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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  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: [email protected]

  Stewart Bryant
  Cisco Systems
  EMail: [email protected]























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