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RFC 6230 - Media Control Channel Framework


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Internet Engineering Task Force (IETF)                        C. Boulton
Request for Comments: 6230                               NS-Technologies
Category: Standards Track                                   T. Melanchuk
ISSN: 2070-1721                                               Rainwillow
                                                            S. McGlashan
                                                         Hewlett-Packard
                                                                May 2011

                    Media Control Channel Framework

Abstract

   This document describes a framework and protocol for application
   deployment where the application programming logic and media
   processing are distributed.  This implies that application
   programming logic can seamlessly gain access to appropriate resources
   that are not co-located on the same physical network entity.  The
   framework uses the Session Initiation Protocol (SIP) to establish an
   application-level control mechanism between application servers and
   associated external servers such as media servers.

   The motivation for the creation of this framework is to provide an
   interface suitable to meet the requirements of a centralized
   conference system, where the conference system can be distributed, as
   defined by the XCON working group in the IETF.  It is not, however,
   limited to this scope.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6230.

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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  4
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Control Channel Setup  . . . . . . . . . . . . . . . . . . . . 10
     4.1.  Control Client SIP UAC Behavior  . . . . . . . . . . . . . 10
     4.2.  Control Server SIP UAS Behavior  . . . . . . . . . . . . . 13
   5.  Establishing Media Streams - Control Client SIP UAC
       Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   6.  Control Framework Interactions . . . . . . . . . . . . . . . . 15
     6.1.  General Behavior for Constructing Requests . . . . . . . . 17
     6.2.  General Behavior for Constructing Responses  . . . . . . . 17
     6.3.  Transaction Processing . . . . . . . . . . . . . . . . . . 18
       6.3.1.  CONTROL Transactions . . . . . . . . . . . . . . . . . 18
       6.3.2.  REPORT Transactions  . . . . . . . . . . . . . . . . . 19
       6.3.3.  K-ALIVE Transactions . . . . . . . . . . . . . . . . . 21
       6.3.4.  SYNC Transactions  . . . . . . . . . . . . . . . . . . 22
   7.  Response Code Descriptions . . . . . . . . . . . . . . . . . . 24
     7.1.  200 Response Code  . . . . . . . . . . . . . . . . . . . . 25
     7.2.  202 Response Code  . . . . . . . . . . . . . . . . . . . . 25
     7.3.  400 Response Code  . . . . . . . . . . . . . . . . . . . . 25
     7.4.  403 Response Code  . . . . . . . . . . . . . . . . . . . . 25
     7.5.  405 Response Code  . . . . . . . . . . . . . . . . . . . . 25
     7.6.  406 Response Code  . . . . . . . . . . . . . . . . . . . . 25
     7.7.  420 Response Code  . . . . . . . . . . . . . . . . . . . . 25
     7.8.  421 Response Code  . . . . . . . . . . . . . . . . . . . . 25
     7.9.  422 Response Code  . . . . . . . . . . . . . . . . . . . . 25
     7.10. 423 Response Code  . . . . . . . . . . . . . . . . . . . . 25
     7.11. 481 Response Code  . . . . . . . . . . . . . . . . . . . . 26
     7.12. 500 Response Code  . . . . . . . . . . . . . . . . . . . . 26
   8.  Control Packages . . . . . . . . . . . . . . . . . . . . . . . 26
     8.1.  Control Package Name . . . . . . . . . . . . . . . . . . . 26

     8.2.  Framework Message Usage  . . . . . . . . . . . . . . . . . 26
     8.3.  Common XML Support . . . . . . . . . . . . . . . . . . . . 27
     8.4.  CONTROL Message Bodies . . . . . . . . . . . . . . . . . . 27
     8.5.  REPORT Message Bodies  . . . . . . . . . . . . . . . . . . 27
     8.6.  Audit  . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     8.7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 28
   9.  Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . . . 28
     9.1.  Control Framework Formal Syntax  . . . . . . . . . . . . . 28
     9.2.  Control Framework Dialog Identifier SDP Attribute  . . . . 31
   10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
   11. Extensibility  . . . . . . . . . . . . . . . . . . . . . . . . 35
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 36
     12.1. Session Establishment  . . . . . . . . . . . . . . . . . . 36
     12.2. Transport-Level Protection . . . . . . . . . . . . . . . . 36
     12.3. Control Channel Policy Management  . . . . . . . . . . . . 37
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
     13.1. Control Packages Registration Information  . . . . . . . . 38
       13.1.1. Control Package Registration Template  . . . . . . . . 39
     13.2. Control Framework Method Names . . . . . . . . . . . . . . 39
     13.3. Control Framework Status Codes . . . . . . . . . . . . . . 39
     13.4. Control Framework Header Fields  . . . . . . . . . . . . . 40
     13.5. Control Framework Port . . . . . . . . . . . . . . . . . . 40
     13.6. Media Type Registrations . . . . . . . . . . . . . . . . . 40
       13.6.1. Registration of MIME Media Type application/cfw  . . . 41
       13.6.2. Registration of MIME Media Type
               application/framework-attributes+xml . . . . . . . . . 42
     13.7. 'cfw-id' SDP Attribute . . . . . . . . . . . . . . . . . . 42
     13.8. URN Sub-Namespace for
           urn:ietf:params:xml:ns:control:framework-attributes  . . . 43
     13.9. XML Schema Registration  . . . . . . . . . . . . . . . . . 43
   14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 44
   15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 44
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 44
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 44
     16.2. Informative References . . . . . . . . . . . . . . . . . . 46
   Appendix A.  Common Package Components . . . . . . . . . . . . . . 47
     A.1.  Common Dialog/Multiparty Reference Schema  . . . . . . . . 47

1.  Introduction

   Real-time media applications are often developed using an
   architecture where the application logic and media processing
   activities are distributed.  Commonly, the application logic runs on
   "application servers", but the processing runs on external servers,
   such as "media servers".  This document focuses on the framework and
   protocol between the application server and external processing
   server.  The motivation for this framework comes from a set of
   requirements for Media Server Control, which can be found in "Media
   Server Control Protocol Requirements" [RFC5167].  While the Framework
   is not specific to media server control, it is the primary driver and
   use case for this work.  It is intended that the framework contained
   in this document be able to be used for a variety of device control
   scenarios (for example, conference control).

   This document does not define a particular SIP extension for the
   direct control of external components.  Rather, other documents,
   known as "Control Packages", extend the Control Framework described
   by this document.  Section 8 provides a comprehensive set of
   guidelines for creating such Control Packages.

   Current IETF device control protocols, such as Megaco [RFC5125],
   while excellent for controlling media gateways that bridge separate
   networks, are troublesome for supporting media-rich applications in
   SIP networks.  This is because Megaco duplicates many of the
   functions inherent in SIP.  Rather than using a single protocol for
   session establishment and application media processing, application
   developers need to translate between two separate mechanisms.
   Moreover, the model provided by the framework presented here, using
   SIP, better matches the application programming model than does
   Megaco.

   SIP [RFC3261] provides the ideal rendezvous mechanism for
   establishing and maintaining control connections to external server
   components.  The control connections can then be used to exchange
   explicit command/response interactions that allow for media control
   and associated command response results.

2.  Conventions and Terminology

   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, [RFC2119], as
   scoped to those conformance targets.

   The following additional terms are defined for use in this document:

   User Agent Client (UAC):  As specified in [RFC3261].

   User Agent Server (UAS):  As specified in [RFC3261].

   B2BUA:  A B2BUA is a Back-to-Back SIP User Agent.

   Control Server:  A Control Server is an entity that performs a
      service, such as media processing, on behalf of a Control Client.
      For example, a media server offers mixing, announcement, tone
      detection and generation, and play and record services.  The
      Control Server has a direct Real-Time Transport Protocol (RTP)
      [RFC3550] relationship with the source or sink of the media flow.
      In this document, we often refer to the Control Server simply as
      "the Server".

   Control Client:  A Control Client is an entity that requests
      processing from a Control Server.  Note that the Control Client
      might not have any processing capabilities whatsoever.  For
      example, the Control Client may be an application server (B2BUA)
      or other endpoint requesting manipulation of a third party's media
      stream that terminates on a media server acting in the role of a
      Control Server.  In this document, we often refer to the Control
      Client simply as "the Client".

   Control Channel:  A Control Channel is a reliable connection between
      a Client and Server that is used to exchange Framework messages.
      The term "Connection" is used synonymously within this document.

   Framework Message:  A Framework message is a message on a Control
      Channel that has a type corresponding to one of the Methods
      defined in this document.  A Framework message is often referred
      to by its method, such as a "CONTROL message".

   Method:  A Method is the type of a Framework message.  Four Methods
      are defined in this document: SYNC, CONTROL, REPORT, and K-ALIVE.

   Control Command:  A Control Command is an application-level request
      from a Client to a Server.  Control Commands are carried in the
      body of CONTROL messages.  Control Commands are defined in
      separate specifications known as "Control Packages".

   Framework Transaction:  A Framework Transaction is defined as a
      sequence composed of a Control Framework message originated by
      either a Control Client or Control Server and responded to with a
      Control Framework response code message.  Note that the Control
      Framework has no "provisional" responses.  A Control Framework

      transaction is referenced throughout the document as a
      'Transaction-Timeout'.

   Transaction-Timeout:  The maximum allowed time between a Control
      Client or Server issuing a Framework message and it arriving at
      the destination.  The value for 'Transaction-Timeout' is 10
      seconds.

3.  Overview

   This document details mechanisms for establishing, using, and
   terminating a reliable transport connection channel using SIP and the
   Session Description Protocol offer/answer [RFC3264] exchange.  The
   established connection is then used for controlling an external
   server.  The following text provides a non-normative overview of the
   mechanisms used.  Detailed, normative guidelines are provided later
   in the document.

   Control Channels are negotiated using standard SIP mechanisms that
   would be used in a similar manner to creating a SIP multimedia
   session.  Figure 1 illustrates a simplified view of the mechanism.
   It highlights a separation of the SIP signaling traffic and the
   associated Control Channel that is established as a result of the SIP
   interactions.

   Initial analysis into the Control Framework, as documented in
   [MSCL-THOUGHTS], established the following.  One might ask, "If all
   we are doing is establishing a TCP connection to control the media
   server, why do we need SIP?"  This is a reasonable question.  The key
   is that we use SIP for media session establishment.  If we are using
   SIP for media session establishment, then we need to ensure the URI
   used for session establishment resolves to the same node as the node
   for session control.  Using the SIP routing mechanism, and having the
   server initiate the TCP connection back, ensures this works.  For
   example, the URI sip:myserver.example.com may resolve to sip:
   server21.farm12.northeast.example.net, whereas the URI
   http://myserver.example.com may resolve to
   http://server41.httpfarm.central.example.net.  That is, the host part
   is not necessarily unambiguous.

   The use of SIP to negotiate the Control Channel provides many
   inherent capabilities, which include:

   o  Service location - Use SIP Proxies and Back-to-Back User Agents
      for locating Control Servers.

   o  Security mechanisms - Leverage established security mechanisms
      such as Transport Layer Security (TLS) and Client Authentication.

   o  Connection maintenance - The ability to re-negotiate a connection,
      ensure it is active, and so forth.

   o  Application agnostic - Generic protocol allows for easy extension.

   As mentioned in the previous list, one of the main benefits of using
   SIP as the session control protocol is the "Service Location"
   facilities provided.  This applies both at a routing level, where
   [RFC3263] provides the physical location of devices, and at the
   service level, using Caller Preferences [RFC3840] and Callee
   Capabilities [RFC3841].  The ability to select a Control Server based
   on service-level capabilities is extremely powerful when considering
   a distributed, clustered architecture containing varying services
   (for example, voice, video, IM).  More detail on locating Control
   Server resources using these techniques is outlined in Section 4.1 of
   this document.

           +--------------SIP Traffic--------------+
          |                                       |
          v                                       v
       +-----+                                 +--+--+
       | SIP |                                 | SIP |
       |Stack|                                 |Stack|
   +---+-----+---+                         +---+-----+---+
   |   Control   |                         |   Control   |
   |   Client    |<----Control Channel---->|   Server    |
   +-------------+                         +-------------+

                       Figure 1: Basic Architecture

   The example from Figure 1 conveys a 1:1 connection between the
   Control Client and the Control Server.  It is possible, if required,
   for the client to request multiple Control Channels using separate
   SIP INVITE dialogs between the Control Client and the Control Server
   entities.  Any of the connections created between the two entities
   can then be used for Server control interactions.  The control
   connections are orthogonal to any given media session.  Specific
   media session information is incorporated in control interaction
   commands, which themselves are defined in external packages, using
   the XML schema defined in Appendix A.  The ability to have multiple
   Control Channels allows for stronger redundancy and the ability to
   manage high volumes of traffic in busy systems.

   Consider the following simple example for session establishment
   between a Client and a Server.  (Note: Some lines in the examples are
   removed for clarity and brevity.)  Note that the roles discussed are
   logical and can change during a session, if the Control Package
   allows.

   The Client constructs and sends a standard SIP INVITE request, as
   defined in [RFC3261], to the external Server.  The Session
   Description Protocol (SDP) payload includes the required information
   for Control Channel negotiation and is the primary mechanism for
   conveying support for this specification.  The application/cfw MIME
   type is defined in this document to convey the appropriate SDP format
   for compliance to this specification.  The Connection-Oriented Media
   (COMEDIA) [RFC4145] specification for setting up and maintaining
   reliable connections is used as part of the negotiation mechanism
   (more detail available in later sections).  The Client also includes
   the 'cfw-id' SDP attribute, as defined in this specification, which
   is a unique identifier used to correlate the underlying Media Control
   Channel with the offer/answer exchange.

   Client Sends to External Server:

   INVITE sip:External-Server@example.com SIP/2.0
   To: <sip:External-Server@example.com>
   From: <sip:Client@example.com>;tag=64823746
   Via: SIP/2.0/UDP client.example.com;branch=z9hG4bK72d
   Call-ID: 7823987HJHG6
   Max-Forwards: 70
   CSeq: 1 INVITE
   Contact: <sip:Client@clientmachine.example.com>
   Content-Type: application/sdp
   Content-Length: [..]

   v=0
   o=originator 2890844526 2890842808 IN IP4 controller.example.com
   s=-
   c=IN IP4 controller.example.com
   m=application 49153 TCP cfw
   a=setup:active
   a=connection:new
   a=cfw-id:H839quwhjdhegvdga

   On receiving the INVITE request, an external Server supporting this
   mechanism generates a 200 OK response containing appropriate SDP and
   formatted using the application/cfw MIME type specified in this
   document.  The Server inserts its own unique 'cfw-id' SDP attribute,
   which differs from the one received in the INVITE (offer).

   External Server Sends to Client:

SIP/2.0 200 OK
To: <sip:External-Server@example.com>;tag=28943879
From: <sip:Client@example.com>;tag=64823746
Via: SIP/2.0/UDP client.example.com;branch=z9hG4bK72d;received=192.0.2.4

Call-ID: 7823987HJHG6
CSeq: 1 INVITE
Contact: <sip:External-Server@servermachine.example.com>
Content-Type: application/sdp
Content-Length: [..]

v=0
o=responder 2890844526 2890842808 IN IP4 server.example.com
s=-
c=IN IP4 mserver.example.com
m=application 7563 TCP cfw
a=setup:passive
a=connection:new
a=cfw-id:U8dh7UHDushsdu32uha

   The Control Client receives the SIP 200 OK response and extracts the
   relevant information (also sending a SIP ACK).  It creates an
   outgoing (as specified by the SDP 'setup' attribute of 'active') TCP
   connection to the Control Server.  The connection address (taken from
   'c=') and port (taken from 'm=') are used to identify the remote port
   in the new connection.

   Once established, the newly created connection can be used to
   exchange requests and responses as defined in this document.  If
   required, after the Control Channel has been set up, media sessions
   can be established using standard SIP Third Party Call Control (3PCC)
   [RFC3725].

   Figure 2 provides a simplified example where the framework is used to
   control a User Agent's RTP session.

                         +--------Control SIP Dialog(1)---------+
                         |                                      |
                         v                                      v
                      +-----+                                +--+--+
     +------(2)------>| SIP |---------------(2)------------->| SIP |
     |                |Stack|                                |Stack|
     |            +---+-----+---+                        +---+-----+---+
     |            |             |                        |             |
     |            |   Control   |<--Control Channel(1)-->|             |
     |            |   Client    |                        |   Control   |
     |            +-------------+                        |   Server    |
  +--+--+                                                |             |
  |User |                                                |             |
  |Agent|<=====================RTP(2)===================>|             |
  +-----+                                                +-------------+

                    Figure 2: Participant Architecture

   The link (1) represents the SIP INVITE dialog usage and dedicated
   Control Channel previously described in this overview section.  The
   link (2) from Figure 2 represents the User Agent SIP INVITE dialog
   usage interactions and associated media flow.  A User Agent creates a
   SIP INVITE dialog usage with the Control Client entity.  The Control
   Client entity then creates a SIP INVITE dialog usage to the Control
   Server, using B2BUA type functionality.  Using the interaction
   illustrated by (2), the Control Client negotiates media capabilities
   with the Control Server, on behalf of the User Agent, using SIP 3PCC.
   [RFC3725].

4.  Control Channel Setup

   This section describes the setup, using SIP, of the dedicated Control
   Channel.  Once the Control Channel has been established, commands can
   be exchanged (as discussed in Section 6).

4.1.  Control Client SIP UAC Behavior

   When a UAC wishes to establish a Control Channel, it MUST construct
   and transmit a new SIP INVITE request for Control Channel setup.  The
   UAC MUST construct the INVITE request as defined in [RFC3261].

   If a reliable response is received (as defined in [RFC3261] and
   [RFC3262]), the mechanisms defined in this document are applicable to
   the newly created SIP INVITE dialog usage.

   The UAC SHOULD include a valid session description (an 'offer' as
   defined in [RFC3264]) in an INVITE request using the Session
   Description Protocol defined in [RFC4566] but MAY choose an offer-
   less INVITE as per [RFC3261].  The SDP SHOULD be formatted in
   accordance with the steps below and using the MIME type application/
   cfw, which is registered in Section 13.  The following information
   defines the composition of specific elements of the SDP payload the
   offerer MUST adhere to when used in a SIP-based offer/answer exchange
   using SDP and the application/cfw MIME type.  The SDP being
   constructed MUST contain only a single occurrence of a Control
   Channel definition outlined in this specification but can contain
   other media lines if required.

   The Connection Data line in the SDP payload is constructed as
   specified in [RFC4566]:

   c=<nettype> <addrtype> <connection-address>

   The first sub-field, <nettype>, MUST equal the value "IN".  The
   second sub-field, <addrtype>, MUST equal either "IP4" or "IP6".  The
   third sub-field for Connection Data is <connection-address>.  This

   supplies a representation of the SDP originator's address, for
   example, DNS/IP representation.  The address is the address used for
   connections.

   Example:

   c=IN IP4 controller.example.com

   The SDP MUST contain a corresponding Media Description entry:

   m=<media> <port> <proto> <fmt>

   The first "sub-field", <media>, MUST equal the value "application".
   The second sub-field, <port>, MUST represent a port on which the
   constructing client can receive an incoming connection if required.
   The port is used in combination with the address specified in the
   Connection Data line defined previously to supply connection details.
   If the entity constructing the SDP can't receive incoming
   connections, it must still enter a valid port entry.  The use of the
   port value '0' has the same meaning as defined in a SIP offer/answer
   exchange [RFC3264].  The Control Framework has a default port defined
   in Section 13.5.  This value is default, although a client is free to
   choose explicit port numbers.  However, SDP SHOULD use the default
   port number, unless local policy prohibits its use.  Using the
   default port number allows network administrators to manage firewall
   policy for Control Framework interactions.  The third sub-field,
   <proto>, compliant to this specification, MUST support the values
   "TCP" and "TCP/TLS".  Implementations MUST support TLS as a
   transport-level security mechanism for the Control Channel, although
   use of TLS in specific deployments is optional.  Control Framework
   implementations MUST support TCP as a transport protocol.  When an
   entity identifies a transport value but is not willing to establish
   the session, it MUST respond using the appropriate SIP mechanism.
   The <fmt> sub-field MUST contain the value "cfw".

   The SDP MUST also contain a number of SDP media attributes (a=) that
   are specifically defined in the COMEDIA [RFC4145] specification.  The
   attributes provide connection negotiation and maintenance parameters.
   It is RECOMMENDED that a Controlling UAC initiate a connection to an
   external Server but that an external Server MAY negotiate and
   initiate a connection using COMEDIA, if network topology prohibits
   initiating connections in a certain direction.  An example of the
   COMEDIA attributes is:

                           a=setup:active
                           a=connection:new

   This example demonstrates a new connection that will be initiated
   from the owner of the SDP payload.  The connection details are
   contained in the SDP answer received from the UAS.  A full example of
   an SDP payload compliant to this specification can be viewed in
   Section 3.  Once the SDP has been constructed along with the
   remainder of the SIP INVITE request (as defined in [RFC3261]), it can
   be sent to the appropriate location.  The SIP INVITE dialog usage and
   appropriate control connection is then established.

   A SIP UAC constructing an offer MUST include the 'cfw-id' SDP
   attribute as defined in Section 9.2.  The 'cfw-id' attribute
   indicates an identifier that can be used within the Control Channel
   to correlate the Control Channel with this SIP INVITE dialog usage.
   The 'cfw-id' attribute MUST be unique in the context of the
   interaction between the UAC and UAS and MUST NOT clash with instances
   of the 'cfw-id' used in other SIP offer/answer exchanges.  The value
   chosen for the 'cfw-id' attribute MUST be used for the entire
   duration of the associated SIP INVITE dialog usage and not be changed
   during updates to the offer/answer exchange.  This applies
   specifically to the 'connection' attribute as defined in [RFC4145].
   If a SIP UAC wants to change some other parts of the SDP but reuse
   the already established connection, it uses the value of 'existing'
   in the 'connection' attribute (for example, a=connection:existing).
   If it has noted that a connection has failed and wants to re-
   establish the connection, it uses the value of 'new' in the
   'connection' attribute (for example, a=connection:new).  Throughout
   this, the connection identifier specified in the 'cfw-id' SDP
   parameter MUST NOT change.  One is simply negotiating the underlying
   TCP connection between endpoints but always using the same Control
   Framework session, which is 1:1 for the lifetime of the SIP INVITE
   dialog usage.

   A non-2xx-class final SIP response (3xx, 4xx, 5xx, and 6xx) received
   for the INVITE request indicates that no SIP INVITE dialog usage has
   been created and is treated as specified by SIP [RFC3261].
   Specifically, support of this specification is negotiated through the
   presence of the media type defined in this specification.  The
   receipt of a SIP error response such as "488" indicates that the
   offer contained in a request is not acceptable.  The inclusion of the
   media line associated with this specification in such a rejected
   offer indicates to the client generating the offer that this could be
   due to the receiving client not supporting this specification.  The
   client generating the offer MUST act as it would normally on
   receiving this response, as per [RFC3261].  Media streams can also be
   rejected by setting the port to "0" in the "m=" line of the session
   description, as defined in [RFC3264].  A client using this
   specification MUST be prepared to receive an answer where the "m="
   line it inserted for using the Control Framework has been set to "0".

   In this situation, the client will act as it would for any other
   media type with a port set to "0".

4.2.  Control Server SIP UAS Behavior

   On receiving a SIP INVITE request, an external Server (SIP UAS)
   inspects the message for indications of support for the mechanisms
   defined in this specification.  This is achieved through inspection
   of the session description of the offer message and identifying
   support for the application/cfw MIME type in the SDP.  If the SIP UAS
   wishes to construct a reliable response that conveys support for the
   extension, it MUST follow the mechanisms defined in [RFC3261].  If
   support is conveyed in a reliable SIP provisional response, the
   mechanisms in [RFC3262] MUST also be used.  It should be noted that
   the SDP offer is not restricted to the initial INVITE request and MAY
   appear in any series of messages that are compliant to [RFC3261],
   [RFC3262], [RFC3311], and [RFC3264].

   When constructing an answer, the SDP payload MUST be constructed
   using the semantic (connection, media, and attribute) defined in
   Section 4.1 using valid local settings and also with full compliance
   to the COMEDIA [RFC4145] specification.  For example, the SDP
   attributes included in the answer constructed for the example offer
   provided in Section 4.1 would look as follows:

                           a=setup:passive
                           a=connection:new

   A client constructing an answer MUST include the 'cfw-id' SDP
   attribute as defined in Section 9.2.  This attribute MUST be unique
   in the context of the interaction between the UAC and UAS and MUST
   NOT clash with instances of the 'cfw-id' used in other SIP offer/
   answer exchanges.  The 'cfw-id' MUST be different from the 'cfw-id'
   value received in the offer as it is used to uniquely identify and
   distinguish between multiple endpoints that generate SDP answers.
   The value chosen for the 'cfw-id' attribute MUST be used for the
   entire duration of the associated SIP INVITE dialog usage and not be
   changed during updates to the offer/answer exchange.

   Once the SDP answer has been constructed, it is sent using standard
   SIP mechanisms.  Depending on the contents of the SDP payloads that
   were negotiated using the offer/answer exchange, a reliable
   connection will be established between the Controlling UAC and
   External Server UAS entities.  The newly established connection is
   now available to exchange Control Command primitives.  The state of
   the SIP INVITE dialog usage and the associated Control Channel are
   now implicitly linked.  If either party wishes to terminate a Control
   Channel, it simply issues a SIP termination request (for example, a

   SIP BYE request or appropriate response in an early SIP INVITE dialog
   usage).  The Control Channel therefore lives for the duration of the
   SIP INVITE dialog usage.

   A UAS receiving a SIP OPTIONS request MUST respond appropriately as
   defined in [RFC3261].  The UAS MUST include the media types supported
   in the SIP 200 OK response in a SIP 'Accept' header to indicate the
   valid media types.

5.  Establishing Media Streams - Control Client SIP UAC Behavior

   It is intended that the Control Framework will be used within a
   variety of architectures for a wide range of functions.  One of the
   primary functions will be the use of the Control Channel to apply
   multiple specific Control Package commands to media sessions
   established by SIP INVITE dialogs (media dialogs) with a given remote
   server.  For example, the Control Server might send a command to
   generate audio media (such as an announcement) on an RTP stream
   between a User Agent and a media server.

   SIP INVITE dialogs used to establish media sessions (see Figure 2) on
   behalf of User Agents MAY contain more than one Media Description (as
   defined by "m=" in the SDP).  The Control Client MUST include a media
   label attribute, as defined in [RFC4574], for each "m=" definition
   received that is to be directed to an entity using the Control
   Framework.  This allows the Control Client to later explicitly direct
   commands on the Control Channel at a specific media line (m=).

   This framework identifies the referencing of such associated media
   dialogs as extremely important.  A connection reference attribute has
   been specified that can optionally be imported into any Control
   Package.  It is intended that this will reduce the repetitive
   specifying of dialog reference language.  The schema can be found in
   Appendix A.1.

   Similarly, the ability to identify and apply commands to a group of
   associated media dialogs (multiparty) is also identified as a common
   structure that could be defined and reused, for example, playing a
   prompt to all participants in a Conference.  The schema for such
   operations can also be found in Appendix A.1.

   Support for both the common attributes described here is specified as
   part of each Control Package definition, as detailed in Section 8.

6.  Control Framework Interactions

   In this document, the use of the COMEDIA specification allows for a
   Control Channel to be set up in either direction as a result of a SIP
   INVITE transaction.  SIP provides a flexible negotiation mechanism to
   establish the Control Channel, but there needs to be a mechanism
   within the Control Channel to correlate it with the SIP INVITE dialog
   usage implemented for its establishment.  A Control Client receiving
   an incoming connection (whether it be acting in the role of UAC or
   UAS) has no way of identifying the associated SIP INVITE dialog usage
   as it could be simply listening for all incoming connections on a
   specific port.  The following steps, which implementations MUST
   support, allow a connecting UA (that is, the UA with the active role
   in COMEDIA) to identify the associated SIP INVITE dialog usage that
   triggered the connection.  Unless there is an alternative dialog
   association mechanism used, the UAs MUST carry out these steps before
   any other signaling on the newly created Control Channel.

   o  Once the connection has been established, the UA acting in the
      active role (active UA) to initiate the connection MUST send a
      Control Framework SYNC request.  The SYNC request MUST be
      constructed as defined in Section 9.1 and MUST contain the
      'Dialog-ID' message header.

   o  The 'Dialog-ID' message header is populated with the value of the
      local 'cfw-id' media-level attribute that was inserted by the same
      client in the SDP offer/answer exchange to establish the Control
      Channel.  This allows for a correlation between the Control
      Channel and its associated SIP INVITE dialog usage.

   o  On creating the SYNC request, the active UA MUST follow the
      procedures outlined in Section 6.3.3.  This provides details of
      connection keep-alive messages.

   o  On creating the SYNC request, the active UA MUST also follow the
      procedures outlined in Section 6.3.4.2.  This provides details of
      the negotiation mechanism used to determine the Protocol Data
      Units (PDUs) that can be exchanged on the established Control
      Channel connection.

   o  The UA in the active role for the connection creation MUST then
      send the SYNC request.  If the UA in the active role for the
      connection creation is a SIP UAS and has generated its SDP
      response in a 2xx-class SIP response, it MUST wait for an incoming
      SIP ACK message before issuing the SYNC.  If the UA in the active
      role for the connection creation is a SIP UAS and has generated
      its SDP response in a reliable 1XX class SIP response, it MUST
      wait for an incoming SIP PRACK message before issuing the SYNC.

      If the UA in the active role for the connection creation is a SIP
      UAC, it MUST send the SYNC message immediately on establishment of
      the Control Channel.  It MUST then wait for a period of at least
      2*'Transaction-Timeout' to receive a response.  It MAY choose a
      longer time to wait, but it MUST NOT be shorter than 'Transaction-
      Timeout'.  In general, a Control Framework transaction MUST
      complete within 20 (2*'Transaction-Timeout') seconds and is
      referenced throughout the document as 'Transaction-Timeout'.

   o  If no response is received for the SYNC message, a timeout occurs
      and the Control Channel is terminated along with the associated
      SIP INVITE dialog usage.  The active UA MUST issue a BYE request
      to terminate the SIP INVITE dialog usage.

   o  If the active UA receives a 481 response from the passive UA, this
      means the SYNC request was received, but the associated SIP INVITE
      dialog usage specified in the SYNC message does not exist.  The
      active client MUST terminate the Control Channel.  The active UA
      MUST issue a SIP BYE request to terminate the SIP INVITE dialog
      usage.

   o  All other error responses received for the SYNC request are
      treated as detailed in this specification and also result in the
      termination of the Control Channel and the associated SIP INVITE
      dialog usage.  The active UA MUST issue a BYE request to terminate
      the SIP INVITE dialog usage.

   o  The receipt of a 200 response to a SYNC message implies that the
      SIP INVITE dialog usage and control connection have been
      successfully correlated.  The Control Channel can now be used for
      further interactions.

   SYNC messages can be sent at any point while the Control Channel is
   open from either side, once the initial exchange is complete.  If
   present, the contents of the 'Keep-Alive' and 'Dialog-ID' headers
   MUST NOT change.  New values of the 'Keep-Alive' and 'Dialog-ID'
   headers have no relevance as they are negotiated for the lifetime of
   the Media Control Channel Framework session.

   Once a successful Control Channel has been established, as defined in
   Sections 4.1 and 4.2, and the connection has been correlated, as
   described in previous paragraphs, the two entities are now in a
   position to exchange Control Framework messages.  The following sub-
   sections specify the general behavior for constructing Control
   Framework requests and responses.  Section 6.3 specifies the core
   Control Framework methods and their transaction processing.

6.1.  General Behavior for Constructing Requests

   An entity acting as a Control Client that constructs and sends
   requests on a Control Channel MUST adhere to the syntax defined in
   Section 9.  Note that either entity can act as a Control Client
   depending on individual package requirements.  Control Commands MUST
   also adhere to the syntax defined by the Control Packages negotiated
   in Sections 4.1 and 4.2 of this document.  A Control Client MUST
   create a unique transaction and associated identifier for insertion
   in the request.  The transaction identifier is then included in the
   first line of a Control Framework message along with the method type,
   as defined in the ABNF in Section 9.  The first line starts with the
   "CFW" token for the purpose of easily extracting the transaction
   identifier.  The transaction identifier MUST be unique in the context
   of the interaction between the Control Client and Control Server.
   This unique property helps avoid clashes when multiple client
   entities could be creating transactions to be carried out on a single
   receiving server.  All required, mandatory, and optional Control
   Framework headers are then inserted into the request with appropriate
   values (see relevant individual header information for explicit
   detail).  A 'Control-Package' header MUST also be inserted with the
   value indicating the Control Package to which this specific request
   applies.  Multiple packages can be negotiated per Control Channel
   using the SYNC message discussed in Section 6.3.4.2.

   Any Framework message that contains an associated payload MUST also
   include the 'Content-Type' and 'Content-Length' message headers,
   which indicate the MIME type of the payload specified by the
   individual Control Framework packages and the size of the message
   body represented as a whole decimal number of octets, respectively.
   If no associated payload is to be added to the message, the 'Content-
   Length' header MUST have a value of '0'.

   A Server receiving a Framework message request MUST respond with an
   appropriate response (as defined in Section 6.2).  Control Clients
   MUST wait for a minimum of 2*'Transaction-Timeout' for a response
   before considering the transaction a failure and tidying state
   appropriately depending on the extension package being used.

6.2.  General Behavior for Constructing Responses

   An entity acting as a Control Server, on receiving a request, MUST
   generate a response within the 'Transaction-Timeout', as measured
   from the Control Client.  The response MUST conform to the ABNF
   defined in Section 9.  The first line of the response MUST contain
   the transaction identifier used in the first line of the request, as

   defined in Section 6.1.  Responses MUST NOT include the 'Status' or
   'Timeout' message headers, and these MUST be ignored if received by a
   Client in a response.

   A Control Server MUST include a status code in the first line of the
   response.  If there is no error, the Server responds with a 200
   Control Framework status code, as defined in Section 7.1.  The 200
   response MAY include message bodies.  If the response contains a
   payload, the message MUST include the 'Content-Length' and 'Content-
   Type' headers.  When the Control Client receives a 2xx-class
   response, the Control Command transaction is complete.

   If the Control Server receives a request, like CONTROL, that the
   Server understands, but the Server knows processing the command will
   exceed the 'Transaction-Timeout', then the Server MUST respond with a
   202 status code in the first line of the response.  Following the
   initial response, the server will send one or more REPORT messages as
   described in Section 6.3.2.  A Control Package MUST explicitly define
   the circumstances under which the server sends 200 and 202 messages.

   If a Control Server encounters problems with a Control Framework
   request (like REPORT or CONTROL), an appropriate error code MUST be
   used in the response, as listed in Section 7.  The generation of a
   non-2xx-class response code to a Control Framework request (like
   CONTROL or REPORT) will indicate failure of the transaction, and all
   associated transaction state and resources MUST be terminated.  The
   response code may provide an explicit indication of why the
   transaction failed, which might result in a re-submission of the
   request depending on the extension package being used.

6.3.  Transaction Processing

   The Control Framework defines four types of requests (methods):
   CONTROL, REPORT, K-ALIVE, and SYNC.  Implementations MUST support
   sending and receiving these four methods.

   The following sub-sections specify each Control Framework method and
   its associated transaction processing.

6.3.1.  CONTROL Transactions

   A CONTROL message is used by the Control Client to pass control-
   related information to a Control Server.  It is also used as the
   event-reporting mechanism in the Control Framework.  Reporting events
   is simply another usage of the CONTROL message, which is permitted to
   be sent in either direction between two participants in a session,
   carrying the appropriate payload for an event.  The message is
   constructed in the same way as any standard Control Framework

   message, as discussed in Section 6.1 and defined in Section 9.  A
   CONTROL message MAY contain a message body.  The explicit Control
   Command(s) of the message payload contained in a CONTROL message are
   specified in separate Control Package specifications.  Separate
   Control Package specifications MUST conform to the format defined in
   Section 8.4.  A CONTROL message containing a payload MUST include a
   'Content-Type' header.  The payload MUST be one of the payload types
   defined by the Control Package.  Individual packages MAY allow a
   CONTROL message that does not contain a payload.  This could in fact
   be a valid message exchange within a specific package; if it's not,
   an appropriate package-level error message MUST be generated.

6.3.2.  REPORT Transactions

   A 'REPORT' message is used by a Control Server when processing of a
   CONTROL command extends beyond the 'Transaction-Timeout', as measured
   from the Client.  In this case, the Server returns a 202 response.
   The Server returns status updates and the final results of the
   command in subsequent REPORT messages.

   All REPORT messages MUST contain the same transaction ID in the
   request start line that was present in the original CONTROL
   transaction.  This correlates extended transactions with the original
   CONTROL transaction.  A REPORT message containing a payload MUST
   include the 'Content-Type' and 'Content-Length' headers indicating
   the payload MIME type [RFC2045] defined by the Control Package and
   the length of the payload, respectively.

6.3.2.1.  Reporting the Status of Extended Transactions

   On receiving a CONTROL message, a Control Server MUST respond within
   'Transaction-Timeout' with a status code for the request, as
   specified in Section 6.2.  If the processing of the command completes
   within that time, a 200 response code MUST be sent.  If the command
   does not complete within that time, the response code 202 MUST be
   sent indicating that the requested command is still being processed
   and the CONTROL transaction is being extended.  The REPORT method is
   then used to update and terminate the status of the extended
   transaction.  The Control Server should not wait until the last
   possible opportunity to make the decision of issuing a 202 response
   code and should ensure that it has plenty of time for the response to
   arrive at the Control Client.  If it does not have time, transactions
   will be terminated (timed out) at the Control Client before
   completion.

   A Control Server issuing a 202 response MUST ensure the message
   contains a 'Timeout' message header.  This header MUST have a value
   in seconds that is the amount of time the recipient of the 202
   message MUST wait before assuming that there has been a problem and
   terminating the extended transaction and associated state.

   The initial REPORT message MUST contain a 'Seq' (Sequence) message
   header with a value equal to '1'.  Note: the 'Seq' numbers at both
   Control Client and Control Server for Framework messages are
   independent.

   All REPORT messages for an extended CONTROL transaction MUST contain
   a 'Timeout' message header.  This header will contain a value in
   seconds that is the amount of time the recipient of the REPORT
   message MUST wait before assuming that there has been a problem and
   terminating the extended transaction and associated state.  On
   receiving a REPORT message with a 'Status' header of 'update', the
   Control Client MUST reset the timer for the associated extended
   CONTROL transaction to the indicated timeout period.  If the timeout
   period approaches and no intended REPORT messages have been
   generated, the entity acting as a Control Framework UAS for the
   interaction MUST generate a REPORT message containing, as defined in
   this paragraph, a 'Status' header of 'update' with no associated
   payload.  Such a message acts as a timeout refresh and in no way
   impacts the extended transaction because no message body or semantics
   are permitted.  It is RECOMMENDED that a minimum value of 10 and a
   maximum value of 15 seconds be used for the value of the 'Timeout'
   message header.  It is also RECOMMENDED that a Control Server refresh
   the timeout period of the CONTROL transaction at an interval that is
   not too close to the expiry time.  A value of 80% of the timeout
   period could be used.  For example, if the timeout period is 10
   seconds, the Server would refresh the transaction after 8 seconds.

   Subsequent REPORT messages that provide additional information
   relating to the extended CONTROL transaction MUST also include and
   increment by 1 the 'Seq' header value.  A REPORT message received
   that has not been incremented by 1 MUST be responded to with a 406
   response and the extended transaction MUST be considered terminated.
   On receiving a 406 response, the extended transaction MUST be
   terminated.  REPORT messages MUST also include a 'Status' header with
   a value of 'update'.  These REPORT messages sent to update the
   extended CONTROL transaction status MAY contain a message body, as
   defined by individual Control Packages and specified in Section 8.5.
   A REPORT message sent updating the extended transaction also acts as
   a timeout refresh, as described earlier in this section.  This will
   result in a transaction timeout period at the initiator of the
   original CONTROL request being reset to the interval contained in the
   'Timeout' message header.

   When all processing for an extended CONTROL transaction has taken
   place, the entity acting as a Control Server MUST send a terminating
   REPORT message.  The terminating REPORT message MUST increment the
   value in the 'Seq' message header by the value of '1' from the
   previous REPORT message.  It MUST also include a 'Status' header with
   a value of 'terminate' and MAY contain a message body.  It MUST also
   contain a 'Timeout' message header with a valid value.  The inclusion
   of the 'Timeout' header is for consistency, and its value is ignored.
   A Control Framework UAC can then clean up any pending state
   associated with the original CONTROL transaction.

6.3.3.  K-ALIVE Transactions

   The protocol defined in this document may be used in various network
   architectures.  This includes a wide range of deployments where the
   clients could be co-located in a secured, private domain, or spread
   across disparate domains that require traversal of devices such as
   Network Address Translators (NATs) and firewalls.  A keep-alive
   mechanism enables the Control Channel to be kept active during times
   of inactivity.  This is because many firewalls have a timeout period
   after which connections are closed.  This mechanism also provides the
   ability for application-level failure detection.  It should be noted
   that the following procedures apply only to the Control Channel being
   created.  For details relating to the SIP keep-alive mechanism,
   implementers should seek guidance from SIP Outbound [RFC5626].

   The following keep-alive procedures MUST be implemented.  Specific
   deployments MAY choose not to use the keep-alive mechanism if both
   entities are in a co-located domain.  Note that choosing not to use
   the keep-alive mechanism defined in this section, even when in a co-
   located architecture, will reduce the ability to detect application-
   level errors, especially during long periods of inactivity.

   Once the SIP INVITE dialog usage has been established and the
   underlying Control Channel has been set up, including the initial
   correlation handshake using SYNC as discussed in Section 6, both
   entities acting in the active and passive roles, as defined in
   COMEDIA [RFC4145], MUST start a keep-alive timer equal to the value
   negotiated during the Control Channel SYNC request/response exchange.
   This is the value from the 'Keep-Alive' header in seconds.

6.3.3.1.  Behavior for an Entity in an Active Role

   When in an active role, a K-ALIVE message MUST be generated before
   the local keep-alive timer fires.  An active entity is free to send
   the K-ALIVE message whenever it chooses.  It is RECOMMENDED for the
   entity to issue a K-ALIVE message after 80% of the local keep-alive
   timer.  On receiving a 200 OK Control Framework message for the

   K-ALIVE request, the active entity MUST reset the local keep-alive
   timer.  If no 200 OK response is received to the K-ALIVE message, or
   a transport-level problem is detected by some other means, before the
   local keep-alive timer fires, the active entity MAY use COMEDIA re-
   negotiation procedures to recover the connection.  Otherwise, the
   active entity MUST tear down the SIP INVITE dialog and recover the
   associated Control Channel resources.

6.3.3.2.  Behavior for an Entity in a Passive Role

   When acting as a passive entity, a K-ALIVE message must be received
   before the local keep-alive timer fires.  When a K-ALIVE request is
   received, the passive entity MUST generate a 200 OK Control Framework
   response and reset the local keep-alive timer.  No other Control
   Framework response is valid.  If no K-ALIVE message is received (or a
   transport level problem is detected by some other means) before the
   local keep-alive timer fires, the passive entity MUST tear down the
   SIP INVITE dialog and recover the associated Control Channel
   resources.

6.3.4.  SYNC Transactions

   The initial SYNC request on a Control Channel is used to negotiate
   the timeout period for the Control Channel keep-alive mechanism and
   to allow clients and servers to learn the Control Packages that each
   supports.  Subsequent SYNC requests MAY be used to change the set of
   Control Packages that can be used on the Control Channel.

6.3.4.1.  Timeout Negotiation for the Initial SYNC Transaction

   The initial SYNC request allows the timeout period for the Control
   Channel keep-alive mechanism to be negotiated.  The following rules
   MUST be followed for the initial SYNC request:

   o  If the Client initiating the SDP offer has a COMEDIA 'setup'
      attribute equal to active, the 'Keep-Alive' header MUST be
      included in the SYNC message generated by the offerer.  The value
      of the 'Keep-Alive' header SHOULD be in the range of 95 to 120
      seconds (this is consistent with SIP Outbound [RFC5626]).  The
      value of the 'Keep-Alive' header MUST NOT exceed 600 seconds.  The
      client that generated the SDP "Answer" (the passive client) MUST
      copy the 'Keep-Alive' header into the 200 response to the SYNC
      message with the same value.

   o  If the Client initiating the SDP offer has a COMEDIA 'setup'
      attribute equal to passive, the 'Keep-Alive' header parameter MUST
      be included in the SYNC message generated by the answerer.  The
      value of the 'Keep-Alive' header SHOULD be in the range of 95 to

      120 seconds.  The client that generated the SDP offer (the passive
      client) MUST copy the 'Keep-Alive' header into the 200 response to
      the SYNC message with the same value.

   o  If the Client initiating the SDP offer has a COMEDIA 'setup'
      attribute equal to 'actpass', the 'Keep-Alive' header parameter
      MUST be included in the SYNC message of the entity who is the
      active participant in the SDP session.  If the client generating
      the subsequent SDP answer places a value of 'active' in the
      COMEDIA SDP 'setup' attribute, it will generate the SYNC request
      and include the 'Keep-Alive' header.  The value SHOULD be in the
      range 95 to 120 seconds.  If the client generating the subsequent
      SDP answer places a value of 'passive' in the COMEDIA 'setup'
      attribute, the original UA making the SDP will generate the SYNC
      request and include the 'Keep-Alive' header.  The value SHOULD be
      in the range 95 to 120 seconds.

   o  If the initial negotiated offer/answer results in a COMEDIA
      'setup' attribute equal to 'holdconn', the initial SYNC mechanism
      will occur when the offer/answer exchange is updated and the
      active/passive roles are resolved using COMEDIA.

   The previous steps ensure that the entity initiating the Control
   Channel connection is always the one specifying the keep-alive
   timeout period.  It will always be the initiator of the connection
   who generates the K-ALIVE messages.

   Once negotiated, the keep-alive timeout applies for the remainder of
   the Control Framework session.  Any subsequent SYNC messages
   generated in the Control Channel do not impact the negotiated keep-
   alive property of the session.  The 'Keep-Alive' header MUST NOT be
   included in subsequent SYNC messages, and if it is received, it MUST
   be ignored.

6.3.4.2.  Package Negotiation

   As part of the SYNC message exchange, a client generating the request
   MUST include a 'Packages' header, as defined in Section 9.  The
   'Packages' header contains a list of all Control Framework packages
   that can be supported within this control session, from the
   perspective of the client creating the SYNC message.  All Channel
   Framework package names MUST be tokens that adhere to the rules set
   out in Section 8.  The 'Packages' header of the initial SYNC message
   MUST contain at least one value.

   A server receiving the initial SYNC request MUST examine the contents
   of the 'Packages' header.  If the server supports at least one of the
   packages listed in the request, it MUST respond with a 200 response

   code.  The response MUST contain a 'Packages' header that lists the
   supported packages that are in common with those from the 'Packages'
   header of the request (either all or a subset).  This list forms a
   common set of Control Packages that are supported by both parties.
   Any Control Packages supported by the server that are not listed in
   the 'Packages' header of the SYNC request MAY be placed in the
   'Supported' header of the response.  This provides a hint to the
   client that generated the SYNC request about additional packages
   supported by the server.

   If no common packages are supported by the server receiving the SYNC
   message, it MUST respond with a 422 error response code.  The error
   response MUST contain a 'Supported' header indicating the packages
   that are supported.  The initiating client can then choose to either
   re-submit a new SYNC message based on the 422 response or consider
   the interaction a failure.  This would lead to termination of the
   associated SIP INVITE dialog by sending a SIP BYE request, as per
   [RFC3261].

   Once the initial SYNC transaction is completed, either client MAY
   choose to send a subsequent new SYNC message to re-negotiate the
   packages that are supported within the Control Channel.  A new SYNC
   message whose 'Packages' header has different values from the
   previous SYNC message can effectively add and delete the packages
   used in the Control Channel.  If a client receiving a subsequent SYNC
   message does not wish to change the set of packages, it MUST respond
   with a 421 Control Framework response code.  Subsequent SYNC messages
   MUST NOT change the value of the 'Dialog-ID' and 'Keep-Alive' Control
   Framework headers that appeared in the original SYNC negotiation.

   An entity MAY honor Control Framework commands relating to a Control
   Package it no longer supports after package re-negotiation.  When the
   entity does not wish to honor such commands, it MUST respond to the
   request with a 420 response.

7.  Response Code Descriptions

   The following response codes are defined for transaction responses to
   methods defined in Section 6.1.  All response codes in this section
   MUST be supported and can be used in response to both CONTROL and
   REPORT messages except that a 202 MUST NOT be generated in response
   to a REPORT message.

   Note that these response codes apply to Framework Transactions only.
   Success or error indications for Control Commands MUST be treated as
   the result of a Control Command and returned in either a 200 response
   or REPORT message.

7.1.  200 Response Code

   The framework protocol transaction completed successfully.

7.2.  202 Response Code

   The framework protocol transaction completed successfully and
   additional information will be provided at a later time through the
   REPORT mechanism defined in Section 6.3.2.

7.3.  400 Response Code

   The request was syntactically incorrect.

7.4.  403 Response Code

   The server understood the request, but is refusing to fulfill it.
   The client SHOULD NOT repeat the request.

7.5.  405 Response Code

   Method not allowed.  The primitive is not supported.

7.6.  406 Response Code

   Message out of sequence.

7.7.  420 Response Code

   Intended target of the request is for a Control Package that is not
   valid for the current session.

7.8.  421 Response Code

   Recipient does not wish to re-negotiate Control Packages at this
   moment in time.

7.9.  422 Response Code

   Recipient does not support any Control Packages listed in the SYNC
   message.

7.10.  423 Response Code

   Recipient has an existing transaction with the same transaction ID.

7.11.  481 Response Code

   The transaction of the request does not exist.  In response to a SYNC
   request, the 481 response code indicates that the corresponding SIP
   INVITE dialog usage does not exist.

7.12.  500 Response Code

   The recipient does not understand the request.

8.  Control Packages

   Control Packages specify behavior that extends the capability defined
   in this document.  Control Packages MUST NOT weaken statements of
   "MUST" and "SHOULD" strength in this document.  A Control Package MAY
   strengthen "SHOULD", "RECOMMENDED", and "MAY" to "MUST" if justified
   by the specific usage of the framework.

   In addition to the usual sections expected in Standards-Track RFCs
   and SIP extension documents, authors of Control Packages need to
   address each of the issues detailed in the following sub-sections.
   The following sections MUST be used as a template and included
   appropriately in all Control-Package specifications.  To reiterate,
   the following sections do not solely form the basis of all Control-
   Package specifications but are included as a minimum to provide
   essential package-level information.  A Control-Package specification
   can take any valid form it wishes as long as it includes at least the
   following information listed in this section.

8.1.  Control Package Name

   This section MUST be present in all extensions to this document and
   provides a token name for the Control Package.  The section MUST
   include information that appears in the IANA registration of the
   token.  Information on registering Control Package tokens is
   contained in Section 13.

8.2.  Framework Message Usage

   The Control Framework defines a number of message primitives that can
   be used to exchange commands and information.  There are no
   limitations restricting the directionality of messages passed down a
   Control Channel.  This section of a Control Package document MUST
   explicitly detail the types of Framework messages (Methods) that can
   be used as well as provide an indication of directionality between
   entities.  This will include which role type is allowed to initiate a
   request type.

8.3.  Common XML Support

   This optional section is only included in a Control Package if the
   attributes for media dialog or conference reference are required, as
   defined and discussed in Appendix A.1.  The Control Package will make
   strong statements (using language from RFC 2119 [RFC2119]) if the XML
   schema defined in Appendix A.1 is to be supported.  If only part of
   the schema is required (for example, just 'connectionid' or
   'conferenceid'), the Control Package will make equally strong
   statements (using language from RFC 2119 [RFC2119]).

8.4.  CONTROL Message Bodies

   This mandatory section of a Control Package defines the control body
   that can be contained within a CONTROL command request, as defined in
   Section 6, or that no Control Package body is required.  This section
   MUST indicate the location of detailed syntax definitions and
   semantics for the appropriate MIME [RFC2045] body type that apply to
   a CONTROL command request and, optionally, the associated 200
   response.  For Control Packages that do not have a Control Package
   body, making such a statement satisfies the "MUST" strength of this
   section in the Control Package document.

8.5.  REPORT Message Bodies

   This mandatory section of a Control Package defines the REPORT body
   that can be contained within a REPORT command request, as defined in
   Section 6, or that no report package body is required.  This section
   MUST indicate the location of detailed syntax definitions and
   semantics for the appropriate MIME [RFC2045] body type.  It should be
   noted that the Control Framework specification does allow for
   payloads to exist in 200 responses to CONTROL messages (as defined in
   this document).  An entity that is prepared to receive a payload type
   in a REPORT message MUST also be prepared to receive the same payload
   in a 200 response to a CONTROL message.  For Control Packages that do
   not have a Control Package body, stating such satisfies the "MUST"
   strength of this section in the Control Package document.

8.6.  Audit

   Auditing of various Control Package properties such as capabilities
   and resources (package-level meta-information) is extremely useful.
   Such meta-data usually has no direct impact on Control Framework
   interactions but allows for contextual information to be learnt.
   Control Packages are encouraged to make use of Control Framework
   interactions to provide relevant package audit information.

   This section SHOULD include the following information:

   o  If an auditing capability is available in this package.

   o  How auditing information is triggered (for example, using a
      Control Framework CONTROL message) and delivered (for example, in
      a Control Framework 200 response).

   o  The location of the audit query and response format for the
      payload (for example, it could be a separate XML schema OR part of
      a larger XML schema).

8.7.  Examples

   It is strongly RECOMMENDED that Control Packages provide a range of
   message flows that represent common flows using the package and this
   framework document.

9.  Formal Syntax

9.1.  Control Framework Formal Syntax

   The Control Framework interactions use the UTF-8 transformation
   format as defined in [RFC3629].  The syntax in this section uses the
   Augmented Backus-Naur Form (ABNF) as defined in [RFC5234] including
   types 'DIGIT', 'CRLF', and 'ALPHA'.

   Unless otherwise stated in the definition of a particular header
   field, field values, parameter names, and parameter values are not
   case-sensitive.

  control-req-or-resp = control-request / control-response
  control-request = control-req-start *headers CRLF [control-content]
  control-response = control-resp-start *headers CRLF [control-content]
  control-req-start  = pCFW SP trans-id SP method CRLF
  control-resp-start = pCFW SP trans-id SP status-code CRLF

  pCFW = %x43.46.57; CFW in caps
  trans-id = alpha-num-token
  method = mCONTROL / mREPORT / mSYNC / mK-ALIVE / other-method
  mCONTROL = %x43.4F.4E.54.52.4F.4C ; CONTROL in caps
  mREPORT = %x52.45.50.4F.52.54     ; REPORT in caps
  mSYNC = %x53.59.4E.43             ; SYNC in caps
  mK-ALIVE = %x4B.2D.41.4C.49.56.45 ; K-ALIVE in caps

  other-method = 1*UPALPHA
  status-code = 3*DIGIT ; any code defined in this and other documents

  headers = header-name CRLF

  header-name = (Content-Length
   /Content-Type
   /Control-Package
   /Status
   /Seq
   /Timeout
   /Dialog-ID
   /Packages
   /Supported
   /Keep-alive
   /ext-header)

  Content-Length = "Content-Length:" SP 1*DIGIT
  Control-Package = "Control-Package:" SP 1*alpha-num-token
  Status = "Status:" SP ("update" / "terminate" )
  Timeout = "Timeout:" SP 1*DIGIT
  Seq = "Seq:" SP 1*DIGIT
  Dialog-ID = "Dialog-ID:" SP dialog-id-string
  Packages = "Packages:" SP package-name *(COMMA package-name)
  Supported = "Supported:" SP supprtd-alphanum *(COMMA supprtd-alphanum)
  Keep-alive = "Keep-Alive:" SP kalive-seconds

  dialog-id-string = alpha-num-token
  package-name = alpha-num-token
  supprtd-alphanum = alpha-num-token
  kalive-seconds = 1*DIGIT

  alpha-num-token = ALPHANUM  3*31alpha-num-tokent-char
  alpha-num-tokent-char = ALPHANUM / "." / "-" / "+" / "%" / "=" / "/"

  control-content = *OCTET

  Content-Type = "Content-Type:" SP media-type
  media-type = type "/" subtype *(SP ";" gen-param )
  type = token    ; Section 4.2 of RFC 4288
  subtype = token ; Section 4.2 of RFC 4288

  gen-param = pname [ "=" pval ]
  pname = token
  pval  = token / quoted-string

  token = 1*(%x21 / %x23-27 / %x2A-2B / %x2D-2E
             / %x30-39 / %x41-5A / %x5E-7E)

  quoted-string = DQUOTE *(qdtext / qd-esc) DQUOTE
  qdtext = SP / HTAB / %x21 / %x23-5B / %x5D-7E
              / UTF8-NONASCII
  qd-esc = (BACKSLASH BACKSLASH) / (BACKSLASH DQUOTE)
  BACKSLASH = "\"
  UPALPHA  = %x41-5A
  ALPHANUM = ALPHA / DIGIT

  ext-header = hname ":" SP hval CRLF

  hname = ALPHA *token
  hval = utf8text

  utf8text = *(HTAB / %x20-7E / UTF8-NONASCII)

  UTF8-NONASCII = UTF8-2 / UTF8-3 / UTF8-4 ; From RFC 3629

   The following table details a summary of the headers that can be
   contained in Control Framework interactions.

         Header field        Where    CONTROL REPORT SYNC  K-ALIVE
         ___________________________________________________________
         Content-Length                  o      o      -      -
         Control-Package       R         m      -      -      -
         Seq                             -      m      -      -
         Status                R         -      m      -      -
         Timeout               R         -      m      -      -
         Timeout              202        -      m      -      -
         Dialog-ID             R         -      -      m      -
         Packages                        -      -      m      -
         Supported             r         -      -      o      -
         Keep-Alive            R         -      -      o      -
         Content-Type                    o      o      -      -

       Table 1: Summary of Headers in Control Framework Interactions

   The notation used in Table 1 is as follows:

 R: header field may only appear in requests.
 r: header field may only appear in responses.
 2xx, 4xx, etc.: response codes with which the header field can be used.
 [blank]: header field may appear in either requests or responses.
 m: header field is mandatory.
 o: header field is optional.
 -: header field is not applicable (ignored if present).

9.2.  Control Framework Dialog Identifier SDP Attribute

   This specification defines a new media-level value attribute:
   'cfw-id'.  Its formatting in SDP is described by the following ABNF
   [RFC5234].

            cfw-dialog-id = "a=cfw-id:" 1*(SP cfw-id-name) CRLF

            cfw-id-name   = token

            token         = 1*(token-char)

            token-char    = %x21 / %x23-27 / %x2A-2B / %x2D-2E / %x30-39
                            / %x41-5A / %x5E-7E

   The token-char and token elements are defined in [RFC4566] but
   included here to provide support for the implementer of this SDP
   feature.

10.  Examples

   The following examples provide an abstracted flow of Control Channel
   establishment and Control Framework message exchange.  The SIP
   signaling is prefixed with the token 'SIP'.  All other messages are
   Control Framework interactions defined in this document.

   In this example, the Control Client establishes a Control Channel,
   SYNCs with the Control Server, and issues a CONTROL request that
   can't be completed within the 'Transaction-Timeout', so the Control
   Server returns a 202 response code to extend the transaction.  The
   Control Server then follows with REPORTs until the requested action
   has been completed.  The SIP INVITE dialog is then terminated.

            Control Client                                Control Server
                   |                                             |
                   |       (1) SIP INVITE                        |
                   |  ---------------------------------------->  |
                   |                                             |
                   |       (2) SIP 200                           |
                   |  <---------------------------------------   |
                   |                                             |
                   |       (3) SIP ACK                           |
                   |  ---------------------------------------->  |
                   |                                             |
                   |==>=======================================>==|
                   |         Control Channel Established         |
                   |==>=======================================>==|
                   |                                             |
                   |       (4) SYNC                              |
                   |  ---------------------------------------->  |
                   |                                             |
                   |       (5) 200                               |
                   |  <---------------------------------------   |
                   |                                             |
                   |       (6) CONTROL                           |
                   |  ---------------------------------------->  |
                   |                                             |

   (1)   Control Client-->Control Server (SIP): INVITE
         sip:control-server@example.com

   INVITE sip:control-server@example.com SIP/2.0
   To: <sip:control-server@example.com>
   From: <sip:control-client@example.com>;tag=8937498
   Via: SIP/2.0/UDP client.example.com;branch=z9hG4bK123
   CSeq: 1 INVITE
   Max-Forwards: 70
   Call-ID: 893jhoeihjr8392@example.com
   Contact: <sip:control-client@pc1.example.com>
   Content-Type: application/sdp
   Content-Length: 206

   v=0
   o=originator 2890844526 2890842808 IN IP4 controller.example.com
   s=-
   c=IN IP4 control-client.example.com
   m=application 49153 TCP cfw
   a=setup:active
   a=connection:new
   a=cfw-id:fndskuhHKsd783hjdla

   (2)   Control Server-->Control Client (SIP): 200 OK

SIP/2.0 200 OK
To: <sip:control-server@example.com>;tag=023983774
From: <sip:control-client@example.com>;tag=8937498
Via: SIP/2.0/UDP client.example.com;branch=z9hG4bK123;received=192.0.2.5
CSeq: 1 INVITE
Call-ID: 893jhoeihjr8392@example.com
Contact: <sip:control-server@pc2.example.com>
Content-Type: application/sdp
Content-Length: 203

v=0
o=responder 2890844600 2890842900 IN IP4 controller.example.com
s=-
c=IN IP4 control-server.example.com
m=application 49153 TCP cfw
a=setup:passive
a=connection:new
a=cfw-id:7JeDi23i7eiysi32

   (3)   Control Client-->Control Server (SIP): ACK

   (4)   Control Client opens a TCP connection to the Control Server.
         The connection can now be used to exchange Control Framework
         messages.  Control Client-->Control Server (Control Framework
         message): SYNC.

   CFW 8djae7khauj SYNC
   Dialog-ID: fndskuhHKsd783hjdla
   Keep-Alive: 100
   Packages: msc-ivr-basic/1.0

   (5)   Control Server-->Control Client (Control Framework message):
         200.

   CFW 8djae7khauj 200
   Keep-Alive: 100
   Packages: msc-ivr-basic/1.0
   Supported: msc-ivr-vxml/1.0,msc-conf-audio/1.0

   (6)   Once the SYNC process has completed, the connection can now be
         used to exchange Control Framework messages.  Control
         Client-->Control Server (Control Framework message): CONTROL.

   CFW i387yeiqyiq CONTROL
   Control-Package: <package-name>
   Content-Type: example_content/example_content

   Content-Length: 11

   <XML BLOB/>

   (7)   Control Server-->Control Client (Control Framework message):
         202.

   CFW i387yeiqyiq 202
   Timeout: 10

   (8)   Control Server-->Control Client (Control Framework message):
         REPORT.

   CFW i387yeiqyiq REPORT
   Seq: 1
   Status: update
   Timeout: 10

   (9)   Control Client-->Control Server (Control Framework message):
         200.

   CFW i387yeiqyiq 200
   Seq: 1

   (10)  Control Server-->Control Client (Control Framework message):
         REPORT.

   CFW i387yeiqyiq REPORT
   Seq: 2
   Status: update
   Timeout: 10
   Content-Type: example_content/example_content
   Content-Length: 11

   <XML BLOB/>

   (11)  Control Client-->Control Server (Control Framework message):
         200.

   CFW i387yeiqyiq 200
   Seq: 2

   (12)  Control Server-->Control Client (Control Framework message):
         REPORT.

   CFW i387yeiqyiq REPORT
   Seq: 3
   Status: terminate

   Timeout: 10
   Content-Type: example_content/example_content
   Content-Length: 11

   <XML BLOB/>

   (13)  Control Client-->Control Server (Control Framework message):
         200.

   CFW i387yeiqyiq 200
   Seq: 3

   (14)  Control Client-->Control Server (SIP): BYE

   BYE sip:control-server@pc2.example.com SIP/2.0
   To: <sip:control-server@example.com>;tag=023983774
   From: <sip:client@example.com>;tag=8937498
   Via: SIP/2.0/UDP client.example.com;branch=z9hG4bK234
   CSeq: 2 BYE
   Max-Forwards: 70
   Call-ID: 893jhoeihjr8392@example.com
   Contact: <sip:control-client@pc1.example.com>
   Content-Length: 0

   (15)  Control Server-->Control Client (SIP): 200 OK

SIP/2.0 200 OK
To: <sip:control-server@example.com>;tag=023983774
From: <sip:client@example.com>;tag=8937498
Via: SIP/2.0/UDP client.example.com;branch=z9hG4bK234;received=192.0.2.5
CSeq: 2 BYE
Call-ID: 893jhoeihjr8392@example.com
Contact: <sip:control-server@pc1.example.com>
Content-Length: 0

11.  Extensibility

   The Media Control Channel Framework was designed to be only minimally
   extensible.  New methods, header fields, and status codes can be
   defined in Standards-Track RFCs.  The Media Control Channel Framework
   does not contain a version number or any negotiation mechanism to
   require or discover new features.  If an extension is specified in
   the future that requires negotiation, the specification will need to
   describe how the extension is to be negotiated in the encapsulating
   signaling protocol.  If a non-interoperable update or extension
   occurs in the future, it will be treated as a new protocol, and it
   MUST describe how its use will be signaled.

   In order to allow extension header fields without breaking
   interoperability, if a Media Control Channel device receives a
   request or response containing a header field that it does not
   understand, it MUST ignore the header field and process the request
   or response as if the header field was not present.  If a Media
   Control Channel device receives a request with an unknown method, it
   MUST return a 500 response.

12.  Security Considerations

   The Channel Framework provides confidentiality and integrity for the
   messages it transfers.  It also provides assurances that the
   connected host is the host that it meant to connect to and that the
   connection has not been hijacked, as discussed in the remainder of
   this section.

   In design, the Channel Framework complies with the security-related
   requirements documented in "Media Server Control Protocol
   Requirements" [RFC5167] -- more specifically, REQ-MCP-11, REQ-MCP-12,
   REQ-MCP-13, and REQ-MCP-14.  Specific security measures employed by
   the Channel Framework are summarized in the following sub-sections.

12.1.  Session Establishment

   Channel Framework sessions are established as media sessions
   described by SDP within the context of a SIP INVITE dialog.  In order
   to ensure secure rendezvous between Control Framework clients and
   servers, the Media Channel Control Framework should make full use of
   mechanisms provided by SIP.  The use of the 'cfw-id' SDP attribute
   results in important session information being carried across the SIP
   network.  For this reason, SIP clients using this specification MUST
   use appropriate security mechanisms, such as TLS [RFC5246] and SMIME
   [RFC5751], when deployed in open networks.

12.2.  Transport-Level Protection

   When using only TCP connections, the Channel Framework security is
   weak.  Although the Channel Framework requires the ability to protect
   this exchange, there is no guarantee that the protection will be used
   all the time.  If such protection is not used, anyone can see data
   exchanges.

   Sensitive data, such as private and financial data, is carried over
   the Control Framework channel.  Clients and servers must be properly
   authenticated/authorized and the Control Channel must permit the use
   of confidentiality, replay protection, and integrity protection for
   the data.  To ensure Control Channel protection, Control Framework
   clients and servers MUST support TLS and SHOULD use it by default

   unless alternative Control Channel protection is used or a protected
   environment is guaranteed by the administrator of the network.
   Alternative Control Channel protection MAY be used if desired (e.g.,
   IPsec [RFC5246]).

   TLS is used to authenticate devices and to provide integrity, replay
   protection, and confidentiality for the header fields being
   transported on the Control Channel.  Channel Framework elements MUST
   implement TLS and MUST also implement the TLS ClientExtendedHello
   extended hello information for server name indication as described in
   [RFC5246].  A TLS cipher-suite of TLS_RSA_WITH_AES_128_CBC_SHA
   [RFC3261] MUST be supported.  Other cipher-suites MAY also be
   supported.

   When a TLS client establishes a connection with a server, it is
   presented with the server's X.509 certificate.  Authentication
   proceeds as described in Section 7.3 ("Client Behavior") of RFC 5922
   [RFC5922].

   A TLS server conformant to this specification MUST ask for a client
   certificate; if the client possesses a certificate, it will be
   presented to the server for mutual authentication, and authentication
   proceeds as described in Section 7.4 ("Server Behavior") of RFC 5922
   [RFC5922].

12.3.  Control Channel Policy Management

   This specification permits the establishment of a dedicated Control
   Channel using SIP.  It is also permitted for entities to create
   multiple channels for the purpose of failover and redundancy.  As a
   general solution, the ability for multiple entities to create
   connections and have access to resources could be the cause of
   potential conflict in shared environments.  It should be noted that
   this document does not carry any specific mechanism to overcome such
   conflicts but will provide a summary of how to do so.

   It can be determined that access to resources and use of Control
   Channels relate to policy.  It can be considered implementation and
   deployment detail that dictates the level of policy that is adopted.
   The authorization and associated policy of a Control Channel can be
   linked to the authentication mechanisms described in this section.
   For example, strictly authenticating a Control Channel using TLS
   authentication allows entities to protect resources and ensure the
   required level of granularity.  Such policy can be applied at the
   package level or even as low as a structure like a conference
   instance (Control Channel X is not permitted to issue commands for
   Control Package y OR Control Channel A is not permitted to issue
   commands for conference instance B).  Systems should ensure that, if

   required, an appropriate policy framework is adopted to satisfy the
   requirements for implemented packages.  The most robust form of
   policy can be achieved using a strong authentication mechanism such
   as mutual TLS authentication on the Control Channel.  This
   specification provides a Control Channel response code (403) to
   indicate to the issuer of a command that it is not permitted.  The
   403 response MUST be issued to Control Framework requests that are
   not permitted under the implemented policy.  If a 403 response is
   received, a Control Framework client MAY choose to re-submit the
   request with differing requirements or to abandon the request.  The
   403 response does not provide any additional information on the
   policy failure due to the generic nature of this specification.
   Individual Control Packages can supply additional information if
   required.  The mechanism for providing such additional information is
   not mandated in this specification.  It should be noted that
   additional policy requirements to those covered in this section might
   be defined and applied in individual packages that specify a finer
   granularity for access to resources, etc.

13.  IANA Considerations

   IANA has created a new registry for SIP Control Framework parameters.
   The "Media Control Channel Framework Parameters" registry is a
   container for sub-registries.  This section further introduces sub-
   registries for control packages, method names, status codes, header
   field names, and port and transport protocol.

   Additionally, Section 13.6 registers a new MIME type for use with
   SDP.

   For all registries and sub-registries created by this document, the
   policy applied when creating a new registration is also applied when
   changing an existing registration.

13.1.  Control Packages Registration Information

   This specification establishes the Control Packages sub-registry
   under Media Control Channel Framework Packages.  New parameters in
   this sub-registry must be published in an RFC (either in the IETF
   stream or Independent Submission stream), using the IANA policy
   [RFC5226] "RFC Required".

   As this document specifies no package or template-package names, the
   initial IANA registration for Control Packages will be empty.  The
   remainder of the text in this section gives an example of the type of
   information to be maintained by the IANA.

   The table below lists the Control Packages defined in the "Media
   Control Channel Framework".

    Package Name      Reference
    ------------      ---------
    example1          [RFCXXXX]

13.1.1.  Control Package Registration Template

      Package Name:

          (Package names must conform to the syntax described in
          Section 8.1.)

      Published Specification(s):

          (Control Packages require an RFC.)

      Person & email address to contact for further information:

13.2.  Control Framework Method Names

   This specification establishes the Method Names sub-registry under
   Media Control Channel Framework Parameters and initiates its
   population as follows.  New parameters in this sub-registry must be
   published in an RFC (either in the IETF stream or Independent
   Submission stream).

    CONTROL - [RFC6230]
    REPORT  - [RFC6230]
    SYNC    - [RFC6230]
    K-ALIVE - [RFC6230]

   The following information MUST be provided in an RFC in order to
   register a new Control Framework method:

   o  The method name.

   o  The RFC number in which the method is registered.

13.3.  Control Framework Status Codes

   This specification establishes the Status Code sub-registry under
   Media Control Channel Framework Parameters.  New parameters in this
   sub-registry must be published in an RFC (either in the IETF stream
   or Independent Submission stream).  Its initial population is defined
   in Section 9.  It takes the following format:

    Code Description Reference

   The following information MUST be provided in an RFC in order to
   register a new Control Framework status code:

   o  The status code number.

   o  The RFC number in which the method is registered.

   o  A brief description of the status code.

13.4.  Control Framework Header Fields

   This specification establishes the Header Field sub-registry under
   Media Control Channel Framework Parameters.  New parameters in this
   sub-registry must be published in an RFC (either in the IETF stream
   or Independent Submission stream).  Its initial population is defined
   as follows:

      Control-Package - [RFC6230]
      Status - [RFC6230]
      Seq - [RFC6230]
      Timeout - [RFC6230]
      Dialog-ID - [RFC6230]
      Packages - [RFC6230]
      Supported - [RFC6230]
      Keep-Alive - [RFC6230]
      Content-Type - [RFC6230]
      Content-Length - [RFC6230]

   The following information MUST be provided in an RFC in order to
   register a new Channel Framework header field:

   o  The header field name.

   o  The RFC number in which the method is registered.

13.5.  Control Framework Port

   The Control Framework uses TCP port 7563, from the "registered" port
   range.  Usage of this value is described in Section 4.1.

13.6.  Media Type Registrations

   This section describes the media types and names associated with
   payload formats used by the Control Framework.  The registration uses
   the templates defined in [RFC4288].  It follows [RFC4855].

13.6.1.  Registration of MIME Media Type application/cfw

    Type name: application

    Subtype name: cfw

    Required parameters: None

    Optional parameters: None

    Encoding considerations: Binary and see Section 4 of RFC 6230

    Security considerations: See Section 12 of RFC 6230

    Interoperability considerations:
       Endpoints compliant to this specification must
       use this MIME type.  Receivers who cannot support
       this specification will reject using appropriate
       protocol mechanism.

    Published specification: RFC 6230

    Applications that use this media type:
       Applications compliant with Media Control Channels.

     Additional Information:
       Magic number(s): (none)
       File extension(s): (none)
       Macintosh file type code(s): (none)

    Person & email address to contact for further information:
       Chris Boulton <chris@ns-technologies.com>

    Intended usage: COMMON

    Restrictions on usage:
       Should be used only in conjunction with this specification,
       RFC 6230.

    Author: Chris Boulton

    Change controller:
       IETF MEDIACTRL working group, delegated from the IESG.

13.6.2.  Registration of MIME Media Type application/
         framework-attributes+xml

    Type name:  application

    Subtype name:  framework-attributes+xml

    Required parameters:  (none)

    Optional parameters: Same as charset parameter of application/xml as
       specified in RFC 3023 [RFC3023].

    Encoding considerations:  Same as encoding considerations of
       application/xml as specified in RFC 3023 [RFC3023].

    Security considerations:  No known security considerations outside
       of those provided by core Media Control Channel Framework.

    Interoperability considerations:  This content type provides common
       constructs for related Media Control Channel packages.

    Published specification:  RFC 6230

    Applications that use this media type:  Implementations of
       appropriate Media Control Channel packages.

    Additional information:
       Magic number(s): (none)
       File extension(s): (none)
       Macintosh file type code(s): (none)

    Person & email address to contact for further information:
       Chris Boulton <chris@ns-technologies.com>

    Intended usage:  LIMITED USE

    Author/Change controller:  The IETF

    Other information:  None.

13.7.  'cfw-id' SDP Attribute

   Contact name:          Chris Boulton <chris@ns-technologies.com>

   Attribute name:        "cfw-id".

   Type of attribute      Media level.

   Subject to charset:    Not.

   Purpose of attribute:  The 'cfw-id' attribute indicates an
      identifier that can be used to correlate the Control Channel with
      the SIP INVITE dialog used to negotiate it, when the attribute
      value is used within the Control Channel.

   Allowed attribute values:  A token.

13.8.  URN Sub-Namespace for
       urn:ietf:params:xml:ns:control:framework-attributes

   IANA has registered a new XML namespace,
   "urn:ietf:params:xml:ns:control:framework-attributes", per the
   guidelines in RFC 3688 [RFC3688].

  URI: urn:ietf:params:xml:ns:control:framework-attributes

  Registrant Contact: IETF MEDIACTRL working group <mediactrl@ietf.org>,
     Chris Boulton <chris@ns-technologies.com>.

  XML:

     BEGIN
     <?xml version="1.0"?>
     <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
         "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
      <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
       <head>
        <title>Media Control Channel attributes</title>
       </head>
       <body>
        <h1>Namespace for Media Control Channel attributes</h1>
        <h2>urn:ietf:params:xml:ns:control:framework-attributes</h2>
          <p>See <a href="http://www.rfc-editor.org/rfc/rfc6230.txt">
             RFC 6230</a>.</p>
       </body>
      </html>
     END

13.9.  XML Schema Registration

   This section registers an XML schema as per the guidelines in RFC
   3688 [RFC3688].

  URI:  urn:ietf:params:xml:ns:control:framework-attributes

  Registrant Contact: IETF MEDIACTRL working group <mediactrl@ietf.org>,
     Chris Boulton <chris@ns-technologies.com>.

  Schema:  The XML for this schema can be found in Appendix A.1 of this
     document.

14.  Contributors

   Asher Shiratzky from Radvision provided valuable support and
   contributions to the early versions of this document.

15.  Acknowledgments

   The authors would like to thank Ian Evans of Avaya, Michael
   Bardzinski and John Dally of NS-Technologies, Adnan Saleem of
   Radisys, and Dave Morgan for useful review and input to this work.
   Eric Burger contributed to the early phases of this work.

   Expert review was also provided by Spencer Dawkins, Krishna Prasad
   Kalluri, Lorenzo Miniero, and Roni Even.  Hadriel Kaplan provided
   expert guidance on the dialog association mechanism.  Lorenzo Miniero
   has constantly provided excellent feedback based on his work.

   Ben Campbell carried out the RAI expert review on this document and
   provided a great deal of invaluable input.  Brian Weis carried out a
   thorough security review.  Jonathan Lennox carried out a thorough SDP
   review that provided some excellent modifications.  Text from Eric
   Burger was used in the introduction in the explanation for using SIP.

16.  References

16.1.  Normative References

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

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

   [RFC3261]  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.

   [RFC3262]  Rosenberg, J. and H. Schulzrinne, "Reliability of
              Provisional Responses in Session Initiation Protocol
              (SIP)", RFC 3262, June 2002.

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3311]  Rosenberg, J., "The Session Initiation Protocol (SIP)
              UPDATE Method", RFC 3311, October 2002.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
              the Session Description Protocol (SDP)", RFC 4145,
              September 2005.

   [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", BCP 13, RFC 4288, December 2005.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4574]  Levin, O. and G. Camarillo, "The Session Description
              Protocol (SDP) Label Attribute", RFC 4574, August 2006.

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, February 2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, January 2010.

   [RFC5922]  Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
              Certificates in the Session Initiation Protocol (SIP)",
              RFC 5922, June 2010.

16.2.  Informative References

   [MSCL-THOUGHTS]
              Burger, E., "Media Server Control Language and Protocol
              Thoughts", Work in Progress, June 2006.

   [RFC3023]  Murata, M., St. Laurent, S., and D. Kohn, "XML Media
              Types", RFC 3023, January 2001.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3725]  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.

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

   [RFC3841]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
              Preferences for the Session Initiation Protocol (SIP)",
              RFC 3841, August 2004.

   [RFC5125]  Taylor, T., "Reclassification of RFC 3525 to Historic",
              RFC 5125, February 2008.

   [RFC5167]  Dolly, M. and R. Even, "Media Server Control Protocol
              Requirements", RFC 5167, March 2008.

   [RFC5626]  Jennings, C., Mahy, R., and F. Audet, "Managing Client-
              Initiated Connections in the Session Initiation Protocol
              (SIP)", RFC 5626, October 2009.

Appendix A.  Common Package Components

   During the creation of the Control Framework, it has become clear
   that there are a number of components that are common across multiple
   packages.  It has become apparent that it would be useful to collect
   such reusable components in a central location.  In the short term,
   this appendix provides the placeholder for the utilities, and it is
   the intention that this section will eventually form the basis of an
   initial 'Utilities Document' that can be used by Control Packages.

A.1.  Common Dialog/Multiparty Reference Schema

   The following schema provides some common attributes for allowing
   Control Packages to apply specific commands to a particular SIP media
   dialog (also referred to as "Connection") or conference.  If used
   within a Control Package, the Connection and multiparty attributes
   will be imported and used appropriately to specifically identify
   either a SIP dialog or a conference instance.  If used within a
   package, the value contained in the 'connectionid' attribute MUST be
   constructed by concatenating the 'Local' and 'Remote' SIP dialog
   identifier tags as defined in [RFC3261].  They MUST then be separated
   using the ':' character.  So the format would be:

               'Local Dialog tag' + ':' + 'Remote Dialog tag'

   As an example, for an entity that has a SIP Local dialog identifier
   of '7HDY839' and a Remote dialog identifier of 'HJKSkyHS', the
   'connectionid' attribute for a Control Framework command would be:

                 7HDY839:HJKSkyHS

   It should be noted that Control Framework requests initiated in
   conjunction with a SIP dialog will produce a different 'connectionid'
   value depending on the directionality of the request; for example,
   Local and Remote tags are locally identifiable.

   As with the Connection attribute previously defined, it is useful to
   have the ability to apply specific Control Framework commands to a
   number of related dialogs, such as a multiparty call.  This typically
   consists of a number of media dialogs that are logically bound by a
   single identifier.  The following schema allows for Control Framework
   commands to explicitly reference such a grouping through a
   'conferenceid' XML container.  If used by a Control Package, any
   control XML referenced by the attribute applies to all related media
   dialogs.  Unlike the dialog attribute, the 'conferenceid' attribute
   does not need to be constructed based on the overlying SIP dialog.
   The 'conferenceid' attribute value is system specific and should be
   selected with relevant context and uniqueness.

   It should be noted that the values contained in both the
   'connectionid' and 'conferenceid' identifiers MUST be compared in a
   case-sensitive manner.

   The full schema follows:

 <?xml version="1.0" encoding="UTF-8"?>

 <xsd:schema
   targetNamespace="urn:ietf:params:xml:ns:control:framework-attributes"
   xmlns:xsd="http://www.w3.org/2001/XMLSchema"
   xmlns="urn:ietf:params:xml:ns::control:framework-attributes"
   elementFormDefault="qualified" attributeFormDefault="unqualified">

        <xsd:attributeGroup name="framework-attributes">
          <xsd:annotation>
            <xsd:documentation>
              SIP Connection and Conf Identifiers
            </xsd:documentation>
          </xsd:annotation>

          <xsd:attribute name="connectionid" type="xsd:string"/>

          <xsd:attribute name="conferenceid" type="xsd:string"/>

        </xsd:attributeGroup>
 </xsd:schema>

Authors' Addresses

   Chris Boulton
   NS-Technologies

   EMail: chris@ns-technologies.com

   Tim Melanchuk
   Rainwillow

   EMail: timm@rainwillow.com

   Scott McGlashan
   Hewlett-Packard
   Gustav III:s boulevard 36
   SE-16985 Stockholm, Sweden

   EMail: smcg.stds01@mcglashan.org

 

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