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Internet Engineering Task Force (IETF)                             H. LiRequest for Comments: 6456                                      R. ZhengCategory: Informational                              Huawei TechnologiesISSN: 2070-1721                                                A. Farrel                                                      Old Dog Consulting                                                           November 2011Multi-Segment Pseudowires in Passive Optical NetworksAbstract   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   (PSN).   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.Li, et al.                    Informational                     [Page 1]

RFC 6456            Multi-Segment Pseudowires in PON       November 2011Status 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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6456.Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Table of Contents1. Introduction ....................................................22. Terminology for G-PON/XG-PON ....................................53. Multi-Segment Pseudowire over PON Network Reference Model .......64. Label Provisioning for Pseudowires over PON .....................95. Security Considerations .........................................96. References .....................................................106.1. Normative References ......................................106.2. Informative References ....................................111.  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].Li, et al.                    Informational                     [Page 2]

RFC 6456            Multi-Segment Pseudowires in PON       November 2011   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   used.   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 aLi, et al.                    Informational                     [Page 3]

RFC 6456            Multi-Segment Pseudowires in PON       November 2011   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 poppingLi, et al.                    Informational                     [Page 4]

RFC 6456            Multi-Segment Pseudowires in PON       November 2011   (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      [G.984.3amd1].   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 theLi, et al.                    Informational                     [Page 5]

RFC 6456            Multi-Segment Pseudowires in PON       November 2011      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.Li, et al.                    Informational                     [Page 6]

RFC 6456            Multi-Segment Pseudowires in PON       November 2011   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 ModelLi, et al.                    Informational                     [Page 7]

RFC 6456            Multi-Segment Pseudowires in PON       November 2011   Base   ----AC-- TPE1--PW over PON--SPE1--PW over PSN--TPE2--AC------   Station                          ----------        ----------           --------      |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.Li, et al.                    Informational                     [Page 8]

RFC 6456            Multi-Segment Pseudowires in PON       November 20114.  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   deployed.   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.Li, et al.                    Informational                     [Page 9]

RFC 6456            Multi-Segment Pseudowires in PON       November 2011   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.  References6.1.  Normative References   [G.984.1]   ITU-T, "Gigabit-capable passive optical networks (GPON):               General characteristics", March 2008,               <http://www.itu.int/rec/T-REC-G.984.1-200803-I>.   [G.984.3amd1]               ITU-T, "Gigabit-capable Passive Optical Networks (G-PON):               Transmission convergence layer specification", February               2009, <http://www.itu.int/rec/T-REC-               G.984.3-200902-I!Amd1>.   [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.987-201010-I>.   [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.987.3-201010-I/en>.   [G.988]     ITU-T, "ONU management and control interface (OMCI)               specification", October 2010, <http://www.itu.int/rec/T-REC-G.988-201010-I>.   [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.Li, et al.                    Informational                    [Page 10]

RFC 6456            Multi-Segment Pseudowires in PON       November 2011   [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",RFC5659, October 2009.6.2.  Informative References   [G.984.4amd2]               ITU-T, "Gigabit-capable passive optical networks (G-PON):               ONT management and control interface specification",               November 2009, <http://www.itu.int/rec/T-REC-               G.984.4-200911-I!Amd2>.   [RFC6073]   Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.               Aissaoui, "Segmented Pseudowire",RFC 6073, January 2011.Li, et al.                    Informational                    [Page 11]

RFC 6456            Multi-Segment Pseudowires in PON       November 2011Authors' Addresses   Hongyu Li   Huawei Technologies   Huawei Industrial Base   Shenzhen   China   EMail: hongyu.lihongyu@huawei.com   Ruobin Zheng   Huawei Technologies   Huawei Industrial Base   Shenzhen   China   EMail: robin@huawei.com   Adrian Farrel   Old Dog Consulting   EMail: adrian@olddog.co.ukLi, et al.                    Informational                    [Page 12]