Microsoft pptp vpn vulnerabilities Exploits in Action

GRE (Generic Routing Encapsulation Protocol)
RFC 1701 
http://www.ietf.org/rfc/rfc1701.txt
describes GRE in detail. A 
(very) brief summary follows:
RFC Abstract: 
“This document specifies a protocol for performing 
encapsulation of an arbitrary network layer protocol over another arbitrary 
network layer protocol.“
The three key parts of GRE transport are:
Delivery Header
•
GRE header (describes the GRE packet and protocol carried within it), 
•
encapsulated inside another protocol (like IP) to be carried to it’s 
destination at the other end of the VPN connection where it will then be 
disassembled at that remote network and the information from it’s 
payload packet as well
Payload packet (the entire packet in whatever protocol – IP, IPX, 
•
NetBEUI, etc. – with it’s data and possibly it’s route) encapsulated inside 
the GRE packet.