RFC 4479 PROPOSED STANDARD PRESENCE · DATA MODEL · SIP
Presence data model: A Data Model for Presence
What the parts of a presence document mean: the person, the services on offer and the devices those services run on.
RFC EDITOR
Rosenberg, J., "A Data Model for Presence", RFC 4479, DOI 10.17487/RFC4479, July 2006, <https://www.rfc-editor.org/info/rfc4479>.
BIBTEX
@misc{rfc4479,
series = {Request for Comments},
number = 4479,
howpublished = {RFC 4479},
publisher = {RFC Editor},
doi = {10.17487/RFC4479},
url = {https://www.rfc-editor.org/info/rfc4479},
author = {J. Rosenberg},
title = {{A Data Model for Presence}},
pagetotal = 35,
year = 2006,
month = jul,
} RFC text
Contents
Network Working Group J. Rosenberg
Request for Comments: 4479 Cisco Systems
Category: Standards Track July 2006
A Data Model for Presence
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines the underlying presence data model used by
Session Initiation Protocol (SIP) for Instant Messaging and Presence
Leveraging Extensions (SIMPLE) presence agents. The data model
provides guidance on how to map various communications systems into
presence documents in a consistent fashion.
Table of Contents
1. Introduction ....................................................2
2. Definitions .....................................................3
3. The Model .......................................................5
3.1. Presentity URI .............................................6
3.2. Person .....................................................7
3.3. Service ....................................................8
3.3.1. Characteristics .....................................9
3.3.2. Reach Information ..................................10
3.3.3. Relative Information ...............................13
3.3.4. Status .............................................13
3.4. Device ....................................................15
3.5. Modeling Ambiguity ........................................17
3.6. The Meaning of Nothing ....................................19
3.7. Status vs. Characteristics ................................19
3.8. Presence Document Properties ..............................20
4. Motivation for the Model .......................................21
5. Encoding .......................................................22
5.1. XML Schemas ...............................................24
5.1.1. Common Schema ......................................24
5.1.2. Data Model .........................................25
6. Extending the Presence Model ...................................26
7. Example Presence Document ......................................26
7.1. Basic IM Client ...........................................27
8. Security Considerations ........................................29
9. Internationalization Considerations ............................29
10. IANA Considerations ...........................................30
10.1. URN Sub-Namespace Registration ...........................30
10.2. XML Schema Registrations .................................31
10.2.1. Common Schema .....................................31
10.2.2. Data Model ........................................31
11. Acknowledgements ..............................................31
12. References ....................................................32
12.1. Normative References .....................................32
12.2. Informative References ...................................32
1. Introduction
Presence conveys the ability and willingness of a user to communicate
across a set of devices. RFC 2778 [10] defines a model and
terminology for describing systems that provide presence information.
RFC 3863 [1] defines an XML [5] [6] [7] document format for
representing presence information. In these specifications, presence
information is modeled as a series of tuples, each of which contains
a status, communications address, and other markup. However, neither
specification gives guidance on exactly what a tuple is meant to
model, or how to map real-world communications systems (and in
particular, those built around the Session Initiation Protocol (SIP)
[11]) into a presence document.
In particular, several important concepts are not clearly modeled or
well delineated by RFCs 2778 and 3863. These are the following:
Service: A communications service, such as instant messaging (IM) or
telephony, is a system for interaction between users that provides
certain modalities or content.
Device: A communications device is a physical component that a user
interacts with in order to make or receive communications.
Examples are a phone, PDA, or PC.
Person: A person is the end user, and for the purposes of presence,
is characterized by states, such as "busy" or "sad", that impact
their ability and willingness to communicate.
This specification defines these concepts more fully by means of a
presence data model, and concretely defines how to take real-world
systems and map them into presence documents using that model. This
data model is defined in terms of an extension to RFC 3863.
2. Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [9].
This document makes use of many additional terms beyond those defined
in RFC 2778 and RFC 3863. These new terms are as follows:
Device: A device models the physical environment in which services
manifest themselves for users. Devices have characteristics that
are useful in allowing a user to make a choice about which
communications service to use.
Service: A service models a form of communication that can be used
to interact with the user.
Person: A person models the human user and their states that are
relevant to presence systems.
Occurrence: A single description of a particular service, a
particular device, or a person. There may be multiple occurrences
for a particular service or device, or multiple person occurrences
in a Presence Information Data Format (PIDF) document, in cases
where there is ambiguity that is best resolved by the watcher.
Presentity: A presentity combines devices, services, and person
information for a complete picture of a user's presence status on
the network.
Presentity URI: A URI that acts as a unique identifier for a
presentity and provides a handle for obtaining presence
information about that presentity.
Data Component: One of the device, service, or person parts of a
presence document.
Status: Presence information about a service, person, or device that
typically changes over time, in contrast to characteristics, which
are generally static.
Characteristics: Presence information about a service, person, or
device that is usually fixed over time, and descriptive in nature.
Characteristics are useful in providing context that identifies
the service or device as different from another service or device.
Attribute: A status or characteristic. It represents a single piece
of presence information.
Presence Attribute: A synonym for attribute.
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.
3. The Model
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Figure 1
The data model for presence is shown in Figure 1. The model seeks to
describe the presentity, identified by a presentity URI. There are
three components in the model: the person, the service, and the
device. These three data components contain information (called
attributes) that provide a description of some aspect of the service,
person, or device. It is central to this model that each attribute
is affiliated with the service, person, or device because they
describe that service, presentity, or device. This is in contrast to
a model whereby the attributes are associated with the service,
presentity, or device because they were reported by that service,
presentity, or device. As an example, if a cell phone reports that a
user is in a meeting, this would be done by including an attribute as
part of the person information, indicating a status of
"in-a-meeting". The presence information may also include
information on the cell phone as a device. However, even though it
is the device that is reporting that the user is in a meeting, "in a
meeting" is a fact that describes the human user, not their physical
device. Consequently, this attribute is placed in the person
component of the document.
3.1. Presentity URI
The identifier for the presentity is a URI. For each unique
presentity in the network, there is one or more presentity URIs. A
presentity may have multiple URIs because they are identified by both
a URI from the Presence (pres) scheme [12] and a protocol-specific
URI, such as a SIP URI [11] or an Extensible Messaging and Presence
Protocol Internationalized Resource Identifier (XMPP IRI) [13]. Or,
it can be because a user has several aliases in a domain, all of
which are equivalent identifiers for the presentity.
When a document is constructed, the presentity URI is ideally set to
the identifier used to request the document in the first place. For
example, if a document was requested through a SIP SUBSCRIBE request,
the presentity URI would match the Request URI of the SUBSCRIBE
request. This follows the principle of least surprise, since the
entity requesting the document may not be aware of the other
identifiers for the presentity.
Irrespective of the scheme from which the URI is taken, the
presentity URI is independent of any of the services or devices that
the presentity possesses. However, the URI is not just a name - it
represents a resource that can be subscribed to, in order to find out
the status of the user. When the URI is a SIP URI, it will often be
the Address of Record for the user, to which SIP calls can be
directed. This equivalence is not mandated by this specification,
but is a recommended configuration for easing the burden of
remembering and storing identifiers for users.
3.2. Person
The person data component models information about the user whom the
presence data is trying to describe. This information consists of
characteristics of the user, and their status.
Characteristics of a person are the static information about a user
that does not change under normal circumstances. Such information
might include physical characteristics, such as age and height.
Another example of a person characteristic is an alias. An alias is
a URI that identities the same user, but with a different presentity
URI. For example, a presentity "sip:bob@example.com" might have a
presence document with a person component that indicates an alias of
"sip:robert@example.com" and "sip:r.smith@example.com".
Status information about a presentity represents the dynamic
information about a user. This typically consists of things the
*user* is doing, places the *user* is at, feelings the *user* has,
and so on. Examples of typical person status are "in a meeting", "on
the phone", "out to lunch", "happy", and "writing Internet Drafts".
The line between static status information and dynamic status
information is fuzzy, and it is not important that a line be drawn.
The model does not differentiate in a syntactically or semantically
meaningful way between these two types of attributes.
In the model, there can be only one person component per presentity.
In other words, the person component models a single human being, and
includes characteristics and statuses that are related to the
communication states for a single human being. Of course, the system
has no way to verify that the human described by the person component
is actually a single human being, as opposed to a group of users, or
even a dog for that matter. As the saying goes, "on the Internet, no
one knows you are a dog", and the same is true here. The person
component is a facade for a single person; anything that can be made
to look like a single person can be modeled with that facade.
As an example, consider the task of using a presence document to
describe a customer support help desk. The person component can be
considered to be "busy" if none of the support staff are available,
and "at lunch" if the help desk department has a group lunch
together. The watcher that receives the document will consider the
help desk to be a single person; nothing in the document (except
perhaps the note element, should its value be "help desk" or
something similar) conveys information that would indicate that the
person in question is actually a help desk.
However, there can be multiple occurrences of the person component.
This happens in cases where the state of the person component is
ambiguous, as discussed in Section 3.5.
3.3. Service
Each presentity has access to a number of services. Each of these
represents a point of reachability for communications that can be
used to interact with the user. Examples of services are telephony
(that is, traditional circuit-based telephone service), push-to-talk,
instant messaging, Short Message Service (SMS), and Multimedia
Message Service (MMS).
It is difficult to give a precise definition for service. One
reasonable approach is to model each software or hardware agent in
the system as a service. If a user starts a softphone application on
their PC, then that represents a service. If a user has a videophone
device, then that represents another service. This is effectively a
physical view of services. This definition, however, starts to fall
apart when a service is spread across multiple software agents or
devices. For example, a SIP URI representing an address-of-record
can be routed to a softphone or a videophone, or both. In that case,
one might attempt instead to define a service based on its address on
the network. This definition also falls apart when modeling devices
or applications that receive calls and dispatch them to different
"helpers" based on potentially complex logic. For example, a
cellular telephone might house multiple SIP applications, each of
which can "register" different handlers based on the method or even
body type of the request. Each of those applications or handlers can
rightfully be considered a service, but it doesn't have an address on
the network distinct from the others.
Because of this inherent difficulty in precisely defining a service,
the data model doesn't try to constrain what can be considered a
service. Rather, anything can be considered a service so long as it
exhibits a set of key properties defined by this model. In
particular, each service is associated with characteristics that
identify the nature and capabilities of that service, with reach
information that indicates how to connect to the service, with status
information representing the state of that service, and relative
information that describes the ways in which that service relates to
others associated with the presentity.
As a consequence, in this model, services are not explicitly
enumerated. There is no central registry where one finds identifiers
for each service. Consequently, each service does not have a single
"service" attribute with values such as "ptt" or "telephony". That
doesn't mean that these consolidated monikers aren't useful; indeed,
they represent an essential summary of what the service is. Such
summarization is useful in creating icons that allow a user to choose
one service over another. A watcher is free to create such
summarization information from any of the information associated with
a service. The reach information often provides valuable information
for creating such a summarization. Oftentimes, the scheme of the URI
is synonymous with the view of what a service is. An "sms" URI [14]
clearly indicates SMS, for example. For some URIs, there may be many
services available, for example, SIP or tel [15], in which case the
scheme is less meaningful as a way of creating a summary. The reach
information could also indicate that certain application software has
to be invoked (such as a videogame), in which case that aspect of the
reach information would be useful for generating an iconic
representation of the game.
3.3.1. Characteristics
Each service is adorned with characteristics that describe the nature
and capabilities of the service that will be experienced when a
watcher invokes that URI. The nature of a service is a set of
properties that are relatively static across communication sessions
established to that service. The nature of a service tends to be
descriptive. Examples of the nature of a service are that it
represents an interactive voice response or voicemail server, that it
is an automaton, or that it is a telephony service used for the
purposes of work. Capabilities, on the other hand, represent
properties that might be exhibited, and whether they are exhibited
depends on negotiation and other dynamic functions that take place
during session establishment. Examples of such capabilities are the
type of media that might be used, the directionality of
communications that are permitted, the SIP extensions supported, and
so on. Capabilities can be very complex; for example, RFC 2533 [16]
describes a model for representing capabilities through N-ary boolean
functions. It is difficult to differentiate a capability with one
modality (e.g., this service only does voice) from a characteristic
that represents the nature of a service. However, it is not
important to do so.
Characteristics are important when multiple services are indicated.
That is because the purpose of listing multiple services in a
presence document is to give the watcher a *choice*. That is, the
presentity is explicitly offering the watcher an opportunity to
contact them using a multiplicity of different services. To help the
watcher make a decision, the presence document includes
characteristics of each service that help differentiate the services
from each other and give the watcher the context in which to make a
choice.
Because their purpose is primarily to facilitate choice, capabilities
do not impose a requirement on the way in which a user reaches that
service. For example, if a presence document includes two services,
and one supports audio only while the other supports only video, this
does not mean that, when contacting the first service, a user has to
offer only an audio stream, or when contacting the second service, a
user has to offer only a video stream. A user can use local policy
at its discretion in determining what capabilities or communications
modalities are offered when they choose to connect with a service.
It is not necessary for a watcher to add SIP caller preferences [2]
to request routing of the request to a service with the
characteristics described in the presence document.
If, in order to reach a service, the user agent must generate a
request that exhibits a particular capability or contains a specific
header, then this is indicated separately in the reach information,
described below.
One important characteristic of each service is the list of devices
on which that service executes. Each device is identified uniquely
by a device ID. As such, the service characteristics can include a
list of device IDs. A presence document might also contain
information on each device, but this is a separate part of the
document. Indeed, the information on each device might not even be
present in the document. In that case, the device IDs listed for
each service are nothing more than correlation identifiers, useful
for determining when two services run on the same device. The
benefit of this model is that information on the devices can be
filtered out of a presence document, yet the service information,
which includes the device IDs, remains useful and meaningful.
It is perfectly valid for a presence document to contain just a
single service. This is permitted even if the presentity actually
has multiple services at their disposal. The lack of multiple
services in the document merely means that the presentity is not
offering a choice to the watcher. In such a case, the service
characteristics are less important, but may be helpful in allowing a
watcher to decide if they wish to communicate at all.
3.3.2. Reach Information
The reach information for a service provides the instructions for the
recipient of a document on how to correctly contact that service.
When a service is accessible over a communications network, reach
information includes a URI that can be "hit" to access the service.
This URI is called the service URI. However, some services are not
accessible over a communications network (such as in-person
communications or a written letter), and as such, may not utilize a
URI.
Even for services reachable over a communications network, the URI
alone may not be sufficient. For example, two applications may be
running within a cellular telephone, both of which are reachable
through the user's SIP Address of Record. However, one application
is launched when the INVITE request contains a body of a particular
type, and the other is launched for other body types. As another
example, a service may provide complex application logic that
operates correctly only when contacted from matching application
software. In such a case, even though the communications between
instances utilizes a standard protocol (such as SIP), the user
experience will not be correct unless the applications are matched.
When the URI is not sufficient, additional attributes of the service
can be present that define the instructions on how the service is to
be reached. These attributes must be understood for the service to
be utilized. If a watcher receives a presence document containing
reach information it does not understand, it should discard the
service information.
The reach information is an important part of the service. When the
watcher makes a decision about which service of the presentity they
wish to access, the watcher utilizes the reach information for that
service. For this reason, each service has to have a unique set of
reach information. If this was not the case, the user would have no
way to choose between the services. This means that the reach
information represents a unique identifier for the service. However,
a presence document can contain multiple occurrences of a particular
service, each of which contains the same reach information, but
differs in its occurrence identifier. Multiple occurrences of a
service exist in a document when the state of the service is
ambiguous, as discussed in Section 3.5.
Because the reach information serves as an identifier for a service,
it also serves as a way to figure out whether a communications
capability should be represented as one service or more. Something
cannot be a service unless there is a way to reach it separately from
another service. As an example, consider a softphone application
that is capable of audio and video. It is not possible to describe
this softphone as two services - one capable of just audio, and one
capable of just video. That's because there is no way to reach the
video-only service; for example, sending a SIP INVITE with just a
video stream doesn't suffice, since one can always add the audio
stream later and it will work. Video and audio, in this case,
represent capabilities for a single service.
The reach information represents a weak form of contract; the
presentity tells the watcher that, if the watcher utilizes the reach
information included in the presence document, the watcher might be
connected to a service described by the characteristics included in
the presence document. It is important to stress that this is not a
guarantee in any way. It cannot be a guarantee for two reasons.
First, the service in the document might actually be modelling a
number of actual services used by the user, and it may not be
possible to connect the watcher to a service with all of the
characteristics described in the presence document. Second, the
preferences of the presentity always take precedence. The caller
might ask to be connected to the video service, but it is permissible
to connect them to a different service if that is the wish of the
presentity.
This loose contract also provides some guidance on the type of URI
that is most ideally suited for the service URI. A URN [3] can be
used as the service URI. However, since a URN could be resolved to
potentially any number of different URIs, the characteristics,
status, and relative information need to be sensible for all of the
URIs that can be resolved from the URN. As the URN becomes
increasingly "vague" in terms of the service it identifies, the
number of presence attributes that can be included decreases
correspondingly.
The tel URI [11] shares similar properties with a URN, and the same
considerations apply. If, for example, the telephone number exists
in ENUM [18] and multiple ENUM services are defined, including voice
and messaging, it is likely that very little characteristic
information can be included in that service. If, however, a tel URI
has only a single ENUM service defined, and it refers to a telephone
service on the Public Switched Telephone Network (PSTN), more can be
said about its characteristics, status, and relative priority.
It is important to point out that there can be a many-to-one mapping
of reach information to a service. That is, a particular service can
potentially be reachable through an infinite number of reach
information sets. This is true even if the reach information is just
the service URI; it is permissible for multiple service URIs to reach
the same service. Within any particular document, for a particular
service, there will be a single service URI. However, it is allowed
and even valuable to provide different service URIs to different
watchers, or to change the service URIs provided to a particular
watcher over time. Doing so affords many benefits, in fact. It can
allow the recipient of a communications attempt to determine the
context for that attempt - that the attempt was made as a result of
trying to reach a particular service in a particular presence
document. This can be used as a technique for preventing
communications spam, for example [19].
It is also possible for a presence document to contain a service that
has no reach information at all. In such a case, the presentity is
indicating that the service exists, but is electing not to offer the
watcher the opportunity to connect to it. One such example would be
to let a watcher know that a user has a telephony service, and that
they are busy, but in order to avoid receipt of a call, no reach
information is provided.
In an ideal system, the URI alone would represent sufficient reach
information for each service. A URI is supposed to provide
sufficient context for reaching the resource associated with the URI,
and thus in theory there is no need for additional context. However,
sometimes, additional information is needed. Since the reach
information has to be understood in order for the service to be
utilized, reach information beyond the URI should be defined and used
sparingly. Extensions to PIDF that define attributes that are reach
information should clearly call those attributes out as such.
3.3.3. Relative Information
Each service is also associated with a priority, which represents the
preference that the user has for usage of one service over another.
This does not mean that, when a watcher wishes to communicate with
the presentity, that they should always use the service with the
highest priority. If that were the case, there would be no point in
including multiple services in the presence document. Rather, the
priority says, "If you, the watcher, cannot decide which of these to
use, or if it is not important to you, this is the order in which I
would like you to contact me. However, I am giving you a choice."
The priorities are relative to each other, and have no meaning as
absolute numbers. If there are two services, and they have
priorities of 1 and .5, respectively, this is identical to giving
them priorities of .2 and .1, respectively.
3.3.4. Status
Each service also has a status. Status represents generally dynamic
information about the availability of communications using that
service. This is in contrast to characteristics, which describe
fairly static properties of the various services. The simplest form
of status is the basic status, which is a binary indicator of
availability for communications using that service. It can have
values of either "closed" or "open". "Closed" means that
communication to the service will, in all likelihood, fail, will not
reach the intended party, or will not result in communications as
described by the characteristics of the service. As an example, if a
call is forwarded to voicemail if the user is busy or unavailable,
the service is marked as "closed". Similarly, a presentity may
include a hotel phone number as a service URI. After checkout, the
phone number will still ring, but reach the chambermaid or the next
guest. Thus, it would be declared "closed" by that presentity. As
another example, if a user has a SIP URI as their service URI that
points to a SIP softphone application, and the PC shuts down, calls
to that SIP URI will return a 480 response code. This service would
also be declared "closed". "Open" implies the opposite - that
communications to this service will likely succeed and reach the
desired target.
It is also possible to have status information that is dependent on
the characteristics of the communications session that eventually
gets set up. For example, a status attribute can be defined that
indicates that a softphone service is available if instant messaging
is used, but unavailable if audio is used.
Other status information might indicate more details on why the
service is available or unavailable. For example, a telephony
service might have additional status to indicate that the user is on
the phone, or that the user is handling 3 calls for that service.
Services inherently have a lot of dynamic state associated with them.
For example, consider a wireless telephony service (i.e., a cell
phone). There are many dynamic statuses of this service - whether or
not the phone is registered, whether or not it is roaming, which
provider it has roamed into, its signal strength, how many calls it
has, what the state of those calls are, how long the user has been in
a call, and so on. As another example, consider an IM service. The
statuses in this service include whether the user is registered, how
long they have been registered, whether they have an IM conversation
in progress, how many IM conversations are in progress, whether the
user is typing, to whom they are typing, and so on.
However, not all of this dynamic state is appropriate to include
within a service data component of a presence document. Information
is included only when it has a bearing on helping the watcher decide
whether to initiate communications with that service, or helping the
watcher decide when to initiate it, if not now. As an example,
whether a cell phone has strong signal strength or just good signal
strength does not pass the litmus test. Knowing this is not likely
to have an impact on a decision to use this service.
3.4. Device
Devices model the physical operating environment in which services
execute. Examples of devices include cell phones, PCs, laptops,
PDAs, consumer telephones, enterprise PBX extensions, and operator
dispatch consoles.
The mapping of services to devices are many to many. A single
service can execute in multiple devices. Consider a SIP telephony
service. Two SIP phones can register against a single Address of
Record for this service. As a result, the SIP service is associated
with two devices. Similarly, a single device can support a
multiplicity of services. A cell phone can support a SIP telephony
service, an SMS service, and an MMS service. Similarly, a PC can
support a SIP telephony service and a SIP videophone service.
Furthermore, a single device can support no services. In such a
case, the device has no useful presence information by itself.
However, when composed with other documents that describe this same
device in relation to a service, a richer presence document can be
created. For example, consider a Radio Frequency ID (RFID) tag as a
device. This device does not execute any services. However, as a
device, it has properties, such as location, and it may have network
connectivity with which it can report its status and characteristics.
If a video telephone were to report that it was running a video
service, and one of its properties was that it was tagged with that
RFID, a compositor could combine the two documents together, and use
the location of the RFID to say something about the location of the
video telephony device.
Devices are identified with a device ID. A device ID is a URI that
is a globally and temporally unique identifier for the device. In
particular, a device ID is a URN. The URN has to be unique across
all other devices for a particular presentity. However, it is also
highly desirable that it be persistent across time, globally unique,
and computable in a fashion so that different systems are likely to
refer to the device using the same ID. With these properties,
differing sources of presence information based on device status can
be combined. The last of these three properties - readily computable
- is particularly useful. It allows for a compositor to combine
disparate sources of information about a device, all linked by a
common device ID that each source has independently used to identify
the device in question.
Unfortunately, due to the variety of different devices in existence,
it is difficult for a single URN scheme to be used that will have
these properties. It is anticipated that multiple schemes will be
defined, with different ones appropriate for different types of
devices. For cellular telephones, the Electronic Serial Number
(ESN), for example, is a good identifier. For IP devices, the MAC
address is another good one. The MAC address has the property of
being readily computable, but lacks persistence across time (it would
change if the interface card on a device were to change). In any
case, neither of these are associated with URN schemes at this time.
In the interim, the Universally Unique IDentifier (UUID) URN [20] can
be used. For devices with a MAC address, version 1 UUIDs are
RECOMMENDED, as they result in a time-based identifier that makes use
of the MAC address. For devices without a MAC, a version 4 UUID is
RECOMMENDED. This is a purely random identifier, providing
uniqueness. The UUID for a device would typically be chosen at the
time of fabrication in the device, and then persisted in the device
within flash or some other kind of non-volatile storage. The UUID
URN has the properties of being globally and temporally unique, but
because of its random component, it is not at all readily computable,
and therefore useless as a correlation ID with other presence sources
on a network. It is anticipated that future specifications will be
developed that provide additional, superior device IDs.
Though each device is identified by a unique device ID, there can be
multiple occurrences of a particular device represented in a
document. Each one will share the same device ID, but differ in its
occurrence identifier. Multiple occurrences of a device exist in a
document when the state of the device is ambiguous, as discussed in
Section 3.5.
Though this document does not mandate a particular implementation
approach, the device ID is most useful when all of the services on
the device have a way to obtain the device ID and get the same value
for it. This would argue for its placement as an operating system
feature. Operating system developers interested in implementing this
specification are encouraged to provide APIs that allow applications
to obtain the device ID. Absent such APIs, applications that report
presence information about their devices will have to generate their
own device IDs. This leads to the possibility that the applications
may choose different device IDs, using different algorithms or data.
In the worst case, these may mean that two services that run on the
same device, do not appear to.
Like services and person data components, device data components have
generally static characteristics and generally dynamic status.
Characteristics of a device include its physical dimensions and
capabilities - the size of its display, the speed of its CPU, and the
amount of memory. Status information includes dynamic information
about the device. This includes whether the device is powered on or
off, the amount of battery power that remains in the device, the
geographic location of the device, and so on.
The characteristics and status information reported about a device
are for the purposes of choice - to allow the user to choose the
service based on knowledge of what the device is. The device
characteristics and status cannot, in any reliable way, be used to
extract information about the nature of the service that will be
received on the device. For example, if the device characteristics
include the speed of the CPU, and the speed is sufficient to support
high-quality video compression, this cannot be interpreted to mean
that video quality would be good for a video service on that device.
Other constraints on the system may reduce the amount of CPU
available to that service. If there is a desire to indicate that
higher-quality video is available on a device, that should be done by
including service characteristics that say just that. The speed of
the CPU might be useful in helping the watcher differentiate between
a device that is a PC and one that is a cell phone, in the case where
the watcher wishes to call the user's cell phone.
Similarly, if there is dynamic device status (such as whether the
device is on or off), and this state impacts the state of the
service, this is represented by adjusting the state of the service.
Unless a consumer of a presence document has a priori knowledge
indicating otherwise (note that presence agents often do), the state
of a device has no bearing on the state of the service.
Just like services, there is no enumeration of device types - PCs,
PDAs, cell phones, etc. Rather, the device is defined by its
characteristics, from which a watcher can extrapolate whether the
device is a PDA, cell phone, or what have you.
It is important to point out that the device is a *model* of the
underlying physical systems in which services execute. There is
nothing that says that this model cannot be used to talk about
systems where services run in virtualized systems, rather than real
ones. For example, if a PC is executing a virtual machine and
running services within that virtual machine, it is perfectly
acceptable to use this model to talk about that PC as being composed
of two separate devices.
3.5. Modeling Ambiguity
Ambiguity is a reality of a presence system, and it is explicitly
modeled by this specification. Ambiguity exists when there are
multiple pieces of information about a person, a particular device,
or a particular service. This ambiguity naturally arises when
multiple elements publish information about the person, a particular
service, or a particular device. In some cases, a compositor can
resolve the ambiguity in an automated way, and combine the data about
the person, device, or service into a single coherent description.
In other cases, it cannot, perhaps because the compositor lacks the
ability to do so.
However, in many cases, the resolution of this ambiguity is best left
to the watcher that consumes the document. This consumer could be an
application with more information than the compositor, and thus be
able to do a better job of resolving the ambiguity. Or, it may be
presented to the human user, and the human can often resolve the
ambiguity. Unsurprisingly, a human can often do this far better than
an automaton can.
To model ambiguity, the model allows each service, each device, or
the person component to contain multiple occurrences. Each
occurrence has a unique identifier, called the occurrence identifier.
This identifier is unique across all other occurrence identifiers for
any service, device, or person. That is, its uniqueness is scoped
within all of the services, devices, and person elements for a
particular presentity. The identifier ideally persists over time,
since it serves as a valuable handle for setting composition and
authorization policies. Even if there is a single occurrence for a
particular device, service, or person, the occurrence has an
occurrence identifier.
The occurrence identifier is not to be confused with the instance ID
defined in the SIP Outbound specification [27]. A user agent
instance is best modeled as a service, and indeed, a Globally
Routable User Agent URI (GRUU) [22], which is derived from the
instance ID, represents a reasonable choice for a service URI.
However, if the status of such a UA instance could not be determined
unambiguously, a presence document could include two or more
occurrences of the service modeling that UA instance. In such a
case, each occurrence has a unique occurrence ID, but they share the
same service URI, and consequently, the same instance ID.
When multiple occurrences exist in a document, it is important that
some of the attributes of the device, service, or person help the
recipient resolve the ambiguity. For humans, the note field and
timestamp serve as valuable tools. For an automaton, nearly any
attribute of the device, service, or person can be used to resolve
the ambiguity. The timestamp in particular is very useful for both
humans and automatons. As described in RFC 3863 [1], the timestamp
provides the time of most recent change for the tuple. This
specification defines the timestamp for person and device components
as well, with the same meaning. Absent other information, the
person, device, or service that most recently changed can be used as
the more reliable source of data. However, such a resolution
algorithm is not normatively required in any way.
3.6. The Meaning of Nothing
It is clear that the existence of a presence attribute in a document
tells something to a watcher about the value of that presence
attribute. However, what does the absence of a presence attribute
say? This data model follows the lead of RFC 3840 [17], which is
used to define capabilities for SIP user agents. In that
specification, if a capability declaration omits a particular feature
tag, it means that the agent is making no definitive statement either
way about whether this feature tag is supported. The same is true
here - the absence of a presence attribute from a document means that
a watcher cannot make any definitive statement about the value for
that presence attribute. It may be absent because it is being
withheld from the watcher, or it may be absent because that attribute
is not supported by the presentity's software. Neither conclusion
can be drawn.
Because the absence of a presence attribute conveys no information
whatsoever, presence documents achieve their maximum value when they
have as many presence attributes as possible. As such, it is
RECOMMENDED that a presence document contain as many presence
attributes as the presentity is willing to and able to provide to a
watcher.
3.7. Status vs. Characteristics
The data model tries to separate status information from
characteristics, generally by defining status as a relatively dynamic
state about a person, device, or service, whereas a characteristic is
relatively static. However, this distinction is often artificial.
Almost any characteristic can change over time, and sometimes
characteristics can change relatively quickly. As a result, the
distinction between status and characteristics is merely a conceptual
one to facilitate understanding about the different types of presence
information. Nothing in a presence document indicates whether an
element is a characteristic vs. a status, and when a presence
attribute is defined, there is no need for it to be declared one or
the other. Presence documents allow any presence attribute, whether
it can be thought of as a characteristic or a status, to change at
any time.
Unfortunately, the original PIDF specification did have a separate
part of a tuple for describing status, and the basic status was
defined to exist within that part of the tuple. This specification
does not change PIDF; however, all future presence attributes MUST be
defined as children of the <tuple> and not the <status> element.
Furthermore, the schemas defined here do not contain a <status>
element for either the <person> or <device> elements.
3.8. Presence Document Properties
The overall presence document has several important properties that
are essential to this model.
First, a presence document has a concrete meaning independent of how
it is transported or where it is found. The semantics of a document
are the same regardless of whether a document is published by a
presence user agent to its compositor, or whether it is distributed
from a presence agent to watchers. There are no required or implied
behaviors for a recipient of a document. Rather, there are well-
defined semantics for the document itself, and a recipient of a
document can take whatever actions it chooses based on those
semantics.
A corollary of this property is that presence systems are infinitely
composeable. A presence user agent can publish a document to its
presence server. That presence server can compose it with other
documents, and place the result in a notification to a watcher. That
watcher can actually be another presence agent, combining that
document with others it has received, and placing those results in
yet another notify.
Yet another corollary of this property is that implied behaviors in
reaction to the document cannot ever be assumed. For example, just
because a service indicates that it supports audio does not mean that
a watcher will offer audio in a communications attempt to that
service. If doing so is necessary to reach the service, this must be
indicated explicitly through reach information.
It is also important to understand that the role of the presence
document is to help a user make a choice amongst a set of services,
and furthermore, to know ahead of time with as much certainty as
possible whether a communications attempt will succeed or fail.
Success is a combination of many factors: Does the watcher understand
the service URI? Can it act on all of the reach information? Does
it support a subset of the capabilities associated with the service?
Does the person information indicate that the user is likely to
answer? All of these checks should ideally be made before attempting
communication.
Because the presence document serves to help a user to choose and
establish communications, the presentity URI - as the index to that
document - represents a form of "one-number" communications.
Starting from this URI, all of the communications modalities and
their URIs for a user can be discovered, and then used to invoke a
particular communications service. Rather than having to give out a
separate phone number, email address, IM address, Voice over Internet
Protocol (VoIP) address, and so on, the presentity URI can be
provided, and all of the others can be learned from there.
4. Motivation for the Model
Presence is defined in [21] as the ability, willingness, or desire to
communicate across a set of devices. The core of this definition is
the conveyance of information about the ability, willingness, or
desire for communications. Thus, the presence data model needs to be
tailored around conveying information that achieves this goal.
The person data component is targeted at conveying willingness and
desire for communications. It is used to represent information about
the users themselves that affects willingness and desire to
communicate. Whether I am in a meeting, whether I am on the phone -
each of these says something about my willingness to communicate, and
thus makes sense for inclusion in a presence document.
The service component of the data model aims to convey information on
the ability to communicate. The ability to communicate is defined by
the services by which a user is reachable. Thus, including them is
essential.
How do devices fit in? For many users, devices represent the ability
to communicate, not services. Frequently, users make statements
like, "Call me on my cell phone" or "I'm at my desk". These are
statements for preference for communications using a specific device,
as opposed to a service. Thus, it is our expectation that users will
want to represent devices as part of the presence data.
Furthermore, the concept of device adds the ability to correlate
services together. The device models the underlying platform that
supports all of the services on the phone. Its state therefore
impacts all services. For example, if a presence server can
determine that a cell phone is off, this says something about the
services that run on that device: they are all not available. Thus,
if services include indicators about the devices on which they run,
device state can be obtained and thus used to compute the state of
the services on the device.
The data model tries hard to separate device, service, and person as
different concepts. Part of this differentiation is that many
attributes will be applicable to some of these, but not others. For
example, geographic location is a meaningful attribute of the person
(the user has a location) and of a device (the device has a
location), but not of a service (services don't inherently have
locations). Based on this, geographic location information should
only appear as part of device or person, never service. Furthermore,
it is possible and meaningful for location information to be conveyed
for both device and person, and for these locations to be different.
The fact that the presence system might try to determine the location
of the person by extrapolation from the location of one of the
devices is irrelevant from a data modeling perspective. Person
location and device location are not the same thing.
[25] defines the <geopriv> XML element for conveying location
information, and indicates that it is carried as a child of the
<tuple> element in a PIDF document. [25] was developed prior to this
specification, and unfortunately, its recommendation to include
location objects underneath <tuple> runs contrary to the
recommendations here. As such, implementations based on this
specification SHOULD include <geopriv> location objects as part of
person and/or device components of the document, but SHOULD be
prepared to receive presence documents with that object as a child to
<tuple>. A <geopriv> location object would be included in a person
component when the document means to convey the location of the user,
and within a device component when it means to convey the location of
the device.
5. Encoding
Information represented according to the data model described above
needs to be mapped into an on-the-wire format for transport and
storage. The Presence Information Data Format [1] is used for
representation of presence data.
The <presence> element contains the presence information for the
presentity. The "entity" attribute of this element contains the
presentity URI.
The existing <tuple> element in the PIDF document is used to
represent the service. This is consistent with the original intent
of RFC 2778 and RFC 3863, and achieves backward compatibility with
implementations developed before the model described here was
complete. The <contact> element in the <tuple> element is used to
encode the service URI. New presence attributes, whether they
represent dynamic status or static characteristics, appear directly
as children of <tuple>. However, attributes defined prior to
publication of this specification that were defined as children of
<status> (such as <basic>) remain as children of <status>, for
purposes of backward compatibility. Consequently, a presence
attribute describing a service could appear as either a child of
<status> or directly as a child of <tuple>, but never both.
The "id" attribute of the <tuple> element conveys the service
occurrence. Each <tuple> element with the same <contact> URI
represents a different occurrence of a particular service.
This specification introduces the <person> element, which can appear
as a child to <presence>. There can be zero or more occurrences of
this element per document. Each one has a mandatory "id" attribute,
which contains the occurrence identifier for the person. Each
<person> element contains any number of elements that indicate status
and characteristic information. This is followed by zero or more
optional <note> elements and an optional <timestamp>. Multiple
<note> elements would appear to convey the same note in multiple
languages.
RFC 3863 defines a <note> element, zero or more of which can be
present as a child to <presence>. As it relates to the model defined
here, these note elements, if present in a document, apply to all
person occurrences that do not have any of their own <note> elements.
In other words, if a <person> element has one or more <note>
elements, those are the <note> elements for that <person> element.
If a <person> element does not have any of its own <note> elements,
the <note> elements that are the direct children of <presence> are
the <note> elements for that <person>. If there are no <note>
elements underneath the <person> element, and there are no <note>
elements that are a direct child of <presence>, then that <person>
element has no <note> elements.
This specification also introduces the <device> element, which can
appear as a child to <presence>. There can be zero or more
occurrences of this element per document. The <device> element can
appear either before or after the <person> element; there are no
constraints on order. Each <device> element has a mandatory "id"
attribute, which contains the occurrence identifier for the device.
Like <person>, <device> contains any number of elements that indicate
status and characteristic information. This is followed by
<deviceID>, which contains the URN for the device ID for this device.
This is followed by zero or more optional <note> elements and an
optional <timestamp>. Multiple <note> elements would appear to
convey the same note in multiple languages.
A client that receives a PIDF document containing the <device> and
<person> elements, but does not understand them (because it doesn't
implement this specification), will ignore them. Furthermore, since
the semantics of service as defined here are aligned with the meaning
of a tuple as defined in RFC 2778 and RFC 3863, documents
incorporating the concepts defined in this model are compliant with
older implementations.
It's important to note that the mapping of the presence data model
into a PIDF document is merely an exercise in syntax.
Presence documents created according to this model MUST be valid,
with the following exception. A compositor is permitted to create a
presence document that it cannot fully validate but that otherwise
validates when processed according to the lax processing rules
allowed by the schema of the compositor. However, it is not expected
that entities receiving these documents would perform schema
validation; rather, they would merely access the information from the
document in the places they were expecting it to be. Implementations
SHOULD be prepared to receive documents that are not valid, and
extract whatever information from them that they can parse.
5.1. XML Schemas
The XML schemas are broken into a common schema, called common-
schema.xsd, which contains common type definitions, and the rest of
the data model, data-model.xsd.
5.1.1. Common Schema
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified" attributeFormDefault="unqualified">
<xs:import namespace="http://www.w3.org/XML/1998/namespace"
schemaLocation="http://www.w3.org/2001/xml.xsd"/>
<xs:simpleType name="Timestamp_t">
<xs:annotation>
<xs:documentation>Timestamp type</xs:documentation>
</xs:annotation>
<xs:restriction base="xs:dateTime"/>
</xs:simpleType>
<xs:simpleType name="deviceID_t">
<xs:annotation>
<xs:documentation>Device ID, a URN</xs:documentation>
</xs:annotation>
<xs:restriction base="xs:anyURI"/>
</xs:simpleType>
<xs:complexType name="Note_t">
<xs:annotation>
<xs:documentation>Note type</xs:documentation>
</xs:annotation>
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attribute ref="xml:lang"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
<xs:attributeGroup name="fromUntil">
<xs:attribute name="from" type="xs:dateTime"/>
<xs:attribute name="until" type="xs:dateTime"/>
</xs:attributeGroup>
<xs:complexType name="empty"/>
</xs:schema>
5.1.2. Data Model
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema targetNamespace="urn:ietf:params:xml:ns:pidf:data-model"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns="urn:ietf:params:xml:ns:pidf:data-model"
elementFormDefault="qualified" attributeFormDefault="unqualified">
<xs:include schemaLocation="common-schema.xsd"/>
<xs:element name="deviceID" type="deviceID_t">
<xs:annotation>
<xs:documentation>Device ID, a URN</xs:documentation>
</xs:annotation>
</xs:element>
<xs:element name="device">
<xs:annotation>
<xs:documentation>Contains information about the
device</xs:documentation>
</xs:annotation>
<xs:complexType>
<xs:sequence>
<xs:any namespace="##other" processContents="lax"
minOccurs="0" maxOccurs="unbounded"/>
<xs:element ref="deviceID"/>
<xs:element name="note" type="Note_t" minOccurs="0"
maxOccurs="unbounded"/>
<xs:element name="timestamp" type="Timestamp_t" minOccurs="0"/>
</xs:sequence>
<xs:attribute name="id" type="xs:ID" use="required"/>
</xs:complexType>
</xs:element>
<xs:element name="person">
<xs:annotation>
<xs:documentation>Contains information about the human
user</xs:documentation>
</xs:annotation>
<xs:complexType>
<xs:sequence>
<xs:any namespace="##other" processContents="lax"
minOccurs="0" maxOccurs="unbounded">
<xs:annotation>
<xs:documentation>Characteristic and status
information</xs:documentation>
</xs:annotation>
</xs:any>
<xs:element name="note" type="Note_t" minOccurs="0"
maxOccurs="unbounded"/>
<xs:element name="timestamp" type="Timestamp_t" minOccurs="0"/>
</xs:sequence>
<xs:attribute name="id" type="xs:ID" use="required"/>
</xs:complexType>
</xs:element>
</xs:schema>
6. Extending the Presence Model
When new presence attributes are added, any such extension has to
consider the following questions:
1. Is the new attribute applicable to person, service, or device
data components? If it is applicable to more than one, what is
its meaning in each context? An extension should strive to have
each attribute concisely defined for each area of applicability,
so that a source can clearly determine to which type of data
component it should be applied.
2. Does it belong in a new namespace, or an existing one?
Generally, new presence attributes defined within the same
specification SHOULD belong to the same namespace. Presence
attributes defined in separate specifications, but produced in a
coordinated way by a centralized administration, MAY be placed in
the same namespace. Doing so, however, requires the centralized
administration to ensure that there are no collisions of element
names across those specifications. Furthermore, if a new
extension has elements meant to be placed as the children of
another element at a point of extensibility defined by <any
namespace="##other">, the new extension MUST use a different
namespace than that of its parent elements.
3. Does the extension itself require extensibility? If so, points
of extension MUST be defined in the schema, and SHOULD be done
using the <any namespace="##other"> construct.
7. Example Presence Document
In this section, we give an example of a physical system, present the
model of that system using the concepts described here, and then show
the resulting presence document. The example makes use of presence
attributes defined in [23] and [24].
7.1. Basic IM Client
In this scenario, a provider is offering a service very similar to
the instant messaging services offered today by the public providers
like AOL, Yahoo!, and MSN. In this service, each user has a "screen
name" that identifies the user in the service. A single client,
generally a PC application, connects to the service at a time. When
the client connects, this fact is made available to other watchers of
that user in the system. The user has the ability to set a textual
note that describes what they are doing, and this note is seen by the
watchers in the system. The user can set one of several status
messages (busy, in a meeting, etc.), which are pre-defined notes that
the system understands. If a user does not type anything on their
keyboard for some time, the user's status changes to idle on the
screens of the various watchers of the system. The system also
indicates the amount of time that the user has been idle.
Whenever a user is connected to the system, they are capable of
receiving instant messages. A user can set their status to
"invisible", which means that they appear as offline to other users.
However, if an IM is sent to them, it will still be delivered.
This system is modeled by representing each presentity in the system
with three data components: a person component, a service component,
and a device component. The person component describes the state of
the user, including the note and the pre-defined status messages.
These represent information about the human user, so they are
included in the person component. The service tuple represents the
IM service. No characteristics are included. The service URI
published by the client is set to the client's Address of Record
(AOR). The device component is used to model the PC. The device
component includes the <user-input> element [23], since the idleness
refers to usage of the device, not the service.
The document published by the client would look like this:
<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:dm="urn:ietf:params:xml:ns:pidf:data-model"
xmlns:rp="urn:ietf:params:xml:ns:pidf:rpid"
xmlns:caps="urn:ietf:params:xml:ns:pidf:caps"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<tuple id="sg89ae">
<status>
<basic>open</basic>
</status>
<dm:deviceID>mac:8asd7d7d70</dm:deviceID>
<caps:servcaps>
<caps:extensions>
<caps:supported>
<caps:pref/>
</caps:supported>
</caps:extensions>
<caps:methods>
<caps:supported>
<caps:MESSAGE/>
<caps:OPTIONS/>
</caps:supported>
</caps:methods>
</caps:servcaps>
<contact>sip:someone@example.com</contact>
</tuple>
<dm:person id="p1">
<rp:activities>
<rp:on-the-phone/>
</rp:activities>
</dm:person>
<dm:device id="pc122">
<rp:user-input>idle</rp:user-input>
<dm:deviceID>mac:8asd7d7d70</dm:deviceID>
</dm:device>
</presence>
It is worth commenting further on the value of having a separate
device element just to convey the idle indicator. The idle
indication of interest is really an indicator that the device is
idle. By making that explicit, the idle indicator can be used by the
presence server to affect the state of other services running on the
same device. For example, let's say there is a VoIP application
running on the same device. This application reports its presence
state separately, but indicates that it runs on the same device.
Since it has indicated that it runs on the same device, the presence
server can use the status of the service to further refine the idle
indicator of the device. Specifically, if the user is using its VoIP
application, the presence server knows that the device is in use,
even if the IM application reports that the device is idle.
Typically, idleness is determined by lack of keyboard or mouse input,
neither of which might be used during a VoIP call.
In a more simplistic case, reporting the idle indicator as part of
the device status allows that indicator to be used for other services
on the same device. Taking, again, the example of the VoIP
application on the same device, if the VoIP application does not
report any device information, and a watcher is not provided
information on the IM service, the presence document sent to the
watcher can include the device status. Because of the usage of the
device IDs and the device information, the presence server can
correlate the device status as reported by the IM application with
the VoIP service, and use them together.
8. Security Considerations
The presence information described by the model defined here is very
sensitive. It is for this reason that privacy filtering plays a key
role in the processing of presence data. Privacy filtering is the
act of applying permissions to a presence document for the purposes
of removing information that a watcher is not authorized to see. In
more general terms, privacy filtering is a form of authorization.
Privacy filtering can also ensure that a watcher cannot see any
presence data for a presentity, and indeed, it can even ensure that
the presentity doesn't know that it is being blocked. The SIP
presence specifications (RFC 3856 [21]) require that such
authorization processing be performed before divulging presence
information. Specifications have also been defined for conveying
authorization policies to presence servers [26].
Integrity of presence information is also critical. Modification of
presence data by an attacker can lead to diverted communications, for
example. Protocols used to transport presence data, such as SIP for
presence, are used to provide necessary integrity functions.
9. Internationalization Considerations
This specification defines a data model that contains mostly tokens
that are meant for consumption by programs, not directly by humans.
Programs are expected to translate those tokens into language-
appropriate text strings according to the preferences of the watcher.
However, this specification defines a <note> element that can contain
free text. This element and other ones defined by extensions to PIDF
that can contain free text SHOULD be labeled with the 'xml:lang'
attribute to indicate their language and script. This specification
allows multiple occurrences of the <note> element so that the
presentity can convey the note in multiple scripts and languages. If
no 'xml:lang' attribute is provided, the default value is "i-default"
[8].
Since the presence model is represented in XML, it provides native
support for encoding information using the Unicode character set and
its more compact representations including UTF-8. Conformant XML
processors recognize both UTF-8 and UTF-16. Though XML includes
provisions to identify and use other character encodings through use
of an "encoding" attribute in an <?xml?> declaration, use of UTF-8 is
RECOMMENDED in environments where parser encoding support
incompatibility exists.
10. IANA Considerations
There are several IANA considerations associated with this
specification.
10.1. URN Sub-Namespace Registration
This section registers a new XML namespace, per the guidelines in [4]
URI: The URI for this namespace is
urn:ietf:params:xml:ns:pidf:data-model.
Registrant Contact: IETF, SIMPLE working group, (simple@ietf.org),
Jonathan Rosenberg (jdrosen@jdrosen.net).
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>A Data Model for Presence</title>
</head>
<body>
<h1>Namespace for Presence Data Model</h1>
<h2>urn:ietf:params:xml:ns:pidf:data-model</h2>
<p>See <a href="http://www.rfc-editor.org/rfc/rfc4479.txt">
RFC4479</a>.</p>
</body>
</html>
END
10.2. XML Schema Registrations
This section registers two XML schemas per the procedures in [4].
10.2.1. Common Schema
URI: urn:ietf:params:xml:schema:pidf:common-schema.
Registrant Contact: IETF, SIMPLE working group, (simple@ietf.org),
Jonathan Rosenberg (jdrosen@jdrosen.net).
The XML for this schema can be found as the sole content of
Section 5.1.1.
10.2.2. Data Model
URI: urn:ietf:params:xml:schema:pidf:data-model.
Registrant Contact: IETF, SIMPLE working group, (simple@ietf.org),
Jonathan Rosenberg (jdrosen@jdrosen.net).
The XML for this schema can be found as the sole content of
Section 5.1.2.
11. Acknowledgements
This document is really a distillation of many ideas discussed over a
long period of time. These ideas were contributed by many
participants in the SIMPLE working group. 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. Example presence documents came
from Robert Sparks' example presence documents specification, and
ideas on defining services through characteristics, rather than
enumeration, came from Adam Roach's service features document. A
special thanks to Steve Donovan for discussions on the topics
discussed here, and to Elwyn Davies for his final review of the
document.
12. References
12.1. Normative References
[1] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and
J. Peterson, "Presence Information Data Format (PIDF)", RFC
3863, August 2004.
[2] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)", RFC
3841, August 2004.
[3] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[4] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, January
2004.
[5] Yergeau, F., Paoli, J., Sperberg-McQueen, C., Bray, T., and E.
Maler, "Extensible Markup Language (XML) 1.0 (Third Edition)",
W3C REC REC-xml-20040204, February 2004.
[6] Maloney, M., Beech, D., Thompson, H., and N. Mendelsohn, "XML
Schema Part 1: Structures Second Edition", W3C REC REC-
xmlschema-1-20041028, October 2004.
[7] Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes Second
Edition", W3C REC REC-xmlschema-2-20041028, October 2004.
[8] Alvestrand, H., "IETF Policy on Character Sets and Languages",
BCP 18, RFC 2277, January 1998.
[9] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
12.2. Informative References
[10] Day, M., Rosenberg, J., and H. Sugano, "A Model for Presence
and Instant Messaging", RFC 2778, February 2000.
[11] 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.
[12] Peterson, J., "Common Profile for Presence (CPP)", RFC 3859,
August 2004.
[13] Saint-Andre, P., "Internationalized Resource Identifiers (IRIs)
and Uniform Resource Identifiers (URIs) for the Extensible
Messaging and Presence Protocol (XMPP)", Work in Progress,
December 2005.
[14] Wilde, E. and A. Vaha-Sipila, "URI Scheme for GSM Short Message
Service", Work in Progress, February 2006.
[15] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966,
December 2004.
[16] Klyne, G., "A Syntax for Describing Media Feature Sets", RFC
2533, March 1999.
[17] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
User Agent Capabilities in the Session Initiation Protocol
(SIP)", RFC 3840, August 2004.
[18] Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource
Identifiers (URI) Dynamic Delegation Discovery System (DDDS)
Application (ENUM)", RFC 3761, April 2004.
[19] Rosenberg, J., "The Session Initiation Protocol (SIP) and
Spam", Work in Progress, March 2006.
[20] Leach, P., Mealling, M., and R. Salz, "A Universally Unique
IDentifier (UUID) URN Namespace", RFC 4122, July 2005.
[21] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
[22] Rosenberg, J., "Obtaining and Using Globally Routable User
Agent (UA) URIs (GRUU) in the Session Initiation Protocol
(SIP)", Work in Progress, October 2005.
[23] Schulzrinne, H., "RPID: Rich Presence Extensions to the
Presence Information Data Format (PIDF)", RFC 4480, July 2006.
[24] Lonnfors, M. and K. Kiss, "Session Initiation Protocol (SIP)
User Agent Capability Extension to Presence Information Data
Format (PIDF)", Work in Progress, January 2006.
[25] Peterson, J., "A Presence-based GEOPRIV Location Object
Format", RFC 4119, December 2005.
[26] Rosenberg, J., "Presence Authorization Rules", Work in
Progress, March 2006.
[27] Jennings C. and R. Mahy, "Managing Client Initiated Connections
in the Session Initiation Protocol (SIP)", Work in Progress,
March 2006.
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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ISO Plain Language
On this page
PROBLEMPresence documents said how to format status information but not what each part of a document describes, so real communications systems had no consistent way to map themselves into one.
MECHANISMThe RFC defines a data model with three components, the person, the service and the device, gives each a set of properties, and maps them onto the PIDF XML format with new person and device elements.
WHY IT MATTERSA presence document now states which services are on offer, how to reach each one and which device it runs on, and a reader can combine documents from several sources.
§1Why a Presence Data Model
Presence is the ability and willingness of a user to communicate across a set of devices. RFC 2778 defines a model and the terminology for presence systems, and RFC 3863 defines the XML document format used to carry presence information. Both model presence information as a series of tuples, where each tuple holds a status, a communications address and other markup. Neither says what a tuple is meant to describe, nor how a real communications system maps into a presence document.
RFC 4479 answers both questions. It defines a presence data model with three data components, the person, the service and the device, and it shows how to map a communications system into a presence document using that model. The model is defined as an extension to the PIDF document format of RFC 3863, so documents that follow it stay compatible with implementations built before the model was complete.
The three components answer three different questions. The person component carries the willingness and the desire to communicate, such as being in a meeting or out to lunch. The service component carries the ability to communicate, because the services are what a user is reachable through. The device component carries the physical platform, which lets a reader correlate services that run on the same hardware. Geographic location illustrates the split: a location is a fact about a person and about a device, but services have no location of their own, so location belongs in the person or device component and never in the service component.
Presence tells others whether you are willing to talk and how to reach you. Earlier rules described the file format but left unclear what the parts of it were for. This RFC names three parts: the person, the services on offer and the devices those services run on. Each part answers a different question: are you willing, how do I reach you, and where does it run.
§2The Vocabulary of the Model
Beyond the terms of RFC 2778 and RFC 3863, the document defines the words it uses for the parts of a presence document.
- Device: a physical component a user interacts with to make or receive communications, such as a phone, a PDA or a PC.
- Service: a form of communication used to interact with the user, such as instant messaging or telephony.
- Person: the end user, described by states such as busy or sad that affect the ability and willingness to communicate.
- Occurrence: a single description of one service, one device or one person. There may be several occurrences of the same thing in one document.
- Presentity: the combination of devices, services and person information that gives a complete picture of a user's presence on the network.
- Presentity URI: a URI that uniquely identifies a presentity and provides a handle for obtaining presence information about it.
- Data component: one of the device, service or person parts of a presence document.
- Status: information about a service, person or device that typically changes over time. Characteristics: information about the same things that is usually fixed over time and descriptive in nature, such as the size of a display.
- Attribute: a status or a characteristic, in other words a single piece of presence information. A presence attribute means the same thing.
- Composition: the act of combining presence and event data about a presentity into a coherent picture of its state.
The document names the parts of a presence document and says which of them change over time. The word to learn is presentity, the thing whose presence is being described, and data component, one of the three parts of the document: person, service or device.
§3Person, Service and Device
Figure 1 draws the model. A presentity, identified by a presentity URI, holds three kinds of data component: the person, the service and the device. Each component carries attributes, and every attribute belongs to the component it describes rather than to the component that reported it. If a cell phone reports that the user is in a meeting, the status in-a-meeting belongs to the person, because it is a fact about the human and not about the handset. The document may also carry information about the cell phone as a device.
The person component models the user, and there is only one per presentity, so it is a facade for a single human being. A customer support help desk can be modelled this way, treated as one person who is busy when no member of staff is available. Person characteristics are the static facts, such as an alias, which is a URI that identifies the same user under a different presentity URI. Person status is the dynamic part: what the user is doing, where the user is, how the user feels. The document notes that the line between static and dynamic information is fuzzy, and the model does not draw it in a way a program could rely on.
Ambiguity is modelled rather than hidden. When several sources describe the same service, device or person, each component can carry several occurrences, and each occurrence has its own occurrence identifier, unique across the services, devices and person elements of that document. The identifier is meant to persist over time, because it is used to set composition and authorization policies. Notes and timestamps help a reader resolve the ambiguity: the timestamp gives the time of the most recent change, and the component that changed most recently can be treated as the more reliable source. The document does not require any particular resolution algorithm.
The presentity URI identifies the whole thing. A presentity may have more than one, for example a URI from the pres scheme, a SIP URI or an XMPP IRI, or several aliases in one domain that are all equivalent identifiers. When a document is built, the presentity URI is ideally the identifier that was used to request the document, since the requester may not know about the other identifiers. The URI is independent of the services and devices the presentity has, and it is not only a name: it represents a resource that can be subscribed to, which is what makes the presentity URI a form of one-number communications.
Each part of a presence document is about one thing: the person, one service or one device. Information goes with the thing it describes, not with the thing that happened to report it. When two sources disagree about the same service or device, the document can hold several versions of it, each with its own identifier, and leaves the choice to the reader.
- Presentity (URI) identifies the user's presence
- Person the human user
- Service telephony, instant messaging, push-to-talk
- Device cell phone
- Service instant messaging
- Service push-to-talk
- Device PC
- Service telephony, instant messaging, push-to-talk
- Person the human user
§3.3What a Service Carries
The document does not fix a list of service types, because a service is hard to define precisely. One definition fails when a service is spread across several programs or devices, and another fails when several programs share one address. Instead, anything may be treated as a service as long as it carries the properties the model requires: characteristics that identify its nature and capabilities, reach information that says how to contact it, status that gives its current state, and relative information that relates it to the other services of the presentity. Because of this, there is no central registry of services and no single attribute with values such as telephony.
Characteristics describe what a watcher gets when the service is used. Their purpose is to help the watcher choose, because listing several services in one document is an explicit offer of a choice. Capabilities, such as the media that might be used or the direction of communication allowed, are part of the characteristics. One important characteristic is the list of devices on which the service runs, identified by device ID, which lets a reader tell which services run on the same hardware even when the document carries no device information of its own. A document may also list a single service only, which means the presentity is not offering a choice.
Relative information is the priority the user gives each service. The priorities are relative to each other and have no meaning as absolute numbers, so two services with priorities of 1 and .5 are the same as two services with priorities of .2 and .1. The priority says: if you cannot decide which to use, use these in this order.
Status is the dynamic part of a service. The simplest form is a basic status with two values, closed and open. Closed means a communication attempt will probably fail, will not reach the intended party, or will not produce the communications the characteristics describe. A call forwarded to voicemail when the user is busy is closed, and so is a hotel room telephone number that still rings after checkout, because the call reaches the next guest. Open means the opposite, that the attempt will probably succeed and reach the desired target. Status can also depend on the session that is set up, so a softphone service can be available for instant messaging and unavailable for audio. Only dynamic state that helps a watcher decide whether and when to contact the service belongs in the document: whether a cell phone has strong or merely good signal does not pass that test.
A service is anything the user can be reached through, such as a phone line or an instant messaging account. The document does not list service types. Each service instead has to say what it is, how to reach it, how it is doing, and how the user prefers it compared with the other services. Its status is simply open or closed.
| Aspect | Characteristic | Status |
|---|---|---|
| What it holds | The nature and capabilities of a service | The current availability of a service |
| How it changes | Usually fixed over time | Changes over time |
| Example | A voicemail server | Open or closed |
§3.3.2Reach Information and the Service URI
Reach information tells the reader how to contact a service. For a service reachable over a network it includes a service URI that can be used to reach the service. A URI alone is often enough, but not always. Two applications in one mobile phone may share one SIP address of record, and the application that answers depends on the type of body in the request. A service may also hold application logic that works only when contacted from matching software. When more than the URI is needed, the service carries extra attributes, and these must be understood for the service to be used at all. A watcher that receives reach information it does not understand should discard the service information.
Reach information identifies the service, so each service in a document needs a distinct set of it, otherwise the reader would have no way to choose between them. This is also the test for whether something counts as one service or several: something cannot be a service unless there is a way to reach it separately from another service. A softphone that handles both audio and video is one service, because a video-only version could not be reached on its own. A service may still be listed with no reach information at all, which says the service exists but is not on offer, for example a telephone number given only so watchers know the user is busy.
The reach information is a weak contract, not a guarantee. The document may model a group of real services, so a connection with all the described characteristics may not be possible, and the preferences of the presentity always come first, so a caller asking for the video service may be connected to another one. The same service may be reachable through many URIs, and different watchers may be given different URIs, or the URI for one watcher may change over time. The document notes that this lets the recipient of a communications attempt work out the context in which the attempt was made, which can be used as a technique against unwanted communications.
Reach information is how to contact one service, usually a single address such as a SIP URI. When the address alone is not enough, the service carries extra instructions that the reader has to understand, and a reader that does not understand them should drop that service. The address is a promise, not a guarantee, because the user can still prefer to connect the caller somewhere else.
§3.4Devices and Device Identifiers
A device models the physical environment in which services run: a cell phone, a PC, a laptop, a consumer telephone, a PBX extension or a dispatch console. The mapping between services and devices is many to many. One service can run on several devices, as when two SIP phones register against a single address of record, and one device can carry several services, as a cell phone carrying telephony, SMS and MMS. A device may carry no service at all, which is how an RFID tag can be described: it has properties such as location, and a compositor that finds a video telephone tagged with the same RFID can use the tag's location to say something about where the video telephone is. The device is a model of the system, so a PC running services inside a virtual machine can be described as two devices.
Each device has a device ID, a URI that is a globally and temporally unique identifier, and in this model a URN. The identifier should also be readily computable, so that different systems are likely to refer to the same device with the same ID, because that is what allows information about a device from several sources to be combined. No single URN scheme suits every device, so more than one is expected. The Electronic Serial Number works for cellular telephones and the MAC address for IP devices, but the MAC address changes if the interface card changes, and neither is yet tied to a URN scheme.
In the meantime a UUID URN is used. For devices with a MAC address, version 1 UUIDs are recommended, since they are time based and make use of the MAC address; for devices without one, version 4 UUIDs are recommended, since they are purely random and give uniqueness. The UUID would normally be chosen at manufacture and stored in flash or other non-volatile memory. A UUID has the properties of being globally and temporally unique, but because of its random component it cannot be readily computed, so it cannot serve as a correlation identifier with other presence sources on the network. The document expects later specifications to provide better device identifiers.
Device characteristics describe the physical device, such as its display size, processor speed and memory. Device status is the dynamic part, such as whether the device is powered on, how much battery power remains and where the device is. These are reported so the user can choose a service, and they cannot be used to infer the nature of the service: a fast processor does not mean the video quality on that device will be good, because other constraints may reduce the processor available to that service. Likewise, the state of a device does not by itself set the state of a service, unless the consumer of the document knows otherwise.
A device is the box the services run on: a phone, a PC, a console. Each device gets a device ID, which is a URN, and the ID should be easy for different programs to calculate the same way, so information from several sources can be joined. The document suggests a UUID URN for now and notes that it cannot be calculated, so joining needs another method.
§5Mapping the Model onto PIDF
Presence data is carried in the PIDF format of RFC 3863. The presence element holds the information for the presentity, and its entity attribute holds the presentity URI. The existing tuple element represents a service, which keeps the mapping compatible with implementations built before this model existed, and the contact element inside the tuple holds the service URI. New presence attributes appear directly as children of tuple. Attributes defined before this document, such as basic, stay as children of status, so a service attribute can appear under either element but never under both. The id attribute of the tuple carries the service occurrence.
The document introduces a person element and a device element, both of which may appear zero or more times as children of presence, each with a mandatory id attribute holding its occurrence identifier. The person element carries status and characteristic elements, then optional note elements and an optional timestamp. The device element carries its own elements, then a deviceID element holding the URN for the device, then notes and a timestamp. Notes that are direct children of presence apply to every person occurrence that has no note of its own. A client that does not understand the new elements ignores them, and because the meaning of a service matches the meaning of a tuple in RFC 2778 and RFC 3863, documents using this model stay compliant with older implementations. The two schemas are split into a common schema, common-schema.xsd, and the data model, data-model.xsd.
Two rules about attributes matter. A missing attribute says nothing at all, following the same idea as the capability declarations of RFC 3840: the watcher cannot conclude anything about its value, whether it was withheld or is unsupported, so a presence document is at its best when it carries as many attributes as the presentity is willing and able to provide. And the line between status and characteristics is conceptual only. Nothing in a document marks an attribute as one or the other, and any presence attribute may change at any time, but all future presence attributes must be defined as children of tuple rather than of status, and the schemas here define no status element for the person and device elements.
Extensions add presence attributes, and the document sets out three questions an extension has to answer: which data components the new attribute applies to and what it means in each, whether it belongs in a new namespace or an existing one, and whether the extension itself needs points of extension, which must then be marked in the schema with the any construct. The worked example in Section 7 models an instant messaging client, where the note and the pre-defined status messages belong to the person, the service tuple publishes the client's address of record and carries no characteristics, and the idle indicator belongs to the device because idleness describes use of the device rather than of the service.
The model is written into the XML format defined earlier for presence documents. A service becomes a tuple element, a person and a device each get their own element with an identifier, and the device carries its identifier in a deviceID element. A missing attribute means nothing at all, so a document should carry as much as it can.
Presence document for an instant messaging client
<?xml version="1.0" encoding="UTF-8"?>
<presence xmlns="urn:ietf:params:xml:ns:pidf"
xmlns:dm="urn:ietf:params:xml:ns:pidf:data-model"
xmlns:rp="urn:ietf:params:xml:ns:pidf:rpid"
xmlns:caps="urn:ietf:params:xml:ns:pidf:caps">
<tuple id="sg89ae">
<status>
<basic>open</basic>
</status>
<dm:deviceID>mac:8asd7d7d70</dm:deviceID>
<contact>sip:someone@example.com</contact>
</tuple>
<dm:person id="p1">
<rp:activities>
<rp:on-the-phone/>
</rp:activities>
</dm:person>
<dm:device id="pc122">
<rp:user-input>idle</rp:user-input>
<dm:deviceID>mac:8asd7d7d70</dm:deviceID>
</dm:device>
</presence>§8Security and Privacy
The presence information described by this model is very sensitive, and privacy filtering plays a key role in processing it. Privacy filtering is the act of applying permissions to a presence document in order to remove information a watcher is not authorized to see, and in more general terms it is a form of authorization. It can also ensure that a watcher sees no presence data at all for a presentity, and it can even ensure that the presentity does not know it is being blocked. The SIP presence specifications of RFC 3856 require this authorization processing to be performed before presence information is divulged, and separate specifications define how authorization policies are conveyed to presence servers.
Integrity of presence information is equally critical. An attacker who modifies presence data can divert communications, for example by changing the address a service is reachable at. The protocols used to transport presence data, such as SIP for presence, are relied on to provide the necessary integrity functions.
The document also has internationalization considerations. The data model mostly contains tokens meant for programs rather than for people, and programs are expected to translate those tokens into text in the language the watcher prefers. The note element can hold free text, so it and other free-text elements should carry the xml:lang attribute, and the document allows several note elements so the same note can be given in several languages and scripts. Without that attribute the default value is i-default. Because the model is XML, it carries the Unicode character set natively, and use of UTF-8 is recommended where parser encoding support is inconsistent.
Presence information says where someone is and how to reach them, so it is treated as sensitive. Before a document is sent, a server filters it, removing the parts the watcher is not allowed to see, and it can even hide the fact that the watcher is blocked. The document also has to arrive unaltered, because a changed address would send calls to the wrong place.
§9References
[1]RFC 3863Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr, W., and J. Peterson, "Presence Information Data Format (PIDF)", RFC 3863, August 2004.August 2004[4]RFC 3688Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, January 2004.January 2004[8]RFC 2277Alvestrand, H., "IETF Policy on Character Sets and Languages", BCP 18, RFC 2277, January 1998.January 1998[10]RFC 2778Day, M., Rosenberg, J., and H. Sugano, "A Model for Presence and Instant Messaging", RFC 2778, February 2000.February 2000[11]RFC 3261Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002.July 2002[15]RFC 3966Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966, December 2004.December 2004[17]RFC 3840Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating User Agent Capabilities in the Session Initiation Protocol (SIP)", RFC 3840, August 2004.August 2004[18]RFC 3761Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource Identifiers (URI) Dynamic Delegation Discovery System (DDDS) Application (ENUM)", RFC 3761, April 2004.April 2004[20]RFC 4122Leach, P., Mealling, M., and R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, July 2005.July 2005[21]RFC 3856Rosenberg, J., "A Presence Event Package for the Session Initiation Protocol (SIP)", RFC 3856, August 2004.August 2004[23]RFC 4480Schulzrinne, H., "RPID: Rich Presence Extensions to the Presence Information Data Format (PIDF)", RFC 4480, July 2006.July 2006[25]RFC 4119Peterson, J., "A Presence-based GEOPRIV Location Object Format", RFC 4119, December 2005.December 2005