Kamailio

Kamailio implements the SIP signaling protocol defined in RFC 3261 and functions as a registrar, proxy, router and firewall for real‑time communications. It is built for high scalability, capable of handling thousands of call setups per second, and can be deployed in large carrier or IP‑telephony environments as well as in smaller enterprise or personal setups that require VoIP, instant messaging, presence, WebRTC or MSRP services.

The server offers extensive protocol support—including asynchronous TCP, UDP, SCTP, TLS, WebSocket, IPv4/IPv6, and IMS extensions for VoLTE—along with features such as least‑cost routing, load balancing, fail‑over, authentication, accounting and a range of backend integrations (MySQL, PostgreSQL, Oracle, Radius, LDAP, Redis, Cassandra, MongoDB, Memcached). Control interfaces include JSON, XML‑RPC and theMP monitoring.

Kamailio is released under GPL‑2.0 (with some BSD‑licensed modules), is self‑hostable, free‑tier, and has been in continuous development since 2001. Its modular architecture, strong security, and broad feature set make it a widely used open‑source solution for building robust SIP‑based communication platforms.