One document matched: draft-ietf-mpls-ldp-mrt-01.xml
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<rfc category="std" docName="draft-ietf-mpls-ldp-mrt-01" 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="LDP Extensions to Support MRT">LDP Extensions to Support 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="Kishore Tiruveedhula" initials="K." surname="Tiruveedhula">
<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>kishoret@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="Jeff Tantsura" initials="J.T." surname="Tantsura">
<organization>Ericsson</organization>
<address>
<postal>
<street>300 Holger Way</street>
<city>San Jose</city>
<region>CA</region>
<code>95134</code>
<country>USA</country>
</postal>
<email>jeff.tantsura@ericsson.com</email>
</address>
</author>
<author fullname="IJsbrand Wijnands" initials="IJ.W." surname="Wijnands">
<organization>Cisco Systems, Inc.</organization>
<address>
<email>ice@cisco.com</email>
</address>
</author>
<date day="4" month="July" year="2015"/>
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<!-- Meta-data Declarations -->
<area>Routing</area>
<workgroup>MPLS Working Group</workgroup>
<abstract>
<t>This document specifies extensions to the Label Distribution
Protocol(LDP) to support the
creation of label-switched paths for Maximally Redundant Trees
(MRT). A prime use of MRTs is for unicast and multicast IP/LDP
Fast-Reroute, which we will refer to as MRT-FRR. </t>
<t>The sole protocol extension to LDP is simply the ability to
advertise an MRT Capability. This document describes that
extension and the associated behavior expected for LSRs and LERs
advertising the MRT Capability.</t>
<t>MRT-FRR uses LDP multi-topology extensions and requires three
different multi-topology IDs to be allocated from the LDP MT-ID
space.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>This document describes the LDP signaling extension and
associated behavior necessary to support the architecture that
defines how IP/LDP Fast-Reroute can use MRTs <xref
target="I-D.ietf-rtgwg-mrt-frr-architecture"/>. It is necessary
to be familiar with the architecture in <xref
target="I-D.ietf-rtgwg-mrt-frr-architecture"/> to understand how
and why the LDP extensions for behavior are needed.</t>
<t>At least one common standardized algorithm (e.g. the
MRT Lowpoint algorithm explained and fully documented in <xref
target="I-D.ietf-rtgwg-mrt-frr-algorithm"/>) is required so
that the routers supporting MRT computation consistently compute
the same MRTs. LDP depends on an IGP for computation of MRTs and
alternates. Extensions to OSPF are defined in <xref
target="I-D.ietf-ospf-mrt"/>. Extension to IS-IS are defined in
<xref target="I-D.ietf-isis-mrt"/>. </t>
<t>MRT can also be used to protect multicast traffic (signalled via
PIM or mLDP) using either
global protection or local protection <xref
target="I-D.atlas-rtgwg-mrt-mc-arch"/>. An MRT path can be used
to provide node-protection for mLDP traffic via the mechanisms
described in <xref
target="I-D.wijnands-mpls-mldp-node-protection"/>; an MRT path
can also be used to provide link protection for mLDP traffic.</t>
<t>For each destination, IP/LDP Fast-Reroute with MRT (MRT-FRR)
creates two alternate destination-based trees separate from the
shortest path forwarding used during stable operation. LDP
uses the multi-topology extensions <xref target="RFC7307"/> to signal
Forwarding Equivalency Classes (FECs) for
these two sets of forwarding trees, MRT-Blue and
MRT-Red.</t>
<t>In order to create MRT paths and support IP/LDP Fast-Reroute,
a new capability extension is needed for LDP. An LDP
implementation supporting MRT MUST also follow the
rules described here for originating and
managing FECs related to MRT, as
indicated by their multi-topology ID. Network reconvergence is
described in <xref
target="I-D.ietf-rtgwg-mrt-frr-architecture"/> and the
worst-case network convergence time can be flooded via the
extension in Section 7 of <xref
target="I-D.ietf-ospf-mrt"/>.</t>
<t>IP/LDP Fast-Reroute using MRTs can provide 100% coverage for
link and node failures in an arbitrary network topology where
the failure doesn't partition the network. It can also be deployed
incrementally; an MRT Island is formed of connected supporting
routers and the MRTs are computed inside that island.</t>
</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>For ease of reading, some of the terminology defined in <xref
target="I-D.ietf-rtgwg-mrt-frr-architecture"/> is repeated here.</t>
<t><list style="hanging">
<t hangText="Redundant Trees (RT): ">A pair of trees where the
path from any node X to the root R along the first tree is
node-disjoint with the path from the same node X to the root
along the second tree. These can be computed in 2-connected
graphs.</t>
<t hangText="Maximally Redundant Trees (MRT): ">A pair of trees
where the path from any node X to the root R along the first tree
and the path from the same node X to the root along the second
tree share the minimum number of nodes and the minimum number of
links. Each such shared node is a cut-vertex. Any shared links
are cut-links. Any RT is an MRT but many MRTs are not RTs. The
two MRTs are referred to as MRT-Blue and MRT-Red.</t>
<t hangText="MRT-Red: "> MRT-Red is used to describe one of the
two MRTs; it is used to described the associated forwarding
topology and 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 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 MT-ID: "> It is useful to have an MT-ID that
refers to the multiple MRT topologies and to the default
topology. This is referred to as the Rainbow MRT 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: "> From the computing router, the set of
routers that support a particular MRT profile and are connected
via MRT-eligible links.</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>
</list></t>
</section>
<section title="Overview of LDP Signaling Extensions for MRT">
<t>Routers need to know which of their LDP neighbors support MRT.
This is communicated using the MRT Capability Advertisement.
Supporting MRT indicates several different aspects of behavior, as
listed below.
<list style="numbers">
<t>Sending and receiving multi-topology FEC elements, as defined
in <xref target="RFC7307"/>.</t>
<t>Understanding the Rainbow MRT MT-ID and applying the associated labels
to all relevant MT-IDs.</t>
<t>Advertising the Rainbow MRT FEC to the appropriate neighbors for
the appropriate prefix.</t>
<t>If acting as LDP egress for a prefix in the default topology, also acting as egress
for the same prefix in MRT-Red and MRT-Blue.</t>
<t>For a FEC learned from a neighbor that does not support MRT,
originating FECs for MRT-Red and MRT-Blue with the same prefix.
This MRT Island egress behavior is to support an MRT Island
that does not include all routers in the area/level.</t>
</list></t>
<section title="MRT Capability Advertisement">
<t>A new MRT Capability Parameter TLV is defined in
accordance with LDP Capability definition guidelines<xref
target="RFC5561"/>.</t>
<t>The LDP MRT capability can be advertised during LDP session
initialization or after the LDP session is established. Advertisement
of the MRT capability indicates support of the procedures for
establishing the MRT-Blue and MRT-Red LSP paths detailed in this document.
If the peer has not advertised the MRT capability, then it
indicates that LSR does not support MRT procedures.</t>
<t> If a router advertises the LDP MRT capability to its peer, but
the peer has not advertised the MRT capability, then the router
MUST NOT advertise MRT-related FEC-label bindings to that peer.</t>
<t>The following is the format of the MRT Capability Parameter.</t>
<figure title="MRT Capability TLV Format">
<artwork align="center"><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| MRT Capability (IANA) | Length (= 1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| Reserved |
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>Where:
<list style="hanging">
<t hangText="U-bit: "> The unknown TLV bit MUST be 1.
A router that does not recognize the MRT Capability TLV will silently
ignore the TLV and process the rest of the message as if the unknown TLV did not exist.</t>
<t hangText="F-bit: "> The forward unknown TLV bit MUST be 0 as required by
Section 3 of <xref target="RFC5561"/>.</t>
<t hangText="MRT Capability: "> TBA-MRT-LDP-1 (To Be Allocated by IANA)</t>
<t hangText="Length: "> The length (in octets) of TLV. Its value is
1.</t>
<t hangText="S-bit: "> The State bit MUST be 1 if used in LDP "Initialization"
message. MAY be set to 0 or 1 in dynamic "Capability" message to
advertise or withdraw the capability respectively, as described in <xref target="RFC5561"/>.</t>
</list></t>
<section title="Interaction of MRT Capability and MT Capability">
<t> An LSR advertising the LDP MRT Capability MUST also advertise
the LDP Multi-topology (MT) capability. If an LSR negotiates
LDP MRT Capability with an LDP neighbor without also negotiating
the LDP MT Capability, the LSR MUST behave as if LDP MRT Capability
has not been negotiated and respond with the "MRT Capability negotiated
without MT Capability" status code in the LDP Notification message
(defined in the document).
The E-bit of this Notification should be set to 0 to indicate that
this is an Advisory Notification. The LDP session SHOULD NOT be terminated.
</t>
</section>
<section title="Interaction of LDP MRT Capability with IPv4 and IPv6">
<t>The MRT LDP Capability Advertisement does not distinguish between IPv4 and IPv6
address families.
An LSR which advertises the MRT LDP capability is expected to advertise
MRT-related FEC-label bindings for the same address families for which it
advertises shortest-path FEC-label bindings. Therefore, an LSR advertising
MRT LDP capability and shortest path FEC-label bindings
for IPv4 only (or IPv6 only) would be expected to advertise MRT-related
FEC-label binding for IPv4 only (or IPv6 only).
An LSR advertising the MRT LDP capability and shortest-path FEC label bindings
for BOTH IPv4 and IPv6 is expected to advertise
MRT-related FEC-label bindings for BOTH IPv4 and IPv6. In this scenario,
advertising MRT-related FEC-label bindings only for IPv4 only (or only for IPv6)
is not supported.
</t>
</section>
</section>
<section title="Use of the Rainbow MRT MT-ID">
<t>Section 10.1 of <xref target="I-D.ietf-rtgwg-mrt-frr-architecture"/>
describes the need for an area border router (ABR) to have different
neighbors use different MPLS labels when sending traffic to the ABR
for the same FEC. More detailed discussion
of the Rainbow MRT MT-ID is provided in
<xref target="sec_rainbow"/>.
</t>
<t>Another use for the Rainbow MRT MT-ID is for an LSR to send
the Rainbow MRT MT-ID with an IMPLICIT_NULL label to indicate
penultimate-hop-popping for all three types of FECs (shortest path, red,
and blue). The EXPLICIT_NULL label advertised using the
Rainbow MRT MT-ID similarly applies to all the types of FECs.
Note that the only scenario in which it is generally useful
to advertise the implicit or explicit null label for all three
FEC types is when the FEC refers to the LSR itself. See <xref target="sec_egress_lsr"/> for more details.
</t>
<t> The value of the Rainbow MRT MT-ID (TBA-MRT-LDP-2)
will be assigned by IANA from the LDP MT-ID space. Prototype experiments have
used the value 3999.</t>
</section>
<section title="MRT-Blue and MRT-Red FECs">
<t>To provide MRT support in LDP, the MT Prefix FEC is used.
<xref target="I-D.ietf-rtgwg-mrt-frr-architecture"/>
contains the IANA request for the MRT-Red and MRT-Blue MT-IDs
associated with the Default MRT Profile.</t>
<t>The MT Prefix FEC encoding is defined in <xref target="RFC7307"/>
and is used without alteration for advertising label mappings for
MRT-Blue, MRT-Red and Rainbow MRT FECs.</t>
</section>
</section>
<section title="LDP MRT FEC Advertisements">
<t>This sections describes how and when labels for MRT-Red and
MRT-Blue FECs are advertised. The associated LSPs must be created
before a failure occurs, in order to provide protection paths which
are immediately usable by the point of local repair in the
event of a failure.</t>
<t> In this section, we will use the term "shortest path FEC" to refer to
the usual FEC associated with the shortest path destination-based
forwarding tree for a given prefix as determined by the IGP.
We will use the terms "red FEC" and "blue FEC" to refer to FECs associated
with the MRT-Red and MRT-Blue destination-based forwarding trees
for a given prefix as determined by a particular MRT algorithm.
</t>
<t> We first describe label distribution
behavior specific to MRT. Then we provide the
correct interpretation of several important
concepts in <xref target="RFC5036"/> in the context of
MRT FEC label distribution.</t>
<section title="MRT-specific behavior">
<section anchor="sec_rainbow" title="ABR behavior and use of the Rainbow FEC">
<t>Section 10.1 of <xref target="I-D.ietf-rtgwg-mrt-frr-architecture"/>
describes the need for an area border router (ABR) to have different
neighbors use different MPLS labels when sending traffic to the ABR
for the same FEC. The method to accomplish this using the Rainbow MRT MT-ID
is described in detail in
<xref target="I-D.ietf-rtgwg-mrt-frr-architecture"/>. Here we provide a
brief summary. To those LDP peers in the same area as the best route to the
destination, the ABR advertises two different labels corresponding
to the MRT-Red and MRT-Blue forwarding trees for the destination.
An LDP peer receiving these advertisements forwards MRT traffic to the ABR using
these two different labels, depending on the FEC of the traffic.
We refer to this as best-area advertising and forwarding behavior, which is
identical to normal MRT behavior.
</t>
<t>For all other LDP peers supporting MRT,
the ABR advertises a FEC-label binding for the Rainbow MRT MT-ID scoped FEC
with the label corresponding to the default forwarding tree for the destination.
An LDP peer receiving this advertisement forwards MRT traffic to the ABR using this
label, for both MRT Red and MRT Blue traffic.
We refer to this as non-best-area advertising and forwarding behavior.
</t>
<t>The use of the Rainbow-FEC by the ABR for non-best-area
advertisements is RECOMMENDED. An ABR MAY advertise the label
for the default topology in separate MRT-Blue and MRT-Red advertisements.
An LSR advertising the MRT capability MUST recognize the Rainbow
MRT MT-ID and associate the advertised label with the specific prefix
with the MRT-Red and MRT-Blue MT-IDs associated with all MRT Profiles
that advertise LDP as the forwarding mechanism.</t>
<t> Due to changes in topology or configuration, an ABR
and a given LDP peer may need to transition from best-area
advertising and forwarding behavior to non-best-area behavior for
a given destination, and vice versa. When the ABR requires best-area
behavior for a red(blue) FEC, it MUST withdraw any existing label mappings
advertisements for the corresponding rainbow FEC
and advertise label mappings for the
red(blue) FEC. When the ABR requires non-best-area
behavior for a red(blue) FEC, it MUST withdraw any existing label
mappings for both red and blue FECs and
advertise label mappings for the corresponding Rainbow FEC label binding.
</t>
<t> If an LSR receives a label mapping advertisement for a rainbow FEC from an MRT LDP peer
while it still retains a label mapping for the corresponding red or blue FEC,
the LSR MUST continue to use the label mapping for the red or blue FEC, and it MUST
send a Label Release Message corresponding to the rainbow FEC label advertisement.
If an LSR receives a label mapping advertisement for red or blue FEC
while it still retains a label mapping for the corresponding rainbow FEC,
the LSR MUST continue to use the label mapping for the rainbow FEC, and it MUST
send a Label Release Message corresponding to the red or blue FEC label advertisement.
</t>
</section>
<section anchor="sec_proxy_node" title="Proxy-node attachment router behavior">
<t>Section 11.2 of <xref target="I-D.ietf-rtgwg-mrt-frr-architecture"/>
describes how MRT provides FRR protection for multi-homed prefixes using calculations
involving a
named proxy-node. This covers the scenario where a prefix is originated by a router
in the same area as the MRT Island, but outside of the MRT Island. It also covers
the scenario of a prefix being advertised by a multiple routers in the MRT Island.
</t>
<t>In the named proxy-node calculation, each multi-homed prefix
is represented by a conceptual proxy-node which is attached to
two real proxy-node attachment routers.
(A single proxy-node attachment router is allowed
in the case of a prefix advertised by a same area router outside of the
MRT Island which is singly connected to the MRT Island.) All routers in the
MRT Island perform the same calculations to determine the
same two proxy-node attachment routers for each multi-homed prefix.
The resulting graph in the computation consists of the MRT Island with
the proxy-node representing the multi-homed prefix directly attached to the
two proxy-node attachment routers. Conceptually, one then runs the MRT algorithm
on this simplified graph to determine the MRT-red and blue next-hops to
reach the proxy-node, which gives the next-hops to reach the prefix. In this manner,
one can see that one of the two proxy-node attachment routers will always
have a MRT-red next-hop to the proxy-node while the other will always have the
MRT-blue next-hop to the proxy-node. We will refer to these as the
red and blue proxy-node attachment routers respectively. (In practice,
the MRT-red and blue next-hops to reach the proxy-node
can then be determined in a more computationally efficient manner
based on the MRT-red and blue next-hops to
reach the proxy-node attachment routers, as described in
<xref target="I-D.ietf-rtgwg-mrt-frr-algorithm"/>.)
</t>
<t> In terms of LDP behavior, a red proxy-node attachment router for a
given prefix MUST
originate a label mapping for the red FEC for that prefix, while the
a blue proxy-node attachment router for a given prefix MUST
originate a label mapping for the blue FEC for that prefix.
If the red(blue) proxy-node attachment router is an Island Border Router (IBR),
then when it receives a packet
with the label corresponding to the red(blue) FEC for a prefix,
it MUST forward the packet to the Island Neighbor (IN) whose
whose cost was used in the selection of the IBR as a proxy-node attachment router.
The IBR MUST swap the incoming label for the outgoing label corresponding to the
shortest path FEC for the prefix advertised by the IN. In the case where
the IN does not support LDP, the IBR MUST pop the incoming label and forward
the packet to the IN.
</t>
<t>
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>
<section title="LDP protocol procedures in the context of MRT label distribution">
<t> <xref target="RFC5036"/> specifies the LDP label
distribution procedures for shortest path FECs. In general,
the same procedures can be applied to the distribution of
label mappings for red and blue FECs, provided that the
procedures are interpreted in the context of MRT FEC label
distribution. The correct interpretation of several important
concepts in <xref target="RFC5036"/> in the context of
MRT FEC label distribution is provided below.</t>
<section title="LDP peer in RFC5036">
<t> In the context of distributing label mappings for
red and blue FECs, we restrict LDP peer in <xref target="RFC5036"/>
to mean LDP peers for which the LDP MRT capability has been negotiated. In
order to make this distinction clear, in this document
we will use the term "MRT LDP peer" to refer to an LDP peer
for which the LDP MRT capability has been negotiated.</t>
</section>
<section title="Next hop in RFC5036">
<t> Several procedures in <xref target="RFC5036"/> use
the next hop of a (shortest path) FEC to determine behavior. The next hop
of the shortest path FEC is based on the shortest path forwarding tree to
the prefix associated with the FEC. When the procedures of
<xref target="RFC5036"/> are used to distribute label mapping for
red and blue FECs, the next hop for the red/blue FEC is based on
the MRT-Red/Blue forwarding tree to the prefix associated with the FEC. </t>
<t> For example, Appendix A.1.7. of <xref target="RFC5036"/> specifies
the response by an LSR to a change in the next hop for a FEC. For a
shortest path FEC, the next hop may change as the result of
the LSR running a shortest path computation on a modified IGP topology database.
For the red and blue FECs, the red and blue next hops
may change as the result of the LSR running a particular MRT algorithm
on a modified IGP topology database. </t>
<t> As another example, Section 2.6.1.2 of <xref target="RFC5036"/> specifies
how that when an LSR is using LSP Ordered Control, it may initiate the transmission
of a label mapping only for a (shortest path) FEC for which it has a label mapping
for the FEC next hop, or for which the LSR is the egress. The FEC next hop
for a shortest path FEC is based on the shortest path forwarding tree to the prefix
associated with the FEC. In the context of distributing MRT LDP labels,
this procedure is understood to mean the following.
When an LSR is using LSP Ordered Control, it may initiate the transmission
of a label mapping only for a red(blue) FEC for which it has a label mapping
for the red(blue) FEC next hop, or for which the LSR is the egress. The
red or blue FEC next hop is based on the MRT-Red or Blue forwarding tree to the
prefix associated with the FEC.</t>
</section>
<section anchor="sec_egress_lsr" title="Egress LSR in RFC5036">
<t>
Procedures in <xref target="RFC5036"/> related to Ordered Control
label distribution mode rely on
whether or not an LSR may act as an egress LSR for a particular FEC
in order to determine whether or not the LSR
may originate a label mapping for that FEC. The status of being an egress LSR
for a particular FEC is also used in loop detection procedures in
<xref target="RFC5036"/>.
Section 2.6.1.2 of <xref target="RFC5036"/> specifies the
conditions under which an LSR may act as an egress LSR with
respect to a particular (shortest path) FEC.
<list style="numbers">
<t>The (shortest path) FEC refers to the LSR itself (including one of its directly
attached interfaces). </t>
<t>The next hop router for the (shortest path) FEC is outside of the Label
Switching Network. </t>
<t>(Shortest path) FEC elements are reachable by crossing a routing domain
boundary.</t>
</list>
</t>
<t> The conditions for determining an egress LSR with respect to
a red or blue FEC need to be modified. An LSR may act as an egress LSR with
respect to a particular red(blue) FEC under any of
the following conditions:
<list style="numbers">
<t>The prefix associated with the red(blue) FEC refers
to the LSR itself (including one of its directly attached interfaces). </t>
<t>The LSR is the red(blue) proxy-node attachment router with
respect to the multi-homed prefix
associated with the red(blue) FEC. This includes the
degenerate case of a single
red and blue proxy-node attachment router for a single-homed prefix.</t>
<t>The LSR is an area border router (ABR) AND the MRT LDP peer
requires non-best-area advertising and forwarding behavior for the prefix
associated with the FEC.</t>
</list>
</t>
<t> Note that condition(3) scopes an LSR's status as an egress LSR with
respect to a particular FEC to a particular MRT LDP peer. Therefore, the
condition "Is LSR egress for FEC?" that occurs in several procedures
in <xref target="RFC5036"/> needs to be
interpreted as "Is LSR egress for FEC with respect to Peer?" </t>
<t> Also note that there is no explicit condition that allows an LSR
to be classified as an egress LSR with respect a red or blue FEC based
only on the primary next-hop for the shortest path FEC not supporting LDP,
or not supporting LDP MRT capability. These situations are covered by the
proxy-node attachment router and ABR conditions (conditions 2 and 3). In
particular, an Island Border Router is not the egress LSR for a red(blue) FEC
unless it is also the red(blue) proxy-node attachment router for that FEC.
</t>
<t> Also note that in general a proxy-node attachment router for a given prefix
should not advertise an implicit or explicit null label for the corresponding
red or blue FEC, even though it may be an egress LSR for the shortest path
FEC. In general, the proxy-node attachment router needs to forward red
or blue traffic for that prefix to a particular loop free island neighbor,
which may be different
from the shortest path next-hop. The proxy-node attachment router needs to
receive the red or blue traffic with a non-null label to correctly forward it.
</t>
</section>
<section title="Use of Rainbow FEC to satisfy label mapping existence requirements in RFC5036">
<t>Several procedures in <xref target="RFC5036"/> require the LSR to
determine if it has previously received and retained a label
mapping for a FEC from the next hop. In the case of an LSR that has
received and retained a label mapping for a Rainbow FEC from an ABR, the label mapping
for the Rainbow FEC satisfies the label mapping existence requirement
for the corresponding red and blue FECs. Label mapping existence requirements
in the context of MRT LDP label distribution are modified as:
"Has LSR previously received and retained a label mapping for
the red(blue) FEC (or the corresponding Rainbow FEC)
from the red(blue) next hop?"
</t>
<t>As an example, this behavior allows an LSR which has
received and retained a label mapping for the Rainbow FEC to
advertise label mappings for the corresponding red and blue FECs
when operating in Ordered Control label distribution mode.
</t>
</section>
<section title="Validating FECs in routing table">
<t> In <xref target="RFC5036"/> an LSR uses its routing table to
validate prefixes associated with shortest path FECs. For example,
section 3.5.7.1 of <xref target="RFC5036"/> specifies that
"an LSR receiving a Label Mapping message from a downstream LSR for a Prefix SHOULD NOT use the label for forwarding unless its routing
table contains an entry that exactly matches the FEC Element."
In the context of MRT FECs, a red or blue FEC element matches
a routing table entry if the corresponding shortest path FEC element
matches a routing table entry.
</t>
</section>
<section title="Recognizing new FECs">
<t> Section A.1.6 of <xref target="RFC5036"/> describes the response of
an LSR to the "Recognize New FEC" event, which occurs when
an LSR learns a new (shortest path) FEC via the routing table.
In the context of MRT FECs, when MRT LDP capability has been enabled,
when an LSR learns a new shortest path FEC, it should generate
"Recognize New FEC" events for the corresponding red and blue FECs,
in addition to the "Recognize New FEC" event for the shortest path
FEC.
</t>
</section>
<section title="Not propagating Rainbow FEC label mappings">
<t> A label mapping for the Rainbow FEC should only be originated by an ABR
under the conditions described in <xref target="sec_rainbow"/>.
A neighbor of the ABR that receives a label mapping for the Rainbow FEC
MUST NOT propagate a label mapping for that Rainbow FEC.
</t>
</section>
</section>
</section>
<section title="Security Considerations">
<t>The labels distributed by the extensions in this document create
additional forwarding paths that do not following shortest path routes.
The transit label swapping operations defining these alternative
forwarding paths are created during normal operations (before a failure occurs).
Therefore, a malicious packet with an appropriate label
injected into the network from a compromised location would be forwarded
to a destinations 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>
</section>
<section title="IANA Considerations">
<t>IANA is requested to allocate a value for the new LDP Capability TLV
(the first free value in the range 0x0500 to 0x05FF) from the
LDP registry "TLV Type Name Space": MRT Capability TLV (TBA-MRT-LDP-1).
</t>
<figure>
<artwork align="center"><![CDATA[
Value Description Reference Notes / Reg. Date
------------- ------------------ ------------ -----------------
TBA-MRT-LDP-1 MRT Capability TLV [This draft]
]]></artwork>
</figure>
<t>IANA is requested to allocate a value for the new LDP Status Code
(the first free value in the range 0x00000032-0x00000036) from the
LDP registry "Status Code Name Space": "MRT Capability negotiated
without MT Capability" (TBA-MRT-LDP-3).
</t>
<figure>
<artwork align="center"><![CDATA[
Value E Description Reference Notes / Reg. Date
------------- - ------------------ ------------ -----------------
TBA-MRT-LDP-3 0 MRT Capability [This draft]
negotiated without
MT Capability
]]></artwork>
</figure>
<t>IANA is requested to allocate a value from the
MPLS Multi-Topology Identifiers Name Space
<xref target="RFC7307"/>: Rainbow MRT MT-ID (TBA-MRT-LDP-2).
</t>
<figure>
<artwork align="center"><![CDATA[
Value Purpose Reference
------------- ------------------ ------------
TBA-MRT-LDP-2 Rainbow MRT MT-ID [This draft]
]]></artwork>
</figure>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>The authors would like to thank Ross Callon, Loa Andersson,
Stewart Bryant, Mach Chen, and Greg Mirsky for their suggestions.</t>
</section>
</middle>
<!-- *****BACK MATTER ***** -->
<back>
<references title="Normative References">
&RFC5561;
&RFC5036;
&RFC7307;
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.draft-ietf-rtgwg-mrt-frr-architecture-05.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.draft-ietf-rtgwg-mrt-frr-algorithm-05.xml"?>
</references>
<references title="Informative References">
&RFC2119;
&I-D.atlas-rtgwg-mrt-mc-arch;
&I-D.wijnands-mpls-mldp-node-protection;
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.draft-ietf-isis-mrt-00.xml"?>
<?rfc include="http://xml.resource.org/public/rfc/bibxml3/reference.I-D.draft-ietf-ospf-mrt-00.xml"?>
</references>
<!-- Change Log
v00 2013-07-02 AKA Initial version
v01 2014-05-30 CJB Update
v02 2014-10-22 CJB Update incorporating feedback
v03 2014-12-08 CJB Incorporating MPLS-RT feedback
-->
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-22 08:34:09 |