Internet Engineering Task Force (IETF) Q. Zhao

Internet Engineering Task Force (IETF) Q. Zhao
Request for Comments: 7307 Huawei Technology
Category: Standards Track K. Raza
ISSN: 2070-1721 C. Zhou
Cisco Systems
L. Fang
Microsoft
L. Li
China Mobile
D. King
Old Dog Consulting
July 2014

LDP Extensions for Multi-Topology

Abstract

Multi-Topology (MT) routing is supported in IP networks with the use
of MT-aware IGPs. In order to provide MT routing within
Multiprotocol Label Switching (MPLS) Label Distribution Protocol
(LDP) networks, new extensions are required.

This document describes the LDP protocol extensions required to
support MT routing in an MPLS environment.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7307.

Zhao, et al. Standards Track [Page 1]

RFC 7307 LDP Multi-Topology Extensions July 2014

Copyright Notice

Copyright (c) 2014 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
(http://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.

Zhao, et al. Standards Track [Page 2]

RFC 7307 LDP Multi-Topology Extensions July 2014

Table of Contents

1. Introduction ....................................................4
2. Terminology .....................................................4
3. Signaling Extensions ............................................5
3.1. Topology-Scoped Forwarding Equivalence Class (FEC) .........5
3.2. New Address Families: MT IP ................................5
3.3. LDP FEC Elements with MT IP AF .............................6
3.4. IGP MT-ID Mapping and Translation ..........................7
3.5. LDP MT Capability Advertisement ............................7
3.5.1. Protocol Extension ..................................7
3.5.2. Procedures ..........................................9
3.6. Label Spaces ..............................................10
3.7. Reserved MT-ID Values .....................................10
4. MT Applicability on FEC-Based Features .........................10
4.1. Typed Wildcard FEC Element ................................10
4.2. Signaling LDP Label Advertisement Completion ..............11
4.3. LSP Ping ..................................................11
4.3.1. New FEC Sub-Types ..................................11
4.3.2. MT LDP IPv4 FEC Sub-TLV ............................12
4.3.3. MT LDP IPv6 FEC Sub-TLV ............................13
4.3.4. Operation Considerations ...........................13
5. Error Handling .................................................14
5.1. MT Error Notification for Invalid Topology ID .............14
6. Backwards Compatibility ........................................14
7. MPLS Forwarding in MT ..........................................14
8. Security Considerations ........................................14
9. IANA Considerations ............................................15
10. Manageability Considerations ..................................17
10.1. Control of Function and Policy ...........................17
10.2. Information and Data Models ..............................17
10.3. Liveness Detection and Monitoring ........................17
10.4. Verify Correct Operations ................................17
10.5. Requirements on Other Protocols ..........................17
10.6. Impact on Network Operations .............................17
11. Contributors ..................................................18
12. Acknowledgements ..............................................19
13. References ....................................................19
13.1. Normative References .....................................19
13.2. Informative References ...................................19

Zhao, et al. Standards Track [Page 3]

RFC 7307 LDP Multi-Topology Extensions July 2014

1. Introduction

Multi-Topology (MT) routing is supported in IP networks with the use
of MT-aware IGPs. It would be advantageous for Communications
Service Providers (CSPs) to support an MPLS Multi-Topology (MPLS-MT)
environment. The benefits of MPLS-MT technology are features for
various network scenarios, including:

o A CSP may want to assign varying Quality of Service (QoS) profiles
to different traffic classes, based on a specific topology in an
MT routing network;

o Separate routing and MPLS domains may be used to isolate multicast
and IPv6 islands within the backbone network;

o Specific IP address space could be routed across an MT based on
security or operational isolation requirements;

o Low-latency links could be assigned to an MT for delay-sensitive
traffic;

o Management traffic may be divided from customer traffic using
different MTs utilizing separate links, thus ensuring that
management traffic is separated from customer traffic.

This document describes the Label Distribution Protocol (LDP)
procedures and protocol extensions required to support MT routing in
an MPLS environment.

This document defines two new Forwarding Equivalence Class (FEC)
types for use in Label Switched Path (LSP) ping [RFC4379].

2. Terminology

This document uses MPLS terminology defined in [RFC5036]. Additional
terms are defined below:

o MT-ID: A 16-bit value used to represent the Multi-Topology ID.

o Default MT Topology: A topology that is built using the MT-ID
default value of 0.

o MT Topology: A topology that is built using the corresponding MT-
ID.

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].

Zhao, et al. Standards Track [Page 4]

RFC 7307 LDP Multi-Topology Extensions July 2014

3. Signaling Extensions

3.1. Topology-Scoped Forwarding Equivalence Class (FEC)

LDP assigns and binds a label to a FEC, where a FEC is a list of one
or more FEC elements. To set up LSPs for unicast IP routing paths,
LDP assigns local labels for IP prefixes and advertises these labels
to its peers so that an LSP is set up along the routing path. To set
up MT LSPs for IP prefixes under a given topology scope, the LDP
prefix-related FEC element must be extended to include topology
information. This implies that the MT-ID becomes an attribute of the
prefix-related FEC element, and all FEC-Label binding operations are
performed under the context of a given topology (MT-ID).

The following section ("New Address Families: MT IP") defines the
extension required to bind the prefix-related FEC to a topology.

3.2. New Address Families: MT IP

Section 2.1 of the LDP base specification [RFC5036] defines the
Address Prefix FEC element. The Prefix encoding is defined for a
given "Address Family" (AF), and has length (in bits) specified by
the "PreLen" field.

To extend IP address families for MT, two new Address Families named
"MT IP" and "MT IPv6" are used to specify IPv4 and IPv6 prefixes
within a topology scope.

The format of data associated with these new Address Families is
described below:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | MT-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: MT IP Address Family Format

Zhao, et al. Standards Track [Page 5]

RFC 7307 LDP Multi-Topology Extensions July 2014

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 Address |
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | MT-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: MT IPv6 Address Family Format

Where "IP Address" is an IPv4 and IPv6 address/prefix for "MT IP" and
"MT IPv6" AF respectively, and the field "MT-ID" corresponds to the
16-bit Topology ID for a given address.

The definition and usage for the remaining fields in the FEC elements
are as defined for IP/IPv6 AF. The value of MT-ID 0 corresponds to
the default topology and MUST be ignored on receipt so as to not
cause any conflict/confusion with existing non-MT procedures.

The defined FEC elements with "MT IP" Address Family can be used in
any LDP message and procedures that currently specify and allow the
use of FEC elements with IP/IPv6 Address Family.

3.3. LDP FEC Elements with MT IP AF

The following section specifies the format extensions of the existing
LDP FEC elements to support MT. The "Address Family" of these FEC
elements will be set to "MT IP" or "MT IPv6".

The MT Prefix FEC element encoding is as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (2) | Address Family (MT IP/MT IPv6)| PreLen |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | MT-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: MT Prefix FEC Element Format

Zhao, et al. Standards Track [Page 6]

RFC 7307 LDP Multi-Topology Extensions July 2014

The MT Typed Wildcard FEC element encoding is as follows:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Typed Wcard (5)| FEC Type | Len = 6 | AF = MT IP ..|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|... or MT IPv6 | MT-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: MT Typed Wildcard FEC Element

The above format can be used for any LDP FEC element that allows use
of the IP/IPv6 Address Family. In the scope of this document, the
allowed "FEC Type" in a MT Typed Wildcard FEC element is the Prefix
FEC element.

3.4. IGP MT-ID Mapping and Translation

The non-reserved non-special IGP MT-ID values can be used and carried
in LDP without the need for translation. However, there is a need
for translating reserved or special IGP MT-ID values to corresponding
LDP MT-IDs. The assigned, unassigned, and special LDP MT-ID values
have been assigned as described in Section 9 ("IANA Considerations").

How future LDP MT-ID values are allocated is outside the scope of
this document. Instead, a separate document will be created to
detail the allocation policy and process for requesting new MT-ID
values.

3.5. LDP MT Capability Advertisement

3.5.1. Protocol Extension

We specify a new LDP capability, named "Multi-Topology (MT)", which
is defined in accordance with the LDP capability guidelines
[RFC5561]. The LDP "MT" capability can be advertised by an LDP
speaker to its peers either during the LDP session initialization or
after the LDP session is set up. The advertisement is to announce
the capability of the Label Switching Router (LSR) to support MT for
the given IP address family. An LDP speaker MUST NOT send messages
containing MT FEC elements unless the peer has said it can handle it.

The MT capability is specified using the Multi-Topology Capability
TLV. The Multi-Topology Capability TLV format is in accordance with
the LDP capability guidelines as defined in [RFC5561]. To be able to

Zhao, et al. Standards Track [Page 7]

RFC 7307 LDP Multi-Topology Extensions July 2014

specify IP address family, the capability-specific data (i.e., the
"Capability Data" field of Capability TLV) is populated using the
"Typed Wildcard FEC element" as defined in [RFC5918].

The format of the Multi-Topology Capability TLV is as follows:

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| Multi-Topology Cap.(IANA) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| Reserved | |
+-+-+-+-+-+-+-+-+ |
~ Typed Wildcard FEC element(s) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 5: Multi-Topology Capability TLV Format

Where:

o U-bit: MUST be 1 so that the TLV will be silently ignored by a
recipient if it is unknown, according to the rules of [RFC5036].

o F-bit: MUST be 0 as per Section 3 ("Specifying Capabilities in LDP
Messages") of LDP Capabilities [RFC5561].

o Multi-Topology Capability: Capability TLV type (IANA assigned)

o S-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.

o Typed Wildcard FEC element(s): One or more elements specified as
the "Capability data".

o Length: length of Value field, starting from the S-bit, in octets.

o The encoding of the Typed Wildcard FEC element, as defined in
[RFC5918], is defined in Section 4.1 ("Typed Wildcard FEC
element") of this document. The MT-ID field of the MT Typed
Wildcard FEC element MUST be set to "Wildcard Topology" when it is
specified in the MT Capability TLV.

Zhao, et al. Standards Track [Page 8]

RFC 7307 LDP Multi-Topology Extensions July 2014

3.5.2. Procedures

To announce its MT capability for an IP address family, LDP FEC type,
and Multi-Topology, an LDP speaker sends an "MT Capability" including
the exact Typed Wildcard FEC element with the corresponding
"AddressFamily" field (i.e., set to "MT IP" for IPv4 and set to "MT
IPv6" for IPv6 address family), corresponding "FEC Type" field (i.e.,
set to "Prefix"), and corresponding "MT-ID". To announce its MT
capability for both the IPv4 and IPv6 address family, or for multiple
FEC types, or for multiple Multi-Topologies, an LDP speaker sends an
"MT Capability" with one or more MT Typed FEC elements in it.

o The capability for supporting multi-topology in LDP can be
advertised during LDP session initialization stage by including
the LDP MT capability TLV in LDP Initialization message. After an
LDP session is established, the MT capability can also be
advertised or withdrawn using the Capability message (only if the
"Dynamic Capability Announcement" capability [RFC5561] has already
been successfully negotiated).

o If an LSR has not advertised MT capability, its peer MUST NOT send
to this LSR any LDP messages with FEC elements that include an MT
identifier.

o If an LSR is changed from non-MT capable to MT capable, it sets
the S-bit in the MT capability TLV and advertises via the
Capability message (if it supports Dynamic Capability
Announcement). The existing LSP is treated as an LSP for default
MT (ID 0).

o If an LSR is changed from LDP-MT capable to non-MT capable, it
initiates withdrawal of all label mapping for existing LSPs of all
non-default MTs. It also cleans up all the LSPs of all non-
default MTs locally. Then, it clears the S-bit in the MT
capability TLV and advertises via the Capability message (if it
supports Dynamic Capability Announcement). When an LSR knows the
peer node is changed from LDP-MT capable to non-MT capable, it
cleans up all the LSPs of all non-default MTs locally and
initiates withdrawal of all label mapping for existing LSPs of all
non-default MTs. Each side of the node sends a label release to
its peer once it receives the label release messages even though
each side has already cleaned up all the LSPs locally.

o If an LSR does not support "Dynamic Capability Announcement", it
MUST reset the session with its peer whenever the LSR changes its
local capability with regards to supporting LDP MT.

Zhao, et al. Standards Track [Page 9]

RFC 7307 LDP Multi-Topology Extensions July 2014

o If an LSR is changed from IGP-MT capable to non-MT capable, it may
wait until the routes update to withdraw the FEC and release the
label mapping for existing LSPs of a specific MT.

3.6. Label Spaces

The use of multiple topologies for LDP does not require different
label spaces for each topology. An LSR can use the same label space
for all MT FECs as for the default topology.

Similarly, signaling for different topologies can and should be done
within a single LDP session.

3.7. Reserved MT-ID Values

Certain MT topologies are assigned to serve predetermined purposes.

In Section 9 ("IANA Considerations"), this document defines a new
IANA registry "MPLS Multi-Topology Identifiers" to keep LDP MT-ID
reserved values.

If an LSR receives a FEC element with an "MT-ID" value that is
"Unassigned" for future use (and not IANA allocated yet), the LSR
MUST abort the processing of the FEC element and SHOULD send a
notification message with status code "Invalid Topology ID" to the
sender.

4. MT Applicability on FEC-Based Features

4.1. Typed Wildcard FEC Element

[RFC5918] extends base LDP and defines the Typed Wildcard FEC element
framework. The Typed Wildcard FEC element can be used in any LDP
message to specify a wildcard operation/action for a given type of
FEC.

The MT extensions defined in this document do not require any
extension to procedures for the Typed Wildcard FEC element, and these
procedures apply as is to MT wildcarding. The MT extensions, though,
allow use of "MT IP" or "MT IPv6" in the Address Family field of the
Typed Wildcard FEC element in order to use wildcard operations in the
context of a given topology. The use of MT-scoped address family
also allows us to specify MT-ID in these operations.

The defined format in Section 4.1 ("Typed Wildcard FEC element")
allows an LSR to perform wildcard FEC operations under the scope of a
topology. If an LSR wishes to perform a wildcard operation that
applies to all topologies, it can use a "Wildcard Topology" MT-ID.

Zhao, et al. Standards Track [Page 10]

RFC 7307 LDP Multi-Topology Extensions July 2014

For example, upon local de-configuration of a topology "x", an LSR
may send a typed wildcard Label Withdraw message with MT-ID "x" to
withdraw all its labels from the peer that advertised under the scope
of topology "x". Additionally, upon a global configuration change,
an LSR may send a typed wildcard Label Withdraw message with the
MT-ID set to "Wildcard Topology" to withdraw all its labels under all
topologies from the peer.

4.2. Signaling LDP Label Advertisement Completion

[RFC5919] specifies extensions and procedures for an LDP speaker to
signal its convergence for a given FEC type towards a peer. The
procedures defined in [RFC5919] apply as they are to an MT FEC
element. This allows an LDP speaker to signal its IP convergence
using Typed Wildcard FEC element, and its MT IP convergence per
topology using a MT Typed Wildcard FEC element.

4.3. LSP Ping

[RFC4379] defines procedures to detect data-plane failures in MPLS
LSPs via LSP ping. That specification defines a "Target FEC Stack"
TLV that describes the FEC stack being tested. This TLV is sent in
an MPLS Echo Request message towards the LSP's egress LSR and is
forwarded along the same data path as other packets belonging to the
FEC.

"Target FEC Stack" TLV contains one or more sub-TLVs pertaining to
different FEC types. Section 3.2 of [RFC4379] defines the Sub-Types
and format of the FEC. To support LSP ping for MT LDP LSPs, this
document defines the following extensions to [RFC4379].

4.3.1. New FEC Sub-Types

We define two new FEC types for LSP ping:

o MT LDP IPv4 FEC

o MT LDP IPv6 FEC

We also define the following new sub-types for sub-TLVs to specify
these FECs in the "Target FEC Stack" TLV of [RFC4379]:

Sub-Type Length Value Field
-------- ------ -----------------
31 8 MT LDP IPv4 prefix
32 20 MT LDP IPv6 prefix

Figure 6: New Sub-Types for Sub-TLVs

Zhao, et al. Standards Track [Page 11]

RFC 7307 LDP Multi-Topology Extensions July 2014

The rules and procedures of using these sub-TLVs in an MPLS echo
request message are the same as defined for LDP IPv4/IPv6 FEC sub-TLV
types in [RFC4379].

4.3.2. MT LDP IPv4 FEC Sub-TLV

The format of the "MT LDP IPv4 FEC" sub-TLV to be used in a "Target
FEC Stack" [RFC4379] is:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 31 (MT LDP IPv4 FEC) | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | MBZ | MT-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 7: MT LDP IPv4 FEC Sub-TLV

The format of this sub-TLV is similar to the LDP IPv4 FEC sub-TLV as
defined in [RFC4379]. In addition to "IPv4 prefix" and "Prefix
Length" fields, this new sub-TLV also specifies the MT-ID (Multi-
Topology ID). The Length for this sub-TLV is 5.

The term "Must Be Zero" (MBZ) is used in object descriptions for
reserved fields. These fields MUST be set to zero when sent and
ignored on receipt.

Zhao, et al. Standards Track [Page 12]

RFC 7307 LDP Multi-Topology Extensions July 2014

4.3.3. MT LDP IPv6 FEC Sub-TLV

The format of the "MT LDP IPv6 FEC" sub-TLV to be used in a "Target
FEC Stack" [RFC4379] is:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 32 (MT LDP IPv6 FEC) | Length = 20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| IPv6 prefix |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | MBZ | MT-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 8: MT LDP IPv6 FEC Sub-TLV

The format of this sub-TLV is similar to the LDP IPv6 FEC sub-TLV as
defined in [RFC4379]. In addition to the "IPv6 prefix" and "Prefix
Length" fields, this new sub-TLV also specifies the MT-ID (Multi-
Topology ID). The Length for this sub-TLV is 17.

4.3.4. Operation Considerations

To detect data-plane failures using LSP ping for a specific topology,
the router will initiate an LSP ping request with the target FEC
stack TLV containing the LDP MT IP Prefix Sub-TLV in the Echo Request
packet. The Echo Request packet is sent with the label bound to the
IP Prefix in the topology. Once the Echo Request packet reaches the
target router, it will process the packet and perform checks for the
LDP MT IP Prefix sub-TLV present in the Target FEC Stack as described
in [RFC4379] and respond according to the processing rules in
[RFC4379]. For the case that the LSP ping with return path is not
specified, the reply packet must go through the default topology
instead of the topology where the Echo Request goes through.

It should be noted that the existing MIB modules for an MPLS LSR
[RFC3813] and MPLS LDP managed objects [RFC3815] do not provide the
necessary information to support the extensions in this document.
For example, the absence of the MT-ID as an index into the MIB
modules means that there is no way to disambiguate different topology
instances.

Zhao, et al. Standards Track [Page 13]

RFC 7307 LDP Multi-Topology Extensions July 2014

5. Error Handling

The extensions defined in this document utilize the existing LDP
error handling defined in [RFC5036]. If an LSR receives an error
notification from a peer for a session, it terminates the LDP session
by closing the TCP transport connection for the session and
discarding all multi-topology label mappings learned via the session.

5.1. MT Error Notification for Invalid Topology ID

An LSR should respond with an "Invalid Topology ID" status code in
the LDP Notification message when it receives an LDP message with a
FEC element specifying an MT-ID that is not locally known or not
supported. The LSR MUST also discard the entire message before
sending the Notification message.

6. Backwards Compatibility

The MPLS-MT solution is backwards compatible with existing LDP
enhancements defined in [RFC5036], including message authenticity,
integrity of message, and topology loop detection.

The legacy node that does not support MT should not receive any
MT-related LDP messages. In case bad things happen, according to
[RFC5036], processing of such messages should be aborted.

7. MPLS Forwarding in MT

Although forwarding is out of the scope of this document, we include
some forwarding consideration for informational purposes here.

The specified signaling mechanisms allow all the topologies to share
the platform-specific label space. This feature allows the existing
data-plane techniques to be used. Also, there is no way for the data
plane to associate a received packet with any one topology, meaning
that topology-specific label spaces cannot be used.

8. Security Considerations

The use of MT over existing MPLS solutions does not offer any
specific security benefit.

General LDP communication security threats and how these may be
mitigated are described in [RFC5036]; these threats include:

o spoofing

o privacy

Zhao, et al. Standards Track [Page 14]

RFC 7307 LDP Multi-Topology Extensions July 2014

o denial of service

For further discussion regarding possible LDP communication threats
and mitigation techniques, see [RFC5920].

9. IANA Considerations

This document introduces the following new protocol elements, which
have been assigned by IANA:

o New LDP Capability TLV: "Multi-Topology Capability" TLV (0x050C)
from the LDP Parameters registry "TLV Type Name Space".

o New Status Code: "Invalid Topology ID" (0x00000031) from the LDP
Parameters registry "Status Code Name Space").

Registry:
Range/Value Description
-------------- ------------------------------
0x00000031 Invalid Topology ID

Figure 9: New Code Point for LDP Multi-Topology Extensions

o New address families under the IANA registry "Address Family
Numbers":

Number Description
-------- ------------------------------------
29 MT IP: Multi-Topology IP version 4
30 MT IPv6: Multi-Topology IP version 6

Figure 10: Address Family Numbers

Zhao, et al. Standards Track [Page 15]

RFC 7307 LDP Multi-Topology Extensions July 2014

o New registry "MPLS Multi-Topology Identifiers".

This is a registry of the "Multiprotocol Label Switching
Architecture (MPLS)" category.

The initial registrations and allocation policies for this
registry are:

Range/Value Purpose Reference
----------- ------------------------------------- ----------
0 Default/standard topology RFC 7307
1 IPv4 in-band management RFC 7307
2 IPv6 routing topology RFC 7307
3 IPv4 multicast topology RFC 7307
4 IPv6 multicast topology RFC 7307
5 IPv6 in-band management RFC 7307
6-3995 Unassigned for future IGP topologies RFC 7307
Assigned by Standards Action RFC 7307
3996-4095 Experimental RFC 7307
4096-65534 Unassigned for MPLS topologies RFC 7307
Assigned by Standards Action
65535 Wildcard Topology RFC 7307

Figure 11: MPLS Multi-Topology Identifier Registry

o New Sub-TLV Types for LSP ping: The following new sub-type values
under TLV type 1 (Target FEC Stack) have been registered from the
"Sub-TLVs for TLV Types 1, 16, and 21" sub-registry within the
"Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs)
Ping Parameters" registry.

Sub-Type Value Field
-------- ------------------
31 MT LDP IPv4 prefix
32 MT LDP IPv6 prefix

Figure 12: New Sub-TLV Types for LSP Ping

As highlighted at the end of Section 3.4 ("IGP MT-ID Mapping and
Translation"), a new document will be created to detail the policy
and process for allocating new MT-ID values.

Zhao, et al. Standards Track [Page 16]

RFC 7307 LDP Multi-Topology Extensions July 2014

10. Manageability Considerations

10.1. Control of Function and Policy

There are capabilities that should be configurable to enable good
manageability. One such example is to allow that the LDP Multi-
Topology capability be enabled or disabled. It is assumed that the
mapping of the LDP MT-ID and IGP MT-ID is manually configured on
every router by default. If an automatic mapping between IGP MT-IDs
and LDP MT-IDs is needed, there must be explicit configuration to do
so.

10.2. Information and Data Models

Any extensions that may be required for existing MIBs are beyond the
scope of this document.

10.3. Liveness Detection and Monitoring

Mechanisms defined in this document do not imply any new liveness
detection and monitoring requirements.

10.4. Verify Correct Operations

In order to debug an LDP-MT-enabled network, it may be necessary to
associate between the LDP label advertisement and the IGP routing
advertisement. In this case, the user MUST understand the mapping
mechanism to convert the IGP MT-ID to the LDP MT-ID. The method and
type of mapping mechanism is out of the scope of this document.

10.5. Requirements on Other Protocols

If the LDP MT-ID has an implicit dependency on IGP MT-ID, then the
corresponding IGP MT features will need to be supported.

10.6. Impact on Network Operations

Mechanisms defined in this document do not have any impact on network
operations.

Zhao, et al. Standards Track [Page 17]

RFC 7307 LDP Multi-Topology Extensions July 2014

11. Contributors

Ning So
Tata Communications
2613 Fairbourne Cir.
Plano, TX 75082
USA

EMail: [email protected]

Raveendra Torvi
Juniper Networks
10 Technology Park Drive
Westford, MA 01886-3140
US

EMail: [email protected]

Huaimo Chen
Huawei Technology
125 Nagog Technology Park
Acton, MA 01719
US

Emily Chen
2717 Seville Blvd, Apt. 1205
Clearwater, FL 33764
US

EMail: [email protected]

Chen Li
China Mobile
53A, Xibianmennei Ave.
Xunwu District, Beijing 01719
China

EMail: [email protected]

Lu Huang
China Mobile
53A, Xibianmennei Ave.
Xunwu District, Beijing 01719
China

Zhao, et al. Standards Track [Page 18]

RFC 7307 LDP Multi-Topology Extensions July 2014

12. Acknowledgements

The authors would like to thank Dan Tappan, Nabil Bitar, Huang Xin,
Eric Rosen, IJsbrand Wijnands, Dimitri Papadimitriou, Yiqun Chai,
Pranjal Dutta, George Swallow, Curtis Villamizar, Adrian Farrel, Alia
Atlas, and Loa Anderson for their valuable comments on this document.

13. References

13.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
February 2006.

[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, October 2007.

[RFC5561] Thomas, B., Raza, K., Aggarwal, S., Aggarwal, R., and JL.
Le Roux, "LDP Capabilities", RFC 5561, July 2009.

[RFC5918] Asati, R., Minei, I., and B. Thomas, "Label Distribution
Protocol (LDP) 'Typed Wildcard' Forward Equivalence Class
(FEC)", RFC 5918, August 2010.

[RFC5919] Asati, R., Mohapatra, P., Chen, E., and B. Thomas,
"Signaling LDP Label Advertisement Completion", RFC 5919,
August 2010.

13.2. Informative References

[RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.

[RFC3813] Srinivasan, C., Viswanathan, A., and T. Nadeau,
"Multiprotocol Label Switching (MPLS) Label Switching
Router (LSR) Management Information Base (MIB)", RFC 3813,
June 2004. Srinivasan, C., Viswanathan, A., and T.
Nadeau,

[RFC3815] Cucchiara, J., Sjostrand, H., and J. Luciani, "Definitions
of Managed Objects for the Multiprotocol Label Switching
(MPLS), Label Distribution Protocol (LDP)", RFC 3815, June
2004.

Zhao, et al. Standards Track [Page 19]

RFC 7307 LDP Multi-Topology Extensions July 2014

Authors' Addresses

Quintin Zhao
Huawei Technology
125 Nagog Technology Park
Acton, MA 01719
US
EMail: [email protected]

Kamran Raza
Cisco Systems
2000 Innovation Drive
Kanata, ON K2K-3E8
Canada
EMail: [email protected]

Chao Zhou
Cisco Systems
300 Beaver Brook Road
Boxborough, MA 01719
US
EMail: [email protected]

Luyuan Fang
Microsoft
5600 148th Ave NE
Redmond, WA 98052
US
EMail: [email protected]

Lianyuan Li
China Mobile
53A, Xibianmennei Ave.
Xunwu District, Beijing 01719
China
EMail: [email protected]

Daniel King
Old Dog Consulting
EMail: [email protected]

Zhao, et al. Standards Track [Page 20]