RFC 6230
Media Control Channel Framework
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 FrameworkAbstract
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.
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