1.3.2 Tunnel Protocol Overview

PPTP requires the establishment of a tunnel for each communicating PNS-PAC pair.  This tunnel is used to carry all user session PPP packets for sessions involving a given PNS-PAC pair.  A key which present in the GRE header indicates which session a particular PPP packet belongs to.  In this manner, PPP packets are multiplexed and demultiplexed over a single tunnel between a given PNS-PAC pair. The value to use in the key field is established by the call establishment procedure which takes place on the control connection.

The GRE header also contains acknowledgment and sequencing information that is used to perform some level of congestion-control and error detection over the tunnel.  Again the control connection is used to determine rate and buffering parameters that are used to regulate the flow of PPP packets for a particular session over the tunnel.  PPTP does not specify the particular algorithms to use for congestion-control and flow-control.  Suggested algorithms for the determination of adaptive time-outs to recover from dropped data or acknowledgments on the tunnel are included in Appendix A of this document.