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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                                               dynamicsoft
Expires: December 31, 2004                                  July 2, 2004


  Obtaining and Using Globally Routable User Agent (UA) URIs (GRUU) in
                 the Session Initiation Protocol (SIP)
                         draft-ietf-sip-gruu-02

Status of this Memo

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   This Internet-Draft will expire on December 31, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   Several applications of the Session Initiation Protocol (SIP) require
   a user agent (UA) to construct and distribute a URI which can be used
   by anyone on the Internet to route a call to that specific UA
   instance.  A URI which 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  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Defining a GRUU  . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     4.1   REFER  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     4.2   Conferencing . . . . . . . . . . . . . . . . . . . . . . .  4
     4.3   Presence . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  5
   6.  Creation of a GRUU . . . . . . . . . . . . . . . . . . . . . .  6
   7.  Obtaining a GRUU . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1   Through Registrations  . . . . . . . . . . . . . . . . . .  9
       7.1.1   User Agent Behavior  . . . . . . . . . . . . . . . . .  9
       7.1.2   Registrar Behavior . . . . . . . . . . . . . . . . . . 11
     7.2   Administratively . . . . . . . . . . . . . . . . . . . . . 12
   8.  Using the GRUU . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1   Sending a Message Containing a GRUU  . . . . . . . . . . . 13
     8.2   Sending a Message to a GRUU  . . . . . . . . . . . . . . . 14
     8.3   Receiving a Request Sent to a GRUU . . . . . . . . . . . . 14
     8.4   Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . 15
   9.  425 (Instance Conflict) Response Code  . . . . . . . . . . . . 15
   10.   Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   11.   Requirements . . . . . . . . . . . . . . . . . . . . . . . . 16
   12.   Example Call Flow  . . . . . . . . . . . . . . . . . . . . . 17
   13.   Security Considerations  . . . . . . . . . . . . . . . . . . 23
   14.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 23
     14.1  Header Field Parameter . . . . . . . . . . . . . . . . . . 23
     14.2  Response Code  . . . . . . . . . . . . . . . . . . . . . . 23
     14.3  URI Parameter  . . . . . . . . . . . . . . . . . . . . . . 24
     14.4  Media Feature Tag  . . . . . . . . . . . . . . . . . . . . 24
     14.5  SIP Option Tag . . . . . . . . . . . . . . . . . . . . . . 25
   15.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
   16.   References . . . . . . . . . . . . . . . . . . . . . . . . . 25
   16.1  Normative References . . . . . . . . . . . . . . . . . . . . 25
   16.2  Informative References . . . . . . . . . . . . . . . . . . . 26
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 27
   A.  Example GRUU Construction Algorithms . . . . . . . . . . . . . 27
     A.1   Encrypted Instance ID and AOR  . . . . . . . . . . . . . . 27
     A.2   Hashed Indices . . . . . . . . . . . . . . . . . . . . . . 28
       Intellectual Property and Copyright Statements . . . . . . . . 29










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1.  Introduction

   Several applications of the Session Initiation Protocol (SIP) [1]
   require a user agent (UA) to construct and distribute a URI which can
   be used by anyone on the Internet to route a call to that specific UA
   instance.  An example of such an application is call transfer [18],
   based on the REFER method [5].  Another application is the usage of
   endpoint-hosted conferences within the conferencing  framework [14].
   We call these URIs Globally Routable UA URIs (GRUU).  This
   specification provides a mechanism for obtaining and using GRUUs.

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 [3] and
   indicate requirement levels for compliant implementations.

3.  Defining a GRUU

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

   Routes to a Single Instance: It routes to a specific UA instance, and
      never forks.  In that regard, it is unlike an address-of-record.
      When a request is sent to a normal AOR which represents a user,
      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 GRUU, the routing logic is
      dictated by the properties of a GRUU.  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 GRUU.  This
      does not mean that a GRUU represents a fundamentally different
      type of URI; it only means that the logic a proxy applies to a
      GRUU is going to generally be simpler than that it applies to a
      normal AOR.

4.  Use Cases

   We have encountered several use cases for a GRUU.

4.1  REFER

   Consider a blind transfer application [18].  User A is talking to



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   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 identifiers 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 which user B is using to talk to user A.
   If it didn't, the transfer service would 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 is a GRUU.

4.2  Conferencing

   A similar need arises in conferencing [14].  In that framework, a
   conference is described by a URI which 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 [2] on both dialogs, and provides user B and user
   C with an updated Contact URI that now holds the conference URI.  The
   Contact URI also has a callee capabilities [9] 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 didn't, basic conferencing features would fail.
   Therefore, this URI is a GRUU.

4.3  Presence

   In a SIP-based presence [19] system, the Presence Agent (PA)
   generates notifications about the state of a user.  This state is
   represented with the Presence Information Document Format (PIDF)



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   [17].  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 contact URI, which is a SIP URI representing that
   device.  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 is 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
   them.  Similarly, a UA can obtain a GRUU but never use it.  This
   specification defines two mechanisms for obtaining a GRUU - through
   registrations, and through administrative operation.  Only the former
   requires protocol operations.

   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" media
   feature tag in the Contact header field of each Contact for which a
   GRUU is desired.  This media feature tag 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 instace.  That GRUU is guaranteed to remain valid for the
   duration of the registration.  The GRUU is bound to the UA instace.
   Should the client change its Contact URI, 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.



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   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
   normally place the GRUU into the Contact header field of dialog
   creating requests and responses it generates.  However, it is
   important for the UA receiving the message to know whether the
   Contact URI 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 Contact URI is a GRUU.

   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
   corresponding Contact URI, the server will copy the grid parameter
   into the Contact URI.  As a result, when the UA receives the request,
   the Request URI will contain the grid parameter it placed in the
   corresponding GRUU.

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 MUST maintain certain
   information associated with the GRUU.  This information, and its
   relationship with the GRUU, are modeled in Figure 2.


                           +-------------+
                           |             |
                           |             |
                           |    GRUU     |----------------------+
                           |             |                      |
                           |             |                      |
                           +-------------+                      |
                                  | 0..1                        |
                                  |                             |
                                  | associated-with             |
                                  |                             |
                                  |                             |
                                  | 1                           |
                          +----------------+                    |
                          |                |                    |
                 +--------|  instance ID/  |------+             |
                 |        |    AOR Pair    |      |             |
                 |        |                |      |             |
                 |        +----------------+      |             |
                 |                                |             |



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                 |                                |             |
                 |                                |             |translates
                 V                                V             |to
          +--------------+                  +-----------+       |
          |              |                  |           |       |
          |   instance   |                  |    AOR    |       |
          |      ID      |                  |           |       |
          |              |                  +-----------+       |
          +--------------+                        |             |
                 ^                                |             |
                 |                                |             |
                 |                                |             |
                 |                                |is-bound-to  |
                 |        +----------------+      |             |
                 |        |                |      |             |
                 |        |                |      |             |
                 +--------|  Contact URI   |<-----+             |
                          |                | 0..*               |
                          |                |                    |
                          +----------------+                    |
                             0..1 ^                             |
                                  |                             |
                                  +-----------------------------+

                                Figure 2

   The instance ID plays a key role in this specification.  It is an
   indentifier, represented by a URI, that uniquely identifies a SIP
   user agent amongst all other user agents with a Contact URI bound to
   an Address of Record (AOR).  The instance ID allows a domain to
   create a GRUU that maps to the same UA instance, even if the Contact
   URI of that instance changes.  Furthermore, the instance ID allows a
   domain to enforce the restriction that a specific UA instance can
   only be registered once against an AOR.  When elements compliant to
   this specification compare two instance IDs for equality, the
   comparison is done using the equality rules for the scheme associated
   with that URI.

   A GRUU is associated, in a one-to-one fashion, with the combination
   of an Address of Record (AOR) and instance ID.  The GRUU is said to
   be associated with the combination, and the combination is associated
   with the GRUU.  This combination is referred to as an instance ID/AOR
   pair.  The instance ID/AOR pair serve 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 this uniqueness is over the
   instance ID/AOR pair, not just the instance ID.  For example, if a
   user registered the Contact
   sip:ua@pc.example.com;+sip.instance="urn:foo:1", representing a
   device with instance ID urn:foo:1, to the AOR sip:user@example.com,
   and also registered the same Contact, representing the same instance
   ID - sip:ua@pc.example.com;+sip.instance="urn:foo:1" to a second AOR,
   say sip:boss@example.com, each of those UA instances would have a
   different GRUU, since they belong to different AORs.

   A GRUU translates to zero or one Contact URIs.  The Contact URI is a
   temporary URI that can be used to reach the instance ID/AOR pair.
   This URI can change due to changes in the IP address associated with
   the instance ID/AOR pair.  If the instance ID associated with the
   GRUU is the instance ID of a Contact URI currently bound to the AOR
   associated with that GRUU, then the GRUU translates to that Contact
   URI.  If, however, the instance ID associated with the GRUU is not an
   instance ID of a Contact URI currently bound to the AOR associated
   with the GRUU (possibly because there are no Contact URIs bound to
   the AOR), the GRUU maps to no Contact URI, and the GRUU is said to be
   invalid.

   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, exists in the global DNS and
      corresponds to an IP address present on the public Internet.
   o  When a request is sent to this URI, it routes to a proxy server in
      the same domain as that of the registrar.
   o  A proxy server in the domain can determine that the URI is a GRUU.
   o  When a proxy server in this domain receives a request sent to a
      URI that is a GRUU, that URI MUST be translated to the Contact URI
      currently bound to the AOR associated with that GRUU whose
      instance ID is the one associated with the GRUU.

   Once an association from an instance ID/AOR to a GRUU is created,
   that mapping MUST remain in existence, and valid, as long as there
   exists any Contact bound to that AOR whose instance ID is that
   instance ID.  If, through a de-registration or expiration, there is
   no longer any Contact bound to that AOR whose instance ID is that
   instance ID, the registrar MUST remove the mapping, and invalidate
   the GRUU.  However, at any time in the future, should a Contact
   become bound to that same AOR, and that Contact is associated with
   the same instance ID, the domain SHOULD create the same GRUU that was
   previously associated with that instance ID/AOR pair.  Indeed, this
   requirement would ideally be a MUST if it was achieveable, but even
   with the stateless algorithm described above, key rotation or server



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   failures may cause the GRUU associated with an instance ID/AOR pair
   to change.  The value of associating the GRUU with an instance ID/AOR
   pair, as opposed to a Contact URI/AOR pair, is that the association
   can transcend changes in IP address.  As a result, domains SHOULD
   make every effort possible to maintain the association for as long as
   possible.

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
   therefore registers to the network to bind itself to an AOR, a GRUU
   is provided to the user agent through SIP REGISTER messages.

7.1.1  User Agent Behavior

   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 [9].  As described in [9], 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 URI
   [7].  [9] defines equality rules for callee capabilities parameters,
   and according to that specification, the "sip.instance" media feature
   tag will be compared by case sensitive string comparison.  Those
   equality rules apply only to the generic usages defined there and in
   the caller preferences specification [16].  When the instance ID is
   used in this specification, it is effectively "extracted" from the
   value in the "sip.instance" media feature tag, and thus equality
   comparisons are performed using the rules for URI equality specific
   to the scheme in the URI.

   It is RECOMMENDED that the URI be a Uniform Resource Name (URN) [8].
   This specification makes no normative recommendation on the specific
   URI or URN that is to be used.  However, the URI MUST be selected
   such that the instance can be certain that no other instance
   registering against the same AOR would choose the same URI value.
   Usage of a URN is RECOMMENDED since it provides a persistent and
   unique name for the UA instance, allowing it to obtain the same GRUU



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   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.

   One URN that readily meets the requirements of this specification is
   the UUID URN [20], which allows for non-centralized computation of a
   URN based on time, unique names (such as a MAC address) or a random
   number generator.  An example of a URN that would not meet the
   requirements of this specification is the national bibliographic
   number [13].  Since there is no clear relationship between an 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.

   Besides the presence of the "gruu" option tag in the Supported header
   field and the "+sip.instance" Contact header field parameter, the
   REGISTER request is constructed identically to the case where this
   extension was not understood.  Specifically, the Contact URI in the
   REGISTER request SHOULD NOT contain the gruu Contact header field
   parameter.  Any such parameters are ignored by the registrar, as the
   UA cannot propose a GRUU for usage with the Contact URI.

   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.

   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 URI that represents
   a GRUU corresponding to the UA instance that registered the contact.
   Any requests sent to the GRUU URI will be routed by the domain to the
   Contact URI currently bound to that instance ID.  The GRUU will not
   normally 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, 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 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



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   invalidate the GRUU.

   If the response to the REGISTER request was a 425, it means that one
   of the Contact URI in the REGISTER request contained an instance ID
   that was already associated with a different registered Contact.  It
   is up to the client to resolve this conflict.  The conflict normally
   arises when a client registers a Contact with its instance ID,
   crashes, and reboots.  After reboot, it obtains a new IP address, and
   attempts to register a Contact for that address, containing the same
   instance ID.  In such a case, the proper course of action is to
   remove the old registration.  To do that, the client can send a
   REGISTER request with no Contacts.  The 200 OK contains the list of
   currently registered Contacts, including their instance IDs.  The
   client can find the existing contact that matches its instance ID,
   and then send a new REGISTER request.  This request would include the
   old Contact, with the instance ID, and an expires value of 0.  Then,
   the client can retry its failed registration.

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.

   When a registrar compliant to this specification receives a REGISTER
   request, it checks for the presence of the Require header field in
   the request.  If present, and if it contains the "gruu" option tag,
   the registrar MUST follow the procedures in the remainder of this
   section (that is, the procedures which result in the creation of new
   GRUUs for Contacts indicating an instance ID, and the listing of
   GRUUs in the REGISTER response).  If not present, but a Supported
   header field was present with the "gruu" option tag, the registrar
   SHOULD follow the procedures in the remainder of this section.  If
   the Supported header field was not present, or it if was present but
   did not contain the value "gruu", the registrar SHOULD NOT follow the
   procedures in the remainder of this section.

   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 additionally checks whether each contact contains a
   "+sip.instance" header field parameter.  If it does, the registrar
   takes the value of that parameter as an instance ID.  The registrar
   checks to see if there is any other contact bound to the same AOR
   with the same instance ID (recall that equality is computed using URI
   equality for the scheme in question).  If there is, this is an error
   condition.  Only a single Contact URI at a time can be registered for



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   each instance ID.  As a result, the registrar MUST reject the request
   with a 425 (Instance Conflict) error response.  This response code
   informs the client that its registration failed because the instance
   ID provided in the request is already registered to a different
   Contact.  It is up to the client to decide how to proceed.

   If there is no other contact bound to the same AOR with the same
   instance ID, the server allocates and/or creates a GRUU for that
   instance ID/AOR pair 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.  In addition to storing the contact
   URI, the server MUST store the instance ID.

   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 URI, the server MUST add a "gruu" Contact header field
   parameter.  This parameter contains the instance ID for the user
   agent.  The value of the gruu parameter is a quoted string containing
   the URI that is the GRUU for the associated instance ID/AOR pair.

   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 ones with a specific
   instance ID, are deleted.

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

7.2  Administratively

   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 URI it should be translated to.  It is not
   strictly necessary to also configure the instance ID and AOR, since



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   the translation can be done directly.  However, they serve as a
   useful tool for determining which resource and UA instance the GRUU
   is supposed to map to.

   In addition to configuring the GRUU and its associated Contact URI in
   the proxy serving the domain, the GRUU will also need to be
   configured into the UA instance associated with the 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 URI.
   However, a UA compliant to this specification MUST use a GRUU when
   populating the Contact header field of dialog-creating requests and
   responses.  This includes the INVITE request and its 2xx response,
   the SUBSCRIBE [4] request, its 2xx response, the NOTIFY request, and
   the REFER [5] request and its 2xx response.  Similarly, in those
   requests and responses where the GRUU is used in the Contact header
   field, 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 or not its peer in the dialog uses a GRUU before inserting
   one into the Contact header field.

   When placing a GRUU into the Contact header field of a request or
   response, a UA MAY add the "grid" URI parameter to the GRUU.  This
   parameter MAY take on any value permitted by the grammar for the
   parameter.  Note that there are no limitations on the size of this
   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 corresponding Contact URI.  However, it 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 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.  It is anticipated that this limitation will be
   addressed in future specifications.

   Once a UA knows that the Contact URI provided by its peer is a GRUU,



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   it can use it 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 will know that the Contact header field contained a GRUU
   if the request or response had a Supported header field that included
   the value "gruu".  The UA can take the GRUU, and send a request to
   it, and then be sure that it is delivered to the UA instance which
   sent the request or response.

   Since 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 [16] which 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, and do not work
   with the some reliability as GRUU.  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 UAS receives a request sent to its GRUU, the incoming request
   URI will be equal to the Contact URI that was registered (through
   REGISTER or some other action) by that UA instance.  If the user
   agent had previously handed out its GRUU with 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 whether the request was sent to the AOR or to the GRUU.
   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.






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8.4  Proxy Behavior

   When a proxy server receives a request, and the proxy owns the domain
   in the Request URI, and the proxy 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 MUST check if the Request URI is a
   GRUU created by that domain.

   If the URI is a GRUU, the proxy MUST determine if there is still a
   Contact URI bound to AOR associated with the GRUU, whose instance ID
   is the instance ID associated with the GRUU.  If that AOR no longer
   has any contacts bound to it, or if it does have contacts bound to
   it, but none of them have an instance ID equal to the instance ID
   associated with the GRUU, the proxy MUST generate a 480 (Temorarily
   Unavailable) response to the request.  If, however, the proxy does
   not recognize the GRUU as one it had constructed previously for the
   domain, the proxy MUST generate a 404 (Not Found) response to the
   request.

   Otherwise, the proxy MUST populate the target set with a single URI.
   This URI MUST be equal to the Contact URI that is translated from the
   GRUU.  Furthermore, if the GRUU contained a "grid" URI parameter, the
   URI in the target set MUST also contain the same parameter with the
   same value.

   A proxy MAY apply other processing to the request, such as execution
   of called party features.  In particular, it is RECOMMENDED that
   non-routing called party features, such as call logging and
   screening, that are associated with the AOR are also applied to
   requests for all GRUUs associated with that AOR.

   In many cases, a proxy will record-route an initial INVITE request,
   and the user agents will insert a GRUU into the Contact header field.
   When this happens, a mid-dialog request will arrive at the proxy with
   a Route header field that was inserted by the proxy, and a
   Request-URI that represents a GRUU.  Proxies follow normal processing
   in this case; they will strip the Route header field, and then
   process the Request URI as described above.

   The procedures of RFC 3261 are then followed to proxy the request.
   The request SHOULD NOT be redirected in this case.  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.

9.  425 (Instance Conflict) Response Code

   This specification defines a new response code for SIP.  The response



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   code is 425, and it has a default reason phrase of "Instance
   Conflict".  This response code is valid only for REGISTER responses.
   It informs the UA that its registration failed because the instance
   ID provided in the request is already registered to a different
   Contact.

10.  Grammar

   This specification defines two new Contact header field parameters,
   gruu and +sip.instance, and a new URI parameter, grid.  The grammar
   for string-value is obtained from [9], and the grammar for uric is
   defined in RFC 2396 [7].


   contact-params    =  c-p-q / c-p-expires / c-p-gruu / cp-instance
                         / contact-extension
   c-p-gruu          =  "gruu" EQUAL DQUOTE SIP-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
                        / other-param
   grid-param        = "grid=" 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, each one of which routes to that UA instance.  This is
      needed to support ad-hoc conferencing, for example, where a 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 which was used to invoke the
      request.  This is necessary as a consequence of requirement 4.
   REQ 6: It MUST be possible for a UA to add opaque content to a GRUU,
      which is not interpreted or altered by the network, and 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



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      state embedded within it.
   REQ 7: It MUST be possible for a proxy to execute services and
      features on behalf of a UA instace 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
      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 requirement 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, authoritative for a domain,
      to construct a GRUU which routes to a UA instace 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.


          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          |
             |                     |<--------------------|



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             |(12) 200 OK          |                     |
             |<--------------------|                     |
             |                     |(13) NOTIFY          |
             |                     |<--------------------|
             |(14) NOTIFY          |                     |
             |<--------------------|                     |
             |(15) 200 OK          |                     |
             |-------------------->|                     |
             |                     |(16) 200 OK          |
             |                     |-------------------->|
             |                     |                     |Crashes, Reboots
             |                     |(17) REGISTER        |
             |                     |<--------------------|
             |                     |(18) 425             |
             |                     |-------------------->|
             |                     |(19) REGISTER        |
             |                     |<--------------------|
             |                     |(20) 200 OK          |
             |                     |-------------------->|
             |                     |(21) REGISTER        |
             |                     |<--------------------|
             |                     |(22) 200 OK          |
             |                     |-------------------->|
             |                     |(23) REGISTER        |
             |                     |<--------------------|
             |                     |(24) 200 OK          |
             |                     |-------------------->|

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


   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:






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   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
   Call-ID: 1j9FpLxk3uxtm8tn@192.0.2.1
   CSeq: 1 REGISTER
   Contact: <sip:callee@192.0.2.1>
     ;gruu="sip:hha9s8d=-999a@example.com"
     ;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
     ;expires=3600
   Content-Length: 0

   Note how the Contact header field in the REGISTER response contains
   the gruu parameter with the URI sip:hha9s8d=-999a@example.com.  This
   represents a GRUU that translates to the Contact URI
   sip:callee@192.0.2.1.

   The INVITE from the caller is a normal SIP INVITE.  The 200 OK
   generated by the callee, however, now contains a GRUU in the Contact
   header field.  The UA has also chosen to include a grid URI parameter
   into the GRUU.


   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
   Contact: <sip:hha9s8d=-999a@example.com;grid=99a>
   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 [15] at that specific UA.  To do that, it
   generates a SUBSCRIBE request (message 9), but directs it towards the
   GRUU contained in the Contact header field.









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   SUBSCRIBE sip:hha9s8d=-999a@example.com;grid=99a SIP/2.0
   Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz8
   From: Caller <sip:caller@example.com>;tag=kkaz-
   To: Callee <sip:callee@example.com>
   Call-ID: faif9a@host.example.com
   CSeq: 2 SUBSCRIBE
   Supported: gruu
   Event: dialog
   Allow: INVITE, OPTIONS, CANCEL, BYE, ACK
   Contact: <sip:bad998asd8asd0000a0@example.com>
   Content-Length: 0

   In this example, the caller itself supports the GRUU extension, and
   is using its own GRUU to populate the Contact header field of the
   SUBSCRIBE.

   This request is routed to the proxy, which proceeds to perform a
   location lookup on the request URI.  It is translated into the
   Contact URI of that GRUU, and then proxied there (message 10 below).
   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-
   To: Callee <sip:callee@example.com>
   Call-ID: faif9a@host.example.com
   CSeq: 2 SUBSCRIBE
   Supported: gruu
   Event: dialog
   Allow: INVITE, OPTIONS, CANCEL, BYE, ACK
   Contact: <sip:bad998asd8asd0000a0@example.com>
   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:











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   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.  Thus, it rejects
   the request in message 18, below:


   SIP/2.0 425 Instance Conflict
   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=776554
   Call-ID: hf8asxzff8s7f@192.0.2.2
   CSeq: 1 REGISTER

   Next, the client formulates a new REGISTER request, to query for the
   existing set of registrations (message 19, below):


   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.2;branch=z9hG4bKnasbbb
   Max-Forwards: 70
   From: Callee <sip:callee@example.com>;tag=ha8d777f1
   Supported: gruu
   To: Callee <sip:callee@example.com>
   Call-ID: hf8asxzff8s7g@192.0.2.2
   CSeq: 2 REGISTER

   This generates a 200 (OK) response (message 20, below) that includes
   the existing contact:


   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.0.2.2;branch=z9hG4bKnasbbb
   From: Callee <sip:callee@example.com>;tag=ha8d777f1
   To: Callee <sip:callee@example.com>;tag=8asd7d666
   Call-ID: hf8asxzff8s7g@192.0.2.2
   CSeq: 2 REGISTER
   Contact: <sip:callee@192.0.2.1>



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     ;gruu="sip:hha9s8d=-999a@example.com"
     ;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
     ;expires=2000

   The client realizes that a different IP address is registered with
   the same instance ID.  Since the client knows that its instance ID is
   globally unique, it deletes that registration (message 21, below):


   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.2;branch=z9hG4bKnasbbc
   Max-Forwards: 70
   From: Callee <sip:callee@example.com>;tag=ha8d777f2
   Supported: gruu
   To: Callee <sip:callee@example.com>
   Call-ID: hf8asxzff8s7g@192.0.2.2
   CSeq: 3 REGISTER
   Contact: <sip:callee@192.0.2.1>
     ;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
     ;expires=0

   This deletes the contact, as indicated by the lack of of the Contact
   header field in the resulting 200 OK (message 22, below):


   SIP/2.0 200 OK
   Via: SIP/2.0/UDP 192.0.2.2;branch=z9hG4bKnasbbc
   From: Callee <sip:callee@example.com>;tag=ha8d777f2
   To: Callee <sip:callee@example.com>;tag=7asdnj7d6f
   Call-ID: hf8asxzff8s7g@192.0.2.2
   CSeq: 3 REGISTER

   Finally, the client can retry its original registration (message 23,
   below):


   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.2;branch=z9hG4bKnasbbd
   Max-Forwards: 70
   From: Callee <sip:callee@example.com>;tag=ha8d777f3
   Supported: gruu
   To: Callee <sip:callee@example.com>
   Call-ID: hf8asxzff8s7g@192.0.2.2
   CSeq: 4 REGISTER
   Contact: <sip:callee@192.0.2.2>
     ;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"

   This time, the registration succeeds, and the client is registered.



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   The response, message 24, is shown below:


   REGISTER sip:example.com SIP/2.0
   Via: SIP/2.0/UDP 192.0.2.2;branch=z9hG4bKnasbbd
   From: Callee <sip:callee@example.com>;tag=ha8d777f3
   To: Callee <sip:callee@example.com>;tag=asd7salll
   Call-ID: hf8asxzff8s7g@192.0.2.2
   CSeq: 4 REGISTER
   Contact: <sip:callee@192.0.2.2>
     ;+sip.instance="<urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6>"
     ;expires=3600


13.  Security Considerations

   GRUUs do not provide a complete or reliable 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 [12].

   It is important for a UA to be assured of the integrity of a GRUU
   when it is given one 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 when
   registering.

14.  IANA Considerations

   This specification defines a new Contact header field parameter, a
   new SIP response code, a SIP URI parameter, 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 [10].  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  Response Code

   This specification defines the new SIP response code, 425, per the



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   guidelines in Section 27.4 of RFC 3261.
   RFC Number: This specification, RFC XXXX [[NOTE to IANA: Please
      replace XXXX with the RFC number for this specification.]].
   Response Code Number: 425
   Default Reason Phrase: Instance Conflict

14.3  URI Parameter

   This specification defines a new SIP URI parameter, as per the
   registry created by [11].
   Name of the Parameter grid
   RFC Reference RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
      RFC number of this specification.]]

14.4  Media Feature Tag

   This section registers a new media feature tag, per the procedures
   defined in RFC 2506 [6].  The tag is placed into the sip tree, which
   is defined in [9].
   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 URI, and ideally 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 [16] 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 of this,
      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.






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14.5  SIP Option Tag

   This specification registers a new SIP option tag, as per the
   guidelines in Section 27.1 of RFC 3261.
   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., "The Session Initiation Protocol (SIP) UPDATE
         Method", RFC 3311, October 2002.

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

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

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

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

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

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

   [9]   Rosenberg, J., "Indicating User Agent Capabilities in the



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         Session Initiation Protocol  (SIP)",
         draft-ietf-sip-callee-caps-03 (work in progress), January 2004.

   [10]  Camarillo, G., "The Internet Assigned Number Authority (IANA)
         Header Field Parameter  Registry for the Session Initiation
         Protocol (SIP)", draft-ietf-sip-parameter-registry-02 (work in
         progress), June 2004.

   [11]  Camarillo, G., "The Internet Assigned Number Authority (IANA)
         Universal Resource Identifier  (URI) Parameter Registry for the
         Session Initiation Protocol (SIP)",
         draft-ietf-sip-uri-parameter-reg-02 (work in progress), June
         2004.

16.2  Informative References

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

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

   [14]  Rosenberg, J., "A Framework for Conferencing with the Session
         Initiation Protocol",
         draft-ietf-sipping-conferencing-framework-01 (work in
         progress), October 2003.

   [15]  Rosenberg, J. and H. Schulzrinne, "An INVITE Inititiated Dialog
         Event Package for the Session Initiation  Protocol (SIP)",
         draft-ietf-sipping-dialog-package-04 (work in progress),
         February 2004.

   [16]  Rosenberg, J., Schulzrinne, H. and P. Kyzivat, "Caller
         Preferences for the Session Initiation Protocol (SIP)",
         draft-ietf-sip-callerprefs-10 (work in progress), October 2003.

   [17]  Sugano, H. and S. Fujimoto, "Presence Information Data Format
         (PIDF)", draft-ietf-impp-cpim-pidf-08 (work in progress), May
         2003.

   [18]  Sparks, R. and A. Johnston, "Session Initiation Protocol Call
         Control - Transfer", draft-ietf-sipping-cc-transfer-02 (work in
         progress), February 2004.

   [19]  Rosenberg, J., "A Presence Event Package for the Session
         Initiation Protocol (SIP)", draft-ietf-simple-presence-10 (work
         in progress), January 2003.




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   [20]  Mealling, M., "A UUID URN Namespace",
         draft-mealling-uuid-urn-03 (work in progress), March 2004.


Author's Address

   Jonathan Rosenberg
   dynamicsoft
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone: +1 973 952-5000
   EMail: jdrosen@dynamicsoft.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.  Others are, of course, permitted, as long as they
   meet the constraints defined for a GRUU.

A.1  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
   or not 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 implies concatenation.  The single space (" ") between
   components is used as a delimeter, so that the components can easily
   be extracted after decryption.  Salt represents a random string that
   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.




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   The benefit of this mechanism is that a server need not store
   additional information on mapping a GRUU to its corresponding Contact
   URI.  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 matching
   the instance ID encoded in the request URI.  The Contact URI whose
   instance ID is that instance ID is then used as the translated
   version of the URI.  Encryption is needed to prevent attacks whereby
   the server is sent requests with faked GRUU, 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 one 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.  The approach in the following section
   produces smaller results, at the cost of additional structures in the
   database.

A.2  Hashed Indices

   As an alternative approach, the server can construct the GRUU by
   computing a cryptographic hash of the AOR and instance ID, taking 64
   bits of the result, and placing a string representation of those 64
   bits into the user part of the URI.

   When a GRUU is created through registration or administrative action,
   the server computes this hash and stores the hash in the database.
   This hash acts the primary key, with the columns of the table
   providing the instance ID, AOR and Contact.  When the registration is
   deleted, the corresponding row from the table is removed.  When a
   request arrives to a proxy, the user part of the URI is looked up in
   the database, and the Contact, AOR and instance ID can be extracted.

   This approach produces GRUUs of relatively short length.  However, it
   requires additional structures to be created and stored in a database
   that would be used by the registrar (at least, new structures are
   needed for efficient operation).  However, it does not require the
   registrar to store anything for longer than the duration of the
   registration.









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