Mandatory Use of Control Word for PWE3 Encapsulations

Versions: 00                                                            
Network Working Group                                  N. Del Regno, Ed.
Internet-Draft                                    Verizon Communications
Intended status: Standards Track                               T. Nadeau
Expires: April 18, 2011                                           Huawei
                                                               V. Manral
                                                             IP Infusion
                                                                 D. Ward
                                                        Juniper Networks
                                                        October 15, 2010

         Mandatory Use of Control Word for PWE3 Encapsulations


   Of the many variations of PWE3 Encapsulations and Modes (e.g.
   Ethernet, Port Mode, VLAN Mode, etc), only five have the Control Word
   (CW) as being optional.  As a result, this causes an issue with VCCV
   Control Channel selection.  This draft endeavors to resolve the issue
   going forward by making the Control Word, and subsequently the CW-
   based VCCV Control Channel, mandatory for all PWE3 Encapsulations.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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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   This Internet-Draft will expire on April 18, 2011.

Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Mandatory Control Word  . . . . . . . . . . . . . . . . . . . . 4
   3.  Backward Compatibility  . . . . . . . . . . . . . . . . . . . . 4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6

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1.  Introduction

   The PWE3 working group has defined many encapsulations of various
   Layer 1 and Layer 2 links.  Within these encapsulations, there are
   often several modes of encapsulation which have differing
   requirements in order to fully emulate the service.  As such, the use
   of the PWE3 Control Word is mandated in many of the encapsulations,
   but not all.  This can present interoperability issues related to A)
   Control Word use and B) VCCV Control Channel negotiation in mixed
   implementation environments.

   In the various encapsulations where the Control Word is optional, the
   language from [RFC4385] is consistently referenced: "The features
   that the control word provides may not be needed for a given PW.  For
   example, ECMP may not be present or active on a given MPLS network,
   strict frame sequencing may not be required, etc.  If this is the
   case, the control word provides little value and is therefore
   optional."  As such, early implementations may not have supported the
   Control Word for those encapsulations which didn't require it.
   However, as recent discussions have shown [CBIT], the lack of the
   Control Word opens up other issues related to control-word
   negotiation (e.g. preferred vs. not- preferred) and VCCV Contol
   Channel negotiation and selection [DEL].

   The encapsulations and modes for which the Control Word is currently
   optional are:

   o  Ethernet Tagged Mode

   o  Ethernet Raw Mode

   o  PPP

   o  HDLC

   o  Frame Relay Port Mode

   o  ATM (N:1 Cell Mode)

   While the encapsulation for PPP, HDLC and Frame Relay Port Mode are
   the same encap, the services which they emulate may have different
   requirements, and are therefore listed separately.

   Unfortunately, some early implementations of PWE3 standard (and/or
   prestandard) encapsulations are limited in their support for Control
   Word for the above encapsulations due to A) hardware deficiencies, B)
   software deficiencies or C) a combination of the two.  In other
   cases, deployed implementations support control word, but the service

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   provider has had no impetus to suffer the minor loss of overhead
   efficiency.  However, this document asserts based on operational
   feedback of the PWE3 protocols in actual deployments, that the
   benefits of requiring a mandatory control word in the PWE3 standards
   outweigh the minor efficiencies gained when not using it.

   One of the major benefits of consistent use of the Control Word
   pertains to the choice of the VCCV Control Channel.  As identified in
   [DEL], Control Channel Type 1 is the only "in-band" PWE3 control
   channel.  This provides the advantage of proper VCCV forwarding
   behavior in the presence of ECMP.  Further, while the sequencing
   supported by the Control Word is not mandatory, the use of the
   Control Word enables the use of sequencing without forcing the
   renegotiation of the PW.

   All increases in the amount of overhead used to provide service
   should be weighed versus their perceived gain, especially when that
   overhead is large in comparison to the data being carried.  This is a
   common concern with the ATM N:1 encapsulation.  In theory, if only a
   single cell is encapsulated per PSN packet, not only is the inherent
   overhead inacceptably large, the additon of 4 bytes only compounds
   the problem.  However, in practice, the PDUs, or groups of PDUs,
   carried in encapsulations above, including ATM (N:1 Cell Mode), are
   sufficiently large that the additional 4-bytes of CW overhead
   represent a relatively minor increase in the total overhead

2.  Mandatory Control Word

   The Control Word SHALL be mandatory for all PWE3 encapsulations.  The
   use of the sequence number remains OPTIONAL.

   As a result of the Control Word being Mandatory, all implementations
   of the PWE3 encapsulations SHALL follow Section 6.1 of [RFC4447]
   wherein the "PWs MUST have c=1".  This requirement SHALL remain until
   such time, if ever, RFC4447 is superceded and the support for Control
   Word negotiation is removed as a result of this mandate.

3.  Backward Compatibility

   This Control Word mandate will not support backward compatibility
   with implementations which cannot support Control Word.  For those
   implementations, CW negotiation identified in [RFC4447] will result
   in the PW negotiation never completing since the end which cannot
   support CW will ignore the Label Mapping message with c=1.  However,
   for those implementations which currently support Control Word, the
   Control Word mandate will be supported as long as CW is set to

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   PREFERRED and the subsequent c=1 is negotiated.

4.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

5.  Security Considerations

   This document specifies the mandatory behavior which must be
   supported by implementations of PWE3 encapsulations.  As the Control
   Word is either already mandated by various encapsulations or is
   optional, this mandate does not introduce any security issues not
   already addressed by the encapsulation definitions, if any.  Further,
   the mandate of Control Word use may improve the security of related
   protocol behaviors, such as VCCV Control Word (e.g. no need for
   Router Alert Label support).

6.  Acknowledgements

7.  Normative References

   [CBIT]     Jin, L., Key, R., Delord, S., Nadeau, T., and V. Manral,
              "Pseudowire Control Word Negotiation Mechanism Analysis
              and Update", October 2010.

   [DEL]      Del Regno, N., Manral, V., Kunze, R., Paul, M., and T.
              Nadeau, "Mandatory Features of Virtual Circuit
              Connectivity Verification Implementations", October 2010.

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

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", February 2006.

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

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Authors' Addresses

   Nick Del Regno (editor)
   Verizon Communications


   Thomas Nadeau


   Vishwas Manral
   IP Infusion


   David Ward
   Juniper Networks


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