PPVPN Working Group
Internet Draft
Document: draft-ietf-l2vpn-vpls-requirements-00.txt
Category: Informational
October 2002                                                 Waldemar Augustyn
Expires: April 2003                                                   (editor)

   Giles Heron                                                  Pascal Menezes
   PacketExchange Ltd                                                 Terabeam

   Vach Kompella                                             Hamid Ould-Brahim
   TiMetra Networks                                            Nortel Networks

   Marc Lasserre                                            Tissa Senevirathne
   Riverstone Networks

           Requirements for Virtual Private LAN Services (VPLS)

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   For potential updates to the above required-text see:

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1  Abstract

   This draft describes service requirements related to emulating a
   virtual private LAN over an IP or MPLS network infrastructure. The
   service is called VPLS. It is a class of Provider Provisioned
   Virtual Private Network [2]. The general requirements can be found
   in [3].

2  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in RFC-2119 [4].

3  Definitions

3.1 VPLS

   Virtual Private LAN Service, a case of L2VPN service distinguished
   by the support of L2 broadcast.  The term is also used, when clear
   from the context, to refer to a particular instance of VPLS service.

   A VPLS service allows the connection of multiple sites in a single
   broadcast domain over a provider managed IP or MPLS network. All
   customer sites in the VPLS appear to be on the same LAN regardless
   of their location.

3.2 VPLS Domain

   A Layer 2 VPN that is composed of a community of interest of L2 MAC
   addresses and VLANs. Each VPLS Domain MAY have multiple VLANs in it.

3.3 VLAN

   A customer VLAN identification using some scheme such as IEEE 802.1Q
   tags, port configuration or any other means. A VPLS service can be
   extended to recognize customer VLANs as specified in 6.1 .

3.4 VLAN Flooding Scope (VLAN Broadcast Domain)

   The scope of flooding for a given VLAN. In a VPLS service, a VLAN
   flooding scope is identical to the flooding scope of the VPLS it is
   part of. If a VPLS service is extended to recognize customer VLANs,
   the VLAN flooding scope is limited to the broadcast domain of each
   recognized VLAN.

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3.5 VSI

   Virtual Switching Instance. A virtual layer 2 forwarding entity that
   is closed to a VPLS domain membership. VSI forwarding can be based
   on MAC addresses, VLAN tags, policies, topologies, filters, QoS
   parameters, and other relevant information on a per VPLS basis.

4  Introduction

   Traditionally, the typical connectivity between a service provider
   and a customer is a WAN link with some type of a point-to-point
   protocol. This arrangement was borne out of the necessity to
   traverse TDM circuits originally designed for voice traffic. The
   introduction of WAN links to network architectures significantly
   increased the complexity of network topologies and required highly
   skilled personnel to manage and maintain the network.

   One solution to the above has been for service providers to deploy
   emulated LAN services known in this context as "Transparent LAN"
   services. These have typically been offered using a mesh of ATM PVCs
   between locations.  While this technique reduced complexity for the
   customer, it proved inadequate in the area of scaling and ease of
   management on the provider side.

   The aim of this effort is to develop a Virtual Private LAN Service,
   VPLS, that scales well, is simple to manage, and is based on the
   existing MPLS or IP backbone.

   VPLS emulates a flat LAN with learning and switching capabilities.
   In a given LAN, there is a reasonably small set of MAC devices with
   a limited number of MAC addresses to learn and manage. There is no
   need for additional routing protocol support between the CE and the
   PE devices.  In the VPLS model, the service is transparent to the
   customer's choice of routing protocol. Moreover, VPLS services also
   benefit from being transparent to higher layer protocols, so the
   same technology can transport, for example, IPv4, IPv6, MPLS as well
   as legacy protocols such as IPX and OSI.

   The VPLS model, while offering significant benefits for both
   customers and service providers, retains all the quintessential
   characteristics of L2 networks including their well known
   limitations e.g. the maximum practical number of hosts on a single
   LAN, etc.  A likely application of this model is to connect a few
   sites with only a single customer router at each site, or a small
   number of customer hosts, at each site, connected via the VPLS.

   The scope of this document will be limited to supporting Ethernet as
   the access framing technology for VPLS implementation.

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5  VPLS Reference Model

   The following diagram shows a VPLS reference model where PE devices
   that are VPLS-capable provide a logical interconnect such that CE
   devices belonging to a specific VPLS appear to be connected by a
   single logical Ethernet bridge. A VPLS can contain a single VLAN or
   multiple, tagged VLANs.

    +-----+                                       +-----+
    + CE1 +--+                                +---| CE2 |
    +-----+  |    ........................    |   +-----+
     VPLS A  |  +----+                +----+  |    VPLS A
             +--| PE |--- Service  ---| PE |--+
                +----+    Provider    +----+
               /  .       Backbone       .  \     -   /\-_
    +-----+   /   .          |           .   \   / \ /   \     +-----+
    + CE  +--+    .          |           .    +--\ Access \----| CE  |
    +-----+       .        +----+        .       | Network |   +-----+
     VPLS B       .........| PE |.........        \       /     VPLS B
                           +----+     ^            -------
                             |        |
                             |        |
                          +-----+     |
                          | CE3 |     +-- Logical bridge
                           VPLS A

   Separate L2 broadcast domains are maintained on a per VPLS basis by
   PE devices. Such domains are then mapped onto tunnels in the service
   provider network. These tunnels can either be specific to a VPLS
   (e.g. as with IP) or shared among several VPLSs (e.g. as with MPLS
   tunnel LSPs). In the above diagram, the top PE routers maintain
   separate forwarding instances for VPLS A and VPLS B.

   The CE-to-PE links can either be direct physical links, e.g.
   100BaseTX, or logical links, e.g. ATM PVC, T1/E1 TDM, or RFC1490-
   encapsulated links, over which bridged Ethernet traffic is carried.

   The PE-to-PE links carry tunneled Ethernet frames using different
   tunneling technologies (e.g., GRE, IPSec, MPLS, L2TP, etc.).

   Each PE device learns remote MAC addresses, and is responsible for
   proper forwarding of the customer traffic to the appropriate end
   nodes. It is responsible for guaranteeing each VPLS topology is loop

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6  VPLS General Requirements

6.1 Layer 2 Domain representation

   A VPLS system MUST distinguish different customer domains. Each of
   these customer domains MUST appear as a L2 broadcast domain network
   behaving like a LAN (Local Area Network). These domains are referred
   to as VPLS domains.

   A VPLS MAY span multiple service providers. Each VPLS MUST carry a
   unique identification within a VPLS system. It is RECOMMENDED that
   VPLS identification be globally unique.

   Each VPLS domain MUST be capable of learning and forwarding based on
   MAC addresses thus emulating an Ethernet virtual switch to the
   customer CE devices attached to PEs.

   A VPLS system MAY recognize customer VLAN identification. In that
   case, a VLAN MUST be recognized in the context of the VPLS it is
   part of.  If customer VLANs are recognized, separate VLAN broadcast
   domains SHOULD be maintained.

   A provider's implementation of a VPLS system SHOULD NOT constrain
   the customer's ability to configure LAN topologies, tags, 802.1 p-
   bits, or any other Layer 2 parameters.

6.2 VPLS Topology

   The VPLS system MAY be realized using one or more network tunnel
   topologies to interconnect PEs, ranging  from simple point-to-point
   to distributed hierarchical arrangements. The typical topologies

     o point-to-point
     o point-to-multipoint, a.k.a. hub and spoke
     o any-to-any, a.k.a. full mesh
     o mixed, a.k.a. partial mesh
     o hierarchical

   Regardless of the topology employed, the service to the customers
   MUST retain the typical LAN any-to-any connectivity.  This
   requirement does not imply that all traffic characteristics (such as
   bandwidth, QoS, delay, etc.) be necessarily the same between any two
   end points.

6.3 Redundancy and Failure Recovery

   The VPLS infrastructure SHOULD provide redundant paths to assure
   high availability.  The reaction to failures SHOULD result in an
   attempt to restore the service using alternative paths.

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   The intention is to keep the restoration time small. It is
   RECOMMENDED that the restoration time be less than the time it takes
   the CE devices, or customer L2 control protocols, to detect a
   failure in the VPLS.

   In cases where the provider knows a priori about impending changes
   in network topology, the network SHOULD have the capability to
   reconfigure without a loss, duplication, or re-ordering of customer
   packets.  This situation typically arises with planned network
   upgrades or scheduled maintenance activities.

6.4 Policy Constraints

   A VPLS system MAY employ policy constraints governing various
   interconnection attributes for VPLS domains. Typical attributes

     o Selection of available network infrastructure
     o QoS services needed
     o Protection services needed
     o Availability of higher level service access points (see 9.7 )

   Policy attributes SHOULD be advertised via the VPLS system's control

6.5 PE nodes

   The PE nodes are the devices in the VPLS system that store
   information related to customer VPLS domains and employ methods to
   forward customer traffic based on that information. In this
   document, the PE nodes are meant in logical sense.  In the actual
   implementations, the PE nodes may be comprised of several physical
   devices. Conversely, a single physical device may contain more than
   one PE node.

   All forwarding decisions related to customer VPLS traffic MUST be
   made by PE nodes.  This requirement prohibits any other network
   components from altering decisions made by PE nodes.

6.6 PE-PE Interconnection and Tunneling

   A VPLS system MUST provide for connectivity between each pair of PE
   nodes.  The connectivity is referred to as transport tunneling or
   simply tunneling.

   There are several choices for implementing transport tunnels. Some
   popular choices include MPLS, IP in IP tunnels, variations of

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   802.1Q, etc.  Regardless of the choice, the existence of the tunnels
   and their operations MUST be transparent to the customers.

6.7 PE-CE Interconnection and Profiles

   A VPLS system MUST provide for connectivity between PE nodes and CE
   nodes.  That connectivity is referred to as an Attachment Circuit
   (AC). Attachment Circuits MAY span networks of other providers or
   public networks.

   There are several choices for implementing ACs. Some popular choices
   include Ethernet, ATM (DSL), Frame Relay, MPLS-based virtual
   circuits etc. Regardless of the choice, the ACs MUST use Ethernet
   frames as the Service Protocol Data Unit (SPDU).

   A CE access connection over an AC MUST be bi-directional in nature.

   PE devices MAY support multiple ACs on a single physical interface.
   In such cases, PE devices MUST NOT rely on customer controlled
   parameters for distinguishing between different access connections.
   For example, if VLAN tags were used for that purpose, the provider
   would be controlling the assignment of the tag values and would
   strictly enforce compliance by the CEs.

   An AC connection, whether direct or virtual, MUST maintain all
   committed characteristics of the customer traffic, such as QoS,
   priorities etc. The characteristics of an AC connection are only
   applicable to that connection.

7  Control Plane Requirements

7.1 Provider Edge Signaling

   The control protocols SHOULD provide methods for signaling between
   PEs. The signaling SHOULD inform of membership, tunneling
   information, and other relevant parameters.

   The infrastructure MAY employ manual configuration methods to
   provide this type of information.

   The infrastructure SHOULD use policies to scope the membership and
   reachability advertisements for a particular VPLS.

7.2 VPLS Membership Discovery

   The control plane and/or the management plane SHOULD provide methods
   to discover the PEs which connect CEs forming a VPLS.

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7.3 Support for Layer 2 control protocols

   The VPLS system's control protocols SHOULD allow transparent
   operation of Layer 2 control protocols employed by customers.

   A VPLS system MUST ensure that loops be prevented. This can be
   accomplished through a loop free topologies or appropriate
   forwarding rules.  Control protocols such as Spanning Tree (STP) or
   similar could be employed.  The system's control protocols MAY use
   indications from customer control protocols, e.g. STP, to improve
   the operation of a VPLS.

7.4 Scaling Requirements

   In a VPLS system, the control plane traffic increases with the
   growth of VPLS membership. Similarly, the control plane traffic
   increases with the number of supported VPLS domains.  The rate of
   growth of the associated control plane traffic SHOULD be linear.

   The use of control plane resources increases with the number of
   hosts connected to a VPLS. The rate of growth of the demand for
   control process resources SHOULD be linear.  The control plane MAY
   offer means for enforcing a limit on the number of customer hosts
   attached to a VPLS.

8  Data Plane Requirements

8.1 Transparency

   VPLS service is intended to be transparent to Layer 2 customer
   networks.  It SHOULD NOT require any special packet processing by
   the end users before sending packets to the provider's network.

8.2 QoS and packet re-ordering

   A VPLS system SHOULD have capabilities to enforce QoS parameters.

   The queuing and forwarding policies SHOULD preserve packet order for
   packets with the same QoS parameters.

   The service SHOULD not duplicate packets.

8.3 Broadcast Domain

   The Broadcast Domain is defined as the flooding scope of a Layer 2
   network. A separate Broadcast Domain MUST be maintained for each

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   In addition to VPLS Broadcast Domains, a VPLS system MAY recognize
   customer VLAN Broadcast Domains. In that case, the system SHOULD
   maintain a separate VLAN Broadcast Domain for each customer VLAN.  A
   VLAN Broadcast Domain MUST be a subset of the owning VPLS Broadcast

8.4 MAC address learning

   A VPLS service SHOULD derive all topology and forwarding information
   from packets originating at customer sites.  Typically, MAC
   addresses learning mechanisms are used for this purpose.

   In a VPLS system, MAC address learning MUST take place on a per
   Virtual Switching Instance (VSI) basis, i.e. in the context of a
   VPLS and, if supported, in the context of VLANs therein.

8.5 Unicast, Unknown Unicast, Multicast, and Broadcast forwarding

   VPLS MUST be aware of the existence and the designated roles of
   special MAC addresses such as Multicast and Broadcast addresses.
   VPLS MUST forward these packets according to their intended
   functional meaning and scope.

   Broadcast packets MUST be flooded to all destinations.

   Multicast packets MUST be flooded to all destinations. However, a
   VPLS system MAY employ multicast snooping techniques, in which case
   multicast packets SHOULD be forwarded only to their intended

   Unicast packets MUST be forwarded to their intended destinations.

   Unknown Unicast packets MUST be flooded to all destinations in the
   flooding scope of the VPLS (or VLAN). If the VPLS service relies on
   MAC learning for its operations, it MUST assure proper forwarding of
   packets with MAC addresses that have not been learned.  Once
   destination MAC addresses are learned, unicast packets SHOULD be
   forwarded only to their intended destinations.

   A provider MAY employ a method to limit the scope of flooding of
   Unknown Unicast packets in cases where a customer desires to
   conserve its bandwidth or wants to implement certain security

8.6 Virtual Switching Instance

   VPLS Provider Edge devices MUST maintain a separate Virtual
   Switching Instance (VSI) per each VPN. Each VSI MUST have

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   capabilities to forward traffic based on customer's traffic
   parameters such as MAC addresses, VLAN tags (if supported), etc. as
   well as local policies.

   VPLS Provider Edge devices MUST have capabilities to classify
   incoming customer traffic into the appropriate VSI.

   Each VSI MUST have flooding capabilities for its Broadcast Domain to
   facilitate proper forwarding of Broadcast, Multicast and Unknown
   Unicast customer traffic.

8.7 Minimum MTU

   The VPLS service MUST support customer frames with payload 1500
   bytes long.  The service MAY offer support for longer frames.

   The service MUST NOT fragment packets.  Packets exceeding committed
   MTU size MUST be discarded.

   The committed minimum MTU size MUST be the same for a given instance
   of VPLS. Different VPLS instances MAY have different committed MTU
   sizes. If VLANs are supported, all VLANs within a given VPLS MUST
   inherit the same MTU size.

8.8 Multilink Access

   The VPLS service SHOULD support multilink access for CE devices.
   The VPLS service MAY support multihome access for CE devices.

8.9 End-point VLAN tag translation

   If VLANs are recognized, the VPLS system MAY support translation of
   customers' VLAN tags. Such service simplifies connectivity of sites
   that want to keep their tag assignments or sites that belong to
   different administrative entities.  In the latter case, the
   connectivity is sometimes referred to as L2 extranet.

8.10 Support for MAC Services

   VPLS are REQUIRED to provide MAC service compliant with IEEE 802.1D
   specification [5] Section 6. Compliance with this section
   facilitates proper operation of 802.1 LAN and seamless integration
   of VPLS with bridged Local Area Networks.  It is also useful to
   compare [6], [7], and [8].

   A MAC service in the context of VPLS is defined as the transfer of
   user data between source and destination end stations via the
   service access points using the information specified in the VSI.

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   1. A PE device that provides VPLS MUST NOT be directly accessed by
      end stations except for explicit management purposes.

   2. All MAC addresses MUST be unique within a given broadcast domain.

   3. The topology and configuration of the VPLS MUST NOT restrict the
      MAC addresses of end stations

9  Management and Operations Requirements

9.1 VPLS configuration and monitoring

   A VPLS system MUST have capabilities to configure, manage, and
   monitor its different components.

   It SHOULD be possible to create several disjoint instances of VPLS
   systems within the same underlying network infrastructures.

   The infrastructure SHOULD monitor all characteristics of the service
   that are reflected in the customer SLA. This includes but is not
   limited to bandwidth usage, packet counts, packet drops, service
   outages, etc.

9.2 VPLS operations

   The operations of a VPLS systems is controlled by an Administrative
   Authority (Admin).  The Admin is the originator of all operational
   parameters of a VPLS system. Conversely, the admin is also the
   ultimate destination for the status of the VPLS system and the
   related statistical information.  A typical VPLS system spans
   several such Admins.

   A VPLS system MUST support proper dissemination of operational
   parameters to all elements of a VPLS system in the presence of
   multiple Admins.

   A VPLS system MUST employ mechanisms for sharing operational
   parameters between different Admins.  These mechanism MUST NOT
   assume any particular structure of the different Admins.  For
   example the VPLS should not be relying on Admins forming a

   A VPLS system SHOULD support policies for proper selection of
   operational parameters coming from different Admins. Similarly, a
   VPLS system SHOULD support policies for selecting information to be
   disseminated to different Admins.

   A VPLS system SHOULD employ discovery mechanisms to minimize the
   amount of operational information maintained by the Admins.  For

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   example, if an admin adds or removes a customer port on a given PE,
   the remaining PEs should determine the necessary actions to take
   without the Admins having to explicitly reconfigure those PEs.

9.3 CE Provisioning

   The VPLS MUST require only minimal or no configuration on the CE
   devices, depending on the CE device that connects into the

9.4 Customer traffic policing

   The VPLS service SHOULD provide the ability to police and/or shape
   customer traffic entering and leaving the VPLS system.

9.5 Dynamic Service Signaling

   A Provider MAY offer to customers an in-band method for selecting
   services from the list specified in the SLA. A Provider MAY use the
   same mechanism for reporting statistical data related to the

9.6 Class of Service Model

   The VPLS service MAY define a graded selection of classes of
   traffic.  These include, but are not limited to

     o range of priorities
     o best effort vs. guaranteed effort
     o range of minimum delay characteristics

9.7 VPLS service access option.

   The VPLS service SHOULD allow for a Provider based Service Access
   for orderly injection of L3 or higher services to the customers'
   VPLS networks.

   In particular, the system SHOULD allow to build L3VPN services,
   including L3 interworking schemes such as ARP mediation or similar.

   As a value added service, a Provider MAY offer access to other
   services such as, IP gateways, storage networks, content delivery

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9.8 Testing

   The VPLS service SHOULD provide the ability to test and verify
   operational and maintenance activities on a per VPLS basis and, if
   supported, on a per VLAN basis.

9.9 Learning information from customer devices

   The VPLS service SHOULD provide means for limiting the amount of
   information learned from customer devices. For example, VPLS
   implementations may limit the number of MAC addresses learned from
   the customers' devices.

10 Security Requirements

10.1 Traffic separation

   A VPLS system MUST provide traffic separation between different VPLS
   domains. If VLANs are supported, the system MUST provide traffic
   separation between customer VLANs within each VPLS domain.

10.2 Provider network protection.

   A VPLS system MUST be immune to malformed or maliciously constructed
   customer traffic. This includes but is not limited to duplicate or
   invalid L2 addresses, customer side loops, short/long packets,
   spoofed management packets, spoofed VLAN tags, etc.

   The VPLS infrastructure devices MUST NOT be accessible from the

10.3 Value added security services

   Value added security services such as encryption and/or
   authentication of customer packets, certificate management, and
   similar are OPTIONAL.

   Security measures employed by the VPLS system SHOULD NOT restrict
   implementation of customer based security add-ons.

11 References

   1. Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.

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   2. Carugi, et al., "Service requirements for Provider Provisioned
      Virtual Private Networks ", Work in progress, December 2001.

   3. Nagarajan et al., " Generic Requirements for Provider Provisioned
      VPN", Work in progress, September 2002.

   4. Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.

   5. ANSI/IEEE Std 802.1D 1998 Edition, "Media Access Control (MAC)
      Bridges", 1998.

   6. IEEE Standard 802.1Q, "IEEE Standards for Local and Metropolitan
      Area Networks: Virtual Bridged Local Area Networks", 1998.

   7. IEEE Standard 802.1u-2001, "IEEE Standard for Local and
      Metropolitan Area Networks: Virtual Bridged Local Area Networks -
      Amendment 1: Technical and editorial corrections", 2001.

   8. IEEE Standard 802.1v-2001, "IEEE Standard for Local and
      Metropolitan Area Networks: Virtual Bridged Local Area Networks -
      Amendment 2: VLAN Classification by Protocol and Port", 2001.

12 Acknowledgments

   We would like to acknowledge extensive comments provided by Loa
   Anderson, Joel Halpern, and Eric Rosen. The authors, also, wish to
   extend appreciations to their respective employers and various other
   people who volunteered to review this work and provided feedback.

13 Authors' Addresses

   Waldemar Augustyn
   Email: waldemar@nxp.com

   Giles Heron
   PacketExchange Ltd.
   The Truman Brewery
   91 Brick Lane
   London E1 6QL
   United Kingdom
   Email: giles@packetexchange.net

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   Vach Kompella
   TiMetra Networks
   274 Ferguson Dr.
   Mountain View, CA 94043
   Email: vkompella@timetra.com

   Marc Lasserre
   Riverstone Networks
   5200 Great America Pkwy
   Santa Clara, CA 95054
   Phone: 408-878-6500
   Email: marc@riverstonenet.com

   Pascal Menezes
   Phone: 206-686-2001
   Email: pascal.menezes@terabeam.com

   Hamid Ould-Brahim
   Nortel Networks
   P.O. Box 3511 Station C
   Ottawa ON K1Y 4H7
   Phone: 613-765-3418
   Email: hbrahim@nortelnetworks.com

   Tissa Senevirathne
   Email: tsenevir@hotmail.com

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