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.

Farrel & Bryant              Standards Track                    [Page 1]
RFC 7026                    Retiring ACH TLVs             September 2013

Copyright Notice

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   document authors.  All rights reserved.

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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.

Farrel & Bryant              Standards Track                    [Page 2]
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