Internet Engineering Task Force (IETF) R. Asati

Internet Engineering Task Force (IETF) R. Asati
Request for Comments: 7552 C. Pignataro
Updates: 5036, 6720 K. Raza
Category: Standards Track Cisco
ISSN: 2070-1721 V. Manral
Ionos Networks
R. Papneja
Huawei
June 2015

Updates to LDP for IPv6

Abstract

The Label Distribution Protocol (LDP) specification defines
procedures to exchange label bindings over either IPv4 or IPv6
networks, or both. This document corrects and clarifies the LDP
behavior when an IPv6 network is used (with or without IPv4). This
document updates RFCs 5036 and 6720.

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/rfc7552.

Asati, et al. Standards Track [Page 1]

RFC 7552 Updates to LDP for IPv6 June 2015

Copyright Notice

Copyright (c) 2015 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
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This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
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material may not have granted the IETF Trust the right to allow
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Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.

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RFC 7552 Updates to LDP for IPv6 June 2015

Table of Contents

1. Introduction ....................................................4
1.1. Topology Scenarios for Dual-Stack Environment ..............5
1.2. Single-Hop vs. Multi-Hop LDP Peering .......................6
2. Specification Language ..........................................6
3. LSP Mapping .....................................................7
4. LDP Identifiers .................................................8
5. Neighbor Discovery ..............................................8
5.1. Basic Discovery Mechanism ..................................8
5.1.1. Maintaining Hello Adjacencies .......................9
5.2. Extended Discovery Mechanism ..............................10
6. LDP Session Establishment and Maintenance ......................10
6.1. Transport Connection Establishment ........................10
6.1.1. Dual-Stack: Transport Connection Preference
and Role of an LSR .................................12
6.2. LDP Session Maintenance ...................................14
7. Binding Distribution ...........................................15
7.1. Address Distribution ......................................15
7.2. Label Distribution ........................................16
8. LDP Identifiers and Duplicate Next-Hop Addresses ...............17
9. LDP TTL Security ...............................................18
10. IANA Considerations ...........................................18
11. Security Considerations .......................................19
12. References ....................................................19
12.1. Normative References .....................................19
12.2. Informative References ...................................20
Appendix A. Additional Considerations .............................21
A.1. LDPv6 and LDPv4 Interoperability Safety Net ................21
A.2. Accommodating Implementations Not Compliant with RFC 5036 ..21
A.3. Why prohibit IPv4-mapped IPv6 addresses in LDP? ............22
A.4. Why a 32-bit value even for the IPv6 LDP Router Id? ........22
Acknowledgments ...................................................23
Contributors ......................................................23
Authors' Addresses.................................................24

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RFC 7552 Updates to LDP for IPv6 June 2015

1. Introduction

The LDP specification [RFC5036] defines procedures and messages for
exchanging FEC-label bindings over either IPv4 or IPv6 networks, or
both (i.e., Dual-stack networks).

However, RFC 5036 has the following deficiencies (i.e., lacks
details) in regard to IPv6 usage (with or without IPv4):

1. Label Switched Path (LSP) Mapping: No rule for mapping a
particular packet to a particular LSP that has an Address Prefix
Forwarding Equivalence Class (FEC) element containing the IPv6
address of the egress router

2. LDP Identifier: No details specific to IPv6 usage

3. LDP Discovery: No details for using a particular IPv6 destination
(multicast) address or the source address

4. LDP Session Establishment: No rule for handling both IPv4 and IPv6
Transport Address optional objects in a Hello message, and
subsequently two IPv4 and IPv6 transport connections

5. LDP Address Distribution: No rule for advertising IPv4 and/or IPv6
address bindings over an LDP session

6. LDP Label Distribution: No rule for advertising IPv4 and/or IPv6
FEC-label bindings over an LDP session, or for handling the
coexistence of IPv4 and IPv6 FEC Elements in the same FEC TLV

7. Next-Hop Address Resolution: No rule for accommodating the usage
of duplicate link-local IPv6 addresses

8. LDP Time to Live (TTL) Security: No rule for a built-in
Generalized TTL Security Mechanism (GTSM) in LDP with IPv6 (this
is a deficiency in [RFC6720])

This document addresses the above deficiencies by specifying the
desired behavior/rules/details for using LDP in IPv6-enabled networks
(IPv6-only or Dual-stack networks). This document closes the IPv6
MPLS gap discussed in Sections 3.2.1, 3.2.2, and 3.3.1.1 of
[RFC7439].

Note that this document updates [RFC5036] and [RFC6720].

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1.1. Topology Scenarios for Dual-Stack Environment

Two Label Switching Routers (LSRs) may involve Basic and/or Extended
LDP Discovery in IPv6 and/or IPv4 address families in various
topology scenarios.

This document addresses the following three topology scenarios in
which the LSRs may be connected via one or more Dual-stack
LDP-enabled interfaces (Figure 1), or one or more Single-stack
LDP-enabled interfaces (Figures 2 and 3):

R1------------------R2
IPv4+IPv6

Figure 1: LSRs Connected via a Dual-Stack Interface

IPv4
R1=================R2
IPv6

Figure 2: LSRs Connected via Two Single-Stack Interfaces

R1------------------R2---------------R3
IPv4 IPv6

Figure 3: LSRs Connected via a Single-Stack Interface

Note that the topology scenario illustrated in Figure 1 also covers
the case of a Single-stack LDP-enabled interface (say, IPv4) being
converted to a Dual-stack LDP-enabled interface (by enabling IPv6
routing as well as IPv6 LDP), even though the LDP-over-IPv4
(LDPoIPv4) session may already be established between the LSRs.

Note that the topology scenario illustrated in Figure 2 also
covers the case of two routers getting connected via an additional
Single-stack LDP-enabled interface (IPv6 routing and IPv6 LDP), even
though the LDPoIPv4 session may already be established between the
LSRs over the existing interface(s).

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This document also addresses the scenario in which the LSRs do the
Extended Discovery in IPv6 and/or IPv4 address families:

IPv4
R1-------------------R2
IPv6

Figure 4: LSRs Involving IPv4 and IPv6 Address Families

1.2. Single-Hop vs. Multi-Hop LDP Peering

The LDP TTL Security mechanism specified by this document applies
only to single-hop LDP peering sessions, not to multi-hop LDP peering
sessions, in line with Section 5.5 of [RFC5082]. [RFC5082] describes
the Generalized TTL Security Mechanism (GTSM).

As a consequence, any LDP feature that relies on a multi-hop LDP
peering session would not work with GTSM and will warrant (statically
or dynamically) disabling GTSM. Please see Section 9.

2. Specification 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 [RFC2119].

Abbreviations:

LDP Label Distribution Protocol

LDPoIPv4 LDP-over-IPv4 transport connection

LDPoIPv6 LDP-over-IPv6 transport connection

FEC Forwarding Equivalence Class

TLV Type Length Value

LSR Label Switching Router

LSP Label Switched Path

LSPv4 IPv4-signaled Label Switched Path

LSPv6 IPv6-signaled Label Switched Path

AFI Address Family Identifier

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LDP Id LDP Identifier

Single-stack LDP LDP supporting just one address family
(for discovery, session setup, address/label
binding exchange, etc.)

Dual-stack LDP LDP supporting two address families
(for discovery, session setup, address/label
binding exchange, etc.)

Dual-stack LSR LSR supporting Dual-stack LDP for a peer

Single-stack LSR LSR supporting Single-stack LDP for a peer

Note that an LSR can be a Dual-stack and Single-stack LSR at the same
time for different peers. This document loosely uses the term
"address family" to mean "IP address family".

3. LSP Mapping

Section 2.1 of [RFC5036] specifies the procedure for mapping a
particular packet to a particular LSP using three rules. Quoting the
third rule from [RFC5036]:

If it is known that a packet must traverse a particular egress
router, and there is an LSP that has an Address Prefix FEC element
that is a /32 address of that router, then the packet is mapped to
that LSP.

This rule is correct for IPv4 (to set up LSPv4), but not for IPv6
(to set up LSPv6), since an IPv6 router may even have a /64 or /96
or /128 (or whatever prefix length) address. Hence, that rule is
updated here to use IPv4 or IPv6 addresses instead of /32 or /128
addresses, as shown below:

If it is known that a packet must traverse a particular egress
router, and there is an LSP that has an Address Prefix FEC element
that is an IPv4 or IPv6 address of that router, then the packet is
mapped to that LSP.

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4. LDP Identifiers

In line with Section 2.2.2 of [RFC5036], this document specifies the
usage of a 32-bit (unsigned non-zero integer) LSR Id on an
IPv6-enabled LSR (with or without Dual-stacking).

This document also qualifies the first sentence of the last paragraph
of Section 2.5.2 of [RFC5036] to be per address family.

From Section 2.5.2 of [RFC5036]:

An LSR MUST advertise the same transport address in all Hellos
that advertise the same label space.

Updated by this document, as follows:

For a given address family, an LSR MUST advertise the same
transport address in all Hellos that advertise the same label
space.

This rightly enables the per-platform label space to be shared
between IPv4 and IPv6.

In summary, this document mandates the usage of a common LDP
Identifier (the same LSR Id and label space id) for both IPv4 and
IPv6 address families.

5. Neighbor Discovery

If Dual-stack LDP is enabled (i.e., LDP enabled in both IPv6 and IPv4
address families) on an interface or for a targeted neighbor, then
the LSR MUST transmit both IPv6 and IPv4 LDP (Link or targeted)
Hellos and include the same LDP Identifier (assuming per-platform
label space usage) in them.

If Single-stack LDP is enabled (i.e., LDP enabled in either an IPv6
or IPv4 address family), then the LSR MUST transmit either IPv6 or
IPv4 LDP (Link or targeted) Hellos, respectively.

5.1. Basic Discovery Mechanism

Section 2.4.1 of [RFC5036] defines the Basic Discovery mechanism for
directly connected LSRs. Following this mechanism, LSRs periodically
send LDP Link Hellos destined to the "all routers on this subnet"
group multicast IP address.

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Interestingly enough, per the IPv6 addressing architecture [RFC4291],
IPv6 has three "all routers on this subnet" multicast addresses:

ff01:0:0:0:0:0:0:2 = Interface-local scope

ff02:0:0:0:0:0:0:2 = Link-local scope

ff05:0:0:0:0:0:0:2 = Site-local scope

[RFC5036] does not specify which particular IPv6 "all routers on this
subnet" group multicast IP address should be used by LDP Link Hellos.

This document specifies the usage of link-local scope (i.e.,
ff02:0:0:0:0:0:0:2) as the destination multicast IP address in IPv6
LDP Link Hellos. An LDP Link Hello packet received on any of the
other destination addresses MUST be dropped. Additionally, the
link-local IPv6 address MUST be used as the source IP address in IPv6
LDP Link Hellos.

Also, the LDP Link Hello packets MUST have their IPv6 Hop Limit set
to 255, be checked for the same upon receipt (before any LDP-specific
processing), and be handled as specified in Section 3 of [RFC5082].
The built-in inclusion of GTSM automatically protects IPv6 LDP from
off-link attacks.

More importantly, if an interface is a Dual-stack LDP interface
(i.e., LDP enabled in both IPv6 and IPv4 address families), then the
LSR MUST periodically transmit both IPv6 and IPv4 LDP Link Hellos
(using the same LDP Identifier per Section 4) on that interface and
be able to receive them. This facilitates discovery of IPv6-only,
IPv4-only, and Dual-stack peers on the interface's subnet and ensures
successful subsequent peering using the appropriate (address family)
transport on a multi-access or broadcast interface.

5.1.1. Maintaining Hello Adjacencies

In the case of a Dual-stack LDP-enabled interface, the LSR SHOULD
maintain Link Hello adjacencies for both IPv4 and IPv6 address
families. This document, however, allows an LSR to maintain
Receive-side Link Hello adjacencies only for the address family that
has been used for the establishment of the LDP session (whether an
LDPoIPv4 or LDPoIPv6 session).

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5.2. Extended Discovery Mechanism

The Extended Discovery mechanism (defined in Section 2.4.2 of
[RFC5036]), in which the targeted LDP Hellos are sent to a unicast
IPv6 address destination, requires only one IPv6-specific
consideration: the link-local IPv6 addresses MUST NOT be used as the
targeted LDP Hello packet's source or destination addresses.

6. LDP Session Establishment and Maintenance

Section 2.5.1 of [RFC5036] defines a two-step process for LDP session
establishment, once the neighbor discovery has completed (i.e., LDP
Hellos have been exchanged):

1. Transport connection establishment

2. Session initialization

Section 6.1 discusses the LDP considerations for IPv6 and/or
Dual-stacking in the context of session establishment, whereas
Section 6.2 discusses the LDP considerations for IPv6 and/or
Dual-stacking in the context of session maintenance.

6.1. Transport Connection Establishment

Section 2.5.2 of [RFC5036] specifies the use of a Transport Address
optional object (TLV) in LDP Hello messages to convey the transport
(IP) address; however, it does not specify the behavior of LDP if
both IPv4 and IPv6 Transport Address objects (TLVs) are sent in a
Hello message or separate Hello messages. More importantly, it does
not specify whether both IPv4 and IPv6 transport connections should
be allowed if both IPv4 and IPv6 Hello adjacencies were present prior
to session establishment.

This document specifies the following:

1. An LSR MUST NOT send a Hello message containing both IPv4 and IPv6
Transport Address optional objects. In other words, there MUST be
at most one Transport Address optional object in a Hello message.
An LSR MUST include only the transport address whose address
family is the same as that of the IP packet carrying the Hello
message.

2. An LSR SHOULD accept the Hello message that contains both IPv4 and
IPv6 Transport Address optional objects but MUST use only the
transport address whose address family is the same as that of the
IP packet carrying the Hello message. An LSR SHOULD accept only
the first Transport Address optional object for a given address

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family in the received Hello message and ignore the rest if the
LSR receives more than one Transport Address optional object for a
given address family.

3. An LSR MUST send separate Hello messages (each containing either
an IPv4 or IPv6 Transport Address optional object) for each IP
address family if Dual-stack LDP is enabled (for an interface or
neighbor).

4. An LSR MUST use a global unicast IPv6 address in an IPv6 Transport
Address optional object of outgoing targeted Hellos and check for
the same in incoming targeted Hellos (i.e., MUST discard the
targeted Hello if it failed the check).

5. An LSR MUST prefer using a global unicast IPv6 address in an
IPv6 Transport Address optional object of outgoing Link Hellos if
it had to choose between a global unicast IPv6 address and a
unique-local or link-local IPv6 address.

6. A Single-stack LSR MUST establish either an LDPoIPv4 or LDPoIPv6
session with a remote LSR as per the enabled address family.

7. A Dual-stack LSR MUST NOT initiate or accept the request for a TCP
connection for a new LDP session with a remote LSR if it already
has an LDPoIPv4 or LDPoIPv6 session for the same LDP Identifier
established with that remote LSR.

This means that only one transport connection is established,
regardless of IPv6 and/or IPv4 Hello adjacencies present between
two LSRs.

8. A Dual-stack LSR SHOULD prefer establishing an LDPoIPv6 session
(instead of an LDPoIPv4 session) with a remote Dual-stack LSR by
following the 'transport connection role' determination logic in
Section 6.1.1.

Additionally, to ensure the above preference in the case where
Dual-stack LDP is enabled on an interface, it would be desirable
that IPv6 LDP Link Hellos are transmitted before IPv4 LDP Link
Hellos, particularly when an interface is coming into service or
being reconfigured.

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6.1.1. Dual-Stack: Transport Connection Preference and Role of an LSR

Section 2.5.2 of [RFC5036] specifies the rules for determining
active/passive roles in setting up a TCP connection. These rules are
clear for Single-stack LDP but not for Dual-stack LDP, in which an
LSR may assume different roles for different address families,
causing the LDP session to not get established.

To ensure a deterministic transport connection (active/passive) role
in the case of Dual-stack LDP, this document specifies that the
Dual-stack LSR conveys its transport connection preference in every
LDP Hello message. This preference is encoded in a new TLV, named
the "Dual-Stack capability" TLV, as defined 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| Dual-Stack capability | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|TR | Reserved | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 5: Dual-Stack Capability TLV

Where:

U and F bits: 1 and 0 (as specified by [RFC5036])

Dual-Stack capability: TLV code point (Ox0701)

TR: Transport Connection Preference

This document defines the following two values:

0100: LDPoIPv4 connection

0110: LDPoIPv6 connection (default)

Reserved
This field is reserved. It MUST be set to zero on
transmission and ignored on receipt.

A Dual-stack LSR (i.e., an LSR supporting Dual-stack LDP for a peer)
MUST include the Dual-Stack capability TLV in all of its LDP Hellos
and MUST set the "TR" field to announce its preference for either an
LDPoIPv4 or LDPoIPv6 transport connection for that peer. The default
preference is LDPoIPv6.

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A Dual-stack LSR MUST always check for the presence of the Dual-Stack
capability TLV in the received Hello messages and take appropriate
action, as follows:

1. If the Dual-Stack capability TLV is present and the remote
preference does not match the local preference (or does not get
recognized), then the LSR MUST discard the Hello message and log
an error.

If an LDP session was already in place, then the LSR MUST send a
fatal Notification message with status code of 'Transport
Connection Mismatch' (0x00000032) and reset the session.

2. If the Dual-Stack capability TLV is present and the remote
preference matches the local preference, then:

a) If TR=0100 (LDPoIPv4), then determine the active/passive roles
for the TCP connection using an IPv4 transport address as
defined in Section 2.5.2 of RFC 5036.

b) If TR=0110 (LDPoIPv6), then determine the active/passive roles
for the TCP connection by using an IPv6 transport address as
defined in Section 2.5.2 of RFC 5036.

3. If the Dual-Stack capability TLV is NOT present and

a) only IPv4 Hellos are received, then the neighbor is deemed as a
legacy IPv4-only LSR (supporting Single-stack LDP); hence, an
LDPoIPv4 session SHOULD be established (similar to that of 2a
above).

However, if IPv6 Hellos are also received at any time during
the life of the session from that neighbor, then the neighbor
is deemed as a noncompliant Dual-stack LSR (similar to that of
3c below), resulting in any established LDPoIPv4 session being
reset and a fatal Notification message being sent (with status
code of 'Dual-Stack Noncompliance', 0x00000033).

b) only IPv6 Hellos are received, then the neighbor is deemed as
an IPv6-only LSR (supporting Single-stack LDP) and an LDPoIPv6
session SHOULD be established (similar to that of 2b above).

However, if IPv4 Hellos are also received at any time during
the life of the session from that neighbor, then the neighbor
is deemed as a noncompliant Dual-stack LSR (similar to that of
3c below), resulting in any established LDPoIPv6 session being
reset and a fatal Notification message being sent (with status
code of 'Dual-Stack Noncompliance', 0x00000033).

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c) both IPv4 and IPv6 Hellos are received, then the neighbor is
deemed as a noncompliant Dual-stack neighbor and is not allowed
to have any LDP session. A Notification message should be sent
(with status code of 'Dual-Stack Noncompliance', 0x00000033).

A Dual-stack LSR MUST convey the same transport connection preference
("TR" field value) in all (link and targeted) Hellos that advertise
the same label space to the same peer and/or on the same interface.
This ensures that two LSRs linked by multiple Hello adjacencies using
the same label spaces play the same connection establishment role for
each adjacency.

A Dual-stack LSR MUST follow Section 2.5.5 of [RFC5036] and check for
matching Hello messages from the peer (either all Hellos also include
the Dual-Stack capability (with the same TR value) or none do).

A Single-stack LSR does not need to use the Dual-Stack capability in
Hello messages and SHOULD ignore this capability if received.

An implementation may provide an option to favor one AFI (say, IPv4)
over another AFI (say, IPv6) for the TCP transport connection, so as
to use the favored IP version for the LDP session and force
deterministic active/passive roles.

Note: An alternative to this new capability TLV could be a new Flag
value in an LDP Hello message; however, it would be used even in
Single-stack IPv6 LDP networks and linger on forever, even though
Dual-stack will not. Hence, the idea of this alternative has been
discarded.

6.2. LDP Session Maintenance

This document specifies that two LSRs maintain a single LDP session,
regardless of the number of Link or targeted Hello adjacencies
between them, as described in Section 6.1. This is independent of
whether:

- they are connected via a Dual-stack LDP-enabled interface(s) or via
two (or more) Single-stack LDP-enabled interfaces;

- a Single-stack LDP-enabled interface is converted to a Dual-stack
LDP-enabled interface (see Figure 1) on either LSR;

- an additional Single-stack or Dual-stack LDP-enabled interface is
added or removed between two LSRs (see Figure 2).

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If the last Hello adjacency for a given address family goes down
(e.g., due to Dual-stack LDP-enabled interfaces being converted into
Single-stack LDP-enabled interfaces on one LSR) and that address
family is the same as the one used in the transport connection, then
the transport connection (LDP session) MUST be reset. Otherwise, the
LDP session MUST stay intact.

If the LDP session is torn down for whatever reason (LDP disabled for
the corresponding transport, Hello adjacency expiry, preference
mismatch, etc.), then the LSRs SHOULD initiate the establishment of a
new LDP session as per the procedures described in Section 6.1 of
this document.

7. Binding Distribution

LSRs by definition can be enabled for Dual-stack LDP globally and/or
per peer so as to exchange the address and label bindings for both
IPv4 and IPv6 address families, independent of any LDPoIPv4 or
LDPoIPv6 session between them.

However, there might be some legacy LSRs that are fully compliant
with RFC 5036 for IPv4 but are noncompliant for IPv6 (for example,
see Section 3.5.5.1 of RFC 5036), causing them to reset the session
upon receiving IPv6 address bindings or IPv6 FEC (Prefix) label
bindings from a peer compliant with this document. This is somewhat
undesirable, as clarified further in Appendices A.1 and A.2.

To help maintain backward compatibility (i.e., accommodate IPv4-only
LDP implementations that may not be compliant with RFC 5036,
Section 3.5.5.1), this specification requires that an LSR MUST NOT
send any IPv6 bindings to a peer if the peer has been determined to
be a legacy LSR.

The Dual-Stack capability TLV, which is defined in Section 6.1.1, is
also used to determine whether or not a peer is a legacy (IPv4-only
Single-stack) LSR.

7.1. Address Distribution

An LSR MUST NOT advertise (via an Address message) any IPv4-mapped
IPv6 addresses (as defined in Section 2.5.5.2 of [RFC4291]) and MUST
ignore such addresses if ever received. Please see Appendix A.3.

If an LSR is enabled with Single-stack LDP for any peer, then it MUST
advertise (via an Address message) its local IP addresses as per the
enabled address family to that peer and process received Address
messages containing IP addresses as per the enabled address family
from that peer.

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If an LSR is enabled with Dual-stack LDP for a peer and

1. does not find the Dual-Stack capability TLV in the incoming IPv4
LDP Hello messages from that peer, then the LSR MUST NOT advertise
its local IPv6 addresses to the peer.

2. finds the Dual-Stack capability TLV in the incoming IPv4 (or IPv6)
LDP Hello messages from that peer, then it MUST advertise (via an
Address message) its local IPv4 and IPv6 addresses to that peer.

3. does not find the Dual-Stack capability TLV in the incoming IPv6
LDP Hello messages, then it MUST advertise (via an Address
message) only its local IPv6 addresses to that peer.

This last point helps to maintain forward compatibility (no need
to require this TLV in the case of IPv6 Single-stack LDP).

7.2. Label Distribution

An LSR MUST NOT allocate and MUST NOT advertise FEC-label bindings
for link-local or IPv4-mapped IPv6 addresses (defined in
Section 2.5.5.2 of [RFC4291]), and it MUST ignore such bindings if
ever received. Please see Appendix A.3.

If an LSR is enabled with Single-stack LDP for any peer, then it MUST
advertise (via a Label Mapping message) FEC-label bindings for the
enabled address family to that peer and process received FEC-label
bindings for the enabled address family from that peer.

If an LSR is enabled with Dual-stack LDP for a peer and

1. does not find the Dual-Stack capability TLV in the incoming IPv4
LDP Hello messages from that peer, then the LSR MUST NOT advertise
IPv6 FEC-label bindings to the peer (even if IP capability
negotiation for the IPv6 address family was done).

2. finds the Dual-Stack capability TLV in the incoming IPv4 (or IPv6)
LDP Hello messages from that peer, then it MUST advertise
FEC-label bindings for both IPv4 and IPv6 address families to that
peer.

3. does not find the Dual-Stack capability TLV in the incoming IPv6
LDP Hello messages, then it MUST advertise FEC-label bindings for
IPv6 address families to that peer.

This last point helps to maintain forward compatibility (no need
to require this TLV for IPv6 Single-stack LDP).

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An LSR MAY further constrain the advertisement of FEC-label bindings
for a particular address family by negotiating the IP capability for
a given address family, as specified in [RFC7473]. This allows an
LSR pair to neither advertise nor receive the undesired FEC-label
bindings on a per-address-family basis to a peer.

If an LSR is configured to change an interface or peer from
Single-stack LDP to Dual-stack LDP, then an LSR SHOULD use Typed
Wildcard FEC procedures [RFC5918] to request the label bindings for
the enabled address family. This helps to relearn the label bindings
that may have been discarded before, without resetting the session.

8. LDP Identifiers and Duplicate Next-Hop Addresses

RFC 5036, Section 2.7 specifies the logic for mapping the IP routing
next hop (of a given FEC) to an LDP peer so as to find the correct
label entry for that FEC. The logic involves using the IP routing
next-hop address as an index into the (peer address) database (which
is populated by the Address message containing a mapping between each
peer's local addresses and its LDP Identifier) to determine the LDP
peer.

However, this logic is insufficient to deal with duplicate IPv6
(link-local) next-hop addresses used by two or more peers. The
reason is that all interior IPv6 routing protocols (can) use
link-local IPv6 addresses as the IP routing next hops, and
"IP Version 6 Addressing Architecture" [RFC4291] allows a link-local
IPv6 address to be used on more than one link.

Hence, this logic is extended by this specification to use not only
the IP routing next-hop address but also the IP routing next-hop
interface to uniquely determine the LDP peer(s). The next-hop
address-based LDP peer mapping is to be done through the LDP peer
address database (populated by Address messages received from the LDP
peers), whereas next-hop interface-based LDP peer mapping is to be
done through the LDP Hello adjacency/interface database (populated by
Hello messages received from the LDP peers).

This extension solves the problem of two or more peers using the same
link-local IPv6 address (in other words, duplicate peer addresses) as
the IP routing next hops.

Lastly, for better scale and optimization, an LSR may advertise only
the link-local IPv6 addresses in the Address message, assuming that
the peer uses only the link-local IPv6 addresses as static and/or
dynamic IP routing next hops.

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9. LDP TTL Security

This document mandates the use of the Generalized TTL Security
Mechanism (GTSM) [RFC6720] for LDP Link Hello packets over IPv6 (see
Section 5.1).

This document further recommends enabling GTSM for the LDP/TCP
transport connection over IPv6 (i.e., LDPoIPv6). This GTSM inclusion
is intended to automatically protect IPv6 LDP peering sessions from
off-link attacks.

[RFC6720] allows for the implementation to statically (via
configuration) and/or dynamically override the default behavior
(enable/disable GTSM) on a per-peer basis. Such an option could be
set on either LSR in a peering session (since GTSM negotiation would
ultimately disable GTSM between the LSR and its peer(s)).

LDP Link Hello packets MUST have their IPv6 Hop Limit set to 255 and
be checked for the same upon receipt before any further processing,
as per Section 3 of [RFC5082].

10. IANA Considerations

This document defines a new optional parameter for the LDP Hello
message and two new status codes for the LDP Notification message.

The "Dual-Stack capability" parameter has been assigned a code point
(0x0701) from the "TLV Type Name Space" registry. IANA has allocated
this code point from the IETF Consensus range 0x0700-0x07ff for the
Dual-Stack capability TLV.

The 'Transport Connection Mismatch' status code has been assigned a
code point (0x00000032) from the "Status Code Name Space" registry.
IANA has allocated this code point from the IETF Consensus range and
marked the E bit column with a '1'.

The 'Dual-Stack Noncompliance' status code has been assigned a code
point (0x00000033) from the "Status Code Name Space" registry. IANA
has allocated this code point from the IETF Consensus range and
marked the E bit column with a '1'.

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11. Security Considerations

The extensions defined in this document only clarify the behavior of
LDP; they do not define any new protocol procedures. Hence, this
document does not add any new security issues to LDP.

While the security issues relevant for [RFC5036] are relevant for
this document as well, this document reduces the chances of off-link
attacks when using an IPv6 transport connection by including the use
of GTSM procedures [RFC5082]. Please see Section 9 for LDP TTL
Security details.

Moreover, this document allows the use of IPsec [RFC4301] for IPv6
protection; hence, LDP can benefit from the additional security as
specified in [RFC7321] as well as [RFC5920].

12. References

12.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291,
February 2006, <http://www.rfc-editor.org/info/rfc4291>.

[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
October 2007, <http://www.rfc-editor.org/info/rfc5036>.

[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
Pignataro, "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,
<http://www.rfc-editor.org/info/rfc5082>.

[RFC5918] Asati, R., Minei, I., and B. Thomas, "Label Distribution
Protocol (LDP) 'Typed Wildcard' Forward Equivalence Class
(FEC)", RFC 5918, DOI 10.17487/RFC5918, August 2010,
<http://www.rfc-editor.org/info/rfc5918>.

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12.2. Informative References

[RFC4038] Shin, M-K., Ed., Hong, Y-G., Hagino, J., Savola, P., and
E. Castro, "Application Aspects of IPv6 Transition",
RFC 4038, DOI 10.17487/RFC4038, March 2005,
<http://www.rfc-editor.org/info/rfc4038>.

[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <http://www.rfc-editor.org/info/rfc4301>.

[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<http://www.rfc-editor.org/info/rfc5340>.

[RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
<http://www.rfc-editor.org/info/rfc5920>.

[RFC6286] Chen, E. and J. Yuan, "Autonomous-System-Wide Unique BGP
Identifier for BGP-4", RFC 6286, DOI 10.17487/RFC6286,
June 2011, <http://www.rfc-editor.org/info/rfc6286>.

[RFC6720] Pignataro, C. and R. Asati, "The Generalized TTL Security
Mechanism (GTSM) for the Label Distribution Protocol
(LDP)", RFC 6720, DOI 10.17487/RFC6720, August 2012,
<http://www.rfc-editor.org/info/rfc6720>.

[RFC7321] McGrew, D. and P. Hoffman, "Cryptographic Algorithm
Implementation Requirements and Usage Guidance for
Encapsulating Security Payload (ESP) and Authentication
Header (AH)", RFC 7321, DOI 10.17487/RFC7321, August 2014,
<http://www.rfc-editor.org/info/rfc7321>.

[RFC7439] George, W., Ed., and C. Pignataro, Ed., "Gap Analysis for
Operating IPv6-Only MPLS Networks", RFC 7439,
DOI 10.17487/RFC7439, January 2015,
<http://www.rfc-editor.org/info/rfc7439>.

[RFC7473] Raza, K. and S. Boutros, "Controlling State Advertisements
of Non-negotiated LDP Applications", RFC 7473,
DOI 10.17487/RFC7473, March 2015,
<http://www.rfc-editor.org/info/rfc7473>.

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Appendix A. Additional Considerations

A.1. LDPv6 and LDPv4 Interoperability Safety Net

It is not safe to assume that implementations compliant with RFC 5036
have supported the handling of an IPv6 address family (IPv6
FEC-label) in a Label Mapping message all along.

If a router upgraded per this specification advertised both IPv4 and
IPv6 FECs in the same Label Mapping message, then an IPv4-only peer
(not knowing how to process such a message) may abort processing the
entire Label Mapping message (thereby discarding even the IPv4
FEC-labels), as per Section 3.4.1.1 of [RFC5036].

This would result in LDPv6 being somewhat undeployable in existing
production networks.

Section 7 of this document provides a good safety net and makes LDPv6
incrementally deployable without making any such assumption on the
routers' support for IPv6 FEC processing in current production
networks.

A.2. Accommodating Implementations Not Compliant with RFC 5036

It is not safe to assume that implementations have been [RFC5036]
compliant in gracefully handling an IPv6 address family (IPv6 Address
List TLV) in an Address message all along.

If a router upgraded per this specification advertised IPv6 addresses
(with or without IPv4 addresses) in an Address message, then an
IPv4-only peer (not knowing how to process such a message) may not
follow Section 3.5.5.1 of [RFC5036] and may tear down the LDP
session.

This would result in LDPv6 being somewhat undeployable in existing
production networks.

Sections 6 and 7 of this document provide a good safety net and make
LDPv6 incrementally deployable without making any such assumption on
the routers' support for IPv6 FEC processing in current production
networks.

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A.3. Why prohibit IPv4-mapped IPv6 addresses in LDP?

Per discussion with the 6MAN and V6OPS working groups, the
overwhelming consensus was to not promote IPv4-mapped IPv6 addresses
appearing in the routing table, as well as in LDP (address and label)
databases.

Also, [RFC4038], Section 4.2 suggests that IPv4-mapped IPv6-addressed
packets should never appear on the wire.

A.4. Why a 32-bit value even for the IPv6 LDP Router Id?

The first four octets of the LDP Identifier, the 32-bit LSR Id (i.e.,
LDP router Id), identify the LSR and provide a globally unique value
within the MPLS network, regardless of the address family used for
the LDP session.

Please note that the 32-bit LSR Id value would not map to any IPv4
address in an IPv6-only LSR (i.e., Single-stack), nor would there be
an expectation of it being IP routable or DNS resolvable. In IPv4
deployments, the LSR Id is typically derived from an IPv4 address,
generally assigned to a loopback interface. In IPv6-only
deployments, this 32-bit LSR Id must be derived by some other means
that guarantees global uniqueness within the MPLS network, similar to
that of the BGP Identifier [RFC6286] and the OSPF router Id
[RFC5340].

This document reserves 0.0.0.0 as the LSR Id and prohibits its usage
with IPv6, in line with the OSPF router Id in OSPF version 3
[RFC5340].

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Acknowledgments

We acknowledge the authors of [RFC5036], since some text in this
document is borrowed from [RFC5036].

Thanks to Bob Thomas for providing critical feedback to improve this
document early on.

Many thanks to Eric Rosen, Lizhong Jin, Bin Mo, Mach Chen, Shane
Amante, Pranjal Dutta, Mustapha Aissaoui, Matthew Bocci, Mark Tinka,
Tom Petch, Kishore Tiruveedhula, Manoj Dutta, Vividh Siddha, Qin Wu,
Simon Perreault, Brian E. Carpenter, Santosh Esale, Danial Johari,
and Loa Andersson for thoroughly reviewing this document and for
providing insightful comments and multiple improvements.

Contributors

The following individuals contributed to this document:

Nagendra Kumar
Cisco Systems, Inc.
7200 Kit Creek Road
Research Triangle Park, NC 27709, United States
EMail: [email protected]

Andre Pelletier
Cisco Systems, Inc.
2000 Innovation Drive
Kanata, ON K2K-3E8, Canada
EMail: [email protected]

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Authors' Addresses

Rajiv Asati
Cisco Systems, Inc.
7025 Kit Creek Road
Research Triangle Park, NC 27709-4987
United States

EMail: [email protected]

Carlos Pignataro
Cisco Systems, Inc.
7200 Kit Creek Road
Research Triangle Park, NC 27709-4987
United States

EMail: [email protected]

Kamran Raza
Cisco Systems, Inc.
2000 Innovation Drive
Ottawa, ON K2K-3E8
Canada

EMail: [email protected]

Vishwas Manral
Ionos Networks
4100 Moorpark Ave., Ste. #122
San Jose, CA 95117
United States
Phone: +1 408 447 1497

EMail: [email protected]

Rajiv Papneja
Huawei Technologies
2330 Central Expressway
Santa Clara, CA 95050
United States
Phone: +1 571 926 8593

EMail: [email protected]

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