Figure 2.12 Session invitation
User Joe uses address sip:bob@b.com to call Bob. Joe's user agent does not know how to route the invitation itself but it is configured to send all outbound traffic to the company SIP Proxy Server proxy.a.com.The proxy server figures out that user sip:bob@b.com is in a different company so it will look up B's SIP Proxy Server and send the invitation there. B's proxy server can be either pre-configured at proxy.a.com or the proxy will use DNS SRV records to find B's proxy server.The invitation reaches proxy.bo.com.The proxy knows that Bob is currently sitting in his office and is reachable through phone on his desk, which has IP address 1.2.3.4, so the proxy will send the invitation there.
2.2.2.2.3 Registrar
Its has been mentioned that the SIP Proxy at proxy.b.com knows current Bob's location but have not mentioned yet how a proxy can learn current location of a user. Bob's user agent (SIP
phone) must register with a registrar.The registrar is a special SIP entity that receives registrations from users, extracts information about their current location (IP address, port and username in this case) and stores the information into a location database.The purpose of the location database is to map sip:bob@b.com to something like sip:bob@1.2.3.4:5060.The location database is then used by B's proxy server.When the proxy receives an invitation for sip:bob@b.com it will search the location database. It finds sip:bob@1.2.3.4:5060 and will send the invitation there.
A registrar is very often a logical entity only. Because of their tight coupling with proxies, registrars are usually co-located with proxy servers.
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[IP Telephony Cookbook] / Technological Background
Figure 2.13 shows a typical SIP registration. A REGISTER message containing Address of Record sip:jan@iptel.org and contact address sip:jan@1.2.3.4:5060 where 1.2.3.4 is IP
address of the phone is sent to the registrar.The registrar extracts this information and stores it into the location database. If everything went well then the registrar sends a 200 OK response to the phone and the process of registration is finished.
Location Database
Record in Location Database
User Agent
Registrar