Retiring TLVs from the Associated Channel Header of the MPLS Generic Associated Channel
draft-ietf-mpls-retire-ach-tlv-02

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Network Working Group                                          A. Farrel
Internet Draft                                          Juniper Networks
Category: Standards Track                                      S. Bryant
Updates: 5586 (if approved)                                Cisco Systems
Expires: December 28, 2013                                 June 28, 2013

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

                draft-ietf-mpls-retire-ach-tlv-02.txt

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 Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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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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   described in the Simplified BSD License.

1.  Introduction and Scope

   RFC4385 [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, known as the PWACH or more generally 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 live Internet-Drafts that utilize ACH TLVs.

   Furthermore, G-ACh packets are intended to be substantially processed
   in hardware, however, processing TLVs in hardware can be hard because

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   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