|PPTP, Point-to-Point Tunneling Protocol|
|Protocol type:||Application layer tunneling protocol.|
|Port:||1723 (TCP) control.|
Designed as an extension of PPP, PPTP encapsulates PPP packets for transmission over IP.
PPTP was proposed as a standard in March 1996 by Microsoft, Ascend Communications, ECI Telematics, 3Com/Primary Access and U.S.Robotics.
PPTP can be used to tunnel the IP, IPX and NetBEUI protocols.
|MAC header||IP header||TCP header||PPTP header||Data :::|
Total length of the PPTP message in bytes.
Message type. 16 bits.
Always set to 0x1A2B3C4D.
Data. Variable length.
Control Message Type. 16 bits.
ATO, Adaptive Time-Out.
(RFC 2637) The time that must elapse before an acknowledgment is considered lost. After a time-out, the sliding window is partially closed and the ATO is backed off.
NAS, Network Access Server.
(RFC 2637). A device providing temporary, on-demand network access to users. This access is point-to-point using PSTN or ISDN lines.
PAC, PPTP Access Concentrator.
(RFC 2637). A device attached to one or more PSTN or ISDN lines capable of PPP operation and of handling the PPTP protocol. The PAC need only implement TCP/IP to pass traffic to one or more PNSs. It may also tunnel non-IP protocols.
PNS, PPTP Network Server.
(RFC 2637). A PNS is envisioned to operate on general-purpose computing/server platforms. The PNS handles the server side of the PPTP protocol. Since PPTP relies completely on TCP/IP and is independent of the interface hardware, the PNS may use any combination of IP interface hardware including LAN and WAN devices.
PPD, Packet Processing Delay.
(RFC 2637). The amount of time required for each side to process the maximum amount of data buffered in their receive packet sliding window. The PPD is the value exchanged between the PAC and PNS when a call is established. For the PNS, this number should be small. For a PAC making modem connections, this number could be significant.
RTT, Round-Trip Time.
(RFC 2637). The estimated round-trip time for an Acknowledgment to be received for a given transmitted packet. When the network link is a local network, this delay will be minimal (if not zero). When the network link is the Internet, this delay could be substantial and vary widely. RTT is adaptive: it will adjust to include the PPD and whatever shifting network delays contribute to the time between a packet being transmitted and receiving its acknowledgment.
(RFC 2637) A tunnel is defined by a PNS-PAC pair. The tunnel protocol is defined by a modified version of GRE. The tunnel carries PPP datagrams between the PAC and the PNS. Many sessions are multiplexed on a single tunnel. A control connection operating over TCP controls the establishment, release, and maintenance of sessions and of the tunnel itself.
[RFC 2637] Point-to-Point Tunneling Protocol (PPTP).