Description
The Interrogating Call Session Control Function (I-CSCF) is a SIP (Session Initiation Protocol) proxy server defined within the 3GPP IMS architecture. Its primary operational domain is at the border of an IMS operator's network. When a SIP request, such as an INVITE for a voice or video call, arrives from an external network (like another IMS network or a circuit-switched network via a Media Gateway Control Function), it is first received by the I-CSCF. The I-CSCF's first major task is to query the Home Subscriber Server (HSS) using the Diameter Cx interface. It provides the user's public identity (e.g., SIP URI) to the HSS, which responds with the required capabilities for the user's Serving-CSCF (S-CSCF) or, if the user is already registered, may directly provide the address of the assigned S-CSCF.
Based on the information from the HSS, the I-CSCF selects an appropriate S-CSCF for the user if one is not already assigned. This selection is based on the S-CSCF capabilities required by the user's service profile. The I-CSCF then forwards the SIP request to the chosen S-CSCF. A crucial security and topology-hiding function of the I-CSCF is network hiding. It can act as a Topology Hiding Inter-network Gateway (THIG), removing internal network topology information (like the addresses of specific S-CSCFs) from SIP messages before they leave the home network, thereby enhancing security.
In the reverse direction, for sessions originating within its home network and destined for an external network, the I-CSCF may also be the exit point. It routes the outgoing SIP request towards the destination network based on DNS (Domain Name System) queries for the target domain. The I-CSCF is stateless in the sense that it does not maintain session state for the duration of a call, but it is involved in the initial routing decision. Its presence is mandatory in multi-operator environments and is essential for enabling roaming, inter-operator connectivity, and maintaining a secure and scalable IMS core network architecture.
Purpose & Motivation
The I-CSCF was introduced to solve critical routing, scalability, and security challenges in the transition to all-IP service delivery via the IMS. In a pure SIP network without such a functional element, external entities would need to know the direct address of a user's serving proxy (S-CSCF), exposing internal network architecture and creating a single point of failure and a scaling bottleneck. The I-CSCF abstracts this internal detail.
Its creation was motivated by the need for a standardized, secure method to handle incoming sessions from other networks, especially for roaming scenarios. It acts as a designated border control function for SIP signaling. By centralizing the HSS query and S-CSCF selection logic, it allows for load balancing across multiple S-CSCF pools and enables network operators to hide their internal topology from potential threats or competitive analysis, a concept known as topology hiding.
Furthermore, the I-CSCF provides a clear separation of concerns within the IMS core. While the S-CSCF handles session control and service logic for registered users, and the P-CSCF is the user's first contact point, the I-CSCF specializes in inter-network routing and initial interrogation. This modular architecture allows operators to scale, secure, and manage these functions independently, which was essential for the commercial deployment of large-scale, carrier-grade VoIP and multimedia services.
Key Features
- Acts as the first point of contact for incoming SIP sessions from external networks
- Queries the HSS via the Diameter Cx interface to obtain user service profile and S-CSCF assignment information
- Selects an appropriate S-CSCF based on required capabilities for unregistered users
- Forwards SIP requests to the assigned S-CSCF for further session processing
- Provides topology hiding by masking internal S-CSCF addresses from external networks
- Facilitates load balancing across a pool of S-CSCF servers
Evolution Across Releases
Introduced as a core component of the IMS architecture defined in 3GPP Release 5, but Release 8 solidified its role within the EPS (Evolved Packet System). The initial architecture established the I-CSCF with its fundamental functions: interrogating the HSS, selecting the S-CSCF, and acting as a THIG. It was defined to work with the Diameter-based Cx interface for HSS communication.
Defining Specifications
| Specification | Title |
|---|---|
| TS 28.702 | 3GPP TS 28.702 |
| TS 32.632 | 3GPP TR 32.632 |
| TS 32.732 | 3GPP TR 32.732 |