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RFC 2637 - Point-to-Point Tunneling Protocol (PPTP)


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Network Working Group                                          K. Hamzeh
Request for Comments: 2637                         Ascend Communications
Category: Informational                                          G. Pall
                                                   Microsoft Corporation
                                                             W. Verthein
                                                                    3Com
                                                               J. Taarud
                                                Copper Mountain Networks
                                                               W. Little
                                                          ECI Telematics
                                                                 G. Zorn
                                                   Microsoft Corporation
                                                               July 1999

                Point-to-Point Tunneling Protocol (PPTP)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

IESG Note

   The PPTP protocol was developed by a vendor consortium. The
   documentation of PPTP is provided as information to the Internet
   community. The PPP WG is currently defining a Standards Track
   protocol (L2TP) for tunneling PPP across packet-switched networks.

Abstract

   This document specifies a protocol which allows the Point to Point
   Protocol (PPP) to be tunneled through an IP network.  PPTP does not
   specify any changes to the PPP protocol but rather describes a new
   vehicle for carrying PPP.  A client-server architecture is defined in
   order to decouple functions which exist in current Network Access
   Servers (NAS) and support Virtual Private Networks (VPNs).  The PPTP
   Network Server (PNS) is envisioned to run on a general purpose
   operating system while the client, referred to as a PPTP Access
   Concentrator (PAC) operates on a dial access platform.  PPTP
   specifies a call-control and management protocol which allows the
   server to control access for dial-in circuit switched calls
   originating from a PSTN or ISDN or to initiate outbound circuit-

   switched connections.  PPTP uses an enhanced GRE (Generic Routing
   Encapsulation) mechanism to provide a flow- and congestion-controlled
   encapsulated datagram service for carrying PPP packets.

Specification of Requirements

   In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
   "recommended", "SHOULD", and "SHOULD NOT" are to be interpreted as
   described in [12].

   The words "silently discard", when used in reference to the behavior
   of an implementation upon receipt of an incoming packet, are to be
   interpreted as follows: the implementation discards the datagram
   without further processing, and without indicating an error to the
   sender.  The implementation SHOULD provide the capability of logging
   the error, including the contents of the discarded datagram, and
   SHOULD record the event in a statistics counter.

Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   1.1.  Protocol Goals and Assumptions . . . . . . . . . . . . . .   4
   1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .   5
   1.3.  Protocol Overview  . . . . . . . . . . . . . . . . . . . .   6
   1.3.1.  Control Connection Overview  . . . . . . . . . . . . . .   7
   1.3.2.  Tunnel Protocol Overview . . . . . . . . . . . . . . . .   7
   1.4.  Message Format and Protocol Extensibility  . . . . . . . .   8
   2.  Control Connection Protocol Specification  . . . . . . . . .  10
   2.1.  Start-Control-Connection-Request . . . . . . . . . . . . .  10
   2.2.  Start-Control-Connection-Reply . . . . . . . . . . . . . .  12
   2.3.  Stop-Control-Connection-Request  . . . . . . . . . . . . .  15
   2.4.  Stop-Control-Connection-Reply  . . . . . . . . . . . . . .  16
   2.5.  Echo-Request . . . . . . . . . . . . . . . . . . . . . . .  17
   2.6.  Echo-Reply . . . . . . . . . . . . . . . . . . . . . . . .  18
   2.7.  Outgoing-Call-Request  . . . . . . . . . . . . . . . . . .  19
   2.8.  Outgoing-Call-Reply  . . . . . . . . . . . . . . . . . . .  22
   2.9.  Incoming-Call-Request  . . . . . . . . . . . . . . . . . .  25
   2.10.  Incoming-Call-Reply . . . . . . . . . . . . . . . . . . .  28
   2.11.  Incoming-Call-Connected . . . . . . . . . . . . . . . . .  29
   2.12.  Call-Clear-Request  . . . . . . . . . . . . . . . . . . .  31
   2.13.  Call-Disconnect-Notify  . . . . . . . . . . . . . . . . .  32
   2.14.  WAN-Error-Notify  . . . . . . . . . . . . . . . . . . . .  33
   2.15.  Set-Link-Info . . . . . . . . . . . . . . . . . . . . . .  35
   2.16.  General Error Codes . . . . . . . . . . . . . . . . . . .  36
   3.  Control Connection Protocol Operation  . . . . . . . . . . .  36
   3.1.  Control Connection States  . . . . . . . . . . . . . . . .  37
   3.1.1.  Control Connection Originator (may be PAC or PNS)  . . .  37
   3.1.2.  Control connection Receiver (may be PAC or PNS)  . . . .  39

   3.1.3.  Start Control Connection Initiation Request Collision  .  40
   3.1.4.  Keep Alives and Timers . . . . . . . . . . . . . . . . .  40
   3.2.  Call States  . . . . . . . . . . . . . . . . . . . . . . .  41
   3.2.1.  Timing considerations  . . . . . . . . . . . . . . . . .  41
   3.2.2.  Call ID Values . . . . . . . . . . . . . . . . . . . . .  41
   3.2.3.  Incoming Calls . . . . . . . . . . . . . . . . . . . . .  41
   3.2.3.1.  PAC Incoming Call States . . . . . . . . . . . . . . .  42
   3.2.3.2.  PNS Incoming Call States . . . . . . . . . . . . . . .  43
   3.2.4.  Outgoing Calls . . . . . . . . . . . . . . . . . . . . .  44
   3.2.4.1.  PAC Outgoing Call States . . . . . . . . . . . . . . .  45
   3.2.4.2.  PNS Outgoing Call States . . . . . . . . . . . . . . .  46
   4.  Tunnel Protocol Operation  . . . . . . . . . . . . . . . . .  47
   4.1.  Enhanced GRE header  . . . . . . . . . . . . . . . . . . .  47
   4.2.  Sliding Window Protocol  . . . . . . . . . . . . . . . . .  49
   4.2.1.  Initial Window Size  . . . . . . . . . . . . . . . . . .  49
   4.2.2.  Closing the Window . . . . . . . . . . . . . . . . . . .  49
   4.2.3.  Opening the Window . . . . . . . . . . . . . . . . . . .  50
   4.2.4.  Window Overflow  . . . . . . . . . . . . . . . . . . . .  50
   4.2.5.  Multi-packet Acknowledgment  . . . . . . . . . . . . . .  50
   4.3.  Out-of-sequence Packets  . . . . . . . . . . . . . . . . .  50
   4.4.  Acknowledgment Time-Outs . . . . . . . . . . . . . . . . .  51
   4.4.1.  Calculating Adaptive Acknowledgment Time-Out . . . . . .  53
   4.4.2.  Congestion Control: Adjusting for Time-Out . . . . . . .  54
   5.  Security Considerations  . . . . . . . . . . . . . . . . . .  54
   6.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  55
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . .  56
   8.  Full Copyright Statement . . . . . . . . . . . . . . . . . .  57

1.  Introduction

   PPTP allows existing Network Access Server (NAS) functions to be
   separated using a client-server architecture. Traditionally, the
   following functions are implemented by a NAS:

      1) Physical native interfacing to PSTN or ISDN and control of
         external modems or terminal adapters.

         A NAS may interface directly to a telco analog or digital
         circuit or attach via an external modem or terminal adapter.
         Control of a circuit-switched connection is accomplished with
         either modem control or DSS1 ISDN call control protocols.

         The NAS, in conjunction with the modem or terminal adapters,
         may perform rate adaption, analog to digital conversion, sync
         to async conversion or a number of other alterations of data
         streams.

      2) Logical termination of a Point-to-Point-Protocol (PPP) Link
         Control Protocol (LCP) session.

      3) Participation in PPP authentication protocols [3,9,10].

      4) Channel aggregation and bundle management for PPP Multilink
         Protocol.

      5) Logical termination of various PPP network control protocols
         (NCP).

      6) Multiprotocol routing and bridging between NAS interfaces.

   PPTP divides these functions between the PAC and PNS. The PAC is
   responsible for functions 1, 2, and possibly 3. The PNS may be
   responsible for function 3 and is responsible for functions 4, 5, and
   6.  The protocol used to carry PPP protocol data units (PDUs) between
   the PAC and PNS, as well as call control and management is addressed
   by PPTP.

   The decoupling of NAS functions offers these benefits:

      Flexible IP address management. Dial-in users may maintain a
      single IP address as they dial into different PACs as long as they
      are served from a common PNS. If an enterprise network uses
      unregistered addresses, a PNS associated with the enterprise
      assigns addresses meaningful to the private network.

      Support of non-IP protocols for dial networks behind IP networks.
      This allows Appletalk and IPX, for example to be tunneled through
      an IP-only provider. The PAC need not be capable of processing
      these protocols.

      A solution to the "multilink hunt-group splitting" problem.
      Multilink PPP, typically used to aggregate ISDN B channels,
      requires that all of the channels composing a multilink bundle be
      grouped at a single NAS.  Since a multilink PPP bundle can be
      handled by a single PNS, the channels comprising the bundle may be
      spread across multiple PACs.

1.1.  Protocol Goals and Assumptions

   The PPTP protocol is implemented only by the PAC and PNS. No other
   systems need to be aware of PPTP. Dial networks may be connected to a
   PAC without being aware of PPTP. Standard PPP client software should
   continue to operate on tunneled PPP links.

   PPTP can also be used to tunnel a PPP session over an IP network. In
   this configuration the PPTP tunnel and the PPP session runs between
   the same two machines with the caller acting as a PNS.

   It is envisioned that there will be a many-to-many relationship
   between PACs and PNSs.  A PAC may provide service to many PNSs. For
   example, an Internet service provider may choose to support PPTP for
   a number of private network clients and create VPNs for them. Each
   private network may operate one or more PNSs. A single PNS may
   associate with many PACs to concentrate traffic from a large number
   of geographically diverse sites.

   PPTP uses an extended version of GRE to carry user PPP packets. These
   enhancements allow for low-level congestion and flow control to be
   provided on the tunnels used to carry user data between PAC and PNS.
   This mechanism allows for efficient use of the bandwidth available
   for the tunnels and avoids unnecessary retransmisions and buffer
   overruns.  PPTP does not dictate the particular algorithms to be used
   for this low level control but it does define the parameters that
   must be communicated in order to allow such algorithms to work.
   Suggested algorithms are included in section 4.

1.2.  Terminology

   Analog Channel

      A circuit-switched communication path which is intended to carry
      3.1 Khz audio in each direction.

   Digital Channel

      A circuit-switched communication path which is intended to carry
      digital information in each direction.

   Call

      A connection or attempted connection between two terminal
      endpoints on a PSTN or ISDN -- for example, a telephone call
      between two modems.

   Control Connection

      A control connection is created for each PAC, PNS pair and
      operates over TCP [4]. The control connection governs aspects of
      the tunnel and of sessions assigned to the tunnel.

   Dial User

      An end-system or router attached to an on-demand PSTN or ISDN
      which is either the initiator or recipient of a call.

   Network Access Server (NAS)

      A device providing temporary, on-demand network access to users.
      This access is point-to-point using PSTN or ISDN lines.

   PPTP Access Concentrator (PAC)

      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.

   PPTP Network Server (PNS)

      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.

   Session

      PPTP is connection-oriented.  The PNS and PAC maintain state for
      each user that is attached to a PAC.  A session is created when
      end-to-end PPP connection is attempted between a dial user and the
      PNS.  The datagrams related to a session are sent over the tunnel
      between the PAC and PNS.

   Tunnel

      A tunnel is defined by a PNS-PAC pair.  The tunnel protocol is
      defined by a modified version of GRE [1,2].  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.

1.3.  Protocol Overview

   There are two parallel components of PPTP: 1) a Control Connection
   between each PAC-PNS pair operating over TCP and 2) an IP tunnel
   operating between the same PAC-PNS pair which is used to transport
   GRE encapsulated PPP packets for user sessions between the pair.

1.3.1.  Control Connection Overview

   Before PPP tunneling can occur between a PAC and PNS, a control
   connection must be established between them. The control connection
   is a standard TCP session over which PPTP call control and management
   information is passed. The control session is logically associated
   with, but separate from, the sessions being tunneled through a PPTP
   tunnel.  For each PAC-PNS pair both a tunnel and a control connection
   exist. The control connection is responsible for establishment,
   management, and release of sessions carried through the tunnel. It is
   the means by which a PNS is notified of an incoming call at an
   associated PAC, as well as the means by which a PAC is instructed to
   place an outgoing dial call.

   A control connection can be established by either the PNS or the PAC.
   Following the establishment of the required TCP connection, the PNS
   and PAC establish the control connection using the Start-Control-
   Connection-Request and -Reply messages.  These messages are also used
   to exchange information about basic operating capabilities of the PAC
   and PNS.  Once the control connection is established, the PAC or PNS
   may initiate sessions by requesting outbound calls or responding to
   inbound requests. The control connection may communicate changes in
   operating characteristics of an individual user session with a Set-
   Link-Info message.  Individual sessions may be released by either the
   PAC or PNS, also through Control Connection messages.

   The control connection itself is maintained by keep-alive echo
   messages.  This ensures that a connectivity failure between the PNS
   and the PAC can be detected in a timely manner. Other failures can be
   reported via the

   Wan-Error-Notify message, also on the control connection.

   It is intended that the control connection will also carry management
   related messages in the future, such as a message allowing the PNS to
   request the status of a given PAC; these message types have not yet
   been defined.

1.3.2.  Tunnel Protocol Overview

   PPTP requires the establishment of a tunnel for each communicating
   PNS-PAC pair.  This tunnel is used to carry all user session PPP
   packets for sessions involving a given PNS-PAC pair.  A key which is
   present in the GRE header indicates which session a particular PPP
   packet belongs to.

   In this manner, PPP packets are multiplexed and demultiplexed over a
   single tunnel between a given PNS-PAC pair.  The value to use in the
   key field is established by the call establishment procedure which
   takes place on the control connection.

   The GRE header also contains acknowledgment and sequencing
   information that is used to perform some level of congestion-control
   and error detection over the tunnel.  Again the control connection is
   used to determine rate and buffering parameters that are used to
   regulate the flow of PPP packets for a particular session over the
   tunnel.  PPTP does not specify the particular algorithms to use for
   congestion-control and flow-control.  Suggested algorithms for the
   determination of adaptive time-outs to recover from dropped data or
   acknowledgments on the tunnel are included in section 4.4 of this
   document.

1.4.  Message Format and Protocol Extensibility

   PPTP defines a set of messages sent as TCP data on the control
   connection between a PNS and a given PAC.  The TCP session for the
   control connection is established by initiating a TCP connection to
   port 1723 [6]. The source port is assigned to any unused port number.

   Each PPTP Control Connection message begins with an 8 octet fixed
   header portion.  This fixed header contains the following: the total
   length of the message, the PPTP Message Type indicator, and a "Magic
   Cookie".

   Two Control Connection message types are indicated by the PPTP
   Message Type field:

         1 - Control Message
         2 - Management Message

   Management messages are currently not defined.

   The Magic Cookie is always sent as the constant 0x1A2B3C4D.  Its
   basic purpose is to allow the receiver to ensure that it is properly
   synchronized with the TCP data stream.  It should not be used as a
   means for resynchronizing the TCP data stream in the event that a
   transmitter issues an improperly formatted message.  Loss of
   synchronization must result in immediate closing of the control
   connection's TCP session.

   For clarity, all Control Connection message templates in the next
   section include the entire PPTP Control Connection message header.
   Numbers preceded by 0x are hexadecimal values.

The currently defined Control Messages, grouped by function, are:

         Control Message                        Message Code

         (Control Connection Management)

         Start-Control-Connection-Request            1
         Start-Control-Connection-Reply              2
         Stop-Control-Connection-Request             3
         Stop-Control-Connection-Reply               4
         Echo-Request                                5
         Echo-Reply                                  6

         (Call Management)

         Outgoing-Call-Request                       7
         Outgoing-Call-Reply                         8
         Incoming-Call-Request                       9
         Incoming-Call-Reply                        10
         Incoming-Call-Connected                    11
         Call-Clear-Request                         12
         Call-Disconnect-Notify                     13

         (Error Reporting)

         WAN-Error-Notify                           14

         (PPP Session Control)

         Set-Link-Info                              15

   The Start-Control-Connection-Request and -Reply messages determine
   which version of the Control Connection protocol will be used.  The
   version number field carried in these messages consists of a version
   number in the high octet and a revision number in the low octet.
   Version handling is described in section 2. The current value of the
   version number field is 0x0100 for version 1, revision 0.

   The use of the GRE-like header for the encapsulation of PPP user
   packets is specified in section 4.1.

   The MTU for the user data packets encapsulated in GRE is 1532 octets,
   not including the IP and GRE headers.

2.  Control Connection Protocol Specification

   Control Connection messages are used to establish and clear user
   sessions.  The first set of Control Connection messages are used to
   maintain the control connection itself.  The control connection is
   initiated by either the PNS or PAC after they establish the
   underlying TCP connection.  The procedure and configuration
   information required to determine which TCP connections are
   established is not covered by this protocol.

   The following Control Connection messages are all sent as user data
   on the established TCP connection between a given PNS-PAC pair.  Note
   that care has been taken to ensure that all word (2 octet) and
   longword (4 octet) values begin on appropriate boundaries.  All data
   is sent in network order (high order octets first).  Any "reserved"
   fields MUST be sent as 0 values to allow for protocol extensibility.

2.1.  Start-Control-Connection-Request

   The Start-Control-Connection-Request is a PPTP control message used
   to establish the control connection between a PNS and a PAC.  Each
   PNS-PAC pair requires a dedicated control connection to be
   established.  A control connection must be established before any
   other PPTP messages can be issued.  The establishment of the control
   connection can be initiated by either the PNS or PAC.  A procedure
   which handles the occurrence of a collision between PNS and PAC
   Start-Control-Connection-Requests is described in section 3.1.3.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Length            |       PPTP Message Type       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message Type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Protocol Version        |           Reserved1           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Framing Capabilities                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Bearer Capabilities                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Maximum Channels        |       Firmware Revision       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                     Host Name (64 octets)                     +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                   Vendor String (64 octets)                   +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Length                   Total length in octets of this PPTP
                               message, including the entire PPTP
                               header.

      PPTP Message Type        1 for Control Message.

      Magic Cookie             0x1A2B3C4D. This constant value is used
                               as a sanity check on received messages
                               (see section 1.4).

      Control Message Type     1 for Start-Control-Connection-Request.

      Reserved0                This field MUST be 0.

      Protocol Version         The version of the PPTP protocol that the
                               sender wishes to use.

      Reserved1                This field MUST be 0.

   Framing Capabilities     A set of bits indicating the type of framing
                            that the sender of this message can provide.
                            The currently defined bit settings are:

                               1 - Asynchronous Framing supported

                               2 - Synchronous Framing supported

   Bearer Capabilities      A set of bits indicating the bearer
                            capabilities that the sender of this message
                            can provide.  The currently defined bit
                            settings are:

                               1 - Analog access supported

                               2 - Digital access supported

   Maximum Channels         The total number of individual PPP sessions
                            this PAC can support.  In Start-Control-
                            Connection-Requests issued by the PNS, this
                            value SHOULD be set to 0.  It MUST be
                            ignored by the PAC.

   Firmware Revision        This field contains the firmware revision
                            number of the issuing PAC, when issued by
                            the PAC, or the version of the PNS PPTP
                            driver if issued by the PNS.

   Host Name                A 64 octet field containing the DNS name of
                            the issuing PAC or PNS.  If less than 64
                            octets in length, the remainder of this
                            field SHOULD be filled with octets of value
                            0.

   Vendor Name              A 64 octet field containing a vendor
                            specific string describing the type of PAC
                            being used, or the type of PNS software
                            being used if this request is issued by the
                            PNS.  If less than 64 octets in length, the
                            remainder of this field SHOULD be filled
                            with octets of value 0.

2.2.  Start-Control-Connection-Reply

   The Start-Control-Connection-Reply is a PPTP control message sent in
   reply to a received Start-Control-Connection-Request message.  This
   message contains a result code indicating the result of the control
   connection establishment attempt.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Length            |       PPTP Message Type       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message Type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Protocol Version        |  Result Code  |  Error Code   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Framing Capability                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Bearer Capability                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Maximum Channels        |       Firmware Revision       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                     Host Name (64 octets)                     +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                   Vendor String (64 octets)                   +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length                   Total length in octets of this PPTP message,
                            including the entire PPTP header.

   PPTP Message Type        1 for Control Message.

   Magic Cookie             0x1A2B3C4D.

   Control Message Type     2 for Start-Control-Connection-Reply.

   Reserved0                This field MUST be 0.

   Protocol Version         The version of the PPTP protocol that the
                            sender wishes to use.

   Result Code              Indicates the result of the command channel
                            establishment attempt.  Current valid Result
                            Code values are:

                                  1 - Successful channel establishment

                                  2 - General error -- Error Code
                                      indicates the problem

                                  3 - Command channel already exists;

                                  4 - Requester is not authorized to
                                      establish a command channel

                                  5 - The protocol version of the
                                      requester is not supported

   Error Code               This field is set to 0 unless a "General
                            Error" exists, in which case Result Code is
                            set to 2 and this field is set to the value
                            corresponding to the general error condition
                            as specified in section 2.2.

   Framing Capabilities     A set of bits indicating the type of framing
                            that the sender of this message can provide.
                            The currently defined bit settings are:

                                  1 - Asynchronous Framing supported

                                  2 - Synchronous Framing supported.

   Bearer Capabilities      A set of bits indicating the bearer
                            capabilities that the sender of this message
                            can provide.  The currently defined bit
                            settings are:

                                  1 - Analog access supported

                                  2 - Digital access supported

   Maximum Channels         The total number of individual PPP sessions
                            this PAC can support.  In a Start-Control-
                            Connection-Reply issued by the PNS, this
                            value SHOULD be set to 0 and it must be
                            ignored by the PAC. The PNS MUST NOT use
                            this value to try to track the remaining
                            number of PPP sessions that the PAC will
                            allow.

   Firmware Revision        This field contains the firmware revision
                            number of the issuing PAC, or the version of
                            the PNS PPTP driver if issued by the PNS.

   Host Name                A 64 octet field containing the DNS name of
                            the issuing PAC or PNS.  If less than 64
                            octets in length, the remainder of this
                            field SHOULD be filled with octets of value
                            0.

   Vendor Name              A 64 octet field containing a vendor
                            specific string describing the type of PAC
                            being used, or the type of PNS software
                            being used if this request is issued by the
                            PNS.  If less than 64 octets in length, the
                            remainder of this field SHOULD be filled
                            with octets of value 0.

2.3.  Stop-Control-Connection-Request

   The Stop-Control-Connection-Request is a PPTP control message sent by
   one peer of a PAC-PNS control connection to inform the other peer
   that the control connection should be closed.  In addition to closing
   the control connection, all active user calls are implicitly cleared.
   The reason for issuing this request is indicated in the Reason field.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Length            |       PPTP Message Type       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message Type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Reason     |   Reserved1   |           Reserved2           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length                   Total length in octets of this PPTP message,
                            including the entire PPTP header.

   PPTP Message Type        1 for Control Message.

   Magic Cookie             0x1A2B3C4D.

   Control Message Type     3 for Stop-Control-Connection-Request.

   Reserved0                This field MUST be 0.

   Reason                   Indicates the reason for the control
                            connection being closed. Current valid
                            Reason values are:

                                  1 (None) - General request to clear
                                    control connection

                                  2 (Stop-Protocol) - Can't support
                                    peer's version of the protocol

                                  3 (Stop-Local-Shutdown) - Requester is
                                    being shut down

      Reserved1, Reserved2     These fields MUST be 0.

2.4.  Stop-Control-Connection-Reply

   The Stop-Control-Connection-Reply is a PPTP control message sent by
   one peer of a PAC-PNS control connection upon receipt of a Stop-
   Control-Connection-Request from the other peer.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Length             |       PPTP Message Type       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message Type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Result Code  |   Error Code  |           Reserved1           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length                   Total length in octets of this PPTP message,
                            including the entire PPTP header.

   PPTP Message Type        1 for Control Message.

   Magic Cookie             0x1A2B3C4D.

   Control Message Type     4 for Stop-Control-Connection-Reply.

   Reserved0                This field MUST be 0.

   Result Code              Indicates the result of the attempt to close
                            the control connection. Current valid Result
                            Code values are:

                                  1 (OK) - Control connection closed

                                  2 (General Error) - Control connection
                                    not closed for reason indicated in
                                    Error Code

   Error Code               This field is set to 0 unless a "General
                            Error" exists, in which case Result Code is
                            set to 2 and this field is set to the value
                            corresponding to the general error condition
                            as specified in section 2.2.

   Reserved1                This field MUST be 0.

2.5.  Echo-Request

   The Echo-Request is a PPTP control message sent by either peer of a
   PAC-PNS control connection. This control message is used as a "keep-
   alive" for the control connection.  The receiving peer issues an
   Echo-Reply to each Echo-Request received. As specified in section
   3.1.4, if the sender does not receive an Echo-Reply in response to an
   Echo-Request, it will eventually clear the control connection.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Length             |       PPTP Message Type       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message Type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Identifier                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length                   Total length in octets of this PPTP message,
                            including the entire PPTP header.

   PPTP Message Type        1 for Control Message.

   Magic Cookie             0x1A2B3C4D.

   Control Message Type     5 for Echo-Request.

   Reserved0                This field MUST be 0.

   Identifier               A value set by the sender of the Echo-
                            Request that is used to match the reply with
                            the corresponding request.

2.6.  Echo-Reply

   The Echo-Reply is a PPTP control message sent by either peer of a
   PAC-PNS control connection in response to the receipt of an Echo-
   Request.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Length             |      PPTP Message Type        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message Type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Identifier                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Result Code  |   Error Code  |           Reserved1           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length                   Total length in octets of this PPTP message,
                            including the entire PPTP header.

   PPTP Message Type        1 for Control Message.

   Magic Cookie             0x1A2B3C4D.

   Control Message Type     6 for Echo-Reply.

   Reserved0                This field MUST be 0.

   Identifier               The contents of the identify field from the
                            received Echo-Request is copied to this
                            field.

   Result Code              Indicates the result of the receipt of the
                            Echo-Request. Current valid Result Code
                            values are:

                                  1 (OK) - The Echo-Reply is valid

                                  2 (General Error) - Echo-Request not
                                    accepted for the reason indicated in
                                    Error Code

   Error Code               This field is set to 0 unless a "General
                            Error" condition exists, in which case
                            Result Code is set to 2 and this field is
                            set to the value corresponding to the
                            general error condition as specified in
                            section 2.2.

   Reserved1                This field MUST be 0.

2.7.  Outgoing-Call-Request

   The Outgoing-Call-Request is a PPTP control message sent by the PNS
   to the PAC to indicate that an outbound call from the PAC is to be
   established.  This request provides the PAC with information required
   to make the call. It also provides information to the PAC that is
   used to regulate the transmission of data to the PNS for this session
   once it is established.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Length             |       PPTP Message Type       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message Type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Call ID            |      Call Serial Number       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Minimum BPS                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Maximum BPS                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Bearer Type                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Framing Type                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Packet Recv. Window Size    |    Packet Processing Delay    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Phone Number Length      |           Reserved1           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                   Phone Number (64 octets)                    +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                    Subaddress (64 octets)                     +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length                   Total length in octets of this PPTP message,
                            including the entire PPTP header.

   PPTP Message Type        1 for Control Message.

   Magic Cookie             0x1A2B3C4D.

   Control Message Type     7 for Outgoing-Call-Request.

   Reserved0                This field MUST be 0.

   Call ID                  A unique identifier, unique to a particular
                            PAC-PNS pair assigned by the PNS to this
                            session.  It is used to multiplex and
                            demultiplex data sent over the tunnel
                            between the PNS and PAC involved in this
                            session.

   Call Serial Number       An identifier assigned by the PNS to this
                            session for the purpose of identifying this
                            particular session in logged session
                            information.  Unlike the Call ID, both the
                            PNS and PAC associate the same Call Serial
                            Number with a given session. The combination
                            of IP address and call serial number SHOULD
                            be unique.

   Minimum BPS              The lowest acceptable line speed (in
                            bits/second) for this session.

   Maximum BPS              The highest acceptable line speed (in
                            bits/second) for this session.

   Bearer Type              A value indicating the bearer capability
                            required for this outgoing call.  The
                            currently defined values are:

                                  1 - Call to be placed on an analog
                                      channel

                                  2 - Call to be placed on a digital
                                      channel

                                  3 - Call can be placed on any type of
                                      channel

   Framing Type             A value indicating the type of PPP framing
                            to be used for this outgoing call.

                                  1 - Call to use Asynchronous framing

                                  2 - Call to use Synchronous framing

                                  3 - Call can use either type of
                                      framing

   Packet Recv. Window Size The number of received data packets the PNS
                            will buffer for this session.

   Packet Processing Delay  A measure of the packet processing delay
                            that might be imposed on data sent to the
                            PNS from the PAC.  This value is specified
                            in units of 1/10 seconds.  For the PNS this
                            number should be very small.  See section
                            4.4 for a description of how this value is
                            determined and used.

   Phone Number Length      The actual number of valid digits in the
                            Phone Number field.

   Reserved1                This field MUST be 0.

   Phone Number             The number to be dialed to establish the
                            outgoing session.  For ISDN and analog calls
                            this field is an ASCII string.  If the Phone
                            Number is less than 64 octets in length, the
                            remainder of this field is filled with
                            octets of value 0.

   Subaddress               A 64 octet field used to specify additional
                            dialing information.  If the subaddress is
                            less than 64 octets long, the remainder of
                            this field is filled with octets of value 0.

2.8.  Outgoing-Call-Reply

   The Outgoing-Call-Reply is a PPTP control message sent by the PAC to
   the PNS in response to a received Outgoing-Call-Request message.  The
   reply indicates the result of the outgoing call attempt.  It also
   provides information to the PNS about particular parameters used for
   the call.  It provides information to allow the PNS to regulate the
   transmission of data to the PAC for this session.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Length             |      PPTP Message Type        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message Type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Call ID            |       Peer's Call ID          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Result Code  |  Error Code   |          Cause Code           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Connect Speed                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Packet Recv. Window Size    |    Packet Processing Delay    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Physical Channel ID                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length                   Total length in octets of this PPTP message,
                            including the entire PPTP header.

   PPTP Message Type        1 for Control Message.

   Magic Cookie             0x1A2B3C4D.

   Control Message Type     8 for Outgoing-Call-Reply.

   Reserved0                This field MUST be 0.

   Call ID                  A unique identifier for the tunnel, assigned
                            by the PAC to this session.  It is used to
                            multiplex and demultiplex data sent over the
                            tunnel between the PNS and PAC involved in
                            this session.

   Peer's Call ID           This field is set to the value received in
                            the Call ID field of the corresponding
                            Outgoing-Call-Request message.  It is used
                            by the PNS to match the Outgoing-Call-Reply
                            with the Outgoing-Call-Request it issued. It
                            also is used as the value sent in the GRE
                            header for mux/demuxing.

   Result Code              This value indicates the result of the
                            Outgoing-Call-Request attempt.  Currently
                            valid values are:

                                  1 (Connected) - Call established with
                                    no errors

                                  2 (General Error) - Outgoing Call not
                                    established for the reason indicated
                                    in Error Code

                                  3 (No Carrier) - Outgoing Call failed
                                    due to no carrier detected

                                  4 (Busy) - Outgoing Call failed due to
                                    detection of a busy signal

                                  5 (No Dial Tone) - Outgoing Call
                                    failed due to lack of a dial tone

                                  6 (Time-out) - Outgoing Call was not
                                    established within time allotted by
                                    PAC

                                  7 (Do Not Accept) - Outgoing Call
                                    administratively prohibited

   Error Code               This field is set to 0 unless a "General
                            Error" condition exists, in which case
                            Result Code is set to 2 and this field is
                            set to the value corresponding to the
                            general error condition as specified in
                            section 2.2.

   Cause Code               This field gives additional failure
                            information.  Its value can vary depending
                            upon the type of call attempted.  For ISDN
                            call attempts it is the Q.931 cause code.

   Connect Speed            The actual connection speed used, in
                            bits/second.

   Packet Recv. Window Size The number of received data packets the PAC
                            will buffer for this session.

   Packet Processing Delay  A measure of the packet processing delay
                            that might be imposed on data sent to the
                            PAC from the PNS.  This value is specified
                            in units of 1/10 seconds.  For the PAC, this
                            number is related to the size of the buffer
                            used to hold packets to be sent to the
                            client and to the speed of the link to the
                            client.  This value should be set to the
                            maximum delay that can normally occur
                            between the time a packet arrives at the PAC
                            and is delivered to the client.  See section
                            4.4 for an example of how this value is
                            determined and used.

   Physical Channel ID      This field is set by the PAC in a vendor-
                            specific manner to the physical channel
                            number used to place this call.  It is used
                            for logging purposes only.

2.9.  Incoming-Call-Request

   The Incoming-Call-Request is a PPTP control message sent by the PAC
   to the PNS to indicate that an inbound call is to be established from
   the PAC.  This request provides the PNS with parameter information
   for the incoming call.

   This message is the first in the "three-way handshake" used by PPTP
   for establishing incoming calls.  The PAC may defer answering the
   call until it has received an Incoming-Call-Reply from the PNS
   indicating that the call should be established. This mechanism allows
   the PNS to obtain sufficient information about the call before it is
   answered to determine whether the call should be answered or not.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Length             |       PPTP Message Type       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message Type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Call ID            |      Call Serial Number       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Call Bearer Type                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Physical Channel ID                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Dialed Number Length      |     Dialing Number Length     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                   Dialed Number (64 octets)                   +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                  Dialing Number (64 octets)                   +
      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                    Subaddress (64 octets)                     +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length                   Total length in octets of this PPTP message,
                            including the entire PPTP header.

   PPTP Message Type        1 for Control Message.

   Magic Cookie             0x1A2B3C4D.

   Control Message Type     9 for Incoming-Call-Request.

   Reserved0                This field MUST be 0.

   Call ID                  A unique identifier for this tunnel,
                            assigned by the PAC to this session.  It is
                            used to multiplex and demultiplex data sent
                            over the tunnel between the PNS and PAC
                            involved in this session.

   Call Serial Number       An identifier assigned by the PAC to this
                            session for the purpose of identifying this
                            particular session in logged session
                            information.  Unlike the Call ID, both the
                            PNS and PAC associate the same Call Serial
                            Number to a given session. The combination
                            of IP address and call serial number should
                            be unique.

   Bearer Type              A value indicating the bearer capability
                            used for this incoming call.  Currently
                            defined values are:

                                  1 - Call is on an analog channel

                                  2 - Call is on a digital channel

   Physical Channel ID      This field is set by the PAC in a vendor-
                            specific manner to the number of the
                            physical channel this call arrived on.

   Dialed Number Length     The actual number of valid digits in the
                            Dialed Number field.

   Dialing Number Length    The actual number of valid digits in the
                            Dialing Number field.

   Dialed Number            The number that was dialed by the caller.
                            For ISDN and analog calls this field is an
                            ASCII string.  If the Dialed Number is less
                            than 64 octets in length, the remainder of
                            this field is filled with octets of value 0.

   Dialing Number           The number from which the call was placed.
                            For ISDN and analog calls this field is an
                            ASCII string.  If the Dialing Number is less
                            than 64 octets in length, the remainder of
                            this field is filled with octets of value 0.

   Subaddress               A 64 octet field used to specify additional
                            dialing information.  If the subaddress is
                            less than 64 octets long, the remainder of
                            this field is filled with octets of value 0.

2.10.  Incoming-Call-Reply

   The Incoming-Call-Reply is a PPTP control message sent by the PNS to
   the PAC in response to a received Incoming-Call-Request message.  The
   reply indicates the result of the incoming call attempt.  It also
   provides information to allow the PAC to regulate the transmission of
   data to the PNS for this session.

   This message is the second in the three-way handshake used by PPTP
   for establishing incoming calls.  It indicates to the PAC whether the
   call should be answered or not.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Length             |       PPTP Message Type       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message Type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Call ID            |       Peer's Call ID          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Result Code  |  Error Code   |   Packet Recv. Window Size    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Packet Transmit Delay     |           Reserved1           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length                   Total length in octets of this PPTP message,
                            including the entire PPTP header.

   PPTP Message Type        1 for Control Message.

   Magic Cookie             0x1A2B3C4D.

   Control Message Type     10 for Incoming-Call-Reply.

   Reserved0                This field MUST be 0.

   Call ID                  A unique identifier for this tunnel assigned
                            by the PNS to this session.  It is used to
                            multiplex and demultiplex data sent over the
                            tunnel between the PNS and PAC involved in
                            this session.

   Peer's Call ID           This field is set to the value received in
                            the Call ID field of the corresponding
                            Incoming-Call-Request message. It is used by

                            the PAC to match the Incoming-Call-Reply
                            with the Incoming-Call-Request it issued.
                            This value is included in the GRE header of
                            transmitted data packets for this session.

   Result Code              This value indicates the result of the
                            Incoming-Call-Request attempt.  Current
                            valid Result Code values are:

                                  1 (Connect) - The PAC should answer
                                    the incoming call

                                  2 (General Error) - The Incoming Call
                                    should not be established due to the
                                    reason indicated in Error Code

                                  3 (Do Not Accept) - The PAC should not
                                    accept the incoming call.  It should
                                    hang up or issue a busy indication

   Error Code               This field is set to 0 unless a "General
                            Error" condition exists, in which case
                            Result Code is set to 2 and this field is
                            set to the value corresponding to the
                            general error condition as specified in
                            section 2.2.

   Packet Recv. Window Size The number of received data packets the PAC
                            will buffer for this session.

   Packet Transmit Delay    A measure of the packet processing delay
                            that might be imposed on data sent to the
                            PAC from the PNS.  This value is specified
                            in units of 1/10 seconds.

   Reserved1                This field MUST be 0.

2.11.  Incoming-Call-Connected

   The Incoming-Call-Connected message is a PPTP control message sent by
   the PAC to the PNS in response to a received Incoming-Call-Reply.  It
   provides information to the PNS about particular parameters used for
   the call.  It also provides information to allow the PNS to regulate
   the transmission of data to the PAC for this session.

   This message is the third in the three-way handshake used by PPTP for
   establishing incoming calls.  It provides a mechanism for providing
   the PNS with additional information about the call that cannot, in

   general, be obtained at the time the Incoming-Call-Request is issued
   by the PAC.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Length             |      PPTP Message Type        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message Type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Peer's Call ID          |           Reserved1           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Connect Speed                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Packet Recv. Window Size    |     Packet Transmit Delay     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Framing Type                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length                   Total length in octets of this PPTP message,
                            including the entire PPTP header.

   PPTP Message Type        1 for Control Message.

   Magic Cookie             0x1A2B3C4D.

   Control Message Type     11 for Incoming-Call-Connected.

   Reserved0                This field MUST be 0.

   Peer's Call ID           This field is set to the value received in
                            the Call ID field of the corresponding
                            Incoming-Call-Reply message.  It is used by
                            the PNS to match the Incoming-Call-Connected
                            with the Incoming-Call-Reply it issued.

   Connect Speed            The actual connection speed used, in
                            bits/second.

   Packet Recv. Window Size The number of received data packets the PAC
                            will buffer for this session.

   Packet Transmit Delay    A measure of the packet processing delay
                            that might be imposed on data sent to the
                            PAC from the PNS.  This value is specified
                            in units of 1/10 seconds.

   Framing Type             A value indicating the type of PPP framing
                            being used by this incoming call.

                                  1 - Call uses asynchronous framing

                                  2 - Call uses synchronous framing

2.12.  Call-Clear-Request

   The Call-Clear-Request is a PPTP control message sent by the PNS to
   the PAC indicating that a particular call is to be disconnected.  The
   call being cleared can be either an incoming or outgoing call, in any
   state.  The PAC responds to this message with a Call-Disconnect-
   Notify message.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Length             |      PPTP Message Type        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message Type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Call ID            |           Reserved1           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length                   Total length in octets of this PPTP message,
                            including the entire PPTP header.

   PPTP Message Type        1 for Control Message.

   Magic Cookie             0x1A2B3C4D.

   Control Message Type     12 for Call-Clear-Request.

   Reserved0                This field MUST be 0.

   Call ID                  The Call ID assigned by the PNS to this
                            call.  This value is used instead of the
                            Peer's Call ID because the latter may not be
                            known to the PNS if the call must be aborted
                            during call establishment.

   Reserved1                This field MUST be 0.

2.13.  Call-Disconnect-Notify

   The Call-Disconnect-Notify message is a PPTP control message sent by
   the PAC to the PNS.  It is issued whenever a call is disconnected,
   due to the receipt by the PAC of a Call-Clear-Request or for any
   other reason.  Its purpose is to inform the PNS of both the
   disconnection and the reason for it.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Length             |      PPTP Message Type        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message Type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Call ID            |  Result Code  |  Error Code   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Cause Code           |           Reserved1           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +              Call Statistics (128 octets)                     +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length                   Total length in octets of this PPTP message,
                            including the entire PPTP header.

   PPTP Message Type        1 for Control Message.

   Magic Cookie             0x1A2B3C4D.

   Control Message Type     13 for Call-Disconnect-Notify.

   Reserved0                This field MUST be 0.

   Call ID                  The value of the Call ID assigned by the PAC
                            to this call.  This value is used instead of
                            the Peer's Call ID because the latter may
                            not be known to the PNS if the call must be
                            aborted during call establishment.

   Result Code              This value indicates the reason for the
                            disconnect. Current valid Result Code values
                            are:

                                  1 (Lost Carrier) - Call disconnected
                                    due to loss of carrier

                                  2 (General Error) - Call disconnected
                                    for the reason indicated in Error
                                    Code

                                  3 (Admin Shutdown) - Call disconnected
                                    for administrative reasons

                                  4 (Request) - Call disconnected due to
                                    received Call-Clear-Request

   Error Code               This field is set to 0 unless a "General
                            Error" condition exists, in which case the
                            Result Code is set to 2 and this field is
                            set to the value corresponding to the
                            general error condition as specified in
                            section 2.2.

   Cause Code               This field gives additional disconnect
                            information.  Its value varies depending on
                            the type of call being disconnected.  For
                            ISDN calls it is the Q.931 cause code.

   Call Statistics          This field is an ASCII string containing
                            vendor-specific call statistics that can be
                            logged for diagnostic purposes.  If the
                            length of the string is less than 128, the
                            remainder of the field is filled with octets
                            of value 0.

2.14.  WAN-Error-Notify

   The WAN-Error-Notify message is a PPTP control message sent by the
   PAC to the PNS to indicate WAN error conditions (conditions that
   occur on the interface supporting PPP).  The counters in this message
   are cumulative.  This message should only be sent when an error
   occurs, and not more than once every 60 seconds.  The counters are
   reset when a new call is established.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Length             |      PPTP Message Type        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Peer's Call ID         |           Reserved1           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          CRC Errors                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Framing Errors                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Hardware Overruns                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Buffer Overruns                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Time-out Errors                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Alignment Errors                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length                   Total length in octets of this PPTP message,
                            including the entire PPTP header.

   PPTP Message Type        1 for Control Message.

   Magic Cookie             0x1A2B3C4D.

   Control Message Type     14 for WAN-Error-Notify.

   Reserved0                This field MUST be 0.

   Peer's Call ID           Th Call ID assigned by the PNS to this call.

   CRC Errors               Number of PPP frames received with CRC
                            errors since session was established.

   Framing Errors           Number of improperly framed PPP packets
                            received.

   Hardware Overruns        Number of receive buffer over-runs since
                            session was established.

   Buffer Overruns          Number of buffer over-runs detected since
                            session was established.

   Time-out Errors          Number of time-outs since call was
                            established.

   Alignment Errors         Number of alignment errors since call was
                            established.

2.15.  Set-Link-Info

   The Set-Link-Info message is a PPTP control message sent by the PNS
   to the PAC to set PPP-negotiated options.  Because these options can
   change at any time during the life of the call, the PAC must be able
   to update its internal call information dynamically and perform PPP
   negotiation on an active PPP session.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Length             |      PPTP Message Type        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Magic Cookie                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Control Message type      |           Reserved0           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Peer's Call ID         |           Reserved1           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Send ACCM                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Receive ACCM                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length                   Total length in octets of this PPTP message,
                            including the entire PPTP header.

   PPTP Message Type        1 for Control Message.

   Magic Cookie             0x1A2B3C4D.

   Control Message Type     15 for Set-Link-Info.

   Reserved0                This field MUST be 0.

   Peer's Call ID           The value of the Call ID assigned by the PAC
                            to this call.

   Reserved1                This field MUST be 0.

   Send ACCM                The send ACCM value the client should use to
                            process outgoing PPP packets.  The default
                            value used by the client until this message
                            is received is 0XFFFFFFFF.  See [7].

   Receive ACCM             The receive ACCM value the client should use
                            to process incoming PPP packets. The default
                            value used by the client until this message
                            is received is 0XFFFFFFFF.  See [7].

2.16.  General Error Codes

   General error codes pertain to types of errors which are not specific
   to any particular PPTP request, but rather to protocol or message
   format errors.  If a PPTP reply indicates in its Result Code that a
   general error occurred, the General Error value should be examined to
   determined what the error was.  The currently defined General Error
   codes and their meanings are:

      0 (None)          - No general error

      1 (Not-Connected) - No control connection exists yet for this
                          PAC-PNS pair

      2 (Bad-Format)    - Length is wrong or Magic Cookie value is
                          incorrect

      3 (Bad-Value)     - One of the field values was out of range or
                          reserved field was non-zero

      4 (No-Resource)   - Insufficient resources to handle this command
                          now

      5 (Bad-Call ID)    - The Call ID is invalid in this context

      6 (PAC-Error)     - A generic vendor-specific error occurred in
                          the PAC

3.  Control Connection Protocol Operation

   This section describes the operation of various PPTP control
   connection functions and the Control Connection messages which are
   used to support them.  The protocol operation of the control
   connection is simplified because TCP is used to provide a reliable
   transport mechanism.  Ordering and retransmission of messages is not
   a concern at this level.  The TCP connection itself, however, can
   close at any time and an appropriate error recovery mechanism must be
   provided to handle this case.

   Some error recovery procedures are common to all states of the
   control connection.  If an expected reply does not arrive within 60
   seconds, the control connection is closed, unless otherwise
   specified.  Appropriate logging should be implemented for easy
   determination of the problems and the reasons for closing the control
   connection.

   Receipt of an invalid or malformed Control Connection message should
   be logged appropriately, and the control connection should be closed
   and restarted to ensure recovery into a known state.

3.1.  Control Connection States

   The control connection relies on a standard TCP connection for its
   service.  The PPTP control connection protocol is not distinguishable
   between the PNS and PAC, but is distinguishable between the
   originator and receiver. The originating peer is the one which first
   attempts the TCP open. Since either PAC or PNS may originate a
   connection, it is possible for a TCP collision to occur.  See section
   3.1.3 for a description of this situation.

3.1.1.  Control Connection Originator (may be PAC or PNS)

                TCP Open Indication
                /Send Start Control
                  Connection Request       +-----------------+
     +------------------------------------>|  wait_ctl_reply |
     |                                     +-----------------+
     |     Collision/See (4.1.3) Close TCP   V  V  V   Receive Start Ctl
     |       +-------------------------------+  |  |   Connection Reply
     |       |                                  |  |   Version OK
     ^       V                                  |  V
+-----------------+          Receive Start Ctl  | +-----------------+
|      idle       |          Connection Reply   | |   established   |
+-----------------+          Version Not OK     | +-----------------+
     ^                                          |  V   Local Terminate
     |         Receive Stop Control             |  |   /Send Stop
     |         Connection Request               |  |    Control Request
     |         /Send Stop Control Reply         V  V
     |          Close TCP                  +-----------------+
     +-------------------------------------| wait_stop_reply |
                                           +-----------------+

   idle
      The control connection originator attempts to open a TCP
      connection to the peer during idle state. When the TCP connection
      is open, the originator transmits a send Start-Control-
      Connection-Request and then enters the wait_ctl_reply state.

   wait_ctl_reply
      The originator checks to see if another TCP connection has been
      requested from the same peer, and if so, handles the collision
      situation described in section 3.1.3.

      When a Start-Control-Connection-Reply is received, it is examined
      for a compatible version. If the version of the reply is lower
      than the version sent in the request, the older (lower) version
      should be used provided it is supported.  If the version in the
      reply is earlier and supported, the originator moves to the
      established state. If the version is earlier and not supported, a
      Stop-Control-Connection-Request SHOULD be sent to the peer and the
      originator moves into the wait_stop_reply state.

   established
      An established connection may be terminated by either a local
      condition or the receipt of a Stop-Control-Connection-Request. In
      the event of a local termination, the originator MUST send a
      Stop-Control-Connection-Request and enter the wait_stop_reply
      state.

      If the originator receives a Stop-Control-Connection-Request it
      SHOULD send a Stop-Control-Connection-Reply and close the TCP
      connection making sure that the final TCP information has been
      "pushed" properly.

   wait_stop_reply
      If a Stop-Control-Connection-Reply is received, the TCP connection
      SHOULD be closed and the control connection becomes idle.

3.1.2.  Control connection Receiver (may be PAC or PNS)

Receive Start Control Connection Request
Version Not OK/Send Start Control Connection
Reply with Error
  +--------+
  |        |         Receive Control Connection Request Version OK
  |        |         /Send Start Control Connection Reply
  |        |   +----------------------------------------+
  ^        V   ^                                        V
+-----------------+             Receive Start Ctl    +-----------------+
|      Idle       |             Connection Request   |   Established   |
+-----------------+             /Send Stop Reply     +-----------------+
        ^      ^                 Close TCP           V  V Local Terminate
        |      +-------------------------------------+  | /Send Stop
        |                                               |  Control Conn.
        |                                               V  Request
        |                                     +-----------------+
        +-------------------------------------| Wait-Stop-Reply |
                 Receive Stop Control         +-----------------+
                 Connection Reply
                 /Close TCP

   idle
      The control connection receiver waits for a TCP open attempt on
      port 1723. When notified of an open TCP connection, it should
      prepare to receive PPTP messages.  When a Start-Control-
      Connection-Request is received its version field should be
      examined. If the version is earlier than the receiver's version
      and the earlier version can be supported by the receiver, the
      receiver SHOULD send a Start-Control-Connection-Reply. If the
      version is earlier than the receiver's version and the version
      cannot be supported, the receiver SHOULD send a Start-Connection-
      Reply message, close the TCP connection and remain in the idle
      state.  If the receiver's version is the same as earlier than the
      peer's, the receiver SHOULD send a Start-Control-Connection-Reply
      with the receiver's version and enter the established state.

   established
      An established connection may be terminated by either a local
      condition or the reception of a Stop-Control-Connection-Request.
      In the event of a local termination, the originator MUST send a
      Stop-Control-Connection-Request and enter the wait_stop_reply
      state.

      If the originator receives a Stop-Control-Connection-Request it
      SHOULD send a Stop-Control-Connection-Reply and close the TCP
      connection, making sure that the final TCP information has been
      "pushed" properly.

   wait_stop_reply
      If a Stop-Control-Connection-Reply is received, the TCP connection
      SHOULD be closed and the control connection becomes idle.

3.1.3.  Start Control Connection Initiation Request Collision

   A PAC and PNS must have only one control connection between them. It
   is possible, however, for a PNS and a PAC to simultaneously attempt
   to establish a control connection to each other. When a Start-
   Control-Connection-Request is received on one TCP connection and
   another Start-Control-Connection-Request has already been sent on
   another TCP connection to the same peer, a collision has occurred.

   The "winner" of the initiation race is the peer with the higher IP
   address (compared as 32 bit unsigned values, network number more
   significant). For example, if the peers 192.33.45.17 and 192.33.45.89
   collide, the latter will be declared the winner.  The loser will
   immediately close the TCP connection it initiated, without sending
   any further PPTP control messages on it and will respond to the
   winner's request with a Start-Control-Connection-Reply message. The
   winner will wait for the Start-Control-Connection-Reply on the
   connection it initiated and also wait for a TCP termination
   indication on the connection the loser opened.  The winner MUST NOT
   send any messages on the connection the loser initiated.

3.1.4.  Keep Alives and Timers

   A control connection SHOULD be closed by closing the underlying TCP
   connection under the following circumstances:

   1. If a control connection is not in the established state (i.e.,
      Start-Control-Connection-Request and Start-Control-Connection-
      Reply have not been exchanged), a control connection SHOULD be
      closed after 60 seconds by a peer waiting for a Start-Control-
      Connection-Request or Start-Control-Connection-Reply message.

   2. If a peer's control connection is in the established state and has
      not received a control message for 60 seconds, it SHOULD send a
      Echo-Request message. If an Echo-Reply is not received 60 seconds
      after the Echo-Request message transmission, the control
      connection SHOULD be closed.

3.2.  Call States

3.2.1.  Timing considerations

   Because of the real-time nature of telephone signaling, both the PNS
   and PAC should be implemented with multi-threaded architectures such
   that messages related to multiple calls are not serialized and
   blocked. The transit delay between the PAC and PNS should not exceed
   one second. The call and connection state figures do not specify
   exceptions caused by timers. The implicit assumption is that since
   the TCP-based control connection is being verified with keep-alives,
   there is less need to maintain strict timers for call control
   messages.

   Establishing outbound international calls, including the modem
   training and negotiation sequences, may take in excess of 1 minute so
   the use of short timers is discouraged.

   If a state transition does not occur within 1 minute (except for
   connections in the idle or established states), the integrity of the
   protocol processing between the peers is suspect and the ENTIRE
   CONTROL CONNECTION should be closed and restarted. All Call IDs are
   logically released whenever a control connection is started. This
   presumably also helps in preventing toll calls from being "lost" and
   never cleared.

3.2.2.  Call ID Values

   Each peer assigns a Call ID value to each user session it requests or
   accepts. This Call ID value MUST be unique for the tunnel between the
   PNS and PAC to which it belongs. Tunnels to other peers can use the
   same Call ID number so the receiver of a packet on a tunnel needs to
   associate a user session with a particular tunnel and Call ID.  It is
   suggested that the number of potential Call ID values for each tunnel
   be at least twice as large as the maximum number of calls expected on
   a given tunnel.

   A session is defined by the triple (PAC, PNS, Call ID).

3.2.3.  Incoming Calls

   An Incoming-Call-Request message is generated by the PAC when an
   associated telephone line rings. The PAC selects a Call ID and serial
   number and indicates the call bearer type.  Modems should always
   indicate analog call type.  ISDN calls should indicate digital when
   unrestricted digital service or rate adaption is used and analog if

   digital modems are involved. Dialing number, dialed number, and
   subaddress may be included in the message if they are available from
   the telephone network.

   Once the PAC sends the Incoming-Call-Request, it waits for a response
   from the PNS but does not answer the call from the telephone network.
   The PNS may choose not to accept the call if:

      -  No resources are available to handle more sessions

      -  The dialed, dialing, or subaddress fields are not indicative of
         an authorized user

      -  The bearer service is not authorized or supported

   If the PNS chooses to accept the call, it responds with an Incoming-
   Call-Reply which also indicates window sizes (see section 4.2). When
   the PAC receives the Outgoing-Call-Reply, it attempts to connect the
   call, assuming the calling party has not hung up. A final call
   connected message from the PAC to the PNS indicates that the call
   states for both the PAC and the PNS should enter the established
   state.

   When the dialed-in client hangs up, the call is cleared normally and
   the PAC sends a Call-Disconnect-Notify message. If the PNS wishes to
   clear a call, it sends a Call-Clear-Request message and then waits
   for a Call-Disconnect-Notify.

3.2.3.1.  PAC Incoming Call States

    Ring/Send Incoming Call Request          +-----------------+
  +----------------------------------------->|    wait_reply   |
  |                                          +-----------------+
  |           Receive Incoming Call Reply    V  V  V
  |           Not Accepting                  |  |  |   Receive Incoming
  |         +--------------------------------+  |  |   Call Reply Accept-
  |         |    +------------------------------+  |   ing/Answer call;
  |         |    |     Abort/Send Call             |   Send Call
  ^         V    V     Disconnect Notify           V   Connected
+-----------------+                              +-----------------+
|      idle       |<-----------------------------|   established   |
+-----------------+  Receive Clear Call Request  +-----------------+
                     or telco call dropped
                     or local disconnect
                     /Send Call Disconnect Notify

The states associated with the PAC for incoming calls are:

   idle
      The PAC detects an incoming call on one of its telco interfaces.
      Typically this means an analog line is ringing or an ISDN TE has
      detected an incoming Q.931 SETUP message. The PAC sends an
      Incoming-Call-Request message and moves to the wait_reply state.

   wait_reply
      The PAC receives an Incoming-Call-Reply message indicating non-
      willingness to accept the call (general error or don't accept) and
      moves back into the idle state. If the reply message indicates
      that the call is accepted, the PAC sends an Incoming-Call-
      Connected message and enters the established state.

   established
      Data is exchanged over the tunnel.  The call may be cleared
      following:

         An event on the telco connection. The PAC sends a
         Call-Disconnect-Notify message

         Receipt of a Call-Clear-Request.  The PAC sends a
         Call-Disconnect-Notify message

         A local reason. The PAC sends a Call-Disconnect-Notify message.

3.2.3.2.  PNS Incoming Call States

  Receive Incoming Call Request
  /Send Incoming Call Reply                  +-----------------+
   Not Accepting if Error                    |   Wait-Connect  |
  +-----+                                    +-----------------+
  |     |     Receive Incoming Call Req.     ^  V  V
  |     |     /Send Incoming Call Reply OK   |  |  |   Receive Incoming
  |     |   +--------------------------------+  |  |   Call Connect
  ^     V   ^    V------------------------------+  V
+-----------------+  Receive Call Disconnect     +-----------------+
|      Idle       |  Notify                   +- |   Established   |
+-----------------+                           |  +-----------------+
        ^        ^                            |   V   Local Terminate
        |        +----------------------------+   |   /Send Call Clear
        |            Receive Call Disconnect      |    Request
        |            Notify                       V
        |                                      +-----------------+
        +--------------------------------------| Wait-Disconnect |
                     Receive Call Disconnect   +-----------------+
                     Notify

The states associated with the PNS for incoming calls are:

   idle
      An Incoming-Call-Request message is received. If the request is
      not acceptable, an Incoming-Call-Reply is sent back to the PAC and
      the PNS remains in the idle state.  If the Incoming-Call-Request
      message is acceptable, an Incoming-Call-Reply is sent indicating
      accept in the result code. The session moves to the wait_connect
      state.

   wait_connect
      If the session is connected on the PAC, the PAC sends an incoming
      call connect message to the PNS which then moves into established
      state. The PAC may send a Call-Disconnect-Notify to indicate that
      the incoming caller could not be connected.  This could happen,
      for example, if a telephone user accidently places a standard
      voice call to a PAC resulting in a handshake failure on the called
      modem.

   established
      The session is terminated either by receipt of a Call-Disconnect-
      Notify message from the PAC or by sending a Call-Clear-Request.
      Once a Call-Clear-Request has been sent, the session enters the
      wait_disconnect state.

   wait_disconnect
      Once a Call-Disconnect-Notify is received the session moves back
      to the idle state.

3.2.4.  Outgoing Calls

   Outgoing messages are initiated by a PNS and instruct a PAC to place
   a call on a telco interface. There are only two messages for outgoing
   calls: Outgoing-Call-Request and Outgoing-Call-Reply. The PNS sends
   an Outgoing-Call-Request specifying the dialed party phone number and
   subaddress as well as speed and window parameters. The PAC MUST
   respond to the Outgoing-Call-Request message with an Outgoing-Call-
   Reply message once the PAC determines that:

      The call has been successfully connected

      A call failure has occurred for reasons such as: no interfaces are
      available for dial-out, the called party is busy or does not
      answer, or no dial tone is detected on the interface chosen for
      dialing

3.2.4.1.  PAC Outgoing Call States

Receive Outgoing Call Request in Error
/Send Outgoing Call Reply with Error
  |--------+
  |        |         Receive Outgoing Call Request No Error
  |        |         /Off Hook; Dial
  |        |   +-----------------------------------------
  ^        V   ^                                        V
+-----------------+           Incomplete Call        +-----------------+
|      idle       |           /Send Outgoing Call    |   wait_cs_ans   |
+-----------------+            Reply with Error      +-----------------+
        ^      ^           or Recv. Call Clear Req.  V  V Telco Answer
        |      |              /Send Disconnect Notify|  | /Send Outgoing
        |      +-------------------------------------+  |  Call Reply.
        |                                               V
        |                                     +-----------------+
        +-------------------------------------|   established   |
                 Receive Call Clear Request   +-----------------+
                 or local terminate
                 or telco disconnect
                 /Hangup call and send
                 Call Disconnect Notify

   The states associated with the PAC for outgoing calls are:

   idle
      Received Outgoing-Call-Request. If this is received in error,
      respond with an Outgoing-Call-Reply with error condition set.
      Otherwise, allocate physical channel to dial on. Place the
      outbound call, wait for a connection, and move to the wait_cs_ans
      state.

   wait_cs_ans
      If the call is incomplete, send an Outgoing-Call-Reply with a
      non-zero Error Code. If a timer expires on an outbound call, send
      back an Outgoing-Call-Reply with a non-zero Error Code. If a
      circuit switched connection is established, send an Outgoing-
      Call-Reply indicating success.

   established
      If a Call-Clear-Request is received, the telco call SHOULD be
      released via appropriate mechanisms and a Call-Disconnect-Notify
      message SHOULD BE sent to the PNS. If the call is disconnected by
      the client or by the telco interface, a Call-Disconnect-Notify
      message SHOULD be sent to the PNS.

3.2.4.2.  PNS Outgoing Call States

                Open Indication                              Abort/Send
                /Send Outgoing Call                          Call Clear
                 Request                  +-----------------+Request
        +-------------------------------->|    Wait-Reply   |----------+
        |                                 +-----------------+          |
        |     Receive Outgoing Call Reply   V     V   Receive Outgoing |
        |     with Error                    |     |   Call Reply       |
        |   +-------------------------------+     |   No Error         |
        ^   V                                     V                    |
+-----------------+                              +-----------------+   |
|      Idle       |<-----------------------------|   Established   |   |
+-----------------+    Receive Call Disconnect   +-----------------+   |
        ^              Notify                     V   Local Terminate  |
        |                                         |   /Send Call Clear |
        |                                         |    Request         |
        |     Receive Call Disconnect             V                    |
        |     Notify                      +-----------------+          |
        +---------------------------------| Wait-Disconnect |<---------+
                                          +-----------------+

The states associated with the PNS for outgoing calls are:

   idle
      An Outgoing-Call-Request message is sent to the PAC and the
      session moves into the wait_reply state.

   wait_reply
      An Outgoing-Call-Reply is received which indicates an error. The
      session returns to idle state. No telco call is active. If the
      Outgoing-Call-Reply does not indicate an error, the telco call is
      connected and the session moves to the established state.

   established
      If a Call-Disconnect-Notify is received, the telco call has been
      terminated for the reason indicated in the Result and Cause Codes.
      The session moves back to the idle state. If the PNS chooses to
      terminate the session, it sends a Call-Clear-Request to the PAC
      and then enters the wait_disconnect state.

   wait_disconnect
      A session disconnection is waiting to be confirmed by the PAC.
      Once the PNS receives the Call-Disconnect-Notify message, the
      session enters idle state.

4.  Tunnel Protocol Operation

   The user data carried by the PPTP protocol are PPP data packets.  PPP
   packets are carried between the PAC and PNS, encapsulated in GRE
   packets which in turn are carried over IP.  The encapsulated PPP
   packets are essentially PPP data packets less any media specific
   framing elements.  No HDLC flags, bit insertion, control characters,
   or control character escapes are included. No CRCs are sent through
   the tunnel. The IP packets transmitted over the tunnels between a PAC
   and PNS has the following general structure:

      +--------------------------------+
      |          Media Header          |
      +--------------------------------+
      |           IP Header            |
      +--------------------------------+
      |           GRE Header           |
      +--------------------------------+
      |           PPP Packet           |
      +--------------------------------+

4.1.  Enhanced GRE header

   The GRE header used in PPTP is enhanced slightly from that specified
   in the current GRE protocol specification [1,2].  The main difference
   involves the definition of a new Acknowledgment Number field, used to
   determine if a particular GRE packet or set of packets has arrived at
   the remote end of the tunnel.  This Acknowledgment capability is not
   used in conjunction with any retransmission of user data packets.  It
   is used instead to determine the rate at which user data packets are
   to be transmitted over the tunnel for a given user session.  The
   format of the enhanced GRE header is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |C|R|K|S|s|Recur|A| Flags | Ver |         Protocol Type         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Key (HW) Payload Length    |       Key (LW) Call ID        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Sequence Number (Optional)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Acknowledgment Number (Optional)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   C
      (Bit 0) Checksum Present.  Set to zero (0).

   R
      (Bit 1) Routing Present.  Set to zero (0).

   K
      (Bit 2) Key Present.  Set to one (1).
   S
      (Bit 3) Sequence Number Present.  Set to one (1) if a payload
      (data) packet is present.  Set to zero (0) if payload is not
      present (GRE packet is an Acknowledgment only).

   s
      (Bit 4) Strict source route present.  Set to zero (0).

   Recur
      (Bits 5-7) Recursion control.  Set to zero (0).

   A
      (Bit 8) Acknowledgment sequence number present.  Set to one (1) if
      packet contains Acknowledgment Number to be used for acknowledging
      previously transmitted data.

   Flags
      (Bits 9-12) Must be set to zero (0).

   Ver
      (Bits 13-15) Must contain 1 (enhanced GRE).

   Protocol Type
      Set to hex 880B [8].

   Key
      Use of the Key field is up to the implementation.  PPTP uses it as
      follows:
         Payload Length
            (High 2 octets of Key) Size of the payload, not including
            the GRE header

         Call ID
            (Low 2 octets) Contains the Peer's Call ID for the session
            to which this packet belongs.

         Sequence Number
            Contains the sequence number of the payload.  Present if S
            bit (Bit 3) is one (1).

         Acknowledgment Number
            Contains the sequence number of the highest numbered GRE
            packet received by the sending peer for this user session.
            Present if A bit (Bit 8) is one (1).

         The payload section contains a PPP data packet without any
         media specific framing elements.

         The sequence numbers involved are per packet sequence numbers.
         The sequence number for each user session is set to zero at
         session startup.  Each packet sent for a given user session
         which contains a payload (and has the S bit (Bit 3) set to one)
         is assigned the next consecutive sequence number for that
         session.

         This protocol allows acknowledgments to be carried with the
         data and makes the overall protocol more efficient, which in
         turn requires less buffering of packets.

4.2.  Sliding Window Protocol

   The sliding window protocol used on the PPTP data path is used for
   flow control by each side of the data exchange.  The enhanced GRE
   protocol allows packet acknowledgments to be piggybacked on data
   packets.  Acknowledgments can also be sent separately from data
   packets.  Again, the main purpose of the sliding window protocol is
   for flow control--retransmissions are not performed by the tunnel
   peers.

4.2.1.  Initial Window Size

   Although each side has indicated the maximum size of its receive
   window, it is recommended that a conservative approach be taken when
   beginning to transmit data.  The initial window size on the
   transmitter is set to half the maximum size the receiver requested,
   with a minimum size of one packet.  The transmitter stops sending
   packets when the number of packets awaiting acknowledgment is equal
   to the current window size.  As the receiver successfully digests
   each window, the window size on the transmitter is bumped up by one
   packet until the maximum is reached.  This method prevents a system
   from flooding an already congested network because no history has
   been established.

4.2.2.  Closing the Window

   When a time-out does occur on a packet, the sender adjusts the size
   of the transmit window down to one half its value when it failed.
   Fractions are rounded up, and the minimum window size is one.

4.2.3.  Opening the Window

   With every successful transmission of a window's worth of packets
   without a time-out, the transmit window size is increased by one
   packet until it reaches the maximum window size that was sent by the
   other side when the call was connected.  As stated earlier, no
   retransmission is done on a time-out. After a time-out, the
   transmission resumes with the window starting at one half the size of
   the transmit window when the time-out occurred and adjusting upward
   by one each time the transmit window is filled with packets that are
   all acknowledged without time-outs.

4.2.4.  Window Overflow

   When a receiver's window overflows with too many incoming packets,
   excess packets are thrown away.  This situation should not arise if
   the sliding window procedures are being properly followed by the
   transmitter and receiver. It is assumed that, on the transmit side,
   packets are buffered for transmission and are no longer accepted from
   the packet source when the transmit buffer fills.

4.2.5.  Multi-packet Acknowledgment

   One feature of the PPTP sliding window protocol is that it allows the
   acknowledgment of multiple packets with a single acknowledgment. All
   outstanding packets with a sequence number lower or equal to the
   acknowledgment number are considered acknowledged. Time-out
   calculations are performed using the time the packet corresponding to
   the highest sequence number being acknowledged was transmitted.

   Adaptive time-out calculations are only performed when an
   Acknowledgment is received.  When multi-packet acknowledgments are
   used, the overhead of the adaptive time-out algorithm is reduced. The
   PAC is not required to transmit multi-packet acknowledgments; it can
   instead acknowledge each packet individually as it is delivered to
   the PPP client.

4.3.  Out-of-sequence Packets

   Occasionally packets lose their sequencing across a complicated
   internetwork.  Say, for example that a PNS sends packets 0 to 5 to a
   PAC.  Because of rerouting in the internetwork, packet 4 arrives at
   the PAC before packet 3. The PAC acknowledges packet 4, and may
   assume packet 3 is lost. This acknowledgment grants window credit
   beyond packet 4.

   When the PAC does receive packet 3, it MUST not attempt to transmit
   it to the corresponding PPP client.  To do so could cause problems,
   as proper PPP protocol operation is premised upon receiving packets
   in sequence.  PPP does properly deal with the loss of packets, but
   not with reordering so out of sequence packets between the PNS and
   PAC MUST be silently discarded, or they may be reordered by the
   receiver.  When packet 5 comes in, it is acknowledged by the PAC
   since it has a higher sequence number than 4, which was the last
   highest packet acknowledged by the PAC.  Packets with duplicate
   sequence numbers should never occur since the PAC and PNS never
   retransmit GRE packets.  A robust implementation will silently
   discard duplicate GRE packets, should it receive any.

4.4.  Acknowledgment Time-Outs

   PPTP uses sliding windows and time-outs to provide both user session
   flow-control across the internetwork and to perform efficient data
   buffering to keep the PAC-PNS data channels full without causing
   receive buffer overflow.  PPTP requires that a time-out be used to
   recover from dropped data or acknowledgment packets.  The exact
   implementation of the time-out is vendor-specific.  It is suggested
   that an adaptive time-out be implemented with backoff for congestion
   control.  The time-out mechanism proposed here has the following
   properties:

      Independent time-outs for each session. A device (PAC or PNS) will
      have to maintain and calculate time-outs for every active session.

      An administrator-adjustable maximum time-out, MaxTimeOut, unique
      to each device.

      An adaptive time-out mechanism that compensates for changing
      throughput.  To reduce packet processing overhead, vendors may
      choose not to recompute the adaptive time-out for every received
      acknowledgment.  The result of this overhead reduction is that the
      time-out will not respond as quickly to rapid network changes.

      Timer backoff on time-out to reduce congestion. The backed-off
      timer value is limited by the configurable maximum time-out value.
      Timer backoff is done every time an acknowledgment time-out
      occurs.

   In general, this mechanism has the desirable behavior of quickly
   backing off upon a time-out and of slowly decreasing the time-out
   value as packets are delivered without time-outs.

   Some definitions:

      Packet Processing Delay (PPD) is 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.

      Sample is the actual amount of time incurred receiving an
      acknowledgment for a packet. The Sample is measured, not
      calculated.

      Round-Trip Time (RTT) is 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.

      Adaptive Time-Out (ATO) is 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.

   The Packet Processing Delay (PPD) parameter is a 16-bit word
   exchanged during the Call Control phase that represents tenths of a
   second (64 means 6.4 seconds). The protocol only specifies that the
   parameter is exchanged, it does not specify how it is calculated. The
   way values for PPD are calculated is implementation-dependent and
   need not be variable (static time-outs are allowed). The PPD must be
   exchanged in the call connect sequences, even if it remains constant
   in an implementation. One possible way to calculate the PPD is:

   PPD' = ((PPP_MAX_DATA_MTU - Header) * WindowSize * 8) / ConnectRate
   PPD = PPD' + PACFudge

   Header is the total size of the IP and GRE headers, which is 36. The
   MTU is the overall MTU for the internetwork link between the PAC and
   PNS.  WindowSize represents the number of packets in the sliding
   window, and is implementation-dependent. The latency of the
   internetwork could be used to pick a window size sufficient to keep
   the current session's pipe full. The constant 8 converts octets to
   bits (assuming ConnectRate is in bits per second).  If ConnectRate is
   in bytes per second, omit the 8.  PACFudge is not required but can be
   used to take overall processing overhead of the PAC into account.

   The value of PPD is used to seed the adaptive algorithm with the
   initial RTT[n-1] value.

4.4.1.  Calculating Adaptive Acknowledgment Time-Out

   We still must decide how much time to allow for acknowledgments to
   return. If the time-out is set too high, we may wait an unnecessarily
   long time for dropped packets. If the time-out is too short, we may
   time out just before the acknowledgment arrives. The acknowledgment
   time-out should also be reasonable and responsive to changing network
   conditions.

   The suggested adaptive algorithm detailed below is based on the TCP
   1989 implementation and is explained in [11].  'n' means this
   iteration of the calculation, and 'n-1' refers to values from the
   last calculation.

      DIFF[n] = SAMPLE[n] - RTT[n-1]
      DEV[n] = DEV[n-1] + (beta * (|DIFF[n]| - DEV[n-1]))
      RTT[n] = RTT[n-1] + (alpha * DIFF[n])
      ATO[n] = MAX (MinTimeOut, MIN (RTT[n] +
               (chi * DEV[n]), MaxTimeOut))

      DIFF represents the error between the last estimated round-trip
      time and the measured time. DIFF is calculated on each iteration.

      DEV is the estimated mean deviation. This approximates the
      standard deviation.  DEV is calculated on each iteration and
      stored for use in the next iteration. Initially, it is set to 0.

      RTT is the estimated round-trip time of an average packet. RTT is
      ycalculated on each iteration and stored for use in the next
      iteration.  Initially, it is set to PPD.

      ATO is the adaptive time-out for the next transmitted packet. ATO
      is calculated on each iteration.  Its value is limited, by the MIN
      function, to be a maximum of the configured MaxTimeOut value.

      Alpha is the gain for the average and is typically 1/8 (0.125).

      Beta is the gain for the deviation and is typically 1/4 (0.250).

      Chi is the gain for the time-out and is typically set to 4.

   To eliminate division operations for fractional gain elements, the
   entire set of equations can be scaled. With the suggested gain
   constants, they should be scaled by 8 to eliminate all division.  To
   simplify calculations, all gain values are kept to powers of two so
   that shift operations can be used in place of multiplication or
   division.

4.4.2.  Congestion Control: Adjusting for Time-Out

   This section describes how the calculation of ATO is modified in the
   case where a time-out does occur.  When a time-out occurs, the time-
   out value should be adjusted rapidly upward.  Although the GRE
   packets are not retransmitted when a time-out occurs, the time-out
   should be adjusted up toward a maximum limit.  To compensate for
   shifting internetwork time delays, a strategy must be employed to
   increase the time-out when it expires (notice that in addition to
   increasing the time-out, we are also shrinking the size of the window
   as described in the next section).  For an interval in which a time-
   out occurs, the new
   ATO is calculated as:

      RTT[n] = delta * RTT[n-1]
      DEV[n] = DEV[n-1]
      ATO[n] = MAX (MinTimeOut, MIN (RTT[n] +
               (chi * DEV[n]), MaxTimeOut))

   In this calculation of ATO, only the two values that both contribute
   to ATO and are stored for the next iteration are calculated.  RTT is
   scaled by delta, and DEV is unmodified.  DIFF is not carried forward
   and is not used in this scenario.  A value of 2 for Delta, the time-
   out gain factor for RTT, is suggested.

5.  Security Considerations

   The security of user data passed over the tunneled PPP connection is
   addressed by PPP, as is authentication of the PPP peers.

   Because the PPTP control channel messages are neither authenticated
   nor integrity protected, it might be possible for an attacker to
   hijack the underlying TCP connection.  It is also possible to
   manufacture false control channel messages and alter genuine messages
   in transit without detection.

   The GRE packets forming the tunnel itself are not cryptographically
   protected.  Because the PPP negotiations are carried out over the
   tunnel, it may be possible for an attacker to eavesdrop on and modify
   those negotiations.

   Unless the PPP payload data is cryptographically protected, it can be
   captured and read or modified.

6.  Authors' Addresses

   Kory Hamzeh
   Ascend Communications
   1275 Harbor Bay Parkway
   Alameda, CA 94502

   EMail: kory@ascend.com

   Gurdeep Singh Pall
   Microsoft Corporation
   Redmond, WA

   EMail: gurdeep@microsoft.com

   William Verthein
   U.S. Robotics/3Com

   Jeff Taarud
   Copper Mountain Networks

   W. Andrew Little
   ECI Telematics

   Glen Zorn
   Microsoft Corporation
   Redmond, WA

   EMail: glennz@microsoft.com

7.  References

   [1]  Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing
        Encapsulation (GRE)", RFC 1701, October 1994.

   [2]  Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing
        Encapsulation (GRE) over IPv4 Networks", RFC 1702, October 1994.

   [3]  Lloyd, B. and W. Simpson, "PPP Authentication Protocols", RFC
        1334, October 1992.

   [4]  Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
        September 1981.

   [5]  Postel, J., "User Data Protocol", STD 6, RFC 768, August 1980.

   [6]  Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
        October 1994.  See also: http://www.iana.org/numbers.html

   [7]  Simpson, W., editor, "The Point-to-Point Protocol (PPP)", STD
        51, RFC 1661, July 1994.

   [8]  Ethertype for PPP, Reserved with Xerox Corporation.

   [9]  Simpson, W., "PPP Challenge Handshake Authentication Protocol
        (CHAP)", RFC 1994, August 1996.

   [10] Blunk, L. and J Vollbrecht, "PPP Extensible Authentication
        Protocol (EAP)", RFC 2284, March 1998.

   [11] Stevens, R., "TCP/IP Illustrated, Volume 1", p. 300, Addison-
        Wesley, 1994.

   [12] Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

8. Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
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   The limited permissions granted above are perpetual and will not be
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

 

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