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Considerations For Using Unique Local Addresses

Document Type Active Internet-Draft (v6ops WG)
Authors Sheng Jiang , Bing Liu , Nick Buraglio
Last updated 2024-05-17
Replaces draft-ietf-v6ops-ula-usage-recommendations
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Internet Engineering Task Force                                 S. Jiang
Internet-Draft                                                      BUPT
Intended status: Informational                                    B. Liu
Expires: 18 November 2024                            Huawei Technologies
                                                             N. Buraglio
                                                    ForwardingPlane, LLC
                                                            17 May 2024

            Considerations For Using Unique Local Addresses


   This document provides considerations for using IPv6 Unique Local
   Addresses (ULAs).  Based on an analysis of different ULA usage
   scenarios, this document identifies use cases where ULA addresses are
   helpful as well as potential problems caused by using them.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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."

   This Internet-Draft will expire on 18 November 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  General Considerations For Using ULAs . . . . . . . . . . . .   3
     3.1.  Do Not Treat ULA Equal to RFC1918 . . . . . . . . . . . .   3
     3.2.  Using ULAs in a Limited Scope . . . . . . . . . . . . . .   4
   4.  Analysis and Operational Considerations for Scenarios Using
           ULAs  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  ULA-only in Isolated Networks . . . . . . . . . . . . . .   4
     4.2.  ULA+PA in Connected Networks  . . . . . . . . . . . . . .   5
     4.3.  ULA-Only in Connected Networks  . . . . . . . . . . . . .   7
     4.4.  Some Specific Use Cases . . . . . . . . . . . . . . . . .   8
       4.4.1.  Special Routing . . . . . . . . . . . . . . . . . . .   8
       4.4.2.  Used as Identifier  . . . . . . . . . . . . . . . . .   8
     4.5.  IPv4 Co-existence Considerations  . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Unique Local Addresses (ULA) is defined in [RFC4193], and it is an
   alternative to site-local address (deprecated in [RFC3879]).  ULAs
   have the following features:

   -  Automatically Generated

      ULA prefixes can be automatically generated using the algorithms
      described in [RFC4193].  This feature allows automatic prefix
      allocation.  Thus one can get a network working immediately
      without applying for prefix(es) from an RIR/LIR (Regional Internet
      Registry/Local Internet Registry).

   -  Globally Unique

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      ULAs are defined as a global scope address space.  However, they
      are not intended to be used globally on the public Internet; in
      contrast, they are mostly used locally, for example, in isolated
      networks, internal networks, or VPNs.

      ULAs are intended to have an extremely low probability of
      collision.  The randomization of 40 bits in a ULA prefix is
      considered sufficient enough to ensure a high degree of uniqueness
      (refer to [RFC4193] Section 3.2.3 for details) and simplifies
      merging of networks by avoiding the need to renumber overlapping
      IP address space.

   -  Provider Independent Address Space

      ULAs can be used for internal communications even without Internet
      connectivity.  They need no registration, so they can support on-
      demand usage and do not carry any RIR/LIR burden of documentation
      or fees.

   -  Well Known Prefix

      The prefixes of ULAs are well known thus they are easily
      identified and filtered.

   This document aims to introduce the usage of ULAs in various
   scenarios, provide some operational considerations, and clarify the
   advantages and disadvantages of the usage in each scenario.  Thus,
   the administrators could choose to use ULAs in a certain way that
   considered benificial for them.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  General Considerations For Using ULAs

3.1.  Do Not Treat ULA Equal to RFC1918

   ULA and [RFC1918] are similar in some aspects.  The most obvious one
   is as described in Section 3.1.3 that ULA provides an internal
   address independence capability in IPv6 that is similar to how
   [RFC1918] is commonly used.  ULA allows administrators to configure
   the internal network of each platform the same way it is configured
   in IPv4.  Many organizations have security policies and architectures

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   based around the local-only routing of [RFC1918] addresses and those
   policies may directly map to ULA [RFC4864].

   But this does not mean that ULA is equal to an IPv6 version of
   [RFC1918] deployment.  [RFC1918] usually combines with NAT/NAPT for
   global connectivity.  But it is not necessary to combine ULAs with
   any kind of NAT.  Operators can use ULA for local communications
   along with global addresses for global communications (see
   Section 4.2).  This is a big advantage brought by default support of
   multiple-addresses-per-interface feature in IPv6.  (People may still
   have a requirement for NAT with ULA, this is discussed in
   Section 4.3.  But people also need to keep in mind that ULA is not
   intentionally designed for this kind of use case.)

   Another important difference is the ability to merge two ULA networks
   without renumbering (because of the uniqueness), which is a big
   advantage over [RFC1918].

3.2.  Using ULAs in a Limited Scope

   A ULA is by definition a prefix that is never advertised outside a
   given domain, and is used within that domain by agreement of those
   networked by the domain.

   So when using ULAs in a network, the administrators need to clearly
   set the scope of the ULAs and configure ACLs on relevant border
   routers to block them out of the scope.  And if internal DNS is
   enabled, the administrators might also need to use internal-only DNS
   names for ULAs and might need to split the DNS so that the internal
   DNS server includes records that are not presented in the external
   DNS server.

4.  Analysis and Operational Considerations for Scenarios Using ULAs

4.1.  ULA-only in Isolated Networks

   IP is used ubiquitously.  Some networks like industrial control bus
   (e.g.  [RS-485], [SCADA], or even non-networked digital interfaces
   like [MIL-STD-1397] have begun to use IP.  In these kinds of
   networks, the system may lack the ability to communicate with the
   public networks.

   As another example, there may be some networks in which the equipment
   has the technical capability to connect to the Internet, but is
   prohibited by administration.  These networks may include data center
   networks, separate financial networks, lab networks.  machine-to-
   machine (e.g. vehicle networks), sensor networks, or even normal
   LANs, and can include very large numbers of addresses.

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   ULA is a straightforward way to assign the IP addresses in the kinds
   of networks just described, with minimal administrative cost or
   burden.  Also, ULAs fit in multiple subnet scenarios, in which each
   subnet has its own ULA prefix.  For example, when assigning vehicles
   with ULAs, it is then possible to separate in-vehicle embedded
   networks into different subnets depending on real-time situation.

   However, each isolated network has the possibility to be connected in
   the future.  Administrators need to consider the following before
   deciding whether to use ULAs:

   *  If the network eventually connects to another isolated or private
      network, the potential for address collision arises.  However, if
      the ULAs were generated in the standard way, this will not be a
      big problem.

   *  If the network eventually connects to the global Internet, then
      the operator will need to add a new global prefix and ensure that
      the address selection policy is properly set up on all interfaces.

   Operational considerations:

   *  Prefix generation: randomly generated according to the algorithms
      defined in [RFC4193] or manually assigned.  Normally, automatic
      generation of the prefixes is recommended, following [RFC4193].
      If there are some specific reasons that call for manual
      assignment, administrators have to plan the prefixes carefully to
      avoid collision.

   *  Prefix announcement: in some cases, networks might need to
      announce prefixes to each other.  For example, in vehicle networks
      with infrastructure-less settings such as Vehicle-to-Vehicle (V2V)
      communication, prior knowledge of the respective prefixes is
      unlikely.  Hence, a prefix announcement mechanism is needed to
      enable inter-vehicle communications based on IP.  As one
      possibility, such announcements could rely on extensions to the
      Router Advertisement message of the Neighbor Discovery Protocol

4.2.  ULA+PA in Connected Networks

   Two classes of network might need to use ULA with PA (Provider
   Aggregated) addresses:

   *  Home network.  Home networks are normally assigned with one or
      more globally routed PA prefixes to connect to the uplink of an
      ISP.  In addition, they may need internal routed networking even
      when the ISP link is down.  Then ULA is a proper tool to fit the

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      requirement.  [RFC7084] requires the CPE to support ULA.  Note:
      ULAs provide more benefit for multiple-segment home networks; for
      home networks containing only one segment, link-local addresses
      are better alternatives.

   *  Enterprise network.  An enterprise network is usually a managed
      network with one or more PA prefixes or with a PI prefix, all of
      which are globally routed.  The ULA can be used to improve
      internal connectivity and make it more resilient, or to isolate
      certain functions like OAM for servers.

   Benefits of Using ULAs in this scenario:

   *  Separated local communication plane: for either home networks or
      enterprise networks, the main purpose of using ULAs along with PA
      addresses is to provide a logically local routing plane separated
      from the global routing plane.  The benefit is to ensure stable
      and specific local communication regardless of the ISP uplink
      failure.  This benefit is especially meaningful for the home
      network or for private OAM function in an enterprise.

   *  Renumbering: in some special cases such as renumbering, enterprise
      administrators may want to avoid the need to renumber their
      internal-only, private nodes when they have to renumber the PA
      addresses of the rest of the network because they are changing
      ISPs, because the ISP has restructured its address allocations, or
      for some other reason.  In these situations, ULA is an effective
      tool for addressing internal-only nodes.  Even public nodes can
      benefit from ULA for renumbering, on their internal interfaces.
      When renumbering, as [RFC4192] suggests, old prefixes continue to
      be valid until the new prefix(es) is(are) stable.  In the process
      of adding new prefix(es) and deprecating old prefix(es), it is not
      easy to keep local communication disentangled from global routing
      plane change.  If we use ULAs for local communication, the
      separated local routing plane can isolate the effects of global
      routing change.


   *  Operational Complexity: there are some arguments that in practice
      the use of ULA+PA creates additional operational complexity.  This
      is not a ULA-specific problem; the multiple-addresses-per-
      interface is an important feature of IPv6 protocol.  Nevertheless,
      running multiple prefixes needs more operational consideration
      than running a single one.

   Operational considerations:

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   *  Default Routing: connectivity may be broken if ULAs are used as
      default route.  When using RIO (Route Information Option) in
      [RFC4191], specific routes can be added without a default route,
      thus avoiding bad user experience due to timeouts on ICMPv6
      redirects.  This behavior was well documented in [RFC7084] as rule
      ULA-5 "An IPv6 CE router MUST NOT advertise itself as a default
      router with a Router Lifetime greater than zero whenever all of
      its configured and delegated prefixes are ULA prefixes." and along
      with rule L-3 "An IPv6 CE router MUST advertise itself as a router
      for the delegated prefix(es) (and ULA prefix if configured to
      provide ULA addressing) using the "Route Information Option"
      specified in Section 2.3 of [RFC4191].  This advertisement is
      independent of having or not having IPv6 connectivity on the WAN
      interface.".  However, it needs to be noticed that current OSes
      don't all support [RFC4191].

   *  SLAAC/DHCPv6 co-existing: Since SLAAC and DHCPv6 might be enabled
      in one network simultaneously; the administrators need to
      carefully plan how to assign ULA and PA prefixes in accordance
      with the two mechanisms.  The administrators need to know the
      current issue of the SLAAC/DHCPv6 interaction.

   *  Address selection: As mentioned in [RFC5220], there is a
      possibility that the longest matching rule will not be able to
      choose the correct address between ULAs and global unicast
      addresses for correct intra-site and extra-site communication.
      [RFC6724] claims that a site-specific policy entry can be used to
      cause ULAs within a site to be preferred over global addresses.

   *  DNS relevant: if administrators choose not to do reverse DNS
      delegation inside of their local control of ULA prefixes, a
      significant amount of information about the ULA population may
      leak to the outside world.  Because reverse queries will be made
      and naturally routed to the global reverse tree, so external
      parties will be exposed to the existence of a population of ULA
      addresses.  [ULA-IN-WILD] provides more detailed situations on
      this issue.  Administrators may need a split DNS to separate the
      queries from internal and external for ULA entries and GUA

4.3.  ULA-Only in Connected Networks

   In theory, a site numbered with ULAs only can get connected via a
   NPTv6[RFC6296] (which is an experimental specification that provides
   a stateless one-to-one mapping between internal addresses and
   external addresses) or application-layer proxy.  This approach could
   get provider independent addresses or get connected from the isolated
   stage without applying to any RIRs/LIRs.  This might make small

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   organizations saving time and address fee.

   However, this approach breaks the end-to-end transparence.  People
   have suffered from the NAT/Proxy middle boxes so much in the IPv4
   ear, there is no reason to continue the suffering when IPv6 is
   available.  This document does not consider ULA+NPTv6/Proxy as a good
   choice for normal cases.  Rather, this document considers ULA+PA
   (Provider Aggregated) as a better approach to connect to the global
   network when ULAs are expected to be retained.

4.4.  Some Specific Use Cases

   Along with the general scenarios, this section provides some specific
   use cases that could benefit from using ULA.

4.4.1.  Special Routing

   For various reasons the administrators may want to have private
   routing be controlled and separated from other routing.  For example,
   in the business-to-business case, two companies might want to use
   direct connectivity that only connects stated machines, such as a
   silicon foundry with client engineers that use it.  A ULA provides a
   simple way to assign prefixes that would be used in accordance with
   an agreement between the parties.

4.4.2.  Used as Identifier

   ULAs could be self-generated and easily grabbed from the standard
   IPv6 stack.  And ULAs don't need to be changed as the GUA prefixes
   do.  So they are very suitable to be used as identifiers by the up
   layer applications.  And since ULA is not intended to be globally
   routed, it is not harmful to the routing system.

   Such kind of benefit has been utilized in real implementations.  For
   example, in [RFC6281], the protocol BTMM (Back To My Mac) needs to
   assign a topology-independent identifier to each client host
   according to the following considerations:

   *  TCP connections between two end hosts wish to survive in network

   *  Sometimes one needs a constant identifier to be associated with a
      key so that the Security Association can survive the location

   It needs to be noticed again that in theory ULA has the possibility
   of collision.  However, the probability is desirably small enough and
   can be ignored in most cases when ULAs are used as identifiers.

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4.5.  IPv4 Co-existence Considerations

   Generally, this document does not consider IPv4 to be in scope.  But
   regarding ULA, there is a special case needs to be recognized, which
   is described in Section 3.2.2 of [RFC5220].  When an enterprise has
   IPv4 Internet connectivity but does not yet have IPv6 Internet
   connectivity, and the enterprise wants to provide site-local IPv6
   connectivity, a ULA is the best choice for site-local IPv6
   connectivity.  Each employee host will have both an IPv4 global or
   private address and a ULA.  Here, when this host tries to connect to
   an outside node that has registered both A and AAAA records in the
   DNS, the host will choose AAAA as the destination address and the ULA
   for the source address according to the IPv6 preference of the
   default policy table defined in the old address selection standard
   [RFC3484].  This will clearly result in a connection failure.  The
   new address selection standard [RFC6724] has corrected this behavior
   by preferring IPv4 than ULAs in the default policy table.  However,
   there are still lots of hosts using the old standard [RFC3484], thus
   this could be an issue in real networks.

   Happy Eyeballs [RFC8305] solves this connection failure problem, but
   unwanted timeouts will obviously lower the user experience.  One
   possible approach to eliminating the timeouts is to deprecate the
   IPv6 default route and simply configure a scoped route on hosts (in
   the context of this document, only configure the ULA prefix routes).
   Another alternative is to configure IPv4 preference on the hosts, and
   not include DNS A records but only AAAA records for the internal
   nodes in the internal DNS server.  Then outside nodes have both A and
   AAAA records and can be connected through IPv4 as default and
   internal nodes can always connect through IPv6.  But since IPv6
   preference is default, changing the default in all nodes is not
   suitable at scale.

5.  Security Considerations

   Security considerations regarding ULAs, in general, please refer to
   the ULA specification [RFC4193].  Also refer to [RFC4864], which
   shows how ULAs help with local network protection.

   As mentioned in Section 4.2, when using NPTv6, the administrators
   need to know where the firewall is located to set proper filtering

   Also as mentioned in Section 4.2, if administrators choose not to do
   reverse DNS delegation inside their local control of ULA prefixes, a
   significant amount of information about the ULA population may leak
   to the outside world.

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6.  IANA Considerations

   This memo has no actions for IANA.

7.  Acknowledgements

   Many valuable comments were received in the IETF v6ops WG mail list,
   especially from Cameron Byrne, Fred Baker, Brian Carpenter, Lee
   Howard, Victor Kuarsingh, Alexandru Petrescu, Mikael Abrahamsson, Tim
   Chown, Jen Linkova, Christopher Palmer Jong-Hyouk Lee, Mark Andrews,
   Lorenzo Colitti, Ted Lemon, Joel Jaeggli, David Farmer, Doug Barton,
   Owen Delong, Gert Doering, Bill Jouris, Bill Cerveny, Dave Thaler,
   Nick Hilliard, Jan Zorz, Randy Bush, Anders Brandt, , Sofiane Imadali
   and Wesley George.

   Some test of using ULA in the lab was done by our research partner
   BNRC-BUPT (Broad Network Research Centre in Beijing University of
   Posts and Telecommunications).  Thanks for the work of Prof.
   Xiangyang Gong and student Dengjia Xu.

   Tom Taylor did a language review and revision throught the whole
   document.  The authors appreciate a lot for his help.

   This document was produced using the xml2rfc tool [RFC7991]
   (initially prepared using

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,

   [RFC7991]  Hoffman, P., "The "xml2rfc" Version 3 Vocabulary",
              RFC 7991, DOI 10.17487/RFC7991, December 2016,

8.2.  Informative References

              "Military Standard, Input/Output Interfaces, Standard
              Digital Data, Navy Systems (MIL-STD-1397B), 3 March 1989".

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   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <>.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484,
              DOI 10.17487/RFC3484, February 2003,

   [RFC3879]  Huitema, C. and B. Carpenter, "Deprecating Site Local
              Addresses", RFC 3879, DOI 10.17487/RFC3879, September
              2004, <>.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
              November 2005, <>.

   [RFC4192]  Baker, F., Lear, E., and R. Droms, "Procedures for
              Renumbering an IPv6 Network without a Flag Day", RFC 4192,
              DOI 10.17487/RFC4192, September 2005,

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,

   [RFC4864]  Van de Velde, G., Hain, T., Droms, R., Carpenter, B., and
              E. Klein, "Local Network Protection for IPv6", RFC 4864,
              DOI 10.17487/RFC4864, May 2007,

   [RFC5220]  Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
              "Problem Statement for Default Address Selection in Multi-
              Prefix Environments: Operational Issues of RFC 3484
              Default Rules", RFC 5220, DOI 10.17487/RFC5220, July 2008,

   [RFC6281]  Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
              "Understanding Apple's Back to My Mac (BTMM) Service",
              RFC 6281, DOI 10.17487/RFC6281, June 2011,

   [RFC6296]  Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
              Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,

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   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,

   [RFC7084]  Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
              Requirements for IPv6 Customer Edge Routers", RFC 7084,
              DOI 10.17487/RFC7084, November 2013,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,

   [RS-485]   "Electronic Industries Association (1983). Electrical
              Characteristics of Generators and Receivers for Use in
              Balanced Multipoint Systems. EIA Standard RS-485.".

   [SCADA]    "Boyer, Stuart A. (2010). SCADA Supervisory Control and
              Data Acquisition. USA: ISA - International Society of

              "G. Michaelson, "

Authors' Addresses

   Sheng Jiang
   Beijing University of Posts and Telecommunications
   No. 10 Xitucheng Road
   Haidian District

   Bing Liu
   Huawei Technologies
   Q14, Huawei Campus, No.156 Beiqing Road
   Hai-Dian District, Beijing, 100095
   P.R. China

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   Nick Buraglio
   ForwardingPlane, LLC

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