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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.

Nikander, et al. Experimental [Page 1] RFC 4843 Cryptographic Hash IDentifiers (ORCHID) April 2007

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.

Nikander, et al. Experimental [Page 2] RFC 4843 Cryptographic Hash IDentifiers (ORCHID) April 2007

 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

Nikander, et al. Experimental [Page 3] RFC 4843 Cryptographic Hash IDentifiers (ORCHID) April 2007

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].

Nikander, et al. Experimental [Page 4] RFC 4843 Cryptographic Hash IDentifiers (ORCHID) April 2007

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

Nikander, et al. Experimental [Page 5] RFC 4843 Cryptographic Hash IDentifiers (ORCHID) April 2007

 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

Nikander, et al. Experimental [Page 6] RFC 4843 Cryptographic Hash IDentifiers (ORCHID) April 2007

 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

Nikander, et al. Experimental [Page 7] RFC 4843 Cryptographic Hash IDentifiers (ORCHID) April 2007

 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.

Nikander, et al. Experimental [Page 8] RFC 4843 Cryptographic Hash IDentifiers (ORCHID) April 2007

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

Nikander, et al. Experimental [Page 9] RFC 4843 Cryptographic Hash IDentifiers (ORCHID) April 2007

 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.

Nikander, et al. Experimental [Page 10] RFC 4843 Cryptographic Hash IDentifiers (ORCHID) April 2007

 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.

Nikander, et al. Experimental [Page 11] RFC 4843 Cryptographic Hash IDentifiers (ORCHID) April 2007

 [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.

Nikander, et al. Experimental [Page 12] RFC 4843 Cryptographic Hash IDentifiers (ORCHID) April 2007

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

Nikander, et al. Experimental [Page 13] RFC 4843 Cryptographic Hash IDentifiers (ORCHID) April 2007

Full Copyright Statement

 Copyright (C) The IETF Trust (2007).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Nikander, et al. Experimental [Page 14]

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