3.2.2.3 KeepAlive Timer

HTTP Encapsulation protocols do not support a native KeepAlive timer, but rely on the encapsulated protocol to provide a KeepAlive mechanism. Encapsulated protocols SHOULD implement their own KeepAlive mechanisms. The SSTP protocol provides its own KeepAlive mechanism using the SSTP_NOOP command<15>. This data serves to keep the LongLived connection from being closed by firewalls and proxies. All LongLived connections SHOULD use KeepAlive timers, regardless of whether or not the client detects if a connection is a proxy connection, as some firewalls and proxies are undetectable. The default server KeepAlive timeout value is 45 seconds. The maximum KeepAlive value is limited by proxy implementations. The KeepAlive timer event processing is handled as specified in section 3.2.6.3.