| Rfc | 5629 |
|---|
| Title | A Framework for Application Interaction in the Session Initiation
Protocol (SIP) |
|---|
| Author | J. Rosenberg |
|---|
| Date | October 2009 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Network Working Group J. Rosenberg
Request for Comments: 5629 Cisco Systems
Category: Standards Track October 2009
A Framework for Application Interaction
in the Session Initiation Protocol (SIP)
Abstract
This document describes a framework for the interaction between users
and Session Initiation Protocol (SIP) based applications. By
interacting with applications, users can guide the way in which they
operate. The focus of this framework is stimulus signaling, which
allows a user agent (UA) to interact with an application without
knowledge of the semantics of that application. Stimulus signaling
can occur to a user interface running locally with the client, or to
a remote user interface, through media streams. Stimulus signaling
encompasses a wide range of mechanisms, ranging from clicking on
hyperlinks, to pressing buttons, to traditional Dual-Tone Multi-
Frequency (DTMF) input. In all cases, stimulus signaling is
supported through the use of markup languages, which play a key role
in this framework.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. A Model for Application Interaction . . . . . . . . . . . . . 7
4.1. Functional vs. Stimulus . . . . . . . . . . . . . . . . . 9
4.2. Real-Time vs. Non-Real-Time . . . . . . . . . . . . . . . 10
4.3. Client-Local vs. Client-Remote . . . . . . . . . . . . . . 10
4.4. Presentation-Capable vs. Presentation-Free . . . . . . . . 11
5. Interaction Scenarios on Telephones . . . . . . . . . . . . . 11
5.1. Client Remote . . . . . . . . . . . . . . . . . . . . . . 12
5.2. Client Local . . . . . . . . . . . . . . . . . . . . . . . 12
5.3. Flip-Flop . . . . . . . . . . . . . . . . . . . . . . . . 13
6. Framework Overview . . . . . . . . . . . . . . . . . . . . . . 13
7. Deployment Topologies . . . . . . . . . . . . . . . . . . . . 16
7.1. Third-Party Application . . . . . . . . . . . . . . . . . 16
7.2. Co-Resident Application . . . . . . . . . . . . . . . . . 17
7.3. Third-Party Application and User Device Proxy . . . . . . 18
7.4. Proxy Application . . . . . . . . . . . . . . . . . . . . 19
8. Application Behavior . . . . . . . . . . . . . . . . . . . . . 19
8.1. Client-Local Interfaces . . . . . . . . . . . . . . . . . 20
8.1.1. Discovering Capabilities . . . . . . . . . . . . . . . 20
8.1.2. Pushing an Initial Interface Component . . . . . . . . 20
8.1.3. Updating an Interface Component . . . . . . . . . . . 22
8.1.4. Terminating an Interface Component . . . . . . . . . . 22
8.2. Client-Remote Interfaces . . . . . . . . . . . . . . . . . 23
8.2.1. Originating and Terminating Applications . . . . . . . 23
8.2.2. Intermediary Applications . . . . . . . . . . . . . . 24
9. User Agent Behavior . . . . . . . . . . . . . . . . . . . . . 24
9.1. Advertising Capabilities . . . . . . . . . . . . . . . . . 24
9.2. Receiving User Interface Components . . . . . . . . . . . 25
9.3. Mapping User Input to User Interface Components . . . . . 26
9.4. Receiving Updates to User Interface Components . . . . . . 27
9.5. Terminating a User Interface Component . . . . . . . . . . 27
10. Inter-Application Feature Interaction . . . . . . . . . . . . 27
10.1. Client-Local UI . . . . . . . . . . . . . . . . . . . . . 28
10.2. Client-Remote UI . . . . . . . . . . . . . . . . . . . . . 29
11. Intra Application Feature Interaction . . . . . . . . . . . . 29
12. Example Call Flow . . . . . . . . . . . . . . . . . . . . . . 30
13. Security Considerations . . . . . . . . . . . . . . . . . . . 36
14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 36
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 36
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
16.1. Normative References . . . . . . . . . . . . . . . . . . . 36
16.2. Informative References . . . . . . . . . . . . . . . . . . 37
1. Introduction
The Session Initiation Protocol (SIP) [2] provides the ability for
users to initiate, manage, and terminate communications sessions.
Frequently, these sessions will involve a SIP application. A SIP
application is defined as a program running on a SIP-based element
(such as a proxy or user agent) that provides some value-added
function to a user or system administrator. Examples of SIP
applications include prepaid calling card calls, conferencing, and
presence-based [12] call routing.
In order for most applications to properly function, they need input
from the user to guide their operation. As an example, a prepaid
calling card application requires the user to input their calling
card number, their PIN code, and the destination number they wish to
reach. The process by which a user provides input to an application
is called "application interaction".
Application interaction can be either functional or stimulus.
Functional interaction requires the user device to understand the
semantics of the application, whereas stimulus interaction does not.
Stimulus signaling allows for applications to be built without
requiring modifications to the user device. Stimulus interaction is
the subject of this framework. The framework provides a model for
how users interact with applications through user interfaces, and how
user interfaces and applications can be distributed throughout a
network. This model is then used to describe how applications can
instantiate and manage user interfaces.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [1]
3. Definitions
SIP Application: A SIP application is defined as a program running
on a SIP-based element (such as a proxy or user agent) that
provides some value-added function to a user or system
administrator. Examples of SIP applications include prepaid
calling card calls, conferencing, and presence-based [12] call
routing.
Application Interaction: The process by which a user provides input
to an application.
Real-Time Application Interaction: Application interaction that
takes place while an application instance is executing. For
example, when a user enters their PIN number into a prepaid
calling card application, this is real-time application
interaction.
Non-Real-Time Application Interaction: Application interaction that
takes place asynchronously with the execution of the application.
Generally, non-real-time application interaction is accomplished
through provisioning.
Functional Application Interaction: Application interaction is
functional when the user device has an understanding of the
semantics of the interaction with the application.
Stimulus Application Interaction: Application interaction is
stimulus when the user device has no understanding of the
semantics of the interaction with the application.
User Interface (UI): The user interface provides the user with
context to make decisions about what they want. The user
interacts with the device, which conveys the user input to the
user interface. The user interface interprets the information and
passes it to the application.
User Interface Component: A piece of user interface that operates
independently of other pieces of the user interface. For example,
a user might have two separate web interfaces to a prepaid calling
card application: one for hanging up and making another call, and
another for entering the username and PIN.
User Device: The software or hardware system that the user directly
interacts with to communicate with the application. An example of
a user device is a telephone. Another example is a PC with a web
browser.
User Device Proxy: A software or hardware system that a user
indirectly interacts through to communicate with the application.
This indirection can be through a network. An example is a
gateway from IP to the Public Switched Telephone Network (PSTN).
It acts as a user device proxy, acting on behalf of the user on
the circuit network.
User Input: The "raw" information passed from a user to a user
interface. Examples of user input include a spoken word or a
click on a hyperlink.
Client-Local User Interface: A user interface that is co-resident
with the user device.
Client-Remote User Interface: A user interface that executes
remotely from the user device. In this case, a standardized
interface is needed between the user device and the user
interface. Typically, this is done through media sessions: audio,
video, or application sharing.
Markup Language: A markup language describes a logical flow of
presentation of information to the user, collection of information
from the user, and transmission of that information to an
application.
Media Interaction: A means of separating a user and a user interface
by connecting them with media streams.
Interactive Voice Response (IVR): An IVR is a type of user interface
that allows users to speak commands to the application, and hear
responses to those commands prompting for more information.
Prompt-and-Collect: The basic primitive of an IVR user interface.
The user is presented with a voice option, and the user speaks
their choice.
Barge-In: The act of entering information into an IVR user interface
prior to the completion of a prompt requesting that information.
Focus: A user interface component has focus when user input is
provided to it, as opposed to any other user interface components.
This is not to be confused with the term "focus" within the SIP
conferencing framework, which refers to the center user agent in a
conference [14].
Focus Determination: The process by which the user device determines
which user interface component will receive the user input.
Focusless Device: A user device that has no ability to perform focus
determination. An example of a focusless device is a telephone
with a keypad.
Presentation-Capable UI: A user interface that can prompt the user
with input, collect results, and then prompt the user with new
information based on those results.
Presentation-Free UI: A user interface that cannot prompt the user
with information.
Feature Interaction: A class of problems that result when multiple
applications or application components are trying to provide
services to a user at the same time.
Inter-Application Feature Interaction: Feature interactions that
occur between applications.
DTMF: Dual-Tone Multi-Frequency. DTMF refers to a class of tones
generated by circuit-switched telephony devices when the user
presses a key on the keypad. As a result, DTMF and keypad input
are often used synonymously, when in fact one of them (DTMF) is
merely a means of conveying the other (the keypad input) to a
client-remote user interface (the switch, for example).
Application Instance: A single execution path of a SIP application.
Originating Application: A SIP application that acts as a User Agent
Client (UAC), making a call on behalf of the user.
Terminating Application: A SIP application that acts as a User Agent
Server (UAS), answering a call generated by a user. IVR
applications are terminating applications.
Intermediary Application: A SIP application that is neither the
caller or callee, but rather a third party involved in a call.
4. A Model for Application Interaction
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| | | | | | | |
| | | U | | U | | A |
| | Input | s | Input | s | Results | p |
| | ---------> | e | ---------> | e | ----------> | p |
| U | | r | | r | | l |
| s | | | | | | i |
| e | | D | | I | | c |
| r | Output | e | Output | f | Update | a |
| | <--------- | v | <--------- | a | <.......... | t |
| | | i | | c | | i |
| | | c | | e | | o |
| | | e | | | | n |
| | | | | | | |
+---+ +---+ +---+ +---+
Figure 1: Model for Real-Time Interactions
Figure 1 presents a general model for how users interact with
applications. Generally, users interact with a user interface
through a user device. A user device can be a telephone, or it can
be a PC with a web browser. Its role is to pass the user input from
the user to the user interface. The user interface provides the user
with context in order to make decisions about what they want. The
user interacts with the device, causing information to be passed from
the device to the user interface. The user interface interprets the
information, and passes it as a user interface event to the
application. The application may be able to modify the user
interface based on this event. Whether or not this is possible
depends on the type of user interface.
User interfaces are fundamentally about rendering and interpretation.
Rendering refers to the way in which the user is provided context.
This can be through hyperlinks, images, sounds, videos, text, and so
on. Interpretation refers to the way in which the user interface
takes the "raw" data provided by the user, and returns the result to
the application as a meaningful event, abstracted from the
particulars of the user interface. As an example, consider a prepaid
calling card application. The user interface worries about details
such as what prompt the user is provided, whether the voice is male
or female, and so on. It is concerned with recognizing the speech
that the user provides, in order to obtain the desired information.
In this case, the desired information is the calling card number, the
PIN code, and the destination number. The application needs that
data, and it doesn't matter to the application whether it was
collected using a male prompt or a female one.
User interfaces generally have real-time requirements towards the
user. That is, when a user interacts with the user interface, the
user interface needs to react quickly, and that change needs to be
propagated to the user right away. However, the interface between
the user interface and the application need not be that fast. Faster
is better, but the user interface itself can frequently compensate
for long latencies between the user interface and the application.
In the case of a prepaid calling card application, when the user is
prompted to enter their PIN, the prompt should generally stop
immediately once the first digit of the PIN is entered. This is
referred to as "barge-in". After the user interface collects the
rest of the PIN, it can tell the user to "please wait while
processing". The PIN can then be gradually transmitted to the
application. In this example, the user interface has compensated for
a slow UI to application interface by asking the user to wait.
The separation between user interface and application is absolutely
fundamental to the entire framework provided in this document. Its
importance cannot be overstated.
With this basic model, we can begin to taxonomize the types of
systems that can be built.
4.1. Functional vs. Stimulus
The first way to taxonomize the system is to consider the interface
between the UI and the application. There are two fundamentally
different models for this interface. In a functional interface, the
user interface has detailed knowledge about the application and is,
in fact, specific to the application. The interface between the two
components is through a functional protocol, capable of representing
the semantics that can be exposed through the user interface.
Because the user interface has knowledge of the application, it can
be optimally designed for that application. As a result, functional
user interfaces are almost always the most user friendly, the
fastest, and the most responsive. However, in order to allow
interoperability between user devices and applications, the details
of the functional protocols need to be specified in standards. This
slows down innovation and limits the scope of applications that can
be built.
An alternative is a stimulus interface. In a stimulus interface, the
user interface is generic -- that is, totally ignorant of the details
of the application. Indeed, the application may pass instructions to
the user interface describing how it should operate. The user
interface translates user input into "stimulus", which are data
understood only by the application, and not by the user interface.
Because they are generic, and because they require communications
with the application in order to change the way in which they render
information to the user, stimulus user interfaces are usually slower,
less user friendly, and less responsive than a functional
counterpart. However, they allow for substantial innovation in
applications, since no standardization activity is needed to build a
new application, as long as it can interact with the user within the
confines of the user interface mechanism. The web is an example of a
stimulus user interface to applications.
In SIP systems, functional interfaces are provided by extending the
SIP protocol to provide the needed functionality. For example, the
SIP caller preferences specification [15] provides a functional
interface that allows a user to request applications to route the
call to specific types of user agents. Functional interfaces are
important, but are not the subject of this framework. The primary
goal of this framework is to address the role of stimulus interfaces
to SIP applications.
4.2. Real-Time vs. Non-Real-Time
Application interaction systems can also be real-time or non-real-
time. Non-real-time interaction allows the user to enter information
about application operation asynchronously with its invocation.
Frequently, this is done through provisioning systems. As an
example, a user can set up the forwarding number for a call-forward
on no-answer application using a web page. Real-time interaction
requires the user to interact with the application at the time of its
invocation.
4.3. Client-Local vs. Client-Remote
Another axis in the taxonomization is whether the user interface is
co-resident with the user device (which we refer to as a client-local
user interface), or the user interface runs in a host separated from
the client (which we refer to as a client-remote user interface). In
a client-remote user interface, there exists some kind of protocol
between the client device and the UI that allows the client to
interact with the user interface over a network.
The most important way to separate the UI and the client device is
through media interaction. In media interaction, the interface
between the user and the user interface is through media: audio,
video, messaging, and so on. This is the classic mode of operation
for VoiceXML [5], where the user interface (also referred to as the
voice browser) runs on a platform in the network. Users communicate
with the voice browser through the telephone network (or using a SIP
session). The voice browser interacts with the application using
HTTP to convey the information collected from the user.
In the case of a client-local user interface, the user interface runs
co-located with the user device. The interface between them is
through the software that interprets the user's input and passes it
to the user interface. The classic example of this is the Web. In
the Web, the user interface is a web browser, and the interface is
defined by the HTML document that it's rendering. The user interacts
directly with the user interface running in the browser. The results
of that user interface are sent to the application (running on the
web server) using HTTP.
It is important to note that whether or not the user interface is
local or remote (in the case of media interaction) is not a property
of the modality of the interface, but rather a property of the
system. As an example, it is possible for a Web-based user interface
to be provided with a client-remote user interface. In such a
scenario, video- and application-sharing media sessions can be used
between the user and the user interface. The user interface, still
guided by HTML, now runs "in the network", remote from the client.
Similarly, a VoiceXML document can be interpreted locally by a client
device, with no media streams at all. Indeed, the VoiceXML document
can be rendered using text, rather than media, with no impact on the
interface between the user interface and the application.
It is also important to note that systems can be hybrid. In a hybrid
user interface, some aspects of it (usually those associated with a
particular modality) run locally, and others run remotely.
4.4. Presentation-Capable vs. Presentation-Free
A user interface can be capable of presenting information to the user
(a presentation-capable UI), or it can be capable only of collecting
user input (a presentation-free UI). These are very different types
of user interfaces. A presentation-capable UI can provide the user
with feedback after every input, providing the context for collecting
the next input. As a result, presentation-capable user interfaces
require an update to the information provided to the user after each
input. The Web is a classic example of this. After every input
(i.e., a click), the browser provides the input to the application
and fetches the next page to render. In a presentation-free user
interface, this is not the case. Since the user is not provided with
feedback, these user interfaces tend to merely collect information as
it's entered, and pass it to the application.
Another difference is that a presentation-free user interface cannot
easily support the concept of a focus. Selection of a focus usually
requires a means for informing the user of the available
applications, allowing the user to choose, and then informing them
about which one they have chosen. Without the first and third steps
(which a presentation-free UI cannot provide), focus selection is
very difficult. Without a selected focus, the input provided to
applications through presentation-free user interfaces is more of a
broadcast or notification operation.
5. Interaction Scenarios on Telephones
In this section, we apply the model of Section 4 to telephones.
In a traditional telephone, the user interface consists of a 12-key
keypad, a speaker, and a microphone. Indeed, from here forward, the
term "telephone" is used to represent any device that meets, at a
minimum, the characteristics described in the previous sentence.
Circuit-switched telephony applications are almost universally
client-remote user interfaces. In the Public Switched Telephone
Network (PSTN), there is usually a circuit interface between the user
and the user interface. The user input from the keypad is conveyed
using Dual-Tone Multi-Frequency (DTMF), and the microphone input as
Pulse Code Modulated (PCM) encoded voice.
In an IP-based system, there is more variability in how the system
can be instantiated. Both client-remote and client-local user
interfaces to a telephone can be provided.
In this framework, a PSTN gateway can be considered a User Device
Proxy. It is a proxy for the user because it can provide, to a user
interface on an IP network, input taken from a user on a circuit-
switched telephone. The gateway may be able to run a client-local
user interface, just as an IP telephone might.
5.1. Client Remote
The most obvious instantiation is the "classic" circuit-switched
telephony model. In that model, the user interface runs remotely
from the client. The interface between the user and the user
interface is through media, which is set up by SIP and carried over
the Real Time Transport Protocol (RTP) [18]. The microphone input
can be carried using any suitable voice-encoding algorithm. The
keypad input can be conveyed in one of two ways. The first is to
convert the keypad input to DTMF, and then convey that DTMF using a
suitable encoding algorithm (such as PCMU). An alternative, and
generally the preferred approach, is to transmit the keypad input
using RFC 4733 [19], which provides an encoding mechanism for
carrying keypad input within RTP.
In this classic model, the user interface would run on a server in
the IP network. It would perform speech recognition and DTMF
recognition to derive the user intent, feed them through the user
interface, and provide the result to an application.
5.2. Client Local
An alternative model is for the entire user interface to reside on
the telephone. The user interface can be a VoiceXML browser, running
speech recognition on the microphone input, and feeding the keypad
input directly into the script. As discussed above, the VoiceXML
script could be rendered using text instead of voice, if the
telephone has a textual display.
For simpler phones without a display, the user interface can be
described by a Keypad Markup Language request document [8]. As the
user enters digits in the keypad, they are passed to the user
interface, which generates user interface events that can be
transported to the application.
5.3. Flip-Flop
A middle-ground approach is to flip back and forth between a client-
local and client-remote user interface. Many voice applications are
of the type that listen to the media stream and wait for some
specific trigger that kicks off a more complex user interaction. The
long pound in a prepaid calling card application is one example.
Another example is a conference recording application, where the user
can press a key at some point in the call to begin recording. When
the key is pressed, the user hears a whisper to inform them that
recording has started.
The ideal way to support such an application is to install a client-
local user interface component that waits for the trigger to kick off
the real interaction. Once the trigger is received, the application
connects the user to a client-remote user interface that can play
announcements, collect more information, and so on.
The benefit of flip-flopping between a client-local and client-remote
user interface is cost. The client-local user interface will
eliminate the need to send media streams into the network just to
wait for the user to press the pound key on the keypad.
The Keypad Markup Language (KPML) was designed to support exactly
this kind of need [8]. It models the keypad on a phone and allows an
application to be informed when any sequence of keys has been
pressed. However, KPML has no presentation component. Since user
interfaces generally require a response to user input, the
presentation will need to be done using a client-remote user
interface that gets instantiated as a result of the trigger.
It is tempting to use a hybrid model, where a prompt-and-collect
application is implemented by using a client-remote user interface
that plays the prompts, and a client-local user interface, described
by KPML, that collects digits. However, this only complicates the
application. Firstly, the keypad input will be sent to both the
media stream and the KPML user interface. This requires the
application to sort out which user inputs are duplicates, a process
that is very complicated. Secondly, the primary benefit of KPML is
to avoid having a media stream towards a user interface. However,
there is already a media stream for the prompting, so there is no
real savings.
6. Framework Overview
In this framework, we use the term "SIP application" to refer to a
broad set of functionality. A SIP application is a program running
on a SIP-based element (such as a proxy or user agent) that provides
some value-added function to a user or system administrator. SIP
applications can execute on behalf of a caller, a called party, or a
multitude of users at once.
Each application has a number of instances that are executing at any
given time. An instance represents a single execution path for an
application. It is established as a result of some event. That
event can be a SIP event, such as the reception of a SIP INVITE
request, or it can be a non-SIP event, such as a web form post or
even a timer. Application instances also have an end time. Some
instances have a lifetime that is coupled with a SIP transaction or
dialog. For example, a proxy application might begin when an INVITE
arrives, and terminate when the call is answered. Other applications
have a lifetime that spans multiple dialogs or transactions. For
example, a conferencing application instance may exist so long as
there are dialogs connected to it. When the last dialog terminates,
the application instance terminates. Other applications have a
lifetime that is completely decoupled from SIP events.
It is fundamental to the framework described here that multiple
application instances may interact with a user during a single SIP
transaction or dialog. Each instance may be for the same
application, or different applications. Each of the applications may
be completely independent, in that each may be owned by a different
provider, and may not be aware of each other's existence. Similarly,
there may be application instances interacting with the caller, and
instances interacting with the callee, both within the same
transaction or dialog.
The first step in the interaction with the user is to instantiate one
or more user interface components for the application instance. A
user interface component is a single piece of the user interface that
is defined by a logical flow that is not synchronously coupled with
any other component. In other words, each component runs
independently.
A user interface component can be instantiated in one of the user
agents in a dialog (for a client-local user interface), or within a
network element (for a client-remote user interface). If a client-
local user interface is to be used, the application needs to
determine whether or not the user agent is capable of supporting a
client-local user interface, and in what format. In this framework,
all client-local user interface components are described by a markup
language. A markup language describes a logical flow of presentation
of information to the user, a collection of information from the
user, and a transmission of that information to an application.
Examples of markup languages include HTML, Wireless Markup Language
(WML), VoiceXML, and the Keypad Markup Language (KPML) [8].
Unlike an application instance, which has a very flexible lifetime, a
user interface component has a very fixed lifetime. A user interface
component is always associated with a dialog. The user interface
component can be created at any point after the dialog (or early
dialog) is created. However, the user interface component terminates
when the dialog terminates. The user interface component can be
terminated earlier by the user agent, and possibly by the
application, but its lifetime never exceeds that of its associated
dialog.
There are two ways to create a client-local interface component. For
interface components that are presentation capable, the application
sends a REFER [7] request to the user agent. The Refer-To header
field contains an HTTP URI that points to the markup for the user
interface, and the REFER contains a Target-Dialog header field [10]
which identifies the dialog associated with the user interface
component. For user interface components that are presentation free
(such as those defined by KPML), the application sends a SUBSCRIBE
request to the user agent. The body of the SUBSCRIBE request
contains a filter, which, in this case, is the markup that defines
when information is to be sent to the application in a NOTIFY. The
SUBSCRIBE does not contain the Target-Dialog header field, since
equivalent information is conveyed in the Event header field.
If a user interface component is to be instantiated in the network,
there is no need to determine the capabilities of the device on which
the user interface is instantiated. Presumably, it is on a device on
which the application knows a UI can be created. However, the
application does need to connect the user device to the user
interface. This will require manipulation of media streams in order
to establish that connection.
The interface between the user interface component and the
application depends on the type of user interface. For presentation-
capable user interfaces, such as those described by HTML and
VoiceXML, HTTP form POST operations are used. For presentation-free
user interfaces, a SIP NOTIFY is used. The differing needs and
capabilities of these two user interfaces, as described in
Section 4.4, are what drives the different choices for the
interactions. Since presentation-capable user interfaces require an
update to the presentation every time user data is entered, they are
a good match for HTTP. Since presentation-free user interfaces
merely transmit user input to the application, a NOTIFY is more
appropriate.
Indeed, for presentation-free user interfaces, there are two
different modalities of operation. The first is called "one shot".
In the one-shot role, the markup waits for a user to enter some
information and, when they do, reports this event to the application.
The application then does something, and the markup is no longer
used. In the other modality, called "monitor", the markup stays
permanently resident, and reports information back to an application
until termination of the associated dialog.
7. Deployment Topologies
This section presents some of the network topologies in which this
framework can be instantiated.
7.1. Third-Party Application
+-------------+
/---| Application |
/ +-------------+
/
SUB/ / REFER/
NOT / HTTP
/
+--------+ SIP (INVITE) +-----+
| UI A--------------------X |
|........| | SIP |
| User | RTP | UA |
| Device B--------------------Y |
+--------+ +-----+
Figure 2: Third-Party Topology
In this topology, the application that is interested in interacting
with the users exists outside of the SIP dialog between the user
agents. In that case, the application learns about the initiation
and termination of the dialog, along with the dialog identifiers,
through some out-of-band means. One such possibility is the dialog
event package [16]. Dialog information is only revealed to trusted
parties, so the application would need to be trusted by one of the
users in order to obtain this information.
At any point during the dialog, the application can instantiate user
interface components on the user device of the caller or callee. It
can do this using either SUBSCRIBE or REFER, depending on the type of
user interface (presentation capable or presentation free).
7.2. Co-Resident Application
+--------+ SIP (INVITE) +-----+
| User A--------------------X SIP |
| Device | RTP | UA |
|........B--------------------Y |
| | SUB/NOT | App)|
| UI A'-------------------X' |
+--------+ REFER/HTTP +-----+
Figure 3: Co-Resident Topology
In this deployment topology, the application is co-resident with one
of the user agents (the one on the right in the picture above). This
application can install client-local user interface components on the
other user agent, which is acting as the user device. These
components can be installed using either SUBSCRIBE, for presentation-
free user interfaces, or REFER, for presentation-capable ones. This
situation typically arises when the application wishes to install UI
components on a presentation-capable user interface. If the only
user input is via keypad input, the framework is not needed per se,
because the UA/application will receive the input via RFC 4733 in the
RTP stream.
If the application resides in the called party, it is called a
"terminating application". If it resides in the calling party, it is
called an "originating application".
This kind of topology is common in protocol converter and gateway
applications.
7.3. Third-Party Application and User Device Proxy
+-------------+
/---| Application |
/ +-------------+
/
SUB/ / REFER/
NOT / HTTP
/
+-----+ SIP +---M----+ SIP +-----+
| V--------------------C A--------------------X |
| SIP | | UI | | SIP |
| UAa | RTP | | RTP | UAb |
| W--------------------D B--------------------Y |
+-----+ +--------+ +-----+
User User
Device Device
Proxy
Figure 4: User Device Proxy Topology
In this deployment topology, there is a third-party application as in
Section 7.1. However, instead of installing a user interface
component on the end user device, the component is installed in an
intermediate device, known as a User Device Proxy. From the
perspective of the actual user device (on the left), the User Device
Proxy is a client remote user interface. As such, media, typically
transported using RTP (including RFC 4733 for carrying user input),
is sent from the user device to the client remote user interface on
the User Device Proxy. As far as the application is concerned, it is
installing what it thinks is a client-local user interface on the
user device, but it happens to be on a user device proxy that looks
like the user device to the application.
The user device proxy will need to terminate and re-originate both
signaling (SIP) and media traffic towards the actual peer in the
conversation. The User Device Proxy is a media relay in the
terminology of RFC 3550 [18]. The User Device Proxy will need to
monitor the media streams associated with each dialog, in order to
convert user input received in the media stream to events reported to
the user interface. This can pose a challenge in multi-media
systems, where it may be unclear on which media stream the user input
is being sent. As discussed in RFC 3264 [20], if a user agent has a
single media source and is supporting multiple streams, it is
supposed to send that source to all streams. In cases where there
are multiple sources, the mapping is a matter of local policy. In
the absence of a way to explicitly identify or request which sources
map to which streams, the user device proxy will need to do the best
job it can. This specification RECOMMENDS that the User Device Proxy
monitor the first stream (defined in terms of ordering of media
sessions within a session description). As such, user agents SHOULD
send their user input on the first stream, absent a policy to direct
it otherwise.
7.4. Proxy Application
+----------+
SUB/NOT | App | SUB/NOT
+--------------->| |<-----------------+
| REFER/HTTP |..........| REFER/HTTP |
| | SIP | |
| | Proxy | |
| +----------+ |
V ^ | V
+----------+ | | +----------+
| UI | INVITE | | INVITE | UI |
| |------------+ +------------>| |
|......... | |..........|
| SIP |...................................| SIP |
| UA | | UA |
+----------+ RTP +----------+
User Device User Device
Figure 5: Proxy Application Topology
In this topology, the application is co-resident with a transaction
stateful, record-routing proxy server on the call path between two
user devices. The application uses SUBSCRIBE or REFER to install
user interface components on one or both user devices.
This topology is common in routing applications, such as a web-
assisted call-routing application.
8. Application Behavior
The behavior of an application within this framework depends on
whether it seeks to use a client-local or client-remote user
interface.
8.1. Client-Local Interfaces
One key component of this framework is support for client-local user
interfaces.
8.1.1. Discovering Capabilities
A client-local user interface can only be instantiated on a user
agent if the user agent supports that type of user interface
component. Support for client-local user interface components is
declared by both the UAC and UAS in their Allow, Accept, Supported,
and Allow-Event header fields of dialog-initiating requests and
responses. If the Allow header field indicates support for the SIP
SUBSCRIBE method, and the Allow-Event header field indicates support
for the KPML package [8], and the Supported header field indicates
support for the Globally Routable UA URI (GRUU) [9] specification
(which, in turn, means that the Contact header field contains a
GRUU), it means that the UA can instantiate presentation-free user
interface components. In this case, the application can push
presentation-free user interface components according to the rules of
Section 8.1.2. The specific markup languages that can be supported
are indicated in the Accept header field.
If the Allow header field indicates support for the SIP REFER method,
and the Supported header field indicates support for the Target-
Dialog header field [10], and the Contact header field contains UA
capabilities [6] that indicate support for the HTTP URI scheme, it
means that the UA supports presentation-capable user interface
components. In this case, the application can push presentation-
capable user interface components to the client according to the
rules of Section 8.1.2. The specific markups that are supported are
indicated in the Accept header field.
A third-party application that is not present on the call path will
not be privy to these header fields in the dialog-initiating requests
that pass by. As such, it will need to obtain this capability
information in other ways. One way is through the registration event
package [21], which can contain user agent capability information
provided in REGISTER requests [6].
8.1.2. Pushing an Initial Interface Component
Generally, we anticipate that interface components will need to be
created at various different points in a SIP session. Clearly, they
will need to be pushed during session setup, or after the session is
established. A user interface component is always associated with a
specific dialog, however.
An application MUST NOT attempt to push a user interface component to
a user agent until it has determined that the user agent has the
necessary capabilities and a dialog has been created. In the case of
a UAC, this means that an application MUST NOT push a user interface
component for an INVITE-initiated dialog until the application has
seen a request confirming the receipt of a dialog-creating response.
This could be an ACK for a 200 OK, or a PRACK for a provisional
response [3]. For SUBSCRIBE-initiated dialogs, the application MUST
NOT push a user interface component until the application has seen a
200 OK to the NOTIFY request. For a user interface component on a
UAS, the application MUST NOT push a user interface component for an
INVITE-initiated dialog until it has seen a dialog-creating response
from the UAS. For a SUBSCRIBE-initiated dialog, it MUST NOT push a
user interface component until it has seen a NOTIFY request from the
notifier.
To create a presentation-capable UI component on the UA, the
application sends a REFER request to the UA. This REFER MUST be sent
to the GRUU [9] advertised by that UA in the Contact header field of
the dialog-initiating request or response sent by that UA. Note that
this REFER request creates a separate dialog between the application
and the UA. The Refer-To header field of the REFER request MUST
contain an HTTP URI that references the markup document to be
fetched.
Furthermore, it is essential for the REFER request to be correlated
with the dialog to which the user interface component will be
associated. This is necessary for authorization and for terminating
the user interface components when the dialog terminates. To provide
this context, the REFER request MUST contain a Target-Dialog header
field identifying the dialog with which the user interface component
is associated. As discussed in [10], this request will also contain
a Require header field with the tdialog option tag.
To create a presentation-free user interface component, the
application sends a SUBSCRIBE request to the UA. The SUBSCRIBE MUST
be sent to the GRUU advertised by the UA. This SUBSCRIBE request
creates a separate dialog. The SUBSCRIBE request MUST use the KPML
[8] event package. The body of the SUBSCRIBE request contains the
markup document that defines the conditions under which the
application wishes to be notified of user input.
In both cases, the REFER or SUBSCRIBE request SHOULD include a
display name in the From header field that identifies the name of the
application. For example, a prepaid calling card might include a
From header field that looks like:
From: "Prepaid Calling Card" <sip:prepaid@example.com>
Any of the SIP identity assertion mechanisms that have been defined,
such as [11] and [13], are applicable to these requests as well.
8.1.3. Updating an Interface Component
Once a user interface component has been created on a client, it can
be updated. The means for updating it depends on the type of UI
component.
Presentation-capable UI components are updated using techniques
already in place for those markups. In particular, user input will
cause an HTTP POST operation to push the user input to the
application. The result of the POST operation is a new markup that
the UI is supposed to use. This allows the UI to be updated in
response to user action. Some markups, such as HTML, provide the
ability to force a refresh after a certain period of time, so that
the UI can be updated without user input. Those mechanisms can be
used here as well. However, there is no support for an asynchronous
push of an updated UI component from the application to the user
agent. A new REFER request to the same GRUU would create a new UI
component rather than update any components already in place.
For presentation-free UI, the story is different. The application
MAY update the filter at any time by generating a SUBSCRIBE refresh
with the new filter. The UA will immediately begin using this new
filter.
8.1.4. Terminating an Interface Component
User interface components have a well-defined lifetime. They are
created when the component is first pushed to the client. User
interface components are always associated with the SIP dialog on
which they were pushed. As such, their lifetime is bound by the
lifetime of the dialog. When the dialog ends, so does the interface
component.
However, there are some cases where the application would like to
terminate the user interface component before its natural termination
point. For presentation-capable user interfaces, this is not
possible. For presentation-free user interfaces, the application MAY
terminate the component by sending a SUBSCRIBE with Expires equal to
zero. This terminates the subscription, which removes the UI
component.
A client can remove a UI component at any time. For presentation-
capable UI, this is analogous to the user dismissing the web form
window. There is no mechanism provided for reporting this kind of
event to the application. The application MUST be prepared to time
out and never receive input from a user. The duration of this
timeout is application dependent. For presentation-free user
interfaces, the UA can explicitly terminate the subscription. This
will result in the generation of a NOTIFY with a Subscription-State
header field equal to "terminated".
8.2. Client-Remote Interfaces
As an alternative to, or in conjunction with client-local user
interfaces, an application can make use of client-remote user
interfaces. These user interfaces can execute co-resident with the
application itself (in which case no standardized interfaces between
the UI and the application need to be used), or they can run
separately. This framework assumes that the user interface runs on a
host that has a sufficient trust relationship with the application.
As such, the means for instantiating the user interface is not
considered here.
The primary issue is to connect the user device to the remote user
interface. Doing so requires the manipulation of media streams
between the client and the user interface. Such manipulation can
only be done by user agents. There are two types of user agent
applications within this framework: originating/terminating
applications, and intermediary applications.
8.2.1. Originating and Terminating Applications
Originating and terminating applications are applications that are
themselves the originator or the final recipient of a SIP invitation.
They are "pure" user agent applications, not back-to-back user
agents. The classic example of such an application is an interactive
voice response (IVR) application, which is typically a terminating
application. It is a terminating application because the user
explicitly calls it; i.e., it is the actual called party. An example
of an originating application is a wakeup call application, which
calls a user at a specified time in order to wake them up.
Because originating and terminating applications are a natural
termination point of the dialog, manipulation of the media session by
the application is trivial. Traditional SIP techniques for adding
and removing media streams, modifying codecs, and changing the
address of the recipient of the media streams can be applied.
8.2.2. Intermediary Applications
Intermediary applications are, at the same time, more common than
originating/terminating applications and more complex. Intermediary
applications are applications that are neither the actual caller nor
the called party. Rather, they represent a "third party" that wishes
to interact with the user. The classic example is the ubiquitous
prepaid calling card application.
In order for the intermediary application to add a client-remote user
interface, it needs to manipulate the media streams of the user agent
to terminate on that user interface. This also introduces a
fundamental feature interaction issue. Since the intermediary
application is not an actual participant in the call, the user will
need to interact with both the intermediary application and its peer
in the dialog. Doing both at the same time is complicated and is
discussed in more detail in Section 10.
9. User Agent Behavior
9.1. Advertising Capabilities
In order to participate in applications that make use of stimulus
interfaces, a user agent needs to advertise its interaction
capabilities.
If a user agent supports presentation-capable user interfaces, it
MUST support the REFER method. It MUST include, in all dialog-
initiating requests and responses, an Allow header field that
includes the REFER method. The user agent MUST support the target
dialog specification [10], and MUST include the "tdialog" option tag
in the Supported header field of dialog-forming requests and
responses. Furthermore, the UA MUST support the SIP user agent
capabilities specification [6]. The UA MUST be capable of being
REFERed to an HTTP URI. It MUST include, in the Contact header field
of its dialog-initiating requests and responses, a "schemes" Contact
header field parameter that includes the HTTP URI scheme. The UA
MUST include, in all dialog-initiating requests and responses, an
Accept header field listing all of those markups supported by the UA.
It is RECOMMENDED that all user agents that support presentation-
capable user interfaces support HTML.
If a user agent supports presentation-free user interfaces, it MUST
support the SUBSCRIBE [4] method. It MUST support the KPML [8] event
package. It MUST include, in all dialog-initiating requests and
responses, an Allow header field that includes the SUBSCRIBE method.
It MUST include, in all dialog-initiating requests and responses, an
Allow-Events header field that lists the KPML event package. The UA
MUST include, in all dialog-initiating requests and responses, an
Accept header field listing those event filters it supports. At a
minimum, a UA MUST support the "application/kpml-request+xml" MIME
type.
For either presentation-free or presentation-capable user interfaces,
the user agent MUST support the GRUU [9] specification. The Contact
header field in all dialog-initiating requests and responses MUST
contain a GRUU. The UA MUST include a Supported header field that
contains the "gruu" option tag and the "tdialog" option tag.
Because these headers are examined by proxies that may be executing
applications, a UA that wishes to support client-local user
interfaces should not encrypt them.
9.2. Receiving User Interface Components
Once the UA has created a dialog (in either the early or confirmed
states), it MUST be prepared to receive a SUBSCRIBE or REFER request
against its GRUU. If the UA receives such a request prior to the
establishment of a dialog, the UA MUST reject the request.
A user agent SHOULD attempt to authenticate the sender of the
request. The sender will generally be an application; therefore, the
user agent is unlikely to ever have a shared secret with it, making
digest authentication useless. However, authenticated identities can
be obtained through other means, such as the Identity mechanism [11].
A user agent MAY have pre-defined authorization policies that permit
applications which have authenticated themselves with a particular
identity to push user interface components. If such a set of
policies is present, it is checked first. If the application is
authorized, processing proceeds.
If the application has authenticated itself but is not explicitly
authorized or blocked, this specification RECOMMENDS that the
application be automatically authorized if it can prove that it was
either on the call path, or is trusted by one of the elements on the
call path. An application proves this to the user agent by
demonstrating that it knows the dialog identifiers. That occurs by
including them in a Target-Dialog header field for REFER requests, or
in the Event header field parameters of the KPML SUBSCRIBE request.
Because the dialog identifiers serve as a tool for authorization, a
user agent compliant to this framework SHOULD use dialog identifiers
that are cryptographically random, with at least 128 bits of
randomness. It is recommended that this randomness be split between
the Call-ID and From header field tags in the case of a UAC.
Furthermore, to ensure that only applications resident in or trusted
by on-path elements can instantiate a user interface component, a
user agent compliant to this specification SHOULD use the Session
Initiation Protocol Secure (SIPS) URI scheme for all dialogs it
initiates. This will guarantee secure links between all the elements
on the signaling path.
If the dialog was not established with a SIPS URI, or the user agent
did not choose cryptographically random dialog identifiers, then the
application MUST NOT automatically be authorized, even if it
presented valid dialog identifiers. A user agent MAY apply any other
policies in addition to (but not instead of) the ones specified here
in order to authorize the creation of the user interface component.
One such mechanism would be to prompt the user, informing them of the
identity of the application and the dialog it is associated with. If
an authorization policy requires user interaction, the user agent
SHOULD respond to the SUBSCRIBE or REFER request with a 202. In the
case of SUBSCRIBE, if authorization is not granted, the user agent
SHOULD generate a NOTIFY to terminate the subscription. In the case
of REFER, the user agent MUST NOT act upon the URI in the Refer-To
header field until user authorization is obtained.
If an application does not present a valid dialog identifier in its
REFER or SUBSCRIBE request, the user agent MUST reject the request
with a 403 response.
If a REFER request to an HTTP URI is authorized, the UA executes the
URI and fetches the content to be rendered to the user. This
instantiates a presentation-capable user interface component. If a
SUBSCRIBE was authorized, a presentation-free user interface
component is instantiated.
9.3. Mapping User Input to User Interface Components
Once the user interface components are instantiated, the user agent
must direct user input to the appropriate component. In the case of
presentation-capable user interfaces, this process is known as focus
selection. It is done by means that are specific to the user
interface on the device. In the case of a PC, for example, the
window manager would allow the user to select the appropriate user
interface component to which their input is directed.
For presentation-free user interfaces, the situation is more
complicated. In some cases, the device may support a mechanism that
allows the user to select a "line", and thus the associated dialog.
Any user input on the keypad while this line is selected are fed to
the user interface components associated with that dialog.
Otherwise, for client-local user interfaces, the user input is
assumed to be associated with all user interface components. For
client-remote user interfaces, the user device converts the user
input to media, typically conveyed using RFC 4733, and sends this to
the client-remote user interface. This user interface then needs to
map user input from potentially many media streams into user
interface events. The process for doing this is described in
Section 7.3.
9.4. Receiving Updates to User Interface Components
For presentation-capable user interfaces, updates to the user
interface occur in ways specific to that user interface component.
In the case of HTML, for example, the document can tell the client to
fetch a new document periodically. However, this framework does not
provide any additional machinery to asynchronously push a new user
interface component to the client.
For presentation-free user interfaces, an application can push an
update to a component by sending a SUBSCRIBE refresh with a new
filter. The user agent will process these according to the rules of
the event package.
9.5. Terminating a User Interface Component
Termination of a presentation-capable user interface component is a
trivial procedure. The user agent merely dismisses the window (or
its equivalent). The fact that the component is dismissed is not
communicated to the application. As such, it is purely a local
matter.
In the case of a presentation-free user interface, the user might
wish to cease interacting with the application. However, most
presentation-free user interfaces will not have a way for the user to
signal this through the device. If such a mechanism did exist, the
UA SHOULD generate a NOTIFY request with a Subscription-State header
field equal to "terminated" and a reason of "rejected". This tells
the application that the component has been removed and that it
should not attempt to re-subscribe.
10. Inter-Application Feature Interaction
The inter-application feature interaction problem is inherent to
stimulus signaling. Whenever there are multiple applications, there
are multiple user interfaces. The system has to determine to which
user interface any particular input is destined. That question is
the essence of the inter-application feature interaction problem.
Inter-application feature interaction is not an easy problem to
resolve. For now, we consider separately the issues for client-local
and client-remote user interface components.
10.1. Client-Local UI
When the user interface itself resides locally on the client device,
the feature interaction problem is actually much simpler. The end
device knows explicitly about each application, and therefore can
present the user with each one separately. When the user provides
input, the client device can determine to which user interface the
input is destined. The user interface to which input is destined is
referred to as the "application in focus", and the means by which the
focused application is selected is called "focus determination".
Generally speaking, focus determination is purely a local operation.
In the PC universe, focus determination is provided by window
managers. Each application does not know about focus; it merely
receives the user input that has been targeted to it when it's in
focus. This basic concept applies to SIP-based applications as well.
Focus determination will frequently be trivial, depending on the user
interface type. Consider a user that makes a call from a PC. The
call passes through a prepaid calling card application and a call-
recording application. Both of these wish to interact with the user.
Both push an HTML-based user interface to the user. On the PC, each
user interface would appear as a separate window. The user interacts
with the call-recording application by selecting its window, and with
the prepaid calling card application by selecting its window. Focus
determination is literally provided by the PC window manager. It is
clear to which application the user input is targeted.
As another example, consider the same two applications, but on a
"smart phone" that has a set of buttons, and next to each button,
there is an LCD display that can provide the user with an option.
This user interface can be represented using the Wireless Markup
Language (WML), for example.
The phone would allocate some number of buttons to each application.
The prepaid calling card would get one button for its "hangup"
command, and the recording application would get one for its "start/
stop" command. The user can easily determine which application to
interact with by pressing the appropriate button. Pressing a button
determines focus and provides user input, both at the same time.
Unfortunately, not all devices will have these advanced displays. A
PSTN gateway, or a basic IP telephone, may only have a 12-key keypad.
The user interfaces for these devices are provided through the Keypad
Markup Language (KPML). Considering once again the feature
interaction case above, the prepaid calling card application and the
call-recording application would both pass a KPML document to the
device. When the user presses a button on the keypad, to which
document does the input apply? The device does not allow the user to
select. A device where the user cannot provide focus is called a
"focusless device". This is quite a hard problem to solve. This
framework does not make any explicit normative recommendation, but it
concludes that the best option is to send the input to both user
interfaces unless the markup in one interface has indicated that it
should be suppressed from others. This is a sensible choice by
analogy -- it's exactly what the existing circuit-switched telephone
network will do. It is an explicit non-goal to provide a better
mechanism for feature interaction resolution than the PSTN on devices
that have the same user interface as they do on the PSTN. Devices
with better displays, such as PCs or screen phones, can benefit from
the capabilities of this framework, allowing the user to determine
which application they are interacting with.
Indeed, when a user provides input on a focusless device, the input
must be passed to all client-local user interfaces AND all client-
remote user interfaces, unless the markup tells the UI to suppress
the media. In the case of KPML, key events are passed to remote user
interfaces by encoding them as described in RFC 4733 [19]. Of
course, since a client cannot determine whether or not a media stream
terminates in a remote user interface, these key events are passed in
all audio media streams unless the KPML request document is used to
suppress them.
10.2. Client-Remote UI
When the user interfaces run remotely, the determination of focus can
be much, much harder. There are many architectures that can be
deployed to handle the interaction. None are ideal. However, all
are beyond the scope of this specification.
11. Intra Application Feature Interaction
An application can instantiate a multiplicity of user interface
components. For example, a single application can instantiate two
separate HTML components and one WML component. Furthermore, an
application can instantiate both client-local and client-remote user
interfaces.
The feature interaction issues between these components within the
same application are less severe. If an application has multiple
client user interface components, their interaction is resolved
identically to the inter-application case -- through focus
determination. However, the problems in focusless user devices (such
as a keypad on a telephone) generally won't exist, since the
application can generate user interfaces that do not overlap in their
usage of an input.
The real issue is that the optimal user experience frequently
requires some kind of coupling between the differing user interface
components. This is a classic problem in multi-modal user
interfaces, such as those described by Speech Application Language
Tags (SALT). As an example, consider a user interface where a user
can either press a labeled button to make a selection, or listen to a
prompt, and speak the desired selection. Ideally, when the user
presses the button, the prompt should cease immediately, since both
of them were targeted at collecting the same information in parallel.
Such interactions are best handled by markups that natively support
such interactions, such as SALT, and thus require no explicit support
from this framework.
12. Example Call Flow
This section shows the operation of a call-recording application.
This application allows a user to record the media in their call by
clicking on a button in a web form. The application uses a
presentation-capable user interface component that is pushed to the
caller. The conventions of [17] are used to describe representation
of long message lines.
A Recording App B
|(1) INVITE | |
|----------------------->| |
| |(2) INVITE |
| |----------------------->|
| |(3) 200 OK |
| |<-----------------------|
|(4) 200 OK | |
|<-----------------------| |
|(5) ACK | |
|----------------------->| |
| |(6) ACK |
| |----------------------->|
|(7) REFER | |
|<-----------------------| |
|(8) 200 OK | |
|----------------------->| |
|(9) NOTIFY | |
|----------------------->| |
|(10) 200 OK | |
|<-----------------------| |
|(11) HTTP GET | |
|----------------------->| |
|(12) 200 OK | |
|<-----------------------| |
|(13) NOTIFY | |
|----------------------->| |
|(14) 200 OK | |
|<-----------------------| |
|(15) HTTP POST | |
|----------------------->| |
|(16) 200 OK | |
|<-----------------------| |
Figure 6
First, the caller, A, sends an INVITE to set up a call (message 1).
Since the caller supports the framework and can handle presentation-
capable user interface components, it includes the Supported header
field indicating that the GRUU extension and the Target-Dialog header
field are understood, the Allow header field indicating that REFER is
understood, and the Contact header field that includes the "schemes"
header field parameter.
INVITE sip:B@example.com SIP/2.0
Via: SIP/2.0/TLS host.example.com;branch=z9hG4bK9zz8
From: Caller <sip:A@example.com>;tag=kkaz-
To: Callee <sip:B@example.org>
Call-ID: fa77as7dad8-sd98ajzz@host.example.com
CSeq: 1 INVITE
Max-Forwards: 70
Supported: gruu, tdialog
Allow: INVITE, OPTIONS, BYE, CANCEL, ACK, REFER
Accept: application/sdp, text/html
<allOneLine>
Contact: <sip:A@example.com;gr=urn:uuid:f81d4fae
-7dec-11d0-a765-00a0c91e6bf6>;schemes="http,sip"
</allOneLine>
Content-Length: ...
Content-Type: application/sdp
--SDP not shown--
The proxy acts as a recording server, and forwards the INVITE to the
called party (message 2). It strips the Record-Route it would
normally insert due to the presence of the GRUU in the INVITE:
INVITE sip:B@pc.example.com SIP/2.0
Via: SIP/2.0/TLS app.example.com;branch=z9hG4bK97sh
Via: SIP/2.0/TLS host.example.com;branch=z9hG4bK9zz8
From: Caller <sip:A@example.com>;tag=kkaz-
To: Callee <sip:B@example.org>
Call-ID: fa77as7dad8-sd98ajzz@host.example.com
CSeq: 1 INVITE
Max-Forwards: 70
Supported: gruu, tdialog
Allow: INVITE, OPTIONS, BYE, CANCEL, ACK, REFER
Accept: application/sdp, text/html
<allOneLine>
Contact: <sip:A@example.com;gr=urn:uuid:f81d4fae
-7dec-11d0-a765-00a0c91e6bf6>;schemes="http,sip"
</allOneLine>
Content-Length: ...
Content-Type: application/sdp
--SDP not shown--
B accepts the call with a 200 OK (message 3). It does not support
the framework, so the various header fields are not present.
SIP/2.0 200 OK
Via: SIP/2.0/TLS app.example.com;branch=z9hG4bK97sh
Via: SIP/2.0/TLS host.example.com;branch=z9hG4bK9zz8
From: Caller <sip:A@example.com>;tag=kkaz-
To: Callee <sip:B@example.com>;tag=7777
Call-ID: fa77as7dad8-sd98ajzz@host.example.com
CSeq: 1 INVITE
Contact: <sip:B@pc.example.com>
Content-Length: ...
Content-Type: application/sdp
--SDP not shown--
This 200 OK is passed back to the caller (message 4):
SIP/2.0 200 OK
Record-Route: <sip:app.example.com;lr>
Via: SIP/2.0/TLS host.example.com;branch=z9hG4bK9zz8
From: Caller <sip:A@example.com>;tag=kkaz-
To: Callee <sip:B@example.com>;tag=7777
Call-ID: fa77as7dad8-sd98ajzz@host.example.com
CSeq: 1 INVITE
Contact: <sip:B@pc.example.com>
Content-Length: ...
Content-Type: application/sdp
--SDP not shown--
The caller generates an ACK (message 5).
ACK sip:B@pc.example.com
Route: <sip:app.example.com;lr>
Via: SIP/2.0/TLS host.example.com;branch=z9hG4bK9zz9
From: Caller <sip:A@example.com>;tag=kkaz-
To: Callee <sip:B@example.com>;tag=7777
Call-ID: fa77as7dad8-sd98ajzz@host.example.com
CSeq: 1 ACK
The ACK is forwarded to the called party (message 6).
ACK sip:B@pc.example.com
Via: SIP/2.0/TLS app.example.com;branch=z9hG4bKh7s
Via: SIP/2.0/TLS host.example.com;branch=z9hG4bK9zz9
From: Caller <sip:A@example.com>;tag=kkaz-
To: Callee <sip:B@example.com>;tag=7777
Call-ID: fa77as7dad8-sd98ajzz@host.example.com
CSeq: 1 ACK
Now, the application decides to push a user interface component to
user A. So, it sends it a REFER request (message 7):
<allOneLine>
REFER sip:A@example.com;gr=urn:uuid:f81d4fae
-7dec-11d0-a765-00a0c91e6bf6 SIP/2.0
</allOneLine>
Refer-To: https://app.example.com/script.pl
Target-Dialog: fa77as7dad8-sd98ajzz@host.example.com
;remote-tag=7777;local-tag=kkaz-
Require: tdialog
Via: SIP/2.0/TLS app.example.com;branch=z9hG4bK9zh6
Max-Forwards: 70
From: Recorder Application <sip:app.example.com>;tag=jhgf
<allOneLine>
To: Caller <sip:A@example.com;gr=urn:uuid:f81d4fae
-7dec-11d0-a765-00a0c91e6bf6>
</allOneLine>
Require: tdialog
Allow: INVITE, OPTIONS, BYE, CANCEL, ACK, REFER
Call-ID: 66676776767@app.example.com
CSeq: 1 REFER
Event: refer
Contact: <sip:app.example.com>
Since the recording application is the same as the authoritative
proxy for the domain, it resolves the Request URI to the registered
contact of A, and then sent there. The REFER is answered by a 200 OK
(message 8).
SIP/2.0 200 OK
Via: SIP/2.0/TLS app.example.com;branch=z9hG4bK9zh6
From: Recorder Application <sip:app.example.com>;tag=jhgf
To: Caller <sip:A@example.com>;tag=pqoew
Call-ID: 66676776767@app.example.com
Supported: gruu, tdialog
Allow: INVITE, OPTIONS, BYE, CANCEL, ACK, REFER
<allOneLine>
Contact: <sip:A@example.com;gr=urn:uuid:f81d4fae
-7dec-11d0-a765-00a0c91e6bf6>;schemes="http,sip"
</allOneLine>
CSeq: 1 REFER
User A sends a NOTIFY (message 9):
NOTIFY sip:app.example.com SIP/2.0
Via: SIP/2.0/TLS host.example.com;branch=z9hG4bK9320394238995
To: Recorder Application <sip:app.example.com>;tag=jhgf
From: Caller <sip:A@example.com>;tag=pqoew
Call-ID: 66676776767@app.example.com
CSeq: 1 NOTIFY
Max-Forwards: 70
<allOneLine>
Contact: <sip:A@example.com;gr=urn:uuid:f81d4fae
-7dec-11d0-a765-00a0c91e6bf6>;schemes="http,sip"
</allOneLine>
Event: refer;id=93809824
Subscription-State: active;expires=3600
Content-Type: message/sipfrag;version=2.0
Content-Length: 20
SIP/2.0 100 Trying
And the recording server responds with a 200 OK (message 10).
SIP/2.0 200 OK
Via: SIP/2.0/TLS host.example.com;branch=z9hG4bK9320394238995
To: Recorder Application <sip:app.example.com>;tag=jhgf
From: Caller <sip:A@example.com>;tag=pqoew
Call-ID: 66676776767@app.example.com
CSeq: 1 NOTIFY
The REFER request contained a Target-Dialog header field parameter
with a valid dialog identifier. Furthermore, all of the signaling
was over TLS and the dialog identifiers contain sufficient
randomness. As such, the caller, A, automatically authorizes the
application. It then acts on the Refer-To URI, fetching the script
from app.example.com (message 11). The response, message 12,
contains a web application that the user can click on to enable
recording. Because the client executed the URL in the Refer-To, it
generates another NOTIFY to the application, informing it of the
successful response (message 13). This is answered with a 200 OK
(message 14). When the user clicks on the link (message 15), the
results are posted to the server, and an updated display is provided
(message 16).
13. Security Considerations
There are many security considerations associated with this
framework. It allows applications in the network to instantiate user
interface components on a client device. Such instantiations need to
be from authenticated applications, and also need to be authorized to
place a UI into the client. Indeed, the stronger requirement is
authorization. It is not as important to know the name of the
provider of the application, as it is to know that the provider is
authorized to instantiate components.
This specification defines specific authorization techniques and
requirements. Automatic authorization is granted if the application
can prove that it is on the call path, or is trusted by an element on
the call path. As documented above, this can be accomplished by the
use of cryptographically random dialog identifiers and the usage of
SIPS for message confidentiality. It is RECOMMENDED that SIPS be
implemented by user agents compliant to this specification. This
does not represent a change from the requirements in RFC 3261.
14. Contributors
This document was produced as a result of discussions amongst the
application interaction design team. All members of this team
contributed significantly to the ideas embodied in this document.
The members of this team were:
Eric Burger
Cullen Jennings
Robert Fairlie-Cuninghame
15. Acknowledgements
The authors would like to thank Martin Dolly and Rohan Mahy for their
input and comments. Thanks to Allison Mankin for her support 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.
[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] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[5] McGlashan, S., Lucas, B., Porter, B., Rehor, K., Burnett, D.,
Carter, J., Ferrans, J., and A. Hunt, "Voice Extensible Markup
Language (VoiceXML) Version 2.0", W3C CR CR-voicexml20-
20030220, February 2003.
[6] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
User Agent Capabilities in the Session Initiation Protocol
(SIP)", RFC 3840, August 2004.
[7] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[8] Burger, E. and M. Dolly, "A Session Initiation Protocol (SIP)
Event Package for Key Press Stimulus (KPML)", RFC 4730,
November 2006.
[9] Rosenberg, J., "Obtaining and Using Globally Routable User
Agent URIs (GRUUs) in the Session Initiation Protocol (SIP)",
RFC 5627, October 2009.
[10] Rosenberg, J., "Request Authorization through Dialog
Identification in the Session Initiation Protocol (SIP)",
RFC 4538, June 2006.
16.2. Informative References
[11] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
RFC 4474, August 2006.
[12] Day, M., Rosenberg, J., and H. Sugano, "A Model for Presence
and Instant Messaging", RFC 2778, February 2000.
[13] Jennings, C., Peterson, J., and M. Watson, "Private Extensions
to the Session Initiation Protocol (SIP) for Asserted Identity
within Trusted Networks", RFC 3325, November 2002.
[14] Rosenberg, J., "A Framework for Conferencing with the Session
Initiation Protocol (SIP)", RFC 4353, February 2006.
[15] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
[16] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session Initiation
Protocol (SIP)", RFC 4235, November 2005.
[17] Sparks, R., Hawrylyshen, A., Johnston, A., Rosenberg, J., and
H. Schulzrinne, "Session Initiation Protocol (SIP) Torture Test
Messages", RFC 4475, May 2006.
[18] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[19] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF Digits,
Telephony Tones, and Telephony Signals", RFC 4733, December
2006.
[20] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[21] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Registrations", RFC 3680, March 2004.
Author's Address
Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
EMail: jdrosen@cisco.com
URI: http://www.jdrosen.net