3.8.2.1 SSTP KeepAlive Timer

The HTTP Encapsulation protocols do not define a KeepAlive timer. The underlying encapsulated protocol MUST implement a KeepAlive timer. The SSTP protocol uses the KeepAlive mechanism provided by the SSTP_NOOP command<57>.

The KeepAlive message keeps the Secure Tunnel connection from being closed by firewalls and proxies because of connection inactivity. All Secure Tunnel connections SHOULD use KeepAlive timers, regardless of whether the client detects if a connection is a proxy connection or not, as some firewall and proxies are undetectable. The recommended client KeepAlive timeout value is 45 seconds<58>.