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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
 Provisions Relating to IETF Documents
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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

Farrel & Bryant Standards Track [Page 5]