RFC 2637
Point-to-Point Tunneling Protocol (PPTP)
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 collision to occur. See section 3.1.3 for a description of this situation.3.1.1. Control Connection Originator (may be PAC or PNS)
TCP Open Indication /Send Start Control Connection Request +-----------------+ +------------------------------------>| wait_ctl_reply | | +-----------------+ | Collision/See (4.1.3) Close TCP V V V Receive Start Ctl | +-------------------------------+ | | Connection Reply | | | | Version OK ^ V | V +-----------------+ Receive Start Ctl | +-----------------+ | idle | Connection Reply | | established | +-----------------+ Version Not OK | +-----------------+ ^ | V Local Terminate | Receive Stop Control | | /Send Stop | Connection Request | | Control Request | /Send Stop Control Reply V V | Close TCP +-----------------+ +-------------------------------------| wait_stop_reply | +-----------------+
idle
The control connection originator attempts to open a TCP
connection to the peer during idle state. When the TCP connection
is open, the originator transmits a send Start-Control-
Connection-Request and then enters the wait_ctl_reply state.
wait_ctl_reply
The originator checks to see if another TCP connection has been
requested from the same peer, and if so, handles the collision
situation described in section 3.1.3.
When a Start-Control-Connection-Reply is received, it is examined
for a compatible version. If the version of the reply is lower
than the version sent in the request, the older (lower) version
should be used provided it is supported. If the version in the
reply is earlier and supported, the originator moves to the
established state. If the version is earlier and not supported, a
Stop-Control-Connection-Request SHOULD be sent to the peer and the
originator moves into the wait_stop_reply state.
established
An established connection may be terminated by either a local
condition or the receipt of a Stop-Control-Connection-Request. In
the event of a local termination, the originator MUST send a
Stop-Control-Connection-Request and enter the wait_stop_reply
state.
If the originator receives a Stop-Control-Connection-Request it
SHOULD send a Stop-Control-Connection-Reply and close the TCP
connection making sure that the final TCP information has been
"pushed" properly.
wait_stop_reply
If a Stop-Control-Connection-Reply is received, the TCP connection
SHOULD be closed and the control connection becomes idle.
3.1.2. Control connection Receiver (may be PAC or PNS)
Receive Start Control Connection Request Version Not OK/Send Start Control Connection Reply with Error +--------+ | | Receive Control Connection Request Version OK | | /Send Start Control Connection Reply | | +----------------------------------------+ ^ V ^ V +-----------------+ Receive Start Ctl +-----------------+ | Idle | Connection Request | Established | +-----------------+ /Send Stop Reply +-----------------+ ^ ^ Close TCP V V Local Terminate | +-------------------------------------+ | /Send Stop | | Control Conn. | V Request | +-----------------+ +-------------------------------------| Wait-Stop-Reply | Receive Stop Control +-----------------+ Connection Reply /Close TCP idle The control connection receiver waits for a TCP open attempt on port 1723. When notified of an open TCP connection, it should prepare to receive PPTP messages. When a Start-Control- Connection-Request is received its version field should be examined. If the version is earlier than the receiver's version and the earlier version can be supported by the receiver, the receiver SHOULD send a Start-Control-Connection-Reply. If the version is earlier than the receiver's version and the version cannot be supported, the receiver SHOULD send a Start-Connection- Reply message, close the TCP connection and remain in the idle state. If the receiver's version is the same as earlier than the peer's, the receiver SHOULD send a Start-Control-Connection-Reply with the receiver's version and enter the established state. established An established connection may be terminated by either a local condition or the reception of a Stop-Control-Connection-Request. In the event of a local termination, the originator MUST send a Stop-Control-Connection-Request and enter the wait_stop_reply state.
If the originator receives a Stop-Control-Connection-Request it
SHOULD send a Stop-Control-Connection-Reply and close the TCP
connection, making sure that the final TCP information has been
"pushed" properly.
wait_stop_reply
If a Stop-Control-Connection-Reply is received, the TCP connection
SHOULD be closed and the control connection becomes idle.
3.1.3. Start Control Connection Initiation Request Collision
A PAC and PNS must have only one control connection between them. It
is possible, however, for a PNS and a PAC to simultaneously attempt
to establish a control connection to each other. When a Start-
Control-Connection-Request is received on one TCP connection and
another Start-Control-Connection-Request has already been sent on
another TCP connection to the same peer, a collision has occurred.
The "winner" of the initiation race is the peer with the higher IP
address (compared as 32 bit unsigned values, network number more
significant). For example, if the peers 192.33.45.17 and 192.33.45.89
collide, the latter will be declared the winner. The loser will
immediately close the TCP connection it initiated, without sending
any further PPTP control messages on it and will respond to the
winner's request with a Start-Control-Connection-Reply message. The
winner will wait for the Start-Control-Connection-Reply on the
connection it initiated and also wait for a TCP termination
indication on the connection the loser opened. The winner MUST NOT
send any messages on the connection the loser initiated.
3.1.4. Keep Alives and Timers
A control connection SHOULD be closed by closing the underlying TCP
connection under the following circumstances:
1. If a control connection is not in the established state (i.e.,
Start-Control-Connection-Request and Start-Control-Connection-
Reply have not been exchanged), a control connection SHOULD be
closed after 60 seconds by a peer waiting for a Start-Control-
Connection-Request or Start-Control-Connection-Reply message.
2. If a peer's control connection is in the established state and has
not received a control message for 60 seconds, it SHOULD send a
Echo-Request message. If an Echo-Reply is not received 60 seconds
after the Echo-Request message transmission, the control
connection SHOULD be closed.
3.2. Call States
3.2.1. Timing considerations
Because of the real-time nature of telephone signaling, both the PNS and PAC should be implemented with multi-threaded architectures such that messages related to multiple calls are not serialized and blocked. The transit delay between the PAC and PNS should not exceed one second. The call and connection state figures do not specify exceptions caused by timers. The implicit assumption is that since the TCP-based control connection is being verified with keep-alives, there is less need to maintain strict timers for call control messages. Establishing outbound international calls, including the modem training and negotiation sequences, may take in excess of 1 minute so the use of short timers is discouraged. If a state transition does not occur within 1 minute (except for connections in the idle or established states), the integrity of the protocol processing between the peers is suspect and the ENTIRE CONTROL CONNECTION should be closed and restarted. All Call IDs are logically released whenever a control connection is started. This presumably also helps in preventing toll calls from being "lost" and never cleared.3.2.2. Call ID Values
Each peer assigns a Call ID value to each user session it requests or accepts. This Call ID value MUST be unique for the tunnel between the PNS and PAC to which it belongs. Tunnels to other peers can use the same Call ID number so the receiver of a packet on a tunnel needs to associate a user session with a particular tunnel and Call ID. It is suggested that the number of potential Call ID values for each tunnel be at least twice as large as the maximum number of calls expected on a given tunnel. A session is defined by the triple (PAC, PNS, Call ID).3.2.3. Incoming Calls
An Incoming-Call-Request message is generated by the PAC when an associated telephone line rings. The PAC selects a Call ID and serial number and indicates the call bearer type. Modems should always indicate analog call type. ISDN calls should indicate digital when unrestricted digital service or rate adaption is used and analog if
digital modems are involved. Dialing number, dialed number, and
subaddress may be included in the message if they are available from
the telephone network.
Once the PAC sends the Incoming-Call-Request, it waits for a response
from the PNS but does not answer the call from the telephone network.
The PNS may choose not to accept the call if:
- No resources are available to handle more sessions
- The dialed, dialing, or subaddress fields are not indicative of
an authorized user
- The bearer service is not authorized or supported
If the PNS chooses to accept the call, it responds with an Incoming-
Call-Reply which also indicates window sizes (see section 4.2). When
the PAC receives the Outgoing-Call-Reply, it attempts to connect the
call, assuming the calling party has not hung up. A final call
connected message from the PAC to the PNS indicates that the call
states for both the PAC and the PNS should enter the established
state.
When the dialed-in client hangs up, the call is cleared normally and
the PAC sends a Call-Disconnect-Notify message. If the PNS wishes to
clear a call, it sends a Call-Clear-Request message and then waits
for a Call-Disconnect-Notify.
3.2.3.1. PAC Incoming Call States
Ring/Send Incoming Call Request +-----------------+
+----------------------------------------->| wait_reply |
| +-----------------+
| Receive Incoming Call Reply V V V
| Not Accepting | | | Receive Incoming
| +--------------------------------+ | | Call Reply Accept-
| | +------------------------------+ | ing/Answer call;
| | | Abort/Send Call | Send Call
^ V V Disconnect Notify V Connected
+-----------------+ +-----------------+
| idle |<-----------------------------| established |
+-----------------+ Receive Clear Call Request +-----------------+
or telco call dropped
or local disconnect
/Send Call Disconnect Notify
The states associated with the PAC for incoming calls are:
idle
The PAC detects an incoming call on one of its telco interfaces.
Typically this means an analog line is ringing or an ISDN TE has
detected an incoming Q.931 SETUP message. The PAC sends an
Incoming-Call-Request message and moves to the wait_reply state.
wait_reply
The PAC receives an Incoming-Call-Reply message indicating non-
willingness to accept the call (general error or don't accept) and
moves back into the idle state. If the reply message indicates
that the call is accepted, the PAC sends an Incoming-Call-
Connected message and enters the established state.
established
Data is exchanged over the tunnel. The call may be cleared
following:
An event on the telco connection. The PAC sends a
Call-Disconnect-Notify message
Receipt of a Call-Clear-Request. The PAC sends a
Call-Disconnect-Notify message
A local reason. The PAC sends a Call-Disconnect-Notify message.
3.2.3.2. PNS Incoming Call States
Receive Incoming Call Request
/Send Incoming Call Reply +-----------------+
Not Accepting if Error | Wait-Connect |
+-----+ +-----------------+
| | Receive Incoming Call Req. ^ V V
| | /Send Incoming Call Reply OK | | | Receive Incoming
| | +--------------------------------+ | | Call Connect
^ V ^ V------------------------------+ V
+-----------------+ Receive Call Disconnect +-----------------+
| Idle | Notify +- | Established |
+-----------------+ | +-----------------+
^ ^ | V Local Terminate
| +----------------------------+ | /Send Call Clear
| Receive Call Disconnect | Request
| Notify V
| +-----------------+
+--------------------------------------| Wait-Disconnect |
Receive Call Disconnect +-----------------+
Notify
The states associated with the PNS for incoming calls are:
idle
An Incoming-Call-Request message is received. If the request is
not acceptable, an Incoming-Call-Reply is sent back to the PAC and
the PNS remains in the idle state. If the Incoming-Call-Request
message is acceptable, an Incoming-Call-Reply is sent indicating
accept in the result code. The session moves to the wait_connect
state.
wait_connect
If the session is connected on the PAC, the PAC sends an incoming
call connect message to the PNS which then moves into established
state. The PAC may send a Call-Disconnect-Notify to indicate that
the incoming caller could not be connected. This could happen,
for example, if a telephone user accidently places a standard
voice call to a PAC resulting in a handshake failure on the called
modem.
established
The session is terminated either by receipt of a Call-Disconnect-
Notify message from the PAC or by sending a Call-Clear-Request.
Once a Call-Clear-Request has been sent, the session enters the
wait_disconnect state.
wait_disconnect
Once a Call-Disconnect-Notify is received the session moves back
to the idle state.
3.2.4. Outgoing Calls
Outgoing messages are initiated by a PNS and instruct a PAC to place
a call on a telco interface. There are only two messages for outgoing
calls: Outgoing-Call-Request and Outgoing-Call-Reply. The PNS sends
an Outgoing-Call-Request specifying the dialed party phone number and
subaddress as well as speed and window parameters. The PAC MUST
respond to the Outgoing-Call-Request message with an Outgoing-Call-
Reply message once the PAC determines that:
The call has been successfully connected
A call failure has occurred for reasons such as: no interfaces are
available for dial-out, the called party is busy or does not
answer, or no dial tone is detected on the interface chosen for
dialing
3.2.4.1. PAC Outgoing Call States
Receive Outgoing Call Request in Error /Send Outgoing Call Reply with Error |--------+ | | Receive Outgoing Call Request No Error | | /Off Hook; Dial | | +----------------------------------------- ^ V ^ V +-----------------+ Incomplete Call +-----------------+ | idle | /Send Outgoing Call | wait_cs_ans | +-----------------+ Reply with Error +-----------------+ ^ ^ or Recv. Call Clear Req. V V Telco Answer | | /Send Disconnect Notify| | /Send Outgoing | +-------------------------------------+ | Call Reply. | V | +-----------------+ +-------------------------------------| established | Receive Call Clear Request +-----------------+ or local terminate or telco disconnect /Hangup call and send Call Disconnect Notify The states associated with the PAC for outgoing calls are: idle Received Outgoing-Call-Request. If this is received in error, respond with an Outgoing-Call-Reply with error condition set. Otherwise, allocate physical channel to dial on. Place the outbound call, wait for a connection, and move to the wait_cs_ans state. wait_cs_ans If the call is incomplete, send an Outgoing-Call-Reply with a non-zero Error Code. If a timer expires on an outbound call, send back an Outgoing-Call-Reply with a non-zero Error Code. If a circuit switched connection is established, send an Outgoing- Call-Reply indicating success. established If a Call-Clear-Request is received, the telco call SHOULD be released via appropriate mechanisms and a Call-Disconnect-Notify message SHOULD BE sent to the PNS. If the call is disconnected by the client or by the telco interface, a Call-Disconnect-Notify message SHOULD be sent to the PNS.
3.2.4.2. PNS Outgoing Call States
Open Indication Abort/Send /Send Outgoing Call Call Clear Request +-----------------+Request +-------------------------------->| Wait-Reply |----------+ | +-----------------+ | | Receive Outgoing Call Reply V V Receive Outgoing | | with Error | | Call Reply | | +-------------------------------+ | No Error | ^ V V | +-----------------+ +-----------------+ | | Idle |<-----------------------------| Established | | +-----------------+ Receive Call Disconnect +-----------------+ | ^ Notify V Local Terminate | | | /Send Call Clear | | | Request | | Receive Call Disconnect V | | Notify +-----------------+ | +---------------------------------| Wait-Disconnect |<---------+ +-----------------+ The states associated with the PNS for outgoing calls are: idle An Outgoing-Call-Request message is sent to the PAC and the session moves into the wait_reply state. wait_reply An Outgoing-Call-Reply is received which indicates an error. The session returns to idle state. No telco call is active. If the Outgoing-Call-Reply does not indicate an error, the telco call is connected and the session moves to the established state. established If a Call-Disconnect-Notify is received, the telco call has been terminated for the reason indicated in the Result and Cause Codes. The session moves back to the idle state. If the PNS chooses to terminate the session, it sends a Call-Clear-Request to the PAC and then enters the wait_disconnect state. wait_disconnect A session disconnection is waiting to be confirmed by the PAC. Once the PNS receives the Call-Disconnect-Notify message, the session enters idle state.
4. Tunnel Protocol Operation
The user data carried by the PPTP protocol are PPP data packets. PPP packets are carried between the PAC and PNS, encapsulated in GRE packets which in turn are carried over IP. The encapsulated PPP packets are essentially PPP data packets less any media specific framing elements. No HDLC flags, bit insertion, control characters, or control character escapes are included. No CRCs are sent through the tunnel. The IP packets transmitted over the tunnels between a PAC and PNS has the following general structure: +--------------------------------+ | Media Header | +--------------------------------+ | IP Header | +--------------------------------+ | GRE Header | +--------------------------------+ | PPP Packet | +--------------------------------+4.1. Enhanced GRE header
The GRE header used in PPTP is enhanced slightly from that specified in the current GRE protocol specification [1,2]. The main difference involves the definition of a new Acknowledgment Number field, used to determine if a particular GRE packet or set of packets has arrived at the remote end of the tunnel. This Acknowledgment capability is not used in conjunction with any retransmission of user data packets. It is used instead to determine the rate at which user data packets are to be transmitted over the tunnel for a given user session. The format of the enhanced GRE header is as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |C|R|K|S|s|Recur|A| Flags | Ver | Protocol Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Key (HW) Payload Length | Key (LW) Call ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence Number (Optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Acknowledgment Number (Optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
C
(Bit 0) Checksum Present. Set to zero (0).
R
(Bit 1) Routing Present. Set to zero (0).
K
(Bit 2) Key Present. Set to one (1).
S
(Bit 3) Sequence Number Present. Set to one (1) if a payload
(data) packet is present. Set to zero (0) if payload is not
present (GRE packet is an Acknowledgment only).
s
(Bit 4) Strict source route present. Set to zero (0).
Recur
(Bits 5-7) Recursion control. Set to zero (0).
A
(Bit 8) Acknowledgment sequence number present. Set to one (1) if
packet contains Acknowledgment Number to be used for acknowledging
previously transmitted data.
Flags
(Bits 9-12) Must be set to zero (0).
Ver
(Bits 13-15) Must contain 1 (enhanced GRE).
Protocol Type
Set to hex 880B [8].
Key
Use of the Key field is up to the implementation. PPTP uses it as
follows:
Payload Length
(High 2 octets of Key) Size of the payload, not including
the GRE header
Call ID
(Low 2 octets) Contains the Peer's Call ID for the session
to which this packet belongs.
Sequence Number
Contains the sequence number of the payload. Present if S
bit (Bit 3) is one (1).
Acknowledgment Number
Contains the sequence number of the highest numbered GRE
packet received by the sending peer for this user session.
Present if A bit (Bit 8) is one (1).
The payload section contains a PPP data packet without any
media specific framing elements.
The sequence numbers involved are per packet sequence numbers.
The sequence number for each user session is set to zero at
session startup. Each packet sent for a given user session
which contains a payload (and has the S bit (Bit 3) set to one)
is assigned the next consecutive sequence number for that
session.
This protocol allows acknowledgments to be carried with the
data and makes the overall protocol more efficient, which in
turn requires less buffering of packets.
4.2. Sliding Window Protocol
The sliding window protocol used on the PPTP data path is used for
flow control by each side of the data exchange. The enhanced GRE
protocol allows packet acknowledgments to be piggybacked on data
packets. Acknowledgments can also be sent separately from data
packets. Again, the main purpose of the sliding window protocol is
for flow control--retransmissions are not performed by the tunnel
peers.
4.2.1. Initial Window Size
Although each side has indicated the maximum size of its receive
window, it is recommended that a conservative approach be taken when
beginning to transmit data. The initial window size on the
transmitter is set to half the maximum size the receiver requested,
with a minimum size of one packet. The transmitter stops sending
packets when the number of packets awaiting acknowledgment is equal
to the current window size. As the receiver successfully digests
each window, the window size on the transmitter is bumped up by one
packet until the maximum is reached. This method prevents a system
from flooding an already congested network because no history has
been established.
4.2.2. Closing the Window
When a time-out does occur on a packet, the sender adjusts the size
of the transmit window down to one half its value when it failed.
Fractions are rounded up, and the minimum window size is one.
4.2.3. Opening the Window
With every successful transmission of a window's worth of packets without a time-out, the transmit window size is increased by one packet until it reaches the maximum window size that was sent by the other side when the call was connected. As stated earlier, no retransmission is done on a time-out. After a time-out, the transmission resumes with the window starting at one half the size of the transmit window when the time-out occurred and adjusting upward by one each time the transmit window is filled with packets that are all acknowledged without time-outs.4.2.4. Window Overflow
When a receiver's window overflows with too many incoming packets, excess packets are thrown away. This situation should not arise if the sliding window procedures are being properly followed by the transmitter and receiver. It is assumed that, on the transmit side, packets are buffered for transmission and are no longer accepted from the packet source when the transmit buffer fills.4.2.5. Multi-packet Acknowledgment
One feature of the PPTP sliding window protocol is that it allows the acknowledgment of multiple packets with a single acknowledgment. All outstanding packets with a sequence number lower or equal to the acknowledgment number are considered acknowledged. Time-out calculations are performed using the time the packet corresponding to the highest sequence number being acknowledged was transmitted. Adaptive time-out calculations are only performed when an Acknowledgment is received. When multi-packet acknowledgments are used, the overhead of the adaptive time-out algorithm is reduced. The PAC is not required to transmit multi-packet acknowledgments; it can instead acknowledge each packet individually as it is delivered to the PPP client.4.3. Out-of-sequence Packets
Occasionally packets lose their sequencing across a complicated internetwork. Say, for example that a PNS sends packets 0 to 5 to a PAC. Because of rerouting in the internetwork, packet 4 arrives at the PAC before packet 3. The PAC acknowledges packet 4, and may assume packet 3 is lost. This acknowledgment grants window credit beyond packet 4.
When the PAC does receive packet 3, it MUST not attempt to transmit it to the corresponding PPP client. To do so could cause problems, as proper PPP protocol operation is premised upon receiving packets in sequence. PPP does properly deal with the loss of packets, but not with reordering so out of sequence packets between the PNS and PAC MUST be silently discarded, or they may be reordered by the receiver. When packet 5 comes in, it is acknowledged by the PAC since it has a higher sequence number than 4, which was the last highest packet acknowledged by the PAC. Packets with duplicate sequence numbers should never occur since the PAC and PNS never retransmit GRE packets. A robust implementation will silently discard duplicate GRE packets, should it receive any.4.4. Acknowledgment Time-Outs
PPTP uses sliding windows and time-outs to provide both user session flow-control across the internetwork and to perform efficient data buffering to keep the PAC-PNS data channels full without causing receive buffer overflow. PPTP requires that a time-out be used to recover from dropped data or acknowledgment packets. The exact implementation of the time-out is vendor-specific. It is suggested that an adaptive time-out be implemented with backoff for congestion control. The time-out mechanism proposed here has the following properties: Independent time-outs for each session. A device (PAC or PNS) will have to maintain and calculate time-outs for every active session. An administrator-adjustable maximum time-out, MaxTimeOut, unique to each device. An adaptive time-out mechanism that compensates for changing throughput. To reduce packet processing overhead, vendors may choose not to recompute the adaptive time-out for every received acknowledgment. The result of this overhead reduction is that the time-out will not respond as quickly to rapid network changes. Timer backoff on time-out to reduce congestion. The backed-off timer value is limited by the configurable maximum time-out value. Timer backoff is done every time an acknowledgment time-out occurs. In general, this mechanism has the desirable behavior of quickly backing off upon a time-out and of slowly decreasing the time-out value as packets are delivered without time-outs.
Some definitions:
Packet Processing Delay (PPD) is the amount of time required for
each side to process the maximum amount of data buffered in their
receive packet sliding window. The PPD is the value exchanged
between the PAC and PNS when a call is established. For the PNS,
this number should be small. For a PAC making modem connections,
this number could be significant.
Sample is the actual amount of time incurred receiving an
acknowledgment for a packet. The Sample is measured, not
calculated.
Round-Trip Time (RTT) is the estimated round-trip time for an
Acknowledgment to be received for a given transmitted packet. When
the network link is a local network, this delay will be minimal
(if not zero). When the network link is the Internet, this delay
could be substantial and vary widely. RTT is adaptive: it will
adjust to include the PPD and whatever shifting network delays
contribute to the time between a packet being transmitted and
receiving its acknowledgment.
Adaptive Time-Out (ATO) is the time that must elapse before an
acknowledgment is considered lost. After a time-out, the sliding
window is partially closed and the ATO is backed off.
The Packet Processing Delay (PPD) parameter is a 16-bit word
exchanged during the Call Control phase that represents tenths of a
second (64 means 6.4 seconds). The protocol only specifies that the
parameter is exchanged, it does not specify how it is calculated. The
way values for PPD are calculated is implementation-dependent and
need not be variable (static time-outs are allowed). The PPD must be
exchanged in the call connect sequences, even if it remains constant
in an implementation. One possible way to calculate the PPD is:
PPD' = ((PPP_MAX_DATA_MTU - Header) * WindowSize * 8) / ConnectRate
PPD = PPD' + PACFudge
Header is the total size of the IP and GRE headers, which is 36. The
MTU is the overall MTU for the internetwork link between the PAC and
PNS. WindowSize represents the number of packets in the sliding
window, and is implementation-dependent. The latency of the
internetwork could be used to pick a window size sufficient to keep
the current session's pipe full. The constant 8 converts octets to
bits (assuming ConnectRate is in bits per second). If ConnectRate is
in bytes per second, omit the 8. PACFudge is not required but can be
used to take overall processing overhead of the PAC into account.
The value of PPD is used to seed the adaptive algorithm with the initial RTT[n-1] value.4.4.1. Calculating Adaptive Acknowledgment Time-Out
We still must decide how much time to allow for acknowledgments to return. If the time-out is set too high, we may wait an unnecessarily long time for dropped packets. If the time-out is too short, we may time out just before the acknowledgment arrives. The acknowledgment time-out should also be reasonable and responsive to changing network conditions. The suggested adaptive algorithm detailed below is based on the TCP 1989 implementation and is explained in [11]. 'n' means this iteration of the calculation, and 'n-1' refers to values from the last calculation. DIFF[n] = SAMPLE[n] - RTT[n-1] DEV[n] = DEV[n-1] + (beta * (|DIFF[n]| - DEV[n-1])) RTT[n] = RTT[n-1] + (alpha * DIFF[n]) ATO[n] = MAX (MinTimeOut, MIN (RTT[n] + (chi * DEV[n]), MaxTimeOut)) DIFF represents the error between the last estimated round-trip time and the measured time. DIFF is calculated on each iteration. DEV is the estimated mean deviation. This approximates the standard deviation. DEV is calculated on each iteration and stored for use in the next iteration. Initially, it is set to 0. RTT is the estimated round-trip time of an average packet. RTT is ycalculated on each iteration and stored for use in the next iteration. Initially, it is set to PPD. ATO is the adaptive time-out for the next transmitted packet. ATO is calculated on each iteration. Its value is limited, by the MIN function, to be a maximum of the configured MaxTimeOut value. Alpha is the gain for the average and is typically 1/8 (0.125). Beta is the gain for the deviation and is typically 1/4 (0.250). Chi is the gain for the time-out and is typically set to 4.
To eliminate division operations for fractional gain elements, the entire set of equations can be scaled. With the suggested gain constants, they should be scaled by 8 to eliminate all division. To simplify calculations, all gain values are kept to powers of two so that shift operations can be used in place of multiplication or division.4.4.2. Congestion Control: Adjusting for Time-Out
This section describes how the calculation of ATO is modified in the case where a time-out does occur. When a time-out occurs, the time- out value should be adjusted rapidly upward. Although the GRE packets are not retransmitted when a time-out occurs, the time-out should be adjusted up toward a maximum limit. To compensate for shifting internetwork time delays, a strategy must be employed to increase the time-out when it expires (notice that in addition to increasing the time-out, we are also shrinking the size of the window as described in the next section). For an interval in which a time- out occurs, the new ATO is calculated as: RTT[n] = delta * RTT[n-1] DEV[n] = DEV[n-1] ATO[n] = MAX (MinTimeOut, MIN (RTT[n] + (chi * DEV[n]), MaxTimeOut)) In this calculation of ATO, only the two values that both contribute to ATO and are stored for the next iteration are calculated. RTT is scaled by delta, and DEV is unmodified. DIFF is not carried forward and is not used in this scenario. A value of 2 for Delta, the time- out gain factor for RTT, is suggested.5. Security Considerations
The security of user data passed over the tunneled PPP connection is addressed by PPP, as is authentication of the PPP peers. Because the PPTP control channel messages are neither authenticated nor integrity protected, it might be possible for an attacker to hijack the underlying TCP connection. It is also possible to manufacture false control channel messages and alter genuine messages in transit without detection. The GRE packets forming the tunnel itself are not cryptographically protected. Because the PPP negotiations are carried out over the tunnel, it may be possible for an attacker to eavesdrop on and modify those negotiations.
Unless the PPP payload data is cryptographically protected, it can be captured and read or modified.6. Authors' Addresses
Kory Hamzeh Ascend Communications 1275 Harbor Bay Parkway Alameda, CA 94502 EMail: kory@ascend.com Gurdeep Singh Pall Microsoft Corporation Redmond, WA EMail: gurdeep@microsoft.com William Verthein U.S. Robotics/3Com Jeff Taarud Copper Mountain Networks W. Andrew Little ECI Telematics Glen Zorn Microsoft Corporation Redmond, WA EMail: glennz@microsoft.com
7. References
[1] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 1701, October 1994. [2] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic Routing Encapsulation (GRE) over IPv4 Networks", RFC 1702, October 1994. [3] Lloyd, B. and W. Simpson, "PPP Authentication Protocols", RFC 1334, October 1992. [4] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [5] Postel, J., "User Data Protocol", STD 6, RFC 768, August 1980. [6] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700, October 1994. See also: http://www.iana.org/numbers.html [7] Simpson, W., editor, "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [8] Ethertype for PPP, Reserved with Xerox Corporation. [9] Simpson, W., "PPP Challenge Handshake Authentication Protocol (CHAP)", RFC 1994, August 1996. [10] Blunk, L. and J Vollbrecht, "PPP Extensible Authentication Protocol (EAP)", RFC 2284, March 1998. [11] Stevens, R., "TCP/IP Illustrated, Volume 1", p. 300, Addison- Wesley, 1994. [12] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
8. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society.