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+
+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
+
+
+
+Hamzeh, et al [Page 1]
+
+Internet Draft PPTP July 1997
+
+
+ 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:
+
+
+
+
+Hamzeh, et al [Page 2]
+
+Internet Draft PPTP July 1997
+
+
+ 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
+
+
+Hamzeh, et al [Page 3]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+
+Hamzeh, et al [Page 4]
+
+Internet Draft PPTP July 1997
+
+
+ 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
+
+
+
+Hamzeh, et al [Page 5]
+
+Internet Draft PPTP July 1997
+
+
+ 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",
+
+
+
+Hamzeh, et al [Page 6]
+
+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
+
+
+
+Hamzeh, et al [Page 7]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+Hamzeh, et al [Page 8]
+
+Internet Draft PPTP July 1997
+
+
+ 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:
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 9]
+
+Internet Draft PPTP July 1997
+
+
+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).
+
+
+
+Hamzeh, et al [Page 10]
+
+Internet Draft PPTP July 1997
+
+
+ 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
+
+
+
+Hamzeh, et al [Page 11]
+
+Internet Draft PPTP July 1997
+
+
+ issued by the PNS. If less than 64
+ octets in length, the remainder of this
+ field SHOULD be filled with octets of
+ value 0.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 12]
+
+Internet Draft PPTP July 1997
+
+
+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
+
+
+
+Hamzeh, et al [Page 13]
+
+Internet Draft PPTP July 1997
+
+
+ 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
+
+
+
+Hamzeh, et al [Page 14]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 15]
+
+Internet Draft PPTP July 1997
+
+
+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.
+
+
+
+Hamzeh, et al [Page 16]
+
+Internet Draft PPTP July 1997
+
+
+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]
+
+Internet Draft PPTP July 1997
+
+
+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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 18]
+
+Internet Draft PPTP July 1997
+
+
+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
+
+
+
+Hamzeh, et al [Page 19]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 20]
+
+Internet Draft PPTP July 1997
+
+
+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.
+
+
+
+
+Hamzeh, et al [Page 21]
+
+Internet Draft PPTP July 1997
+
+
+ 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
+
+
+
+Hamzeh, et al [Page 22]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 23]
+
+Internet Draft PPTP July 1997
+
+
+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.
+
+
+
+
+Hamzeh, et al [Page 24]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+
+Hamzeh, et al [Page 25]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 26]
+
+Internet Draft PPTP July 1997
+
+
+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.
+
+
+
+Hamzeh, et al [Page 27]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+Hamzeh, et al [Page 28]
+
+Internet Draft PPTP July 1997
+
+
+ Dialing Number The number from which the call was
+ placed. For ISDN and analog calls this
+ field is an ASCII string. If the Dialing
+ Number is less than 64 octets in length,
+ the remainder of this field is filled
+ with octets of value 0.
+
+ Subaddress A 64 octet field used to specify
+ additional dialing information. If the
+ subaddress is less than 64 octets long,
+ the remainder of this field is filled
+ with octets of value 0.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 29]
+
+Internet Draft PPTP July 1997
+
+
+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
+
+
+
+Hamzeh, et al [Page 30]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 31]
+
+Internet Draft PPTP July 1997
+
+
+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
+
+
+
+Hamzeh, et al [Page 32]
+
+Internet Draft PPTP July 1997
+
+
+ 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
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 33]
+
+Internet Draft PPTP July 1997
+
+
+2.12 Call-Clear-Request
+
+ The Call-Clear-Request is a PPTP control message sent by the PNS to
+ the PAC indicating that a particular call is to be disconnected. The
+ call being cleared can be either an incoming or outgoing call, in any
+ state. The PAC responds to this message with a Call-Disconnect-
+ Notify message.
+
+ 0 1 2 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Length | PPTP Message Type |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Magic Cookie |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Control Message Type | Reserved0 |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Call ID | Reserved1 |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+
+ Length Total length in octets of this PPTP
+ message, including the entire PPTP
+ header.
+
+ PPTP Message Type 1 for Control Message.
+
+ Magic Cookie 0x1A2B3C4D.
+
+ Control Message Type 12 for Call-Clear-Request.
+
+ Reserved0 This field MUST be 0.
+
+ Call ID The Call ID assigned by the PNS to this
+ call. This value is used instead of the
+ Peer's Call ID because the latter may not
+ be known to the PNS if the call must be
+ aborted during call establishment.
+
+ Reserved1 This field MUST be 0.
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 34]
+
+Internet Draft PPTP July 1997
+
+
+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
+
+
+
+Hamzeh, et al [Page 35]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 36]
+
+Internet Draft PPTP July 1997
+
+
+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.
+
+
+
+Hamzeh, et al [Page 37]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 38]
+
+Internet Draft PPTP July 1997
+
+
+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].
+
+
+
+
+Hamzeh, et al [Page 39]
+
+Internet Draft PPTP July 1997
+
+
+ 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].
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 40]
+
+Internet Draft PPTP July 1997
+
+
+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
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 41]
+
+Internet Draft PPTP July 1997
+
+
+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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 42]
+
+Internet Draft PPTP July 1997
+
+
+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.
+
+
+
+
+
+
+Hamzeh, et al [Page 43]
+
+Internet Draft PPTP July 1997
+
+
+ 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
+
+
+
+Hamzeh, et al [Page 44]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+
+Hamzeh, et al [Page 45]
+
+Internet Draft PPTP July 1997
+
+
+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
+
+
+
+Hamzeh, et al [Page 46]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 47]
+
+Internet Draft PPTP July 1997
+
+
+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
+
+
+
+
+Hamzeh, et al [Page 48]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+
+
+Hamzeh, et al [Page 49]
+
+Internet Draft PPTP July 1997
+
+
+ established
+
+ The session is terminated either by receipt of a Call-Disconnect-
+ Notify message from the PAC or by sending a Call-Clear-Request. Once
+ a Call-Clear-Request has been sent, the session enters the
+ wait_disconnect state.
+
+ wait_disconnect
+
+ Once a Call-Disconnect-Notify is received the session moves back to
+ the idle state.
+
+3.2.4 Outgoing calls
+
+ Outgoing messages are initiated by a PNS and instruct a PAC to place
+ a call on a telco interface. There are only two messages for outgoing
+ calls: Outgoing-Call-Request and Outgoing-Call-Reply. The PNS sends
+ an Outgoing-Call-Request specifying the dialed party phone number and
+ subaddress as well as speed and window parameters. The PAC MUST
+ respond to the Outgoing-Call-Request message with an Outgoing-Call-
+ Reply message once the PAC determines that:
+
+ The call has been successfully connected
+
+ A call failure has occurred for reasons such as: no interfaces are
+ available for dial-out, the called party is busy or does not
+ answer, or no dial tone is detected on the interface chosen for
+ dialing
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 50]
+
+Internet Draft PPTP July 1997
+
+
+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
+
+
+
+Hamzeh, et al [Page 51]
+
+Internet Draft PPTP July 1997
+
+
+ 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
+
+
+
+Hamzeh, et al [Page 52]
+
+Internet Draft PPTP July 1997
+
+
+ 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:
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 53]
+
+Internet Draft PPTP July 1997
+
+
+
+ 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
+
+
+
+Hamzeh, et al [Page 54]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+
+
+
+
+Hamzeh, et al [Page 55]
+
+Internet Draft PPTP July 1997
+
+
+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.
+
+
+
+
+
+
+
+Hamzeh, et al [Page 56]
+
+Internet Draft PPTP July 1997
+
+
+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.
+
+
+
+
+
+Hamzeh, et al [Page 57]
+
+Internet Draft PPTP July 1997
+
+
+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
+
+
+
+Hamzeh, et al [Page 58]
+
+Internet Draft PPTP July 1997
+
+
+ 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
+
+
+
+Hamzeh, et al [Page 59]
+
+Internet Draft PPTP July 1997
+
+
+ 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
+
+
+
+Hamzeh, et al [Page 60]
+
+Internet Draft PPTP July 1997
+
+
+ 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
+
+
+
+Hamzeh, et al [Page 61]
+
+Internet Draft PPTP July 1997
+
+
+ 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Hamzeh, et al [Page 62]
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