Incoming initial request

Description

An Incoming initial request is a Session Initiation Protocol (SIP) request received by a user's equipment or application server that is intended to initiate either a dialog or a standalone transaction, and is terminated by the served user. In the context of 3GPP IP Multimedia Subsystem (IMS), this term is used to distinguish requests that begin a new communication session or transaction from those that are part of an existing dialog (such as re-INVITE or UPDATE). The served user is the subscriber whose service profile and preferences are applied to handle the request. This concept is critical for service invocation, as it triggers initial filter criteria (iFC) evaluation in the Serving Call Session Control Function (S-CSCF), which determines which application servers should be involved in processing the request.

The architecture for handling incoming initial requests spans several IMS entities. When a SIP request arrives at the Proxy-CSCF (P-CSCF) from the access network, it is forwarded to the S-CSCF based on the user's registration state. The S-CSCF then evaluates the initial filter criteria, which may route the request to one or more application servers (AS) before ultimately delivering it to the terminating user equipment (UE). The request may be an INVITE for a voice or video call, a MESSAGE for instant messaging, or a SUBSCRIBE for presence information. Each of these is considered an initial request because it starts a new dialog or transaction. The handling of these requests is governed by 3GPP specifications such as TS 24.229 (IMS SIP signalling) and the specific service-related specifications listed (e.g., 24.315 for IMS messaging, 24.407 for IMS conferencing).

Key components involved in processing an incoming initial request include the P-CSCF, I-CSCF, S-CSCF, and application servers. The P-CSCF acts as the entry point and may perform policy control and charging (PCC) interactions. The I-CSCF queries the Home Subscriber Server (HSS) to select the appropriate S-CSCF. The S-CSCF authenticates and authorizes the request, applies service triggers, and routes it accordingly. Application servers may modify the request, fork it, or reject it based on service logic. The terminating UE receives the request and responds, completing the dialog establishment. Throughout this process, the concept of an initial request ensures that proper session control and service delivery mechanisms are applied only at the beginning of a communication, avoiding unnecessary re-evaluation for in-dialog requests.

The importance of correctly identifying and handling incoming initial requests lies in enabling a wide range of IMS services, including voice over LTE (VoLTE), video calling, instant messaging, presence, and conferencing. It allows network operators to apply differentiated treatment based on service type, user subscription, and operator policy. For example, an incoming initial INVITE may trigger call forwarding, barring, or simultaneous ringing services. Without a clear definition, service logic could be applied inconsistently, leading to incorrect charging, failed service invocation, or poor user experience. Therefore, 3GPP has standardized the term to ensure interoperability across vendors and networks.

In summary, an Incoming initial request is a foundational concept in IMS that defines the starting point for session or transaction establishment. It enables the IMS core network to apply service logic, enforce policies, and route requests appropriately. Its definition has remained stable across releases, reflecting its fundamental role in SIP-based communications.

Purpose & Motivation

The concept of an Incoming initial request exists to clearly delineate the start of a new communication session or transaction in IMS. It solves the problem of how to trigger service logic, such as call forwarding or conferencing, only when a new session begins, rather than for every SIP message. This distinction is essential for efficient network operation and correct service execution. Before its formal definition, ambiguity could lead to inconsistent service invocation or charging errors. By standardizing the term, 3GPP ensures that all network elements and application servers interpret the beginning of a session uniformly, enabling seamless interoperability and predictable user experiences. It also provides a basis for security and policy enforcement at the session initiation stage.