pptp-draft.txt

pptp第二层隧道模块

Internet Draft                    PPTP                         July 1997


                          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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