Before PPP tunneling can occur between a PAC and PNS, a control connection must be established between them. The control connection is a standard TCP session over which PPTP call control and management information is passed. The control session is logically associated with, but separate from, the sessions being tunneled through a PPTP tunnel. For each PAC-PNS pair both a tunnel and a control connection exist. The control connection is responsible for establishment, management, and release of sessions carried through the tunnel. It is the means by which a PNS is notified of an incoming call at an associated PAC, as well as the means by which a PAC is instructed to place an outgoing dial call.
A control connection can be established by either the PNS or the PAC. Following the establishment of the required TCP connection, the PNS and PAC establish the control connection using the Start-Control- Connection-Request and -Reply messages. These messages are also used to exchange information about basic operating capabilities of the PAC and PNS. Once the control connection is established, the PAC or PNS may initiate sessions by requesting outbound calls or responding to inbound requests. The control connection may communicate changes in operating characteristics of an individual user session with a Set- Link-Info message. Individual sessions may be released by either the PAC or PNS, also through Control Connection messages.
The control connection itself is maintained by keep-alive echo messages. This ensures that a connectivity failure between the PNS and the PAC can be detected in a timely manner. Other failures can be reported via the Wan-Error-Notify message, also on the control connection.
It is intended that the control connection will also carry management related messages in the future, such as a message allowing the PNS to request the status of a given PAC; these message types have not yet been defined.