Network Working Group J. Rosenberg

Network Working Group J. Rosenberg
Request for Comments: 4235 Cisco Systems
Category: Standards Track H. Schulzrinne
Columbia University
R. Mahy, Ed.
SIP Edge LLC
November 2005

An INVITE-Initiated Dialog Event Package for the
Session Initiation Protocol (SIP)

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 01) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

Abstract

This document defines a dialog event package for the SIP Events
architecture, along with a data format used in notifications for this
package. The dialog package allows users to subscribe to another
user and to receive notification of the changes in state of INVITE-
initiated dialog usages in which the subscribed-to user is involved.

Table of Contents

1. Introduction ....................................................3
2. Terminology .....................................................4
3. Dialog Event Package ............................................4
3.1. Event Package Name .........................................4
3.2. Event Package Parameters ...................................4
3.3. SUBSCRIBE Bodies ...........................................5
3.4. Subscription Duration ......................................6
3.5. NOTIFY Bodies ..............................................6
3.6. Notifier Processing of SUBSCRIBE Requests ..................7
3.7. Notifier Generation of NOTIFY Requests .....................8
3.7.1. The Dialog State Machine ............................8
3.7.2. Applying the State Machine .........................11

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3.8. Subscriber Processing of NOTIFY Requests ..................12
3.9. Handling of Forked Requests ...............................12
3.10. Rate of Notifications ....................................13
3.11. State Agents .............................................13
4. Dialog Information Format ......................................13
4.1. Structure of Dialog Information ...........................13
4.1.1. Dialog Element .....................................14
4.1.2. State Element ......................................15
4.1.3. Duration Element ...................................15
4.1.4. Replaces Element ...................................15
4.1.5. Referred-By Element ................................16
4.1.6. Local and Remote Elements ..........................16
4.2. Sample Notification Body ..................................17
4.3. Constructing Coherent State ...............................18
4.4. Schema ....................................................19
5. Definition of New Media Feature Parameters .....................22
5.1. The "sip.byeless" Parameter ...............................22
5.2. The "sip.rendering" parameter .............................23
6. Examples .......................................................24
6.1. Basic Example .............................................24
6.2. Emulating a Shared-Line Phone System ......................26
6.3. Minimal Dialog Information with Privacy ...................31
7. Security Considerations ........................................32
8. IANA Considerations ............................................32
8.1. application/dialog-info+xml MIME Registration .............33
8.2. URN Sub-Namespace Registration for
urn:ietf:params:xml:ns:dialog-info ........................34
8.3. Schema Registration .......................................34
8.4. Media Feature Parameter Registration ......................34
8.4.1. sip.byeless ........................................35
8.4.2. sip.rendering ......................................35
9. Acknowledgements ...............................................36
10. References ....................................................36
10.1. Normative References .....................................36
10.2. Informative References ...................................37

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1. Introduction

The SIP Events framework [1] defines general mechanisms for
subscription to, and notification of, events within SIP networks. It
introduces the notion of a package, which is a specific
"instantiation" of the events mechanism for a well-defined set of
events. Packages have been defined for user presence [16], watcher
information [17], and message waiting indicators [18], amongst
others. This document defines an event package for INVITE-initiated
dialog usages. Dialogs refer to the SIP relationship established
between two SIP peers [2]. Dialogs can be created by many methods,
although RFC 3261 defines only one: the INVITE method. RFC 3265 [1]
defines the SUBSCRIBE and NOTIFY methods, which also create new
dialog usages. However, using this package to model state for non-
session dialog usages is out of the scope of this specification.

A variety of applications are enabled through knowledge of INVITE
dialog usage state. Some examples include:

Automatic Callback: In this basic Public Switched Telephone
Network (PSTN) application, user A calls user B but User B is
busy. User A would like to get a callback when user B hangs
up. When B hangs up, user A's phone rings. When A picks up,
they hear ringing, while they are being connected to B. To
implement this with SIP, a mechanism is required for A to
receive a notification when the dialogs at B are complete.

Presence-Enabled Conferencing: In this application, user A wishes
to set up a conference call with users B and C. Rather than
being scheduled, the call is created automatically when A, B
and C are all available. To do this, the server providing the
application would like to know whether A, B, and C are
"online", not idle, and not in a phone call. Determining
whether or not A, B, and C are in calls can be done in two
ways. In the first, the server acts as a call-stateful proxy
for users A, B, and C, and therefore knows their call state.
This won't always be possible, however, and it introduces
scalability, reliability, and operational complexities. In the
second way, the server subscribes to the dialog state of those
users and receives notifications as this state changes. This
enables the application to be provided in a distributed way;
the server need not reside in the same domain as the users.

IM Conference Alerts: In this application, a user can receive an
Instant Message (IM) on their phone whenever someone joins a
conference that the phone is involved in. The IM alerts are
generated by an application separate from the conference
server.

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In general, the dialog package allows for construction of distributed
applications, where the application requires information on dialog
state but is not co-resident with the end user on which that state
resides.

This document also defines two new callee capability [10] feature
parameters:

o "sip.byeless", which indicates that a SIP user agent (UA) is not
capable of terminating a session itself (for example, in some
announcement or recording services, or in some call centers) in
which the UA is no longer interested in participating; and

o "sip.rendering", which positively describes whether the user
agent is rendering any of the media it is receiving. These
feature parameters are useful in many of the same applications
that motivated the dialog package, such as conferencing,
presence, and the shared-line example described in Section 6.2.

2. Terminology

In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [9] and
indicate requirement levels for compliant implementations.

3. Dialog Event Package

This section provides the details for defining a SIP Events package,
as specified in [1].

3.1. Event Package Name

The name of this event package is "dialog". This package name is
carried in the Event and Allow-Events header fields, as defined in
[1].

3.2. Event Package Parameters

This package defines four Event Package parameters: call-id, to-tag,
from-tag, and include-session-description. If a subscription to a
specific dialog is requested, the first three of these parameters
MUST be present, to identify the dialog that is being subscribed to.
The to-tag is matched against the local tag, the from-tag is matched
against the remote tag, and the call-id is matched against the
Call-ID. The include-session-description parameter indicates whether
the subscriber would like to receive the session descriptions
associated with the subscribed dialog usage or usages.

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It is also possible to subscribe to the set of dialogs created as a
result of a single INVITE sent by a UAC (user agent client). In that
case, the call-id and to-tag MUST be present. The to-tag is matched
against the local tag and the call-id is matched against the Call-ID.

The ABNF for these parameters is shown below. It refers to many
constructions from the ABNF of RFC3261, such as EQUAL, DQUOTE, and
token.

call-id = "call-id" EQUAL ( token / DQUOTE callid DQUOTE )
;; NOTE: any DQUOTEs inside callid MUST be escaped!
from-tag = "from-tag" EQUAL token
to-tag = "to-tag" EQUAL token
with-sessd = "include-session-description"

If any call-ids contain embedded double-quotes, those double-quotes
MUST be escaped using the backslash-quoting mechanism. Note that the
call-id parameter may need to be expressed as a quoted string. This
is because the ABNF for the callid production and the word
production, which is used by callid (both from RFC 3261 [1]), allow
some characters (such as "@", "[", and ":") that are not allowed
within a token.

3.3. SUBSCRIBE Bodies

A SUBSCRIBE request for a dialog package MAY contain a body. This
body defines a filter to be applied to the subscription. Filter
documents are not specified in this document, and at the time of
writing, they are expected to be the subject of future
standardization activity.

A SUBSCRIBE request for a dialog package MAY be sent without a body.
This implies the default subscription filtering policy. The default
policy is:

o If the Event header field contained dialog identifiers, a
notification is generated every time there is a change in the
state of any matching dialogs for the user identified in the
request URI of the SUBSCRIBE.

o If there were no dialog identifiers in the Event header field, a
notification is generated every time there is any change in the
state of any dialogs for the user identified in the request URI of
the SUBSCRIBE with the following exceptions. If the target
(Contact) URI of a subscriber is equivalent to the remote target
URI of a specific dialog, then the dialog element for that dialog
is suppressed for that subscriber. (The subscriber is already a
party in the dialog directly, so these notifications are

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superfluous.) If no dialogs remain after suppressing dialogs, the
entire notification to that subscriber is suppressed and the
version number in the dialog-info element is not incremented for
that subscriber. Implicit filtering for one subscriber does not
affect notifications to other subscribers.

o Notifications do not normally contain full state; rather, they
only indicate the state of the dialog(s) whose state has changed.
The exceptions are a NOTIFY sent in response to a SUBSCRIBE, and a
NOTIFY that contains no dialog elements. These NOTIFYs contain
the complete view of dialog state.

o The notifications contain the identities of the participants in
the dialog, the target URIs, and the dialog identifiers. Session
descriptions are not included unless explicitly requested and
explicitly authorized.

3.4. Subscription Duration

Dialog state changes fairly quickly. Once established, a typical
phone call lasts a few minutes (this is different for other session
types, of course). However, the interval between new calls is
typically long. Clients SHOULD specify an explicit duration.

There are two distinct use cases for dialog state. The first is when
a subscriber is interested in the state of a specific dialog or
dialogs (and they are authorized to find out just the state of those
dialogs). In that case, when the dialogs terminate, so too does the
subscription. In these cases, the value of the subscription duration
is largely irrelevant; it SHOULD be longer than the typical duration
of a dialog. We recommend a default duration of two hours, which is
likely to cover most dialogs.

In another case, a subscriber is interested in the state of all
dialogs for a specific user. In these cases, a shorter interval
makes more sense. The default is one hour for these subscriptions.

3.5. NOTIFY Bodies

As described in RFC 3265 [1], the NOTIFY message will contain bodies
that describe the state of the subscribed resource. This body is in
a format listed in the Accept header field of the SUBSCRIBE, or in a
package-specific default format if the Accept header field was
omitted from the SUBSCRIBE.

In this event package, the body of the notification contains a dialog
information document. This document describes the state of one or
more dialogs associated with the subscribed resource. All

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subscribers and notifiers MUST support the "application/
dialog-info+xml" data format described in Section 4. The subscribe
request MAY contain an Accept header field. If no such header field
is present, it has a default value of "application/dialog-info+xml".
If the header field is present, it MUST include "application/
dialog-info+xml", and it MAY include any other types capable of
representing dialog state.

Of course, the notifications generated by the server MUST be in one
of the formats specified in the Accept header field in the SUBSCRIBE
request.

3.6. Notifier Processing of SUBSCRIBE Requests

The dialog information for a user contains sensitive information.
Therefore, all subscriptions SHOULD be authenticated and then
authorized before approval. All implementors of this package MUST
support the digest authentication mechanism as a baseline. The
authorization policy is at the discretion of the administrator, as
always. However, a few recommendations can be made.

It is RECOMMENDED that, if the policy of user B is that user A is
allowed to call them, dialog subscriptions from user A be allowed.
However, the information provided in the notifications does not
contain any dialog identification information, merely an indication
of whether the user is in at least one call. Specifically, they
should not be able to find out any more information than if they sent
an INVITE. (This concept of a "virtual" dialog is discussed more in
Section 3.7.2, and an example of such a notification body is shown
below).

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="0" state="full"
entity="sip:[email protected]">
<dialog id="as7d900as8">
<state>confirmed</state>
</dialog>
</dialog-info>

A user agent that registers with the address-of-record X SHOULD
authorize subscriptions that come from any entity that can
authenticate itself as X. Complete information on the dialog state
SHOULD be sent in this case. This authorization behavior allows a
group of devices representing a single user to become aware of each
other's state. This is useful for applications such as
single-line-extension, also known as shared lines.

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Note that many implementations of "shared-lines" have a feature
that allows details of calls on a shared address-of-record to be
made private. This is a completely reasonable authorization
policy that could result in notifications that contain only the id
attribute of the dialog element and the state element when
shared-line privacy is requested, and notifications with more
complete information when shared-line privacy is not requested.

3.7. Notifier Generation of NOTIFY Requests

Notifications are generated for the dialog package when an INVITE
request is sent, when a new dialog comes into existence at a UA, or
when the state or characteristics of an existing dialog changes.
Therefore, a model of dialog state is needed in order to determine
precisely when to send notifications, and what their content should
be. The SIP specification has a reasonably well defined lifecycle
for dialogs. However, it is not explicitly modelled. This
specification provides an explicit model of dialog state through a
finite state machine.

It is RECOMMENDED that NOTIFY requests only contain information on
the dialogs whose state or participation information has changed.
However, if a notifier receives a SUBSCRIBE request, the triggered
NOTIFY SHOULD contain the state of all dialogs that the subscriber is
authorized to see.

3.7.1. The Dialog State Machine

Modelling of dialog state is complicated by two factors. The first
is forking, which can cause a single INVITE to generate many dialogs
at a UAC. The second is the differing views of state at the UAC
(user agent client) and UAS (usage agent server). We have chosen to
handle the first issue by extending the dialog finite state machine
(FSM) to include the states between transmission of the INVITE and
the creation of actual dialogs through receipt of 1xx and 2xx
responses. As a result, this specification supports the notion of
dialog state for dialogs before they are fully instantiated.

We have also chosen to use a single FSM for both UAC and UAS.

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+----------+ +----------+
| | 1xx-notag | |
| |----------->| |
| Trying | |Proceeding|-----+
| |---+ +-----| | |
| | | | | | |
+----------+ | | +----------+ |
| | | | | |
| | | | | |
+<--C-----C--+ |1xx-tag |
| | | | |
cancelled| | | V |
rejected| | |1xx-tag +----------+ |
| | +------->| | |2xx
| | | | |
+<--C--------------| Early |-----C---+ 1xx-tag
| | replaced | | | | w/new tag
| | | |<----C---+ (new FSM
| | +----------+ | instance
| | 2xx | | created)
| +----------------+ | |
| | |2xx |
| | | |
V V V |
+----------+ +----------+ |
| | | | |
| | | | |
|Terminated|<-----------| Confirmed|<----+
| | error | |
| | timeout | |
+----------+ replaced +----------+
local-bye | ^
remote-bye | |
| |
+------+
2xx w. new tag
(new FSM instance
created)

Figure 3

The FSM for dialog state is shown in Figure 3. The FSM is best
understood by considering the UAC and UAS cases separately.

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The FSM is created in the Trying state when the UAC sends an INVITE
request. Upon receipt of a 1xx without a tag, the FSM transitions to
the Proceeding state. Note that there is no actual dialog yet, as
defined by the SIP specification. However, there is a "half-dialog",
in the sense that two of the three components of the dialog ID (the
call identifier and local tag) are known. If a 1xx with a tag is
received, the FSM transitions to the Early state. The full dialog
identifier is now defined. Had a 2xx been received, the FSM would
have transitioned to the Confirmed state.

If, after transitioning to the Early or Confirmed states, the UAC
receives another 1xx or 2xx respectively with a different tag,
another instance of the FSM is created, initialized into the Early or
Confirmed state, respectively. The benefit of this approach is that
there will be a single FSM representing the entire state of the
invitation and resulting dialog when dealing in the common case of no
forking.

If the UAC sends a CANCEL and then subsequently receives a 487 to its
INVITE transaction, all FSMs spawned from that INVITE transition to
the Terminated state with the event "cancelled". If the UAC receives
a new invitation (with a Replaces [13] header) that replaces the
current Early or Confirmed dialog, all INVITE transactions spawned
from the replaced invitation transition to the Terminated state with
the event "replaced". If the INVITE transaction terminates with a
non-2xx response for any other reason, all FSMs spawned from that
INVITE transition to the Terminated state with the event "rejected".

Once in the Confirmed state, the call is active. It can transition
to the Terminated state if the UAC sends a BYE or receives a BYE
(corresponding to the "local-bye" and "remote-bye" events as
appropriate), if a mid-dialog request generates a 481 or 408 response
(corresponding to the "error" event), or a mid-dialog request
generates no response (corresponding to the "timeout" event).

From the perspective of the UAS, when an INVITE is received, the FSM
is created in the Trying state. If it sends a 1xx without a tag, the
FSM transitions to the Proceeding state. If a 1xx is sent with a
tag, the FSM transitions to the Early state, and if a 2xx is sent, it
transitions to the Confirmed state. If the UAS receives a CANCEL
request and then generates a 487 response to the INVITE (which can
occur in the Proceeding and Early states), the FSM transitions to the
Terminated state with the event "cancelled". If the UAS generates
any other non-2xx final response to the INVITE request, the FSM
transitions to the Terminated state with the event "rejected". If
the UAS receives a new invitation (with a Replaces [13] header field)
that replaces the current Confirmed dialog, the replaced invitation
transitions to the Terminated state with the event "replaced". Once

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in the Confirmed state, the other transitions to the Terminated state
occur for the same reasons they do in the case of UAC.

There should never be a transition from the Trying state to the
Terminated state with the event "cancelled", since the SIP
specification prohibits transmission of CANCEL until a provisional
response is received. However, this transition is defined in the
FSM just to unify the transitions from Trying, Proceeding, and
Early states to the Terminated state.

3.7.2. Applying the State Machine

The notifier MAY generate a NOTIFY request on any event transition of
the FSM. Whether it does or not is policy dependent. However, some
general guidelines are provided.

When the subscriber is unauthenticated, or it is authenticated but
represents a third party with no specific authorization policies, it
is RECOMMENDED that subscriptions to an individual dialog or to a
specific set of dialogs be forbidden. Only subscriptions to all
dialogs (i.e., there are no dialog identifiers in the Event header
field) are permitted. In that case, actual dialog states across all
dialogs will not be reported. Rather, a single "virtual" dialog FSM
will be used, and event transitions on that FSM will be reported.

If there is any dialog at the UA whose state is Confirmed, the
virtual FSM is in the Confirmed state. If there are no dialogs at
the UA in the Confirmed state but there is at least one in the Early
state, the virtual FSM is in the Early or Confirmed state. If there
are no dialogs in the Confirmed or Early states but there is at least
one in the Proceeding state, the virtual FSM is in the Proceeding,
Early, or Confirmed state. If there are no dialogs in the Confirmed,
Early, or Proceeding states but there is at least one in the Trying
state, the virtual FSM is in the Trying, Proceeding, Early or
Confirmed state. The choice of state to use depends on whether the
UA wishes to let unknown users know that their phone is ringing, as
opposed to being in an active call.

It is RECOMMENDED that, in the absence of any preference, Confirmed
is used in all cases as shown in the example in Section 3.6.
Furthermore, it is RECOMMENDED that the notifications of changes in
the virtual FSM machine not convey any information except the state
of the FSM and its event transitions - no dialog identifiers (which
are ill-defined in this model in any case). The use of this virtual
FSM allows minimal information to be conveyed. A subscriber cannot
know how many calls are in progress, or with whom, just that there
exists a call. This is the same information they would receive if

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they simply sent an INVITE to the user instead; a 486 (Busy Here)
response would indicate that they are on a call.

When the subscriber is authenticated and has authenticated itself
with the same address-of-record that the UA itself uses, if no
explicit authorization policy is defined, it is RECOMMENDED that all
state transitions on dialogs that have been subscribed to be
reported, along with complete dialog IDs. This means either all of
the dialogs, if no dialog identifiers were present in the Event
header field, or the specific set of dialogs identified by the Event
header field parameters.

The notifier SHOULD generate a NOTIFY request on any change in the
characteristics associated with the dialog. Since these include
Contact URIs, Contact parameters, and session descriptions, receipt
of re-INVITEs and UPDATE requests [3] that modify this information
MAY trigger notifications.

3.8. Subscriber Processing of NOTIFY Requests

The SIP Events framework expects packages to specify how a subscriber
processes NOTIFY requests in package-specific ways. In particular, a
package should specify how it uses the NOTIFY requests to construct a
coherent view of the state of the subscribed resource.

Typically, the NOTIFY for the dialog package will contain information
about only those dialogs whose state has changed. To construct a
coherent view of the total state of all dialogs, a subscriber to the
dialog package will need to combine NOTIFYs received over time.

Notifications within this package can convey partial information;
that is, they can indicate information about a subset of the state
associated with the subscription. This means that an explicit
algorithm needs to be defined in order to construct coherent and
consistent state. The details of this mechanism are specific to the
particular document type. See Section 4.3 for information on
constructing coherent information from an application/dialog-info+xml
document.

3.9. Handling of Forked Requests

Since dialog state is distributed across the UA for a particular
user, it is reasonable and useful for a SUBSCRIBE request for dialog
state to fork and to reach multiple UAs.

As a result, a forked SUBSCRIBE request for dialog state can install
multiple subscriptions. Subscribers to this package MUST be prepared

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to install subscription state for each NOTIFY generated as a result
of a single SUBSCRIBE.

3.10. Rate of Notifications

For reasons of congestion control, it is important that the rate of
notifications not be excessive. It is RECOMMENDED that the server
not generate notifications for a single subscriber faster than once
every 1 second.

3.11. State Agents

Dialog state is ideally maintained in the user agents in which the
dialog resides. Therefore, the elements that maintain the dialog are
the ones best suited to handle subscriptions to it. However, in some
cases, a network agent may also know the state of the dialogs held by
a user. Such state agents MAY be used with this package.

4. Dialog Information Format

Dialog information is an XML document [4] that MUST be well-formed
and SHOULD be valid. Dialog information documents MUST be based on
XML 1.0 and MUST be encoded using UTF-8. This specification makes
use of XML namespaces for identifying dialog information documents
and document fragments. The namespace URI for elements defined by
this specification is a URN [5], using the namespace identifier
'ietf' defined by [6] and extended by [7]. This URN is:

urn:ietf:params:xml:ns:dialog-info

A dialog information document begins with the root element tag
"dialog-info".

4.1. Structure of Dialog Information

A dialog information document starts with a dialog-info element.
This element has three mandatory attributes:

o version: This attribute allows the recipient of dialog information
documents to properly order them. Versions start at 0, and
increment by one for each new document sent to a subscriber.
Versions are scoped within a subscription. Versions MUST be
representable using a non-negative 32 bit integer.

o state: This attribute indicates whether the document contains the
full dialog information, or whether it contains only information
on those dialogs that have changed since the previous document
(partial).

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o entity: This attribute contains a URI that identifies the user
whose dialog information is reported in the remainder of the
document. This user is referred to as the "observed user".

The dialog-info element has a series of zero or more dialog sub-
elements. Each of those represents a specific dialog. An example:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="0" notify-state="full"
entity="sip:[email protected]">
</dialog-info>

4.1.1. Dialog Element

The dialog element reports information about a specific dialog or
"half-dialog". It has a single mandatory attribute: id. The id
attribute provides a single string that can be used as an identifier
for this dialog or "half-dialog". This is a different identifier
than the dialog ID defined in RFC 3261 [2], but related to it.

For a caller, the id is created when an INVITE request is sent. When
a 1xx response with a tag, or a 2xx response is received, the dialog
is formally created. The id remains unchanged. However, if an
additional 1xx or 2xx is received, resulting in the creation of
another dialog (and resulting FSM), that dialog is allocated a new
id.

For a callee, the id is created when an INVITE outside of an existing
dialog is received. When a 2xx or a 1xx with a tag is sent, creating
the dialog, the id remains unchanged.

The id MUST be unique amongst all current dialogs at a UA.

There are a number of optional attributes that provide identification
information about the dialog:

o call-id: This attribute is a string that represents the call-id
component of the dialog identifier. (Note that single and
double quotes inside a call-id must be escaped using &quote;
for " and ' for ' .)

o local-tag: This attribute is a string that represents the
local-tag component of the dialog identifier.

o remote-tag: This attribute is a string that represents the
remote-tag component of the dialog identifier. The remote tag
attribute won't be present if there is only a "half-dialog",

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resulting from the generation of an INVITE for which no final
responses or provisional responses with tags has been received.

o direction: This attribute is either initiator or recipient, and
indicates whether the observed user was the initiator of the
dialog, or the recipient of the INVITE that created it.

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="0" state="partial"
entity="sip:[email protected]">
<dialog id="as7d900as8" call-id="a84b4c76e66710"
local-tag="1928301774" direction="initiator">
...
</dialog>
</dialog-info>

The sub-elements of the dialog element provide additional information
about the dialog. Some of these sub-elements provide more detail
about the dialog itself, while the local and remote sub-elements
describe characteristics of the participants involved in the dialog.
The only mandatory sub-element is the state element.

4.1.2. State Element

The "state" element indicates the state of the dialog. Its value is
an enumerated type describing one of the states in the FSM above. It
has an optional event attribute that can be used to indicate the
event that caused any transition into the terminated state, and an
optional code attribute that indicates the response code associated
with any transition caused by a response to the original INVITE.

<state event="rejected" code="486">terminated</state>

4.1.3. Duration Element

The "duration" element contains the amount of time, in seconds, since
the FSM was created.

<duration>145</duration>

4.1.4. Replaces Element

The "replaces" element is used to correlate a new dialog with one it
replaced as a result of an invitation with a Replaces header field.
This element is present in the replacement dialog only (the newer
dialog) and contains attributes with the call-id, local-tag, and
remote-tag of the replaced dialog.

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<replaces call-id="hg287s98s89"
local-tag="6762h7" remote-tag="09278hsb"/>

4.1.5. Referred-By Element

The "referred-by" element is used to correlate a new dialog with a
REFER [12] request that triggered it. The element is present in a
dialog that was triggered by a REFER request that contained a
Referred-By [11] header field and contains the (optional) display
name attribute and the Referred-By URI as its value.

<referred-by display="Bob">sip:[email protected]</referred-by>

4.1.6. Local and Remote Elements

The "local" and "remote" elements are sub-elements of the dialog
element that contain information about the local and remote
participants, respectively. They both have a number of optional
sub-elements that indicate the identity conveyed by the participant,
the target URI, the feature-tags of the target, and the
session-description of the participant.

4.1.6.1. Identity Element

The "identity" element indicates a local or remote URI, as defined in
[2] as appropriate. It has an optional attribute, display, that
contains the display name from the appropriate URI.

Note that multiple identities (for example a sip: URI and a tel:
URI) could be included if they all correspond to the participant.
To avoid repeating identity information in each request, the
subscriber can assume that the identity URIs are the same as in
previous notifications if no identity elements are present in the
corresponding local or remote element. If any identity elements
are present in the local or remote part of a notification, the new
list of identity tags completely supersedes the old list in the
corresponding part.

<identity display="Anonymous">
sip:[email protected]</identity>

4.1.6.2. Target Element

The "target" contains the local or remote target URI constructed by
the user agent for this dialog, as defined in RFC 3261 [2] in a "uri"
attribute.

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It can contain a list of Contact header parameters in param sub-
elements (such as those defined in [10]). The param element contains
two required attributes, pname and pval. Boolean parameters are
represented by the explicit pval values, "true" and "false" (for
example, when a feature parameter is explicitly negated). Parameters
that have no value at all are represented by the explicit pval value
"true". The param element itself has no contents. To avoid
repeating Contact information in each request, the subscriber can
assume that the target URI and parameters are the same as in previous
notifications if no target element is present in the corresponding
local or remote element. If a target element is present in the local
or remote part of a notification, the new target tag and list of
parameter tags completely supersedes the old target and parameter
list in the corresponding part. Note that any quoting (including
extra angle-bracket quoting used to quote string values in [10]) or
backslash escaping MUST be removed before being placed in a pval
attribute. Any remaining single quotes, double quotes, and
ampersands MUST be properly XML escaped.

<target uri="sip:[email protected]">
<param pname="isfocus" pval="true"/>
<param pname="class" pval="business"/>
<param pname="description" pval="Alice's desk & office"/>
<param pname="sip.rendering" pval="no"/>
</target>

4.1.6.3. Session Description Element

The session-description element contains the session description used
by the observed user for its end of the dialog. This element should
generally NOT be included in the notifications, unless it was
explicitly requested by the subscriber. It has a single attribute,
"type", which indicates the MIME media type of the session
description. To avoid repeating session description information in
each request, the subscriber can assume that the session description
is the same as in previous notifications if no session description
element is present in the corresponding local or remote element.

4.2. Sample Notification Body

<?xml version="1.0" encoding="UTF-8"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="urn:ietf:params:xml:ns:dialog-info"
version="1" state="full">
<dialog id="123456">
<state>confirmed</state>
<duration>274</duration>

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<local>
<identity display="Alice">sip:[email protected]</identity>
<target uri="sip:[email protected]">
<param pname="isfocus" pval="true"/>
<param pname="class" pval="personal"/>
</target>
</local>
<remote>
<identity display="Bob">sip:[email protected]</identity>
<target uri="sip:[email protected]"/>
</remote>
</dialog>
</dialog-info>

4.3. Constructing Coherent State

The dialog information subscriber maintains a table listing the
dialogs, with a row for each dialog. Each row is indexed by an ID
that is present in the "id" attribute of the "dialog" element. Each
row contains the state of that dialog, as conveyed in the document.

The table is also associated with a version number. The version
number MUST be initialized with the value of the "version" attribute
from the "dialog-info" element in the first document received. Each
time a new document is received, the value of the local version
number is compared to the "version" attribute in the new document.
If the value in the new document is one higher than the local version
number, the local version number is increased by one and the document
is processed. If the value in the document is more than one higher
than the local version number, the local version number is set to the
value in the new document and the document is processed. If the
document did not contain full state, the subscriber SHOULD generate a
refresh request (SUBSCRIBE) to trigger a full state notification. If
the value in the document is less than the local version, the
document is discarded without processing.

The processing of the dialog information document depends on whether
it contains full or partial state. If it contains full state,
indicated by the value of the "state" attribute in the "dialog-info"
element, the contents of the table are flushed and then repopulated
from the document. A new row in the table is created for each
"dialog" element. If the document contains partial state, as
indicated by the value of the "state" attribute in the "dialog-info"
element, the document is used to update the table. For each "dialog"
element in the document, the subscriber checks to see whether a row
exists for that dialog. This check compares the ID in the "id"
attribute of the "dialog" element with the ID associated with the
row. If the dialog does not exist in the table, a row is added and

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its state is set to the information from that "dialog" element. If
the dialog does exist, its state is updated to be the information
from that "dialog" element. If a row is updated or created, such
that its state is now terminated, that entry MAY be removed from the
table at any time.

4.4. Schema

The following is the schema for the application/dialog-info+xml type:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema
targetNamespace="urn:ietf:params:xml:ns:dialog-info"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:tns="urn:ietf:params:xml:ns:dialog-info"
elementFormDefault="qualified"
attributeFormDefault="unqualified">

<xs:import namespace="http://www.w3.org/XML/1998/namespace"
schemaLocation="http://www.w3.org/2001/03/xml.xsd"/>

<xs:element name="dialog-info">
<xs:complexType>
<xs:sequence>
<xs:element ref="tns:dialog" minOccurs="0"
maxOccurs="unbounded"/>
<xs:any namespace="##other" processContents="lax"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="version" type="xs:nonNegativeInteger"
use="required"/>
<xs:attribute name="state" use="required">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="full"/>
<xs:enumeration value="partial"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="entity" type="xs:anyURI"
use="required"/>
</xs:complexType>
</xs:element>

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<xs:element name="dialog">
<xs:complexType>
<xs:sequence>
<xs:element ref="tns:state" minOccurs="1" maxOccurs="1"/>
<xs:element name="duration" type="xs:nonNegativeInteger"
minOccurs="0" maxOccurs="1"/>
<xs:element name="replaces" minOccurs="0" maxOccurs="1">
<xs:complexType>
<xs:attribute name="call-id" type="xs:string"
use="required"/>
<xs:attribute name="local-tag" type="xs:string"
use="required"/>
<xs:attribute name="remote-tag" type="xs:string"
use="required"/>
</xs:complexType>
</xs:element>
<xs:element name="referred-by" type="tns:nameaddr"
minOccurs="0" maxOccurs="1"/>
<xs:element name="route-set" minOccurs="0" maxOccurs="1">
<xs:complexType>
<xs:sequence>
<xs:element name="hop" type="xs:string"
minOccurs="1" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="local" type="tns:participant"
minOccurs="0" maxOccurs="1"/>
<xs:element name="remote" type="tns:participant"
minOccurs="0" maxOccurs="1"/>
<xs:any namespace="##other" processContents="lax"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="id" type="xs:string" use="required"/>
<xs:attribute name="call-id" type="xs:string"
use="optional"/>
<xs:attribute name="local-tag" type="xs:string"
use="optional"/>
<xs:attribute name="remote-tag" type="xs:string"
use="optional"/>
<xs:attribute name="direction" use="optional">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="initiator"/>
<xs:enumeration value="recipient"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>

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</xs:complexType>
</xs:element>

<xs:complexType name="participant">
<xs:sequence>
<xs:element name="identity" type="tns:nameaddr"
minOccurs="0" maxOccurs="1"/>
<xs:element name="target" minOccurs="0" maxOccurs="1">
<xs:complexType>
<xs:sequence>
<xs:element name="param" minOccurs="0"
maxOccurs="unbounded">
<xs:complexType>
<xs:attribute name="pname" type="xs:string"
use="required"/>
<xs:attribute name="pval" type="xs:string"
use="required"/>
</xs:complexType>
</xs:element>
</xs:sequence>
<xs:attribute name="uri" type="xs:string"
use="required"/>
</xs:complexType>
</xs:element>
<xs:element name="session-description" type="tns:sessd"
minOccurs="0" maxOccurs="1"/>
<xs:element name="cseq" type="xs:nonNegativeInteger"
minOccurs="0" maxOccurs="1"/>
<xs:any namespace="##other" processContents="lax"
minOccurs="0" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>

<xs:complexType name="nameaddr">
<xs:simpleContent>
<xs:extension base="xs:anyURI">
<xs:attribute name="display-name" type="xs:string"
use="optional"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
<xs:complexType name="sessd">
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attribute name="type" type="xs:string"
use="required"/>
</xs:extension>
</xs:simpleContent>

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</xs:complexType>

<xs:element name="state">
<xs:complexType>
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attribute name="event" use="optional">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="cancelled"/>
<xs:enumeration value="rejected"/>
<xs:enumeration value="replaced"/>
<xs:enumeration value="local-bye"/>
<xs:enumeration value="remote-bye"/>
<xs:enumeration value="error"/>
<xs:enumeration value="timeout"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
<xs:attribute name="code" use="optional">
<xs:simpleType>
<xs:restriction base="xs:positiveInteger">
<xs:minInclusive value="100"/>
<xs:maxInclusive value="699"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
</xs:schema>

5. Definition of New Media Feature Parameters

This section defines two new media feature parameters that are useful
as input to user presence, in conferencing applications, and in
applications like the shared-line example described in Section 6.2.
These feature parameters are especially useful in combination with
the dialog package, as they allow an authorized third party to become
aware of these characteristics.

5.1. The "sip.byeless" Parameter

The "sip.byeless" media feature parameter is a new boolean parameter,
defined in this document, that provides a positive indication that
the user agent setting the parameter is unable to terminate sessions
on its own (for example, by sending a BYE request). For example,

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continuous announcement services and certain recording services are
unable to determine when it would be desirable to terminate a
session, and therefore they do not have the ability to terminate
sessions at all. Also, many human call centers are configured so
that they never terminate sessions. (This is to prevent call center
agents from accidentally disconnecting the caller). (Note that per
[10], this parameter name must be preceded by a "+" character when
used in a SIP Contact header field.)

Contact: <sip:[email protected]>
;automaton;+sip.byeless

5.2. The "sip.rendering" Parameter

The "sip.rendering" media feature parameter is a new string
parameter, defined in this document, that can provide a positive
indication whether the user agent setting the parameter is currently
rendering any of the media it is receiving in the context of a
specific session. It MUST only be used in a Contact header field in
a dialog created using the INVITE request.

This parameter has three legal values: "yes", "no", and "unknown".
The value "yes" indicates positive knowledge that the user agent is
rendering at least one of the streams of media that it is receiving.
The value "no" indicates positive knowledge that the user agent is
rendering none of the media that it is receiving. The value
"unknown" indicates that the user agent does not know whether the
media associated with the session is being rendered (which may be the
case if the user agent is acting as a 3pcc (Third Party Call Control)
[19] controller).

The "sip.rendering" parameter is useful in applications such as
shared appearances, conference status monitoring, or as an input to
user presence.

Contact: <sip:[email protected]>
;automaton;+sip.rendering="no"

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6. Examples

6.1. Basic Example

For example, if a UAC sends an INVITE that looks, in part, like:

INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP pc33.example.com;branch=z9hG4bKnashds8
Max-Forwards: 70
To: Bob <sip:[email protected]>
From: Alice <sip:[email protected]>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Contact: <sip:[email protected]>
Content-Type: application/sdp
Content-Length: 142

[SDP not shown]

The XML document in a notification from Alice might look like:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="0"
state="full"
entity="sip:[email protected]">
<dialog id="as7d900as8" call-id="a84b4c76e66710"
local-tag="1928301774" direction="initiator">
<state>trying</state>
</dialog>
</dialog-info>

If the following 180 response is received:

SIP/2.0 180 Ringing
Via: SIP/2.0/UDP pc33.example.com;branch=z9hG4bKnashds8
To: Bob <sip:[email protected]>;tag=456887766
From: Alice <sip:[email protected]>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Contact: <sip:[email protected]>

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The XML document in a notification might look like:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="1"
state="full"
entity="sip:[email protected]">
<dialog id="as7d900as8" call-id="a84b4c76e66710"
local-tag="1928301774" remote-tag="456887766"
direction="initiator">
<state>early</state>
</dialog>
</dialog-info>

If it receives a second 180 with a different tag:

SIP/2.0 180 Ringing
Via: SIP/2.0/UDP pc33.example.com;branch=z9hG4bKnashds8
To: Bob <sip:[email protected]>;tag=hh76a
From: Alice <sip:[email protected]>;tag=1928301774
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Contact: <sip:[email protected]>

This results in the creation of a second dialog:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="2"
state="full"
entity="sip:[email protected]">
<dialog id="as7d900as8" call-id="a84b4c76e66710"
local-tag="1928301774" remote-tag="456887766"
direction="initiator">
<state>early</state>
</dialog>
<dialog id="as7d900as8" call-id="a84b4c76e66710"
local-tag="1928301774" remote-tag="hh76a"
direction="initiator">
<state>early</state>
</dialog>
</dialog-info>

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If a 200 OK response is received on the second dialog, the dialog
moves to confirmed:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="3"
state="partial"
entity="sip:[email protected]">
<dialog id="as7d900as8" call-id="a84b4c76e66710"
local-tag="1928301774" remote-tag="hh76a"
direction="initiator">
<state>confirmed</state>
</dialog>
</dialog-info>

32 seconds later, the other early dialog terminates because no 2xx
response has been received for it. This implies that it was
successfully cancelled, and therefore the following notification is
sent:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="4"
state="partial"
entity="sip:[email protected]">
<dialog id="as7d900as8" call-id="a84b4c76e66710"
local-tag="1928301774" remote-tag="hh76a"
direction="initiator">
<state event="cancelled">terminated</state>
</dialog>
</dialog-info>

6.2. Emulating a Shared-Line Phone System

The following example shows how a SIP telephone user agent can
provide detailed state information and also emulate a shared-line
telephone system (the phone "lies" about having a dialog while it is
merely offhook).

Idle:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="0" state="full"
entity="sip:[email protected]">
</dialog-info>

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Seized:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="1" state="partial"
entity="sip:[email protected]">
<dialog id="as7d900as8">
<state>trying</state>
</dialog>
</dialog-info>

Dialing:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="2" state="partial"
entity="sip:[email protected]">
<dialog id="as7d900as8" call-id="a84b4c76e66710"
local-tag="1928301774" direction="initiator">
<state>trying</state>
<local>
<identity display="Alice Smith">
sip:[email protected]
</identity>
<target uri="sip:[email protected]"/>
</local>
<remote>
<identity>sip:[email protected]</identity>
</remote>
</dialog>
</dialog-info>

Ringing:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="3" state="partial"
entity="sip:[email protected]">
<dialog id="as7d900as8" call-id="a84b4c76e66710"
local-tag="1928301774"
remote-tag="07346y131" direction="initiator">
<state code="180">early</state>
<remote>
<target uri="sip:[email protected]"/>
</remote>
</dialog>
</dialog-info>

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Answered (by voicemail):

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="4" state="partial"
entity="sip:[email protected]">
<dialog id="as7d900as8" call-id="a84b4c76e66710"
local-tag="1928301774"
remote-tag="07346y131" direction="initiator">
<state reason="cancelled">terminated</state>
</dialog>
<dialog id="zxcvbnm3" call-id="a84b4c76e66710"
local-tag="1928301774"
remote-tag="8736347" direction="initiator">
<state code="200">confirmed</state>
<remote>
<target uri="sip:[email protected]">
<param pname="actor" pval="msg-taker"/>
<param pname="automaton" pval="true"/>
<param pname="+sip.byeless" pval="true"/>
</target>
</remote>
</dialog>
</dialog-info>

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Alice would rather talk to Bob's assistant (Cathy Jones) than to
Bob's voicemail. She indicates this preference by pressing a key
(perhaps "0" in North America or "9" in Europe). Bob's voicemail
system then acts on this keypress by transferring [20] Alice's call
to Cathy's AOR.

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="5" state="partial"
entity="sip:[email protected]">
<dialog id="zxcvbnm3" call-id="a84b4c76e66710"
local-tag="1928301774"
remote-tag="8736347" direction="initiator">
<state reason="replaced">terminated</state>
</dialog>
<dialog id="sfhjsjk12" call-id="o34oii1"
local-tag="8903j4"
remote-tag="78cjkus" direction="receiver">
<state reason="replaced">confirmed</state>
<replaces call-id="a84b4c76e66710"
local-tag="1928301774"
remote-tag="8736347"/>
<referred-by>
sip:[email protected]
</referred-by>
<local>
<target uri="sip:[email protected]"/>
<param pname="+sip.rendering" pval="yes"/>
</local>
<remote>
<identity display="Cathy Jones">
sip:[email protected]
</identity>
<target uri="sip:[email protected]">
<param pname="actor" pval="attendant"/>
<param pname="automaton" pval="false"/>
</target>
</remote>
</dialog>
</dialog-info>

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Alice and Cathy talk, Cathy adds Alice to a local conference:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="6" state="partial"
entity="sip:[email protected]">
<dialog id="sfhjsjk12" call-id="o34oii1"
local-tag="8903j4"
remote-tag="78cjkus" direction="receiver">
<state>confirmed</state>
<remote>
<target uri="sip:[email protected]">
<param pname="isfocus" pval="true"/>
</target>
</remote>
</dialog>
</dialog-info>

Alice puts Cathy on hold:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="7" state="partial"
entity="sip:[email protected]">
<dialog id="sfhjsjk12" call-id="o34oii1"
local-tag="8903j4"
remote-tag="78cjkus" direction="receiver">
<state>confirmed</state>
<local>
<target uri="sip:[email protected]"/>
<param pname="+sip.rendering" pval="no"/>
</target>
</local>
</dialog>
</dialog-info>

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Cathy hangs up:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="8" state="partial"
entity="sip:[email protected]">
<dialog id="sfhjsjk12" call-id="o34oii1"
local-tag="8903j4"
remote-tag="78cjkus" direction="receiver">
<state reason="remote-bye">terminated</state>
</dialog>
<dialog id="08hjh1345">
<state>trying</state>
</dialog>
</dialog-info>

Alice hangs up:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="9" state="full"
entity="sip:[email protected]">
</dialog-info>

6.3. Minimal Dialog Information with Privacy

The following example shows the same user agent providing minimal
information to maintain privacy for services like automatic callback.

Onhook:

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="0" state="full"
entity="sip:[email protected]">
</dialog-info>

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RFC 4235 Dialog Package November 2005

Offhook: (implementation/policy choice for Alice to transition to
this "state" when "seized", when Trying, when Proceeding, or when
Confirmed.)

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="1" state="full"
entity="sip:[email protected]">
<dialog id="1">
<state>confirmed</state>
</dialog>
</dialog-info>

Onhook: (implementation/policy choice for Alice to transition to this
"state" when terminated, or when no longer "seized")

<?xml version="1.0"?>
<dialog-info xmlns="urn:ietf:params:xml:ns:dialog-info"
version="2" state="full"
entity="sip:[email protected]">
</dialog-info>

7. Security Considerations

Subscriptions to dialog state can reveal sensitive information. For
this reason, Section 3.6 discusses authentication and authorization
of subscriptions, and provides guidelines on sensible authorization
policies. All implementations of this package MUST support the
digest authentication mechanism.

Since the data in notifications is sensitive as well, end-to-end SIP
encryption mechanisms using S/MIME MAY be used to protect it. User
agents that implement the dialog package SHOULD also implement SIP
over TLS [15] and the sips: scheme.

8. IANA Considerations

This document registers a new MIME type, application/dialog-info+xml;
a new XML namespace; and two new media feature parameters in the SIP
tree.

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RFC 4235 Dialog Package November 2005

8.1. MIME Registration for application/dialog-info+xml Type

MIME media type name: application

MIME subtype name: dialog-info+xml

Mandatory parameters: none

Optional parameters: Same as charset parameter application/xml as
specified in RFC 3023 [8].

Encoding considerations: Same as encoding considerations of
application/xml as specified in RFC 3023 [8].

Security considerations: See Section 10 of RFC 3023 [8] and Section 7
of this specification.

Interoperability considerations: none.

Published specification: This document.

Applications that use this media type: This document type has been
used to support SIP applications such as call return and
auto-conference.

Additional Information:

Magic Number: None
File Extension: .xml
Macintosh file type code: "TEXT"

Personal and email address for further information: Jonathan
Rosenberg, <[email protected]>

Intended usage: COMMON

Author/Change controller: The IETF.

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RFC 4235 Dialog Package November 2005

8.2. URN Sub-Namespace Registration for
urn:ietf:params:xml:ns:dialog-info

This section registers a new XML namespace, per the guidelines in
[7].

URI: The URI for this namespace is
urn:ietf:params:xml:ns:dialog-info.

Registrant Contact: The IESG, <[email protected]>
XML:

BEGIN
<?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN"
"http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="content-type"
content="text/html;charset=iso-8859-1"/>
<title>Dialog Information Namespace</title>
</head>
<body>
<h1>Namespace for Dialog Information</h1>
<h2>urn:ietf:params:xml:ns:dialog-info</h2>
<p>See <a href="ftp://ftp.rfc-editor.org/in-notes/rfc4235.txt">
RFC4235</a>.</p>
</body>
</html>
END

8.3. Schema Registration

This specification registers a schema, per the guidelines in [7].

URI: urn:ietf:params:xml:schema:dialog-info

Registrant Contact: The IESG, <[email protected]>

XML: The XML can be found as the sole content of Section 4.4.

8.4. Media Feature Parameter Registration

This section registers two new media feature tags, per the procedures
defined in RFC 2506 [14]. The tags are placed into the sip tree,
which is defined in [10].

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RFC 4235 Dialog Package November 2005

8.4.1. Media Feature Tag sip.byeless
Media feature tag name sip.byeless

ASN.1 Identifier 19

Summary of the media feature indicated by this tag: This feature tag
is a boolean flag. When set it indicates that the device is
incapable of terminating a session autonomously.

Values appropriate for use with this feature tag: Boolean.

The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms: This
feature tag is most useful in a communications application for
describing the capabilities of an application, such as an
announcement service, recording service, conference, or call center.

Examples of typical use: Call centers and media services.

Related standards or documents: RFC 4235
Security Considerations: This media feature tag can be used in ways
that affect application behaviors or may reveal private information.
For example, a conferencing or other application may decide to
terminate a call prematurely if this media feature tag is set.
Therefore, if an attacker can modify the values of this tag, they may
be able to affect the behavior of applications. As a result of this,
applications that utilize this media feature tag SHOULD provide a
means for ensuring its integrity. Similarly, this feature tag should
only be trusted as valid when it comes from the user or user agent
described by the tag. As a result, protocols for conveying this
feature tag SHOULD provide a mechanism for guaranteeing authenticity.

8.4.2. Media Feature Tag sip.rendering

Media feature tag name: sip.rendering

ASN.1 Identifier: 20

Summary of the media feature indicated by this tag: This feature tag
contains one of three string values indicating if the device is
rendering any media from the current session ("yes"), none of the
media from the current session ("no"), or if this status is not
known to the device ("unknown").

Values appropriate for use with this feature tag: String.

Rosenberg, et al. Standards Track [Page 35]

RFC 4235 Dialog Package November 2005

The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms: This
feature tag is most useful in a communications application, for
describing the state of a device (such as a phone or PDA) during a
multimedia session.

Examples of typical use: Conferencing, telephone shared-line
emulation, and presence applications.

Related standards or documents: RFC 4235

Security Considerations: This media feature tag can be used in ways
that affect application behaviors or may reveal private
information. For example, a conferencing or other application may
decide to terminate a call prematurely if this media feature tag
is set to "no". Therefore, if an attacker can modify the values
of this tag, they may be able to affect the behavior of
applications. As a result of this, applications that utilize this
media feature tag SHOULD provide a means for ensuring its
integrity. Similarly, this feature tag should only be trusted as
valid when it comes from the user or user agent described by the
tag. As a result, protocols for conveying this feature tag SHOULD
provide a mechanism for guaranteeing authenticity.

9. Acknowledgements

The authors would like to thank Sean Olson for his comments.

10. References

10.1. Normative References

[1] Roach, A.B., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.

[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., "The Session Initiation Protocol (SIP) UPDATE
Method", RFC 3311, October 2002.

[4] Paoli, J., Sperberg-McQueen, C., Bray, T., and E. Maler,
"Extensible Markup Language (XML) 1.0 (Second Edition)", W3C
FirstEdition REC-xml-20001006, October 2000.

[5] Moats, R., "URN Syntax", RFC 2141, May 1997.

Rosenberg, et al. Standards Track [Page 36]

RFC 4235 Dialog Package November 2005

[6] Moats, R., "A URN Namespace for IETF Documents", RFC 2648,
August 1999.

[7] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.

[8] Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types",
RFC 3023, January 2001.

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

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

[11] Sparks, R., "The Session Initiation Protocol (SIP) Referred-By
Mechanism", RFC 3892, September 2004.

[12] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.

[13] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
Protocol (SIP) "Replaces" Header", RFC 3891, September 2004.

[14] Holtman, K., Mutz, A., and T. Hardie, "Media Feature Tag
Registration Procedure", BCP 31, RFC 2506, March 1999.

[15] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.

10.2. Informative References

[16] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.

[17] Rosenberg, J., "A Watcher Information Event Template-Package
for the Session Initiation Protocol (SIP)", RFC 3857, August
2004.

[18] Mahy, R., "A Message Summary and Message Waiting Indication
Event Package for the Session Initiation Protocol (SIP)", RFC
3842, August 2004.

[19] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
"Best Current Practices for Third Party Call Control (3pcc) in
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
2004.

Rosenberg, et al. Standards Track [Page 37]

RFC 4235 Dialog Package November 2005

[20] Sparks, R., "Session Initiation Protocol Call Control -
Transfer", Work in Progress, July 2005.

Authors' Addresses

Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
US

Phone: +1 973 952-5000
EMail: [email protected]
URI: http://www.jdrosen.net

Henning Schulzrinne
Columbia University
M/S 0401
1214 Amsterdam Ave.
New York, NY 10027
US

EMail: [email protected]
URI: http://www.cs.columbia.edu/~hgs

Rohan Mahy (editor)
SIP Edge LLC

EMail: [email protected]

Rosenberg, et al. Standards Track [Page 38]

RFC 4235 Dialog Package November 2005

Full Copyright Statement

Copyright (C) The Internet Society (2005).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

Rosenberg, et al. Standards Track [Page 39]