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MAC-Forced Forwarding: A Method for Subscriber Separation on an Ethernet Access Network

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 4562.
Authors Torben Melsen , Steven Blake
Last updated 2018-12-20 (Latest revision 2006-02-03)
RFC stream Independent Submission
Intended RFC status Informational
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IESG IESG state Became RFC 4562 (Informational)
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Responsible AD Mark Townsley
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Personal                                                       T. Melsen
Internet-Draft                                                  S. Blake
Expires: July 31, 2006                                          Ericsson
                                                        January 27, 2006

 MAC-Forced Forwarding: A Method for Traffic Separation on an Ethernet
                             Access Network

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   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
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   The list of current Internet-Drafts can be accessed at

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

   This Internet-Draft will expire on July 31, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document describes a mechanism to ensure layer-2 separation of
   LAN stations accessing an v4IPv4 gateway over a bridged Ethernet

   The mechanism - called "MAC-Forced Forwarding" - implements an ARP
   proxy function that prohibits MAC address resolution between hosts
   located within the same IPv4 subnet but at different customer

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   premises, and in effect directs all upstream traffic to an IPv4
   gateway.  The IPv4 gateway provides IP-layer connectivity between
   these same hosts.

Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Access Network Requirements  . . . . . . . . . . . . . . .  4
     2.2.  Using Ethernet as an Access Network Technology . . . . . .  5
   3.  Solution Aspects . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Obtaining the IP and MAC addresses of the Access Router  .  7
     3.2.  Responding to ARP Requests . . . . . . . . . . . . . . . .  7
     3.3.  Filtering Upstream Traffic . . . . . . . . . . . . . . . .  8
     3.4.  Restricted Access to Application Servers . . . . . . . . .  8
   4.  Access Router Considerations . . . . . . . . . . . . . . . . .  9
   5.  Resiliency Considerations  . . . . . . . . . . . . . . . . . .  9
   6.  Multicast Considerations . . . . . . . . . . . . . . . . . . . 10
   7.  IPv6 Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 11
     10.2. Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14

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1.  Terminology

   Access Node (AN)
      The entity interconnecting individual subscriber lines to the
      shared aggregation network.

   Access Router (AR)
      The entity interconnecting the access network to the Internet or
      other IP-based networks.  The AR provides connectivity between
      hosts on the access network at different customer premises.  It is
      also used to provide security filtering, policing, and accounting
      of customer traffic.

   Application Server (AS)
      A server, usually owned by a service provider, that attaches
      directly to the aggregation network, and is directly reachable at
      layer-2 by customer hosts.

   Ethernet Access Node (EAN)
      An Access Node supporting Ethernet-based subscriber lines and
      uplinks to an Ethernet-based aggregation network, and MAC-Forced
      Forwarding.  For example, for xDSL access, the EAN is an Ethernet-
      centric DSLAM.  The EAN is a special type of filtering bridge that
      does not forward Ethernet broadcast and multicast frames
      originating on a subscriber line to other subscriber lines, but
      either discards them or forwards them upstream (towards the
      aggregation network).  The EAN also discards unicast Ethernet
      frames originating on a subscriber line and not addressed to an

2.  Introduction

   The main purpose of an access network is to provide connectivity
   between customer hosts and service provider access routers (ARs),
   typically offering reachability to the Internet and other IP networks
   and/or IP-based applications.

   An access network may be decomposed into a subscriber line part and
   an aggregation network part.  The subscriber line - often referred to
   as "the first mile" - is characterized by an individual physical (or
   logical, in the case of some wireless technologies) connection to
   each customer premise.  The aggregation network - "the second mile" -
   performs aggregation and concentration of customer traffic.

   The subscriber line and the aggregation network are interconnected by
   an Access Node (AN).  Thus, the AN constitutes the border between
   individual subscriber lines and the common aggregation network.  This

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   is illustrated in the following figure.

        Access       Aggregation  Access    Subscriber    Customer
        Routers      Network      Nodes     Lines         Premise
        +----+           |
      --+ AR +-----------|        +----+
        +----+           |        |    +----------------[]--------
                         |--------+ AN |
                         |        |    +----------------[]--------
                         |        +----+
                         |        +----+
                         |        |    +----------------[]--------
                         |--------+ AN |
                         |        |    +----------------[]--------
                         |        +----+
                         |        +----+
                         |        |    +----------------[]--------
                         |--------+ AN |
        +----+           |        |    +----------------[]--------
      --+ AR +-----------|        +----+
        +----+           |

2.1.  Access Network Requirements

   There are two basic requirements that an access network solution must
   1.  Layer-2 traffic separation between customer premises.
   2.  High IPv4 address assignment efficiency.

   It is required that all traffic to and from customer hosts located at
   different premises (i.e., accessed via different subscriber lines, or
   via different access networks) be forwarded via an AR, and not
   bridged or switched at layer-2 (Requirement 1).  This enables the
   access network service provider to use the AR(s) to perform security
   filtering, policing, and accounting of all customer traffic.  This
   implies that within the access network, layer-2 traffic paths should
   not exist that circumvent an AR (with some exceptions; see
   Section 3.4).

   In ATM-based access networks, the separation of individual customer

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   hosts' traffic is an intrinsic feature achieved by the use of ATM
   permanent virtual connections (PVCs) between the customers' access
   device (e.g., DSL modem) and the AR (typically co-located/integrated
   with access control functionality in a Broadband Remote Access Server
   (BRAS)).  In this case, the AN is an ATM-based Digital Subscriber
   Line Access Multiplexer (DSLAM).

   This document, however, targets Ethernet-based access networks.
   Techniques other than ATM PVCs must be employed to ensure the desired
   separation of traffic to and from individual customer hosts.

   Efficient address assignment is necessary to minimize consumption of
   scarce IPv4 address space (Requirement 2).  See [RFC3069] for further
   discussion.  Address assignment efficiency is improved if host
   addresses are assigned out of one or more large pools, rather than by
   being assigned out of separate, smaller subnet blocks allocated to
   each customer premise.  IPv6 address assignment efficiency is of much
   less concern, and it is anticipated that IPv6 deployments will
   allocate separate IPv6 subnet blocks to each customer premise [v6BB].

2.2.  Using Ethernet as an Access Network Technology

   A major aspect of using Ethernet as an access technology is that
   traffic pertaining to different customer hosts is conveyed over a
   shared broadcast network.  Layer-2 isolation between customer premise
   networks could be provided by implementing access router
   functionality in each EAN, treating each subscriber line as a
   separate IP interface.  However, there are a variety of reasons why
   it is often desirable to avoid IP routing in the access network,
   including the need to satisfy regulatory requirements for direct
   layer-2 accessibility to multiple IP service providers.  In addition,
   this solution would not solve Requirement 2.

   To avoid IP routing within the access network, the Ethernet
   aggregation network is bridged via EANs to individual Ethernet
   networks at the customers' premises.  If the EAN were a standard
   Ethernet bridge then there would be direct layer-2 visibility between
   Ethernet stations (hosts) located at different customers' premises.
   Specifically, hosts located within the same IP subnet would have this
   visibility.  This violates Requirement 1 (Section 2.1) and introduces
   security issues, as malicious end-users could attack hosts at other
   customers' premises directly at the Ethernet layer.

   Existing standardized solutions may be deployed to prevent layer-2
   visibility between stations:
   o  PPP over Ethernet [RFC2516].  The use of PPPoE creates individual
      PPP sessions between hosts and one or more BRASs over a bridged
      Ethernet topology.  Traffic always flows between a BRAS and hosts,

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      never directly between hosts.  The AN can force upstream traffic
      to flow only to the BRAS initially selected by the host.
   o  VLAN per-customer premise network [RFC3069].  Traffic to/from each
      customer premise network can be separated into different VLANs
      across the aggregation network between the AN and the AR.

   Both solutions provide layer-2 isolation between customer hosts, but
   they are not considered optimal for broadband access networks,
   o  PPPoE does not support efficient multicast: packets must be
      replicated on each PPPoE session to hosts listening on a multicast
      group.  This negates one of the major advantages of using Ethernet
      (instead of ATM) as an access technology.  This is an especially
      problematic limitation for services such as IPTV which require
      high bandwidth per-multicast group (channel), and which may often
      have hundreds or thousands of listening customer hosts per-group.
   o  Using VLANs to isolate individual customer premise networks also
      forces multicast packets to be replicated to each VLAN with a
      listening host.  Furthermore, the basic limit of a maximum of 4096
      VLANs per-Ethernet network limits the scalability of the solution.
      This scalability limit can be removed by deploying VLAN stacking
      techniques within the access network, but this approach increases
      provisioning complexity.

   The solution proposed in this document avoids these problems.

3.  Solution Aspects

   The basic property of the solution is that the EAN ensures that
   upstream traffic is always sent to a designated AR, even if the IP
   traffic should ultimately flow between customer hosts located within
   the same IP subnet.

   The solution has three major aspects:
   1.  Initially, the EAN obtains the IP and MAC address of the legal
       target ARs for each customer host.
   2.  The EAN replies to any upstream ARP request [RFC0826] from
       customer hosts with the MAC address of a legal target AR.
   3.  The EAN discards any upstream unicast traffic to MAC addresses
       other than the legal target ARs.  The EAN also discards all non-
       essential broadcast and multicast packets received on subscriber

   These aspects are discussed in the following sections.

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3.1.  Obtaining the IP and MAC addresses of the Access Router

   An access network may contain multiple ARs, and different hosts may
   be assigned to different (groups of) ARs.  This implies that the EAN
   must register the assigned AR addresses on a per-customer host basis.

   For each customer host, one of the ARs is acting as the default
   gateway.  If a customer has simultaneous access to multiple ARs, the
   other ARs typically will provide access to other IP networks.

   The EAN learns the IPv4 address of the legal target ARs in one of two
   ways, depending on the host IPv4 address assignment method.  For each
   host using DHCP, the EAN learns the AR IPv4 addresses dynamically by
   snooping the DHCPACK reply to a host [RFC2131].  If a host using DHCP
   shall have simultaneous access to multiple ARs, DHCP option 121
   [RFC3442] or DHCP option 33 [RFC2132] must be used to specify them to
   that host.  If static address assignment is used instead of DHCP,
   then AR IPv4 addresses must be pre-provisioned in the EAN by the
   network operator.  In both cases, the EAN will need to ARP to
   determine the ARs' corresponding MAC addresses.  This can be done
   immediately after the IPv4 addresses are learned, or when the MAC
   addresses are first required.

   The DHCP server can associate customer hosts with subscriber lines if
   the EAN uses the DHCP Relay Agent Information Option (82) to convey a
   subscriber line identifier to the DHCP server in DHCP messages
   flowing upstream from the customer host [RFC3046].

3.2.  Responding to ARP Requests

   If all customer networks were assigned individual IP subnet blocks
   (and if routing protocols were blocked inside the access network),
   then all upstream traffic would normally go to an AR (typically the
   default gateway), and the EAN could validate all upstream traffic by
   checking that the destination MAC address matched an AR's.

   However, to comply with Requirement 2 of Section 2.1, residential
   customer networks are not (usually) assigned individual IPv4 subnet
   blocks.  In other words, several hosts located at different premises
   are within the same IPv4 subnet.  Consequently, if a host wishes to
   communicate with a host at another premise, an ARP is issued to
   obtain that host's corresponding MAC address.  This ARP request is
   intercepted by the EAN's ARP proxy, and an ARP reply is sent,
   specifying a legal AR MAC address (typically the default gateway's)
   as the requested layer-2 destination address, in a manner similar to
   the "proxy ARP" mechanism described in [RFC1009].  In this way, the
   ARP table of the requesting host will register an AR MAC address as
   the layer-2 destination for any host within that IPv4 subnet (except

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   those at the same customer premise; see below).

   ARP requests for an IPv4 address of a legal target AR are replied to
   by the EAN's ARP proxy with that AR's MAC address, rather than the
   MAC address of the default gateway AR.

   An exception is made when a host is ARPing for another host located
   within the same premise network.  If this ARP request reaches the
   EAN, it should be discarded, because it is assumed to be answered
   directly by the target host within the premise network.  The EAN must
   keep track of all assigned IPv4 addresses on a subscriber line so
   that it can detect these ARP requests and discard them.

3.3.  Filtering Upstream Traffic

   Since the EAN's ARP proxy will reply always with the MAC address of
   an AR, the requesting host will never learn MAC addresses of hosts
   located at other premises.  However, malicious customers or
   malfunctioning hosts may still try to send traffic using other
   unicast destination MAC addresses.  The EAN must discard all unicast
   frames received on a subscriber line that are not addressed to a
   destination MAC address for a legal AR (with some exceptions; see
   Section 3.4.

   Similarly, broadcast or multicast packets received on a subscriber
   line must never be forwarded on other subscriber lines, but only on
   EAN uplinks to the aggregation network.  An EAN must discard all
   broadcast packets received on subscriber lines, except when DHCP is
   in use, in which case the EAN must forward client-to-server DHCP
   DHCPINFORM) [RFC2131] upstream.  An EAN should rate limit upstream
   broadcast packets.

   Broadcast packets forwarded on an EAN uplink may be forwarded to
   other EANs by the aggregation network.  EANs should discard all
   broadcast packets received from the aggregation network, except
   server-to-client DHCP messages (DHCPOFFER, DHCPACK, DHCPNAK)
   [RFC2131], when DHCP is in use.

   Filtering of multicast packets to and from an EAN uplink is discussed
   in Section 6.

3.4.  Restricted Access to Application Servers

   The previous discussion (Section 3.1) describes how customer hosts
   are allowed direct layer-2 connectivity only to one or more ARs.
   Similarly, a customer host could be allowed direct layer-2 access to
   one or more Application Servers (ASs) which are directly connected to

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   the aggregation network.  There is no functional difference in the
   way that MAC-Forced Forwarding treats access to ARs or ASs.

4.  Access Router Considerations

   Traffic between customer hosts that belong to the same IPv4 subnet
   but are located at different customer premises always will be
   forwarded via an AR.  In this case, the AR will forward the traffic
   to the originating network, i.e., on the same interface from where it
   was received.  This normally results in an ICMP redirect message
   [RFC0792] being sent to the originating host.  To prevent this
   behavior, the ICMP redirect function for aggregation network
   interfaces must be disabled in the AR.

5.  Resiliency Considerations

   The operation of MAC-Forced Forwarding does not interfere with or
   delay IP connectivity recovery in the event of a sustained AR
   failure.  Use of DHCP to configure hosts with information on
   multiple, redundant ARs, or use of VRRP [RFC2338] to implement AR
   redundancy, allows IP connectivity to be maintained.

   MAC-Forced Forwarding is a stateful protocol.  If static IPv4 address
   assignment is used in the access network, then the EAN must be pre-
   provisioned with state information on the customer hosts which may be
   configured on a subscriber line, and the ARs associated with those
   hosts.  In the event of a transient EAN failure, the EAN's state
   database can be quickly recovered from its configuration storage.

   If DHCP is used to assign IPv4 addresses in the access network, then
   MAC-Forced Forwarding operates as a soft-state protocol.  Since the
   DHCP and ARP messages that are snooped to construct the EAN state
   database are usually sent infrequently, a transient failure may not
   be detected by either the AR(s) or the customer hosts.  Therefore, a
   transient failure of an EAN could lead to an extended loss of
   connectivity.  To minimize connectivity loss, an EAN should maintain
   its dynamic state database in resilient storage to permit timely
   database and connectivity restoration.

   The EAN is a single point of attachment between a subscriber line and
   the aggregation network; hence, the EAN is a single point of
   connectivity failure.  Customers seeking more resilient connectivity
   should multi-home.

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6.  Multicast Considerations

   Multicast traffic delivery for streams originating from within the
   aggregation network or further upstream, and delivered to one or more
   customer hosts in an access network, is supported in a scalable
   manner by virtue of Ethernet's native multicast capability.
   Bandwidth efficiency can be enhanced if the EAN behaves as an IGMP
   snooping bridge; e.g., if it snoops on IGMP Membership Report and
   Leave Group messages originating on subscriber lines, to prune the
   set of subscriber lines on which to forward particular multicast
   groups [RFC3376].

   An EAN must discard all IPv4 multicast packets received on a
   subscriber line other than IGMP Membership Report and Leave Group
   messages [RFC3376].  If a customer host wishes to source multicast
   packets to a group, the host must tunnel them to an upstream
   multicast router; e.g., an AR acting as a PIM-SM Designated Router .
   An AR will forward them back into the access network if there are any
   listening customer hosts.

   EAN processing of IPv6 multicast packets is discussed in the next

7.  IPv6 Considerations

   MAC-Forced Forwarding is not directly applicable for IPv6 access
   networks, for the following reasons:
   1.  IPv6 access networks do not require the same efficiency of
       address allocation as IPv4 access networks.  It is expected that
       customer premise networks will be allocated unique network
       prefixes (e.g., /48) accommodating large numbers of customer
       subnets and hosts [v6BB].
   2.  IPv6 nodes do not use ARP, but instead use the Neighbor Discovery
       protocol [RFC2461] for layer-2 address resolution.

   To simultaneously support IPv6 and MAC-Forced Forwarding for IPv4, an
   EAN can still implement the unicast, broadcast, and multicast
   filtering rules described in Section 3.3.  To correctly perform
   unicast filtering, the EAN must learn the IPv6 and MAC addresses of
   the legal ARs for a particular subscriber line.  It can learn these
   addresses either through static configuration, or by snooping Router
   Discovery messages exchanged between the customer premises router and
   one or more ARs [RFC2461].

   Multicast is an intrinsic part of the IPv6 protocol suite.
   Therefore, an EAN must not indiscriminately filter IPv6 multicast
   packets flowing upstream, although it may rate limit them.  Detailed

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   IPv6 multicast filtering rules are not discussed in this document.

8.  Security Considerations

   MAC-Forced Forwarding is by its nature a security function, ensuring
   layer-2 isolation of customer hosts sharing a broadcast access
   medium.  In that sense it provides security equivalent to alternative
   PVC-based solutions.  Security procedures appropriate for any shared
   access medium are equally appropriate when MAC-Forced Forwarding is
   employed.  It does not introduce any additional vulnerabilities over
   those of standard Ethernet bridging.

   In addition to layer-2 isolation, An EAN implementing MAC-Forced
   Forwarding must discard all upstream broadcast packets, except for
   valid DHCP messages.  In particular, the EAN must discard any DHCP
   replies originated on a subscriber line.  Further, an EAN may rate
   limit upstream broadcast DHCP messages.

   An EAN implementing MAC-Forced Forwarding must keep track of IPv4
   addresses allocated on subscriber lines.  The EAN therefore has
   sufficient information to discard upstream traffic with spoofed IPv4
   source addresses.

9.  Acknowledgements

   The authors would like to thank Ulf Jonsson, Thomas Narten, James
   Carlson, Rolf Engstrand, Tomas Thyni, and Johan Kolhi for their
   helpful comments.

10.  References

10.1.  Normative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

   [RFC0826]  Plummer, D., "Ethernet Address Resolution Protocol: Or
              converting network protocol addresses to 48.bit Ethernet
              address for transmission on Ethernet hardware", STD 37,
              RFC 826, November 1982.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor

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              Extensions", RFC 2132, March 1997.

   [RFC2362]  Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering,
              S., Handley, M., and V. Jacobson, "Protocol Independent
              Multicast-Sparse Mode (PIM-SM): Protocol Specification",
              RFC 2362, June 1998.

   [RFC3046]  Patrick, M., "DHCP Relay Agent Information Option",
              RFC 3046, January 2001.

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, October 2002.

   [RFC3442]  Lemon, T., Cheshire, S., and B. Volz, "The Classless
              Static Route Option for Dynamic Host Configuration
              Protocol (DHCP) version 4", RFC 3442, December 2002.

10.2.  Informative References

   [RFC1009]  Braden, R. and J. Postel, "Requirements for Internet
              gateways", RFC 1009, June 1987.

   [RFC2338]  Knight, S., Weaver, D., Whipple, D., Hinden, R., Mitzel,
              D., Hunt, P., Higginson, P., Shand, M., and A. Lindem,
              "Virtual Router Redundancy Protocol", RFC 2338,
              April 1998.

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              December 1998.

   [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, December 1998.

   [RFC2516]  Mamakos, L., Lidl, K., Evarts, J., Carrel, D., Simone, D.,
              and R. Wheeler, "A Method for Transmitting PPP Over
              Ethernet (PPPoE)", RFC 2516, February 1999.

   [RFC3069]  McPherson, D. and B. Dykes, "VLAN Aggregation for
              Efficient IP Address Allocation", RFC 3069, February 2001.

   [v6BB]     Asadullah, S., Ahmed, A., Popoviciu, C., Savola, P., and
              J. Palet, "ISP IPv6 Deployment Scenarios in Broadband
              Access Networks", work in progress.

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Authors' Addresses

   Torben Melsen
   Struer  DK-7600


   Steven Blake
   920 Main Campus Drive
   Suite 500
   Raleigh, NC  27606

   Phone: +1 919 472 9913

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