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Network Working Group                                         C. Boulton
Internet-Draft                             Ubiquity Software Corporation
Expires: June 26, 2006                                      T. Melanchuk
                                                              BlankSpace
                                                            S. McGlashan
                                                         Hewlett-Packard
                                                            A. Shiratzky
                                                               Radvision
                                                       December 23, 2005


     A Control Framework for the Session Initiation Protocol (SIP)
                 draft-boulton-sip-control-framework-00

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on June 26, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   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



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   (SIP) to establish an application-level control mechanism between
   Application Servers and tightly associated external Servers e.g.
   Media Server.

   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.









































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  4
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Locating External Server Resources . . . . . . . . . . . . . .  9
   5.  Controlling UAC Behavior - Control Channel Setup . . . . . . .  9
     5.1.  Controlling UAC Behavior - Media Dialogs . . . . . . . . . 11
   6.  External Server UAS Behavior - Control Channel Setup . . . . . 11
   7.  Control Framework Interactions . . . . . . . . . . . . . . . . 12
     7.1.  Constructing Requests  . . . . . . . . . . . . . . . . . . 13
       7.1.1.  Sending CONTROL  . . . . . . . . . . . . . . . . . . . 13
       7.1.2.  Sending REPORT . . . . . . . . . . . . . . . . . . . . 13
     7.2.  Constructing Responses . . . . . . . . . . . . . . . . . . 15
   8.  Response Code Descriptions . . . . . . . . . . . . . . . . . . 16
     8.1.  200 Response Code  . . . . . . . . . . . . . . . . . . . . 16
     8.2.  202 Response Code  . . . . . . . . . . . . . . . . . . . . 16
     8.3.  400 Response Code  . . . . . . . . . . . . . . . . . . . . 16
     8.4.  500 Response Code  . . . . . . . . . . . . . . . . . . . . 16
   9.  Control Packages . . . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  Control Package Name . . . . . . . . . . . . . . . . . . . 16
     9.2.  Framework Message Usage  . . . . . . . . . . . . . . . . . 17
     9.3.  CONTROL Message Bodies . . . . . . . . . . . . . . . . . . 17
     9.4.  REPORT Message Bodies  . . . . . . . . . . . . . . . . . . 17
     9.5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 17
   10. Network Address Translation (NAT)  . . . . . . . . . . . . . . 17
   11. Formal Syntax  . . . . . . . . . . . . . . . . . . . . . . . . 17
     11.1. SIP Formal Syntax  . . . . . . . . . . . . . . . . . . . . 18
     11.2. Control Framework Formal Syntax  . . . . . . . . . . . . . 18
   12. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
     14.1. IANA Registration of the 'escs' Option Tag . . . . . . . . 21
     14.2. Control Package Registration Information . . . . . . . . . 21
       14.2.1. Control Package Registration Template  . . . . . . . . 21
     14.3. SDP Transport Protocol . . . . . . . . . . . . . . . . . . 21
       14.3.1. TCP/ESCS . . . . . . . . . . . . . . . . . . . . . . . 21
       14.3.2. TCP/TLS/ESCS . . . . . . . . . . . . . . . . . . . . . 21
     14.4. SDP Attribute Names  . . . . . . . . . . . . . . . . . . . 21
     14.5. SIP Response Codes . . . . . . . . . . . . . . . . . . . . 21
   15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 21
     16.2. Informative References . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
   Intellectual Property and Copyright Statements . . . . . . . . . . 24





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1.  Introduction

   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 e.g. "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 and can be found in the 'Media Control Protocol Framework'
   document[8].  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" are described in Section 9.

   Current IETF transport device control protocols, such as megaco [7],
   while excellent for controlling media gateways which bridge separate
   networks are troublesome for supporting media-rich applications in
   SIP networks as 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 [11] to establish media sessions from SIP user agents to a
   media server.  SIP, as defined in RFC 3261 [2], 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.


2.  Conventions and Terminology

   In this document, BCP 14/RFC 2119 [1] 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:




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   B2BUA : A B2BUA is a Back-to-Back SIP User Agent.
   Media Server : A Media Server is an entity that performs media
      processing on behalf of a requesting agent or Media Control
      Client.  In particular, a Media Server offers mixing,
      announcement, tone detection and generation, and object play and
      record services.  The Media Server has a direct RTP [14]
      relationship with the source or sink of the media flow.
   Control Client : A Control Client is an entity that requests
      processing from an external 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.
      In the document, we often refer to this entity simply as "the
      Client".


3.  Overview

   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 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:-
   o  Service location - Use SIP Proxies or Back-to-Back User Agents for
      discovering external Servers.
   o  Security mechanisms - Leverage established security mechanisms
      such as TLS and Client Authentication.
   o  Connection Maintenance - The ability to re-negotiate a connection,
      ensure it is active, audit parameters, etc.
   o  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 at both a routing level where RFC
   3263 [4] provides the physical location of devices and at the Service
   level using Caller Preferences[12] and Callee Capabilities[13].  The
   ability to select an external Server based on Service level
   capabilities is extremely powerful when considering a distributed,



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   clustered architecture containing varying services (e.g.  Voice,
   Video, IM).  More detail on locating external Server resources using
   these techniques is outlined in Section 5 of this document.


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



   Figure 1: Basic Architecture

   The example from Figure 1 conveys a 1:1 connection between the
   Control Client and the external Server.  It is possible, if required,
   for multiple connections using separate SIP dialogs to be established
   between the Media Control Client and the external Server entities.
   Any of the connections created between the two entities can then be
   used for external Server control interactions.  The control
   connections are agnostic to the overlying media sessions and specific
   session information can be incorporated in the control interaction
   commands represented using the defined XML schema (as defined in
   external control packages ).  The ability to have multiple
   connections 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 an external Server (Note: Some lines in the
   examples are removed for clarity and brevity).

   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



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   information for control channel negotiation.  The COMEDIA [6]
   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



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   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 Media 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 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    |                           |   External  |
      |               +-------------+                           |   Server    |
   +--+--+                                                      |             |
   |User |                                                      |             |
   |Agent|<============================RTP(2)==================>|             |
   +-----+                                                      +-------------+


   Figure 4: Participant Architecture

   (2) from Figure 4 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 external Server (B2BUA type
   functionality).  Using the interaction illustrated by (2), the User
   Agent is able to negotiate media capabilities using standard SIP
   mechanisms as defined in RFC 3261 [2] and RFC 3264 [5] with the



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   external Server.

   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 [10]
   should be inserted for every media description (identified as m= line
   as defined in [9]).  This provides flexibility for the Control Client
   as it can generate control messages that specify a particular Media
   stream (between User Agent and external Server) within a SIP media
   dialog.  If a Media label is not included in the Control XML command
   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 are treated as per
   RFC 3261 [2].  One exception to this is the "496" (TODO:need to pick
   an appropriate response code) response code whose operation is
   defined in Section 6


4.  Locating External Server Resources

   Section will describe mechanisms for locating an external Server.


5.  Controlling UAC Behavior - Control Channel Setup

   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' SHOULD NOT be present in any other SIP
   request type, although extensions to SIP MAY allow its usage with
   other request methods.

   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 INVITE MUST include a SIP "Control-Packages" header which MUST
   contain at least one valid entry.

   If a reliable response is received (as defined RFC 3261 [2] and RFC
   3262 [3]) 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 [9].
   The following information defines the composition of some specific



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   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 [9]:

   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 e.g. dns/IP
   representation.  The address will be the network address used for
   connections in this specification.

   Example:

   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 not 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[9].  The third sub-field, <proto>, MUST equal the value
   "TCP/ESCS" as defined in Section 14.3.2 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 [6]
   specification but MAY choose not to support values if it can
   definitely determine they will never be used (e.g. will only ever
   initiate outgoing connections).  It is RECOMMENDED that a Controlling
   UAC initiate a connection to an external Server but an external
   Server MAY negotiate and initiate a connection using COMEDIA, if
   network topology prohibits initiating connections in a certain



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   direction.  An example of the attributes might be:


                           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 RFC 3261 [2]), it
   can be sent to the appropriate location.  The SIP dialog and
   appropriate control connection is then established.

5.1.  Controlling UAC Behavior - Media Dialogs

   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 for the
   manipulation of alternative media dialogs that have been established
   with the remote server e.g. manipulation of media server
   participants.

   A subsequent SIP dialog's SDP (between Control Client and external
   server) that contains more than one Media Description SHOULD include
   a media label attribute, as defined in [10], per "m=" definition.  A
   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.


6.  External Server UAS Behavior - Control Channel Setup

   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 [2] for responding to SIP supported and Require
   headers.  If support is conveyed in a reliable SIP provisional
   response, the mechanisms in RFC 3262 [3] 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 using valid local settings and also with full



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   compliance to the COMEDIA[6] specification.  For example, the SDP
   attributes included in the answer constructed for the example offer
   provided in Section 5 would look as illustrated below:


                           a=setup:passive
                           a=connection:new


   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.

   If the UAS does not support the extension contained in SIP Supported
   or Require header it MUST respond as detailed in RFC 3261 [2].  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 [2] 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 [2].  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.


7.  Control Framework Interactions

   Once a successful control channel has been established, as defined in
   Section 5 and Section 6, the two entities are now in a position to
   exchange relevant control framework commands.  The remainder of this
   section provides details of the core set of commands and responses
   that MUST be supported for the core control framework.  Future
   extensions to the this document MAY define new commands and



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   responses.

7.1.  Constructing Requests

   An entity acting as a controlling UAC is now able to construct and
   send new requests on a control channel.  This is achieved by creating
   a unique transaction and associated identifier per request
   transaction.  The transaction identifier is then included in the
   first line of a control framework request along with the method type
   (as defined in the ABNF in Section 11).  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).

   Any Control Framework message constructed 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 control message, a
   'Content-Length' header with a value of '0' MUST be included.

   When all of the properties have been included in the Control
   Framework message, it is sent down the control channel established in
   Section 5.

   It is a requirement that a Control Framework UAS receiving such a
   request respond immediately with an appropriate response (as
   discussed in Section 7.2).  A Control Client entity needs to wait for
   an arbitrary amount of time for a response before considering the
   transaction a failure.  A wait time of 5 seconds is RECOMMENDED.

   Editors Note: Needs work on transaction timeout - is this good
   enough?

7.1.1.  Sending CONTROL

   A 'CONTROL' message is used by an entity acting as a UAC Control
   Client to invoke control commands on an entity acting as a UAS
   Control Client.  The message is constructed like any standard Control
   Framework message as discussed in Section 7.1 and defined in
   Section 11.  A CONTROL message MAY contain a message body.  The
   explicit detail of message payload contained in a CONTROL message is
   declared in the individual Control Package, as specified by this
   framework (defined in Section 9.3).

7.1.2.  Sending REPORT

   On receiving a CONTROL command, an entity acting as a Control
   Framework UAS MUST respond immediately with a status code for the



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   request, as specified in Section 7.2.  The response code 202
   indicates that although the Control Framework transaction has been
   understood and completed, the requested command is still being
   processed.  The REPORT message is used to update the status of the
   command request.

   A Control Framework UAS entity issuing a 202 response MUST
   immediately issue a REPORT message that contains the same transaction
   ID in the request start line that was present in the original CONTROL
   transaction.  The initial REPORT message MUST also contain a 'Seq'
   (Sequence) message header with a value equal to '1' (It should be
   noted that the 'Seq' numbers at both Controlling UAC client and UAS
   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, it is simply used to establish a subsequent message transaction
   based on the initial CONTROL command.  All REPORT messages for a
   particular 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 entire CONTROL transaction and associated state.  On
   receiving a REPORT message, the Control Framework UAC MUST reset the
   counter to the indicated timeout period.  This is then repeated for
   every REPORT message received for the associated CONTROL transaction
   (as indicated by the unique transaction ID).  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 CONTROL transaction as 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
   Framework UAS 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 e.g, a timeout period of 10
   seconds would be refreshed after 8 seconds.

   Subsequent REPORT messages which provide additional information
   relating to the original CONTROL command MUST also include and
   increment by 1 the 'Seq' header value.  It MUST also include a
   'Status' header with a value of 'update'.  An interim REPORT message
   sent to update the CONTROL command status MAY contain a message body,
   as defined by individual Control Packages and specified in
   Section 9.4.  A REPORT message sent updating the transaction also
   acts as a timeout refresh, as described earlier in this section.
   This will result in transaction timeout period at the initiator of



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   the request being reset to the interval contained in the 'Timeout'
   message header.

   When all processing for a CONTROL command has taken place, the entity
   acting as a Control Framework UAS 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.

7.2.  Constructing Responses

   A Control Framework entity, on receiving a request, will be required
   to immediately generate a response.  A Control Framework response
   MUST be generated and sent immediately and MUST conform to the ABNF
   defined in Section 11.  The first line of the response message MUST
   contain the transaction identifier used in first line of the request,
   as defined in Section 7.1.  The generated response to a Control
   Framework message MUST contain a 'Seq' message header that mirrors
   the value from the original request.  This is then used in
   conjunction with the unique transaction identifier to correlate a
   response to a request.  Responses MUST NOT include the 'Status' or
   'Timeout' message headers - if they are included they have no meaning
   or semantics.  Responses are not permitted to include message bodies
   and so MUST NOT include the 'Content-Length' message header.

   EDITORS NOTE: A large number of scenarios would benefit from allowing
   message bodies in a response.  For the initial version of the
   docuement it is prohibited BUT this will be discussed in detail for
   the next revision.

   A Control Framework entity MUST then include a status code in the
   first line of the constructed response.  A CONTROL request that has
   been understood, and the relevant actions for the control transaction
   have been completed uses the 200 status code as defined in
   Section 8.1.  A client receiving a 200 class response then considers
   the control command completed.  A CONTROL request that is received,
   understood but requires further processing 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.  The specific
   Control Package will explicitly define the circumstances under which
   either 200 or 202 with subsequent processing takes place.

   If the receiving Control Framework entity encounters problems with
   either a REPORT or CONTROL request an appropriate error code should
   be used in the response, as listed in Section 8.  The generation of a



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   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.


8.  Response Code Descriptions

   The following response codes are defined for transactional responses
   to commands defined in .  All response codes in this section MUST be
   supported.

8.1.  200 Response Code

   Editors Note: TODO

8.2.  202 Response Code

   Editors Note: TODO

8.3.  400 Response Code

   Editors Note: TODO

8.4.  500 Response Code

   Editors Note: TODO


9.  Control Packages

   "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.

9.1.  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 which appears in the IANA registration of the



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   token.  Information on registering control package event tokens is
   contained in Section 14

9.2.  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 should
   explicitly detail the control messages that can be used as well as 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.]

9.3.  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 7 (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.

9.4.  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 7 (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.

9.5.  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.


10.  Network Address Translation (NAT)

   [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.]


11.  Formal Syntax





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11.1.  SIP Formal Syntax

   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 = token


11.2.  Control Framework Formal Syntax

   The Control Framework interactions use the UTF-8 transformation
   format as defined in RFC3629 [15].  The syntax in this section uses
   the Augmented Backus-Naur Form (ABNF) as defined in RFC2234 [16].


   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  = method SP transact-id CRLF
   control-resp-start = status-code SP transact-id [SP comment] CRLF
   comment = utf8text

   transact-id = alpha-num-token
   method = mCONTROL / mREPORT / other-method
   mCONTROL = %x43.4F.4E.54.52.4F.4C; CONTROL in caps
   mREPORT = %x50.52.4F.47.52.45.53.53; REPORT in caps

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

   headers = Content-Length
    /Status
    /Seq
    /Timeout
    /ext-header

   Content-Length = "Content-Length:" SP 1*DIGIT
   Status = "Status:" SP ("pending" / "update" / "terminate" )
   Timeout = "Timeout:" SP 1*DIGIT
   Seq = "Seq:" SP 1*DIGIT

   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



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   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



12.  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.








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            Control Client                                Control Server
                   |                                             |
                   |       (1) SIP INVITE                        |
                   |  ---------------------------------------->  |
                   |                                             |
                   |       (2) SIP 200                           |
                   |  <---------------------------------------   |
                   |                                             |
                   |       (3) SIP ACK                           |
                   |  ---------------------------------------->  |
                   |                                             |
                   |=============================================|
                   |         Control Channel Established         |
                   |=============================================|
                   |                                             |
                   |       (4) CONTROL                           |
                   |  ---------------------------------------->  |
                   |                                             |
                   |       (5) 202                               |
                   |  <---------------------------------------   |
                   |                                             |
                   |       (6) REPORT (pending)                  |
                   |  <----------------------------------------  |
                   |                                             |
                   |       (7) 200                               |
                   |  ---------------------------------------->  |
                   |                                             |
                   |       (8) REPORT (update)                   |
                   |  <----------------------------------------  |
                   |                                             |
                   |       (9) 200                               |
                   |  ---------------------------------------->  |
                   |                                             |
                   |       (10) REPORT (terminate)               |
                   |  <----------------------------------------  |
                   |                                             |
                   |       (11) 200                              |
                   |  ---------------------------------------->  |
                   |                                             |
                   |       (12) SIP BYE                          |
                   |  ---------------------------------------->  |
                   |                                             |
                   |       (13) SIP 200                          |
                   |  <---------------------------------------   |
                   |=============================================|
                   |         Control Channel Terminated          |
                   |=============================================|
                   |                                             |



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13.  Security Considerations

   Security Considerations to be included in later versions of this
   document.


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

14.3.1.  TCP/ESCS

14.3.2.  TCP/TLS/ESCS

14.4.  SDP Attribute Names

14.5.  SIP Response Codes


15.  Acknowledgments

   The authors would like to thank Ian Evans and Michael Bardzinski of
   Ubiquity Software and Dave Morgan for useful review and input to this
   work.  Eric Burger contributed to the early phases of this work.


16.  References

16.1.  Normative References

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

16.2.  Informative References

   [2]   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.

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



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

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

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

   [7]   Groves, C., Pantaleo, M., Anderson, T., and T. Taylor, "Gateway
         Control Protocol Version 1", RFC 3525, June 2003.

   [8]   Dolly, M., "Media Control Protocol Framework",
         draft-dolly-xcon-mediacntrlframe-00 (work in progress),
         October 2005.

   [9]   Handley, M., "SDP: Session Description Protocol",
         draft-ietf-mmusic-sdp-new-25 (work in progress), July 2005.

   [10]  Levin, O. and G. Camarillo, "The SDP (Session Description
         Protocol) Label Attribute",
         draft-ietf-mmusic-sdp-media-label-01 (work in progress),
         January 2005.

   [11]  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.

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

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

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

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

   [16]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 2234, November 1997.





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Authors' Addresses

   Chris Boulton
   Ubiquity Software Corporation
   Building 3
   Wern Fawr Lane
   St Mellons
   Cardiff, South Wales  CF3 5EA

   Email: cboulton@ubiquitysoftware.com


   Tim Melanchuk
   BlankSpace

   Email: tim.melanchuk@gmail.com


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

   Email: scott.mcglashan@hp.com


   Asher Shiratzky
   Radvision
   24 Raoul Wallenberg st
   Tel-Aviv, Israel

   Email: ashers@radvision.com



















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