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Versions: 00 01 02 03 04 05 06 07 08 09 10 rfc2637                      
Internet Draft                                  Kory Hamzeh
                                                Ascend Communications
                                                Gurdeep Singh Pall
                                                Microsoft Corporation
                                                William Verthein
                                                U.S. Robotics/3Com
                                                Jeff Taarud
                                                Copper Mountain Networks
                                                W. Andrew Little
                                                ECI Telematics
July 1997
Expire in six months


                  Point-to-Point Tunneling Protocol--PPTP
                       draft-ietf-pppext-pptp-02.txt

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).

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



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        management protocol which allows the server to control
        access for dial-in circuit switched calls originating from
        a PSTN or ISDN or to initiate outbound circuit-switched
        connections. PPTP uses an enhanced GRE (Generic Routing
        Encapsulation) mechanism to provide a flow- and
        congestion-controlled encapsulated datagram service for
        carrying PPP packets.

1. Introduction

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

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

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

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

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

   3) Participation in PPP authentication protocols [3].

   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:




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

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

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


1.1 Protocol Goals and Assumptions

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

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

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

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


1.2 Terminology


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      Analog Channel

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

      Digital Channel

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

      Call

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

      Control Connection

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

      Dial User

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

      Network Access Server (NAS)

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

      PPTP Access Concentrator (PAC)

         A device attached to one or more PSTN or ISDN lines capable of
         PPP operation and of handling the PPTP protocol. The PAC need
         only implement TCP/IP to pass traffic to one or more PNSs. It
         may also tunnel non-IP protocols.

      PPTP Network Server (PNS)

         A PNS is envisioned to operate on general-purpose
         computing/server platforms. The PNS handles the server side of
         the PPTP protocol. Since PPTP relies completely on TCP/IP and
         is independent of the interface hardware, the PNS may use any
         combination of IP interface hardware including LAN and WAN
         devices.




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      Session

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

      Tunnel

         A tunnel is defined by a PNS-PAC pair. The tunnel protocol is
         defined by a modified version of GRE [1,2]. The tunnel carries
         PPP datagrams between the PAC and the PNS.  Many sessions are
         multiplexed on a single tunnel. A control connection operating
         over TCP controls the establishment, release, and maintenance
         of sessions and of the tunnel itself.

1.3 Protocol Overview

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

1.3.1 Control Connection Overview

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

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



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   PAC or PNS, also through Control Connection messages.

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

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

1.3.2 Tunnel Protocol Overview

   PPTP requires the establishment of a tunnel for each communicating
   PNS-PAC pair.  This tunnel is used to carry all user session PPP
   packets for sessions involving a given PNS-PAC pair.  A key which is
   present in the GRE header indicates which session a particular PPP
   packet belongs to.  In this manner, PPP packets are multiplexed and
   demultiplexed over a single tunnel between a given PNS-PAC pair.  The
   value to use in the key field is established by the call
   establishment procedure which takes place on the control connection.

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

1.4 Specification Language

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.

      MUST                This word, or the adjective "required", means
                          that the definition is an absolute requirement
                          of the specification.

      MUST NOT            This phrase means that the definition is an
                          absolute prohibition of the specification.

      SHOULD              This word, or the adjective "recommended",



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                          means that, in some circumstances, valid
                          reasons may exist to ignore this item, but the
                          full implications must be understood and
                          carefully weighed before choosing a different
                          course. Unexpected results may result
                          otherwise.

      MAY                 This word, or the adjective "optional", means
                          that this item is one of an allowed set of
                          alternatives.  An implementation which does
                          not include this option MUST be prepared to
                          interoperate with another implementation which
                          does include the option.

      silently discard    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.


1.5 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



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   synchronization must result in immediate closing of the control
   connection's TCP session.

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

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

           Control Message                        Message Code

           (Control Connection Management)

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

           (Call Management)

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

           (Error Reporting)

           WAN-Error-Notify                           14

           (PPP Session Control)

           Set-Link-Info                              15


   The Start-Control-Connection-Request and -Reply messages determine
   which version of the Control Connection protocol will be used.  The
   version number field carried in these messages consists of a version
   number in the high octet and a revision number in the low octet.
   Version handling is described in Section 3. 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.



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

2.0 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.

   The TCP header is followed by the PPTP fields shown in the following:





























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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.

       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 1.5).



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

      Reserved0                This field MUST be 0.

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

      Reserved1                This field MUST be 0.

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

                                  1 - Asynchronous Framing supported

                                  2 - Synchronous Framing supported

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

                                  1 - Analog access supported

                                  2 - Digital access supported

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

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

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

      Vendor Name              A 64 octet field containing a vendor
                               specific string describing the type of
                               PAC being used, or the type of PNS
                               software being used if this request is



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                               issued by the PNS.  If less than 64
                               octets in length, the remainder of this
                               field SHOULD be filled with octets of
                               value 0.















































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2.2 Start-Control-Connection-Reply

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

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



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

      PPTP Message Type        1 for Control Message.

      Magic Cookie             0x1A2B3C4D.

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

      Reserved0                This field MUST be 0.

      Protocol Version         The version of the PPTP protocol that the



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                               sender wishes to use.

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

                                  1 - Successful channel establishment

                                  2 - General error -- Error Code
                                      indicates the problem

                                  3 - Command channel already exists;

                                  4 - Requester is not authorized to
                                      establish a command channel

                                  5 - The protocol version of the
                                      requester is not supported

      Error Code               This field is set to 0 unless a "General
                               Error" exists, in which case Result Code
                               is set to 2 and this field is set to the
                               value corresponding to the general error
                               condition as specified in Section 2.16.

      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



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                               PNS MUST NOT use this value to try to
                               track the remaining number of PPP
                               sessions that the PAC will allow.

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

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

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




























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2.3 Stop-Control-Connection-Request

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

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



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

      PPTP Message Type        1 for Control Message.

      Magic Cookie             0x1A2B3C4D.

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

      Reserved0                This field MUST be 0.

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

                                  1 (None) - General request to clear
                                    control connection

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

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

      Reserved1, Reserved2     These fields MUST be 0.



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2.4 Stop-Control-Connection-Reply

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

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


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

      PPTP Message Type        1 for Control Message.

      Magic Cookie             0x1A2B3C4D.

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

      Reserved0                This field MUST be 0.

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

                                  1 (OK) - Control connection closed

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

      Error Code               This field is set to 0 unless a "General
                               Error" exists, in which case Result Code
                               is set to 2 and this field is set to the
                               value corresponding to the general error
                               condition as specified in Section 2.16.

      Reserved1                This field MUST be 0.



Hamzeh, et al                                                  [Page 17]


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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,
   if the sender does not receive an Echo Reply in response to an Echo-
   Request, it will eventually clear the control connection.

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


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

      PPTP Message Type        1 for Control Message.

      Magic Cookie             0x1A2B3C4D.

      Control Message Type     5 for Echo-Request.

      Reserved0                This field MUST be 0.

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














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2.6 Echo-Reply

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

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


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

      PPTP Message Type        1 for Control Message.

      Magic Cookie             0x1A2B3C4D.

      Control Message Type     6 for Echo-Reply.

      Reserved0                This field MUST be 0.

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

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

                                  1 (OK) - The Echo-Reply is valid

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

      Error Code               This field is set to 0 unless a "General



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                               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 2.16.

      Reserved1                This field MUST be 0.












































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2.7 Outgoing-Call-Request

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

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


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

      PPTP Message Type        1 for Control Message.




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      Magic Cookie             0x1A2B3C4D.

      Control Message Type     7 for Outgoing-Call-Request.

      Reserved0                This field MUST be 0.

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

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

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

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

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

                                  1 - Call to be placed on an analog
                                      channel

                                  2 - Call to be placed on a digital
                                      channel

                                  3 - Call can be placed on any type of
                                      channel

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

                                  1 - Call to use Asynchronous framing

                                  2 - Call to use Synchronous framing



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                                  3 - Call can use either type of
                                      framing

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

      Packet Processing Delay  A measure of the packet processing delay
                               that might be imposed on data sent to the
                               PNS from the PAC.  This value is
                               specified in units of 1/10 seconds.  For
                               the PNS this number should be very small.
                               See appendix A 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.




















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2.8 Outgoing-Call-Reply

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

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


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

      PPTP Message Type        1 for Control Message.

      Magic Cookie             0x1A2B3C4D.

      Control Message Type     8 for Outgoing-Call-Reply.

      Reserved0                This field MUST be 0.

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




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

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

                                  1 (Connected) - Call established with
                                    no errors

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

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

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

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

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

                                  7 (Do Not Accept) - Outgoing Call
                                    administratively prohibited

      Error Code               This field is set to 0 unless a "General
                               Error" condition exists, in which case
                               Result Code is set to 2 and this field is
                               set to the value corresponding to the
                               general error condition as specified in
                               Section 2.16.

      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.




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

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

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


























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2.9 Incoming-Call-Request

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

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

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


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



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      PPTP Message Type        1 for Control Message.

      Magic Cookie             0x1A2B3C4D.

      Control Message Type     9 for Incoming-Call-Request.

      Reserved0                This field MUST be 0.

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

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

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

                                  1 - Call is on an analog channel

                                  2 - Call is on a digital channel

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

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

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

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



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      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.







































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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 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



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                               in the Call ID field of the corresponding
                               Incoming-Call-Request message. It is used
                               by the PAC to match the Incoming-Call-
                               Reply with the Incoming-Call-Request it
                               issued. This value is included in the GRE
                               header of transmitted data packets for
                               this session.

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

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

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

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

      Error Code               This field is set to 0 unless a "General
                               Error" condition exists, in which case
                               Result Code is set to 2 and this field is
                               set to the value corresponding to the
                               general error condition as specified in
                               Section 2.16.

      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.  See
                               Appendix A for a description of how this
                               value is determined and used.

      Reserved1                This field MUST be 0.










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2.11 Incoming-Call-Connected

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

   This message is the third in the three-way handshake used by PPTP for
   establishing incoming calls.  It provides a mechanism for providing
   the PNS with additional information about the call that cannot, in
   general, be obtained at the time the Incoming-Call-Request is issued
   by the PAC.

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


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

      PPTP Message Type        1 for Control Message.

      Magic Cookie             0x1A2B3C4D.

      Control Message Type     11 for Incoming-Call-Connected.

      Reserved0                This field MUST be 0.

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



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                               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.  See
                               Appendix A for a description of how this
                               value is determined and used.

      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




























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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.











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2.13 Call-Disconnect-Notify

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

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


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

      PPTP Message Type        1 for Control Message.

      Magic Cookie             0x1A2B3C4D.

      Control Message Type     13 for Call-Disconnect-Notify.

      Reserved0                This field MUST be 0.

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

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



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                               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 2.16.

      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.

















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2.14 WAN-Error-Notify

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

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


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

      PPTP Message Type        1 for Control Message.

      Magic Cookie             0x1A2B3C4D.

      Control Message Type     14 for WAN-Error-Notify.

      Reserved0                This field MUST be 0.

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



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

      Framing Errors           Number of improperly framed PPP packets
                               received.

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

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

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

      Alignment Errors         Number of alignment errors since call was
                               established.


































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2.15 Set-Link-Info

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

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


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

      PPTP Message Type        1 for Control Message.

      Magic Cookie             0x1A2B3C4D.

      Control Message Type     15 for Set-Link-Info.

      Reserved0                This field MUST be 0.

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

      Reserved1                This field MUST be 0.

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




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      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].














































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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
























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3.0 Control Connection Protocol Operation

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

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

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


3.1 Control Connection States

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


















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3.1.1 Control Connection Originator (may be PAC or PNS)

                      TCP Open Indication
                      /Send Start Control
                        Connection Request       +-----------------+
           +------------------------------------>|  wait_ctl_reply |
           |                                     +-----------------+
           |     Collision/See (3.1.3) Close TCP   V  V  V   Receive Start Ctl
           |       +-------------------------------+  |  |   Connection Reply
           |       |                                  |  |   Version OK
           ^       V                                  |  V
   +-----------------+             Receive Start Ctl  | +-----------------+
   |      idle       |             Connection Reply   | |   established   |
   +-----------------+             Version Not OK     | +-----------------+
           ^                                          |  V   Local Terminate
           |         Receive Stop Control             |  |   /Send Stop
           |         Connection Request               |  |    Control Request
           |         /Send Stop Control Reply         V  V
           |          Close TCP                  +-----------------+
           +-------------------------------------| wait_stop_reply |
                                                 +-----------------+



   idle

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

   wait_ctl_reply

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

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






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   established

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

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

   wait_stop_reply

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

3.1.2 Control connection Receiver (may be PAC or PNS)


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

   idle

   The control connection receiver waits for a TCP open attempt on port
   1723. When notified of an open TCP connection, it should prepare to
   receive PPTP messages.  When a Start-Control-Connection-Request is
   received its version field should be examined. If the version is
   earlier than the receiver's version and the earlier version can be



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   supported by the receiver, the receiver SHOULD send a Start-Control-
   Connection-Reply. If the version is earlier than the receiver's
   version and the version cannot be supported, the receiver SHOULD send
   a Start- Connection-Reply message, close the TCP connection and
   remain in the idle state.  If the receiver's version is the same as
   earlier than the peer's, the receiver SHOULD send a Start-Control-
   Connection-Reply with the receiver's version and enter the
   established state.

   established

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

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

   wait_stop_reply

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

3.1.3 Start Control Connection Initiation Request Collision

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

   The "winner" of the initiation race is the peer with the higher IP
   address (compared as 32 bit unsigned values, network number more
   significant). For example, if the peers 192.33.45.17 and 192.33.45.89
   collide, the latter will be declared the winner.

   The loser will immediately close the TCP connection it initiated,
   without sending any further PPTP control messages on it and will
   respond to the winner's request with a Start-Control-Connection-Reply
   message. The winner will wait for the Start-Control-Connection-Reply
   on the connection it initiated and also wait for a TCP termination
   indication on the connection the loser opened.  The winner MUST NOT
   send any messages on the connection the loser initiated.




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3.1.3 Keep Alives and Timers

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

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

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

3.2 Call States

3.2.1 Timing considerations

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

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

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

3.2.2 Call ID values

   Each peer assigns a Call ID value to each user session it requests or
   accepts. This Call ID value MUST be unique for the tunnel between the
   PNS and PAC to which it belongs. Tunnels to other peers can use the



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   same Call ID number so the receiver of a packet on a tunnel needs to
   associate a user session with a particular tunnel and Call ID.  It is
   suggested that the number of potential Call ID values for each tunnel
   be at least twice as large as the maximum number of calls expected on
   a given tunnel.

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

3.2.3 Incoming calls

   An Incoming-Call-Request message is generated by the PAC when an
   associated telephone line rings. The PAC selects a Call ID and serial
   number and indicates the call bearer type.  Modems should always
   indicate analog call type.  ISDN calls should indicate digital when
   unrestricted digital service or rate adaption is used and analog if
   digital modems are involved. Dialing number, dialed number, and
   subaddress may be included in the message if they are available from
   the telephone network.

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

      -  No resources are available to handle more sessions

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

      -  The bearer service is not authorized or supported

   If the PNS chooses to accept the call, it responds with an Outgoing-
   Call-Reply which also indicates window sizes (see Appendix B). 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.









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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 Accepting
     |         |    +------------------------------+  |   /Answer call; Send
     |         |    |     Abort/Send Call             |    Call Connected
     ^         V    V     Disconnect Notify           V
   +-----------------+                              +-----------------+
   |      idle       |<-----------------------------|   established   |
   +-----------------+  Receive Clear Call Request  +-----------------+
                        or telco call dropped
                        or local disconnect
                        /Send Call Disconnect Notify



   The states associated with the PAC for incoming calls are:

   idle

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

   wait_reply

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

   established

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

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

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




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

3.2.3.2 PNS Incoming Call States



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



   The states associated with the PNS for incoming calls are:

   idle

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

   wait_connect

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





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   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























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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



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

3.2.4.2 PNS Outgoing Call States


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


   The states associated with the PNS for outgoing calls are:

   idle

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

   wait_reply

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

   established

   If a Call-Disconnect-Notify is received, the telco call has been
   terminated for the reason indicated in the Result and Cause Codes.
   The session moves back to the idle state. If the PNS chooses to



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   terminate the session, it sends a Call-Clear-Request to the PAC and
   then enters the wait_disconnect state.

   wait_disconnect

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

4.0 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:








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


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

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

      K                        (Bit 2) Key Present.  Set to one (1).

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

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

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

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

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

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

      Protocol Type            Set to hex 880B [8].

      Key                      Use of the Key field is up to the



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                               implementation.
                                PPTP uses it as follows:

                               Payload Length
                                  (High 2 octets of Key) Size of the
                                  payload, not including the GRE header

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

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

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

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

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

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


4.2 Sliding Window Protocol

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







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4.3 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.

4.4 Out-of-Sequence Packets

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

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

5.0 Security Considerations

   Security issues are not addressed in this document. End to end
   security is addressed by PPP. Further security considerations will be
   addressed by the next version of PPTP.







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6.0 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

7.0 References

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

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

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

   [4]  Postel, J. B., "Transmission Control Protocol", RFC 791,
        September 1981

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

   [6]  Reynolds, J., Postel, J., "Assigned Numbers", RFC 1700, October,
        1994.

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

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





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Appendix A. Acknowledgment Time-Outs


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

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

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

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

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

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

   Some definitions:

      Packet Processing Delay (PPD) is the amount of time required for
      each side to process the maximum amount of data buffered in their
      receive packet sliding window. The PPD is the value exchanged
      between the PAC and PNS when a call is established. For the PNS,
      this number should be small.  For a PAC making modem connections,
      this number could be significant.

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

      Round-Trip Time (RTT) is the estimated round-trip time for an



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      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.


Packet Processing Delay (PPD)

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

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

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

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

Sample

   Sample is the actual measured time for a returned acknowledgment.

Round-Trip Time (RTT)

   The RTT value represents an estimate of the average time it takes for
   an acknowledgment to be received after a packet is sent to the remote



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   end of the tunnel.


A.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 Richard Steven's book TCP/IP
   Illustrated, Volume 1 (page 300). '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] = 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
      calculated on each iteration and stored for use in the next
      iteration.  Initially, it is set to PPD.

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

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

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

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

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



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   division.


A.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.  A simple formula called
   Karn's Algorithm is used in TCP implementations and may be used in
   implementing the backoff timers for the PNS or the PAC.  Notice that
   in addition to increasing the time-out, we are also shrinking the
   size of the window as described in the next section.

   Karn's timer backoff algorithm, as used in TCP, is:


      NewTIMEOUT = delta * TIMEOUT

   Adapted to our time-out calculations, 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] = MIN (RTT[n] + (chi * DEV[n]), MaxTimeOut)

   In this modified calculation of ATO, only the two values that
   contribute to ATO and that are stored for the next iteration are
   calculated. RTT is scaled by chi, 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.

Appendix B. Acknowledgment Time-Out and Window Adjustment

B.1 Initial Window Size

   Although each side has indicated the maximum size of its receive
   window, it is recommended that a slow start method be used to begin
   transmitting 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



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   congested network because no history has been established.

B.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.

B.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.

B.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.























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