Network Working Group J. Peterson Request for Comments: 4079 NeuStar Category: Informational July 2005
A Presence Architecture for the Distribution of GEOPRIV Location Objects
Status of This Memo
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2005).
GEOPRIV defines the concept of a 'using protocol' -- a protocol that carries GEOPRIV location objects. GEOPRIV also defines various scenarios for the distribution of location objects that require the concepts of subscriptions and asynchronous notifications. This document examines some existing IETF work on the concept of presence, shows how presence architectures map onto GEOPRIV architectures, and moreover demonstrates that tools already developed for presence could be reused to simplify the standardization and implementation of GEOPRIV.
GEOPRIV is a standard for the transmission of location information and privacy policies over the Internet. Location information is a description of a particular spatial location, which may be represented as coordinates (via longitude, latitude, and so on), as civil addresses (such as postal addresses), or in other ways. GEOPRIV focuses on the privacy and security issues, from both a technology perspective and a policy perspective, of sharing location information over the Internet; it essentially defines a secure container class capable of carrying both location information and policy data governing the distribution of this information. GEOPRIV also defines the concept of a 'using protocol' -- a protocol that carries the GEOPRIV location object.
Presence is a service defined in RFC2778  that allows users of a communications service to monitor one another's availability and disposition in order to make decisions about communicating. Presence information is highly dynamic, and it generally characterizes whether a user is online or offline, busy or idle, away from communications devices or nearby, and the like.
This document shows the applicability of presence to GEOPRIV and shows that a presence protocol could be a suitable using protocol for GEOPRIV. This document is not intended to demonstrate that presence is the only method by which GEOPRIV location objects might be distributed. However, there are numerous applications of GEOPRIV that depend on the fundamental subscription/notification architecture that also underlies presence.
The GEOPRIV framework  defines four primary network entities: a Location Generator, a Location Server, a Location Recipient, and a Rule Holder. Three interfaces between these entities are defined, including a publication interface and a notification interface.
GEOPRIV specifies that a 'using protocol' is employed to transport location objects from one place to another. If the publication interface and notification interface are network connections, then a using protocol would be responsible for the transmission of the location object. Location Recipients may request that a Location Server provide them with GEOPRIV location information concerning a particular Target. The Location Generator publishes Location Information to a Location Server, which, in coordination with policies set by the Rule Maker, distributes the location information to Location Recipients as necessary.
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The GEOPRIV requirements document shows three scenarios for the use of the GEOPRIV protocol. In some of these scenarios (such as the third), a Location Recipient sends some kind of message to the Location Server to request the periodic transmission of location information. The location of a GEOPRIV Target is likely to vary over time (if the Target is a person, or something similarly mobile), and consequently the concept of a persistent subscription to the location of a Target resulting in periodic notification is valuable to GEOPRIV. In other scenarios, a Location Recipient may request a one- time notification of the geographical location of the Target.
GEOPRIV places few requirements on using protocols. However, it is clear from the description above that there must be some mechanism allowing Location Recipients to establish a persistent subscription in order to receive regular notification of the geographical location of a Target as their location changes over time. There must also be a way for Location Generators to publish location information to a Location Server that applies further policies for distribution.
This document adopts a model in which the using protocol is responsible for requesting subscriptions, handling publications, and sending notifications. There are other models for GEOPRIV in which these operations might be built into location objects themselves. However, there is a significant amount of pre-existing work in the IETF related to managing publications, subscriptions, and notifications for data sets that vary over time. In fact, these concepts all correspond exactly to architectures for presence that have been developed in support of real-time communications applications such as instant messaging, voice and video sessions.
Note that in some GEOPRIV scenarios, the Location Recipient does not actively request the location of a Target; rather, it receives an unsolicited notification of Target's location. This document focuses on the use of presence only for scenarios in which the Location Recipient actively solicits location information. However, it is possible that many of these base operations of the subscription/notification framework of presence could be reused for cases in which the Location Recipient is passive.
The Common Profile for Presence  (CPP) defines a set of operations for delivery of presence information. These primarily consist of subscription operations and notification operations. A subscription creates a persistent connection between a 'watcher' (which corresponds to the Location Recipient of GEOPRIV) and a 'presentity' (which corresponds roughly to the GEOPRIV target). When a watcher subscribes to a presentity, a persistent connection is created;
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notifications of presence information will henceforth be sent to the watcher as the presence information changes. CPP also supports unsubscriptions (terminating the persistent subscription) and fetches (one-time requests for presence information that do not result in a persistent subscription).
CPP provides a number of attributes of these operations that flesh out the presence system. There is a system for automatically expiring subscriptions if they are not refreshed at user-defined intervals (in order to eliminate stale subscriptions). There are transaction and subscription identifiers used to correlate messages, and a URI scheme ("pres:") is defined to identify watchers and presentities.
The IETF IMPP WG has also defined an XML data format for presence information, called the Presence Information Data Format  (PIDF). PIDF is a body that is carried by presence protocols and that contains presence information, including the current state of a presentity. PIDF is discussed in more detail in Section 4.
At a high level, then, the presence architecture seems to have considerable applicability to the problem of delivering GEOPRIV information. However, the CPP framework is an abstract framework: it doesn't actually specify a protocol, instead it specifies a framework and a set of requirements to which presence protocols must conform. Also, CPP does not define any concept similar to a Location Server.
However, the IETF has standardized protocols that instantiate this framework, such as SIMPLE  and XMPP . XMPP and SIMPLE both have architectural elements comparable to a Location Server: points where presentities register their availability, and where policies for distributing presence can be managed. The presence community has also defined a policy protocol and schema set called XCAP  through which authorization policies can be provisioned in a presence server.
In summary, like GEOPRIV, presence requires an architecture for publication, subscription, and notification for a mutable set of data associated with a principal. Presence has already tackled many of the harder issues associated with subscription management, including subscription expiration, development of identifiers for principals, and defining document formats for presence information. Rather than reinvent work that has been done elsewhere in the IETF, GEOPRIV has reused this existing work by specifying presence protocols as GEOPRIV using protocols. Moreover, the existing foundational presence tools developed in IMPP, such as PIDF, have immediate applicability to the efforts underway in GEOPRIV to develop objects for sharing location information.
As was mentioned above, the presence architecture developed in the IETF IMPP WG has defined a format for presence information called PIDF. PIDF is an XML format that provides presence information about a presentity. Primarily, this consists of status information, but it also optionally includes contact addresses (a way of reaching the presentity), timestamps, and textual notes with arbitrary content.
PIDF is an extensible format. It defines an XML element for representing the status of a presentity (the status element), and it gives some guidance as to how this element might be extended. Although the authors of PIDF viewed geographical location as a potential category of presence information, baseline PIDF defines no format for location information.
PIDF meets the security requirements given in RFC2779  (see especially sections 5.1, 5.2, and 5.3), which parallel those of the GEOPRIV location object given in the GEOPRIV requirements . CPP and PIDF specify mechanisms for mutual authentication of participants in a presence exchange as well as for confidentiality and integrity properties for presence information.
In short, many of the requirements of GEOPRIV objects map well onto the capabilities of PIDF.
GEOPRIV information, like presence information, has very sensitive security requirements. The requirements of RFC2779 , which are instantiated by CPP, PIDF, and XCAP, in addition to the various derivative concrete presence protocols, such as XMPP and SIMPLE, map well onto the security requirements of the GEOPRIV protocol, as defined in the GEOPRIV requirements document and the GEOPRIV threat analysis  document. Specifically, the presence security requirements call for authentication of watchers, integrity and confidentiality properties, and similar measures to prevent abuse of presence information.
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