SIP registrar

An SIP registrar is a server in an Session Initiation Protocol (SIP) network that accepts SIP REGISTER requests. Although SIP endpoints can connect to each other without any additional SIP infrastructure, that is not practical since each endpoint would need to know each others IP-address. The SIP registrar is a location service which registers a number of IP-addresses to a certain URI, usually a "sip:" address but other types are also possible such as "tel:". More than one User agent can register on the same address, with the result that all registered user agents will ring when called on that address.

Registrars in SIP are only logical elements and are commonly co-located with SIP proxies. But it is also possible and often good for network scalability to push this location service into a redirect server instead. A redirect server will consist of a user agent server handling incoming INVITEs and responding to them with a 301 (Moved Permanently) or 302 (Moved Temporarily) response if a user has been registered on the requested URI. There are number of SIP Registrar Servers available and a trial version can be downloaded.