 the implementation, that is, if the implementation fol- lows the FSM specification except for the output produced for certain state transitions. Nevertheless, the w-method and distinguishing sequence method will find all such errors if the number of states of the implementation remains within certain boundaries. The discussion of the fault coverage of the test methods is, therefore, based on the fault model of FSM "output" and "trans- fer" faults. 8.2.1.1 Example: SIP REGISTER Session State Machine The specifications for the UA REGISTER session originate from the high- level state machine shown in Figure 8.4. Following, for example, the transition tour method, a test sequence (called a transition-tour sequence) can be gen- erated by simply applying random inputs to the UA REGISTER FSM until the machine has traversed every transition at least once. A transition-tour sequence for the FMS in Figure 8.4 is reported below: 12341212343 © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security UA REGISTER SRV 200 SRV 401 Pending REGISTER Pending Un-REGISTER SRV 401 SRV 200 UA Un-REGISTER UA REGISTER refresh Registered FIGURE 8.4 SIP REGISTER session state machine. In looking at the test sequence, note that it can be divided into four parts, each one identifying a test: - 1 2 3 : Registration without authentication: A UA in an idle state sends a SIP message, REGISTER, without authentication credentials, receives a 200 OK SIP answer from the registrar server and moves to the regis- tered state. - 341 : Un-registration without authentication: A UA in the registered state sends an un-REGISTER message, obtains a 200 OK SIP answer and moves to the idle state. 1 2 1 2 3 : Registration with authentication: A UA in an idle state sends a SIP REGISTER message without authentication credentials, receives an error answer, 401 "Unauthorized", and replies with a REG- ISTER message with an authentication field, receives a 200 OK SIP answer and moves to the registered state. - 3 4 3 : Un-registration with