SIP Express Router (SER) is a high-performance, configurable, free SIP server licensed under the open-source GNU license . It can act as SIP (RFC 3261) registrar, proxy or redirect server. SER can be configured to serve specialized purposes such as load balancing or SIP front-end to application servers, SEMS  for example. Together with the RTPproxy software it can act as media relay or Session Border Controller.