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Versions: (draft-thomas-mpls-ldp-capabilities) 00 01 02 03 04 RFC 5561

Network Working Group                                        Bob Thomas
Internet Draft                                       Cisco Systems, Inc.
Updates: 5036
Intended Status: Proposed Standard                          S. Aggarwal
Expiration Date: October 22, 2009                      Juniper Networks

                                                             R. Aggarwal
                                                        Juniper Networks

                                                            J.L. Le Roux
                                                          France Telecom

                                                       Syed Kamran Raza
                                                    Cisco Systems, Inc.

                                                         April 23, 2009

                             LDP Capabilities

                  draft-ietf-mpls-ldp-capabilities-04.txt


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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on October 22, 2009.

Copyright

  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
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  Please review these documents carefully, as they describe your rights
  and restrictions with respect to this document.

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.

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"
   interchangably.

Table of Contents

   1. Introduction...................................................3
   2. The LDP Capability Mechanism...................................4
      2.1. Capability Document.......................................5
   3. Specifying Capabilities in LDP Messages........................5
      3.1. Backward Compatibility TLVs...............................7
   4. Capability Message.............................................7
   5. Note on Terminology............................................8
   6. Procedures for Capability Parameters in Initialization Messages8
   7. Procedures for Capability Parameters in Capability Messages...10


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   8. Extensions to Error Handling..................................10
   9. Dynamic Capability Announcement TLV...........................11
   10. Backward Compatibility.......................................11
   11. Security Considerations......................................12
   12. IANA Considerations..........................................12
   13. Acknowledgments..............................................12
   14. References...................................................13
      14.1. Normative References....................................13
      14.2. Informative References..................................13
   15. Author's Addresses...........................................14

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 LSPs [IALDP].  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.

  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 deviationts 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 does but inbound label filtering,
  where a speaker installs forwarding state for only certain FECs,
  does not.






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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:

     - 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 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.

     - 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.

     - There is a well-known capability called Dynamic Capability
      Announcement which an LDP speaker MAY advertise in its
      Initialization message to indicate that it is capable of




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      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:

     - Describes the motivation for the enhancement;

     - Specifies the behavior of LDP when the enhancement is enabled.
      This includes the procedures, parameters, messages, and TLVs
      required by the enhancement;

     - 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.

  The format of a "Capability Parameter" TLV is as follows:








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      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 Capability document associated
       with given capability.

     F-bit:
       Forward unknown TLV bit, as described in [RFC5036]. The value of
       this bit MUST be 0 since a Capability Paramter TLV is sent only
       in Initialization and Capability messages which are not
       forwarded.

     TLV Code Point:
         The TLV type which identifies a specific capability. This is
       IANA assigned code point (from TLV Type namespace) for 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.


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       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 peer before terminating the
  session with peer. The Status Code in the Status TLV of the
  Notification message MUST be Malformed TLV, 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.

  The format of the Capability message is as follows:







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       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 (IANA)        |            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.

  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 advertisted 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 of Capability Parameters present in the Initialization message
  received from the peer.




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  An LDP speaker MAY use a particular capability with its peer after
  the speaker determines that the peer has enabled that capability.

  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.  LDP speaker
  should set U-bit to 1 if the capability document allows to continue
  with a peer that does not understand the enhancement, and set U-bit
  to 0 otherwise. If a speaker receives a message containng unsupported
  capability, it responds according to U-bit setting in the TLV. If U-
  bit is 1, then speaker MUST silently ignore the Capability Parameter
  and allow the session to be established. However, if 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).






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7. Procedures for Capability Parameters in Capability Messages

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

  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.  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.



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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 Announcement (IANA)|            Length (1)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1| Reserved    |
      +-+-+-+-+-+-+-+-+

  The value of U-bit for 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 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 S-bit is always 1
  for 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.



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  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 adhoc mechanism for enabling capabilities at sesssion
  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.

12. IANA Considerations

This document specifies the following which require code points assigned
by IANA:

   - LDP message code point for the Capability message.  The authors
      request message type 0x0202 for the Capability message.

   - LDP TLV code point for the Dynamic Capability Announcemnt TLV.
      The authors request TLV type code 0x0506.

   - LDP TLV code point for the Returned TLVs TLV.  The authors
      request TLV type 0x304.

   - LDP Status Code code point for the Unsupported Capability Status
      Code. The authors request Status Code 0x0000002C.


13. Acknowledgments

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

  This document was prepared using 2-Word-v2.0.template.dot.







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14. References

14.1. Normative References


[RFC5036] Andersson, L., Menei, I., and Thomas, B., Editors, "LDP
          Specification", RFC 5036, September 2007.

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

[RFC3479] Farrel, A., Editor,  "Fault Tolerance for the Label
          Distribution Protocol (LDP)", RFC 3479, February 2003.

14.2. Informative References

[IALDP]   Decraene, B., Le Roux, JL., Minei, I, "LDP Extensions for
          Inter-Area LSPs", draft-decraene-mpls-ldp-interarea-04.txt,
          Work in Progress, March 2007

[MLDP]    Minei, I., Wijnamds, I., Editors, "Label Distribution Protocol
          Extensions for Point-to-Multipoint and Multipoint-to-
          Multipoint Label Switched Paths", draft-minei-wijnands-mpls-
          ldp-p2mp-00.txt, Work in Progress, September 2005

[NNHOP]   Shen, N., Chen, E., Tian, A. "Discovery LDP Next-Nexthop
          Labels", draft-shen-mpls-ldp-nnhop-label-02.txt, Work in
          Progress, May 2005
[RFC4447] L. Martini, Editor, E. Rosen, El-Aawar, T. Smith, G. Heron,
          "Pseudowire Setup and Maintenance using the Label Distribution
          Protocol", RFC 4447, April 2006.

[RFC3478] Leelanivas, M., Rekhter, Y, Aggarwal, R., "Graceful Restart
          Mechanism for Label Distribution Protocol (LDP)", RFC 3478,
          February 2003.

[UPSTREAM_LDP] Aggarwal R., Le Roux, J.L.,  "MPLS Upstream Label
          Assignment for LDP" draft-ietf-mpls-ldp-upstream-00.txt, Work
          in Progress, February 2006.





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[MPLS_SEC] Fang, L. et al., Security Framework for MPLS and GMPLS
          Networks, draft-ietf-mpls-mpls-and-gmpls-security-framework-
          05.txt, Work in Progress, March 2009.

15. Author's Addresses

  Bob Thomas
  Cisco Systems, Inc.
  1414 Massachusetts Ave.
  Boxborough, MA 01719
  E-mail: 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
  E-mail: jeanlouis.leroux@orange-ftgroup.com

  Syed Kamran Raza
  Cisco Systems, Inc.
  2000 Innovation Dr.
  Kanata, ON K2K-3E8, Canada
  E-mail: skraza@cisco.com









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