Internet Engineering Task Force (IETF) H. Li
Request for Comments: 6456 R. Zheng
Category: Informational Huawei Technologies
ISSN: 2070-1721 A. Farrel
Old Dog Consulting
Multi-Segment Pseudowires in Passive Optical Networks
This document describes the application of MPLS multi-segment
pseudowires (MS-PWs) in a dual-technology environment comprising a
Passive Optical Network (PON) and an MPLS Packet Switched Network
PON technology may be used in mobile backhaul networks to support the
end segments closest to the aggregation devices. In these cases,
there may be a very large number of pseudowire (PW) Terminating
Provider Edge (T-PE) nodes. The MPLS control plane could be used to
provision these end segments, but support for the necessary protocols
would complicate the management of the T-PEs and would significantly
increase their expense. Alternatively, static, or management plane,
configuration could be used to configure the end segments, but the
very large number of such segments in a PON places a very heavy
burden on the network manager.
This document describes how to set up the end segment of an end-to-
end MPLS PW over a Gigabit-capable Passive Optical Network (G-PON) or
10 Gigabit-capable Passive Optical Network (XG-PON) using the G-PON
and XG-PON management protocol, Optical Network Termination
Management and Control Interface (OMCI). This simplifies and speeds
up PW provisioning compared with manual configuration.
This document also shows how an MS-PW may be constructed from an end
segment supported over a PON, and switched to one or more segments
supported over an MPLS PSN.
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,
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Table of Contents
1. Introduction ....................................................2
2. Terminology for G-PON/XG-PON ....................................5
3. Multi-Segment Pseudowire over PON Network Reference Model .......6
4. Label Provisioning for Pseudowires over PON .....................9
5. Security Considerations .........................................9
6. References .....................................................10
6.1. Normative References ......................................10
6.2. Informative References ....................................11
The use of PWs in Packet Switched Networks (PSNs) is defined in
[RFC3985]. This architecture is extended in [RFC5659] for multi-
segment pseudowires (MS-PWs) satisfying the requirements in
[RFC5254]. More detail on MS-PWs is provided in [RFC6073].
An MS-PW is a useful technology for certain applications where there
is an aggregation of paths toward a common point in the network,
e.g., mobile backhaul; the segments can be aggregated within tunnels
between PW switching points thus improving scalability and reducing
the number of control plane adjacencies where a control plane is
Segments of an MS-PW in a PSN can be set up using manual provisioning
(static PWs) or using a dynamic control plane such as the Label
Distribution Protocol (LDP) [RFC5036] [RFC4447].
In many scenarios, in access and metro networks, a Passive Optical
Network (PON) provides longer distance, higher bandwidth, and better
economy than other technologies such as point-to-point Ethernet or
Digital Subscriber Line (DSL). Mobile backhaul with PON is already
Figure A depicts the physical infrastructure of an Optical
Distribution Network (ODN).
|<--Optical Distribution Network-->|
| branch main |
+-----+ fibers fiber
Base ------| | | |
Stations ------| ONU |\ | |
------| | \ V |
+-----+ \ |
\ +----------+ |
+-----+ \| | | +-----+
Base ------| | | Optical | V | |
Stations ------| ONU |---------| Splitter |-------------| OLT |
------| | /| | | |
+-----+ / +----------+ +-----+
Base ------| |/
Stations ------| ONU |
Figure A: Typical PON System Architecture
In a PON, the Optical Network Unit (ONU) and Optical Line Termination
(OLT) are adjacent nodes connected by an Optical Distribution Network
(ODN), which consists of optical fibers and optical splitters in a
tree topology. The link between each ONU and OLT is simulated as a
point-to-point link, and there is no path redundancy between them.
The OLT resides in the central office, while ONUs reside in customer
premises. ONUs are deployed in huge numbers and so they are cost
sensitive. More information about ODNs can be found in [G.984.1].
In a mobile backhaul network, many 2G and 3G base stations still use
legacy interfaces such as Time-Division Multiplexing (TDM) and ATM.
Therefore, these native services must be carried across the PON
before they can be carried over the PSN using PWs. This document
describes how MS-PWs can be constructed with end segments that
operate over the PON and are switched to further segments operated
over the PSN. In this case, the base stations are connected by
access circuits (ACs) to the ONUs, which act as Terminating Provider
Edge (T-PE) nodes. The OLT is a Switching Provider Edge (S-PE).
This model is shown in Figure B.
Routing protocols and dynamic label distribution protocols such as
LDP would significantly increase the ONUs' cost and complexity as
they place requirements on both hardware and software. Besides the
coding and maintenance of these new protocols, a much more powerful
CPU and more memory are also necessary for them to run smoothly.
As there is no redundant path between each ONU and the OLT, routing
and path selection are not necessary in the PON. Therefore, static
provisioning of PW labels between ONUs and the OLT is simple and
preferred because it can greatly reduce the cost of an ONU that acts
as a T-PE. However, use of a Network Management System (NMS) to
provision PWs in a PON would require the network manager to configure
each ONU and to configure the OLT once for each PW. Since there may
be very many ONUs (and hence very many PWs) in a PON, this requires a
large amount of operational effort. Additionally, there is an issue
that the configuration of each PW at the OLT and ONU might be
inconsistent since these nodes are configured separately.
[G.988] defines the G-PON/XG-PON management protocol called the "ONT
Management and Control Interface (OMCI)". OMCI is an implementation
requirement for all G-PON/XG-PON systems. If OMCI is used to
configure PWs on an ONU, no upgrade to an ONU's hardware is required
and the extension to the OMCI implementation is negligible. This
provides a way of reducing the cost and complexity of provisioning
PWs in a G-PON/XG-PON.
This document shows how the two technologies (PON and PSN) can be
combined to provide an end-to-end multi-segment MPLS PW. The MPLS
PWs are also carried over the PON in MPLS Label Switched Path (LSP)
tunnels. There is an MPLS LSP tunnel in each direction between each
ONU and the OLT in a one-to-one relationship with the underlying G-
PON/XG-PON channel. The OLT and ONU perform penultimate hop popping
(PHP) [RFC3031] on this single-hop LSP so no labels are used on the
wire for the MPLS LSP tunnel. There is no change to the operation of
MPLS PWs, and MPLS packets are carried by the G-PON link layer
according to ITU-T [G.984.3amd1] or XG-PON link layer according to
2. Terminology for G-PON/XG-PON
We defined the following terms derived from [G.987]:
o Gigabit-capable Passive Optical Network (G-PON). A variant of the
Passive Optical Network (PON) access technology supporting
transmission rates in excess of 1 Gbit/s and based on the ITU-T
G.984.x series of Recommendations [G.984.1], [G.984.4amd2] and
o G-PON Encapsulation Method (GEM). A data frame transport scheme
used in G-PON systems that is connection oriented and that
supports fragmentation of the user data frames into variable sized
o GEM port. An abstraction of the G-PON adaptation layer
representing a logical connection associated with a specific
client packet flow between the OLT and the ONU.
o 10-gigabit-capable Passive Optical Network (XG-PON): A PON system
supporting nominal transmission rates on the order of 10 Gbit/s in
at least one direction, and implementing the suite of protocols
specified in the ITU-T G.987.x series Recommendations.
o XG-PON encapsulation method (XGEM): A data frame transport scheme
used in XG PON systems that is connection oriented and that
supports fragmentation of user data frames into variable-sized
o XGEM port: An abstraction in the XG-PON transmission convergence
(XGTC) service adaptation sublayer representing a logical
connection associated with a specific client packet flow.
o Optical Distribution Network (ODN). In the PON context, a tree of
optical fibers in the access network, supplemented with power or
wavelength splitters, filters, or other passive optical devices.
o Optical Line Termination (OLT). A device that terminates the
common (root) endpoint of an ODN; implements a PON protocol, such
as that defined by ITU-T G.984 series; and adapts PON PDUs for
uplink communications over the provider service interface. The
OLT provides management and maintenance functions for the
subtended ODN and ONUs. In this document, the OLT is a network
element with multiple PON ports and uplinks that provide switching
capability to the PSN.
o Optical Network Termination (ONT). A single subscriber device
that terminates any one of the distributed (leaf) endpoints of an
ODN, implements a PON protocol, and adapts PON PDUs to subscriber
service interfaces. An ONT is a special case of an ONU.
o Optical Network Unit (ONU). A generic term denoting a device that
terminates any one of the distributed (leaf) endpoints of an ODN,
implements a PON protocol, and adapts PON PDUs to subscriber
service interfaces. In some contexts, an ONU implies a multiple
subscriber device. In this document, an ONU is a Provider Edge
(PE) node with one or more ACs that map to the service interfaces.
The ONU acts as a T-PE.
o ONT Management and Control Interface (OMCI). The management and
control channel between OLT and ONT in PON. The OMCI protocol
runs between the OLT Controller and the ONT Controller across a
GEM connection that is established at ONT initialization. The
OMCI protocol is asymmetric: the Controller in the OLT is the
master and the one in the ONT is the slave. A single OLT
Controller using multiple instances of the protocol over separate
control channels may control multiple ONTs. The OMCI protocol is
used to manage the ONT in areas of configuration, fault
management, performance, and security.
o Passive Optical Network (PON). An OLT connected, using an ODN, to
one or more ONUs or ONTs.
3. Multi-Segment Pseudowire over PON Network Reference Model
[RFC5659] provides several pseudowire emulation edge-to-edge (PWE3)
reference architectures for the multi-segment case. These are
general models extended from [RFC3985] to enable point-to-point
pseudowires through multiple PSN tunnels.
A G-PON/XG-PON consists of an OLT, an ODN, and multiple ONUs. The
ODN is actually a fiber tree that provides physical connections
between the OLT and the ONUs. G-PON/XG-PON has its own physical
layer and link layer. A GEM/XGEM port is a logical point-to-point
connection between the OLT and each ONU over GPON Transmission
Convergence (GTC) layer/XG-PON transmission convergence (XGTC) layer.
There can be more than one GEM/XGEM port between the OLT and an
individual ONU. Each GEM/XGEM port can be assigned different Quality
of Service (QoS) and bandwidth.
Figure B shows how the MS-PW architecture is applied to a network
comprising a PON and a PSN. The Terminating PE1 (TPE1) is an ONU and
the Switching PE1 (SPE1) is an OLT. One or more PWs run between the
ONU and the remote end system (TPE2) to provide service emulation
between Customer Edges (CEs) (CE1 and CE2).
In each of the PON and PSN, the PW segments are carried in PSN
tunnels. In the PSN, the tunnel is established and operated as
normal for PWs (see [RFC3985]). In the PON, the tunnel used is a
single-hop MPLS LSP tunnel so that the OLT and ONU are label edge
routers. The OLT and ONU make use of PHP on the MPLS LSP tunnel.
Since this is a single-hop LSP (there are no MPLS-capable nodes
between the OLT and ONU), this means that there is no MPLS
encapsulation for the MPLS LSP tunnel on the wire (that is, no label
or shim header is used). This results in the on-wire encapsulations
shown in Figure C.
Native |<------Multi-Segment Pseudowire------>| Native
Service | GEM/XGEM | Service
(AC) | |<--Port-->| | (AC)
| | | | | |
| | | PSN | PSN | |
| | |<-Tunnel->| |<-Tunnel->| | |
| V V V V V V |
| +----+ +-----+ +----+ |
+----+ | |TPE1|===========|S-PE1|==========|TPE2| | +----+
| |------|..... PW.Seg't1....X....PW.Seg't3.....|-------| |
| CE1| | | | | | | | | |CE2 |
| |------|..... PW.Seg't2....X....PW.Seg't4.....|-------| |
+----+ | | |===========| |==========| | | +----+
Base ^ +----+ +-----+ +----+ ^
Station | Provider Edge 1 ^ Provider Edge 2 |
| ONU | |
| PW switching point |
| OLT |
|<------------------ Emulated Service --------------->|
Figure B: MS-PW over PON Network Reference Model
Base ----AC-- TPE1--PW over PON--SPE1--PW over PSN--TPE2--AC------
-------- |Packetized| |Packetized| --------
|Native | |Native | |Native | |Native |
|Service | |Service | |Service | |Service |
-------- |----------| |----------| --------
|Control | |Control |
|Word | |Word |
|PW Label | |PW Label |
|GEM/XGEM | |MPLS |
|----------| |Tunnel |
|GPON/XGPON| |Label |
|-Phy | | |
Figure C: On-Wire Data Encapsulations for MS-PWs
It should be noted that all PW segments are of the same technology,
which is packet encapsulated.
The use of the PW label enables multiple PWs to be multiplexed over a
single GEM/XGEM port within the MPLS LSP tunnel. This enables the
traffic for multiple base stations to be kept separate and allows
different services and separate ACs for a single base station to be
supported. Furthermore, the multiple ACs at an ONU can belong to
different native services.
At the same time, each ONU can support more than one GEM/XGEM port
(each supporting a single MPLS LSP tunnel) connecting it to the OLT.
This allows greater bandwidth and so more PWs. It may also be used
to provide a simple way to aggregate PWs intended to be routed across
different PSN tunnels in the core network, or even across different
At present, Ethernet over GEM/XGEM is the dominant encapsulation in
G-PON/XG-PON. For fast deployment of MPLS over G-PON/XG-PON, putting
MPLS PWs over Ethernet over GEM/XGEM is an alternative way of
transporting MPLS PWs over G-PON/XG-PON with existing hardware.
4. Label Provisioning for Pseudowires over PON
For an MS-PW with a segment running over a PON, where the OLT acts as
an S-PE and the ONU as a T-PE, PW provisioning can be performed
through static configuration, e.g., from an NMS. However, in this
model, each ONU has to be configured as each PW is set up. The huge
number of ONUs (and PWs) makes this method quite forbidding.
The labor of provisioning static labels at the ONUs for PWs can be
significantly reduced by using a management protocol over PON. This
approach keeps the ONU simple by not requiring the implementation of
a new dynamic control protocol.
The usual management protocol in a G-PON/XG-PON system used to manage
and control ONUs is OMCI. It is used to perform all configuration of
the G-PON/XG-PON physical layer and data GTC/XGTC layer on ONUs. Per
[G.984.4amd2] and [G.988], OMCI can also be used to set up PWs and
the MPLS LSP Tunnels from ONUs to OLT. When using OMCI to provision
PWs in a G-PON/XG-PON, the network manager sends configuration
information to the OLT only. The OLT will select suitable PW labels
and send all PW and MPLS LSP tunnel parameters to the ONUs through
OMCI. The AC can be identified in the OMCI signaling so that the
network manager does not need to configure the PWs at each ONU.
OMCI supports the configuration of a number of PW types including
TDM, ATM, and Ethernet. The protocol can also be used to allow the
ONU to notify the OLT of the status of the AC.
5. Security Considerations
This document describes a variation of a multi-segment pseudowire
running over an MPLS PSN, in which one (or both) of the MPLS PSNs
that provides connectivity between a T-PE and its associated S-PE is
replaced by a G-PON/XG-PON PSN. The security considerations that
apply to the PW itself [RFC3985] [RFC4385] are unchanged by this
change in PSN type. For further considerations of PW security, see
the security considerations section of the specific PW type being
G-PON/XG-PON [G.987.3] [G.984.3amd1] includes security mechanisms
that are as good as those provided in a well-secured MPLS PSN. The
use of a G-PON/XG-PON PSN in place of an MPLS PSN therefore does not
increase the security risk of a multi-segment pseudowire.
Protecting against an attack at the physical or data link layer of
the PON is out of the scope of this document.
The MPLS control plane and management plane mechanisms are unchanged
by this document. This document introduces OMCI as a provisioning
mechanism that runs between the OLT Controller and the ONT Controller
across a GEM connection that is established at ONT initialization.
In other words, the protocol runs on an in-fiber control channel.
That means that injection and modification of OMCI messages would be
very hard (harder, for example, than injection or modification in an
MPLS Associated Channel Header (ACH) that has been accepted to
provide adequate security by isolation ([RFC4385] and [RFC5586]).
6.1. Normative References
[G.984.1] ITU-T, "Gigabit-capable passive optical networks (GPON):
General characteristics", March 2008,
ITU-T, "Gigabit-capable Passive Optical Networks (G-PON):
Transmission convergence layer specification", February
[G.987] ITU-T, "10-Gigabit-capable passive optical network (XG-
PON) systems: Definitions, abbreviations, and acronyms",
October 2010, <http://www.itu.int/rec/T-REC-
[G.987.3] ITU-T, "10-Gigabit-capable passive optical networks (XG-
PON): Transmission convergence (TC) layer specification",
October 2010, <http://www.itu.int/rec/T-REC-
[G.988] ITU-T, "ONU management and control interface (OMCI)
specification", October 2010, <http://www.itu.int/rec/T-
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[RFC3985] Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation
Edge-to-Edge (PWE3) Architecture", RFC 3985, March 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., Ed., Rosen, E., El-Aawar, N., Smith, T., and
G. Heron, "Pseudowire Setup and Maintenance Using the
Label Distribution Protocol (LDP)", RFC 4447, April 2006.
[RFC5036] Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, October 2007.
[RFC5254] Bitar, N., Ed., Bocci, M., Ed., and L. Martini, Ed.,
"Requirements for Multi-Segment Pseudowire Emulation
Edge-to-Edge (PWE3)", RFC 5254, October 2008.
[RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
"MPLS Generic Associated Channel", RFC 5586, June 2009.
[RFC5659] Bocci, M. and S. Bryant, "An Architecture for
Multi-Segment Pseudowire Emulation Edge-to-Edge", RFC
5659, October 2009.
6.2. Informative References
ITU-T, "Gigabit-capable passive optical networks (G-PON):
ONT management and control interface specification",
November 2009, <http://www.itu.int/rec/T-REC-
[RFC6073] Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
Aissaoui, "Segmented Pseudowire", RFC 6073, January 2011.
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