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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: July 6, 2004                                    January 6, 2004


  Obtaining and Using Globally Routable User Agent (UA) URIs (GRUU) in
                 the Session Initiation Protocol (SIP)
                         draft-ietf-sip-gruu-00

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on July 6, 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  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.1  REFER  . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.2  Conferencing . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.3  Presence . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.   Overview of Operation  . . . . . . . . . . . . . . . . . . .   5
   6.   User Agent Behavior  . . . . . . . . . . . . . . . . . . . .   6
   6.1  REGISTER Processing  . . . . . . . . . . . . . . . . . . . .   6
   6.2  Using the GRUU . . . . . . . . . . . . . . . . . . . . . . .   7
   7.   Registrar Behavior . . . . . . . . . . . . . . . . . . . . .   8
   7.1  Creation and Maintenance of GRUUs  . . . . . . . . . . . . .   8
   7.2  Providing GRUUs to User Agents . . . . . . . . . . . . . . .   9
   8.   Proxy Behavior . . . . . . . . . . . . . . . . . . . . . . .  10
   9.   Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   10.  Requirements . . . . . . . . . . . . . . . . . . . . . . . .  11
   11.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   12.  Security Considerations  . . . . . . . . . . . . . . . . . .  15
   13.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  16
   13.1 Header Field Parameter . . . . . . . . . . . . . . . . . . .  16
   13.2 URI Parameter  . . . . . . . . . . . . . . . . . . . . . . .  16
   14.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  16
        Normative References . . . . . . . . . . . . . . . . . . . .  16
        Informative References . . . . . . . . . . . . . . . . . . .  17
        Author's Address . . . . . . . . . . . . . . . . . . . . . .  18
        Intellectual Property and Copyright Statements . . . . . . .  19






















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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, based
   on the REFER method [4]. Another application is the usage of
   endpoint-hosted conferences within the conferencing  framework [8].
   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 [2] and
   indicate requirement levels for compliant implementations.

3. Defining a GRUU

   A GRUU is a SIP URI which has a specific set of 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 call that user. The same is true for a GRUU.

      Temporally Scoped: It may be temporally scoped. In that regard,
      its not like an AOR for a user. The general assumption is that an
      AOR for a user is valid so long as the user resides within that
      domain (of course, policies can be imposed to limit its validity,
      but that is not the default case). However, a GRUU has a limited
      lifetime by default. It can never be valid for longer than the
      duration of the registration of the UA to which it is bound. For
      example, if my PC registers to the SIP network, a GRUU for my PC
      is only valid as long as my PC is registered. If the PC
      unregisters, the GRUU is invalid; calls to it would result in a
      404. If the PC comes back, the GRUU may or may not be valid once
      more. Furthermore, it will frequently be the case that the GRUU
      has a lifetime shorter than the duration of the registration.

      Instance Routing: It routes to a specific UA instance, and never
      forks. In that regard, it is unlike an address-of-record. When a
      call is made to a normal AOR which represents a user, routing
      logic is applied in proxies to deliver the call 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
      call is made to a GRUU, the routing logic is much more static. It



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      has to cause the call to be delivered to a very specific UA
      instance. That UA instance has to be the same UA instance
      throughout the lifetime of the 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 [12]. User A is talking to user
   B. A wants to transfer the call to user C. So, it 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 needs to contain a URI that can be used by C to
   place a call to B. However, this call needs to route to the specific
   UA which B is using to talk to A. If it didn't, the transfer service
   would not execute. This URI is provided to A by B. Because B doesn't
   know who 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 [8]. In that framework, a
   conference is described by a URI which identifies the focus of the
   conference. The focus is a SIP UA at the center of a 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 B.
   They then put B on hold, and call C. Now, A has two separate dialogs
   for two separate calls - one to B, and one to C. A would like to
   conference them. One model is that A morphs itself into a focus. It
   sends a re-INVITE on each existing dialog, and provides both B and C
   with an updated URI that now holds the conference URI. It also has a
   callee capabilities [10] parameter which indicates that this URI is a
   conference URI. A proceeds to mix the media streams from B and C.
   This is called an ad-hoc conference.

   At this point, normal conferencing features can be applied. That
   means that B can send another user, D, the conference URI, perhaps in



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   an email. D can send an INVITE to that URI, and join the conference.
   For this to work, the conference URI used by A in its re-INVITE has
   to be usable by anyone, and it has to route to the specific UA
   instance of A that is acting as the focus. If it didn't, basic
   conferencing features would fail. Therefore, it is a GRUU.

4.3 Presence

   In a SIP-based presence [13] system, the presence agent (PA)
   generates notifications about the state of a user. This state is
   represented with the Presence Information Document Format (PIDF)
   [11]. In a PIDF document, a user is represented by a series of
   tuples, each of which identifies the devices that the user has and
   provides information about them. 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
   device whose presence is represented in the tuple.

   The URI in the presence document therefore has to route to the
   specific UA instance whose presence was reported. 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, in this case, the GRUU needs to be
   constructed by a presence agent. This may be a server in the network,
   or may be on an end-device, such as a PC.

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.

   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. 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
   specific Contact URI. That GRUU is guaranteed to remain valid for the
   duration of the registration.

   Since the GRUU is a URI like any other, it can be handed out by a UA



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   by placing it in any header field which can contain a GRUU. 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. User Agent Behavior

   User agent behavior is divided into two separate parts - REGISTER
   processing, and GRUU usage.

6.1 REGISTER Processing

   When a UA wishes to obtain a GRUU within the domain of its AOR, when
   it 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.
   Besides the presence of this option tag in the Supported header
   field, 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 may contain a
   "gruu" parameter. This parameter contains a SIP URI that represents a
   GRUU corresponding to that Contact URI. Any requests sent to the GRUU
   URI will be routed by the domain to the Contact URI. The GRUU will
   not change in subsequent 2xx responses to REGISTER as long as the UA
   does not let the registration expire. However, if the UA waits until



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   the last moment to refresh its registration, it may cause a race
   condition where the registration expires while the registration is in
   transit. The resulting 200 OK might then contain a different GRUU.
   Since "last moment" is ill defined, it is RECOMMENDED that a UA be
   prepared to handle a change in the GRUU during a registration.
   Handling a change depends on the way in which it has been used. In
   the case where it is included in the Contact URI of a dialog
   initiating request or response, the UA would update the Contact URI
   with a target refresh request.

6.2 Using the GRUU

   A UA first obtains a GRUU  using the procedures of Section 6.1.

   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 [3] request, its 2xx response, and the NOTIFY request, and
   the REFER [4] 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,
   it can use it in any application or SIP extension which requires a



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   globally routable URI to operate. One such example is assisted call
   transfer.

7. Registrar Behavior

   A registrar compliant to this specification is responsible for the
   creation and maintenance of GRUUs, and for providing those GRUU's to
   a UA in response to a REGISTER request.

7.1 Creation and Maintenance of GRUUs

   There is a one-to-one mapping between a Contact URI for a particular
   AOR, and a GRUU. As a result, if two Contact/AOR pairs are different,
   the GRUU for each MUST be different. If two GRUUs are different, the
   Contact/AOR pair for those MUST be different. It is important to
   understand that this uniqueness is over the Contact/AOR pair, not
   just the Contact. For example, if a user registered the Contact
   sip:ua@pc.example.com to the AOR sip:user@example.com, and also
   registered the same Contact - sip:ua@pc.example.com to a second AOR,
   say sip:boss@example.com, each of those Contacts would have a
   different GRUU, since they belong to different AORs.

   A registrar MAY create a GRUU for a particular AOR/Contact 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.

   This specification does not mandate a particular mechanism for
   construction of the GRUU. 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 translates this URI, the result
      is equal to the Contact URI corresponding to the GRUU.

   o  It MUST NOT be possible, based on inspection of the URI, to
      determine the associated Contact URI or Address of Record.

   With these rules, it is possible, though not required, to construct a



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   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 + Contact URI + AOR)))

   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. 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.

   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 can itself contain the Contact 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.

   Once created, the registrar MUST maintain that GRUU for the duration
   over which that Contact remains registered to that AOR at the
   registrar. Furthermore, the registrar MUST NOT change the gruu during
   that duration. This is true even if the Contact is refreshed from a
   rebooted or different UA (known by a change in the Call-ID of the
   REGISTER request). After the Contact expires, the registrar ceases to
   maintain the binding. The registrar is under no obligation to use the
   same GRUU should that Contact be re-registered at a later time. Of
   course, it MAY create the same GRUU if it likes, but this would be an
   implementation choice.

   The implication of these rules is that a registrar is responsible for
   reliable storage of the GRUU for the duration of a registration.

7.2 Providing GRUUs to User Agents

   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 next paragraph for
   inclusion of the "gruu" parameter in a 2xx response to REGISTER. If
   not present, but a Supported header field was present with the "gruu"
   option tag, the registrar SHOULD follow the procedures in the next
   paragraph for inclusion of the "gruu" parameter in a 2xx response to
   REGISTER. If the Supported header field was not present, or it if was



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   present but did not contain the value "gruu", the registrar SHOULD
   NOT follow the procedures of the next paragraph for inclusion of the
   "gruu" parameter in a 2xx response to REGISTER.

   If the register request contained any "gruu" Contact header field
   parameters, these MUST be ignored by the registrar. A UA cannot
   suggest or otherwise provide a GRUU to the registrar.

   A GRUU is provided to a UA by including it in the "gruu" Contact
   header field parameter for a particular Contact URI. The value of
   this parameter is a quoted string containing the URI that is the GRUU
   for the associated Contact URI/AOR pair. If the server does not
   currently have a GRUU associated with the Contact URI, either because
   the Contact URI is being newly registered, or because it is being
   refreshed, but the registrar has not yet computed a GRUU for that
   Contact, one is created according to the procedures of Section 7.1.
   Otherwise, if a GRUU already exists for that AOR/Contact pair, the
   GRUU associated with that pair MUST be placed into the "gruu" Contact
   header field parameter of the REGISTER response.

   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 a
   contact at the request of 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.

8. 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 the Contact URI
   associated with the GRUU is still registered to the AOR it was
   registered to when the GRUU was constructed. If that AOR no longer
   has any registered contacts, or if it does have registered contacts,
   but none of them equal the Contact URI associated with the GRUU, 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 associated with that 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



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   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 the GRUU.

   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 a redirection may prevent such a case from working.

9. Grammar

   This specification defines a new Contact header field parameter,
   gruu, and a new URI parameter, grid.


   contact-params    =  c-p-q / c-p-expires / c-p-gruu
                         / contact-extension
   c-p-gruu          =  "gruu" EQUAL SWS DQUOTE SIP-URI DQUOTE
   uri-parameter     =  transport-param / user-param / method-param
                        / ttl-param / maddr-param / lr-param / grid-param
                        / other-param
   grid-param        = "grid=" pvalue


10. 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.



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   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
      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.


11. Examples

   The following call flow shows a basic registration and call setup,
   followed by a subscription directed to the GRUU.








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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          |
             |                     |-------------------->|

   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 client.example.com;branch=z9hG4bKnashds7
   Max-Forwards: 70
   From: Callee <sip:callee@example.com>;tag=a73kszlfl
   Supported: gruu
   To: Callee <sip:callee@example.com>
   Call-ID: 1j9FpLxk3uxtm8tn@client.example.com
   CSeq: 1 REGISTER
   Contact: <sip:callee@client.example.com>
   Content-Length: 0



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   The REGISTER response would look like:


   SIP/2.0 200 OK
   Via: SIP/2.0/UDP client.example.com;branch=z9hG4bKnashds7
   From: Callee <sip:callee@example.com>;tag=a73kszlfl
   To: Callee <sip:callee@example.com> ;tag=b88sn
   Call-ID: 1j9FpLxk3uxtm8tn@client.example.com
   CSeq: 1 REGISTER
   Contact: <sip:callee@client.example.com>;gruu="sip:hha9s8d=-999a@example.com"
   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 associated with the Contact URI
   sip:callee@client.example.com.

   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 [9] 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 bound to that GRUU, and then proxied there (message 10). Note how
   the grid parameter is maintained.


   SUBSCRIBE sip:callee@client.example.com;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


12. Security Considerations

   Since GRUUs do not reveal information about the identity of the
   associated address-of-record or Contact URI, they provide routability
   without identity. However, 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 [7].



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   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.

13. IANA Considerations

   This specification defines a new Contact header field parameter and
   URI parameter.

13.1 Header Field Parameter

   This specification defines a new header field parameter, as per the
   registry created by [5]. 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.]]


13.2 URI Parameter

   This specification defines a new SIP URI parameter, as per the
   registry created by [6].

   Name of the Parameter grid

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


14. Acknowledgements

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

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



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

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

   [5]  Camarillo, G., "The Internet Assigned Number Authority Header
        Field Parameter Registry for  the Session Initiation Protocol",
        draft-ietf-sip-parameter-registry-01 (work in progress),
        November 2003.

   [6]  Camarillo, G., "The Internet Assigned Number Authority Universal
        Resource Identifier  Parameter Registry for the Session
        Initiation Protocol", draft-ietf-sip-uri-parameter-reg-01 (work
        in progress), November 2003.

Informative References

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

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

   [9]   Rosenberg, J. and H. Schulzrinne, "An INVITE Inititiated Dialog
         Event Package for the Session Initiation  Protocol (SIP)",
         draft-ietf-sipping-dialog-package-03 (work in progress),
         October 2003.

   [10]  Rosenberg, J., "Indicating User Agent Capabilities in the
         Session Initiation Protocol  (SIP)",
         draft-ietf-sip-callee-caps-02 (work in progress), December
         2003.

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

   [12]  Sparks, R. and A. Johnston, "Session Initiation Protocol Call
         Control - Transfer", draft-ietf-sipping-cc-transfer-01 (work in
         progress), February 2003.

   [13]  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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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








































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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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