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

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7026.
Authors Adrian Farrel , Stewart Bryant
Last updated 2018-12-20 (Latest revision 2013-07-31)
Replaces draft-farbryantrel-mpls-retire-ach-tlv
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Loa Andersson
Shepherd write-up Show Last changed 2013-07-11
IESG IESG state Became RFC 7026 (Proposed Standard)
Action Holders
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Spencer Dawkins
Send notices to (None)
IANA IANA review state IANA OK - Actions Needed
IANA action state RFC-Ed-Ack
Network Working Group                                          A. Farrel
Internet Draft                                          Juniper Networks
Category: Standards Track                                      S. Bryant
Updates: 5586 (if approved)                                Cisco Systems
Expires: January 31, 2014                                  July 31, 2013

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



   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

   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

   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
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

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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
   ( 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

   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
   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",
   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 requests IANA to make two changes to the IANA

4.1. Associated Channel Header TLV Registry

   The "Pseudowire Name Spaces (PWE3)" registry has a sub-registry
   called the "Associated Channel Header TLV Registry".  IANA is
   requested to entirely delete this sub-registry, but to leave a
   tombstone record in the top-level list of registries that says:

     Associated Channel Header TLV Registry RFC 5586 RFC xxxx

   Where "xxxx" is the number of this RFC.

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4.2. Pseudowire Associated Channel Types Registry

   The "Pseudowire Name Spaces (PWE3)" registry has a sub-registry
   called the "Pseudowire Associated Channel Types Registry".  This
   sub-registry includes a column marked "TLV Follows".  IANA is
   requested to entirely delete this column leaving no record.

   IANA is requested to add a pointer to this RFC in the definition of
   this sub-registry.

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 sugested 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 responsiblity of the specification that defines G-ACh messages to
   consider the security requirements of those messages which 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., Vigoureux, M., and S. Bryant, "MPLS Generic
               Associated Channel", RFC 5586, June 2009.

8.2.  Informative References

   [IANA-ACH] "Pseudowire Associated Channel Types", IANA,

Authors' Addresses

   Adrian Farrel
   Juniper Networks

   Stewart Bryant
   Cisco Systems

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