Initiating a Session in SIP

   When a user agent client desires to initiate a session (for example,
   audio, video, or a game), it formulates an INVITE request.  The
   INVITE request asks a server to establish a session.  This request
   may be forwarded by proxies, eventually arriving at one or more UAS
   that can potentially accept the invitation.  These UASs will
   frequently need to query the user about whether to accept the
   invitation.  After some time, those UASs can accept the invitation
   (meaning the session is to be established) by sending a 2xx response.
   If the invitation is not accepted, a 3xx, 4xx, 5xx or 6xx response is
   sent, depending on the reason for the rejection.  Before sending a
   final response, the UAS can also send provisional responses (1xx) to
   advise the UAC of progress in contacting the called user.

13.1 Overview

After possibly receiving one or more provisional responses, the UAC
   will get one or more 2xx responses or one non-2xx final response.
   Because of the protracted amount of time it can take to receive final
   responses to INVITE, the reliability mechanisms for INVITE
   transactions differ from those of other requests (like OPTIONS).
   Once it receives a final response, the UAC needs to send an ACK for
   every final response it receives.  The procedure for sending this ACK
   depends on the type of response.  For final responses between 300 and
   699, the ACK processing is done in the transaction layer and follows
   one set of rules (See Section 17).  For 2xx responses, the ACK is
   generated by the UAC core.

   A 2xx response to an INVITE establishes a session, and it also
   creates a dialog between the UA that issued the INVITE and the UA
   that generated the 2xx response.  Therefore, when multiple 2xx
   responses are received from different remote UAs (because the INVITE
   forked), each 2xx establishes a different dialog.  All these dialogs
   are part of the same call.

   This section provides details on the establishment of a session using
   INVITE.  A UA that supports INVITE MUST also support ACK, CANCEL and
   BYE.

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