The SIP specification has been extended over time to support a general mechanism allowing for subscription to asynchronous events. Such events can include SIP proxy statistics changes, presence information, session changes and so on.
A user agent interested in event notification sends a SUBSCRIBE message to an SIP server. The SUBSCRIBE message establishes a dialog and is immediately followed by the server replying with 200 OK response. At this point the dialog is established. The server sends a NOTIFY request to the user every time the event to which the user subscribed changes. NOTIFY messages are sent within the dialog established by the SUBSCRIBE.
ACK method confirms that an entity has received a final response to an INVITE request. In its turn, BYE method signals termination of a dialog and ends a call.
Here is the example:
This blog article will be dedicated to SIP and SIP calls. It is meant to provide an introduction for UC Engineers to the main headers and guide through the debugging of the most popular call scenarios that will be described in the next articles.
As we have seen in SIP and XMPP standards in Unified Communications and Media transfer in Unified Communications – SDP protocol, SIP (Session Initiation Protocol) and SDP-RTP have become the de facto replacement mode for traditional analog and digital lines provided by operators all over the world.
SIP is also the most popular signaling mode to handle VoIP calls.
Being able to understand SIP sessions has become for a Unified Communication Engineer as important as in the past for Telephony Experts to read signaling traces in Q.931 / Q.921 / DS1 / QSIG generated by T1 / E1 PRI – BRI ISDN lines.
Before Starting the Analysis