4.4 SSTP Layer Teardown
The Call Abort message is used in situations in which the SSTP control channel negotiation has encountered a problem such as a time-out, an invalid message, or other similar problems. The Call Abort-based shutdown uses short timers for quick cleanup.
Graceful shutdown occurs when the Call Disconnect message is sent to the far end. The far end responds by sending a Call Disconnect Acknowledge message in order to signal the end of data transfer.
Figure 7: SSTP graceful shutdown