There are several basic SIP methods currently defined for use in the IMS:
?– Ack
?– Bye
?– Cancel
Session Initiation Protocol (SIP) 61
?– Info
?– Invite
?– Message
?– Notify
?– Options
?– Register
?– Subscribe
?– Update
Ack When an INVITE has been sent, the sender awaits a response by the destination.
However, a dialog is not established between the two entities until the originator
of the INVITE sends the ACK method. This method is the final handshake required to
establish the dialog and allow for the session to begin.
If a device receives multiple 2xx responses to an INVITE (as would be the case for a
conference call, for example), it must then generate an ACK for each of the responses.
The ACK contains the same credentials as the INVITE and may also contain a message
body carrying SDP or other content.
The details of how ACK is used to establish a connection are provided in Chapter 5;
there you will find details about forming a dialog and the handshake sequence required
between the two devices.
Bye This is the simplest of methods. To release a session in progress, either device
sends the BYE method to release the session. It can be initiated by either end of the
session. Like all other methods used during a dialog, the BYE does require a response
and acknowledgment prior to the session being released.
Cancel This method is used by a device when it wishes to cancel a request prior to
receiving a response from the destination.
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