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Versions: (draft-ietf-sipping-session-policy-framework) 00 01 02 03 04 05 06 07 08 09 10 RFC 6794

SIPPING Working Group                                            V. Hilt
Internet-Draft                                  Bell Labs/Alcatel-Lucent
Expires: August 13, 2007                                    G. Camarillo
                                                                Ericsson
                                                            J. Rosenberg
                                                           Cisco Systems
                                                        February 9, 2007


   A Framework for Session Initiation Protocol (SIP) Session Policies
               draft-ietf-sip-session-policy-framework-01

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   This Internet-Draft will expire on August 13, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Proxy servers play a central role as an intermediary in the Session
   Initiation Protocol (SIP) as they define and impact policies on call
   routing, rendezvous, and other call features.  This document
   specifies a framework for SIP session policies that provides a
   standard mechanism by which a proxy can define or influence policies



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   on sessions, such as the codecs or media types to be used.  It
   defines a model, an overall architecture and new protocol mechanisms
   for session policies.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Session-Independent Policies . . . . . . . . . . . . . . . . .  5
     3.1.  Architecture and Overview  . . . . . . . . . . . . . . . .  5
     3.2.  Policy Subscription  . . . . . . . . . . . . . . . . . . .  6
   4.  Session-Specific Policies  . . . . . . . . . . . . . . . . . .  7
     4.1.  Architecture . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.3.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 10
       4.3.1.  Offer in Request . . . . . . . . . . . . . . . . . . . 10
       4.3.2.  Offer in Response  . . . . . . . . . . . . . . . . . . 13
     4.4.  UA/Policy Server Rendezvous  . . . . . . . . . . . . . . . 14
       4.4.1.  UAC Behavior . . . . . . . . . . . . . . . . . . . . . 14
       4.4.2.  Proxy Behavior . . . . . . . . . . . . . . . . . . . . 17
       4.4.3.  UAS Behavior . . . . . . . . . . . . . . . . . . . . . 18
       4.4.4.  Caching the Local Policy Server URI  . . . . . . . . . 18
       4.4.5.  Storing Session Policy Server URIs . . . . . . . . . . 19
       4.4.6.  Contacting the Policy Server . . . . . . . . . . . . . 19
       4.4.7.  Header Definition and Syntax . . . . . . . . . . . . . 20
     4.5.  Policy Channel . . . . . . . . . . . . . . . . . . . . . . 22
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
     6.1.  Registration of the "Policy-Id" Header . . . . . . . . . . 24
     6.2.  Registration of the "Policy-Contact" Header  . . . . . . . 24
     6.3.  Registration of the "policy" SIP Option-Tag  . . . . . . . 24
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 24
   Appendix B.  Session-Specific Policies - Call Flows  . . . . . . . 25
     B.1.  Offer in Invite  . . . . . . . . . . . . . . . . . . . . . 25
     B.2.  Offer in Response  . . . . . . . . . . . . . . . . . . . . 27
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 28
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 28
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
   Intellectual Property and Copyright Statements . . . . . . . . . . 30










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1.  Introduction

   The Session Initiation Protocol (SIP) [6] is a signaling protocol for
   creating, modifying and terminating multimedia sessions.  A central
   element in SIP is the proxy server.  Proxy servers are intermediaries
   that are responsible for request routing, rendezvous, authentication
   and authorization, mobility, and other signaling services.  However,
   proxies are divorced from the actual sessions - audio, video, and
   messaging - that SIP establishes.  Details of the sessions are
   carried in the payload of SIP messages, and are usually described
   with the Session Description Protocol (SDP) [7].  Indeed, SIP
   provides end-to-end encryption features using S/MIME, so that all
   information about the sessions can be hidden from eavesdroppers and
   proxies alike.

   However, experience has shown that there is a need for SIP
   intermediaries to impact aspects of a session.  For example, SIP may
   be used in a wireless network, which has limited resources for media
   traffic.  During periods of high activity, the wireless network
   provider wants to restrict the amount of bandwidth available to each
   individual user.  With session policies, an intermediary in the
   wireless network can inform the user agent about the bandwidth it can
   currently count on.  This information enables the user agent to make
   an informed decision about the number of streams, the media types,
   and the codecs it can successfully use in a session.  Similarly, a
   network provider may have a service level agreement with a user that
   defines the set of media types a user can use.  With session
   policies, the network can convey the current set of policies to user
   agents, enabling them to set up sessions without inadvertently
   violating any of the network policies.

   In another example, a SIP user agent is using a network which is
   connected to the public Internet through a firewall or a network
   border device.  The network provider would like to tell the user
   agent that it needs to send its media streams to a specific IP
   address and port on the firewall or border device to reach the public
   Internet.  Knowing this policy enables the user agent to set up
   sessions across the firewall or the network border.  In contrast to
   other methods for inserting a media intermediary, the use of session
   policies does not require the inspection or modification of SIP
   message bodies.

   Domains often enforce the session policies they have in place.  For
   example, a domain might have a policy that disallows the use of video
   and may enforce this policy by dropping all packets that contain a
   video encoding.  Unfortunately, enforcement mechanisms usually do not
   inform the user about the policies they are enforcing.  Instead, they
   silently keep the user from doing anything against them.  This may



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   lead to a malfunctioning of devices that is incomprehensible to the
   user.  With session policies, the user knows about the current
   network policies and can set up policy-compliant sessions or simply
   connect to a domain with less stringent policies.  Thus, session
   policies provide an important combination of consent coupled with
   enforcement.  That is, the user becomes aware of the policy and needs
   to act on it, but the provider still retains the right to enforce the
   policy.

   Two types of session policies exist: session-specific policies and
   session-independent policies.  Session-specific policies are policies
   that are created for one particular session, based on the session
   description of this session.  They enable a network intermediary to
   examine the session description a UA is proposing and to return a
   policy specifically for this session description.  For example, an
   intermediary could open pinholes in a firewall/NAT for each media
   stream in a session and return a policy that replaces the internal IP
   addresses and ports with external ones.  Since session-specific
   policies are tailored to a session, they only apply to the session
   they are created for.  Session-specific policies are created on a
   session-by-session basis at the time the session is established.

   Session-independent policies on the other hand are policies that are
   created independent of a session and generally apply to all SIP
   sessions set up by a user agent.  A session-independent policy can,
   for example, be used to inform user agents about an existing
   bandwidth limit or media type restrictions.  Since these policies are
   not based on a specific session description, they can be created
   independent of an attempt to set up a session and only need to be
   conveyed to the user agent when it initializes (e.g. at the time the
   device is powered on) and when the policies are changed.

   This specification defines a framework for SIP session policies.  It
   specifies a model, the overall architecture and new protocol
   mechanisms that are needed for session-independent and session-
   specific policies.


2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [1] and indicate requirement levels for
   compliant implementations.






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3.  Session-Independent Policies

   Session-independent policies are policies that are created
   independent of a session and generally apply to all sessions a user
   agent is setting up.  They typically remain stable for a longer
   period of time and apply to any session set up while they are valid.
   However, session-independent policies may also change over time.  For
   example, a policy that defines a bandwidth limit for a user may
   change during the day, defining a lower limit during peak hours and
   allow more bandwidth off-peak.

3.1.  Architecture and Overview

                        +-------------+
                 /------|   policy    |
      +----+    /       |  server 1   |
      |    |---/        +-------------+
      | UA |                 ...
      |    |---\        +-------------+
      +----+    \       |   policy    |
                 \------|  server n   |
                        +-------------+


                                 Figure 1

   A SIP UA may receive session-independent policies from one or more
   policy servers.  In a typical configuration, a UA receives session-
   independent policies from a policy server in the access or local
   network domain (i.e. the domain from which the UA receives IP
   service) and possibly the home network domain (i.e. the domain the UA
   registers at).  The local network may have policies that support the
   access network infrastructure.  For example, in a wireless network
   where bandwidth is scarce, a provider may restrict the bandwidth
   available to an individual user.  The home network may have policies
   that are needed to support services or policies that reflect the
   service level agreement with the user.  Thus, in most cases, a UA
   will receive session-independent policies from one or two policy
   servers.

   Setting up session-independent policies involves the following steps:

   1.  A user agent requests session-independent policies from the
       policy servers in the local network and home domain.  A user
       agent typically requests these policies when it starts up or
       connects to a new network domain.





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   2.  The policy server selects the policies that apply to this user
       agent.  The policy server may have general policies that apply to
       all users or maintain separate policies for each individual user.
       The selected policies are returned to the user agent.
   3.  The policy server may update the policies, for example, when
       network conditions change.

3.2.  Policy Subscription

   A UA requests session-independent policies by subscribing to session-
   independent policies on the policy server in a domain.  Subscriptions
   to session-independent policies are established using the "ua-
   profile" event package defined in the Framework for SIP User Agent
   Profile Delivery [4].

   The "ua-profile" event package [4] provides a mechanism to discover
   policy servers in the local network and the home domain.  The "local-
   network" profile-type enables a UA to discover a policy server in the
   local domain.  The "user" profile type enables the discovery of a
   policy server in the home domain.  A UA compliant to this
   specification SHOULD attempt to discover and subscribe to the policy
   servers in these two domains.

   A UA SHOULD (re-)subscribe to session-independent policies when the
   following events occur:

   o  The UA registers a new address-of-record (AoR) or removes a AoR
      from the set of AoRs it has registered.  In these cases, the UA
      SHOULD establish subscriptions for each new AoR using the "user"
      and the "local-network" profile-types.  The UA SHOULD terminate
      all subscriptions for AoRs it has removed.
   o  The UA changes the domain it is connected to.  The UA SHOULD
      terminate all existing subscriptions for the "local-network"
      profile-type.  It SHOULD then create a new subscription for each
      AoR using the "local-network" profile-type.  This way, the UA
      stops receiving policies from the previous local domain and starts
      to receive the policies of the new local domain.  The UA does not
      need to change the subscriptions for "user" profiles.

   If a subscriber is unable to establish a subscription, it SHOULD NOT
   attempt to re-try this subscription, unless one of the above events
   occurs again.  This is to limit the number of SUBSCRIBE requests sent
   within domains that do not support session-independent policies.

   A UA compliant to this specification MUST support the User Agent
   Profile Data Set for Media Policy [3].  To indicate that the UA wants
   to receive session-independent policies, it includes the MIME type
   "application/session-policy+xml" in the Accept header of a SUBSCRIBE



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   request.

   A policy server MAY send a notification to the subscriber every time
   the session-independent policies covered by the subscription change.
   The definition of what causes a policy to change is at the discretion
   of the administrator.  A change in the policy may be triggered, for
   example, by a change in the network status, by the change in the time
   of day or by an update of the service level agreement with the
   customer.  The session-independent policies contained in a
   notification MUST represent a complete session-independent policy.
   Deltas to previous policies or partial policies are not supported.


4.  Session-Specific Policies

   Session-specific policies are policies that are created specifically
   for one particular session of a UA.  Thus, session-specific policies
   will typically be different for different sessions.  The session-
   specific policies for a session may change during the course of the
   session.  For example, a user may run out of credit during a session,
   which will cause the network to disallow the transmission all media
   streams from this point on.

4.1.  Architecture

                           domain 1
                        +-----------+
                 /------|   proxy   |----...
      +----+    /       +-----------+
      |    |---/        +-----------+
      |    |            |  policy   |
      | UA |============|  server   |
      |    |            +-----------+
      |    |****        +-----------+
      +----+    *       |  policy   |
                 *******|enforcement|****...
                        +-----------+

      --- SIP Signaling
      === Policy Channel
      *** Media

                                 Figure 2

   The following entities are needed for session-specific policies (see
   Figure 2): a user agent (UA), a proxy, a policy server and possibly a
   policy enforcement entity.




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   The role of the proxy is to provide a rendezvous mechanism for UAs
   and policy servers.  It ensures that each UA has the URI of the
   policy server in its domain and knows where to retrieve policies
   from.  The proxy conveys the policy server URI to UAs in case they
   have not yet received it (e.g. in a previous call or through other
   means such as configuration).  The proxy does not deliver the actual
   policies to UAs.

   The policy server is a separate logical entity that may be physically
   co-located with the proxy.  The role of the policy server is to
   deliver session policies to UAs.  The policy server receives session
   information from the UA, uses this information to determine the
   policies that apply to the session and returns these policies to the
   UA.  The mechanism for generating policies (i.e. making policy
   decisions) is outside of the scope of this specification.  A policy
   server may, for example, query an external entity to get policies or
   it may directly incorporate a policy decision point and generate
   policies locally.

   A UA receives the URI of a policy server from a proxy.  It uses this
   URI to contact the policy server.  It provides information about the
   current session to the policy server and receives session policies in
   response.  The UA may also receive policy updates from the policy
   server during the course of a session.

   A network may have a policy enforcement infrastructure in place.
   However, this specification does not make any assumptions about the
   enforcement of session policies and the mechanisms defined here are
   orthogonal a policy enforcement infrastructure.  Their goal is to
   provide a mechanism to convey session information to a policy server
   and to return the policies that apply to a session to the UA.

   In principle, each domain that is traversed by SIP signaling messages
   can define session-specific policies for a session.  Each domain
   needs to run a policy server and a proxy that is able to rendezvous a
   UA with the policy server (as shown in Figure 2).  However, it is
   expected that session-specific policies will often only be provided
   by the local domain of the user agent.

4.2.  Overview

   The protocol defined in this specification clearly separates SIP
   signaling and the exchange of policies.  SIP signaling is only used
   to rendezvous the UA with the policy server.  From this point on, UA
   and policy server communicate directly with each other over a
   separate policy channel.  This is opposed to a piggyback model, where
   the exchange of policy information between endpoint and a policy
   server in the network is piggybacked onto the SIP signaling messages



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   that are exchanged between endpoints.

   The main advantage of using a separate policy channel is that it
   decouples the exchange of signaling messages between endpoints from
   the exchange of policies between endpoint and policy server.  This
   decoupling has a number of desirable properties.  It enables the use
   of separate encryption mechanisms on the signaling path to secure the
   communication between endpoints, and on the policy channel to secure
   the communication between endpoint and policy server.  Policies can
   be submitted directly from the policy server to the endpoint and
   never travel along the signaling path, possibly crossing many
   domains.  Endpoints set up a separate policy channel to each policy
   server and can specifically decide which information they want to
   disclose to which policy server.  Finally, policy servers do not need
   to rely on a SIP signaling message flowing by to send policies or
   policy updates to an endpoint.  A policy server can use the policy
   channel at any time to update session policies as needed.  A
   disadvantage of the separate channel model is that it requires
   additional messages for the exchange of policy information.

   Following this model, signaling for session-specific policies
   involves the following two fundamental tasks:

   1.  UA/policy server rendezvous: a UA setting up a session needs to
       be able to discover the policy servers that are relevant to this
       session.
   2.  Policy channel: once the UA has discovered the relevant policy
       servers for a session, it needs to connect to these servers,
       disclose session information and retrieve the policies that apply
       to this session.

   The setting up session-specific policies over the policy channel
   involves the following steps:

   1.  A user agent submits information about the session it is trying
       to establish to the policy server and asks whether a session
       using these parameters is permissible.
   2.  The policy server generates a policy decision for this session
       and returns the decision to the user agent.  Possible policy
       decisions are (1) to deny the session, (2) to propose changes to
       the session parameters with which the session would be
       acceptable, or (3) to accept the session as it was proposed.
   3.  The policy server can update the policy decision at a later time.
       A policy decision update can, for example, propose additional
       changes to the session (e.g. change the available bandwidth) or
       deny a previously accepted session (i.e. disallow the
       continuation of a session).




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   In many cases, the mechanism for session-specific policies will be
   used to disclose session information and return session policies.
   However, some scenarios may only involve the disclosure of session
   information to a network intermediary.  If an intermediary does not
   intend to return a policy, it can simply accept the session as it was
   proposed.  Similarly, some session-specific policies only apply to
   the offer (and therefore only require the disclosure of the offer)
   whereas others apply to offer and answer.  Both types of policies are
   supported by session-specific policy mechanism.

4.3.  Examples

   This section provides two examples to illustrate the overall
   operation of session-specific policies.  The call flows depict the
   rendezvous mechanism between UA and policy server and indicate the
   points at which the UA exchanges policy information with the policy
   server.

   The example is based on the following scenario: there are two domains
   (domain A and domain B), which both have session-specific policies
   for the UAs in their domain.  Both domains do not provide policies to
   the UAs outside of their domain.  The two domains have a proxy (P A
   and P B) and a policy server (PS A and PS B).  The policies in both
   domains involve the session description offer and answer.

4.3.1.  Offer in Request

   The first call flow shown in Figure 3 depicts an INVITE transaction
   with the offer in the request.  It is assumed that this is the first
   INVITE request the UAC creates in this domain and that it therefore
   does not have previous knowledge about the policy server URIs in this
   domain.

   (1) UA A sends an INVITE to proxy P A. P A knows that policies apply
   to this session and (2) returns a 488 to UA A. P A includes the URI
   of PS A in the 488 response.  This step is needed since the UAC has
   no prior knowledge about the URI of PS A. (3) UA A uses the URI to
   contact PS A, discloses the session description offer to PS A and (4)
   receives policies for the offer. (5) UA A reformulates the INVITE
   request under consideration of the received policies and includes a
   Policy-Id header to indicate that it has already contacted PS A. P A
   does not reject the INVITE this time and removes the Policy-Id header
   when forwarding the INVITE.  P B adds a Policy-Contact header
   containing the URI of PS B. (6) UA B uses this URI to contact PS B
   and discloses the offer and the answer it is about to send. (7) UA B
   receives policies from PS B and applies them to the offer and answer
   respectively. (8) UA B returns the updated answer in the 200 OK. (9)
   UA A contacts PS A with the answer and (10) retrieves answer policies



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   from PS A.


















































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    UA A              P A              P B             UA B
     |                 |                |                 |
     | INVITE offer    |                |                 |
     |---------------->|                |                 | (1)
     | 488             |                |                 |
     | + Policy-Contact|                |                 |
     |<----------------|                |                 | (2)
     | ACK             |                |                 |
     |---------------->|                |                 |
     |                 | PS A           |                 |
     |                    |             |                 |
     | PolicyChannel      |             |                 |
     | + InfoOffer        |             |                 |
     |------------------->|             |                 | (3)
     | PolicyChannel      |             |                 |
     | + PolicyOffer      |             |                 |
     |<-------------------|             |                 | (4)
     |                    |             |                 |
     |                 |                |                 |
     | INVITE offer'   | INVITE offer'  | INVITE offer    |
     | + Policy-Id     |                | + Policy-Contact|
     |---------------->|--------------->|---------------->| (5)
     |                 |                |                 |
     |                 |           PS B |                 |
     |                 |             |                    |
     |                 |             | PolicyChannel      |
     |                 |             | + InfoOffer        |
     |                 |             | + InfoAnswer       |
     |                 |             |<-------------------| (6)
     |                 |             | PolicyChannel      |
     |                 |             | + PolicyOffer      |
     |                 |             | + PolicyAnswer     |
     |                 |             |------------------->| (7)
     |                 |             |                    |
     |                 |                |                 |
     | OK answer       | OK answer      | OK answer       |
     |<----------------|<---------------|<----------------| (8)
     | ACK                                                |
     |--------------------------------------------------->|
     |                 |                |                 |
     |                    |             |                 |
     | PolicyChannel      |             |                 |
     | + InfoAnswer       |             |                 |
     |------------------->|             |                 | (9)
     | PolicyChannel      |             |                 |
     | + PolicyAnswer     |             |                 |
     |<-------------------|             |                 | (10)
     |                    |             |                 |



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                                 Figure 3

4.3.2.  Offer in Response

   The call flow shown in Figure 4 depicts an INVITE transaction with
   the offer in the response.

   (1) UA A sends an INVITE without an offer to proxy P A and (2) P A
   returns a 488 response containing the URI of PS A. (3) and (4) UA A
   uses this policy server URI to set up the policy channel.  At this
   time, UA A does not disclose a session description since it does not
   have the offer yet. (5) UA A re-sends the INVITE request and includes
   a Policy-Id header to indicate that it has contacted PS A. P A does
   not reject the INVITE this time and removes the Policy-Id header when
   forwarding the INVITE.  P B adds a Policy-Contact header containing
   the URI of PS B. (6) UA B uses this URI to discloses the offer to PS
   B. (7) UA B receives policies from PS B and applies them to the
   offer. (8) UA B returns the updated offer the 200 OK. (9) and (10) UA
   A contacts PS and discloses the offer and the answer it is about to
   send.  An important difference to the flow in the previous example is
   that UA A performs steps (9) and (10) before returning the answer in
   step (11).  This enables UA A to return the final answer in the ACK,
   which includes all applicable policies.  However, it requires that PS
   A immediately returns a policy to avoid a delay in the transmission
   of the ACK. (12) and (13) UA B also sends the answer to PS B and
   applies the policies it receives to the answer before using it.

    UA A              P A              P B             UA B
     |                 |                |                 |
     | INVITE          |                |                 |
     |---------------->|                |                 | (1)
     | 488             |                |                 |
     | + Policy-Contact|                |                 |
     |<----------------|                |                 | (2)
     | ACK             |                |                 |
     |---------------->|                |                 |
     |                 | PS A           |                 |
     |                    |             |                 |
     | PolicyChannel      |             |                 |
     |------------------->|             |                 | (3)
     | PolicyChannel      |             |                 |
     |<-------------------|             |                 | (4)
     |                    |             |                 |
     |                 |                |                 |
     | INVITE          | INVITE         | INVITE          |
     | + Policy-Id     |                | + Policy-Contact|
     |---------------->|--------------->|---------------->| (5)
     |                 |                |                 |



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     |                 |           PS B |                 |
     |                 |             |                    |
     |                 |             | PolicyChannel      |
     |                 |             | + InfoOffer        |
     |                 |             |<-------------------| (6)
     |                 |             | PolicyChannel      |
     |                 |             | + PolicyOffer      |
     |                 |             |------------------->| (7)
     |                 |             |                    |
     |                 |                |                 |
     | OK offer        | OK offer       | OK offer        |
     |<----------------|<---------------|<----------------| (8)
     |                 |                |                 |
     |                    |             |                 |
     | PolicyChannel      |             |                 |
     | + InfoOffer        |             |                 |
     | + InfoAnswer       |             |                 |
     |------------------->|             |                 | (9)
     | PolicyChannel      |             |                 |
     | + PolicyOffer      |             |                 |
     | + PolicyAnswer     |             |                 |
     |<-------------------|             |                 | (10)
     |                    |             |                 |
     | ACK answer                                         |
     |--------------------------------------------------->| (11)
     |                 |                |                 |
     |                 |             |                    |
     |                 |             | PolicyChannel      |
     |                 |             | + InfoAnswer       |
     |                 |             |<-------------------| (12)
     |                 |             | PolicyChannel      |
     |                 |             | + PolicyAnswer     |
     |                 |             |------------------->| (13)
     |                 |             |                    |

                                 Figure 4

4.4.  UA/Policy Server Rendezvous

   The first step in setting up session-specific policies is to
   rendezvous the UAs with the relevant policy servers.  This is
   achieved by providing the URIs of all policy servers relevant for a
   session to the UAs.

4.4.1.  UAC Behavior

   A UAC compliant to this specification MUST include a Supported header
   field with the option tag "policy" into all requests that can



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   initiate an offer/answer exchange [8] (e.g.  INVITE, UPDATE and PRACK
   requests).  The UA MUST include the "policy" option tag into these
   requests even if the particular request does not contain an offer or
   answer (e.g. an INVITE request without an offer).

   A UAC may receive a 488 response that contains a Policy-Contact
   header field.  The Policy-Contact header is a new header defined in
   this specification.  It contains the URI of a policy server.  A 488
   response with this header is generated by a proxy to convey the URI
   of the local policy server to the UAC.  After receiving a 488
   response with a Policy-Contact header, a UAC compliant to this
   specification needs to decide if it wants to continue with the
   session now knowing that there is a policy server.  If the UAC
   decides to continue, it MUST use the policy server URI to contact the
   policy server using mechanism defined in Section 4.5.  After
   receiving policies from the policy server, the UAC decides if it
   wants to accept these policies or not.  If it accepts these policies,
   it MUST apply them to the current request and resend the updated
   request.  If no changes are required by policies or no policies have
   been received, the request can be resend without any policy-induced
   changes.  If the UAC decides that the list of policy servers or the
   received session policies are unacceptable, it MUST NOT resend the
   request.

   The UAC MUST insert a Policy-Id header into a request if it has
   contacted a policy server and accepted the policies received for this
   request.  The Policy-Id header is a new header that is defined in
   this specification.  The UA MUST create a Policy-Id header value for
   each policy server involved in the preparation of a request.  A
   Policy-Id header value contains two pieces of information: the policy
   server URI and an optional token.  The policy server URI is the URI
   the UA has used to contact the policy server.  The token is an opaque
   string the UAC may have received from the policy server after
   contacting it.  If the UA has received a token from the policy server
   it MUST include the token in the Policy-Id header.  The format of the
   Policy-Id header is defined in Section 4.4.7.

   The Policy-Id header serves two main purposes: first and most
   importantly, it enables a proxy to determine if the UAC already knows
   the URI of its policy server.  A proxy can pass through a request if
   the URI of its policy server is included in the Policy-Id header.  If
   the policy server URI is not yet known to the UAC, the proxy can
   convey this URI to the UAC by rejecting the request with a 488
   response.

   The second purpose of the Policy-Id header is to enable a domain to
   route all requests that belong to the same session (i.e. the initial
   request and requests a UA retransmits after contacting the policy



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   server) to the same proxy and policy server.  This is important if a
   domain has multiple proxy/policy server combinations (e.g. in a
   proxy/policy server farm that receives requests through a load
   balancer) and the proxies/policy servers create per-session state in
   the network.  An example for such a scenario is a policy server that
   is associated with a session border device.  The policy server
   configures the session border device after receiving a session
   description from the UAC via the policy channel.  Retransmitted
   requests for such a session need to be routed to the same proxy/
   policy server as the initial request since this proxy/policy server
   combination that has configured the associated border device for the
   session.

   Routing requests that belong to the same session to the same proxy
   can be achieved by using the Policy-Id header token.  It requires
   that the policy server returns a token to the UAC that uniquely
   identifies the specific proxy/policy server combination.  The UAC
   includes this token in the Policy-Id header and it can be used
   (together with the policy server URI) by proxies in this domain to
   route the request along the desired path.  The format of this token
   does not require standardization.  The only requirement is that the
   token provides sufficient information for proxies to route the
   message inside a domain to the desired proxy/policy server.  The
   token can, for example, be a numeric identifier or an IP address.

      Note: it has been proposed to use the Policy-Id header to provide
      a hint for a proxy that the UAC has actually contacted the policy
      server.  This usage also requires the policy server to return a
      token to the UA.  In addition, the policy server needs to share
      valid tokens with the proxy.  After receiving a request with a
      Policy-Id header, the proxy can determine if the token in the
      Policy-Id header is valid.  If it is valid, the proxy knows that
      the UA has contacted the policy server for this session.  However,
      it is important to note that this token does not provide any proof
      that the UA has actually used the policies it has received from
      the policy server.  A malicious UA may simply contact the policy
      server, discard all policies it receives but still use the token
      in the Policy-Id header.

   In some cases, a request may traverse multiple domains with session-
   policies in place.  Each of these domains may return a 488 response
   containing a policy server URI.  Since the UAC contacts a policy
   server after receiving a 488 response from a domain and before re-
   sending the request, session policies are always applied to a request
   in the order in which the request traverses through the domains.  The
   UAC MUST NOT change this implicit order among policy servers.

   A UAC frequently needs to contact the policy server in the local



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   domain before sending a new request.  To avoid the retransmission of
   the local policy server URI in a 488 for each new request, a UA
   SHOULD maintain a cache that contains the URI of the local policy
   server (see Section 4.4.4).  The UAC SHOULD use the cached policy
   server URI to contact the local policy server before sending the
   initial request that starts an offer/answer exchange (e.g. an INVITE
   request).

   A UA may decide to change the session description of a session and to
   initiate subsequent offer/answer exchanges (e.g. using INVITE, UPDATE
   or PRACK requests) to re-negotiate the session description.  When
   creating such a mid-dialog request, a UA SHOULD contact the same
   policy servers it has contacted during the initial offer/answer
   exchange (see Section 4.4.5) before sending the request.  This avoids
   the retransmission of all policy server URIs in 488 responses for
   mid-dialog requests.

4.4.2.  Proxy Behavior

   A proxy provides rendezvous functionality for UAs and a policy
   server.  This is achieved by conveying the URI of a policy server to
   the UAC or the UAS (or both) when processing INVITE, UPDATE or PRACK
   requests (or any other request that can initiate an offer/answer
   exchange).

   If such a request contains a Supported header field with the option
   tag "policy", the proxy MAY reject the request with a 488 response to
   provide the local policy server URI to the UAC.  Before rejecting a
   request, the proxy MUST verify that the request does not contain a
   Policy-Id header field, which has the local policy server URI as a
   value.  If the request does not contain such a header or the local
   policy server URI is not present in this header, then the proxy MAY
   reject the request with a 488.  The proxy MUST insert a Policy-
   Contact header in the 488 response that contains the URI of its
   associated policy server.  The proxy MAY add the header field
   parameter "non-cacheable" to prevent the UAC from caching this policy
   server URI (see Section 4.4.4).

   If the local policy server URI is present in a Policy-Id header value
   of a request, then the proxy MUST NOT reject the request as described
   above (it may still reject the request for other reasons).  The proxy
   SHOULD remove the Policy-Id header value for this policy server from
   the Policy-Id header field before forwarding the request.  This value
   only increases message size and is not relevant to other proxies on
   the path.  It also would disclose the policy server URI to subsequent
   proxies.

   The proxy MAY insert a Policy-Contact header field into INVITE,



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   UPDATE or PRACK requests (or any other request that can initiate an
   offer/answer exchange) in order to convey the policy server URI to
   the UAS.  If the request already contains a Policy-Contact header
   field, the proxy MUST insert the URI ahead of all existing values at
   the beginning of the list.  A proxy MUST NOT change the order of
   existing Policy-Contact header values.

4.4.3.  UAS Behavior

   A UAS may receive an INVITE, UPDATE or PRACK request (or another
   request that can initiate offer/answer exchanges), which contains a
   Policy-Contact header filed with a list of policy server URIs.  A UAS
   that receives such a request needs to decide if it wants to accept
   the session knowing that there are policy servers involved.  If it
   accepts, it MUST contact all policy server URIs in a Policy-Contact
   header.  The UAS MUST contact the policy server URIs in the order in
   which they were contained in the Policy-Contact header, starting with
   the topmost value.

   If a UAS decides that it does not want to accept a session because
   there are policy servers involved or because one of the session
   policies received from a policy server is not acceptable, the UAS
   MUST reject the request with a 488 response.

   A UAS may receive a token from a policy server via the policy
   channel.  Since the UAS does not create a Policy-ID header, it can
   simply ignore this token.

4.4.4.  Caching the Local Policy Server URI

   A UAC may frequently need to contact the policy server in the local
   domain before sending a request.  To avoid the retransmission of the
   local policy server URI for each new request, a UA SHOULD maintain a
   cache that contains the URI of the local policy server.  A UA may
   receive this URI in a Policy-Contact header of a request or a 488
   response.  The UA may also receive the local policy server URI
   through configuration, for example, via the configuration framework
   [4].  If a UA has received a local policy server URI through
   configuration and receives another local policy server URI in a
   Policy-Contact header, it SHOULD overwrite the configured URI with
   the most recent one received in a Policy-Contact header.

   Domains can prevent a UA from caching the local policy server URI.
   This is useful, for example, if the policy server does not need to be
   involved in all sessions or the policy server URI changes from
   session to session.  A proxy can mark the URI of such a policy server
   as "non-cacheable".  A UA MUST NOT cache a non-cacheable policy
   server URI.  It SHOULD remove the current URI from the cache when



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   receiving a local policy server URI that is marked as "non-
   cacheable".  This is to avoid the use of policy server URIs that are
   outdated.

   The UA SHOULD NOT cache policy server URIs it has received from
   proxies outside of the local domain.  These policy servers may not be
   relevant for subsequent sessions, which may go to a different
   destination, traversing different domains.

   The UA MUST NOT cache tokens it may receive from a policy server.  A
   token is only valid for one request.

4.4.5.  Storing Session Policy Server URIs

   A UA discovers the list of policy servers relevant for a session
   during the initial offer/answer exchange.  It SHOULD store this list
   of policy server URIs, as part of the dialog state.  The UA SHOULD
   maintain this list until the session is terminated.  It SHOULD store
   policy server URIs in this list even if they are marked as "non-
   cacheable".  The non-cacheable parameter only refers to caching
   policy server URIs for re-use between sessions.

   If a UAC has contacted all stored policy servers before sending a
   mid-dialog request and receives a 488 in response to this request
   with a Policy-Contact header containing a new policy server URI, it
   MUST discard the stored policy server URI list for the current
   dialog.  Receiving a 488 response at this point indicates that the
   set of policy servers relevant for the current dialog has changed.
   The UAC SHOULD retry sending the request as if it was the first
   request in a dialog (i.e. without applying any policies except
   policies from the local policy server).  This way, the UAC will re-
   discover the list of policy server URIs relevant for the current
   request.

   If a UAS receives a mid-dialog request with a Policy-Contact header
   containing a list of policy server URIs that is different from the
   list stored for the dialog, then the UAS SHOULD replace the stored
   list with the one received in the Policy-Contact header field.

4.4.6.  Contacting the Policy Server

   A UA compliant to this specification MUST contact the discovered
   policy servers and apply session policies to an offer or answer
   before using the offer or answer.  If the UA does not want to contact
   the policy servers provided or the policies received for a session
   are unacceptable, it MUST NOT continue with the session.  This means
   that, the UA MUST cancel or reject a pending INVITE transaction for
   the session or terminate the session if it is already in progress.



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   A UA that receives a SIP message containing an offer or answer SHOULD
   completely process the message (e.g. according to [6]) before
   contacting the policy server.  The SIP processing of the message
   includes, for example, updating dialog state and timers as well as
   creating ACK or PRACK requests as necessary.  This ensures that
   contacting a policy server does not interfere with SIP message
   processing and timing (e.g. by inadvertently causing timers to
   expire).  This implies, for example, that a UAC which has received a
   response to an INVITE request SHOULD finish the processing of the
   response including transmitting the ACK before it contacts the policy
   server.  An important exception to this rule is discussed in the next
   paragraph.

   In some cases, a UA needs to use the offer/answer it has received in
   a SIP message to create an ACK or PRACK response for this message,
   i.e. it needs to use the offer/answer before finishing the SIP
   machinery for this message.  For example, a UAC that has received an
   offer in the response to an INVITE request needs to apply policies to
   the offer and the answer before it can send the answer in an ACK.  In
   these cases, a UA SHOULD contact the policy server even if this is
   during the processing of a SIP message.  This implies that a UA,
   which has received an offer in the response of an INVITE request,
   SHOULD contact the policy server and apply session policies before
   sending the answer in the ACK.

      Note: this assumes that the policy server can always respond
      immediately to a policy request and does not require manual
      intervention to create a policy.  This will be the case for most
      policy servers.  If, however, a policy server cannot respond with
      a policy right away, it may return a policy that temporarily
      denies the session and update this policy as the actual policy
      decision becomes available.  A delay in the response from the
      policy server to the UA would delay the transmission of the ACK
      and could trigger retransmissions of the INVITE response (also see
      the recommendations for Flow I in [9]).

   A UA MUST inform the policy server when a session is terminated via
   the policy channel.  This enables a policy server to free all
   resources it may have allocated for this session.  A policy server
   can indicate via the policy channel that it does not need to be
   contacted at the end of a session.  In this case, the UA SHOULD NOT
   inform the policy server about the termination of the session.

4.4.7.  Header Definition and Syntax







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4.4.7.1.  Policy-Id Header

   The Policy-Id header field is inserted by the UAC into INVITE, UPDATE
   or PRACK requests (or any other request that can be used to initiate
   an offer/answer exchange).  The Policy-Id header identifies all
   policy servers the UAC has contacted for this request.

   The value of a Policy-Id header consists of a policy server URI.  A
   Policy-Id header value may have an optional token parameter.  The
   token parameter contains a token the UA may receive from the policy
   server.  If the UA has received a token, it MUST be included in the
   Policy-Id header.

   The syntax of the Policy-Id header field is:

     Policy-Id        = "Policy-Id" HCOLON policyURI
                        *(COMMA  policyURI)
     policyURI        = ( SIP-URI / SIPS-URI )
                        [ SEMI token-param ] *( SEMI generic-param )
     token-param      = "token=" token

   The BNF for SIP-URI, SIPS-URI, token and generic-param is defined in
   [6].

4.4.7.2.  Policy-Contact Header

   The Policy-Contact header field can be inserted by a proxy into a 488
   response to INVITE, UPDATE or PRACK requests (or other requests that
   initiate an offer/answer exchange).  It contains a policy server URI
   that needs to be contacted by the UAC.  A proxy MAY add the "non-
   cacheable" header field parameter to indicate that a UA MUST NOT
   cache this policy server URI.

   The Policy-Contact header field can also be inserted by a proxy into
   INVITE, UPDATE and PRACK requests (or other requests that can be used
   to initiate an offer/answer exchange).  It contains an ordered list
   of policy server URIs that need to be contacted by the UAS.  The UAS
   starts to process the header field at the topmost value of this list.
   New header field values are inserted at the top.  The Policy-Contact
   header field effectively forms a stack.

   The syntax of the Policy-Contact header field is:

     Policy-Contact   = "Policy-Contact" HCOLON policyContactURI
                        *(COMMA policyContactURI)
     policyContactURI = ( SIP-URI / SIPS-URI )
                        [ SEMI "non-cacheable" ] *( SEMI generic-param )




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   The BNF for SIP-URI, SIPS-URI and generic-param is defined in [6].

   Table 1 is an extension of Tables 2 and 3 in [6].  The column 'UPD'
   is for the UPDATE method [5].

     Header field          where   proxy ACK BYE CAN INV OPT REG UPD
     _______________________________________________________________
     Policy-Id               R       rd   -   -   -   o   -   -   o
     Policy-Contact          R       a    -   -   -   o   -   -   o
     Policy-Contact         488      a    -   -   -   o   -   -   o
           Table 1: Policy-Id and Policy-Contact Header Fields

4.5.  Policy Channel

   The main task of the policy channel is to enable a UA to submit
   information about the session it is trying to establish (i.e. the
   offer and the answer) to a policy server and to receive the resulting
   session-specific policies and possible updates to these policies in
   response.

   A UA compliant to this specification MUST implement the Event Package
   for Session-Specific Session Policies [2].  It contacts a policy
   server by subscribing to this event package.

   A UA MUST use the policies it has received from the policy server in
   the current session.  If these policies are unacceptable, the UA MUST
   NOT attempt to establish the session.  When a UA receives a
   notification about a change in the current policies, it MUST apply
   the updated policies to the current session or it MUST terminate the
   session.  If the policy update causes a change in the session
   description of a session, the UA may need to re-negotiate the
   modified session description with its peer UA, for example, using a
   re-INVITE or UPDATE request.  For example, if a policy update
   disallows the use of video and video is part of the current session
   description, then the UA will need to create an new session
   description offer without video.  After receiving this offer, the
   peer UA knows that video can't be used any more and responds with the
   corresponding answer.  The re-INVITE or UPDATE message need to be
   generated in accordance to Section 4.4.1.


5.  Security Considerations

   Session policies can significantly change the behavior of a user
   agent and can be used by an attacker to compromise a user agent.  For
   example, session policies can be used to prevent a user agent from
   successfully establishing a session (e.g. by setting the available
   bandwidth to zero).  Such a policy can be submitted to the user agent



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   during a session, which may cause the UA to terminate the session.

   A user agent transmits session information to a policy server for
   session-specific policies.  This session information may contain
   sensitive data the user may not want an eavesdropper or an
   unauthorized policy server to see.  For example, the session
   information may contain the encryption keys for media streams.  Vice
   versa, session policies may also contain sensitive information about
   the network or service level agreements the service provider may not
   want to disclose to an eavesdropper or an unauthorized user agent.

   It is important to secure the communication between the proxy and the
   user agent (for session-specific policies) as well as the user agent
   and the policy server.  The following four discrete attributes need
   to be protected:

   1.  integrity of the policy server URI (for session-specific
       policies),
   2.  authentication of the policy server and, if needed, the user
       agent,
   3.  confidentiality of the messages exchanged between the user agent
       and the policy server and
   4.  ensuring that private information is not exchanged between the
       two parties, even over an confidentiality-assured and
       authenticated session.

   To protect the integrity of the policy server URI, a UA SHOULD use a
   secured transport protocol such as TLS between proxies and the UA.
   Protecting the integrity of the policy server URI is important since
   an attacker could intercept SIP messages between the UA and the proxy
   and remove the policy headers needed for session-specific policies.
   This would impede the rendezvous between UA and policy server and,
   since the UA would not contact the policy server, may prevent a UA
   from setting up a session.

   Instead of removing a policy server URI, an attacker can also modify
   the policy server URI and point the UA to a compromised policy
   server.  To prevent such an attack from being effective, it is
   RECOMMENDED that a UA authenticates policy servers.

   Policy servers SHOULD authenticate UAs to protect the information
   that is contained in a session policy.  However, a policy server may
   also frequently encounter UAs it cannot authenticate.  In these
   cases, the policy server MAY provide a generic policy that does not
   reveal sensitive information to these UAs.

   It is RECOMMENDED that administrators use SIPS URIs as policy server
   URIs so that subscriptions to session policies are transmitted over



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   TLS.

   The above security attributes are important to protect the
   communication between the user agent and policy server.  This
   document does not define the protocol used for the communication
   between user agent and policy server and merely refers to other
   specifications for this purpose.  The security considerations of
   these specifications need to address the above security aspects.


6.  IANA Considerations

6.1.  Registration of the "Policy-Id" Header

      Name of Header: Policy-Id

      Short form: none

      Normative description: Section 4.4.7 of this document

6.2.  Registration of the "Policy-Contact" Header

      Name of Header: Policy-Contact

      Short form: none

      Normative description: Section 4.4.7 of this document

6.3.  Registration of the "policy" SIP Option-Tag

      Name of option: policy

      Description: Support for the Policy-Contact and Policy-Id headers.

      SIP headers defined: Policy-Contact, Policy-Id

      Normative description: This document


Appendix A.  Acknowledgements

   Many thanks to Allison Mankin for the discussions and the suggestions
   for this draft and to Roni Even, Bob Penfield, Mary Barnes and Shida
   Schubert for reviewing the draft and providing feedback.  Many thanks
   to Vijay Gurbani for the comments and feedback.






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Appendix B.  Session-Specific Policies - Call Flows

   The following call flows illustrate the overall operation of session-
   specific policies including the policy channel protocol as defined in
   [2].

   The following abbreviations are used:

      o: offer
      o': offer modified by a policy
      po: offer policy
      a: answer
      a': answer modified by a policy
      pa: answer policy
      ps uri: policy server URI (in Policy-Contact header)
      ps id: policy server id (in Policy-Id header)

B.1.  Offer in Invite

































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   UA A       P A      PS A      PS B       P B      UA B
     |         |         |         |         |         |
     |(1) INV <o>        |         |         |         |
     |-------->|         |         |         |         |
     |(2) 488 <ps uri>   |         |         |         |
     |<--------|         |         |         |         |
     |(3) ACK  |         |         |         |         |
     |-------->|         |         |         |         |
     |(4) SUBSCRIBE <o>  |         |         |         |
     |------------------>|         |         |         |
     |(5) 200 OK         |         |         |         |
     |<------------------|         |         |         |
     |(6) NOTIFY <po>    |         |         |         |
     |<------------------|         |         |         |
     |(7) 200 OK         |         |         |         |
     |------------------>|         |         |         |
     |(8) INV <ps id, o'>|         |         |         |
     |-------->|         |         |         |         |
     |         |(9) INV <o'>       |         |         |
     |         |---------------------------->|         |
     |         |         |         |         |(10) INV <o', ps uri>
     |         |         |         |         |-------->|
     |         |         |         |(11) SUBSCRIBE <o', a>
     |         |         |         |<------------------|
     |         |         |         |(12) 200 OK        |
     |         |         |         |------------------>|
     |         |         |         |(13) NOTIFY <po, pa>
     |         |         |         |------------------>|
     |         |         |         |(14) 200 OK        |
     |         |         |         |<------------------|
     |         |         |         |         |(15) 200 OK <a'>
     |         |         |         |         |<--------|
     |         |(16) 200 OK <a'>   |         |         |
     |         |<----------------------------|         |
     |(17) 200 OK <a'>   |         |         |         |
     |<--------|         |         |         |         |
     |(18) ACK |         |         |         |         |
     |------------------------------------------------>|
     |(19) SUBSCRIBE <o', a'>      |         |         |
     |------------------>|         |         |         |
     |(20) 200 OK        |         |         |         |
     |<------------------|         |         |         |
     |(21) NOTIFY <pa>   |         |         |         |
     |<------------------|         |         |         |
     |(22) 200 OK        |         |         |         |
     |------------------>|         |         |         |
     |         |         |         |         |         |
     |         |         |         |         |         |



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B.2.  Offer in Response

   UA A       P A      PS A      PS B       P B      UA B
     |         |         |         |         |         |
     |(1) INV  |         |         |         |         |
     |-------->|         |         |         |         |
     |(2) 488 <ps uri>   |         |         |         |
     |<--------|         |         |         |         |
     |(3) ACK  |         |         |         |         |
     |-------->|         |         |         |         |
     |(4) SUBSCRIBE      |         |         |         |
     |------------------>|         |         |         |
     |(5) 200 OK         |         |         |         |
     |<------------------|         |         |         |
     |(6) NOTIFY         |         |         |         |
     |<------------------|         |         |         |
     |(7) 200 OK         |         |         |         |
     |------------------>|         |         |         |
     |(8) INV <ps id>    |         |         |         |
     |-------->|         |         |         |         |
     |         |(9) INV  |         |         |         |
     |         |---------------------------->|         |
     |         |         |         |         |(10) INV <ps uri>
     |         |         |         |         |-------->|
     |         |         |         |(11) SUBSCRIBE <o> |
     |         |         |         |<------------------|
     |         |         |         |(12) 200 OK        |
     |         |         |         |------------------>|
     |         |         |         |(13) NOTIFY <po>   |
     |         |         |         |------------------>|
     |         |         |         |(14) 200 OK        |
     |         |         |         |<------------------|
     |         |         |         |         |(15) 200 OK <o'>
     |         |         |         |         |<--------|
     |         |(16) 200 OK <o'>   |         |         |
     |         |<----------------------------|         |
     |(17) 200 OK <o'>   |         |         |         |
     |<--------|         |         |         |         |
     |(18) SUBSCRIBE <o', a>       |         |         |
     |------------------>|         |         |         |
     |(19) 200 OK        |         |         |         |
     |<------------------|         |         |         |
     |(20) NOTIFY <po, pa>         |         |         |
     |<------------------|         |         |         |
     |(21) 200 OK        |         |         |         |
     |------------------>|         |         |         |
     |(22) ACK <a'>      |         |         |         |
     |------------------------------------------------>|



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     |         |         |         |(23) SUBSCRIBE <o', a'>
     |         |         |         |<------------------|
     |         |         |         |(24) 200 OK        |
     |         |         |         |------------------>|
     |         |         |         |(25) NOTIFY <po, pa>
     |         |         |         |------------------>|
     |         |         |         |(26) 200 OK        |
     |         |         |         |<------------------|
     |         |         |         |         |         |
     |         |         |         |         |         |


7.  References

7.1.  Normative References

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

   [2]  Hilt, V. and G. Camarillo, "A Session Initiation Protocol (SIP)
        Event Package for Session-Specific Session Policies.",
        draft-ietf-sipping-policy-package-03 (work in progress),
        January 2007.

   [3]  Hilt, V., Camarillo, G., and J. Rosenberg, "A User Agent Profile
        Data Set for Media Policy",
        draft-ietf-sipping-media-policy-dataset-02 (work in progress),
        October 2006.

   [4]  Petrie, D. and S. Channabasappa, "A Framework for Session
        Initiation Protocol User Agent Profile Delivery",
        draft-ietf-sipping-config-framework-10 (work in progress),
        January 2007.

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

   [6]  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.

7.2.  Informative References

   [7]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [8]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
        Session Description Protocol (SDP)", RFC 3264, June 2002.



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   [9]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
        "Best Current Practices for Third Party Call Control (3pcc) in
        the Session Initiation Protocol (SIP)", BCP 85, RFC 3725,
        April 2004.


Authors' Addresses

   Volker Hilt
   Bell Labs/Alcatel-Lucent
   101 Crawfords Corner Rd
   Holmdel, NJ  07733
   USA

   Email: volkerh@bell-labs.com


   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.com


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

   Email: jdrosen@cisco.com


















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