ate machine for the INVITE client transaction. The state machine is entered when the endpoint sends an INVITE request. When it happens, the machine moves to the calling state. Then, if a 1xx response arrives, it moves to the proceeding state. Instead, if a final 2xx response arrives, it moves to the terminated state. If a response of types 3xx, 4xx, 5xx, or 6xx arrives in the calling or proceeding states, the machine moves to the completed state and waits for the expiration of a timer to go to the terminated state. From the state machine of Figure 8.14, according to the transition tour method, the following sequence can be extracted, which covers all states and transitions: 1122334-134-14-124-1234 - © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security INVITE sent Timer B Timer A INVITE sent Transport CALLING ACK sent PROCEEDING 300-699 ACK sent ACK sent 300-699 ACK sent 300-699 ACK sent Transport COMPLETED Timer D TERMINATED FIGURE 8.14 INVITE client transaction state machine for a SIP UAC starting an invitation process. This sequence can be reproduced by running the following tests: 1. - Corresponding to the subsequence 1122334 The SIP endpoint sends an INVITE request; timer A fires and a new INVITE is sent; a 1xx message is received; a new 1xx message is received; a response with code between 300 and 699 is received, and the ACK is sent; timer D fires. 2. - Corresponding to the subsequence 134- - The SIP endpoint sends an INVITE request; a response with code between 300 and 699 is received, and the ACK is sent; timer D fires. 3. - Corresponding to the subsequence 14- The SIP endpoint sends an INVITE request; a 2xx response is received and an ACK is sent. 4. - Corresponding to the subsequence 124- - The SIP endpoint sends an INVITE request; a 1xx response is received a 2xx response is received and an ACK is sent. C 2009 by Taylor & Francis Group, LLC Compliance and Interoperability Testing of SIP Devices In order to test for the compliance of the invitation process of the SIP 