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Versions: 00 01 02                                                      
Internet Architecture Board                            P. Faltstrom, Ed.
Internet-Draft                                      G. Huston, Eds., Ed.
Expires: June 14, 2005                                               IAB
                                                       December 14, 2004


                    A Survey of Internet Identities
                      draft-iab-identities-02.txt

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on June 14, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This memo provides an overview of the various realms of
   identification used within the Internet protocol suite, with a view
   to noting the interdependencies of the different identifiers and
   consequent implications for updating their specifications or changing
   their infrastructures' operations.





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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1  Desirable properties of Internet Identities  . . . . . . .   3
   2.   A Hierarchy of Identities  . . . . . . . . . . . . . . . . .   5
     2.1  Media Access Addresses . . . . . . . . . . . . . . . . . .   6
       2.1.1  Summary  . . . . . . . . . . . . . . . . . . . . . . .   8
     2.2  IP Addresses . . . . . . . . . . . . . . . . . . . . . . .   8
       2.2.1  Summary  . . . . . . . . . . . . . . . . . . . . . . .   9
     2.3  Service and Session Identities . . . . . . . . . . . . . .  10
       2.3.1  Summary  . . . . . . . . . . . . . . . . . . . . . . .  12
     2.4  Routing and Forwarding Identities  . . . . . . . . . . . .  13
       2.4.1  Summary  . . . . . . . . . . . . . . . . . . . . . . .  14
     2.5  Mobile Identities  . . . . . . . . . . . . . . . . . . . .  15
       2.5.1  Summary  . . . . . . . . . . . . . . . . . . . . . . .  15
     2.6  Opportunistic Identities . . . . . . . . . . . . . . . . .  16
       2.6.1  Summary  . . . . . . . . . . . . . . . . . . . . . . .  17
     2.7  Domain Names . . . . . . . . . . . . . . . . . . . . . . .  17
       2.7.1  Summary  . . . . . . . . . . . . . . . . . . . . . . .  18
     2.8  Uniform Resource Identifiers . . . . . . . . . . . . . . .  19
       2.8.1  Summary for URLs . . . . . . . . . . . . . . . . . . .  22
     2.9  Uniform Resource Names . . . . . . . . . . . . . . . . . .  23
       2.9.1  Summary  . . . . . . . . . . . . . . . . . . . . . . .  23
     2.10   Human Friendly Strings . . . . . . . . . . . . . . . . .  24
       2.10.1   Summary  . . . . . . . . . . . . . . . . . . . . . .  25
   3.   Issues with Identities . . . . . . . . . . . . . . . . . . .  25
     3.1  Overloading the IP Address . . . . . . . . . . . . . . . .  25
     3.2  Dynamic DNS Updates and Nomadism . . . . . . . . . . . . .  27
     3.3  URLs and Persistent Identifiers  . . . . . . . . . . . . .  28
   4.   The DNS in Identity Spaces . . . . . . . . . . . . . . . . .  31
     4.1  The role of the DNS  . . . . . . . . . . . . . . . . . . .  32
     4.2  Changing the DNS . . . . . . . . . . . . . . . . . . . . .  32
     4.3  The DNS is a strict lookup service . . . . . . . . . . . .  33
     4.4  Coherency of the DNS . . . . . . . . . . . . . . . . . . .  34
     4.5  The DNS as an Identity Glue  . . . . . . . . . . . . . . .  35
   5.   Security Considerations  . . . . . . . . . . . . . . . . . .  36
   6.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  36
   7.   Informative References . . . . . . . . . . . . . . . . . . .  36
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  38
   A.   IAB Members  . . . . . . . . . . . . . . . . . . . . . . . .  39
        Intellectual Property and Copyright Statements . . . . . . .  40










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1.  Introduction

   In any communications domain where two parties wish to conduct a
   conversation across a network each party must specify to the network
   sufficient information for the network to identify the other party.
   When the conversation refers to a resource or service that is
   accessible through the network, the only effective way to refer to
   such a resource of service is to use an identifier that can
   subsequently be passed to the network to perform the access.

   Some networks use a single identifier domain to identity all parties
   and services.  Other networks use a collection of discrete identifier
   domains, where each identifier domain has a specific realm of
   discourse or application.  The Internet is an example of a
   multiple-identifier domain network, where there are a number of
   identity domains, each referring to a particular function or area of
   application.  In terms of routing and forwarding IP packets the
   identity domain used is that of IP addresses, while in terms of
   identifying particular services or resources the URI form of identity
   is commonly used.  In terms of human use of identities, the most
   common form of identity in the Internet is based upon the domain
   name.

   This memo examines the role of identities and identifiers, together
   with an overview of the various realms of identity that are used in
   the Internet.  The document then looks in more detail at the Domain
   Name System (DNS) and examines its role in relation to these identity
   realms.

   One of the characteristics of the Internet's multiple identifier
   systems is their heavy interdependence.  This memo notes those
   interrelationships and provides some observations on implications for
   technical or operational evolution of their specifications.

1.1  Desirable properties of Internet Identities

   Before exploring the set of Internet-based identity realms, its
   useful to enumerate a set of desirable characteristics of any useful
   identity system.  The following list is of characteristics and some
   related questions related to properties of the identifier is proposed
   as a useful, although not comprehensive, collection of identity
   attributes:

   Uniqueness:

      In what realm is the identifier unique?

      Can the same identifier be associated with two or more distinct



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      objects within the domain of a single realm?

      Can multiple identifiers in a single realm be associated with the
      same object?

         An identifier can only be used reliably and deterministically
         when there is a unique association with an object.  An identity
         realm is generally useful when the association between
         identities and objects is a relationship where each unique
         identifier references a single unique object.  Note that there
         is no requirement in the reverse direction, in that it
         typically makes little difference to the utility of an identity
         realm if a unique object is associated with multiple
         identities.  In other words it can lead to ambiguities in
         identity resolution if an identity is associated with two or
         more distinct objects, but it generally is not as critical if
         an object is associated with multiple identities (i.e.
         multiple identity aliases for a unique object).

   Consistency:

      Is the identity asserted within a consistent identifier space?

         This avoids an assertion of identity being interpreted by
         another party in an unintended manner.

   Persistence:

      Does the identity remain constant, or are gratuitous changes in
      the mapping from the identifier to the referenced object avoided?

         Constantly changing identities are, at the very least,
         difficult to track.

   Trust:

      Can a particular identity withstand a challenge as to its
      validity?

         Other parties who would like to use this identity would like to
         be reassured that they are not being deceived.  'Use' in this
         context is a generic term that includes actions such as
         resolution to the object identified by the identity value,
         storage of the identity value for subsequent 'use', referral,
         where the token is passed to another party for their 'use'.






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   Robustness:

      Is the identity realm capable of withstanding deliberate or
      unintentional attempts to corrupt it in various ways?
         A robust identity system should be able to withstand efforts to
         undertake identity theft or identity fraud.

   Withholding:

      If the identity is composed of a number of components, are only
      those components of the identity that are essential to support the
      communication exposed to other parties?
         Compound identity systems should not reveal those components of
         the identity structure that are not relevant to the identity
         operation being performed.

   Referential Consistency:

      If the identity is used in the context of a reference, then when
      the referenced object is altered or relocated, does the identifier
      remain a valid reference to the object?
         Referential consistency refers not only to the constancy of the
         reference in the face of changes to the referenced object, but
         also consistemncy when one entity passed the identity value to
         another.  The identity resolution should remain constant in
         such cases.

   Structure:

      Is the token space from which identity values are drawn structured
      or unstructured?

         Structured token spaces allow various forms of retrieval
         operations based on the identity value to be undertaken
         efficiently, while unstructured token spaces allow for more
         flexible generation and use of identities within more
         restrictive realms of discourse.

   This list is not attempting to be a complete enumeration of required
   identifier properties, but instead list the most important desireable
   properties of identifier realms in the context of the Internet.

2.  A Hierarchy of Identities

   In networking models there is a conceptual layering of functionality,
   starting at the layer of bits on the wire at the media access level
   and moving up a stack of layers through internetworking, end-to-end
   transport and application levels.  Each one of these layers creates



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   at least one context in which identifiers are used for the
   communication.  It would appear that from this perspective an
   identity within the Internet is not just a single identity, but an
   collection of various identities, used in a variety of contexts.

2.1  Media Access Addresses

   There are two generic types of base media in this realm.  One is a
   point-to-point medium, a bilateral communications system where all
   Protocol Data Units (PDUs) generated by one party are passed to the
   other party.  In such environments use of media access addresses are
   not strictly required.  The other form of environment is a
   multi-access environment, where a number of parties can communicate
   directly using a common medium.  In this environment the sender must
   specify the intended recipient of the PDU, and to achieve this all
   connected entities must use a unique media access address.  The most
   common of these multi-access media are encompassed within the IEEE
   802 collection of media standards.

   These IEEE 802 technologies share a common structure of Media Access
   Control layer address (MAC address) to uniquely identity devices
   connected within a LAN.  There are two forms of this identity space,
   one using a 48 bit identity space (EUI-48 [21]), and the other a 64
   bit space (EUI-64 [22]).  Both identity spaces can be considered as
   partially-structured identity spaces, where a number of bits within
   this MAC address determines whether the address has been globally or
   locally assigned.  Globally assigned values are globally unique, but
   are structured in such a way that there is no imposed hierarchy
   within the address that could be used for efficient searching, in
   contexts such as, for example, a routing or forwarding application.

   A global MAC address identity certainly passes one of the more basic
   tests of an identity domain, that of uniqueness.  Two parties cannot
   assume the same MAC address value and use this same value as a unique
   identity.  So in a LAN context, a collection of devices can
   distinguish between each other by virtue of this unique MAC address.
   A manufacturer of Ethernet devices is assigned a manufacturer's block
   of Ethernet MAC addresses and uniquely places one address in each
   device.  The end consumer has no need to reconfigure the device with
   a new address, nor is there any need to alter existing MAC addresses
   each time the LAN changes with new devices being added, and the
   address is intended to be globally unique.

   Beyond these attributes there are some real weaknesses in using a MAC
   address as an identity outside the context of a LAN environment.  The
   identity space, while globally unique, has few other distinguishing
   properties.  The structure of the identity space does not reflect its
   current location within a particular network topology, so its of no



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   assistance as a location token.  In the context of equating a device
   identity to this network interface identity, the identity has limited
   persistence, in that it follows the interface hardware, not the host
   computer or its use.  For example, switching a device from a wireless
   to a wired connection changes its MAC identity.  The identity has no
   capability to express any linkage to any other identity domain.  It
   has no internal structure of sub-fields that could be interpreted as
   pointers into other identity fields.  Its precise semantics are to
   define an interface to a network rather than the device itself.  This
   issue of identifier scope comes up in link layer security discussions
   where it may not be the best possible approach to bind master session
   keys to MAC addresses, rather than some other identity.  Another
   example, in IEEE 802.11i it is possible for a host to have multiple
   interfaces and therefore there is a significant difference between
   binding an Master Session Key to a MAC address and binding to a host
   identity.

   This lack of a direct association between an interface's MAC address
   and a host device has undesirable effects when it has been assumed
   that a MAC address equates to a host identity.  In "Authentication
   for DHCP Messages " [13] where the MAC address takes on the role of
   the DHCP client-identifier, or in the administrative model of IEEE
   802.11-1999 Wired Equivalent Privacy (WEP) [23] it can be an
   administrative burden to keep track of all the network interface
   cards, their MAC addresses and their associated secrets.

   Even despite these limitations, the MAC address is regarded as a
   useful identity mechanism in the context of an identity space.  The
   original 48 bit identity specification has been augmented with 16
   padding bits in order to be incorporated into the IEEE 64-bit EUI-64
   global identifier structure, which in turn has been incorporated into
   the IPv6 address architecture as the interface identifier component
   of the unicast address [9].

   It should be noted that this latter action of embedding one identity
   (a MAC address) in another (the IPv6 address) lifts the original
   identity outside its original context.  There have been some concerns
   noted where public disclosure of the MAC address within every IPv6
   address also discloses both the unique identifier and, potentially,
   the role of the device.  For example, a device manufactured by a
   specialized storage manufacturer is more likely to be a very
   expensive storage subsystem housing mission-critical data.  This may
   not be information that is intended to be made public, and a
   follow-up proposal advocated the ability for the interface identifier
   within an IPv6 address to be a temporary randomized value [12].  This
   is illustrative of one of the side-effects of identity
   interdependence where one identity realm is embedded in another.




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2.1.1  Summary

   Uniqueness:
      MAC addresses are globally unique

   Consistency:
      Mac addresses are intended to be a unique token for a network
      interface.  Within this limited scope they are consistent.

   Persistence:
      MAC addresses are associated with an interface, in the hardware,
      and, as such, are persistent.  There are uses of MAC addresses
      that do not assume permanence, but that has more to do with the
      impermanence of binding that address to some other identifier
      (e.g., IP address) than anything else.

   Trust, Robustness:
      Both Trust and Robustness seem to be tied to the use of the MAC
      address, as there is no Internet infrastructure for assigning or
      reporting them.  This raises the question of whether the process
      that asserts the MAC address of a hardware element and the
      transmission of that assertion is trustable, and whether the the
      manufacturing process that embeds MAC addresses into hardware is
      always reliable.

   Withholding:
      MAC addresses are not decomposed, and are completely exposed.

   Referential Consistency:
      The MAC address refers to an interface, rather than a device or an
      endpoint of an application.  If the hardware component is moved
      then the the MAC address is still a valid identifier for the
      interface.  However, from the perspective of dependent identifier
      systems, there may be some consistency issues, in that another
      system may no longer be able to see the interface identified by
      the MAC address.

   Structure:
      MAC addresses are unstructured.

2.2  IP Addresses

   Moving up one level in the protocol stack model provides an identity
   based on the internetworking layer, namely the IP address.  The IPv4
   address is a 32 bit field providing each Internet-connected interface
   with a unique value.  IPv6 uses effectively the same construct, using
   a 128 bit identity domain rather than a 32 bit domain.  In both cases
   the IP address is a structured identity space where there is a



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   globally significant prefix that is used in the context of routing
   and forwarding outside of a particular local domain, and a local part
   that is used to deliver the packet to the correct interface of the
   associated device within the local network.  The fact that the
   structure of the address is based on the requirements of routing, and
   is therefore topologically sensitive, implies that the underlying
   semantics of the IP identity can be most reasonably assumed to be
   temporal rather than persistent.

   As an identity token, an IP address should be unique.  It is
   structured to be useful to forward packets to the addressed device,
   and it's well known, in that it's not a secret value.

   An IP address is not everything one could hope for in an identity.
   The IP address identifies an interface, not a device or its user.  A
   device with multiple active interfaces has multiple IP addresses, and
   while it's obvious to the device itself that it has multiple
   identities, no remote party can tell that the multiple identities are
   in fact pseudonyms, and that the multiple addresses simply reflect
   the potential for multiple paths to reach the same endpoint.

   Furthermore, the IP address is an information-bearing identifier,
   which is structured in such a way that it can be used in routing and
   forwarding.  This is helpful in the sense that there is no need to
   deploy a second identity system that refers only to locality within a
   network, however it compromises the use of the address as an
   identity, since in some circumstances a change in the connectivity of
   a local network will require a renumbering of that network, such that
   the address of each individual device will change.

   This is a specific example of the more generic observation about IP
   addresses, namely that the IP address carries both the identity of
   the endpoint in the IP realm and the location of the endpoint in the
   IP network.  It is a matter of longstanding study that continues
   today as to the merits of delineating these two roles of identity at
   the IP level, creating one identity realm as a means of uniquely
   identifying an instance of a protocol stack within an end device
   (variously called a " stack identifier" or "endpoint identifier" in
   previous studies) and a second identity realm that is used to
   identify the current location of the identity element within the
   network (typically called a "locator" identity) [1][25][5].

2.2.1  Summary

   Uniqueness:
      With the exception of certain identified special cases, such as
      private addresses [4], IP unicast addresses are globally unique.
      In the context of anycast use of IP addresses, an IP anycast



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      address represents a collection of individual entities that
      undertake an equivalent function.  In the context of multicast IP
      addresses, an IP multicast address represents a set of many
      independent hosts.

   Consistency:
      Mostly consistent; private addresses are known by convention, not
      by any internal identifier structure.

   Persistence:
      IP addresses are intended to be persistent.  Becuase of the
      duality of the address as representing both identity and location,
      a change in location, such as in a mobile device, often triggers a
      change in IP address.

   Trust:
      There is no systematic method of validating an assertion of an IP
      address.

   Robustness:
      Attempting to hijack an IP address also requires some form of
      corruption of the routing system on order for other system to be
      informed of the updated location of the corrupted address.

   Withholding:
      IP addresses cannot be decomposed, and are completely exposed.

   Referential Consistency:
      It is normally the case that IP addresses are referentially
      consistent, and one party can pass a reference to a correspondant
      party to any other party by means of passing the IP address.  One
      caveat is that where the IP address is deliberately corrupted, by
      viture of the use of a NAT device, or in the case of dynamically
      addigned addresses, then IP addresses lose their referential
      consistency.  As noted above, anycast addresses  may not preserve
      precise referential integrity.

   Structure:
      The structure of an IP address refers to a structure of topology
      in a locational context.  In order to provide effective
      summarization tools in the context of routing, the IP address is
      structured such that adjacent devices use adjacent address, such
      that a common address prefix can be used to summarize the location
      of a local network of addressed devices.

2.3  Service and Session Identities

   In the TCP/IP protocol suite the next level of identity is that of



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   the transport session.  In order for a system to advertise a
   particular service that is a point of attachment for clients it
   combines three fields: IP server address, transport protocol
   identity, and the address of the local service identity (port number)
   into a compound identity that describes a particular service port on
   a particular device.

   The port address concept, used in TCP and UDP, represent generic
   identities for service rendezvous points.  When combined with an IP
   address they become particular service points, or, identified service
   points, and these compound identification objects (IP address,
   Transport Protocol, Port) are service identifiers.

   The identity concept for transport is further extended by including
   the sender's IP address and port address.  The corresponding 5-tuple
   of (Source IP address, Destination IP address, Transport Protocol,
   Source Port, Destination Port) is an identifier for a particular
   instance of a session.  Not only is this 5-tuple used at the
   destination point to correctly de-multiplex an incoming packet stream
   and send each packet to the correct local instance of the
   application, the session identity can also be used within the network
   to recognize a 'flow' of packets that require identical forwarding
   treatment and may require identical service treatment, if so
   configured.  In the latter case the session identity is being used to
   trigger a particular service response within the network, and the
   assumption being made within such contexts is that this 5-tuple is
   sufficiently unique to identify particular sessions to the relevant
   network elements.  (SCTP also has a port address, but uses a set of
   IP addresses to identify the remote end.  At the network level a
   'flow' or 'stream' is identified as a collection of 5-tuples, rather
   than as a single 5-tuple.)

   There are circumstances where the complete 5-tuple is not visible to
   the network, such as in the use of IPSEC [8].  It is an objective
   when using the Encapsulating Security Payload (ESP) protocol when
   confidentiality is enabled to hide session information.  The
   objective of the deliberate attempt to occlude these details is in
   order to impede traffic analysis or greatly reduce the information
   obtainable via traffic analysis.  When using IPSEC with ESP the user
   has choices about how ESP is deployed.  One choice is to use a
   separate Security Association for each flow, while another choice is
   to use a single Security Association for multiple flows to hide that
   flow information.  It is not uncommon to use multiple already
   encrypted flows and re-encrypt them together using a common Security
   Association.  This technique is very effective in impeding or
   preventing traffic analysis.  The triple (source IP, dest IP,
   Security Parameter Index (SPI)) will identify the full flow
   granularity that the user intended to reveal.  Of course, the SPI



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   value will change at key rollover time, but usually the packet
   patterns (size, frequency of transmission, etc) will reveal which new
   SPI value corresponds with which previous SPI value.  So if an entity
   is trying to identify flows, it is best to use that natural triple in
   the case of IPSEC with ESP.

   Session identities are intended to be unique at any single point in
   time, in that two distinct sessions will not share a common session
   identity.  However, this identity is temporal, in that once the
   session is finished the identifier is no longer of direct relevance,
   and at a subsequent time a different session may use the same
   5-tuple.  As well as impermanence, session level identifiers exhibit
   a very fine level of granularity, and as such are often at a level of
   detail which is too fine to be a useful general identity token across
   the entire Internet realm.  One use is to allow a session to
   construct an identity that refers to itself or its correspondent that
   can then be handed into a quality of service policy controller to
   request a specialized service response for the session.  Other uses
   of session identities can be found in filters, firewalls and network
   address translators, as well as various forms of middleware
   applications.

2.3.1  Summary

   Uniqueness:
      A session identity is unique in a very limited context, such that
      the session identity is only unique between the communicating
      endpoints, and only unique for the lifetime of the session.

   Consistency:
      A session identity is intended to be consistent within the scope
      of the IP-level multiplexing and demultiplexing function performed
      in the endpoints of the session.  Some forms of active middleware
      attempt to use this session level identity as a means of session
      identification.  This use out of intended context is not always
      reliable.

   Persistence:
      Session identities are not persistent

   Trust:
      Session identities are not necessarily trustable.  Additional
      mechanisms would be required to improve the trust attributes of
      session identities.







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   Robustness:
      Session identities are not robust, and some other form of session
      context is required to minimize the risk of session hijacking.

   Withholding:
      Session identities are an instance of withholding, in that an end
      point session state includes a number of additional information
      items relating to packet sequence numbers and endpoint protocol
      state.  These items are withheld from the explicit protocol
      exchange and are inferred at each end from the protocol exchange.

   Referential Consistency:
      Session identities are not referentially consistent.

   Structure:
      Session identities have a number of components, but each component
      is is not internally structured.

2.4  Routing and Forwarding Identities

   As mentioned above, IP addresses provide information required by
   routing and forwarding systems.  Forwarding is undertaken using the
   entire address as the lookup function into a forwarding table, using
   the best match of the address against a table entry as the basis of
   the forwarding decision (where 'best' refers to a precise match
   across the longest sequence of leading bits).  Routing within the
   Internet uses a hierarchy of environments, ranging from a non-routed
   multi-access local network, through a set of locally routed networks
   where routing is based on comprehensive knowledge of local network
   topology, through to the interdomain routing environment, where
   routing is based on a sequence of edge-to-edge transits across
   domains.

   This hierarchy of routing is reflected in the structure of addresses.
   At any point in the routing hierarchy an address is divided into two
   parts, a routing network part and a subnet address part.  Early
   definitions of this address structure used a fixed division, while
   later refinements of classless IPv4 addresses and IPv6 both use an
   explicit prefix length value that is combined with an IP address
   prefix to form the routing identifier.

   Interdomain IP routing incorporates both routing identifiers and
   routing domains, or "autonomous systems".  Within a given routing
   domain, IP routing is performed using only routing identifiers.
   However for routing between domains, IP routing is performed using a
   new identity, the Autonomous System number.  The most common
   implementation of inter-domain routing is a distance vector
   distributed computation of inter-domain topology using vectors of AS



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   numbers as both a loop detection and a path preference mechanism.
   The AS identity space is an unstructured space of numeric values,
   allocated from a single 16-bit identifier space.

   An IP address is located within a routing system by identifying the
   most specific enclosing routing identifier.  Forwarding a packet to a
   specific IP address involves an algorithm of locating the associated
   routing identifier and undertaking the forwarding action associated
   with that object.  Coherency of the routing system demands that
   routing identifiers are managed in a consistent fashion.  The
   overloading of an IP address as both an IP identity and a component
   of a routing identifier implies that a device's location is
   implicitly described by its IP address.  As noted earlier, relocating
   a device to a new network location, or relocating a network to a
   different point in the overall Internet topology necessarily implies
   associating a new IP address with the device.  In the absence of any
   other mechanisms, this new IP address replaces the previous IP
   address, changing the device's IP identity, the device's service
   identities and the device's session identities.

2.4.1  Summary

   Uniqueness:
      Routing identities are intended to be unique, deriving their
      uniqueness from the underlying properties of the IP address space.

   Consistency:
      Routing identities are intended to be a unique token within the
      context of a routing realm.

   Persistence:
      Routing identities are not persistent, and have a liketime
      associated with connectivity of the described entity within the
      routing realm.

   Trust:
      There is no systematic method of validating an assertion of a
      routing identity.

   Robustness:
      The identity structure does not inherantly prevent various forms
      of corruption of routing identities

   Withholding:
      An inter-domain routing identity is a compund entiy consisting of
      an address prefix and an autonomous system number.  There is no
      withholding of elements of this identity.




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   Referential Consistency:
      Routing identities are intended to be consistent within a routing
      realm, and the operation of routing protocols rely on this
      referential consistency of routing identities.

   Structure:
      Routing identities are not internally structured.

2.5   Mobile Identities

   Device and network mobility adds an additional dimension to identity,
   in that mobility implies some level of decoupling of the notion of
   location with that of identity.  In one form of approach to this
   generic space, that of device mobility, a device has an additional IP
   address that acts as a 'current locator' that describes the device's
   current location within the network, while the device also retains a
   constant 'home address' that in effect acts as the device's constant
   identity and also acts as the discovery service point for its current
   location.  With this approach a 'home agent' acts as a proxy agent
   for the device when it is roaming beyond the confines of its local
   network.  The home agent tunnels traffic sent to the home address to
   an address at the host's current topological location, called the
   'care of' address in Mobile IP.  The host is responsible for updating
   the binding between the home address and the care of address in the
   home agent, by sending a binding update message when the care of
   address changes.  The mechanism involved in mapping between the home
   address and care of address is very similar to the mechanism used on
   the local link for the ARP neighbour cache, except IP addresses are
   involved for both.

   This approach raises a critical issue for identities, namely that of
   robustness.  Approaches to mobility need to be aware of a potentially
   hostile environment where third parties may attempt to subvert the
   implicit redirection of traffic by assuming the identity of the
   mobile element through the generation of false updates of the current
   location.

2.5.1  Summary

   Uniqueness:
      A mobile identity is a compound entity using two IP addresses: a
      'home' address that functions as an endpoint identifier and a
      'core of' address, which functions as a current endpoint location
      identifier.  A mobile identity is intended to be unique.







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   Consistency:
      A mobile identity is consistent within the realm of mobile IP
      protocols.

   Persistence:
      A mobile identity is not persistent,.  The endpoint identity value
      is intended to be persistent, while the endpoint location
      identifier is only intended to be valid while the mobile entity is
      located at the specified 'care of' location.

   Trust:
      Of itself a mobile identity is not trustable.  Mobile IP protocols
      add additional communication exchnages between the mobile entity
      and 'agent' entities in order to create trust in a mobile
      identity.

   Robustness:
      A mobile identity is not intrinsically robust.  The protocol
      exchanges between the mobile entity and its agents can create a
      robust mobile identities.

   Withholding:
      Manipulation of mobile identities can include deliberate
      withholding of the current location information, or the persistent
      identity information.

   Referential Consistency:
      Becuase of the temporal nature of the location identifier, mobile
      identities are not consistent over time.

   Structure:
      The structure of a mobile identity is derived fromt he structure
      of the underlying IP address space.

2.6  Opportunistic Identities

   This concept of maintaining some form of identity association in the
   face of a communicating within a potentially hostile environment has
   lead to a proposal for an identity token that has its roots in the
   public / private key pairs.  In this approach the identity token is
   associated with the public key value of a public / private key pair.
   A message encrypted with a private key can be passed to the other
   party where only the originating party's publicly asserted identity
   (or public key) can decrypt the message.

   Such identity realms can serve to support a reliable assertion that
   the received message originated from the same party that originated
   the communication and that the message has not been tampered with



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   while in transit.  The identity systems are opportunistic in that
   they are self- generated identities, and have no external structure.
   The implication is that such identities have no particular structure
   and may not be completely unique.  For this reason their utility in
   other identity applications where persistence or referential
   integrity is required, such as acting as a persistent reference to
   other attributes of a named object, is limited.

2.6.1  Summary

   Uniqueness:
      Opportunistic identities are not unique.

   Consistency:
      Opportunistic identities may not be consistent.

   Persistence:
      Opportunistic identities are not necessarily persistent.

   Trust:
      Opportunistic identities are not trustable in general.  There may
      be limited contexts in which an opportunistic identity may be
      considered trustable..

   Robustness:
      Oppostunistic identities are normally robust in the sense that
      they are not generally divulged, are generated in a manner that is
      systematically predictable by a third party, and are often drawn
      from a sufficient large space that they are resilient to guessing
      techniques.  In this sense an opportunistic identity can be
      considered to be robust.

   Withholding:
      Opportunistic identities can be simple or compound tokens.
      Withholding is possible in the case of compound identity realms.

   Referential Consistency:
      Opportunistic identities are normally bounded by a particular
      context of use, and are not referentially consistent outside of
      this context.

   Structure:
      Opportunistic identities may not necessarily be structured.

2.7  Domain Names

   The set of identities described so far have no particular
   human-visible aspects of their function.  The identity tokens are



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   structured to meet a particular purpose, and are not intended, as
   their primary purpose, to be manipulated by humans nor are they
   intended to be used primarily within the realm of human discourse.
   By contrast, the Domain Name System (DNS) was specifically intended
   to be a name realm that is suitable to be included in human
   discourse, yet at the same time to admit enough structure to be
   manipulated by computer applications in a deterministic fashion.

   The DNS is essentially a hierarchical name space, where the
   hierarchical name structure allows the space to be efficiently
   searched and managed in a distributed fashion, but also supports one
   of the most desirable attributes for an identity space, namely
   uniqueness.  The explicit hierarchy also assists in ensuring
   uniqueness, as DNS names are intended to be unique across the entire
   name string rather than just at the first component, so that "a.b.c"
   is a different identifier to "a.d.e " even though the first token in
   the domain names, "a", is the same in both cases.

   The most common use of the DNS is to map domain names to IP
   addresses, but other uses are possible via mapping a name to a number
   of other defined 'resources'.  The core functionality of the DNS is
   that of a unique, structured, name space and a mapping capability
   that allows a query to be performed to retrieve the mapping
   information for a DNS name for a particular class of resource
   mapping.

   The Domain Name System is more than a set of syntactic rules for
   constructing a well-formed DNS name.  The resultant name, if well
   constructed and properly implemented, can be used as a referral token
   to a service environment.  In this fashion the DNS encompasses a
   translation service that maps from domain names to defined resources,
   including IP addresses.  For example, given a well formed DNS name, a
   DNS lookup can query for a corresponding IP address.  The DNS
   describes a data model, a set of relationships between data objects
   as well as a protocol used to send queries and receive answers.

   As DNS names provide a mapping from a name to a resource, the name
   does not need to change when the resource changes location or some
   other identifying attribute.  The mapping changes, but the name
   remains constant, and for this reason domain names can be considered
   to be stable unique identifiers, residing within a structured space
   that can be efficiently searched and managed in a highly distributed
   manner.

2.7.1  Summary






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   Uniqueness:
      DNS identifiers are unique.

   Consistency:
      DNS identifiers are intended to be consistent.  There are a number
      of issues relating to character equivalence within various
      languages that impinge upon consistency of interpretation in some
      contexts.

   Persistence:
      DNS identities are intended to be persistent.

   Trust:
      The trustability of a DNS identity is based on the integrity of
      delegation within the hierarchy of the DNS identity.

   Robustness:
      The DNS is implemented as a distributed name database, and the
      robustness of the DNS is based on the robustness of this database.

   Withholding:
      DNS identities are capable of wothholding.  A DNS identity can be
      regarded as a DNS name, and an associated set of resource records.
      Resource record values are withheld unless explicitly requested as
      part of a resolution query.

   Referential Consistency:
      DNS identities are intended to the referentially consistent.

   Structure:
      The DNS name space uses a hierarchical structure.

2.8  Uniform Resource Identifiers

   When communicating, applications often need more information than a
   domain name.  For electronic mail, for example, the sending
   application must use a combination of the domain name, the TCP
   protocol, the mail delivery or mail agent's service port and the
   mailbox name of the recipient.  Other applications require different
   compound identification objects, in accordance with their
   characteristics.  This compound identity may be specified in the
   format of a Uniform Resource Locator, or URL.

   Uniform Resource Locators (URLs) are a subset of a more generic form
   of resource identification, Uniform Resource Identifiers (URIs).  As
   an identity space, the URI space is very loosely defined, and it's
   quite remarkable as to the extent to which it has spread across the
   world as a form of object identifier, or identity token.  URLs refer



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   to the subset of URIs that identify resources via a representation of
   their primary access mechanism.  Other forms of URIs provide resource
   identification through a name scheme or by other attributes of the
   resource.

   There are few syntax rules to the Universal Resource Identifier
   space, and only a small amount of common semantic structure.  The
   original IETF documentation, RFC 1630 [2], refers quite simply to a
   syntax of a prefix word, a colon, and a following string.  Where
   there is hierarchy in the following string, slashes are used to
   delineate the hierarchical levels, and the hierarchy runs from left
   to right.  The current generic syntax of URIs is described in RFC
   2396 [7], and the only change to this generic syntax is to refer to
   'schemes', as in "<scheme>:<scheme-specific-part>".

   The common usage of URIs has been more structured than this general
   specification, and most URI schemes do not provide a single string
   that is an alias for an identity, but instead form an identity from
   the instructions that specify how to access the resource, in the same
   way as a postal address is often constructed from the instructions as
   to how to deliver a postal letter to you.  This form of a URI, which
   can be viewed as a location specification, is the basis of the URL
   scheme.  In other words such protocol-scheme URLs consist of what
   could be interpreted as a device selector, an application selector
   and an application-specific string that acts as an object reference.

   Within such protocol-scheme URLs the scheme prefix is an identifier
   that uniquely identifies the service being referenced, or in terms of
   access it references the protocol and port address to be used.  The
   first, or top, level of the hierarchical following string is either
   the DNS name of the server, or the DNS name coupled with some
   specific qualification, such as a mail address.  Any subsequent
   hierarchical components represent service-specific instructions to be
   specified that lead you to the referenced object.  Thus we have
   "mailto:user@domain.example.com" for a mail specification, where the
   scheme prefix "mailto" identifies the use of the TCP transport
   protocol, a port address of 25 and a protocol of SMTP.  The following
   string, "user@domain.example.com" references the mail agent (a DNS
   lookup of "domain.example.com" for an MX resource record) and a value
   to be used in the protocol exchange (delivery to the mailbox
   "user@domain.  example.com").  Similarly,
   "http://www.example.com/directory/hierarchy/index.html" for a
   specific web page uses "http" as a scheme identifier for TCP, port
   80, protocol HTTP, the initial part of the following string to
   reference the server (a DNS lookup for an A or AAAA resource record
   for "www.example.com") and an HTTP protocol request for
   "www.example.com/directory/hierarchy/index.html".




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   In this form of the URL identity system uniqueness is keyed from the
   general use of a DNS name within the URL, and the wrapping around the
   DNS string is taking the general form of the DNS as an alias for an
   IP address, and, additionally, specifying a service point, and then
   arguments that are needed to provide to this service point in order
   to retrieve the referenced resource.  In that way a protocol-scheme
   URL is closer to a description of an algorithm than to an identifier
   whose structure of the identifier is adapted to tasks such as
   sorting, searching or equivalence operations.  There are issues with
   consistency here in that while the hierarchically structured string
   set makes sense to one application it may not make any sense in the
   context of a different application.

   The persistence of protocol-scheme URLs is also an issue, in that the
   resource may change location over time, and the corresponding
   algorithm to locate the resource, or URL, must necessarily change as
   well.  The other major difference between a structured identifier
   space and the protocol-scheme URL approach is that the structured
   identifier space requires some form of lookup to apply the identity
   into a retrieval system.  By changing the outcomes from the lookup
   operation, the identity owner can track changes in the location of
   the resource.  In the protocol-scheme URL approach there is no way to
   understand how widely the identity has circulated, and it is not
   possible to update the in-circulation copies of the URL.  The
   property of the DNS is that in itself, the DNS identities are simple
   structured tokens, and they require a lookup operation to be
   performed in order to produce an algorithm that allows an application
   to refer to a particular object.  While such protocol-scheme URLs are
   widely used as service and resource identities, they pale in
   significance, persistence and utility when compared with DNS names.
   In other words URLs specify "how" to access a service, while generic
   DNS names can be interpreted as identity tokens that can be used to
   identify a resource that may host a service (or "who").

   It is also not surprising from this perspective to see the emergence
   of DNS resource records that refer to URLs, as in NAPTR records [10].
   In this approach the first DNS lookup retrieves one or more URLs that
   have been associated with the DNS name, and a second lookup is used
   to resolve any DNS names as may be referenced in the URL strings.  In
   this framework a service may change its location, or the access
   algorithm may be altered (and by necessity, the URL changed), but the
   DNS identity that maps to this URL remains constant.  This is one of
   the clearer forms of delineating identity from access mechanisms.

   This mapping can also be used for service discovery.  Given the name
   of a domain it is possible to look up NAPTR records to discover what
   URLs can be used for communication with that domain.  This is for
   example used in the ENUM specification [11].  In ENUM a lookup in DNS



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   of NAPTR records for a domain name created from an E.164 number is
   via transformation turned into a list of URLs.  This give an ability
   to know what URLs one can use in order to contact the entity referred
   to by a given E.164 number.  The more general form of this approach
   can use NAPTR resource records to associate a DNS name with one or
   more resources.  The name that has the NAPTR records can be
   considered as an identity token, while the associated NAPTR records
   provide a mapping from this identity to the instantiation of the
   identified service.  This approach has been used in the Archive
   Resource Key (ARK) proposal [26].

   Of course not all URIs are protocol-scheme URLs of the form outlined
   above.  URIs are a very general construct where the initial "scheme"
   part of the URI determines the structure and semantics of the
   remainder of the URI string.  The next section examines that class of
   URIs where persistence of the identity is a specific feature of the
   identity realm, the Uniform Resource Name.

2.8.1  Summary for URLs

   This summary section explicity refers to URLs rather than URIs.  The
   more general case of URIs is one that, in the general case, is
   unclear on all these desireable identity attributes.

   Uniqueness:
      URLs are intended to be unique.

   Consistency:
      URLs are not necessarily consistent.  A URL typically includes the
      specification of an application, an enpoint and some additional
      arguments for the application to apply to the application instance
      on the nominated endpoint.  Inconsistent interpretation of URL
      components by other applications is possible.

   Persistence:
      URLs are not necessarily persistent, as they implicitly identify
      how to access a resource or service, rather than identifying the
      resource or service per se.  If the service or resource changes
      location, a new URL is required to reference the new location.
      In the case where URIs use a DNS identifier as part of the URI
      scheme, as in URLs, for example, such URIs also depend on the
      persistence of the underlying DNS identity for persistence of the
      URL.

   Trust:
      URLs are not necessarily trustable.





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   Robustness:
      URLs are not necessarily robust.

   Withholding:
      URLs are not capable of withholding elements of the URL identity.

   Referential Consistency:
      URLs are intended to be referentially consistent, but are limited
      in terms of their persistence.

   Structure:
      URLs are a structured identity space.

2.9  Uniform Resource Names

   To solve the problem of lack of long term stability for references,
   URNs can be used as an alternative to recursive references into the
   DNS.  URNs are generally considered not to be entirely within a human
   realm as they often include what would appear to be long random
   combination of characters.  URNs are intended to be globally unique,
   and never reused.  As long as a named object exists, it retains that
   name.  An object can have many names.  The object may cease to exist,
   in which case the URN can no longer be resolved, because the
   resolution service (from URN to URI) is no longer working, but, as
   the name exists (virtually), a new service can be created and the
   object re-established if there is need for it.  RFC 3305 [14]
   describes in more detail the different views that exist on the
   relationship between URIs, URLs and URNs.

2.9.1  Summary

   Uniqueness:
      URNs are intended to be unique.

   Consistency:
      URNs are indended to be consistent.

   Persistence:
      URNs are intended to be persistent.

   Trust:
      It is unclear how trust relationships are formed with URNs.

   Robustness:
      As with trust relationships, the robustness properties of URNs are
      unclear.





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   Withholding:
      It is unlikely that URNs can withold parts of the URN.

   Referential Consistency:
      URNs are intended to be referentially consistent.

   Structure:
      URNs included unstructured components.

2.10  Human Friendly Strings

   URIs have a problem that URNs didn't solve, and that is the ability
   for humans to remember them.  Humans act in a context, so global
   uniqueness is not important at this level of abstraction.  Instead,
   when a human uses a name, they normally want a resolution service
   that "does what they want".  In this realm the context of the name is
   an important factor in resolving the name to an object, and global
   uniqueness is neither necessary nor assumed.

   This area of human friendly strings is a topic of ongoing work.  One
   possible goal for a working system is to be able to handle the
   so-called "side of the bus" problem.  A human sees something in an
   advertisement on the side of a bus, remembers it (or remembers part
   of it), and when they come to a computer they try to get more
   information about what they have seen.  This involves complex
   language and localization (and internationalization) problems.

   There has been various ideas connected to "layers above DNS", for
   example mentioned in RFC 3467 [19] (subject of the SIREN Research
   Group in the IRTF).  This topic encompasses an effort to decouple the
   naming realms that makes sense to humans, with their various forms of
   implied context for resolution, from the naming realms that work for
   computers, with the implication of explicit specification of
   resolution, and define a mapping between them.  The DNS can't handle
   the types of names that often make sense to people, because people
   always work in a context (such as a geographical context of
   'locality'), and it's no longer sufficient for people to fit their
   needs into what DNS can handle.  For a some time it was considered
   possible to overload the semantics of the DNS label
   (machine-parseable, vaguely human-recognizable) but it is becoming
   evident that this is not a tenable approach, and some distinction
   needs to be drawn between DNS names and context-sensitive
   human-friendly strings.

   No real human friendly naming system exists today on the Internet.






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2.10.1  Summary

   Uniqueness:
      HFS are intended to be unique within a context of discourse.

   Consistency:
      Unclear.

   Persistence:
      Unclear, although it would appear to be a desireable attribute in
      this context.

   Trust:
      Unclear.

   Robustness:
      Unclear.

   Withholding:
      Unclear.

   Referential Consistency:
      Desireable.

   Structure:
      Unclear.

3.   Issues with Identities

3.1  Overloading the IP Address

   An IP address suffers from semantic overload in attempting to carry
   both location and some form of constant identity.  If a network or
   individual device changes access providers then this is, in effect, a
   change in network location, and if provider-based address aggregation
   is being used, then the local IP address will change.  The same issue
   applies with mobile devices.  This implies that an IP address is not
   necessarily a permanent or truly persistent association with a
   device, and such impermanence is a weakness in any persistent
   identity system.

   Another issue with IP addresses, at least in version 4 of the
   protocol, is that of their total span.  While 32 bits is still a
   large size, encompassing some 4.4 billion unique addresses, there is
   an inevitable level of wastage in deployment, and a completely
   exhausted 32 bit address space may only encompass a connectivity
   realm of perhaps only 1 or 2 billion IP devices.  When this is
   coupled with a world of embedded IP devices in all kinds of



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   industrial and consumer applications, 1 or 2 billion addresses is
   insufficient to provide unique addressing to every possible device.

   In response to these address pressures there has been the
   introduction of a number of technologies that dilute the strong
   binding of IP address with identity.  Such approaches tend to treat
   the IP address purely as a routing and forwarding token without any
   of the other attributes of identity, including persistence and public
   association.  For example, DHCP, or address-lending, is a commonly
   used method of extending a fixed pool of IP addresses over a domain
   where not every device is connected to the network at any given time,
   or when devices enter and leave a local network over time and need
   addresses only for the time they are within the local network's
   domain.  In this form of identity, the association of the device with
   a particular IP address is temporary, and hence there is some
   weakening of the identity concept, as the dynamically-assigned IP
   address is being used primarily for routing and forwarding.  This has
   been taken a further step with the use of dynamic Network Address
   Translation (NAT) approaches, where a single device has a pool of
   public addresses to use, and maps a privately used address to one of
   its public addresses when the private device initiates a session with
   a remote public device.  The private-side device has no direct
   knowledge of the public address that the NAT edge will use for the
   session, nor does the corresponding public-side device necessarily
   know that it is using a temporary identity association to address the
   private device.

   These forms of changes to the original semantics of an IP address are
   significant architectural changes to the concept of identity at the
   level of IP, particularly in the presence of NATs.  The widespread
   deployment of such approaches continues to underline the concept that
   as an identity token there is a lack of persistence in an IP address,
   and the various forms of aliasing weaken its utility as an identity
   system.  The conclusion drawn from these observations is that,
   increasingly, an IP address, in the world of the IPv4 Internet, is
   being seen primarily as a locality token with a very weak association
   with some form of identity.

   Version 6 of IP represents an effort to restructure the address
   field, and the 128 bits of address space represents a very large
   space in which to attempt to place structure.  One of the more
   innovative concepts that was discussed within the development of IPv6
   was extending the concept of the IPv6 interface identifier field of
   the address to be a globally unique identifier.  This had some
   obvious connotations in being able to identify when the connectivity
   for a device has changed, as in such cases the globally unique
   interface identifier could remain constant while the routing prefix
   may have changed.  There was also some potential applications in the



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   area of supporting multi-homed networks, where a local network could
   be seen via different routing prefixes.  At present these aspects of
   IPv6 address architecture are the topic of ongoing work in the IETF.
   One of the fundamental issues with this form of approach is
   management of an interface identifier space that is globally unique
   and persistent, as well as being adequately robust.  Current
   directions of activity in this area indicate that the self-assertion
   of identity using this field within IPv6 are insufficiently robust to
   prevent various forms of redirection attacks.  Approaches currently
   being investigated are looking deeper into various aspects of
   mechanisms that are intended to provide corroboration of identity
   assertion in the face of locator change and additional protocol
   mechanisms appear to be a common feature of the current proposals
   relating to multi-homing and aspects of mobility.

3.2  Dynamic DNS Updates and Nomadism

   An alternative mechanism to revising the semantics of the IP address
   is looking at the concept of moving the role of completing the
   transition of persistent identity into the DNS.  Here the constant
   identity of the device is its DNS name.  In a mobile context, as the
   device or network it roams across the network, and by using a
   sequence of secure dynamic incremental updates to the DNS, update the
   association of the constant DNS name to the new local IP address.
   This approach has possible applications in various multi-homing
   scenarios.

   However, this approach is not without attendant considerations.  Much
   of the leverage of the DNS as an efficient lookup mechanism is based
   on extensive use of local caching of DNS information.  Increasing the
   responsiveness of the DNS to dynamic updates implies that the extent
   to which cached information can be retained is compromised, and any
   cache has to refer more frequently to the primary source to refresh
   the currency of the local cached copy.  The tradeoff here is greater
   DNS traffic loads and increased DNS server query loads in order to
   get a more responsive name system.  Such a mechanism also requires an
   "always available" primary DNS server to accept the incremental
   updates, so that the failure backup mechanism of the DNS with primary
   and secondary servers is compromised in this nomadic model with the
   requirement for primary server availability in order to undertake an
   authoritative update to the DNS.

   An alternative approach is to equip the DNS with an additional
   resource record that contains an identity value in addition to the
   current A or AAAA address values.  This approach can be used in
   conjunction with an additional element within the protocol stack that
   could allow the transport layer to operate using this identity field,
   and a new stack element provides a dynamic mapping between this



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   identity and a 'current' locator value, where the equivalent current
   locator is passed into the IP protocol element.

   An alternative to this approach of changing mappings is to place the
   responsibility for the redirection into the application protocol.
   For example, with SIP, the mobile node could use the REGISTER method
   to change its registration for session setup.  This may not be fast,
   but may be faster than dynamic DNS updates and perhaps even fast
   enough for handling initiating new sessions.  A mobile HTTP server,
   on the other hand, would have to use HTTP Redirect from a fixed
   server whose address was in the DNS.

3.3  URLs and Persistent Identifiers

   URLs are, as their name suggests, locators rather than location
   independent identifiers.  When the resource is relocated, or when
   multiple copies of the same resource exist, the URL scheme cannot
   persist across the change.  Despite the almost universal use of the
   URL within web browsers, URLs are not an ideal candidate for a
   persistent identity.

   This weakness in the URL scheme has lead to the consideration of many
   alternate naming schemes, although the underlying requirements for
   any candidate naming scheme is that it is cleanly mappable into a
   URI-styled format and that there is a robust resolution system
   associated with the name scheme.  Resolution is a critical factor
   here, as without the ability operate in a predictable, robust,
   scalable, trustable and reliable manner when translating an
   identifier into a resource, entity or service access description, the
   identifier scheme is of dubious value.

   The requirement for persistent identifiers is not intended to
   dispense with URLs, or similar forms of locators and service
   descriptors, but to separate the notions of identification and
   location, and to use distinct label space for each concept, and to
   use a resolution mechanism to map from the identifier to the location
   descriptor.

   Work on the development of a unique permanent identifier space has
   proceeded concurrently with the formalization of URL schemes, using
   the name of URN (Uniform Resource Name) schemes.  A specification
   outlining the minimum requirements of the URN can be found at [3].
   The syntax of the URN as expressed in [6] is as follows:



      urn:<Namespace Identifier (NID)>:<Namespace Specific String (NSS)>




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      The NID ensures the global uniqueness of the identifier.  The NSS
      can take any form specified by the naming authority provided that
      it is unique within that namespace.

   The simple structure of the identifier reflects recognition of the
   need to accommodate different requirements and different schemes.
   Because the local, or namespace specific, string can be in any form,
   the identifier structure allows maximum flexibility in the identifier
   while providing a mechanism to assure global uniqueness and
   facilitating interoperability between discrete systems.

   There is a need to distinguish between naming schemes and resolution
   systems.  A naming scheme, as a procedure for creating unique URNs
   that conform to a specific syntax, is independent of the resolution
   service which resolves the URN to locate the resource.  Ideally, a
   naming scheme should not be tied to any specific resolution system
   and a resolution service should be capable of resolving a URN from
   any given name scheme.

   This objective is consistent with the intentions behind the
   development of the URN.  A persistent identifier, especially when
   used for archival data must of necessity be capable of outlasting any
   systems and protocols that are currently in use.  However the lack of
   a commonly agreed upon resolution system is also a major obstacle to
   the wide deployment of URNs.

   A variety of solutions have been proposed, including the NAPTR
   (Naming Authority PoinTeR) DNS resource record [10], that provides
   rules for mapping parts of URIs to domain names and then using these
   domain names as DNS lookup queries to find mapped URIs.  This was
   specification has been further refined as the Dynamic Delegation
   Discovery System (DDDS) [15][16][17][18].  As noted in RFC3404 [18]:


      "For the short term, the Domain Name System (DNS) is the obvious
      candidate for the resolution framework, since it is widely
      deployed and understood.  However, it is not appropriate to use
      DNS to maintain information on a per-resource basis.  First of
      all, DNS was never intended to handle that many records.  Second,
      the limited record size is inappropriate for catalogue
      information.  Third, domain names are not appropriate as URNs.
      Therefore our approach is to use the DDDS to locate "resolvers"
      that can provide information on individual resources, potentially
      including the resource itself."

   There appears to be some residual issues over the status of URNs.
   For URNs to achieve widespread deployment, not only is consensus on
   functional requirements and syntax needed, but the ability to



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   recognize and resolve URNs should be incorporated into the
   application realm.  For example, it would be a reasonable objective
   to incorporate URN support in standard Web browsers.  However a
   pre-requisite for this step is the definition and construction of the
   necessary resolving infrastructure, developed either by leveraging
   off the existing Domain Name System or by some other route.  As long
   as application developers are uncertain of what is to be accepted as
   a standard resolution mechanism, and while naming scheme developers
   are uncertain of how to register their name and resolution schemes
   these issues will not be fully resolved.

   Until the resolution issues are clarified and there is clear
   consensus to adopt a particular specification, implementation of URN
   systems will require some form of application level assistance by way
   of proxy servers.  The implication is that use of URNs will require
   encapsulation in a URL in order to specify the appropriate proxy
   server address.

   This approach has already been undertaken in the specification of
   PURLS [24], which is a naming scheme that incorporates within the
   PURL a conventional URL reference to a resolver to specify a PURL
   resolution service and a name part of the URL that the resolution
   service translates to the resource URL.  In a web-based context this
   is handed back to the client as an HTTP redirect.  The dependency of
   the identifier scheme on the behavior of a particular application
   (namely HTTP in this case) is not the most desireable of attributes
   for an identity scheme.  If the PURL was to be used in a different
   context by a different application, a comparable redirection
   mechanism would be required to support the desired outcome.

   In comparison, the Handle system [20] uses a non-URL name scheme, and
   resolution in applications requires modification of the application.
   The 'handle' itself is a persistent identifier consisting of two
   parts.  The syntax is a two part identifier of "<naming authority>/<
   name>" where the naming authority is an administrative unit
   authorized to create and maintain handles and the name of the
   resource is a string which must be unique to that authority but which
   has no prescribed syntax.  Use of handles can be through standard web
   browsers using a plug-in, or through unmodified web clients using
   proxy servers and embedding the handle within a URL that specifies a
   handle resolver in a manner similar to the PURL approach.  The
   specification of a distinct handle syntax allows handles to be used
   in a broader set of contexts than web browsing as there is
   independence of the identifier to a particular access protocol and
   server location.

   The issue of resolution of the compound identifiers remains
   problematic, and the use of embedding the URN into a proxy URL to



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   undertake redirection can be argued as defeating the purpose of
   having location and protocol independent identifiers, since the
   resultant identifier includes the location of the proxy agent.  The
   full value of persistent identifiers to ensure persistence in
   citations can only be realized if they are actually useful when
   citing documents and objects.  In order to use them, the user must
   know that there is a persistent identifier and must be able to
   discover what it is and how to resolve the identity.  At present this
   is difficult because of the nature of the redirects used in most
   existing systems.

4.  The DNS in Identity Spaces

   How good are any of these identities? Which one should be used in
   which context?

   Each of these digital identities have a context of usage, or realm of
   discourse, and outside of that realm they tend to break down as a
   cohesive and useful identity tool.  Offering a MAC address as an
   email point of contact makes little sense, even though it could
   conceivably be used to form a unique identity in the mail realm.
   Offering an identification at the appropriate level of abstraction
   that provides a description of the mode of contact and identity in a
   form that matches the actions at this level is often used to
   distinguish between identities.  At the level of human interaction we
   commonly identify email addresses using a domain and user name part.
   We do this because this is what you need to enter into your mail
   application in order to send me a message.

   There are considerations when generating identity spaces based on
   generic descriptions of algorithms of how to access the specific
   resource, trigger the particular application or contact a particular
   individual or role's network point of presence.  These
   considerations, commonly found in conjunction with URI's, raise
   consideration of maintaining referential integrity, allowing
   efficient searching and persistence of the identity.  The human
   world, and its digital counterpart, is far from static.  Any identity
   system that aspires to be useful in a human space needs to be able to
   support a maintenance function that allows any implicit reference
   that is contained in an identity space to be updated and refreshed in
   a reliable, trustable and timely manner.  Knowing who you were is a
   less important piece of information as compared to knowing who you
   are right now.  That leads to consideration of structured identity
   spaces whose two major attributes are:
   o  sufficient structure to ensure that specific instances of the
      identity are unique, and





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   o  appropriate structure to allow rapid lookup of the identity to be
      able to retrieve the current set of associated pointers within
      various specified realms.


   There is a good match between these desired attributes and those of
   the DNS, and one perspective to be drawn from this is that the major
   underpinning of useful and lasting digital identities rests within
   the framework of the DNS.  In other words any useful identity space
   is highly likely to have managerial and operational characteristics
   that would largely parallel that of the DNS.

4.1  The role of the DNS

   Different identities are used in the Internet for different purposes.
   IP addresses are essential at the level of forwarding protocol data
   units across the network, but are unwieldy to use in the context of
   naming resources and services at the level of human operation of
   applications.  In the context of URIs, the use of a DNS identity
   within a URI ensures that the identity of a service doesn't have to
   be changed when the IP address changes.  The domain name creates an
   abstraction layer above the IP addresses that allows a service to be
   identified without particular reference to its current location
   within the Internet, and using a name realm that has better
   properties for human use.

   We could use something else, like static tables, databases or more
   similar systems like X.500.  But, none of these alternatives have
   been able to prove they scale as well as DNS.  Both the protocol
   itself and the data model with the distributed delegation has proven
   to be extremely efficient (even though some things could be
   "better").

   The perspective being espoused here is that we don't have any current
   viable alternative to the DNS in terms of a structured identity space
   that supports mapping across identity realms.  Even if we stop using
   domain names in URI's and instead using something else, deploying a
   translation service from this other name to IP addresses would
   inevitably involve recreating much of the functionality of the DNS.

4.2  Changing the DNS

   Because DNS is the service we use for mapping between many of the
   namespaces we use on the Internet, it is extremely important it
   works.  Because of this, changes to DNS must be made with care.  This
   refers to both changes to the protocol as well as the DNS data model.

   Example of changes to the protocol include the need for DNSSEC



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   (signed record sets) which makes it possible for a recipient of a DNS
   response to verify whether it comes from an authoritative source.
   This has been discussed in the IETF for some years, and is
   illustrative of the required level of care in the design of changes
   to the DNS.

   Example of a change to the database structure include a move from an
   hierarchical to a flatter namespace.  The result might be a
   disruptive change of DNS traffic on the global Internet which in turn
   might make further scaling difficult.  Another similar change is
   allocation of names which are not registered properly.  Especially in
   the root zone, this leads to problems such as the inability to later
   allocate and set a policy for the domain, and increased number of
   queries for non-existing names in the root zone when leakage of names
   happens from presumed to be closed networks.  Example of the former
   are the very large TLDs like .com and .de.  Example of the latter is
   the use of the pseudo TLDs '.local' and '.gprs' which are being used
   in private or enterprise contexts without any proper definition or
   registration and their consequent leakage of queries into the
   'public' DNS.

4.3  The DNS is a strict lookup service

   When sending a query to a server, the server is to send the same data
   back regardless of context.  Further, the server should send either a
   "match" which consists of one or more resource records, or a
   "failure" which include the special response "no such domain".

   This implies that two users sending the same query from two different
   locations at the same time should receive the same data in response.
   Or, the same user using two different computers with different
   operating system should receive the same data.

   Having the DNS server doing a "search", undertaking "fuzzy matching"
   or inferring some additional context to a query that guides the
   server to choose a particular response is ill-advised.  The DNS
   server can not know the context of the query, nor should it guess
   what the DNS response is to be used for.  It is always tempting to
   assume that the response is to be used by the most popular operating
   system for the most popular application of the day.  It must though
   be remembered that other operating systems and other applications
   might break when fuzzy matching happens.  For example, instead of
   giving back a "no such response" it is conceivable to give back
   something which pushes a potential error to the application layer by
   returning a synthesized answer that has resource records pointing to
   some form of application- level service.  This implies the DNS server
   must know what application layer protocol is in use, and that a "no"
   at the application layer has the same semantics as a "no" on the DNS



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   (naming) layer.  Often TCP is used at the application layer which
   implies a "no" might only be signalled to the other end by not
   accepting the connection, which means the querying client cannot
   differentiate between "no such (dns) name" and "no response in
   application protocol".

4.4   Coherency of the DNS

   DNS is a bootstrap mechanism that publishes your data in a manner
   that allows queries from others to be answered.  If you make mistakes
   in your local DNS configuration then you don't destroy the utility of
   the DNS for yourself, but you destroy the ability for others to
   contact you.  Someone trying to reach your webpage might not be able
   to do so as they can not find the proper translation from your domain
   name to the IP address of the web server.  It is also the case that
   mis-configurations most often happen in the glue between parent zone
   and child zone, and not in the child zone itself.  Because of this,
   if you know where your nameserver is, you might not see the errors,
   as they have to do with finding the nameserver, and not the content
   of it.

   As mentioned before, it is very important the same response is sent
   back regardless of from where it is sent.  The assumption within the
   DNS is that you should be able to pass a URI with an embedded domain
   name in it to all of your friends, and they all should be able to
   resolve it in an identical fashion.  It is extremely important the
   domain names are globally unique, and lead to the same result every
   time, and from every location.

   Part of the coherence requirement is that the servers must be able to
   give back the same response to the same query.  The implication is
   that all servers have to use the same matching algorithms when
   attempting to locate a match between a query and the local data used
   to form a response.  What matching algorithm is used when looking in
   the data cannot change between servers because then they will give
   back different results for the same query.

   Complications arise when considering this in the context of use of
   various character sets within the DNS.  Having each server use a
   local set of rules that defines equivalence of characters generates
   the situation of the same query generating different responses.  The
   implication is that the consideration of different matching/equality
   rules can be solved by creating "bundles" of characters which are to
   be treated as equal, and solving the problem at the time of
   registration.  This gives a greater choice for the registrant, and it
   can also give a higher freedom regarding context, as the bundles
   possibly look differently depending on such things like (parent)
   domain and language.



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4.5  The DNS as an Identity Glue

   When comparing the desired attributes of a useful identity system to
   the properties of the DNS it is evident that there is a reasonable
   level of fit between the DNS and a generic identity realm.  The DNS
   provides a namespace that ensures uniqueness, is consistent, can
   support persistence, and referential consistency.  The space is
   structured in a manner that supports relatively efficient lookup over
   a large name space that has both hierarchical structuring and within
   that some areas of large flat name spaces.  The DNS can support trust
   models in terms of being able to validate the authenticity of
   responses.  The DNS can support a variety of resource records that
   allow a DNS name token to be used as a search object that can map to
   related values drawn from other identifier realms, as well as
   supporting indirect self-reference through the use of NAPTR records
   and URIs.

   There are obvious trade-offs in the design, protocol and deployment
   of the DNS in terms of resiliency, dynamic behaviours and
   scalability.  While it is not argued here that the DNS represents the
   only optimal trade-off between these properties, it is argued that
   any other identity space with similar properties will be faced with
   precisely the same set of trade-offs.  It is also probable that any
   similar identity space faced with the same requirements of
   scalability, operational performance, accuracy and validity of
   responses and flexibility of mapping the identity space to related
   objects in other identity realms would find a resolution between
   these requirements in a manner that would not differ markedly from
   the DNS.

   The salient observation here is that an identity system acts
   generically as a reference to an initial point of rendezvous in a
   communication transaction.  In this vein the role of the identity
   system is to identify how other parties in the network can refer to
   the identified element using an identity token that is persistent,
   with associated referential mappings into other identity realms that
   reflect the current status of the element.  Once a communications
   state has been established using the rendezvous points, if there are
   characteristics of the application that require the subsequent
   exchange of information (such as location changes in a mobility
   environment, or a server hand-over at the application level) this is
   generally the task of components within the protocol stack, using a
   trust relationship between the communicating parties to alter the
   identity elements used within the stack to match the changing
   characteristics.






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5.  Security Considerations

   Any identity system that provides a mapping from an identity value
   within one realm to an identity value (or set of values) within
   another realm will present a number of considerations with respect to
   security.  The trust model for an identity system is that the mapping
   supported by the identity system is authentic, and that when the
   identity value is used as a key in a query operation, the response
   should be an accurate response that correctly represents the mapping
   originally provided by the assigned holder of that identity value.

   Equally, it is necessary to correctly report responses where an
   invalid or unassigned identity value is used, providing the query
   agent with a clear indication that the identity value is not
   assigned.

   In a hierarchically structured identity space there are a number of
   potential weak points in the identity space, where vulnerabilities
   exist for third parties to intercept queries and substitute a
   non-authentic response.  This could involve misrepresentation of the
   of the root servers for the hierarchy, or misrepresentation of
   delegation points, as well as misrepresentation of responses for
   particular mapping queries.

   Any design of an identity space resolution service should be
   resilient to these forms of attack, by using appropriate mechanisms
   to reduce the risks of interception and misrepresentation in identity
   resolution operations.  However, recognizing the lack of absolute
   assurances that a resolution system is resilient to all forms of
   attack, a resolution services should also be capable of exposing the
   trust model that exists within the identity space, and allow a user
   of the resolution service the ability to validate the response
   against the trust model.  In other words authenticity should be a
   verifiable quality of the identity realm, rather than simply being an
   assertion that is interpretable only as a article of faith.

6.  Acknowledgements

   The editors acknowledge the contributions made by Ran Atkinson, Brian
   Carpenter, Vint Cerf, Leslie Daigle, Joel Halpern and James Kempf in
   the preparation of this document.

7  Informative References

   [1]   Saltzer, J., "On the Naming and Binding of Network
         Destinations", RFC 1498, August 1993.

   [2]   Berners-Lee, T., "Universal Resource Identifiers in WWW: A



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         Unifying Syntax for the Expression of Names and Addresses of
         Objects on the Network as used in the World-Wide Web", RFC
         1630, June 1994.

   [3]   Sollins, K. and L. Masinter, "Functional Requirements for
         Uniform Resource Names", RFC 1737, December 1994.

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

   [5]   Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4 Address
         Behaviour Today", RFC 2101, February 1997.

   [6]   Moats, R., "URN Syntax", RFC 2141, May 1997.

   [7]   Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
         Resource Identifiers (URI): Generic Syntax", RFC 2396, August
         1998.

   [8]   Kent, S. and R. Atkinson, "Security Architecture for the
         Internet Protocol", RFC 2401, November 1998.

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

   [10]  Mealling, M. and R. Daniel, "The Naming Authority Pointer
         (NAPTR) DNS Resource Record", RFC 2915, September 2000.

   [11]  Faltstrom, P., "E.164 number and DNS", RFC 2916, September
         2000.

   [12]  Narten, T. and R. Draves, "Privacy Extensions for Stateless
         Address Autoconfiguration in IPv6", RFC 3041, January 2001.

   [13]  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
         RFC 3118, June 2001.

   [14]  Mealling, M. and R. Denenberg, "Report from the Joint W3C/IETF
         URI Planning Interest Group: Uniform Resource Identifiers
         (URIs), URLs, and Uniform Resource Names (URNs): Clarifications
         and Recommendations", RFC 3305, August 2002.

   [15]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
         One: The Comprehensive DDDS", RFC 3401, October 2002.

   [16]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
         Two: The Algorithm", RFC 3402, October 2002.



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   [17]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
         Three: The Domain Name System (DNS) Database", RFC 3403,
         October 2002.

   [18]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
         Four: The Uniform Resource Identifiers (URI)", RFC 3404,
         October 2002.

   [19]  Klensin, J., "Role of the Domain Name System (DNS)", RFC 3467,
         February 2003.

   [20]  Sun, S., Lannom, L. and B. Boesch, "Handle System Overview",
         RFC 3650, November 2003.

   [21]  IEEE, "Guidelines for use of a 48-bit Global Identifier
         (EUI-48)", December 2003,
         <http://standards.ieee.org/regauth/oui/tutorials/EUI48.html>.

   [22]  IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)
         Registration Authority", December 2003,
         <http://standards.ieee.org/db/oui/tutorials/EUI64.html>.

   [23]  IEEE, "802.11 Wireless", December 2003,
         <http://standards.ieee.org/getieee802/802.11.html>.

   [24]  OCLC, "PURLS: Persistent Uniform Resource Locators", December
         1995, <http://purl.oclc.org/docs/new_purl_summary.html>.

   [25]  Shoch, J., "Internetwork Naming, Addressing, and Routing",
         Proceedings of the 17th IEEE Computer Society International
         Conference pp. 72-79, December 1978.

   [26]  Kunze, J. and R. Rodgers, "The ARK Persistent Identifier
         Scheme", draft-kunze-ark-08 (work in progress), July 2004.


Authors' Addresses

   Patrik Faltstrom (editor)
   Internet Architecture Board

   EMail: paf@cisco.com


   Geoff Huston (editor)
   Internet Architecture Board

   EMail: gih@telstra.net



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Appendix A.  IAB Members

   Internet Architecture Board Members at the time this document was
   drafted were:


      Bernard Aboba
      Harald Alvestrand
      Rob Austein
      Leslie Daigle
      Patrik Faltstrom
      Sally Floyd
      Jun-ichiro Itojun Hagino
      Mark Handley
      Geoff Huston
      Pete Resnick
      Bob Hinden
      Eric Rescorla
      Jonathan Rosenberg
































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