Push Temporary Identifier

Description

The P-TID is a security and privacy identifier defined within the context of the OMA Push-to-talk over Cellular (PoC) service, which is standardized by 3GPP for integration with IMS. It is a temporary alias for the PoC client, generated and managed by the PoC server. When a user subscribes to or activates the PoC service, the PoC server assigns a P-TID to that user's client. This identifier is then used within PoC session signaling (e.g., in SIP messages) and potentially in media plane control packets instead of the user's permanent public identity, such as a SIP URI or MSISDN. The generation of the P-TID should follow principles that prevent tracking of the user across different sessions or correlating it to the permanent identity by unauthorized parties. During a PoC session setup, the calling PoC client includes its P-TID in the invitation, and the PoC server uses it to identify the target user, often mapping it internally to the user's permanent subscription data. The use of P-TID enhances privacy on the Gm reference point (between UE and P-CSCF) and within the PoC service layer itself. It is particularly important for group communications where session control packets might be observed by multiple participants. The management of P-TID, including its lifetime, renewal, and revocation, is handled by the PoC server, and its format is specified by the PoC service provider. The identifier plays a role in lawful interception and charging correlation, where the network operator and PoC service provider can map the P-TID back to the real user identity in their secure domains.

Purpose & Motivation

The P-TID was created to address privacy concerns inherent in push-to-talk group communication services. In a service like PoC, where session control messages (e.g., "user X is talking") are broadcast to all group members, using a permanent user identity like a phone number or SIP URI would unnecessarily expose this sensitive information to all participants and potentially to eavesdroppers on the air interface. The P-TID provides a layer of anonymity, making it difficult for other PoC users to directly identify the speaker's permanent identity. This solves the problem of unwanted user tracking and profiling within the service context. Furthermore, it aligns with regulatory requirements for user privacy in telecommunications. Its introduction was motivated by the specific architecture of PoC, which separates the service layer (PoC server) from the underlying IMS core, allowing for service-specific temporary identifiers. It addresses the limitation of using IMS Private User Identities (IMPI) or Public User Identities (IMPU) directly in service-level signaling, which are more permanent and potentially sensitive.