SIP Session creation across domains

Imagine that a SIP enabled device in a local network needs to talk to a SIP enabled device in a different network domain. On power-up, the devices in both domains tell their proxy server that they are available, and the Registrar server for each domain has stored the IP addresses of all the active devices in that domain.

Lets say that Matt in Domain A uses the SIP phone, and Wayne in Domain B has a PDA. Matt decides to call Wayne, and so this sequence of events occurs.

flow diagram explaining the following numbered text
  • Matt's SIP phone tells the SIP proxy server that he wants to contact Waynes PDA.
  • The Proxy server sees that Wayne's PDA is in a different domain and so asks the Redirect server for The IP address for Richard's phone.
  • The Proxy server gets the address of the SIP phone from the Registrar Server.
  • The proxy server then sends the session invitation to the Proxy server for The other domain.
  • The second proxy server contacts its Registrar server to get Wayne's PDA address.
  • Which is returned to it
  • The Proxy server then sends the invite to communicate to Wayne's PDA.
  • Wayne receives this message and replies to the Proxy server that this is ok. And that communication can start.
  • The Proxy server sends the agreement back to Matt's domain's Proxy server and establishes the session.
  • Matt's domain's Proxy server returnes the agreement to Matt's SIP phone.
  • The users create a point to point connection to begin the conversation.
person at a computer