Kamailio (former OpenSER) is a mature and flexible open source SIP server (RFC3261). It can be used on systems with limited resources as well as on carrier grade servers, scaling to up to thousands call setups per second. It is written in pure C for Unix/Linux-like systems with architecture specific optimizations to offer high performances. It is customizable, being able to feature as fast load balancer; SIP server flavours: registrar, location server, proxy server, redirect server; enum, UDP, TCP, TLS and SCTP; IPv6; VoIP; video and instant messaging; SIMPLE presence and XCAP server; embedded MSRP relay; load balancer; lead cost routing; WebSockets and WebRTC; SNMP monitoring; DoS detection.