4.3 SSTP Layer Establishment
After the bidirectional HTTPS layer is up, the SSTP finite state machine begins. The server initializes the FSM by waiting for the SSTP Call Connect Request message to arrive from the client. After the Call Connect Request message arrives, the server validates the Encapsulated Protocol ID attribute value for PPP. The SSTP server then responds to the client by using the Call Connect Acknowledge message. The client sends the Call Connected message to the server to indicate that it is ready to send data traffic. When the server receives the Call Connected message, it allows bidirectional data transfer on the SSTP connection.
After the control channel is established, the echo timers continue to send Echo Request messages in order to keep the channel alive. Echo responses are sent and received to ensure that the connection is not broken.
Figure 6: SSTP call setup for a non-proxy scenario