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
AbstractThe 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.
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 (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. 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.
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. RFC2119]. 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. RFC5586] [RFC7026]
[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. [IANA-ACH] "Pseudowire Associated Channel Types", IANA, <http://www.iana.org/assignments/pwe3-parameters>