ftp.cc.uoc.gr
rfc2637
This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.

The following 'Verified' errata have been incorporated in this document: EID 4790
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 2.16.

EID 4790 (Verified) is as follows:

Section: 2.2

Original Text:

   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.

Corrected Text:

   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 2.16.
Notes:
Incorrect reference to section 2.2.

Note - Similar pointers occur in Sections 2.4, 2.6, 2.8, 2.10, 2.13.
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. 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