





Internet Engineering Task Force (IETF)                       L. Jin, Ed.
Request for Comments: 6723                                           ZTE
Updates: 4447, 6073                                          R. Key, Ed.
Category: Standards Track                                         Huawei
ISSN: 2070-1721                                                S. Delord
                                                          Alcatel-Lucent
                                                               T. Nadeau
                                                                 Juniper
                                                              S. Boutros
                                                     Cisco Systems, Inc.
                                                          September 2012


      Update of the Pseudowire Control-Word Negotiation Mechanism

Abstract

   The control-word negotiation mechanism specified in RFC 4447 has a
   problem when a PE (Provider Edge) changes the preference for the use
   of the control word from NOT PREFERRED to PREFERRED.  This document
   updates RFC 4447 and RFC 6073 by adding the Label Request message to
   resolve this control-word negotiation issue for single-segment and
   multi-segment pseudowires.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6723.














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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
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   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  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Conventions Used in This Document . . . . . . . . . . . . . . . 3
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Control-Word Renegotiation by Label Request Message . . . . . . 4
     4.1.  Control-Word Renegotiation for Multi-Segment PW . . . . . . 5
     4.2.  Control-Word Renegotiation Use Case . . . . . . . . . . . . 5
   5.  Backward Compatibility  . . . . . . . . . . . . . . . . . . . . 6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . 7
   9.  Normative References  . . . . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Updated Diagram of C-Bit Handling Procedures . . . . . 8

1.  Introduction

   The control-word negotiation mechanism specified in [RFC4447],
   Section 6.2, encounters a problem when a PE changes the preference
   for the use of the control word from NOT PREFERRED to PREFERRED.
   [RFC4447] specifies that if both endpoints prefer the use of the
   control word, then the pseudowire control word should be used.
   However, in the case where a PE changes its preference from NOT
   PREFERRED to PREFERRED and both ends of the PW (pseudowire) PE have
   the use of control word set as PREFERRED, an incorrect negotiated
   result of the control word as "not used" occurs.  This document
   updates the control-word negotiation mechanism in [RFC4447] by adding
   a Label Request message to resolve this negotiation issue for single-
   segment PW.  Multi-segment PW in [RFC6073] inherits the control-word
   negotiation mechanism in [RFC4447], and this document updates
   [RFC6073] by adding the processing of Label Request message on the





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   S-PE (Switching Provider Edge).  When the PE changes the preference
   for the use of control word from PREFERRED to NOT PREFERRED, it
   should follow [RFC4447], and there is no problem.

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 [RFC2119].

3.  Problem Statement

   [RFC4447], Section 6, describes the control-word negotiation
   mechanism.  Each PW endpoint has a configurable parameter that
   specifies whether the use of the control word is PREFERRED or NOT
   PREFERRED.  During control-word negotiation, if one PE advertises a C
   bit set to 0 in the Label Mapping message with its locally configured
   use of control word as PREFERRED, and a corresponding peer PE changes
   its use of control word from NOT PREFERRED to PREFERRED, this causes
   an incorrect negotiated control-word result of "not used".

   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 not
       used, and ultimately PE1 sends the Label Mapping message with C
       bit set to 0 according to [RFC4447], Section 6.2.

   3.  PE2 then changes its use of control-word configuration from NOT
       PREFERRED to PREFERRED, by deleting PW configuration with NOT
       PREFERRED use of control word, and configuring the PW again with
       PREFERRED use of control word.

   4.  PE2 will then send the Label Withdraw message to PE1, and
       correspondingly will receive the Label Release message from PE1.






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   5.  According to the control-word negotiation mechanism, the
       previously received Label Mapping message on PE2 from PE1 carries
       the C bit set to 0; therefore, PE2 will still send the Label
       Mapping message with the C bit set to 0.

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

4.  Control-Word Renegotiation by Label Request Message

   The control-word negotiation mechanism in [RFC4447], Section 6, is
   updated to add the Label Request message described in this section.

   The renegotiation process begins when the local PE has received the
   remote Label Mapping message with the C bit set to 0, and at the
   point its use of control word is changed from NOT PREFERRED to
   PREFERRED.  The following additional procedure will be carried out:

   i.    The local PE MUST send a Label Release message to remote PE.
         If local PE has previously sent a Label Mapping message, it
         MUST send a Label Withdraw message to remote PE and wait until
         it has received a Label Release message from the remote PE.
         Note: the above-mentioned sending of the Label Release message
         and Label Withdraw message does not require a specific
         sequence.

   ii.   The local PE MUST send a Label Request message to the peer PE,
         and then MUST wait until it receives a Label Mapping message
         containing the peer's current configured preference for use of
         control word.

   iii.  After receiving the remote peer PE Label Mapping message with
         the C bit, the local PE MUST follow the procedures defined in
         [RFC4447], Section 6, when sending its Label Mapping message.

   The remote PE will follow [RFC4447], and once the remote PE has
   successfully processed the Label Withdraw message and Label Release
   message, it will reset its use of control word with the locally
   configured preference.  Then, the remote PE will send a Label Mapping
   message with locally configured preference for use of control word as
   a response to Label Request message as specified in [RFC5036].

   Note: for the local PE, before processing new request to change the
   configuration, the above message-exchanging process should be
   finished.  The FEC (Forwarding Equivalence Class) element in the
   Label Request message should be the PE's local PW FEC element.  As a
   response to the Label Request message, the peer PE should send a



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   Label Mapping message with its own local PW FEC element.  The Label
   Request message format and procedure is described in [RFC5036].

4.1.  Control-Word Renegotiation for Multi-Segment PW

   The multi-segment PW case for a T-PE (Terminating Provider Edge)
   operates similarly as the PE in single-segment PW described in the
   above section.  An initial passive role is defined in [RFC6073] for
   the S-PE when processing of the Label Mapping message.  [RFC6073] is
   updated by applying this passive role to the processing of Label
   Request message.  When an S-PE receives a Label Request message from
   one of its adjacent PEs (which may be an 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 an
   S-PE does not have complete information of the interface parameter
   field in the FEC advertisement.  When the S-PE receives a Label
   Release message from remote PE, it MUST send a corresponding Label
   Release message to the other remote PE when it holds a label for the
   PW from the remote PE.

   Note: because the local T-PE will send a Label Withdraw message
   before sending a Label Request message to the remote peer, the S-PE
   MUST process the Label Withdraw message before the Label Request
   message.  When the S-PE receives the Label Withdraw message, it
   should process this message to send a Label Release message as a
   response and a Label Withdraw message to an upstream S-PE/T-PE.  The
   S-PE will then process the next LDP message, e.g. the Label Request
   message.

   When the local PE changes the use of control word from PREFERRED to
   NOT PREFERRED, the local PE would then renegotiate the control word
   so that it is not used by deleting the PW configuration with
   PREFERRED use of control word, and configuring the PW again with NOT
   PREFERRED use of control word.  All of these procedures have been
   defined in [RFC4447], Section 5.4.1.

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

4.2.  Control-Word Renegotiation Use Case

   The procedure of PE1 and PE2 for the use case in Figure 1 will become
   as follows:

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





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   2.  PE2 will then send the Release messages to PE1.  PE2 will also
       send the Label Withdraw message, and wait until it has received
       the Label Release message from PE1.

   3.  PE1 will send the Label Release message in response to the Label
       Withdraw message from PE2.  After processing the Label Release
       from PE2, PE1 will then reset the use of control word to the
       locally configured preference as PREFERRED.

   4.  Upon receipt of the Label Release message from PE1, PE2 will send
       the Label Request message to PE1, and proceed to wait until a
       Label Mapping message is received.

   5.  PE1 will send a Label Mapping message with the C bit set to 1
       again to PE2 in response to the Label Request message.

   6.  PE2 receives the Label Mapping message from PE1 and gets the
       remote label binding information.  PE2 will wait for the PE1
       Label Mapping message before sending its Label Mapping message
       with the C bit set.

   7.  PE2 will send the Label Mapping to PE1 with C bit set to 1, and
       follow procedures defined in [RFC4447], Section 6.

   While it is assumed that PE1 is configured to prefer the use of the
   control word, in step 5, if PE1 doesn't prefer or support the control
   word, PE1 would then send the Label Mapping message with the C bit
   set to 0.  As a result, PE2 in step 7 would send a Label Mapping
   message with the C bit set 0 as per [RFC4447], Section 6.

   By sending a 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 the
   Label Mapping message received from peer PE1 and the locally
   configured preference for use of control word, PE2 should determine
   the use of PW control word according to [RFC4447], Section 6.

5.  Backward Compatibility

   Since control-word negotiation mechanism is updated by adding the
   Label Request message, and still follows the basic procedure
   described in [RFC4447], Section 6, this document is fully compatible
   with existing implementations.  For single-segment pseudowire, the
   remote PE (PE1 in Figure 1) which already implements [RFC4447], and
   the Label Request message as defined in [RFC5036] could be compatible
   with the PE (PE2 in Figure 1) following the mechanism of this





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   document.  For the multi-segment pseudowire, the T-PE is the same as
   PE in single-segment pseudowire; the S-PE should be upgraded with the
   mechanism defined in this document.

6.  Security Considerations

   The security considerations specified in [RFC4447] and [RFC6073] also
   apply to this document, and this document does not introduce any
   additional security constraints.

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.  Contributors

   Vishwas Manral
   Hewlett-Packard Co.
   19111 Pruneridge Ave., Bldg. 44
   Cupertino, CA 95014-0691
   US
   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.





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Appendix A.  Updated Diagram of C-Bit Handling Procedures

   -----------------------------------
   |                                 |
   |                        ------------------
   |                    Y   | Received Label |       N
   |                 -------|  Mapping msg?  |--------------
   |                 |      ------------------             |
   |             --------------                            |
   |             |            |                            |
   |          -------      -------                         |
   |          | C=0 |      | C=1 |                         |
   |          -------      -------                         |
   |             |            |                            |
   |             |    ----------------                     |
   |             |    | Control Word |     N               |
   |             |    |    Capable?  |-----------          |
   |             |    ----------------          |          |
   |             |          Y |                 |          |
   |             |            |                 |          |
   |             |   ----------------           |          |
   |             |   | Control Word |  N        |          |
   |             |   |  Preferred?  |----       |          |
   |             |   ----------------   |       |          |
   |             |          Y |         |       |          |
   |  ---------------------   |         |       |          |
   |  |Control Word change|   |         |       |   ----------------
   |  |from NOT PREFERRED |   |         |       |   | Control Word |
   |  | to PREFERRED?     |   |         |       |   |  Preferred?  |
   |  ---------------------   |         |       |   ----------------
   |     Y |     | N          |         |       |     N |     Y |
   |       | Delete, and      |         |       |       |       |
   |       | configure      Send      Send    Send    Send    Send
   |       | new PW again    C=1       C=0     C=0     C=0     C=1
   |       |                            |       |       |       |
   |  ----------------------------   ----------------------------------
   |  |Send Label Release msg,   |   | If receive the same as sent,   |
   |  |send Label Withdraw msg if|   | PW setup is complete.  If not: |
   |  |has sent Label Mapping msg|   ----------------------------------
   |  ----------------------------          |       |       |       |
   |           |                       ------------------- -----------
   |  -------------------              |     Receive     | | Receive |
   |  | Receive Label   |              |       C=1       | |   C=0   |
   |  | Release message |              ------------------- -----------
   |  -------------------                       |               |






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   |           |                          Wait for the        Send
   |  -------------------                 next message     Wrong C-bit
   |  | Send Label      |                                       |
   |  | Request message |                                  Send Label
   |  -------------------                              Mapping message
   |           |
   -------------

Authors' Addresses

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

   EMail: lizhong.jin@zte.com.cn


   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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