Media Negotiation and Establishing the Session So far we have looked at how a request
is routed through the network, and how the responses are sent in return. Now let??™s look
at the procedures at both the origination and the destination devices for establishing a
session. To maintain clarity, we will repeat some of the previous procedures.
We have already learned that to establish a session with another device, the originating
subscriber device must first initiate a request (usually in the form of an INVITE
message). This is the first step in establishing a dialog with the destination device. This
dialog will be maintained throughout the session and allows both devices to communicate
with one another regarding the established session.
To establish the dialog, the originating device must first receive a final response from
the destination device. A final response in this case must be a successful response (2 xx
class of responses). If any other final response is received, there was an error and the
session cannot be established.
The dialog is not the same as a session. The dialog is established between two entities
and requires a specific sequence of events to take place prior to the dialog being established.
This sequence of events is the acceptance of a request, and successful responses
to that request.
When the originator receives a 200 OK, for example, the device will return an ACK,
establishing the dialog between the two devices.
Pages:
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277