MPLS Working Group                                          Kamran Raza
Internet Draft                                             Sami Boutros
Updates: 5036, 4447 (if approved)                          Luca Martini
Intended status: Standards Track                    Cisco Systems, Inc.
Expires: February 7, 2013
                                                        Nicolai Leymann
                                                       Deutsche Telekom


                                                         August 8, 2012


               Applicability of LDP Label Advertisement Mode

            draft-ietf-mpls-ldp-applicability-label-adv-00.txt

Abstract

   An LDP speaker negotiates the label advertisement mode with its LDP
   peer at the time of session establishment. Although different
   applications sharing the same LDP session may need different modes
   of label distribution and advertisement, there is only one type of
   label advertisement mode that is negotiated and used per LDP
   session. This document clarifies the use and the applicability of
   session's negotiated label advertisement mode, and categorizes LDP
   applications into two broad categories of negotiated mode-bound and
   mode-independent applications. The document also suggests an update
   to RFC 5036 and RFC 4447 to remove any ambiquity and conflict in the
   area of using correct label advertisement mode for a given
   application.

 Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt




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Internet-Draft  Applicability of LDP Label Advertisement Mode  Aug 2012

   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on February 7, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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Table of Contents

  1. Introduction                                                     3
  2. Conventions used in this document                                3
  3. Label Advertisement Mode Applicability                           4
     3.1. Label Advertisement Mode Negotiation                        4
     3.2. Mode-based Categorization of LDP Applications               4
          3.2.1. Session mode-bound Applications                      5
          3.2.2. Session mode-independent Applications                5
     3.3. Unacceptable label advertisement mode                       6
  4. Clarification on Mode Applicability                              6
     4.1. Update to RFC-5036                                          7
     4.2. Update to RFC-4447                                          7
  5. Security Considerations                                          7
  6. IANA Considerations                                              7
  7. References                                                       7
     7.1. Normative References                                        7
     7.2. Informative References                                      8
  8. Acknowledgments                                                  8







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1. Introduction

   The MPLS architecture [RFC3031] defines two modes of label
   advertisement for an LSR:

     1. Downstream-on-Demand

     2. Unsolicited Downstream

   The "Downstream-on-Demand" mode requires an LSR to explicitly
   request the label binding for FECs from its peer, whereas
   "Unsolicited Downstream" mode allows an LSR to distribute the label
   binding for FECs to LSR peers that have not explicitly requested
   them. The MPLS architecture also specifies that on any given label
   distribution adjacency, the upstream LSR and the downstream LSR must
   agree to use a single label advertisement mode.

   Label Distribution Protocol (LDP) [RFC5036] allows label
   advertisement mode negotiation at time of session establishment
   (section 3.5.3 [RFC5036]). To comply with MPLS architecture, LDP
   specification also dictates that only single label advertisement
   mode is agreed and used for a given LDP session between two LSRs.

   With the advent of new LDP applications, such as L2VPN [RFC4447],
   mLDP [RFC6388], ICCP [ICCP], there are situations when an LDP
   session is shared across more than one application to exchange label
   bindings for different types of FEC. Although different applications
   sharing the same LDP session may need a different type of label
   advertisement mode negotiated, there is only one type of label
   advertisement mode that is negotiated and agreed at the time of
   establishment of LDP session.

   This document clarifies the use and the applicability of label
   advertisement mode of a session for each application using the
   session. It also categorizes LDP applications into two broad
   categories of mode-bound and mode-independent applications.

   The document also suggests an update to RFC-5036 and RFC-4447 to
   remove any ambiguity and conflict in the area of using correct label
   advertisement mode for a given LDP application.

2. 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 RFC-2119 [RFC2119].




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   The unqualified term "mode" used in document refers to "label
   advertisement mode".

   Please also note that LDP specification [RFC5036] uses the term
   "Downstream Unsolicited" to refer to "Unsolicited Downstream". The
   LDP specification also uses the terms "label distribution mode" and
   "label advertisement mode" interchangeably. Like LDP specification
   document, this document also uses these terms interchangeably.

3. Label Advertisement Mode Applicability

3.1. Label Advertisement Mode Negotiation

   Label advertisement mode is negotiated between LSR peers at the time
   of session establishment. The label advertisement mode is specified
   in LDP Initialization message's "Common Session Parameter" TLV by
   setting A-bit (Label Advertisement Discipline bit) to 1 or 0 for
   Downstream-on-Demand or Downstream-Unsolicited modes respectively.
   The negotiation of the A-bit is specified in section 3.5.3 of
   [RFC5036] as follows:

     "If one LSR proposes Downstream Unsolicited and the other proposes
     Downstream on Demand, the rules for resolving this difference is:

        -  If the session is for a label-controlled ATM link or a
         label- controlled Frame Relay link, then Downstream on Demand
         MUST be used.

       -  Otherwise, Downstream Unsolicited MUST be used."

   Once label advertisement mode has been negotiated and agreed, both
   LSR peers must use the same mode for label binding exchange.

3.2. Mode-based Categorization of LDP Applications

   The earlier applications, defined and identified at the time of
   standardization of LDP base specification RFC-3036, using LDP to
   exchange their FEC bindings were:

     .  Dynamic Label Switching for IP Prefixes

     .  Label-controlled ATM/FR

   Since then, several new applications have emerged that use LDP to
   signal their FEC bindings and/or application data. These include:

     .  L2VPN P2P PW   ([RFC4447])



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     .  L2VPN P2MP PW  ([P2MP-PW])

     .  mLDP           ([RFC6388])

     .  ICCP           ([ICCP])

   We divide the LDP applications into two broad categories from label
   advertisement mode usage point of view:

   1. Session mode-bound Applications

   2. Session mode-independent Applications

3.2.1. Session mode-bound Applications

  We define a "session mode-bound application" to be an application
  which uses the negotiated label advertisement mode. This means that
  the FEC-label binding exchange for such an LDP applications MUST use
  the label advertisement mode negotiated for the LDP session.

   The early LDP applications "Dynamic Label Switching for IP Prefixes"
   and "Label-controlled ATM/FR" are included into this category.

3.2.2. Session mode-independent Applications

   We define a "session mode-independent application" to be an
   application which does not care about the negotiated label
   advertisement mode. This means that the FEC-label binding, or any
   other application data, exchange for such an LDP application does
   not care about, nor tied to the "negotiated" label advertisement
   mode of the session; rather, the information exchange is driven by
   the application need and procedures as described by its
   specification document. For example, [RFC6388] specifies procedures
   to advertise P2MP FEC label binding in an unsolicited manner,
   irrespective of the negotiated label advertisement mode of the
   session.

   The applications, PW (P2P and P2MP), MLDP, and ICCP, are included
   into this category of LDP applications.

3.2.2.1. Upstream Label Assignment

  As opposed to downstream assigned label advertisement defined by
  [RFC3031], [RFC6389] specification defines new mode of label
  advertisement where label advertisement and distribution occurs for
  upstream assigned labels.




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  As stated earlier in this document, LDP base specification RFC-5036
  only allows specifying Downstream-Unsolicited or Downstream-on-Demand
  mode. This means that any LDP application that requires upstream
  assigned label advertisement also falls under the category of Session
  mode-independent application.

3.3. Unacceptable label advertisement mode

  The procedures related to unacceptable label advertisement mode, as
  defined in RFC-5036 section 3.5.3, continue to apply for any "mode-
  bound" FEC/application. For a "mode-independent" FEC/application,
  mode negotiation does not apply and hence both LSRs MUST operate in
  the mode specified for the given application by the respective
  specification.

  If a session is jointly shared amongst mode-bound and mode-
  independent FEC/applications, session will not be established if the
  label advertisement mode is unacceptable (between the LSRs) for a
  given mode-bound FEC/application type. This is inline with RFC-5036
  section 3.5.3 specification for unacceptable mode.

4. Clarification on Mode Applicability

   To remove any ambiguity and conflict amongst different
   specifications with regards to the use of LDP session's label
   advertisement mode, we propose an update to LDP base specification
   RFC-5036 to clarify the applicability of session's negotiated mode.

   Furthermore, RFC-4447 specifies LDP extensions and procedures to
   exchange label bindings for P2P PW FECs [RFC4447], and dictates the
   use of Downstream-Unsolicited mode for an LDP session related to
   L2VPN PW. This mode dictation creates a direct conflict in
   situations when a PW LDP session is shared with an LDP application
   with Downstream-on-Demand mode (such as Label switching Application
   for IP prefixes). To remove such a conflict, we also propose an
   update to a section of RFC-4447.














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4.1. Update to RFC-5036

   The section 3.5.3 of [RFC5036] is updated to add following two
   statements under the description of "A, Label Advertisement
   Discipline":

   -  The negotiated label advertisement discipline only applies to FEC
     label binding advertisement of "Address Prefix" FECs;

   -  Any new document specifying a new FEC MUST state the
     applicability of the negotiated label advertisement discipline for
     that FEC.

4.2. Update to RFC-4447

   The section 3 of [RFC4447] states:

    "LDP MUST be used in its downstream unsolicited mode."

   Since PW application falls under session mode-independent
   application category, the above statement in [RFC4447] should be
   read to mean as follows:

   "LDP MUST exchange PW FEC label bindings in downstream unsolicited
   manner, independent of the negotiated label advertisement mode of
   the LDP session".

5. Security Considerations

   This document specification only clarifies the applicability of LDP
   session's label advertisement mode, and hence does not add any LDP
   security mechanics and considerations to those already defined in
   LDP specification [RFC5036].

6. IANA Considerations

  None.

7. References

7.1. Normative References

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





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   [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.

   [RFC3031] E. Rosen, A. Viswanathan, and R. Callon, "Multiprotocol
             Label Switching Architecture", RFC 3031, January 2001.

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

7.2. Informative References

   [P2MP-PW] S. Boutros, L. Martini, S. Sivabalan, G. Del Vecchio,
             Kamite, L. Jin, "Signaling Root-Initiated P2MP PWs using
             LDP", draft-ietf-pwe3-p2mp-pw-04.txt, Work in Progress,
             March 2012.

   [RFC6388] I. Minei, I. Wijnand, K. Kompella, B., "LDP Extensions for
             P2MP and MP2MP LSPs", RFC 6388, November 2011.

   [ICCP]    L. Martini, S. Salam, A. Sajassi, and S. Matsushima,
             "Inter-Chassis Communication Protocol for L2VPN PE
             Redundancy", draft-ietf-pwe3-iccp-08.txt, Work in
             Progress, June 2012.

   [RFC6389] R. Aggarwal, and J.L. Le Roux, "MPLS Upstream Label
             Assignment for LDP", RFC 6389, November 2011.

8. Acknowledgments

   We acknowledge the authors of [RFC5036] and [RFC4447] since some of
   the text in this document is borrowed from their specification. We
   also acknowledge Eric Rosen and Rajiv Asati for their review and
   input.

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

Authors' Addresses

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




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Internet-Draft  Applicability of LDP Label Advertisement Mode  Aug 2012

  Sami Boutros
  Cisco Systems, Inc.
  3750 Cisco Way,
  San Jose, CA 95134, USA.
  E-mail: sboutros@cisco.com

  Luca Martini
  Cisco Systems, Inc.
  9155 East Nichols Avenue, Suite 400,
  Englewood, CO 80112, USA.
  E-mail: lmartini@cisco.com

  Nicolai Leymann
  Deutsche Telekom,
  Winterfeldtstrasse 21-27,
  10781 Berlin, Germany.
  E-mail: N.Leymann@telekom.de


































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