IPv6 maintenance Working Group (6man)                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Best Current Practice                           G. Gont
Expires: January 4, 2018                                    SI6 Networks
                                                         M. Garcia Corbo
                                                              C. Huitema
                                                    Private Octopus Inc.
                                                            July 3, 2017

                   IPv6 Address Usage Recommendations


   This document analyzes the security and privacy implications of IPv6
   addresses based on a number of properties such as address scope,
   stability, and usage type.  It analyzes what properties are desirable
   for some popular scenarios, and provides advice regarding the
   configuration and usage of such addresses.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 4, 2018.

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   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  Issues Associated with Sub-optimal IPv6 Address Usage . .   4
       4.1.1.  Testing for the Presence of Node in the Network . . .   4
       4.1.2.  Unexpected Address Discovery  . . . . . . . . . . . .   4
       4.1.3.  Availability Outside the Expected Scope . . . . . . .   5
     4.2.  Current Limitations in the Address Selection API  . . . .   5
   5.  IPv6 Address Considerations . . . . . . . . . . . . . . . . .   6
     5.1.  Address Scope Considerations  . . . . . . . . . . . . . .   6
     5.2.  Address Stability Considerations  . . . . . . . . . . . .   6
     5.3.  Usage Type Considerations . . . . . . . . . . . . . . . .   8
   6.  Possible Approaches for IPv6 Address Usage  . . . . . . . . .   9
   7.  Advice on IPv6 Address Configuration  . . . . . . . . . . . .  10
   8.  Advice on IPv6 Address Usage  . . . . . . . . . . . . . . . .  10
   9.  Future Work . . . . . . . . . . . . . . . . . . . . . . . . .  11
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     13.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   IPv6 hosts typically configure a number of IPv6 addresses, which may
   differ in multiple aspects, such as address scope and stability (e.g.
   stable addresses vs.  temporary addresses).  For example, a host may
   configure one stable and one temporary address per each
   autoconfiguration prefix advertised on the local network.  The
   addresses to be configured typically depend on local system policy
   configuration, with the aforementioned policy being static and
   irrespective of the network the host attaches to.

   There are three parameters that affect the security and privacy
   properties of an address:

   o  Scope

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   o  Stability

   o  Usage type (client-like "outgoing connections" vs. server-like
      "incoming connections")

   Section 5.1, Section 5.2, and Section 5.3 discuss the security and
   privacy implications (and associated tradeoffs) of the scope,
   stability and usage type properties of IPv6 addresses, respectively.

2.  Terminology

   This document employs the definitions of "public address", "stable
   address", and "temporary address" from Section 2 of [RFC7721].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Background

   Predictable IPv6 addresses result in a number of security and privacy
   implications.  For example, [Barnes2012] discusses how patterns in
   network prefixes can be leveraged for IPv6 address scanning.  On the
   other hand, [RFC7707], [RFC7721] and [RFC7217] discuss the security
   and privacy implications of predictable IPv6 Interface Identifiers

   Given the aforementioned previous work in this area, and the formal
   specification update produced by [RFC8064], we expect (and assume in
   the rest of this document) that implementations have replaced any
   schemes that produce predictable addresses with alternative schemes
   that avoid such patterns (e.g., RFC7217 in replacement of the
   traditional SLAAC addresses that embed link-layer addresses).

4.  Problem Statement

   Applications use system API's to select the IPv6 addresses that will
   be used for incoming and outgoing connections.  This choices have
   consequences in terms of privacy, security, stability and

   Default Address Selection for IPv6 is specified in [RFC6724].  The
   selection starts with a set of potential destination addresses, such
   as returned by getaddrinfo(), and the set of potential source
   addresses currently configured for the selected interfaces.  For each
   potential destination address, the algorithm will select the source
   address that provides the best route to the destination, while
   choosing the appropriate scope and preferring temporary addresses.

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   The algorithm will then select the destination address, while giving
   a preference to reachable addresses with the smallest scope.  The
   selection may be affected by system settings.

   We note that [RFC6724] only applies for outgoing connections, such as
   those made by clients trying to use services offered by other hosts.
   When devices provide a service, the common pattern is to just wait
   for connections over all addresses configured on the device.  For
   example, applications using the Sockets API will commonly bind() the
   listening socket to undefined address.  This long-established
   behavior is appropriate for devices providing public services, but
   may have unexpected results for devices providing semi-private
   services, such as various forms of peer-to-peer or local-only

   This behavior leads to three problems: device tracking, discussed in
   Section 4.1.1; unexpected address discovery, discussed in
   Section 4.1.2; and availability outside the expected scope, discussed
   in Section 4.1.3.  These problems are caused in part by the
   limitations of available address selection API, presented in
   Section 4.2.

4.1.  Issues Associated with Sub-optimal IPv6 Address Usage

4.1.1.  Testing for the Presence of Node in the Network

   The stable addresses recommended in [RFC8064] use stable IID defined
   in [RFC7217].  One key part of that algorithm is that if a device
   connects to a given network at different times, it will always
   configure the same IPv6 addresses on that network.  If the device
   hosts a service ready to accept connections on that stable address,
   adversaries can test the presence of the device on the network by
   attempting connections to that stable address.  Stable addresses used
   by listening services will thus enable testing whether a specific
   device is returning to a particular network, which in a number of
   cases will be considered a privacy issue.

4.1.2.  Unexpected Address Discovery

   Systems like DNS-Based Service Discovery [RFC6763] allow clients to
   discover services within a limited scope, that can be defined by a
   domain name.  These services are not advertised outside of that
   scope, and thus do not expect to be discovered by random parties on
   the Internet.  Yet it appears that such services are easily
   discoverable if they listen for connections to IPv6 addresses that a
   client process also uses as source address when connecting to remote

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      An example of such unexpected discovery is described in [Hein].  A
      network manager observed scanning traffic directed at the
      temporary addresses of local devices.  Analysis shows that the
      scanners learned the addresses by observing the device contact an
      NTP service ([RFC5905]).  The remote scanning was possible because
      the local devices were also accepting connections directed to the
      temporary addresses.

   It is obvious from the example that the "attack surface" of the
   services is increased because they are bond to the same IPv6
   addresses that are also used by clients for outgoing communications
   with remote systems.  But the overlap between "client" and "server"
   addresses is only one part of the problem.  Suppose that a devices
   hosts both a video game and a home automation application.  The video
   game users will be able to discover the IPv6 address of the game
   server.  If the home automation server listens to the same IPv6
   addresses, it is now exposed to connection attempts by all these
   users.  That, too, increases the attack surface of the home
   automation server.

4.1.3.  Availability Outside the Expected Scope

   The IPv6 addressing architecture [RFC4291] defines multiple address
   scopes.  In practice, devices are often configured with globally
   reachable unicast addresses, link local addresses, and Unique Local
   IPv6 Unicast Addresses (ULA) [RFC4193].  Availability outside the
   expected scope happens when a service is expected to be only
   available in some local scope, but inadvertently becomes available to
   remote parties.  That could happen for example if a service is meant
   to be available only on a given link, but becomes reachable through
   ULA or through globally reachable addresses, or if a service is meant
   to be available only inside some organization's perimeter and becomes
   reachable through globally reachable addresses.  It will happen in
   particular if a service intended for some local scope is programmed
   to bind to "unspecified" addresses, which in practice means every
   address configured for the device.

4.2.  Current Limitations in the Address Selection API

   Application developers using the Sockets API can "bind" a listening
   socket to a specific address, and ensure that the application is only
   reachable through that address.  In theory, careful selection of the
   binding address could mitigate the three problems mentioned above.
   Binding services to temporary address could mitigate device tracking.
   Binding different services to different addresses could mitigate
   unexpected discovery.  Binding services to link local addresses or
   ULA could mitigate availability outside the expected scope.  However,

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   explicitly managing addresses adds significant complexity to the
   application development.  It requires that application developers
   master addressing architectures subtleties, and implement logic that
   reacts adequately to connectivity events and address changes.
   Experience shows that application developers would probably prefer
   some much simpler solution.

   In addition, we should note that many application developers use high
   level APIs that listen to TLS, HTTP, or some other application
   protocol.  These high level APIs seldom provide detailed access to
   specific IP addresses, and typically default to listening to all
   available addresses.

5.  IPv6 Address Considerations

5.1.  Address Scope Considerations

   The IPv6 address scope can, in some scenarios, limit the attack
   exposure of a node as a result of the implicit isolation provided by
   a non-global address scope.  For example, a node that only employs
   link-local addresses may, in principle, only be exposed to attack
   from other nodes in the local link.  Hosts employing only Unique
   Local Addresses (ULAs) may be more isolated from attack than those
   employing Global Unicast Addresses (GUAs), assuming that proper
   packet filtering is enforced at the network edge.

   The potential protection provided by a non-global addresses should
   not be regarded as a complete security strategy, but rather as a form
   of "prophylactic" security (see

   We note that the use of non-global addresses is usually limited to a
   reduced type of applications/protocols that e.g. are only meant to
   operate on a reduced scope, and hence their applicability may be

   A discussion of ULA usage considerations can be found in

5.2.  Address Stability Considerations

   The stability of an address has two associated security/privacy

   o  Ability of an attacker to correlate network activity

   o  Exposure to attack

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   For obvious reasons, an address that is employed for multiple
   communication instances allows the aforementioned network activities
   to be correlated.  The longer an address is employed (i.e., the more
   stable it is), the longer such correlation will be possible.  In the
   worst-case scenario, a stable address that is employed for multiple
   communication instances over time will allow all such activities to
   be correlated.  On the other hand, if a host were to generate (and
   eventually "throw away") one new address for each communication
   instance (e.g., TCP connection), network activity correlation would
   be mitigated.

      the use of constant IIDs (as in traditional SLAAC) result in
      addresses that, while not constant as a whole (since the prefix
      changes), contain a globally-unique value that leaks out the node
      "identity".  Such addresses result in the worst possible security
      and privacy implications, and their use has been deprecated by

   Typically, when it comes to attack exposure, the longer an address is
   employed the longer an attacker is exposed to attacks (e.g. an
   attacker has more time to find the address in the first place
   [RFC7707]).  While such exposure is traditionally associated with the
   stability of the address, the usage type of the address (see
   Section 5.3) may also have an impact on attack exposure.

   A popular approach to mitigate network activity correlation is the
   use of "temporary addresses" [RFC4941].  Temporary addresses are
   typically configured and employed along with stable addresses, with
   the temporary addresses employed for outgoing communications, and the
   stable addresses employed for incoming communications.

      Ongoing work [I-D.gont-6man-non-stable-iids] aims at updating
      [RFC4941] such that temporary addresses can be employed without
      the need to configure stable addresses.

   We note that the extent to which temporary addresses provide improved
   mitigation of network activity correlation and/or reduced attack
   exposure may be questionable and/or limited in some scenarios.  For
   example, a temporary address that is reachable for, say, a few hours
   has a questionable "reduced exposure" (particularly when automated
   attack tools do not typically require such a long period of time to
   complete their task).  Similarly, if network activity can be
   correlated for the life of such address (e.g., on the order of
   several hours), such period of time might be long enough for the
   attacker to correlate all the network activity he is meaning to

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   In order to better mitigate network activity correlation and/or
   possibly reduce host exposure, an implementation might want to either
   reduce the preferred lifetime of a temporary address, or even better,
   generate one new temporary address for each new transport protocol
   instance.  However, the associated lifetime/stability of an address
   may have a negative impact on the network.  For example, if a node
   were to employ "throw away" IPv6 addresses, or employ temporary
   addresses [RFC4941] with a short preferred lifetime, local nodes
   might need to maintain too many entries in their Neighbor Cache, and
   a number of devices (possibly enforcing security policies) might also
   need to keep such additional state.

   Additionally, enforcing a maximum lifetime on IPv6 addresses may
   cause long-lived TCP connections to fail.  For example, an address
   becoming "Invalid" (after transitioning through the "Preferred" and
   "Deprecated" states) would cause the TCP connections employing them
   to break.  This, in turn, would cause e.g. long-lived SSH sessions to

   In some scenarios, attack exposure may be reduced by limiting the
   usage of temporary addresses to outbound connections, and prevent
   such addresses from being used for inbound connections (please see
   Section 5.3).

5.3.  Usage Type Considerations

   A node that employs one of its addresses to communicate with an
   external server (i.e., to perform an "outgoing connection") may cause
   such address to become exposed to attack.  For example, once the
   external server receives an incoming connection, the corresponding
   server might launch an attack against the aforementioned address.  A
   real-world instance of this type of scenario has been documented in

   However, we note that employing an IPv6 address for an outgoing
   communications need not increase the exposure of local services to
   other parties.  For example, nodes could employ temporary addresses
   only for outgoing connections, but not for incoming connections.
   Thus, external nodes that learn about client's addresses could not
   really leverage such addresses for actively contacting the clients.

   There are multiple ways in which this could possibly be achieved,
   with different implications.  Namely:

   o  Run a host-based or network-based firewall

   o  Bind services to specific (explicit) addresses

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   o  Bind services only to stable addresses

   A client could simply run a host-based firewall that only allows
   incoming connections on the stable addresses.  This is clearly more
   of an operational way of achieving the desired functionality, and may
   require good firewall/host integration (e.g., the firewall should be
   able to tell stable vs. temporary addresses), may require the client
   to run additional firewall software for this specific purpose, etc.
   In other scenarios, a network-based firewall could be configured to
   allow outgoing communications from all internal addresses, but only
   allow incoming communications to stable addresses.  For obvious
   reasons, this is generally only applicable to networks where incoming
   communications are allowed to a limited number of hosts/servers.

   Services could be bound to specific (explicit) addresses, rather than
   to all locally-configured addresses.  However, there are a number of
   short-comings associated with this approach.  Firstly, an application
   would need to be able to learn all of its addresses and associated
   stability properties, something that tends to be non-trivial and non-
   portable, and that also makes applications protocol-dependent,
   unnecessarily.  Secondly, the Sockets API does not really allow a
   socket to be bound to a subset of the node's addresses.  That is,
   sockets can be bound to a single address or to all available
   addresses (wildcard), but not to a subset of all the configured

   Binding services only to stable addresses provides a clean separation
   between addresses employed for client-like outgoing connections and
   server-like incoming connections.  However, we currently lack an
   appropriate API for nodes to be able to specify that a socket should
   only be bound to stable addresses.  Development of such an API should
   be considered for future work.

6.  Possible Approaches for IPv6 Address Usage

   There are a number of ways in which a system or network may affect
   which address (and how) may be employed for different services and
   cases.  Namely,

   o  TCP/IP stack address filtering

   o  Application-based address filtering

   o  Firewall-based address filtering

   Clearly, the most elegant approach for address selection is for
   applications to be able to specify the properties of the addresses
   they are willing to employ by means of an API, such the TCP/IP stack

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   itself can "filter" which addresses are allowed to be employed for
   the given service/application.  This relieves the application from
   dealing with low level details of networking, improves portability,
   and avoids duplicate code in applications.  However, constraints in
   the current APIs (see Section 4.2) may limit the ability of
   application progremmers form leveraging this technique.

   Another possible approach is for applications to e.g. bind services
   to all available addresses, and perform the associated selection/
   filtering at the application level.  While possible this has a number
   of drawbacks.  Firstly, it would require applications to deal with
   low-level networking details, require that all the associated code be
   duplicated in all applications, and also negatively affect
   portability.  Besides, performing address/selection filtering at the
   application level may not mitigate some possible threats.  For
   example, port scanning will still be possible, since the
   aforementioned filtering will only be performed e.g. once UDP packets
   are received or TCP connections are established.

   Finally, a firewall may be employed to filter addresses based on
   their intended usage.  For example, a firewall may block incoming
   requests to all addresses except to some whitelisted addresses (such
   as the stable addresses of the node).  This technique not only
   requires the use of a firewall (which may or may not be present), but
   also implies knowledge of the firewall regarding the desired
   properties of the addresses that each application/service is intended
   to use.

7.  Advice on IPv6 Address Configuration


   TODO: This section is expected to provide advice regarding the
   configuration of different addresses for different typical scenarios.
   e.g., when nodes may want to configure stable-only, temporary-only,
   or stable+temporary.  In the most simple analysis, one might expect
   nodes in a typical enterprise network to employ only stable
   addresses.  General-purpose nodes in a home or "trusted" network may
   want to employ both stable and temporary addresses.  Finally, mobile
   nodes (e.g. when roaming across non-trusted networks) may want to
   employ only temporary addresses).

8.  Advice on IPv6 Address Usage


   TODO: This section is expected to provide recommendations regarding
   the usage of IPv6 addresses.  Among others, it is expected to provide

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   recommendations regarding the usage of IPv6 addresses when providing
   network services.  In the mos simple form, one argue that nodes may
   want to employ only the smallest-scope applicable addresses (if
   available) and, if stable addresses are available, nodes may want to
   accept incoming connections on such addresses but *not* on temporary

9.  Future Work

   Some of the discussion in this document suggest that in order to
   fully benefit from the IPv6 addresses (in terms of e.g. increased
   availability of addresses and address types) additional work may be
   required in this areas:

   o  Sockets API: The API may need to be extended such that a node may
      bind() only a subset of the available addresses, possibly by
      specifying a criteria (e.g. "only stable addresses", "only
      global", "only local", etc.).

   The aforementioned work may be carried out in this document, or as a
   result of spin off documents.

10.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an

11.  Security Considerations

   The security and privacy implications associated with the
   predictability and lifetime of IPv6 addresses has been analyzed in
   [RFC7217] [RFC7721], and [RFC7707].  This document complements and
   extends the aforementioned analysis by considering other IPv6
   properties such as the address scope and address usage type.

   This document also analyzes what properties are desirable for some
   popular scenarios, and provides advice regarding the configuration
   and usage of such addresses.  Finally, it describes possible future
   standards-track work to allow for greater flexibility in IPv6 address

12.  Acknowledgements

   The authors would like to thank (in alphabetical order) Francis
   Dupont, Tatuya Jinmei, and Dave Thaler for providing valuable
   comments on earlier versions of this document.

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13.  References

13.1.  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,

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <http://www.rfc-editor.org/info/rfc4291>.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, DOI 10.17487/RFC4941, September 2007,

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,

   [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,

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,

   [RFC8064]  Gont, F., Cooper, A., Thaler, D., and W. Liu,
              "Recommendation on Stable IPv6 Interface Identifiers",
              RFC 8064, DOI 10.17487/RFC8064, February 2017,

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13.2.  Informative References

              Barnes, R., Altmann, R., and D. Kerr, "Mapping the Great
              Void Smarter scanning for IPv6",  ISMA 2012 AIMS-4 -
              Workshop on Active Internet Measurements, February 2012,

   [Hein]     Hein, B., "The Rising Sophistication of Network Scanning",
               January 2016, <http://netpatterns.blogspot.be/2016/01/

              Gont, F., Huitema, C., Gont, G., and M. Corbo,
              "Recommendation on Temporary IPv6 Interface Identifiers",
              draft-gont-6man-non-stable-iids-01 (work in progress),
              March 2017.

              Gont, F. and F. Baker, "On Firewalls in Network Security",
              draft-gont-opsawg-firewalls-analysis-02 (work in
              progress), February 2016.

              Liu, B. and S. Jiang, "Considerations For Using Unique
              Local Addresses", draft-ietf-v6ops-ula-usage-
              considerations-02 (work in progress), March 2017.

   [RFC7707]  Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,

   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,

Authors' Addresses

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   Fernando Gont
   SI6 Networks / UTN-FRH
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706

   Phone: +54 11 4650 8472
   Email: fgont@si6networks.com
   URI:   http://www.si6networks.com

   Guillermo Gont
   SI6 Networks
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706

   Phone: +54 11 4650 8472
   Email: ggont@si6networks.com
   URI:   https://www.si6networks.com

   Madeleine Garcia Corbo
   Servicios de Informacion del Transporte
   Neptuno 358
   Havana City  10400

   Email: madelen.garcia16@gmail.com

   Christian Huitema
   Private Octopus Inc.
   Friday Harbor, WA  98250

   Email: huitema@huitema.net
   URI:   http://privateoctopus.com

Gont, et al.             Expires January 4, 2018               [Page 14]