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Internet Engineering Task Force (IETF) M. Bocci, Ed.
Request for Comments: 5921 Alcatel-Lucent
Category: Informational S. Bryant, Ed.
ISSN: 2070-1721 D. Frost, Ed.
Cisco Systems
L. Levrau
Alcatel-Lucent
L. Berger
LabN
July 2010
A Framework for MPLS in Transport Networks
Abstract
This document specifies an architectural framework for the
application of Multiprotocol Label Switching (MPLS) to the
construction of packet-switched transport networks. It describes a
common set of protocol functions -- the MPLS Transport Profile (MPLS-
TP) -- that supports the operational models and capabilities typical
of such networks, including signaled or explicitly provisioned
bidirectional connection-oriented paths, protection and restoration
mechanisms, comprehensive Operations, Administration, and Maintenance
(OAM) functions, and network operation in the absence of a dynamic
control plane or IP forwarding support. Some of these functions are
defined in existing MPLS specifications, while others require
extensions to existing specifications to meet the requirements of the
MPLS-TP.
This document defines the subset of the MPLS-TP applicable in general
and to point-to-point transport paths. The remaining subset,
applicable specifically to point-to-multipoint transport paths, is
outside the scope of this document.
This document is a product of a joint Internet Engineering Task Force
(IETF) / International Telecommunication Union Telecommunication
Standardization Sector (ITU-T) effort to include an MPLS Transport
Profile within the IETF MPLS and Pseudowire Emulation Edge-to-Edge
(PWE3) architectures to support the capabilities and functionalities
of a packet transport network as defined by the ITU-T.
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RFC 5921 MPLS Transport Profile Framework July 2010
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see 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/rfc5921.
Copyright Notice
Copyright (c) 2010 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.
Bocci, et al. Informational [Page 2]
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation and Background . . . . . . . . . . . . . . . . 4
1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
1.3.1. Transport Network . . . . . . . . . . . . . . . . . . 7
1.3.2. MPLS Transport Profile . . . . . . . . . . . . . . . . 7
1.3.3. MPLS-TP Section . . . . . . . . . . . . . . . . . . . 7
1.3.4. MPLS-TP Label Switched Path . . . . . . . . . . . . . 7
1.3.5. MPLS-TP Label Switching Router . . . . . . . . . . . . 8
1.3.6. Customer Edge (CE) . . . . . . . . . . . . . . . . . . 10
1.3.7. Transport LSP . . . . . . . . . . . . . . . . . . . . 10
1.3.8. Service LSP . . . . . . . . . . . . . . . . . . . . . 10
1.3.9. Layer Network . . . . . . . . . . . . . . . . . . . . 10
1.3.10. Network Layer . . . . . . . . . . . . . . . . . . . . 10
1.3.11. Service Interface . . . . . . . . . . . . . . . . . . 10
1.3.12. Native Service . . . . . . . . . . . . . . . . . . . . 11
1.3.13. Additional Definitions and Terminology . . . . . . . . 11
2. MPLS Transport Profile Requirements . . . . . . . . . . . . . 11
3. MPLS Transport Profile Overview . . . . . . . . . . . . . . . 12
3.1. Packet Transport Services . . . . . . . . . . . . . . . . 12
3.2. Scope of the MPLS Transport Profile . . . . . . . . . . . 13
3.3. Architecture . . . . . . . . . . . . . . . . . . . . . . . 14
3.3.1. MPLS-TP Native Service Adaptation Functions . . . . . 14
3.3.2. MPLS-TP Forwarding Functions . . . . . . . . . . . . . 15
3.4. MPLS-TP Native Service Adaptation . . . . . . . . . . . . 16
3.4.1. MPLS-TP Client/Server Layer Relationship . . . . . . . 16
3.4.2. MPLS-TP Transport Layers . . . . . . . . . . . . . . . 17
3.4.3. MPLS-TP Transport Service Interfaces . . . . . . . . . 18
3.4.4. Pseudowire Adaptation . . . . . . . . . . . . . . . . 25
3.4.5. Network Layer Adaptation . . . . . . . . . . . . . . . 28
3.5. Identifiers . . . . . . . . . . . . . . . . . . . . . . . 33
3.6. Generic Associated Channel (G-ACh) . . . . . . . . . . . . 33
3.7. Operations, Administration, and Maintenance (OAM) . . . . 36
3.8. Return Path . . . . . . . . . . . . . . . . . . . . . . . 38
3.8.1. Return Path Types . . . . . . . . . . . . . . . . . . 39
3.8.2. Point-to-Point Unidirectional LSPs . . . . . . . . . . 39
3.8.3. Point-to-Point Associated Bidirectional LSPs . . . . . 40
3.8.4. Point-to-Point Co-Routed Bidirectional LSPs . . . . . 40
3.9. Control Plane . . . . . . . . . . . . . . . . . . . . . . 40
3.10. Inter-Domain Connectivity . . . . . . . . . . . . . . . . 43
3.11. Static Operation of LSPs and PWs . . . . . . . . . . . . . 43
3.12. Survivability . . . . . . . . . . . . . . . . . . . . . . 44
3.13. Sub-Path Maintenance . . . . . . . . . . . . . . . . . . . 45
3.14. Network Management . . . . . . . . . . . . . . . . . . . . 47
4. Security Considerations . . . . . . . . . . . . . . . . . . . 48
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49
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6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 50
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 50
7.1. Normative References . . . . . . . . . . . . . . . . . . . 50
7.2. Informative References . . . . . . . . . . . . . . . . . . 51
1. Introduction
1.1. Motivation and Background
This document describes an architectural framework for the
application of MPLS to the construction of packet-switched transport
networks. It specifies the common set of protocol functions that
meet the requirements in [RFC5654], and that together constitute the
MPLS Transport Profile (MPLS-TP) for point-to-point transport paths.
The remaining MPLS-TP functions, applicable specifically to point-to-
multipoint transport paths, are outside the scope of this document.
Historically, the optical transport infrastructure -- Synchronous
Optical Network/Synchronous Digital Hierarchy (SONET/SDH) and Optical
Transport Network (OTN) -- has provided carriers with a high
benchmark for reliability and operational simplicity. To achieve
this, transport technologies have been designed with specific
characteristics:
o Strictly connection-oriented connectivity, which may be long-lived
and may be provisioned manually, for example, by network
management systems or direct node configuration using a command
line interface.
o A high level of availability.
o Quality of service.
o Extensive Operations, Administration, and Maintenance (OAM)
capabilities.
Carriers wish to evolve such transport networks to take advantage of
the flexibility and cost benefits of packet switching technology and
to support packet-based services more efficiently. While MPLS is a
maturing packet technology that already plays an important role in
transport networks and services, not all MPLS capabilities and
mechanisms are needed in, or consistent with, the transport network
operational model. There are also transport technology
characteristics that are not currently reflected in MPLS.
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There are thus two objectives for MPLS-TP:
1. To enable MPLS to be deployed in a transport network and operated
in a similar manner to existing transport technologies.
2. To enable MPLS to support packet transport services with a
similar degree of predictability to that found in existing
transport networks.
In order to achieve these objectives, there is a need to define a
common set of MPLS protocol functions -- an MPLS Transport Profile --
for the use of MPLS in transport networks and applications. Some of
the necessary functions are provided by existing MPLS specifications,
while others require additions to the MPLS tool-set. Such additions
should, wherever possible, be applicable to MPLS networks in general
as well as those that conform strictly to the transport network
model.
This document is a product of a joint Internet Engineering Task Force
(IETF) / International Telecommunication Union Telecommunication
Standardization Sector (ITU-T) effort to include an MPLS Transport
Profile within the IETF MPLS and PWE3 architectures to support the
capabilities and functionalities of a packet transport network as
defined by the ITU-T.
1.2. Scope
This document describes an architectural framework for the
application of MPLS to the construction of packet-switched transport
networks. It specifies the common set of protocol functions that
meet the requirements in [RFC5654], and that together constitute the
MPLS Transport Profile (MPLS-TP) for point-to-point MPLS-TP transport
paths. The remaining MPLS-TP functions, applicable specifically to
point-to-multipoint transport paths, are outside the scope of this
document.
1.3. Terminology
Term Definition
---------- ----------------------------------------------------------
AC Attachment Circuit
ACH Associated Channel Header
Adaptation The mapping of client information into a format suitable
for transport by the server layer
APS Automatic Protection Switching
ATM Asynchronous Transfer Mode
BFD Bidirectional Forwarding Detection
CE Customer Edge
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CL-PS Connectionless - Packet Switched
CM Configuration Management
CO-CS Connection Oriented - Circuit Switched
CO-PS Connection Oriented - Packet Switched
DCN Data Communication Network
EMF Equipment Management Function
FCAPS Fault, Configuration, Accounting, Performance, and
Security
FM Fault Management
G-ACh Generic Associated Channel
GAL G-ACh Label
LER Label Edge Router
LSP Label Switched Path
LSR Label Switching Router
MAC Media Access Control
MCC Management Communication Channel
ME Maintenance Entity
MEG Maintenance Entity Group
MEP Maintenance Entity Group End Point
MIP Maintenance Entity Group Intermediate Point
MPLS Multiprotocol Label Switching
MPLS-TP MPLS Transport Profile
MPLS-TP P MPLS-TP Provider LSR
MPLS-TP PE MPLS-TP Provider Edge LSR
MS-PW Multi-Segment Pseudowire
Native The traffic belonging to the client of the MPLS-TP network
Service
OAM Operations, Administration, and Maintenance (see
[OAM-DEF])
OSI Open Systems Interconnection
OTN Optical Transport Network
PDU Protocol Data Unit
PM Performance Monitoring
PSN Packet Switching Network
PW Pseudowire
SCC Signaling Communication Channel
SDH Synchronous Digital Hierarchy
S-PE PW Switching Provider Edge
SPME Sub-Path Maintenance Element
SS-PW Single-Segment Pseudowire
T-PE PW Terminating Provider Edge
TE LSP Traffic Engineered Label Switched Path
VCCV Virtual Circuit Connectivity Verification
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1.3.1. Transport Network
A Transport Network provides transparent transmission of user traffic
between attached client devices by establishing and maintaining
point-to-point or point-to-multipoint connections between such
devices. The architecture of networks supporting point-to-multipoint
connections is outside the scope of this document. A Transport
Network is independent of any higher-layer network that may exist
between clients, except to the extent required to supply this
transmission service. In addition to client traffic, a Transport
Network may carry traffic to facilitate its own operation, such as
that required to support connection control, network management, and
Operations, Administration, and Maintenance (OAM) functions.
See also the definition of packet transport service in Section 3.1.
1.3.2. MPLS Transport Profile
The MPLS Transport Profile (MPLS-TP) is the subset of MPLS functions
that meet the requirements in [RFC5654]. Note that MPLS is defined
to include any present and future MPLS capability specified by the
IETF, including those capabilities specifically added to support
transport network requirements [RFC5654].
1.3.3. MPLS-TP Section
MPLS-TP sections are defined in [DATA-PLANE]. See also the
definition of "section layer network" in Section 1.2.2 of [RFC5654].
1.3.4. MPLS-TP Label Switched Path
An MPLS-TP Label Switched Path (MPLS-TP LSP) is an LSP that uses a
subset of the capabilities of an MPLS LSP in order to meet the
requirements of an MPLS transport network as set out in [RFC5654].
The characteristics of an MPLS-TP LSP are primarily that it:
1. Uses a subset of the MPLS OAM tools defined in [OAM-FRAMEWORK].
2. Supports 1+1, 1:1, and 1:N protection functions.
3. Is traffic engineered.
4. May be established and maintained via the management plane, or
using GMPLS protocols when a control plane is used.
5. Is either point-to-point or point-to-multipoint. Multipoint-to-
point and multipoint-to-multipoint LSPs are not supported.
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6. It is either unidirectional, associated bidirectional, or co-
routed bidirectional (i.e., the forward and reverse components of
a bidirectional LSP follow the same path, and the intermediate
nodes are aware of their association). These are further defined
in [DATA-PLANE].
Note that an MPLS LSP is defined to include any present and future
MPLS capability, including those specifically added to support the
transport network requirements.
See [DATA-PLANE] for further details on the types and data-plane
properties of MPLS-TP LSPs.
The lowest server layer provided by MPLS-TP is an MPLS-TP LSP. The
client layers of an MPLS-TP LSP may be network-layer protocols, MPLS
LSPs, or PWs. The relationship of an MPLS-TP LSP to its client
layers is described in detail in Section 3.4.
1.3.5. MPLS-TP Label Switching Router
An MPLS-TP Label Switching Router (LSR) is either an MPLS-TP Provider
Edge (PE) router or an MPLS-TP Provider (P) router for a given LSP,
as defined below. The terms MPLS-TP PE router and MPLS-TP P router
describe logical functions; a specific node may undertake only one of
these roles on a given LSP.
Note that the use of the term "router" in this context is historic
and neither requires nor precludes the ability to perform IP
forwarding.
1.3.5.1. Label Edge Router
An MPLS-TP Label Edge Router (LER) is an LSR that exists at the
endpoints of an LSP and therefore pushes or pops the LSP label, i.e.,
does not perform a label swap on the particular LSP under
consideration.
1.3.5.2. MPLS-TP Provider Edge Router
An MPLS-TP Provider Edge (PE) router is an MPLS-TP LSR that adapts
client traffic and encapsulates it to be transported over an MPLS-TP
LSP. Encapsulation may be as simple as pushing a label, or it may
require the use of a pseudowire. An MPLS-TP PE exists at the
interface between a pair of layer networks. For an MS-PW, an MPLS-TP
PE may be either an S-PE or a T-PE, as defined in [RFC5659] (see
below). A PE that pushes or pops an LSP label is an LER for that
LSP.
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The term Provider Edge refers to the node's role within a provider's
network. A provider edge router resides at the edge of a given
MPLS-TP network domain, in which case it has links to another MPLS-TP
network domain or to a CE, except for the case of a pseudowire
switching provider edge (S-PE) router, which is not restricted to the
edge of an MPLS-TP network domain.
1.3.5.3. MPLS-TP Provider Router
An MPLS-TP Provider router is an MPLS-TP LSR that does not provide
MPLS-TP PE functionality for a given LSP. An MPLS-TP P router
switches LSPs that carry client traffic, but does not adapt client
traffic and encapsulate it to be carried over an MPLS-TP LSP. The
term Provider Router refers to the node's role within a provider's
network. A provider router does not have links to other MPLS-TP
network domains.
1.3.5.4. Pseudowire Switching Provider Edge Router (S-PE)
RFC 5659 [RFC5659] defines an S-PE as:
A PE capable of switching the control and data planes of the
preceding and succeeding PW segments in an MS-PW. The S-PE
terminates the PSN tunnels of the preceding and succeeding
segments of the MS-PW. It therefore includes a PW switching point
for an MS-PW. A PW switching point is never the S-PE and the T-PE
for the same MS-PW. A PW switching point runs necessary protocols
to set up and manage PW segments with other PW switching points
and terminating PEs. An S-PE can exist anywhere a PW must be
processed or policy applied. It is therefore not limited to the
edge of a provider network.
Note that it was originally anticipated that S-PEs would only be
deployed at the edge of a provider network where they would be
used to switch the PWs of different service providers. However,
as the design of MS-PW progressed, other applications for MS-PW
were recognized. By this time S-PE had become the accepted term
for the equipment, even though they were no longer universally
deployed at the provider edge.
1.3.5.5. Pseudowire Terminating Provider Edge (T-PE) Router
RFC 5659 [RFC5659] defines a T-PE as:
A PE where the customer-facing attachment circuits (ACs) are bound
to a PW forwarder. A terminating PE is present in the first and
last segments of an MS-PW. This incorporates the functionality of
a PE as defined in RFC 3985.
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1.3.6. Customer Edge (CE)
A Customer Edge (CE) is the client function that sources or sinks
native service traffic to or from the MPLS-TP network. CEs on either
side of the MPLS-TP network are peers and view the MPLS-TP network as
a single link.
1.3.7. Transport LSP
A Transport LSP is an LSP between a pair of PEs that may transit zero
or more MPLS-TP provider routers. When carrying PWs, the Transport
LSP is equivalent to the PSN tunnel LSP in [RFC3985] terminology.
1.3.8. Service LSP
A service LSP is an LSP that carries a single client service.
1.3.9. Layer Network
A layer network is defined in [G.805] and described in [RFC5654]. A
layer network provides for the transfer of client information and
independent operation of the client OAM. A layer network may be
described in a service context as follows: one layer network may
provide a (transport) service to a higher client layer network and
may, in turn, be a client to a lower-layer network. A layer network
is a logical construction somewhat independent of arrangement or
composition of physical network elements. A particular physical
network element may topologically belong to more than one layer
network, depending on the actions it takes on the encapsulation
associated with the logical layers (e.g., the label stack), and thus
could be modeled as multiple logical elements. A layer network may
consist of one or more sublayers.
1.3.10. Network Layer
This document uses the term Network Layer in the same sense as it is
used in [RFC3031] and [RFC3032]. Network-layer protocols are
synonymous with those belonging to Layer 3 of the Open System
Interconnect (OSI) network model [X.200].
1.3.11. Service Interface
The packet transport service provided by MPLS-TP is provided at a
service interface. Two types of service interfaces are defined:
o User-Network Interface (UNI) (see Section 3.4.3.1).
o Network-Network Interface (NNI) (see Section 3.4.3.2).
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A UNI service interface may be a Layer 2 interface that carries only
network layer clients. MPLS-TP LSPs are both necessary and
sufficient to support this service interface as described in
Section 3.4.3. Alternatively, it may be a Layer 2 interface that
carries both network-layer and non-network-layer clients. To support
this service interface, a PW is required to adapt the client traffic
received over the service interface. This PW in turn is a client of
the MPLS-TP server layer. This is described in Section 3.4.2.
An NNI service interface may be to an MPLS LSP or a PW. To support
this case, an MPLS-TP PE participates in the service interface
signaling.
1.3.12. Native Service
The native service is the client layer network service that is
transported by the MPLS-TP network, whether a pseudowire or an LSP is
used for the adaptation (see Section 3.4).
1.3.13. Additional Definitions and Terminology
Detailed definitions and additional terminology may be found in
[RFC5654] and [ROSETTA-STONE].
2. MPLS Transport Profile Requirements
The requirements for MPLS-TP are specified in [RFC5654], [RFC5860],
and [NM-REQ]. This section provides a brief reminder to guide the
reader. It is not normative or intended as a substitute for these
documents.
MPLS-TP must not modify the MPLS forwarding architecture and must be
based on existing pseudowire and LSP constructs.
Point-to-point LSPs may be unidirectional or bidirectional, and it
must be possible to construct congruent bidirectional LSPs.
MPLS-TP LSPs do not merge with other LSPs at an MPLS-TP LSR and it
must be possible to detect if a merged LSP has been created.
It must be possible to forward packets solely based on switching the
MPLS or PW label. It must also be possible to establish and maintain
LSPs and/or pseudowires both in the absence or presence of a dynamic
control plane. When static provisioning is used, there must be no
dependency on dynamic routing or signaling.
OAM and protection mechanisms, and forwarding of data packets, must
be able to operate without IP forwarding support.
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It must be possible to monitor LSPs and pseudowires through the use
of OAM in the absence of control-plane or routing functions. In this
case, information gained from the OAM functions is used to initiate
path recovery actions at either the PW or LSP layers.
3. MPLS Transport Profile Overview
3.1. Packet Transport Services
One objective of MPLS-TP is to enable MPLS networks to provide packet
transport services with a similar degree of predictability to that
found in existing transport networks. Such packet transport services
exhibit a number of characteristics, defined in [RFC5654]:
o In an environment where an MPLS-TP layer network is supporting a
client layer network, and the MPLS-TP layer network is supported
by a server layer network then operation of the MPLS-TP layer
network must be possible without any dependencies on either the
server or client layer network.
o The service provided by the MPLS-TP network to a given client will
not fall below the agreed level as a result of the traffic loading
of other clients.
o The control and management planes of any client network layer that
uses the service is isolated from the control and management
planes of the MPLS-TP layer network, where the client network
layer is considered to be the native service of the MPLS-TP
network.
o Where a client network makes use of an MPLS-TP server that
provides a packet transport service, the level of coordination
required between the client and server layer networks is minimal
(preferably no coordination will be required).
o The complete set of packets generated by a client MPLS(-TP) layer
network using the packet transport service, which may contain
packets that are not MPLS packets (e.g., IP or CLNS
(Connectionless Network Service) packets used by the control/
management plane of the client MPLS(-TP) layer network), are
transported by the MPLS-TP server layer network.
o The packet transport service enables the MPLS-TP layer network
addressing and other information (e.g., topology) to be hidden
from any client layer networks using that service, and vice-versa.
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These characteristics imply that a packet transport service does not
support a connectionless packet-switched forwarding mode. However,
this does not preclude it carrying client traffic associated with a
connectionless service.
3.2. Scope of the MPLS Transport Profile
Figure 1 illustrates the scope of MPLS-TP. MPLS-TP solutions are
primarily intended for packet transport applications. MPLS-TP is a
strict subset of MPLS, and comprises only those functions that are
necessary to meet the requirements of [RFC5654]. This includes MPLS
functions that were defined prior to [RFC5654] but that meet the
requirements of [RFC5654], together with additional functions defined
to meet those requirements. Some MPLS functions defined before
[RFC5654] such as Equal Cost Multi-Path (ECMP), LDP signaling when
used in such a way that it creates multipoint-to-point LSPs, and IP
forwarding in the data plane are explicitly excluded from MPLS-TP by
that requirements specification.
Note that MPLS as a whole will continue to evolve to include
additional functions that do not conform to the MPLS Transport
Profile or its requirements, and thus fall outside the scope of
MPLS-TP.
|<============================== MPLS ==============================>|
{ Post-RFC5654 }
{ non-Transport }
{ Functions }
|<========== Pre-RFC5654 MPLS ===========>|
{ ECMP }
{ LDP/non-TE LSPs }
{ IP forwarding }
|<======== MPLS-TP ============>|
{ Additional }
{ Transport }
{ Functions }
Figure 1: Scope of MPLS-TP
MPLS-TP can be used to construct packet networks and is therefore
applicable in any packet network context. A subset of MPLS-TP is
also applicable to ITU-T-defined packet transport networks, where the
transport network operational model is deemed attractive.
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3.3. Architecture
MPLS-TP comprises the following architectural elements:
o A standard MPLS data plane [RFC3031] as profiled in [DATA-PLANE].
o Sections, LSPs, and PWs that provide a packet transport service
for a client network.
o Proactive and on-demand Operations, Administration, and
Maintenance (OAM) functions to monitor and diagnose the MPLS-TP
network, as outlined in [OAM-FRAMEWORK].
o Control planes for LSPs and PWs, as well as support for static
provisioning and configuration, as outlined in [CP-FRAMEWORK].
o Path protection mechanisms to ensure that the packet transport
service survives anticipated failures and degradations of the
MPLS-TP network, as outlined in [SURVIVE-FWK].
o Control-plane-based restoration mechanisms, as outlined in
[SURVIVE-FWK].
o Network management functions, as outlined in [NM-FRAMEWORK].
The MPLS-TP architecture for LSPs and PWs includes the following two
sets of functions:
o MPLS-TP native service adaptation
o MPLS-TP forwarding
The adaptation functions interface the native service (i.e., the
client layer network service) to MPLS-TP. This includes the case
where the native service is an MPLS-TP LSP.
The forwarding functions comprise the mechanisms required for
forwarding the encapsulated native service traffic over an MPLS-TP
server layer network, for example, PW and LSP labels.
3.3.1. MPLS-TP Native Service Adaptation Functions
The MPLS-TP native service adaptation functions interface the client
layer network service to MPLS-TP. For pseudowires, these adaptation
functions are the payload encapsulation described in Section 4.4 of
[RFC3985] and Section 6 of [RFC5659]. For network layer client
services, the adaptation function uses the MPLS encapsulation format
as defined in [RFC3032].
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The purpose of this encapsulation is to abstract the data plane of
the client layer network from the MPLS-TP data plane, thus
contributing to the independent operation of the MPLS-TP network.
MPLS-TP is itself a client of an underlying server layer. MPLS-TP is
thus also bounded by a set of adaptation functions to this server
layer network, which may itself be MPLS-TP. These adaptation
functions provide encapsulation of the MPLS-TP frames and for the
transparent transport of those frames over the server layer network.
The MPLS-TP client inherits its Quality of Service (QoS) from the
MPLS-TP network, which in turn inherits its QoS from the server
layer. The server layer therefore needs to provide the necessary QoS
to ensure that the MPLS-TP client QoS commitments can be satisfied.
3.3.2. MPLS-TP Forwarding Functions
The forwarding functions comprise the mechanisms required for
forwarding the encapsulated native service traffic over an MPLS-TP
server layer network, for example, PW and LSP labels.
MPLS-TP LSPs use the MPLS label switching operations and Time-to-Live
(TTL) processing procedures defined in [RFC3031], [RFC3032], and
[RFC3443], as profiled in [DATA-PLANE]. These operations are highly
optimized for performance and are not modified by the MPLS-TP
profile.
In addition, MPLS-TP PWs use the SS-PW and optionally the MS-PW
forwarding operations defined in [RFC3985] and [RFC5659].
Per-platform label space is used for PWs. Either per-platform, per-
interface, or other context-specific label space [RFC5331] may be
used for LSPs.
MPLS-TP forwarding is based on the label that identifies the
transport path (LSP or PW). The label value specifies the processing
operation to be performed by the next hop at that level of
encapsulation. A swap of this label is an atomic operation in which
the contents of the packet after the swapped label are opaque to the
forwarder. The only event that interrupts a swap operation is TTL
expiry. This is a fundamental architectural construct of MPLS to be
taken into account when designing protocol extensions (such as those
for OAM) that require packets to be sent to an intermediate LSR.
Further processing to determine the context of a packet occurs when a
swap operation is interrupted in this manner, or a pop operation
exposes a specific reserved label at the top of the stack, or the
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packet is received with the GAL (Section 3.6) at the top of stack.
Otherwise, the packet is forwarded according to the procedures in
[RFC3032].
MPLS-TP supports Quality of Service capabilities via the MPLS
Differentiated Services (Diffserv) architecture [RFC3270]. Both
E-LSP and L-LSP MPLS Diffserv modes are supported.
Further details of MPLS-TP forwarding can be found in [DATA-PLANE].
3.4. MPLS-TP Native Service Adaptation
This document describes the architecture for two native service
adaptation mechanisms, which provide encapsulation and demultiplexing
for native service traffic traversing an MPLS-TP network:
o A PW
o An MPLS LSP
MPLS-TP uses IETF-defined pseudowires to emulate certain services,
for example, Ethernet, Frame Relay, or PPP / High-Level Data Link
Control (HDLC). A list of PW types is maintained by IANA in the
"MPLS Pseudowire Type" registry. When the native service adaptation
is via a PW, the mechanisms described in Section 3.4.4 are used.
An MPLS LSP can also provide the adaptation, in which case any native
service traffic type supported by [RFC3031] and [RFC3032] is allowed.
Examples of such traffic types include IP packets and MPLS-labeled
packets. Note that the latter case includes TE-LSPs [RFC3209] and
LSP-based applications such as PWs, Layer 2 VPNs [RFC4664], and Layer
3 VPNs [RFC4364]. When the native service adaptation is via an MPLS
label, the mechanisms described in Section 3.4.5 are used.
3.4.1. MPLS-TP Client/Server Layer Relationship
The relationship between the client layer network and the MPLS-TP
server layer network is defined by the MPLS-TP network boundary and
the label context. It is not explicitly indicated in the packet. In
terms of the MPLS label stack, when the native service traffic type
is itself MPLS-labeled, then the S bits of all the labels in the
MPLS-TP label stack carrying that client traffic are zero; otherwise,
the bottom label of the MPLS-TP label stack has the S bit set to 1.
In other words, there can be only one S bit set in a label stack.
The data-plane behavior of MPLS-TP is the same as the best current
practice for MPLS. This includes the setting of the S bit. In each
case, the S bit is set to indicate the bottom (i.e., innermost) label
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in the label stack that is contiguous between the MPLS-TP LSP and its
payload, and only one label stack entry (LSE) contains the S bit
(Bottom of Stack bit) set to 1. Note that this best current practice
differs slightly from [RFC3032], which uses the S bit to identify
when MPLS label processing stops and network layer processing starts.
The relationship of MPLS-TP to its clients is illustrated in
Figure 2. Note that the label stacks shown in the figure are divided
between those inside the MPLS-TP network and those within the client
network when the client network is MPLS(-TP). They illustrate the
smallest number of labels possible. These label stacks could also
include more labels.
PW-Based MPLS Labeled IP
Services Services Transport
|------------| |-----------------------------| |------------|
Emulated PW over LSP IP over LSP IP
Service
+------------+
| PW Payload |
+------------+ +------------+ (CLIENTS)
|PW Lbl(S=1) | | IP |
+------------+ +------------+ +------------+ +------------+
| PW Payload | |LSP Lbl(S=0)| |LSP Lbl(S=1)| | IP |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|PW Lbl (S=1)| |LSP Lbl(S=0)| |LSP Lbl(S=0)| |LSP Lbl(S=1)|
+------------+ +------------+ +------------+ +------------+
|LSP Lbl(S=0)| . . .
+------------+ . . . (MPLS-TP)
. . . .
.
.
~~~~~~~~~~~ denotes Client <-> MPLS-TP layer boundary
Figure 2: MPLS-TP - Client Relationship
3.4.2. MPLS-TP Transport Layers
An MPLS-TP network consists logically of two layers: the Transport
Service layer and the Transport Path layer.
The Transport Service layer provides the interface between Customer
Edge (CE) nodes and the MPLS-TP network. Each packet transmitted by
a CE node for transport over the MPLS-TP network is associated at the
receiving MPLS-TP Provider Edge (PE) node with a single logical
point-to-point connection at the Transport Service layer between this
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(ingress) PE and the corresponding (egress) PE to which the peer CE
is attached. Such a connection is called an MPLS-TP Transport
Service Instance, and the set of client packets belonging to the
native service associated with such an instance on a particular CE-PE
link is called a client flow.
The Transport Path layer provides aggregation of Transport Service
Instances over MPLS-TP transport paths (LSPs), as well as aggregation
of transport paths (via LSP hierarchy).
Awareness of the Transport Service layer need exist only at PE nodes.
MPLS-TP Provider (P) nodes need have no awareness of this layer.
Both PE and P nodes participate in the Transport Path layer. A PE
terminates (i.e., is an LER with respect to) the transport paths it
supports, and is responsible for multiplexing and demultiplexing of
Transport Service Instance traffic over such transport paths.
3.4.3. MPLS-TP Transport Service Interfaces
An MPLS-TP PE node can provide two types of interface to the
Transport Service layer. The MPLS-TP User-Network Interface (UNI)
provides the interface between a CE and the MPLS-TP network. The
MPLS-TP Network-Network Interface (NNI) provides the interface
between two MPLS-TP PEs in different administrative domains.
When MPLS-TP is used to provide a transport service for, e.g., IP
services that are a part of a Layer 3 VPN, then packets are
transported in the same manner as specified in [RFC4364].
3.4.3.1. User-Network Interface
The MPLS-TP User-Network interface (UNI) is illustrated in Figure 3.
The UNI for a particular client flow may or may not involve signaling
between the CE and PE, and if signaling is used, it may or may not
traverse the same attachment circuit that supports the client flow.
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: User-Network Interface : MPLS-TP
:<-------------------------------------->: Network <----->
: :
-:------------- --------------:------------------
: | | : Transport |
: | | Transport : Path |
: | | Service : Mux/Demux |
: | | Control : -- |
: | | Plane : | | Transport|
: ---------- | Signaling | ---------- : | | Path |
:|Signaling |_|___________|_|Signaling | : | | --------->
:|Controller| | | |Controller| : | | |
: ---------- | | ---------- : | | --------->
: :......|...........|......: : | | |
: | Control | : | | Transport|
: | Channel | : | | Path |
: | | : | | --------->
: | | : | | -+----------->TSI
: | | Transport : | | | --------->
: | Client | Service : | | | |
: | Traffic | Data Plane : | | | |
: ---------- | Flows | -------------- | | |Transport|
:|Signaling |-|-----------|-|Client/Service|-| |- Path |
:|Controller|=|===========|=| Traffic | | | --------->
: ---------- | | | Processing |=| |===+===========>TSI
: | | | -------------- | | --------->
: |______|___________|______| : | | |
: | Data Link | : | | |
: | | : -- |
: | | : Transport |
: | | : Service |
: | | : Data Plane|
--------------- ---------------------------------
Customer Edge Node MPLS-TP Provider Edge Node
TSI = Transport Service Instance
Figure 3: MPLS-TP PE Containing a UNI
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--------------From UNI-------> :
-------------------------------------------:------------------
| | Client Traffic Unit : |
| Link-Layer-Specific | Link Decapsulation : Service Instance |
| Processing | & : Transport |
| | Service Instance : Encapsulation |
| | Identification : |
-------------------------------------------:------------------
:
:
-------------------------------------------:------------------
| | : Service Instance |
| | : Transport |
| Link-Layer-Specific | Client Traffic Unit : Decapsulation |
| Processing | Link Encapsulation : & |
| | : Service Instance |
| | : Identification |
-------------------------------------------:------------------
<-------------To UNI --------- :
Figure 4: MPLS-TP UNI Client-Server Traffic Processing Stages
Figure 4 shows the logical processing steps involved in a PE both for
traffic flowing from the CE to the MPLS-TP network (left to right),
and from the network to the CE (right to left).
In the first case, when a packet from a client flow is received by
the PE from the CE over the data-link, the following steps occur:
1. Link-layer-specific pre-processing, if any, is performed. An
example of such pre-processing is the PREP function illustrated
in Figure 3 of [RFC3985]. Such pre-processing is outside the
scope of MPLS-TP.
2. The packet is extracted from the data-link frame, if necessary,
and associated with a Transport Service Instance. At this point,
UNI processing has completed.
3. A transport service encapsulation is associated with the packet,
if necessary, for transport over the MPLS-TP network.
4. The packet is mapped to a transport path based on its associated
Transport Service Instance, the transport path encapsulation is
added, if necessary, and the packet is transmitted over the
transport path.
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In the second case, when a packet associated with a Transport Service
Instance arrives over a transport path, the following steps occur:
1. The transport path encapsulation is disposed of.
2. The transport service encapsulation is disposed of and the
Transport Service Instance and client flow identified.
3. At this point, UNI processing begins. A data-link encapsulation
is associated with the packet for delivery to the CE based on the
client flow.
4. Link-layer-specific postprocessing, if any, is performed. Such
postprocessing is outside the scope of MPLS-TP.
3.4.3.2. Network-Network Interface
The MPLS-TP NNI is illustrated in Figure 5. The NNI for a particular
Transport Service Instance may or may not involve signaling between
the two PEs; and if signaling is used, it may or may not traverse the
same data-link that supports the service instance.
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: Network-Network Interface :
:<--------------------------------->:
: :
------------:------------- -------------:------------
| Transport : | | : Transport |
| Path : Transport | | Transport : Path |
| Mux/Demux : Service | | Service : Mux/Demux |
| -- : Control | | Control : -- |
| | | : Plane |Sig- | Plane : | | |
|TP | | : ---------- | naling| ---------- : | | TP|
<--- | | :|Signaling |_|_______|_|Signaling |: | | --->
TSI<-+- | | :|Controller| | | |Controller|: | | |
<--- | | | : ---------- | | ---------- : | | --->
| | | | : :......|.......|......: : | | |
| | | | : |Control| : | | |
|TP | | | : |Channel| : | | TP|
<--- | | | : | | : | | --->
| | | | : | | : | | -+->TSI
<--- | | | : Transport | | Transport : | | | --->
| | | | : Service |Service| Service : | | | |
| | | | : Data Plane |Traffic| Data Plane : | | | |
| | | | ------------- | Flows | ------------- | | | |
|TP -| |-| Service |-|-------|-| Service |-| |- TP|
<--- | | | Traffic | | | | Traffic | | | --->
TSI<=+===| |=| Processing |=|=======|=| Processing |=| |===+=>TSI
<--- | | ------------- | | ------------- | | --->
| | | : |______|_______|______| : | | |
| | | : | Data | : | | |
| -- : | Link | : -- |
| : | | : |
-------------------------- --------------------------
MPLS-TP Provider Edge Node MPLS-TP Provider Edge Node
TP = Transport Path
TSI = Transport Service Instance
Figure 5: MPLS-TP PE Containing an NNI
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:
--------------From NNI-------> :
--------------------------------------------:------------------
| | Service Traffic Unit : |
| Link-Layer-Specific | Link Decapsulation : Service Instance |
| Processing | & : Encapsulation |
| | Service Instance : Normalization |
| | Identification : |
--------------------------------------------:------------------
:
:
--------------------------------------------:------------------
| | : Service Instance |
| | : Identification |
| Link-Layer-Specific | Service Traffic Unit : & |
| Processing | Link Encapsulation : Service Instance |
| | : Encapsulation |
| | : Normalization |
--------------------------------------------:------------------
<-------------To NNI --------- :
Figure 6: MPLS-TP NNI Service Traffic Processing Stages
Figure 6 shows the logical processing steps involved in a PE for
traffic flowing both from the peer PE (left to right) and to the peer
PE (right to left).
In the first case, when a packet from a Transport Service Instance is
received by the PE from the peer PE over the data-link, the following
steps occur:
1. Link-layer specific pre-processing, if any, is performed. Such
pre-processing is outside the scope of MPLS-TP.
2. The packet is extracted from the data-link frame if necessary,
and associated with a Transport Service Instance. At this point,
NNI processing has completed.
3. The transport service encapsulation of the packet is normalized
for transport over the MPLS-TP network. This step allows a
different transport service encapsulation to be used over the NNI
than that used in the internal MPLS-TP network. An example of
such normalization is a swap of a label identifying the Transport
Service Instance.
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4. The packet is mapped to a transport path based on its associated
Transport Service Instance, the transport path encapsulation is
added, if necessary, and the packet is transmitted over the
transport path.
In the second case, when a packet associated with a Transport Service
Instance arrives over a transport path, the following steps occur:
1. The transport path encapsulation is disposed of.
2. The Transport Service Instance is identified from the transport
service encapsulation, and this encapsulation is normalized for
delivery over the NNI (see Step 3 above).
3. At this point, NNI processing begins. A data-link encapsulation
is associated with the packet for delivery to the peer PE based
on the normalized Transport Service Instance.
4. Link-layer-specific postprocessing, if any, is performed. Such
postprocessing is outside the scope of MPLS-TP.
3.4.3.3. Example Interfaces
This section considers some special cases of UNI processing for
particular transport service types. These are illustrative, and do
not preclude other transport service types.
3.4.3.3.1. Layer 2 Transport Service
In this example the MPLS-TP network is providing a point-to-point
Layer 2 transport service between attached CE nodes. This service is
provided by a Transport Service Instance consisting of a PW
established between the associated PE nodes. The client flows
associated with this Transport Service Instance are the sets of all
Layer 2 frames transmitted and received over the attachment circuits.
The processing steps in this case for a frame received from the CE
are:
1. Link-layer specific pre-processing, if any, is performed,
corresponding to the PREP function illustrated in Figure 3 of
[RFC3985].
2. The frame is associated with a Transport Service Instance based
on the attachment circuit over which it was received.
3. A transport service encapsulation, consisting of the PW control
word and PW label, is associated with the frame.
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4. The resulting packet is mapped to an LSP, the LSP label is
pushed, and the packet is transmitted over the outbound interface
associated with the LSP.
For PW packets received over the LSP, the steps are performed in the
reverse order.
3.4.3.3.2. IP Transport Service
In this example, the MPLS-TP network is providing a point-to-point IP
transport service between CE1, CE2, and CE3, as follows. One point-
to-point Transport Service Instance delivers IPv4 packets between CE1
and CE2, and another instance delivers IPv6 packets between CE1 and
CE3.
The processing steps in this case for an IP packet received from CE1
are:
1. No link-layer-specific processing is performed.
2. The IP packet is extracted from the link-layer frame and
associated with a Service LSP based on the source MAC address
(CE1) and the IP protocol version.
3. A transport service encapsulation, consisting of the Service LSP
label, is associated with the packet.
4. The resulting packet is mapped to a tunnel LSP, the tunnel LSP
label is pushed, and the packet is transmitted over the outbound
interface associated with the LSP.
For packets received over a tunnel LSP carrying the Service LSP
label, the steps are performed in the reverse order.
3.4.4. Pseudowire Adaptation
MPLS-TP uses pseudowires to provide a Virtual Private Wire Service
(VPWS), a Virtual Private Local Area Network Service (VPLS), a
Virtual Private Multicast Service (VPMS), and an Internet Protocol
Local Area Network Service (IPLS). VPWS, VLPS, and IPLS are
described in [RFC4664]. VPMS is described in [VPMS-REQS].
If the MPLS-TP network provides a layer 2 interface (that can carry
both network-layer and non-network-layer traffic) as a service
interface, then a PW is required to support the service interface.
The PW is a client of the MPLS-TP LSP server layer. The architecture
for an MPLS-TP network that provides such services is based on the
MPLS [RFC3031] and pseudowire [RFC3985] architectures. Multi-segment
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pseudowires may optionally be used to provide a packet transport
service, and their use is consistent with the MPLS-TP architecture.
The use of MS-PWs may be motivated by, for example, the requirements
specified in [RFC5254]. If MS-PWs are used, then the MS-PW
architecture [RFC5659] also applies.
Figure 7 shows the architecture for an MPLS-TP network using single-
segment PWs. Note that, in this document, the client layer is
equivalent to the emulated service described in [RFC3985], while the
Transport LSP is equivalent to the Packet Switched Network (PSN)
tunnel of [RFC3985].
|<----------------- Client Layer ------------------->|
| |
| |<-------- Pseudowire -------->| |
| | encapsulated, packet | |
| | transport service | |
| | | |
| | Transport | |
| | |<------ LSP ------->| | |
| V V V V |
V AC +----+ +-----+ +----+ AC V
+-----+ | | PE1|=======\ /========| PE2| | +-----+
| |----------|.......PW1.| \ / |............|----------| |
| CE1 | | | | | X | | | | | CE2 |
| |----------|.......PW2.| / \ |............|----------| |
+-----+ ^ | | |=======/ \========| | | ^ +-----+
^ | +----+ ^ +-----+ +----+ | ^
| | Provider | ^ Provider | |
| | Edge 1 | | Edge 2 | |
Customer | | P Router | Customer
Edge 1 | TE LSP | Edge 2
| |
| |
Native service Native service
Figure 7: MPLS-TP Architecture (Single Segment PW)
Figure 8 shows the architecture for an MPLS-TP network when multi-
segment pseudowires are used. Note that as in the SS-PW case,
P-routers may also exist.
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|<--------------------- Client Layer ------------------------>|
| |
| Pseudowire encapsulated, |
| |<---------- Packet Transport Service ------------->| |
| | | |
| | Transport Transport | |
| AC | |<-------- LSP1 --------->| |<--LSP2-->| | AC |
| | V V V V V V | |
V | +----+ +-----+ +----+ +----+ | V
+---+ | |TPE1|===============\ /=====|SPE1|==========|TPE2| | +---+
| |----|......PW1-Seg1.... | \ / | ......X...PW1-Seg2......|----| |
|CE1| | | | | X | | | | | | |CE2|
| |----|......PW2-Seg1.... | / \ | ......X...PW2-Seg2......|----| |
+---+ ^ | |===============/ \=====| |==========| | | ^+---+
| +----+ ^ +-----+ +----+ ^ +----+ |
| | ^ | |
| TE LSP | TE LSP |
| P-router |
Native Service Native Service
PW1-segment1 and PW1-segment2 are segments of the same MS-PW,
while PW2-segment1 and PW2-segment2 are segments of another MS-PW.
Figure 8: MPLS-TP Architecture (Multi-Segment PW)
The corresponding MPLS-TP protocol stacks including PWs are shown in
Figure 9. In this figure, the Transport Service layer [RFC5654] is
identified by the PW demultiplexer (Demux) label, and the Transport
Path layer [RFC5654] is identified by the LSP Demux Label.
Bocci, et al. Informational [Page 27]
RFC 5921 MPLS Transport Profile Framework July 2010
+-------------------+ /===================\ /===================\
| Client Layer | H OAM PDU H H OAM PDU H
/===================\ H-------------------H H-------------------H
H PW Encap H H GACh H H GACh H
H-------------------H H-------------------H H-------------------H
H PW Demux (S=1) H H PW Demux (S=1) H H GAL (S=1) H
H-------------------H H-------------------H H-------------------H
H Trans LSP Demux(s)H H Trans LSP Demux(s)H H Trans LSP Demux(s)H
\===================/ \===================/ \===================/
| Server Layer | | Server Layer | | Server Layer |
+-------------------+ +-------------------+ +-------------------+
User Traffic PW OAM LSP OAM
Note: H(ighlighted) indicates the part of the protocol stack considered
in this document.
Figure 9: MPLS-TP Label Stack Using Pseudowires
PWs and their associated labels may be configured or signaled. See
Section 3.11 for additional details related to configured service
types. See Section 3.9 for additional details related to signaled
service types.
3.4.5. Network Layer Adaptation
MPLS-TP LSPs can be used to transport network-layer clients. This
document uses the term Network Layer in the same sense as it is used
in [RFC3031] and [RFC3032]. The network-layer protocols supported by
[RFC3031] and [RFC3032] can be transported between service
interfaces. Support for network-layer clients follows the MPLS
architecture for support of network-layer protocols as specified in
[RFC3031] and [RFC3032].
With network-layer adaptation, the MPLS-TP domain provides either a
unidirectional or bidirectional point-to-point connection between two
PEs in order to deliver a packet transport service to attached
customer edge (CE) nodes. For example, a CE may be an IP, MPLS, or
MPLS-TP node. As shown in Figure 10, there is an attachment circuit
between the CE node on the left and its corresponding provider edge
(PE) node (which provides the service interface), a bidirectional LSP
across the MPLS-TP network to the corresponding PE node on the right,
and an attachment circuit between that PE node and the corresponding
CE node for this service.
The attachment circuits may be heterogeneous (e.g., any combination
of SDH, PPP, Frame Relay, etc.) and network-layer protocol payloads
arrive at the service interface encapsulated in the Layer 1 / Layer 2
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RFC 5921 MPLS Transport Profile Framework July 2010
encoding defined for that access link type. It should be noted that
the set of network-layer protocols includes MPLS, and hence MPLS-
encoded packets with an MPLS label stack (the client MPLS stack) may
appear at the service interface.
The following figures illustrate the reference models for network-
layer adaptation. The details of these figures are described further
in the following paragraphs.
|<------------- Client Network Layer --------------->|
| |
| |<----------- Packet --------->| |
| | Transport Service | |
| | | |
| | | |
| | Transport | |
| | |<------ LSP ------->| | |
| V V V V |
V AC +----+ +-----+ +----+ AC V
+-----+ | | PE1|=======\ /========| PE2| | +-----+
| |----------|..Svc LSP1.| \ / |............|----------| |
| CE1 | | | | | X | | | | | CE2 |
| |----------|..Svc LSP2.| / \ |............|----------| |
+-----+ ^ | | |=======/ \========| | | ^ +-----+
^ | +----+ ^ +-----+ +----+ | | ^
| | Provider | ^ Provider | |
| | Edge 1 | | Edge 2 | |
Customer | | P Router | Customer
Edge 1 | TE LSP | Edge 2
| |
| |
Native service Native service
Figure 10: MPLS-TP Architecture for Network-Layer Clients
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RFC 5921 MPLS Transport Profile Framework July 2010
|<--------------------- Client Layer ------------------------>|
| |
| |
| |<---------- Packet Transport Service ------------->| |
| | | |
| | Transport Transport | |
| AC | |<-------- LSP1 --------->| |<--LSP2-->| | AC |
| | V V V V V V | |
V | +----+ +-----+ +----+ +----+ | V
+---+ | | PE1|===============\ /=====| PE2|==========| PE3| | +---+
| |----|......svc-lsp1.... | \ / | .....X....svc-lsp1......|----| |
|CE1| | | | | X | | | | | | |CE2|
| |----|......svc-lsp2.... | / \ | .....X....svc-lsp2......|----| |
+---+ ^ | |===============/ \=====| |==========| | | ^+---+
| +----+ ^ +-----+ +----+ ^ +----+ |
| | ^ ^ | |
| TE LSP | | TE LSP |
| P-router | |
Native Service (LSR for | Native Service
T'port LSP1) |
|
LSR for Service LSPs
LER for Transport LSPs
Figure 11: MPLS-TP Architecture for Network Layer Adaptation, Showing
Service LSP Switching
Client packets are received at the ingress service interface. The PE
pushes one or more labels onto the client packets that are then label
switched over the transport network. Correspondingly, the egress PE
pops any labels added by the MPLS-TP networks and transmits the
packet for delivery to the attached CE via the egress service
interface.
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/===================\
H OAM PDU H
+-------------------+ H-------------------H /===================\
| Client Layer | H GACh H H OAM PDU H
/===================\ H-------------------H H-------------------H
H Encap Label H H GAL (S=1) H H GACh H
H-------------------H H-------------------H H-------------------H
H SvcLSP Demux H H SvcLSP Demux (S=0)H H GAL (S=1) H
H-------------------H H-------------------H H-------------------H
H Trans LSP Demux(s)H H Trans LSP Demux(s)H H Trans LSP Demux(s)H
\===================/ \===================/ \===================/
| Server Layer | | Server Layer | | Server Layer |
+-------------------+ +-------------------+ +-------------------+
User Traffic Service LSP OAM LSP OAM
Note: H(ighlighted) indicates the part of the protocol stack considered
in this document.
Figure 12: MPLS-TP Label Stack for IP and LSP Clients
In the figures above, the Transport Service layer [RFC5654] is
identified by the Service LSP (SvcLSP) demultiplexer (Demux) label,
and the Transport Path layer [RFC5654] is identified by the Transport
(Trans) LSP Demux Label. Note that the functions of the
Encapsulation Label (Encap Label) and the Service Label (SvcLSP
Demux) shown above may alternatively be represented by a single label
stack entry. Note that the S bit is always zero when the client
layer is MPLS-labeled. It may be necessary to swap a service LSP
label at an intermediate node. This is shown in Figure 11.
Within the MPLS-TP transport network, the network-layer protocols are
carried over the MPLS-TP network using a logically separate MPLS
label stack (the server stack). The server stack is entirely under
the control of the nodes within the MPLS-TP transport network and it
is not visible outside that network. Figure 12 shows how a client
network protocol stack (which may be an MPLS label stack and payload)
is carried over a network layer client service over an MPLS-TP
transport network.
A label may be used to identify the network-layer protocol payload
type. Therefore, when multiple protocol payload types are to be
carried over a single service LSP, a unique label stack entry needs
to be present for each payload type. Such labels are referred to as
"Encapsulation Labels", one of which is shown in Figure 12. An
Encapsulation Label may be either configured or signaled.
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Both an Encapsulation Label and a Service Label should be present in
the label stack when a particular packet transport service is
supporting more than one network-layer protocol payload type. For
example, if both IP and MPLS are to be carried, then two
Encapsulation Labels are mapped on to a common Service Label.
Note: The Encapsulation Label may be omitted when the service LSP is
supporting only one network-layer protocol payload type. For
example, if only MPLS labeled packets are carried over a service,
then the Service Label (stack entry) provides both the payload type
indication and service identification. The Encapsulation Label
cannot have any of the reserved label values [RFC3032].
Service labels are typically carried over an MPLS-TP Transport LSP
edge-to-edge (or transport path layer). An MPLS-TP Transport LSP is
represented as an LSP Transport Demux label, as shown in Figure 12.
Transport LSP is commonly used when more than one service exists
between two PEs.
Note that, if only one service exists between two PEs, the functions
of the Transport LSP label and the Service LSP Label may be combined
into a single label stack entry. For example, if only one service is
carried between two PEs, then a single label could be used to provide
both the service indication and the MPLS-TP Transport LSP.
Alternatively, if multiple services exist between a pair of PEs, then
a per-client Service Label would be mapped on to a common MPLS-TP
Transport LSP.
As noted above, the Layer 2 and Layer 1 protocols used to carry the
network-layer protocol over the attachment circuits are not
transported across the MPLS-TP network. This enables the use of
different Layer 2 and Layer 1 protocols on the two attachment
circuits.
At each service interface, Layer 2 addressing needs to be used to
ensure the proper delivery of a network-layer packet to the adjacent
node. This is typically only an issue for LAN media technologies
(e.g., Ethernet) that have Media Access Control (MAC) addresses. In
cases where a MAC address is needed, the sending node sets the
destination MAC address to an address that ensures delivery to the
adjacent node. That is, the CE sets the destination MAC address to
an address that ensures delivery to the PE, and the PE sets the
destination MAC address to an address that ensures delivery to the
CE. The specific address used is technology type specific and is not
specified in this document. In some technologies, the MAC address
will need to be configured.
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Note that when two CEs, which peer with each other, operate over a
network layer transport service and run a routing protocol such as
IS-IS or OSPF, some care should be taken to configure the routing
protocols to use point-to-point adjacencies. The specifics of such
configuration is outside the scope of this document. See [RFC5309]
for additional details.
The CE-to-CE service types and corresponding labels may be configured
or signaled.
3.5. Identifiers
Identifiers are used to uniquely distinguish entities in an MPLS-TP
network. These include operators, nodes, LSPs, pseudowires, and
their associated maintenance entities. MPLS-TP defined two types of
sets of identifiers: those that are compatible with IP, and those
that are compatible with ITU-T transport-based operations. The
definition of these sets of identifiers is outside the scope of this
document and is provided by [IDENTIFIERS].
3.6. Generic Associated Channel (G-ACh)
For correct operation of OAM mechanisms, it is important that OAM
packets fate-share with the data packets. In addition, in MPLS-TP it
is necessary to discriminate between user data payloads and other
types of payload. For example, a packet may be associated with a
Signaling Communication Channel (SCC) or a channel used for a
protocol to coordinate path protection state. This is achieved by
carrying such packets in either:
o A generic control channel associated to the LSP, PW, or section,
with no IP encapsulation, e.g., in a similar manner to
Bidirectional Forwarding Detection for Virtual Circuit
Connectivity Verification (VCCV-BFD) with PW ACH encapsulation
[RFC5885]).
o An IP encapsulation where IP capabilities are present, e.g., PW
ACH encapsulation with IP headers for VCCV-BFD [RFC5885] or IP
encapsulation for MPLS BFD [RFC5884].
MPLS-TP makes use of such a generic associated channel (G-ACh) to
support Fault, Configuration, Accounting, Performance, and Security
(FCAPS) functions by carrying packets related to OAM, a protocol used
to coordinate path protection state, SCC, MCC or other packet types
in-band over LSPs, PWs, or sections. The G-ACh is defined in
[RFC5586] and is similar to the Pseudowire Associated Channel
[RFC4385], which is used to carry OAM packets over pseudowires. The
G-ACh is indicated by an Associated Channel Header (ACH), similar to
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RFC 5921 MPLS Transport Profile Framework July 2010
the Pseudowire VCCV control word; this header is present for all
sections, LSPs, and PWs that make use of FCAPS functions supported by
the G-ACh.
As specified in [RFC5586], the G-ACh must only be used for channels
that are an adjunct to the data service. Examples of these are OAM,
a protocol used to coordinate path protection state, MCC, and SCC,
but the use is not restricted to these services. The G-ACh must not
be used to carry additional data for use in the forwarding path,
i.e., it must not be used as an alternative to a PW control word, or
to define a PW type.
At the server layer, bandwidth and QoS commitments apply to the gross
traffic on the LSP, PW, or section. Since the G-ACh traffic is
indistinguishable from the user data traffic, protocols using the
G-ACh need to take into consideration the impact they have on the
user data with which they are sharing resources. Conversely,
capacity needs to be made available for important G-ACh uses such as
protection and OAM. In addition, the security and congestion
considerations described in [RFC5586] apply to protocols using the
G-ACh.
Figure 13 shows the reference model depicting how the control channel
is associated with the pseudowire protocol stack. This is based on
the reference model for VCCV shown in Figure 2 of [RFC5085].
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RFC 5921 MPLS Transport Profile Framework July 2010
+-------------+ +-------------+
| Payload | < FCAPS > | Payload |
+-------------+ +-------------+
| Demux / | < ACH for PW > | Demux / |
|Discriminator| |Discriminator|
+-------------+ +-------------+
| PW | < PW > | PW |
+-------------+ +-------------+
| PSN | < LSP > | PSN |
+-------------+ +-------------+
| Physical | | Physical |
+-----+-------+ +-----+-------+
| |
| ____ ___ ____ |
| _/ \___/ \ _/ \__ |
| / \__/ \_ |
| / \ |
+--------| MPLS-TP Network |---+
\ /
\ ___ ___ __ _/
\_/ \____/ \___/ \____/
Figure 13: PWE3 Protocol Stack Reference Model Showing the G-ACh
PW-associated channel messages are encapsulated using the PWE3
encapsulation, so that they are handled and processed in the same
manner (or in some cases, an analogous manner) as the PW PDUs for
which they provide a control channel.
Figure 14 shows the reference model depicting how the control channel
is associated with the LSP protocol stack.
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RFC 5921 MPLS Transport Profile Framework July 2010
+-------------+ +-------------+
| Payload | < FCAPS > | Payload |
+-------------+ +-------------+
|Discriminator| < ACH on LSP > |Discriminator|
+-------------+ +-------------+
|Demultiplexer| < GAL on LSP > |Demultiplexer|
+-------------+ +-------------+
| PSN | < LSP > | PSN |
+-------------+ +-------------+
| Physical | | Physical |
+-----+-------+ +-----+-------+
| |
| ____ ___ ____ |
| _/ \___/ \ _/ \__ |
| / \__/ \_ |
| / \ |
+--------| MPLS-TP Network |---+
\ /
\ ___ ___ __ _/
\_/ \____/ \___/ \____/
Figure 14: MPLS Protocol Stack Reference Model Showing the LSP
Associated Control Channel
3.7. Operations, Administration, and Maintenance (OAM)
The MPLS-TP OAM architecture supports a wide range of OAM functions
to check continuity, to verify connectivity, to monitor path
performance, and to generate, filter, and manage local and remote
defect alarms. These functions are applicable to any layer defined
within MPLS-TP, i.e., to MPLS-TP sections, LSPs, and PWs.
The MPLS-TP OAM tool-set is able to operate without relying on a
dynamic control plane or IP functionality in the data path. In the
case of an MPLS-TP deployment in a network in which IP functionality
is available, all existing IP/MPLS OAM functions (e.g., LSP Ping,
BFD, and VCCV) may be used. Since MPLS-TP can operate in
environments where IP is not used in the forwarding plane, the
default mechanism for OAM demultiplexing in MPLS-TP LSPs and PWs is
the Generic Associated Channel (Section 3.6). Forwarding based on IP
addresses for OAM or user data packets is not required for MPLS-TP.
[RFC4379] and BFD for MPLS LSPs [RFC5884] have defined alert
mechanisms that enable an 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 for IPv4 and that same range
embedded as IPv4-mapped IPv6 addresses for IPv6 [RFC4379]. When the
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OAM packets are encapsulated in an IP header, these mechanisms are
the default mechanisms for MPLS networks (in general) for identifying
MPLS OAM packets, although the mechanisms defined in [RFC5586] can
also be used. MPLS-TP is able to operate in environments where IP
forwarding is not supported, and thus the G-ACh/GAL is the default
mechanism to demultiplex OAM packets in MPLS-TP in these
environments.
MPLS-TP supports a comprehensive set of OAM capabilities for packet
transport applications, with equivalent capabilities to those
provided in SONET/SDH.
MPLS-TP requires [RFC5860] that a set of OAM capabilities is
available to perform fault management (e.g., fault detection and
localization) and performance monitoring (e.g., packet delay and loss
measurement) of the LSP, PW, or section. The framework for OAM in
MPLS-TP is specified in [OAM-FRAMEWORK].
MPLS-TP OAM packets share the same fate as their corresponding data
packets, and are identified through the Generic Associated Channel
mechanism [RFC5586]. This uses a combination of an Associated
Channel Header (ACH) and a G-ACh Label (GAL) to create a control
channel associated to an LSP, section, or PW.
OAM and monitoring in MPLS-TP is based on the concept of maintenance
entities, as described in [OAM-FRAMEWORK]. A Maintenance Entity (ME)
can be viewed as the association of two Maintenance Entity Group End
Points (MEPs). A Maintenance Entity Group (MEG) is a collection of
one or more MEs that belongs to the same transport path and that are
maintained and monitored as a group. The MEPs that form an ME limit
the OAM responsibilities of an OAM flow to within the domain of a
transport path or segment, in the specific layer network that is
being monitored and managed.
A MEG may also include a set of Maintenance Entity Group Intermediate
Points (MIPs).
A G-ACh packet may be directed to an individual MIP along the path of
an LSP or MS-PW by setting the appropriate TTL in the label stack
entry for the G-ACh packet, as per the traceroute mode of LSP Ping
[RFC4379] and the vccv-trace mode of [SEGMENTED-PW]. Note that this
works when the location of MIPs along the LSP or PW path is known by
the MEP. There may be circumstances where this is not the case,
e.g., following restoration using a facility bypass LSP. In these
cases, tools to trace the path of the LSP may be used to determine
the appropriate setting for the TTL to reach a specific MIP.
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Within an LSR or PE, MEPs and MIPs can only be placed where MPLS
layer processing is performed on a packet. The MPLS architecture
mandates that MPLS layer processing occurs at least once on an LSR.
Any node on an LSP can send an OAM packet on that LSP. Likewise, any
node on a PW can send OAM packets on a PW, including S-PEs.
An OAM packet can only be received to be processed at an LSP
endpoint, a PW endpoint (T-PE), or on the expiry of the TTL in the
LSP or PW label stack entry.
3.8. Return Path
Management, control, and OAM protocol functions may require response
packets to be delivered from the receiver back to the originator of a
message exchange. This section provides a summary of the return path
options in MPLS-TP networks. Although this section describes the
case of an MPLS-TP LSP, it is also applicable to a PW.
In this description, U and D are LSRs that terminate MPLS-TP LSPs
(i.e., LERs), and Y is an intermediate LSR along the LSP. Note that
U is the upstream LER, and D is the downstream LER with respect to a
particular direction of an LSP. This reference model is shown in
Figure 15.
LSP LSP
U ========= Y ========= D
LER LSR LER
---------> Direction of user traffic flow
Figure 15: Return Path Reference Model
The following cases are described for the various types of LSPs:
Case 1 Return path packet transmission from D to U
Case 2 Return path packet transmission from Y to U
Case 3 Return path packet transmission from D to Y
Note that a return path may not always exist (or may exist but be
disabled), and that packet transmission in one or more of the above
cases may not be possible. In general, the existence and nature of
return paths for MPLS-TP LSPs is determined by operational
provisioning.
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3.8.1. Return Path Types
There are two types of return path that may be used for the delivery
of traffic from a downstream node D to an upstream node U. Either:
a. The LSP between U and D is bidirectional, and therefore D has a
path via the MPLS-TP LSP to return traffic back to U, or
b. D has some other unspecified means of directing traffic back to
U.
The first option is referred to as an "in-band" return path, the
second as an "out-of-band" return path.
There are various possibilities for "out-of-band" return paths. Such
a path may, for example, be based on ordinary IP routing. In this
case, packets would be forwarded as usual to a destination IP address
associated with U. In an MPLS-TP network that is also an IP/MPLS
network, such a forwarding path may traverse the same physical links
or logical transport paths used by MPLS-TP. An out-of-band return
path may also be indirect, via a distinct Data Communication Network
(DCN) (provided, for example, by the method specified in [RFC5718]);
or it may be via one or more other MPLS-TP LSPs.
3.8.2. Point-to-Point Unidirectional LSPs
Case 1 If an in-band return path is required to deliver traffic from
D back to U, it is recommended for reasons of operational
simplicity that point-to-point unidirectional LSPs be
provisioned as associated bidirectional LSPs (which may also
be co-routed) whenever return traffic from D to U is
required. Note that the two directions of such an LSP may
have differing bandwidth allocations and QoS characteristics.
The discussion below for such LSPs applies.
As an alternative, an out-of-band return path may be used.
Case 2 In this case, only the out-of-band return path option is
available. However, an additional out-of-band possibility is
worthy of note here: if D is known to have a return path to
U, then Y can arrange to deliver return traffic to U by first
sending it to D along the original LSP. The mechanism by
which D recognizes the need for and performs this forwarding
operation is protocol specific.
Case 3 In this case, only the out-of-band return path option is
available. However, if D has a return path to U, then (in a
manner analogous to the previous case) D can arrange to
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deliver return traffic to Y by first sending it to U along
that return path. The mechanism by which U recognizes the
need for and performs this forwarding operation is protocol
specific.
3.8.3. Point-to-Point Associated Bidirectional LSPs
For Case 1, D has a natural in-band return path to U, the use of
which is typically preferred for return traffic, although out-of-band
return paths are also applicable.
For Cases 2 and 3, the considerations are the same as those for
point-to-point unidirectional LSPs.
3.8.4. Point-to-Point Co-Routed Bidirectional LSPs
For all of Cases 1, 2, and 3, a natural in-band return path exists in
the form of the LSP itself, and its use is preferred for return
traffic. Out-of-band return paths, however, are also applicable,
primarily as an alternative means of delivery in case the in-band
return path has failed.
3.9. Control Plane
A distributed dynamic control plane may be used to enable dynamic
service provisioning in an MPLS-TP network. Where the requirements
specified in [RFC5654] can be met, the MPLS Transport Profile uses
existing standard control-plane protocols for LSPs and PWs.
Note that a dynamic control plane is not required in an MPLS-TP
network. See Section 3.11 for further details on statically
configured and provisioned MPLS-TP services.
Figure 16 illustrates the relationship between the MPLS-TP control
plane, the forwarding plane, the management plane, and OAM for point-
to-point MPLS-TP LSPs or PWs.
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+------------------------------------------------------------------+
| |
| Network Management System and/or |
| |
| Control Plane for Point-to-Point Connections |
| |
+------------------------------------------------------------------+
| | | | | |
.............|.....|... ....|.....|.... ....|.....|............
: +---+ | : : +---+ | : : +---+ | :
: |OAM| | : : |OAM| | : : |OAM| | :
: +---+ | : : +---+ | : : +---+ | :
: | | : : | | : : | | :
\: +----+ +--------+ : : +--------+ : : +--------+ +----+ :/
--+-|Edge|<->|Forward-|<---->|Forward-|<----->|Forward-|<->|Edge|-+--
/: +----+ |ing | : : |ing | : : |ing | +----+ :\
: +--------+ : : +--------+ : : +--------+ :
''''''''''''''''''''''' ''''''''''''''' '''''''''''''''''''''''
Note:
1) NMS may be centralized or distributed. Control plane is
distributed.
2) 'Edge' functions refers to those functions present at
the edge of a PSN domain, e.g., native service processing or
classification.
3) The control plane may be transported over the server
layer, an LSP, or a G-ACh.
Figure 16: MPLS-TP Control Plane Architecture Context
The MPLS-TP control plane is based on existing MPLS and PW control
plane protocols, and is consistent with the Automatically Switched
Optical Network (ASON) architecture [G.8080]. MPLS-TP uses:
o Generalized MPLS (GMPLS) signaling ([RFC3945], [RFC3471],
[RFC3473]) for LSPs, and
o Targeted LDP (T-LDP) signaling ([RFC4447], [SEGMENTED-PW],
[DYN-MS-PW]) for pseudowires.
MPLS-TP requires that any control-plane traffic be capable of being
carried over an out-of-band signaling network or a signaling control
channel such as the one described in [RFC5718]. Note that while
T-LDP signaling is traditionally carried in-band in IP/MPLS networks,
this does not preclude its operation over out-of-band channels.
References to T-LDP in this document do not preclude the definition
of alternative PW control protocols for use in MPLS-TP.
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PW control (and maintenance) takes place separately from LSP tunnel
signaling. The main coordination between LSP and PW control will
occur within the nodes that terminate PWs. The control planes for
PWs and LSPs may be used independently, and one may be employed
without the other. This translates into the four possible scenarios:
(1) no control plane is employed; (2) a control plane is used for
both LSPs and PWs; (3) a control plane is used for LSPs, but not PWs;
(4) a control plane is used for PWs, but not LSPs. The PW and LSP
control planes, collectively, need to satisfy the MPLS-TP control
plane requirements reviewed in the MPLS-TP Control Plane Framework
[CP-FRAMEWORK]. When client services are provided directly via LSPs,
all requirements must be satisfied by the LSP control plane. When
client services are provided via PWs, the PW and LSP control planes
operate in combination, and some functions may be satisfied via the
PW control plane, while others are provided to PWs by the LSP control
plane.
Note that if MPLS-TP is being used in a multi-layer network, a number
of control protocol types and instances may be used. This is
consistent with the MPLS architecture, which permits each label in
the label stack to be allocated and signaled by its own control
protocol.
The distributed MPLS-TP control plane may provide the following
functions:
o Signaling
o Routing
o Traffic engineering and constraint-based path computation
In a multi-domain environment, the MPLS-TP control plane supports
different types of interfaces at domain boundaries or within the
domains. These include the User-Network Interface (UNI), Internal
Network-Network Interface (I-NNI), and External Network-Network
Interface (E-NNI). Note that different policies may be defined that
control the information exchanged across these interface types.
The MPLS-TP control plane is capable of activating MPLS-TP OAM
functions as described in the OAM section of this document
Section 3.7, e.g., for fault detection and localization in the event
of a failure in order to efficiently restore failed transport paths.
The MPLS-TP control plane supports all MPLS-TP data-plane
connectivity patterns that are needed for establishing transport
paths, including protected paths as described in Section 3.12.
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Examples of the MPLS-TP data-plane connectivity patterns are LSPs
utilizing the fast reroute backup methods as defined in [RFC4090] and
ingress-to-egress 1+1 or 1:1 protected LSPs.
The MPLS-TP control plane provides functions to ensure its own
survivability and to enable it to recover gracefully from failures
and degradations. These include graceful restart and hot redundant
configurations. Depending on how the control plane is transported,
varying degrees of decoupling between the control plane and data
plane may be achieved. In all cases, however, the control plane is
logically decoupled from the data plane such that a control-plane
failure does not imply a failure of the existing transport paths.
3.10. Inter-Domain Connectivity
A number of methods exist to support inter-domain operation of
MPLS-TP, including the data-plane, OAM, and configuration aspects,
for example:
o Inter-domain TE LSPs [RFC4726]
o Multi-segment Pseudowires [RFC5659]
o LSP stitching [RFC5150]
o Back-to-back attachment circuits [RFC5659]
An important consideration in selecting an inter-domain connectivity
mechanism is the degree of layer network isolation and types of OAM
required by the operator. The selection of which technique to use in
a particular deployment scenario is outside the scope of this
document.
3.11. Static Operation of LSPs and PWs
A PW or LSP may be statically configured without the support of a
dynamic control plane. This may be either by direct configuration of
the PEs/LSRs or via a network management system. Static operation is
independent for a specific PW or LSP instance. Thus, it should be
possible for a PW to be statically configured, while the LSP
supporting it is set up by a dynamic control plane. When static
configuration mechanisms are used, care must be taken to ensure that
loops are not created. Note that the path of an LSP or PW may be
dynamically computed, while the LSP or PW itself is established
through static configuration.
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3.12. Survivability
The survivability architecture for MPLS-TP is specified in
[SURVIVE-FWK].
A wide variety of resiliency schemes have been developed to meet the
various network and service survivability objectives. For example,
as part of the MPLS/PW paradigms, MPLS provides methods for local
repair using back-up LSP tunnels ([RFC4090]), while pseudowire
redundancy [PW-REDUNDANCY] supports scenarios where the protection
for the PW cannot be fully provided by the underlying LSP (i.e.,
where the backup PW terminates on a different target PE node than the
working PW in dual-homing scenarios, or where protection of the S-PE
is required). Additionally, GMPLS provides a well-known set of
control-plane-driven protection and restoration mechanisms [RFC4872].
MPLS-TP provides additional protection mechanisms that are optimized
for both linear topologies and ring topologies and that operate in
the absence of a dynamic control plane. These are specified in
[SURVIVE-FWK].
Different protection schemes apply to different deployment topologies
and operational considerations. Such protection schemes may provide
different levels of resiliency, for example:
o two concurrent traffic paths (1+1).
o one active and one standby path with guaranteed bandwidth on both
paths (1:1).
o one active path and a standby path the resources of which are
shared by one or more other active paths (shared protection).
The applicability of any given scheme to meet specific requirements
is outside the scope of this document.
The characteristics of MPLS-TP resiliency mechanisms are as follows:
o Optimized for linear, ring, or meshed topologies.
o Use OAM mechanisms to detect and localize network faults or
service degenerations.
o Include protection mechanisms to coordinate and trigger protection
switching actions in the absence of a dynamic control plane.
o MPLS-TP recovery schemes are applicable to all levels in the
MPLS-TP domain (i.e., section, LSP, and PW) providing segment and
end-to-end recovery.
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o MPLS-TP recovery mechanisms support the coordination of protection
switching at multiple levels to prevent race conditions occurring
between a client and its server layer.
o MPLS-TP recovery mechanisms can be data-plane, control-plane, or
management-plane based.
o MPLS-TP supports revertive and non-revertive behavior.
3.13. Sub-Path Maintenance
In order to monitor, protect, and manage a portion (i.e., segment or
concatenated segment) of an LSP, a hierarchical LSP [RFC3031] can be
instantiated. A hierarchical LSP instantiated for this purpose is
called a Sub-Path Maintenance Element (SPME). Note that by
definition an SPME does not carry user traffic as a direct client.
An SPME is defined between the edges of the portion of the LSP that
needs to be monitored, protected or managed. The SPME forms an
MPLS-TP Section [DATA-PLANE] that carries the original LSP over this
portion of the network as a client. OAM messages can be initiated at
the edge of the SPME and sent to the peer edge of the SPME or to a
MIP along the SPME by setting the TTL value of the LSE at the
corresponding hierarchical LSP level. A P router only pushes or pops
a label if it is at the end of a SPME. In this mode, it is an LER
for the SPME.
For example, in Figure 17, two SPMEs are configured to allow
monitoring, protection, and management of the LSP concatenated
segments. One SPME is defined between LER2 and LER3, and a second
SPME is set up between LER4 and LER5. Each of these SPMEs may be
monitored, protected, or managed independently.
|<============================= LSP =============================>|
|<---- Carrier 1 ---->| |<---- Carrier 2 ---->|
|LER1|---|LER2|---|LSR|---|LER3|-------|LER4|---|LSR|---|LER5|---|LER6|
|====== SPME =========| |====== SPME =========|
(Carrier 1) (Carrier 2)
Note 1: LER2, LER3, LER4, and LER5 are with respect to the SPME,
but LSRs with respect to the LSP.
Note 2: The LSP terminates in LERs outside of Carrier 1 and
Carrier 2, for example, LER1 and LER6.
Figure 17: SPMEs in Inter-Carrier Network
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The end-to-end traffic of the LSP, including data traffic and control
traffic (OAM, Protection Switching Control, management and signaling
messages) is tunneled within the hierarchical LSP by means of label
stacking as defined in [RFC3031].
The mapping between an LSP and a SPME can be 1:1, in which case it is
similar to the ITU-T Tandem Connection Element [G.805]. The mapping
can also be 1:N to allow aggregated monitoring, protection, and
management of a set of LSP segments or concatenated LSP segments.
Figure 18 shows a SPME that is used to aggregate a set of
concatenated LSP segments for the LSP from LERx to LERt and the LSP
from LERa to LERd. Note that such a construct is useful, for
example, when the LSPs traverse a common portion of the network and
they have the same Traffic Class.
The QoS aspects of a SPME are network specific. [OAM-FRAMEWORK]
provides further considerations on the QoS aspects of OAM.
|LERx|--|LSRy|-+ +-|LSRz|--|LERt|
| |
| |<---------- Carrier 1 --------->| |
| +-----+ +---+ +---+ +-----+ |
+--| |---| |---| |----| |--+
|LER1 | |LSR| |LSR| |LER2 |
+--| |---| |---| |----| |--+
| +-----+ +---+ + P + +-----+ |
| |============ SPME ==============| |
|LERa|--|LSRb|-+ (Carrier 1) +-|LSRc|--|LERd|
Figure 18: SPME for a Set of Concatenated LSP Segments
SPMEs can be provisioned either statically or using control-plane
signaling procedures. The make-before-break procedures which are
supported by MPLS allow the creation of a SPME on existing LSPs in-
service without traffic disruption, as described in [SURVIVE-FWK]. A
SPME can be defined corresponding to one or more end-to-end LSPs.
New end-to-end LSPs that are tunneled within the SPME can be set up,
which may require coordination across administrative boundaries.
Traffic of the existing LSPs is switched over to the new end-to-end
tunneled LSPs. The old end-to-end LSPs can then be torn down.
Hierarchical label stacking, in a similar manner to that described
above, can be used to implement Sub-Path Maintenance Elements on
pseudowires, as described in [OAM-FRAMEWORK].
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3.14. Network Management
The network management architecture and requirements for MPLS-TP are
specified in [NM-FRAMEWORK] and [NM-REQ]. These derive from the
generic specifications described in ITU-T G.7710/Y.1701 [G.7710] for
transport technologies. They also incorporate the OAM requirements
for MPLS Networks [RFC4377] and MPLS-TP Networks [RFC5860] and expand
on those requirements to cover the modifications necessary for fault,
configuration, performance, and security in a transport network.
The Equipment Management Function (EMF) of an MPLS-TP Network Element
(NE) (i.e., LSR, LER, PE, S-PE, or T-PE) provides the means through
which a management system manages the NE. The Management
Communication Channel (MCC), realized by the G-ACh, provides a
logical operations channel between NEs for transferring management
information. The Network Management System (NMS) can be used to
provision and manage an end-to-end connection across a network.
Maintenance operations are run on a connection (LSP or PW) in a
manner that is independent of the provisioning mechanism. Segments
may be created or managed by, for example, Netconf [RFC4741], SNMP
[RFC3411], or CORBA (Common Object Request Broker Architecture)
interfaces, but not all segments need to be created or managed using
the same type of interface. Where an MPLS-TP NE is managed by an
NMS, at least one of these standard management mechanisms is required
for interoperability, but this document imposes no restriction on
which of these standard management protocols is used. In MPLS-TP,
the EMF needs to support statically provisioning LSPs for an LSR or
LER, and PWs for a PE, as well as any associated MEPs and MIPs, as
per Section 3.11.
Fault Management (FM) functions within the EMF of an MPLS-TP NE
enable the supervision, detection, validation, isolation, correction,
and alarm handling of abnormal conditions in the MPLS-TP network and
its environment. FM needs to provide for the supervision of
transmission (such as continuity, connectivity, etc.), software
processing, hardware, and environment. Alarm handling includes alarm
severity assignment, alarm suppression/aggregation/correlation, alarm
reporting control, and alarm reporting.
Configuration Management (CM) provides functions to control,
identify, collect data from, and provide data to MPLS-TP NEs. In
addition to general configuration for hardware, software protection
switching, alarm reporting control, and date/time setting, the EMF of
the MPLS-TP NE also supports the configuration of maintenance entity
identifiers (such as Maintenance Entity Group Endpoint (MEP) ID and
MEG Intermediate Point (MIP) ID). The EMF also supports the
configuration of OAM parameters as a part of connectivity management
to meet specific operational requirements. These may specify whether
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the operational mode is one-time on-demand or is periodic at a
specified frequency.
The Performance Management (PM) functions within the EMF of an
MPLS-TP NE support the evaluation and reporting of the behavior of
the NEs and the network. One particular requirement for PM is to
provide coherent and consistent interpretation of the network
behavior in a hybrid network that uses multiple transport
technologies. Packet loss measurement and delay measurements may be
collected and used to detect performance degradation. This is
reported via fault management to enable corrective actions to be
taken (e.g., protection switching) and via performance monitoring for
Service Level Agreement (SLA) verification and billing. Collection
mechanisms for performance data should be capable of operating on-
demand or proactively.
4. Security Considerations
The introduction of MPLS-TP into transport networks means that the
security considerations applicable to both MPLS [RFC3031] and PWE3
[RFC3985] apply to those transport networks. When an MPLS function
is included in the MPLS transport profile, the security
considerations pertinent to that function apply to MPLS-TP.
Furthermore, when general MPLS networks that utilize functionality
outside of the strict MPLS Transport Profile are used to support
packet transport services, the security considerations of that
additional functionality also apply.
For pseudowires, the security considerations of [RFC3985] and
[RFC5659] apply.
MPLS-TP nodes that implement the G-ACh create a Control Channel (CC)
associated with a pseudowire, LSP, or section. This control channel
can be signaled or statically configured. Over this control channel,
control channel messages related to network maintenance functions
such as OAM, signaling, or network management are sent. Therefore,
three different areas are of concern from a security standpoint.
The first area of concern relates to control plane parameter and
status message attacks, that is, attacks that concern the signaling
of G-ACh capabilities. MPLS-TP Control Plane security is discussed
in [RFC5920].
A second area of concern centers on data-plane attacks, that is,
attacks on the G-ACh itself. MPLS-TP nodes that implement the G-ACh
mechanisms are subject to additional data-plane denial-of-service
attacks as follows:
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An intruder could intercept or inject G-ACh packets effectively
disrupting the protocols carried over the G-ACh.
An intruder could deliberately flood a peer MPLS-TP node with
G-ACh messages to deny services to others.
A misconfigured or misbehaving device could inadvertently flood a
peer MPLS-TP node with G-ACh messages that could result in denial
of services. In particular, if a node has either implicitly or
explicitly indicated that it cannot support one or all of the
types of G-ACh protocol, but is sent those messages in sufficient
quantity, it could result in a denial of service.
To protect against these potential (deliberate or unintentional)
attacks, multiple mitigation techniques can be employed:
G-ACh message throttling mechanisms can be used, especially in
distributed implementations that have a centralized control-plane
processor with various line cards attached by some control-plane
data path. In these architectures, G-ACh messages may be
processed on the central processor after being forwarded there by
the receiving line card. In this case, the path between the line
card and the control processor may become saturated if appropriate
G-ACh traffic throttling is not employed, which could lead to a
complete denial of service to users of the particular line card.
Such filtering is also useful for preventing the processing of
unwanted G-ACh messages, such as those which are sent on unwanted
(and perhaps unadvertised) control channel types.
A third and last area of concern relates to the processing of the
actual contents of G-ACh messages. It is necessary that the
definition of the protocols using these messages carried over a G-ACh
include appropriate security measures.
Additional security considerations apply to each MPLS-TP solution.
These are discussed further in [SEC-FRAMEWORK].
The security considerations in [RFC5920] apply.
5. IANA Considerations
IANA considerations resulting from specific elements of MPLS-TP
functionality will be detailed in the documents specifying that
functionality.
This document introduces no additional IANA considerations in itself.
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6. Acknowledgements
The editors wish to thank the following for their contributions to
this document:
o Rahul Aggarwal
o Dieter Beller
o Malcolm Betts
o Italo Busi
o John E Drake
o Hing-Kam Lam
o Marc Lasserre
o Vincenzo Sestito
o Nurit Sprecher
o Martin Vigoureux
o Yaacov Weingarten
o The participants of ITU-T SG15
7. References
7.1. Normative References
[G.7710] ITU-T, "Common equipment management function
requirements", ITU-T Recommendation G.7710/Y.1701,
July 2007.
[G.805] ITU-T, "Generic Functional Architecture of Transport
Networks", ITU-T Recommendation G.805, November
1995.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon,
"Multiprotocol Label Switching Architecture", RFC
3031, January 2001.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label
Stack Encoding", RFC 3032, January 2001.
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RFC 5921 MPLS Transport Profile Framework July 2010
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S.,
Vaananen, P., Krishnan, R., Cheval, P., and J.
Heinanen, "Multi-Protocol Label Switching (MPLS)
Support of Differentiated Services", RFC 3270, May
2002.
[RFC3473] Berger, L., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation
Protocol-Traffic Engineering (RSVP-TE) Extensions",
RFC 3473, January 2003.
[RFC3985] Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-
to-Edge (PWE3) Architecture", RFC 3985, March 2005.
[RFC4090] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
May 2005.
[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.
[RFC4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and
G. Heron, "Pseudowire Setup and Maintenance Using
the Label Distribution Protocol (LDP)", RFC 4447,
April 2006.
[RFC4872] Lang, J., Rekhter, Y., and D. Papadimitriou,
"RSVP-TE Extensions in Support of End-to-End
Generalized Multi-Protocol Label Switching (GMPLS)
Recovery", RFC 4872, May 2007.
[RFC5085] Nadeau, T. and C. Pignataro, "Pseudowire Virtual
Circuit Connectivity Verification (VCCV): A Control
Channel for Pseudowires", RFC 5085, December 2007.
[RFC5586] Bocci, M., Vigoureux, M., and S. Bryant, "MPLS
Generic Associated Channel", RFC 5586, June 2009.
7.2. Informative References
[CP-FRAMEWORK] Andersson, L., Berger, L., Fang, L., Bitar, N.,
Takacs, A., Vigoureux, M., Bellagamba, E., and E.
Gray, "MPLS-TP Control Plane Framework", Work in
Progress, March 2010.
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RFC 5921 MPLS Transport Profile Framework July 2010
[DATA-PLANE] Frost, D., Bryant, S., and M. Bocci, "MPLS Transport
Profile Data Plane Architecture", Work in Progress,
July 2010.
[DYN-MS-PW] Martini, L., Bocci, M., Balus, F., Bitar, N., Shah,
H., Aissaoui, M., Rusmisel, J., Serbest, Y., Malis,
A., Metz, C., McDysan, D., Sugimoto, J., Duckett,
M., Loomis, M., Doolan, P., Pan, P., Pate, P.,
Radoaca, V., Wada, Y., and Y. Seo, "Dynamic
Placement of Multi Segment Pseudo Wires", Work in
Progress, October 2009.
[G.8080] ITU-T, "Architecture for the automatically switched
optical network (ASON)", ITU-T Recommendation
G.8080/Y.1304, 2005.
[IDENTIFIERS] Bocci, M. and G. Swallow, "MPLS-TP Identifiers",
Work in Progress, March 2010.
[NM-FRAMEWORK] Mansfield, S., Ed., Gray, E., Ed., and H. Lam, Ed.,
"MPLS-TP Network Management Framework", Work in
Progress, February 2010.
[NM-REQ] Mansfield, S. and K. Lam, "MPLS TP Network
Management Requirements", Work in Progress, October
2009.
[OAM-DEF] Andersson, L., Helvoort, H., Bonica, R., Romascanu,
D., and S. Mansfield, "The OAM Acronym Soup", Work
in Progress, June 2010.
[OAM-FRAMEWORK] Busi, I., Ed., Niven-Jenkins, B., Ed., and D. Allan,
Ed., "MPLS-TP OAM Framework", Work in Progress,
April 2010.
[PW-REDUNDANCY] Muley, P., "Pseudowire (PW) Redundancy", Work in
Progress, May 2010.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T.,
Srinivasan, V., and G. Swallow, "RSVP-TE: Extensions
to RSVP for LSP Tunnels", RFC 3209, December 2001.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network
Management Protocol (SNMP) Management Frameworks",
STD 62, RFC 3411, December 2002.
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RFC 5921 MPLS Transport Profile Framework July 2010
[RFC3443] Agarwal, P. and B. Akyol, "Time To Live (TTL)
Processing in Multi-Protocol Label Switching (MPLS)
Networks", RFC 3443, January 2003.
[RFC3471] Berger, L., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Functional Description",
RFC 3471, January 2003.
[RFC3945] Mannie, E., "Generalized Multi-Protocol Label
Switching (GMPLS) Architecture", RFC 3945, October
2004.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual
Private Networks (VPNs)", RFC 4364, February 2006.
[RFC4377] Nadeau, T., Morrow, M., Swallow, G., Allan, D., and
S. Matsushima, "Operations and Management (OAM)
Requirements for Multi-Protocol Label Switched
(MPLS) Networks", RFC 4377, February 2006.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-
Protocol Label Switched (MPLS) Data Plane Failures",
RFC 4379, February 2006.
[RFC4664] Andersson, L. and E. Rosen, "Framework for Layer 2
Virtual Private Networks (L2VPNs)", RFC 4664,
September 2006.
[RFC4726] Farrel, A., Vasseur, J., and A. Ayyangar, "A
Framework for Inter-Domain Multiprotocol Label
Switching Traffic Engineering", RFC 4726, November
2006.
[RFC4741] Enns, R., "NETCONF Configuration Protocol", RFC
4741, December 2006.
[RFC5150] Ayyangar, A., Kompella, K., Vasseur, JP., and A.
Farrel, "Label Switched Path Stitching with
Generalized Multiprotocol Label Switching Traffic
Engineering (GMPLS TE)", RFC 5150, February 2008.
[RFC5254] Bitar, N., Bocci, M., and L. Martini, "Requirements
for Multi-Segment Pseudowire Emulation Edge-to-Edge
(PWE3)", RFC 5254, October 2008.
[RFC5309] Shen, N. and A. Zinin, "Point-to-Point Operation
over LAN in Link State Routing Protocols", RFC 5309,
October 2008.
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RFC 5921 MPLS Transport Profile Framework July 2010
[RFC5331] Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS
Upstream Label Assignment and Context-Specific Label
Space", RFC 5331, August 2008.
[RFC5654] Niven-Jenkins, B., Brungard, D., Betts, M.,
Sprecher, N., and S. Ueno, "Requirements of an MPLS
Transport Profile", RFC 5654, September 2009.
[RFC5659] Bocci, M. and S. Bryant, "An Architecture for Multi-
Segment Pseudowire Emulation Edge-to-Edge", RFC
5659, October 2009.
[RFC5718] Beller, D. and A. Farrel, "An In-Band Data
Communication Network For the MPLS Transport
Profile", RFC 5718, January 2010.
[RFC5860] Vigoureux, M., Ward, D., and M. Betts, "Requirements
for Operations, Administration, and Maintenance
(OAM) in MPLS Transport Networks", RFC 5860, May
2010.
[RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G.
Swallow, "Bidirectional Forwarding Detection (BFD)
for MPLS Label Switched Paths (LSPs)", RFC 5884,
June 2010.
[RFC5885] Nadeau, T. and C. Pignataro, "Bidirectional
Forwarding Detection (BFD) for the Pseudowire
Virtual Circuit Connectivity Verification (VCCV)",
RFC 5885, June 2010.
[RFC5920] Fang, L., Ed., "Security Framework for MPLS and
GMPLS Networks", RFC 5920, July 2010.
[ROSETTA-STONE] Sprecher, N., "A Thesaurus for the Terminology used
in Multiprotocol Label Switching Transport Profile
(MPLS-TP) drafts/RFCs and ITU-T's Transport Network
Recommendations.", Work in Progress, May 2010.
[SEC-FRAMEWORK] Fang, L. and B. Niven-Jenkins, "Security Framework
for MPLS-TP", Work in Progress, March 2010.
[SEGMENTED-PW] Martini, L., Nadeau, T., Metz, C., Bocci, M., and M.
Aissaoui, "Segmented Pseudowire", Work in Progress,
June 2010.
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[SURVIVE-FWK] Sprecher, N. and A. Farrel, "Multiprotocol Label
Switching Transport Profile Survivability
Framework", Work in Progress, June 2010.
[VPMS-REQS] Kamite, Y., JOUNAY, F., Niven-Jenkins, B., Brungard,
D., and L. Jin, "Framework and Requirements for
Virtual Private Multicast Service (VPMS)", Work in
Progress, October 2009.
[X.200] ITU-T, "Information Technology - Open Systems
Interconnection - Basic reference Model: The Basic
Model", ITU-T Recommendation X.200, 1994.
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Authors' Addresses
Matthew Bocci (editor)
Alcatel-Lucent
Voyager Place, Shoppenhangers Road
Maidenhead, Berks SL6 2PJ
United Kingdom
EMail: matthew.bocci@alcatel-lucent.com
Stewart Bryant (editor)
Cisco Systems
250 Longwater Ave
Reading RG2 6GB
United Kingdom
EMail: stbryant@cisco.com
Dan Frost (editor)
Cisco Systems
EMail: danfrost@cisco.com
Lieven Levrau
Alcatel-Lucent
7-9, Avenue Morane Sulnier
Velizy 78141
France
EMail: lieven.levrau@alcatel-lucent.com
Lou Berger
LabN Consulting, L.L.C.
EMail: lberger@labn.net
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