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 . . . . . . . . . . . . . . . . . . 57
1. Introduction
PPTP allows existing Network Access Server (NAS) functions to be
separated using a client-server architecture. Traditionally, the
following functions are implemented by a NAS:
1) Physical native interfacing to PSTN or ISDN and control of
external modems or terminal adapters.
A NAS may interface directly to a telco analog or digital
circuit or attach via an external modem or terminal adapter.
Control of a circuit-switched connection is accomplished with
either modem control or DSS1 ISDN call control protocols.
The NAS, in conjunction with the modem or terminal adapters,
may perform rate adaption, analog to digital conversion, sync
to async conversion or a number of other alterations of data
streams.
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.
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.
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.
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.
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 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.
Peer"s Call ID This field is set to the value received 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 2.2.
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.
Reserved1 This field MUST be 0.
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 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.
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
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.
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 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 2.2.
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.
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.
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.
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].
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].
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
3. 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 colli
