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UTT Technologies
Chapter 10 VPN
9.1.1 Protocol Overview
There are two parallel components of PPTP:
1. A PPTP Control Connection
It is a logical connection representing the PPTP tunnel that must be created, maintained,
and terminated through a series of PPTP messages. The PPTP control connection traffic
uses a dynamically allocated TCP port on the PPTP client and the registered TCP port
1723 on the PPTP server.
2. GRE encapsulation for data
When data is sent through the PPTP tunnel, PPP frames are encapsulated with a Generic
Routing Encapsulation (GRE) header, which includes information that identifies the
specific PPTP tunnel for the data packet. GRE is described in RFC 1701.
The use of a separate GRE mechanism for PPTP data encapsulation has an interesting
side effect for NAT devices. Most NAT devices can translate TCP-based packets for PPTP
tunnel maintenance. However, many NAT devices or firewalls cannot handle GRE packets,
thus the PPTP data packets with the GRE header cannot pass them. The UTT products
support NAT traversal for PPTP tunnels.
In order for the PPTP tunnel to be established and function properly, the following basic
conditions are necessary:
1) The PPTP client and server should have IP-route reachability between them.
2) The firewalls between the two endpoints of the tunnel should be configured to open
TCP port 1723 and IP protocol 47 (GRE) to allow PPTP traffic.
