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rfc:rfc4479

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.

Rosenberg Standards Track [Page 1] RFC 4479 Presence Data Model July 2006

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

Rosenberg Standards Track [Page 2] RFC 4479 Presence Data Model July 2006

 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.

Rosenberg Standards Track [Page 3] RFC 4479 Presence Data Model July 2006

 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.

Rosenberg Standards Track [Page 4] RFC 4479 Presence Data Model July 2006

3. The Model

  +----------------------------------------------------------------+
  |                                                                |
  |                       +----------------+                       |
  |                      +----------------+|                       |
  |                      |                ||                       |
  |                      |                ||                       |
  |                      |     Person     ||                       |
  |                      |                ||\                      |
  |                     /|                |+ \                     |
  |                    / +----------------+   \                    |
  |                   /           |            \                   |
  |                  /            |             \                  |
  |                 /             |              \                 |
  |                /              |               \                |
  |               /               |                \               |
  |              V                V                 V              |
  |  +----------------+   +----------------+   +----------------+  |
  | +----------------+|  +----------------+|  +----------------+|  |
  | |                ||  |                ||  |                ||  |
  | |                ||  |                ||  |                ||  |
  | |    Service     ||  |    Service     ||  |    Service     ||  |
  | |                ||  |                ||  |                ||  |
  | |                |+  |                |+  |                |+  |
  | +----------------+   +----------------+   +----------------+   |
  |             \              /       \                           |
  |              \            /         \                          |
  |               \          /           \                         |
  |                V        V             V                        |
  |          +----------------+        +----------------+          |
  |         +----------------+|       +----------------+|          |
  |         |                ||       |                ||          |
  |         |                ||       |                ||          |
  |         |    Device      ||       |    Device      ||          |
  |         |                ||       |                ||          |
  |         |                |+       |                |+          |
  |         +----------------+        +----------------+           |
  |                                                                |
  |                                                                |
  | Presentity (URI)                                               |
  +----------------------------------------------------------------+
                               Figure 1

Rosenberg Standards Track [Page 5] RFC 4479 Presence Data Model July 2006

 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.

Rosenberg Standards Track [Page 6] RFC 4479 Presence Data Model July 2006

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.

Rosenberg Standards Track [Page 7] RFC 4479 Presence Data Model July 2006

 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,

Rosenberg Standards Track [Page 8] RFC 4479 Presence Data Model July 2006

 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.

Rosenberg Standards Track [Page 9] RFC 4479 Presence Data Model July 2006

 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

Rosenberg Standards Track [Page 10] RFC 4479 Presence Data Model July 2006

 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.

Rosenberg Standards Track [Page 11] RFC 4479 Presence Data Model July 2006

 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

Rosenberg Standards Track [Page 12] RFC 4479 Presence Data Model July 2006

 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

Rosenberg Standards Track [Page 13] RFC 4479 Presence Data Model July 2006

 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.

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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

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 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.

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 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.

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 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.

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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.

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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

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 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,

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 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.

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 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.

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 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>

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  </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>

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       <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].

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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>

Rosenberg Standards Track [Page 27] RFC 4479 Presence Data Model July 2006

    <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

Rosenberg Standards Track [Page 28] RFC 4479 Presence Data Model July 2006

 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

Rosenberg Standards Track [Page 29] RFC 4479 Presence Data Model July 2006

 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

Rosenberg Standards Track [Page 30] RFC 4479 Presence Data Model July 2006

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.

Rosenberg Standards Track [Page 31] RFC 4479 Presence Data Model July 2006

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.

Rosenberg Standards Track [Page 32] RFC 4479 Presence Data Model July 2006

 [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.

Rosenberg Standards Track [Page 33] RFC 4479 Presence Data Model July 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

Rosenberg Standards Track [Page 34] RFC 4479 Presence Data Model July 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
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 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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Acknowledgement

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 Administrative Support Activity (IASA).

Rosenberg Standards Track [Page 35]

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