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Kamailio provides documented IMS modules and example configurations for the P-CSCF, I-CSCF, and S-CSCF roles, so it can be a foundation for building an IMS core. That does not, by itself, establish 3GPP conformance or production readiness. Treat compliance as a target to demonstrate against a specified 3GPP release and operator profile, with traceable requirements and test results.
What the three CSCF roles do
An IMS core is a set of cooperating network functions, not one interchangeable SIP proxy. Kamailio can be configured for the three CSCF roles, but each has a distinct place in signaling and depends on supporting functions beyond those roles.
P-CSCF: the UE-facing signaling point
The Proxy-CSCF is the first IMS signaling point a user equipment (UE) reaches and remains in the signaling path. Kamailio’s IMS workshop describes functions such as message validation and possible QoS, SigComp, IPsec, and TLS/SRTP translation. The ims_registrar_pcscf module documents REGISTER processing for a P-CSCF and depends on ims_usrloc_pcscf and Kamailio’s internal IMS library. These documented capabilities are not a guarantee that every security or media behavior required by a particular network profile is implemented or enabled.
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I-CSCF: the network-domain entry point
The Interrogating-CSCF receives signaling at the network domain boundary and helps locate the serving CSCF. Kamailio’s workshop describes querying the HSS to find an S-CSCF or select an HSS, then forwarding requests toward the selected S-CSCF. Kamailio’s overview identifies ims_icscf as able to query a third-party HSS.
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S-CSCF: serving, registration, and routing
The Serving-CSCF handles subscriber registration and serving-side routing. The workshop describes registration, application-server forwarding, routing, and charging functions; Kamailio’s overview identifies an S-CSCF registrar module. Which application-server and charging interactions are needed depends on the chosen service and operator profile.
External network functions remain dependencies
The CSCF roles do not provide all the functions around them. The documented architecture relies on an HSS, and QoS or policy and charging integrations may involve separate PCRF and charging systems. Those systems, their interfaces, and their operational availability must be designed and tested as part of the overall service.
What Kamailio’s documented IMS setup establishes—and what it does not
Kamailio publishes IMS modules and role-specific example configurations. Its official “IMS in a box” lab, authored by DI Franz Edler of the University of Applied Sciences / Vienna, places P-CSCF, I-CSCF, S-CSCF, and HSS on one machine for educational use. The document calls it “a stripped-down version of Kamailio-IMS” and states that advanced functions including NAT handling, RTP relay, antiflooding, capturing, and dispatching are omitted or disabled. It is useful for understanding a learning topology and incremental configuration, not evidence of carrier-grade capacity, failover, security, production readiness, or standards conformance.
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The KamailioWorld 2018 workshop describes Linux, DNS, persistent storage, and a network HSS as prerequisites and shows separate role-specific configuration files. Those details describe that workshop’s setup, not a current installation recipe. An older Kamailio IMS installation how-to likewise describes separate P-, I-, and S-CSCF configuration files and Diameter XML settings, but explicitly labels itself outdated. Use it only as historical context; choose installation steps and module settings from documentation for the exact Kamailio release you deploy.
Neither the presence of modules nor an example topology is an independent conformance report. The reviewed Kamailio material does not provide a release-by-release compliance matrix or proof that a given assembled deployment satisfies a selected operator profile.
How to plan a Kamailio IMS implementation
Start by defining interfaces, requirements, and evidence—not by copying a lab configuration into a live network. The following sequence turns the architecture into an implementation plan without implying that any unverified configuration is conformant.
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- Choose the target standards baseline. Identify the 3GPP release and the regional and operator profile you intend to meet. TS 24.229 covers IMS SIP/SDP call control and is maintained under change control. ETSI lists TS 124 229 V19.5.0, corresponding to 3GPP TS 24.229 Release 19, dated February 2026. Record the exact document versions in the implementation guide and re-check the 3GPP catalogue for updates.
- Pin the Kamailio build and module sources. Record the Kamailio release, enabled IMS modules, their source, and the corresponding versioned documentation. Verify module names, dependencies, supported interfaces, and configuration syntax for that release; do not treat old how-tos or examples as current instructions.
- Specify the subscriber and policy integration. Document how the CSCFs reach the HSS and how policy authorization is provided. Kamailio’s versioned 5.7
ims_qosdocumentation describes P-CSCF policy authorization over Rx/Diameter toward a PCRF. It lists dependencies including CDP (Diameter Peer), CDP_AVP, transaction management, dialog, and P-CSCF usrloc modules. Check those dependencies and interfaces against the release actually selected. - Define security and service behavior. Establish required access security, emergency-call handling, charging, QoS, NAT and media handling, and application-server interactions for the target network. The stripped-down lab disables several advanced functions, so these requirements cannot be assumed to be covered by reproducing it.
- Design persistence and resilience. Decide how subscriber and registration state, configuration, and dependent services are persisted and recovered. Define failure behavior and availability targets for the CSCFs, HSS, policy systems, and any media or charging components. An all-in-one educational topology does not demonstrate high availability.
- Build a traceable conformance and interoperability test plan. Map each applicable requirement to configuration, a test case, expected signaling or service behavior, and recorded results. Include tests with the actual UE, HSS/UDM, policy and charging systems, and other operator network elements wherever relevant. Module presence is not a substitute for this evidence.
Does the documented Kamailio QoS path cover 5G policy?
Not on the evidence of the documented ims_qos path alone. That version-specific module documentation describes the Rx/Diameter model and PCRF integration. The 3GPP catalogue also lists TS 29.514 for 5G policy authorization, but a specification’s existence does not establish that a Kamailio module implements its procedures.
A 27 September 2026 post on Kamailio’s sr-users mailing list announced a separately maintained extension proposing 5G policy and SBI/HSS-facing modules. An announcement is not proof that the code has been merged into Kamailio, released, interoperated with an operator’s 5G core, or shown compliant with relevant 3GPP releases. Verify the project status, code provenance, release compatibility, interfaces, and test evidence before treating it as a supported implementation path.
For a 5G deployment, assess the target policy interface and HSS/UDM integration method against the intended architecture and profile. Do not infer 5G support simply because a Kamailio IMS deployment supports IMS signaling or uses a component called a policy module.
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What to verify before production
Production suitability is a deployment-specific conclusion. Before launch, ensure the implementation has evidence for the scope it claims to meet:
- A requirements-to-test matrix tied to a named 3GPP release, current specification versions, and the operator’s regional profile.
- Verified behavior for registration, routing, HSS selection or lookup, application-server forwarding, and any applicable charging procedures.
- Successful integration tests for the selected policy interface and subscriber-data systems, including the actual PCRF/PCF and HSS/UDM path.
- Validated access security and emergency-call behavior, plus NAT, media relay, QoS, and antiflood protections required by the service.
- Operational tests for persistent state, overload, monitoring, upgrades, component failure, and recovery across the CSCFs and their external dependencies.
- Recorded interoperability results with the target devices and network elements, not merely a successful single-machine lab demonstration.
A deployment that has not been assessed against those requirements should be described as an IMS implementation or lab, not as 3GPP-compliant. A compliance claim should identify its scope and be supported by traceable test evidence.
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