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Network Working Group K. HamzehRequest for Comments: 2637 Ascend CommunicationsCategory: 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) . . . . 39Hamzeh, 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 . . . . . . . . . . . . . . . . . . 571. Introduction PPTP allows existing Network Access Server (NAS) functions to be separated using a client-server architecture. Traditionally, the following functions are implemented by a NAS: 1) Physical native interfacing to PSTN or ISDN and control of external modems or terminal adapters. A NAS may interface directly to a telco analog or digital circuit or attach via an external modem or terminal adapter. Control of a circuit-switched connection is accomplished with either modem control or DSS1 ISDN call control protocols. The NAS, in conjunction with the modem or terminal adapters, may perform rate adaption, analog to digital conversion, sync to async conversion or a number of other alterations of data streams.Hamzeh, et al. Informational [Page 3] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999 2) Logical termination of a Point-to-Point-Protocol (PPP) Link Control Protocol (LCP) session. 3) Participation in PPP authentication protocols [3,9,10]. 4) Channel aggregation and bundle management for PPP Multilink Protocol. 5) Logical termination of various PPP network control protocols (NCP). 6) Multiprotocol routing and bridging between NAS interfaces. PPTP divides these functions between the PAC and PNS. The PAC is responsible for functions 1, 2, and possibly 3. The PNS may be responsible for function 3 and is responsible for functions 4, 5, and 6. The protocol used to carry PPP protocol data units (PDUs) between the PAC and PNS, as well as call control and management is addressed by PPTP. The decoupling of NAS functions offers these benefits: Flexible IP address management. Dial-in users may maintain a single IP address as they dial into different PACs as long as they are served from a common PNS. If an enterprise network uses unregistered addresses, a PNS associated with the enterprise assigns addresses meaningful to the private network. Support of non-IP protocols for dial networks behind IP networks. This allows Appletalk and IPX, for example to be tunneled through an IP-only provider. The PAC need not be capable of processing these protocols. A solution to the "multilink hunt-group splitting" problem. Multilink PPP, typically used to aggregate ISDN B channels, requires that all of the channels composing a multilink bundle be grouped at a single NAS. Since a multilink PPP bundle can be handled by a single PNS, the channels comprising the bundle may be spread across multiple PACs.1.1. Protocol Goals and Assumptions The PPTP protocol is implemented only by the PAC and PNS. No other systems need to be aware of PPTP. Dial networks may be connected to a PAC without being aware of PPTP. Standard PPP client software should continue to operate on tunneled PPP links.Hamzeh, et al. Informational [Page 4] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999 PPTP can also be used to tunnel a PPP session over an IP network. In this configuration the PPTP tunnel and the PPP session runs between the same two machines with the caller acting as a PNS. It is envisioned that there will be a many-to-many relationship between PACs and PNSs. A PAC may provide service to many PNSs. For example, an Internet service provider may choose to support PPTP for a number of private network clients and create VPNs for them. Each private network may operate one or more PNSs. A single PNS may associate with many PACs to concentrate traffic from a large number of geographically diverse sites. PPTP uses an extended version of GRE to carry user PPP packets. These enhancements allow for low-level congestion and flow control to be provided on the tunnels used to carry user data between PAC and PNS. This mechanism allows for efficient use of the bandwidth available for the tunnels and avoids unnecessary retransmisions and buffer overruns. PPTP does not dictate the particular algorithms to be used for this low level control but it does define the parameters that must be communicated in order to allow such algorithms to work. Suggested algorithms are included in section 4.1.2. Terminology Analog Channel A circuit-switched communication path which is intended to carry 3.1 Khz audio in each direction. Digital Channel A circuit-switched communication path which is intended to carry digital information in each direction. Call A connection or attempted connection between two terminal endpoints on a PSTN or ISDN -- for example, a telephone call between two modems. Control Connection A control connection is created for each PAC, PNS pair and operates over TCP [4]. The control connection governs aspects of the tunnel and of sessions assigned to the tunnel.Hamzeh, et al. Informational [Page 5] RFC 2637 Point-to-Point Tunneling Protocol (PPTP) July 1999 Dial User An end-system or router attached to an on-demand PSTN or ISDN which is either the initiator or recipient of a call. Network Access Server (NAS) A device providing temporary, on-demand network access to users. This access is point-to-point using PSTN or ISDN lines. PPTP Access Concentrator (PAC) A device attached to one or more PSTN or ISDN lines capable of PPP operation and of handling the PPTP protocol. The PAC need only implement TCP/IP to pass traffic to one or more PNSs. It may also tunnel non-IP protocols. PPTP Network Server (PNS) A PNS is envisioned to operate on general-purpose computing/server platforms. The PNS handles the server side of the PPTP protocol. Since PPTP relies completely on TCP/IP and is independent of the interface hardware, the PNS may use any combination of IP interface hardware including LAN and WAN devices.⌨️ 快捷键说明
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