    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]

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    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
       (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.

       This document may contain material from IETF Documents or IETF
       Contributions published or made publicly available before November
       10, 2008.  The person(s) controlling the copyright in some of this
       material may not have granted the IETF Trust the right to allow
       modifications of such material outside the IETF Standards Process.
       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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    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: naikumar@cisco.com

       Andre Pelletier
       Cisco Systems, Inc.
       2000 Innovation Drive
       Kanata, ON K2K-3E8, Canada
       EMail: apelleti@cisco.com

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    Authors' Addresses

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

       EMail: rajiva@cisco.com

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

       EMail: cpignata@cisco.com

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

       EMail: skraza@cisco.com

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

       EMail: vishwas@ionosnetworks.com

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

       EMail: rajiv.papneja@huawei.com

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