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Internet Engineering Task Force (IETF)                         A. FarrelRequest for Comments: 7026                              Juniper NetworksUpdates:5586                                                  S. BryantCategory: Standards Track                                  Cisco SystemsISSN: 2070-1721                                           September 2013Retiring TLVs from the Associated Channel Headerof the MPLS Generic Associated ChannelAbstract   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 updatesRFC 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 inSection 2 of   RFC 5741.   Information about the current status of this document, any   errata, and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7026.Farrel & Bryant              Standards Track                    [Page 1]

RFC 7026                    Retiring ACH TLVs             September 2013Copyright Notice   Copyright (c) 2013 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 ScopeRFC 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 inRFC 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 toRFC 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 inRFC 5586, are not   useful and might be harmful.  It updatesRFC 5586 by deprecating the   ACH TLV and updating the associated IANA registries as described inSection 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 toRFC 5586Section 3 of RFC 5586 is deleted.   References to ACH TLVs inSection 4 of RFC 5586 should also be   disregarded.  Note that the text inSection 4 currently uses phrases   like "ACH TLV(s), if present" so, with the removal ofSection 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]Farrel & Bryant              Standards Track                    [Page 3]

RFC 7026                    Retiring ACH TLVs             September 20134.2.  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.Farrel & Bryant              Standards Track                    [Page 4]

RFC 7026                    Retiring ACH TLVs             September 20138.  References8.1.  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.comFarrel & Bryant              Standards Track                    [Page 5]