After the PPTP tunnel has been established, user data is transmitted between the client and PPTP server. Data is transmitted in IP datagrams containing PPP packets. The IP datagrams are created using a modified version of the Internet GRE protocol. (GRE is defined in RFCs 1701 and 1702.) The IP datagram created by PPTP is similar to the illustration in the following figure.
The IP delivery header provides the information necessary for the datagram to traverse the Internet. The GRE header is used to encapsulate the PPP packet within the IP datagram. The PPP packet was created by RAS. Note that the PPP packet is just one unintelligible block because it is encrypted. Even if the IP datagram were intercepted, it would be nearly impossible to decrypt the data.