SIMPLE J. Rosenberg
Internet-Draft Cisco Systems
Expires: December 28, 2006 June 26, 2006
A Processing Model for Presence
draft-rosenberg-simple-presence-processing-model-02
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Abstract
This document defines the underlying data processing operations used
by Session Initiation Protocol (SIP) for Instant Messaging Leveraging
Presence Extensions (SIMPLE) presence agents and presence user
agents. The data processing operations described here include
composition, privacy filtering, and watcher filtering.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Publication . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Reporting . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Overriding . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Presence Server Processing . . . . . . . . . . . . . . . . . . 7
4.1. SIP Subscription Processing . . . . . . . . . . . . . . . 7
4.2. Presence Document Processing . . . . . . . . . . . . . . . 7
4.2.1. Collection . . . . . . . . . . . . . . . . . . . . . . 10
4.2.2. Composition . . . . . . . . . . . . . . . . . . . . . 11
4.2.3. Privacy Filtering . . . . . . . . . . . . . . . . . . 17
4.2.4. Watcher Filtering . . . . . . . . . . . . . . . . . . 17
4.2.5. Post-Processing Composition . . . . . . . . . . . . . 18
5. Security Considerations . . . . . . . . . . . . . . . . . . . 18
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
7. Informative References . . . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . . . 21
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1. Introduction
Presence conveys the ability and willingness of a user to communicate
across a set of devices. RFC 2778 [1] defines a model and
terminology for describing systems that provide presence information.
RFC 3863 [3] defines an XML document format for representing presence
information. [6] defines a data model for modeling communications
systems using that document format.
This specification is a companion to the data modeling specifications
described above. Rather than describing the meaning of the
underlying presence data, it describes the processing operations used
by presence agents in processing that data. Other specifications,
such as the presence event package [4] and the PUBLISH method [8]
document the protocol interfaces for moving presence documents
between these entities. However, none of these specifications define
the behaviors these elements can exhibit in terms of processing those
documents. This specification defines those procedures, including
composition, privacy filtering, and watcher filtering, in more
detail. By providing a model for those operations, consistent
interpretation of authorization policies and composition policies
across implementations can be achieved. This allows for consistent
user experiences.
2. Definitions
Subscription Authorization Decision: The process by which a server
determines whether a subscription should be placed into the
accepted, rejected or pending states.
Presence Document Generation Process: The flow of operations followed
by a presence server that takes a set of presence sources, and
based on various policy documents, produces the presence document
sent to a particular watcher.
Composition: The act of combining a set of presence and event data
about a presentity into a coherent picture of the state of that
presentity.
Raw Presence Document: The result of an initial composition
operation, before privacy and watcher filtering operations have
been applied.
Collection: The process of obtaining the set of event state that is
necessary for performing the composition operation.
Merging: Merging is an operation that allows a presence server to
combine together a set of different services or devices into a
single composite service or device.
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Privacy Filtering: The act of applying permissions to a presence
document for the purposes of removing information that a watcher
is not authorized to see.
Watcher filtering The act of removing information from a presence
document at the request of a watcher, to reduce the information
flowing to that watcher.
Pivot: A presence attribute used to select a set of services or
devices that are to be combined as part of a composition
operation.
View: A view represents a stream of presence documents generated by a
presentity after composition and authorization policies have been
applied. Depending on how these policies are structured, each
watcher to a presentity may get a different view, or they may all
get the same view.
Publication: The act of pushing a piece of event state, including
presence, to a state agent, such as a presence server.
Back end subscription: A subscription made from a state agent, such
as a presence server, to a source of presence, for the purpose of
collecting event state in order to perform composition.
Device View: A presence document obtained by composing together
services with the same value of the device ID attribute.
Presentity View: A presence document obtained by composing together
all services into a single tuple.
Service View: A presence document whereby the compositor has not
combined together services, or it has combined them, but not used
the device ID as a pivot.
Splitting: Splitting is the process of taking a single service or
device data element, and splitting into two services or devices.
Reporting: When a service or device publishes presence data about
itself, it is called reporting. Reporting is in contrast to
overriding, where a software agent publishes about a different
service or device.
Overriding: When a service or device publishes presence information
about a different service or device, in an attempt to correct or
modify that data.
3. Publication
Publication is defined as the process by which an event publication
agent (EPA) pushes event state into the network [8]. In this
section, we consider how an EPA for the presence event package would
generate the presence document it will publish.
3.1. Reporting
Reporting is the process whereby a service publishes about itself, an
agent on a device publishes about the device, or an agent
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representing the human user publishes person information elements.
Reporting is in contrast to overriding, where a software agent
attempts to publish information about a different service or device.
An EPA for presence (also known as a Presence User Agent (PUA))
computes the presence document as if it had full knowledge of the
state of the presentity. That is, it represents the complete view of
user presence as understood by that PUA. Frequently, the PUA is a
software agent that acts as a service, and will therefore be
authoritative for the service information it reports. It is
anticipated that services will also frequently report information on
device and person status as well, as this information is sometimes
collected by applications representing services. It is possible that
devices can themselves publish information about a presentity, and
that software agents representing the person, and not their services,
can also publish presence documents. For the remainder of this
discussion, we assume that the entity doing the publishing is a
service.
When a document is created by such a PUA, the presentity URI (encoded
in the "entity" attribute) will typically be a SIP URI, and equal to
the AOR of the presentity. This will also usually be the same as the
request URI in the PUBLISH request itself, but it need not be so.
The URI serve different purposes. As described in [8], the request
URI serves as a means to route the request to the appropriate event
state compositor, and identity the target of the publication. As
such, it is primary a means for targeting the document. The entity
about whom presence is reported is always taken from the "entity" of
the presence document.
A PUA will also publish the services it knows about, and the device
it's associated with. The service URI needs to be a unique
identifier that defines the service as far as the PUA is concerned.
That URI should be a GRUU, as discussed above. The device ID for the
device is obtained from the local operating system.
3.2. Overriding
Overriding is the process whereby a PUA attempts to publish
information in an explicit attempt to have that information take the
place of information published by a different PUA for the same
presentity.
The motivating use case for this feature is as follows. A user has
an office PC and a home PC, both of which run an Instant Messaging
(IM) application. While at work, they set the status of their IM
application to "in a meeting". This information is reported in
publications produced by the PUA on their office PC. When the user
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arrives at home, they realize that their office PC is still reporting
out-of-date information, and they would like to correct it. As such,
the user would like their home PC to publish data that overrides the
information being published by their office PC.
In this specific example, the office PC will be publishing a document
with a person information element indicating that the user is in a
meeting and a service information element indicating availability for
IM communications. The service URI is equal to the GRUU for that
client. The home PC will be publishing a document with a person
information element indicating that the user is at home and a service
information element indicating availability for IM service. That IM
service uses a different service URI than the one at work, since the
two are running on separate UA instances. This presents the presence
server with conflicting person information elements for the same
presentity.
Overriding is ultimately an attempt by a publisher to force the
composition processing in a presence server to resolve a conflict in
a particular way. Ideally, this is done by having a software agent
directly set the composition policy that will be used, and then
publishing information which will be known to "win" the conflict
resolution. In the absence of directly controllable conflict
resolution policies, Section 4.2.2.2 provides guidelines on resolving
conflicts for service, device and person information. Publishers can
attempt to make use of these guidelines to cause an override to
occur.
In most cases, the information that needs to be overriden will be
person information. In the example above, the stale information is
the status "in a meeting", which is a property of the person
information element. Service information is most usually "self
reported" - that is, reported by an agent providing that service.
That agent will likely be authoritative for the service, and it is
unlikely that some other service needs to provide more up to date
information. The situation is more complicated for devices. At the
time of writing, most devices did not contain separate agents that
published information about themselves; the publication happens from
the software agent providing the service. This does present the
possibility that conflicting or incorrect information could be
reported about a device, neccesitating an override. Since a human
being is authoritative about the person information elements, it is
likely that any software agent that reports it will have incorrect
information. It is for this reason that person information elements
are expected to be the most common target for overrides.
Fortunately, overriding person information is easy. The guidelines
in Section 4.2.2.2 recommend that, absent policy or meta-data guiding
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otherwise, the most recently reported status wins. An agent wishing
to override the person status can therefore just publish a person
information element for the presentity. It only needs the presentity
URI to do so.
Overriding service and device information elements is more
complicated, since it requires the service URI or device ID published
by that service or device. The composition operation will often
modify the service URI and device ID before the presence document is
distributed to watchers. The result is that a normal watcher of
presence information will not have enough information at its disposal
to perform an override. At the time of writing, it was anticipated
that new event packages could be defined to facilitate this
discovery, should the need really arise in practice.
4. Presence Server Processing
In this section, we outline the processing done by a presence server.
This processing is broken into two components - the SIP subscription
processing and the presence document processing.
4.1. SIP Subscription Processing
For the most part, processing of SIP subscriptions is described in
RFC 3856 [4]. That specification does leave some hooks for policy-
based decisions in subscription handling, as discussed in Section
6.6.2 of RFC 3856. In particular, when the presence server receives
a SUBSCRIBE request, it needs to make an immediate decision to put
that subscription into one of three states - rejected, successful,
and pending. That decision, called the subscription authorization
decision, is governed by a Presence Authorization document. This
document, discussed below in more detail, also contains information
that guides privacy filtering when the subscription is accepted.
Users can change their presence authorization documents at any time.
As a result, a user could change those policies to alter the state of
the subscription. Changes in the document need to take effect
immediately.
4.2. Presence Document Processing
Once a subscription has been accepted, presence documents are
delivered to the watcher through notifications. This requires the
presence server to generate a presence document for the watcher. The
process for doing that is called the presence document generation
process. This process is invoked by the presence server under
several conditions:
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Subscription Transition to Accepted: When the subscription itself
transitions to the accepted state, the presence server needs to
generate the current state of the presentity and place this in a
NOTIFY to the watcher. To do this, the presence document
generation process is invoked.
SUBSCRIBE refreshes: Once in the accepted state, SUBSCRIBE refreshes
on the SIP dialog request that the server generate a notification
containing the current state of the presentity. To do this, the
presence document generation process is invoked.
Source changes: When one of the sources of presence information for a
user changes, the result may change the state of the presentity.
To determine the new state, the server invokes the presence
document generation process.
Policy changes: When one of the policy documents governing the
presence document generation process changes, the result may
change the state of the presentity. To determine the new state,
the server invokes the presence document generation process.
The basic steps in the presence document generation process are shown
in Figure 1. This is a logical flow, and does not require a server
to implement exactly these steps every time the process is invoked.
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+---------+
|Presence |
| Source |\
+---------+ \ +-----------+
\ | |
\ /------\ | Raw | //------\\
+---------+ \// Create \\ | Presence | || Privacy ||-----+
|Presence |----| View |-->| Document |->|| Filtering|| |
| Source | \\ // | | \\------// |
+---------+ / \------/ | | |
/ ^ +-----------+ ^governs |
/ |governs | |
+---------+ / +------------+ +------------+ |
|Presence |/ | Composition| | Presence | |
| Source | | Policy | selects | Auth | |
+---------+ | |<----------------| | |
governs | | | | |
+--------| | | | |
| +------------+ +------------+ |
| |
V V
------ +-----------+ +-----------+
//// \\\\ | | ------ | |
| Post | | Filtered | /// \\\ | Candidate |
| Processing |<---| Presence |<--| Watcher | | Presence |
| Composition | | Document | | Filter | <---| Document |
\\\\ //// | | \\\ /// | |
------ | | ------ | |
| +-----------+ +-----------+
|
|
|
|
V
+-----------+
| |
| Final |
| Presence |
| Document |
| |
| |
+-----------+
Figure 1
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4.2.1. Collection
The first step is the process of collection. Collection is defined
as the process of obtaining the set of event state that is necessary
for performing the composition operation that creates the initial
view. A view is defined as the particular stream of presence
documents seen by a watcher after the application of policy. In this
case, the initial view is the view of the presentity before the
application of privacy and watcher filtering.
The event state that is collected includes all of the presence
documents that have been published for the presentity. This, by
definition, is the set of documents whose "entity" attribute in the
<presence> element in the presence document is the same as that of
the presentity. However, it may also include other presence
documents for other presentities, in cases where the presence server
knows that the state of one presentity is a function of the state of
another. An example is the helpdesk presentity, whose state is a
function of the state of the users in the help desk.
In addition to presence events, other event state can be used as
well. As an example, registration state [2] has a bearing on
presence, as does dialog state [12], and the state of non-SIP
systems, such as traditional telephony equipment, layer 2 devices,
and so on. This state can be obtained by a presence server in
several ways. Firstly, publishers for that state can send PUBLISH
requests for it to the presence server. In another approach, the
presence server acts as a watcher, and subscribes to the event state
for the resources it needs. This is referred to as a back-end
subscription.
Each of these non-presence events can then be converted into a piece
of presence state (presentity, device or service information) based
on local policy. For example, if the presence server has somehow
obtained information that says that the user's cell-phone is on, this
can be converted into device state (using the device ID of the phone)
along with service state, if the presence server knows about the
services on the device.
Registration state is of particular importance. It can be obtained
by a presence server by having the presence server co-located with
the registrar, or by having the presence server subscribe to the
registration event package for the user [2]. Each registered contact
is considered a service. The service URI (expressed in the <contact>
element in each tuple of the presence document) is obtained from the
GRUU for each contact, if it exists, else it is set to the Contact
URI from the registration. Service parameters can be extracted from
any callee capabilities provided in the registration [9]. The
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presentity URI is set to the address-of-record. This mapping has the
advantage that it is readily correlated to any service information
that might also be PUBLISHed explicitly by that UA. As such, a UA
that registers should also PUBLISH its state, in the event the
presence server cannot access registration information.
Once the non-presence event state is converted into pieces of
presence state, the compositor will have, at its disposal, a set of
presence data, each of which is for the same presentity.
4.2.2. Composition
The next step in the process is the composition operation, which
produces the raw presence document, also known as the initial view,
from the document sources. This document is "raw" because it
contains more information than any watcher might actually see.
Privacy and watcher filtering may eliminate some of the data from the
document.
Composition is governed by the rules defined in the composition
policy. The composition policy is a document selected by the
presence authorization document. Since different composition
policies may have differing implications on privacy, the
authorization rules themselves need to select the composition policy.
As an example, "polite blocking", defined in RFC 2779 [5], is
actually the selection of a specific composition policy - one which
generates a view that falsely represents the watcher as unavailable.
The decision as to whether to invoke this composition policy or not
is dictated by the authorization document.
The authorization rules applicable to the presence document
generation process can themselves depend on the current state of the
presentity, which is derived from the initial view in the raw
presence document. This results in a seemingly circular decision -
the composition policy to generate the raw presence document is based
on authorization policies that are selected by the value of the raw
presence document. However, as discussed below, this problem is
avoided by using a specific composition policy (the default policy)
to compute the view used to assist in the selection of the
authorization policy. That authorization policy can select a
different composition policy to generate the document actually sent
to a watcher.
Composition policies can be complex and rich. However, there are
some general tools and techniques that merit discussion.
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4.2.2.1. Correlation
A key part of composition is using information in one presence
document, describing a person, service or device, to affect
information in another. As an example, if the presence server has a
document indicating that the user has a telephony service that is
busy, the server can use this to extract information about the person
- that they are on the phone. Similarly, if one document indicates
that a device with ID 1 is off, and another document that indicates a
telephony service is running on the device with ID 1, the server can
determine that the telephony service is closed.
The way in which the various input data impact each other are a
matter of local policy. However, a key to performing such
combination operations is the usage of a correlation identifier that
can match a service, device, and person together across input
sources. The presence document provides the service URI, presentity
URI and device ID as correlation identifiers. All three of these
identifiers have uniqueness and temporal persistence properties that
make them useful for purposes of correlation. Indeed, its not just
that the identifiers have temporal persistence; its that they have a
common value that is used persistently across different sources. In
the example above, the device ID of 1 is useful for correlating the
device state to the service state, if, and only if, the source
indicating the device state uses the same device ID as the source
indicating the service state. This makes selection of the device ID
a critically important operation.
4.2.2.2. Conflict Resolution
In some cases, there may be multiple sources that provide conflicting
information about a service, person, or device. In this case,
"conflicting" means that there are multiple person data elements that
say different things, multiple service data elements for the same
service (where the same service is defined as two services with the
same service URI), or multiple device data elements with the same
device ID.
Conflicting person information is very likely. The typical situation
is described in Section 3.2, where a user wishes to change a stale
status set by another software agent no longer under their direct
control.
Ultimately, how to resolve conflicts is under the control of the
composition policy governing the operation of the server. Here, we
discuss approaches that would be typical and appropriate to use.
One way in which conflicts can be resolved is by measuring the
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likelihood that the information from each source is accurate. In
this simple case, the person data element is reported from two IM
clients. However, one IM client may report an idle indicator for the
device, whilst the other (the home IM client) reports that it is not
idle. The presence server can use this information to believe the
device which is not idle.
More generally, when a source publishes information, it publishes its
"world view", including information it thinks it knows about the
person, about the service it is providing, and the device it runs on.
The fact that all of these are reported together in a presence
document is key. It provides additional context that can be used to
determine the level of accuracy of a source for particular
information. For example, if a cell phone reports that the user is
in a meeting, the cell phone's document will include, in addition to
the person status, cell phone device and cell phone service
information. Simimlarly, if a calendaring application acts as a
source, and indicates that the user is in a meeting, it would include
only information about the meeting. The presence server might decide
to trust the information that reports *just* the meeting, more than a
cell phone that reports a meeting.
The presence server may also know the source of the presence data,
based on authenticated identities. For example, in the case above,
the calendaring application may have a separate identity it uses to
authenticate itself to the presence server. The presence server can
be configured to know that the owner of that particular authenticated
identity is a calendar application, and therefore, it can trust its
information on meeting status information more than another source.
[[OPEN ISSUE: do we want a <source> attribute that can be used to
explicitly define information about the publisher of the
information?? How would this be authorized??]].
Without such additional meta-data, the conflict can be resolved by a
simple freshness metric. The presence source which has most recently
begun reporting information for a specific service, device or person
data element, wins. It is imporant not to confuse the time at which
a status is initially reported, from when it is refreshed. The
former occurs when the status of the person, device or service
changes, and the latter occurs for subsequent publications which do
not change the value.
Conflicts of services or devices are less likely to occur in the
model presented here, due to the unique nature of the service URI and
device ID. However, it is possible. Indeed, a client might
explicitly choose to publish with the same service URI as another
client, if its goal is to explicitly override the service of the
other. Using the same service ID is the "hint" to the presence
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server that conflicting data exists, and one needs to be chosen.
4.2.2.3. Merging
Merging is an operation that allows a presence server to combine
together a set of different services or devices into a single
composite service or device. Two services are different if they have
different service URIs, and two devices are different if they have
different device IDs. This operation is a common one in composition
operations.
The merging process involves three steps. The first is to select the
set of services or devices to merge. The second is to combine the
characteristics and status of each. The third is to generate a
composite service URI or device ID.
One way to identify the set of services that will be combined is by
defining a "pivot". A pivot is a particular attribute (either
characteristic or status) of a service that is used as the selector.
All of the services in the raw presence document for whom the pivot
attribute has the same value, are all combined together, and the
resulting service will clearly have that value for that particular
pivot attribute. If the raw presence document has three distinct
values for the pivot attribute, the presence document, after
combination, will have three services.
For example, if the video prescaps [10] attribute is used as the
pivot, then all services that support video will be combined, and all
of those that don't will be combined. The resulting presence
document after merging will have two services - one with a
characteristic of video, and one with a characteristic of no-video.
An important pivot is the SIP address-of-record. When a PUA
publishes a presence document, it includes its GRUU as the service
URI in the <contact> element in the tuple. If the presence server
has access to registrar data, it can determine the AOR associated
with that GRUU (if there is one). By using the implicitly provided
AOR as a pivot, the presence server can combine together all of the
services which are reachable through the same AOR.
Once the set of services or devices is selected, the next step is to
combine their characteristics and status information. How the
characteristics and status are combined will vary for each attribute.
For many attributes, if the value is the same across all services,
the combination operation is easy - use that value. If the attribute
differs across services, it is a matter of local policy as to how
they are combined.
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As an example, consider the <basic> status as defined in [3]. The
most sensible combination operation is the boolean OR operation.
That is, a composite service is said to be available as long as one
of its component services is available.
The final step, combining service URIs, is more complicated. If the
service URIs are GRUUs within the same AOR, they can easily be
combined by using the AOR as the result of the combination function.
Indeed, even if the presence server is not combining multiple
services together, it might make sense to change the GRUU to the AOR
in the presence document delivered to a watcher. If the service URIs
are SIP URIs but are not GRUUs, the presence server may need to
create a URI which represents the collection of services. Requests
made to that URI could fork to the set of services that were combined
together. If the service URIs are not even the same URI scheme, for
example, a mailto and a tel URI, there is little that can be done.
In such a case, the <contact> URI should be removed from the
document. There are some cases where URIs with distinct URI schemes
can be combined. For example, if one service has a tel URI, and the
other has a SIP URI, a combined service can be represented by a SIP
URI generated by the presence server. If the watcher generates a
request towards this SIP URI, the proxy server could fork the request
to the original tel URI and the original SIP URI. This works in this
specific case (sip and tel URI combination) because SIP requests can
sensibly be directed to a tel URI. These cases aside, it is
generally not a good idea to combine services together that have
radically different URIs.
The merging operation takes place for devices identically to the way
it takes place for services. Fortunately, combining of device IDs is
a bit less complicated than combining service URIs. The server can
manufacture new device IDs that represent a "virtual" device that
represents a collection of other devices.
It is perfectly valid for the merging operation to eliminate all
devices from the final document, or to eliminate the person data
element. However, for a presence document to be meaningful, it has
to contain at least one service data element (encoded using a
<tuple>).
If a presence document is obtained by using the device ID within each
service element as a pivot, the result is a device view - there is a
single service in the document for each device. If all of the
services are composed together, so that the final document has a
single service, it is called a presentity view. A service view is
used to describe documents where services are either uncombined, or
are combined using a pivot other than the device ID.
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4.2.2.4. Splitting
Splitting is the process of taking a single service or device data
element, and splitting into two services or devices. This is useful
when the presence server or presence user agent wishes to model a
complex application (such as a voice, video and IM enabled client) by
a multiplicity of distinct services.
The process of splitting involves taking the attributes (both status
and characteristics) for the service, and determine which of the
component services that attribute will describe. In some cases, a
single attribute will be split so that it is present in both
components. For example, if the composite service has an idle
indication, meaning that the service has not been used in some time,
the component services would both inherit the same value for the idle
indicator. In other cases, an attribute gets assigned only to one
service, or in other cases, its value is changed as it is split up.
The way in which this is done is a matter of local policy.
In all cases, it is important to remember that the purpose of having
multiple services or devices described in a document is to give the
watcher choice about what service to use. Therefore, the splitting
operation should result in multiple services that have sufficient
characteristics associated with them to differentiate them to a
watcher.
Splitting of a service URI is a relatively simple operation. The
entity performing the split creates two new service URIs, each of
which, should a request be received for that URI, would get
translated to, or routed to, the composite service URI. If a
presence user agent is performing the split, it can use the grid
parameter of the GRUU to manufacture an infinite supply of URIs that
all get routed to itself. If a presence server is doing the split,
it can manufacture an entirely new URI (in conjunction with the
domain owner, of course) as needed.
When a service is split, there is usually no reason to split the
device as well. The component services all run on the same device,
and there is much benefit to indicating that this is the case. For
example, it would allow a presence server that is compositing the
presence document for the presentity, to determine that all of the
component services are inactive if the device should fail.
4.2.2.5. Default Composition Policy
Unless a user specifies otherwise through an explicit composition
policy statement, it is RECOMMENDED that presence servers follow the
default composition policy described here. By following this
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default, the processing of the presence server becomes more
predictable by users and their agents, allowing them to set their
presence status in ways that result in the desired predictable
output. If a different default is used, users may be surprised by
the results of their actions.
TODO: place default policy here
4.2.3. Privacy Filtering
Once the merging operation has been applied, the next step is to
perform privacy filtering. Privacy filtering is a process by which
information is removed or transformed in a raw presence document, for
the purposes of withholding sensitive information about the
presentity. Typically, the filtering operation runs at the bequest
of the presentity, in order to protect their own privacy. However,
privacy filtering can be instantiated by the operator, in order to
execute domain filtering policies, or even third parties that are
authorized to specify filtering.
The exact privacy filtering operation that takes place depends on the
identity of the watcher, and can also depend on other variables, such
as time of day, the weather in Helsinki, and so on. The set of
information that dictates the way in which privacy filtering is
executed is called authorization policy. Authorization policy is
expressed using the document format defined in [7].
These rules describe how a series of authorization documents are
matched to the subscription, combined together, and then applied.
This matching process is based on conditions described in each
authorization document. These conditions can include the presence
state of the presentity itself. The presence state used to determine
these authorization policies is different than the presence state
sent to the watcher. To compute this presence state, the presence
server runs the presence document generation process using the
default composition policy described above, and then stops the
process once the raw presence document is generated. This raw
presence document is used for any presence states needed to select
the authorization policies applicable to the watcher.
4.2.4. Watcher Filtering
Watcher filtering is the process by which information is further
removed from the presence document. However, it is the watcher which
specifies the information subset that they would like to receive.
Watcher filtering is accomplished by including filter documents in
subscription requests. These filters are then bound to the
subscription, and applied to any presence document generated during
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the lifetime of that subscription.
Filters are described using the document format defined in [11].
4.2.5. Post-Processing Composition
After the privacy and watcher filtering operations have been applied,
the resulting presence document may contain service or device
elements which no longer contain enough information to differentiate
one from another. As discussed above, the purpose of having multiple
services or devices described in a document is to give the watcher
choice about which service to invoke. If the services defined in a
document all appear the same, differing only in the service URI,
there is no reason for a user to choose one over another. In such a
case, composition rules, and in particular, merging of services, will
need to be done. The result is the final presence document that can
be delivered to watchers.
5. Security Considerations
6. Acknowledgements
This document is really a distillation of many ideas discussed over a
long period of time. These ideas were contributed by many different
participants in the SIMPLE working group. Henning Schulzrinne
initially described the "pivot" operation described above for
composition. Brian Rosen deserves credit for the "presentity view".
Aki Niemi, Paul Kyzivat, Cullen Jennings, Ben Campbell, Robert
Sparks, Dean Willis, Adam Roach, Hisham Khartabil, and Jon Peterson
contributed many of the concepts that are described here. A special
thanks to Steve Donovan for discussions on the topics discussed here.
7. Informative References
[1] Day, M., Rosenberg, J., and H. Sugano, "A Model for Presence
and Instant Messaging", RFC 2778, February 2000.
[2] Rosenberg, J., "A Session Initiation Protocol (SIP) Event
Package for Registrations", RFC 3680, March 2004.
[3] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and
J. Peterson, "Presence Information Data Format (PIDF)",
RFC 3863, August 2004.
[4] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
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[5] Day, M., Aggarwal, S., Mohr, G., and J. Vincent, "Instant
Messaging / Presence Protocol Requirements", RFC 2779,
February 2000.
[6] Rosenberg, J., "A Data Model for Presence",
draft-ietf-simple-presence-data-model-07 (work in progress),
January 2006.
[7] Rosenberg, J., "Presence Authorization Rules",
draft-ietf-simple-presence-rules-07 (work in progress),
June 2006.
[8] Niemi, A., "Session Initiation Protocol (SIP) Extension for
Event State Publication", RFC 3903, October 2004.
[9] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
User Agent Capabilities in the Session Initiation Protocol
(SIP)", RFC 3840, August 2004.
[10] Lonnfors, M. and K. Kiss, "Session Initiation Protocol (SIP)
User Agent Capability Extension to Presence Information Data
Format (PIDF)", draft-ietf-simple-prescaps-ext-06 (work in
progress), January 2006.
[11] Khartabil, H., "An Extensible Markup Language (XML) Based
Format for Event Notification Filtering",
draft-ietf-simple-filter-format-05 (work in progress),
March 2005.
[12] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session Initiation
Protocol (SIP)", RFC 4235, November 2005.
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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
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