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IETF RFC 6423
Last modified on Tuesday, November 1st, 2011
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Internet Engineering Task Force (IETF) H. Li
Request for Comments: 6423 China Mobile
Updates: 5586 L. Martini
Category: Standards Track Cisco System
ISSN: 2070-1721 J. He
Huawei
F. Huang
Alcatel-Lucent
November 2011
Using the Generic Associated Channel Label for Pseudowire in
the MPLS Transport Profile (MPLS-TP)
 Abstract
This document describes the requirements for using the Generic
Associated Channel Label (GAL) in pseudowires (PWs) in MPLS Transport
Profile (MPLS-TP) networks, and provides an update to the description
of GAL usage in RFC 5586 by removing the restriction that is imposed
on using GAL for PWs, especially in MPLS-TP environments.
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/RFC 6423.
Li, et al. Standards Track PAGE 1 
RFC 6423 Using GAL in MPLS-TP PW November 2011
Copyright Notice
Copyright (c) 2011 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.
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
2.1. Terminology ................................................3
3. GAL Usage for MPLS-TP PW ........................................3
4. Security Considerations .........................................4
5. Acknowledgments .................................................4
6. References ......................................................5
6.1. Normative References .......................................5
6.2. Informative References .....................................5
1. Introduction
[RFC 5586] generalizes the Associated Channel mechanism of [RFC 5085]
to be used for Sections, Label Switched Paths (LSPs), and Pseudowires
(PWs) in MPLS networks. [RFC 5085] defines the Associated Channel
Header (ACH), and [RFC 5586] generalizes this for use as the Generic
Associated Channel (G-ACh).
[RFC 5586] defines a generalized label-based exception mechanism using
the Generic Associated Channel Label (GAL) to work together with the
ACH for use with LSPs but prohibits GAL usage with PWs.
This document removes the restriction imposed by [RFC 5586].
2. Conventions Used in This Document
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 [RFC 2119].
Li, et al. Standards Track PAGE 2
RFC 6423 Using GAL in MPLS-TP PW November 2011
2.1. Terminology
ACH Associated Channel Header
CW Control Word
G-ACh Generic Associated Channel
GAL Generic Associated Channel Label
MPLS-TP MPLS Transport Profile
OAM Operation, Administration, and Maintenance
3. GAL Usage for MPLS-TP PW
According to the MPLS-TP requirements document [RFC 5654], it is
necessary that MPLS-TP mechanisms and capabilities be able to
interoperate with the existing IETF MPLS [RFC 3031] and IETF PWE3
[RFC 3985] architectures as appropriate. [RFC 5586] differentiates
between the usage of the GAL with PWs in MPLS and MPLS-TP
environments in Section 4.2 as follows:
In MPLS-TP, the GAL MUST be used with packets on a G-ACh on LSPs,
Concatenated Segments of LSPs, and with Sections, and MUST NOT be
used with PWs.
This indicates that the GAL can be used for MPLS-TP LSPs and
Sections, but not for PWs in an MPLS-TP network.
However, there is no restriction imposed on the usage of the GAL in
MPLS PWs, which is described immediately afterwards in the same
section (Section 4.2) of [RFC 5586]:
However, in other MPLS environments, this document places no
restrictions on where the GAL may appear within the label stack or
its use with PWs.
The inconsistency between the usage of the GAL with MPLS PWs and
MPLS-TP PWs may cause unnecessary implementation differences and is
in disagreement with the MPLS-TP requirements.
Therefore, this document specifies that the GAL can be used with
packets on a G-ACh on LSPs, Concatenated Segments of LSPs, Sections,
and PWs in both MPLS and MPLS-TP environments without discrimination.
Li, et al. Standards Track PAGE 3
RFC 6423 Using GAL in MPLS-TP PW November 2011
[RFC 5586] is updated by removing the restrictions on using GAL for PW
as follows:
- Section 1 (Introduction) in [RFC 5586], the original text:
The GAL mechanism is defined to work together with the ACH for
LSPs and MPLS Sections.
is replaced by:
The GAL mechanism is defined to work together with the ACH for
LSPs and MPLS Sections, and for PWs.
- Section 4.2. (GAL Applicability and Usage) in [RFC 5586], the
original text:
In MPLS-TP, the GAL MUST be used with packets on a G-ACh on
LSPs, Concatenated Segments of LSPs, and with Sections, and
MUST NOT be used with PWs. It MUST always be at the bottom of
the label stack (i.e., S bit set to 1). However, in other MPLS
environments, this document places no restrictions on where the
GAL may appear within the label stack or its use with PWs.
is replaced by:
In MPLS-TP, the GAL MUST be used with packets on a G-ACh on
LSPs, Concatenated Segments of LSPs, and with Sections, and MAY
be used with PWs. The presence of a GAL indicates that an ACH
immediately follows the MPLS label stack.
4. Security Considerations
There are no further security considerations than those in [RFC 5586].
5. Acknowledgments
The authors would like to thank Luyuan Fang, Adrian Farrel, Haiyan
Zhang, Guanghui Sun, Italo Busi, and Matthew Bocci for their
contributions to this work.
The authors would also like to thank the authors of [RFC 5586] and
people who were involved in the development of [RFC 5586].
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RFC 6423 Using GAL in MPLS-TP PW November 2011
6. References
6.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997
[RFC 3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC 3985] Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation
Edge-to-Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC 5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
"MPLS Generic Associated Channel", RFC 5586, June 2009.
6.2. Informative References
[RFC 5085] Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire Virtual
Circuit Connectivity Verification (VCCV): A Control Channel
for Pseudowires", RFC 5085, December 2007.
[RFC 5654] Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
Sprecher, N.,and S. Ueno, "Requirements of an MPLS
Transport Profile", RFC 5654, September 2009.
Authors' Addresses
Han Li
China Mobile Communications Corporation
EMail: lihan@chinamobile.com
Luca Martini
Cisco Systems, Inc.
EMail: lmartini@cisco.com
Jia He
Huawei Technologies Co., Ltd.
EMail: hejia@huawei.com
Feng Huang
Alcatel-Lucent shanghai Bell
EMail: feng.f.huang@alcatel-sbell.com.cn
Li, et al. Standards Track PAGE 5
RFC TOTAL SIZE: 9343 bytes
PUBLICATION DATE: Tuesday, November 1st, 2011
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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