The dialog is a specific
set of communications between two unique devices regarding the specified session.
The CALL ID identifies the session itself, and differentiates one session from another
for all devices involved. The dialog ID is only known by two entities communicating
with one another and is not shared by more than two devices.
Now that you understand how a dialog is established, let??™s discuss resource negotiation.
Resource negotiation is the first part of the dialog, where the destination device
examines the received request and determines whether or not it will be able to support
the media type and other requirements for the requested session.
If the device is able to support the session, the destination device must agree to the
session. This is done through the use of a final response. The destination device looks
at the SDP in the message body to determine if the requested session can be supported
by the device. This is the first phase of session negotiation. If the device cannot support
the requested session, the device will return a response of 488 Not Acceptable Here
and reject the request.
When the INVITE is received by the called subscriber, there are two options. The first
option is for the device to process the INVITE and send the appropriate response, given
the requirements set forth in the INVITE SDP (codecs required, media type supported,
etc.
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