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SIPCORE Working Group                                        C. Holmberg
Internet-Draft                                               I. Sedlacek
Intended status: Standards Track                                Ericsson
Expires: June 7, 2012                                   December 5, 2011


    Requirements for indication of features supported by a SIP proxy
              draft-ietf-sipcore-proxy-feature-reqs-03.txt

Abstract

   The Session Initiation Protocol (SIP) "Caller Preferences" extension
   defined in RFC 3840 provides a mechanism that allows a SIP message to
   convey information relating to the originator's supported features/
   capabilities.  This document defines requirements for a mechanism
   that would allow SIP proxies to convey information relating to the
   proxy's supported features/capabilities.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 7, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as



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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Use-case: IMS Service Continuity, handover of session
           in alerting state . . . . . . . . . . . . . . . . . . . . . 3
     1.2.  Use-case: IMS Enhanced Service Continuity . . . . . . . . . 3
       1.2.1.  Use-case: IMS Enhanced Service Continuity, ATCF
               discovery . . . . . . . . . . . . . . . . . . . . . . . 4
       1.2.2.  Use-case: IMS Enhanced Service Continuity,
               identifying sessions subject to handover  . . . . . . . 4
       1.2.3.  Use-case: IMS Enhanced Service Continuity,
               indicating handover subfeature support  . . . . . . . . 4
     1.3.  Use-case: IMS Inter-UE Transfer . . . . . . . . . . . . . . 5
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . . . 5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   7.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9

























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1.  Introduction

   The Session Initiation Protocol (SIP) "Caller Preferences" extension
   defined in RFC 3840 [RFC3840] provides a mechanism that allows a SIP
   message to convey information relating to the originator's supported
   features/capabilities.

   It can be useful for SIP proxies to indicate supported feature/
   capabilities, that might trigger actions and enable functions in
   other SIP entities.

   This document defines requirements for a mechanism that would allow
   SIP proxies to convey information related to the proxy's supported
   features/capabilities.

1.1.  Use-case: IMS Service Continuity, handover of session in alerting
      state

   The 3rd Generation Partnership Project (3GPP) defines a IP Multimedia
   Subsystem (IMS) Service Continuity mechanism [3GPP.23.237] for
   handover of Packet Switched (PS) sessions to Circuit Switched (CS)
   calls.

   The handover is controlled by a Service Centralization and Continuity
   Application Server (SCC AS).  When a session is established the User
   Equipment (UE) needs to determine whether SCC AS in signalling path
   of the session supports handover of session in alerting state (i.e.
   180 Ringing response has already been sent or received but the dialog
   is not confirmed dialog yet) or not.

   When handover occurs and a session in alerting state exists and both
   UE and SCC AS indicated support of the handover of session in
   alerting state, then the UE and SCC AS perform handover for the
   session in alerting state.

   NOTE: The UE indicates the support of the handover of session in
   alerting state by the feature tag included in Contact header field.

1.2.  Use-case: IMS Enhanced Service Continuity

   The 3rd Generation Partnership Project (3GPP) defines a IP Multimedia
   Subsystem (IMS) Service Continuity mechanism [3GPP.23.237] for
   handover of Packet Switched (PS) sessions to Circuit Switched (CS)
   calls.  The handover can be performed by a Service Centralization and
   Continuity Application Server (SCC AS), or by a SCC AS together with
   an Access Transfer Control Function (ATCF), that acts as a SIP proxy.
   Delegating part of the session handover functionality to an ATCF
   provides advantages related to voice interruption during session



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   handover etc, since the ATCF is located in the same network as the
   user.

1.2.1.  Use-case: IMS Enhanced Service Continuity, ATCF discovery

   In order for an SCC AS to delegate part of the session handover
   functionality to an ATCF, when the SCC AS is informed by the
   registrar about an accepted REGISTER transaction, the SCC AS needs to
   determine whether a proxy supporting the ATCF functionality is in the
   registration path.

1.2.2.  Use-case: IMS Enhanced Service Continuity, identifying sessions
        subject to handover

   In order for ATCF to perform the delegated part of the session
   handover functionality, when a session is set up, the ATCF needs to
   determine whether a SIP proxy supporting the SCC AS functionality is
   in the signalling path of the session.

1.2.3.  Use-case: IMS Enhanced Service Continuity, indicating handover
        subfeature support

   As the session handover functionality has been specified over several
   3GPP releases, some subfeatures of the handover functionality are
   optional.  Examples are:

   - The handover of sessions with audio on hold (called the MSC server
   assisted mid-call feature); and

   - The handover of sessions where a 180 Ringing response to the
   initial SIP INVITE request has already been sent or received but a
   final response has not been sent or received yet (called the SRVCC
   for calls in alerting phase).

   The SCC AS needs to be aware of support of those subfeatures in ATCF,
   in order for the UE and the SCC AS to execute the correct handling
   when the handover occurs.

   When ATCF receives a SIP REGISTER request, the ATCF indicates the
   support of those subfeatures along the indication of ATCF
   functionality.

   When SCC AS is informed about the new/updated binding where a proxy
   indicated support of ATCF functionality along with support of those
   subfeatures, the SCC AS discovers the support of those subfeatures in
   the ATCF.





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1.3.  Use-case: IMS Inter-UE Transfer

   The 3rd Generation Partnership Project (3GPP) defines inter-UE
   transfer enhancements [3GPP.24.837] which enhance delivery of media
   of a session to several User Equipments (UE).

   The Service Centralization and Continuity Application Server (SCC AS)
   serving one of the UEs acts as local hub for the session.  The UE
   controls the media of the session and is called controller UE.

   Triggered by requests from the controller UE, the SCC AS serving the
   controller UE transfers media of the session to other UEs, called
   controlee UEs, by sending INVITE request offering the media to be
   transferred.

   When an INVITE request is routed to the UE, the SCC AS serving the UE
   needs to determine whether a SIP proxy supporting the inter-UE
   transfer enhancements functionality (i.e.  SCC AS of the controller
   UE) is already in the signalling path.

   If so, the SCC AS proxies the signalling without further handling as
   there is already an existing local hub for the session.

   If not, the SCC AS acts as local hub for the session.


2.  Conventions

   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 BCP 14, RFC 2119
   [RFC2119].


3.  Requirements

   REQ-1: It MUST be possible for a SIP proxy to indicate, and convey to
   other SIP entities in the signalling path of a registration request,
   support of a particular feature/capability.

   REQ-2: It MUST be possible for a SIP proxy to indicate, and convey to
   other SIP entities in the signalling path of a dialog-forming
   request, support of a particular feature/capability.

   REQ-3: It MUST be possible for a SIP proxy to indicate that the
   indicated support of a feature/capability only applies to other SIP
   entities in the direction towards one of the SIP endpoints in the
   signalling path.



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   REQ-4: A SIP proxy MUST NOT, when indicating support of a feature/
   capability, make any assumptions that SIP entities in the signalling
   path that receive the indicator will support, or understand the
   meaning of, the feature/capability, or even support the proxy
   feature/capability indication mechanism as a whole.

   REQ-5: A SIP proxy MUST be able to indicate support of a feature/
   capability to other SIP entities in the signaling path, even if some
   SIP entities in the signaling path (possibly including the UAC and/or
   UAS) do not support, or understand the meaning of, the feature/
   capability, or even support the proxy feature/capability indication
   mechanism as a whole.

   REQ-6: It MUST be possible to indicate whether indicated support of a
   feature/capability applies to specific registration, to a specific
   dialog, or to all dialogs created as part of INVITE transaction.

   NOTE: This requirement might be fully implemented as part of the
   protocol mechanism, or parts might be left to be specified in a
   feature/capability specification, or it might be left to be specified
   in a feature/capability specification completely.

   REQ-7: It MUST be possible to assign additional parameters (either as
   a single value, or a list of values) to a feature/capability
   indicator, in order to provide additional information about the
   feature/capability.

   REQ-8: If a SIP entity that supports the proxy feature/capability
   indication mechanism receives a feature support indication that it
   does not understand, it MUST act as if it hadn't received the
   indication.

   REQ-9: If a SIP entity that does not support the proxy feature/
   capability indication mechanism receives a feature support
   indication, it MUST act as if it hadn't received the indication.

   REQ-10: SIP entities on the path of the SIP message MUST be able to
   inspect the feature/capability support indicators introduced by other
   entities.

   REQ-11: A feature/capability support indicator MUST only be used to
   indicate support of a feature/capability, and MUST NOT be used to
   indicate whether procedures associated with the feature/capability
   have been applied or not.

   REQ-12: It MUST be possible to determine which features/capabilities
   are supported by the same proxy.




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   REQ-13: A SIP proxy that indicates support of a feature/capability
   associated with a dialog MUST take the necessary steps to ensure it
   is part of the signaling path of the remainder that dialog, by
   indicating the support of the feature/capability as part of a dialog
   forming transaction, or as part of a target refresh request within a
   dialog.

   REQ-14: A procedure for registering feature/capability indication
   values, which prevents name collisions of indicators, with IANA MUST
   be defined.


4.  Security Considerations

   Feature/capability support indications can provide sensitive
   information about a SIP entity.  RFC 3840 cautions against providing
   sensitive information to another party.  Once this information is
   given out, any use may be made of it.


5.  IANA Considerations

   None identified.


6.  Acknowledgements

   Thanks to Paul Kyzivat and Robert Sparks for their comments and
   guidance on the mailing list.  Thanks to Andrew Allen and Dale Worley
   for providing text on additional use-cases.  Thanks to Cullen
   Jennings for providing text on additional requirements.  Thanks to
   Dale Worley for providing comments and text improvement suggestions.
   Thanks to Hadriel Kaplan for telling us how SBCs mess up the
   mechanisms we try to specify.

   Thanks to Robert Sparks, Adam Roach and Paul Kyzivat for giving
   working procedure guidance.


7.  Change Log

   [RFC EDITOR NOTE: Please remove this section when publishing]

   Changes from draft-ietf-sipcore-proxy-feature-reqs-02
   o  Changes based on comments from Robert Sparks at IETF#82 (http://
      www.ietf.org/mail-archive/web/sipcore/current/msg04473.html).





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   o  REQ-10: now talks about having access to the information, rather
      than talking specifically about making routing decisions.
   o  REQ-8: editorial modification, to better distinguish it from
      REQ-9.
   o  New REQ-13 (old REQ-13 becomes REQ-14) added.
   o  REQ-14: indication that the IANA registration procedure will
      prevent name collision between feature indications.
   Changes from draft-ietf-sipcore-proxy-feature-reqs-01
   o  New REQ-12 added (old REQ-12 becomes REQ-13).
   o  New use-case added (section 1.2.3).
   Changes from draft-ietf-sipcore-proxy-feature-reqs-00
   o  New REQ-5 added (IETF#81).
   o  New REQ-9 added (Dale Worley).
   o  Text added to REQ-4 and REQ-5, indicating that the requirement
      applies also in cases where an entity does not support the
      mechanism as a whole (Dale Worley).
   o  Usage of "session establishment transactions" terminology in
      REQ-6, in order to avoid misunderstanding of "session" (Dale
      Worley).
   o  Editorial correction in REQ-7: "additional parameter"->"additional
      parameters"
   o  Editorial clarifications to use-cases.


8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2.  Informative References

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840, August 2004.

   [3GPP.23.237]
              3GPP, "IP Multimedia Subsystem (IMS) Service Continuity;
              Stage 2", 3GPP TS 23.237 10.7.0, September 2011.

   [3GPP.24.837]
              3GPP, "IP Multimedia (IM) Core Network (CN) subsystem
              inter-UE transfer enhancements; Stage 3", 3GPP TR 24.837
              10.0.0, April 2011.






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Authors' Addresses

   Christer Holmberg
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: christer.holmberg@ericsson.com


   Ivo Sedlacek
   Ericsson
   Scheelevaegen 19C
   Lund  22363
   Sweden

   Email: ivo.sedlacek@ericsson.com

































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