MPLS Working Group M. Bocci, Ed.
Internet-Draft Alcatel-Lucent
Intended status: Standards Track S. Bryant, Ed.
Expires: April 19, 2010 D. Frost
Cisco Systems
L. Levrau
Alcatel-Lucent
October 16, 2009
A Framework for MPLS in Transport Networks
draft-ietf-mpls-tp-framework-06
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Abstract
This document specifies an architectural framework for the
application of Multi Protocol Label Switching (MPLS) in transport
networks, by enabling the construction of packet switched equivalents
to traditional circuit switched carrier 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 for point-to-point paths, including signaled
or explicitly provisioned bi-directional 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 exist in existing MPLS specifications, while
others require extensions to existing specifications to meet the
requirements of the MPLS-TP.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119].
Although this document is not a protocol specification, these key
words are to be interpreted as instructions to the protocol designers
producing solutions that satisfy the architectural concepts set out
in this document.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation and Background . . . . . . . . . . . . . . . . 4
1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.1. MPLS Transport Profile. . . . . . . . . . . . . . . . 6
1.3.2. MPLS-TP Section . . . . . . . . . . . . . . . . . . . 6
1.3.3. MPLS-TP Label Switched Path . . . . . . . . . . . . . 6
1.3.4. MPLS-TP Label Switching Router (LSR) and Label
Edge Router (LER) . . . . . . . . . . . . . . . . . . 7
1.3.5. MPLS-TP Customer Edge (CE) . . . . . . . . . . . . . . 8
1.3.6. Additional Definitions and Terminology . . . . . . . . 8
1.4. Applicability . . . . . . . . . . . . . . . . . . . . . . 8
2. Introduction to Requirements . . . . . . . . . . . . . . . . . 10
3. Transport Profile Overview . . . . . . . . . . . . . . . . . . 11
3.1. Packet Transport Services . . . . . . . . . . . . . . . . 11
3.2. Scope of MPLS Transport Profile . . . . . . . . . . . . . 12
3.3. Architecture . . . . . . . . . . . . . . . . . . . . . . . 12
3.3.1. MPLS-TP Adaptation . . . . . . . . . . . . . . . . . . 13
3.3.2. MPLS-TP Forwarding Functions . . . . . . . . . . . . . 13
3.4. MPLS-TP Client Adaptation . . . . . . . . . . . . . . . . 15
3.4.1. Adaptation using Pseudowires . . . . . . . . . . . . . 15
3.4.2. Network Layer Clients . . . . . . . . . . . . . . . . 18
3.5. Identifiers . . . . . . . . . . . . . . . . . . . . . . . 21
3.6. Operations, Administration and Maintenance (OAM) . . . . . 22
3.6.1. OAM Architecture . . . . . . . . . . . . . . . . . . . 22
3.6.2. OAM Functions . . . . . . . . . . . . . . . . . . . . 25
3.7. Generic Associated Channel (G-ACh) . . . . . . . . . . . . 26
3.8. Control Plane . . . . . . . . . . . . . . . . . . . . . . 29
3.8.1. PW Control Plane . . . . . . . . . . . . . . . . . . . 31
3.8.2. LSP Control Plane . . . . . . . . . . . . . . . . . . 31
3.9. Static Operation of LSPs and PWs . . . . . . . . . . . . . 32
3.10. Survivability . . . . . . . . . . . . . . . . . . . . . . 32
3.11. Network Management . . . . . . . . . . . . . . . . . . . . 33
4. Security Considerations . . . . . . . . . . . . . . . . . . . 34
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
7. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 36
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
8.1. Normative References . . . . . . . . . . . . . . . . . . . 36
8.2. Informative References . . . . . . . . . . . . . . . . . . 38
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1. Introduction
1.1. Motivation and Background
This document describes a framework for a Multiprotocol Label
Switching Transport Profile (MPLS-TP). It presents the architectural
framework for MPLS-TP, defining those elements of MPLS applicable to
supporting the requirements in [RFC5654] and what new protocol
elements are required.
Historically the optical transport infrastructure (Synchronous
Optical Networking (SONET)/Synchronous Digital Hierarchy (SDH),
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 or by network management.
o A high level of protection and availability.
o Quality of service.
o Extended OAM capabilities.
Carriers wish to evolve such transport networks to support packet
based services, and to take advantage of the flexibility and cost
benefits of packet switching technology. While MPLS is a maturing
packet technology that is already playing an important role in
transport networks and services, not all of MPLS's capabilities and
mechanisms are needed and/or consistent with transport network
operations. There are also transport technology characteristics that
are not currently reflected in MPLS.
The types of packet transport services delivered by transport
networks are very similar to Layer 2 Virtual Private Networks defined
by the IETF.
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.
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In order to achieve these objectives, there is a need to create a
common set of new functions that are applicable to both MPLS networks
in general, and those belonging to the MPLS-TP profile.
MPLS-TP therefore defines a profile of MPLS targeted at transport
applications and networks. This profile specifies the specific MPLS
characteristics and extensions required to meet transport
requirements.
1.2. Scope
This document describes an architectural framework for the
application of MPLS to 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).
The architecture for point-to-point MPLS-TP paths is described. The
architecture for point-to-multipoint paths is outside the scope of
this document.
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1.3. Terminology
Term Definition
---------------- ------------------------------------------
LSP Label Switched Path
MPLS-TP MPLS Transport profile
SDH Synchronous Digital Hierarchy
ATM Asynchronous Transfer Mode
OTN Optical Transport Network
cl-ps Connectionless - Packet Switched
co-cs Connection Oriented - Circuit Switched
co-ps Connection Oriented - Packet Switched
OAM Operations, Administration and Maintenance
G-ACh Generic Associated Channel
GAL Generic Alert Label
MEP Maintenance End Point
MIP Maintenance Intermediate Point
APS Automatic Protection Switching
SCC Signaling Communication Channel
MCC Management Communication Channel
EMF Equipment Management Function
FM Fault Management
CM Configuration Management
PM Performance Management
LSR Label Switch Router.
MPLS-TP PE MPLS-TP Provider Edge
MPLS-TP P Router An MPLS-TP Provider (P) router
PW Pseudowire
1.3.1. 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 the
transport network requirement [RFC5654].
1.3.2. MPLS-TP Section
An MPLS-TP Section is defined in Section 1.1.2 of [RFC5654].
1.3.3. 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:
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1. Uses a subset of the MPLS OAM tools defined as described in
[I-D.ietf-mpls-tp-oam-framework].
2. Supports only 1+1, 1:1, and 1:N protection functions.
3. Is traffic engineered.
4. Is established and maintained using GMPLS protocols when a
control plane is used.
5. LSPs can only be point to point or point to multipoint, i.e. the
merging of LSPs is not permitted.
Note that an MPLS LSP is defined to include any present and future
MPLS capability include those specifically added to support the
transport network requirements.
1.3.4. MPLS-TP Label Switching Router (LSR) and Label Edge Router (LER)
An MPLS-TP Label Switching Router (MPLS-TP LSR) is either an MPLS-TP
Provider Edge (MPLS-TP PE) or an MPLS-TP Provider (MPLS-TP P Router)
router for a given LSP, as defined below. The terms MPLS-TP PE and
MPLS-TP P router describe functions and specific node may undertake
both roles.
Note that the use of the term "router" in this context is historic
and neither requires nor precludes the ability to perform IP
forwarding.
1.3.4.1. MPLS-TP Provider Edge Router (PE)
An MPLS-TP Provider Edge Router is an MPLS-TP LSR that adapts client
traffic and encapsulates it to be carried 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.
A layer network is defined in [G.805].
1.3.4.2. MPLS-TP Provider Router (P)
An MPLS-TP Provider router is an MPLS-TP LSR that does not provide
MPLS-TP PE functionality. An MPLS-TP P router switches LSPs which
carry client traffic, but do not adapt the client traffic and
encapsulate it to be carried over an MPLS-TP LSP.
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1.3.5. MPLS-TP Customer Edge (CE)
An MPLS-TP Customer Edge is the client function sourcing or sinking
client 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 point to point or point to multi-point link. These clients
have no knowledge of the presence of the interveining MPLS-TP
network.
1.3.6. Additional Definitions and Terminology
Detailed definitions and additional terminology may be found in
[RFC5654].
1.4. Applicability
MPLS-TP can be used to construct a packet transport networks and is
therefore applicable in any packet transport network application. It
is also as an alternative architecture for subsets of a packet
network where the transport network model is deemed attractive. The
following are examples of MPLS-TP applicability models:
1. MPLS-TP provided by a network that only supports MPLS-TP, acting
as a server for other layer 1, layer 2 and layer 3 networks
(Figure 1).
2. MPLS-TP provided by a network that also supports non-MPLS-TP
functions, acting as a server for other layer 1, layer 2 and
layer 3 networks (Figure 2).
3. MPLS-TP as a server layer for client layer traffic of IP or MPLS
networks which do not use functions of the MPLS transport profile
(Figure 3).
These models are not mutually exclusive.
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MPLS-TP LSP, provided by a network that only supports MPLS-TP, acting as a server
for other layer 1, layer 2 and layer 3 networks.
|<-- L1/2/3 -->|<-- MPLS-TP-->|<-- L1/2/3 -->|
Only
MPLS-TP
+---+ LSP +---+
+---+ Client | |----------| | Client +---+
|CE1|==Traffic=|PE2|==========|PE3|=Traffic==|CE1|
+---+ | |----------| | +---+
+---+ +---+
Example a) [Ethernet] [Ethernet] [Ethernet]
layering [ PW ]
[-TP LSP ]
b) [ IP ] [ IP ] [ IP ]
[ LSP ]
[-TP LSP ]
Figure 1: MPLS-TP Server Layer Example
MPLS-TP LSP, provided by a network that also supports non-MPLS-TP functions,
acting as a server for other layer 1, layer 2 and layer 3 networks.
|<-- L1/2/3 -->|<-- MPLS -->|<-- L1/2/3 -->|
MPLS-TP
+---+ LSP +---+
+---+ Client | |----------| | Client +---+
|CE1|==Traffic=|PE2|==========|PE3|=Traffic==|CE1|
+---+ | |----------| | +---+
+---+ +---+
Example a) [Ethernet] [Ethernet] [Ethernet]
layering [ PW ]
[-TP LSP ]
b) [ IP ] [ IP ] [ IP ]
[ LSP ]
[-TP LSP ]
Figure 2: MPLS-TP in MPLS Network Example
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MPLS-TP as a server layer for client layer traffic of IP or MPLS networks which
do not use functions of the MPLS transport profile.
|<-- MPLS ---->|<-- MPLS-TP-->|<--- MPLS --->|
Only
+---+ +---+ Non-TP +---+ MPLS-TP +---+ Non-TP +---+ +---+
|CE1|---|PE1|====LSP===|PE2|====LSP===|PE3|====LSP===|PE4|-----|CE2|
+---+ +---+ +---+ +---+ +---+ +---+
(a) [ Eth ] [ Eth ] [ Eth ] [ Eth ] [ Eth ]
[ MS-PW ] [ MS-PW ] [ MS-PW ]
[ LSP ] [-TP LSP ] [ LSP ]
(a) [ IP ] [ IP ] [ IP ] [ IP ] [ IP ]
[ LSP ] [-TP LSP ] [ LSP ]
Figure 3: MPLS-TP Transporting Client Service Traffic
2. Introduction to Requirements
The requirements for MPLS-TP are specified in [RFC5654],
[I-D.ietf-mpls-tp-oam-requirements], and [I-D.ietf-mpls-tp-nm-req].
This section provides a brief reminder to guide the reader and is
therefore not normative. It is not 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 bi-directional, and it
must be possible to construct congruent Bi-directional 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, protection and forwarding of data packets must be able to
operate without IP forwarding support.
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
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case information gained from the OAM functions is used to initiate
path recovery actions at either the PW or LSP layers.
3. 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
inherit 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 the server or
client layer network.
o The service provided by the MPLS-TP network to the client is
guaranteed not to fall below the agreed level regardless of other
client activity.
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.
o Where a client network makes use of an MPLS-TP server that
provides a packet transport service, the level of co-ordination
required between the client and server layer networks is minimal
(preferably no co-ordination 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 CNLS 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.
Therefore, a packet transport service doe not support a
connectionless packet switched forwarding mode. However, this does
not preclude it carrying client traffic associated with a
connectionless service.
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3.2. Scope of MPLS Transport Profile
Figure 4 illustrates the scope of MPLS-TP. MPLS-TP solutions are
primarily intended for packet transport applications. MPLS-TP is a
strict sub-set of MPLS, and comprises those functions that 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] e.g.
Equal Cost Multi-Path, LDP signaling used in such a way that it
creates multi-point to point LSPs, and IP forwarding in the data
plane are explicitly excluded from MPLS-TP by that requirements
specification.
Note that this does not preclude the future definition of MPLS
functions that do not meet the requirements of [RFC5654] and thus
fall outside the scope of MPLS-TP as defined by this document.
{Additional Transport Functions}
|<============== MPLS-TP ==================>|
{ ECMP, mp2p LDP, IP fwd }
|<====== Pre-RFC5654 MPLS ===========>|
|<============================== MPLS ==============================>|
Figure 4: Scope of MPLS-TP
3.3. Architecture
MPLS-TP comprises the following
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. e.g.
connectivity check, connectivity verification, and performance
monitoring.
o Optional control planes for LSPs and PWs, as well as static
configuration.
o Path protection mechanisms to ensure that the packet transport
service survives anticipated failures and degradations of the
MPLS-TP network.
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o Network management functions.
The MPLS-TP architecture for LSPs and PWs includes the the following
two sets of functions:
o MPLS-TP adaptation
o MPLS-TP forwarding
The adaptation functions interface the client service to MPLS-TP.
This includes the case where the client service is an MPLS-TP LSP.
The forwarding functions comprise the mechanisms required for
forwarding the encapsulated client over an MPLS-TP server layer
network E.g. PW label and LSP label.
3.3.1. MPLS-TP Adaptation
The MPLS-TP adaptation interfaces the client service to MPLS-TP. For
pseudowires, these adaptation functions are the payload encapsulation
shown in Figure 7 of [RFC3985] and Figure 7 of
[I-D.ietf-pwe3-ms-pw-arch]. For network layer client services, the
adaptation function uses the MPLS encapsulation format as defined in
RFC 3032[RFC3032].
The purpose of this encapsulation is to abstract the client service
data plane 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 QoS from the MPLS-TP network, which
in turn inherits its QoS from the server layer. The server layer
must therefore provide the necessary Quality of Service (QoS) to
ensure that the MPLS-TP client QoS commitments are satisfied.
3.3.2. MPLS-TP Forwarding Functions
The forwarding functions comprise the mechanisms required for
forwarding the encapsulated client over an MPLS-TP server layer
network E.g. PW label and LSP label.
MPLS-TP LSPs use the MPLS label switching operations and TTL
processing procedures defined in [RFC3031] and [RFC3032]. These
operations are highly optimized for performance and are not modified
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by the MPLS-TP profile.
In addition, MPLS-TP PWs use the PW and MS-PW forwarding operations
defined in[RFC3985] and [I-D.ietf-pwe3-ms-pw-arch]. The PW label is
processed by a PW forwarder and is always at the bottom of the label
stack for a given MPLS-TP layer network.
Per-platform label space is used for PWs. Either per-platform, per-
interface or other context-specific label space 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 design protocol extensions that requires
packets (e.g. OAM 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. Otherwise
the packet is forwarded according to the procedures in [RFC3032].
Point to point MPLS-TP LSPs can be either unidirectional or
bidirectional.
It MUST be possible to configure an MPLS-TP LSP such that the forward
and backward directions of a bidirectional MPLS-TP LSP are co-routed
i.e. they follow the same path. The pairing relationship between the
forward and the backward directions must be known at each LSR or LER
on a bidirectional LSP.
In normal conditions, all the packets sent over a PW or an LSP follow
the same path through the network and those that belong to a common
ordered aggregate are delivered in order. For example per-packet
equal cost multi-path (ECMP) load balancing is not applicable to
MPLS-TP LSPs.
Penultimate hop popping (PHP) is disabled on MPLS-TP LSPs by default.
Both E-LSP and L-LSP are supported in MPLS-TP, as defined in
[RFC3270].
The Traffic Class field (formerly the MPLS EXP field) follows the
definition and processing rules of [RFC5462] and [RFC3270].
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Only the pipe and short-pipe models are supported in MPLS-TP.
3.4. MPLS-TP Client Adaptation
This document specifies the architecture for two types of client
adaptation:
o A PW
o An MPLS Label
When the client is a PW, the MPLS-TP client adaptation functions
include the PW encapsulation mechanisms, including the PW control
word. When the client is operating at the network layer the
mechanism described in Section 3.4.2 is used.
3.4.1. Adaptation using Pseudowires
The architecture for a transport profile of MPLS (MPLS-TP) that uses
PWs is based on the MPLS [RFC3031] and pseudowire [RFC3985]
architectures. If multi-segment pseudowires are used to provide a
packet transport service, motivated by, for example, the requirements
specified in [RFC5254] then the MS-PW architecture
[I-D.ietf-pwe3-ms-pw-arch] also applies.
Figure 5 shows the architecture for an MPLS-TP network using single-
segment PWs.
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|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudowire ------->| |
| | encapsulated | |
| | Pkt Xport Service | |
| | | |
| | |<-- PSN Tunnel -->| | |
| V V V V |
V AC +----+ +---+ +----+ AC V
+-----+ | | PE1|======:=X=:=======| PE2| | +-----+
| |----------|...........:PW1:............|----------| |
| CE1 | | | | | : | | | | CE2 |
| |----------|...........:PW2:............|----------| |
+-----+ ^ | | |======:=X=:=======| | | ^ +-----+
^ | +----+ +---+ +----+ | | ^
| | Provider Edge 1 ^ Provider Edge 2 | |
| | | | |
Customer | P Router | Customer
Edge 1 | | Edge 2
| |
| |
Native service Native service
Figure 5: MPLS-TP Architecture (Single Segment PW)
Figure 6 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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|<-------------------Pseudowire-------------------->|
| encapsulated |
| Pkt Xport Service |
| |
| PSN |
AC | |<------- PSN tun1------>| |<--tun2-->| | AC
| V V V V V V |
| +----+ +-----+ +----+ +----+ |
+---+ | |TPE1|===============\ /=====|SPE1|==========|TPE2| | +---+
| |---|......PW.Seg't1... | \ / | ......X...PW.Seg't3.....|---| |
|CE1| | | | | X | | | | | | |CE2|
| |---|......PW.Seg't2... | / \ | ......X...PW.Seg't4.....|---| |
+---+ | | |===============/ \=====| |==========| | | +---+
^ +----+ ^ +-----+ +----+ ^ +----+ ^
| | ^ | |
| TE LSP | TE LSP |
| P-router |
| |
|<-------------------- Emulated Service ------------------->|
Figure 6: MPLS-TP Architecture (Multi-Segment PW)
The corresponding domain of the MPLS-TP protocol stack including PWs
is shown in Figure 7.
+-------------------+
| Client Layer |
/===================\ /===================\
H PW Encap H H PW OAM H
H-------------------H H-------------------H /===================\
H PW Demux (S=1) H H PW Demux (S=1) H H LSP OAM H
H-------------------H H-------------------H H-------------------H
H LSP Demux(s) H H LSP Demux(s) H H LSP Demux(s) H
\===================/ \===================/ \===================/
| Server Layer | | Server Layer | | Server Layer |
+-------------------+ +-------------------+ +-------------------+
User Traffic PW OAM LSP OAM
Note: Transport Service Layer = PW Demux
Transport Path Layer = LSP Demux
Figure 7: MPLS-TP Layer Network using Pseudowires
When providing a Virtual Private Wire Service (VPWS), Virtual Private
Local Area Network Service (VPLS), Virtual Private Multicast Service
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(VPMS) or Internet Protocol Local Area Network Service (IPLS),
pseudowires MUST be used to carry the client service. These PWs can
be configured either statically or via the control plane defined in
[RFC4447].
Note that in MPLS-TP environments where IP is used for control or OAM
purposes, IP MAY be carried over the LSP demultiplexers as per
RFC3031 [RFC3031], or directly over the server.
3.4.2. Network Layer Clients
MPLS-TP LSPs can be used to transport network layer clients. Any
network layer protocol can be transported between service interfaces.
Examples of network layer protocols include IP, MPLS and MPLS-TP.
With network layer transport, the MPLS-TP domain provides a
bidirectional point-to-point connection between two customer edge
(CE) nodes. Note that a CE may be an an IP, MPLS or MPLS-TP node.
As shown in Figure 8, there is an attachment circuit between the CE
node on the left and its corresponding provider edge (PE) node that
provides the service interface, a bidirectional LSP across the
MPLS-TP service 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.
|<------------- Client Network Layer-------------->|
| |
| |<---- Pkt Xport Service --->|
| | | |
| | |<-- PSN Tunnel -->| | |
| V V V V |
V AC +----+ +---+ +----+ AC V
+-----+ | | PE1|======:=X=:=======| PE2| | +-----+
| |----------|...........:LSP:............|----------| |
| CE1 | | | | | : | | | | CE2 |
| |----------|...........: IP:............|----------| |
+-----+ ^ | | |======:=X=:=======| | | ^ +-----+
^ | +----+ +---+ +----+ | | ^
| | Provider Edge 1 ^ Provider Edge 2 | |
| | | | |
Customer | P Router | Customer
Edge 1 | | Edge 2
| |
| |
Native service Native service
Figure 8: MPLS-TP Architecture for Network Layer Clients
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At the ingress service interface the PE transforms the ingress packet
to the format that will be carried over the transport network, and
similarly the corresponding service interface at the egress PE
transforms the packet to the format needed by the attached CE. 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 Layer1/Layer2 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.
+-------------------+
| Client Layer |
/===================\ /===================\
H Encap Label (S=1) H H SvcLSP OAM H
H-------------------H H-------------------H /===================\
H SvcLSP Demux H H SvcLSP Demux (S=1)H H LSP OAM H
H-------------------H H-------------------H H-------------------H
H LSP Demux(s) H H LSP Demux(s) H H LSP Demux(s) H
\===================/ \===================/ \===================/
| Server Layer | | Server Layer | | Server Layer |
+-------------------+ +-------------------+ +-------------------+
User Traffic Service LSP OAM LSP OAM
Note: Transport Service Layer = SvcLSP Demux
Transport Path Layer = LSP Demux
Note that the functions of the Encap label and the Service Label may represented
by a single label
Figure 9: Domain of MPLS-TP Layer Network for IP and LSP Clients
Within the MPLS-TP transport network, the network layer protocols are
carried over the MPLS-TP LSP using a 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. In accordance with [RFC3032], the bottom
label, with the 'bottom of stack' bit set to '1', defines the network
layer protocol being transported. Figure 9 shows how an a client
network protocol stack (which may be an MPLS label stack and payload)
is carried over as a network layer transport service over an MPLS-TP
transport network.
A label per network layer protocol payload type that is to be
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transported is REQUIRED. Such labels are referred to as "Service
Labels", one of which is shown in Figure 9. The mapping between
protocol payload type and Service Label is either configured or
signaled.
Service labels are typically carried over an MPLS-TP edge-to-edge
LSP, which is also shown in Figure 9. The use of an edge-to-edge LSP
is RECOMMENDED when more than one protocol payload type is to be
transported. For example, if only MPLS is carried then a single
Service Label would be used to provided both payload type indication
and the MPLS-TP edge-to-edge LSP. Alternatively, if both IP and MPLS
is to be carried then two Service Labels would be mapped on to a
common MPLS-TP edge-to-edge LSP.
As noted above, any layer 2 and layer 1 protocols used to carry the
network layer protocol over the attachment circuit is terminated at
the service interface and is not transported across the MPLS-TP
network. This enables the use of different layer 2 / layer 1
technologies at two service interfaces.
At each service interface, Layer 2 addressing must 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) which have Media Access Control (MAC) addresses. In cases
where a MAC address is needed, the sending node MUST set 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
covered in this document. In some technologies the MAC address will
need to be configured (Examples for the Ethernet case include a
configured unicast MAC address for the adjacent node, or even using
the broadcast MAC address when the CE-PE service interface is
dedicated. The configured address is then used as the MAC
destination address for all packets sent over the service interface.)
Note that when the two CEs operating over the network layer transport
service are running a routing protocol such as ISIS 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.
[Editors Note we need to confer with ISIS and OSPF WG to verify that
the cautionary note above is necessary and sufficient.]
The CE to CE service types and corresponding labels may be configured
or signaled. When they are signaled the CE to PE control channel may
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be either out-of-band or in-band. An out-of-band control channel
uses standard GMPLS out-of-band signaling techniques. There are a
number of methods that can be used to carry this signalling:
o It can be carried via an out-of-band control channel. (As is
commonly done in today's GMPLS controlled transport networks.)
o It could be carried over the attachment circuit with MPLS using a
reserved label.
o It could be carried over the attachment circuit with MPLS using a
normal label that is agreed between CE and PE.
o It could be carried over the attachment circuit in an ACH.
o It could be carried over the attachment circuit in IP.
In the MPLS and ACH cases above, this label value is used to carry
LSP signaling without any further encapsulation. This signaling
channel is always point-to-point and MUST use local CE and PE
addressing.
The method(s) to be used will be described in a future version of the
document.
3.5. Identifiers
Identifiers to be used in within MPLS-TP where compatibility with
existing MPLS control plane conventions are necessary are described
in [draft-swallow-mpls-tp-identifiers-00]. The MPLS-TP requirements
[RFC5654] require that the elements and objects in an MPLS-TP
environment are able to be configured and managed without a control
plane. In such an environment many conventions for defining
identifiers are possible. However it is also anticipated that
operational environments where MPLS-TP objects, LSPs and PWs will be
signaled via existing protocols such as the Label Distribution
Protocol [RFC4447] and the Resource Reservation Protocol as it is
applied to Generalized Multi-protocol Label Switching ( [RFC3471] and
[RFC3473]) (GMPLS). [draft-swallow-mpls-tp-identifiers-00] defines a
set of identifiers for MPLS-TP which are both compatible with those
protocols and applicable to MPLS-TP management and OAM functions.
MPLS-TP distinguishes between addressing used to identify nodes in
the network, and identifiers used for demultiplexing and forwarding.
Whilst IP addressing is used by default, MPLS-TP must be able to
operate in environments where IP is not used in the forwarding plane.
Therefore, the default mechanism for OAM demultiplexing in MPLS-TP
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LSPs and PWs is the generic associated channel. Forwarding based on
IP addresses for user or OAM packets is not REQUIRED for MPLS-TP.
[RFC4379]and BFD for MPLS LSPs [I-D.ietf-bfd-mpls] 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. These mechanisms are the
default mechanisms for MPLS networks in general for identifying MPLS
OAM packets when the OAM packets are encapsulated in an IP header.
MPLS-TP is unable to rely on the availability of IP and thus uses the
GACH/GAL to demultiplex OAM packets.
3.6. Operations, Administration and Maintenance (OAM)
MPLS-TP supports a comprehensive set of OAM capabilities for packet
transport applications, with equivalent capabilities to those
provided in SONET/SDH.
MPLS-TP defines mechanisms to differentiate specific packets (e.g.
OAM, APS, MCC or SCC) from those carrying user data packets on the
same LSP. These mechanisms are described in [RFC5586].
MPLS-TP requires [I-D.ietf-mpls-tp-oam-requirements] 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 [I-D.ietf-mpls-tp-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 Generic Alert Label (GAL) to create a
control channel associated to an LSP, Section or PW.
3.6.1. OAM Architecture
OAM and monitoring in MPLS-TP is based on the concept of maintenance
entities, as described in [I-D.ietf-mpls-tp-oam-framework]. A
Maintenance Entity can be viewed as the association of two (or more)
Maintenance End Points (MEPs) (see example in Figure 10 ). The MEPs
that form an ME should be configured and managed to limit the OAM
responsibilities of an OAM flow within a network or sub- network, or
a transport path or segment, in the specific layer network that is
being monitored and managed.
Each OAM flow is associated with a single ME. Each MEP within an ME
resides at the boundaries of that ME. An ME may also include a set
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of zero or more Maintenance Intermediate Points (MIPs), which reside
within the Maintenance Entity. Maintenance end points (MEPs) are
capable of sourcing and sinking OAM flows, while maintenance
intermediate points (MIPs) can only sink or respond to OAM flows.
========================== End to End LSP OAM ==========================
..... ..... ..... .....
-----|MIP|---------------------|MIP|---------|MIP|------------|MIP|-----
''''' ''''' ''''' '''''
|<-------- Carrier 1 --------->| |<--- Carrier 2 ----->|
---- --- --- ---- ---- --- ----
NNI | | | | | | | | NNI | | | | | | NNI
-----| PE |---| P |---| P |----| PE |--------| PE |---| P |---| PE |----
| | | | | | | | | | | | | |
---- --- --- ---- ---- --- ----
==== Segment LSP OAM ====== == Seg't == === Seg't LSP OAM ===
(Carrier 1) LSP OAM (Carrier 2)
(inter-carrier)
..... ..... ..... .......... .......... ..... .....
|MEP|---|MIP|---|MIP|--|MEP||MEP|---|MEP||MEP|--|MIP|----|MEP|
''''' ''''' ''''' '''''''''' '''''''''' ''''' '''''
<------------ ME ----------><--- ME ----><------- ME -------->
Note: MEPs for End-to-end LSP OAM exist outside of the scope
of this figure.
Figure 10: Example of MPLS-TP OAM
Figure 11 illustrates how the concept of Maintenance Entities can be
mapped to sections, LSPs and PWs in an MPLS-TP network that uses MS-
PWs.
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Native |<-------------------- PW15 --------------------->| Native
Layer | | Layer
Service | |<-PSN13->| |<-PSN3X->| |<-PSNXZ->| | Service
(AC1) V V LSP V V LSP V V LSP V V (AC2)
+----+ +-+ +----+ +----+ +-+ +----+
+---+ |TPE1| | | |SPE3| |SPEX| | | |TPEZ| +---+
| | | |=========| |=========| |=========| | | |
|CE1|------|........PW1.....X..|...PW3...|.X......PW5........|-----|CE2|
| | | |=========| |=========| |=========| | | |
+---+ | 1 | |2| | 3 | | X | |Y| | Z | +---+
+----+ +-+ +----+ +----+ +-+ +----+
|<- Subnetwork 123->| |<- Subnetwork XYZ->|
.------------------- PW15 PME -------------------.
.---- PW1 PTCME ----. .---- PW5 PTCME ---.
.---------. .---------.
PSN13 LME PSNXZ LME
.--. .--. .--------. .--. .--.
Sec12 SME Sec23 SME Sec3X SME SecXY SME SecYZ SME
TPE1: Terminating Provider Edge 1 SPE2: Switching Provider Edge 3
TPEX: Terminating Provider Edge X SPEZ: Switching Provider Edge Z
.---. ME . MEP ==== LSP .... PW
SME: Section Maintenance Entity
LME: LSP Maintenance Entity
PME: PW Maintenance Entity
Figure 11: MPLS-TP OAM archtecture
The following MPLS-TP MEs are specified in
[I-D.ietf-mpls-tp-oam-framework]:
o A Section Maintenance Entity (SME), allowing monitoring and
management of MPLS-TP Sections (between MPLS LSRs).
o A LSP Maintenance Entity (LME), allowing monitoring and management
of an end-to-end LSP (between LERs).
o A PW Maintenance Entity (PME), allowing monitoring and management
of an end-to-end SS/MS-PWs (between T-PEs).
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o An LSP Tandem Connection Maintenance Entity (LTCME), allowing
estimation of OAM fault and performance metrics of a single LSP
segment or of an aggregate of LSP segments. It also enables any
OAM function applied to segment(s) of an LSP to be independent of
the OAM function(s) operated on the end-to-end LSP. This can be
achieved by including a label representing the LTCME on one or
more LSP label stacks for 1:1 or N:1 monitoring of LSPs,
respectively. Note that the term Tandem Connection Monitoring has
historical significance dating back to the early days of the
telephone network, but is equally applicable to the hierarchal
architectures commonly employed in todays packet networks.
Individual MIPs along the path of an LSP or PW are addressed by
setting the appropriate TTL in the label for the OAM packet, as per
[I-D.ietf-pwe3-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
cases where this is not the case in general MPLS networks 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.
Within an LSR or PE, MEPs and MIPs can only be placed where MPLS
layer processing is performed on a packet. The architecture mandates
that this must occur at least once.
There is only one MIP on an LSP or PW in each node. That MIP is for
all applicable OAM functions on its associated LSP or PW. This
document does not specify the default position of the MIP within the
node. Therefore, this document does not specify where the exception
mechanism resides (i.e. at the ingress interface, the egress
interface, or some other location within the node). An optional
protocol may be developed that sets the position of a MIP along the
path of an LSP or PW within the node (and thus determines the
exception processing location).
MEPs may only act as a sink of OAM packets when the label associated
with the LSP or PW for that ME is popped. MIPs can only be placed
where an exception to the normal forwarding operation occurs. A MEP
may act as a source of OAM packets whereever a label is pushed or
swapped. For example, on a MS-PW, a MEP may source OAM within an
S-PE or a T-PE, but a MIP may only be associated with a S-PE and a
sink MEP can only be associated with a T-PE.
3.6.2. OAM Functions
The MPLS-TP OAM architecture support a wide range of OAM functions,
including the following
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o Continuity Check
o Connectivity Verification
o Performance monitoring (e.g. loss and delay)
o Alarm suppression
o Remote Integrity
These are applicable to any layer defined within MPLS-TP, i.e. MPLS
Section, LSP and PW.
The MPLS-TP OAM toolset needs to be able to operate without relying
on a dynamic control plane or IP functionality in the datapath. In
the case of MPLS-TP deployment with IP functionality, all existing
IP-MPLS OAM functions, e.g. LSP-Ping, BFD and VCCV, may be used.
This does not preclude the use of other OAM tools in an IP
addressable network.
One use of OAM mechanisms is to detect link failures, node failures
and performance outside the required specification which then may be
used to trigger recovery actions, according to the requirements of
the service.
3.7. Generic Associated Channel (G-ACh)
For correct operation of the OAM it is important that the OAM packets
fate share with the data packets. In addition in MPSL-TP it is
necessary to discriminate between user data payloads and other types
of payload. For example the packet may contain a Signaling
Communication Channel (SCC), or a channel used for Automatic
Protection Switching (APS) data. Such packets are carried on a
control channel associated to the LSP, Section or PW. This is
achieved by carrying such packets on a generic control channel
associated to the LSP, PW or section.
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, APS, SCC, MCC
or other packet types in band over LSPs or PWs. The G-ACH is defined
in [RFC5586] and it is similar to the Pseudowire Associated Channel
[RFC4385], which is used to carry OAM packets across pseudowires.
The G-ACH is indicated by a generic associated channel header (ACH),
similar to the Pseudowire VCCV control word, and this is present for
all Sections, LSPs and PWs making use of FCAPS functions supported by
the G-ACH.
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For pseudowires, the G-ACh use the first nibble of the pseudowire
control word to provide the initial discrimination between data
packets a packets belonging to the associated channel, as described
in[RFC4385]. When the first nibble of a packet, immediately
following the label at the bottom of stack, has a value of one, then
this packet belongs to a G-ACh. The first 32 bits following the
bottom of stack label then have a defined format called an associated
channel header (ACH), which further defines the content of the
packet. The ACH is therefore both a demultiplexer for G-ACh traffic
on the PW, and a discriminator for the type of G-ACh traffic.
When the OAM, or a similar message is carried over an LSP, rather
than over a pseudowire, it is necessary to provide an indication in
the packet that the payload is something other than a user data
packet. This is achieved by including a reserved label with a value
of 13 in the label stack. This reserved label is referred to as the
'Generic Alert Label (GAL)', and is defined in [RFC5586]. When a GAL
is found anywhere within the label stack it indicates that the
payload begins with an ACH. The GAL is thus a demultiplexer for
G-ACh traffic on the LSP, and the ACH is a discriminator for the type
of traffic carried on the G-ACh. Note however that MPLS-TP
forwarding follows the normal MPLS model, and that a GAL is invisible
to an LSR unless it is the top label in the label stack. The only
other circumstance under which the label stack may be inspected for a
GAL is when the TTL has expired. Any MPLS-TP component that
intentionally performs this inspection must assume that it is
asynchronous with respect to the forwarding of other packets. All
operations on the label stack are in accordance with [RFC3031] and
[RFC3032].
In MPLS-TP, the 'Generic Alert Label (GAL)' always appears at the
bottom of the label stack (i.e. S bit set to 1), however this does
not preclude its use elsewhere in the label stack in other
applications.
The G-ACH MUST only be used for channels that are an adjunct to the
data service. Examples of these are OAM, APS, MCC and SCC, but the
use is not restricted to those names 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.
Since the G-ACh traffic is indistinguishable from the user data
traffic at the server layer, bandwidth and QoS commitments apply to
the gross traffic on the LSP, PW or section. Protocols using the
G-ACh must therefore take into consideration the impact they have on
the user data that they are sharing resources with. In addition,
protocols using the G-ACh MUST conform to the security and congestion
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considerations described in [RFC5586]. .
Figure 12 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].
+-------------+ +-------------+
| Payload | < Service / FCAPS > | Payload |
+-------------+ +-------------+
| Demux / | < CW / ACH for PWs > | Demux / |
|Discriminator| |Discriminator|
+-------------+ +-------------+
| PW | < PW > | PW |
+-------------+ +-------------+
| PSN | < LSP > | PSN |
+-------------+ +-------------+
| Physical | | Physical |
+-----+-------+ +-----+-------+
| |
| ____ ___ ____ |
| _/ \___/ \ _/ \__ |
| / \__/ \_ |
| / \ |
+--------| MPLS/MPLS-TP Network |---+
\ /
\ ___ ___ __ _/
\_/ \____/ \___/ \____/
Figure 12: PWE3 Protocol Stack Reference Model including 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 13 shows the reference model depicting how the control channel
is associated with the LSP protocol stack.
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+-------------+ +-------------+
| Payload | < Service > | Payload |
+-------------+ +-------------+
|Discriminator| < ACH on LSP > |Discriminator|
+-------------+ +-------------+
|Demultiplexer| < GAL on LSP > |Demultiplexer|
+-------------+ +-------------+
| PSN | < LSP > | PSN |
+-------------+ +-------------+
| Physical | | Physical |
+-----+-------+ +-----+-------+
| |
| ____ ___ ____ |
| _/ \___/ \ _/ \__ |
| / \__/ \_ |
| / \ |
+--------| MPLS/MPLS-TP Network |---+
\ /
\ ___ ___ __ _/
\_/ \____/ \___/ \____/
Figure 13: MPLS Protocol Stack Reference Model including the LSP
Associated Control Channel
3.8. Control Plane
MPLS-TP should be capable of being operated with centralized Network
Management Systems (NMS). The NMS may be supported by a distributed
control plane, but MPLS-TP can operated in the absence of such a
control plane. A distributed control plane may be used to enable
dynamic service provisioning in multi-vendor and multi-domain
environments using standardized protocols that guarantee
interoperability. Where the requirements specified in [RFC5654] can
be met, the MPLS transport profile uses existing control plane
protocols for LSPs and PWs.
Figure 14 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 centralised or distributed. Control plane is
distributed
2) 'Edge' functions refers to those functions present at
the edge of a PSN domain, e.g. NSP or classification.
3) The control plane may be transported over the server
layer, and LSP or a G-ACh.
Figure 14: MPLS-TP Control Plane Architecture Context
The MPLS-TP control plane is based on a combination of the LDP-based
control plane for pseudowires [RFC4447] and the RSVP-TE based control
plane for MPLS-TP LSPs [RFC3471]. Some of the RSVP-TE functions that
are required for LSP signaling for MPLS-TP are based on GMPLS.
The distributed MPLS-TP control plane provides 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 Node Interface (I-NNI), and External Network Node Interface
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(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 3.6 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 the survivability
section Section 3.10 of this document. 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.
3.8.1. PW Control Plane
An MPLS-TP network provides many of its transport services using
single-segment or multi-segment pseudowires, in compliance with the
PWE3 architecture ([RFC3985] and [I-D.ietf-pwe3-ms-pw-arch] ). The
setup and maintenance of single-segment or multi- segment pseudowires
uses the Label Distribution Protocol (LDP) as per [RFC4447] and
extensions for MS-PWs [I-D.ietf-pwe3-segmented-pw] and
[I-D.ietf-pwe3-dynamic-ms-pw].
3.8.2. LSP Control Plane
MPLS-TP provider edge nodes aggregate multiple pseudowires and carry
them across the MPLS-TP network through MPLS-TP tunnels (MPLS-TP
LSPs). Applicable functions from the Generalized MPLS (GMPLS)
protocol suite supporting packet-switched capable (PSC) technologies
are used as the control plane for MPLS-TP transport paths (LSPs).
The LSP control plane includes:
o RSVP-TE for signalling
o OSPF-TE or ISIS-TE for routing
RSVP-TE signaling in support of GMPLS, as defined in [RFC3473], is
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used for the setup, modification, and release of MPLS-TP transport
paths and protection paths. It supports unidirectional, bi-
directional and multicast types of LSPs. The route of a transport
path is typically calculated in the ingress node of a domain and the
RSVP explicit route object (ERO) is utilized for the setup of the
transport path exactly following the given route. GMPLS based
MPLS-TP LSPs must be able to inter-operate with RSVP-TE based MPLS-TE
LSPs, as per [RFC5146]
OSPF-TE routing in support of GMPLS as defined in [RFC4203] is used
for carrying link state information in a MPLS-TP network. ISIS-TE
routing in support of GMPLS as defined in [RFC5307] is used for
carrying link state information in a MPLS-TP network.
3.9. 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. The collateral
damage that loops can cause during the time taken to detect the
failure may be severe. When static configuration mechanisms are
used, care must be taken to ensure that loops to not form.
3.10. Survivability
Survivability requirements for MPLS-TP are specified in
[I-D.ietf-mpls-tp-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 [I-D.ietf-pwe3-redundancy] supports scenarios where the
protection for the PW can not be fully provided by the PSN layer
(i.e. where the backup PW terminates on a different target PE node
than the working PW). Additionally, GMPLS provides a well known set
of control plane driven protection and restoration mechanisms
[RFC4872]. MPLS-TP provides additional protection mechanisms that
are optimised for both linear topologies and ring topologies, and
that operate in the absence of a dynamic control plane. These are
specified in [I-D.ietf-mpls-tp-survive-fwk].
Different protection schemes apply to different deployment topologies
and operational considerations. Such protection schemes may provide
different levels of resiliency. For example, two concurrent traffic
paths (1+1), one active and one standby path with guaranteed
bandwidth on both paths (1:1) or one active path and a standby path
that is shared by one or more other active paths (shared protection).
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The applicability of any given scheme to meet specific requirements
is outside the current scope of this document.
The characteristics of MPLS-TP resiliency mechanisms are listed
below.
o Optimised 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. This
is known as an Automatic Protection Switching (APS) mechanism.
o MPLS-TP recovery schemes are applicable to all levels in the
MPLS-TP domain (i.e. MPLS section, LSP and PW), providing segment
and end-to- end recovery.
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.
3.11. Network Management
The network management architecture and requirements for MPLS-TP are
specified in [I-D.ietf-mpls-tp-nm-req]. It derives from the generic
specifications described in ITU-T G.7710/Y.1701 [G.7710] for
transport technologies. It also incorporates the OAM requirements
for MPLS Networks [RFC4377] and MPLS-TP Networks
[I-D.ietf-mpls-tp-oam-requirements] and expands on those requirements
to cover the modifications necessary for fault, configuration,
performance, and security in a transport network.
The Equipment Management Function (EMF) of a 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. For the management interface from a management system
to a MPLS-TP NE, there is no restriction on which management protocol
should be used. It is used to provision and manage an end-to-end
connection across a network where some segments are create/managed,
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for examples by Netconf or SNMP and other segments by XML or CORBA
interfaces. Maintenance operations are run on a connection (LSP or
PW) in a manner that is independent of the provisioning mechanism.
An MPLS-TP NE is not required to offer more than one standard
management interface. In MPLS-TP, the EMF must be capable of
statically provisioning LSPs for an LSR or LER, and PWs for a PE, as
per Section 3.9.
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 must 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 MEP ID and 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
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 behaviour of the
NEs and the network. One particular requirement for PM is to provide
coherent and consistent interpretation of the network behaviour 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 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 and PWE3 apply to
those transport networks. Furthermore, when general MPLS networks
that utilise functionality outside of the strict MPLS-TP profile are
used to support packet transport services, the security
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considerations of that additional functionality also apply.
For pseudowires, the security considerations of [RFC3985] and
[I-D.ietf-pwe3-ms-pw-arch] apply.
Packets that arrive on an interface with a given label value should
not be forwarded unless that label value was previously assigned to
an LSP or PW to a peer LSR or PE that it reachable via that
interface.
Each MPLS-TP solution must specify the additional security
considerations that 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.
6. Acknowledgements
The editors wish to thank the following for their contribution to
this document:
o Rahul Aggarwal
o Dieter Beller
o Lou Berger
o Malcolm Betts
o Italo Busi
o John E Drake
o Hing-Kam Lam
o Marc Lasserre
o Vincenzo Sestito
o Martin Vigoureux
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7. Open Issues
This section contains a list of issues that must be resolved before
last call.
o Add addition detail on survivability architectures.
o Consider whether there is too much detail in the OAM, network
management, identifiers and control plane sections. Should this
framework document reduce the discussion on these topics in order
to minimise the dependency on other components not yet ready for
publication.
o There is some text missing from the network layer clients section.
Text is invited covering the use of out of band signaling on the
AC.
o Need text to address how the LSR next hop MAC address is
determined for Ethernet link layers when no IP (i.e. ARP) is
available. If statically configured, what is the default?
o Are there any other invariants of a typical LSR/PE architecture
that need to be clarified in the context of MPLS-TP.
8. References
8.1. Normative References
[G.7710] "ITU-T Recommendation G.7710/
Y.1701 (07/07), "Common
equipment management function
requirements"", 2005.
[G.805] "ITU-T Recommendation G.805
(11/95), "Generic Functional
Architecture of Transport
Networks"", November 1995.
[RFC2119] Bradner, S., "Key words for use
in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119,
March 1997.
[RFC3031] Rosen, E., Viswanathan, A., and
R. Callon, "Multiprotocol Label
Switching Architecture",
RFC 3031, January 2001.
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[RFC3032] Rosen, E., Tappan, D., Fedorkow,
G., Rekhter, Y., Farinacci, D.,
Li, T., and A. Conta, "MPLS
Label Stack Encoding", RFC 3032,
January 2001.
[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.
[RFC3471] Berger, L., "Generalized Multi-
Protocol Label Switching (GMPLS)
Signaling Functional
Description", RFC 3471,
January 2003.
[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.
[RFC4203] Kompella, K. and Y. Rekhter,
"OSPF Extensions in Support of
Generalized Multi-Protocol Label
Switching (GMPLS)", RFC 4203,
October 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,
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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.
[RFC5307] Kompella, K. and Y. Rekhter,
"IS-IS Extensions in Support of
Generalized Multi-Protocol Label
Switching (GMPLS)", RFC 5307,
October 2008.
[RFC5332] Eckert, T., Rosen, E., Aggarwal,
R., and Y. Rekhter, "MPLS
Multicast Encapsulations",
RFC 5332, August 2008.
[RFC5462] Andersson, L. and R. Asati,
"Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP"
Field Renamed to "Traffic Class"
Field", RFC 5462, February 2009.
[RFC5586] Bocci, M., Vigoureux, M., and S.
Bryant, "MPLS Generic Associated
Channel", RFC 5586, June 2009.
8.2. Informative References
[I-D.ietf-bfd-mpls] Aggarwal, R., Kompella, K.,
Nadeau, T., and G. Swallow, "BFD
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For MPLS LSPs",
draft-ietf-bfd-mpls-07 (work in
progress), June 2008.
[I-D.ietf-l2vpn-arp-mediation] Rosen, E., Shah, H., Heron, G.,
and V. Kompella, "ARP Mediation
for IP Interworking of Layer 2
VPN", draft-ietf-l2vpn-arp-
mediation-12 (work in progress),
June 2009.
[I-D.ietf-mpls-tp-nm-req] Gray, E., Mansfield, S., and K.
Lam, "MPLS TP Network Management
Requirements",
draft-ietf-mpls-tp-nm-req-05
(work in progress),
September 2009.
[I-D.ietf-mpls-tp-oam-framework] Busi, I. and B. Niven-Jenkins,
"MPLS-TP OAM Framework and
Overview", draft-ietf-mpls-tp-
oam-framework-01 (work in
progress), July 2009.
[I-D.ietf-mpls-tp-oam-requirements] Vigoureux, M., Ward, D., and M.
Betts, "Requirements for OAM in
MPLS Transport Networks", draft-
ietf-mpls-tp-oam-requirements-03
(work in progress), August 2009.
[I-D.ietf-mpls-tp-rosetta-stone] Helvoort, H., Andersson, L., and
N. Sprecher, "A Thesaurus for
the Terminology used in
Multiprotocol Label Switching
Transport Profile (MPLS-TP)
drafts/RFCs and ITU-T's
Transport Network
Recommendations.", draft-ietf-
mpls-tp-rosetta-stone-00 (work
in progress), June 2009.
[I-D.ietf-mpls-tp-survive-fwk] Sprecher, N., Farrel, A., and H.
Shah, "Multiprotocol Label
Switching Transport Profile
Survivability Framework", draft-
ietf-mpls-tp-survive-fwk-00
(work in progress), April 2009.
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[I-D.ietf-pwe3-dynamic-ms-pw] Martini, L., Bocci, M., Bitar,
N., Shah, H., Aissaoui, M., and
F. Balus, "Dynamic Placement of
Multi Segment Pseudo Wires",
draft-ietf-pwe3-dynamic-ms-pw-09
(work in progress), March 2009.
[I-D.ietf-pwe3-ms-pw-arch] Bocci, M. and S. Bryant, "An
Architecture for Multi-Segment
Pseudowire Emulation Edge-to-
Edge",
draft-ietf-pwe3-ms-pw-arch-07
(work in progress), July 2009.
[I-D.ietf-pwe3-redundancy] Muley, P. and M. Bocci,
"Pseudowire (PW) Redundancy",
draft-ietf-pwe3-redundancy-01
(work in progress),
September 2008.
[I-D.ietf-pwe3-segmented-pw] Martini, L., Nadeau, T., Metz,
C., Duckett, M., Bocci, M.,
Balus, F., and M. Aissaoui,
"Segmented Pseudowire",
draft-ietf-pwe3-segmented-pw-13
(work in progress), August 2009.
[RFC0826] Plummer, D., "Ethernet Address
Resolution Protocol: Or
converting network protocol
addresses to 48.bit Ethernet
address for transmission on
Ethernet hardware", STD 37,
RFC 826, November 1982.
[RFC2390] Bradley, T., Brown, C., and A.
Malis, "Inverse Address
Resolution Protocol", RFC 2390,
September 1998.
[RFC2461] Narten, T., Nordmark, E., and W.
Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461,
December 1998.
[RFC3122] Conta, A., "Extensions to IPv6
Neighbor Discovery for Inverse
Discovery Specification",
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RFC 3122, June 2001.
[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.
[RFC5146] Kumaki, K., "Interworking
Requirements to Support
Operation of MPLS-TE over GMPLS
Networks", RFC 5146, March 2008.
[RFC5254] Bitar, N., Bocci, M., and L.
Martini, "Requirements for
Multi-Segment Pseudowire
Emulation Edge-to-Edge (PWE3)",
RFC 5254, October 2008.
[RFC5654] Niven-Jenkins, B., Brungard, D.,
Betts, M., Sprecher, N., and S.
Ueno, "Requirements of an MPLS
Transport Profile", RFC 5654,
September 2009.
Authors' Addresses
Matthew Bocci (editor)
Alcatel-Lucent
Voyager Place, Shoppenhangers Road
Maidenhead, Berks SL6 2PJ
United Kingdom
Phone:
EMail: matthew.bocci@alcatel-lucent.com
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Stewart Bryant (editor)
Cisco Systems
250 Longwater Ave
Reading RG2 6GB
United Kingdom
Phone:
EMail: stbryant@cisco.com
Dan Frost
Cisco Systems
Phone:
Fax:
EMail: danfrost@cisco.com
URI:
Lieven Levrau
Alcatel-Lucent
7-9, Avenue Morane Sulnier
Velizy 78141
France
Phone:
EMail: lieven.levrau@alcatel-lucent.com
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