Update When a device wishes to establish a session, it uses the INVITE method. To
update the destination regarding changes to the session, the requestor would typically
64 Chapter 3
use the re-invite (sending an INVITE again using the same CALL-ID). The changes to
the session are then contained in the new INVITE.
However, this also changes the dialog between the two devices, which would not be
desirable if the initial INVITE had not yet been acknowledged. If an INVITE is sent,
and an update to that INVITE is needed, then the UPDATE method can be used to
send new information about the session without altering the dialog (which is yet to be
established).
In essence this is a method used to notify the destination of session changes before
the destination has a chance to accept the original session. To fully understand this
concept, you must first understand the concept of a dialog, which is explained in full
detail in Chapter 5. The dialog is not the same as the session but rather comprises communications
between two devices about a session.
SIP Requests
In any communication, there is a request and a response. The same is true in SIP
networks. We already discussed the concept of the request and response when we described
the various methods used in SIP.
The request is a call to establish a dialog. A dialog is a virtual connection between
two endpoints.
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