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An IPv6 Prefix for Overlay Routable Cryptographic Hash Identifiers (ORCHID)
RFC 4843

Document Type RFC - Experimental (April 2007)
Obsoleted by RFC 7343
Was draft-laganier-ipv6-khi (individual in int area)
Authors Pekka Nikander , Julien Laganier , Francis Dupont
Last updated 2018-12-20
RFC stream Internet Engineering Task Force (IETF)
IESG Responsible AD Jari Arkko
Send notices to (None)
RFC 4843
Network Working Group                                        P. Nikander
Request for Comments: 4843                 Ericsson Research Nomadic Lab
Category: Experimental                                       J. Laganier
                                                        DoCoMo Euro-Labs
                                                               F. Dupont
                                                              April 2007

                          An IPv6 Prefix for
        Overlay Routable Cryptographic Hash Identifiers (ORCHID)

Status of This Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document introduces Overlay Routable Cryptographic Hash
   Identifiers (ORCHID) as a new, experimental class of IPv6-address-
   like identifiers.  These identifiers are intended to be used as
   endpoint identifiers at applications and Application Programming
   Interfaces (API) and not as identifiers for network location at the
   IP layer, i.e., locators.  They are designed to appear as application
   layer entities and at the existing IPv6 APIs, but they should not
   appear in actual IPv6 headers.  To make them more like vanilla IPv6
   addresses, they are expected to be routable at an overlay level.
   Consequently, while they are considered non-routable addresses from
   the IPv6 layer point-of-view, all existing IPv6 applications are
   expected to be able to use them in a manner compatible with current
   IPv6 addresses.

   This document requests IANA to allocate a temporary prefix out of the
   IPv6 addressing space for Overlay Routable Cryptographic Hash
   Identifiers.  By default, the prefix will be returned to IANA in
   2014, with continued use requiring IETF consensus.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Rationale and Intent . . . . . . . . . . . . . . . . . . .  3
     1.2.  ORCHID Properties  . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Expected use of ORCHIDs  . . . . . . . . . . . . . . . . .  4
     1.4.  Action Plan  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.5.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Cryptographic Hash Identifier Construction . . . . . . . . . .  5
   3.  Routing Considerations . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Overlay Routing  . . . . . . . . . . . . . . . . . . . . .  6
   4.  Collision Considerations . . . . . . . . . . . . . . . . . . .  7
   5.  Design Choices . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 11

1.  Introduction

   This document introduces Overlay Routable Cryptographic Hash
   Identifiers (ORCHID), a new class of IP address-like identifiers.
   These identifiers are intended to be globally unique in a statistical
   sense (see Section 4), non-routable at the IP layer, and routable at
   some overlay layer.  The identifiers are securely bound, via a secure
   hash function, to the concatenation of an input bitstring and a
   context tag.  Typically, but not necessarily, the input bitstring
   will include a suitably encoded public cryptographic key.

1.1.  Rationale and Intent

   These identifiers are expected to be used at the existing IPv6
   Application Programming Interfaces (API) and application protocols
   between consenting hosts.  They may be defined and used in different
   contexts, suitable for different overlay protocols.  Examples of
   these include Host Identity Tags (HIT) in the Host Identity Protocol
   (HIP) [HIP-BASE] and Temporary Mobile Identifiers (TMI) for Mobile
   IPv6 Privacy Extension [PRIVACYTEXT].

   As these identifiers are expected to be used along with IPv6
   addresses at both applications and APIs, co-ordination is desired to
   make sure that an ORCHID is not inappropriately taken for a vanilla
   IPv6 address and vice versa.  In practice, allocation of a separate
   prefix for ORCHIDs seems to suffice, making them compatible with IPv6
   addresses at the upper layers while simultaneously making it trivial
   to prevent their usage at the IP layer.

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   While being technically possible to use ORCHIDs between consenting
   hosts without any co-ordination with the IETF and the IANA, the
   authors would consider such practice potentially dangerous.  A
   specific danger would be realised if the IETF community later decided
   to use the ORCHID prefix for some different purpose.  In that case,
   hosts using the ORCHID prefix would be, for practical purposes,
   unable to use the prefix for the other new purpose.  That would lead
   to partial balkanisation of the Internet, similar to what has
   happened as a result of historical hijackings of non-RFC 1918
   [RFC1918] IPv4 addresses for private use.

   The whole need for the proposed allocation grows from the desire to
   be able to use ORCHIDs with existing applications and APIs.  This
   desire leads to the potential conflict, mentioned above.  Resolving
   the conflict requires the proposed allocation.

   One can argue that the desire to use these kinds of identifiers via
   existing APIs is architecturally wrong, and there is some truth in
   that argument.  Indeed, it would be more desirable to introduce a new
   API and update all applications to use identifiers, rather than
   locators, via that new API.  That is exactly what we expect to happen
   in the long run.

   However, given the current state of the Internet, we do not consider
   it viable to introduce any changes that, at once, require
   applications to be rewritten and host stacks to be updated.  Rather
   than that, we believe in piece-wise architectural changes that
   require only one of the existing assets to be touched.  ORCHIDs are
   designed to address this situation: to allow people to experiment
   with protocol stack extensions, such as secure overlay routing, HIP,
   or Mobile IP privacy extensions, without requiring them to update
   their applications.  The goal is to facilitate large-scale
   experiments with minimum user effort.

   For example, there already exists, at the time of this writing, HIP
   implementations that run fully in user space, using the operating
   system to divert a certain part of the IPv6 address space to a user
   level daemon for HIP processing.  In practical terms, these
   implementations are already using a certain IPv6 prefix for
   differentiating HIP identifiers from IPv6 addresses, allowing them
   both to be used by the existing applications via the existing APIs.

   This document argues for allocating an experimental prefix for such
   purposes, thereby paving the way for large-scale experiments with
   cryptographic identifiers without the dangers caused by address-space

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1.2.  ORCHID Properties

   ORCHIDs are designed to have the following properties:

   o  Statistical uniqueness; also see Section 4

   o  Secure binding to the input parameters used in their generation
      (i.e., the context identifier and a bitstring).

   o  Aggregation under a single IPv6 prefix.  Note that this is only
      needed due to the co-ordination need as indicated above.  Without
      such co-ordination need, the ORCHID namespace could potentially be
      completely flat.

   o  Non-routability at the IP layer, by design.

   o  Routability at some overlay layer, making them, from an
      application point of view, semantically similar to IPv6 addresses.

   As mentioned above, ORCHIDs are intended to be generated and used in
   different contexts, as suitable for different mechanisms and
   protocols.  The context identifier is meant to be used to
   differentiate between the different contexts; see Section 4 for a
   discussion of the related API and kernel level implementation issues,
   and Section 5 for the design choices explaining why the context
   identifiers are used.

1.3.  Expected use of ORCHIDs

   Examples of identifiers and protocols that are expected to adopt the
   ORCHID format include Host Identity Tags (HIT) in the Host Identity
   Protocol [HIP-BASE] and the Temporary Mobile Identifiers (TMI) in the
   Simple Privacy Extension for Mobile IPv6 [PRIVACYTEXT].  The format
   is designed to be extensible to allow other experimental proposals to
   share the same namespace.

1.4.  Action Plan

   This document requests IANA to allocate an experimental prefix out of
   the IPv6 addressing space for Overlay Routable Cryptographic Hash

1.5.  Terminology

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

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2.  Cryptographic Hash Identifier Construction

   An ORCHID is generated using the algorithm below.  The algorithm
   takes a bitstring and a context identifier as input and produces an
   ORCHID as output.

   Input      :=  any bitstring
   Hash Input :=  Context ID | Input
   Hash       :=  Hash_function( Hash Input )
   ORCHID     :=  Prefix | Encode_100( Hash )


   |               : Denotes concatenation of bitstrings

   Input           : A bitstring that is unique or statistically unique
                     within a given context. The bitstring is intended
                     to be associated with the to-be-created ORCHID in
                     the given context.

   Context ID      : A randomly generated value defining the expected
                     usage context for the particular ORCHID and the
                     hash function to be used for generation of ORCHIDs
                     in this context.  These values are allocated out of
                     the namespace introduced for CGA Type Tags; see RFC
                     3972 and

   Hash_function   : The one-way hash function (i.e., hash function with
                     pre-image resistance and second pre-image
                     resistance) to be used according to the document
                     defining the context usage identified by the
                     Context ID.  For example, the current version of
                     the HIP specification defines SHA1 [RFC3174] as
                     the hash function to be used to generate ORCHIDs
                     used in the HIP protocol [HIP-BASE].

   Encode_100( )   : An extraction function in which output is obtained
                     by extracting the middle 100-bit-long bitstring
                     from the argument bitstring.

   Prefix          : A constant 28-bit-long bitstring value

   To form an ORCHID, two pieces of input data are needed.  The first
   piece can be any bitstring, but is typically expected to contain a
   public cryptographic key and some other data.  The second piece is a

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   context identifier, which is a 128-bit-long datum, allocated as
   specified in Section 7.  Each specific experiment (such as HIP HITs
   or MIP6 TMIs) is expected to allocate their own, specific context

   The input bitstring and context identifier are concatenated to form
   an input datum, which is then fed to the cryptographic hash function
   to be used according to the document defining the context usage
   identified by the Context ID.  The result of the hash function is
   processed by an encoding function, resulting in a 100-bit-long value.
   This value is prepended with the 28-bit ORCHID prefix.  The result is
   the ORCHID, a 128-bit-long bitstring that can be used at the IPv6
   APIs in hosts participating to the particular experiment.

   The ORCHID prefix is allocated under the IPv6 global unicast address
   block.  Hence, ORCHIDs are indistinguishable from IPv6 global unicast
   addresses.  However, it should be noted that ORCHIDs do not conform
   with the IPv6 global unicast address format defined in Section 2.5.4
   of [RFC4291] since they do not have a 64-bit Interface ID formatted
   as described in Section 2.5.1. of [RFC4291].

3.  Routing Considerations

   ORCHIDs are designed to serve as location independent endpoint-
   identifiers rather than IP-layer locators.  Therefore, routers MAY be
   configured not to forward any packets containing an ORCHID as a
   source or a destination address.  If the destination address is an
   ORCHID but the source address is a valid unicast source address,
   routers MAY be configured to generate an ICMP Destination
   Unreachable, Administratively Prohibited message.

   Due to the experimental nature of ORCHIDs, router software MUST NOT
   include any special handling code for ORCHIDs.  In other words, the
   non-routability property of ORCHIDs, if implemented, MUST be
   implemented via configuration and NOT by hardwired software code.  At
   this time, it is RECOMMENDED that the default router configuration
   not handle ORCHIDs in any special way.  In other words, there is no
   need to touch existing or new routers due to this experiment.  If
   such a reason should later appear, for example, due to a faulty
   implementation leaking ORCHIDs to the IP layer, the prefix can be and
   should be blocked by a simple configuration rule.

3.1.  Overlay Routing

   As mentioned multiple times, ORCHIDs are designed to be non-routable
   at the IP layer.  However, there are multiple ongoing research
   efforts for creating various overlay routing and resolution
   mechanisms for flat identifiers.  For example, the Host Identity

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   Indirection Infrastructure (Hi3) [Hi3] and Node Identity
   Internetworking Architecture (NodeID) [NodeID] proposals, outline
   ways for using a Distributed Hash Table to forward HIP packets based
   on the Host Identity Tag.

   What is common to the various research proposals is that they create
   a new kind of resolution or routing infrastructure on top of the
   existing Internet routing structure.  In practical terms, they allow
   delivery of packets based on flat, non-routable identifiers,
   utilising information stored in a distributed database.  Usually, the
   database used is based on Distributed Hash Tables.  This effectively
   creates a new routing network on top of the existing IP-based routing
   network, capable of routing packets that are not addressed by IP
   addresses but some other kind of identifiers.

   Typical benefits from overlay routing include location independence,
   more scalable multicast, anycast, and multihoming support than in IP,
   and better DoS resistance than in the vanilla Internet.  The main
   drawback is typically an order of magnitude of slower performance,
   caused by an easily largish number of extra look-up or forwarding
   steps needed.  Consequently, in most practical cases, the overlay
   routing system is used only during initial protocol state set-up (cf.
   TCP handshake), after which the communicating endpoints exchange
   packets directly with IP, bypassing the overlay network.

   The net result of the typical overlay routing approaches is a
   communication service whose basic functionality is comparable to that
   provided by classical IP but provides considerably better resilience
   that vanilla IP in dynamic networking environments.  Some experiments
   also introduce additional functionality, such as enhanced security or
   ability to effectively route through several IP addressing domains.

   The authors expect ORCHIDs to become fully routable, via one or more
   overlay systems, before the end of the experiment.

4.  Collision Considerations

   As noted above, the aim is that ORCHIDs are globally unique in a
   statistical sense.  That is, given the ORCHID referring to a given
   entity, the probability of the same ORCHID being used to refer to
   another entity elsewhere in the Internet must be sufficiently low so
   that it can be ignored for most practical purposes.  We believe that
   the presented design meets this goal; see Section 5.

   Consider next the very rare case that some ORCHID happens to refer to
   two different entities at the same time, at two different locations
   in the Internet.  Even in this case, the probability of this fact
   becoming visible (and therefore a matter of consideration) at any

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   single location in the Internet is negligible.  For the vast majority
   of cases, the two simultaneous uses of the ORCHID will never cross
   each other.  However, while rare, such collisions are still possible.
   This section gives reasonable guidelines on how to mitigate the
   consequences in the case that such a collision happens.

   As mentioned above, ORCHIDs are expected to be used at the legacy
   IPv6 APIs between consenting hosts.  The context ID is intended to
   differentiate between the various experiments, or contexts, sharing
   the ORCHID namespace.  However, the context ID is not present in the
   ORCHID itself, but only in front of the input bitstring as an input
   to the hash function.  While this may lead to certain implementation-
   related complications, we believe that the trade-off of allowing the
   hash result part of an ORCHID being longer more than pays off the

   Because ORCHIDs are not routable at the IP layer, in order to send
   packets using ORCHIDs at the API level, the sending host must have
   additional overlay state within the stack to determine which
   parameters (e.g., what locators) to use in the outgoing packet.  An
   underlying assumption here, and a matter of fact in the proposals
   that the authors are aware of, is that there is an overlay protocol
   for setting up and maintaining this additional state.  It is assumed
   that the state-set-up protocol carries the input bitstring, and that
   the resulting ORCHID-related state in the stack can be associated
   back with the appropriate context and state-set-up protocol.

   Even though ORCHID collisions are expected to be extremely rare, two
   kinds of collisions may still happen.  First, it is possible that two
   different input bitstrings within the same context may map to the
   same ORCHID.  In this case, the state-set-up mechanism is expected to
   resolve the conflict, for example, by indicating to the peer that the
   ORCHID in question is already in use.

   A second type of collision may happen if two input bitstrings, used
   in different usage contexts, map to the same ORCHID.  In this case,
   the main confusion is about which context to use.  In order to
   prevent these types of collisions, it is RECOMMENDED that
   implementations that simultaneously support multiple different
   contexts maintain a node-wide unified database of known ORCHIDs, and
   indicate a conflict if any of the mechanisms attempt to register an
   ORCHID that is already in use.  For example, if a given ORCHID is
   already being used as a HIT in HIP, it cannot simultaneously be used
   as a TMI in Mobile IP.  Instead, if Mobile IP attempts to use the
   ORCHID, it will be notified (by the kernel) that the ORCHID in
   question is already in use.

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5.  Design Choices

   The design of this namespace faces two competing forces:

   o  As many bits as possible should be preserved for the hash result.

   o  It should be possible to share the namespace between multiple

   The desire to have a long hash result requires that the prefix be as
   short as possible, and use few (if any) bits for additional encoding.
   The present design takes this desire to the maxim: all the bits
   beyond the prefix are used as hash output.  This leaves no bits in
   the ORCHID itself available for identifying the context.
   Additionally, due to security considerations, the present design
   REQUIRES that the hash function used in constructing ORCHIDs be
   constant; see Section 6.

   The authors explicitly considered including a hash-extension
   mechanism, similar to the one in CGA [RFC3972], but decided to leave
   it out.  There were two reasons: desire for simplicity, and the
   somewhat unclear IPR situation around the hash-extension mechanism.
   If there is a future revision of this document, we strongly advise
   the future authors to reconsider the decision.

   The desire to allow multiple mechanisms to share the namespace has
   been resolved by including the context identifier in the hash-
   function input.  While this does not allow the mechanism to be
   directly inferred from a ORCHID, it allows one to verify that a given
   input bitstring and ORCHID belong to a given context, with high-
   probability; but also see Section 6.

6.  Security Considerations

   ORCHIDs are designed to be securely bound to the Context ID and the
   bitstring used as the input parameters during their generation.  To
   provide this property, the ORCHID generation algorithm relies on the
   second-preimage resistance (a.k.a. one-way) property of the hash
   function used in the generation [RFC4270].  To have this property and
   to avoid collisions, it is important that the allocated prefix is as
   short as possible, leaving as many bits as possible for the hash

   For a given Context ID, all mechanisms using ORCHIDs MUST use exactly
   the same mechanism for generating an ORCHID from the input bitstring.
   Allowing different mechanisms, without explicitly encoding the
   mechanism in the Context ID or the ORCHID itself, would allow so-
   called bidding-down attacks.  That is, if multiple different hash

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   functions were allowed to construct ORCHIDs valid for the same
   Context ID, and if one of the hash functions became insecure, that
   would allow attacks against even those ORCHIDs valid for the same
   Context ID that had been constructed using the other, still secure
   hash functions.

   Due to the desire to keep the hash output value as long as possible,
   the hash function is not encoded in the ORCHID itself, but rather in
   the Context ID.  Therefore, the present design allows only one method
   per given Context ID for constructing ORCHIDs from input bitstrings.
   If other methods (perhaps using more secure hash functions) are later
   needed, they MUST use a different Context ID.  Consequently, the
   suggested method to react to the hash result becoming too short, due
   to increased computational power, or to the used hash function
   becoming insecure due to advances in cryptology, is to allocate a new
   Context ID and cease to use the present one.

   As of today, SHA1 [RFC3174] is considered as satisfying the second-
   preimage resistance requirement.  The current version of the HIP
   specification defines SHA1 [RFC3174] as the hash function to be used
   to generate ORCHIDs for the Context ID used by the HIP protocol

   In order to preserve a low enough probability of collisions (see
   Section 4), each method MUST utilize a mechanism that makes sure that
   the distinct input bitstrings are either unique or statistically
   unique within that context.  There are several possible methods to
   ensure this; for example, one can include into the input bitstring a
   globally maintained counter value, a pseudo-random number of
   sufficient entropy (minimum 100 bits), or a randomly generated public
   cryptographic key.  The Context ID makes sure that input bitstrings
   from different contexts never overlap.  These together make sure that
   the probability of collisions is determined only by the probability
   of natural collisions in the hash space and is not increased by a
   possibility of colliding input bitstrings.

7.  IANA Considerations

   IANA allocated a temporary non-routable 28-bit prefix from the IPv6
   address space.  By default, the prefix will be returned to IANA in
   2014, continued use requiring IETF consensus.  As per [RFC4773], the
   28-bit prefix was drawn out of the IANA Special Purpose Address
   Block, namely 2001:0000::/23, in support of the experimental usage
   described in this document.  IANA has updated the IPv6 Special
   Purpose Address Registry.

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   During the discussions related to this document, it was suggested
   that other identifier spaces may be allocated from this block later.
   However, this document does not define such a policy or allocations.

   The Context Identifier (or Context ID) is a randomly generated value
   defining the usage context of an ORCHID and the hash function to be
   used for generation of ORCHIDs in this context.  This document
   defines no specific value.

   We propose sharing the name space introduced for CGA Type Tags.
   Hence, defining new values would follow the rules of Section 8 of
   [RFC3972], i.e., on a First Come First Served basis.

8.  Acknowledgments

   Special thanks to Geoff Huston for his sharp but constructive
   critique during the development of this memo.  Tom Henderson helped
   to clarify a number of issues.  This document has also been improved
   by reviews, comments, and discussions originating from the IPv6,
   Internet Area, and IETF communities.

   Julien Laganier is partly funded by Ambient Networks, a research
   project supported by the European Commission under its Sixth
   Framework Program.  The views and conclusions contained herein are
   those of the authors and should not be interpreted as necessarily
   representing the official policies or endorsements, either expressed
   or implied, of the Ambient Networks project or the European

9.  References

9.1.  Normative References

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

   [RFC3972]      Aura, T., "Cryptographically Generated Addresses
                  (CGA)", RFC 3972, March 2005.

9.2.  Informative References

   [HIP-BASE]     Moskowitz, R., "Host Identity Protocol", Work
                  in Progress, February 2007.

   [Hi3]          Nikander, P., Arkko, J., and B. Ohlman, "Host Identity
                  Indirection Infrastructure (Hi3)", November 2004.

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   [NodeID]       Ahlgren, B., Arkko, J., Eggert, L., and J. Rajahalme,
                  "A Node Identity Internetworking Architecture
                  (NodeID)", April 2006.

   [PRIVACYTEXT]  Dupont, F., "A Simple Privacy Extension for Mobile
                  IPv6", Work in Progress, July 2006.

   [RFC1918]      Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G.,
                  and E. Lear, "Address Allocation for Private
                  Internets", BCP 5, RFC 1918, February 1996.

   [RFC3174]      Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
                  (SHA1)", RFC 3174, September 2001.

   [RFC4270]      Hoffman, P. and B. Schneier, "Attacks on Cryptographic
                  Hashes in Internet Protocols", RFC 4270,
                  November 2005.

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

   [RFC4773]      Huston, G., "Administration of the IANA Special
                  Purpose IPv6 Address Block", RFC 4773, December 2006.

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Authors' Addresses

   Pekka Nikander
   Ericsson Research Nomadic Lab
   JORVAS  FI-02420

   Phone: +358 9 299 1

   Julien Laganier
   DoCoMo Communications Laboratories Europe GmbH
   Landsberger Strasse 312
   Munich  80687

   Phone: +49 89 56824 231

   Francis Dupont


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Full Copyright Statement

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