    Internet Engineering Task Force (IETF)                         A. Farrel
    Request for Comments: 7026                              Juniper Networks
    Updates: 5586                                                  S. Bryant
    Category: Standards Track                                  Cisco Systems
    ISSN: 2070-1721                                           September 2013

                Retiring TLVs from the Associated Channel Header
                     of the MPLS Generic Associated Channel

    Abstract

       The MPLS Generic Associated Channel (G-ACh) is a generalization of
       the applicability of the pseudowire (PW) Associated Channel Header
       (ACH).  RFC 5586 defines the concept of TLV constructs that can be
       carried in messages on the G-ACh by placing them in the ACH between
       the fixed header fields and the G-ACh message.  These TLVs are called
       ACH TLVs

       No Associated Channel Type yet defined uses an ACH TLV.  Furthermore,
       it is believed that handling TLVs in hardware introduces significant
       problems to the fast path, and since G-ACh messages are intended to
       be processed substantially in hardware, the use of ACH TLVs is
       undesirable.

       This document updates RFC 5586 by retiring ACH TLVs and removing the
       associated registry.

    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/rfc7026.

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    Copyright Notice

       Copyright (c) 2013 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.

    1.  Introduction and Scope

       RFC 4385 [RFC4385] says that if the first nibble of a PW packet
       carried over an MPLS network has a value of 1, then the packet starts
       with a specific header format called the Pseudowire Associated
       Channel Header (PWACH) or more generally known as the ACH.  This
       mechanism creates an Associated Channel that is a message channel
       associated with a specific pseudowire (PW).

       The applicability of the ACH is generalized in RFC 5586 [RFC5586] to
       define the MPLS Generic Associated Channel (G-ACh).  This creates a
       common encapsulation header for control channel messages associated
       with MPLS Sections, Label Switching Paths (LSPs), and PWs.

       As part of making the ACH fully generic, RFC 5586 defines ACH TLV
       constructs.  According to RFC 5586:

         In some applications of the generalized associated control channel,
         it is necessary to include one or more ACH TLVs to provide
         additional context information to the G-ACh packet.

       RFC 5586 goes on to say:

         If the G-ACh message MAY be preceded by one or more ACH TLVs, then
         this MUST be explicitly specified in the definition of an ACH
         Channel Type.

       However, at the time of writing, of the 18 ACH Channel Types defined,
       none allows the use of ACH TLVs [IANA-ACH].  At the time of writing,
       there are no unexpired Internet-Drafts that utilize ACH TLVs.

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    RFC 7026                    Retiring ACH TLVs             September 2013

       Furthermore, G-ACh packets are intended to be substantially processed
       in hardware; however, processing TLVs in hardware can be difficult
       because of the unpredictable formats and lengths that they introduce
       to the normal ACH format.

       This document states that ACH TLVs, as specified in RFC 5586, are not
       useful and might be harmful.  It updates RFC 5586 by deprecating the
       ACH TLV and updating the associated IANA registries as described in
       Section 4 of this document.  This document makes no comment about the
       use of TLVs in other places.  In particular, proposals to use TLVs
       within ACH messages or as an appendage to ACH messages, are not in
       scope of this document.

    1.1.  Specification of Requirements

       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].

    2.  Update to RFC 5586

       Section 3 of RFC 5586 is deleted.

       References to ACH TLVs in Section 4 of RFC 5586 should also be
       disregarded.  Note that the text in Section 4 currently uses phrases
       like "ACH TLV(s), if present" so, with the removal of Section 3 that
       used to define ACH TLVs, they will not be present.

    3.  Implication for the ACH

       A G-ACh message MUST NOT be preceded by an ACH TLV.

    4.  IANA Considerations

       This document details two changes to the IANA registries.

    4.1.  Associated Channel Header TLV Registry

       The "Pseudowire Name Spaces (PWE3)" registry has a subregistry called
       the "Associated Channel Header TLV Registry".  IANA has entirely
       deleted this subregistry but has left a tombstone record in the top-
       level list of registries that says:

         Associated Channel Header TLV Registry (DELETED)

         Reference
         [RFC5586] [RFC7026]

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    4.2.  Pseudowire Associated Channel Types Registry

         The "Pseudowire Name Spaces (PWE3)" registry has a subregistry
         called the "Pseudowire Associated Channel Types" registry.  This
         subregistry previously included a column marked "TLV Follows".
         IANA has entirely deleted this column leaving no record.

    5.  Manageability Considerations

         This document will have no impact on network or device
         manageability because there are no ACH Types that allow the use of
         TLVs.  The document removes a feature that might have been used to
         enhance management messages, and especially Operations, Management,
         and Administration (OAM) messages.  However, given the considerable
         experience in defining MPLS OAM messages in the last few years, it
         would appear that this feature is not useful.

         It is possible that packet sniffers that have already been
         implemented will look for ACH TLVs.  The deletion of the construct
         will not have a negative impact.

    6.  Security Considerations

         Deleting the ACH TLV has a marginal positive effect on security
         because it removes a feature that might have been used as an attack
         vector to carry false information or to bloat G-ACh messages.

         On the other hand, it had been suggested that the ACH TLV could
         have been used to carry security parameters to secure the messages
         on the G-ACh in a generic way.  However, no mechanisms have been
         proposed at the time of writing, and it has generally been
         considered that it is the responsibility of the specification that
         defines G-ACh messages to consider the security requirements of
         those messages that may be different for the different
         applications.

         Otherwise, this document has no implications for security.

    7.  Acknowledgements

         Thanks to Eric Osborne, Thomas Morin, Lizhong Jin, Greg Mirsky, Jia
         He, and Pearl Liang for suggestions to improve the text.

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    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.

       [RFC4385]   Bryant, S., Swallow, G., Martini, L., and D. McPherson,
                  "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
                  Use over an MPLS PSN", RFC 4385, February 2006.

       [RFC5586]   Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
                  "MPLS Generic Associated Channel", RFC 5586, June 2009.

    8.2.  Informative References

       [IANA-ACH] "Pseudowire Associated Channel Types", IANA,
                  <http://www.iana.org/assignments/pwe3-parameters>

    Authors' Addresses

       Adrian Farrel
       Juniper Networks
       EMail: adrian@olddog.co.uk

       Stewart Bryant
       Cisco Systems
       EMail: stbryant@cisco.com

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