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rfc:rfc5561

Network Working Group B. Thomas Request for Comments: 5561 K. Raza Updates: 5036 Cisco Systems, Inc. Category: Standards Track S. Aggarwal

                                                           R. Aggarwal
                                                      Juniper Networks
                                                           JL. Le Roux
                                                        France Telecom
                                                             July 2009
                         LDP Capabilities

Abstract

 A number of enhancements to the Label Distribution Protocol (LDP)
 have been proposed.  Some have been implemented, and some are
 advancing toward standardization.  It is likely that additional
 enhancements will be proposed in the future.  This document defines a
 mechanism for advertising LDP enhancements at session initialization
 time, as well as a mechanism to enable and disable enhancements after
 LDP session establishment.

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (c) 2009 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 in effect on the date of
 publication of this document (http://trustee.ietf.org/license-info).
 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document.
 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

Thomas, et al. Standards Track [Page 1] RFC 5561 LDP Capabilities July 2009

 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.

Table of Contents

 1. Introduction ....................................................2
    1.1. Conventions Used in This Document ..........................3
 2. The LDP Capability Mechanism ....................................3
    2.1. Capability Document ........................................4
 3. Specifying Capabilities in LDP Messages .........................4
    3.1. Backward Compatibility TLVs ................................6
 4. Capability Message ..............................................6
 5. Note on Terminology .............................................7
 6. Procedures for Capability Parameters in Initialization
    Messages ........................................................7
 7. Procedures for Capability Parameters in Capability Messages .....8
 8. Extensions to Error Handling ....................................9
 9. Dynamic Capability Announcement TLV .............................9
 10. Backward Compatibility ........................................10
 11. Security Considerations .......................................10
 12. IANA Considerations ...........................................11
 13. Acknowledgments ...............................................11
 14. References ....................................................11
    14.1. Normative References .....................................11
    14.2. Informative References ...................................11

1. Introduction

 A number of enhancements to LDP as specified in [RFC5036] have been
 proposed.  These include LDP Graceful Restart [RFC3478], Fault
 Tolerant LDP [RFC3479], multicast extensions [MLDP], signaling for
 Layer 2 circuits [RFC4447], a method for learning labels advertised
 by next-next-hop routers in support of fast reroute node protection
 [NNHOP], upstream label allocation [UPSTREAM_LDP], and extensions for
 signaling inter-area Label Switched Paths (LSPs) [RFC5283].  Some
 have been implemented, and some are advancing toward standardization.
 It is also likely that additional enhancements will be implemented
 and deployed in the future.
 This document proposes and defines a mechanism for advertising LDP
 enhancements at session initialization time.  It also defines a
 mechanism to enable and disable these enhancements after LDP session
 establishment.

Thomas, et al. Standards Track [Page 2] RFC 5561 LDP Capabilities July 2009

 LDP capability advertisement provides means for an LDP speaker to
 announce what it can receive and process.  It also provides means for
 a speaker to inform peers of deviations from behavior specified by
 [RFC5036].  An example of such a deviation is LDP Graceful Restart,
 where a speaker retains MPLS forwarding state for LDP-signaled LSPs
 when its LDP control plane goes down.  It is important to point out
 that not all LDP enhancements require capability advertisement.  For
 example, upstream label allocation requires capability advertisement,
 but inbound label filtering, where a speaker installs forwarding
 state for only certain Forwarding Equivalence Classes (FECs), does
 not.

1.1. Conventions Used in This Document

 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].
 This document uses the terms "LDP speaker" and "speaker"
 interchangeably.

2. The LDP Capability Mechanism

 Enhancements are likely to be announced during LDP session
 establishment as each LDP speaker advertises capabilities
 corresponding to the enhancements it desires.
 Beyond that, capability advertisements may be used to dynamically
 modify the characteristics of the session to suit the changing
 conditions.  For example, an LSR capable of a particular enhancement
 in support of some "feature" may not have advertised the
 corresponding capability to its peers at session establishment time
 because the feature was disabled at that time.  Later, an operator
 may enable the feature, at which time the LSR would react by
 advertising the corresponding capability to its peers.  Similarly,
 when an operator disables a feature associated with a capability, the
 LSR reacts by withdrawing the capability advertisement from its
 peers.
 The LDP capability advertisement mechanism operates as follows:
  1. Each LDP speaker is assumed to implement a set of enhancements,

each of which has an associated capability. At any time, a speaker

   may have none, one, or more of those enhancements "enabled".  When
   an enhancement is enabled, the speaker advertises the associated
   capability to its peers.  By advertising the capability to a peer,
   the speaker asserts that it shall perform the protocol actions
   specified for the associated enhancement.  For example, the actions

Thomas, et al. Standards Track [Page 3] RFC 5561 LDP Capabilities July 2009

   may require the LDP speaker to receive and process enhancement-
   specific messages from its peer.  Unless the capability has been
   advertised, the speaker will not perform protocol actions specified
   for the corresponding enhancement.
  1. At session establishment time, an LDP speaker MAY advertise a

particular capability by including an optional parameter associated

   with the capability in its Initialization message.
  1. There is a well-known capability called Dynamic Capability

Announcement that an LDP speaker MAY advertise in its

   Initialization message to indicate that it is capable of processing
   capability announcements following a session establishment.
   If a peer had advertised the Dynamic Capability Announcement
   capability in its Initialization message, then at any time
   following session establishment, an LDP speaker MAY announce
   changes in its advertised capabilities to that peer.  To do this,
   the LDP speaker sends the peer a Capability message that specifies
   the capabilities being advertised or withdrawn.

2.1. Capability Document

 When the capability advertisement mechanism is in place, an LDP
 enhancement requiring LDP capability advertisement will be specified
 by a document that:
  1. Describes the motivation for the enhancement;
  1. Specifies the behavior of LDP when the enhancement is enabled.

This includes the procedures, parameters, messages, and TLVs

      required by the enhancement;
  1. Includes an IANA considerations section that requests IANA

assignment of a code point (from TLV Type namespace) for the

      optional capability parameter corresponding to the enhancement.
      The capability document MUST also describe the interpretation
      and processing of associated capability data, if present.

3. Specifying Capabilities in LDP Messages

 This document uses the term "Capability Parameter" to refer to an
 optional parameter that may be included in Initialization and
 Capability messages to advertise a capability.

Thomas, et al. Standards Track [Page 4] RFC 5561 LDP Capabilities July 2009

 The format of a "Capability Parameter" TLV is as follows:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |U|F| TLV Code Point            |            Length             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S| Reserved    |                                               |
    +-+-+-+-+-+-+-+-+       Capability Data                         |
    |                                               +-+-+-+-+-+-+-+-+
    |                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 where:
    U-bit:
      Unknown TLV bit, as described in [RFC5036].  The value could be
      either 0 or 1 as specified in the Capability document associated
      with the given capability.
    F-bit:
      Forward unknown TLV bit, as described in [RFC5036].  The value
      of this bit MUST be 0 since a Capability Parameter TLV is sent
      only in Initialization and Capability messages, which are not
      forwarded.
    TLV Code Point:
      The TLV type that identifies a specific capability.  This is an
      IANA-assigned code point (from TLV Type namespace) for a given
      capability as requested in the associated capability document.
    S-bit:
      The State Bit.  It indicates whether the sender is advertising
      or withdrawing the capability corresponding to the TLV code
      point.  The State Bit value is used as follows:
        1 - The TLV is advertising the capability specified by the TLV
            code point.
        0 - The TLV is withdrawing the capability specified by the TLV
            code point.
    Capability Data:
      Information, if any, about the capability in addition to the TLV
      code point required to fully specify the capability.

Thomas, et al. Standards Track [Page 5] RFC 5561 LDP Capabilities July 2009

      The method for interpreting and processing this data is specific
      to the TLV code point and MUST be described in the document
      specifying the capability.
 An LDP speaker MUST NOT include more than one instance of a
 Capability Parameter (as identified by the same TLV code point) in an
 Initialization or Capability message.  If an LDP speaker receives
 more than one instance of the same Capability Parameter type in a
 message, it SHOULD send a Notification message to the peer before
 terminating the session with the peer.  The Status Code in the Status
 TLV of the Notification message MUST be Malformed TLV value, and the
 message SHOULD contain the second Capability Parameter TLV of the
 same type (code point) that is received in the message.

3.1. Backward Compatibility TLVs

 LDP extensions that require advertisement or negotiation of some
 capability at session establishment time typically use TLVs that are
 included in an Initialization message.  To ensure backward
 compatibility with existing implementations, such TLVs continue to be
 supported in an Initialization message and are known in this document
 as "Backward Compatibility TLVs".  A Backward Compatibility TLV plays
 the role of a "Capability Parameter" TLV; that is, the presence of a
 Backward Compatibility TLV has the same meaning as a Capability
 Parameter TLV with the S-bit set for the same capability.
 One example of a Backward Capability TLV is the "FT Session TLV" that
 is exchanged in an Initialization message between peers to announce
 LDP Fault Tolerance [RFC3479] capability.

4. Capability Message

 The LDP Capability message is used by an LDP speaker to announce
 changes in the state of one or more of its capabilities subsequent to
 session establishment.

Thomas, et al. Standards Track [Page 6] RFC 5561 LDP Capabilities July 2009

 The format of the Capability message is as follows:
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0|    Capability (0x0202)      |            Length             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Message ID                                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     TLV_1                                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     . . .                                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     TLV_N                                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 where TLV_1 through TLV_N are Capability Parameter TLVs.  The S-bit
 of each of the TLVs specifies the new state for the corresponding
 capability.
 Note that Backward Compatibility TLVs (see Section 3.1) MUST NOT be
 included in Capability messages.  An LDP speaker that receives a
 Capability message from a peer that includes Backward Compatibility
 TLVs SHOULD silently ignore these Backward Compatibility TLVs and
 continue processing the rest of the message.

5. Note on Terminology

 The following sections in this document talk about enabling and
 disabling capabilities.  The terminology "enabling (or disabling) a
 capability" is short hand for "advertising (or withdrawing) a
 capability associated with an enhancement".  Bear in mind that it is
 an LDP enhancement that is being enabled or disabled, and that it is
 the corresponding capability that is being advertised or withdrawn.

6. Procedures for Capability Parameters in Initialization Messages

 The S-bit of a Capability Parameter in an Initialization message MUST
 be 1 and SHOULD be ignored on receipt.  This ensures that any
 Capability Parameter in an Initialization message enables the
 corresponding capability.
 An LDP speaker determines the capabilities of a peer by examining the
 set of Capability Parameters present in the Initialization message
 received from the peer.
 An LDP speaker MAY use a particular capability with its peer after
 the speaker determines that the peer has enabled that capability.

Thomas, et al. Standards Track [Page 7] RFC 5561 LDP Capabilities July 2009

 These procedures enable an LDP speaker S1, that advertises a specific
 LDP capability C, to establish an LDP session with speaker S2 that
 does not advertise C.  In this situation, whether or not capability C
 may be used for the session depends on the semantics of the
 enhancement associated with C.  If the semantics do not require both
 S1 and S2, advertise C to one another, then S2 could use it; i.e.,
 S1's advertisement of C permits S2 to send messages to S1 used by the
 enhancement.
 It is the responsibility of the capability designer to specify the
 behavior of an LDP speaker that has enabled a certain enhancement,
 advertised its capability and determines that its peer has not
 advertised the corresponding capability.  The document specifying
 procedures for the capability MUST describe the behavior in this
 situation.  If the specified procedure is to terminate the session,
 then the LDP speaker SHOULD send a Notification message to the peer
 before terminating the session.  The Status Code in the Status TLV of
 the Notification message MUST be Unsupported Capability, and the
 message SHOULD contain the unsupported capability (see Section 8 for
 more details).
 An LDP speaker that supports capability advertisement and includes a
 Capability Parameter in its Initialization message MUST set the TLV
 U-bit to 0 or 1, as specified by Capability document.  The LDP
 speaker should set the U-bit to 1 if the capability document allows
 it to continue with a peer that does not understand the enhancement,
 and set the U-bit to 0 otherwise.  If a speaker receives a message
 containing unsupported capability, it responds according to the U-bit
 setting in the TLV.  If the U-bit is 1, then the speaker MUST
 silently ignore the Capability Parameter and allow the session to be
 established.  However, if the U-bit is 0, then speaker SHOULD send a
 Notification message to the peer before terminating the session.  The
 Status Code in the Status TLV of the Notification message MUST be
 Unsupported Capability, and the message SHOULD contain the
 unsupported capability (see Section 8 for more details).

7. Procedures for Capability Parameters in Capability Messages

 An LDP speaker MUST NOT send a Capability message to a peer unless
 its peer advertised the Dynamic Capability Announcement capability in
 its session Initialization message.  An LDP speaker MAY send a
 Capability message to a peer if its peer advertised the Dynamic
 Capability Announcement capability in its session Initialization
 message (see Section 9).

Thomas, et al. Standards Track [Page 8] RFC 5561 LDP Capabilities July 2009

 An LDP speaker determines the capabilities enabled by a peer by
 determining the set of capabilities enabled at session initialization
 (as specified in Section 6) and tracking changes to that set made by
 Capability messages from the peer.
 An LDP speaker that has enabled a particular capability MAY use the
 enhancement corresponding to the capability with a peer after the
 speaker determines that the peer has enabled the capability.

8. Extensions to Error Handling

 This document defines a new LDP status code named Unsupported
 Capability.  The E-bit of the Status TLV carried in a Notification
 message that includes this status code MUST be set to 0.
 In addition, this document defines a new LDP TLV, named Returned
 TLVs, that MAY be carried in a Notification message as an Optional
 Parameter.  The U-bit setting for a Returned TLVs TLV in a
 Notification message SHOULD be 1, and the F-bit setting SHOULD be 0.
 When the Status Code in a Notification message is Unsupported
 Capability, the message SHOULD specify the capabilities that are
 unsupported.  When the Notification message specifies the unsupported
 capabilities, it MUST include a Returned TLVs TLV.  The Returned TLVs
 TLV MUST include only the Capability Parameters for unsupported
 capabilities, and the Capability Parameter for each such capability
 SHOULD be encoded as received from the peer.
 When the Status Code in a Notification Message is Unknown TLV, the
 message SHOULD specify the TLV that was unknown.  When the
 Notification message specifies the TLV that was unknown, it MUST
 include the unknown TLV in a Returned TLVs TLV.

9. Dynamic Capability Announcement TLV

 The Dynamic Capability Announcement TLV is a Capability Parameter
 defined by this document with following format:
     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| DynCap Ann. (0x0506)      |            Length (1)         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |1| Reserved    |
    +-+-+-+-+-+-+-+-+

Thomas, et al. Standards Track [Page 9] RFC 5561 LDP Capabilities July 2009

 The value of the U-bit for the Dynamic Capability Announcement
 Parameter TLV MUST be set to 1 so that a receiver MUST silently
 ignore this TLV if unknown to it, and continue processing the rest of
 the message.  There is no "Capability Data" associated with this TLV
 and hence the TLV length MUST be set to 1.
 The Dynamic Capability Announcement Parameter MAY be included by an
 LDP speaker in an Initialization message to signal its peer that the
 speaker is capable of processing Capability messages.
 An LDP speaker MUST NOT include the Dynamic Capability Announcement
 Parameter in Capability messages sent to its peers.  Once enabled
 during session initialization, the Dynamic Capability Announcement
 capability cannot be disabled.  This implies that the S-bit is always
 1 for the Dynamic Capability Announcement.
 An LDP speaker that receives a Capability message from a peer that
 includes the Dynamic Capability Announcement Parameter SHOULD
 silently ignore the parameter and process any other Capability
 Parameters in the message.

10. Backward Compatibility

 From the point of view of the LDP capability advertisement mechanism,
 an [RFC5036]-compliant peer has label distribution for IPv4 enabled
 by default.  To ensure compatibility with an [RFC5036]-compliant
 peer, LDP implementations that support capability advertisement have
 label distribution for IPv4 enabled until it is explicitly disabled
 and MUST assume that their peers do as well.
 Section 3.1 introduces the concept of Backward Compatibility TLVs
 that may appear in an Initialization message in the role of a
 Capability Parameter.  This permits existing LDP enhancements that
 use an ad hoc mechanism for enabling capabilities at session
 initialization time to continue to do so.

11. Security Considerations

 [MPLS_SEC] describes the security framework for MPLS networks,
 whereas [RFC5036] describes the security considerations that apply to
 the base LDP specification.  The same security framework and
 considerations apply to the capability mechanism described in this
 document.

Thomas, et al. Standards Track [Page 10] RFC 5561 LDP Capabilities July 2009

12. IANA Considerations

 This document specifies the following code points assigned by IANA:
  1. LDP message code point for the Capability message (0x0202).
  1. LDP TLV code point for the Dynamic Capability Announcement TLV

(0x0506).

  1. LDP TLV code point for the Returned TLVs TLV (0x0304).
  1. LDP Status Code code point for the Unsupported Capability Status

Code (0x0000002E).

13. Acknowledgments

 The authors wish to thank Enke Chen, Vanson Lim, Ina Minei, Bin Mo,
 Yakov Rekhter, and Eric Rosen for their comments.

14. References

14.1. Normative References

 [RFC5036]      Andersson, L., Ed., Minei, I., Ed., and B. Thomas,
                Ed., "LDP Specification", RFC 5036, October 2007.
 [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3479]      Farrel, A., Ed., "Fault Tolerance for the Label
                Distribution Protocol (LDP)", RFC 3479, February 2003.

14.2. Informative References

 [RFC5283]      Decraene, B., Le Roux, JL., and I. Minei, "LDP
                Extension for Inter-Area Label Switched Paths (LSPs)",
                RFC 5283, July 2008.
 [MLDP]         Minei, I., Ed., Kompella, K., Wijnands, I., Ed., and
                B. Thomas, "Label Distribution Protocol Extensions for
                Point-to-Multipoint and Multipoint-to-Multipoint Label
                Switched Paths", Work in Progress, April 2009.
 [NNHOP]        Shen, N., Chen, E., and A. Tian, "Discovery LDP Next-
                Nexthop Labels", Work in Progress, May 2005.

Thomas, et al. Standards Track [Page 11] RFC 5561 LDP Capabilities July 2009

 [RFC4447]      Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T.,
                and G. Heron, "Pseudowire Setup and Maintenance Using
                the Label Distribution Protocol (LDP)", RFC 4447,
                April 2006.
 [RFC3478]      Leelanivas, M., Rekhter, Y., and R. Aggarwal,
                "Graceful Restart Mechanism for Label Distribution
                Protocol", RFC 3478, February 2003.
 [UPSTREAM_LDP] Aggarwal R., and J.L. Le Roux, "MPLS Upstream Label
                Assignment for LDP" Work in Progress, July 2008.
 [MPLS_SEC]     Fang, L., Ed., "Security Framework for MPLS and GMPLS
                Networks", Work in Progress, March 2009.

Authors' Addresses

 Bob Thomas
 Cisco Systems, Inc.
 1414 Massachusetts Ave.
 Boxborough, MA 01719
 EMail: bobthomas@alum.mit.edu
 Shivani Aggarwal
 Juniper Networks
 1194 North Mathilda Ave.
 Sunnyvale, CA 94089
 EMail: shivani@juniper.net
 Rahul Aggarwal
 Juniper Networks
 1194 North Mathilda Ave.
 Sunnyvale, CA 94089
 EMail: rahul@juniper.net
 Jean-Louis Le Roux
 France Telecom
 2, Avenue Pierre-Marzin
 22307 Lannion Cedex, France
 EMail: jeanlouis.leroux@orange-ftgroup.com
 Kamran Raza
 Cisco Systems, Inc.
 2000 Innovation Dr.
 Kanata, ON K2K 3E8, Canada
 EMail: skraza@cisco.com

Thomas, et al. Standards Track [Page 12]

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