One document matched: draft-ietf-rtgwg-mrt-frr-architecture-10.xml
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<rfc category="std" docName="draft-ietf-rtgwg-mrt-frr-architecture-10" 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." 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="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>
<date day="5" month="February" year="2016"/>
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<!-- Meta-data Declarations -->
<area>Routing</area>
<workgroup>Routing Area Working Group</workgroup>
<abstract>
<t>This document defines the architecture for IP and LDP Fast-Reroute using
Maximally Redundant Trees (MRT-FRR). 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>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>This document describes a solution for IP/LDP fast-reroute <xref
target="RFC5714"/>. MRT-FRR creates two alternate forwarding trees which
are distinct 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.
</t>
<t> <xref target="I-D.ietf-rtgwg-mrt-frr-algorithm"/> describes how
to compute maximally redundant trees using a specific algorithm,
the MRT Lowpoint algorithm. The MRT Lowpoint algorithm
is used by a router that supports the Default MRT Profile,
as specified in this document. </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
the MRTs by one point of local repair (PLR), that traffic is not subject to further repair
actions by another PLR, even in the event of multiple simultaneous
failures. Therefore, traffic repaired by MRT-FRR will not loop between
different PLRs responding to different simultaneous failures. </t>
<t> While MRT provides 100% protection for a single link or node
failure, it may not protect traffic in the event of multiple
simultaneous failures, nor does take into account Shared Risk Link
Groups (SRLGs). Also, while the MRT Lowpoint algorithm is
computationally efficient, it is also new. In order for MRT-FRR to
function properly, all of the other nodes in the network that support
MRT must correctly compute next-hops based on the same algorithm, and
install the corresponding forwarding state. This is in contrast to other
FRR methods where the calculation of backup paths generally involves
repeated application of the simpler and widely-deployed shortest path
first (SPF) algorithm, and backup paths themselves re-use the forwarding
state used for shortest path forwarding of normal traffic. <xref
target="sec_OAM"/> provides operational guidance related to verification
of MRT forwarding paths.</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"/>. However, the current document
does not address the multicast applications of MRTs. </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 loop-free alternates (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
accommodate natural network topology changes.
</t>
<t>When a network needs to use Ordered FIB<xref
target="RFC6976"/> or Nearside Tunneling<xref
target="RFC5715"/> as a micro-loop prevention
mechanism <xref target="RFC5715"/>, then the whole IGP area needs to have
alternates available. This allows the micro-loop prevention
mechanism, which requires slower network convergence, to 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. See <xref target="sec_micro_loop"/> for more
discussion of micro-loop prevention and MRTs.
</t>
</section>
<section title="Partial Deployment and Backwards Compatibility">
<t>MRT-FRR supports partial deployment. Routers advertise their
ability 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. MRT Islands are
discussed in detail in <xref target="sec_island"/>, and partial
deployment is discussed in more detail in
<xref target="sec_partial_deployment"/>.
</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="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="block: ">Either a 2-connected cluster, a cut-edge,
or an isolated vertex.</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. Redundant trees can always
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. In graphs that are not 2-connected,
it is not possible to compute RTs. However, it is possible to compute
MRTs. MRTs are maximally redundant in the sense that they are as
redundant as possible given the constraints of the network graph.
</t>
<t hangText="Directed Acyclic Graph (DAG): ">A graph where all
links are directed and there are no cycles in it.</t>
<t hangText="Almost Directed Acyclic Graph (ADAG): ">A graph
with one node designated as the root. The graph has the property
that if all links incoming to the root were removed, then resulting
graph would be a DAG.</t>
<t hangText="Generalized ADAG (GADAG): ">A graph that is the
combination of the ADAGs of all blocks.</t>
<t hangText="MRT-Red: "> MRT-Red is used to describe one of the
two MRTs; it is used to describe the associated forwarding
topology and MPLS multi-topology identifier (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 forwarding topologies and to the default
forwarding topology. This is referred to as the Rainbow MRT MPLS MT-ID and is
used by LDP to reduce signaling and permit the same label to
always be advertised to all peers for the same (MT-ID,
Prefix).</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="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 (Forwarding Equivalence Class) 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 MRT Lowpoint algorithm 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. 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 (SPT) to
all destinations. From the perspective of a particular destination,
D, this looks like a reverse SPT. 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="RFC6981"/> 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 looping among alternates. 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.</t>
</section>
<section anchor="sec_uni_forwarding" title="Unicast Forwarding with MRT Fast-Reroute">
<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 an 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="Introduction to MRT Forwarding Options">
<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. In general, this approach will
also increase the time needed to install the FRR entries in the Forwarding Information Base (FIB)
and hence the time needed before the next failure can be protected. </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
will be described elsewhere.</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 <xref target="RFC5331"/>. However, the precise
LDP behavior required to support this option for MRT has not been
specified.</t>
</section>
<section title="Compatibility of MRT LDP Label Options 1A and 1B">
<t> MRT transit forwarding based on MRT LDP Label options 1A and 1B can
coexist in the same network, with a packet being forwarded 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. However, to simplify
the process of MRT Island formation we require that all routers in the
MRT Island support at least one common forwarding mechanism. As an example, the
Default MRT Profile requires support for the MRT LDP Label Option 1A
forwarding mechanism. This ensures that the routers in an MRT island
supporting the Default MRT Profile will be able to establish MRT
forwarding paths based on MRT LDP Label Option 1A. However, an
implementation supporting Option 1A may also support Option 1B. If the
scaling or performance characteristics for the two options differ in
this implementation, then it may be desirable for a pair of adjacent
routers to use Option 1B labels instead of the Option 1A labels.
If those routers successfully negotiate the use of Option 1B labels,
they are free to use them. This can occur without any of the other
routers in the MRT Island being made aware of it.
</t>
<t> Note that this document only defines the Default MRT Profile which
requires support for the MRT LDP Label Option 1A forwarding mechanism.
</t>
</section>
<section title="Required support for MRT LDP Label options">
<t> If a router supports a profile that includes the MRT LDP Label
Option 1A for the MRT transit forwarding mechanism, then it MUST support
option 1A, which encodes topology-scoped FECs using a single label. The
router MAY also support option 1B. </t>
<t> If a router supports a profile that includes the MRT LDP Label
Option 1B for the MRT transit forwarding mechanism, then it MUST support
option 1B, which encodes the topology and FEC using a two label stack.
The router MAY also support option 1A. </t>
</section>
</section>
<section anchor="sec_MRT_IP_tunnel" 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.
For example, an MRT ingress router can cause a packet to be tunneled
along the MRT-red path to router X by encapsulating the packet using
the MRT-red loopback address advertised by router X. Upon receiving the
packet, router X would remove the encapsulation header and forward the
packet based on the original destination address.</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 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.
As described below, the two MRT forwarding mechanisms that
use LDP labels do not require targeted LDP sessions.</t>
<section title="Forwarding LDP traffic using MRT LDP Label 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 Label 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 can be done
without the use of targeted LDP sessions when an MRT path to the
destination FEC is used. The alternates selected in <xref
target="I-D.ietf-rtgwg-mrt-frr-algorithm"/> use the MRT path to the
destination FEC, so targeted LDP sessions are not needed. If instead one
found 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, 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>
</section>
<section title="Required support for LDP traffic ">
<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 IPv4 traffic, there is no currently practical alternative
except tunneling to gain the bits needed to indicate the MRT-Blue or
MRT-Red forwarding topology. For IPv6 traffic, in principle one could
define bits in the IPv6 options header to indicate the MRT-Blue or
MRT-Red forwarding topology. However, in this document, we have chosen
not to define a solution that would work for IPv6 traffic but not for
IPv4 traffic.</t>
<t>The choice of tunnel egress is
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.</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 Area Border Router (ABR)
or Level Border Router (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 Label 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 Label 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 variation that uses
a single label as opposed to two labels. 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 for IP traffic">
<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 Label 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 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
consistent 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 information
already exists in the IGPs and can be used by MRT
in Island formation, subject to the interpretation defined here.
</t>
<t>Other information needs to be communicated between routers for which
there do not currently exist protocol extensions. This new information
needs to be shared among all routers in an IGP area, so defining
extensions to existing IGPs to carry this information makes sense. These
new protocol extensions will be defined elsewhere.
</t>
<t> Deployment scenarios using multi-topology OSPF or IS-IS, or
running both IS-IS 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>
<t> At a high level, an MRT Island is defined as the set of routers
supporting the same MRT profile, in the same IGP area/level and the
bi-directional links interconnecting those routers. More detailed
descriptions of these criteria are given below. </t>
<section title="IGP Area or Level">
<t> All links in an MRT Island are bidirectional and belong to
the same IGP area or level. For IS-IS, 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 support the same MRT profile. A
router advertises support for a given MRT profile using an 8-bit MRT
Profile ID value. The registry for the MRT Profile ID is defined in this
document. The protocol extensions for advertising the MRT Profile ID
value will be defined elsewhere. 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>
<t>The process of MRT Island formation takes place independently
for each MRT profile advertised by a given router. For example, consider
a network with 40 connected routers in the same area advertising support
for MRT Profile A and MRT Profile B. Two distinct MRT Islands will be
formed corresponding to Profile A and Profile B, with each island
containing all 40 routers. A complete set of maximally redundant trees
will be computed for each island following the rules defined for each
profile. If we add a third MRT Profile to this example, with Profile C
being advertised by a connected subset of 30 routers, there will be a
third MRT Island formed corresponding to those 30 routers, and a third
set of maximally redundant trees will be computed. In this example,
40 routers would compute and install two sets of MRT transit forwarding
entries corresponding to Profiles A and B, while 30 routers would
compute and install three sets of MRT transit forwarding
entries corresponding to Profiles A, B, and C.</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 IS-IS and OSPF. In IS-IS, 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 <xref target="RFC5443"/>.
</t>
<t> Mechanisms also already exist in IS-IS and OSPF to discourage
or prevent transit
traffic from using a particular router. In IS-IS, the overload
bit is prevents transit traffic from using a router.
</t>
<t> For OSPFv2 and OSPFv3, <xref target="RFC6987"/> specifies
setting all outgoing interface metrics to 0xFFFF to discourage
transit traffic from using a router.( <xref target="RFC6987"/>
defines the metric value 0xFFFF
as MaxLinkMetric, a fixed architectural value for OSPF.)
For OSPFv3, <xref target="RFC5340"/> specifies that a router be
excluded from the intra-area shortest path tree computation
if the V6-bit or R-bit of the LSA options is not set in the Router LSA.
</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 or prevented 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 IS-IS) 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 IS-IS), or it is
advertised with metric values of 0xFFFF on all of its outgoing
interfaces (for OSPFv2 and OSPFv3).</t>
<t> A router MUST be excluded from an MRT Island if it is
advertised with either the V6-bit or R-bit of the LSA options
not set in the Router LSA.</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. The protocol extensions for advertising
that a link is MRT-Ineligible will be defined elsewhere. A link with
either interface advertised as MRT-Ineligible MUST be excluded from an
MRT Island. Note that an interface advertised as MRT-Ineligible 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="Algorithm for MRT Island Identification">
<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 5.2 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>
<t>This document specifies the values and options that correspond
to the Default MRT Profile (Profile ID = 0). Future documents may
define other MRT Profiles by specifying the MRT Profile Options below. </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
algorithm for computing maximally redundant trees
used by the router for this profile.</t>
<t hangText="MRT-Red MT-ID: ">This specifies the MPLS MT-ID to be
associated with the MRT-Red forwarding topology. It is
allocated from the MPLS Multi-Topology Identifiers Registry.</t>
<t hangText="MRT-Blue MT-ID: ">This specifies the MPLS MT-ID to be
associated with the MRT-Blue forwarding topology. It is
allocated from the MPLS Multi-Topology Identifiers Registry.</t>
<t hangText="GADAG Root Selection Policy: ">This specifies 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 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 Label Option 1A,
MRT LDP Label Option 1B, IPv4 Tunneling, IPv6
Tunneling, and None. If IPv4 is supported, then both MRT-Red and MRT-Blue
IPv4 Loopback Addresses SHOULD be specified. If IPv6 is supported,
both MRT-Red and MRT-Blue IPv6 Loopback Addresses SHOULD be
specified.</t>
<t hangText="Recalculation: ">Recalculation specifies the process and timing
by which new MRTs are computed after the topology has been modified.
</t>
<t hangText="Area/Level Border Behavior: "> This specifies how
traffic traveling on the MRT-Blue or MRT-Red in one area should be
treated when it passes into another area. </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>When a new MRT Profile is defined, new and unique values should be
allocated from the MPLS Multi-Topology Identifiers Registry,
corresponding to the MRT-Red and MRT-Blue MT-ID values for the new MRT
Profile .</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.</t>
<t>The ability of MRT-FRR to support transit forwarding entries for
multiple profiles can be used to facilitate a smooth transition from an
existing deployed MRT Profile to a new MRT Profile. The new profile can
be activated in parallel with the existing profile, installing the
transit forwarding entries for the new profile without affecting the
transit forwarding entries for the existing profile. Once the new
transit forwarding state has been verified, the router can be configured
to use the alternates computed by the new profile in the event of a
failure.</t>
</section>
<section title="Router-specific MRT paramaters">
<t>For some profiles, additional router-specific MRT parameters may
need to be advertised. 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 or IPv6. Note that
this parameter is not needed to support the Default MRT profile.</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. Note that
this parameter is not needed to support the Default MRT profile.</t>
</list></t>
<t> Protocol extensions for advertising a router's GADAG Root Selection
Priority value will be defined in other documents. Protocol extensions for the
advertising a router's MRT-Red and MRT-Blue Loopback Addresses will be
defined elsewhere.
</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: "> This value will be allocated from
the MPLS Multi-Topology Identifiers Registry. The IANA request
for this allocation will be in another document.</t>
<t hangText="MRT-Blue MPLS MT-ID: "> This value will be allocated from
the MPLS Multi-Topology Identifiers Registry. The IANA request
for this allocation will be in another document.</t>
<t hangText="GADAG Root Selection Policy: "> Among the routers in the MRT
Island with the lowest numerical value advertised for GADAG Root
Selection Priority, an implementation MUST pick the router with the
highest Router ID to be the GADAG root. Note that a lower numerical
value for GADAG Root Selection Priority indicates a higher preference
for selection.
</t>
<t hangText="Forwarding Mechanisms: ">MRT LDP Label Option 1A</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 will be defined in
another document. 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 to different LDP neighbors 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> The value of the Rainbow MRT MPLS MT-ID will be allocated from
the MPLS Multi-Topology Identifiers Registry. The IANA request
for this allocation will be in another document.
</t>
</section>
<section anchor= "sec_abr_forwarding" title="Inter-area Forwarding Behavior">
<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 a single router failure that manifests
itself in multiple areas (as would be the case with an ABR/LBR failure)
to be separately identified and
repaired around. Third, the packet can be fast-rerouted again, if
necessary, due to a second distinct failure in a different area.</t>
<t>In OSPF, an ABR 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 OSPF 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> The above description applies to OSPF. The same essential behavior
also applies to IS-IS if one substitutes IS-IS level for OSPF area.
However, the analogy with OSPF is not exact. An interface in OSPF can
only be in one area, whereas an interface in IS-IS can be in both
Level-1 and Level-2. Therefore, to avoid confusion and address this
difference, we explicitly describe the behavior for IS-IS in <xref
target="sec_inter_level_isis"/>. In the following sections only the OSPF
terminology is used. </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 is responsible for advertising the proper label to each
neighbor. Assume that an ABR 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 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 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 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 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 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 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 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 implementations and
doesn't have other obvious uses.
</t>
<t>Many existing LDP implementations 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> An ABR may choose to either advertise the Rainbow-FEC or
advertise separate MRT-Blue and MRT-Red advertisements. This is a local choice.
A router that supports the MRT LDP Label Option 1A 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 behavior is dependent
upon the outermost IP address. If the outermost IP address is an MRT
loopback address of the ABR, 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, 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 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 MRT 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 would need to determine
whether the ABR 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.</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 IS-IS 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, protocol extensions could
be developed to address this issue.</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, 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>
See <xref target="sec_area_abstraction"/> for a discussion of a general issue
with multi-homed prefixes connected in two different areas.
</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"/>. In the inequalities below,
D_opt(X,Y) means the shortest distance from node X to node Y, and
D_opt(X,p) means the shortest distance from node X to prefix p. </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 will be
defined elsewhere. The details of what
label-bindings must be originated will be described in another document.</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 (see <xref target="RFC2328"/> Section
16.5) does incremental updates for new summary-LSAs.</t>
<t>The remaining 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
in the MRT Island 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 given in <xref target="I-D.ietf-rtgwg-mrt-frr-algorithm"/>.
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>
<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 anchor="sec_micro_loop" title="Micro-loop prevention and MRTs">
<t> A micro-loop is a transient packet forwarding loop among two or more
routers that can occur during convergence of IGP forwarding state.
<xref target="RFC5715"/>
discusses several techniques for preventing micro-loops. This section
discusses how MRT-FRR relates to two of the micro-loop prevention
techniques discussed in <xref target="RFC5715"/>, Nearside Tunneling
and Farside Tunneling. </t>
<t> In Nearside Tunneling, a router (PLR) adjacent to a failure perform
local repair and inform remote routers of the failure. The remote
routers initially tunnel affected traffic to the nearest PLR, using
tunnels which are unaffected by the failure. Once the forwarding state
for normal shortest path routing has converged, the remote routers
return the traffic to shortest path forwarding. MRT-FRR is relevant for
Nearside Tunneling for the following reason. The process of tunneling
traffic to the PLRs and waiting a sufficient amount of time for IGP
forwarding state convergence with Nearside Tunneling means that traffic
will generally be relying on the local repair at the PLR for longer than
it would in the absence of Nearside Tunneling. Since MRT-FRR provides
100% coverage for single link and node failure, it may be an attractive
option to provide the local repair paths when Nearside Tunneling is
deployed. </t>
<t> MRT-FRR is also relevant for the Farside Tunneling micro-loop
prevention technique. In Farside Tunneling, remote routers tunnel
traffic affected by a failure to a node downstream of the failure with
respect to traffic destination. This node can be viewed as being on the
farside of the failure with respect to the node initiating the tunnel.
Note that the discussion of Farside Tunneling in <xref
target="RFC5715"/> focuses on the case where the farside node is
immediately adjacent to a failed link or node. However, the farside node
may be any node downstream of the failure with respect to traffic
destination, including the destination itself. The tunneling mechanism
used to reach the farside node must be unaffected by the failure. The
alternative forwarding paths created by MRT-FRR have the potential to be
used to forward traffic from the remote routers upstream of the failure
all the way to the destination. In the event of failure, either the
MRT-Red or MRT-Blue path from the remote upstream router to the
destination is guaranteed to avoid a link failure or inferred node
failure. The MRT forwarding paths are also guaranteed to
not be subject to micro-loops because they are locked to the topology
before the failure.</t>
<t> We note that the computations in <xref
target="I-D.ietf-rtgwg-mrt-frr-algorithm"/> address the case of a PLR
adjacent to a failure determining which choice of MRT-Red or MRT-Blue
will avoid a failed link or node. More computation may be required for
an arbitrary remote upstream router to determine whether to choose
MRT-Red or MRT-Blue for a given destination and failure.</t>
</section>
<section anchor="sec_recalculation" title="MRT Recalculation for the Default MRT Profile">
<t>This section describes how the MRT recalculation SHOULD be performed for
the Default MRT Profile. This is intended to support FRR applications.
Other approaches are possible, but they are not specified in this
document. </t>
<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 the new SPT in the FIB.</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 in the FIB.</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>
<t> New protocol extensions for advertising the time needed to recompute
shortest path routes and install them in the FIB will be defined
elsewhere.</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 </t>
<t>Implementation description: MRT-FRR
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-FRR
includes Island identification, GADAG root selection, MRT 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 and IS-IS extensions for MRT. </t>
<t>Implementation description: The MRT-FRR using 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, MRT 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 IS-IS extensions work
for MRT-FRR. </t>
<t>Contact information: lizhenbin@huawei.com, eric.wu@huawei.com </t>
</list>
</t>
</section>
<section anchor="sec_OAM" title="Operational Considerations">
<t> The following aspects of MRT-FRR are useful to consider
when deploying the technology in different operational environments
and network topologies.</t>
<section title="Verifying Forwarding on MRT Paths">
<t> The forwarding paths created by MRT-FRR are not used by normal
(non-FRR) traffic. They are only used to carry FRR traffic for a short
period of time after a failure has been detected. It is RECOMMENDED that
an operator proactively monitor the MRT forwarding paths in order to be
certain that the paths will be able to carry FRR traffic when needed.
Therefore, an implementation SHOULD provide an operator with the ability
to test MRT paths with Operations, Administration, and Maintenance (OAM) traffic.
For example, when MRT paths are
realized using LDP labels distributed for topology-scoped FECs, an
implementation can use the MPLS ping and traceroute as defined in <xref
target="RFC4379"/> and extended in <xref target="RFC7307"/> for
topology-scoped FECs. </t>
</section>
<section anchor="sec_traffic_on_backup_paths" title="Traffic Capacity on Backup Paths">
<t> During a fast-reroute event initiated by a PLR in response to a
network failure, the flow of traffic in the network will generally not
be identical to the flow of traffic after the IGP forwarding state has
converged, taking the failure into account. Therefore, even if a network
has been engineered to have enough capacity on the appropriate links to
carry all traffic after the IGP has converged after the failure, the
network may still not have enough capacity on the appropriate links to
carry the flow of traffic during a fast-reroute event. This can result
in more traffic loss during the fast-reroute event than might otherwise
be expected.</t>
<t> Note that there are two somewhat distinct aspects to this
phenomenon. The first is that the path from the PLR to the destination
during the fast-reroute event may be different from the path after the
IGP converges. In this case, any traffic for the destination that
reaches the PLR during the fast-reroute event will follow a different
path from the PLR to the destination than will be followed after IGP
convergence.</t>
<t> The second aspect is that the amount of traffic arriving at the PLR
for affected destinations during the fast-reroute event may be larger
than the amount of traffic arriving at the PLR for affected destinations
after IGP convergence. Immediately after a failure, any non-PLR routers
that were sending traffic to the PLR before the failure will continue
sending traffic to the PLR, and that traffic will be carried over backup
paths from the PLR to the destinations. After IGP convergence,
upstream non-PLR routers may direct some traffic away from the PLR.</t>
<t> In order to reduce or eliminate the potential for transient traffic
loss due to inadequate capacity during fast-reroute events, an operator
can model the amount of traffic taking different paths during a
fast-reroute event. If it is determined that there is not enough
capacity to support a given fast-reroute event, the operator can address
the issue either by augmenting capacity on certain links or modifying
the backup paths themselves. </t>
<t> The MRT Lowpoint algorithm produces a pair of diverse paths to each
destination. These paths are generated by following the directed links
on a common GADAG. The decision process for constructing the GADAG in the
MRT Lowpoint algorithm takes into account individual IGP link metrics. At
any given node, links are explored in order from lowest IGP metric to
highest IGP metric. Additionally, the process for constructing the MRT-Red
and Blue trees uses SPF traversals of the GADAG. Therefore, the IGP
link metric values affect the computed backup paths. However, adjusting the
IGP link metrics is not a generally applicable tool for modifying the
MRT backup paths. Achieving a desired set of MRT backup paths by adjusting
IGP metrics while at the same time maintaining the desired flow of traffic
along the shortest paths is not possible in general. </t>
<t> MRT-FRR allows an operator to exclude a link from the
MRT Island, and thus the GADAG, by advertising it as MRT-Ineligible.
Such a link will not be used on the MRT forwarding path for any
destination. Advertising links as MRT-Ineligible is the main tool
provided by MRT-FRR for keeping backup traffic off of lower bandwidth
links during fast-reroute events. </t>
<t> Note that all of the backup paths produced by the MRT Lowpoint
algorithm are closely tied to the common GADAG computed as part of that
algorithm. Therefore, it is generally not possible to modify a subset of
paths without affecting other paths. This precludes more fine-grained
modification of individual backup paths when using only paths computed
by the MRT Lowpoint algorithm. </t>
<t>However, it may be desirable to allow an operator to use MRT-FRR
alternates together with alternates provided by other FRR technologies.
A policy-based alternate selection process can allow an operator to
select the best alternate from those provided by MRT and other FRR
technologies. As a concrete example, it may be desirable to implement a
policy where a downstream LFA (if it exists for a given failure mode and
destination) is preferred over a given MRT alternate. This combination
gives the operator the ability to affect where traffic flows during a
fast-reroute event, while still producing backup paths that use no additional
labels for LDP traffic and will not loop under multiple failures. This
and other choices of alternate selection policy can be evaluated in the
context of their effect on fast-reroute traffic flow and available
capacity, as well as other deployment considerations. </t>
<t> Note that future documents may define MRT profiles in addition to
the default profile defined here. Different MRT profiles will generally
produce alternate paths with different properties. An implementation may
allow an operator to use different MRT profiles instead of or in
addition to the default profile. </t>
</section>
<section title="MRT IP Tunnel Loopback Address Management">
<t> As described in <xref target="sec_MRT_IP_tunnel"/>,
if an implementation uses IP tunneling as the mechanism to
realize MRT forwarding paths, each node must advertise an MRT-Red and
an MRT-Blue loopback address. These IP addresses must be unique within
the routing domain to the extent that they do not overlap with each other
or with any other routing table entries. It is expected that
operators will use existing tools and processes for managing
infrastructure IP addresses to manage these additional MRT-related
loopback addresses.
</t>
</section>
<section title="MRT-FRR in a Network with Degraded Connectivity">
<t> Ideally, routers is a service provider network using MRT-FRR will be
initially deployed in a 2-connected topology, allowing MRT-FRR to find
completely diverse paths to all destinations. However, a network can
differ from an ideal 2-connected topology for many possible reasons,
including network failures and planned maintenance events.</t>
<t> MRT-FRR is designed to continue to function properly when network
connectivity is degraded. When a network contains cut-vertices or
cut-links dividing the network into different 2-connected blocks,
MRT-FRR will continue to provide completely diverse paths for
destinations within the same block as the PLR. For a destination in a
different block from the PLR, the redundant paths created by MRT-FRR
will be link and node diverse within each block, and the paths will only
share links and nodes that are cut-links or cut-vertices in the
topology. </t>
<t> If a network becomes partitioned with one set of routers having no
connectivity to another set of routers, MRT-FRR will function
independently in each set of connected routers, providing redundant
paths to destinations in same set of connected routers as a given
PLR.</t>
</section>
<section anchor="sec_partial_deployment" title="Partial Deployment of MRT-FRR in a Network">
<t> A network operator may choose to deploy MRT-FRR only on a subset of
routers in an IGP area. MRT-FRR is designed to accommodate this partial
deployment scenario. Only routers that advertise support for a given MRT
profile will be included in a given MRT Island. For a PLR within the MRT
Island, MRT-FRR will create redundant forwarding paths to all
destinations with the MRT Island using maximally redundant trees all the
way to those destinations. For destinations outside of the MRT Island,
MRT-FRR creates paths to the destination which use forwarding state
created by MRT-FRR within the MRT Island and shortest path forwarding
state outside of the MRT Island. The paths created by MRT-FRR to
non-Island destinations are guaranteed to be diverse within the MRT
Island (if topologically possible). However, the part of the paths
outside of the MRT Island may not be diverse.</t>
</section>
</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, Janos Farkas, Rob Shakir, Stewart Bryant, and Alvaro
Retana for their suggestions and review.</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>IANA is requested to create a registry entitled
"MRT Profile Identifier Registry". The range is 0 to 255.
The Default MRT Profile defined in this document has value 0.
Values 1-200 are allocated by Standards Action.
Values 201-220 are for Experimental Use.
Values 221-254 are for Private Use.
Value 255 is reserved for future registry extension.
(The allocation and use policies are
described in <xref target="RFC5226"/>.)
</t>
<t> The initial registry is shown below.
<figure>
<artwork align="left"><![CDATA[
Value Description Reference
------- ---------------------------------------- ------------
0 Default MRT Profile [This draft]
1-200 Unassigned
201-220 Experimental Use
221-254 Private Use
255 Reserved (for future registry extension)
]]></artwork>
</figure>
</t>
<t> The MRT Profile Identifier Registry is a new registry in the IANA Matrix.
Following existing conventions,
http://www.iana.org/protocols should display a new header entitled
"Maximally Redundant Tree (MRT) Parameters". Under that header, there should
be an entry for "MRT Profile Identifier Registry" with a link to the registry
itself at http://www.iana.org/assignments/mrt-parameters/mrt-parameters.xhtml#mrt-profile-registry.
</t>
</section>
<section anchor="Security" title="Security Considerations">
<t>In general, MRT forwarding paths do not follow shortest paths.
The transit forwarding state corresponding to the MRT paths is
created during normal operations (before a failure occurs). Therefore, a
malicious packet with an appropriate header injected into the network
from a compromised location would be forwarded to a destination along a
non-shortest path. When this technology is deployed, a network security
design should not rely on assumptions about potentially malicious
traffic only following shortest paths.</t>
<t>It should be noted that the creation of non-shortest forwarding
paths is not unique to MRT.</t>
<t> MRT-FRR requires that routers advertise information used in the
formation of MRT backup paths. While this document does not specify the
protocol extensions used to advertise this information, we discuss
security considerations related to the information itself. Injecting
false MRT-related information could be used to direct some MRT backup
paths over compromised transmission links. Combined with the ability to
generate network failures, this could be used to send traffic over
compromised transmission links during a fast-reroute event. In order to
prevent this potential exploit, a receiving router needs to be able to
authenticate MRT-related information that claims to have been advertised by
another router.</t>
</section>
<section anchor="sec_contributors" title="Contributors">
<figure>
<artwork align="left"><![CDATA[
Robert Kebler
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Email: rkebler@juniper.net
Andras Csaszar
Ericsson
Konyves Kalman krt 11
Budapest 1097
Hungary
Email: Andras.Csaszar@ericsson.com
Jeff Tantsura
Ericsson
300 Holger Way
San Jose, CA 95134
USA
Email: jeff.tantsura@ericsson.com
Russ White
VCE
Email: russw@riw.us
]]></artwork>
</figure>
</section>
</middle>
<back>
<references title="Normative References">
&RFC2119;
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.7307.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.draft-ietf-rtgwg-mrt-frr-algorithm-06.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5226.xml"?>
</references>
<references title="Informative References">
&RFC5714;
&RFC5286;
&RFC2328;
&RFC5443;
&RFC5715;
&RFC6982;
&RFC7490;
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5331.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.5340.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.6976.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.6981.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.6987.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml/reference.RFC.4379.xml"?>
&I-D.atlas-rtgwg-mrt-mc-arch;
<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 anchor="sec_inter_level_isis" title="Inter-level Forwarding Behavior for IS-IS">
<t> In the description below, we use the terms "Level-1-only
interface", "Level-2-only interface", and "Level-1-and-Level-2
interface" to mean in interface which has formed only a Level-1
adjacency, only a Level-2 adjacency, or both Level-1 and Level-2
adjacencies. Note that IS-IS also defines the concept of areas. A router
is configured with an IS-IS area identifier, and a given router may be
configured with multiple IS-IS area identifiers. For an IS-IS Level-1
adjacency to form between two routers, at least one IS-IS area
identifier must match. IS-IS Level-2 adjacencies to not require any area
identifiers to match. The behavior described below does not explicitly
refer to IS-IS area identifiers. However, IS-IS area identifiers will indirectly
affect the behavior by affecting the formation of Level-1 adjacencies.
</t>
<t>First consider a packet destined to Z on MRT-Red or MRT-Blue received
on a Level-1-only interface. If the best shortest path route to Z was
learned from a Level-1 advertisement, then the packet should continue to
be forwarded along MRT-Red or MRT-Blue. If instead the best route was
learned from a Level-2 advertisement, then the packet should be removed
from MRT-Red or MRT-Blue and forwarded on the shortest-path default
forwarding topology. </t>
<t>Now consider a packet destined to Z on MRT-Red or MRT-Blue received
on a Level-2-only interface. If the best route to Z was learned from a
Level-2 advertisement, then the packet should continue to be forwarded
along MRT-Red or MRT-Blue. If instead the best route was learned from
a Level-1 advertisement, then the packet should be removed from MRT-Red
or MRT-Blue and forwarded on the shortest-path default forwarding
topology. </t>
<t>Finally, consider a packet destined to Z on MRT-Red or MRT-Blue
received on a Level-1-and-Level-2 interface. This packet should continue
to be forwarded along MRT-Red or MRT-Blue, regardless of which level the
route was learned from. </t>
<t> An implementation may simplify the decision-making process above by
using the interface of the next-hop for the route to Z to determine the
level that the best route to Z was learned from. If the next-hop points
out a Level-1-only interface, then the route was learned from a Level-1
advertisement. If the next-hop points out a Level-2-only interface, then
the route was learned from a Level-2 advertisement. A next-hop that
points out a Level-1-and-Level-2 interface does not provide enough
information to determine the source of the best route. With this
simplification, an implementation would need to continue forwarding
along MRT-Red or MRT-Blue when the next-hop points out a
Level-1-and-Level-2 interface. Therefore, a packet on MRT-Red or
MRT-Blue going from Level-1 to Level-2 (or vice versa) that traverses a
Level-1-and-Level-2 interface in the process will remain on MRT-Red or
MRT-Blue. This simplification may not always produce the optimal
forwarding behavior, but it does not introduce interoperability
problems. The packet will stay on an MRT backup path longer than
necessary, but it will still reach its destination. </t>
</section>
<section anchor="sec_area_abstraction" 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 11:05:15 |