SIP Express Router (SER) is a high-performance, configurable, free SIP server licensed under the open-source GNU GPL license. It can act as SIP registrar, proxy or redirect server. SER can be configured to serve specialized purposes such as load balancing or SIP front-end to application servers. Unlike Asterisk PBX, it is really only a SIP server – it offers functions provided by SIP. Additional functions and services can be provided by separate modules or application servers. This makes its logic much simpler. Being modular and written in C, it is lightweight and fast. Developers claim that it can handle thousands of calls per second being established and torn down on a single dual-CPU PC. Also, it is scalable to virtually unlimited number of subscribers.
This makes SER good choice for deploying as a SIP proxy on a backbone of a SIP infrastructure where many calls per second are signaled. The routing logic can be very simple or very complex, depending on the specific needs.
In 2004, OpenSER project was started by two core developers of SER after moving of the team from Fraunhofer institute to company iptel.org. In 2008, it was renamed to Kamailio because of trademark issues. Shortly thereafter, the OpenSIPS project was created as a Kamailio fork because of a conflict between the core developers. Later in 2008, the Kamailio developers announced to join the original SER developers to form the sip-router project which aims to create a common collaboration framework for all projects related to SER.