Retiring TLVs from the Associated Channel Header of the MPLS Generic Associated Channel
RFC 7026

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
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   publication of this document.  Please review these documents
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   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: adrian@olddog.co.uk

   Stewart Bryant
   Cisco Systems
   EMail: stbryant@cisco.com

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