One document matched: draft-ietf-rtgwg-mrt-frr-architecture-07.xml
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<rfc category="std" docName="draft-ietf-rtgwg-mrt-frr-architecture-07" ipr="trust200902">
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<!-- ***** FRONT MATTER ***** -->
<front>
<!-- The abbreviated title is used in the page header - it is only necessary if the
full title is longer than 39 characters -->
<title abbrev="MRT Unicast FRR Architecture">An Architecture for IP/LDP Fast-Reroute Using Maximally Redundant Trees</title>
<!-- add 'role="editor"' below for the editors if appropriate -->
<!-- Another author who claims to be an editor -->
<author fullname="Alia Atlas" initials="A.K.A." role="editor" surname="Atlas">
<organization>Juniper Networks</organization>
<address>
<postal>
<street>10 Technology Park Drive</street>
<city>Westford</city>
<region>MA</region>
<code>01886</code>
<country>USA</country>
</postal>
<email>akatlas@juniper.net</email>
</address>
</author>
<author fullname="Robert Kebler" initials="R.K." surname="Kebler">
<organization>Juniper Networks</organization>
<address>
<postal>
<street>10 Technology Park Drive</street>
<city>Westford</city>
<region>MA</region>
<code>01886</code>
<country>USA</country>
</postal>
<email>rkebler@juniper.net</email>
</address>
</author>
<author fullname="Chris Bowers" initials="C." surname="Bowers">
<organization>Juniper Networks</organization>
<address>
<postal>
<street>1194 N. Mathilda Ave.</street>
<city>Sunnyvale</city>
<region>CA</region>
<code>94089</code>
<country>USA</country>
</postal>
<email>cbowers@juniper.net</email>
</address>
</author>
<author fullname="Gábor Sándor Enyedi" initials="G.S.E." surname="Enyedi">
<organization>Ericsson</organization>
<address>
<postal>
<street>Konyves Kalman krt 11.</street>
<city>Budapest</city>
<country>Hungary</country>
<code>1097</code>
</postal>
<email>Gabor.Sandor.Enyedi@ericsson.com</email>
</address>
</author>
<author fullname="András Császár" initials="A.C." surname="Császár">
<organization>Ericsson</organization>
<address>
<postal>
<street>Konyves Kalman krt 11</street>
<city>Budapest</city>
<country>Hungary</country>
<code>1097</code>
</postal>
<email>Andras.Csaszar@ericsson.com</email>
</address>
</author>
<author fullname="Jeff Tantsura" initials="J.T." surname="Tantsura">
<organization>Ericsson</organization>
<address>
<postal>
<street>300 Holger Way</street>
<city>San Jose</city>
<region>CA</region>
<code>95134</code>
<country>USA</country>
</postal>
<email>jeff.tantsura@ericsson.com</email>
</address>
</author>
<author fullname="Russ White" initials="R.W." surname="White">
<organization>VCE</organization>
<address>
<email>russw@riw.us</email>
</address>
</author>
<date day="15" month="October" year="2015"/>
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<!-- Meta-data Declarations -->
<area>Routing</area>
<workgroup>Routing Area Working Group</workgroup>
<abstract>
<t>With increasing deployment of Loop-Free Alternates (LFA) <xref
target="RFC5286"/>, it is clear that a complete solution for IP
and LDP Fast-Reroute is required. This specification provides
that solution. IP/LDP Fast-Reroute with Maximally Redundant
Trees (MRT-FRR) is a technology that gives link-protection and
node-protection with 100% coverage in any network topology that
is still connected after the failure.
</t>
<t>MRT removes all need to engineer for coverage. MRT is also
extremely computationally efficient. For any router in the
network, the MRT computation is less than the LFA computation for
a node with three or more neighbors.
</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>This document gives a complete solution for IP/LDP
fast-reroute <xref target="RFC5714"/>. MRT-FRR creates two
alternate trees separate from the primary next-hop forwarding
used during stable operation. These two trees are maximally
diverse from each other, providing link and node protection for
100% of paths and failures as long as the failure does not cut
the network into multiple pieces. This document defines the
architecture for IP/LDP fast-reroute with MRT. The associated
protocol extensions are defined in <xref
target="I-D.ietf-ospf-mrt"/> and <xref
target="I-D.ietf-mpls-ldp-mrt"/>. The exact MRT algorithm is
defined in <xref target="I-D.ietf-rtgwg-mrt-frr-algorithm"/>.
</t>
<t>IP/LDP Fast-Reroute with MRT (MRT-FRR) uses two maximally
diverse forwarding topologies to provide alternates. A primary
next-hop should be on only one of the diverse forwarding
topologies; thus, the other can be used to provide an alternate.
Once traffic has been moved to one of MRTs, it is not subject to
further repair actions. Thus, the traffic will not loop even if
a worse failure (e.g. node) occurs when protection was only
available for a simpler failure (e.g. link).
</t>
<t>In addition to supporting IP and LDP unicast fast-reroute, the
diverse forwarding topologies and guarantee of 100% coverage
permit fast-reroute technology to be applied to multicast traffic
as described in <xref target="I-D.atlas-rtgwg-mrt-mc-arch"/>.
</t>
<t>Other existing or proposed solutions are partial solutions
or have significant issues, as described below.</t>
<texttable anchor="table_comparison">
<preamble>Summary Comparison of IP/LDP FRR Methods</preamble>
<ttcol align='center'>Method</ttcol>
<ttcol align='center'>Coverage</ttcol>
<ttcol align='center'>Alternate Looping?</ttcol>
<ttcol align='center'>Computation (in SPFs)</ttcol>
<c>MRT-FRR</c><c>100% Link/Node</c> <c>None</c> <c> less than 3</c>
<c/><c/><c/><c/>
<c>LFA</c> <c>Partial Link/Node</c> <c>Possible</c> <c>per neighbor</c>
<c/><c/><c/><c/>
<c>Remote LFA</c> <c>Partial Link/Node</c> <c> Possible</c>
<c> per neighbor (link) or neighbor's neighbor (node) </c>
<c/><c/><c/><c/>
<c>Not-Via</c><c>100% Link/Node</c> <c>None</c> <c>per link and node</c>
<c/><c/><c/><c/>
<c>TI-LFA</c><c>100% Link/Node</c> <c>Possible</c>
<c> per neighbor (link) or neighbor's neighbor (node) </c>
</texttable>
<t><list style="hanging">
<t hangText="Loop-Free Alternates (LFA): "> LFAs <xref
target="RFC5286"/> provide limited topology-dependent coverage
for link and node protection. Restrictions on choice of
alternates can be relaxed to improve coverage, but this can
cause forwarding loops if a worse failure is experienced than
protected against. Augmenting a network to provide better
coverage is NP-hard <xref target="LFARevisited"/>. <xref
target="RFC6571"/> discusses the applicability of LFA to
different topologies with a focus on common PoP
architectures.</t>
<t hangText="Remote LFA: " > Remote LFAs <xref
target="RFC7490"/> improve coverage over LFAs
for link protection but still cannot guarantee complete
coverage. The trade-off of looping traffic to improve coverage
is still made. Remote LFAs can provide node-protection <xref
target="I-D.ietf-rtgwg-rlfa-node-protection"/> but not
guaranteed coverage and the computation required is quite high
(an SPF for each PQ-node evaluated). <xref
target="I-D.bryant-ipfrr-tunnels"/> describes additional
mechanisms to further improve coverage, at the cost of added
complexity.</t>
<t hangText="Not-Via: ">Not-Via <xref
target="I-D.ietf-rtgwg-ipfrr-notvia-addresses"/> is the only
other solution that provides 100% coverage for link and node
failures and does not have potential looping. However, the
computation is very high (an SPF per failure point) and
academic implementations <xref target="LightweightNotVia"/>
have found the address management complexity to be high.</t>
<t hangText="TI-LFA: ">Topology Independent Loop-free Alternate
Fast Re-route (TI-LFA) <xref
target="I-D.francois-spring-segment-routing-ti-lfa"/> aims to
provide link and node protection of node and adjacency segments
within the Segment Routing (SR) framework. It guarantees
complete coverage. The TI-LFA computation for link-protection
is fairly straightforward, while the computation for
node-protection is more complex. For link-protection with
symmetric link costs, TI-LFA can provide complete coverage by
pushing up to two additional labels. For node protection on
arbitrary topologies, the label stack size can grow
significantly based on repair path. Note that TI-LFA requires
shortest path forwarding based on SR Node-SIDs, as opposed to
LDP labels, in order to construct label stacks for backups
paths without relying on a large number of targeted LDP
sessions to learn remote FEC-label bindings. It also requires
the use of Adj-SIDs to achieve 100% coverage. </t>
</list></t>
<section title="Importance of 100% Coverage">
<t>Fast-reroute is based upon the single failure assumption -
that the time between single failures is long enough for a
network to reconverge and start forwarding on the new shortest
paths. That does not imply that the network will only
experience one failure or change.
</t>
<t>It is straightforward to analyze a particular network
topology for coverage. However, a real network does not always
have the same topology. For instance, maintenance events will
take links or nodes out of use. Simply costing out a link can
have a significant effect on what LFAs are
available. Similarly, after a single failure has happened, the
topology is changed and its associated coverage. Finally, many
networks have new routers or links added and removed; each of
those changes can have an effect on the coverage for
topology-sensitive methods such as LFA and Remote LFA. If
fast-reroute is important for the network services provided,
then a method that guarantees 100% coverage is important to
accomodate natural network topology changes.
</t>
<t>Asymmetric link costs are also a common aspect of networks.
There are at least three common causes for them. First, any
broadcast interface is represented by a pseudo-node and has
asymmetric link costs to and from that pseudo-node. Second,
when routers come up or a link with LDP comes up, it is
recommended in <xref target="RFC5443"/> and <xref
target="RFC3137"/> that the link metric be raised to the
maximum cost; this may not be symmetric and for <xref
target="RFC3137"/> is not expected to be. Third, techniques
such as IGP metric tuning for traffic-engineering can result in
asymmetric link costs. A fast-reroute solution needs to handle
network topologies with asymmetric link costs.
</t>
<t>When a network needs to use a micro-loop prevention
mechanism <xref target="RFC5715"/> such as Ordered FIB<xref
target="I-D.ietf-rtgwg-ordered-fib"/> or Farside Tunneling<xref
target="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 adversely impacting traffic.
Without complete coverage, traffic to the unprotected
destinations will be dropped for significantly longer than with
current convergence - where routers individually converge as
fast as possible.
</t>
</section>
<section title="Partial Deployment and Backwards Compatibility">
<t>MRT-FRR supports partial deployment. As with many new
features, the protocols (OSPF, LDP, ISIS) indicate their
capability to support MRT. Inside the MRT-capable connected
group of routers (referred to as an MRT Island), the MRTs are
computed. Alternates to destinations outside the MRT Island
are computed and depend upon the existence of a loop-free
neighbor of the MRT Island for that destination.
</t>
</section>
</section><!-- End of Introduction !-->
<section title="Requirements Language">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
<xref target="RFC2119"/></t>
</section>
<section title="Terminology">
<t><list style="hanging">
<t hangText="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.</t>
<t hangText="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.</t>
<t hangText="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.</t>
<t hangText="MRT-Red: "> MRT-Red is used to describe one of the
two MRTs; it is used to described the associated forwarding
topology and MPLS MT-ID. Specifically, MRT-Red is the decreasing
MRT where links in the GADAG are taken in the direction from a
higher topologically ordered node to a lower one.</t>
<t hangText="MRT-Blue: "> MRT-Blue is used to describe one of
the two MRTs; it is used to described the associated forwarding
topology and MPLS MT-ID. Specifically, MRT-Blue is the increasing
MRT where links in the GADAG are taken in the direction from a
lower topologically ordered node to a higher one.</t>
<t hangText="Rainbow MRT: "> It is useful to have an MPLS MT-ID that
refers to the multiple MRT topologies and to the default
topology. This is referred to as the Rainbow MRT MPLS MT-ID and is
used by LDP to reduce signalling and permit the same label to
always be advertised to all peers for the same (MT-ID,
Prefix).</t>
<t hangText="MRT Island: "> The set of routers that support a
particular MRT profile and the links connecting them that
support MRT. </t>
<t hangText="Island Border Router (IBR): "> A router in the MRT
Island that is connected to a router not in the MRT Island and
both routers are in a common area or level.</t>
<t hangText="Island Neighbor (IN): ">A router that is not in
the MRT Island but is adjacent to an IBR and in the same
area/level as the IBR.</t>
<t hangText="cut-link: ">A link whose removal partitions the
network. A cut-link by definition must be connected between
two cut-vertices. If there are multiple parallel links, then
they are referred to as cut-links in this document if removing
the set of parallel links would partition the network
graph. </t>
<t hangText="cut-vertex: ">A vertex whose removal partitions
the network graph.</t>
<t hangText="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.</t>
<t hangText="2-connected cluster: ">A maximal set of nodes that
are 2-connected.</t>
<t hangText="2-edge-connected: ">A network graph where at least
two links must be removed to partition the network.</t>
<t hangText="block: ">Either a 2-connected cluster, a cut-edge,
or an isolated vertex.</t>
<t hangText="DAG: ">Directed Acyclic Graph - a graph where all
links are directed and there are no cycles in it.</t>
<t hangText="ADAG: ">Almost Directed Acyclic Graph - a graph
that, if all links incoming to the root were removed, would be
a DAG.</t>
<t hangText="GADAG: ">Generalized ADAG - a graph that is the
combination of the ADAGs of all blocks.</t>
<t hangText="named proxy-node: ">A proxy-node can represent a
destination prefix that can be attached to the MRT Island via
at least two routers. It is named if there is a way that
traffic can be encapsulated to reach specifically that proxy
node; this could be because there is an LDP FEC for the
associated prefix or because MRT-Red and MRT-Blue IP addresses
are advertised in an undefined fashion for that proxy-node.</t>
</list></t>
</section>
<section title="Maximally Redundant Trees (MRT)">
<t>A pair of Maximally Redundant Trees is a pair of directed
spanning trees that provides maximally disjoint paths towards
their common root. Only links or nodes whose failure would
partition the network (i.e. cut-links and cut-vertices) are
shared between the trees. The algorithm to compute MRTs is given
in <xref target="I-D.ietf-rtgwg-mrt-frr-algorithm"/>. This
algorithm can be computed in O(e + n log n); it is less than
three SPFs. Modeling results comparing the alternate path
lengths obtained with MRT to other approaches are described in
<xref target="I-D.ietf-rtgwg-mrt-frr-algorithm"/>. This document
describes how the MRTs can be used and not how to compute
them.</t>
<t>MRT provides destination-based trees for each destination.
Each router stores its normal primary next-hop(s) as well as
MRT-Blue next-hop(s) and MRT-Red next-hop(s) toward each
destination. The alternate will be selected between the MRT-Blue
and MRT-Red.</t>
<t>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.</t>
<t>For example, in <xref target="fig_example_2_connected"/>,
there is a network graph that is 2-connected in (a) and
associated MRTs in (b) and (c). One can consider the paths from
B to R; on the Blue MRT, the paths are B->F->D->E->R
or B->C->D->E->R. On the Red MRT, the path is
B->A->R. These are clearly link and node-disjoint. These
MRTs are redundant trees because the paths are disjoint.</t>
<figure anchor="fig_example_2_connected" title="A 2-connected Network" align="center">
<artwork align="center"><![CDATA[
[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
]]></artwork>
</figure>
<t>By contrast, in <xref
target="Non-2-connected_Network_Example"/>, the network in (a)
is not 2-connected. If F, G or the link F<->G failed,
then the network would be partitioned. It is clearly impossible
to have two link-disjoint or node-disjoint paths from G, I or J
to R. The MRTs given in (b) and (c) offer paths that are as
disjoint as possible. For instance, the paths from B to R are
the same as in <xref target="fig_example_2_connected"/> 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.</t>
<figure anchor="Non-2-connected_Network_Example"
title="A non-2-connected network" align="center">
<artwork align="center"><![CDATA[
[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] | | [J]
[A]-->[B]---| [A]<--[B]<--|
(b) (c)
Blue MRT towards R Red MRT towards R
]]></artwork>
</figure>
</section>
<section anchor="mrt_and_frr" title="Maximally Redundant Trees (MRT) and Fast-Reroute">
<t>In normal IGP routing, each router has its shortest-path-tree to
all destinations. From the perspective of a particular destination,
D, this looks like a reverse SPT (rSPT). To use maximally redundant
trees, in addition, each destination D has two MRTs associated with
it; by convention these will be called the MRT-Blue and MRT-Red.
MRT-FRR is realized by using multi-topology forwarding. There is a
MRT-Blue forwarding topology and a MRT-Red forwarding topology.</t>
<t>Any IP/LDP fast-reroute technique beyond LFA requires an
additional dataplane procedure, such as an additional forwarding
mechanism. The well-known options are multi-topology forwarding
(used by MRT-FRR), tunneling (e.g. <xref
target="I-D.ietf-rtgwg-ipfrr-notvia-addresses"/> or <xref
target="RFC7490"/>), and per-interface forwarding
(e.g. Loop-Free Failure Insensitive Routing in <xref
target="EnyediThesis"/>).</t>
<t>When there is a link or node failure affecting, but not
partitioning, the network, each node will still have at least one
path via one of the MRTs to reach the destination D. For example,
in <xref target="Non-2-connected_Network_Example"/>, C would
normally forward traffic to R across the C<->R link. If that
C<->R link fails, then C could use the Blue MRT path
C->D->E->R.</t>
<t>As is always the case with fast-reroute technologies, forwarding
does not change until a local failure is detected. Packets are
forwarded along the shortest path. The appropriate alternate to use
is pre-computed. <xref target="I-D.ietf-rtgwg-mrt-frr-algorithm"/>
describes exactly how to determine whether the MRT-Blue next-hops or
the MRT-Red next-hops should be the MRT alternate next-hops for a
particular primary next-hop to a particular destination.</t>
<t>MRT alternates are always available to use. It is a local
decision whether to use an MRT alternate, a Loop-Free Alternate or
some other type of alternate.</t>
<t>As described in <xref target="RFC5286"/>, when a worse failure
than is anticipated happens, using LFAs that are not downstream
neighbors can cause micro-looping. Section 1.1 of <xref
target="RFC5286"/> gives an example of link-protecting alternates
causing a loop on node failure. Even if a worse failure than
anticipated happens, the use of MRT alternates will not cause
looping. Therefore, while node-protecting LFAs may be preferred,
the certainty that no alternate-induced looping will occur is an
advantage of using MRT alternates when the available node-protecting
LFA is not a downstream path.</t>
</section>
<section anchor="sec_uni_forwarding" title="Unicast Forwarding with MRT Fast-Reroute">
<t>As mentioned before, MRT FRR needs multi-topology forwarding.
Unfortunately, neither IP nor LDP provides extra bits for a packet
to indicate its topology. Once the MRTs are computed, the two sets
of MRTs can be used as two additional forwarding topologies. The
same considerations apply for forwarding along the MRTs as for
handling multiple topologies.</t>
<t> There are three possible types of routers involved in forwarding
a packet along an MRT path. At the MRT ingress router, the packet
leaves the shortest path to the destination and follows an MRT path
to the destination. In a FRR application, the MRT ingress router is
the PLR. An MRT transit router takes a packet that arrives already
associated with the particular MRT, and forwards it on that same
MRT. In some situations (to be discussed later), the packet will
need to leave the MRT path and return to the shortest path. This
takes place at the MRT egress router. The MRT ingress and egress
functionality may depend on the underlying type of packet being
forwarded (LDP or IP). The MRT transit functionality is independent
of the type of packet being forwarded. We first consider several
MRT transit forwarding mechanisms. Then we look at how these
forwarding mechanisms can be applied to carrying LDP and IP
traffic.</t>
<section anchor="sec_mrt_forwarding_mechanisms" title="MRT Forwarding Mechanisms">
<t> The following options for MRT forwarding mechanisms are
considered.</t>
<t>
<list style="numbers">
<t>MRT LDP Labels
<list style="letters">
<t> Topology-scoped FEC encoded using a single label</t>
<t> Topology and FEC encoded using a two label stack</t>
</list>
</t>
<t> MRT IP Tunnels
<list style="letters">
<t> MRT IPv4 Tunnels</t>
<t> MRT IPv6 Tunnels</t>
</list>
</t>
</list>
</t>
<section title="MRT LDP labels">
<t> We consider two options for the MRT forwarding mechanisms using
MRT LDP labels.</t>
<section anchor="sec_option_1a" title="Topology-scoped FEC encoded using a single label (Option 1A)">
<t><xref target="RFC7307"/> provides a
mechanism to distribute FEC-Label bindings scoped to a given
MPLS topology (represented by MPLS MT-ID). To use multi-topology LDP to
create MRT forwarding topologies, we associate two MPLS MT-IDs with the
MRT-Red and MRT-Blue forwarding topologies, in addition to the
default shortest path forwarding topology with MT-ID=0.</t>
<t> With this forwarding mechanism, a single label is distributed
for each topology-scoped FEC. For a given FEC in the default
topology (call it default-FEC-A), two additional topology-scoped
FECs would be created, corresponding to the Red and Blue MRT
forwarding topologies (call them red-FEC-A and blue-FEC-A). A
router supporting this MRT transit forwarding mechanism advertises
a different FEC-label binding for each of the three
topology-scoped FECs. When a packet is received with a label
corresponding to red-FEC-A (for example), an MRT transit router
will determine the next-hop for the MRT-Red forwarding topology
for that FEC, swap the incoming label with the outgoing label
corresponding to red-FEC-A learned from the MRT-Red next-hop
router, and forward the packet. </t>
<t>This forwarding mechanism has the useful property that the FEC
associated with the packet is maintained in the labels at each hop
along the MRT. We will take advantage of this property when
specifying how to carry LDP traffic on MRT paths using
multi-topology LDP labels.</t>
<t>This approach is very simple for hardware to support. However,
it reduces the label space for other uses, and it increases the
memory needed to store the labels and the communication required
by LDP to distribute FEC-label bindings.</t>
<t> This forwarding option uses the LDP signaling extensions
described in <xref target="RFC7307"/>.
The MRT-specific LDP extensions required to support this option
are described in <xref target="I-D.ietf-mpls-ldp-mrt"/>. </t>
</section>
<section anchor="sec_option_1b" title="Topology and FEC encoded using a two label stack (Option 1B)">
<t> With this forwarding mechanism, a two label stack is used to
encode the topology and the FEC of the packet. The top label
(topology-id label) identifies the MRT forwarding topology, while
the second label (FEC label) identifies the FEC. The top label
would be a new FEC type with two values corresponding to MRT Red
and Blue topologies. </t>
<t> When an MRT transit router receives a packet with a
topology-id label, the router pops the top label and uses that it
to guide the next-hop selection in combination with the next label
in the stack (the FEC label). The router then swaps the FEC
label, using the FEC-label bindings learned through normal LDP
mechanisms. The router then pushes the topology-id label for the
next-hop.</t>
<t>As with Option 1A, this forwarding mechanism also has the
useful property that the FEC associated with the packet is
maintained in the labels at each hop along the MRT.</t>
<t>This forwarding mechanism 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.</t>
<t> This forwarding option is consistent with context-specific
label spaces, as described in [RFC 5331]. However, the precise
LDP behavior required to support this option for MRT has not been
specified.</t>
</section>
<section title="Compatibility of Option 1A and 1B">
<t> In principle, MRT transit forwarding mechanisms 1A and 1B can
coexist in the same network, with a packet being forwarding along
a single MRT path using the single label of option 1A for some
hops and the two label stack of option 1B for other hops.
</t>
</section>
<section title="Mandatory support for MRT LDP Label option 1A">
<t> If a router supports a profile that includes the MRT LDP Label
option for MRT transit forwarding mechanism, then it MUST support
option 1A, which encodes topology-scoped FECs using a single
label.
</t>
</section>
</section>
<section title="MRT IP tunnels (Options 2A and 2B)">
<t> IP tunneling can also be used as an MRT transit forwarding
mechanism. Each router supporting this MRT transit forwarding
mechanism announces 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. The
special loopback addresses allow the transit nodes to identify the
traffic as being forwarded along either the MRT-blue or MRT-red
topology to reach the tunnel destination. Announcements of these
two additional loopback addresses per router with their MRT color
requires IGP extensions, which have not been defined.</t>
<t> Either IPv4 (option 2A) or IPv6 (option 2B) can be used as the
tunneling mechanism.</t>
<t>Note that the two forwarding mechanisms using LDP Label options
do not require additional loopbacks per router, as is required by
the IP tunneling mechanism. This is because LDP labels are used on a
hop-by-hop basis to identify MRT-blue and MRT-red forwarding
topologies.</t>
</section>
</section>
<section anchor="sec_ldp_uni_forward" title="Forwarding LDP Unicast Traffic over MRT Paths">
<t>In the previous section, we examined several options for
providing MRT transit forwarding functionality, which is independent
of the type of traffic being carried. We now look at the MRT
ingress functionality, which will depend on the type of traffic
being carried (IP or LDP). We start by considering LDP traffic. </t>
<t>We also simplify the initial discussion by assuming that the
network consists of a single IGP area, and that all routers in the
network participate in MRT. Other deployment scenarios that require
MRT egress functionality are considered later in this document.</t>
<t>In principle, it is possible to carry LDP traffic in MRT IP
tunnels. However, for LDP traffic, it is very desirable to avoid
tunneling. Tunneling LDP traffic to a remote node requires
knowledge of remote FEC-label bindings so that the LDP traffic can
continue to be forwarded properly when it leaves the tunnel. This
requires targeted LDP sessions which can add management complexity.
The two MRT LDP Label forwarding mechanisms have the useful property
that the FEC associated with the packet is maintained in the labels
at each hop along the MRT, as long as an MRT to the originator of
the FEC is used. The MRT IP tunneling mechanism does not have this
useful property. Therefore, this document only considers the two
MRT LDP Label forwarding mechanisms for protecting LDP traffic with
MRT fast-reroute.
</t>
<section title="Forwarding LDP traffic using MRT LDP Labels (Option 1A)">
<t> The MRT LDP Label option 1A forwarding mechanism uses
topology-scoped FECs encoded using a single label as described in
section <xref target="sec_option_1a"/>. When a PLR receives an
LDP packet that needs to be forwarded on the Red MRT (for
example), it does a label swap operation, replacing the usual LDP
label for the FEC with the Red MRT label for that FEC received
from the next-hop router in the Red MRT computed by the PLR. When
the next-hop router in the Red MRT receives the packet with the
Red MRT label for the FEC, the MRT transit forwarding
functionality continues as described in <xref
target="sec_option_1a"/>. In this way the original FEC associated
with the packet is maintained at each hop along the MRT. </t>
</section>
<section title="Forwarding LDP traffic using MRT LDP Labels (Option 1B)">
<t>The MRT LDP Label option 1B forwarding mechanism encodes the
topology and the FEC using a two label stack as described in <xref
target="sec_option_1b"/>. When a PLR receives an LDP packet that
needs to be forwarded on the Red MRT, it first does a normal LDP
label swap operation, replacing the incoming normal LDP label
associated with a given FEC with the outgoing normal LDP label for
that FEC learned from the next-hop on the Red MRT. In addition,
the PLR pushes the topology-identification label associated with
the Red MRT, and forward the packet to the appropriate next-hop on
the Red MRT. When the next-hop router in the Red MRT receives the
packet with the Red MRT label for the FEC, the MRT transit
forwarding functionality continues as described in <xref
target="sec_option_1b"/>. As with option 1A, the original FEC
associated with the packet is maintained at each hop along the
MRT.</t>
</section>
<section title="Other considerations for forwarding LDP traffic using MRT LDP Labels ">
<t> Note that forwarding LDP traffic using MRT LDP Labels requires
that an MRT to the originator of the FEC be used. For example,
one might find it desirable to have the PLR use an MRT to reach
the primary next-next-hop for the FEC, and then continue
forwarding the LDP packet along the shortest path tree from the
primary next-next-hop. However, this would require tunneling to
the primary next-next-hop and a targeted LDP session for the PLR
to learn the FEC-label binding for primary next-next-hop to
correctly forward the packet.</t>
<t>For greatest hardware compatibility, routers implementing MRT
fast-reroute of LDP traffic MUST support Option 1A of encoding the
MT-ID in the labels (See <xref target="sec_proto_ldp"/>). </t>
</section>
</section>
<section title="Forwarding IP Unicast Traffic over MRT Paths">
<t>For IP traffic, there is no currently practical alternative
except tunneling to gain the bits needed to indicate the MRT-Blue or
MRT-Red forwarding topology. The choice of tunnel egress MAY be
flexible since any router closer to the destination than the
next-hop can work. This architecture assumes that the original
destination in the area is selected (see <xref
target="sec_multi_homed_prefixes"/> for handling of multi-homed
prefixes); another possible choice is the next-next-hop towards the
destination. As discussed in the previous section, for LDP traffic,
using the MRT to the original destination simplifies MRT-FRR by
avoiding the need for targeted LDP sessions to the next-next-hop.
For IP, that consideration doesn't apply. However, consistency with
LDP is RECOMMENDED.</t>
<t>Some situations require tunneling IP traffic along an MRT to a
tunnel endpoint that is not the destination of the IP traffic.
These situations will be discussed in detail later. We note here
that an IP packet with a destination in a different IGP area/level
from the PLR should be tunneled on the MRT to the ABR/LBR on the
shortest path to the destination. For a destination outside of the
PLR's MRT Island, the packet should be tunneled on the MRT to a
non-proxy-node immediately before the named proxy-node on that
particular color MRT.
</t>
<section title="Tunneling IP traffic using MRT LDP Labels">
<t>An IP packet can be tunneled along an MRT path by pushing the
appropriate MRT LDP label(s). Tunneling using LDP labels, as
opposed to IP headers, has the the advantage that more installed
routers can do line-rate encapsulation and decapsulation using LDP
than using IP. Also, no additional IP addresses would need to be
allocated or signaled.</t>
<section title="Tunneling IP traffic using MRT LDP Labels (Option 1A)">
<t>The MRT LDP Label option 1A forwarding mechanism uses
topology-scoped FECs encoded using a single label as described
in section <xref target="sec_option_1a"/>. When a PLR receives
an IP packet that needs to be forwarded on the Red MRT to a
particular tunnel endpoint, it does a label push operation. The
label pushed is the Red MRT label for a FEC originated by the
tunnel endpoint, learned from the next-hop on the Red MRT.
</t>
</section>
<section title="Tunneling IP traffic using MRT LDP Labels (Option 1B)">
<t>The MRT LDP Label option 1B forwarding mechanism encodes the
topology and the FEC using a two label stack as described in
<xref target="sec_option_1b"/>. When a PLR receives an IP
packet that needs to be forwarded on the Red MRT to a particular
tunnel endpoint, the PLR pushes two labels on the IP packet.
The first (inner) label is the normal LDP label learned from the
next-hop on the Red MRT, associated with a FEC originated by the
tunnel endpoint. The second (outer) label is the
topology-identification label associated with the Red MRT.
</t>
<t> For completeness, we note here a potential optimization. In order to tunnel
an IP packet over an MRT to the destination of the IP packet (as opposed
to an arbitrary tunnel endpoint), then we could just push a
topology-identification label directly onto the packet. An MRT transit
router would need to pop the topology-id label, do an IP route lookup
in the context of that topology-id , and push the topology-id label.
</t>
</section>
</section>
<section title="Tunneling IP traffic using MRT IP Tunnels">
<t>In order to tunnel over the MRT to a particular tunnel
endpoint, the PLR encapsulates the original IP packet with an
additional IP header using the MRT-Blue or MRT-Red loopack address
of the tunnel endpoint.</t>
</section>
<section title="Required support">
<t>For greatest hardware compatibility and ease in removing the
MRT-topology marking at area/level boundaries, routers that
support MPLS and implement IP MRT fast-reroute MUST support
tunneling of IP traffic using MRT LDP Labels Option 1A
(topology-scoped FEC encoded using a single label). </t>
</section>
</section>
</section>
<section anchor="sec_island" title="MRT Island Formation">
<t> The purpose of communicating support for MRT in the IGP is to
indicate that the MRT-Blue and MRT-Red forwarding topologies are
created for transit traffic. The MRT architecture allows for
different, potentially incompatible options. In order to create
constistent MRT forwarding topologies, the routers participating in
a particular MRT Island need to use the same set of options. These
options are grouped into MRT profiles. In addition, the routers in
an MRT Island all need to use the same set of nodes and links within
the Island when computing the MRT forwarding topologies. This
section describes the information used by a router to determine the
nodes and links to include in a particular MRT Island. Some of this
information is shared among routers using the newly-defined IGP
signaling extensions for MRT described in <xref
target="I-D.ietf-ospf-mrt"/> and <xref target="I-D.ietf-isis-mrt"/>.
Other information already exists in the IGPs and can be used by MRT
in Island formation, subject to the interpretation defined here.</t>
<t> Deployment scenarios using multi-topology OSPF or IS-IS, or
running both ISIS and OSPF on the same routers is out of scope for
this specification. As with LFA, it is expected that OSPF Virtual
Links will not be supported.</t>
<section title="IGP Area or Level">
<t> All links in an MRT Island MUST be bidirectional and belong to
the same IGP area or level. For ISIS, a link belonging to both
level 1 and level 2 would qualify to be in multiple MRT Islands.
A given ABR or LBR can belong to multiple MRT Islands,
corresponding to the areas or levels in which it participates.
Inter-area forwarding behavior is discussed in <xref
target="sec_abr_forwarding"/>.</t>
</section>
<section title="Support for a specific MRT profile">
<t> All routers in an MRT Island MUST support the same MRT
profile. A router advertises support for a given MRT profile
using the IGP extensions defined in <xref
target="I-D.ietf-ospf-mrt"/> and <xref target="I-D.ietf-isis-mrt"/>
using an 8-bit Profile ID value. A given router can support
multiple MRT profiles and participate in multiple MRT Islands.
The options that make up an MRT profile, as well as the default
MRT profile, are defined in <xref target="sec_mrt_profile"/>.
</t>
</section>
<section title="Excluding additional routers and interfaces from the MRT Island">
<t> MRT takes into account existing IGP mechanisms for
discouraging traffic from using particular links and routers, and
it introduces an MRT-specific exclusion mechanism for links.
</t>
<section title="Existing IGP exclusion mechanisms">
<t> Mechanisms for discouraging traffic from using particular
links already exist in ISIS and OSPF. In ISIS, an interface
configured with a metric of 2^24-2 (0xFFFFFE) will only be used
as a last resort. (An interface configured with a metric of
2^24-1 (0xFFFFFF) will not be advertised into the topology.) In
OSPF, an interface configured with a metric of 2^16-1 (0xFFFF)
will only be used as a last resort. These metrics can be
configured manually to enforce administrative policy, or they
can be set in an automated manner as with LDP IGP
synchronization [RFC5443].
</t>
<t> Mechanisms also exist in ISIS and OSPF to prevent transit
traffic from using a particular router. In ISIS, the overload
bit is used for this purpose. In OSPF, [RFC3137] specifies
setting all outgoing interface metrics to 0xFFFF to accomplish
this.
</t>
<t> The following rules for MRT Island formation ensure that MRT
FRR protection traffic does not use a link or router that is
discouraged from carrying traffic by existing IGP mechanisms.
<list style="numbers">
<t> A bidirectional link MUST be excluded from an MRT Island
if either the forward or reverse cost on the link is 0xFFFFFE
(for ISIS) or 0xFFFF for OSPF.</t>
<t> A router MUST be excluded from an MRT Island if it is
advertised with the overload bit set (for ISIS), or it is
advertised with metric values of 0xFFFF on all of its outgoing
interfaces (for OSPF).</t>
</list>
</t>
</section>
<section title="MRT-specific exclusion mechanism">
<t> This architecture also defines a means of excluding an
otherwise usable link from MRT Islands. <xref
target="I-D.ietf-ospf-mrt"/> and <xref
target="I-D.ietf-isis-mrt"/> define the IGP extensions for OSPF
and ISIS used to advertise that a link is MRT-Ineligible. A
link with either interface advertised as MRT-Ineligible MUST be
excluded from an MRT Island. Note that an interface advertised
as MRT-Ineligigle by a router is ineligible with respect to all
profiles advertised by that router.
</t>
</section>
</section>
<section title="Connectivity">
<t> All of the routers in an MRT Island MUST be connected by
bidirectional links with other routers in the MRT Island.
Disconnected MRT Islands will operate independently of one
another.</t>
</section>
<section title="Example algorithm">
<t>An algorithm that allows a computing router to identify the
routers and links in the local MRT Island satisfying the above rules
is given in section 5.1 of <xref
target="I-D.ietf-rtgwg-mrt-frr-algorithm"/>. </t>
</section>
</section>
<section anchor="sec_mrt_profile" title="MRT Profile">
<t>An MRT Profile is a set of values and options related to MRT
behavior. The complete set of options is designated by the
corresponding 8-bit Profile ID value. </t>
<section anchor="sec_mrt_profile_options" title="MRT Profile Options">
<t>Below is a description of the values and options that define an
MRT Profile.</t>
<t><list style="hanging">
<t hangText="MRT Algorithm: ">This identifies the particular MRT
algorithm used by the router for this profile. Algorithm
transitions can be managed by advertising multiple MRT profiles.</t>
<t hangText="MRT-Red MT-ID: ">This specifies the MT-ID to be
associated with the MRT-Red forwarding topology. It is needed for
use in LDP signaling. All routers in the MRT Island MUST agree on a
value.</t>
<t hangText="MRT-Blue MT-ID: ">This specifies the MT-ID to be
associated with the MRT-Blue forwarding topology. It is needed for
use in LDP signaling. All routers in the MRT Island MUST agree on a
value.</t>
<t hangText="GADAG Root Selection Policy: ">This specifes the manner
in which the GADAG root is selected. All routers in the MRT island
need to use the same GADAG root in the calculations used construct
the MRTs. A valid GADAG Root Selection Policy MUST be such that
each router in the MRT island chooses the same GADAG root based on
information available to all routers in the MRT island. GADAG Root
Selection Priority values, advertised in the IGP as router-specific
MRT parameters, MAY be used in a GADAG Root Selection Policy.</t>
<t hangText="MRT Forwarding Mechanism: ">This specifies which
forwarding mechanism the router uses to carry transit traffic along
MRT paths. A router which supports a specific MRT forwarding
mechanism must program appropriate next-hops into the forwarding
plane. The current options are MRT LDP Labels, IPv4 Tunneling, IPv6
Tunneling, and None. If the MRT LDP Labels option is supported,
then option 1A and the appropriate signaling extensions MUST be
supported. If IPv4 is supported, then both MRT-Red and MRT-Blue
IPv4 Loopback Addresses SHOULD be specified. If IPv6 is supported,
both MRT-Red and MRT-Blue IPv6 Loopback Addresses SHOULD be
specified. The None option in may be useful for multicast global
protection.</t>
<t hangText="Recalculation: ">As part of what process and timing
should the new MRTs be computed on a modified topology? <xref
target="sec_recalculation"/> describes the minimum behavior required
to support fast-reroute.</t>
<t hangText="Area/Level Border Behavior: ">Should inter-area traffic
on the MRT-Blue or MRT-Red be put back onto the shortest path tree?
Should it be swapped from MRT-Blue or MRT-Red in one area/level to
MRT-Red or MRT-Blue in the next area/level to avoid the potential
failure of an ABR? (See <xref target="I-D.atlas-rtgwg-mrt-mc-arch"/>
for use-case details.</t>
<t hangText="Other Profile-Specific Behavior: "> Depending upon the
use-case for the profile, there may be additional profile-specific
behavior.</t>
</list></t>
<t>If a router advertises support for multiple MRT profiles, then it
MUST create the transit forwarding topologies for each of those,
unless the profile specifies the None option for MRT Forwarding
Mechanism. A router MUST NOT advertise multiple MRT profiles that
overlap in their MRT-Red MT-ID or MRT-Blue MT-ID.</t>
</section>
<section title="Router-specific MRT paramaters">
<t>For some profiles, additional router-specific MRT parameters may
need to be distributed via the IGP. While the set of options
indicated by the MRT Profile ID must be identical for all routers in
an MRT Island, these router-specific MRT parameters may differ
between routers in the same MRT island. Several such parameters are
described below.</t>
<t><list style="hanging">
<t hangText="GADAG Root Selection Priority: "> A GADAG Root Selection
Policy MAY rely on the GADAG Root Selection Priority values advertised
by each router in the MRT island. A GADAG Root Selection Policy may
use the GADAG Root Selection Priority to allow network operators to
configure a parameter to ensure that the GADAG root is selected from a
particular subset of routers. An example of this use of the GADAG
Root Selection Priority value by the GADAG Root Selection Policy is
given in the Default MRT profile below.
</t>
<t hangText="MRT-Red Loopback Address: ">This provides the router's
loopback address to reach the router via the MRT-Red forwarding
topology. It can be specified for either IPv4 and IPv6.</t>
<t hangText="MRT-Blue Loopback Address: ">This provides the router's
loopback address to reach the router via the MRT-Blue forwarding
topology. It can be specified for either IPv4 and IPv6.</t>
</list></t>
<t>The extensions to OSPF and ISIS for advertising a router's GADAG
Root Selection Priority value are defined in <xref
target="I-D.ietf-ospf-mrt"/> and <xref target="I-D.ietf-isis-mrt"/>.
IGP extensions for the advertising a router's MRT-Red and MRT-Blue
Loopback Addresses have not been defined.
</t>
</section>
<section title="Default MRT profile">
<t>The following set of options defines the default MRT Profile. The
default MRT profile is indicated by the MRT Profile ID value of 0.</t>
<t><list style="hanging">
<t hangText="MRT Algorithm: ">MRT Lowpoint algorithm defined in <xref
target="I-D.ietf-rtgwg-mrt-frr-algorithm"/>.</t>
<t hangText="MRT-Red MPLS MT-ID: "> <xref target="I-D.ietf-mpls-ldp-mrt"/>
contains the IANA request for allocation of this value from
the MPLS Multi-Topology Identifiers Registry. Prototype experiments have
used a value of 3997.</t>
<t hangText="MRT-Blue MPLS MT-ID: "> <xref target="I-D.ietf-mpls-ldp-mrt"/>
contains the IANA request for allocation of this value from
the MPLS Multi-Topology Identifiers Registry. Prototype experiments have
used a value of 3998.</t>
<t hangText="GADAG Root Selection Policy: ">Among the routers in the
MRT Island and with the highest priority advertised, an implementation
MUST pick the router with the highest Router ID to be the GADAG
root.</t>
<t hangText="Forwarding Mechanisms: ">MRT LDP Labels</t>
<t hangText="Recalculation: ">Recalculation of MRTs SHOULD occur as
described in <xref target="sec_recalculation"/>. This allows the MRT
forwarding topologies to support IP/LDP fast-reroute traffic.</t>
<t hangText="Area/Level Border Behavior: ">As described in <xref
target="sec_abr_forwarding"/>, ABRs/LBRs SHOULD ensure that traffic
leaving the area also exits the MRT-Red or MRT-Blue forwarding
topology.</t>
</list></t>
</section>
</section>
<section anchor="sec_proto_ldp" title="LDP signaling extensions and considerations">
<t>The protocol extensions for LDP are defined in <xref
target="I-D.ietf-mpls-ldp-mrt"/>. A router must indicate that it
has the ability to support MRT; having this explicit allows the use
of MRT-specific processing, such as special handling of FECs sent
with the Rainbow MRT MT-ID.</t>
<t>A FEC sent with the Rainbow MRT MT-ID indicates that the FEC
applies to all the MRT-Blue and MRT-Red MT-IDs in supported MRT
profiles. The FEC-label bindings for the default shortest-path
based MT-ID 0 MUST still be sent (even though it could be inferred
from the Rainbow FEC-label bindings) to ensure continuous operation
of normal LDP forwarding. The Rainbow MRT MT-ID is defined to
provide an easy way to handle the special signaling that is needed
at ABRs or LBRs. It avoids the problem of needing to signal
different MPLS labels for the same FEC. Because the Rainbow MRT
MT-ID is used only by ABRs/LBRs or an LDP egress router, it is not
MRT profile specific.</t>
<t> <xref target="I-D.ietf-mpls-ldp-mrt"/> contains the IANA
request for the Rainbow MRT MPLS MT-ID.
</t>
</section>
<section anchor= "sec_abr_forwarding" title="Inter-area Forwarding Behavior">
<t>Unless otherwise specified, in this section we will use the terms
area and ABR to indicate either an OSPF area and OSPF ABR or ISIS
level and ISIS LBR.</t>
<t>An ABR/LBR has two forwarding roles. First, it forwards traffic
within areas. Second, it forwards traffic from one area into another.
These same two roles apply for MRT transit traffic. Traffic on
MRT-Red or MRT-Blue destined inside the area needs to stay on MRT-Red
or MRT-Blue in that area. However, it is desirable for traffic
leaving the area to also exit MRT-Red or MRT-Blue and return to
shortest path forwarding.</t>
<t>For unicast MRT-FRR, the need to stay on an MRT forwarding topology
terminates at the ABR/LBR whose best route is via a different
area/level. It is highly desirable to go back to the default
forwarding topology when leaving an area/level. There are three basic
reasons for this. First, the default topology uses shortest paths;
the packet will thus take the shortest possible route to the
destination. Second, this allows failures that might appear in
multiple areas (e.g. ABR/LBR failures) to be separately identified and
repaired around. Third, the packet can be fast-rerouted again, if
necessary, due to a failure in a different area.</t>
<t>An ABR/LBR that receives a packet on MRT-Red or MRT-Blue towards
destination Z should continue to forward the packet along MRT-Red or
MRT-Blue only if the best route to Z is in the same area as the
interface that the packet was received on. Otherwise, the packet
should be removed from MRT-Red or MRT-Blue and forwarded on the
shortest-path default forwarding topology.</t>
<t>To avoid per-interface forwarding state for MRT-Red and MRT-Blue,
the ABR/LBR needs to arrange that packets destined to a different area
arrive at the ABR/LBR already not marked as MRT-Red or MRT-Blue.</t>
<section title="ABR Forwarding Behavior with MRT LDP Label Option 1A">
<t>For LDP forwarding where a single label specifies (MT-ID, FEC), the
ABR/LBR is responsible for advertising the proper label to each
neighbor. Assume that an ABR/LBR has allocated three labels for a
particular destination; those labels are L_primary, L_blue, and L_red.
To those routers in the same area as the best route to the
destination, the ABR/LBR advertises the following FEC-label bindings:
L_primary for the default topology, L_blue for the MRT-Blue MT-ID and
L_red for the MRT-Red MT-ID, as expected. However, to routers in
other areas, the ABR/LBR advertises the following FEC-label bindings:
L_primary for the default topology, and L_primary for the Rainbow MRT
MT-ID. Associating L_primary with the Rainbow MRT MT-ID causes the
receiving routers to use L_primary for the MRT-Blue MT-ID and for the
MRT-Red MT-ID.</t>
<t>The ABR/LBR installs all next-hops for the best area: primary
next-hops for L_primary, MRT-Blue next-hops for L_blue, and MRT-Red
next-hops for L_red. Because the ABR/LBR advertised (Rainbow MRT
MT-ID, FEC) with L_primary to neighbors not in the best area, packets
from those neighbors will arrive at the ABR/LBR with a label L_primary
and will be forwarded into the best area along the default topology.
By controlling what labels are advertised, the ABR/LBR can thus
enforce that packets exiting the area do so on the shortest-path
default topology.</t>
<section title="Motivation for Creating the Rainbow-FEC">
<t> The desired forwarding behavior could be achieved in the above
example without using the Rainbow-FEC. This could be done by having
the ABR/LBR advertise the following FEC-label bindings to neighbors
not in the best area: L1_primary for the default topology, L1_primary
for the MRT-Blue MT-ID, and L1_primary for the MRT-Red MT-ID. Doing
this would require machinery to spoof the labels used in FEC-label
binding advertisements on a per-neighbor basis. Such label-spoofing
machinery does not currently exist in most LDP implmentations and
doesn't have other obvious uses.
</t>
<t>Many existing LDP implmentations do however have the ability to
filter FEC-label binding advertisements on a per-neighbor basis. The
Rainbow-FEC allows us to re-use the existing per-neighbor FEC
filtering machinery to achieve the desired result. By introducing the
Rainbow FEC, we can use per-neighbor FEC-filtering machinery to
advertise the FEC-label binding for the Rainbow-FEC (and filter those
for MRT-Blue and MRT-Red) to non-best-area neighbors of the ABR.</t>
<t>The use of the Rainbow-FEC by the ABR for non-best-area
advertisements is RECOMMENDED. An ABR MAY advertise the label for the
default topology in separate MRT-Blue and MRT-Red advertisements.
However, a router that supports the LDP Label MRT Forwarding Mechanism
MUST be able to receive and correctly interpret the Rainbow-FEC.
</t>
</section>
</section>
<section title="ABR Forwarding Behavior with IP Tunneling (option 2)">
<t>If IP tunneling is used, then the ABR/LBR behavior is dependent
upon the outermost IP address. If the outermost IP address is an MRT
loopback address of the ABR/LBR, then the packet is decapsulated and
forwarded based upon the inner IP address, which should go on the
default SPT topology. If the outermost IP address is not an MRT
loopback address of the ABR/LBR, then the packet is simply forwarded
along the associated forwarding topology. A PLR sending traffic to a
destination outside its local area/level will pick the MRT and use the
associated MRT loopback address of the selected ABR/LBR advertising
the lowest cost to the external destination.</t>
<t>Thus, for these two MRT Forwarding Mechanisms( MRT LDP Label
option 1A and IP tunneling option 2), there is
no need for additional computation or per-area forwarding state.</t>
</section>
<section title="ABR Forwarding Behavior with LDP Label option 1B">
<t>The other MRT forwarding mechanism described in <xref
target="sec_uni_forwarding"/> uses two labels, a topology-id label,
and a FEC-label. This mechanism would require that any router whose
MRT-Red or MRT-Blue next-hop is an ABR/LBR would need to determine
whether the ABR/LBR would forward the packet out of the area/level.
If so, then that router should pop off the topology-identification
label before forwarding the packet to the ABR/LBR.</t>
<t> For example, in <xref target="fig_abr_mrt"/>, if node H fails,
node E has to put traffic towards prefix p onto MRT-Red. But since
node D knows that ABR1 will use a best route from another area, it is
safe for D to pop the Topology-Identification Label and just forward
the packet to ABR1 along the MRT-Red next-hop. ABR1 will use the
shortest path in Area 10.</t>
<t>In all cases for ISIS and most cases for OSPF, the penultimate
router can determine what decision the adjacent ABR will make. The
one case where it can't be determined is when two ASBRs are in
different non-backbone areas attached to the same ABR, then the ASBR's
Area ID may be needed for tie-breaking (prefer the route with the
largest OPSF area ID) and the Area ID isn't announced as part of the
ASBR link-state advertisement (LSA). In this one case, suboptimal
forwarding along the MRT in the other area would happen. If that
becomes a realistic deployment scenario, OSPF extensions could be
considered. This is not covered in <xref
target="I-D.ietf-ospf-mrt"/>.</t>
<figure anchor="fig_abr_mrt" title="ABR Forwarding Behavior and MRTs"
align="center">
<artwork align="center"><![CDATA[
+----[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
]]></artwork>
</figure>
</section>
</section>
<section anchor="sec_multi_homed_prefixes" title="Prefixes Multiply Attached to the MRT Island">
<t>How a computing router S determines its local MRT Island for each
supported MRT profile is already discussed in <xref
target="sec_island"/>.</t>
<t>There are two types of prefixes or FECs that may be multiply
attached to an MRT Island. The first type are multi-homed prefixes
that usually connect at a domain or protocol boundary. The second
type represent routers that do not support the profile for the MRT
Island. The key difference is whether the traffic, once out of the
MRT Island, remains in the same area/level and might re-enter the MRT
Island if a loop-free exit point is not selected.</t>
<t>FRR using LFA has the useful property that it is able 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 be accomplished
with MRT FRR as well.</t>
<t>If ASBR protection is desired, this has additional 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.</t>
<t>As discussed in <xref target="RFC5286"/>, a multi-homed prefix
could be:
<list style="symbols">
<t>An out-of-area prefix announced by more than one ABR,</t>
<t>An AS-External route announced by 2 or more ASBRs,</t>
<t>A prefix with iBGP multipath to different ASBRs,</t>
<t>etc.</t>
</list></t>
<t>There are also two different approaches to protection. The first
is tunnel endpoint selection where the PLR picks a router to tunnel to
where that router is loop-free with respect to the failure-point.
Conceptually, the set of candidate routers to provide LFAs expands to
all routers that can be reached via an MRT alternate, attached to the
prefix.</t>
<t>The second is to use a proxy-node, that can be named via MPLS label
or IP address, and pick the appropriate label or IP address to reach
it on either MRT-Blue or MRT-Red as appropriate to avoid the failure
point. A proxy-node can represent a destination prefix that can be
attached to the MRT Island via at least two routers. It is termed a
named proxy-node if there is a way that traffic can be encapsulated to
reach specifically that proxy-node; this could be because there is an
LDP FEC for the associated prefix or because MRT-Red and MRT-Blue IP
addresses are advertised (in an as-yet undefined fashion) for that
proxy-node. Traffic to a named proxy-node may take a different path
than traffic to the attaching router; traffic is also explicitly
forwarded from the attaching router along a predetermined interface
towards the relevant prefixes.</t>
<t>For IP traffic, multi-homed prefixes can use tunnel endpoint
selection. For IP traffic that is destined to a router outside the
MRT Island, if that router is the egress for a FEC advertised into the
MRT Island, then the named proxy-node approach can be used.</t>
<t>For LDP traffic, there is always a FEC advertised into the MRT
Island. The named proxy-node approach should be used, unless the
computing router S knows the label for the FEC at the selected tunnel
endpoint.</t>
<t>If a FEC is advertised from outside the MRT Island into the MRT
Island and the forwarding mechanism specified in the profile includes
LDP, then the routers learning that FEC MUST also advertise labels for
(MRT-Red, FEC) and (MRT-Blue, FEC) to neighbors inside the MRT Island.
Any router receiving a FEC corresponding to a router outside the MRT
Island or to a multi-homed prefix MUST compute and install the transit
MRT-Blue and MRT-Red next-hops for that FEC. The FEC-label bindings
for the topology-scoped FECs ((MT-ID 0, FEC), (MRT-Red, FEC), and
(MRT-Blue, FEC)) MUST also be provided via LDP to neighbors inside the
MRT Island.</t>
<section title="Protecting Multi-Homed Prefixes using Tunnel Endpoint Selection">
<t>Tunnel endpoint selection is a local matter for a router in the MRT
Island since it pertains to selecting and using an alternate and does
not affect the transit MRT-Red and MRT-Blue forwarding
topologies. </t>
<t>Let the computing router be S and the next-hop F be the node whose
failure is to be avoided. Let the destination be prefix p. Have A be
the router to which the prefix p is attached for S's shortest path to
p. </t>
<t>The candidates for tunnel endpoint selection are those to which the
destination prefix is attached in the area/level. For a particular
candidate B, it is necessary to determine if B is loop-free to reach p
with respect to S and F for node-protection or at least with respect
to S and the link (S, F) for link-protection. If B will always prefer
to send traffic to p via a different area/level, then this is
definitional. Otherwise, distance-based computations are necessary
and an SPF from B's perspective may be necessary. The following
equations give the checks needed; the rationale is similar to that
given in <xref target="RFC5286"/>.</t>
<t>Loop-Free for S: D_opt(B, p) < D_opt(B, S) + D_opt(S, p)</t>
<t>Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(F, p)</t>
<t>The latter is equivalent to the following, which avoids the need to
compute the shortest path from F to p.</t>
<t>Loop-Free for F: D_opt(B, p) < D_opt(B, F) + D_opt(S, p) - D_opt(S, F)</t>
<t>Finally, the rules for Endpoint selection are given below. The
basic idea is to repair to the prefix-advertising router selected for
the shortest-path and only to select and tunnel to a different
endpoint if necessary (e.g. A=F or F is a cut-vertex or the link (S,F)
is a cut-link).</t>
<t><list style="numbers">
<t>Does S have a node-protecting alternate to A? If so, select that.
Tunnel the packet to A along that alternate. For example, if LDP is
the forwarding mechanism, then push the label (MRT-Red, A) or
(MRT-Blue, A) onto the packet. </t>
<t>If not, then is there a router B that is loop-free to reach p while
avoiding both F and S? If so, select B as the end-point. Determine
the MRT alternate to reach B while avoiding F. Tunnel the packet to B
along that alternate. For example, with LDP, push the label (MRT-Red,
B) or (MRT-Blue, B) onto the packet.</t>
<t>If not, then does S have a link-protecting alternate to A? If so,
select that.</t>
<t>If not, then is there a router B that is loop-free to reach p while
avoiding S and the link from S to F? If so, select B as the endpoint
and the MRT alternate for reaching B from S that avoid the link
(S,F).</t>
</list></t>
<t>The tunnel endpoint selected will receive a packet destined to
itself and, being the egress, will pop that MPLS label (or have
signaled Implicit Null) and forward based on what is underneath. This
suffices for IP traffic since the tunnel endpoint can use the IP
header of the original packet to continue forwarding the packet.
However, tunnelling of LDP traffic requires targeted LDP
sessions for learning the FEC-label binding at the tunnel
endpoint.</t>
</section>
<section title="Protecting Multi-Homed Prefixes using Named Proxy-Nodes">
<t> Instead, the named proxy-node method works with LDP traffic
without the need for targeted LDP sessions. It also has a clear
advantage over tunnel endpoint selection, in that it is possible to
explicitly forward from the MRT Island along an interface to a
loop-free island neighbor when that interface may not be a primary
next-hop.</t>
<t>A named proxy-node represents one or more destinations and, for LDP
forwarding, has a FEC associated with it that is signalled into the MRT
Island. Therefore, it is possible to explicitly label packets to go
to (MRT-Red, FEC) or (MRT-Blue, FEC); at the border of the MRT Island,
the label will swap to meaning (MT-ID 0, FEC). It would be possible
to have named proxy-nodes for IP forwarding, but this would require
extensions to signal two IP addresses to be associated with MRT-Red
and MRT-Blue for the proxy-node. A named proxy-node can be uniquely
represented by the two routers in the MRT Island to which it is
connected. The extensions to signal such IP addresses are not defined
in <xref target="I-D.ietf-ospf-mrt"/>. The details of what
label-bindings must be originated are described in <xref
target="I-D.ietf-mpls-ldp-mrt"/>.</t>
<t>Computing the MRT next-hops to a named proxy-node and the MRT
alternate for the computing router S to avoid a particular failure
node F is straightforward. The details of the simple constant-time
functions, Select_Proxy_Node_NHs() and Select_Alternates_Proxy_Node(),
are given in <xref target="I-D.ietf-rtgwg-mrt-frr-algorithm"/>. A key
point is that computing these MRT next-hops and alternates can be done
as new named proxy-nodes are added or removed without requiring a new
MRT computation or impacting other existing MRT paths. This maps very
well to, for example, how OSPFv2 [<xref target="RFC2328"/> Section
16.5] does incremental updates for new summary-LSAs.</t>
<t>The key question is how to attach the named proxy-node to the MRT
Island; all the routers in the MRT Island MUST do this consistently.
No more than 2 routers in the MRT Island can be selected; one should
only be selected if there are no others that meet the necessary
criteria. The named proxy-node is logically part of the
area/level.</t>
<t>There are two sources for candidate routers in the MRT Island to
connect to the named proxy-node. The first set are those routers that
are advertising the prefix; the named-proxy-cost assigned to each
prefix-advertising router is the announced cost to the prefix. The
second set are those routers in the MRT Island that are connected to
routers not in the MRT Island but in the same area/level; such routers
will be defined as Island Border Routers (IBRs). The routers
connected to the IBRs that are not in the MRT Island and are in the
same area/level as the MRT island are Island Neighbors(INs).</t>
<t>Since packets sent to the named proxy-node along MRT-Red or
MRT-Blue may come from any router inside the MRT Island, it is
necessary that whatever router to which an IBR forwards the packet be
loop-free with respect to the whole MRT Island for the destination.
Thus, an IBR is a candidate router only if it possesses at least one
IN whose shortest path to the prefix does not enter the MRT Island. A
method for identifying loop-free Island Neighbors(LFINs) is discussed
below. The named-proxy-cost assigned to each (IBR, IN) pair is
cost(IBR, IN) + D_opt(IN, prefix).</t>
<t>From the set of prefix-advertising routers and the set of IBRs with
at least one LFIN, the two routers with the lowest named-proxy-cost
are selected. Ties are broken based upon the lowest Router ID. For
ease of discussion, the two selected routers will be referred to as
proxy-node attachment routers.</t>
<t>A proxy-node attachment router has a special forwarding role. When
a packet is received destined to (MRT-Red, prefix) or (MRT-Blue,
prefix), if the proxy-node attachment router is an IBR, it MUST swap
to the shortest path forwarding topology (e.g. swap to the label for (MT-ID 0, prefix)
or remove the outer IP encapsulation) and forward the packet to the IN
whose cost was used in the selection. If the proxy-node attachment
router is not an IBR, then the packet MUST be removed from the MRT
forwarding topology and sent along the interface(s) that caused the
router to advertise the prefix; this interface might be out of the
area/level/AS.</t>
<section title="Computing if an Island Neighbor (IN) is loop-free">
<t>As discussed above, the Island Neighbor needs to be loop-free with
respect to the whole MRT Island for the destination. This can be
accomplished by running the usual SPF algorithm while keeping track of
which shortest paths have passed through the MRT island. Pseudo-code for
this is shown in <xref target="fig_island_marking_spf"/>. The
Island_Marking_SPF() is run for each IN that needs to be evaluated for
the loop-free condition, with the IN as the spf_root. Whether or not an
IN is loop-free with respect to the MRT island can then be determined by
evaluating node.PATH_HITS_ISLAND for each destination of interest.</t>
<figure anchor="fig_island_marking_spf" align="center">
<artwork align="center"><![CDATA[
Island_Marking_SPF(spf_root)
Initialize spf_heap to empty
Initialize nodes' spf_metric to infinity and next_hops to empty
and PATH_HITS_ISLAND to false
spf_root.spf_metric = 0
insert(spf_heap, spf_root)
while (spf_heap is not empty)
min_node = remove_lowest(spf_heap)
foreach interface intf of min_node
path_metric = min_node.spf_metric + intf.metric
if path_metric < intf.remote_node.spf_metric
intf.remote_node.spf_metric = path_metric
if min_node is spf_root
intf.remote_node.next_hops = make_list(intf)
else
intf.remote_node.next_hops = min_node.next_hops
if intf.remote_node.IN_MRT_ISLAND
intf.remote_node.PATH_HITS_ISLAND = true
else
intf.remote_node.PATH_HITS_ISLAND =
min_node.PATH_HITS_ISLAND
insert_or_update(spf_heap, intf.remote_node)
else if path_metric == intf.remote_node.spf_metric
if min_node is spf_root
add_to_list(intf.remote_node.next_hops, intf)
else
add_list_to_list(intf.remote_node.next_hops,
min_node.next_hops)
if intf.remote_node.IN_MRT_ISLAND
intf.remote_node.PATH_HITS_ISLAND = true
else
intf.remote_node.PATH_HITS_ISLAND =
min_node.PATH_HITS_ISLAND
]]></artwork>
</figure>
<t>It is also possible that a given prefix is originated by a
combination of non-island routers and island routers. The results of the
Island_Marking_SPF computation can be used to determine if the shortest
path from an IN to reach that prefix hits the MRT island. The shortest
path for the IN to reach prefix P is determined by the total cost to
reach prefix P, which is the sum of the cost for the IN to reach a
prefix-advertising node and the cost with which that node advertises the
prefix. The path with the minimum total cost to prefix P is chosen. If
the prefix-advertising node for that minimum total cost path has
PATH_HITS_ISLAND set to True, then the IN is not loop-free with respect
to the MRT Island for reaching prefix P. If there multiple minimum total
cost paths to reach prefix P, then all of the prefix-advertising routers
involved in the minimum total cost paths MUST have PATH_HITS_ISLAND set
to False for the IN to be considered loop-free to reach P. </t>
<t>Note that there are other computations that could be used to
determine if paths from a given IN _might_ pass through the MRT Island
for a given prefix or destination. For example, a previous version of
this draft specified running the SPF algorithm on modified topology
which treats the MRT island as a single node (with intra-island links
set to zero cost) in order to provide input to computations to determine
if the path from IN to non-island destination hits the MRT island in
this modified topology. This computation is enough to guarantee that a
path will not hit the MRT island in the original topology. However, it
is possible that a path which is disqualified for hitting the MRT island
in the modified topology will not actually hit the MRT Island in the
original topology. The algorithm described in Island_Marking_SPF() above
does not modify the original topology, and will only disqualify a path
if the actual path does in fact hit the MRT island. </t>
<t>Since all routers need to come to the same conclusion about which
routers qualify as LFINs, this specification requires that all routers
computing LFINs MUST use an algorithm whose result is identical to that
of the Island_Marking_SPF() in <xref target="fig_island_marking_spf"/>.
</t>
</section>
</section>
<section title="MRT Alternates for Destinations Outside the MRT Island">
<t>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.</t>
<t>Destinations inside the local island can obviously use MRT
alternates. Destinations outside the local island can be treated like
a multi-homed prefix and either Endpoint Selection or Named
Proxy-Nodes can be used. Named Proxy-Nodes MUST be supported when LDP
forwarding is supported and a label-binding for the destination is
sent to an IBR.</t>
<t>Naturally, there are more complicated options to improve coverage,
such as connecting multiple MRT islands across tunnels, but the need
for the additional complexity has not been justified.</t>
</section>
</section>
<section title="Network Convergence and Preparing for the Next Failure">
<t>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.</t>
<section title="Micro-forwarding loop prevention and MRTs">
<t>As is well known<xref target="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.</t>
<t>There are two possible micro-loop prevention mechanisms discussed
in <xref target="RFC5715"/>. The first is Ordered FIB <xref
target="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.</t>
<t>Since MRTs provide an alternate topology through which traffic can
be sent and which can be manipulated separately from the SPT, it is
possible that MRTs could be used to support Farside Tunneling.
Details of how to do so are outside the scope of this document.</t>
<t>Micro-loop mitigation mechanisms can also work when combined with
MRT.</t>
</section>
<section anchor="sec_recalculation" title="MRT Recalculation">
<t>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.</t>
<t>At each router, therefore, the sequence is as follows:
<list style="numbers">
<t>Receive failure notification</t>
<t>Recompute SPT</t>
<t>Install new SPT</t>
<t>If the network was stable before the failure occured, wait a
configured (or advertised) period for all routers to be using their SPTs
and traffic to drain from the MRTs.</t>
<t>Recompute MRTs</t>
<t>Install new MRTs.</t>
</list>
</t>
<t>While the recomputed MRTs are not installed in the FIB, protection
coverage is lowered. Therefore, it is important to recalculate the
MRTs and install them quickly.</t>
</section>
</section>
<section title="Implementation Status">
<t>
[RFC Editor: please remove this section prior to publication.]
</t>
<t>This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in <xref
target="RFC6982"/>. The description of implementations in this
section is intended to assist the IETF in its decision processes in
progressing drafts to RFCs. Please note that the listing of any
individual implementation here does not imply endorsement by the IETF.
Furthermore, no effort has been spent to verify the information
presented here that was supplied by IETF contributors. This is not
intended as, and must not be construed to be, a catalog of available
implementations or their features. Readers are advised to note that
other implementations may exist.</t>
<t>According to <xref target="RFC6982"/>, "this will allow reviewers
and working groups to assign due consideration to documents that have
the benefit of running code, which may serve as evidence of valuable
experimentation and feedback that have made the implemented protocols
more mature. It is up to the individual working groups to use this
information as they see fit".</t>
<t> Juniper Networks Implementation
<list style="symbols">
<t>Organization responsible for the implementation:
Juniper Networks</t>
<t>Implementation name: MRT-FRR algorithm </t>
<t>Implementation description: The MRT-FRR algorithm
using OSPF as the IGP has been implemented and verified. </t>
<t>The implementation's level of maturity: prototype </t>
<t>Protocol coverage: This implementation of the MRT algorithm
includes Island identification, GADAG root selection, Lowpoint
algorithm, augmentation of GADAG with additional links, and
calculation of MRT transit next-hops alternate next-hops based on
draft "draft-ietf-rtgwg-mrt-frr-algorithm-00". This implementation
also includes the M-bit in OSPF based on "draft-atlas-ospf-mrt-01" as
well as LDP MRT Capability based on
"draft-atlas-mpls-ldp-mrt-00". </t>
<t>Licensing: proprietary </t>
<t>Implementation experience: Implementation was useful for
verifying functionality and lack of gaps. It has also been useful for
improving aspects of the algorithm. </t>
<t>Contact information: akatlas@juniper.net,
shraddha@juniper.net, kishoret@juniper.net </t>
</list>
</t>
<t> Huawei Technology Implementation
<list style="symbols">
<t>Organization responsible for the implementation:
Huawei Technology Co., Ltd.</t>
<t>Implementation name: MRT-FRR algorithm and IS-IS extensions for MRT. </t>
<t>Implementation description: The MRT-FRR algorithm, IS-IS extensions
for MRT and LDP multi-topology have been implemented and verified.</t>
<t>The implementation's level of maturity: prototype </t>
<t>Protocol coverage: This implementation of the
MRT algorithm includes Island
identification, GADAG root selection, Lowpoint algorithm,
augmentation of GADAG with additional links, and calculation of
MRT transit next-hops alternate next-hops based on
draft "draft-enyedi-rtgwg-mrt-frr-algorithm-03". This implementation
also includes IS-IS extension for MRT
based on "draft-li-mrt-00". </t>
<t>Licensing: proprietary </t>
<t>Implementation experience: It is important produce a second
implementation to verify the algorithm is implemented correctly
without looping. It is important to verify the ISIS extensions work
for MRT-FRR. </t>
<t>Contact information: lizhenbin@huawei.com, eric.wu@huawei.com </t>
</list>
</t>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>The authors would like to thank Mike Shand for his valuable
review and contributions.</t>
<t>The authors would like to thank Joel Halpern, Hannes Gredler, Ted
Qian, Kishore Tiruveedhula, Shraddha Hegde, Santosh Esale, Nitin
Bahadur, Harish Sitaraman, Raveendra Torvi, Anil Kumar SN, Bruno Decraene,
Eric Wu, and Janos Farkas for their suggestions and review.</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>Please create an MRT Profile registry for the MRT Profile TLV. The
range is 0 to 255. The default MRT Profile has value 0. Values 1-200
are by Standards Action. Values 201-220 are for experimentation.
Values 221-255 are for vendor private use.</t>
</section>
<section anchor="Security" title="Security Considerations">
<t>This architecture is not currently believed to introduce new security concerns.</t>
</section>
</middle>
<back>
<references title="Normative References">
&RFC5714;
&RFC5286;
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.draft-ietf-rtgwg-mrt-frr-algorithm-05.xml"?>
</references>
<references title="Informative References">
&RFC2119;
&RFC2328;
&RFC3137;
&RFC5443;
&RFC5715;
&RFC6571;
&RFC6982;
&RFC7490;
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.7307.xml"?>
&I-D.atlas-rtgwg-mrt-mc-arch;
&I-D.bryant-ipfrr-tunnels;
&I-D.ietf-rtgwg-ipfrr-notvia-addresses;
&I-D.ietf-rtgwg-ordered-fib;
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.draft-francois-spring-segment-routing-ti-lfa-01.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.draft-ietf-isis-mrt-00.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.draft-ietf-mpls-ldp-mrt-00.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.draft-ietf-ospf-mrt-00.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.draft-ietf-rtgwg-rlfa-node-protection-02.xml"?>
<reference anchor="LightweightNotVia"
target="http://mycite.omikk.bme.hu/doc/71691.pdf">
<front>
<title>IP Fast ReRoute: Lightweight Not-Via without Additional Addresses</title>
<author fullname="Gábor Sándor Enyedi" initials="G.S.E." surname="Enyedi"/>
<author fullname="Gabor Retvari" initials="G.R." surname="Retvari"/>
<author fullname="Peter Szilagyi" initials="P.S." surname="Szilagyi"/>
<author fullname="András Császár" initials="A.C." surname="Császár"/>
<date year="2009" />
</front>
<seriesInfo name="Proceedings of IEEE INFOCOM" value=""/>
<format type='PDF' target="http://mycite.omikk.bme.hu/doc/71691.pdf"/>
</reference>
<reference anchor="LFARevisited"
target="http://opti.tmit.bme.hu/~tapolcai/papers/retvari2011lfa_infocom.pdf">
<front>
<title>IP Fast ReRoute: Loop Free Alternates Revisited</title>
<author fullname="Gabor Retvari" initials="G.R." surname="Retvari"/>
<author fullname="Janos Tapolcai" initials="J.T." surname="Tapolcai"/>
<author fullname="Gábor Sándor Enyedi" initials="G.S.E." surname="Enyedi"/>
<author fullname="András Császár" initials="A.C." surname="Császár"/>
<date year="2011" />
</front>
<seriesInfo name="Proceedings of IEEE INFOCOM" value=""/>
<format type='PDF' target="http://opti.tmit.bme.hu/~tapolcai/papers/retvari2011lfa_infocom.pdf"/>
</reference>
<reference anchor="EnyediThesis"
target="http://timon.tmit.bme.hu/theses/thesis_book.pdf">
<front>
<title>Novel Algorithms for IP Fast Reroute</title>
<author fullname="Gábor Sándor Enyedi" initials="G.S.E." surname="Enyedi"/>
<date month="February" year="2011"/>
</front>
<seriesInfo name="Department of Telecommunications and Media Informatics, Budapest University of Technology and Economics" value="Ph.D. Thesis"/>
<format type='PDF' target="http://www.omikk.bme.hu/collections/phd/Villamosmernoki_es_Informatikai_Kar/2011/Enyedi_Gabor/ertekezes.pdf" />
</reference>
</references>
<section title="General Issues with Area Abstraction">
<t>When a multi-homed prefix is connected in two different areas, it
may be impractical to protect them without adding the complexity of
explicit tunneling. This is also a problem for LFA and Remote-LFA.</t>
<figure anchor="fig_mhp_areas" title="AS external prefixes in different areas">
<artwork align="center"><![CDATA[
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
]]></artwork>
</figure>
<t>Consider the network in <xref target="fig_mhp_areas"/> and assume
there is a richer connective topology that isn't shown, where the same
prefix is announced by ASBR X and ASBR Y which are in different
non-backbone areas. If the link from A to ASBR X fails, then an MRT
alternate could forward the packet to ABR 1 and ABR 1 could forward it
to D, but then D would find the shortest route is back via ABR 1 to
Area 20. This problem occurs because the routers, including the ABR,
in one area are not yet aware of the failure in a different area.</t>
<t>The only way to get it from A to ASBR Y is to explicitly tunnel it
to ASBR Y. If the traffic is unlabeled or the appropriate MPLS labels
are known, then explicit tunneling MAY be used as long as the
shortest-path of the tunnel avoids the failure point. In that case, A
must determine that it should use an explicit tunnel instead of an MRT
alternate.</t>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 00:16:02 |