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RFC 6456

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Multi-Segment Pseudowires in Passive Optical Networks


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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
                                                           November 2011

         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.

Page 2 
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

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   2. 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

1.  Introduction

   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].

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   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
   being deployed.

   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

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   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

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   (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
   ITU-T [G.987.3].

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
      transmission fragments.

   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
      transmission fragments.

   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

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      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.

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   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

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   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      |      |          |
                          ----------       |----------|
                                           |Link Layer|
                                           |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
   core networks.

   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.

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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.

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   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.  References

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
               2009, <

   [G.987]     ITU-T, "10-Gigabit-capable passive optical network (XG-
               PON) systems: Definitions, abbreviations, and acronyms",
               October 2010, <

   [G.987.3]   ITU-T, "10-Gigabit-capable passive optical networks (XG-
               PON): Transmission convergence (TC) layer specification",
               October 2010, <

   [G.988]     ITU-T, "ONU management and control interface (OMCI)
               specification", October 2010, <

   [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.

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   [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, <

   [RFC6073]   Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
               Aissaoui, "Segmented Pseudowire", RFC 6073, January 2011.

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Authors' Addresses

   Hongyu Li
   Huawei Technologies
   Huawei Industrial Base


   Ruobin Zheng
   Huawei Technologies
   Huawei Industrial Base


   Adrian Farrel
   Old Dog Consulting