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Copyright © The Internet Society (2006).
This document describes a Framework and protocol for application deployment where the application logic and processing are distributed. 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 distributed, centralized conference system, as defined by the XCON work group of the IETF. It is not, however, limited to this scope and it is envisioned that this generic Framework will be used for a wide variety of de-coupled control architectures between network entities.
2. Conventions and Terminology
4. Locating External Server Resources
5. Control Client SIP UAC Behavior - Control Channel Setup
5.1. Control Client SIP UAC Behavior - Media Dialogs
6. Control Server SIP UAS Behavior - Control Channel Setup
7. Control Framework Interactions
7.1. Constructing Requests
7.1.1. Sending CONTROL
7.1.2. Sending REPORT
7.2. Constructing Responses
8. Response Code Descriptions
8.1. 200 Response Code
8.2. 202 Response Code
8.3. 400 Response Code
8.4. 403 Response Code
8.5. 481 Response Code
8.6. 500 Response Code
9. Control Packages
9.1. Control Package Name
9.2. Framework Message Usage
9.3. Common XML Support
9.4. CONTROL Message Bodies
9.5. REPORT Message Bodies
10. Network Address Translation (NAT)
11. Formal Syntax
11.1. SIP Formal Syntax
11.2. Control Framework Formal Syntax
13. Security Considerations
14. IANA Considerations
14.1. IANA Registration of the 'escs' Option Tag
14.2. Control Package Registration Information
14.2.1. Control Package Registration Template
14.3. SDP Transport Protocol
14.4. SDP Attribute Names
14.5. SIP Response Codes
16. Appendix A
16.1. Common Dialog/Multiparty Reference Schema
17.1. Normative References
17.2. Informative References
§ Authors' Addresses
§ Intellectual Property and Copyright Statements
Applications are often developed using an architecture where the application logic and processing activities are distributed. Commonly, the application logic runs on "application servers" whilst 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 the 'Media Control Protocol Framework' document (Dolly, M., “Media Control Protocol Requirements,” September 2006.). While the Framework is not media server control specific, it is the primary driver and use case for this work. It is intended that the framework contained in this document will be used for a plethora of appropriate device control scenarios.
This document does not define a SIP based extension that can be used directly for the control of external components. The framework mechanism must be extended by other documents that are known as "Control Packages". A comprehensive set of guidelines for creating "Control Packages" is described in Section 9 (Control Packages).
Current IETF transport device control protocols, such as megaco (Groves, C., Pantaleo, M., Anderson, T., and T. Taylor, “Gateway Control Protocol Version 1,” June 2003.) , while excellent for controlling media gateways that bridge separate networks, are troublesome for supporting media-rich applications in SIP networks, because they duplicate many of the functions inherent in SIP. Rather than relying on single protocol session establishment, application developers need to translate between two separate mechanisms.
Application servers traditionally use SIP third party call control RFC 3725 (Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo, “Best Current Practices for Third Party Call Control (3pcc) in the Session Initiation Protocol (SIP),” April 2004.)  to establish media sessions from SIP user agents to a media server. SIP, as defined in RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) , also 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.
In this document, BCP 14/RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.)  defines the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL". In addition, BCP 15 indicates requirement levels for compliant implementations.
The following additional terms are defined for use in this document:
- 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 in this case, has a direct RTP (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.)  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 may 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".
- A Method is the type of a framework message. Three Methods are defined in this document: SYNCH, CONTROL, and REPORT.
- 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 MUST complete within Transaction-Timeout time.
- extended framework transaction:
- An extended framework transaction is used to extend the lifetime of a CONTROL method transaction when the Control Command it carries cannot be completed within Command-Timeout milliseconds. A Server extends the lifetime of a CONTROL method transaction by sending a 202 response code followed by one or more REPORT transactions as specified in Section 7.1.2 (Sending REPORT). Extended framework transactions allow command failures to be discovered at the transaction layer.
- the maximum allowed time between a control Client or Server issuing a framework message and receiving a corresponding response. The value for the timeout should be based on a multiple of the network RTT plus Command-Timeout milliseconds to allow for message parsing and processing.
- [timm: Do we want to differentiate between Control and Report transaction times - the latter does need to allow for command processing. Do we even need a transaction time for REPORT messages or is it sufficient to simply have transaction times for CONTROL messages and rely on TCP for REPORT?] What about SYNCH? It currently has its own independent timing.
This document details mechanisms for establishing, using, and terminating a reliable channel using SIP for the purpose of 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 voice session. Figure 1 (Basic Architecture) illustrates a simplified view of the proposed 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.
The use of SIP for the specified mechanism provides many inherent capabilities which include:-
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 at both a routing level, where RFC 3263 (Rosenberg, J. and H. Schulzrinne, “Session Initiation Protocol (SIP): Locating SIP Servers,” June 2002.)  provides the physical location of devices, and at the Service level, using Caller Preferences (Rosenberg, J., Schulzrinne, H., and P. Kyzivat, “Indicating User Agent Capabilities in the Session Initiation Protocol (SIP),” August 2004.) and Callee Capabilities (Rosenberg, J., Schulzrinne, H., and P. Kyzivat, “Caller Preferences for the Session Initiation Protocol (SIP),” August 2004.). 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 5 (Control Client SIP UAC Behavior - Control Channel Setup) 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 (Basic Architecture) conveys a 1:1 connection between the Control Client and the Control Server. It is possible, if required, for multiple control channels using separate SIP dialogs to be established 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 agnostic to any media sessions. Specific media session information can be incorporated in control interaction commands (which themselves are defined in external packages) using the XML schema defined in Section 16 (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.
[Editors Note: Still under discussion. How does an app server know, when there are multiple external servers, which specific server has any given media session? Next version of the draft will discuss the correlation procedures. The App server needs a control channel with the media server and needs to know which channel to use once the media session has been established. Sounds like a GRUU usage?]
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 SIP INVITE request to the external Server. The request contains the SIP option tag "escs" in a SIP "Require" header for the purpose of forcing the use of the mechanism described in this document. The SDP payload includes the required information for control channel negotiation. The COMEDIA (Yon, D. and G. Camarillo, “TCP-Based Media Transport in the Session Description Protocol (SDP),” September 2005.)  specification for setting up and maintaining reliable connections is used (more detail available in later sections).
The client MUST include details of control packages that are supported and, more specifically, that will be used within the control channel created. This is achieved through the inclusion of a SIP "Control-Packages" header. The "Control-Packages" header is defined and described later in this document.
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 Require: escs Control-Packages: <example-package> Call-ID: 7823987HJHG6 Content-Type: application/sdp v=0 o=originator 2890844526 2890842808 IN IP4 controller.example,com s=- c=IN IP4 controller.example.com m=application 7575 TCP/ESCS a=setup:active a=connection:new
On receiving the INVITE request, the external Server supporting this mechanism generates a 200 OK response containing appropriate SDP.
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 Call-ID: 7823987HJHG6 Content-Type: application/sdp v=0 o=originator 2890844526 2890842808 IN IP4 controller.example,com s=- c=IN IP4 mserver.example.com m=application 7563 TCP/ESCS a=setup:passive a=connection:new
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) TCP connection to the Control Server. The connection address (taken from 'c=') and port (taken from 'm=')are used to identify the remote part in the new connection.
Once established, the newly created connection can be used to exchange control language requests and responses. If required, after the control channel has been setup, media sessions can be established using standard SIP third party call control.
[Editors Note: See previous note:this is where we may need to mention how an App Server knows which external Server is responsible for any given media session.]
Figure 4 (Participant Architecture) provides a simplified example where the proposed framework is used to control a User Agent's RTP session. (1) in brackets represents the SIP dialog and dedicated control channel previously described in this overview section.
+--------Control SIP Dialog(1)---------+ | | v v +-----+ +--+--+ +------(2)------>| SIP |---------------(2)------------->| SIP | | |Stack| |Stack| | +---+-----+---+ +---+-----+---+ | | | | | | | Control |<--Control Channel(1)-->| | | | Client | | Control | | +-------------+ | Server | +--+--+ | | |User | | | |Agent|<=====================RTP(2)===================>| | +-----+ +-------------+
| Figure 4: Participant Architecture |
(2) from Figure 4 (Participant Architecture) represents the User Agent SIP dialog interactions and associated media flow. A User Agent would create a SIP dialog with the Control Client entity. The Control Client entity will also create a related dialog to the Control Server (B2BUA type functionality). Using the interaction illustrated by (2), the User Agent is able to negotiate media capabilities with the Control Server using standard SIP mechanisms as defined in RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.)  and RFC 3264 (Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” June 2002.) .
If not present in the SDP received by the Control Client from the User Agent(2), a media label SDP attribute, which is defined in  (Levin, O. and G. Camarillo, “The SDP (Session Description Protocol) Label Attribute,” January 2005.), should be inserted for every media description (identified as m= line as defined in  (Handley, M., “SDP: Session Description Protocol,” January 2006.)). This provides flexibility for the Control Client, because it can generate control messages that specify a particular Media stream (between User Agent and Control Server) within a SIP media dialog. If a Media label is not included in the control message, it applies to all media associated with the dialog.
A non 2xx class SIP response received for the INVITE request indicates that no SIP dialog has been created and is treated as specified RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) . One exception to this is the "496" (TODO:need to pick an appropriate response code) response code whose operation is defined in Section 6 (Control Server SIP UAS Behavior - Control Channel Setup).
Section will describe mechanisms for locating an external Server.
On creating a new SIP INVITE request, a UAC can insist on using the mechanisms defined in this document. This is achieved by inserting a SIP Require header containing the option tag 'escs'. A SIP Require header with the value 'escs' MUST NOT be present in any other SIP request type.
If on creating a new SIP INVITE request, a UAC does not want to insist on the usage of the mechanisms defined in this document but merely that it supports them, a SIP Supported header MUST be included in the request with the option tag 'escs'.
The SIP INVITE MUST include a SIP "Control-Packages" header which MUST contain at least one valid entry and can contain multiple control packages if required.
If a reliable response is received (as defined RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.)  and RFC 3262 (Rosenberg, J. and H. Schulzrinne, “Reliability of Provisional Responses in Session Initiation Protocol (SIP),” June 2002.) ) that contains a SIP Require header containing the option tag 'escs', the mechanisms defined in this document are applicable to the newly created dialog.
Before the UAC can send a request, it MUST include a valid session description using the Session Description Protocol defined in  (Handley, M., “SDP: Session Description Protocol,” January 2006.). The following information defines the composition of some specific elements of the SDP payload that MUST be adhered to for compliancy to this specification.
The Connection Data line in the SDP payload is constructed as specified in  (Handley, M., “SDP: Session Description Protocol,” January 2006.):
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 originators address, for example dns/IP representation. The address will be the network address used for connections in this specification.
c=IN IP4 controller.example.com
The SDP MUST contain a corresponding Media Description entry for compliance to this specification:
m=<media> <port> <proto>
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 constructing client can't receive incoming connections it MUST still enter a valid port range entry. The use of the port value '0' has the same meaning as defined in the SDP specification (Handley, M., “SDP: Session Description Protocol,” January 2006.). The third sub-field, <proto>, MUST equal the value "TCP/ESCS" as defined in Section 14.3.2 (TCP/TLS/ESCS) of this document.
[Editors note: Need to cover other protocols so not TCP specific]
The SDP MUST also contain a number of SDP media attributes(a=) that are specifically defined in the COMEDIA specification. The attributes provide connection negotiation and maintenance parameters. A client conforming to this specification SHOULD support all the possible values defined for media attributes from the COMEDIA (Yon, D. and G. Camarillo, “TCP-Based Media Transport in the Session Description Protocol (SDP),” September 2005.)  specification but MAY choose not to support values if it can definitely determine they will never be used (for example will only ever initiate outgoing connections). 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 attributes is:
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 (Overview). Once the SDP has been constructed along with the remainder of the SIP INVITE request (as defined in RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) ), it can be sent to the appropriate location. The SIP dialog and appropriate control connection is then established.
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 specific Control package commands to co-existing SIP dialogs that have been established with the same remote server, for example the manipulation of audio dialogs connected to a media server.
Such co-existing dialogs will pass through the Control Client (see Figure 4 (Participant Architecture)) entity and may contain more than one Media Description (as defined by "m=" in the SDP). The Control Client SHOULD include a media label attribute (B2BUA functionality), as defined in  (Levin, O. and G. Camarillo, “The SDP (Session Description Protocol) Label Attribute,” January 2005.), for each "m=" definition. A Control Client constructing the SDP MAY choose not to include the media label SDP attribute if it does not require direct control on a per media stream basis.
This framework identifies the common re-use of referencing media dialogs and has specified a connection reference attribute that can optionally be imported into any Control Package. It is intended that this will reduce repetitive specifying of dialog reference language. The schema can be found in Section 16.1 (Common Dialog/Multiparty Reference Schema) in Appendix A.
Similarly, the ability to identify and apply commands to a group of media dialogs is also identified as a common structure that could be defined and re-used (for example playing a prompt to all participants in a Conference). The schema for such operations can also be found in Section 16.1 (Common Dialog/Multiparty Reference Schema) in Appendix A.
Support for both the common attributes described here is specified as part of each Control Package definition, as detailed in Section 9 (Control Packages).
On receiving a SIP INVITE request, an external Server(UAS) inspects the message for indications of support for the mechanisms defined in this specification. This is achieved through the presence of the SIP Supported and Require headers containing the option tag 'escs'. If the external Server wishes to construct a reliable response that conveys support for the extension, it should follow the mechanisms defined in RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.)  for responding to SIP supported and Require headers. If support is conveyed in a reliable SIP provisional response, the mechanisms in RFC 3262 (Rosenberg, J. and H. Schulzrinne, “Reliability of Provisional Responses in Session Initiation Protocol (SIP),” June 2002.)  MUST also be used.
When constructing a SIP success response, the SDP payload MUST be constructed using the semantics(Connection, Media and attribute) defined in Section 5 (Control Client SIP UAC Behavior - Control Channel Setup) using valid local settings and also with full compliance to the COMEDIA (Yon, D. and G. Camarillo, “TCP-Based Media Transport in the Session Description Protocol (SDP),” September 2005.) specification. For example, the SDP attributes included in the answer constructed for the example offer provided in Section 5 (Control Client SIP UAC Behavior - Control Channel Setup) would look as illustrated below:
Once the SIP success response 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 connection is now available to exchange commands, as defined in "Control Packages" and described in Section 9 (Control Packages). The state of the SIP Dialog and the associated Control channel are now explicitly linked. If either party wishes to terminate a Control channel is simply issues a SIP termination request (SIP BYE request). The Control Channel therefore lives for the duration of the SIP dialog.
If the UAS does not support the extension contained in a SIP Supported or Require header it MUST respond as detailed in RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) . If the UAS does support the SIP extension contained in a SIP Require or Supported header but does not support one or more of the Control packages, as represented in the "Control-Packages" SIP header, it MUST respond with a SIP "496 Unknown Control Package" response code. The error response MUST conform to RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.)  and MUST also include a "Control-Packages" SIP header which lists the control packages from the request that the UAS does not support. This provides the Controlling UAC with an explicit reason for failure and allows for re-submission of the request without the un-supported control package.
A SIP entity receiving a SIP OPTIONS request MUST respond appropriately as defined in RFC 3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) . This involves providing information relating to supported SIP extensions in the 'Supported' message header. For this extension a value of 'escs' MUST be included. Additionally, a SIP entity MUST include all the additional control packages that are associated with the Control channel. This is achieved by including a 'Control-Packages' SIP message header listing all relevant supported Control package tokens.
The use of the COMEDIA specification in this document allows for a Control Channel to be set up in either direction as a result of the SIP INVITE transaction. While providing a flexible negotiation mechanism, it does provide certain correlation problems between the channel and the overlying SIP dialog. Remember that the two are implicitly linked and so need a robust correlation mechanism. 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 dialog as it could be simply listening for all incoming connections on a specific port. As a consequence, some rules are applied to allow a connecting (defined as 'active' role in COMEDIA) client to identify the associated SIP dialog that triggered the connection. The following steps provide an identification mechanism that MUST be carried out before any other signaling is carried out on the newly created Control channel.
Once a successful control channel has been established, as defined in Section 5 (Control Client SIP UAC Behavior - Control Channel Setup) and Section 6 (Control Server SIP UAS Behavior - Control Channel Setup) (and the connection has been correlated, as described in previous paragraph), the two entities are now in a position to exchange relevant control framework messages. The remainder of this section provides details of the core set of methods and responses that MUST be supported for the core control framework. Future extensions to this document MAY define new methods and responses.
An entity acting as a Control Client is now able to construct and send new requests on a control channel and MUST adhere to the syntax defined in Section 11 (Formal Syntax). Control Commands MUST also adhere to the syntax defined by the Control Packages negotiated in Section 5 (Control Client SIP UAC Behavior - Control Channel Setup) and Section 6 (Control Server SIP UAS Behavior - Control Channel Setup) of this document. A Control Client MUST create a unique transaction and associated identifier per request transaction. 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 11 (Formal Syntax)). The first line starts with the SCFW token for the purpose of easily extracting the transaction identifier. The transaction identifier MUST be globally unique over space and time. All required mandatory and optional control framework headers are then inserted into the control message 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).
Any framework message that contains an associated payload MUST also include a 'Content-Length' message header which represents the size of the message body in decimal number of octets. If no associated payload is to be added to the message, a 'Content-Length' header with a value of '0' MUST be included.
When all of the headers have been included in the framework message, it is sent down the control channel established in Section 5 (Control Client SIP UAC Behavior - Control Channel Setup).
It is a requirement that a Server receiving such a request respond quickkly with an appropriate response (as defined in Section 7.2 (Constructing Responses)). A Control Client entity needs to wait for Transaction-Time time for a response before considering the transaction a failure.
A 'CONTROL' message is used by Control Client to invoke control commands on a Control Server. The message is constructed in the same way as any standard Control Framework message, as discussed previously in Section 7.1 (Constructing Requests) and defined in Section 11 (Formal Syntax). 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. These specifications MUST conform to the format defined in Section 9.4 (CONTROL Message Bodies).
A 'REPORT' message is used by a Control Server in two situations. The first situation occurs when processing of a Control Command extends beyond a Command-Timeout. In this case a 202 response is returned. Status updates and the final results of the command are then returned in subsequent REPORT messages. The second situation allows REPORT to be used as an event notification mechanism where events are correlated with the original CONTROL message. In this case, REPORT messages may be sent after the original transaction or extended transaction has completed.
All REPORT messages MUST contain the same transaction ID in the request start line that was present in the original CONTROL transaction. This allows both extended transactions and event notifications to be correlated with the original CONTROL transaction.
On receiving a CONTROL message, a Control Server MUST respond within Command-Timeout with a status code for the request, as specified in Section 7.2 (Constructing Responses). If the command completed within that time, a 200 response code would have been sent. If the command did not complete within that time, the response code 202 would have been sent indicating that the requested command is still being processed and the CONTROL transaction is being extended. The REPORT method is used to update the status of the extended transaction.
A Control Server issuing a 202 response MUST immediately issue a REPORT message. The initial REPORT message MUST contain a 'Seq' (Sequence) message header with a value equal to '1' (It should be noted that the 'Seq' numbers at both Control Client Control Server for framework messages are independent). The initial REPORT message MUST also contain a 'Status' message header with a value of 'pending'. This initial REPORT message MUST NOT contain a message body and is primarily used to establish a subsequent message transaction based on the initial CONTROL message.
All REPORT messages for an extended CONTROL transaction MUST contain a 'Timeout' message header. This header will contain a value in delta seconds that represents 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 'pending' or 'update', the Control Client MUST reset the counter for the associated extended CONTROL transaction to the indicated timeout period. If the timeout period approaches with no intended REPORT messages being 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 'pending'. 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 of ?? is 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 a timeout period of 10 seconds would be refreshed 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. They 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 9.5. 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 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. A Control Framework UAC can then clean up any pending state associated with the original control transaction.
Commands that are carried in CONTROL messages can request that the Server notify the Client about events that occur sometime in the future. It is not desirable to use extended Control transactions for these types of commands for two reasons. First, an event never occurring is often correct behavior. Second, events may occur long after the original request for their notification.
REPORT messages can be used to notify events. REPORT messages that notify events MUST contain a 'Status' header of 'Notify'. They MUST NOT contain either a 'Timeout' or 'Seq' header and any such headers MUST be ignored when the REPORT message has a 'Status' of 'notify'. The REPORT message MAY contain a message body.
A Control Client or Server, on receiving a request, MUST generate a response within Command-Time time. The response MUST conform to the ABNF defined in Section 11 (Formal Syntax). The first line of the response MUST contain the transaction identifier used in first line of the request, as defined in Section 7.1 (Constructing Requests). Responses MUST NOT include the 'Status' or 'Timeout' message headers - if they are included they have no meaning or semantics.
A Control Client or Server MUST then include a status code in the first line of the constructed response. A CONTROL request that has been understood, and either the relevant actions for the control command have completed or a control command error is detected, uses the 200 status code as defined in Section 8.1 (200 Response Code). A 200 response MAY include message bodies. If a 200 response does contain a payload it MUST include a Content-Length header. A 200 is the only response defined in this specification that allows a message body to be included. A client receiving a 200 class response then considers the control command completed. A CONTROL request that is received and understood but requires processing that extends beyond Command-Time time will return a 202 status code in the response. This will be followed immediately by an initial REPORT message as defined in Section 7.1.2 (Sending REPORT). A Control Package SHOULD explicitly define the circumstances under which either 200 or 202 with subsequent processing takes place.
If a Control Client or Server encounters problems with either a REPORT or CONTROL request, an appropriate error code should be used in the response, as listed in Section 8 (Response Code Descriptions). The generation of a non 2xx class response code to either a CONTROL or REPORT message will result in failure of the transaction, and all associated state and resources should 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.
[timm]: how can an error response provide an explicit indication of the reason for the transaction failure when only a 200 response allows message bodies?
The following response codes are defined for transaction responses to methods defined in Section 7.1 (Constructing Requests). 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.
The 200 code indicates the completion of a successful transaction.
The 202 response code indicates the completion of a successful transaction with additional information to be provided at a later time through the REPORT mechanism defined in Section 7.1.2 (Sending REPORT).
The 400 response indicates that the request was syntactically incorrect.
The 400 response indicates that the requested operation is illegal.
The 481 response indicates that the intended target of the request does not exist.
The 500 response indicates that the recipient does not understand the request
"Control Packages" are intended to specify behavior that extends the the capability defined in this document. "Control Packages" are not allowed to weaken "MUST" and "SHOULD" strength statements that are detailed in this document. A "Control Package" may strengthen "SHOULD" to "MUST" if justified by the specific usage of the framework.
In addition to normal sections expected in a standards-track RFC and SIP extension documents, authors of "Control Packages" need to address each of the issues detailed in the following subsections. The following sections MUST be used as a template and included appropriately in all Control-Packages.
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 event tokens is contained in Section 14 (IANA Considerations). The package name MUST also register a version number for the package. This enables updates to the package to be registered where appropriate. An initial version of a package MUST start with the value '1.0'. Subsequent versions MUST increment this number if the same package name is to be used. The exact increment is left to the discretion of the package author.
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 should explicitly detail the control messages 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.
[Editors Note: Need to examine text.]
This optional section is only included in a Control Package if the attributes for media dialog or Conference reference are required. The Control Package will make strong statements (MUST strength) if the XML schema defined in Section 16.1 (Common Dialog/Multiparty Reference Schema) in Appendix A is to be supported. If only part of the schema is required (for example just 'connection-id' or just conf-id), the Control Package will make equally strong (MUST strength) statements.
This mandatory section of a Control Package defines the control body that can be contained within a CONTROL command request, as defined in Section 7 (Control Framework Interactions) (or that no control package body is required). This section should indicate the location of detailed syntax definitions and semantics for the appropriate body types.
This mandatory section of a Control Package defines the REPORT body that can be contained within a REPORT command request, as defined in Section 7 (Control Framework Interactions) (or that no report package body is required). This section should indicate the location of detailed syntax definitions and semantics for the appropriate body types. 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.
A Control Package can optionally include one or more subscriptions that allow a controlling client to receive specific event updates in REPORT message bodies. The mechanisms that installs/un-installs subscriptions is not specified in document and is considered out of scope. Event notifications are always carried in REPORT messages MUST conform to the rules detailed in Section 188.8.131.52 (Reporting Asynchronous Events). This section of a Control Package definition MUST specify details of the payload expected to be received from subscriptions that have been installed.
[Editors Note: Ongoing discussions relating to a generic subscription/event mechanism across all packages.]
It is strongly RECOMMENDED that Control Packages provide a range of message flows that represent common flows using the package and this framework document.
[Editors Note: This section will look at geographically distributed systems where NAT traversal might be an issue. It will look at both the SIP media dialog traversal and the control channel traversal.]
The ABNF for the "Control-Packages" SIP header is as follows:
Control-Packages = "Control-Packages" HCOLON control-package-value *(COMMA control-package-value) control-package-value = control-package-name "/" control-package-version control-package-name = token control-package-version = 1*DIGIT "." 1*DIGIT
The Control Framework interactions use the UTF-8 transformation format as defined in RFC3629 (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.) . The syntax in this section uses the Augmented Backus-Naur Form (ABNF) as defined in RFC2234 (Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” November 1997.) .
control-req-or-resp = control-request / control-response control-request = control-req-start headers [control-content] control-response = control-resp-start headers control-req-start = pSCFW SP transact-id SP method CRLF control-resp-start = pSCFW SP transact-id SP status-code [SP comment] CRLF comment = utf8text pSCFW = %x184.108.40.206; SCFW in caps transact-id = alpha-num-token method = mCONTROL / mREPORT / mSYNCH / other-method mCONTROL = %x43.4F.4E.54.52.4F.4C; CONTROL in caps mREPORT = %x220.127.116.11F.52.54; REPORT in caps mSYNCH = %x53.59.4E.43.48; SYNCH in caps other-method = 1*UPALPHA status-code = 3DIGIT ; any code defined in this and other documents headers = Content-Length /Control-Package /Status /Seq /Timeout /Dialog-id /ext-header Content-Length = "Content-Length:" SP 1*DIGIT Control-Package = "Control-Package:" SP 1*alpha-num-token Status = "Status:" SP ("pending" / "update" / "terminate" ) Timeout = "Timeout:" SP 1*DIGIT Seq = "Seq:" SP 1*DIGIT Dialog-id = "Dialog-id:" SP dialog-id-string dialog-id-string = alpha-num-token "~" alpha-num-token ["~" alpha-num-token] alpha-num-token = alphanum 3*31alpha-num-tokent-char alpha-num-tokent-char = alphanum / "." / "-" / "+" / "%" / "=" control-content = Content-Type 2CRLF data CRLF Content-Type = "Content-Type:" SP media-type media-type = type "/" subtype *( ";" gen-param ) type = token subtype = token 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) ; token is compared case-insensitive 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 data = *OCTET ext-header = hname ":" SP hval CRLF hname = ALPHA *token hval = utf8text utf8text = *(HTAB / %x20-7E / UTF8-NONASCII) UTF8-NONASCII = %xC0-DF 1UTF8-CONT / %xE0-EF 2UTF8-CONT / %xF0-F7 3UTF8-CONT / %xF8-Fb 4UTF8-CONT / %xFC-FD 5UTF8-CONT UTF8-CONT = %x80-BF
The following table details a summary of the headers that can be contained in Control Framework interactions. The "where" columns details where headers can be used:
R: header field may only appear in requests; r: header field may only appear in responses; 2xx, 4xx, etc.: A numerical value or range indicates response codes with which the header field can be used; An empty entry in the "where" column indicates that the header field may be present in all requests and responses.
The remaining columns list the specified methods and the presence of a specific header:
m: The header field is mandatory. o: The header field is optional.
Header field Where CONTROL REPORT SYNCH ___________________________________________________ Content-Length o o - Control-Package R m - - Seq - m - Status R - m - Timeout R - m - Dialog-id R - - m
| Figure 11: Table 1 |
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.
Control Client Control Server | | | (1) SIP INVITE | | ----------------------------------------> | | | | (2) SIP 200 | | <--------------------------------------- | | | | (3) SIP ACK | | ----------------------------------------> | | | |==>=======================================>==| | Control Channel Established | |==>=======================================>==| | | | (4) SYNCH | | ----------------------------------------> | | | | (5) 200 | | <--------------------------------------- | | | | (6) CONTROL | | ----------------------------------------> | | | | (7) 202 | | <--------------------------------------- | | | | (8) REPORT (pending) | | <---------------------------------------- | | | | (9) 200 | | ----------------------------------------> | | | | (10) REPORT (update) | | <---------------------------------------- | | | | (11) 200 | | ----------------------------------------> | | | | (12) REPORT (terminate) | | <---------------------------------------- | | | | (13) 200 | | ----------------------------------------> | | | | (14) SIP BYE | | ----------------------------------------> | | | | (15) SIP 200 | | <--------------------------------------- | |=============================================| | Control Channel Terminated | |=============================================| | |
INVITE sip:firstname.lastname@example.org SIP/2.0 To: <sip:email@example.com> From: <sip:firstname.lastname@example.org>;tag=8937498 Via: SIP/2.0/UDP control-client.example.com;branch=z9hG412345678 CSeq: 1 INVITE Require: escs Control-Packages: <example-package> Call-ID: email@example.com Contact: <sip:firstname.lastname@example.org> Content-Type: application/sdp v=0 o=originator 2890844526 2890842808 IN IP4 controller.example,com s=- c=IN IP4 control-client.example.com m=application 7575 TCP/ESCS a=setup:active a=connection:new
SIP/2.0 200 OK To: <sip:email@example.com>;tag=023983774 From: <sip:firstname.lastname@example.org>;tag=8937498 Via: SIP/2.0/UDP control-client.example.com;branch=z9hG412345678 CSeq: 1 INVITE Require: escs Control-Packages: <example-package> Call-ID: email@example.com Contact: <sip:firstname.lastname@example.org> Content-Type: application/sdp v=0 o=originator 2890844526 2890842808 IN IP4 controller.example,com s=- c=IN IP4 control-server.example.com m=application 7575 TCP/ESCS a=setup:passive a=connection:new
SCFW 8djae7khauj SYNCH Dialog-id: email@example.com~023983774
SCFW 8djae7khauj 200
SCFW i387yeiqyiq CONTROL Control-Package: <package-name> Content-Length: 11 <XML BLOB/>
SCFW i387yeiqyiq 202
SCFW i387yeiqyiq REPORT Seq: 1 Status: pending Timeout: 10
SCFW i387yeiqyiq 200 Seq: 1
SCFW i387yeiqyiq REPORT Seq: 2 Status: update Timeout: 10 <XML BLOB/>
SCFW i387yeiqyiq 200 Seq: 2
SCFW i387yeiqyiq REPORT Seq: 3 Status: terminate Timeout: 10 <XML BLOB/>
SCFW i387yeiqyiq 200 Seq: 3
BYE sip:firstname.lastname@example.org SIP/2.0 To: <sip:email@example.com> From: <sip:firstname.lastname@example.org>;tag=8937498 Via: SIP/2.0/UDP control-client.example.com;branch=z9hG423456789 CSeq: 2 BYE Require: escs Control-Packages: <example-package> Call-ID: email@example.com
SIP/2.0 200 OK To: <sip:firstname.lastname@example.org>;tag=023983774 From: <sip:email@example.com>;tag=8937498 Via: SIP/2.0/UDP control-client.example.com;branch=z9hG423456789 CSeq: 2 BYE Require: escs Control-Packages: <example-package> Call-ID: firstname.lastname@example.org
Security Considerations to be included in later versions of this document.
The authors would like to thank Ian Evans and Michael Bardzinski of Ubiquity Software, Adnan Saleem of Convedia, and Dave Morgan for useful review and input to this work. Eric Burger contributed to the early phases of this work.
During the creation of the Control Framework it has become clear that there are number of components that are common across multiple packages. It has become apparent that it would be useful to collect such re-usable components in a central location. In the short term this appendix provides the place holder 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.
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 'connection-id' attribute MUST be constructed by concatenating the 'Local' and 'Remote' SIP dialog identifier tags as defined in RFC3261 (Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, “SIP: Session Initiation Protocol,” June 2002.) . 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 'connection-id' attribute for a Control Framework command would be:
If a session description has more than one media description (as identified by 'm=' in  (Handley, M., “SDP: Session Description Protocol,” January 2006.)) it is possible to explicitly reference them individually. When constructing the 'connection-id' attribute for a command that applies to a specific media ('m=') in an SDP description, an optional third component can be concatenated to the Connection reference key. It is again separated using the '~' character and uses the 'label' attribute as specified in  (Levin, O. and G. Camarillo, “The SDP (Session Description Protocol) Label Attribute,” January 2005.). So the format would be:
'Local Dialog tag' + '~' + 'Remote Dialog tag' + '~' + 'Label Attribute'
As an example, for an entity that has a SIP Local dialog identifier of '7HDY839', a Remote dialog identifier of 'HJKSkyHS' and an SDP label attribute of 'HUwkuh7ns', the 'connection-id' attribute for a Control Framework command would be:
It should be noted that Control Framework requests initiated in conjunction with a SIP dialog will produce a different 'connection-id' 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 also 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 'conf' 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 'conf-id' attribute does not need to be constructed based on the overlying SIP dialog. The 'conf-id' attribute value is system specific and should be selected with relevant context and uniqueness.
The full schema follows:
<?xml version="1.0" encoding="UTF-8"?> <xsd:schema targetNamespace="urn:ietf:params:xml:ns:control:framework-attributes" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns="urn:ietf:params:xml:ns::control:framework-attributes" elementFormDefault="qualified" attributeFormDefault="unqualified"> <!-- xs:include schemaLocation="common-schema.xsd"/ --> <xsd:attributeGroup name="framework-attributes"> <xsd:annotation> <xsd:documentation>SIP Connection and Conf Identifiers</xsd:documentation> </xsd:annotation> <xsd:attribute name="connection-id" type="xsd:string"/> <xsd:attribute name="conf-id" type="xsd:string"/> </xsd:attributeGroup> </xs:schema>
|||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).|
|||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.|
|||Rosenberg, J. and H. Schulzrinne, “Reliability of Provisional Responses in Session Initiation Protocol (SIP),” RFC 3262, June 2002.|
|||Rosenberg, J. and H. Schulzrinne, “Session Initiation Protocol (SIP): Locating SIP Servers,” RFC 3263, June 2002.|
|||Rosenberg, J. and H. Schulzrinne, “An Offer/Answer Model with Session Description Protocol (SDP),” RFC 3264, June 2002.|
|||Yon, D. and G. Camarillo, “TCP-Based Media Transport in the Session Description Protocol (SDP),” RFC 4145, September 2005.|
|||Groves, C., Pantaleo, M., Anderson, T., and T. Taylor, “Gateway Control Protocol Version 1,” RFC 3525, June 2003.|
|||Dolly, M., “Media Control Protocol Requirements,” draft-dolly-xcon-mediacntrlframe-02 (work in progress), September 2006.|
|||Handley, M., “SDP: Session Description Protocol,” draft-ietf-mmusic-sdp-new-26 (work in progress), January 2006.|
|||Levin, O. and G. Camarillo, “The SDP (Session Description Protocol) Label Attribute,” draft-ietf-mmusic-sdp-media-label-01 (work in progress), January 2005.|
|||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.|
|||Rosenberg, J., Schulzrinne, H., and P. Kyzivat, “Indicating User Agent Capabilities in the Session Initiation Protocol (SIP),” RFC 3840, August 2004.|
|||Rosenberg, J., Schulzrinne, H., and P. Kyzivat, “Caller Preferences for the Session Initiation Protocol (SIP),” RFC 3841, August 2004.|
|||Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” STD 64, RFC 3550, July 2003 (TXT, PS, PDF).|
|||Yergeau, F., “UTF-8, a transformation format of ISO 10646,” STD 63, RFC 3629, November 2003.|
|||Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” RFC 2234, November 1997 (TXT, HTML, XML).|
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