MPLS Working Group                                          Rajiv Asati
Internet Draft                                                    Cisco
Updates: 5036 (if approved)
Intended status: Standards Track                         Vishwas Manral
Expires: July 23, December 8, 2012                         Hewlett-Packard, Inc.

                                                          Rajiv Papneja

                                                       Carlos Pignataro

                                                       January 23,

                                                           June 8, 2012

                          Updates to LDP for IPv6

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

Table of Contents

   1. Introduction...................................................3
      1.1. Scope.....................................................4
         1.1.1. Topology Scenarios...................................4
         1.1.2. LDP TTL Security.....................................5
   2. Specification Language.........................................5
   3. LSP Mapping....................................................6
   4. LDP Identifiers................................................6
   5. Peer Discovery.................................................7
      5.1. Basic Discovery Mechanism.................................7
      5.2. Extended Discovery Mechanism..............................8
   6. LDP Session Establishment and Maintenance......................8
      6.1. Transport connection establishment........................9
      6.2. Maintaining Hello Adjacencies............................10
      6.3. Maintaining LDP Sessions.................................11
   7. Label Distribution............................................11
   8. LDP Identifiers and Next Hop Addresses........................12
   9. LDP TTL Security..............................................13
   10. IANA Considerations..........................................14
   11. Security Considerations......................................14
   12. Acknowledgments..............................................14
   13. Additional Contributors......................................15
   14. References...................................................16
      14.1. Normative References....................................16
      14.2. Informative References..................................16
   Author's Addresses...............................................17

1. Introduction

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

   However, RFC5036 specification has the following deficiencies in
   regards to IPv6 usage:

   1) LSP Mapping: No rule defined for mapping a particular packet to a
      particular LSP that has an Address Prefix FEC element containing
      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 (with or without IPv4

   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 Label Distribution: No rule for advertising IPv4 or/and IPv6
      FEC-label bindings over an LDP session, and denying the co-
      existence of IPv4 and IPv6 FEC Elements in the same FEC TLV

   6) Next Hop Address & LDP Identifier: No rule for accommodating the
      usage of duplicate link-local IPv6 addresses

   7) LDP TTL Security: No rule for built-in Generalized TTL Security
      Mechanism (GTSM) in LDP

   This document addresses the above deficiencies by specifying the
   desired behavior/rules/details for using LDP in IPv6 enabled
   networks. It also clarifies the scope (section 1.1).

   Note that this document updates RFC5036.

1.1. Scope

1.1.1. Topology Scenarios

   The following scenarios in which the LSRs may be inter-connected via
   one or more dual-stack interfaces (figure 1), or two or more single-
   stack interfaces (figure 2 and figure 3) are addressed by this


            Figure 1 LSRs connected via a Dual-stack Interface


          Figure 2 LSRs connected via two single-stack Interfaces

                       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 interface (IPv4, say) being converted to
   a dual-stacked interface by enabling IPv6 as well as IPv6 LDP, even
   though the IPv4 LDP session may already be established between the

   Note that the topology scenario illustrated in figure 2 also covers
   the case of two routers getting connected via an additional single-
   stack interface (IPv6, say), even though the IPv4 LDP session may
   already be established between the LSRs over the existing interface.

1.1.2. LDP TTL Security

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

   As a consequence, any LDP feature that relies on multi-hop LDP
   peering session would not work with GTSM and will warrant
   (statically or dynamically) disabling GTSM. Please see section 8.

2. Specification Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].


   LDP      - Label Distribution Protocol

   LDPv4    - LDP for enabling IPv4 MPLS forwarding

   LDPv6    - LDP for enabling IPv6 MPLS forwarding

   LDPoIPv4 - LDP over IPv4 transport session

   LDPoIPv6 - LDP over IPv6 transport session

   FEC      - Forwarding Equivalence Class

   TLV      - Type Length Value

   LSR      - Label Switch Router

   LSP      - Label Switched Path

   LSPv4    - IPv4-signaled Label Switched Path [RFC4798]

   LSPv6    - IPv6-signaled Label Switched Path [RFC4798]

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
   3rd 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."

   Suffice to say, this rule is correct for IPv4, but not for IPv6,
   since an IPv6 router may not have any /32 address.

   This document proposes to modify this rule by also including a /128
   address (for IPv6). In fact, it should be reasonable to just say
   IPv4 or IPv6 address instead of /32 or /128 addresses as shown below
   in the updated rule:

     "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."

   Additionally, it is desirable that a packet is forwarded to an LSP
   of an egress router, only if LSP's address-family (e.g. LSPv4 or
   LSPv6) matches with that of the LDP hello adjacency on the next-hop

4. LDP Identifiers

   Section 2.2.2 of [RFC5036] specifies formulating at least one LDP
   Identifier, however, it doesn't provide any consideration in case of
   IPv6 (with or without dual-stacking). Additionally, section 2.5.2 of
   [RFC5036] implicitly prohibits using the same label space for both
   IPv4 and IPv6 FEC-label bindings.

   The first four octets of the LDP identifier, the 32-bit LSR Id, Id (e.g.
   (i.e. LDP Router Id), identify the LSR and is a globally unique value.
   value within the MPLS network. This is regardless of the address
   family used for the LDP session. Hence, this document preserves the
   usage of 32-bit (unsigned non-zero integer) LSR Id on an IPv6 only LSR.
   LSR (note that BGP has also mandated using 32-bit BGP Router ID on
   an IPv6 only Router [RFC6286]).

     Please note that 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 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. uniqueness within the MPLS network, similar to that of BGP
     Identifier [RFC6286].

   This document qualifies the first sentence of last paragraph of
   Section 2.5.2 of [RFC5036] to be per address family and therefore
   updates that sentence to the following: "For a given address family
   over which a Hello is sent, and a given label space, an LSR MUST
   advertise the same transport address." This rightly enables the per-
   platform label space to be shared between IPv4 and IPv6.

   In summary, this document not only allows the usage of a common LDP
   identifier i.e. same LSR-Id, LSR-Id (aka LDP Router-Id), but also the common
   Label space id for both IPv4 and IPv6 on a dual-stack LSR.

   This document reserves as the LSR-Id, and prohibits its

5. Peer Discovery

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
   sends LDP Link Hellos destined to "all routers on this subnet" group
   multicast IP address.

   Interesting 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

   This document specifies the usage of link-local scope e.g.
   FF02:0:0:0:0:0:0:2 as the destination multicast IP address in IPv6
   LDP Link Hellos. An LDP 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

   Also, the 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 specified in Generalized TTL Security Mechanism
   (GTSM)[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
   (e.g. enabled with both IPv4 and IPv6 LDP), then the LSR must
   periodically send both IPv4 and IPv6 LDP Link Hellos (using the same
   LDP Identifier per section 4) and must separately maintain the Hello
   adjacency for IPv4 and IPv6 on that interface.

   In summary, the IPv4 and IPv6 LDP Link Hellos must carry the same
   LDP identifier (assuming per-platform label space usage).

5.2. Extended Discovery Mechanism

   Suffice to say, the extended discovery mechanism (defined in section
   2.4.2 of [RFC5036]) doesn't require any additional IPv6 specific
   consideration, since the targeted LDP Hellos are sent to a pre-
   configured (unicast) destination IPv6 address.

   The link-local IP addresses MUST NOT be used as the source or
   destination IPv6 addresses in extended discovery.

6. LDP Session Establishment and Maintenance

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

     1. Transport connection establishment
     2. Session initialization

   The forthcoming sub-sections discuss the LDP consideration for IPv6
   and/or dual-stacking in the context of session establishment and

6.1. Transport connection establishment

   Section 2.5.2 of [RFC5036] specifies the use of an optional
   transport address object (TLV) in LDP Link Hello message to convey
   the transport (IP) address, however, it does not specify the
   behavior of LDP if both IPv4 and IPv6 transport address objects
   (TLV) 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 there were Hello
   adjacencies for both IPv4 and IPv6 whether over a single interface
   or multiple interfaces.

   This document specifies that:

     1. An LSR MUST NOT send a Hello containing both IPv4 and IPv6
        transport address optional objects. In other words, there MUST
        be at most one optional Transport Address 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

     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 Hello.

     3. An LSR MUST send separate Hellos (each containing either IPv4
        or IPv6 transport address optional object) for each IP address-
        family, if LDP was enabled for both IP address-families.

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

     5. An LSR MUST prefer using global unicast IPv6 address for an LDP
        session with a remote LSR, if it had to choose between global
        unicast IPv6 address and link-local IPv6 address (pertaining to
        the same LDP Identifier) for the transport connection.

     6. An LSR SHOULD NOT create (or honor the request for creating) a
        TCP connection for a new LDP session with a remote LSR, if they
        already have an LDP session (for the same LDP Identifier)
        established over whatever IP version transport.

        This means that only one transport connection is established,
        even if there are two Hello adjacencies (one for IPv4 and
        another for IPv6). This is independent of whether the Hello
        Adjacencies are created over a single interface (scenarios (scenario 1 in
        section 1.1) or multiple interfaces (scenario 2 in section 1.1)
        between two LSRs.

     7. An LSR SHOULD prefer the LDP/TCP connection over IPv6 for a new
        LDP session with a remote LSR, if it has both IPv4 and IPv6
        hello adjacencies for the same LDP Identifier (over a dual-
        stack interface, or two or more single-stack IPv4 and IPv6
        interfaces). This applies to the section 2.5.2 of RFC5036.

     8. An LSR SHOULD prefer the LDP/TCP connection over IPv6 for a new
        LDP session with a remote LSR, if they attempted two TCP
        connections using IPv4 and IPv6 transport addresses

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

6.2. Maintaining Hello Adjacencies

   As outlined in section 2.5.5 of RFC5036, this draft describes that
   if an LSR has a dual-stack interface, which is enabled with both
   IPv4 and IPv6 LDP, then the LSR must periodically send both IPv4 and
   IPv6 LDP Link Hellos and must separately maintain the Hello
   adjacency for IPv4 and IPv6 on that interface.

     This ensures successful labeled IPv4 and labeled IPv6 traffic
     forwarding on a dual-stacked interface, as well as successful LDP
     peering using the appropriate transport on a multi-access
     interface (even if there are IPv4-only, IPv6-only and dual-stack
     LSRs connected to that multi-access interface).

6.3. Maintaining LDP Sessions

   Two LSRs maintain a single LDP session between them, them (i.e. not tear
   down an existing session), as described in section 6.1, whether

   - they are connected via a dual-stack LDP enabled interface or via
     two single-stack LDP enabled interfaces. This is
   also true when interfaces;
   - a single-stack interface is converted to a dual-stack interface
     (e.g. figure 1), 1) on either LSR;
   - an additional single-stack or when another dual-stack interface is added or
     removed between two LSRs (e.g. figure 2).

   Needless to say that the procedures defined in section 6.1 would
   always should
   result in preferring LDPoIPv6 session only after the loss of an
   existing LDP session (because of link failure, node failure, reboot

   On the other hand, if a dual-stack interface is converted to a
   single-stack interface (by disabling IPv4 or IPv6 routing), then the
   LDP session should be torn down ONLY if the disabled IP version was
   the same as that of the transport connection. Otherwise, the LDP
   session should stay intact.

   If the LDP session is torn down for whatever reason (LDP disabled
   for the corresponding transport, hello adjacency expiry etc.), then
   the LSRs should initiate establishing a new LDP session as per the
   procedures described in section 6.1 of this document along with

7. Label Distribution

   An LSR MAY SHOULD NOT advertise both IPv4 and IPv6 FEC-label bindings
   (as well as interface addresses via ADDRESS message) from/to the
   peer over an LDP session (using whatever transport), unless it has
   valid IPv4 and IPv6 Hello Adjacencies for that peer, as specified in
   section 6.2.

     Another solution for getting the same result as above is by
     negotiating the IP Capability for a given AFI, as specified in

   An LSR MUST NOT allocate and advertise FEC-Label bindings for link-
   local IPv6 address, and ignore such bindings, if ever received. An
   LSR MUST treat the IPv4-mapped IPv6 address, defined in section
   2.5.1 of [RFC4291], the same as that of a global IPv6 address and
   not mix it with the 'corresponding' IPv4 address.

   Additionally, to ensure backward compatibility (and interoperability
   with IPv4-only LDP implementations), this document specifies that -

     1. An LSR MUST NOT send a label mapping message with a FEC TLV
        containing FEC Elements of different address-family. In other
        words, a FEC TLV in the label mapping message MUST contain the
        FEC Elements belonging to the same address-family.
     2. An LSR MUST NOT send an Address message (or Address Withdraw
        message) with an Address List TLV containing IP addresses of
        different address-family. In other words, an Address List TLV
        in the Address (or Address Withdraw) message MUST contain the
        addresses belonging to the same address-family.

8. LDP Identifiers and Next Hop Addresses

   RFC5036 section 2.7 specifies logic for mapping between a peer LDP
   Identifier and the peer's addresses to find the correct LIB entry
   for any prefix by using a database populated by the Address message.
   However, this logic is insufficient to deal with overlapping IPv6
   (link-local) addresses used by two or more peers. One may note that
   all interior IP routing protocols specify using link-local IPv6
   addresses as the next-hops.

   This document specifies that the logic is enhanced with the usage of
   (Hello Adjacency) database populated by the Hello messages. This
   additional database lookup is useful only if/when two or more peers use
   the same link-local IPv6 address as the IP routing next-hops
   (causing duplicate next-hop entries).

   Specifically, this document specifies that an LSR should (continue
   to) use the machinery described in RFC5036 section 2.7 to map
   between a peer LDP Identifier and the peer's addresses (learned via
   ADDRESS message) for any prefix. However, if this mapping fails (for
   reasons such as the one described earlier), then an LSR can find the
   peer LDP Identifier by checking for the particular link-local IPv6
   address and interface (corresponding to the next-hop in the unicast
   routing table) in the hello adjacency database.

   If an LSR can't find such a mapping in either database, then LSR
   should follow procedures specified in RFC5036 (e.g. not resolve the

   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.

9. LDP TTL Security

   This document also specifies that the LDP/TCP transport connection
   over IPv6 (i.e. LDPoIPv6) must follow the Generalized TTL Security
   Mechanism (GTSM) procedures (Section 3 of [RFC5082]) for an LDP
   session peering established between the adjacent LSRs using Basic
   Discovery, by default.

   In other words, GTSM is enabled by default for an IPv6 LDP peering
   session using Basic Discovery. This means that the 'IP Hop Limit' in
   IPv6 packet is set to 255 upon sending, and checked to be 255 upon
   receipt. The IPv6 packet must be dropped failing such a check upon

   The reason GTSM is enabled for Basic Discovery by default, but not
   for Extended Discovery is that the usage of Basic Discovery
   typically results in a single-hop LDP peering session, whereas the
   usage of Extended Discovery typically results in a multi-hop LDP
   peering session. While the latter is deemed out of scope (section
   1.2), in line with GTSM [RFC5082], it is worth clarifying the
   following exceptions that may occur with Basic or Extended Discovery

     a) Two adjacent LSRs (i.e. back-to-back PE routers) forming a
       single-hop LDP peering session after doing an Extended Discovery
       (for Pseudowire, say)
     b) Two adjacent LSRs forming a multi-hop LDP peering session after
       doing a Basic Discovery, due to the way IP routing changes
       between them (temporarily (e.g. session protection) or
     c) Two adjacent LSRs (i.e. back-to-back PE routers) forming a
       single-hop LDP peering session after doing both Basic and
       Extended Discovery

   In (a), GTSM is not enabled for the LDP peering session by default,
   hence, it would not do any harm or good.

   In (b), GTSM is enabled by default for the LDP peering session by
   default and enforced, hence, it would prohibit the LDP peering
   session from getting established.

   In (c), GTSM is enabled by default for Basic Discovery and enforced
   on the subsequent LDP peering. However, if each LSR uses the same
   IPv6 transport address object value in both Basic and Extended
   discoveries, then it would result in a single LDP peering session
   and that would be enabled with GTSM. Otherwise, GTSM would not be
   enforced on the 2nd LDP peering session corresponding to the
   Extended Discovery.

   This document allows for the implementation to provide an option to
   statically (configuration) and/or dynamically override the default
   behavior (enable/disable GTSM) on a per-peer basis. This would also
   address the exception (b) above. Suffice to say that such an option
   could be set on either LSR (since GTSM negotiation would ultimately
   disable GTSM between LSR and its peer(s)).

   The built-in GTSM inclusion is intended to automatically protect
   IPv6 LDP peering session from off-link attacks.

10. IANA Considerations


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 the [RFC5036] are relevant
   for this document as well, this document reduces the chances of off-
   link attacks when using IPv6 transport connection by including the
   use of GTSM procedures [RFC5082].

   Moreover, this document allows the use of IPsec [RFC4301] for IPv6
   protection, hence, LDP can benefit from the additional security as
   specified in [RFC4835] as well as [RFC5920].

12. Acknowledgments

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

   Thanks to Bob Thomas for providing critical feedback to improve this
   document early on. Thanks to Eric Rosen, Lizhong Jin, Bin Mo, Mach
   Chen, and Kishore Tiruveedhula for reviewing this document. The
   authors also acknowledge the help of Manoj Dutta and Vividh Siddha.

   Also, thanks to Andre Pelletier (who brought up the issue about
   active/passive determination, and helped us craft the appropriate

   This document was prepared using

13. Additional Contributors

   The following individuals contributed to this document:

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

   Nagendra Kumar
   Cisco Systems, Inc.
   SEZ Unit, Cessna Business Park,
   Bangalore, KT, India

14. References

14.1. Normative References

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

   [RFC4291] Hinden, R. and S. Deering, "Internet Protocol Version 6
             (IPv6) Addressing Architecture", RFC 4291, February 2006.

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

   [RFC5082] Pignataro, C., Gill, V., Heasley, J., Meyer, D., and
             Savola, P., "The Generalized TTL Security Mechanism
             (GTSM)", RFC 5082, October 2007.

14.2. Informative References

   [RFC4301] Kent, S. and K. Seo, "Security Architecture and Internet
             Protocol", RFC 4301, December 2005.

   [RFC4835] Manral, V., "Cryptographic Algorithm Implementation
             Requirements for Encapsulating Security Payload (ESP) and
             Authentication Header (AH)", RFC 4835, April 2007.

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

   [RFC4798] De Clercq, et al., "Connecting IPv6 Islands over IPv4 MPLS
             Using IPv6 Provider Edge Routers (6PE)", RFC 4798,
             February 2007.

   [IPPWCap] Raza, K., "LDP IP and PW Capability", draft-ietf-mpls-ldp-
             ip-pw-capability, June 2011.

Author's Addresses

   Vishwas Manral
   Hewlet-Packard, Inc.
   19111 Pruneridge Ave., Cupertino, CA, 95014
   Phone: 408-447-1497

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

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

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