rfc2637.txt

来自「RFC 的详细文档！」· 文本 代码 · 共 1,522 行 · 第 1/5 页

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


                Point-to-Point Tunneling Protocol (PPTP)

Status of this Memo

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

Copyright Notice

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

IESG Note

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

Abstract

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



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


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

Specification of Requirements

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

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

Table of Contents

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



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


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

1.  Introduction

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

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

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

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





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


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

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

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

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

      6) Multiprotocol routing and bridging between NAS interfaces.

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

   The decoupling of NAS functions offers these benefits:

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

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

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

1.1.  Protocol Goals and Assumptions

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





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


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

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

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

1.2.  Terminology

   Analog Channel

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

   Digital Channel

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

   Call

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

   Control Connection

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






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


   Dial User

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

   Network Access Server (NAS)

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

   PPTP Access Concentrator (PAC)

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

   PPTP Network Server (PNS)