|INVITE                                 
                                  |pass INV to TU                         
               INVITE             V send 100 if TU won't in 200ms         
               send response+-----------+                                 
                   +--------|           |--------+101-199 from TU         
                   |        | Proceeding|        |send response           
                   +------->|           |<-------+                        
                            |           |          Transport Err.         
                            |           |          Inform TU              
                            |           |--------------->+                
                            +-----------+                |                
               300-699 from TU |     |2xx from TU        |                
               send response   |     |send response      |                
                               |     +------------------>+                
                               |                         |                
               INVITE          V          Timer G fires  |                
               send response+-----------+ send response  |                
                   +--------|           |--------+       |                
                   |        | Completed |        |       |                
                   +------->|           |<-------+       |                
                            +-----------+                |                
                               |     |                   |                
                           ACK |     |                   |                
                           -   |     +------------------>+                
                               |        Timer H fires    |                
                               V        or Transport Err.|                
                            +-----------+  Inform TU     |                
                            |           |                |                
                            | Confirmed |                |                
                            |           |                |                
                            +-----------+                |                
                                  |                      |                
                                  |Timer I fires         |                
                                  |-                     |                
                                  |                      |                
                                  V                      |                
                            +-----------+                |                
                            |           |                |                
                            | Terminated|<---------------+                
                            |           |                                 
                            +-----------+