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Versions: (draft-jin-pwe3-cbit-negotiation) 00 01 02 03 04 05 RFC 6723

Network Working Group                                        L. Jin(ed.)
Internet-Draft                                                       ZTE
Updates: 4447 6073(if approved)                              R. Key(ed.)
Intended status: Standards Track                                  Huawei
Expires: December 17, 2012                                     S. Delord
                                                          Alcatel-Lucent
                                                               T. Nadeau
                                                                 Juniper
                                                              S. Boutros
                                                     Cisco Systems, Inc.
                                                           June 15, 2012



          Pseudowire Control Word Negotiation Mechanism Update
                draft-ietf-pwe3-cbit-negotiation-04.txt

Abstract

   The control word negotiation mechanism specified in RFC4447 has a
   problem when PE changes the preference for the use of control word
   from PREFERRED to NOT-PREFERRED.  This draft updates RFC4447 by
   introducing a message exchanging mechanism to resolve this control
   word negotiation issue.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 17, 2012.

Copyright Notice

   Copyright (c) 2012 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


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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Control word re-negotiation by label request message  . . . . . 4
     3.1.  Control word re-negotiation use case  . . . . . . . . . . . 5
   4.  Backward Compatibility  . . . . . . . . . . . . . . . . . . . . 6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   8.  Contributing Authors  . . . . . . . . . . . . . . . . . . . . . 6
   9.  Normative references  . . . . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Updated C-bit Handling Procedures Diagram  . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8


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].





















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1.  Introduction

   The control word negotiation mechanism specified in [RFC4447] section
   6.2 has a problem when PE changes the preference for the use of
   control word from PREFERRED to NOT-PREFERRED.  There would be a wrong
   negotiated result of the control word as "not used" while both ends
   of pseudowire PE have the use of control word as PREFERRED.  This
   draft updates [RFC4447] by introducing a message exchanging mechanism
   to resolve this negotiation issue.


2.  Problem Statement

   [RFC4447] section 6 describes the control word negotiation mechanism.
   Each PW endpoint has the capability of being configurable with a
   parameter that specifies whether the use of the control word is
   PREFERRED or NOT PREFERRED.  While in some case of control word
   negotiation, PE will advertise C-bit=0 in label mapping message with
   its locally configured use of control word as PREFERRED.  This kind
   of behavior will cause some problem when peer PE changes the use of
   control word from NOT PREFERRED to PREFERRED.

   The following case will describe the negotiation problem in detail:


                +-------+                    +-------+
                |       |         PW         |       |
                |  PE1  |====================|  PE2  |
                |       |                    |       |
                +-------+                    +-------+

                                 Figure 1

   1.  Initially, the use of control word on PE1 is configured as
       PREFERRED, and on PE2 as NOT PREFERRED.

   2.  The negotiation result for the control word of this PW is used,
       and PE1 sends label mapping with C-bit=0 finally according to
       [RFC4447] section 6.2.

   3.  PE2 then changes its use of control word configuration to
       PREFERRED.

   4.  PE2 will then send label withdraw message to PE1.

   5.  According to the control word negotiation mechanism, the received
       label mapping on PE2 from PE1 indicates C-bit=0, therefore PE2
       will still send label mapping with C-bit=0.



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   The negotiation result for the control word is still not used, even
   though the use of control word configuration on both PE1 and PE2 is
   PREFERRED.


3.  Control word re-negotiation by label request message

   In order to resolve above problem, the control word re-negotiation
   mechanism as described in [RFC4447] section 6 is updated by adding
   label request message.

   When Local PE changes its use of control word from NOT PREFERRED to
   PREFERRED and only if it already received the remote label mapping
   message with C-bit=0, additional procedure will be added as follow:

      -i.  Local PE MUST send a label withdraw message to remote PE if
      it has previously sent a label mapping, and label release message
      to remote PE, and wait until receiving a label release from the
      remote PE.

      -ii.  Local PE MUST send a label request message to remote PE, and
      wait until receiving a label mapping message containing the remote
      PE locally configured preference for use of control word.

      -iii.  After receiving remote PE label mapping with C-bit, Local
      PE MUST follow procedures defined in [RFC4447] section 6 when
      sending its label mapping message.

   When the remote PE successfully processed the label withdraw and
   label release, and removed the remote label binding, it MUST reset
   its use of control word with the locally configured preference, and
   send label mapping as a response of label request with locally
   configured preference for use of control word.

   Note: the FEC element in label request message should be the PE's
   local FEC element.  Only if FEC element in label request message
   could bind together with peer PE's local FEC element, the peer PE
   sends label mapping with its bound local FEC element and label as a
   response.  The label request message format and procedure is
   described in [RFC5036].

   The multi-segment PW case for T-PE is same.  S-PE SHOULD assume an
   initial passive role as defined in [RFC6073].  When an S-PE receives
   a label request message from one of its adjacent PEs (be it S-PE or
   another T-PE), it MUST send a matching label request message to other
   adjacent PE (again, it may be an S-PE or a T-PE).  This is necessary
   since S-PE does not have full information of interface parameter
   field in the FEC advertisement.  When S-PE receives a label release



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   message from remote PE, it MUST send a corresponding label release to
   the other remote PE where it has received label mapping.

   As local T-PE will send label withdraw before sending label request
   to remote peer, the S-PE MUST send the label withdraw upstream before
   it advertises the label request.  When S-PE receives the label
   withdraw, it should process this message to send a label release as a
   response and a label withdraw to upstream S-PE/T-PE, then process the
   next LDP message, e.g. the label request message.

   When Local PE changes the use of control word from PREFERRED to NOT
   PREFERRED, Local PE would be able to re-negotiate the Control Word to
   be not used following the procedures defined in [RFC4447], and no
   label request message to peer PE will be needed.  In that case, Local
   PE will always send new label mapping with C-bit reflecting the
   locally configured preference for use of Control Word.

   The diagram in Appendix A in this document updates the control word
   negotiation diagram in [RFC4447] Appendix A.

3.1.  Control word re-negotiation use case

   The procedure of PE1 and PE2 for the use case in figure 1 should be
   as follows:

   1.  PE2 changes locally configured preference for use of control word
       to PREFERRED.

   2.  PE2 will then send label withdraw and release messages to PE1,
       and wait until receiving label release from PE1.

   3.  PE1 will send label release in reply to label withdraw message
       from PE2.  After that and processing the label release from PE2,
       it would reset the use of control word to the locally configured
       preference as PREFERRED.

   4.  Upon receipt of Label release message from PE1, PE2 would reset
       use of control word to the locally configured preference as
       PREFERRED, and MUST send label request message to PE1, and wait
       until receiving a label mapping message.

   5.  PE1 must send label mapping message with C-bit=1 again to PE2 as
       response of label request.

   6.  PE2 receives the label mapping from PE1 and gets the remote label
       binding information.  PE2 should wait for PE1 label binding
       before sending its label binding with C-bit set.




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   7.  PE2 will send label mapping to PE1 with C-bit=1, and follow
       procedures defined in [RFC4447] section 6.

   It is to be noted that the above assume that PE1 is configured to
   prefer the use of control word, however in step 5 if PE1 doesn't
   prefer or support control word, PE1 would send the label mapping
   message with C-bit=0, this would result in PE2 in step 7 sending a
   label mapping with C-bit=0 as per [RFC4447] section 6.

   By sending label request message, PE2 will get the locally configured
   preference for use of control word of peer PE1 in the received label
   mapping message.  By using the new C-bit from label mapping message
   received from peer PE1 and locally configured preference for use of
   control word, PE2 should determine the use of PW control word
   according to [RFC4447] section 6.


4.  Backward Compatibility

   Since control word negotiation mechanism is updated by adding label
   request message, and still follows the basic procedure described in
   [RFC4447] section 6, it is fully compatible with existing
   implementations.  The remote PE (PE1 in figure 1) which already
   implement label request message could be compatible with the PE (PE2
   in figure 1) following the mechanism of this document.


5.  Security Considerations

   This document does not introduce any additional security constraints.


6.  IANA Considerations

   This document does not require IANA assignment.


7.  Acknowledgements

   The authors would like to thank Stewart Bryant, Andrew Malis, Nick
   Del Regno, Luca Martini, Venkatesan Mahalingam, Alexander Vainshtein,
   Adrian Farrel and Spike Curtis for their discussion and comments.


8.  Contributing Authors

   Vishwas Manral
   Hewlett-Packard Co.



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   19111 Pruneridge Ave, Bldg 44,
   Cupertino, CA 95014-0691
   Email: vishwas.manral@hp.com

   Reshad Rahman
   Cisco Systems, Inc.
   2000 Innovation Drive Ottawa,
   Ontario K2K 3E8
   CANADA
   Email: rrahman@cisco.com

9.  Normative references

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

   [RFC4447]  Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
              Heron, "Pseudowire Setup and Maintenance Using the Label
              Distribution Protocol (LDP)", RFC 4447, April 2006.

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

   [RFC6073]  Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
              Aissaoui, "Segmented Pseudowire", RFC 6073, January 2011.


Appendix A. Updated C-bit Handling Procedures Diagram

    -----------------------------------
    |                                 |
    |                        ------------------
    |                    Y   | Received Label |       N
    |                 -------|  Mapping Msg?  |--------------
    |                 |      ------------------             |
    |             --------------                            |
    |             |            |                            |
    |          -------      -------                         |
    |          | C=0 |      | C=1 |                         |
    |          -------      -------                         |
    |             |            |                            |
    |             |    ----------------                     |
    |             |    | Control Word |     N               |
    |             |    |    Capable?  |-----------          |
    |             |    ----------------          |          |
    |             |          Y |                 |          |
    |             |            |                 |          |
    |             |   ----------------           |          |



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    |             |   | Control Word |  N        |          |
    |             |   |  Preferred?  |----       |          |
    |             |   ----------------   |       |          |
    |             |          Y |         |       |          |
    |  ---------------------   |         |       |          |
    |  | Control Word      |   |         |       |   ----------------
    |  | change Preferred  |   |         |       |   | Control Word |
    |  | to not-Preferred? |   |         |       |   |  Preferred?  |
    |  ---------------------   |         |       |   ----------------
    |     Y |     | N          |         |       |     N |     Y |
    |       |     |            |         |       |       |       |
    |       |   Send         Send      Send    Send    Send    Send
    |       |    C=0          C=1       C=0     C=0     C=0     C=1
    |       |                            |       |       |       |
    |  -------------------            ----------------------------------
    |  | Send withdraw   |            | If receive the same as sent,   |
    |  | if already sent |            | PW setup is complete. If not:  |
    |  | label mapping,  |            ----------------------------------
    |  | and release msg |               |       |       |       |
    |  -------------------              ------------------- -----------
    |           |                       |     Receive     | | Receive |
    |  -------------------              |       C=1       | |   C=0   |
    |  | Receiving label |              ------------------- -----------
    |  | release message |                       |               |
    |  -------------------                 Wait for the        Send
    |           |                          next message     Wrong C-bit
    |  -------------------                                       |
    |  | Send label      |                                  Send Label
    |  | request message |                               Mapping Message
    |  -------------------
    |           |
    -------------




Authors' Addresses

   Lizhong Jin (editor)
   ZTE Corporation
   889, Bibo Road
   Shanghai, 201203, China

   Email: lizhong.jin@zte.com.cn







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   Raymond Key (editor)
   Huawei

   Email: raymond.key@ieee.org


   Simon Delord
   Alcatel-Lucent

   Email: simon.delord@gmail.com


   Thomas Nadeau
   Juniper

   Email: tnadeau@juniper.net


   Sami Boutros
   Cisco Systems, Inc.
   3750 Cisco Way
   San Jose, California 95134, USA

   Email: sboutros@cisco.com



























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