10.4 What you still need around Kamailio¶

[!IMPORTANT] Kamailio is the CSCF signalling plane, and nothing else. It registers users, routes SIP, and queries Diameter peers. It does not store subscribers, decide policy, rate calls, or touch RTP. A working IMS is Kamailio plus the surrounding systems below.

Scope¶

Kamailio (CSCF) does	Kamailio does not do
SIP registration & routing (P/I/S-CSCF)	Store the subscriber database
Authentication challenge (using HSS vectors)	Generate the auth vectors
Trigger services via iFC/ISC	Be the application server
Ask for QoS/media policy (Rx)	Decide or enforce policy
Ask for charging (Ro/Rf)	Rate or bill the call
—	Relay or transcode media (RTP)

Everything in the right-hand column is a separate system.

The surrounding components¶

flowchart TB
    subgraph SIG["Signalling — Kamailio"]
        P[P-CSCF]
        I[I-CSCF]
        S[S-CSCF]
    end
    subgraph SUB["Subscriber & policy"]
        HSS[(HSS)]
        PCRF[(PCRF / PCF)]
        OCS[(OCS / CHF)]
        DRA{{DRA / SLF}}
    end
    subgraph MEDIA["Media & access"]
        RTP[rtpengine]
        CORE[EPC / 5GC<br/>SMF · UPF]
        DNS[DNS<br/>ENUM/NAPTR/SRV]
        DB[(DB<br/>MariaDB / Valkey)]
    end

    P --- I --- S
    I -.Cx.-> DRA -.-> HSS
    S -.Cx.-> DRA
    P -.Rx.-> PCRF
    S -.Ro/Rf.-> OCS
    S --- DB
    P --- RTP
    CORE --- P
    P --- DNS

    classDef cscf fill:#1f6feb,stroke:#1f6feb,color:#fff
    classDef sub fill:#bf8700,stroke:#bf8700,color:#fff
    classDef media fill:#238636,stroke:#238636,color:#fff
    class P,I,S cscf
    class HSS,PCRF,OCS,DRA sub
    class RTP,CORE,DNS,DB media

HSS — Home Subscriber Server. The subscriber database and the auth-vector generator. Kamailio talks to it over Cx. Open-source choice that works: open5gs (its HSS function).

PCRF / PCF — Policy. Decides whether a media flow is allowed and what QoS it gets, and tells the core to set up a dedicated bearer. P-CSCF talks to it over Rx. open5gs again.

OCS / CHF — Charging. Online charging (prepaid balance, credit reservation) and offline (CDR generation). S-CSCF talks to it over Ro/Rf. Open-source choice: CGRateS, a capable real-time rating engine with a Diameter agent.

DRA / SLF — Diameter routing. In anything beyond a single-HSS lab you put a Diameter Routing Agent between the CSCFs and the HSS/PCRF/OCS, so the CSCF has one Diameter peer to point at instead of N. Open-source choice: freeDiameter as a relay/DRA.

rtpengine — media. Relays (and optionally transcodes/encrypts) RTP, and handles ICE/SRTP for WebRTC access. Kamailio steers it via the rtpengine module but never carries media itself.

DNS. IMS routing leans hard on DNS: NAPTR/SRV to find CSCFs, ENUM to turn tel: numbers into SIP URIs. A misconfigured resolver breaks call routing in ways that look like CSCF bugs. CoreDNS is a clean choice (and it's already the default in Kubernetes).

DB. S-CSCF registration state needs persistence to survive restarts. MariaDB/MySQL is the tested path for ims_usrloc_scscf (hardcoded SQL — see 10.2); Valkey/Redis is lighter and is what rtpengine wants for HA, but needs schema handling on the Kamailio side.

EPC / 5GC core. The device needs an IP bearer and a route to the P-CSCF before any of the above applies. That's the packet core — SMF/UPF in 5G, PGW in 4G — which also discovers the P-CSCF address and (with the PCRF) sets up the dedicated voice bearer. open5gs covers this too.

Monitoring. Diameter and SIP both fail quietly; you want Prometheus/Grafana for metrics and Loki-style log aggregation from day one, plus packet capture (sip || diameter || gtpv2) for the inevitable flow debugging.

A minimal complete stack¶

Kamailio (P/I/S-CSCF) + open5gs (HSS, PCRF, SMF, UPF) + CGRateS (OCS) + freeDiameter (DRA) + rtpengine (media) + CoreDNS + a DB + Prometheus/Grafana/Loki + a test UE.

This is the component list assembled in the lab in 10.5.

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