-local-area MRT
ABR is reached. The penultimate router can determine that the ABR immediately before the ABR/LBR is reached. The penultimate router
will forward the packet out of area and, in that case, the can determine that the ABR/LBR will forward the packet out of area/
penultimate router can remove the MRT marking but still forward the level and, in that case, the penultimate router can remove the MRT
packet along the MRT next-hop to reach the ABR. For instance, in marking but still forward the packet along the MRT next-hop to reach
Figure 4, if node H fails, node E has to put traffic towards prefix p the ABR. For instance, in Figure 4, if node H fails, node E has to
onto the red MRT. But since node D knows that ABR1 will use a best put traffic towards prefix p onto the red MRT. But since node D
from another area, it is safe for D to remove the MRT marking and knows that ABR1 will use a best from another area, it is safe for D
just send the packet to ABR1 still on the red MRT but unmarked. ABR1 to remove the MRT marking and just send the packet to ABR1 still on
will use the shortest path in Area 10. 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 In all cases for ISIS and most cases for OSPF, the penultimate router
can determine what decision the adjacent ABR will make. The one case can determine what decision the adjacent ABR will make. The one case
where it can't be determined is when two ASBRs are in different non- where it can't be determined is when two ASBRs are in different non-
backbone areas attached to the same ABR, then the ASBR's Area ID may backbone areas attached to the same ABR, then the ASBR's Area ID may
be needed for tie-breaking (prefer the route with the largest OPSF be needed for tie-breaking (prefer the route with the largest OPSF
area ID) and the Area ID isn't announced as part of the ASBR link- area ID) and the Area ID isn't announced as part of the ASBR link-
state advertisement (LSA). In this one case, suboptimal forwarding state advertisement (LSA). In this one case, suboptimal forwarding
along the MRT in the other area would happen. If this is a realistic along the MRT in the other area would happen. If this is a realistic
deployment scenario, OSPF extensions could be considered. deployment scenario, OSPF extensions could be considered.
4.2.4. Issues with Area Abstraction 9. Issues with Area Abstraction
MRT fast-reroute provides complete coverage in a area that is MRT fast-reroute provides complete coverage in a area that is
2-connected. Where a failure would partition the network, of course, 2-connected. Where a failure would partition the network, of course,
no alternate can protect against that failure. Similarly, there are no alternate can protect against that failure. Similarly, there are
ways of connecting multi-homed prefixes that make it impractical to ways of connecting multi-homed prefixes that make it impractical to
protect them without excessive complexity. protect them without excessive complexity.
50 50
|----[ASBR Y]---[B]---[ABR 2]---[C] Backbone Area 0: |----[ASBR Y]---[B]---[ABR 2]---[C] Backbone Area 0:
| | ABR 1, ABR 2, C, D | | ABR 1, ABR 2, C, D
skipping to change at page 16, line 26 skipping to change at page 19, line 30
p ---[ASBR X]-X-[A]---[B]---[ABR 1]---[D]---[ASBR Y]--- p p ---[ASBR X]-X-[A]---[B]---[ABR 1]---[D]---[ASBR Y]--- p
BGP prefers ASBR X for prefix p BGP prefers ASBR X for prefix p
Figure 6: Failure of path towards ASBR preferred by BGP Figure 6: Failure of path towards ASBR preferred by BGP
The fine details of how to solve multi-area external prefix cases, or The fine details of how to solve multi-area external prefix cases, or
identifying certain cases as too unlikely and too complex to protect identifying certain cases as too unlikely and too complex to protect
is for further consideration. is for further consideration.
4.2.5. Partial Deployment and Islands of Compatible MRT FRR routers 10. Partial Deployment and Islands of Compatible MRT FRR routers
A natural concern with new functionality is how to have it be useful 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 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 MRT FRR, where it provides alternates when appropriate LFAs aren't
available, there are also deployment scenarios where it may make available, there are also deployment scenarios where it may make
sense to only enable some routers in an area with MRT FRR. A simple 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 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. is connected via two routers to the rest of the area.
First, a computing router S must determine its local island of First, a computing router S must determine its local island of
skipping to change at page 17, line 7 skipping to change at page 20, line 13
like a multi-homed prefix with caveats to avoid looping. For LDP like a multi-homed prefix with caveats to avoid looping. For LDP
labels including both destination and topology, the routers at the labels including both destination and topology, the routers at the
borders of the local island need to originate labels for the original borders of the local island need to originate labels for the original
FEC and the associated MRT-specific labels. Packets sent to an LDP 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 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 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 to one for the destination and forward the packet along the outgoing
interface on the MRT towards a router outside the local island that interface on the MRT towards a router outside the local island that
was represented by the proxy-node. was represented by the proxy-node.
For IP in IP encapsulations, remote destinations may not be For IP in IP encapsulations, remote destinations' loopback addresses
advertising additional IP loopback addresses for the MRTs. In that for the MRTs cannot be used, even if they were available. Instead,
case, a router attached to a proxy-node, which represents the MRT loopback address of the router attached to a proxy-node,
destinations outside the local island, must advertise IP addresses which represents destinations outside the local island, can be used.
associated with that proxy-node. Packets sent to an address Packets sent to the router's MRT loopback address will have their
associated with a proxy-node will have their outer IP header removed outer IP header removed and will need to be explicitly forwarded
by the router attached to the proxy-node and be forwarded by the along the outgoing interface on the MRT towards a router outside the
router along the outgoing interface on the MRT towards a router local island that was represented by the proxy-node. This behavior
outside the local island that was represented by the proxy-node. 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.
4.2.6. Network Convergence and Preparing for the Next Failure 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.
[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 After a failure, MRT detours ensure that packets reach their intended
destination while the IGP has not reconverged onto the new topology. destination while the IGP has not reconverged onto the new topology.
As link-state updates reach the routers, the IGP process calculates As link-state updates reach the routers, the IGP process calculates
the new shortest paths. Two things need attention: micro-loop the new shortest paths. Two things need attention: micro-loop
prevention and MRT re-calculation. prevention and MRT re-calculation.
4.2.6.1. Micro-forwarding loop prevention and MRTs 11.1. Micro-forwarding loop prevention and MRTs
As is well known[RFC5715], micro-loops can occur during IGP As is well known[RFC5715], micro-loops can occur during IGP
convergence; such loops can be local to the failure or remote from convergence; such loops can be local to the failure or remote from
the failure. Managing micro-loops is an orthogonal issue to having the failure. Managing micro-loops is an orthogonal issue to having
alternates for local repair, such as MRT fast-reroute provides. alternates for local repair, such as MRT fast-reroute provides.
There are two possible micro-loop prevention mechanism discussed in There are two possible micro-loop prevention mechanism discussed in
[RFC5715]. The first is Ordered FIB [I-D.ietf-rtgwg-ordered-fib]. [RFC5715]. The first is Ordered FIB [I-D.ietf-rtgwg-ordered-fib].
The second is Farside Tunneling which requires tunnels or an The second is Farside Tunneling which requires tunnels or an
alternate topology to reach routers on the farside of the failure. alternate topology to reach routers on the farside of the failure.
Since MRTs provide an alternate topology through which traffic can be Since MRTs provide an alternate topology through which traffic can be
sent and which can be manipulated separately from the SPT, it is sent and which can be manipulated separately from the SPT, it is
possible that MRTs could be used to support Farside Tunneling. possible that MRTs could be used to support Farside Tunneling.
Details of how to do so are outside of this document. Details of how to do so are outside of this document.
4.2.6.2. MRT Recalculation 11.2. MRT Recalculation
When a failure event happens, traffic is put by the PLRs onto the MRT When a failure event happens, traffic is put by the PLRs onto the MRT
topologies. After that, each router recomputes its shortest path topologies. After that, each router recomputes its shortest path
tree (SPT) and moves traffic over to that. Only after all the PLRs tree (SPT) and moves traffic over to that. Only after all the PLRs
have switched to using their SPTs and traffic has drained from the have switched to using their SPTs and traffic has drained from the
MRT topologies should each router install the recomputed MRTs into MRT topologies should each router install the recomputed MRTs into
the FIBs. the FIBs.
At each router, therefore, the sequence is as follows: At each router, therefore, the sequence is as follows:
skipping to change at page 18, line 22 skipping to change at page 22, line 38
5. Wait configured period for all routers to be using their SPTs and 5. Wait configured period for all routers to be using their SPTs and
traffic to drain from the MRTs. traffic to drain from the MRTs.
6. Install new MRTs. 6. Install new MRTs.
While the recomputed MRTs are not installed in the FIB, protection While the recomputed MRTs are not installed in the FIB, protection
coverage is lowered. Therefore, it is important to recalculate the coverage is lowered. Therefore, it is important to recalculate the
MRTs and install them as quickly as possible. MRTs and install them as quickly as possible.
It is for further study whether MRT re-calculation is possible in an The installation of the MRTs can be staged such that the affected or
incremental fashion, such that the sections of the MRT in use after a broken MRTs are updated first and then the unbroken.
failure are not changed.
5. Acknowledgements 12. Acknowledgements
The authors would like to thank Hannes Gredler, Jeff Tantsura, Ted The authors would like to thank Hannes Gredler, Jeff Tantsura, Ted
Qian, Kishore Tiruveedhula, Santosh Esale, Nitin Bahadur, Harish Qian, Kishore Tiruveedhula, Santosh Esale, Nitin Bahadur, Harish
Sitaraman and Raveendra Torvi for their suggestions and review. Sitaraman and Raveendra Torvi for their suggestions and review.
6. IANA Considerations 13. IANA Considerations
This doument includes no request to IANA. This doument includes no request to IANA.
7. Security Considerations 14. Security Considerations
This architecture is not currently believed to introduce new security This architecture is not currently believed to introduce new security
concerns. concerns.
8. References 15. References
8.1. Normative References 15.1. Normative References
[I-D.enyedi-rtgwg-mrt-frr-algorithm] [I-D.enyedi-rtgwg-mrt-frr-algorithm]
Atlas, A., Envedi, G., and A. Csaszar, "Algorithms for Enyedi, G., Atlas, A. and A. Csaszar, "Algorithms for
computing Maximally Redundant Trees for IP/LDP Fast- computing Maximally Redundant Trees for IP/LDP Fast-
Reroute", draft-enyedi-rtgwg-mrt-frr-algorithm-00 (work in Reroute", draft-enyedi-rtgwg-mrt-frr-algorithm-01 (work
progress), October 2011. in progress), March 2012.
[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast [RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
Reroute: Loop-Free Alternates", RFC 5286, September 2008. Reroute: Loop-Free Alternates", RFC 5286, September 2008.
[RFC5384] Boers, A., Wijnands, I., and E. Rosen, "The Protocol
Independent Multicast (PIM) Join Attribute Format",
RFC 5384, November 2008.
[RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework", [RFC5714] Shand, M. and S. Bryant, "IP Fast Reroute Framework",
RFC 5714, January 2010. RFC 5714, January 2010.
8.2. Informative References 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] [I-D.ietf-rtgwg-ipfrr-notvia-addresses]
Bryant, S., Previdi, S., and M. Shand, "IP Fast Reroute Bryant, S., Previdi, S., and M. Shand, "IP Fast Reroute
Using Not-via Addresses", Using Not-via Addresses",
draft-ietf-rtgwg-ipfrr-notvia-addresses-08 (work in draft-ietf-rtgwg-ipfrr-notvia-addresses-08 (work in
progress), December 2011. progress), December 2011.
[I-D.ietf-rtgwg-lfa-applicability] [I-D.ietf-rtgwg-lfa-applicability]
Filsfils, C. and P. Francois, "LFA applicability in SP Filsfils, C. and P. Francois, "LFA applicability in SP
networks", draft-ietf-rtgwg-lfa-applicability-06 (work in networks", draft-ietf-rtgwg-lfa-applicability-06 (work in
skipping to change at page 20, line 22 skipping to change at page 25, line 4
Email: akatlas@juniper.net Email: akatlas@juniper.net
Robert Kebler Robert Kebler
Juniper Networks Juniper Networks
10 Technology Park Drive 10 Technology Park Drive
Westford, MA 01886 Westford, MA 01886
USA USA
Email: rkebler@juniper.net Email: rkebler@juniper.net
Maciek Konstantynowicz
Juniper Networks
Email: maciek@juniper.net
Gabor Sandor Enyedi Gabor Sandor Enyedi
Ericsson Ericsson
Konyves Kalman krt 11. Konyves Kalman krt 11.
Budapest 1097 Budapest 1097
Hungary Hungary
Email: Gabor.Sandor.Enyedi@ericsson.com Email: Gabor.Sandor.Enyedi@ericsson.com
Andras Csaszar Andras Csaszar
Ericsson Ericsson
Konyves Kalman krt 11 Konyves Kalman krt 11
Budapest 1097 Budapest 1097
Hungary Hungary
Email: Andras.Csaszar@ericsson.com Email: Andras.Csaszar@ericsson.com
Maciek Konstantynowicz
Cisco Systems
Email: maciek@bgp.nu
Russ White Russ White
Cisco Systems Cisco Systems
Email: russwh@cisco.com Email: russwh@cisco.com
Mike Shand Mike Shand
Email: mike@mshand.org.uk Email: mike@mshand.org.uk
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