SIP Registrars

A registrar is a server that accepts registrations from users and places these registrations, (which are essentially location information), in a database known as a Location Service. The process of registration associates a user with a particular location, (IP address); this association is known as a ‘binding’ in SIP. When there is an incoming session for a user within a domain, the proxy server will interrogate the Location Server to determine the route for the signaling messages.

Although the proxy and the registrar may be collocated, for the purposes of the SIP protocol they are logically two distinct elements.

This figure shows a SIP Registrar that is used to gather and store registration data into a database to provide Location Service. In this example, an Internet telephone user agent is connected to the Internet. The Internet telephone had been previously programmed to register with a specific registrar service on the detection of new service (sensing an Internet connection). When the Internet telephone sends its registration information to the registrar (with its dynamically assigned IP address), the registrar updates the Location Service database to allow calls to be routed correctly for SIP communication sessions.

Diagram of how a SIP uses Registrars to track availability of Devices

SIP Registrar Operation Diagram

There are a number of ways in which a User Agent may discover a Registrar. The first of these is to pre-configure the address of the Registrar in the UA, the second method is to use the host portion of the users SIP address to route the registration messages, so if a user had a SIP address of SIP:bill@example.com the UA would route registrations to SIP: example.com. The final method that a UA may use to discover a Registrar is achieved using a multicast address that is sent out by the UA and would be responded to by Registrars that received this multicast.

Session Initiation Protocol - SIP Books

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