Description
The Interrogating-Call Session Control Function (I-CSCF) is a fundamental SIP proxy within the 3GPP IP Multimedia Subsystem (IMS) core, defined across numerous specifications starting from 3GPP Release 5. It is located at the edge of an operator's home IMS network. When a User Equipment (UE) registers with the IMS network, the registration request, after passing through the Proxy-CSCF (P-CSCF), reaches the I-CSCF. The I-CSCF's key task is to query the Home Subscriber Server (HSS) via the Cx interface (using Diameter protocol) to determine which Serving-CSCF (S-CSCF) should be assigned to the user or to discover the S-CSCF already assigned.
During registration, the I-CSCF receives the subscriber's public or private user identity. It sends a User Authorization Request (UAR) to the HSS. The HSS responds with the capabilities required for the S-CSCF (S-CSCF capabilities) or, if an S-CSCF is already assigned, its address. The I-CSCF then uses this information to either select an appropriate S-CSCF from a set based on the required capabilities or to forward the registration request directly to the already-assigned S-CSCF. This process ensures load balancing and efficient assignment of S-CSCFs.
For incoming session requests (e.g., an incoming VoIP call), the I-CSCF acts as the entry point into the home network from an external network (like another operator's IMS). The request is routed to the I-CSCF based on DNS lookups of the home network's domain. The I-CSCF then queries the HSS with a Location Information Request (LIR) to obtain the address of the S-CSCF currently serving the called party. Once it receives the S-CSCF address, it proxies the SIP INVITE request to that S-CSCF for further processing and delivery to the UE.
Additionally, the I-CSCF can provide topology hiding functionality. By acting as a single point of contact, it can hide the configuration, capacity, and topology of the internal network (the S-CSCFs) from external networks. It may also have a role in enforcing network security policies at the border of the home network. Its operation is stateless with respect to individual sessions, focusing on routing and HSS interaction.
Purpose & Motivation
The I-CSCF was created as part of the original IMS architecture (3GPP R5) to solve critical routing and scalability problems in a large-scale, multi-operator SIP-based service network. In a pure SIP network without such a function, finding the server responsible for a specific user would be challenging and could expose internal network structure.
Its primary purpose is to provide a discoverable, stable entry point into a home operator's IMS network. This allows other networks to route sessions to a user without needing to know the specific internal server (S-CSCF) managing that user's registration. It decouples the user's publicly routable address (the home network domain) from the internal, possibly changing, server assignment. This enables load balancing across multiple S-CSCFs and allows for S-CSCF failure recovery without impacting external routing.
Furthermore, the I-CSCF, through its interaction with the HSS, centralizes the logic for assigning S-CSCFs based on user service profiles and network capabilities. This allows for optimized resource allocation. It also fulfills a vital security and privacy role by hiding the internal network topology from outside entities, preventing potential attackers from mapping the network or targeting specific internal servers directly. It was a key innovation that made IMS a viable architecture for carrier-grade, interoperable multimedia services.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (46 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, no specific new functionality was introduced for the I-CSCF itself based on the provided Change Request titles and grounding context. The listed enhancements, such as those for multi-talker control, MMTel access, and P-CSCF restoration for 5GC, pertain to other network functions and service layer capabilities. The technical details in the grounding context exclusively describe charging, policy control, and bearer management architectures from earlier releases, with no mention of I-CSCF updates.
- Introduction of multi-talker control TS 23.280CR0094
- Functional alias controlling role of MC service server TS 23.280CR0106
- Introduction of the multi-talker control TS 23.379CR0086
- Unified Access Control for MMTel TS 24.173CR0126
- No unified access control check when adding or removing media during MMTEL session TS 24.173CR0130
- P-CSCF restoration for 5GC TS 29.228CR0689
In Release 16, enhancements for the I-CSCF were focused on supporting the restoration procedures for other CSCF elements, specifically the P-CSCF and S-CSCF, to improve network reliability. This included defining mechanisms for S-CSCF restoration after registration timer expiry and incorporating P-CSCF subscription information into restoration data. Furthermore, the release extended these restoration capabilities to 5GS and supported PCRF-based P-CSCF restoration.
- Multicast application level control signal clarification TS 23.280CR0151
- Completing availability of talker location with floor control request TS 23.379CR0149
- Pre-established session corrections TS 23.379CR0168
- MCPTT ID in interconnection floor control TS 23.379CR0174
- Making functional alias optional in floor control messages TS 23.379CR0235
- P-CSCF Restoration TS 29.562CR0021
+ 9 more changes
In Release 17, specific enhancements for the I-CSCF included support for procedures related to a Failed P-CSCF and mechanisms for S-CSCF Restoration Information Retrieval. These updates introduced new capabilities for handling network element failures and restoring session control functions within the IMS architecture. The changes also involved interactions for S-CSCF reselection in eIMS to maintain service continuity.
- Providing activated functional alias information to the group controlling server TS 23.280CR0207
- Request for network resources at session establishment from the MC service server TS 23.280CR0278
- Support for signed attestation for emergency and priority IMS sessions TS 29.165CR1029
- Failed P-CSCF TS 29.228CR0697
- Failed P-CSCF TS 29.229CR0298
- S-CSCF Restoration Information Retrieval TS 29.562CR0084
+ 1 more changes
In Release 18, the I-CSCF's functionality was enhanced to support floor control signalling over specific QoS Class Identifiers, namely QCI 69 and QCI 65. Furthermore, the procedures for the floor participant and floor control server were formally aligned within the specification text of section 10.9.1.3. These updates integrate floor control mechanisms more concretely into the IMS architecture's service-based policy and charging framework.
In Release 19, updates to the I-CSCF function primarily involved enhanced overload control mechanisms for the Cx/Dx interfaces and support for IMS Application Server registration with re-assignment for new sessions. The release also introduced procedures for Multi-talker floor control and audio mixing for MCPTT services, involving both UE and network-based mixing. Furthermore, corrections and notifications were specified for S-CSCF registration and deregistration procedures.
- Add the regroup group member update during MCX session TS 23.280CR0441
- MC Recording procedures using Session Recording Protocol TS 23.280CR0630
- Support of multi-talker floor control for ad hoc group calls TS 23.379CR0358
- MCPTT control plane user plane separation TS 23.379CR0447
- Overload Control over Cx/Dx TS 29.228CR0702
- Overload Control over Cx/Dx TS 29.229CR0311
+ 7 more changes
In Release 20, the I-CSCF function was updated with a correction to the multi-talker control configuration. Furthermore, the capability for simultaneous sessions was removed, aligning its operation with revised service flow management procedures.
Explore further
Broader topics and technologies where I-CSCF plays a role.
Defining Specifications
3GPP specifications that define or reference I-CSCF, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.125 v1700 | Flow Based Charging Architecture | Rel-7 |
| TS 23.141 vj00 | Presence Service Stage 2 Architecture | Rel-19 |
| TS 23.179 vd50 | MCPTT Functional Architecture | Rel-13 |
| TS 23.218 vj00 | IMS Call Model Specification | Rel-19 |
| TS 23.278 vj00 | CAMEL for IMS Stage 2 Specification | Rel-19 |
| TS 23.280 vk10 | Common Architecture for Mission Critical Services | Rel-20 |
| TS 23.379 vk00 | MCPTT Functional Architecture | Rel-20 |
| TS 23.417 v1700 | IMS Core Component for NGN Architecture | Rel-7 |
| TS 23.517 v1800 | IMS Core Component for NGN Architecture | Rel-8 |
| TS 23.701 vc00 | WebRTC Access to IMS Architecture Study | Rel-12 |
| TR 23.794 vh00 | Study on enhanced IMS to 5GC integration | Rel-17 |
| TS 23.849 vb00 | Study on IMS Roaming Media Optimization | Rel-11 |
| TR 23.981 vj00 | IPv4 IMS Interworking and Migration Study | Rel-19 |
| TS 24.141 vj00 | Presence Service Protocol Details | Rel-19 |
| TS 24.147 vj00 | IMS Conferencing Protocol Details | Rel-19 |
| TS 24.173 vj00 | Multimedia Telephony Service and Supplementary Services in IMS | Rel-19 |
| TS 24.229 vj50 | IMS call control protocol based on SIP and SDP | Rel-19 |
| TS 24.405 v1700 | Conference Service Protocol Description | Rel-7 |
| TS 24.406 v810 | Message Waiting Indication (MWI) Protocol | Rel-8 |
| TS 24.416 v1700 | Malicious Call Identification Service | Rel-7 |
| TS 24.428 v1700 | Common Basic Communication Procedures | Rel-7 |
| TS 24.505 v810 | Protocol Description of the Conference Service | Rel-8 |
| TS 24.508 v820 | TIP and TIR Service Protocol Description | Rel-8 |
| TS 24.516 v830 | MCID Protocol Specification for NGN | Rel-8 |
| TS 24.523 vj00 | NGCN-NGN Interconnection Scenarios | Rel-19 |
| TS 24.524 vj00 | Hosted Enterprise Services Architecture | Rel-19 |
| TS 24.525 vj00 | Business Trunking Architecture & Requirements | Rel-19 |
| TS 24.528 v830 | Common Basic Communication Procedures for IMS Services | Rel-8 |
| TS 24.606 vj00 | MWI Service Protocol Description | Rel-19 |
| TS 24.802 vc10 | IMS II-NNI Traversal Scenario Determination Study | Rel-12 |
| TS 24.819 v1700 | IMS Services via Fixed Broadband Access | Rel-7 |
| TS 24.841 v1600 | Presence Service IP Multimedia Subsystem | Rel-6 |
| TR 24.930 vj00 | IMS Session Setup Signalling Flows | Rel-19 |
| TS 28.705 vj00 | IMS NRM IRP Information Service | Rel-19 |
| TS 29.162 vj00 | IMS-IP Network Interworking | Rel-19 |
| TS 29.165 vj10 | Inter-IMS Network to Network Interface (NNI) | Rel-19 |
| TS 29.228 vj20 | Cx and Dx Interface Signaling Flows | Rel-19 |
| TS 29.229 vj10 | Diameter Protocol for Cx/Dx Interfaces | Rel-19 |
| TS 29.562 vj40 | HSS Services for IMS & GBA Interworking | Rel-19 |
| TR 29.949 vj00 | VoLTE IMS Roaming Architecture & Procedures | Rel-19 |
| TS 32.102 vj00 | Telecom Management Physical Architecture Framework | Rel-19 |
| TS 32.409 vj00 | IMS Performance Management Measurements | Rel-19 |
| TS 33.203 vj10 | IMS Security Specification | Rel-19 |
| TR 33.978 v1800 | Interim Security for Early IMS | Rel-8 |