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
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
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
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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
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.
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)
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
Otherwise, this document has no implications for security.
Thanks to Eric Osborne, Thomas Morin, Lizhong Jin, Greg Mirsky, Jia
He, and Pearl Liang for suggestions to improve the text.