The dialog is identified by a combination
of headers and parameters, and each device uses the values of these headers and
parameters to calculate a dialog ID. Only INVITE, SUBSCRIBE, and REFER can be
used to create a dialog between two devices (note that the device may be an application
server and is not always a subscriber device).
The dialog ID is calculated by using the TAG value from the TO and FROM headers,
and the CALL ID. When the request is created, the originator will calculate its own tag
value and append it to the FROM header. The destination device will calculate its own
tag value and attach it to the TO header upon responding. The 200 OK response carries
both tags and of course the CALL ID that was sent in the
Establishing a Session in the IMS 133
The tag values are used by each of the receiving devices to correlate requests with
responses throughout the life of the session (and the associated dialog). The dialog ID
is used to correlate requests and responses between two devices. They are separate,
since a device could establish a session with multiple devices, all containing the same
CALL ID. The originator will correlate responses to sessions using the CALL ID, while
correlating responses from each device using the dialog ID (the only difference is the
tag values sent by each device).
It is this nuance that differentiates a dialog ID from the CALL ID.
Pages:
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278