Mandatory Use of Control Word for PWE3 Encapsulations
draft-delregno-pwe3-mandatory-control-word-00

Abstract

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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [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
2.  Mandatory Control Word
3.  Backward Compatibility
4.  IANA Considerations
5.  Security Considerations
6.  Acknowledgements
7.  Normative References
§  Authors' Addresses

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] (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.) 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] (Jin, L., Key, R., Delord, S., Nadeau, T., and V. Manral, “Pseudowire Control Word Negotiation Mechanism Analysis and Update,” October 2010.), 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] (Del Regno, N., Manral, V., Kunze, R., Paul, M., and T. Nadeau, “Mandatory Features of Virtual Circuit Connectivity Verification Implementations,” October 2010.).

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

• Ethernet Tagged Mode
• Ethernet Raw Mode
• PPP
• HDLC
• Frame Relay Port Mode
• 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 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] (Del Regno, N., Manral, V., Kunze, R., Paul, M., and T. Nadeau, “Mandatory Features of Virtual Circuit Connectivity Verification Implementations,” October 2010.), 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] (Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G. Heron, “Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP),” April 2006.) 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] (Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G. Heron, “Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP),” April 2006.) 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 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 RFC.

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

Authors' Addresses