Routing Area Working Group A. Atlas, Ed.
Internet-Draft R. Kebler
Intended status: Standards Track M. Konstantynowicz
Expires: July 29, 2012 Juniper Networks
G. Enyedi
A. Csaszar
Ericsson
R. White
Cisco Systems
M. Shand
January 26, 2012
An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees
draft-ietf-rtgwg-mrt-frr-architecture-00
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 July 29, 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
Provisions Relating to IETF Documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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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
4.1. Multi-homed Prefixes . . . . . . . . . . . . . . . . . . . 9
4.2. Unicast Forwarding with MRT Fast-Reroute . . . . . . . . . 10
4.2.1. LDP Unicast Forwarding - Avoid Tunneling . . . . . . . 11
4.2.1.1. Protocol Extensions and Considerations: LDP . . . 12
4.2.2. IP Unicast Traffic . . . . . . . . . . . . . . . . . . 12
4.2.2.1. Protocol Extensions and Considerations: OSPF
and ISIS . . . . . . . . . . . . . . . . . . . . . 13
4.2.3. Inter-Area and ABR Forwarding Behavior . . . . . . . . 13
4.2.4. Issues with Area Abstraction . . . . . . . . . . . . . 15
4.2.5. Partial Deployment and Islands of Compatible MRT
FRR routers . . . . . . . . . . . . . . . . . . . . . 16
4.2.6. Network Convergence and Preparing for the Next
Failure . . . . . . . . . . . . . . . . . . . . . . . 17
4.2.6.1. Micro-forwarding loop prevention and MRTs . . . . 17
4.2.6.2. MRT Recalculation . . . . . . . . . . . . . . . . 17
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Normative References . . . . . . . . . . . . . . . . . . . 18
8.2. Informative References . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
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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.
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 could 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,
o enable gradual introduction of the new scheme and backward
compatibility,
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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 algorithm 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,
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->F->C->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-conneted. 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.
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.
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
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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,
there are advantages to using MRT alternates when such a node-
protecting LFA is not a downstream path.
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
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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
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
MRT Fast-Reroute is different depending upon whether IP or LDP
unicast traffic is considered.
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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
forwarding plane as essentially two additional topologies. The same
considerations apply for forwarding along the MRTs as for handling
multiple topologies.
4.2.1. 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 Topology in Labels: In addition to sending a
single label for a FEC, a router would provide two additional
labels with their associated MRT colors. This is simple, but
reduces 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 MTR, first swap the LDP label and then
push the topology-identification label for that MTR 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 as are required in
the IP unicast forwarding case. This is because LDP labels are used
on a hop-by-hop basis to identify MRT-blue and MRT-red forwarding
trees.
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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
describes the topology for MRT and the associated MRT color.
4.2.2. 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
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 ].
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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 proxy-nodes associated with one or more multi-homed prefixes, the
problem is harder because there is no router associated with the
proxy-node, so its loopbacks can't be known or used. In this case,
each router attached to the proxy-node could announce two common IP
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
This captures an initial understanding of what may need to be
specified.
o Capabilities: Does a router support MRT? Does the router do MRT
tunneling with LDP or IP or GRE or...?
o Topology Association: A router needs to advertise a loopback and
associate it with an MRT whether blue or red. Additional
flexibility for future uses would be good.
o Proxy-nodes for Multi-homed Prefixes: We need a way to advertise
common addresses with MRT for multi-homed prefixes' proxy-nodes.
Currently, those proxy-nodes aren't named or considered.
As with LFA, it is expected that OSPF Virtual Links will not be
supported.
4.2.3. 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.
The only option that doesn't require forwarding based upon incoming
interface is to forward an MRT marked packet in the area with the
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best route along its associated MRT. If the packet came from that
area, this correctly avoids the failure. If the packet came from a
different area, at least this gets the packet to the destination even
though it is along an MRT rather than the shortest-path.
+----[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
To avoid using an out-of-area MRT, special action can be taken by the
penultimate router along the in-local-area MRT immediately before the
ABR is reached. The penultimate router can determine that the ABR
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will forward the packet out of area 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.
4.2.4. 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
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.
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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.
4.2.5. 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
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
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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 may not be
advertising additional IP loopback addresses for the MRTs. In that
case, a router attached to a proxy-node, which represents
destinations outside the local island, must advertise IP addresses
associated with that proxy-node. Packets sent to an address
associated with a proxy-node will have their outer IP header removed
by the router attached to the proxy-node and be forwarded by the
router along the outgoing interface on the MRT towards a router
outside the local island that was represented by the proxy-node.
4.2.6. 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.
4.2.6.1. 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.
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.
4.2.6.2. 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:
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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.
It is for further study whether MRT re-calculation is possible in an
incremental fashion, such that the sections of the MRT in use after a
failure are not changed.
5. 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.
6. IANA Considerations
This doument includes no request to IANA.
7. Security Considerations
This architecture is not currently believed to introduce new security
concerns.
8. References
8.1. Normative References
[I-D.enyedi-rtgwg-mrt-frr-algorithm]
Atlas, A., Envedi, G., and A. Csaszar, "Algorithms for
computing Maximally Redundant Trees for IP/LDP Fast-
Reroute", draft-enyedi-rtgwg-mrt-frr-algorithm-00 (work in
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progress), October 2011.
[RFC5286] Atlas, A. and A. Zinin, "Basic Specification for IP Fast
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",
RFC 5714, January 2010.
8.2. Informative References
[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
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.
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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
Maciek Konstantynowicz
Juniper Networks
Email: maciek@juniper.net
Gabor Sandor Enyedi
Ericsson
Konyves Kalman krt 11.
Budapest 1097
Hungary
Email: Gabor.Sandor.Enyedi@ericsson.com
Andras Csaszar
Ericsson
Konyves Kalman krt 11
Budapest 1097
Hungary
Email: Andras.Csaszar@ericsson.com
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Russ White
Cisco Systems
Email: russwh@cisco.com
Mike Shand
Email: mike@mshand.org.uk
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