MPLS Working Group A. Atlas
Internet-Draft K. Tiruveedhula
Intended status: Standards Track C. Bowers
Expires: May 19, 2018 Juniper Networks
J. Tantsura
Individual
IJ. Wijnands
Cisco Systems, Inc.
November 15, 2017
LDP Extensions to Support Maximally Redundant Trees
draft-ietf-mpls-ldp-mrt-07
Abstract
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.
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 (Label Switching Routers) and
LERs (Label Edge Routers) advertising the MRT Capability.
MRT-FRR uses LDP multi-topology extensions and requires three
different multi-topology IDs to be allocated from the MPLS MT-ID
space.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 19, 2018.
Atlas, et al. Expires May 19, 2018 [Page 1]
Internet-Draft LDP Extensions to Support MRT November 2017
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Overview of LDP Signaling Extensions for MRT . . . . . . . . 5
4.1. MRT Capability Advertisement . . . . . . . . . . . . . . 5
4.1.1. Interaction of MRT Capability and MT Capability . . . 6
4.1.2. Interaction of LDP MRT Capability with IPv4 and IPv6 7
4.2. Use of the Rainbow MRT MT-ID . . . . . . . . . . . . . . 7
4.3. MRT-Blue and MRT-Red FECs . . . . . . . . . . . . . . . . 7
4.4. Interaction of MRT-related LDP advertisements with the
MRT topology and computations . . . . . . . . . . . . . . 8
5. LDP MRT FEC Advertisements . . . . . . . . . . . . . . . . . 8
5.1. MRT-specific behavior . . . . . . . . . . . . . . . . . . 9
5.1.1. ABR behavior and use of the Rainbow FEC . . . . . . . 9
5.1.2. Proxy-node attachment router behavior . . . . . . . . 10
5.2. LDP protocol procedures in the context of MRT label
distribution . . . . . . . . . . . . . . . . . . . . . . 11
5.2.1. LDP peer in RFC 5036 . . . . . . . . . . . . . . . . 11
5.2.2. Next hop in RFC 5036 . . . . . . . . . . . . . . . . 12
5.2.3. Egress LSR in RFC 5036 . . . . . . . . . . . . . . . 12
5.2.4. Use of Rainbow FEC to satisfy label mapping existence
requirements in RFC 5036 . . . . . . . . . . . . . . 14
5.2.5. Validating FECs in routing table . . . . . . . . . . 14
5.2.6. Recognizing new FECs . . . . . . . . . . . . . . . . 14
5.2.7. Not propagating Rainbow FEC label mappings . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 15
7. Potential restrictions on MRT-related MT-ID values imposed
by RFC 6420 . . . . . . . . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
Atlas, et al. Expires May 19, 2018 [Page 2]
Internet-Draft LDP Extensions to Support MRT November 2017
10.1. Normative References . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
This document describes the LDP signaling extensions and associated
behavior necessary to support the architecture that defines how IP/
LDP Fast-Reroute can use MRTs [RFC7812]. The current document
provides a brief description of the MRT-FRR architecture, focusing on
the aspects most directly related to LDP signaling. The complete
description and specification of the MRT-FRR architecture can be
found in [RFC7812].
At least one common standardized algorithm (e.g., the MRT Lowpoint
algorithm explained and fully documented in [RFC7811]) is required to
be deployed 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 [I-D.ietf-ospf-mrt]. Extensions to IS-IS are defined in
[I-D.ietf-isis-mrt].
MRT can also be used to protect multicast traffic (signalled via PIM
or mLDP) using either global protection or local protection as
described in [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 [RFC7715]; an MRT path can also be used to provide link
protection for mLDP traffic.
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 [RFC7307] to signal Forwarding Equivalency Classes (FECs)
for these two sets of forwarding trees, MRT-Blue and MRT-Red.
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 [RFC7812]
and the worst-case network convergence time can be flooded via the
extension in Section 7 of [I-D.ietf-ospf-mrt].
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.
Atlas, et al. Expires May 19, 2018 [Page 3]
Internet-Draft LDP Extensions to Support MRT November 2017
2. Requirements Language
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 RFC 2119[RFC2119]
3. Terminology
For ease of reading, some of the terminology defined in [RFC7812] is
repeated here. Please refer to the Section 3 of [RFC7812] for a more
complete list.
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.
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.
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).
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.
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).
MRT Island: The set of routers that support a particular MRT
profile and the links connecting them that support MRT.
Island Border Router (IBR): A router in the MRT Island that is
connected to a router not in the MRT Island, both of which are in
a common area or level.
Atlas, et al. Expires May 19, 2018 [Page 4]
Internet-Draft LDP Extensions to Support MRT November 2017
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..
There are several places in this document where the construction
"red(blue) FEC" is used to cover the case of the red FEC and the case
of the blue FEC, independently. As an example, consider the sentence
"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." This sentence should be read as applying to red
FECs. Then it should be read as applying to blue FECs.
4. Overview of LDP Signaling Extensions for MRT
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.
1. Sending and receiving multi-topology FEC elements, as defined in
[RFC7307].
2. Understanding the Rainbow MRT MT-ID and applying the associated
labels to all relevant MT-IDs.
3. Advertising the Rainbow MRT FEC to the appropriate neighbors for
the appropriate prefix.
4. 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.
5. 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.
4.1. MRT Capability Advertisement
A new MRT Capability Parameter TLV is defined in accordance with LDP
Capability definition guidelines[RFC5561].
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.
Atlas, et al. Expires May 19, 2018 [Page 5]
Internet-Draft LDP Extensions to Support MRT November 2017
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.
The following is the format of the MRT Capability Parameter.
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 |
+-+-+-+-+-+-+-+-+
MRT Capability TLV Format
Where:
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.
F-bit: The forward unknown TLV bit MUST be 0 as required by
Section 3 of [RFC5561].
MRT Capability: TBD-MRT-LDP-1 (To Be Allocated by IANA)
Length: The length (in octets) of TLV. Its value is 1.
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
[RFC5561].
4.1.1. Interaction of MRT Capability and MT Capability
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.
Atlas, et al. Expires May 19, 2018 [Page 6]
Internet-Draft LDP Extensions to Support MRT November 2017
4.1.2. Interaction of LDP MRT Capability with IPv4 and IPv6
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.
4.2. Use of the Rainbow MRT MT-ID
Section 10.1 of [RFC7812] 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 Section 5.1.1 of
the current document.
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 Section 5.2.3 for more details.
The value of the Rainbow MRT MPLS MT-ID (TBD-MRT-LDP-3) will be
assigned by IANA from the MPLS MT-ID space.
4.3. MRT-Blue and MRT-Red FECs
To provide MRT support in LDP, the MT Prefix FEC is used. [RFC7812]
defines the Default MRT Profile. Section 8 of the current document
specifies the values in the MPLS Multi-Topology Identifiers Registry
for the MRT-Red and MRT-Blue MPLS MT-IDs associated with the Default
MRT Profile (TBD-MRT-LDP-4 and TBD-MRT-LDP-5).
As described in Section 8.1 of [RFC7812], 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.
Atlas, et al. Expires May 19, 2018 [Page 7]
Internet-Draft LDP Extensions to Support MRT November 2017
The MT Prefix FEC encoding is defined in [RFC7307] and is used
without alteration for advertising label mappings for MRT-Blue, MRT-
Red and Rainbow MRT FECs.
4.4. Interaction of MRT-related LDP advertisements with the MRT
topology and computations
[RFC7811] and [RFC7812] describe how the MRT topology is created
based on information in IGP advertisements. The MRT topology and
computations rely on IGP advertisements. The presence or absence of
MRT-related LDP advertisements does not affect the MRT topology or
the MRT-Red and MRT-Blue next-hops computed for that topology.
As an example, consider a network where all nodes are running MRT IGP
extensions to determine the MRT-topology, which is then used to
compute MRT-Red and MRT-Blue next-hops. The network operator also
configures the nodes in this network to exchange MRT-related LDP
advertisements in order to distribute MPLS labels corresponding to
those MRT next-hops. Suppose that, due to a misconfiguration on one
particular link, the MRT-related LDP advertisements are not being
properly exchanged for that link. Since the MRT-related IGP
advertisements for the link are still being distributed, the link is
still included in the MRT topology and computations. In this
scenario, there will be missing MPLS forwarding entries corresponding
to paths that use the misconfigured link.
Note that the situation is analogous to the interaction of normal LDP
advertisements and IGP advertisements for shortest path forwarding.
Deactivating the distribution of labels for normal shortest path FECs
on a link does not change the topology on which the SPF algorithm is
run by the IGP.
[RFC5443] "LDP IGP Synchronization" addresses the issue of the LDP
topology not matching the IGP topology by the advertising the maximum
IGP cost on links where LDP is not fully operational. This makes the
IGP topology match the LDP topology. As described in Section 7.3.1
of [RFC7812], MRT is designed to be compatible with the LDP IGP
synchronization mechanism. When the IGP advertises the maximum cost
on a link where LDP is not fully operational, the link is excluded
from MRT Island formation, which prevents the MRT algorithm from
creating any paths using that link.
5. LDP MRT FEC Advertisements
This sections describes how and when labels for MRT-Red and MRT-Blue
FECs are advertised. In order to provide protection paths which are
immediately usable by the point of local repair in the event of a
Atlas, et al. Expires May 19, 2018 [Page 8]
Internet-Draft LDP Extensions to Support MRT November 2017
failure, the associated LSPs need to be created before a failure
occurs.
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.
We first describe label distribution behavior specific to MRT. Then
we provide the correct interpretation of several important concepts
in [RFC5036] in the context of MRT FEC label distribution.
[RFC5036] specifies two different Label Distribution Control Modes
(Independent and Ordered), two different Label Retention Modes
(Conservative and Liberal), and two different Label Advertisement
Modes (Downstream Unsolicited and Downstream on Demand). The current
specification for LDP MRT requires that the same Label Distribution
Control, Label Retention, and Label Advertisement modes be used for
the shortest path FECs and the MRT FECs.
5.1. MRT-specific behavior
5.1.1. ABR behavior and use of the Rainbow FEC
Section 10.1 of [RFC7812] 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
[RFC7812]. 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.
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.
The use of the Rainbow-FEC by the ABR for non-best-area
advertisements is RECOMMENDED. An ABR MAY advertise the label for
Atlas, et al. Expires May 19, 2018 [Page 9]
Internet-Draft LDP Extensions to Support MRT November 2017
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.
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.
In this transition, an ABR should never advertise a red(blue) FEC
before withdrawing the corresponding rainbow FEC (or vice versa).
However, should this situation occur, the expected behavior of an LSR
receiving these conflicting advertisements is defined as follows. 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.
5.1.2. Proxy-node attachment router behavior
Section 11.2 of [RFC7812] 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.
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
Atlas, et al. Expires May 19, 2018 [Page 10]
Internet-Draft LDP Extensions to Support MRT November 2017
multi-homed prefix. Section 5.9 of [RFC7811] describes the procedure
for identifying one proxy-node attachment router as "red" and one as
"blue" with respect to the multi-homed prefix, and computing the MRT
red and blue next-hops to reach those red and blue proxy-node
attachment routers.
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 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.
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.
5.2. LDP protocol procedures in the context of MRT label distribution
[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 [RFC5036] in the context of MRT FEC label distribution is
provided below.
5.2.1. LDP peer in RFC 5036
In the context of distributing label mappings for red and blue FECs,
we restrict LDP peer in [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.
Atlas, et al. Expires May 19, 2018 [Page 11]
Internet-Draft LDP Extensions to Support MRT November 2017
5.2.2. Next hop in RFC 5036
Several procedures in [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 [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.
For example, Appendix A.1.7 of [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.
As another example, Section 2.6.1.2 of [RFC5036] specifies 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.
5.2.3. Egress LSR in RFC 5036
Procedures in [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 [RFC5036]. Section 2.6.1.2 of [RFC5036] specifies the
conditions under which an LSR may act as an egress LSR with respect
to a particular (shortest path) FEC.
1. The (shortest path) FEC refers to the LSR itself (including one
of its directly attached interfaces).
2. The next hop router for the (shortest path) FEC is outside of the
Label Switching Network.
Atlas, et al. Expires May 19, 2018 [Page 12]
Internet-Draft LDP Extensions to Support MRT November 2017
3. (Shortest path) FEC elements are reachable by crossing a routing
domain boundary.
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:
1. The prefix associated with the red(blue) FEC refers to the LSR
itself (including one of its directly attached interfaces).
2. 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.
3. 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.
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 [RFC5036] needs to be interpreted as "Is LSR egress for
FEC with respect to Peer?"
Also note that there is no explicit condition that allows an LSR to
be classified as an egress LSR with respect to 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.
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.
Atlas, et al. Expires May 19, 2018 [Page 13]
Internet-Draft LDP Extensions to Support MRT November 2017
5.2.4. Use of Rainbow FEC to satisfy label mapping existence
requirements in RFC 5036
Several procedures in [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?"
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.
5.2.5. Validating FECs in routing table
In [RFC5036] an LSR uses its routing table to validate prefixes
associated with shortest path FECs. For example, section 3.5.7.1 of
[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.
5.2.6. Recognizing new FECs
Section A.1.6 of [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.
5.2.7. Not propagating Rainbow FEC label mappings
A label mapping for the Rainbow FEC should only be originated by an
ABR under the conditions described in Section 5.1.1. A neighbor of
the ABR that receives a label mapping for the Rainbow FEC MUST NOT
propagate a label mapping for that Rainbow FEC.
Atlas, et al. Expires May 19, 2018 [Page 14]
Internet-Draft LDP Extensions to Support MRT November 2017
6. Security Considerations
The labels distributed by the extensions in this document create
additional forwarding paths that do not follow 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 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.
It should be noted that the creation of non-shortest forwarding paths
is not unique to MRT. For example, RSVP-TE [RFC3209] can be used to
construct forwarding paths that do not follow the shortest path.
7. Potential restrictions on MRT-related MT-ID values imposed by RFC
6420
As discussed in the introduction, in addition to unicast forwarding
applications, MRT can be used to provide disjoint trees for multicast
traffic distribution. In the case of PIM, this is accomplished by
using the MRT red and blue next-hops as the PIM RPF topology, the
collection of routes used by PIM to perform the RPF operation when
building source trees. The PIM Multi-Topology ID (MT-ID) Join
Attribute defined in section 5.2 of [RFC6420] can be used to
establish MRT-based multicast distribution trees. [RFC6420] limits
the values of the PIM MT-ID from 1 through 4095.
For the purpose of reducing management overhead and simplifying
troubleshooting, it is desirable to be able to use the same numerical
value for the PIM MT-ID as for the MPLS MT-ID, for multicast and
unicast application using MRT routes constructed using the same MRT
profile. In order to enable this simplification, the MPLS MT-ID
values assigned in this document need to fall in the range 1 through
4095. The IANA request below reflects this by requesting that the
MPLS MT-ID values from 3945 through 3995 be used for MRT-related MPLS
MT-ID values. This allows for 51 MRT-related MPLS MT-ID values which
can be directly mapped to PIM MT-ID values, which accommodates 25 MRT
profiles with red and blue MT-ID pairs, with one extra for the
rainbow MPLS MT-ID value. [RFC7307] designates the MPLS MT-ID range
6-3995 as "Unassigned(for future IGP topologies)". The IANA request
below changes the guidance for MT-ID range 3948-3995 to "Unassigned
(for future MRT-related values)".
Atlas, et al. Expires May 19, 2018 [Page 15]
Internet-Draft LDP Extensions to Support MRT November 2017
8. IANA Considerations
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 Label
Distribution Protocol (LDP) Parameters registry "TLV Type Name
Space": MRT Capability TLV (TBD-MRT-LDP-1).
Value Description Reference Notes / Reg. Date
------------- ------------------ ------------ -----------------
TBD-MRT-LDP-1 MRT Capability TLV [This draft]
IANA is requested to allocate a value for the new LDP Status Code
(the first free value in the range 0x00000032-0x00000036) from the
Label Distribution Protocol (LDP) Parameters registry "Status Code
Name Space": "MRT Capability negotiated without MT Capability" (TBD-
MRT-LDP-2). The Status Code E-bit is set to 0.
Value E Description Reference Notes / Reg. Date
-------------- - ------------------ ------------ -----------------
TBD-MRT-LDP-2 0 MRT Capability [This draft]
negotiated without
MT Capability
IANA is requested to allocate three values from the MPLS Multi-
Topology Identifiers Registry [RFC7307].
Rainbow MRT MPLS MT-ID (TBD-MRT-LDP-3) with suggested value: 3945
Default Profile MRT-Red MPLS MT-ID (TBD-MRT-LDP-4) with suggested
value: 3946
Default Profile MRT-Blue MPLS MT-ID (TBD-MRT-LDP-5) with suggested
value: 3947
IANA is also requested to change the purpose field of the MPLS Multi-
Topology Identifiers Registry for MT-ID range 3948-3995 to
"Unassigned (for future MRT-related values)", assuming the above
suggested values are assigned. The Registration procedure for the
entire registry remains "Standards Action". The entire registry
after implementing the above requests is shown below.
Atlas, et al. Expires May 19, 2018 [Page 16]
Internet-Draft LDP Extensions to Support MRT November 2017
Value Purpose Reference
------------- ---------------------- ------------
0 Default/standard topology [RFC7307]
1 IPv4 in-band management [RFC7307]
2 IPv6 routing topology [RFC7307]
3 IPv4 multicast topology [RFC7307]
4 IPv6 multicast topology [RFC7307]
5 IPv6 in-band management [RFC7307]
6-3944 Unassigned (for future IGP topologies)
TBD-MRT-LDP-3 Rainbow MRT MPLS MT-ID [This draft]
TBD-MRT-LDP-4 Default Profile MRT-Red MPLS MT-ID [This draft]
TBD-MRT-LDP-5 Default Profile MRT-Blue MPLS MT-ID [This draft]
3948-3995 Unassigned (for future MRT-related values) [This draft]
3996-4095 Reserved for Experimental Use [RFC7307]
4096-65534 Unassigned (for MPLS topologies)
65535 Wildcard Topology [RFC7307]
9. Acknowledgements
The authors would like to thank Ross Callon, Loa Andersson, Stewart
Bryant, Mach Chen, Greg Mirsky, Uma Chunduri and Tony Przygienda for
their comments and suggestions.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
October 2007, <https://www.rfc-editor.org/info/rfc5036>.
[RFC5561] Thomas, B., Raza, K., Aggarwal, S., Aggarwal, R., and JL.
Le Roux, "LDP Capabilities", RFC 5561,
DOI 10.17487/RFC5561, July 2009,
<https://www.rfc-editor.org/info/rfc5561>.
[RFC6420] Cai, Y. and H. Ou, "PIM Multi-Topology ID (MT-ID) Join
Attribute", RFC 6420, DOI 10.17487/RFC6420, November 2011,
<https://www.rfc-editor.org/info/rfc6420>.
Atlas, et al. Expires May 19, 2018 [Page 17]
Internet-Draft LDP Extensions to Support MRT November 2017
[RFC7307] Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D.
King, "LDP Extensions for Multi-Topology", RFC 7307,
DOI 10.17487/RFC7307, July 2014,
<https://www.rfc-editor.org/info/rfc7307>.
[RFC7811] Enyedi, G., Csaszar, A., Atlas, A., Bowers, C., and A.
Gopalan, "An Algorithm for Computing IP/LDP Fast Reroute
Using Maximally Redundant Trees (MRT-FRR)", RFC 7811,
DOI 10.17487/RFC7811, June 2016,
<https://www.rfc-editor.org/info/rfc7811>.
[RFC7812] Atlas, A., Bowers, C., and G. Enyedi, "An Architecture for
IP/LDP Fast Reroute Using Maximally Redundant Trees (MRT-
FRR)", RFC 7812, DOI 10.17487/RFC7812, June 2016,
<https://www.rfc-editor.org/info/rfc7812>.
10.2. Informative References
[I-D.atlas-rtgwg-mrt-mc-arch]
Atlas, A., Kebler, R., Wijnands, I., Csaszar, A., and G.
Envedi, "An Architecture for Multicast Protection Using
Maximally Redundant Trees", draft-atlas-rtgwg-mrt-mc-
arch-02 (work in progress), July 2013.
[I-D.ietf-isis-mrt]
Li, Z., Wu, N., <>, Q., Atlas, A., Bowers, C., and J.
Tantsura, "Intermediate System to Intermediate System (IS-
IS) Extensions for Maximally Redundant Trees (MRT)",
draft-ietf-isis-mrt-02 (work in progress), May 2016.
[I-D.ietf-ospf-mrt]
Atlas, A., Hegde, S., Bowers, C., Tantsura, J., and Z. Li,
"OSPF Extensions to Support Maximally Redundant Trees",
draft-ietf-ospf-mrt-02 (work in progress), May 2016.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.
[RFC5443] Jork, M., Atlas, A., and L. Fang, "LDP IGP
Synchronization", RFC 5443, DOI 10.17487/RFC5443, March
2009, <https://www.rfc-editor.org/info/rfc5443>.
[RFC7715] Wijnands, IJ., Ed., Raza, K., Atlas, A., Tantsura, J., and
Q. Zhao, "Multipoint LDP (mLDP) Node Protection",
RFC 7715, DOI 10.17487/RFC7715, January 2016,
<https://www.rfc-editor.org/info/rfc7715>.
Atlas, et al. Expires May 19, 2018 [Page 18]
Internet-Draft LDP Extensions to Support MRT November 2017
Authors' Addresses
Alia Atlas
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Email: akatlas@juniper.net
Kishore Tiruveedhula
Juniper Networks
10 Technology Park Drive
Westford, MA 01886
USA
Email: kishoret@juniper.net
Chris Bowers
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, CA 94089
USA
Email: cbowers@juniper.net
Jeff Tantsura
Individual
USA
Email: jefftant.ietf@gmail.com
IJsbrand Wijnands
Cisco Systems, Inc.
Email: ice@cisco.com
Atlas, et al. Expires May 19, 2018 [Page 19]
