MPLS M. Bocci, Ed.
Internet-Draft M. Vigoureux, Ed.
Updates: 3032, 4385, 5085 Alcatel-Lucent
(if approved) G. Swallow
Intended status: Standards Track D. Ward
Expires: July 10, 2009 Cisco
R. Aggarwal
Juniper Networks
January 6, 2009
MPLS Generic Associated Channel
draft-ietf-mpls-tp-gach-gal-01
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Abstract
This document generalises the applicability of the pseudowire
Associated Channel Header (ACH), enabling the realization of a
control channel associated to MPLS Label Switched Paths (LSP), MPLS
pseudowires (PW) and MPLS Sections. In order to identify the
presence of the Generic ACH (G-ACH), this document also assigns of
one of the reserved MPLS label values to the 'Generic Associated
channel header Label (GAL)', to be used as a label based exception
mechanism.
Requirements Language
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 [1].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Contributing Authors . . . . . . . . . . . . . . . . . . . 5
1.2. Objectives . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Generic Associated Channel Header . . . . . . . . . . . . . . 6
2.1. Allocation of Channel Types . . . . . . . . . . . . . . . 7
3. Generalised Exception Mechanism . . . . . . . . . . . . . . . 7
3.1. Relationship with Existing MPLS OAM Alert Mechanisms . . . 8
3.2. GAL Applicability and Usage . . . . . . . . . . . . . . . 8
3.2.1. GAL Processing . . . . . . . . . . . . . . . . . . . . 8
3.3. Relationship wth RFC 3429 . . . . . . . . . . . . . . . . 11
4. Compatability . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Congestion Considerations . . . . . . . . . . . . . . . . . . 12
6. Security Consderations . . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
There is a need for Operations, Administration and Maintenance (OAM)
mechanisms that can be used for edge-to-edge (i.e. between
originating and terminating LSRs or T-PEs) and segment (e.g. between
any two LSRs or T-PEs/S-PEs along the path of a LSP or PW [15]) fault
detection, diagnostics, maintenance and other functions for a PW and
a LSP. Some of these functions can be supported using tools such as
VCCV [2], BFD [3], or LSP-Ping [4]. However, a requirement has been
indicated to augment the set of maintenance functions, in particular
where MPLS networks are used for packet transport services and
network operations [16]. Examples include performance monitoring,
automatic protection switching, and support for management and
signaling communication channels. These tools must be applicable to,
and function in essentially the same manner (from an operational
point of view) on both MPLS PWs and MPLS LSPs. They must also
operate in-band on the PW or LSP such that they do not depend on PSN
routing, user data traffic or ultimately on PSN or other dynamic
control plane functions.
Virtual Circuit Connectivity Verification (VCCV) can use an
associated channel to provide a control channel between a PW's
ingress and egress points and over which OAM and other control
messages can be exchanged. In this document, we propose a generic
associated channel header (G-ACH) to enable the same control channel
mechanism be used for MPLS Sections, LSPs and PWs. The associated
channel header (ACH) specified in RFC 4385 [5] is used with
additional code points to support additional MPLS maintenance
functions.
Generalizing the ACH mechanism to MPLS LSPs and MPLS Sections also
requires a method to identify that a packet contains a G-ACH followed
by a non-service payload. This document therefore also defines a
label based exception mechanism (the Generic Associated channel
header Label, or GAL) that serves to inform an LSR that a packet that
it receives on an LSP or section belongs to an associated channel.
RFC 4379 [4] and BFD for MPLS LSPs [3] have defined alert mechanisms
that enable a MPLS LSR to identify and process MPLS OAM packets when
the OAM packets are encapsulated in an IP header. These alert
mechanisms are based on TTL expiration and/or use an IP destination
address in the range 127/8. These mechanisms are the default
mechanisms for identifying MPLS OAM packets when the OAM packets are
encapsulated in an IP header. However it may not always be possible
to use these mechanisms in some MPLS applications, (e.g. MPLS-TP
[15]) particularly when IP based demultiplexing cannot be used. This
document proposes an OPTIONAL mechanism that is RECOMMENDED for
identifying and demultiplexing MPLS OAM and other maintenance
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messages when IP based mechanisms such as those in [4] and [3] are
not available.
The G-ACH and GAL mechanisms are defined to work together.
Note that, in this document, maintenace functions and packets should
be understood in the broad sense, that is, as a set of FCAPS
mechanisms that include OAM, Automatic Protection Switching (APS),
Signalling Communication Channel (SCC) and Management Communication
Channel (MCC) messages.
Note that the GAL and G-ACH are applicable to MPLS in general. Their
applicability to specific applications is outside the scope of this
document. For example, the applicability of the GAL and G-ACH to
MPLS-TP is described in [15] and [17].
1.1. Contributing Authors
The editors gratefully acknowledge the contibution of Stewart Bryant,
Italo Busi, Marc Lasserre, and Lieven Levrau.
1.2. Objectives
This document proposes a mechanism to provide for the extended
maintenance needs of emerging applications for MPLS. It creates a
generic control channel identification mechanism that may be applied
to all MPLS LSPs, while maintaining compatibility with the PW
associated channel header (ACH) . It also normalizes the use of the
ACH for PWs in a transport context.
1.3. Scope
This document defines the encapsulation header for LSP, MPLS Section
and PW associated channel messages.
It does not define how associated channel capabilities are signaled
or negotiated between LSRs or PEs, the operation of various OAM
functions, nor how the messages transmitted on the associated
channel.
This document does not deprecate existing MPLS and PW OAM mechanisms.
1.4. Terminology
G-ACH: Generic Associated Channel Header
GAL: Generic Associated Channel Header Label
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MPLS Section: A network segment between two LSRs that are immediately
adjacent at the MPLS layer
Maintenance Packet: Any packet containing a message belonging to a
maintenace protocol that is carried on a PW, LSP or MPLS Section
associated channel. Examples of such maintenance protocols include
OAM functions, signaling communications or management communications.
2. Generic Associated Channel Header
VCCV [2] defines three MPLS Control Channel (CC) Types that may be
used to multiplex OAM messages onto a PW: CC Type 1 uses an
associated channel header and is referred to as "In-band VCCV"; CC
Type 2 uses the MPLS Router Alert Label to indicate VCCV packets and
is referred to as "Out of Band VCCV"; CC Type 3 uses the TTL to force
the packet to be processed by the targeted router control plane and
is referred to as "MPLS PW Label with TTL == 1".
The use of the CC Type 1, currently limited to MPLS PWs, is here
extended to apply to MPLS LSPs as well as to MPLS Sections. This
associated channel header is called the Generic Associated Channel
Header (G- ACH). The PWE3 control word MUST be present in the
encapsulation of user packets when the G-ACH is used to demultiplex
the associated channel packet on a PW.
The CC Type 1 channel header is depicted in figure below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 1|Version|A| Reserved | Channel Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Generic Associated Channel Header
In the above figure, the first nibble is set to 0001b to indicate a
channel associated with a PW, a LSP or a Section. The Version field
is set to 0, as specified in RFC 4385 [5]. This draft allocates Bit
8 of the ACH to the ACH TLV bit. This bit is set to 1 to indicate
that an object defined in the ACH TLV registry immediately follows
the G-ACH, otherwise it is set to 0. Bits 8 to 14 of the G-ACH are
reserved and MUST be set to 0..
Note that VCCV also includes mechanisms for negotiating the Control
Channel and Connectivity Verification (i.e. OAM functions) Types
between PEs. It is anticipated that similar mechanisms will be
applied to existing MPLS LSPs. Such application will require further
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specification. However, such specification is beyond the scope of
this document.
2.1. Allocation of Channel Types
The Channel Type field indicates the type of message carried on the
associated channel e.g. IPv4 or IPv6 if IP demultiplexing is used
for messages on the G-ACH, or OAM or other FCAPS function if IP
demultiplexing is not used. For G-ACH packets where IP is not used
as the multiplexer, the Channel Type SHOULD indicate the specific
maintenance protocol carried in the associated channel.
Values for the Channel Type field currently used for VCCV are
specified in RFC 4446 [6]. The functionality of any additional
channel types will be defined in another document. Each associated
channel protocol solution document must specify the value to use for
any additional channel types.
Note that these values are allocated from the PW Associated Channel
Type registry, but this document modifies the existing policy to
accomodate a level of experimentation. See Section 7 for further
details.
3. Generalised Exception Mechanism
The above mechanism enables the multiplexing of various maintenace
packets onto a PW, LSP or Section and provides information on the
type of function being performed. In the case of a PW, the use of a
control word is negotiated or configured at the time of the PW
establishment. A special case of the control word (the G-ACH) is
used to identify packets belonging to a PW associated channel.
Generalizing the ACH mechanism to MPLS LSPs and MPLS Sections also
requires a method to identify that a packet contains a G-ACH followed
by a non-service payload. This document specifies that a label be
used and calls this special label the 'Generic Associated channel
header Label (GAL)'. One of the reserved label values defined in RFC
3032 [7] is assigned for this purpose. The value of the label is to
be allocated by IANA; this document suggests the value 13.
The GAL provides a generalised exception mechanism to:
o Differentiate specific packets (e.g. those containing OAM
messages) from others, such as normal user-plane ones,
o Indicate that the Generic Associated Channel Header (G-ACH)
appears immediately after the bottom of the label stack.
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The 'Generic Associated channel header Label (GAL)' MUST only be used
where both of these purposes are applicable.
3.1. Relationship with Existing MPLS OAM Alert Mechanisms
RFC 4379 [4] and BFD for MPLS LSPs [3] have defined alert mechanisms
that enable a MPLS LSR to identify and process MPLS OAM packets when
the OAM packets are encapsulated in an IP header. These alert
mechanisms are based on TTL expiration and/or use an IP destination
address in the range 127/8.
These alert mechanisms SHOULD preferably be used in non MPLS-TP
environments. The mechanism defined in this document MAY also be
used.
3.2. GAL Applicability and Usage
The 'Generic Associated channel header Label (GAL)' MUST only be used
with Label Switched Paths (LSPs), with their associated Tandem
Connection Monitoring Entities (see [17] for definitions of TCMEs)
and with MPLS Sections.
The GAL applies to both P2P and P2MP LSPs, unless otherwise stated.
In MPLS-TP, the GAL 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.
The GAL MUST NOT appear in the label stack when transporting normal
user-plane packets. Furthermore, the GAL MUST only appear once in
the label stack for packets on the generic associated channel.
3.2.1. GAL Processing
The Traffic Class (TC) field (formerly known as the EXP field) of the
label stack entry containing the GAL follows the definition and
processing rules specified and referenced in [8].
The Time-To-Live (TTL) field of the label stack entry that contains
the GAL follows the definition and processing rules specified in [9].
3.2.1.1. MPLS Section
The following figure (Figure 2) depicts two MPLS LSRs immediately
adjacent at the MPLS layer.
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+---+ +---+
| A |-------------| Z |
+---+ +---+
Figure 2: Maintenance over an MPLS Section Associated Channel
With regards to the MPLS Section, both LERs are Maintenance End
Points (see [17] for definitions of MEPs).
The following figure (Figure 3) depicts the format of a labelled OAM
packet on an associated channel when used for MPLS Section
maintenance.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GAL | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Generic-ACH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
. Maintenance Message .
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Maintenance Packet Format for MPLS Section
To send a MPLS-TP maintenance packet on an associated channel of the
MPLS Section, the head-end LSR (A) of the MPLS Section generates a
maintenance packet with a G-ACH to which it pushes a GAL.
o The TTL field of the GAL SHOULD be set to 1.
o The S bit of the GAL MUST be set to 1 in MPLS-TP.
The maintenance packet, the G-ACH and the GAL SHOULD NOT be modified
towards the tail-end LSR (Z). Upon reception of the labelled packet,
the tail-end LSR (Z), after having checked the GAL fields, SHOULD
pass the whole packet to the appropriate processing entity.
3.2.1.2. Label Switched Paths
The following figure (Figure 4) depicts four LSRs. A LSP is
established from A to D and switched in B and C.
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+---+ +---+ +---+ +---+
| A |-------------| B |-------------| C |-------------| D |
+---+ +---+ +---+ +---+
Figure 4: Maintenance over an LSP Associated Channel
LERs A and D are Maintenance End Points (MEPs) with respect to this
LSP. Furthermore, LSRs B and C could also be Maintenance
Intermediate Points (MIPs) with respect to this LSP (see [17] for
definitions of MEPs and MIPs).
The following figure (Figure 5) depicts the format of a labelled
maintenance packet when used for a MPLS-TP LSP.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSP Label | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| GAL | TC |S| TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Generic-ACH |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .
. Maintenance Message .
. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Maintenance Packet Format for MPLS-TP LSP
Note that it is possible that the LSP MAY also be tunnelled in
another LSP (e.g. if a MPLS Tunnel exists between B and C), and as
such other labels MAY be present above it in the label stack.
To send a maintenance packet on the LSP associated channel, the head-
end LSR (A) generates a OAM message with a G-ACH on which it first
pushes a GAL followed by the LSP label.
o The TTL field of the GAL SHOULD be set to 1.
o The S bit of the GAL SHOULD be set to 1 in MPLS-TP.
The maintenance message, the G-ACH or the GAL SHOULD NOT be modified
towards the targeted destination. Upon reception of the labelled
packet, the targeted destination, after having checked both the LSP
label and GAL fields, SHOULD pass the whole packet to the appropriate
processing entity.
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3.2.1.3. Tandem Connection Monitoring Entity
Tandem Connection Monitoring will be specified in a separate
document.
3.3. Relationship wth RFC 3429
RFC 3429 [18] describes the assignment of one of the reserved label
values, defined in RFC 3032 [7], to the 'OAM Alert Label' that is
used by user-plane MPLS OAM functions for the identification of MPLS
OAM packets. The value of 14 is used for that purpose.
Both this document and RFC 3429 [18] therefore describe the
assignment of reserved label values for similar purposes. The
rationale for the assignment of a new reserved label can be
summarized as follows:
o Unlike the mechanisms described and referenced in RFC 3429 [18],
MPLS-TP OAM packet payloads will not reside immediately after the
GAL but instead behind the G-ACH, which itself resides immediately
after the bottom of the label stack when the GAL is present. This
ensures that OAM using the generic associated channel complies
with RFC 4928 [10].
o The set of maintenance functions potentially operated in the
context of the generic associated channel is wider than the set of
OAM functions referenced in RFC 3429 [18].
o It has been reported that there are existing implementations and
running deployments using the 'OAM Alert Label' as described in
RFC 3429 [18]. It is therefore not possible to modify the 'OAM
Alert Label' allocation, purpose or usage. Nevertheless, it is
RECOMMENDED by this document that no further OAM extensions based
on 'OAM Alert Label' (Label 14) usage be specified or developed.
4. Compatability
An LER, LSR or PE MUST discard received G-ACH packets if it is not G-
ACH capable, if it is not capable of processing packets on the
indicated G-ACH channel, or if it has not, through means outside the
scope of this document, indicated to the sending LSR, LER or PE that
it will process G-ACH packets received on the indicated channel. The
LER, LSR or PE MAY increment an error counter and MAY also optionally
issue a system and/or SNMP notification.
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5. Congestion Considerations
The congestion considerations detailed in RFC 5085 [2] apply.
Further generic associated channel-specific congestion considerations
will be detailed in a future revision of this document.
6. Security Consderations
The security considerations detailed in RFC 5085 [2], the MPLS
architecture [11], the PWE3 architecture [12] and the MPLS-TP
framework [15] apply.
7. IANA Considerations
This document requests that IANA allocates a Label value, to the
'Generic Associated channel header Label (GAL)', from the pool of
reserved labels, and suggests this value to be 13.
Channel Types for the Generic Associated Channel are allocated from
the IANA PW Associated Channel Type registry [6]. The PW Associated
Channel Type registry is currently allocated based on the IETF
consensus process, described in[13]. This allocation process was
chosen based on the consensus reached in the PWE3 working group that
pseudowire associated channel mechanisms should be reviewed by the
IETF and only those that are consistent with the PWE3 architecture
and requirements should be allocated a code point.
However, a requirement has emerged (see [16]) to allow for
optimizations or extensions to OAM and other control protocols
running in an associated channel to be experimented with without
resorting to the IETF standards process, by supporting experimental
code points. This would prevent code points used for such functions
from being used from the range allocated through the IETF standards
and thus protects an installed base of equipment from potential
inadvertent overloading of code points. In order to support this
requirement, this document requests that the code-point allocation
scheme for the PW Associated Channel Type be changed as follows:
0 - 32751 : IETF Consensus
32752 - 32767 : Experimental
Code points in the experimental range MUST be used according to the
guidelines of RFC 3692 [14]. Experimental OAM functions MUST be
disabled by default. The channel type value used for a given
experimental OAM function MUST be configurable, and care MUST be
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taken to ensure that different OAM functions that are not
interoperable are configured to use different channel type values.
8. Acknowledgements
The authors would like to thank all members of the teams (the Joint
Working Team, the MPLS Interoperability Design Team in IETF and the
T-MPLS Ad Hoc Group in ITU-T) involved in the definition and
specification of MPLS Transport Profile.
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit
Connectivity Verification (VCCV): A Control Channel for
Pseudowires", RFC 5085, December 2007.
[3] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, "BFD
For MPLS LSPs", draft-ietf-bfd-mpls-07 (work in progress),
June 2008.
[4] Kompella, K. and G. Swallow, "Detecting Multi-Protocol Label
Switched (MPLS) Data Plane Failures", RFC 4379, February 2006.
[5] 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.
[6] Martini, L., "IANA Allocations for Pseudowire Edge to Edge
Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
[7] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci,
D., Li, T., and A. Conta, "MPLS Label Stack Encoding",
RFC 3032, January 2001.
[8] Andersson, L. and R. Asati, "Multi-Protocol Label Switching
(MPLS) label stack entry: "EXP" field renamed to "Traffic
Class" field", draft-ietf-mpls-cosfield-def-08 (work in
progress), December 2008.
[9] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing in
Multi-Protocol Label Switching (MPLS) Networks", RFC 3443,
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January 2003.
[10] Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal Cost
Multipath Treatment in MPLS Networks", BCP 128, RFC 4928,
June 2007.
[11] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label
Switching Architecture", RFC 3031, January 2001.
[12] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-Edge
(PWE3) Architecture", RFC 3985, March 2005.
[13] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[14] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, January 2004.
9.2. Informative References
[15] Bocci, M., Bryant, S., and L. Levrau, "A Framework for MPLS in
Transport Networks", draft-ietf-mpls-tp-framework-00 (work in
progress), November 2008.
[16] Vigoureux, M., Ward, D., and M. Betts, "Requirements for OAM in
MPLS Transport Networks",
draft-ietf-mpls-tp-oam-requirements-00 (work in progress),
December 2008.
[17] Busi, I. and B. Niven-Jenkins, "MPLS-TP OAM Framework and
Overview", draft-busi-mpls-tp-oam-framework-00 (work in
progress), October 2008.
[18] Ohta, H., "Assignment of the 'OAM Alert Label' for
Multiprotocol Label Switching Architecture (MPLS) Operation and
Maintenance (OAM) Functions", RFC 3429, November 2002.
Authors' Addresses
Matthew Bocci (editor)
Alcatel-Lucent
Email: matthew.bocci@alcatel-lucent.com
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Martin Vigoureux (editor)
Alcatel-Lucent
Email: martin.vigoureux@alcatel-lucent.com
George Swallow
Cisco
Email: swallow@cisco.com
David Ward
Cisco
Email: dward@cisco.com
Rahul Aggarwal
Juniper Networks
Email: rahul@juniper.net
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