<pre>Network Working Group                                       J. Rosenberg
Request for Comments: 4538                                 Cisco Systems
Category: Standards Track                                      June 2006


          <span class="h1">Request Authorization through Dialog Identification</span>
                <span class="h1">in the Session Initiation Protocol (SIP)</span>

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This specification defines the Target-Dialog header field for the
   Session Initiation Protocol (SIP), and the corresponding option tag,
   tdialog.  This header field is used in requests that create SIP
   dialogs.  It indicates to the recipient that the sender is aware of
   an existing dialog with the recipient, either because the sender is
   on the other side of that dialog, or because it has access to the
   dialog identifiers.  The recipient can then authorize the request
   based on this awareness.





















<span class="grey">Rosenberg                   Standards Track                     [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
      <a href="#section-1.1">1.1</a>. Terminology ................................................<a href="#page-4">4</a>
   <a href="#section-2">2</a>. Overview of Operation ...........................................<a href="#page-4">4</a>
   <a href="#section-3">3</a>. User Agent Client (UAC) Behavior ................................<a href="#page-5">5</a>
   <a href="#section-4">4</a>. User Agent Server Behavior ......................................<a href="#page-7">7</a>
   <a href="#section-5">5</a>. Proxy Behavior ..................................................<a href="#page-8">8</a>
   <a href="#section-6">6</a>. Extensibility Considerations ....................................<a href="#page-8">8</a>
   <a href="#section-7">7</a>. Header Field Definition .........................................<a href="#page-9">9</a>
   <a href="#section-8">8</a>. Security Considerations .........................................<a href="#page-9">9</a>
   <a href="#section-9">9</a>. Relationship with In-Reply-To ..................................<a href="#page-10">10</a>
   <a href="#section-10">10</a>. Example Call Flow .............................................<a href="#page-10">10</a>
   <a href="#section-11">11</a>. IANA Considerations ...........................................<a href="#page-13">13</a>
      <a href="#section-11.1">11.1</a>. Header Field .............................................<a href="#page-13">13</a>
      <a href="#section-11.2">11.2</a>. Header Field Parameters ..................................<a href="#page-13">13</a>
           <a href="#section-11.2.1">11.2.1</a>. local-tag .........................................<a href="#page-13">13</a>
           <a href="#section-11.2.2">11.2.2</a>. remote-tag ........................................<a href="#page-13">13</a>
      <a href="#section-11.3">11.3</a>. SIP Option Tag ...........................................<a href="#page-14">14</a>
   <a href="#section-12">12</a>. Acknowledgements ..............................................<a href="#page-14">14</a>
   <a href="#section-13">13</a>. References ....................................................<a href="#page-14">14</a>
      <a href="#section-13.1">13.1</a>. Normative References .....................................<a href="#page-14">14</a>
      <a href="#section-13.2">13.2</a>. Informative References ...................................<a href="#page-15">15</a>




























<span class="grey">Rosenberg                   Standards Track                     [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   The Session Initiation Protocol (SIP) [<a href="#ref-2" title="&quot;SIP: Session Initiation Protocol&quot;">2</a>] defines the concept of a
   dialog as a persistent relationship between a pair of user agents.
   Dialogs provide context, including sequence numbers, proxy routes,
   and dialog identifiers.  Dialogs are established through the
   transmission of SIP requests with particular methods.  Specifically,
   the INVITE, REFER [<a href="#ref-8" title="&quot;The Session Initiation Protocol (SIP) Refer Method&quot;">8</a>], and SUBSCRIBE [<a href="#ref-3" title="&quot;Session Initiation Protocol (SIP)-Specific Event Notification&quot;">3</a>] requests all create dialogs.

   When a user agent receives a request that creates a dialog, it needs
   to decide whether to authorize that request.  For some requests,
   authorization is a function of the identity of the sender, the
   request method, and so on.  However, many situations have been
   identified in which a user agent's authorization decision depends on
   whether the sender of the request is currently in a dialog with that
   user agent, or whether the sender of the request is aware of a dialog
   the user agent has with another entity.

   One such example is call transfer, accomplished through REFER.  If
   user agents A and B are in an INVITE dialog, and user agent A wishes
   to transfer user agent B to user agent C, user agent A needs to send
   a REFER request to user agent B, asking user agent B to send an
   INVITE request to user agent C.  User agent B needs to authorize this
   REFER.  The proper authorization decision is that user agent B should
   accept the request if it came from a user with whom B currently has
   an INVITE dialog relationship.  Current implementations deal with
   this by sending the REFER on the same dialog as the one in place
   between user agents A and B.  However, this approach has numerous
   problems [<a href="#ref-12" title="&quot;Multiple Dialog Usages in the Session Initiation Protocol&quot;">12</a>].  These problems include difficulties in determining
   the lifecycle of the dialog and its usages and in determining which
   messages are associated with each application usage.  Instead, a
   better approach is for user agent A to send the REFER request to user
   agent B outside of the dialog.  In that case, a means is needed for
   user agent B to authorize the REFER.

   Another example is the application interaction framework [<a href="#ref-14" title="&quot;A Framework for Application Interaction in the Session Initiation Protocol (SIP)&quot;">14</a>].  In
   that framework, proxy servers on the path of a SIP INVITE request can
   place user interface components on the user agent that generated or
   received the request.  To do this, the proxy server needs to send a
   REFER request to the user agent, targeted to its Globally Routable
   User Agent URI (GRUU) [<a href="#ref-13" title="&quot;Obtaining and Using Globally Routable User Agent (UA) URIs (GRUU) in the Session Initiation Protocol (SIP)&quot;">13</a>], asking the user agent to fetch an HTTP
   resource containing the user interface component.  In such a case, a
   means is needed for the user agent to authorize the REFER.  The
   application interaction framework recommends that the request be
   authorized if it was sent from an entity on the path of the original
   dialog.  This can be done by including the dialog identifiers in the





<span class="grey">Rosenberg                   Standards Track                     [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


   REFER, which prove that the user agent that sent the REFER is aware
   of those dialog identifiers (this needs to be secured against
   eavesdroppers through the sips mechanism, of course).

   Another example is if two user agents share an INVITE dialog, and an
   element on the path of the INVITE request wishes to track the state
   of the INVITE.  In such a case, it sends a SUBSCRIBE request to the
   GRUU of the user agent, asking for a subscription to the dialog event
   package.  If the SUBSCRIBE request came from an element on the INVITE
   request path, it should be authorized.

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Terminology</span>

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in <a href="./rfc2119">RFC 2119</a> [<a href="#ref-1" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">1</a>].

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Overview of Operation</span>

                   +--------+            +--------+
                   |        |   INVITE   |        |
                   | Server |-----------&gt;| Server |
                   |   A    |            |   B    |
                   |        |...........&gt;|        |
                   +--------+            +--------+
                      ^          REFER     .   \
                     /                      .   \
                    /                        .   \
                   /                          .   \
                  /                            .   \
                 /                              V   V
           +--------+                            +--------+
           |        |                            |        |
           | User   |                            | User   |
           | Agent  |                            | Agent  |
           |   A    |                            |   B    |
           +--------+                            +--------+

                                 Figure 1

   Figure 1 shows the basic model of operation.  User agent A sends an
   INVITE to user agent B, traversing two servers, server A and server
   B.  Both servers act as proxies for this transaction.  User B sends a
   200 OK response to the INVITE.  This 200 OK includes a Supported
   header field indicating support for this specification (through the
   presence of the tdialog option tag).  The 200 OK response establishes
   a dialog between the two user agents.




<span class="grey">Rosenberg                   Standards Track                     [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


   Next, an entity that was present along the request path (server A,
   for example) wishes to send a dialog-forming request (such as REFER)
   to user agent A or B (user B for example).  So, the entity acts as a
   user agent and sends the request to user agent B.  This request is
   addressed to the URI of user agent B, which server A learned from
   inspecting the Contact header field in the 200 OK of the INVITE
   request.  If this URI has the GRUU [<a href="#ref-11" title="&quot;Randomness Requirements for Security&quot;">11</a>] property (it can be used by
   any element on the Internet, such as server A, to reach the specific
   user agent instance that generated that 200 OK to the INVITE), then
   the mechanism will work across NAT boundaries.

   The request generated by server A will contain a Target-Dialog header
   field.  This header field contains the dialog identifiers for the
   INVITE dialog between user agents A and B, composed of the Call-ID,
   local tag, and remote tag.  Server A knew to include the Target-
   Dialog header field in the REFER request because it knows that user
   agent B supports it.

   When the request arrives at user agent B, it needs to make an
   authorization decision.  Because the INVITE dialog was established
   using a sips URI, and because the dialog identifiers are
   cryptographically random [<a href="#ref-2" title="&quot;SIP: Session Initiation Protocol&quot;">2</a>], no entity except for user agent A or
   the proxies on the path of the initial INVITE request can know the
   dialog identifiers.  Thus, because the request contains those dialog
   identifiers, user agent B can be certain that the request came from
   user agent A, the two proxies, or an entity to whom the user agent or
   proxies gave the dialog identifiers.  As such, it authorizes the
   request and performs the requested actions.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  User Agent Client (UAC) Behavior</span>

   A UAC SHOULD include a Target-Dialog header field in a request if the
   following conditions are all true:

   1.  The request is to be sent outside of any existing dialog.

   2.  The user agent client believes that the request may not be
       authorized by the user agent server unless the user agent client
       can prove that it is aware of the dialog identifiers for some
       other dialog.  Call this dialog the target dialog.

   3.  The request does not otherwise contain information that indicates
       that the UAC is aware of those dialog identifiers.








<span class="grey">Rosenberg                   Standards Track                     [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


   4.  The user agent client knows that the user agent server supports
       the Target-Dialog header field.  It can know this if it has seen
       a request or response from the user agent server within the
       target dialog that contained a Supported header field that
       included the tdialog option tag.

   If the fourth condition is not met, the UAC SHOULD NOT use this
   specification.  Instead, if it is currently within a dialog with the
   User Agent Server (UAS), it SHOULD attempt to send the request within
   the existing target dialog.

   The following are examples of use cases in which these conditions are
   met:

   o  A REFER request is sent according to the principles of [<a href="#ref-14" title="&quot;A Framework for Application Interaction in the Session Initiation Protocol (SIP)&quot;">14</a>].
      These REFER are sent outside of a dialog and do not contain any
      other information that indicates awareness of the target dialog.
      [<a href="#ref-14" title="&quot;A Framework for Application Interaction in the Session Initiation Protocol (SIP)&quot;">14</a>] also mandates that the REFER be sent only if the UA indicates
      support for the target dialog specification.

   o  User A is in separate calls with users B and C. User A decides to
      start a three way call, and so morphs into a focus [<a href="#ref-17" title="&quot;A Framework for Conferencing with the Session Initiation Protocol (SIP)&quot;">17</a>].  User B
      would like to learn the other participants in the conference.  So,
      it sends a SUBSCRIBE request to user A (who is now acting as the
      focus) for the conference event package [<a href="#ref-16" title="&quot;A Session Initiation Protocol (SIP) Event Package for Conference State&quot;">16</a>].  It is sent outside
      of the existing dialog between user B and the focus, and it would
      be authorized by A if user B could prove that it knows the dialog
      identifiers for its existing dialog with the focus.  Thus, the
      Target-Dialog header field would be included in the SUBSCRIBE.

   The following are examples of use cases in which these conditions are
   not met:

   o  A server acting as a proxy is a participant in an INVITE dialog
      that establishes a session.  The server would like to use the
      Keypad Markup Language (KPML) event package [<a href="#ref-18" title="&quot;A Session Initiation Protocol (SIP) Event Package for Key Press Stimulus (KPML)&quot;">18</a>] to find out about
      keypresses from the originating user agent.  To do this, it sends
      a SUBSCRIBE request.  However, the Event header field of this
      SUBSCRIBE contains event parameters that indicate the target
      dialog of the subscription.  As such, the request can be
      authorized without additional information.

   o  A server acting as a proxy is a participant in an INVITE dialog
      that establishes a session.  The server would like to use the
      dialog event package [<a href="#ref-15" title="&quot;An INVITE- Initiated Dialog Event Package for the Session Initiation Protocol (SIP)&quot;">15</a>] to find out about dialogs at the
      originating user agent.  To do this, it sends a SUBSCRIBE request.
      However, the Event header field of this SUBSCRIBE contains event
      parameters that indicate the target dialog of the subscription.



<span class="grey">Rosenberg                   Standards Track                     [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


      As such, the request can be authorized without additional
      information.

   Specifications that intend to make use of the Target-Dialog header
   field SHOULD discuss specific conditions in which it is to be
   included.

   Assuming it is to be included, the value of the callid production in
   the Target-Dialog header field MUST be equal to the Call-ID of the
   target dialog.  The "remote-tag" header field parameter MUST be
   present and MUST contain the tag that would be viewed as the remote
   tag from the perspective of the recipient of the new request.  The
   "local-tag" header field parameter MUST be present and MUST contain
   the tag that would be viewed as the local tag from the perspective of
   the recipient of the new request.

   The request sent by the UAC SHOULD include a Require header field
   that includes the tdialog option tag.  This request should, in
   principle, never fail with a 420 (Bad Extension) response, because
   the UAC would not have sent the request unless it believed the UAS
   supported the extension.  If a Require header field was not included,
   and the UAS didn't support the extension, it would normally reject
   the request because it was unauthorized, probably with a 403.
   However, without the Require header field, the UAC would not be able
   to differentiate between the following:

   o  a 403 that arrived because the UAS didn't actually understand the
      Target-Dialog header field (in which case the client should send
      the request within the target dialog if it can)

   o  a 403 that arrived because the UAS understood the Target-Dialog
      header field, but elected not to authorize the request despite the
      fact that the UAC proved its awareness of the target dialog (in
      which case the client should not resend the request within the
      target dialog, even if it could).

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  User Agent Server Behavior</span>

   If a user agent server receives a dialog-creating request and wishes
   to authorize the request, and if that authorization depends on
   whether or not the sender has knowledge of an existing dialog with
   the UAS, and information outside of the Target-Dialog header field
   does not provide proof of this knowledge, the UAS SHOULD check the
   request for the existence of the Target-Dialog header field.  If this
   header field is not present, the UAS MAY still authorize the request
   by other means.





<span class="grey">Rosenberg                   Standards Track                     [Page 7]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-8" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


   If the header field is present, and the value of the callid
   production, the "remote-tag", and "local-tag" values match the
   Call-ID, remote tag, and local tag of an existing dialog, and the
   dialog that they match was established using a sips URI, the UAS
   SHOULD authorize the request if it would authorize any entity on the
   path of the request that created that dialog, or any entity trusted
   by an entity on the path of the request that created that dialog.

   If the dialog identifiers match, but they match a dialog not created
   with a sips URI, the UAS MAY authorize the request if it would
   authorize any entity on the path of the request that created that
   dialog, or any entity trusted by an entity on the path of the request
   that created that dialog.  However, in this case, any eavesdropper on
   the original dialog path would have access to the dialog identifiers,
   and thus the authorization is optional.

   If the dialog identifiers don't match, or if they don't contain both
   a "remote-tag" and "local-tag" parameter, the header field MUST be
   ignored, and authorization MAY be determined by other means.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Proxy Behavior</span>

   Proxy behavior is unaffected by this specification.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Extensibility Considerations</span>

   This specification depends on a user agent client knowing, ahead of
   sending a request to a user agent server, whether or not that user
   agent server supports the Target-Dialog header field.  As discussed
   in <a href="#section-3">Section 3</a>, the UAC can know this because it saw a request or
   response sent by that UAS within the target dialog that contained the
   Supported header field whose value included the tdialog option tag.

   Because of this requirement, it is especially important that user
   agents compliant to this specification include a Supported header
   field in all dialog forming requests and responses.  Inclusion of the
   Supported header fields in requests is at SHOULD strength per <a href="./rfc3261">RFC</a>
   <a href="./rfc3261">3261</a>.  This specification does not alter that requirement.  However,
   implementers should realize that, unless the tdialog option tag is
   placed in the Supported header field of requests and responses, this
   extension is not likely to be used, and instead, the request is
   likely to be re-sent within the existing target dialog (assuming the
   sender is the UA on the other side of the target dialog).  As such,
   the conditions in which the SHOULD would not be followed would be
   those rare cases in which the UA does not want to enable usage of
   this extension.





<span class="grey">Rosenberg                   Standards Track                     [Page 8]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-9" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Header Field Definition</span>

   The grammar for the Target-Dialog header field is defined as follows:

   Target-Dialog      =     "Target-Dialog" HCOLON callid *(SEMI
                                td-param)    ;callid from <a href="./rfc3261">RFC 3261</a>
   td-param           =     remote-param / local-param /
                            generic-param
   remote-param       =     "remote-tag" EQUAL token
   local-param        =     "local-tag" EQUAL token
                               ;token and generic-param from <a href="./rfc3261">RFC 3261</a>

   Figures 3 and 4 are an extension of Tables 2 and 3 in <a href="./rfc3261">RFC 3261</a> [<a href="#ref-2" title="&quot;SIP: Session Initiation Protocol&quot;">2</a>]
   for the Target-Dialog header field.  The column "INF" is for the INFO
   method [<a href="#ref-4" title="&quot;The SIP INFO Method&quot;">4</a>], "PRA" is for the PRACK method [<a href="#ref-5" title="&quot;Reliability of Provisional Responses in Session Initiation Protocol (SIP)&quot;">5</a>], "UPD" is for the
   UPDATE method [<a href="#ref-6" title="&quot;The Session Initiation Protocol (SIP) UPDATE Method&quot;">6</a>], "SUB" is for the SUBSCRIBE method [<a href="#ref-3" title="&quot;Session Initiation Protocol (SIP)-Specific Event Notification&quot;">3</a>], "NOT" is
   for the NOTIFY method [<a href="#ref-3" title="&quot;Session Initiation Protocol (SIP)-Specific Event Notification&quot;">3</a>], "MSG" is for the MESSAGE method [<a href="#ref-7" title="&quot;Session Initiation Protocol (SIP) Extension for Instant Messaging&quot;">7</a>], "REF"
   is for the REFER method [<a href="#ref-8" title="&quot;The Session Initiation Protocol (SIP) Refer Method&quot;">8</a>], and "PUB" is for the PUBLISH method [<a href="#ref-9" title="&quot;Session Initiation Protocol (SIP) Extension for Event State Publication&quot;">9</a>].

   Header field          where  proxy  ACK BYE CAN INV OPT REG PUB

   Target-Dialog           R      -     -   -   -   o   -   -   -

                Figure 3: Allowed Methods for Target-Dialog


   Header field          where  proxy  PRA UPD SUB NOT INF MSG REF

   Target-Dialog           R      -     -   -   o   -   -   -   o

                Figure 4: Allowed Methods for Target-Dialog

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Security Considerations</span>

   The Target-Dialog header field is used to authorize requests based on
   the fact that the sender of the request has access to information
   that only certain entities have access to.  In order for such an
   authorization decision to be secure, two conditions have to be met.
   Firstly, no eavesdroppers can have access to this information.  That
   requires the original SIP dialog to be established using a sips URI,
   which provides TLS on each hop.  With a sips URI, only the user
   agents and proxies on the request path will be able to know the
   dialog identifiers.  The second condition is that the dialog
   identifiers be sufficiently cryptographically random that they cannot
   be guessed.  <a href="./rfc3261">RFC 3261</a> requires global uniqueness for the Call-ID and
   32 bits of cryptographic randomness for each tag (there are two tags
   for a dialog).  Given the short duration of a typical dialog (perhaps
   as long as a day), this amount of randomness appears adequate for



<span class="grey">Rosenberg                   Standards Track                     [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


   preventing guessing attacks.  However, it's important to note that
   this specification requires true cryptographic randomness as set
   forth in <a href="./rfc4086">RFC 4086</a> [<a href="#ref-11" title="&quot;Randomness Requirements for Security&quot;">11</a>].  Weaker pseudorandom identifiers reduce the
   probability of collision, but because they are guessable, they are
   not sufficient to prevent an attacker from observing a sequence of
   identifiers, guessing the next one, and then using this specification
   to launch an attack.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Relationship with In-Reply-To</span>

   <a href="./rfc3261">RFC 3261</a> defines the In-Reply-To header field.  It provides a list of
   Call-IDs for calls that the current request references or returns.
   It was meant to serve a similar purpose as the Reply-To in email: to
   facilitate the construction of "threads" of conversations in a user
   interface.  Target-Dialog is similar, in that it also references a
   previous session.  Due to their similarities, it is important to
   understand the differences, as these two header fields are not
   substitutes for each other.

   Firstly, In-Reply-To is meant for consumption by a human or a user
   interface widget, for providing the user with a context that allows
   them to decide what a call is about and whether they should take it.
   Target-Dialog, on the other hand, is meant for consumption by the
   user agent itself, to facilitate authorization of session requests in
   specific cases where authorization is not a function of the user, but
   rather the underlying protocols.  A UA will authorize a call
   containing Target-Dialog based on a correct value of the Target-
   Dialog header field.

   Secondly, Target-Dialog references a specific dialog that must be
   currently in progress.  In-Reply-To references a previous call
   attempt, most likely one that did not result in a dialog.  This is
   why In-Reply-To uses a Call-ID, and Target-Dialog uses a set of
   dialog identifiers.

   Finally, In-Reply-To implies cause and effect.  When In-Reply-To is
   present, it means that the request is being sent because of the
   previous request that was delivered.  Target-Dialog does not imply
   cause and effect, merely awareness for the purposes of authorization.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Example Call Flow</span>

   In this example, user agent A and user agent B establish an INVITE-
   initiated dialog through Server-A and Server-B, each of which acts as
   a proxy for the INVITE.  Server B would then like to use the
   application interaction framework [<a href="#ref-14" title="&quot;A Framework for Application Interaction in the Session Initiation Protocol (SIP)&quot;">14</a>] to request that user agent A
   fetch an HTML user interface component.  To do that, it sends a REFER
   request to A's URI.  The flow for this is shown in Figure 5.  The



<span class="grey">Rosenberg                   Standards Track                    [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


   conventions of [<a href="#ref-19" title="&quot;Session Initiation Protocol (SIP) Torture Test Messages&quot;">19</a>] are used to describe representation of long
   message lines.

             A        Server-A     Server-B         B
             |(1) INVITE  |            |            |
             |-----------&gt;|            |            |
             |            |(2) INVITE  |            |
             |            |-----------&gt;|            |
             |            |            |(3) INVITE  |
             |            |            |-----------&gt;|
             |            |            |(4) 200 OK  |
             |            |            |&lt;-----------|
             |            |(5) 200 OK  |            |
             |            |&lt;-----------|            |
             |(6) 200 OK  |            |            |
             |&lt;-----------|            |            |
             |(7) ACK     |            |            |
             |-------------------------------------&gt;|
             |            |(8) REFER   |            |
             |            |&lt;-----------|            |
             |(9) REFER   |            |            |
             |&lt;-----------|            |            |
             |(10) 200 OK |            |            |
             |-----------&gt;|            |            |
             |            |(11) 200 OK |            |
             |            |-----------&gt;|            |

                                 Figure 5

   First, the caller sends an INVITE, as shown in message 1.

   INVITE sips:B@example.com SIP/2.0
   Via: SIP/2.0/TLS host.example.com;branch=z9hG4bK9zz8
   From: Caller &lt;sip:A@example.com&gt;;tag=kkaz-
   To: Callee &lt;sip:B@example.org&gt;
   Call-ID: fa77as7dad8-sd98ajzz@host.example.com
   CSeq: 1 INVITE
   Max-Forwards: 70
   Supported: tdialog
   Allow: INVITE, OPTIONS, BYE, CANCEL, ACK, REFER
   Accept: application/sdp, text/html
   &lt;allOneLine&gt;
   Contact: &lt;sips:A@example.com;gruu;opaque=urn:uuid:f81d4f
   ae-7dec-11d0-a765-00a0c91e6bf6;grid=99a&gt;;schemes="http,sip,sips"
   &lt;/allOneLine&gt;
   Content-Length: ...
   Content-Type: application/sdp




<span class="grey">Rosenberg                   Standards Track                    [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


   --SDP not shown--

   The INVITE indicates that the caller supports GRUU (note its presence
   in the Contact header field of the INVITE) and the Target-Dialog
   header field.  This INVITE is forwarded to the callee (messages 2-3),
   which generates a 200 OK response that is forwarded back to the
   caller (message 4-5).  Message 5 might look like:

   SIP/2.0 200 OK
   Via: SIP/2.0/TLS host.example.com;branch=z9hG4bK9zz8
   From: Caller &lt;sip:A@example.com&gt;;tag=kkaz-
   To: Callee &lt;sip:B@example.org&gt;;tag=6544
   Call-ID: fa77as7dad8-sd98ajzz@host.example.com
   CSeq: 1 INVITE
   Contact: &lt;sips:B@pc.example.org&gt;
   Content-Length: ...
   Content-Type: application/sdp

   --SDP not shown--

   In this case, the called party does not support GRUU or the Target-
   Dialog header field.  The caller generates an ACK (message 7).
   Server B then decides to send a REFER to user A:


   &lt;allOneLine&gt;
   REFER sips:A@example.com;gruu;opaque=urn:uuid:f81d4f
   ae-7dec-11d0-a765-00a0c91e6bf6;grid=99a SIP/2.0
   &lt;/allOneLine&gt;
   Via: SIP/2.0/TLS serverB.example.org;branch=z9hG4bK9zz10
   From: Server B &lt;sip:serverB.example.org&gt;;tag=mreysh
   &lt;allOneLine&gt;
   To: Caller &lt;sips:A@example.com;gruu;opaque=urn:uuid:f81d4f
   ae-7dec-11d0-a765-00a0c91e6bf6;grid=99a&gt;
   &lt;/allOneLine&gt;
   Target-Dialog: fa77as7dad8-sd98ajzz@host.example.com
     ;local-tag=kkaz-
     ;remote-tag=6544
   Refer-To: http://serverB.example.org/ui-component.html
   Call-ID: 86d65asfklzll8f7asdr@host.example.com
   CSeq: 1 REFER
   Max-Forwards: 70
   Require: tdialog
   Allow: INVITE, OPTIONS, BYE, CANCEL, ACK, NOTIFY
   Contact: &lt;sips:serverB.example.org&gt;
   Content-Length: 0





<span class="grey">Rosenberg                   Standards Track                    [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


   This REFER will be delivered to server A because it was sent to the
   GRUU.  From there, it is forwarded to user agent A (message 9) and
   authorized because of the presence of the Target-Dialog header field.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  IANA Considerations</span>

   This specification registers a new SIP header field, a new option tag
   according to the processes of <a href="./rfc3261">RFC 3261</a> [<a href="#ref-2" title="&quot;SIP: Session Initiation Protocol&quot;">2</a>], and two new header field
   parameters according to the processes of <a href="./rfc3968">RFC 3968</a> [<a href="#ref-10" title="&quot;The Internet Assigned Number Authority (IANA) Header Field Parameter Registry for the Session Initiation Protocol (SIP)&quot;">10</a>].

<span class="h3"><a class="selflink" id="section-11.1" href="#section-11.1">11.1</a>.  Header Field</span>

   RFC Number:  <a href="./rfc4538">RFC 4538</a>

   Header Field Name:  Target-Dialog

   Compact Form:  none

<span class="h3"><a class="selflink" id="section-11.2" href="#section-11.2">11.2</a>.  Header Field Parameters</span>

   This section registers two header field parameters according to the
   processes of <a href="./rfc3968">RFC 3968</a> [<a href="#ref-10" title="&quot;The Internet Assigned Number Authority (IANA) Header Field Parameter Registry for the Session Initiation Protocol (SIP)&quot;">10</a>].

<span class="h4"><a class="selflink" id="section-11.2.1" href="#section-11.2.1">11.2.1</a>.  local-tag</span>

   Header Field:  Target-Dialog

   Header Field Parameter:  local-tag

   Predefined Values:  None

   RFC:  <a href="./rfc4538">RFC 4538</a>

<span class="h4"><a class="selflink" id="section-11.2.2" href="#section-11.2.2">11.2.2</a>.  remote-tag</span>

   Header Field:  Target-Dialog

   Header Field Parameter:  remote-tag

   Predefined Values:  None

   RFC:  <a href="./rfc4538">RFC 4538</a>









<span class="grey">Rosenberg                   Standards Track                    [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


<span class="h3"><a class="selflink" id="section-11.3" href="#section-11.3">11.3</a>.  SIP Option Tag</span>

   This specification registers a new SIP option tag per the guidelines
   in <a href="./rfc3261#section-27.1">Section&nbsp;27.1 of RFC 3261</a>.

   Name:  tdialog

   Description:  This option tag is used to identify the target dialog
      header field extension.  When used in a Require header field, it
      implies that the recipient needs to support the Target-Dialog
      header field.  When used in a Supported header field, it implies
      that the sender of the message supports it.

<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>.  Acknowledgements</span>

   This specification is based on a header field first proposed by
   Robert Sparks in the dialog usage draft [<a href="#ref-12" title="&quot;Multiple Dialog Usages in the Session Initiation Protocol&quot;">12</a>].  John Elwell provided
   helpful comments.

<span class="h2"><a class="selflink" id="section-13" href="#section-13">13</a>.  References</span>

<span class="h3"><a class="selflink" id="section-13.1" href="#section-13.1">13.1</a>.  Normative References</span>

   [<a id="ref-1">1</a>]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-2">2</a>]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", <a href="./rfc3261">RFC 3261</a>, June 2002.

   [<a id="ref-3">3</a>]   Roach, A., "Session Initiation Protocol (SIP)-Specific Event
         Notification", <a href="./rfc3265">RFC 3265</a>, June 2002.

   [<a id="ref-4">4</a>]   Donovan, S., "The SIP INFO Method", <a href="./rfc2976">RFC 2976</a>, October 2000.

   [<a id="ref-5">5</a>]   Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
         Responses in Session Initiation Protocol (SIP)", <a href="./rfc3262">RFC 3262</a>,
         June 2002.

   [<a id="ref-6">6</a>]   Rosenberg, J., "The Session Initiation Protocol (SIP) UPDATE
         Method", <a href="./rfc3311">RFC 3311</a>, October 2002.

   [<a id="ref-7">7</a>]   Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and
         D. Gurle, "Session Initiation Protocol (SIP) Extension for
         Instant Messaging", <a href="./rfc3428">RFC 3428</a>, December 2002.

   [<a id="ref-8">8</a>]   Sparks, R., "The Session Initiation Protocol (SIP) Refer
         Method", <a href="./rfc3515">RFC 3515</a>, April 2003.



<span class="grey">Rosenberg                   Standards Track                    [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


   [<a id="ref-9">9</a>]   Niemi, A., "Session Initiation Protocol (SIP) Extension for
         Event State Publication", <a href="./rfc3903">RFC 3903</a>, October 2004.

   [<a id="ref-10">10</a>]  Camarillo, G., "The Internet Assigned Number Authority (IANA)
         Header Field Parameter Registry for the Session Initiation
         Protocol (SIP)", <a href="https://www.rfc-editor.org/bcp/bcp98">BCP 98</a>, <a href="./rfc3968">RFC 3968</a>, December 2004.

<span class="h3"><a class="selflink" id="section-13.2" href="#section-13.2">13.2</a>.  Informative References</span>

   [<a id="ref-11">11</a>]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
         Requirements for Security", <a href="https://www.rfc-editor.org/bcp/bcp106">BCP 106</a>, <a href="./rfc4086">RFC 4086</a>, June 2005.

   [<a id="ref-12">12</a>]  Sparks, R., "Multiple Dialog Usages in the Session Initiation
         Protocol", Work in Progress, March 2006.

   [<a id="ref-13">13</a>]  Rosenberg, J., "Obtaining and Using Globally Routable User
         Agent (UA) URIs (GRUU) in the Session Initiation Protocol
         (SIP)", Work in Progress, May 2006.

   [<a id="ref-14">14</a>]  Rosenberg, J., "A Framework for Application Interaction in the
         Session Initiation Protocol  (SIP)", Work in Progress,
         July 2005.

   [<a id="ref-15">15</a>]  Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
         Initiated Dialog Event Package for the Session Initiation
         Protocol (SIP)", <a href="./rfc4235">RFC 4235</a>, November 2005.

   [<a id="ref-16">16</a>]  Rosenberg, J., "A Session Initiation Protocol (SIP) Event
         Package for Conference State", Work in Progress, July 2005.

   [<a id="ref-17">17</a>]  Rosenberg, J., "A Framework for Conferencing with the Session
         Initiation Protocol (SIP)", <a href="./rfc4353">RFC 4353</a>, February 2006.

   [<a id="ref-18">18</a>]  Burger, E., "A Session Initiation Protocol (SIP) Event Package
         for Key Press Stimulus  (KPML)", Work in Progress,
         December 2004.

   [<a id="ref-19">19</a>]  Sparks, R., Ed., Hawrylyshen, A., Johnston, A., Rosenberg, J.,
         and H. Schulzrinne, "Session Initiation Protocol (SIP) Torture
         Test Messages", <a href="./rfc4475">RFC 4475</a>, May 2006.











<span class="grey">Rosenberg                   Standards Track                    [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


Author's Address

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

   Phone: +1 973 952-5000
   EMail: jdrosen@cisco.com
   URI:   <a href="http://www.jdrosen.net">http://www.jdrosen.net</a>








































<span class="grey">Rosenberg                   Standards Track                    [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc4538">RFC 4538</a>                     Target Dialog                     June 2006</span>


Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a>, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and <a href="https://www.rfc-editor.org/bcp/bcp79">BCP 79</a>.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   <a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).







Rosenberg                   Standards Track                    [Page 17]
</pre>