Network Working Group C. Boulton
Internet-Draft Ubiquity Software Corporation
Expires: December 28, 2006 T. Melanchuk
BlankSpace
S. McGlashan
Hewlett-Packard
A. Shiratzky
Radvision
June 26, 2006
A Control Framework for the Session Initiation Protocol (SIP)
draft-boulton-sip-control-framework-03
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Copyright (C) The Internet Society (2006).
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, for
example 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.
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 . . . . . . . 10
5.1. Controlling UAC Behavior - Media Dialogs . . . . . . . . . 11
6. External Server UAS Behavior - Control Channel Setup . . . . . 12
7. Control Framework Interactions . . . . . . . . . . . . . . . . 13
7.1. Constructing Requests . . . . . . . . . . . . . . . . . . 15
7.1.1. Sending CONTROL . . . . . . . . . . . . . . . . . . . 15
7.1.2. Sending REPORT . . . . . . . . . . . . . . . . . . . . 16
7.2. Constructing Responses . . . . . . . . . . . . . . . . . . 17
8. Response Code Descriptions . . . . . . . . . . . . . . . . . . 18
8.1. 200 Response Code . . . . . . . . . . . . . . . . . . . . 18
8.2. 202 Response Code . . . . . . . . . . . . . . . . . . . . 18
8.3. 400 Response Code . . . . . . . . . . . . . . . . . . . . 18
8.4. 403 Response Code . . . . . . . . . . . . . . . . . . . . 18
8.5. 481 Response Code . . . . . . . . . . . . . . . . . . . . 18
8.6. 500 Response Code . . . . . . . . . . . . . . . . . . . . 18
9. Control Packages . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Control Package Name . . . . . . . . . . . . . . . . . . . 19
9.2. Framework Message Usage . . . . . . . . . . . . . . . . . 19
9.3. Common XML Support . . . . . . . . . . . . . . . . . . . . 19
9.4. CONTROL Message Bodies . . . . . . . . . . . . . . . . . . 19
9.5. REPORT Message Bodies . . . . . . . . . . . . . . . . . . 20
9.6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 20
10. Network Address Translation (NAT) . . . . . . . . . . . . . . 20
11. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 20
11.1. SIP Formal Syntax . . . . . . . . . . . . . . . . . . . . 20
11.2. Control Framework Formal Syntax . . . . . . . . . . . . . 20
12. Common XML Component Definitions . . . . . . . . . . . . . . . 23
12.1. Common Dialog/Conference Reference Schema . . . . . . . . 23
13. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
14. Security Considerations . . . . . . . . . . . . . . . . . . . 29
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15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
15.1. IANA Registration of the 'escs' Option Tag . . . . . . . . 29
15.2. Control Package Registration Information . . . . . . . . . 29
15.2.1. Control Package Registration Template . . . . . . . . 29
15.3. SDP Transport Protocol . . . . . . . . . . . . . . . . . . 29
15.3.1. TCP/ESCS . . . . . . . . . . . . . . . . . . . . . . . 29
15.3.2. TCP/TLS/ESCS . . . . . . . . . . . . . . . . . . . . . 30
15.4. SDP Attribute Names . . . . . . . . . . . . . . . . . . . 30
15.5. SIP Response Codes . . . . . . . . . . . . . . . . . . . . 30
16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30
17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
17.1. Normative References . . . . . . . . . . . . . . . . . . . 30
17.2. Informative References . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32
Intellectual Property and Copyright Statements . . . . . . . . . . 33
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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, 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[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" is described in Section 9.
Current IETF transport device control protocols, such as megaco [7],
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 [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 Transport Layer Security (TLS) and Client Authentication.
o Connection maintenance - The ability to re-negotiate a connection,
ensure it is active, audit parameters, and so forth.
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
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distributed, clustered architecture containing varying services (for
example 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 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 this document
and utilised 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). 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
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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 [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:
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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 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.
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+--------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 with the external
server using standard SIP mechanisms as defined in RFC 3261 [2] and
RFC 3264 [5].
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, because 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 is treated as
specified 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.
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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' 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 [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
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, for example
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>
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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[9]. The third sub-field, <proto>, MUST equal the value
"TCP/ESCS" as defined in Section 15.3.2 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 (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:
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 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.
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Such co-existing dialogs will pass through the Control Client (see
Figure 4) 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 [10], 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 12.1.
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 12.1.
Support for both the common attributes described here is specified as
part of each Control Package definition, as detailed in Section 9.
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
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
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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. 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
[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
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)
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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.
o Once connected, the client initiating the connection (as
determined by COMEDIA) MUST immediately send a Control Framework
SYNCH request. The SYNCH request will be constructed as defined
in Section 11.2 and MUST only contain one message header,
'dialog-id', which contains the SIP dialog information.
o The 'dialog-id' message header is constructed by concatenating the
Local-tag, Call-ID and Remote-tag (as defined in Section 11.2)
from the SIP dialog and separating with a '~'. See syntax defined
in Section 12.1 and examples in Section 9.6. For example, if the
SIP dialog had values of 'Local-tag=HKJDH', 'Remote-tag=JJSUSHJ'
and 'Call-ID=8shKUHSUKHW@example.com' - the 'dialog-id' header
would look like this:
'dialog-id=HKJDH~8shKUHSUKHW@example.com~JJSUSHJ'.
o The client who initiated the connection MUST then send the SYNCH
request. It should then wait for a period of 5 seconds to receive
a response. It MAY choose a longer time to wait but it should not
be shorter than 5 seconds.
o If no response is received for the SYNCH control message, a
timeout occurs and the control channel is terminated along with
the associated SIP dialog (issue a BYE request).
o If the client who initiated a connection receives a 481 response,
this implies that the SYNCH request was received but no associated
SIP dialog exists. This also results in the control channel being
terminated along with the associated SIP dialog (issue a BYE
request).
o All other error responses received for the SYNCH request are
treated as detailed in this specification and also result in the
termination of the control channel and the associated SIP dialog
(issue a BYE request).
o The receipt of a 200 response to a SYNCH message implies that the
SIP dialog and control connection have been successfully
correlated. The control channel can now be used for further
interactions.
Once a successful control channel has been established, as defined in
Section 5 and Section 6 (and the connection has been correlated, as
described in previous paragraph), 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 this document MAY define new
commands and responses.
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7.1. Constructing Requests
An entity acting as a controlling UAC is now able to construct and
send new requests on a control channel and MUST adhere to the syntax
defined in Section 11. A request MUST also adhere to the syntax
defined by the Control Packages negotiated in Section 5 and Section 6
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
request along with the method type (as defined in the ABNF in
Section 11). 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 containing the value
of the Control Package to which this specific request applies
(Multiple packages can be negotiated per control channel).
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 15 seconds is RECOMMENDED.
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 in the same way as 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 the message payload contained in a CONTROL
message is declared in the individual Control Package, as specified
by this framework (defined in Section 9.4).
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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
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 and is primarily 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,
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 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,
for example a timeout period of 10 seconds would be refreshed after 8
seconds.
Subsequent REPORT messages that provide additional information
relating to the original CONTROL command MUST also include and
increment by 1 the 'Seq' header value. They MUST also include a
'Status' header with a value of 'update'. An interim REPORT message
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sent to update the CONTROL command status MAY contain a message body,
as defined by individual Control Packages and specified in
Section 9.5. A REPORT message sent updating the 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 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. Responses MUST NOT include the 'Status'
or 'Timeout' message headers - if they are included they have no
meaning or semantics. A 200 response MAY include message bodies if
the entity responding is able to provide the specified Control
Package information without the request transaction timing out. 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 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
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 and 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 Section 7.1. All response codes in this
section MUST be supported and can be used in response to both CONTROL
and REPORT messages, the exception being that you MUST NOT generate a
202 response to a REPORT message.
8.1. 200 Response Code
The 200 code indicates the completion of a successful transaction.
8.2. 202 Response Code
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.
8.3. 400 Response Code
The 400 response indicates that the request was syntactically
incorrect.
8.4. 403 Response Code
The 400 response indicates that the requested operation is illegal.
8.5. 481 Response Code
The 481 response indicates that the intended target of the request
does not exist.
8.6. 500 Response Code
The 500 response indicates that the recipient does not understand the
request
9. Control Packages
"Control Packages" are intended to specify behavior that extends the
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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.
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 that appears in the IANA registration of the
token. Information on registering control package event tokens is
contained in Section 15
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
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.]
9.3. Common XML Support
This optional section is only included in a Control Package if the
attributes for media dialog or Conference reference are required.
The Control Package will make strong statements (MUST strength) if
the XML schema defined in Section 12.1 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.
9.4. CONTROL Message Bodies
This mandatory section of a Control Package defines the control body
that can be contained within a CONTROL command request, as defined in
Section 7 (or that no control package body is required). This
section should indicate the location of detailed syntax definitions
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and semantics for the appropriate body types.
9.5. REPORT Message Bodies
This mandatory section of a Control Package defines the REPORT body
that can be contained within a REPORT command request, as defined in
Section 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. 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.
9.6. 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
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
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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 = %x53.43.46.57; 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 = %x52.45.50.4F.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
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/ %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.
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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
12. Common XML Component Definitions
This section provides the XML schema definitions for the commonly
used components that can be used in Control Packages.
12.1. Common Dialog/Conference Reference Schema
The following schema provides some common attributes for allowing
Control Packages to apply specific commands to a particular SIP media
dialog (also referred to as Connection) or conference. If used
within a Control Package the Connection and Conference 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 [2]. 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:
7HDY839~HJKSkyHS
If a session description has more than one media description (as
identified by 'm=' in [9]) 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 [10]. So
the format would be:
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'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:
7HDY839~HJKSkyHS~HUwkuh7ns
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 conference. 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>
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13. 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.
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) |
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| <---------------------------------------- |
| |
| (13) 200 |
| ----------------------------------------> |
| |
| (14) SIP BYE |
| ----------------------------------------> |
| |
| (15) SIP 200 |
| <--------------------------------------- |
|=============================================|
| Control Channel Terminated |
|=============================================|
| |
1. Control Client->Control Server (SIP): INVITE
sip:control-server@example.com
INVITE sip:control-server@example.com SIP/2.0
To: <sip:control-server@example.com>
From: <sip:control-client@example.com>;tag=8937498
Via: SIP/2.0/UDP control-client.example.com;branch=z9hG412345678
CSeq: 1 INVITE
Require: escs
Control-Packages: <example-package>
Call-ID: 893jhoeihjr8392@example.com
Contact: <sip:control-client@pc1.example.com>
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
2. Control Server->Control Client (SIP): 200 OK
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SIP/2.0 200 OK
To: <sip:control-server@example.com>;tag=023983774
From: <sip:control-client@example.com>;tag=8937498
Via: SIP/2.0/UDP control-client.example.com;branch=z9hG412345678
CSeq: 1 INVITE
Require: escs
Control-Packages: <example-package>
Call-ID: 893jhoeihjr8392@example.com
Contact: <sip:control-client@pc2.example.com>
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
3. Control Client->Control Server (SIP): ACK
4. Control Client opens a TCP connection to the Control Server.
The connection can now be used to exchange control framework
messages. Control Client-->Control Server (Control Framework
Message): SYNCH.
SCFW 8djae7khauj SYNCH
Dialog-id: 8937498~893jhoeihjr8392@example.com~023983774
5. Control Server-->Control Client (Control Framework Message):
200.
SCFW 8djae7khauj 200
6. Control Client opens a TCP connection to the Control Server.
The connection can now be used to exchange control framework
messages. Control Client-->Control Server (Control Framework
Message): CONTROL.
SCFW i387yeiqyiq CONTROL
Control-Package: <package-name>
Content-Length: 11
<XML BLOB/>
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7. Control Server-->Control Client (Control Framework Message):
202.
SCFW i387yeiqyiq 202
8. Control Server-->Control Client (Control Framework Message):
REPORT.
SCFW i387yeiqyiq REPORT
Seq: 1
Status: pending
Timeout: 10
9. Control Client-->Control Server (Control Framework Message):
200.
SCFW i387yeiqyiq 200
Seq: 1
10. Control Server-->Control Client (Control Framework Message):
REPORT.
SCFW i387yeiqyiq REPORT
Seq: 2
Status: update
Timeout: 10
<XML BLOB/>
11. Control Client-->Control Server (Control Framework Message):
200.
SCFW i387yeiqyiq 200
Seq: 2
12. Control Server-->Control Client (Control Framework Message):
REPORT.
SCFW i387yeiqyiq REPORT
Seq: 3
Status: terminate
Timeout: 10
<XML BLOB/>
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13. Control Client-->Control Server (Control Framework Message):
200.
SCFW i387yeiqyiq 200
Seq: 3
14. Control Client->Control Server (SIP): BYE
BYE sip:control-client@pc2.example.com SIP/2.0
To: <sip:control-server@example.com>
From: <sip:control-client@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: 893jhoeihjr8392@example.com
15. Control Server->Control Client (SIP): 200 OK
SIP/2.0 200 OK
To: <sip:control-server@example.com>;tag=023983774
From: <sip:control-client@example.com>;tag=8937498
Via: SIP/2.0/UDP control-client.example.com;branch=z9hG423456789
CSeq: 2 BYE
Require: escs
Control-Packages: <example-package>
Call-ID: 893jhoeihjr8392@example.com
14. Security Considerations
Security Considerations to be included in later versions of this
document.
15. IANA Considerations
15.1. IANA Registration of the 'escs' Option Tag
15.2. Control Package Registration Information
15.2.1. Control Package Registration Template
15.3. SDP Transport Protocol
15.3.1. TCP/ESCS
15.3.2. TCP/TLS/ESCS
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15.4. SDP Attribute Names
15.5. SIP Response Codes
16. Acknowledgments
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.
17. References
17.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
17.2. 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.
[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 Requirements",
draft-dolly-xcon-mediacntrlframe-01 (work in progress),
February 2006.
[9] Handley, M., "SDP: Session Description Protocol",
draft-ietf-mmusic-sdp-new-26 (work in progress), January 2006.
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[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.
Boulton, et al. Expires December 28, 2006 [Page 31]
Internet-Draft SIP Control Framework June 2006
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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Internet-Draft SIP Control Framework June 2006
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