Pat R. Calhoun
Internet Draft US Robotics Access Corp.
expires in six months July 1996
Virtual Tunnel Protocol
Layer 2 Protocol Extension
<draft-calhoun-vtp-ext-l2-00.txt>
Status of this Memo
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Abstract
The VTP Protocol specification defines a generic tunneling mechanism,
as well as extensions for IP and IPX protocols. This specification
will detail a method to establish Level 2 (PPP and SLIP) tunnels
between two peers.
This strawman proposal is intended as a recommendation for using VTP
as a generic all-purpose tunnel scheme, as opposed to having multiple
tunneling mechanism for layer 2 and 3 protocols.
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1. Introduction
The Secure Dynamic Tunneling Protocol was designed with flexibility in
mind. However, the protocol only defines a method for tunneling IP and
IPX. It is envisioned that further specification would be published
which would describe the method for tunneling additional protocols.
This is one such document. This specification will detail the method
required for tunneling Layer 2 (PPP and SLIP) protocols within VTP.
It is assumed that the reader has existing knowledge of the PPTP [1]
and L2F [2] proposals. However there is a significant difference in the
terminology used in this document. This document does not differenciate
the PAC (PPTP Access Concentrator) and PNS (PPTP Network Server). The
terms Mobile Node as used as the device which initiates the tunnel and
the term Home Agent is used as the Tunnel termininator.
2. Protocol Overview
In the existing VTP specification extensions for IP and IPX have been
defined. This specification details the extensions necessary for
tunneling Layer 2 protocols (i.e. PPP, SLIP).
As stated earlier, the PPTP concept of the PAC and the PNS are no
longer used, but have been replaced by the terms Mobile Node and Home
Agent which are consistent with the existing VTP specification.
In the case where a NAS receives an incoming PPP or SLIP call, it acts
as a proxy Mobile Node by issuing a Registration Request to the Home
Agent in behalf of the dial-up user. However, an issue remains where
the Mobile Node must get the IP address of the Home Agent in order to
establish the tunnel.
In the existing PPTP specification, there is no method defined for
learning the IP address of the Home Agent. There are two methods of
determining the address of the Home Agent, they are:
- The Mobile Node receives the DNIS for the call and passes this
on to some central registry (i.e. RADIUS) which returns the IP
address of the Home Agent. Although this method does work, it
does not scale in large networks.
- The Mobile Node negotiates LCP and enters the authentication
phase. Once the user name/password pair is received it passes
this on to a central registry (i.e. RADIUS) which will return
the IP address of the Home Agent.
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In order to support this, it is assumed that the RADIUS Server
has the ability to authenticate the realm only (meaning the
user name is a fully qualified name, user@realm). Another less
elegant method is to predefine all users in the RADIUS server,
but this means that the user information must be duplicated
both in the RADIUS server and in the Home Agent.
Once the tunnel has been established, the Home Agent must
re-open LCP and restart the authentication phase. Although
technically feasible, this solution causes many problems with
certain existing PPP stacks.
In the L2F specification, there is an option where the Mobile Node
performs the authentication phase with the PPP client and passes the
result to the Home Agent for user authentication.
Agent.
The Layer 2 Forwarding extension to VTP supports all of the approaches
defined above. Note that in order for a Home Agent to conform to this
specification, it must have the ability to authentication the user
locally.
2.1. Dynamic Tunnel Establishment
2.1.1. Incoming Call Tunnel Establishment
Once a PPP session has been identified by the Mobile Node as a
candidate for Layer 2 tunneling and the IP address of the Home Agent
is known, a Registration Request is forwarded to the Home Agent. The
message MUST include the VTP Tunnel-Identifier Extension, the L2TP
Extension and the Incoming-Call-Request Extension.
If the Mobile Node has already answered the incoming call the
Incoming-Call-Established Extension must be present. The Home Agent is
then responsible for reponding with a Registration Response which
includes the VTP Tunnel-Identifier Extension and the
Incoming-Call-Confirm Extension and begin PPP negotiation.
In the case where the Mobile Node wishes to perform the authentication
with the client and pass the result to the Home Agent for
authentication the User-Identification Extension and the
Link-Control-Protocol Extension must be present. In this case, the Home
Agent must authenticate the user and if successful respond with a
Registration Response.
In the case where the Mobile Node wishes to receive confirmation that
it should accept the call from the Home Agent, the Registration Request
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should only contain the VTP Tunnel-Identifier Extension and the
L2TP Extension. If the Home Agent wishes the Mobile Node to answer the
call, a successful Registration Response must be returned which
includes the Incoming-Call-Request Extension. Once the Mobile Node has
accepted the call, a Refresh Refresh message is sent to the Home Agent
which includes the Incoming-Call-Established Extension.
2.1.2. Outgoing Call Tunnel Establishment
If a Mobile Node wishes to establish an outgoing call with a Home
Agent, a Registration Request must be sent which includes the VTP
Tunnel-Identifier Extension and the L2TP Extension and the
Outgoing-Call-Request Extension (which includes dialing information).
If the Home Agent has the ability of dialing the requested number on
the specified bearer type, it must return a Registration Response with
the VTP Tunnel-Identifier extension and the Outgoing-Call-Pending
extension.
Once the outgoing call has been completed, the Home Agent must send
a Refresh Request with the VTP Tunnel-Identifier extension as well as
the Outgoing-Call-Established Extension.
2.2. Dynamic Tunnel Refresh
As described in [3], the tunnel refresh mechanism is used as a keep
alive to inform each peer that the tunnel is still valid. However, this
specification makes use of the tunnel refresh messages to pass along
additional information.
In [3], the tunnel refreshes are typically initiated by the Mobile
Node, however this specification does require that both the Mobile Node
and the Home Agent be able to initiate the message.
As described above, the Refresh Request may be used by the Mobile Node
in the case where it is requesting permission to accept an incoming
call from the Home Agent. A successful Registration Response will
cause the Mobile Node to initiate a Refresh Request with the
VTP Tunnel-Identifier Extension and the Incoming-Call-Established
Extension.
In the case of an outgoing call, a Refresh Request is sent by the
Home Agent in order to notify the Mobile Node that the call either
failed or was successfully completed.
The Mobile Node may send the WAN-Error Extension in the tunnel refresh
Calhoun [Page 4]
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if any errors had occurred since the last refresh. Note that the
counters are cumulative and are never reset while the connection is
established.
If a request to change the ACCM has been received by the Home Agent
from the PPP client, the Home Agent may send a refresh message to the
Mobile Node with the VTP Tunnel-Identifier Extension and the
Set-Link-Info Extension. This extension will inform the Mobile Node of
the new character mapping table. The Home Agent may include this
extension in a Refresh Response if necessary.
2.3. Dynamic Tunnel Shutdown
As described in [3], the Deregistration Request is sent by the Mobile
Node to the Home Agent to inform of a session disconnect. However,
since the PPP session is terminated on the Home Agent, the Mobile Node
may not know that it must terminate the connection (unless it is
examining each LCP packets from the PPP client and the Home Agent).
In this extension, the Deregistration Request must be able to be
initiated by either peer.
If the initiator of the Deregistration Request is the Mobile Node, the
message must include the VTP Tunnel-Identifier Extension and the
Disconnect-Information Extension. If the initiator is the Home Agent,
the L2TP-Disconnect-Request Extension must be present.
If the Mobile Node is to respond with a Deregistration Response it must
include the VTP Tunnel-Identifier Extension and the
Disconnect-Information Extension.
2.4. Security Implications
Since [3] proposes the use of the Mobile-Home Authentication Extension,
it is possible to use this authentication scheme as well with the L2TP
extension. However, since the user authentication is performed by the
Home Agent, it becomes a challenge to retrieve a session key for the
tunnel.
Since the user is not authenticated, the session key distribution
center will have to generate keys for unauthenticated users. This may
not be a problem since the key is not of much use if the username and
password pair are invalid.
Therefore the issue of whether the authentication is necessary or not
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is a point which must be addressed. One way of thinking would be to
abolish the extension and use the AH [4] header instead.
3. Protocol Extensions
The following sections will define the extensions to the protocol which
are necessary for Layer 2 tunneling.
Note that this document only contains extensions which are not already
defined in [3]. It is mandatory that all messages contain the
Foreign-Home Authentication Extension.
Although the underlying protocol used for tunnel management is the
same for both Layer 2 and Layer 3 [3], a tunnel between two peers
MUST not contain both layers. This means that if a layer 2 tunnel is to
be established between two endpoints and a layer 3 tunnel already
exists, the Mobile Node MUST establish a NEW tunnel.
3.1. Registration Request
This section will define the protocol extensions which are necessary
during the tunnel registration process.
The Registration Request message must always include the VTP
Tunnel-Identifier extension as defined in [3]. In addition, if
a secure tunnel is required the Mobile-Home Authentication Extension
as defined in [3] should also be used.
3.1.1. L2TP Extension
This extension should be added to a Registration Request message when
a Layer 2 tunnel is to be initiated. This extension defines local
configuration information.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Protocol Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Call ID | Call Serial Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Framing Capabilities |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bearer Capabilities |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max Channels | Firmware Revision |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor String |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 15 (VTP L2TP Extension)
Sub-Length 10
Flag Bit 1 MUST be set (mandatory support)
Protocol Version This field defines the local version of
the L2TP protocol stack.
Call ID A unique identifier, unique to a tunnel
assigned by the Mobile Node. It is used
to multiplex and demultiplex data sent
over the tunnel.
Call Serial Number An identfier assigned by the Mobile Node
to this session for the purposes of
identifying this particular session in
logged session information. Unlike the
Call ID, both the Mobile Node and the
Hom Agent associate the same Call
Serial Number with a given session. The
combination of IP address and call serial
number SHOULD be unique.
Framing Capabilities This bitmask defines the framing support
of the Mobile Node. The following values
are currently supported:
VTP_FRAMING_ASYNC 0x0001
VTP_FRAMING_SYNC 0x0002
Bearer Capabilities This bitmask defines the bearer
capabilities of the Mobile Node. The
following values are currently supported:
VTP_BEARER_ANALOG 0x0001
VTP_BEARER_DIGITAL 0x0002
Maximum Channels This field contains the maximum number of
sessions the Mobile Node can support. A
zero value indicates that the Mobile Node
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does not support incoming calls.
Firmware Revisions This field contains the firmware revision
of the Mobile Node.
Vendor Name A 64 octet field which contains a vendor
specific string describing the type of
Mobile Node. This field should be padded
with zeros (0).
3.1.2. Incoming-Call-Request Extension
This extension is present when the Mobile Node has answered an incoming
call and wishes the Home Agent to handle the session. The extension
format is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Dialed Length |Dialing Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Call Bearer Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Physical Channel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Dialed Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Dialing Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subaddress |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 16 (Incoming-Call-Request Extension)
Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
Dialed Length Contains the length of the Dialed Number
Field.
Dialing Length Contains the length of the Dialing Number
Field.
Call Bearer Type This field contains the bearer type used
for the incoming call. The following
values have been defined:
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VTP_BEARER_ANALOG 0x0001
VTP_BEARER_DIGITAL 0x0002
Physical Channel ID This field contains the Mobile Node's
Channel Identifier for this call. The
format of this field is vendor specific.
Dialed Number The number which was dialed by the
caller, represented as an ASCII string.
Dialing Number The number from which the call was
initiated, represented as an ASCII string.
Subaddress This field contains a zero padded string
of octet which contain additional dialing
information.
3.1.3. User-Identification Extension
This extension is present when the Mobile Node initiated user
authentication by collecting the username and password and is now
passing the information along to the Home Agent for final
authentication.
When present, the Mobile Node has already initiated LCP and is
passing along the LCP negotiated parameters. However, the LCP
information is present in the Link Control Protocol Extension.
Note that for SLIP, all of the LCP and CHAP specific lengths are
to be set to zero(0).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Authentication Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|UserName Length|Password Length| Challenge Len | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| User Name |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Password |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CHAP Challenge |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 17 (User-Identification Extension)
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Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
Autentication Type This bitmask field represents the
authentication protocol used with the
PPP client. The following values have
been reserved:
VTP_AUTH_CHAP 0x0001
VTP_AUTH_PAP 0x0002
VTP_AUTH_EAP 0x0003
(Note: This models breaks with EAP since
EAP should ideally simply be forwarded
by the Mobile Node to the authenticating
server).
UserName Length Length of the User Name Field
Password Length Length of the Password Field
Challenge Length Length of the CHAP Challenge Field
UserName String representing the User Name. This
field may be up to 63 characters.
Password String representing the User's Password.
If Authentication Type was set to CHAP,
then this field contains the CHAP Response.
This field may be up to 128 octets.
CHAP Challenge Set to the challenge sent to the client by
the NAS.
3.1.4. Link-Control-Protocol Extension
This extension is present when the Mobile Node initiated user
authentication by collecting the username and password and the
layer protocol has been identified as PPP (since LCP was
negotiated).
Note that for SLIP connections, this extension is not present.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LCP1 Length | LCP2 Length | LCP3 Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Client LCP ConfAck Packet |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MN LCP ConfAck Packet |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Client LCP ConfReq Packet |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 18 (Link-Control-Protocol Extension)
Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
LCP1 Length Length of the Client LCP ConfAck Field
LCP2 Length Length of the MN LCP ConfAck Field
LCP3 Length Length of the Client LCP ConfReq Field
Client LCP ConfAck Contains a copy of the closing ConfAck
received from the client.
MN LCP ConfAck Contains a copy of the closing ConfAck
sent to the client by the Mobile Node.
Client LCP ConfReq This may be used by the Home Agent to
detect capabilities of the client which
were negotiated away while starting LCP
with the Mobile Node. Detection of such
options may be used by the Home Agent to
decide to renegotiate LCP.
3.1.4. Incoming-Call-Established Extension
This extension is present in the Registration Request ONLY if the call
has already been accepted by the Mobile Node. The absence of this
extension in the message indicates to the Home Agent that the
Registration Response will result in the call being accepted.
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This is especially useful in cases where the Home Agent is to accept
the call based on the DNIS/ANI (which is available on Digital calls),
or when the Home Agent has been placed into an adminitrative mode
whereby it cannot accept new calls.
The format of the extension is as follow:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Peer's Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Connect Speed |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Recv. Window Size | Packet Transmit Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Framing Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 19 (Incoming-Call-Established Extension)
Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
Call ID This field is set to the
Connect Speed This field contains the speed at which
the connection was established.
Recv. Window Size This field may optionally be set if
flow control is used at the GRE layer. If
set, this field contains the number of
packets the Mobile Node will buffer for
this session.
Packet Transmit Delay A measure of the packet processing delay
that might be imposed on data sent to the
Mobile Node from the Home Agent. This
value is specified in units of 1/10
seconds. See Appendix A for a description
of how this value is determined and used.
Framing Type A value indicating the type of PPP
framing being used by this incoming call.
1 - Call uses asynchronous framing
2 - Call uses synchronous framing
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3.1.5. Outgoing-Call-Request Extension
The Outgoing-Call-Request Extension is added to the Registration
Request if the Mobile Node wishes to establish an outgoing call with
the Home Agent. This extension contains information required in order
to for the Home Agent to make the call.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Phone Number Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Minimum BPS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum BPS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bearer Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Framing Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Recv. Window Size | Packet Processing Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Phone Number (64 octets) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Subaddress (64 octets) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 20 (Outgoing-Call-Request Extension)
Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
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.
Call Bearer Type This field contains the bearer type used
for the incoming call. The following
values have been defined:
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VTP_BEARER_ANALOG 0x0001
VTP_BEARER_DIGITAL 0x0002
VTP_BEARER_ANY 0x0003
Framing Type A value indicating the type of PPP
framing being used by this incoming call.
VTP_FRAMING_ASYNC 0x0001
VTP_FRAMING_SYNC 0x0002
VTP_FRAMING_ANY 0x0003
Recv. Window Size This field may optionally be set if
flow control is used at the GRE layer. If
set, this field contains the number of
packets the Mobile Node will buffer for
this session.
Packet Processing Delay A measure of the packet processing delay
that might be imposed on data sent to the
Mobile Node from the Home Agent. This
value is specified in units of 1/10
seconds. See Appendix A for a description
of how this value is determined and used.
Phone Number Length The actual number of valid digits in the
Phone Number field.
Reserved1 This field MUST be 0.
Phone Number The number to be dialed to establish the
outgoing session. For ISDN and analog
calls this field is an ASCII string.
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.
3.2. Registration Response
This section will define the protocol extensions which are necessary
during the Registration Response process.
The Registration Response message must always include the VTP
Tunnel-Identifier extension as defined in [3]. In addition, if
a secure tunnel is required the Mobile-Home Authentication Extension
Calhoun [Page 14]
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as defined in [3] should also be used.
3.2.1. Incoming-Call-Confirm Extension
This extension is present in the Registration Response to inform the
Mobile Node of Home Agent specific configuration.
The format of the extension is as follow:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Reserved | Error Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Call ID | Peer's Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Recv. Window Size | Packet Transmit Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 21 (Incoming-Call-Confirm Extension)
Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
Error Code This field is set to a non-zero value if
a L2TP specific error occurred. If this
field is set to a value, the VTP header
will have the Code field set to reason
unspecified (32).
Refer to section 5 for the complete list
of L2TP error codes.
Call ID A unique identifier, unique to a tunnel
assigned by the Home Agent. It is used
to multiplex and demultiplex data sent
over the tunnel.
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 Mobile Node to match the
Incoming-Call-Reply with the
Incoming-Call-Request it issued. This
value is included in the GRE header of
Calhoun [Page 15]
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transmitted data packets for this session.
Connect Speed This field contains the speed at which
the connection was established.
Recv. Window Size This field may optionally be set if
flow control is used at the GRE layer. If
set, this field contains the number of
packets the Mobile Node will buffer for
this session.
Packet Transmit Delay A measure of the packet processing delay
that might be imposed on data sent to the
Mobile Node from the Home Agent. This
value is specified in units of 1/10
seconds. See Appendix A for a description
of how this value is determined and used.
3.2.2. Outgoing-Call-Pending Extension
The Outgoing-Call-Pending extension is sent by the Home Agent to the
Mobile Node in order to acknowledge the Outgoing-Call-Request. It must
be followed by a Refresh Request message with the
Outgoing-Call-Established before the tunnel expires. This means that
in this case, the Home Agent is responsible for sending the Refresh
Request to the Mobile Node before the number of seconds which were
specified in the lifetime field of the Registration Request.
The format of the extension is as follow:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Reserved | Error Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Call ID | Peer's Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 28 (Outgoing-Call-Pending Extension)
Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
Error Code This field is set to a non-zero value if
a L2TP specific error occurred. If this
field is set to a value, the VTP header
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will have the Code field set to reason
unspecified (32).
Refer to section 5 for the complete list
of L2TP error codes.
Call ID A unique identifier, unique to a tunnel
assigned by the Home Agent. It is used
to multiplex and demultiplex data sent
over the tunnel.
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 Mobile Node 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.
3.3. Refresh Request
This section will define the protocol extensions which may be present
in the tunnel refresh process.
The Refresh Request message must always include the VTP
Tunnel-Identifier extension as defined in [3]. In addition, if a secure
tunnel is required the Mobile-Home Authentication Extension as defined
in [3] should also be used.
3.3.1. Incoming-Call-Established Extension
This extension is present in the Refresh Request if the Registration
Request did not contain this extension. This SHOULD only occur if the
Registration Request from the Mobile Node was an indication that an
incoming was present and was requesting confirmation from the Home
Agent to answer the call.
Once the call has been answered, a refresh request will be sent from the
Mobile Node with this extension present.
The format of the extension is as follow:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Reserved | Error Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Call ID | Peer's Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Connect Speed |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Recv. Window Size | Packet Transmit Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Framing Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 23 (Incoming-Call-Established Extension)
Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
Error Code This field is set to a non-zero value if
a L2TP specific error occurred. If this
field is set to a value, the VTP header
will have the Code field set to reason
unspecified (32).
Refer to section 5 for the complete list
of L2TP error codes.
Call ID A unique identifier, unique to a tunnel
assigned by the Home Agent. It is used
to multiplex and demultiplex data sent
over the tunnel.
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 Mobile Node 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.
Connect Speed This field contains the speed at which
the connection was established.
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Recv. Window Size This field may optionally be set if
flow control is used at the GRE layer. If
set, this field contains the number of
packets the Mobile Node will buffer for
this session.
Packet Transmit Delay A measure of the packet processing delay
that might be imposed on data sent to the
Mobile Node from the Home Agent. This
value is specified in units of 1/10
seconds. See Appendix A for a description
of how this value is determined and used.
Framing Type A value indicating the type of PPP
framing being used by this incoming call.
VTP_FRAMING_ASYNC 0x0001
VTP_FRAMING_SYNC 0x0002
3.3.2. Outgoing-Call-Established Extension
The Outgoing-Call-Established Extension is sent by the Home Agent to
the Mobile Node once the outgoing call was either successfully
completed or failed. It provides information to the Mobile Node about
particular parameters used for the call. It provides information to
allow the Mobile Node to regulate the transmission of data to the Home
Agent for this session.
The format of the extension is as follow:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Reserved | Error Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Call ID | Peer's Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Connect Speed |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Recv. Window Size | Packet Processing Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Physical Channel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 22 (Outgoing-Call-Established Extension)
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Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
Call ID A unique identifier, unique to a tunnel
assigned by the Home Agent. It is used
to multiplex and demultiplex data sent
over the tunnel.
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 Mobile Node 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.
Error Code This field is set to a non-zero value if
a L2TP specific error occurred. If this
field is set to a value, the VTP header
will have the Code field set to reason
unspecified (32).
Refer to section 5 for the complete list
of L2TP error codes.
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 This field contains the speed at which
the connection was established.
Recv. Window Size This field may optionally be set if
flow control is used at the GRE layer. If
set, this field contains the number of
packets the Mobile Node will buffer for
this session.
Packet Transmit Delay A measure of the packet processing delay
that might be imposed on data sent to the
Mobile Node from the Home Agent. This
value is specified in units of 1/10
seconds. See Appendix A for a description
of how this value is determined and used.
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This value should be set to the maximum
delay that can normally occur between the
time a packet arrives at the Mobile Node
and is delivered to the client. See
Appendix A for an example of how this
value is determined and used.
Framing Type A value indicating the type of PPP
framing being used by this incoming call.
VTP_FRAMING_ASYNC 0x0001
VTP_FRAMING_SYNC 0x0002
Physical Channel ID This field contains the Mobile Node's
Channel Identifier for this call. The
format of this field is vendor specific.
3.3.3. WAN-Error Extension
This extension MAY be present in the Refresh Request message from the
Mobile Node if any errors had occured since the last refresh request
message.
The counters are cumulative and are never cleared while the connection
is established.
The format of the message is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Peer's Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CRC Errors |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Framing Errors |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hardware Overruns |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Buffer Overruns |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time-out Errors |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Alignment Errors |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 24 (WAN-Error Extension)
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Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
Peer's Call ID This field is set to the Call ID which was
assigned by the receiver of this message.
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.
3.3.3. Set-Link-Info Extension
The Set Link Info Extension may be added to the VTP Refresh Request
Message if the sender wishes to change PPP-negotiated options.
Because these options may be changed at any time during the call, this
message is used in order to inform the peer of the change in options.
The format of the message is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Peer's Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Send ACCM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receive ACCM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 25 (Set-Link-Info Extension)
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Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
Peer's Call ID This field is set to the Call ID which was
assigned by the receiver of this message.
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.
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.
3.4. Refresh Response
The following is the extension which may be present in the tunnel
refresh response.
The Refresh Response message must always include the VTP
Tunnel-Identifier extension as defined in [3]. In addition, if a secure
tunnel is required the Mobile-Home Authentication Extension as defined
in [3] should also be used.
3.4.1. Set-Link-Info Extension
The Set Link Info Extension may be added to the VTP Refresh Response
Message if the sender wishes to change PPP-negotiated options. Because
these options may be changed at any time during the call, this message
is used in order to inform the peer of the change in options.
The format of the message is as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Peer's Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Send ACCM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receive ACCM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Sub-Type 25 (Set-Link-Info Extension)
Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
Peer's Call ID This field is set to the Call ID which was
assigned by the receiver of this message.
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.
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.
3.5. Deregistration Request
This section will define the protocol extensions which are necessary
during the tunnel deregistration process.
The Deregistration Request message must always include the VTP
Tunnel-Identifier Extension as defined in [3]. In addition, if a secure
tunnel is required the Mobile-Home Authentication Extension as defined
in [3] should also be used.
3.5.2. L2TP Disconnect-Request Extension
This extension must be present in the Deregistration Request if the
initiator of the deregistration is the Home Agent. This message
includes the Call ID which is necessary for the Mobile Node in order
to determine the call to disconnect.
The format of the extension is as follow:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 26 (L2TP Disconnect-Request Extension)
Calhoun [Page 24]
DRAFT VTP Layer 2 Protocol Tunnel Extension July 1996
Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
Call ID The Call ID assigned by the receiver of
this message. This value is used instead
of the Peer's Call ID because the latter
may not be known to the peer if the call
must be aborted during call establishment.
3.5.1. Disconnect-Info Extension
This extension is present in the Deregistration Request if the Mobile
Node is the initiator of this message.
The format of the extension is as follow:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Reserved | Error Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Call ID | Cause Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Call Statistics (128 octets) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 27 (Disconnect-Info Extension)
Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
Error Code This field is set to a non-zero value if
a L2TP specific error occurred. If this
field is set to a value, the VTP header
will have the Code field set to reason
unspecified (32).
Refer to section 5 for the complete list
of L2TP error codes.
Call ID The Call ID assigned by the receiver of
this message. This value is used instead
Calhoun [Page 25]
DRAFT VTP Layer 2 Protocol Tunnel Extension July 1996
of the Peer's Call ID because the latter
may not be known to the peer if the call
must be aborted during call establishment.
Cause Code This field contains disconnection
specific information. The value is
specific to the Bearer Type (for Digital
calls, the value should contain the Q.931
cause code).
Call Statistics This field contains a vendor specific
string of 128 octets representing the
call statistics. This field MUST be
padded with zero (0).
3.6. Deregistration Response
This section will define the protocol extensions which are necessary
during the tunnel deregistration process.
3.6.1. Disconnect-Info Extension
This extension is present in the Deregistration Response if the Mobile
Node is the initiator of this message.
The format of the extension is as follow:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Sub-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Length | flag | Reserved | Error Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Call ID | Cause Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Call Statistics (128 octets) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Type 27 (Disconnect-Info Extension)
Sub-Length Variable
Flag Bit 1 MUST be set (mandatory support)
Error Code This field is set to a non-zero value if
a L2TP specific error occurred. If this
Calhoun [Page 26]
DRAFT VTP Layer 2 Protocol Tunnel Extension July 1996
field is set to a value, the VTP header
will have the Code field set to reason
unspecified (32).
Refer to section 5 for the complete list
of L2TP error codes.
Call ID The Call ID assigned by the receiver of
this message. This value is used instead
of the Peer's Call ID because the latter
may not be known to the peer if the call
must be aborted during call establishment.
Cause Code This field contains disconnection
specific information. The value is
specific to the Bearer Type (for Digital
calls, the value should contain the Q.931
cause code).
Call Statistics This field contains a vendor specific
string of 128 octets representing the
call statistics. This field MUST be
padded with zero (0).
5. Error Codes
The Error Code field in the L2TP extensions are defined in this
appendix. The field should be set to a non-zero value when the Code
field of the VTP header is set to 32 (reason unspecified). The
following values have been defined:
VTP_NOT_CONNECTED 0x01
No control connection exists yet for this Mobile Node-Home
Agent pair.
VTP_BAD_FORMAT 0x02
Length is wrong.
VTP_BAD_VALUE 0x03
One of the field values was out of range or reserved
field was non-zero.
VTP_NO_RESOURCES 0x04
Insufficient resources to handle this command now.
VTP_BAD_CALL_ID 0x05
The Call ID is invalid in this context.
Calhoun [Page 27]
DRAFT VTP Layer 2 Protocol Tunnel Extension July 1996
VTP_NO_CARRIER 0x06
Outgoing Call failed due to no carrier detected or call
lost due to lost of carrier.
VTP_BUSY 0x07
Outgoing Call failed due to detection of a busy signal.
VTP_NO_DIAL_TONE 0x08
Outgoing Call failed due to lack of a dial tone.
VTP_CALL_PROHIBITED 0x09
Outgoing Call administratively prohibited or the Mobile
Node should not accept the incoming call. It should
hang up or issue a busy indication.
VTP_ADMIN_SHUTDOWN 0x0a
Call disconnecte for administrative reasons.
VTP_DISC_REQUESTED 0x0b
Call disconnected due to received L2TP Disconnect-Request.
5. Data Encapsulation
Once a "tunnel" has been established via a success Registration
Request, all data is encapsulated within a GRE header.
This section defines the encapsulation header which is required for the
VTP and L2TP extension.
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|T|L|Flg| Ver | Protocol Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CheckSum (Opt) | Offset(Opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key (Opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (Opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Routing (Opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tunnel Identifier (Opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP (HW) Payload Length | L2TP (LW) Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number (Optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Calhoun [Page 28]
DRAFT VTP Layer 2 Protocol Tunnel Extension July 1996
C Checksum Present. Set to zero (0).
R Routing Present. Set to zero (0).
K Key Present. Set to zero (1) if the
packet is authenticated.
S 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 Sequence Number Present. Set to one (1).
s Strict Source Route Present. Set to
zero (0).
A Acknowledgment sequence number present.
Set to one (1) if packet contains
Acknowledgment Number to be used for
acknowledging previously transmitted
data.
T Tunnel Identifier. Set to one (1).
L L2TP Tunnel Information. Set to one (1)
for L2TP tunnels.
Recur Recursion control. set to zero (0).
Flg (Flags) Must be set to zero.
Ver Must contain 0.
Protocol Type Contains the Assigned Protocol ID
(see assigned numbers RFC).
Key Key field is zero if each packet is not
authenticated.
Sequence Number Contains the sequence number of the
payload. Present if 'S' bit is set.
Tunnel Identifier Contains the Tunnel Identifier which was
negotiated during the Registration
process. Present if the 'T' bit is set.
Calhoun [Page 29]
DRAFT VTP Layer 2 Protocol Tunnel Extension July 1996
This field is used in order to identify
which tunnel the data belongs to.
L2TP The L2TP field contains two fields and
is present if the 'L' bit is set. The
sub-fields are:
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.
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 is set.
Note that an encapsulated packet which contains an invalid tunnel
identifier in the Sequence Number field MUST be dropped.
Acknowledgements
I would like to thank the following people for their help and support:
Andy Valencia, Kory Hamzeh, Gurdeep Singh Pall, Tim Mortsolf, Tom
Stoner
References
[1] Valencia, Littlewood, Kolar,
"Layer Two Forwarding (Protocol) "L2F"", Internet-Draft,
draft-ietf-pppext-l2f-02.txt, April 1996.
[2] Hamzeh, et al, "Point-to-Point Tunneling Protocol--PPTP",
Internet-Draft, draft-ietf-pppext-pptp-00.txt, July 1996.
[3] Pat R. Calhoun, "Virtual Tunneling Protocol (VTP)",
Internet-Draft, draft-calhoun-vtp-protocol-00.txt, July 1996.
[4] R. Atkinson, "Security Architecture for the Internet Protocol",
RFC-1825, August 1995
Calhoun [Page 30]
DRAFT VTP Layer 2 Protocol Tunnel Extension July 1996
<Appendix A and B were literally pulled out from [2]>
Appendix A. Acknowledgment Time-Outs
L2TP 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 Mobile Node-Home Agent data channels full without
causing receive buffer overflow. L2TP 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 VTP device (Mobile Node or
Home Agent) 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 Mobile Node and Home Agent when a call is established.
For the Home Agent, this number should be small. For a Mobile Node
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
Calhoun [Page 31]
DRAFT VTP Layer 2 Protocol Tunnel Extension July 1996
(if not zero). When the network link is the Internet, this delay
could be substantial and vary widely. RTT is adaptive: it will
adjust to include the PPD and whatever shifting network delays
contribute to the time between a packet being transmitted and
receiving its acknowledgment.
Adaptive Time-Out (ATO) is the time that must elapse before an
acknowledgment is considered lost. After a time-out, the sliding
window is partially closed and the ATO is backed off.
Packet Processing Delay (PPD)
The PPD parameter is a 16-bit word exchanged during the Call Control
phase that represents tenths of a second (64 means 6.4 seconds). The
protocol only specifies that the parameter is exchanged, it does not
specify how it is calculated. The way values for PPD are calculated
is implementation-dependent and need not be variable (static time-
outs are allowed). The PPD must be exchanged in the call connect
sequences, even if it remains constant in an implementation. One
possible way to calculate the PPD is:
PPD' = ((PPP_MAX_DATA_MTU - Header) * WindowSize * 8) / ConnectRate
PPD = PPD' + MobileNodeFudge
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 Mobile
Node and Home Agent. 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. MobileNodeFudge is not required but
can be used to take overall processing overhead of the Mobile Node into
account. The value of PPD is used to seed the adaptive algorithm with
the initial RTT[n-1] value.
Sample
Sample is the actual measured time for a returned acknowledgment.
Round-Trip Time (RTT)
The RTT value represents an estimate of the average time it takes for
an acknowledgment to be received after a packet is sent to the remote
end of the tunnel.
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DRAFT VTP Layer 2 Protocol Tunnel Extension July 1996
A.1 Calculating Adaptive Acknowledgment Time-Out
We still must decide how much time to allow for acknowledgments to
return. If the time-out is set too high, we may wait an unnecessarily
long time for dropped packets. If the time-out is too short, we may
time out just before the acknowledgment arrives. The acknowledgment
time-out should also be reasonable and responsive to changing network
conditions.
The suggested adaptive algorithm detailed below is based on the TCP
1989 implementation and is explained in Richard Steven's book TCP/IP
Illustrated, Volume 1 (page 300). 'n' means this iteration of the
calculation, and 'n-1' refers to values from the last calculation.
DIFF[n] = SAMPLE[n] - RTT[n-1]
DEV[n] = DEV[n-1] + (beta * (|DIFF[n]| - DEV[n-1]))
RTT[n] = RTT[n-1] + (alpha * DIFF[n])
ATO[n] = MIN (RTT[n] + (chi * DEV[n]), MaxTimeOut)
DIFF represents the error between the last estimated round-trip
time and the measured time. DIFF is calculated on each iteration.
DEV is the estimated mean deviation. This approximates the
standard deviation. DEV is calculated on each iteration and
stored for use in the next iteration. Initially, it is set to 0.
RTT is the estimated round-trip time of an average packet. RTT is
calculated on each iteration and stored for use in the next
iteration. Initially, it is set to PPD.
ATO is the adaptive time-out for the next transmitted packet. ATO
is calculated on each iteration. Its value is limited, by the MIN
function, to be a maximum of the configured MaxTimeOut value.
Alpha is the gain for the average and is typically 1/8 (0.125).
Beta is the gain for the deviation and is typically 1/4 (0.250).
Chi is the gain for the time-out and is typically set to 4.
To eliminate division operations for fractional gain elements, the
entire set of equations can be scaled. With the suggested gain
constants, they should be scaled by 8 to eliminate all division. To
simplify calculations, all gain values are kept to powers of two so
that shift operations can be used in place of multiplication or
division.
A.2 Congestion Control: Adjusting for Time-Out
This section describes how the calculation of ATO is modified in the
case where a time-out does occur. When a time-out occurs, the time-
out value should be adjusted rapidly upward. Although the GRE
packets are not retransmitted when a time-out occurs, the time-out
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should be adjusted up toward a maximum limit. To compensate for
shifting internetwork time delays, a strategy must be employed to
increase the time-out when it expires. A simple formula called
Karn's Algorithm is used in TCP implementations and may be used in
implementing the backoff timers for the Mobile Node or the Home Agent.
Notice that in addition to increasing the time-out, we are also
shrinking the size of the window as described in the next section.
Karn's timer backoff algorithm, as used in TCP, is:
NewTIMEOUT = delta * TIMEOUT
Adapted to our time-out calculations, for an interval in which a
time-out occurs, the new ATO is calculated as:
RTT[n] = delta * RTT[n-1]
DEV[n] = DEV[n-1]
ATO[n] = MIN (RTT[n] + (chi * DEV[n]), MaxTimeOut)
In this modified calculation of ATO, only the two values that
contribute to ATO and that are stored for the next iteration are
calculated. RTT is scaled by chi, and DEV is unmodified. DIFF is not
carried forward and is not used in this scenario. A value of 2 for
Delta, the time-out gain factor for RTT, is suggested.
Appendix B. Acknowledgment Time-Out and Window Adjustment
B.1 Initial Window Size
Although each side has indicated the maximum size of its receive
window, it is recommended that a slow start method be used to begin
transmitting data. The initial window size on the transmitter is set
to half the maximum size the receiver requested, with a minimum size
of one packet. The transmitter stops sending packets when the number
of packets awaiting acknowledgment is equal to the current window
size. As the receiver successfully digests each window, the window
size on the transmitter is bumped up by one packet until the maximum
is reached. This method prevents a system from flooding an already
congested network because no history has been established.
B.2 Closing the Window
When a time-out does occur on a packet, the sender adjusts the size
of the transmit window down to one half its value when it failed.
Fractions are rounded up, and the minimum window size is one.
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B.3 Opening the Window
With every successful transmission of a window's worth of packets
without a time-out, the transmit window size is increased by one
packet until it reaches the maximum window size that was sent by the
other side when the call was connected. As stated earlier, no
retransmission is done on a time-out. After a time-out, the
transmission resumes with the window starting at one half the size of
the transmit window when the time-out occurred and adjusting upward
by one each time the transmit window is filled with packets that are
all acknowledged without time-outs.
B.4 Window Overflow
When a receiver's window overflows with too many incoming packets,
excess packets are thrown away. This situation should not arise if
the sliding window procedures are being properly followed by the
transmitter and receiver. It is assumed that, on the transmit side,
packets are buffered for transmission and are no longer accepted from
the packet source when the transmit buffer fills.
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