IPv6 maintenance Working Group (6man)                            F. Gont
Internet-Draft                                    SI6 Networks / UTN-FRH
Intended status: Standards Track                          March 22,                          April 12, 2013
Expires: September 23, October 14, 2013

  A method for Generating Stable Privacy-Enhanced Addresses with IPv6
              Stateless Address Autoconfiguration (SLAAC)


   This document specifies a method for generating IPv6 Interface
   Identifiers to be used with IPv6 Stateless Address Autoconfiguration
   (SLAAC), such that addresses configured using this method are stable
   within each subnet, but the Interface Identifier changes when hosts
   move from one network to another.  The aforementioned method is meant
   to be an alternative to generating Interface Identifiers based on
   IEEE identifiers, such that the benefits of stable addresses can be
   achieved without sacrificing the privacy of users.

Status of this Memo

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   This Internet-Draft will expire on September 23, October 14, 2013.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Design goals . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Algorithm specification  . . . . . . . . . . . . . . . . . . .  7
   4.  Resolving Duplicate Address Detection (DAD) conflicts  . . . . 10
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  Privacy issues still present with RFC 4941  . . . . . 16
     A.1.  Host tracking  . . . . . . . . . . . . . . . . . . . . . . 16
       A.1.1.  Tracking hosts across networks #1  . . . . . . . . . . 16
       A.1.2.  Tracking hosts across networks #2  . . . . . . . . . . 16
       A.1.3.  Revealing the identity of devices performing
               server-like functions  . . . . . . . . . . . . . . . . 17
     A.2.  Address scanning attacks . . . . . . . . . . . . . . . . . 17
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.  Introduction

   [RFC4862] specifies the Stateless Address Autoconfiguration (SLAAC)
   for IPv6 [RFC2460], which typically results in hosts configuring one
   or more "stable" addresses composed of a network prefix advertised by
   a local router, and an Interface Identifier (IID) that typically
   embeds a hardware address (e.g., using IEEE identifiers) [RFC4291].

   Generally, stable addresses are thought to simplify network
   management, since they simplify Access Control Lists (ACLs) and
   logging.  However, since IEEE identifiers are typically globally
   unique, the resulting IPv6 addresses can be leveraged to track and
   correlate the activity of a node over time and across multiple
   subnets and networks, thus negatively affecting the privacy of users.

   The "Privacy Extensions for Stateless Address Autoconfiguration in
   IPv6" [RFC4941] were introduced to complicate the task of
   eavesdroppers and other information collectors to correlate the
   activities of a node, and basically result in temporary (and random)
   Interface Identifiers that are typically more difficult to leverage
   than those based on IEEE identifiers.  When privacy extensions are
   enabled, "privacy addresses" are employed for "outgoing
   communications", while the traditional IPv6 addresses based on IEEE
   identifiers are still used for "server" functions (i.e., receiving
   incoming connections).

      As noted in [RFC4941], "anytime a fixed identifier is used in
      multiple contexts, it becomes possible to correlate seemingly
      unrelated activity using this identifier".  Therefore, since
      "privacy addresses" [RFC4941] do not eliminate the use of fixed
      identifiers for server-like functions, they only *partially*
      mitigate the correlation of host activities (see Appendix A for
      some example attacks that are still possible with privacy
      addresses).  Therefore, it is vital that the privacy
      characteristics of "stable" addresses are improved such that the
      ability of an attacker correlating host activities across networks
      is reduced.

      Another important aspect not mitigated by "Privacy Addresses"
      [RFC4941] is that of host IPv6 address scanning.  Since IPv6 addresses
      that embed IEEE identifiers have specific patterns, an attacker
      could leverage such patterns to greatly reduce the search space
      for "live" hosts.  Since "privacy addresses" do not eliminate the
      use of IPv6 addresses that embed IEEE identifiers, host address
      scanning attacks are still feasible even if "privacy extensions"
      are employed [Gont-DEEPSEC2011] [CPNI-IPv6].  This is yet another
      motivation to improve the privacy characteristics of "stable"
      addresses (currently embedding IEEE identifiers).

   Privacy/temporary addresses can be challenging in a number of areas.
   For example, from a network-management point of view, they tend to
   increase the complexity of event logging, trouble-shooting, and
   enforcing access controls and quality of service, etc.  As a result,
   some organizations disable the use of privacy addresses even at the
   expense of reduced privacy [Broersma].  Also, they result in
   increased complexity, which might not be possible or desirable in
   some implementations (e.g., some embedded devices).

   In scenarios in which "Privacy Extensions" are deliberately not used
   (possibly for any of the aforementioned reasons), all a host is left
   with is the addresses that have been generated using e.g.  IEEE
   identifiers, and this is yet another case in which it is also vital
   that the privacy characteristics of these stable addresses are

   We note that in most (if not all) of those scenarios in which
   "Privacy Extensions" are disabled, there is usually no actual desire
   to negatively affect user privacy, but rather a desire to simplify
   operation of the network (simplify the use of ACLs, logging, etc.).

   This document specifies a method to generate interface identifiers
   that are stable/constant for each network interface within each
   subnet, but that change as hosts move from one network to another,
   thus keeping the "stability" properties of the interface identifiers
   specified in [RFC4291], while still mitigating host-scanning address-scanning
   attacks and preventing correlation of the activities of a node as it
   moves from one network to another.

   We note that this method is incrementally deployable, since it does
   not pose any interoperability implications when deployed on networks
   where other nodes do not implement or employ it.

   This document does not update or modify IPv6 StateLess Address Auto-
   Configuration (SLAAC) [RFC4862] itself, but rather only specifies an
   alternative algorithm to generate Interface IDs.  Therefore, the
   usual address lifetime properties (as specified in the corresponding
   Prefix Information Options) apply when IPv6 addresses are generated
   as a result of employing the algorithm specified in this document
   with SLAAC [RFC4862].  Additionally, from the point of view of
   renumbering, we note that these addresses behave like the traditional
   IPv6 addresses (that embed a hardware address) resulting from SLAAC

   For nodes that currently disable "Privacy extensions" [RFC4941] for
   some of the reasons stated above, this mechanism provides stable
   privacy-enhanced addresses which may already address most of the
   privacy concerns related to addresses that embed IEEE identifiers

   [RFC4291].  On the other hand, in scenarios in which "Privacy
   Extensions" are employed, implementation of the mechanism described
   in this document would mitigate host-scanning attacks and also
   mitigate correlation of host activities.

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

2.  Design goals

   This document specifies a method for selecting interface identifiers
   to be used with IPv6 SLAAC, with the following goals:

   o  The resulting interface identifiers remain constant/stable for
      each prefix used with SLAAC within each subnet.  That is, the
      algorithm generates the same interface identifier when configuring
      an address belonging to the same prefix within the same subnet.

   o  The resulting interface identifiers do not depend on the
      underlying hardware (e.g. link-layer address).  This means that
      e.g. replacing a Network Interface Card (NIC) will not have the
      (generally undesirable) effect of changing the IPv6 addresses used
      for that network interface.

   o  The resulting interface identifiers do change when addresses are
      configured for different prefixes.  That is, if different
      autoconfiguration prefixes are used to configure addresses for the
      same network interface card, the resulting interface identifiers
      must be (statistically) different.

   o  It must be difficult for an outsider to predict the interface
      identifiers that will be generated by the algorithm, even with
      knowledge of the interface identifiers generated for configuring
      other addresses.

   o  The aforementioned interface identifiers are meant to be an
      alternative to those based on e.g.  IEEE identifiers, such as
      those specified in [RFC2464].

   We note that of use of the algorithm specified in this document is
   (to a large extent) orthogonal to the use of "Privacy Extensions"
   [RFC4941].  Hosts that do not implement/use "Privacy Extensions"
   would have the benefit that they would not be subject to the host-
   tracking and host address scanning issues discussed in the previous
   section.  On the other hand, in the case of hosts employing "Privacy
   Extensions", the method specified in this document would prevent host
   address scanning attacks and correlation of node activities across
   networks (see Appendix A).

3.  Algorithm specification

   IPv6 implementations conforming to this specification MUST generate
   interface identifiers using the algorithm specified in this section
   in replacement of any other algorithms used for generating "stable"
   addresses (such as that specified in [RFC2464]).  The aforementioned
   algorithm MUST be employed for generating the interface identifiers
   for all of the IPv6 addresses configured with SLAAC for a given
   interface, including IPv6 link-local addresses.

      This means that this document does not formally obsolete or
      deprecate any of the existing algorithms to generate Interface IDs
      (e.g. such as that specified in [RFC2464]).  However, those IPv6
      implementations that employ this specification must generate all
      of their "stable" addresses as specified in this document.

   Implementations conforming to this specification SHOULD provide the
   means for a system administrator to enable or disable the use of this
   algorithm for generating Interface Identifiers.  Implementations
   conforming to this specification MAY employ the algorithm specified
   in [RFC4941] to generate temporary addresses in addition to the
   addresses generated with the algorithm specified in this document.

   Unless otherwise noted, all of the parameters included in the
   expression below MUST be included when generating an Interface ID.

   1.  Compute a random (but stable) identifier with the expression:

       RID = F(Prefix, Interface_Index, Network_ID, DAD_Counter,


          Random (but stable) Interface Identifier

          A pseudorandom function (PRF) that is not computable from the
          outside (without knowledge of the secret key).  The PRF could
          be implemented as a cryptographic hash of the concatenation of
          each of the function parameters.

          The prefix to be used for SLAAC, as learned from an ICMPv6
          Router Advertisement message.

          The interface index [RFC3493] [RFC3542] corresponding to this
          network interface.

          Some network specific data that identifies the subnet to which
          this interface is attached.  For example the IEEE 802.11
          Service Set Identifier (SSID) corresponding to the network to
          which this interface is associated.  This parameter is

          A counter that is employed to resolve Duplicate Address
          Detection (DAD) conflicts.  It MUST be initialized to 0, and
          incremented by 1 for each new tentative address that is
          configured as a result of a DAD conflict.  Implementations
          that record DAD_Counter in non-volatile memory for each
          {Prefix, Interface_Index, Network_ID} tuple MUST initialize
          DAD_Counter to the recorded value if such an entry exists in
          non-volatile memory).  See Section 4 for additional details.

          A secret key that is not known by the attacker.  The secret
          key MUST be initialized at system installation time to a
          pseudo-random number (see [RFC4086] for randomness
          requirements for security).  An implementation MAY provide the
          means for the user to change the secret key.

   2.  The Interface Identifier is finally obtained by taking the
       leftmost 64 bits of the RID value computed in the previous step.
       The resulting Interface Identifier should be compared against the
       Subnet-Router Anycast [RFC4291] and the Reserved Subnet Anycast
       Addresses [RFC2526], and against those interface identifiers
       already employed in an address of the same network interface and
       the same network prefix.  In the event that an unacceptable
       identifier has been generated, this situation should be handled
       in the same way as the case of duplicate addresses (see
       Section 4).

   This document does not require the use of any specific PRF for the
   function F() above, since the choice of such PRF is usually a trade-
   off between a number of properties (processing requirements, ease of
   implementation, possible intellectual property rights, etc.), and
   since the best possible choice for F() might be different for
   different types of devices (e.g. embedded systems vs. regular
   servers) and might possibly change over time.

   Note that the result of F() in the algorithm above is no more secure
   than the secret key.  If an attacker is aware of the PRF that is
   being used by the victim (which we should expect), and the attacker
   can obtain enough material (i.e. addresses configured by the victim),
   the attacker may simply search the entire secret-key space to find
   matches.  To protect against this, the secret key should be of a
   reasonable length.  Key lengths of at least 128 bits should be
   adequate.  The secret key is initialized at system installation time
   to a pseudo-random number, thus allowing this mechanism to be
   enabled/used automatically, without user intervention.

   Including the SLAAC prefix in the PRF computation causes the
   Interface ID to vary across networks that employ different prefixes,
   thus mitigating host-tracking attacks and any other attacks that
   benefit from predictable Interface IDs (such as host address scanning).

   The Interface Index is expected to remain constant across system
   reboots and other events.  However, we note that it might change upon
   removal or installation of a network interface (typically one with a
   smaller value for the Interface Index, when such a naming scheme is
   used).  When such change occurs, the IPv6 addresses resulting from
   this algorithm for the corresponding interface will change, thus
   affecting the stability property of this method.  We note that we
   expect these scenarios to be unusual.  Some implementations are known
   to provide configuration knobs to set the Interface Index for a given
   interface.  Such configuration knobs could be employed to prevent the
   Interface Index from changing (e.g. as a result of the removal of a
   network interface).

   Including the optional Network_ID parameter when computing the RID
   value above would cause the algorithm to produce a different
   Interface Identifier when connecting to different networks, even when
   configuring addresses belonging to the same prefix.  This means that
   a host would employ a different Interface ID as it moves from one
   network to another even for IPv6 link-local addresses or Unique Local
   Addresses (ULAs).

   The DAD_Counter parameter provides the means to intentionally cause
   this algorithm produce a different IPv6 addresses (all other
   parameters being the same).  This could be necessary to resolve
   Duplicate Address Detection (DAD) conflicts, as discussed in detail
   in Section 4.

4.  Resolving Duplicate Address Detection (DAD) conflicts

   If as a result of performing Duplicate Address Detection (DAD)
   [RFC4862] a host finds that the tentative address generated with the
   algorithm specified in Section 3 is a duplicate address, it SHOULD
   resolve the address conflict by trying a new tentative address as

   o  DAD_Counter is incremented by 1.

   o  A new Interface ID is generated with the algorithm specified in
      Section 3, using the incremented DAD_Counter value.

   This procedure may be repeated a number of times until the address
   conflict is resolved.  We RECOMMEND hosts to try at least
   IDGEN_RETRIES (hereby specified as "3") tentative addresses if DAD
   fails for successive generated addresses, in the hopes of resolving
   the address conflict.  We also note that hosts MUST limit the number
   of tentative addresses that are tried (rather than indefinitely try a
   new tentative address until the conflict is resolved).

   In those (unlikely) scenarios in which duplicate addresses are
   detected and in which the order in which the conflicting nodes
   configure their addresses may vary (e.g., because they may be
   bootstrapped in different order), the algorithm specified in this
   section for resolving DAD conflicts could lead to addresses that are
   not stable within the same subnet.  In order to mitigate this
   potential problem, nodes MAY record the DAD_Counter value employed
   for a specific {Prefix, Interface_Index, Network_ID} tuple in non-
   volatile memory, such that the same DAD_Counter value is employed
   when configuring an address for the same Prefix and subnet at any
   other point in time.

   In the event that a DAD conflict cannot be solved (possibly after
   trying a number of different addresses), address configuration would
   fail.  In those scenarios, nodes MUST NOT automatically fall back to
   employing other algorithms for generating interface identifiers.

5.  IANA Considerations

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

6.  Security Considerations

   This document specifies an algorithm for generating interface
   identifiers to be used with IPv6 Stateless Address Autoconfiguration
   (SLAAC), as an alternative to e.g. interface identifiers that embed
   IEEE identifiers (such as those specified in [RFC2464]).  When
   compared to such identifiers, the identifiers specified in this
   document have a number of advantages:

   o  They prevent trivial host-tracking, since when a host moves from
      one network to another the network prefix used for
      autoconfiguration and/or the Network ID (e.g., IEEE 802.11 SSID)
      will typically change, and hence the resulting interface
      identifier will also change (see Appendix A.

   o  They mitigate address-scanning techniques which leverage
      predictable interface identifiers (e.g., known Organizational
      Unique Identifiers) [I-D.ietf-opsec-ipv6-host-scanning].

   o  They result in IPv6 addresses that are independent of the
      underlying hardware (i.e. the resulting IPv6 addresses do not
      change if a network interface card is replaced).

   We note that this algorithm is meant to be an alternative to
   interface identifiers such as those specified in [RFC2464], but is
   not meant as an alternative to temporary Interface IDs (such as those
   specified in [RFC4941]).  Clearly, temporary addresses may help to
   mitigate the correlation of activities of a node within the same
   network, and may also reduce the attack exposure window (since
   privacy/temporary addresses are short-lived when compared to the
   addresses generated with the method specified in this document).  We
   note that implementation of this algorithm would still benefit those
   hosts employing "Privacy Addresses", since it would mitigate host-
   tracking vectors still present when privacy addresses are used (see
   Appendix A), and would also mitigate host-scanning techniques that
   leverage patterns in IPv6 addresses that embed IEEE identifiers.

   Finally, we note that the method described in this document may
   mitigate most of the privacy concerns arising from the use of IPv6
   addresses that embed IEEE identifiers, without the use of temporary
   addresses, thus possibly offering an interesting trade-off for those
   scenarios in which the use of temporary addresses is not feasible.

7.  Acknowledgements

   The algorithm specified in this document has been inspired by Steven
   Bellovin's work ([RFC1948]) in the area of TCP sequence numbers.

   The author would like to thank (in alphabetical order) Karl Auer,
   Steven Bellovin, Matthias Bethke, Brian Carpenter, Tassos
   Chatzithomaoglou, Dominik Elsbroek, Brian Haberman, Bob Hinden,
   Christian Huitema, Ray Hunter, Jong-Hyouk Lee, Michael Richardson,
   Mark Smith, and Ole Troan, for providing valuable comments on earlier
   versions of this document.

   This document is based on the technical report "Security Assessment
   of the Internet Protocol version 6 (IPv6)" [CPNI-IPv6] authored by
   Fernando Gont on behalf of the UK Centre for the Protection of
   National Infrastructure (CPNI).

   Fernando Gont would like to thank CPNI (http://www.cpni.gov.uk) for
   their continued support.

8.  References

8.1.  Normative References

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

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

   [RFC2526]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
              Addresses", RFC 2526, March 1999.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

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

8.2.  Informative References

   [RFC1948]  Bellovin, S., "Defending Against Sequence Number Attacks",
              RFC 1948, May 1996.

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, December 1998.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, February 2003.

   [RFC3542]  Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
              "Advanced Sockets Application Program Interface (API) for
              IPv6", RFC 3542, May 2003.

              Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", draft-ietf-opsec-ipv6-host-scanning-00 (work in
              progress), December 2012.

              Gont, "Results of a Security Assessment of the Internet
              Protocol version 6 (IPv6)",  DEEPSEC 2011 Conference,
              Vienna, Austria, November 2011, <http://

              Gont, "Recent Advances in IPv6 Security",  BRUCON 2012
              Conference, Ghent, Belgium, September 2012, <http://

              Broersma, R., "IPv6 Everywhere: Living with a Fully IPv6-
              enabled environment",  Australian IPv6 Summit 2010,
              Melbourne, VIC Australia, October 2010,

              Gont, F., "Security Assessment of the Internet Protocol
              version 6 (IPv6)",  UK Centre for the Protection of
              National Infrastructure, (available on request).

Appendix A.  Privacy issues still present with RFC 4941

   This section aims to clarify the motivation of using the method
   specified in this document even when privacy/temporary addresses
   [RFC4941] are employed.  It discusses a (non-exaustive) number of
   scenarios in which host privacy is still sacrificed even when
   privacy/temporary addresses [RFC4941] are employed, as a result of
   employing interface identifiers that are constant across networks
   (e.g., those resulting from embedding IEEE identifiers).

A.1.  Host tracking

   This section describes one possible attack scenario that illustrates
   that host-tracking may still be possible when privacy/temporary
   addresses [RFC4941] are employed.

A.1.1.  Tracking hosts across networks #1

   A host configures its stable addresses with the constant
   Interface-ID, and runs any application that needs to perform a
   server-like function (e.g. a peer-to-peer application).  As a result
   of that, an attacker/user participating in the same application
   (e.g., P2P) would learn the constant Interface-ID used by the host
   for that network interface.

   Some time later, the same host moves to a completely different
   network, and employs the same P2P application, probably even with a
   different username.  The attacker now interacts with the same host
   again, and hence can learn its newly-configured stable address.
   Since the interface ID is the same as the one used before, the
   attacker can infer that it is communicating with the same device as

   This is just *one* possible attack scenario, which should remind us
   that one should not disclose more than it is really needed for
   achieving a specific goal (and an Interface-ID that is constant
   across different networks does exactly that: it discloses more
   information than is needed for providing a stable address).

A.1.2.  Tracking hosts across networks #2

   Once an attacker learns the constant Interface-ID employed by the
   victim host for its stable address, the attacker is able to "probe" a
   network for the presence of such host at any given network.

      See Appendix A.1.1 for just one example of how an attacker could
      learn such value.  Other examples include being able to share the
      same network segment at some point in time (e.g. a conference
      network or any public network), etc.

   For example, if an attacker learns that in one network the victim
   used the Interface-ID 1111:2222:3333:4444 for its stable addresses,
   then he could subsequently probe for the presence of such device in
   the network 2011:db8::/64 by sending a probe packet (ICMPv6 Echo
   Request, or any other probe packet) to the address 2001:db8::1111:

A.1.3.  Revealing the identity of devices performing server-like

   Some devices, such as storage devices or printers, may typically
   perform server-like functions and may be usually moved from one
   network to another.  Such devices are likely to simply disable (or
   not even implement) privacy/temporary addresses [RFC4941].  If the
   aforementioned devices employ Interface-IDs that are constant across
   networks, it would be trivial for an attacker to tell whether the
   same device is being used across networks by simply looking at the
   Interface ID.  Clearly, performing server-like functions should not
   imply that a device discloses its identity (i.e., that the attacker
   can tell whether it is the same device providing some function in two
   different networks, at two different points in time).

   The scheme proposed in this document prevents such information
   leakage by causing nodes to generate different Interface-IDs when
   moving to one network to another, thus mitigating this kind of
   privacy attack.

A.2.  Address scanning attacks

   While it is usually assumed that IPv6 address-scanning attacks are
   unfeasible, an attacker could leverage patterns in IPv6 addresses to
   greatly reduce the search space [I-D.ietf-opsec-ipv6-host-scanning]

   As noted earlier in this document, privacy/temporary addresses do not
   eliminate the use of IPv6 addresses that embed IEEE identifiers, and
   hence such hosts would still be vulnerable to address-scanning
   attacks.  The method specified in this document eliminates such
   patterns and would thus mitigate the aforementioned address-scanning

Author's Address

   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