GRE Protocol 47 Packet Description and Use

The vpn Generic Route Encapsulation protocol is used in conjunction with Point-to-Point Tunneling Protocol, because it select best route.to create virtual private networks, when we create a connection between clients and servers. this is very important service when making a connection. One another implementation is to use Microsoft's VPN technology between two Routing and Remote Access Services servers that are configured for LAN-to-LAN routing.

we can understand the use of Generic Route Encapsulation in the creation and use of VPNs, it is helpful to understand the packet structure. After the PPTP control session has been established, GRE is used to encapsulate the data or payload in a secure manner. For additional information about PPTP The GRE packet format that Microsoft uses for encapsulating data has the fallow general process.

The vpn Generic Route Encapsulation protocol that is work through the tunnel is given a PPP header and then placed inside a Generic Route Encapsulation packet. Generic Route Encapsulation packet send the data between the two tunnel to destination points. After the GRE packet has arrived at the final destination from source to destination. it is discarded and the encapsulated packet is then transmitted to its final destination.


We are Using the diagram at the top of this section, an Internet Protocol (IP) packet from Lclient is first transmitted to the L-RRAS server.The IP packet is encrypted, given an additional PPP header, and then placed inside a GRE packet. The diagram below says "PPP stub" and not "PPP header" because the PPP header is also encrypted along with the data.

The Generic Route Encapsulation protocol is configured to know that a PPP header is already here. The GRE packet with the encapsulated and encrypted data is sent. these time when user data send or recive, across the Internet with a final destination of "R-RRAS server." The R-RRAS server strips off the GRE header and PPP header, and then transmits the decrypted data (IP packet) to Rclient.


Source: http://support.microsoft.com/kb/241251/