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rfc:rfc2637

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-

Hamzeh, et al. Informational [Page 1] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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

Hamzeh, et al. Informational [Page 2] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 3] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

    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.

Hamzeh, et al. Informational [Page 4] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 5] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 6] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

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.

Hamzeh, et al. Informational [Page 7] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 8] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

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.

Hamzeh, et al. Informational [Page 9] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

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.

Hamzeh, et al. Informational [Page 10] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

     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.

Hamzeh, et al. Informational [Page 11] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 12] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

     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

Hamzeh, et al. Informational [Page 13] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

                                3 - Command channel already exists;
                                4 - Requester is not authorized to
                                    establish a command channel
                                5 - The protocol version of the
                                    requester is not supported
 Error Code               This field is set to 0 unless a "General
                          Error" exists, in which case Result Code is
                          set to 2 and this field is set to the value
                          corresponding to the general error condition
                          as specified in section 2.2.
 Framing Capabilities     A set of bits indicating the type of framing
                          that the sender of this message can provide.
                          The currently defined bit settings are:
                                1 - Asynchronous Framing supported
                                2 - Synchronous Framing supported.
 Bearer Capabilities      A set of bits indicating the bearer
                          capabilities that the sender of this message
                          can provide.  The currently defined bit
                          settings are:
                                1 - Analog access supported
                                2 - Digital access supported
 Maximum Channels         The total number of individual PPP sessions
                          this PAC can support.  In a Start-Control-
                          Connection-Reply issued by the PNS, this
                          value SHOULD be set to 0 and it must be
                          ignored by the PAC. The PNS MUST NOT use
                          this value to try to track the remaining
                          number of PPP sessions that the PAC will
                          allow.
 Firmware Revision        This field contains the firmware revision
                          number of the issuing PAC, or the version of
                          the PNS PPTP driver if issued by the PNS.

Hamzeh, et al. Informational [Page 14] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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:

Hamzeh, et al. Informational [Page 15] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

                                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:

Hamzeh, et al. Informational [Page 16] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

                                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.

Hamzeh, et al. Informational [Page 17] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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

Hamzeh, et al. Informational [Page 18] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 19] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

     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.

Hamzeh, et al. Informational [Page 20] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 21] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 22] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

     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.

Hamzeh, et al. Informational [Page 23] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 24] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 25] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

     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.

Hamzeh, et al. Informational [Page 26] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 27] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

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

Hamzeh, et al. Informational [Page 28] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

                          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

Hamzeh, et al. Informational [Page 29] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 30] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 31] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

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.

Hamzeh, et al. Informational [Page 32] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 33] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

     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.

Hamzeh, et al. Informational [Page 34] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 35] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 36] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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 |
                                         +-----------------+

Hamzeh, et al. Informational [Page 37] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 38] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

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.

Hamzeh, et al. Informational [Page 39] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

    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.

Hamzeh, et al. Informational [Page 40] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

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

Hamzeh, et al. Informational [Page 41] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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:
  1. No resources are available to handle more sessions
  1. The dialed, dialing, or subaddress fields are not indicative of

an authorized user

  1. 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

Hamzeh, et al. Informational [Page 42] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

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

Hamzeh, et al. Informational [Page 43] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

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

Hamzeh, et al. Informational [Page 44] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

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.

Hamzeh, et al. Informational [Page 45] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

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.

Hamzeh, et al. Informational [Page 46] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

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)                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Hamzeh, et al. Informational [Page 47] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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).

Hamzeh, et al. Informational [Page 48] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

       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.

Hamzeh, et al. Informational [Page 49] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

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.

Hamzeh, et al. Informational [Page 50] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 51] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 52] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 53] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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.

Hamzeh, et al. Informational [Page 54] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

 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

Hamzeh, et al. Informational [Page 55] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

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.

Hamzeh, et al. Informational [Page 56] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999

8. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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

Hamzeh, et al. Informational [Page 57]

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