Technical Overview of PPTP
Because Cisco routers do not support compulsory tunnel mode with PPTP, only voluntary tunnel mode is examined in this chapter.
Voluntary tunnel mode, unlike compulsory tunnel mode, is initiated by the remote access client itself. In this mode of operation, the remote access client acts as the PNS, with the tunnel being established directly to the PAC.
PPTP provides for two distinct channels of communication between the remote access client/PNS and the PAC:
The control connection/channel— The control connection is used to exchange control messages between the remote access client/PNS and the PAC. These control messages allow the establishment and termination of client sessions and perform such functions as keepalive, ...