Presence Information Data Format

Description

The Presence Information Data Format (PIDF) is an extensible markup language (XML) document format that encapsulates the presence state of a user or entity, known as a presentity. Within the 3GPP IMS architecture, PIDF is the standardized payload carried by SIP (Session Initiation Protocol) messages such as PUBLISH, SUBSCRIBE, and NOTIFY to enable the presence service. A PIDF document contains a mandatory root <presence> element, which includes one or more <tuple> elements. Each <tuple> represents a specific communication service or address (like a SIP URI) and its status, described by a <status> element containing sub-elements like <basic> (open/closed).

How the PIDF works in the IMS presence service involves several entities. The Presentity's User Equipment (UE) acts as a Presence User Agent (PUA), gathering local presence information (e.g., device status, user activity). It aggregates this into a PIDF document and sends it via a SIP PUBLISH request to the Presence Server (PS). The PS, which is the central repository, stores and manages the PIDF for each presentity. When a Watcher (another user subscribed to the presentity's presence) wants updates, it sends a SIP SUBSCRIBE. The PS then generates notifications (SIP NOTIFY) containing the current PIDF document of the presentity to all authorized watchers.

The PIDF is designed for rich expression. Beyond the basic status, it can include optional elements such as <contact> priority, <note> for free-text human-readable messages, and <timestamp> indicating when the presence information was generated. 3GPP and IETF extensions, like Presence Data Model (PDM) and Rich Presence Information Data (RPID), define additional XML elements that can be embedded within PIDF to convey more detailed context such as user activity (meeting, traveling), mood, or geographic location. This extensibility makes PIDF a powerful and future-proof foundation for presence-enabled communication services.

Purpose & Motivation

PIDF was created to solve the problem of fragmented, non-interoperable presence information in early internet communication services. Before its standardization, instant messaging and presence systems (like those from different vendors) used proprietary data formats, creating walled gardens. The IETF's development of PIDF, primarily through RFC 3863, aimed to establish a common, extensible, and universal format for exchanging presence data, which is a prerequisite for ubiquitous, cross-network presence services.

3GPP adopted PIDF as the cornerstone of its IMS Presence Service to enable rich, network-based presence features for mobile users. The motivation was to leverage the convergence of cellular and IP technologies to offer advanced communication services beyond voice. A standardized format was necessary to ensure that presence information generated by a device from one manufacturer could be correctly interpreted and displayed by a device or application from another, and properly processed by network servers. It addressed the limitations of previous cellular services, which lacked a standardized, service-agnostic mechanism for real-time availability sharing.

Thus, PIDF exists to enable interoperability and rich service creation. It provides a well-defined semantic for the core concept of 'presence,' allowing networks, devices, and applications to share a common understanding of a user's state. This facilitates innovative services like enhanced address books with live statuses, context-aware call routing, and integrated messaging experiences. By being XML-based and extensible, it also future-proofed the standard, allowing new attributes and capabilities to be added without breaking existing implementations, thereby supporting the long-term evolution of communication services.