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

Network Working Group                             Lizhong Jin (ed.), ZTE
Internet-Draft                                Raymond Key (ed.), Telstra
Updates: 4447 (if approved)                 Simon Delord, Alcatel-Lucent
Category: Standards Track                 Thomas Nadeau, CA Technologies
Expires: January 5, 2012                 Vishwas Manral, Hewlett-Packard
                                                     Sami Boutros, Cisco
                                                    Reshad Rahman, Cisco


                                                            July 5, 2011


          Pseudowire Control Word Negotiation Mechanism Update
                  draft-ietf-pwe3-cbit-negotiation-01


Abstract

   This document describes the problem of control word negotiation
   mechanism specified in [RFC4447].  Based on the problem analysis, a
   message exchanging mechanism is introduced to solve this control word
   negotiation issue.  This document is to update [RFC4447] control word
   negotiation mechanism.


Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 5, 2012.







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Copyright Notice

   Copyright (c) 2011 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 (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
   4. Backward Compatibility . . . . . . . . . . . . . . . . . . . . . 6
   5. Security Considerations  . . . . . . . . . . . . . . . . . . . . 6
   6. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . 6
   7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
   8. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
   8.1. Normative References . . . . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7
   Appendix A. Updated C-bit Handling Procedures Diagram . . . . . . . 8
   Appendix B. Alternative proposal by "Label Release" message . . . . 9


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

   This document describes the problem of control word negotiation
   mechanism specified in [RFC4447].  Based on the problem analysis, a
   message exchanging mechanism is introduced to solve this negotiation
   issue. The control word negotiation mechanism in this document is to
   update [RFC4447] section 6.2 "PW Types for Which the Control Word is
   NOT Mandatory".

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 control word PREFERRED.  This kind of behavior
   will cause some problem when peer PE changes its control word from
   NOT PREFERRED to PREFERRED.

   This following case will describe the negotiation problem in detail:

             +-------+                    +-------+
             |       |         PW         |       |
             |  PE1  |====================|  PE2  |
             |       |                    |       |
             +-------+                    +-------+
                            Figure 1

       1. Initially, the control word on PE1 is configured to PREFERRED,
          and on PE2 to NOT PREFERRED.

       2. The negotiation result for the control word for this PW is
          "not supported", and PE1 send label mapping with C-bit=0
          finally.

       3. PE2 then changes its 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.

   The negotiation result for the PW control word is still "not
   supported", even though the control word configuration on both PE1
   and PE2 is set to PREFERRED.







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3. Control word re-negotiation by label request message

   In order to solve this problem, the control word re-negotiation is
   operated by adding label request message.  The control word
   negotiation mechanism can still follow the procedure described in
   [RFC4447] section 6.

   When Local PE changes its 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 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 configured control word setting.

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

   When the peer PE successfully processed the label withdraw and
   removed the remote label binding, it MUST send label mapping as a
   response of label request with locally configured control word
   parameter.

   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, and the request message
   MUST be processed in ordered mode.  When S-PE receives a label
   request message from a remote peer, it MUST advertise the request
   message to the other remote PE.  This is necessary since S-PE does
   not have full information of interface parameter field in the FEC
   advertisement.

   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.






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   When Local PE changes its control word from PREFERRED to NOT
   PREFERRED, Local PE would be able to re-negotiate the Control Word to
   be NOT PREFERRED 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
   local Control Word configuration.

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

       1. PE2 changes locally configured control word to PREFERRED.

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

       3. PE1 will send label release in reply to label withdraw message
          from PE2.

       4. Upon receipt of Label release message from PE1, PE2 MUST send
          label request messages to PE1 although it already received the
          label mapping with C-bit=0.

       5. PE1 MUST send label mapping message with C-bit=1 again to PE2
          (Note: PE1 MUST send label mapping with locally configured CW
          parameter).

       6. PE2 receives the label mapping from PE1 and updates the remote
          label binding information.  PE2 MUST wait for PE1 label
          binding before sending its label binding with C-bit set, only
          if it previously had a label binding with C-bit=0 from PE1.

       7. PE2 will send label mapping to PE1 with C-bit=1.

   It is to be noted that the above assume that PE1 is configured to
   support CW, however in step 5 if PE1 doesn't support CW, 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] CW
   negotiation procedure.

   By sending label request message, PE2 will get the configured CW
   parameter of peer PE1 in the received label mapping message.  By
   using the new CW parameter from label mapping message received from
   peer PE1 and locally configured CW, PE2 should determine the PW CW
   parameter according to [RFC4447] section 6.

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








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4. Backward Compatibility

   Since control word re-negotiation is operated by adding label request
   message, and still follows the 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. References

8.1. Normative References

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

   [RFC4447]    Martini, L., and al, Pseudowire Setup and Maintenance
                Using the Label Distribution Protocol (LDP), RFC 4447,
                April 2006

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


















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Authors' Addresses

   Lizhong Jin (editor)
   ZTE Corporation
   889, Bibo Road
   Shanghai, 201203, China
   Email: lizhong.jin@zte.com.cn

   Raymond Key (editor)
   Telstra
   Email: raymond.key@ieee.org

   Simon Delord
   Alcatel-Lucent
   Email: simon.delord@gmail.com

   Thomas Nadeau
   CA Technologies
   273 Corporate Drive
   Portsmouth, NH, USA
   Email: thomas.nadeau@ca.com

   Vishwas Manral
   Hewlett-Packard Co.
   19111 Pruneridge Ave, Bldg 44,
   Cupertino, CA 95014-0691
   Email: vishwas.manral@hp.com

   Sami Boutros
   Cisco Systems, Inc.
   3750 Cisco Way
   San Jose, California 95134
   USA
   Email: sboutros@cisco.com

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













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Appendix A. Updated C-bit Handling Procedures Diagram

    -----------------------------------
    |                                 |
    |                        ------------------
    |                    Y   | Received Label |       N
    |                 -------|  Mapping Msg?  |--------------
    |                 |      ------------------             |
    |             --------------                            |
    |             |            |                            |
    |          -------      -------                         |
    |          | C=0 |      | C=1 |                         |
    |          -------      -------                         |
    |             |            |                            |
    |             |    ----------------                     |
    |             |    | Control Word |     N               |
    |             |    |    Capable?  |-----------          |
    |             |    ----------------          |          |
    |             |          Y |                 |          |
    |             |            |                 |          |
    |             |   ----------------           |          |
    |             |   | 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 wait until  |               |       |       |       |
    |  | receiving label |              ------------------- -----------
    |  | release         |              |     Receive     | | Receive |
    |  -------------------              |       C=1       | |   C=0   |
    |           |                       ------------------- -----------
    |  -------------------                       |               |
    |  | Send label      |                 Wait for the        Send
    |  | request message |                 next message     Wrong C-bit
    |  -------------------                                       |
    |           |                                           Send Label
    |           |                                        Mapping Message
    -------------



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Appendix B. Alternative proposal by "Label Release" message

   We refer to the proposal in section 3 as "Label Request" message
   proposal. There is an alternative proposal to re-negotiate control
   word by sending label release with a new LDP status error code "PW
   Reset C-bit negotiation".  When this label release is received by the
   S-PE in MS-PW case, it must forward it back to the T-PE. When the
   T-PE receives the label release with this error code, it must send a
   label withdraw followed by a new label mapping even if its C-bit
   remains the same.

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

       1. PE2 changes locally configured control word to PREFERRED.

       2. PE2 will then send label withdraw message to PE1, and label
          release with a new LDP status error code "PW Reset C-bit
          negotiation" to PE1.

       3. PE2 will then send label mapping with C-bit=1 as per RFC4447.

       4. PE1 will send label release in reply to label withdraw message
          from PE2.

       5. Upon receipt of label release with error code "PW Reset C-bit
          negotiation" from PE2, PE1 MUST withdraw its label mapping and
          re-advertise with the configured C-bit=1.

       6. PE2 receives the label mapping from PE1 with C-bit=1, and
          re-negotiates to get the result of C-bit=1 as per RFC4447.

   Both "Label Request" and "Label Release" message proposals work to
   resolve the control word negotiation problem described in this
   document.  We do not choose the "Label Release" message proposal for
   the following backward compatible reasons:

       1. PE1 always needs to be updated, while the "label request"
          message proposal has better backward compatibility as
          described in section 4.

       2. S-PE needs to be updated in MS-PW case, while it is not
          necessary in the "label request" message proposal.











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   The following table makes a simple comparison between the two
   proposals.
              +---------+------------+-----------+---------+---------+
              | Has new | Change     | PE1       | PE2     | S-PE    |
              | Defined | LDP        | need      | need    | need    |
              | Status? | RFC5036?   | update?   | update? | update? |
   +----------+---------+------------+-----------+---------+---------+
   | Label    |         | Yes (add   | No (see   |         |         |
   | Request  | No      | FEC update | section 4)| Yes     | No      |
   | Proposal |         | at PE2)    |           |         |         |
   +----------+---------+------------+-----------+---------+---------+
   | Label    |         | Yes (change|           |         |         |
   | Release  | Yes     | label-rel  | Yes       | Yes     | Yes     |
   | Proposal |         | procedure) |           |         |         |
   +----------+---------+------------+-----------+---------+---------+







































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