Tunneling Protocol (PPTP) is a network protocol that enables the secure
transfer of data from a remote client to an enterprise server by creating a VPN
across TCP/IP-based data networks. PPTP encapsulates PPP packets into IP
datagrams for transmission over the Internet or other public TCP/IP-based
PPTP establishes a
tunnel for each communicating PPTP network server (PNS)-PPTP Access
Concentrator (PAC) pair. After the tunnel is set up, PPP packets are exchanged
using enhanced generic routing encapsulation (GRE). A call ID present in the
GRE header indicates the session to which a particular PPP packet belongs.
Translation (NAT) translates only the IP address and the port number of a PPTP
message. Static and dynamic NAT configurations work with PPTP without the
requirement of the PPTP application layer gateway (ALG). However, Port Address
Translation (PAT) configuration requires the PPTP ALG to parse the PPTP header
and facilitate the translation of call IDs in PPTP control packets. NAT then
parses the GRE header and translates call IDs for PPTP data sessions. The PPTP
ALG does not translate any embedded IP address in the PPTP payload. The PPTP
ALG is enabled by default when NAT is configured.
NAT recognizes PPTP
packets that arrive on the default TCP port, 1723, and invokes the PPTP ALG to
parse control packets. NAT translates the call ID parsed by the PPTP ALG by
assigning a global address or port number. Based on the client and server call
IDs, NAT creates two doors based on the request of the PPTP ALG. ( A door is
created when there is insufficient information to create a complete NAT-session
entry. A door contains information about the source IP address and the
destination IP address and port.) Two NAT sessions are created (one with the
server call ID and the other with the client call ID) for two-way data
communication between the client and server. NAT translates the GRE packet
header for data packets that complies with RFC 2673.
PPTP is a TCP-based
protocol. Therefore, when NAT recognizes a TCP packet as a PPTP packet, it
invokes the PPTP ALG parse-callback function. The PPTP ALG fetches the embedded
call ID from the PPTP header and creates a translation token for the header.
The PPTP ALG also creates data channels for related GRE tunnels. After ALG
parsing, NAT processes the tokens created by the ALG.
PPTP Default Timer
The default timer for PPTP is 24 hours. This means that a generic
routing encapsulation (GRE) session will live for 24 hours when deploying
static and dynamic NAT. Based on your PPTP configuration and scaling
requirement, you adjust the PPTP default timer.
Some PPTP clients and servers send keepalive messages to keep GRE
sessions alive. You can adjust the NAT session timer for PPTP sessions by using