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Versions: (draft-rosenberg-sip-gruu) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 5627

SIP                                                         J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: April 23, 2006                                 October 20, 2005


Obtaining and Using Globally Routable User Agent (UA) URIs (GRUU) in the
                   Session Initiation Protocol (SIP)
                         draft-ietf-sip-gruu-06

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   This Internet-Draft will expire on April 23, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   Several applications of the Session Initiation Protocol (SIP) require
   a user agent (UA) to construct and distribute a URI that can be used
   by anyone on the Internet to route a call to that specific UA
   instance.  A URI that routes to a specific UA instance is called a
   Globally Routable UA URI (GRUU).  This document describes an
   extension to SIP for obtaining a GRUU from a server and for
   communicating a GRUU to a peer within a dialog.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Defining a GRUU  . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1   REFER  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.2   Conferencing . . . . . . . . . . . . . . . . . . . . . . .  6
     4.3   Presence . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  7
   6.  Creation of a GRUU . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Obtaining a GRUU . . . . . . . . . . . . . . . . . . . . . . . 13
     7.1   Through Registrations  . . . . . . . . . . . . . . . . . . 13
       7.1.1   User Agent Behavior  . . . . . . . . . . . . . . . . . 13
       7.1.2   Registrar Behavior . . . . . . . . . . . . . . . . . . 15
     7.2   Through Administrative Operation . . . . . . . . . . . . . 17
   8.  Using the GRUU . . . . . . . . . . . . . . . . . . . . . . . . 17
     8.1   Sending a Message Containing a GRUU  . . . . . . . . . . . 17
     8.2   Sending a Message to a GRUU  . . . . . . . . . . . . . . . 19
     8.3   Receiving a Request Sent to a GRUU . . . . . . . . . . . . 19
     8.4   Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 20
       8.4.1   Request Targeting  . . . . . . . . . . . . . . . . . . 20
       8.4.2   Record Routing . . . . . . . . . . . . . . . . . . . . 21
   9.  The opaque SIP URI Parameter . . . . . . . . . . . . . . . . . 25
   10.   Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   11.   Requirements . . . . . . . . . . . . . . . . . . . . . . . . 27
   12.   Example Call Flow  . . . . . . . . . . . . . . . . . . . . . 28
   13.   Security Considerations  . . . . . . . . . . . . . . . . . . 33
   14.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 34
     14.1  Header Field Parameter . . . . . . . . . . . . . . . . . . 34
     14.2  URI Parameters . . . . . . . . . . . . . . . . . . . . . . 34
     14.3  Media Feature Tag  . . . . . . . . . . . . . . . . . . . . 35
     14.4  SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . 35
   15.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36
   16.   References . . . . . . . . . . . . . . . . . . . . . . . . . 36
     16.1  Normative References . . . . . . . . . . . . . . . . . . . 36
     16.2  Informative References . . . . . . . . . . . . . . . . . . 37
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 39
   A.  Example GRUU Construction Algorithms . . . . . . . . . . . . . 39
     A.1   Instance ID in "opaque" URI Parameter  . . . . . . . . . . 39
     A.2   Encrypted Instance ID and AOR  . . . . . . . . . . . . . . 39
       Intellectual Property and Copyright Statements . . . . . . . . 41









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1.  Introduction

   The Session Initiation Protocol, RFC 3261 [1], is used to establish
   and maintain a dialog between a pair of user agents in order to
   manage a communications session.  Messages within the dialog are sent
   from one user agent to another using a series of proxy hops called
   the route set.  They are eventually delivered to the remote target
   (the user agent on the other side of the dialog).  This remote target
   is identified by a SIP URI obtained from the value of the Contact
   header field in INVITE requests and responses.

   RFC 3261 mandates that a user agent populate the Contact header field
   in INVITE requests and responses with a URI that is global (meaning
   that it can be used from any element connected to the Internet) and
   that routes to the user agent which inserted it.  RFC 3261 also
   mandates that this URI be valid for requests sent outside of the
   dialog in which the Contact URI was inserted.

   In practice, these requirements have proven very difficult to meet.
   Few endpoints have a hostname that is is present in DNS.  Many
   endpoints have an IP address that is private because the client is
   behind a NAT.  Techniques like the Simple Traversal of UDP Through
   NAT (STUN) [15] can be used to obtain IP addresses on the public
   Internet.  However, many firewalls will prohibit incoming SIP
   requests from reaching a client unless they first pass through a
   proxy sitting in the DMZ of the network.  Thus, URIs using STUN-
   obtained IP addresses often do not work.

   Because of these difficulties, most clients have actually been
   inserting URIs into the Contact header field of requests and
   responses with the form sip:<IP-address>.  These have the property of
   routing to the client, but they are generally only reachable from the
   proxy to which the user is directly connected.  This limitation does
   not prevent SIP calls to an Address-of-Record (AOR) from proceeding
   because the user's proxy can usually reach these private addresses,
   and the proxy itself is generally reachable over the public network.
   However, this issue has impacted the ability of several other SIP
   mechanisms and applications to work properly.

   An example of such an application is call transfer [25], based on the
   REFER method [7].  Another application is the usage of endpoint-
   hosted conferences within the conferencing  framework [17].  Both of
   these mechanisms require that the endpoint be able to construct a URI
   that not only routes to that user agent, but is usable by entities
   anywhere on the Internet as a target for new SIP requests.

   This specification formally defines a type of URI called a Globally
   Routable User Agent URI (GRUU) which has the properties of routing to



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   the UA and being reachable from anywhere.  Furthermore, it defines a
   new mechanism by which a client can obtain a GRUU from its SIP
   provider, allowing it to use that URI in the Contact header fields of
   its dialog-forming requests and responses.  Because the GRUU is
   provided by the user's SIP provider, the GRUU properties can be
   guaranteed by the provider.  As a result, the various applications
   which require the GRUU property, including transfer, presence, and
   conferencing, can work reliably.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [5] and
   indicate requirement levels for compliant implementations.

   This specification defines the following additional terms:

   contact: The term "contact", when used in all lowercase, refers to a
      URI that is bound to an AOR or GRUU by means of a registration.  A
      contact is usually a SIP URI, and is bound to the AOR and GRUU
      through a REGISTER request by appearing as the value of the
      Contact header field.

   remote target: The term "remote target" refers to a URI that a user
      agent uses to identify itself for receipt of both mid-dialog and
      out-of-dialog requests.  A remote target is established by placing
      a URI in the Contact header field of a dialog-forming request or
      response.

   Contact header field: The term "Contact header field", with a
      capitalized C, refers to the header field which can appear in
      REGISTER requests and responses, redirects, or in dialog-creating
      requests and responses.  Depending on the semantics, the Contact
      header field sometimes conveys a contact, and sometimes conveys a
      remote target.


3.  Defining a GRUU

   URIs have properties, which are granted to the URI based on the
   policies of the domain that owns the URI.  Those properties are not
   visible by inspection of the URI.  In this context, the domain that
   owns the URI is the one indicated in the host part of the SIP URI.
   Some of the properties that a domain can confer upon a URI are:






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   The AOR property: A URI has the Address of Record (AOR) property if a
      domain will allow it to appear in the To header field of REGISTER
      request.

   The alias property: A URI is an alias if its treatment by the domain
      is identical to another URI.

   The service treatment property: A URI has the service treatment
      property if the domain will apply applications, features, and
      services to calls made by, or made to, that URI, possibly based on
      associating that URI with a user that has "subscribed" to various
      features.

   The anonymous property: A URI has the anonymous property when it is
      not possible, by inspection of the URI, to discern the user with
      whom the URI is associated.

   The identity property: A URI is considered an identity when the
      domain will authorize it as a valid value in the >From header
      field of a request, such that an authentication service will sign
      a request with that URI [20].

   This specification focuses on a property, called the Globally
   Routable User Agent URI (GRUU) property.  A URI possesses this
   property when the following is true:

   Global: It can be used by any User Agent Client (UAC) connected to
      the Internet.  In that regard, it is like the address-of-record
      (AOR) property.  A URI with the AOR property (for example,
      sip:joe@example.com), is meant to be used by anyone to reach that
      user.  The same is true for a URI with the GRUU property.

   Routes to a Single Instance: A request sent to that URI will be
      routed to a specific UA instance.  In that regard, it is unlike
      the address-of-record property.  When a request is sent to a URI
      with the AOR property, routing logic is applied in proxies to
      deliver the request to one or more UAs.  That logic can result in
      a different routing decision based on the time of day, or the
      identity of the caller.  However, when a request is made to a URI
      with the GRUU property, the routing logic is dictated by the GRUU
      property.  The request has to be delivered to a very specific UA
      instance.  That UA instance has to be the same UA instance for all
      requests sent to that URI.

   Long Lived: The URI with the GRUU property persists for relatively
      long periods of time, ideally being valid for the duration of
      existence of the AOR itself.  This property cannot be completely
      guaranteed, but providers are supposed to do their best to make



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      sure that a GRUU remains viable indefinitely.

   A URI can have any combination of these properties.  It is the
   responsibility of the domain which mints the URI to determine what
   properties are conferred upon that URI.  This specification imposes
   requirements on a domain that mints a URI with the GRUU property.

   For convenience, a URI that possesses the GRUU property is also
   referred to as a GRUU.

4.  Use Cases

   There are several use cases where the GRUU properties are truly
   needed in order for a SIP application to operate.

4.1  REFER

   Consider a blind transfer application [25].  User A is talking to
   user B. User A wants to transfer the call to user C. So, user A
   sends a REFER to user C. That REFER looks like, in part:

   REFER sip:C@example.com SIP/2.0
   From: sip:A@example.com;tag=99asd
   To: sip:C@example.com
   Refer-To: (URI that identifies B's UA)

   The Refer-To header field needs to contain a URI that can be used by
   user C to place a call to user B. However, this call needs to route
   to the specific UA instance that user B is using to talk to user A.
   If it doesn't, the transfer service will not execute properly.  This
   URI is provided to user A by user B. Because user B doesn't know who
   user A will transfer the call to, the URI has to be usable by anyone.
   Therefore, it needs to be a GRUU.

4.2  Conferencing

   A similar need arises in conferencing [17].  In that framework, a
   conference is described by a URI that identifies the focus of the
   conference.  The focus is a SIP UA that acts as the signaling hub for
   the conference.  Each conference participant has a dialog with the
   focus.  One case described in the framework is where a user A has
   made a call to user B. User A puts user B on hold, and calls user C.
   Now, user A has two separate dialogs for two separate calls -- one to
   user B, and one to user C. User A would like to conference them.  To
   do this, user A's user agent morphs itself into a focus.  It sends a
   re-INVITE or UPDATE [4] on both dialogs, and provides user B and user
   C with an updated remote target that now holds the conference URI.
   The URI in the Contact header field also has a callee capabilities



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   [11] parameter which indicates that this URI is a conference URI.
   User A proceeds to mix the media streams received from user B and
   user C. This is called an ad-hoc conference.

   At this point, normal conferencing features can be applied.  That
   means that user B can send another user, user D, the conference URI,
   perhaps in an email.  User D can send an INVITE to that URI, and join
   the conference.  For this to work, the conference URI used by user A
   in its re-INVITE or UPDATE has to be usable by anyone, and it has to
   route to the specific UA instance of user A that is acting as the
   focus.  If it doesn't, basic conferencing features will fail.
   Therefore, this URI has to be a GRUU.

4.3  Presence

   In a SIP-based presence [27] system, the Presence Agent (PA)
   generates notifications about the state of a user.  This state is
   represented with the Presence Information Document Format (PIDF)
   [24].  In a PIDF document, a user is represented by a series of
   tuples, each of which describes the services that the user has.  Each
   tuple also has a URI in the <contact> element, which is a SIP URI
   representing that service.  A watcher can make a call to that URI,
   with the expectation that the call is routed to the service whose
   presence is represented in the tuple.

   In some cases, the service represented by a tuple may exist on only a
   single user agent associated with a user.  In such a case, the URI in
   the presence document has to route to that specific UA instance.
   Furthermore, since the presence document could be used by anyone who
   subscribes to the user, the URI has to be usable by anyone.  As a
   result, it has to be a GRUU.

   It is interesting to note that the GRUU may need to be constructed by
   a presence agent, depending on how the presence document is computed
   by the server.

5.  Overview of Operation

   This section is tutorial in nature, and does not specify any
   normative behavior.

   This extension allows a UA to obtain a GRUU, and to use a GRUU.
   These two mechanisms are separate, in that a UA can obtain a GRUU in
   any way it likes, and use the mechanisms in this specification to use
   it.  This specification defines two mechanisms for obtaining a GRUU
   -- through registrations and through administrative operation.  Only
   the former requires protocol operations.




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   A UA can obtain a GRUU by generating a normal REGISTER request, as
   specified in RFC 3261 [1].  This request contains a Supported header
   field with the value "gruu", indicating to the registrar that the UA
   supports this extension.  The UA includes a "+sip.instance" Contact
   header field parameter of each contact for which a GRUU is desired.
   This Contact parameter contains a globally unique ID that identifies
   the UA instance.  If the domain that the user is registering against
   also supports GRUU, the REGISTER responses will contain the "gruu"
   parameter in each Contact header field.  This parameter contains a
   GRUU which the domain guarantees will route to that UA instance.  The
   GRUU is associated with the UA instance.  Should the client change
   its contact, but indicate that it represents the same instance ID,
   the server would provide the same GRUU.  Furthermore, if the
   registration for the contact expires, and the UA registers the
   contact at a later time with the same instance identifier, the server
   would provide the same GRUU.

   Since the GRUU is a URI like any other, it can be handed out by a UA
   by placing it in any header field which can contain a URI.  A UA will
   place the GRUU into the Contact header field of dialog creating
   requests and responses it generates.  RFC 3261 mandates that the
   Contact header field have the GRUU property, and this specification
   provides a reliable way for a UA to obtain one.  In other words,
   clients use the GRUU as a remote target.  However, since the remote
   target used by clients to date has typically not had the GRUU
   properties, implementations have adapted their behaviors (oftentimes
   in proprietary ways) to compensate.  To facilitate a transition away
   from these behaviors, it is necessary for a UA receiving the message
   to know whether the remote target is a GRUU or not.  To make this
   determination, the UA looks for the presence of the Supported header
   field in the request or response.  If it is present with a value of
   "gruu", it means that the remote target is a GRUU.

   A domain can construct a GRUU in any way it chooses.  However, it is
   sometimes desirable to construct GRUUs so that any entity that
   receives a GRUU can determine the AOR for the subscriber associated
   with the UA instance.  To facilitate that, the GRUU can be
   constructed such that it is identical to the subscriber's AOR, but
   includes the "opaque" URI parameter.  The "opaque" URI parameter
   provides a general facility to construct a URI (such as a GRUU or a
   voicemail inbox for a user) that is related to an AOR, so that any
   element can extract the AOR from the constructed URI by removing the
   "opaque" parameter.  For example:

   AOR:   sip:alice@example.com
   GRUU:  sip:alice@example.com;opaque="kjh29x97us97d"

   When a proxy in the domain constructs the GRUU, it would set the



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   value of the "opaque" URI parameter such that it includes the
   instance ID.  As such, when that proxy receives a request sent to the
   GRUU, it can extract the AOR and instance ID, both of which are
   needed to process the request.

   When a UA uses a GRUU, it has the option of adding the "grid" URI
   parameter to the GRUU.  This parameter is opaque to the proxy server
   handling the domain.  However, when the server maps the GRUU to the
   contact bound to it, the server will add the "grid" parameter into
   the registered contact, and use the result in the Request URI.  As a
   result, when the UA receives the request, the Request URI will
   contain the "grid" parameter it placed in the corresponding GRUU.

   The "grid" and "opaque" URI parameters play similar roles, but
   complement each other.  The "opaque" parameter is added by the owner
   of the domain to correlate the GRUU to its instance ID, and to easily
   recognize that the URI has the GRUU property.  The "grid" parameter
   is added by the UA instance so that, when a request is received by
   that instance, it can determine the context of the request.

6.  Creation of a GRUU

   A GRUU is a URI that is created and maintained by a server
   authoritative for the domain in which the GRUU resides.
   Independently of whether the GRUU is created as a result of a
   registration or some other means, a server maintains certain
   information associated with the GRUU.  This information, and its
   relationship with the GRUU, is modeled in Figure 1.























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         +-----------+                 +-----------+
         |           |    associated   |           |
         |           |1   with        n|           |
         |    AOR    |<----------------|   GRUU    |
         |           |                 |           |
         |           |                 |           |
         +-----------+                 +-----------+
               ^1               is    ^^     |n
               |             bound  //0..1   |
             is|                to//         |associated
          bound|                //           |with
             to|              //             |
               |            //               |
               |0..n      //                 V1
         +-----------+  //             +-----------+
         |           | / 0..n          |           |
         |           |                 |           |
         |  contact  |---------------->|  Instance |
         |           |1      has   0..1|     ID    |
         |           |                 |           |
         +-----------+                 +-----------+


                                 Figure 1

   The instance ID plays a key role in this specification.  It is an
   identifier, represented with a URN, that uniquely identifies a SIP
   user agent amongst all other user agents associated with an AOR.  For
   hardware-based user agents, the instance ID would typically be burned
   into the device at the factory, similar to the way a unique serial
   number is encoded into each device.  For software-based user agents,
   each installation represents a unique instance.  As such, the
   identifier could be generated on installation and then stored on disk
   for persistence.

   A GRUU is associated, in a many-to-one fashion, with the combination
   of an instance ID and an AOR.  This combination is referred to as an
   instance ID/AOR pair.  For each GRUU, there is one instance ID/AOR
   pair, and for each instance ID/AOR pair, there can be one or more
   GRUU.  More than one GRUU might be defined in order to have aliases
   or URI that are anonymous or have other URI properties.  However,
   this specification doesn't define any way for the client to learn
   about or use more than a single GRUU for each instance ID/AOR pair.
   The instance ID/AOR pair serves to uniquely identify a user agent
   instance servicing a specific AOR.  The AOR identifies a resource,
   such as a user or service within a domain, and the instance ID
   identifies a specific UA instance servicing requests for that
   resource.



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   It is important to understand that GRUU is associated with the
   instance ID/AOR pair, not just the instance ID.  For example, let's
   say a user registered the contact sip:ua@pc.example.com to the AOR
   sip:user@example.com, and included a +sip.instance="urn:foo:1"
   parameter in the Contact header field.  If the user also registered
   the contact sip:ua-112@pc.example.com with the same +sip.instance
   Contact header field parameter to a second AOR (say
   sip:boss@example.com), each of those UA instances would have a
   different GRUU because they belong to different AORs.  That is the
   reason why a single instance ID can be associated with multiple GRUU;
   there would be one such association for each AOR.  The same goes for
   the association of AOR to GRUU; there would be one such association
   for each instance ID.

   The contacts that are bound to the GRUU are always the ones that have
   an instance ID associated with that GRUU.  If none of the contacts
   bound to the AOR have the instance ID associated with the GRUU, then
   there are no contacts bound to the GRUU.  If a contact should become
   registered to the AOR that has an instance ID equal to the one
   associated with the GRUU, that contact also becomes bound to the
   GRUU.  If that contact should expire, it will no longer be bound to
   the AOR, and similarly, it will no longer be bound to the GRUU.  The
   URI of the contact is irrelevant in determining whether it is bound
   to a particular GRUU; only the instance ID and AOR are important.

   This specification does not mandate a particular mechanism for
   construction of the GRUU.  Several example approaches are given in
   Appendix A.  However, the GRUU MUST exhibit the following properties:

   o  The domain part of the URI is an IP address present on the public
      Internet, or, if it is a host name, the resolution procedures of
      RFC 3263 [2], once applied, result in an IP address on the public
      Internet.

   o  When a request is sent to the GRUU, it routes to a server that can
      make sure the request is delivered to the UA instance.  For GRUUs
      created through registrations, this means that the GRUU has to
      route to a proxy server with access to registration data.

   o  A server in the domain can determine that the URI is a GRUU.

   o  For each GRUU, both the SIP and Session Initiation Protocol Secure
      (SIPS) versions MUST exist.  The SIPS URI may not always work,
      particularly if the proxy cannot establish a secure connection to
      the client.  However, the SIPS URI always exists.

   Section 8.4 defines additional behaviors that a proxy must exhibit on
   receipt of a GRUU.



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   When a domain constructs a URI with the GRUU properties, it MAY
   confer other properties upon this URI as a matter of domain policy.
   A domain can elect to confer properties like identity, anonymity, and
   service treatment.  There is nothing in this specification that can
   allow the recipient of the GRUU to determine which of these
   properties (besides the GRUU property itself) have been conferred to
   the URI.

   The service treatment property merits further discussion.  Typically,
   the services a proxy executes upon receipt of a request sent to a
   GRUU will be a subset of those executed when a request is sent to the
   AOR.  For requests that are outside of a dialog, it is RECOMMENDED to
   apply screening types of functions, both automated (such as black and
   white list screening) and interactive (such as interactive voice
   response (IVR) applications that confer with the user to determine
   whether to accept a call).  In many cases, the new request is related
   to an existing dialog, and may be an attempt to join it (using the
   Join header field [30]) or replace it (using the Replaces header
   field [31]).  In such cases, the UA will typically make its own
   authorization decisions, allowing the reuqest if the sender can prove
   it knows the dialog identifiers [19].  In such cases, bypassing
   screening services might make sense, but it needs to be carefully
   considered by network designers, as it depends on the specific type
   of screening service.

   However, forwarding services, such as call forwarding, SHOULD NOT be
   provided for requests sent to a GRUU.  The intent of the GRUU is to
   target a specific UA instance, and this is incompatible with
   forwarding operations.

   Mid-dialog requests will also be sent to GRUUs, as they are included
   as the remote-target in dialog-forming requests and responses.
   However, in those cases, a proxy SHOULD only apply services that are
   meaningful for mid-dialog requests, generally speaking.  This
   excludes screening functions, as well as forwarding ones.

   The "opaque" URI parameter, defined in Section 9, provides a means
   for a domain to construct a GRUU such that the AOR associated with
   the GRUU is readily extractable from the GRUU.  Unless the GRUU is
   meant to also possess the anonymity property, it is RECOMMENDED that
   GRUUs be constructed using this parameter.

   Because the GRUU is associated with both the instance ID and AOR, for
   any particular AOR there can be a potentially infinite number of
   GRUUs, and potentially more than one for each instance ID.  However,
   the instance IDs are only known to the network when an instance
   actually registers with one.  As a result, it is RECOMMENDED that a
   GRUU be created at the time a contact with an instance ID is first



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   registered to an AOR (even if that registration indicates that the
   registering UA doesn't even support GRUUs), until the time that the
   AOR is no longer valid in the domain.  In this context, the GRUU
   exists if the domain, upon receiving a request for that GRUU,
   recognizes it as a GRUU, can determine the AOR and instance ID
   associated with it, and translate the GRUU to a contact if there is
   one with that instance ID currently registered.  This property of the
   GRUU (existing from the time the first registration until removal of
   the AOR) can be difficult to achieve through software failures and
   power outages within a network, and for this reason, providing the
   property is at RECOMMENDED strength, and not MUST.

7.  Obtaining a GRUU

   A GRUU can be obtained in many ways.  This document defines two --
   through registrations and through administrative operation.

7.1  Through Registrations

   When a GRUU is associated with a user agent that comes and goes, and
   registers itself to the network to bind a contact to an AOR, a GRUU
   is provided to the user agent through SIP REGISTER messages.

7.1.1  User Agent Behavior

7.1.1.1  Generating a REGISTER Request

   When a UA compliant to this specification generates a REGISTER
   request (initial or refresh), it MUST include the Supported header
   field in the request.  The value of that header field MUST include
   "gruu" as one of the option tags.  This alerts the registrar for the
   domain that the UA supports the GRUU mechanism.

   Furthermore, for each contact for which the UA desires to obtain a
   GRUU, the UA MUST include a "sip.instance" media feature tag as a UA
   characteristic [11].  The instance ID MUST identify the UA that is
   performing the registration.  As described in [11], this media
   feature tag will be encoded in the Contact header field as the
   "+sip.instance" Contact header field parameter.  The value of this
   parameter MUST be a URN [10].  Usage of a URN is a MUST since it
   provides a persistent and unique name for the UA instance, allowing
   it to obtain the same GRUU over time.  It also provides an easy way
   to guarantee uniquess within the AOR.  However, this specification
   does not require a long-lived and persistent instance identifier to
   properly function, and in some cases, there may be cause to use an
   identifier with weaker temporal persistence.

   [11] defines equality rules for callee capabilities parameters, and



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   according to that specification, the "sip.instance" media feature tag
   will be compared by case-sensitive string comparison.  This means
   that the URN will be encapsulated by angle brackets "<" and ">" when
   it is placed within the quoted string value of the +sip.instance
   Contact header field parameter.  The case-sensitive matching rules
   apply only to the generic usages defined in RFC 3840 [11] and in the
   caller preferences specification [23].  When the instance ID is used
   in this specification, it is effectively "extracted" from the value
   in the "sip.instance" media feature tag.  Thus, equality comparisons
   are performed using the rules for URN equality that are specific to
   the scheme in the URN.  If the element performing the comparisons
   does not understand the URN scheme, it performs the comparisons using
   the lexical equality rules defined in RFC 2141 [10].  Lexical
   equality may result in two URNs being considered unequal when they
   are actually equal.  In this specific usage of URNs, the only element
   which provides the URN is the SIP UA instance identified by that URN.
   As a result, the UA instance SHOULD provide lexically equivalent URNs
   in each registration it generates.  This is likely to be normal
   behavior in any case; clients are not likely to modify the value of
   the instance ID so that it remains functionally equivalent yet
   lexigraphically different to previous registrations.

   A registering UAC SHOULD use a UUID URN [28] in its "+sip.instance"
   Contact header field parameter.  The UUID URN allows for non-
   centralized computation of a URN based on time, unique names (such as
   a MAC address) or a random number generator.  However, if a different
   URN scheme is used, the URN MUST be selected such that the instance
   can be certain that no other instance registering against the same
   AOR would choose the same URN value.  An example of a URN that would
   not meet the requirements of this specification is the national
   bibliographic number [16].  Since there is no clear relationship
   between a SIP UA instance and a URN in this namespace, there is no
   way a selection of a value can be performed that guarantees that
   another UA instance doesn't choose the same value.

   If a UA instance is registering against multiple AORs, it is
   RECOMMENDED that a UA instance provide a different contact URI for
   each AOR.  This is needed for the UA to determine which GRUU to use
   as the remote target in responses to incoming dialog-forming
   requests, as discussed in Section 8.1.

   Besides the procedures discussed above, the REGISTER request is
   constructed as it is in the case where this extension was not
   understood.  Specifically, the contact in the REGISTER request SHOULD
   NOT contain the gruu Contact header field parameter, and the contact
   URI itself SHOULD NOT contain the "grid" parameter defined below.
   Any such parameters are ignored by the registrar, as the UA cannot
   propose a GRUU for usage with the contact.



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   If a UA wishes to guarantee that the request is not processed unless
   the domain supports and uses this extension, it MAY include a Require
   header field in the request with a value that contains the "gruu"
   option tag.

7.1.1.2  Processing the REGISTER Response

   If the response is a 2xx, each Contact header field that contained
   the "+sip.instance" Contact header field parameter may also contain a
   "gruu" parameter.  This parameter contains a SIP or SIPS URI that
   represents a GRUU corresponding to the UA instance that registered
   the contact.  The URI will be a SIP URI if the To header field in the
   REGISTER request contained a SIP URI, else (if the To header field in
   the REGISTER request contained a SIPS URI) it will be a SIPS URI.
   Any requests sent to the GRUU URI will be routed by the domain to a
   contact with that instance ID.  Normally, the GRUU will not change in
   subsequent 2xx responses to REGISTER.  Indeed, even if the UA lets
   the contact expire, when it re-registers it at any later time, the
   registrar will normally provide the same GRUU for the same address-
   of-record and instance ID.  However, as discussed above, this
   property cannot be completely guaranteed, as network failures may
   make it impossible to provide an identifier that persists for all
   time.  As a result, a UA MUST be prepared to receive a different GRUU
   for the same instance ID/AOR pair in a subsequent registration
   response.

   A non-2xx response to the REGISTER request has no impact on any
   existing GRUU previously provided to the UA.  Specifically, if a
   previously successful REGISTER request provided the UA with a GRUU, a
   subsequent failed request does not remove, delete, or otherwise
   invalidate the GRUU.

7.1.2  Registrar Behavior

   A registrar MAY create a GRUU for a particular instance ID/AOR pair
   at any time.  Of course, if a UA requests a GRUU in a registration,
   and the registrar has not yet created one, it will need to do so in
   order to respond to the registration request.  However, the registrar
   can create the GRUU in advance of any request from a UA.

   A registrar MUST create both the SIP and SIPS versions of the GRUU,
   such that if the GRUU exists, both URI exist.

7.1.2.1  Processing a REGISTER Request

   A REGISTER request might contain a Require header field; this
   indicates that the registration has to understand this extension in
   order to process the request.



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   As the registrar is processing the contacts in the REGISTER request
   according to the procedures of step 7 in Section 10.3 of RFC 3261,
   the registrar checks whether each Contact header field in the
   REGISTER message contains a "+sip.instance" header field parameter.
   If present, the contact is processed further.  If the registrar had
   not yet created a GRUU for that instance ID/AOR pair, it MUST do so
   at this time according to the procedures of Section 6.  If the
   contact contained a "gruu" Contact header field parameter, it MUST be
   ignored by the registrar.  A UA cannot suggest or otherwise provide a
   GRUU to the registrar.

   Registration processing then continues as defined in RFC 3261.  If,
   after that processing, that contact is bound to the AOR, it also
   becomes bound to the GRUU associated with that instance ID/AOR pair.
   If, after that processing, the contact was not bound to the AOR (due,
   for example, to an expiration of zero), the contact is not bound to
   the GRUU either.  The registrar MUST store the instance ID along with
   the contact.

   When generating the 200 (OK) response to the REGISTER request, the
   procedures of step 8 of Section 10.3 of RFC 3261 are followed.
   Furthermore, for each Contact header field value placed in the
   response, if the registrar has stored an instance ID associated with
   that contact, that instance ID is returned as a Contact header field
   parameter.  If the REGISTER request contained a Supported header
   field that included the "gruu" option tag, the server MUST add a
   "gruu" Contact header field parameter to that Contact header field.
   The value of the gruu parameter is a quoted string containing the URI
   that is the GRUU for the associated instance ID/AOR pair.  If the To
   header field in the REGISTER request contains a SIP URI, the SIP
   version of the GRUU is returned.  If the To header field in the
   REGISTER request contains a SIPS URI, the SIPS version of the GRUU is
   returned.

   If the REGISTER response contains a gruu Contact header field
   parameter in any of the contacts, the REGISTER response MUST contain
   a Require header field with the value "gruu".  This is because the
   client needs to extract its GRUU from the REGISTER response, and
   utilize a GRUU as the remote target of dialog-initiating requests and
   responses.

   Note that handling of a REGISTER request containing a Contact header
   field with value "*" and an expiration of 0 still retains the meaning
   defined in RFC 3261 -- all contacts, not just those with a specific
   instance ID, are deleted.  This removes the binding of each contact
   to the AOR and the binding of each contact to a GRUU.

   Inclusion of a GRUU in the "gruu" Contact header field parameter of a



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   REGISTER response is separate from the computation and storage of the
   GRUU.  It is possible that the registrar has computed a GRUU for one
   UA, but a different UA that queries for the current set of
   registrations doesn't understand GRUU.  In that case, the REGISTER
   response sent to that second UA would not contain the "gruu" Contact
   header field parameter, even though the UA has a GRUU for that
   contact.

7.1.2.2  Timing Out a Registration

   When a registered contact expires, its binding to the AOR is removed
   as usual.  In addition, its binding to the GRUU is removed at the
   same time.

7.2  Through Administrative Operation

   Administrative creation of GRUUs is useful when a UA instance is a
   network server that is always available, and therefore doesn't
   register to the network.  Examples of such servers are voicemail
   servers, application servers, and gateways.

   There are no protocol operations required to administratively create
   a GRUU.  The proxy serving the domain is configured with the GRUU,
   and with the contact to which it should be translated.  It is not
   strictly necessary to also configure the instance ID and AOR, since
   the translation can be done directly.  However, they serve as useful
   tools for determining to which resource and UA instance the GRUU is
   supposed to map.

   In addition to configuring the GRUU and its associated contact in the
   proxy serving the domain, the GRUU will also need to be configured
   into the UA instance associated with the GRUU.

   It is also reasonable to model certain network servers as logically
   containing both a proxy and a UA instance.  The proxy receives the
   request from the network, and passes it internally to the UA
   instance.  In such a case, the GRUU routes directly to the server,
   and there is no need for a translation of the GRUU to a contact.  The
   server itself would construct its own GRUU.

8.  Using the GRUU

8.1  Sending a Message Containing a GRUU

   A UA first obtains a GRUU using the procedures of Section 7, or by
   other means outside the scope of this specification.

   A UA can use the GRUU in the same way it would use any other SIP or



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   SIPS URI.  However, a UA compliant to this specification MUST use a
   GRUU when populating the Contact header field of dialog-creating
   requests and responses.  In other words, a UA compliant to this
   specification MUST use its GRUU as its remote target.  This includes
   the INVITE request, its 2xx response, the SUBSCRIBE [6] request, its
   2xx response, the NOTIFY request, and the REFER [7] request, and its
   2xx response.

   If the UA instance has obtained multiple GRUUs for different AORs as
   a result of a registration, it MUST use one corresponding to the AOR
   used to send or receive the request.  For sending a request, this
   means that the GRUU corresponds to the AOR present in the From header
   field.  Furthermore, this means that the credentials used for
   authentication of the request correspond to the ones associated with
   that AOR.  When receiving a request, the GRUU in the response
   corresponds to the AOR to which the original request was targeted.
   However, that AOR will be rewritten by the proxy to correspond to the
   UA's registered contact.  It is for this reason that different
   contacts are needed for each AOR that an instance registers against.
   In this way, when an incoming request arrives, the Request URI can be
   examined.  It will be equal to a registered contact.  That contact
   can be used to map directly to the AOR, and from there, the correct
   GRUU can be selected.

   In those requests and responses where a GRUU is used as the remote
   target, the UA MUST include a Supported header field that contains
   the option tag "gruu".  However, it is not necessary for a UA to know
   whether its peer in the dialog supports this specification before the
   UA uses a GRUU as a remote target.

   When using a GRUU as a remote target, a UA MAY add the "grid" URI
   parameter to the GRUU.  This parameter MAY take any value permitted
   by the grammar for the parameter.  When a UA sends a request to the
   GRUU, the proxy for the domain that owns the GRUU will translate the
   GRUU in the Request-URI, replacing it with the contact bound to that
   GRUU.  However, the proxy will retain the "grid" parameter when this
   translation is performed.  As a result, when the UA receives the
   request, the Request-URI will contain the "grid" created by the UA.
   This allows the UA to effectively manufacture an infinite supply of
   GRUU, each of which differs by the value of the "grid" parameter.
   When a UA receives a request that was sent to the GRUU, it will be
   able to tell which GRUU was invoked by looking at the "grid"
   parameter.

   An implication of this behavior is that all mid-dialog requests will
   be routed through intermediate proxies.  There will never be direct,
   UA-to-UA signaling unless the UA is co-resident with the proxy (which
   is the case for administratively constructed GRUUs).  It is



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   anticipated that this limitation will be addressed in future
   specifications.

   Once a UA knows that the remote target provided by its peer is a
   GRUU, it can use the GRUU in any application or SIP extension which
   requires a globally routable URI to operate.  One such example is
   assisted call transfer.

8.2  Sending a Message to a GRUU

   There is no new behavior associated with sending a request to a GRUU.
   A GRUU is a URI like any other.  When a UA receives a request or
   response, it can know that the remote target is a GRUU if the request
   or response had a Supported header field that included the value
   "gruu".  The UA can take the GRUU, send a request to it, and then be
   sure that the request is delivered to the UA instance which sent the
   request or response.

   If the GRUU contains the "opaque" URI parameter, a UA can obtain the
   AOR for the user by stripping the parameter.  The resulting URI is
   the AOR.  If the GRUU does not have the "opaque" URI parameter, there
   is no mechanism defined for determining the AOR from the GRUU.
   Extraction of the AOR from the GRUU is useful for call logs and other
   accounting functions where it is desirable to know the user to whom
   the request was directed.

   Because the instance ID is a callee capabilities parameter, a UA
   might be tempted to send a request to the AOR of a user, and include
   an Accept-Contact header field [23] that indicates a preference for
   routing the request to a UA with a specific instance ID.  Although
   this would appear to have the same effect as sending a request to the
   GRUU, it does not.  The caller preferences expressed in the Accept-
   Contact header field are just preferences.  Its efficacy depends on a
   UA constructing an Accept-Contact header field that interacts with
   domain-processing logic for an AOR, to cause a request to route to a
   particular instance.  Given the variability in routing logic in a
   domain (for example, time-based routing to only selected contacts),
   this doesn't work for many domain-routing policies.  However, this
   specification does not forbid a client from attempting such a
   request, as there may be cases where the desired operation truly is a
   preferential routing request.

8.3  Receiving a Request Sent to a GRUU

   When a User Agent Server (UAS) receives a request sent to its GRUU,
   the incoming request URI will be equal to the contact that was
   registered (through REGISTER or some other action) by that UA
   instance.  If the user agent had previously handed out its GRUU with



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   a "grid" parameter, the incoming request URI may contain that
   parameter.  This indicates to the UAS that the request is being
   received as a result of a request sent by the UAC to that GRUU/grid
   combination.  This specification makes no normative statements about
   when to use a "grid" parameter, or what to do when receiving a
   request made to a GRUU/grid combination.  Generally, any differing
   behaviors are a matter of local policy.

   It is important to note that, when a user agent receives a request,
   and the request URI does not have a "grid" parameter, the user agent
   cannot tell by inspection of the Request URI whether the request was
   sent to the AOR or to the GRUU.  The To header field might be
   different, but, due to forwarding, cannot be depended on as a
   differentiator.  As such, the UAS will process such requests
   identically.  If a user agent needs to differentiate its behavior
   based on these cases, it will need to use a "grid" parameter.

8.4  Proxy Behavior

   Proxy behavior is fully defined in Section 16 of RFC 3261 [1].  GRUU
   processing impacts that processing in two places -- request targeting
   and record routing.

8.4.1  Request Targeting

   When a proxy server receives a request, owns the domain in the
   Request URI, and is supposed to access a Location Service in order to
   compute request targets (as specified in Section 16.5 of RFC 3261
   [1]), the proxy examines the Request URI.  If the Request URI is an
   AOR against which there are multiple registered contacts with the
   same instance ID parameter, the proxy MUST populate the target set so
   that there is never more than one contact at a time with a given
   instance ID.  It is RECOMMENDED that the most recently registered
   contact be used.  It is expected that standards track extensions will
   be developed that provide additional criteria for selecting which
   contact to use [18].

      NOTE: The contact that is the most recently registered is the one
      that has been bound to the AOR for the shortest period of time.
      This corresponds to the minimum value for the "duration-
      registered" attribute from the registration event package [29].
      It is important to note that a refresh of the contact in a
      REGISTER message does not reset the duration it has been
      registered to zero.  For example, if a softphone is started at 9
      am when a user logs into their computer, and the softphone
      refreshes its registration every hour, by 12:30 pm the contact has
      been registered for three and a half hours.




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   If the request URI is within the domain of the proxy, and the URI has
   been constructed by the domain such that the proxy is able to
   determine that it has the form of a GRUU for an AOR that is unknown
   within the domain, the proxy rejects the request with a 404 (Not
   Found).  If the request URI is within the domain of the proxy, and
   the URI has been constructed by the domain such that the proxy is
   able to determine that it has the form of a GRUU for an AOR that is
   known within the domain, but the instance ID is unknown, the proxy
   SHOULD generate a 480 (Temporarily Unavailable).

   If the GRUU does exist, handling of the GRUU proceeds as specified in
   RFC 3261 Section 16.  For GRUUs, the abstract location service
   described in Section 16.5 is utilized, producing a set of zero or
   more contacts, each of which is associated with the same instance ID.
   If there is more than one contact bound to the GRUU, the proxy MUST
   populate the target set such that there is never more than one
   contact at a time.  It is RECOMMENDED that the most recently
   registered contact be used.  This produces zero or one contacts.  The
   server MUST copy the "grid" parameter from the Request URI (if
   present) into the new target URI obtained from the registered
   contact.  If the grid was already present in the contact bound to the
   GRUU, it is overwritten in this process.  If no contacts were bound
   to the GRUU, the lookup of the GRUU in the abstract location service
   will result in zero target URIs, eventually causing the proxy to
   reject the request with a 480 (Temorarily Unavailable) response.

   If the contact was registered using a Path header field [3], then
   that Path is used to construct the route set for reaching the contact
   through the GRUU, as well as through the AOR, using the procedures
   specified in RFC 3327 [3].  However, support for GRUU at a registrar
   does not require support for RFC 3327.

   A proxy MAY apply other processing to the request, such as execution
   of called party features, as discussed in Section 6.

   A request sent to a GRUU SHOULD NOT be redirected.  In many
   instances, a GRUU is used by a UA in order to assist in the traversal
   of NATs and firewalls, and a redirection may prevent such a case from
   working.

8.4.2  Record Routing

   As described above, a user agent uses its GRUU as a remote target.
   This has an impact on the path taken by subsequent mid-dialog
   requests.  Depending on the desires of the proxies involved, this may
   impact record route processing.

   Two cases can be considered.  The first is shown in Figure 2.  In



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   this case, there is a single proxy in the user's domain.  An incoming
   INVITE request arrives for the users AOR (message 1) and is forwarded
   to the user agent at its registered contact C1 (message 2).  The
   proxy inserts a Record-Route header field into the proxied request,
   with a value of R1.  The user agent generates a 200 OK to the
   request, using its GRUU G1 as the remote target.


        (1) + (2): initial INVITE
        (2) + (3): mid-dialog request




       (1)  +-----------+       (2)      +-----------+
     ------>|   Proxy   |--------------->|   User    |
            |           |                |   Agent   |
       (3)  |           |       (4)      |           |
     ------>|R1         |--------------->|G1         |
            |           |                |           |
            +-----------+                +-----------+

                                 Figure 2

   When a mid-dialog request shows up destined for the user agent
   (message 3), it will arrive at the proxy in the following form:


   INVITE G1
   Route: R1

   Since the top Route header field value identifies the proxy, the
   proxy removes it.  As there are no more Route header field values,
   the proxy processes the request URI.  However, the request URI is a
   GRUU, and is therefore a domain under the control of the proxy.  The
   proxy will need to perform the processing of Section 8.4.1, which
   will result in the translation of the GRUU into the contact C1,
   followed by transmission of the request to the user agent (message
   4).

   This sequence of processing in the proxy is somewhat unusual, in that
   mid-dialog requests (that is, requests with a Route header field that
   a proxy inserted as a result of a Record-Route operation) do not
   normally cause a proxy to invoke a location service to process the
   request URI.  Because this processing is unusual, it is explicitly
   called out here.

   The previous case assumed that there was a single proxy in the



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   domain.  In more complicated cases, there can be two or more proxies
   within a domain on the initial request path.  This is shown in
   Figure 3.  In this figure, there is a home proxy to which requests
   targeted to the AOR are sent.  The home proxy executes the abstract
   location service and runs user features.  The edge proxy acts as the
   outbound proxy for users, performs authentication, manages TCP/
   Transport Layer Security (TLS) connections to the client, and does
   other functions associated with the transition from the provider
   proxy network to the client.  This specific division of
   responsibilities between home and edge proxy is just for the purposes
   of illustration; the discussion applies to a disaggregation of proxy
   logic into any number of proxies.  In such a configuration,
   registrations from the user agent would pass through the edge proxy,
   which would insert a Path header field [3] for itself.


       (1) + (2) + (3): initial INVITE
       (4) through (9): mid-dialog request





      (1) +-----------+    (2) +-----------+     (3) +-----------+
     ---->|           |------->|           |-------->|           |
      (4) |           |    (5) |           |         |           |
     ---->|H1   Home  |------->|E1   Edge  |         |     User  |
      (7) |     Proxy |    (8) |     Proxy |     (9) |     Agent |
      +-->|G1         |------->|           |-------->|           |
      |   +-----------+        +-----------+         +-----------+
      |                              |
      |                              |
      +------------------------------+
                     (6)

                                 Figure 3

   When an incoming request arrives for the AOR (message 1), the home
   proxy would look it up, discover the registered contact and Path, and
   then send the request to the edge proxy as a result of the Route
   header field inserted with the Path value.  The home proxy record
   routes with the URI H1.  The edge proxy would forward the request to
   the request URI (which points to the client), and insert a Record-
   Route header field value with the URI E1 (message 2).  This request
   is accepted by the user agent, which inserts its GRUU G1 as the
   remote target.

   When the peer in the dialog sends a mid-dialog request, it will have



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   the following form:


   INVITE G1
   Route: H1, E1

   This request will arrive at the home proxy (due to H1 in the Route
   header field) (message 4).  The home proxy will forward it to the
   edge proxy (due to E1 in the Route header field) (message 5).  The
   edge proxy, seeing no more Route header field values, sends the
   request to the Request URI.  The Request URI will contain a GRUU,
   causing the edge proxy to route the request back around to the home
   proxy (message 6).  The home proxy performs the GRUU processing of
   Section 8.4.1, causing the request to be forwarded to the edge proxy
   a second time (this time, as a result of a Route header field value
   obtained from the Path header in the registration) (message 7), and
   then delivered to the client (message 8).

   Although this flow works, it is highly inefficient because it causes
   each mid-dialog request to spiral route.  This behavior is not
   desirable.  To prevent it, the following procedures SHOULD be
   followed.  When a client generates a REGISTER request, this request
   passes through the edge proxy on its way to the home proxy.  The
   REGISTER request will contain the AOR of the user (in the To header
   field).  If the client is registering an instance, there will be one
   or more Contact header field values with the +sip.instance Contact
   header field parameter, and a Supported header field that includes
   the value "gruu".  Normally there would be just one Contact header
   field value with an instance ID, but it is possible for there to be
   more than one.  However, all Contact header field values are required
   to identify the instance performing the registration.

   The proxy can decide to insert itself on the Path for a particular
   AOR on a case-by-case basis.  However, if it does so for one
   registration, it SHOULD do so for all registrations for the same AOR
   that contain an instance ID in the contact.  The value of the Path
   header field inserted by the proxy SHOULD be constructed so that it
   indicates whether the proxy inserted itself on the Path for this AOR.

   When a request arrives from the home proxy towards the client, the
   proxy inspects the Route header field.  This header field will
   contain the URI the edge proxy had placed into the Path.  If the
   value indicates that the edge proxy had put itself on the Path for
   the registration from this client, there is no need for the proxy to
   retain its record-route in the response.  The proxy MAY remove its
   record-route value from the 200 OK response in this case.  If the
   value indicates that the proxy had not put itself on the Path, it
   would retain the Record-Route in the response.



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   Similarly, if a request arrives from the client towards the home
   proxy, the edge proxy would look at the identity of the sender of the
   request.  If the proxy knows that it is placing itself on the Path
   for registrations from that AOR, and the request contains a Supported
   header field with the value "gruu", the edge proxy would insert a
   Record-Route into the request, and then remove it in the response.
   Similarly, if the identity of the sender of the request is one for
   which the client has not put itself on the Path, the edge proxy would
   keep its Record-Route in the response.

   Removing its Record-Route value from the response will result in the
   route set being seen differently by the caller and callee; the callee
   (which is the user agent in the figure) will have a route set entry
   for its edge proxy, while the caller will not.  The caller will have
   a route set entry for its edge proxy, while the callee will not.

   In such a case, a mid-dialog request that arrives at the home proxy
   will be of the form:


   INVITE G1
   Route: H1

   This does the "right thing" and causes the request to be routed from
   the home proxy to the edge proxy to the client, without the
   additional spiral.

9.  The opaque SIP URI Parameter

   This specification defines a new SIP URI parameter, "opaque".  The
   "opaque" URI parameter is used to construct a URI (called the derived
   URI) that is related to another URI (called the base URI, frequently
   an AOR) in some way.  In this specification, the parameter is used to
   construct the GRUU (the derived URI) from the AOR (the base URI).
   However, there are many other applications outside of GRUU.  It can
   be used, for example, to construct a URI for a voicemail inbox (the
   derived URI) from a subscriber's AOR (the base URI), or the URI for a
   video service advertised via presence [26] (the derived URI) from the
   subscriber's AOR (the base URI).

   To construct a derived URI, the owner of the domain adds the "opaque"
   URI parameter to the base URI, resulting in the derived URI.  In
   fact, these are the only semantics associated with the "opaque" URI
   parameter: a URI containing the parameter MUST be related to another
   URI, obtained by stripping the "opaque" URI parameter.  Because the
   "opaque" URI parameter implies a relationship, any element (including
   those outside the domain that owns the URI) that receives a URI with
   the "opaque" URI parameter will know definitively that it is a



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   derived URI, and can strip it to obtain the base URI.

   The value of the "opaque" URI parameter is not relevant to anyone
   except for the owner of the domain.  It typically contains
   information needed by the owner of the domain to correctly process a
   request targeted to that URI according to the desired semantics of
   the URI.  As such, the parameter is a form of cookie.  In the case of
   a GRUU, the "opaque" URI parameter contains enough information for
   the owner of the domain to determine the instance ID.  Since the
   structure of its value is not subject to standardization, it can only
   be interpreted by the same proxy or cluster of proxies that created
   the derived URI.  For this reason, a proxy or cluster of proxies MUST
   NOT create a derived URI unless a request sent to the base URI (and
   consequently the derived URI) will be routed back to that same proxy
   or cluster of proxies without any upstream proxies requiring
   interpretation of the "opaque" URI parameter.  Simply put, a request
   sent to a derived URI has to get back to the same proxy farm that
   created the derived URI.

   The presence of the "opaque" URI parameter in a URI implies a
   relationship between that URI and its base URI.  However, the nature
   of that relationship cannot be determined from inspection of the URI
   alone.  In some cases, there may be no way to know the relationship
   outside of the domain that constructed the URI.  In other cases, the
   nature of the relationship is determined from the context in which
   the URI was obtained.  When used to construct a GRUU, it means that
   the URI formed by stripping the "opaque" parameter corresponds to the
   AOR associated with the GRUU.  The recipient of a GRUU cannot
   determine that it is a GRUU by direct examination of the URI.
   However, the recipient may know if it received the GRUU in the
   Contact header field of a SIP request or in a response that contained
   a Supported header field with the option tag "gruu".  If it knows its
   a GRUU because it received it in such a SIP message, and the GRUU
   contains the "opaque" parameter, the UA knows it can obtain the AOR
   by removing the "opaque" parameter.

10.  Grammar

   This specification defines two new Contact header field parameters,
   gruu and +sip.instance, and two new URI parameters, "grid" and
   "opaque".  The grammar for string-value is obtained from [11], and
   the grammar for uric is defined in RFC 3986 [9].









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   contact-params  =  c-p-q / c-p-expires / c-p-gruu / cp-instance
                       / contact-extension
   c-p-gruu        =  "gruu" EQUAL DQUOTE (SIP-URI / SIPS-URI) DQUOTE
   cp-instance     =  "+sip.instance" EQUAL LDQUOT "<"
                      instance-val ">" RDQUOT
   uri-parameter   =  transport-param / user-param / method-param
                      / ttl-param / maddr-param / lr-param / grid-param
                      / opaque-param / other-param
   grid-param      = "grid=" pvalue        ; defined in RFC3261
   opaque-param    = "opaque=" pvalue      ; defined in RFC3261
   instance-val    = *uric ; defined in RFC 2396


11.  Requirements

   This specification was created in order to meet the following
   requirements:

   REQ 1: When a UA invokes a GRUU, it MUST cause the request to be
      routed to the specific UA instance to which the GRUU refers.

   REQ 2: It MUST be possible for a GRUU to be invoked from anywhere on
      the Internet, and still cause the request to be routed
      appropriately.  That is, a GRUU MUST NOT be restricted to use
      within a specific addressing realm.

   REQ 3: It MUST be possible for a GRUU to be constructed without
      requiring the network to store additional state.

   REQ 4: It MUST be possible for a UA to obtain a multiplicity of GRUUs
      that each route to that UA instance.  For example, this is needed
      to support ad-hoc conferencing where a UA instance needs a
      different URI for each conference it is hosting.

   REQ 5: When a UA receives a request sent to a GRUU, it MUST be
      possible for the UA to know the GRUU that was used to invoke the
      request.  This is necessary as a consequence of REQ 4.

   REQ 6: It MUST be possible for a UA to add opaque content to a GRUU.
      This content is not interpreted or altered by the network, and is
      used only by the UA instance to whom the GRUU refers.  This
      provides a basic cookie type of functionality, allowing a UA to
      build a GRUU with the state embedded.

   REQ 7: It MUST be possible for a proxy to execute services and
      features on behalf of a UA instance represented by a GRUU.  As an
      example, if a user has call blocking features, a proxy may want to
      apply those call blocking features to calls made to the GRUU, in



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      addition to calls made to the user's AOR.

   REQ 8: It MUST be possible for a UA in a dialog to inform its peer of
      its GRUU, and for the peer to know that the URI represents a GRUU.
      This is needed for the conferencing and dialog reuse applications
      of GRUUs, where the URIs are transferred within a dialog.

   REQ 9: When transferring a GRUU per REQ 8, it MUST be possible for
      the UA receiving the GRUU to be assured of its integrity and
      authenticity.

   REQ 10: It MUST be possible for a server that is authoritative for a
      domain to construct a GRUU which routes to a UA instance bound to
      an AOR in that domain.  In other words, the proxy can construct a
      GRUU, too.  This is needed for the presence application.


12.  Example Call Flow

   The following call flow shows a basic registration and call setup,
   followed by a subscription directed to the GRUU.  It then shows a
   failure of the callee, followed by a re-registration.  The
   conventions of [22] are used to describe representation of long
   message lines.



























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          Caller                 Proxy                Callee
             |                     |(1) REGISTER         |
             |                     |<--------------------|
             |                     |(2) 200 OK           |
             |                     |-------------------->|
             |(3) INVITE           |                     |
             |-------------------->|                     |
             |                     |(4) INVITE           |
             |                     |-------------------->|
             |                     |(5) 200 OK           |
             |                     |<--------------------|
             |(6) 200 OK           |                     |
             |<--------------------|                     |
             |(7) ACK              |                     |
             |-------------------->|                     |
             |                     |(8) ACK              |
             |                     |-------------------->|
             |(9) SUBSCRIBE        |                     |
             |-------------------->|                     |
             |                     |(10) SUBSCRIBE       |
             |                     |-------------------->|
             |                     |(11) 200 OK          |
             |                     |<--------------------|
             |(12) 200 OK          |                     |
             |<--------------------|                     |
             |                     |(13) NOTIFY          |
             |                     |<--------------------|
             |(14) NOTIFY          |                     |
             |<--------------------|                     |
             |(15) 200 OK          |                     |
             |-------------------->|                     |
             |                     |(16) 200 OK          |
             |                     |-------------------->|
             |                     |                     |Crashes,
             |                     |(17) REGISTER        | Reboots
             |                     |<--------------------|
             |                     |(18) 200 OK          |
             |                     |-------------------->|


                                 Figure 4

   The Callee supports the GRUU extension.  As such, its REGISTER (1)
   looks like:







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   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: Callee <sip:callee@example.com>;tag=a73kszlfl
   Supported: gruu
   To: Callee <sip:callee@example.com>
   Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
   CSeq: 1 REGISTER
   Contact: <sip:callee@192.0.2.1>
     ;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
   Content-Length: 0

   The REGISTER response would look like:


   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.0.2.1;branch=z9hG4bKnashds7
   From: Callee <sip:callee@example.com>;tag=a73kszlfl
   To: Callee <sip:callee@example.com> ;tag=b88sn
   Require: gruu
   Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
   CSeq: 1 REGISTER
   <allOneLine>
   Contact: <sip:callee@192.0.2.1>
    ;gruu="sip:callee@example.com;
   opaque=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
   ;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
   ;expires=3600
   </allOneLine>
   Content-Length: 0

   Note how the Contact header field in the REGISTER response contains
   the gruu parameter with the URI sip:callee@
   example.com;opaque=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6.
   This represents a GRUU that translates to the contact
   sip:callee@192.0.2.1.

   The INVITE from the caller is a normal SIP INVITE.  However, the 200
   OK generated by the callee (message 5) now contains a GRUU as the
   remote target.  The UA has also chosen to include a "grid" URI
   parameter into the GRUU.










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   SIP/2.0 200 OK
   Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bKnaa8
   Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK99a
   From: Caller <sip:caller@example.com>;tag=n88ah
   To: Callee <sip:callee@example.com> ;tag=a0z8
   Call-ID: 1j9FpLxk3uxtma7@host.example.com
   CSeq: 1 INVITE
   Supported: gruu
   Allow: INVITE, OPTIONS, CANCEL, BYE, ACK
   <allOneLine>
   Contact:
   <sip:callee@example.com
   ;opaque=urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6;grid=99a>
   </allOneLine>
   Content-Length: --
   Content-Type: application/sdp

   [SDP Not shown]

   At some point later in the call, the caller decides to subscribe to
   the dialog event package [21] at that specific UA.  To do that, it
   generates a SUBSCRIBE request (message 9), but directs it towards the
   remote target, which is a GRUU:


   <allOneLine>
   SUBSCRIBE sip:callee@example.com;opaque=urn:uuid:f8
   1d4fae-7dec-11d0-a765-00a0c91e6bf6;grid=99a
    SIP/2.0
   </allOneLine>
   Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz8
   From: Caller <sip:caller@example.com>;tag=kkaz-
   <allOneLine>
   To: sip:callee@example.com;opaque=urn:uuid:f8
   1d4fae-7dec-11d0-a765-00a0c91e6bf6;grid=99a
   </allOneLine>
   Call-ID: faif9a@host.example.com
   CSeq: 2 SUBSCRIBE
   Supported: gruu
   Event: dialog
   Allow: INVITE, OPTIONS, CANCEL, BYE, ACK
   Contact: <sip:caller@example.com;opaque=hdg7777ad7aflzig8sf7>
   Content-Length: 0

   In this example, the caller itself supports the GRUU extension, and
   is using its own GRUU to populate its remote target.

   This request is routed to the proxy, which proceeds to perform a



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   location lookup on the request URI.  It is translated into the
   contact for that instance, and then proxied to that contact.  Note
   how the "grid" parameter is maintained.


   SUBSCRIBE sip:callee@192.0.2.1;grid=99a SIP/2.0
   Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4bK9555
   Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz8
   From: Caller <sip:caller@example.com>;tag=kkaz-
   <allOneLine>
   To: sip:callee@example.com;opaque=urn:uuid:f8
   1d4fae-7dec-11d0-a765-00a0c91e6bf6;grid=99a
   </allOneLine>
   Call-ID: faif9a@host.example.com
   CSeq: 2 SUBSCRIBE
   Supported: gruu
   Event: dialog
   Allow: INVITE, OPTIONS, CANCEL, BYE, ACK
   Contact: <sip:caller@example.com;opaque=hdg7777ad7aflzig8sf7>
   Content-Length: 0

   At some point after message 16 is received, the callee's machine
   crashes and recovers.  It obtains a new IP address, 192.0.2.2.
   Unaware that it had previously had an active registration, it creates
   a new one (message 17 below).  Notice how the instance ID remains the
   same, as it persists across reboot cycles:


   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.2;branch=z9hG4bKnasbba
   Max-Forwards: 70
   From: Callee <sip:callee@example.com>;tag=ha8d777f0
   Supported: gruu
   To: Callee <sip:callee@example.com>
   Call-ID: hf8asxzff8s7f@192.0.2.2
   CSeq: 1 REGISTER
   Contact: <sip:callee@192.0.2.2>
     ;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
   Content-Length: 0

   The registrar notices that a different contact, sip:callee@192.0.2.1,
   is already associated with the same instance ID.  It registers the
   new one too and returns both in the REGISTER response.  Both have the
   same GRUU.  However, only this new contact (the most recently
   registered one) will be used by the proxy for population in the
   target set.  The registrar then generates the following response:





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   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.0.2.2;branch=z9hG4bKnasbba
   From: Callee <sip:callee@example.com>;tag=ha8d777f0
   To: Callee <sip:callee@example.com>;tag=99f8f7
   Require: gruu
   Call-ID: hf8asxzff8s7f@192.0.2.2
   CSeq: 1 REGISTER
   <allOneLine>
   Contact: <sip:callee@192.0.2.2>
   ;gruu="sip:callee@example.com;opaque=urn:
   uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
   ;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
   ;expires=3600
   </allOneLine>
   Contact: <sip:callee@192.0.2.1>
   ;gruu="sip:callee@example.com;opaque=urn:
   uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
   ;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
   ;expires=400
   </allOneLine>
   Content-Length: 0


13.  Security Considerations

   It is important for a UA to be assured of the integrity of a GRUU
   given in a REGISTER response.  If the GRUU is tampered with by an
   attacker, the result could be denial of service to the UA.  As a
   result, it is RECOMMENDED that a UA use the SIPS URI scheme in the
   Request-URI when registering.  Proxies and registrars MUST support
   the sips URI and MUST support TLS.  Note that this does not represent
   a change from the requirements in RFC 3261.

   The example GRUU construction algorithm in Appendix A.1 makes no
   attempt to create a GRUU that hides the AOR and instance ID
   associated with the GRUU.  In general, determination of the AOR
   associated with a GRUU is considered a good property, since it allows
   for easy tracking of the target of a particular call.  Learning the
   instance ID provides little benefit to an attacker.  To register or
   otherwise impact registrations for the user, an attacker would need
   to obtain the credentials for the user.  Knowing the instance ID is
   insufficient.

   The example GRUU construction algorithm in Appendix A.1 makes no
   attempt to create a GRUU that prevents users from guessing a GRUU
   based on knowledge of the AOR and instance ID.  A user that is able
   to do that will be able to direct a new request at a particular
   instance.  However, this specification recommends that service



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   treatment (in particular, screening features) be given to requests
   that are sent to a GRUU.  That treatment will make sure that the GRUU
   does not provide a back door for attackers to contact a user that has
   tried to block the attacker.

   GRUUs do not provide a solution for privacy.  In particular, since
   the GRUU does not change during the lifetime of a registration, an
   attacker could correlate two calls as coming from the same source,
   which in and of itself reveals information about the caller.
   Furthermore, GRUUs do not address other aspects of privacy, such as
   the addresses used for media transport.  For a discussion of how
   privacy services are provided in SIP, see RFC 3323 [14].

14.  IANA Considerations

   This specification defines a new Contact header field parameter, two
   SIP URI parameters, a media feature tag, and a SIP option tag.

14.1  Header Field Parameter

   This specification defines a new header field parameter, as per the
   registry created by [12].  The required information is as follows:

   Header field in which the parameter can appear: Contact

   Name of the Parameter: gruu

   RFC Reference: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
      RFC number of this specification.]]


14.2  URI Parameters

   This specification defines two new SIP URI parameters, as per the
   registry created by [13].

   Name of the Parameter: grid

   RFC Reference: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
      RFC number of this specification.]]

   Name of the Parameter: opaque

   RFC Reference: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
      RFC number of this specification.]]






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14.3  Media Feature Tag

   This section registers a new media feature tag, per the procedures
   defined in RFC 2506 [8].  The tag is placed into the sip tree, which
   is defined in [11].

   Media feature tag name: sip.instance

   ASN.1 Identifier: New assignment by IANA.

   Summary of the media feature indicated by this tag: This feature tag
      contains a string containing a URN that indicates a unique
      identifier associated with the UA instance registering the
      Contact.

   Values appropriate for use with this feature tag: String.

   The feature tag is intended primarily for use in the following
   applications, protocols, services, or negotiation mechanisms: This
      feature tag is most useful in a communications application, for
      describing the capabilities of a device, such as a phone or PDA.

   Examples of typical use: Routing a call to a specific device.

   Related standards or documents: RFC XXXX [[Note to IANA: Please
      replace XXXX with the RFC number of this specification.]]

   Security Considerations: This media feature tag can be used in ways
      which affect application behaviors.  For example, the SIP caller
      preferences extension [23] allows for call routing decisions to be
      based on the values of these parameters.  Therefore, if an
      attacker can modify the values of this tag, they may be able to
      affect the behavior of applications.  As a result, applications
      which utilize this media feature tag SHOULD provide a means for
      ensuring its integrity.  Similarly, this feature tag should only
      be trusted as valid when it comes from the user or user agent
      described by the tag.  As a result, protocols for conveying this
      feature tag SHOULD provide a mechanism for guaranteeing
      authenticity.


14.4  SIP Option Tag

   This specification registers a new SIP option tag, as per the
   guidelines in Section 27.1 of RFC 3261.






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   Name: gruu

   Description: This option tag is used to identify the Globally
      Routable User Agent URI (GRUU) extension.  When used in a
      Supported header, it indicates that a User Agent understands the
      extension, and has included a GRUU in the Contact header field of
      its dialog-initiating requests and responses.  When used in a
      Require header field of a REGISTER request, it indicates that the
      registrar should assign a GRUU to the Contact URI.


15.  Acknowledgements

   The author would like to thank Rohan Mahy, Paul Kyzivat, Alan
   Johnston, and Cullen Jennings for their contributions to this work.

16.  References

16.1  Normative References

   [1]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [2]   Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
         (SIP): Locating SIP Servers", RFC 3263, June 2002.

   [3]   Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP)
         Extension Header Field for Registering Non-Adjacent Contacts",
         RFC 3327, December 2002.

   [4]   Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
         Method", RFC 3311, October 2002.

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

   [6]   Roach, A., "Session Initiation Protocol (SIP)-Specific Event
         Notification", RFC 3265, June 2002.

   [7]   Sparks, R., "The Session Initiation Protocol (SIP) Refer
         Method", RFC 3515, April 2003.

   [8]   Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag
         Registration Procedure", BCP 31, RFC 2506, March 1999.

   [9]   Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
         Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,



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         January 2005.

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

   [11]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
         User Agent Capabilities in the Session Initiation Protocol
         (SIP)", RFC 3840, August 2004.

   [12]  Camarillo, G., "The Internet Assigned Number Authority (IANA)
         Header Field Parameter Registry for the Session Initiation
         Protocol (SIP)", BCP 98, RFC 3968, December 2004.

   [13]  Camarillo, G., "The Internet Assigned Number Authority (IANA)
         Uniform Resource Identifier (URI) Parameter Registry for the
         Session Initiation Protocol (SIP)", BCP 99, RFC 3969,
         December 2004.

16.2  Informative References

   [14]  Peterson, J., "A Privacy Mechanism for the Session Initiation
         Protocol (SIP)", RFC 3323, November 2002.

   [15]  Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN
         - Simple Traversal of User Datagram Protocol (UDP) Through
         Network Address Translators (NATs)", RFC 3489, March 2003.

   [16]  Hakala, J., "Using National Bibliography Numbers as Uniform
         Resource Names", RFC 3188, October 2001.

   [17]  Rosenberg, J., "A Framework for Conferencing with the Session
         Initiation Protocol",
         draft-ietf-sipping-conferencing-framework-05 (work in
         progress), May 2005.

   [18]  Jennings, C. and R. Mahy, "Managing Client Initiated
         Connections in the Session Initiation Protocol  (SIP)",
         draft-ietf-sip-outbound-00 (work in progress), July 2005.

   [19]  Rosenberg, J., "Request Authorization through Dialog
         Identification in the Session  Initiation Protocol (SIP)",
         draft-ietf-sip-target-dialog-01 (work in progress), July 2005.

   [20]  Peterson, J. and C. Jennings, "Enhancements for Authenticated
         Identity Management in the Session Initiation  Protocol (SIP)",
         draft-ietf-sip-identity-05 (work in progress), May 2005.

   [21]  Rosenberg, J., "An INVITE Inititiated Dialog Event Package for
         the Session Initiation  Protocol (SIP)",



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         draft-ietf-sipping-dialog-package-06 (work in progress),
         April 2005.

   [22]  Sparks, R., "Session Initiation Protocol Torture Test
         Messages", draft-ietf-sipping-torture-tests-07 (work in
         progress), May 2005.

   [23]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
         Preferences for the Session Initiation Protocol (SIP)",
         RFC 3841, August 2004.

   [24]  Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and
         J. Peterson, "Presence Information Data Format (PIDF)",
         RFC 3863, August 2004.

   [25]  Sparks, R., "Session Initiation Protocol Call Control -
         Transfer", draft-ietf-sipping-cc-transfer-05 (work in
         progress), July 2005.

   [26]  Rosenberg, J., "A Data Model for Presence",
         draft-ietf-simple-presence-data-model-04 (work in progress),
         August 2005.

   [27]  Rosenberg, J., "A Presence Event Package for the Session
         Initiation Protocol (SIP)", RFC 3856, August 2004.

   [28]  Leach, P., Mealling, M., and R. Salz, "A Universally Unique
         IDentifier (UUID) URN Namespace", RFC 4122, July 2005.

   [29]  Rosenberg, J., "A Session Initiation Protocol (SIP) Event
         Package for Registrations", RFC 3680, March 2004.

   [30]  Mahy, R. and D. Petrie, "The Session Initiation Protocol (SIP)
         "Join" Header", RFC 3911, October 2004.

   [31]  Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
         Protocol (SIP) "Replaces" Header", RFC 3891, September 2004.














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Author's Address

   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone: +1 973 952-5000
   Email: jdrosen@cisco.com
   URI:   http://www.jdrosen.net

Appendix A.  Example GRUU Construction Algorithms

   The mechanism for constructing a GRUU is not subject to
   specification.  This appendix provides two examples that can be used
   by a registar.  Of course, others are permitted, as long as they meet
   the constraints defined for a GRUU.

A.1  Instance ID in "opaque" URI Parameter

   The most basic approach for constructing a GRUU is to utilize the
   "opaque" URI parameter.  The user and domain portions of the URI are
   equal to the AOR, and the "opaque" parameter is populated with the
   instance ID.

A.2  Encrypted Instance ID and AOR

   In many cases, it will be desirable to construct the GRUU in such a
   way that it will not be possible, based on inspection of the URI, to
   determine the Contact URI that the GRUU translates to.  It may also
   be desirable to construct it so that it will not be possible to
   determine the instance ID/AOR pair associated with the GRUU.  Whether
   a GRUU should be constructed with this property is a local policy
   decision.

   With these rules, it is possible to construct a GRUU without
   requiring the maintenance of any additional state.  To do that, the
   URI would be constructed in the following fashion:

      user-part = "GRUU" | BASE64(E(K, (salt | " " | AOR | " " |
      instance ID)))

   Where E(K,X) represents a suitable encryption function (such as AES
   with 128-bit keys) with key K applied to data block X, and the "|"
   operator signifies concatenation.  The single space (" ") between
   components is used as a delimiter, so that the components can easily
   be extracted after decryption.  Salt represents a random string that



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   prevents a client from obtaining pairs of known plaintext and
   ciphertext.  A good choice would be at least 128 bits of randomness
   in the salt.

   This mechanism uses the user-part of the SIP URI to convey the
   encrypted AOR and instance ID.  The user-part is used instead of the
   "opaque" URI parameter because it has the desired anonymity
   properties.

   The benefit of this mechanism is that a server need not store
   additional information on mapping a GRUU to its corresponding
   contact.  The user-part of the GRUU contains the instance ID and AOR.
   Assuming that the domain stores registrations in a database indexed
   by the AOR, the proxy processing the GRUU would look up the AOR,
   extract the currently registered contacts, and find the one that
   matches the instance ID encoded in the request URI.  The contact
   whose instance ID is that instance ID is then used as the translated
   version of the GRUU.  Encryption is needed to prevent attacks whereby
   the server is sent requests with fake GRUUs, causing the server to
   direct requests to any named URI.  Even with encryption, the proxy
   should validate the user part after decryption.  In particular, the
   AOR should be managed by the proxy in that domain.  Should a UA send
   a request with a fake GRUU, the proxy would decrypt and then discard
   it because there would be no URI or an invalid URI inside.

   While this approach has many benefits, it has the drawback of
   producing fairly long GRUUs.
























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