Internet DRAFT - draft-ietf-mpls-retire-ach-tlv
draft-ietf-mpls-retire-ach-tlv
Network Working Group A. Farrel
Internet Draft Juniper Networks
Category: Standards Track S. Bryant
Updates: 5586 (if approved) Cisco Systems
Expires: January 31, 2014 July 31, 2013
Retiring TLVs from the Associated Channel Header of the MPLS Generic
Associated Channel
draft-ietf-mpls-retire-ach-tlv-03.txt
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 Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright Notice
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This document is subject to BCP 78 and the IETF Trust's Legal
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1. Introduction and Scope
RFC4385 [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, 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 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 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 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 requests IANA to make two changes to the IANA
registries.
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 sub-registry, but to leave a
tombstone record in the top-level list of registries that says:
Associated Channel Header TLV Registry RFC 5586 RFC xxxx
Where "xxxx" is the number of this RFC.
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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
sub-registry includes a column marked "TLV Follows". IANA is
requested to entirely delete this column leaving no record.
IANA is requested to add a pointer to this RFC in the definition of
this sub-registry.
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 sugested 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 responsiblity of the specification that defines G-ACh messages to
consider the security requirements of those messages which 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., Vigoureux, M., and S. Bryant, "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/pwe3-
parameters.xml#pwe3-parameters-10
Authors' Addresses
Adrian Farrel
Juniper Networks
EMail: adrian@olddog.co.uk
Stewart Bryant
Cisco Systems
EMail: stbryant@cisco.com
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