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

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Document Type Active Internet-Draft (individual)
Authors Adrian Farrel  , Stewart Bryant 
Last updated 2013-05-07
Replaced by RFC 7026, RFC 7026
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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: November 7, 2013                                    May 7, 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 Type-Length-Variable (TLV)
   constructs that can be carried in messages on the G-ACh by placing
   them in the ACH.

   No Associated Channel Type yet defined uses a 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 TLVs in

   This document updates RFC 5586 by retiring ACH TLVs and removing the
   associated registry.

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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1.  Introduction and Scope

   RFC 4385 [RFC4385] defines that is the first nibble of a packet
   carried over an MPLS packet switched network has a value of 1, then
   the packet starts with a specific header format call 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 Type-
   Length-Variable (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, of the 18 ACH Channel Types currently defined none allows
   the use of ACH TLVs [IANA-ACH].  There are no currently 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 determines that ACH TLVs 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

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.

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 registry leaving no record.

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
   registry includes a column marked "TLV Follows".  IANA is
   requested to entirely delete 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

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   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.

   Otherwise, this document has no implications for security.

7.  References

7.1.  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.

7.2.  Informative References

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

Authors' Addresses

   Adrian Farrel
   Juniper Networks

   Stewart Bryant
   Cisco Systems

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