RFC 2637
Point-to-Point Tunneling Protocol (PPTP)
Network Working Group K. Hamzeh Request for Comments: 2637 Ascend Communications Category: Informational G. Pall Microsoft Corporation W. Verthein 3Com J. Taarud Copper Mountain Networks W. Little ECI Telematics G. Zorn Microsoft Corporation July 1999 Point-to-Point Tunneling Protocol (PPTP) Status of this Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. IESG Note The PPTP protocol was developed by a vendor consortium. The documentation of PPTP is provided as information to the Internet community. The PPP WG is currently defining a Standards Track protocol (L2TP) for tunneling PPP across packet-switched networks.Abstract
This document specifies a protocol which allows the Point to Point Protocol (PPP) to be tunneled through an IP network. PPTP does not specify any changes to the PPP protocol but rather describes a new vehicle for carrying PPP. A client-server architecture is defined in order to decouple functions which exist in current Network Access Servers (NAS) and support Virtual Private Networks (VPNs). The PPTP Network Server (PNS) is envisioned to run on a general purpose operating system while the client, referred to as a PPTP Access Concentrator (PAC) operates on a dial access platform. PPTP specifies a call-control and management protocol which allows the server to control access for dial-in circuit switched calls originating from a PSTN or ISDN or to initiate outbound circuit-
switched connections. PPTP uses an enhanced GRE (Generic Routing Encapsulation) mechanism to provide a flow- and congestion-controlled encapsulated datagram service for carrying PPP packets. Specification of Requirements In this document, the key words "MAY", "MUST, "MUST NOT", "optional", "recommended", "SHOULD", and "SHOULD NOT" are to be interpreted as described in [12]. The words "silently discard", when used in reference to the behavior of an implementation upon receipt of an incoming packet, are to be interpreted as follows: the implementation discards the datagram without further processing, and without indicating an error to the sender. The implementation SHOULD provide the capability of logging the error, including the contents of the discarded datagram, and SHOULD record the event in a statistics counter.Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Protocol Goals and Assumptions . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 1.3. Protocol Overview . . . . . . . . . . . . . . . . . . . . 6 1.3.1. Control Connection Overview . . . . . . . . . . . . . . 7 1.3.2. Tunnel Protocol Overview . . . . . . . . . . . . . . . . 7 1.4. Message Format and Protocol Extensibility . . . . . . . . 8 2. Control Connection Protocol Specification . . . . . . . . . 10 2.1. Start-Control-Connection-Request . . . . . . . . . . . . . 10 2.2. Start-Control-Connection-Reply . . . . . . . . . . . . . . 12 2.3. Stop-Control-Connection-Request . . . . . . . . . . . . . 15 2.4. Stop-Control-Connection-Reply . . . . . . . . . . . . . . 16 2.5. Echo-Request . . . . . . . . . . . . . . . . . . . . . . . 17 2.6. Echo-Reply . . . . . . . . . . . . . . . . . . . . . . . . 18 2.7. Outgoing-Call-Request . . . . . . . . . . . . . . . . . . 19 2.8. Outgoing-Call-Reply . . . . . . . . . . . . . . . . . . . 22 2.9. Incoming-Call-Request . . . . . . . . . . . . . . . . . . 25 2.10. Incoming-Call-Reply . . . . . . . . . . . . . . . . . . . 28 2.11. Incoming-Call-Connected . . . . . . . . . . . . . . . . . 29 2.12. Call-Clear-Request . . . . . . . . . . . . . . . . . . . 31 2.13. Call-Disconnect-Notify . . . . . . . . . . . . . . . . . 32 2.14. WAN-Error-Notify . . . . . . . . . . . . . . . . . . . . 33 2.15. Set-Link-Info . . . . . . . . . . . . . . . . . . . . . . 35 2.16. General Error Codes . . . . . . . . . . . . . . . . . . . 36 3. Control Connection Protocol Operation . . . . . . . . . . . 36 3.1. Control Connection States . . . . . . . . . . . . . . . . 37 3.1.1. Control Connection Originator (may be PAC or PNS) . . . 37 3.1.2. Control connection Receiver (may be PAC or PNS) . . . . 39
3.1.3. Start Control Connection Initiation Request Collision . 40 3.1.4. Keep Alives and Timers . . . . . . . . . . . . . . . . . 40 3.2. Call States . . . . . . . . . . . . . . . . . . . . . . . 41 3.2.1. Timing considerations . . . . . . . . . . . . . . . . . 41 3.2.2. Call ID Values . . . . . . . . . . . . . . . . . . . . . 41 3.2.3. Incoming Calls . . . . . . . . . . . . . . . . . . . . . 41 3.2.3.1. PAC Incoming Call States . . . . . . . . . . . . . . . 42 3.2.3.2. PNS Incoming Call States . . . . . . . . . . . . . . . 43 3.2.4. Outgoing Calls . . . . . . . . . . . . . . . . . . . . . 44 3.2.4.1. PAC Outgoing Call States . . . . . . . . . . . . . . . 45 3.2.4.2. PNS Outgoing Call States . . . . . . . . . . . . . . . 46 4. Tunnel Protocol Operation . . . . . . . . . . . . . . . . . 47 4.1. Enhanced GRE header . . . . . . . . . . . . . . . . . . . 47 4.2. Sliding Window Protocol . . . . . . . . . . . . . . . . . 49 4.2.1. Initial Window Size . . . . . . . . . . . . . . . . . . 49 4.2.2. Closing the Window . . . . . . . . . . . . . . . . . . . 49 4.2.3. Opening the Window . . . . . . . . . . . . . . . . . . . 50 4.2.4. Window Overflow . . . . . . . . . . . . . . . . . . . . 50 4.2.5. Multi-packet Acknowledgment . . . . . . . . . . . . . . 50 4.3. Out-of-sequence Packets . . . . . . . . . . . . . . . . . 50 4.4. Acknowledgment Time-Outs . . . . . . . . . . . . . . . . . 51 4.4.1. Calculating Adaptive Acknowledgment Time-Out . . . . . . 53 4.4.2. Congestion Control: Adjusting for Time-Out . . . . . . . 54 5. Security Considerations . . . . . . . . . . . . . . . . . . 54 6. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 55 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 56 8. Full Copyright Statement . . . . . . . . . . . . . . . . . . 571. Introduction
PPTP allows existing Network Access Server (NAS) functions to be separated using a client-server architecture. Traditionally, the following functions are implemented by a NAS: 1) Physical native interfacing to PSTN or ISDN and control of external modems or terminal adapters. A NAS may interface directly to a telco analog or digital circuit or attach via an external modem or terminal adapter. Control of a circuit-switched connection is accomplished with either modem control or DSS1 ISDN call control protocols. The NAS, in conjunction with the modem or terminal adapters, may perform rate adaption, analog to digital conversion, sync to async conversion or a number of other alterations of data streams.
2) Logical termination of a Point-to-Point-Protocol (PPP) Link
Control Protocol (LCP) session.
3) Participation in PPP authentication protocols [3,9,10].
4) Channel aggregation and bundle management for PPP Multilink
Protocol.
5) Logical termination of various PPP network control protocols
(NCP).
6) Multiprotocol routing and bridging between NAS interfaces.
PPTP divides these functions between the PAC and PNS. The PAC is
responsible for functions 1, 2, and possibly 3. The PNS may be
responsible for function 3 and is responsible for functions 4, 5, and
6. The protocol used to carry PPP protocol data units (PDUs) between
the PAC and PNS, as well as call control and management is addressed
by PPTP.
The decoupling of NAS functions offers these benefits:
Flexible IP address management. Dial-in users may maintain a
single IP address as they dial into different PACs as long as they
are served from a common PNS. If an enterprise network uses
unregistered addresses, a PNS associated with the enterprise
assigns addresses meaningful to the private network.
Support of non-IP protocols for dial networks behind IP networks.
This allows Appletalk and IPX, for example to be tunneled through
an IP-only provider. The PAC need not be capable of processing
these protocols.
A solution to the "multilink hunt-group splitting" problem.
Multilink PPP, typically used to aggregate ISDN B channels,
requires that all of the channels composing a multilink bundle be
grouped at a single NAS. Since a multilink PPP bundle can be
handled by a single PNS, the channels comprising the bundle may be
spread across multiple PACs.
1.1. Protocol Goals and Assumptions
The PPTP protocol is implemented only by the PAC and PNS. No other
systems need to be aware of PPTP. Dial networks may be connected to a
PAC without being aware of PPTP. Standard PPP client software should
continue to operate on tunneled PPP links.
PPTP can also be used to tunnel a PPP session over an IP network. In this configuration the PPTP tunnel and the PPP session runs between the same two machines with the caller acting as a PNS. It is envisioned that there will be a many-to-many relationship between PACs and PNSs. A PAC may provide service to many PNSs. For example, an Internet service provider may choose to support PPTP for a number of private network clients and create VPNs for them. Each private network may operate one or more PNSs. A single PNS may associate with many PACs to concentrate traffic from a large number of geographically diverse sites. PPTP uses an extended version of GRE to carry user PPP packets. These enhancements allow for low-level congestion and flow control to be provided on the tunnels used to carry user data between PAC and PNS. This mechanism allows for efficient use of the bandwidth available for the tunnels and avoids unnecessary retransmisions and buffer overruns. PPTP does not dictate the particular algorithms to be used for this low level control but it does define the parameters that must be communicated in order to allow such algorithms to work. Suggested algorithms are included in section 4.1.2. Terminology
Analog Channel A circuit-switched communication path which is intended to carry 3.1 Khz audio in each direction. Digital Channel A circuit-switched communication path which is intended to carry digital information in each direction. Call A connection or attempted connection between two terminal endpoints on a PSTN or ISDN -- for example, a telephone call between two modems. Control Connection A control connection is created for each PAC, PNS pair and operates over TCP [4]. The control connection governs aspects of the tunnel and of sessions assigned to the tunnel.
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.
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 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 section 4.4 of this document.1.4. 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 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 2. 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 4.1.
The MTU for the user data packets encapsulated in GRE is 1532 octets,
not including the IP and GRE headers.
2. 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.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 3.1.3.
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 1.4).
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 issued by the
PNS. If less than 64 octets in length, the
remainder of this field SHOULD be filled
with octets of value 0.
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
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 2.2.
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 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.
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.
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 2.2.
Reserved1 This field MUST be 0.
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
3.1.4, 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.
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
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 2.2.
Reserved1 This field MUST be 0.
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.
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
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 section
4.4 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.
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.
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 2.2.
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.
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 section
4.4 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.
(page 25 continued on part 2)
