Internet Draft

Internet Draft
Network Working Group W. Townsley
Internet-Draft A. Valencia
Category: Standards Track cisco Systems
<draft-ietf-l2tpext-l2tpbis-00.txt> A. Rubens
Ascend Communications
G. Pall
G. Zorn
Microsoft Corporation
B. Palter
Redback Networks
July 2000

Layer Two Tunneling Protocol "L2TP"

Status of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of section 10 of RFC2026.

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The distribution of this memo is unlimited. It is filed as and expires January 31, 2001. Please
send comments to the L2TP mailing list (l2tp@ipsec.org).

Abstract

This document describes the Layer Two Tunneling Protocol (L2TP). RFC
1661 specifies multi-protocol access via PPP [RFC1661]. L2TP

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facilitates the tunneling of PPP packets across an intervening
network in a way that is as transparent as possible to both end-users
and applications.

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Contents

Status of this Memo.......................................... 1
1.0 Introduction.......................................... 4
1.1 Specification of Requirements......................... 5
1.2 Terminology........................................... 5
2.0 Topology.............................................. 8
3.0 Protocol Overview..................................... 9
3.1 L2TP Header Format.................................... 10
3.2 Control Message Types................................. 12
4.0 Control Message Attribute Value Pairs................. 13
4.1 AVP Format............................................ 13
4.2 Mandatory AVPs........................................ 15
4.3 Hiding of AVP Attribute Values........................ 15
4.4 AVP Summary........................................... 17
5.0 Protocol Operation.................................... 41
5.1 Control Connection Establishment...................... 42
5.2 Session Establishment................................. 43
5.3 Forwarding PPP Frames................................. 44
5.4 Using Sequence Numbers on the Data Channel............ 44
5.5 Keepalive (Hello)..................................... 45
5.6 Session Teardown...................................... 46
5.7 Control Connection Teardown........................... 46
5.8 Reliable Delivery of Control Messages................. 46
6.0 Control Connection Protocol Specification............. 48
6.1 Start-Control-Connection-Request (SCCRQ).............. 49
6.2 Start-Control-Connection-Reply (SCCRP)................ 49
6.3 Start-Control-Connection-Connected (SCCCN)............ 50
6.4 Stop-Control-Connection-Notification (StopCCN)........ 50
6.5 Hello (HELLO)......................................... 50
6.6 Incoming-Call-Request (ICRQ).......................... 51
6.7 Incoming-Call-Reply (ICRP)............................ 52
6.8 Incoming-Call-Connected (ICCN)........................ 52
6.9 Outgoing-Call-Request (OCRQ).......................... 53
6.10 Outgoing-Call-Reply (OCRP)........................... 53
6.11 Outgoing-Call-Connected (OCCN)....................... 54
6.12 Call-Disconnect-Notify (CDN)......................... 54
6.13 WAN-Error-Notify (WEN)............................... 54
6.14 Set-Link-Info (SLI).................................. 55
7.0 Control Connection State Machines..................... 55
7.1 Control Connection Protocol Operation................. 55
7.2 Control Connection States............................. 57
7.2.1 Control Connection Establishment................. 57
7.3 Timing considerations................................. 59
7.4 Incoming calls........................................ 59
7.4.1 LAC Incoming Call States......................... 60
7.4.2 LNS Incoming Call States......................... 61
7.5 Outgoing calls........................................ 62

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7.5.1 LAC Outgoing Call States......................... 63
7.5.2 LNS Outgoing Call States......................... 64
7.6 Tunnel Disconnection.................................. 65
8.0 L2TP Over Specific Media.............................. 66
8.1 L2TP over UDP/IP...................................... 66
8.2 IP.................................................... 68
9.0 Security Considerations............................... 68
9.1 Tunnel Endpoint Security.............................. 68
9.2 Packet Level Security................................. 68
9.3 End to End Security................................... 69
9.4 L2TP and IPsec........................................ 69
9.5 Proxy PPP Authentication.............................. 69
10.0 IANA Considerations.................................. 70
10.1 AVP Attributes....................................... 70
10.2 Message Type AVP Values.............................. 70
10.3 Result Code AVP Values............................... 70
10.3.1 Result Code Field Values........................ 70
10.3.2 Error Code Field Values......................... 70
10.4 Framing Capabilities & Bearer Capabilities........... 70
10.5 Proxy Authen Type AVP Values......................... 71
10.6 AVP Header Bits...................................... 71
11.0 References........................................... 71
12.0 Acknowledgments...................................... 72
13.0 Authors' Addresses................................... 73

Appendix A: Control Channel Slow Start and Congestion Avoidance 74

Appendix B: Control Message Examples......................... 74

Appendix C: Intellectual Property Notice..................... 76

1.0 Introduction

PPP [RFC1661] defines an encapsulation mechanism for transporting
multiprotocol packets across layer 2 (L2) point-to-point links.
Typically, a user obtains a L2 connection to a Network Access Server
(NAS) using one of a number of techniques (e.g., dialup POTS, ISDN,
ADSL, etc.) and then runs PPP over that connection. In such a
configuration, the L2 termination point and PPP session endpoint
reside on the same physical device (i.e., the NAS).

L2TP extends the PPP model by allowing the L2 and PPP endpoints to
reside on different devices interconnected by a packet-switched
network. With L2TP, a user has an L2 connection to an access
concentrator (e.g., modem bank, ADSL DSLAM, etc.), and the
concentrator then tunnels individual PPP frames to the NAS. This
allows the actual processing of PPP packets to be divorced from the
termination of the L2 circuit.

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One obvious benefit of such a separation is that instead of requiring
the L2 connection terminate at the NAS (which may require a long-
distance toll charge), the connection may terminate at a (local)
circuit concentrator, which then extends the logical PPP session over
a shared infrastructure such as frame relay circuit or the Internet.
From the user's perspective, there is no functional difference
between having the L2 circuit terminate in a NAS directly or using
L2TP.

L2TP may also solve the multilink hunt-group splitting problem.
Multilink PPP [RFC1990] requires that all channels composing a
multilink bundle be grouped at a single Network Access Server (NAS).
Due to its ability to project a PPP session to a location other than
the point at which it was physically received, L2TP can be used to
make all channels terminate at a single NAS. This allows multilink
operation even when the calls are spread across distinct physical
NASs.

This document defines the necessary control protocol for on-demand
creation of tunnels between two nodes and the accompanying
encapsulation for multiplexing multiple, tunneled PPP sessions.

1.1 Specification of Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

1.2 Terminology

Analog Channel

A circuit-switched communication path which is intended to carry
3.1 kHz audio in each direction.

Attribute Value Pair (AVP)

The variable length concatenation of a unique Attribute
(represented by an integer) and a Value containing the actual
value identified by the attribute. Multiple AVPs make up Control
Messages which are used in the establishment, maintenance, and
teardown of tunnels.

Call

A connection (or attempted connection) between a Remote System and
LAC. For example, a telephone call through the PSTN. A Call
(Incoming or Outgoing) which is successfully established between a

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Remote System and LAC results in a corresponding L2TP Session
within a previously established Tunnel between the LAC and LNS.
(See also: Session, Incoming Call, Outgoing Call).

Called Number

An indication to the receiver of a call as to what telephone
number the caller used to reach it.

Calling Number

An indication to the receiver of a call as to the telephone number
of the caller.

CHAP

Challenge Handshake Authentication Protocol [RFC1994], a PPP
cryptographic challenge/response authentication protocol in which
the cleartext password is not passed over the line.

Control Connection

A control connection operates in-band over a tunnel to control the
establishment, release, and maintenance of sessions and of the
tunnel itself.

Control Messages

Control messages are exchanged between LAC and LNS pairs,
operating in-band within the tunnel protocol. Control messages
govern aspects of the tunnel and sessions within the tunnel.

Digital Channel

A circuit-switched communication path which is intended to carry
digital information in each direction.

DSLAM

Digital Subscriber Line (DSL) Access Module. A network device used
in the deployment of DSL service. This is typically a concentrator
of individual DSL lines located in a central office (CO) or local
exchange.

Incoming Call

A Call received at an LAC to be tunneled to an LNS (see Call,
Outgoing Call).

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L2TP Access Concentrator (LAC)

A node that acts as one side of an L2TP tunnel endpoint and is a
peer to the L2TP Network Server (LNS). The LAC sits between an LNS
and a remote system and forwards packets to and from each. Packets
sent from the LAC to the LNS requires tunneling with the L2TP
protocol as defined in this document. The connection from the LAC
to the remote system is either local (see: Client LAC) or a PPP
link.

L2TP Network Server (LNS)

A node that acts as one side of an L2TP tunnel endpoint and is a
peer to the L2TP Access Concentrator (LAC). The LNS is the logical
termination point of a PPP session that is being tunneled from the
remote system by the LAC.

Management Domain (MD)

A network or networks under the control of a single
administration, policy or system. For example, an LNS's Management
Domain might be the corporate network it serves. An LAC's
Management Domain might be the Internet Service Provider that owns
and manages it.

Network Access Server (NAS)

A device providing local network access to users across a remote
access network such as the PSTN. An NAS may also serve as an LAC,
LNS or both.

Outgoing Call

A Call placed by an LAC on behalf of an LNS (see Call, Incoming
Call).

Peer

When used in context with L2TP, peer refers to either the LAC or
LNS. An LAC's Peer is an LNS and vice versa. When used in context
with PPP, a peer is either side of the PPP connection.

POTS

Plain Old Telephone Service.

Remote System

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An end-system or router attached to a remote access network (i.e.
a PSTN), which is either the initiator or recipient of a call.
Also referred to as a dial-up or virtual dial-up client.

Session

L2TP is connection-oriented. The LNS and LAC maintain state for
each Call that is initiated or answered by an LAC. An L2TP Session
is created between the LAC and LNS when an end-to-end PPP
connection is established between a Remote System and the LNS.
Datagrams related to the PPP connection are sent over the Tunnel
between the LAC and LNS. There is a one to one relationship
between established L2TP Sessions and their associated Calls. (See
also: Call).

Tunnel

A Tunnel exists between a LAC-LNS pair. The Tunnel consists of a
Control Connection and zero or more L2TP Sessions. The Tunnel
carries encapsulated PPP datagrams and Control Messages between
the LAC and the LNS.

Zero-Length Body (ZLB) Message

A control packet with only an L2TP header. ZLB messages are used
for explicitly acknowledging packets on the reliable control
channel.

2.0 Topology

The following diagram depicts a typical L2TP scenario. The goal is to
tunnel PPP frames between the Remote System or LAC Client and an LNS
located at a Home LAN.

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[Home LAN]
[LAC Client]----------+ |
____|_____ +--[Host]
| | |
[LAC]---------| Internet |-----[LNS]-----+
| |__________| |
_____|_____ :
| |
| PSTN |
[Remote]--| Cloud |
[System] | | [Home LAN]
|___________| |
| ______________ +---[Host]
| | | |
[LAC]-------| Frame Relay |---[LNS]-----+
| or ATM Cloud | |
|______________| :

The Remote System initiates a PPP connection across the PSTN Cloud to
an LAC. The LAC then tunnels the PPP connection across the Internet,
Frame Relay, or ATM Cloud to an LNS whereby access to a Home LAN is
obtained. The Remote System is provided addresses from the HOME LAN
via PPP NCP negotiation. Authentication, Authorization and Accounting
may be provided by the Home LAN's Management Domain as if the user
were connected to a Network Access Server directly.

A LAC Client (a Host which runs L2TP natively) may also participate
in tunneling to the Home LAN without use of a separate LAC. In this
case, the Host containing the LAC Client software already has a
connection to the public Internet. A "virtual" PPP connection is then
created and the local L2TP LAC Client software creates a tunnel to
the LNS. As in the above case, Addressing, Authentication,
Authorization and Accounting will be provided by the Home LAN's
Management Domain.

3.0 Protocol Overview

L2TP utilizes two types of messages, control messages and data
messages. Control messages are used in the establishment, maintenance
and clearing of tunnels and calls. Data messages are used to
encapsulate PPP frames being carried over the tunnel. Control
messages utilize a reliable Control Channel within L2TP to guarantee
delivery (see section 5.1 for details). Data messages are not
retransmitted when packet loss occurs.

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Figure 3.0 L2TP Protocol Structure

Figure 3.0 depicts the relationship of PPP frames and Control
Messages over the L2TP Control and Data Channels. PPP Frames are
passed over an unreliable Data Channel encapsulated first by an L2TP
header and then a Packet Transport such as UDP, Frame Relay, ATM,
etc. Control messages are sent over a reliable L2TP Control Channel
which transmits packets in-band over the same Packet Transport.

Sequence numbers are required to be present in all control messages
and are used to provide reliable delivery on the Control Channel.
Data Messages may use sequence numbers to reorder packets and detect
lost packets.

All values are placed into their respective fields and sent in
network order (high order octets first).

3.1 L2TP Header Format

L2TP packets for the control channel and data channel share a common
header format. In each case where a field is optional, its space does
not exist in the message if the field is marked not present. Note
that while optional on data messages, the Length, Ns, and Nr fields
marked as optional below, are required to be present on all control
messages.

This header is formatted:

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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T|L|x|x|S|x|O|P|x|x|x|x| Ver | Length (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tunnel ID | Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ns (opt) | Nr (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset Size (opt) | Offset pad... (opt)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3.1 L2TP Message Header

The Type (T) bit indicates the type of message. It is set to 0 for a
data message and 1 for a control message.

If the Length (L) bit is 1, the Length field is present. This bit
MUST be set to 1 for control messages.

The x bits are reserved for future extensions. All reserved bits MUST
be set to 0 on outgoing messages and ignored on incoming messages.

If the Sequence (S) bit is set to 1 the Ns and Nr fields are present.
The S bit MUST be set to 1 for control messages.

If the Offset (O) bit is 1, the Offset Size field is present. The O
bit MUST be set to 0 (zero) for control messages.

If the Priority (P) bit is 1, this data message should receive
preferential treatment in its local queuing and transmission. LCP
echo requests used as a keepalive for the link, for instance, should
generally be sent with this bit set to 1. Without it, a temporary
interval of local congestion could result in interference with
keepalive messages and unnecessary loss of the link. This feature is
only for use with data messages. The P bit MUST be set to 0 for all
control messages.

Ver MUST be 2, indicating the version of the L2TP data message header
described in this document. The value 1 is reserved to permit
detection of L2F [RFC2341] packets should they arrive intermixed with
L2TP packets. Packets received with an unknown Ver field MUST be
discarded.

The Length field indicates the total length of the message in octets.

Tunnel ID indicates the identifier for the control connection. L2TP
tunnels are named by identifiers that have local significance only.

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That is, the same tunnel will be given different Tunnel IDs by each
end of the tunnel. Tunnel ID in each message is that of the intended
recipient, not the sender. Tunnel IDs are selected and exchanged as
Assigned Tunnel ID AVPs during the creation of a tunnel.

Session ID indicates the identifier for a session within a tunnel.
L2TP sessions are named by identifiers that have local significance
only. That is, the same session will be given different Session IDs
by each end of the session. Session ID in each message is that of the
intended recipient, not the sender. Session IDs are selected and
exchanged as Assigned Session ID AVPs during the creation of a
session.

Ns indicates the sequence number for this data or control message,
beginning at zero and incrementing by one (modulo 2**16) for each
message sent. See section 5.8 and 5.4 for more information on using
this field.

Nr indicates the sequence number expected in the next control message
to be received. Thus, Nr is set to the Ns of the last in-order
message received plus one (modulo 2**16). In data messages, Nr is
reserved and, if present (as indicated by the S-bit), MUST be ignored
upon receipt. See section 5.8 for more information on using this
field in control messages.

The Offset Size field specifies the number of octets past the L2TP
header at which the payload data is expected to start. If the offset
field is present, the L2TP header ends after the last octet of the
offset padding. The offset field itself is considered part of the
l2tp header, not part of the offset padding. Thus, an offset field
with a value of zero will add two octets to the header length.
Actual data within the offset padding is undefined. A recommended
maximum for the offset value is 7, which allows for 8 octet
alignment.

3.2 Control Message Types

The Message Type AVP (see section 4.4.1) defines the specific type of
control message being sent. Recall from section 3.1 that this is only
for control messages, that is, messages with the T-bit set to 1.

This document defines the following control message types (see
section 6.1 through 6.14 for details on the construction and use of
each message):

Control Connection Management

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0 (reserved)

1 (SCCRQ) Start-Control-Connection-Request
2 (SCCRP) Start-Control-Connection-Reply
3 (SCCCN) Start-Control-Connection-Connected
4 (StopCCN) Stop-Control-Connection-Notification
5 (reserved)
6 (HELLO) Hello

Call Management

7 (OCRQ) Outgoing-Call-Request
8 (OCRP) Outgoing-Call-Reply
9 (OCCN) Outgoing-Call-Connected
10 (ICRQ) Incoming-Call-Request
11 (ICRP) Incoming-Call-Reply
12 (ICCN) Incoming-Call-Connected
13 (reserved)
14 (CDN) Call-Disconnect-Notify

Error Reporting

15 (WEN) WAN-Error-Notify

PPP Session Control

16 (SLI) Set-Link-Info

4.0 Control Message Attribute Value Pairs

To maximize extensibility while still permitting interoperability, a
uniform method for encoding message types and bodies is used
throughout L2TP. This encoding will be termed AVP (Attribute-Value
Pair) in the remainder of this document.

4.1 AVP Format

Each AVP is encoded as:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H| rsvd | Length | Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute Type | Attribute Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
[until Length is reached]... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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The first six bits are a bit mask, describing the general attributes
of the AVP.

Two bits are defined in this document, the remaining are reserved for
future extensions. Reserved bits MUST be set to 0. An AVP received
with a reserved bit set to 1 MUST be treated as an unrecognized AVP.

Mandatory (M) bit: Controls the behavior required of an
implementation which receives an AVP which it does not recognize. If
the M bit is set on an unrecognized AVP within a message associated
with a particular session, the session associated with this message
MUST be terminated. If the M bit is set on an unrecognized AVP within
a message associated with the overall tunnel, the entire tunnel (and
all sessions within) MUST be terminated. If the M bit is not set, an
unrecognized AVP MUST be ignored. The control message must then
continue to be processed as if the AVP had not been present.

Hidden (H) bit: Identifies the hiding of data in the Attribute Value
field of an AVP. This capability can be used to avoid the passing of
sensitive data, such as user passwords, as cleartext in an AVP.
section 4.3 describes the procedure for performing AVP hiding.

Length: Encodes the number of octets (including the Overall Length
and bitmask fields) contained in this AVP. The Length may be
calculated as 6 + the length of the Attribute Value field in octets.
The field itself is 10 bits, permitting a maximum of 1023 octets of
data in a single AVP. The minimum Length of an AVP is 6. If the
length is 6, then the Attribute Value field is absent.

Vendor ID: The IANA assigned "SMI Network Management Private
Enterprise Codes" [RFC1700] value. The value 0, corresponding to
IETF adopted attribute values, is used for all AVPs defined within
this document. Any vendor wishing to implement their own L2TP
extensions can use their own Vendor ID along with private Attribute
values, guaranteeing that they will not collide with any other
vendor's extensions, nor with future IETF extensions. Note that there
are 16 bits allocated for the Vendor ID, thus limiting this feature
to the first 65,535 enterprises.

Attribute Type: A 2 octet value with a unique interpretation across
all AVPs defined under a given Vendor ID.

Attribute Value: This is the actual value as indicated by the Vendor
ID and Attribute Type. It follows immediately after the Attribute
Type field, and runs for the remaining octets indicated in the Length
(i.e., Length minus 6 octets of header). This field is absent if the
Length is 6.

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4.2 Mandatory AVPs

Receipt of an unknown AVP that has the M-bit set is catastrophic to
the session or tunnel it is associated with. Thus, the M bit should
only be defined for AVPs which are absolutely crucial to proper
operation of the session or tunnel. Further, in the case where the
LAC or LNS receives an unknown AVP with the M-bit set and shuts down
the session or tunnel accordingly, it is the full responsibility of
the peer sending the Mandatory AVP to accept fault for causing an
non-interoperable situation. Before defining an AVP with the M-bit
set, particularly a vendor-specific AVP, be sure that this is the
intended consequence.

When an adequate alternative exists to use of the M-bit, it should be
utilized. For example, rather than simply sending an AVP with the M-
bit set to determine if a specific extension exists, availability may
be identified by sending an AVP in a request message and expecting a
corresponding AVP in a reply message.

Use of the M-bit with new AVPs (those not defined in this document)
MUST provide the ability to configure the associated feature off,
such that the AVP is either not sent, or sent with the M-bit not set.

4.3 Hiding of AVP Attribute Values

The H bit in the header of each AVP provides a mechanism to indicate
to the receiving peer whether the contents of the AVP are hidden or
present in cleartext. This feature can be used to hide sensitive
control message data such as user passwords or user IDs.

The H bit MUST only be set if a shared secret exists between the LAC
and LNS. The shared secret is the same secret that is used for tunnel
authentication (see section 5.1.1). If the H bit is set in any
AVP(s) in a given control message, a Random Vector AVP must also be
present in the message and MUST precede the first AVP having an H bit
of 1.

Hiding an AVP value is done in several steps. The first step is to
take the length and value fields of the original (cleartext) AVP and
encode them into a Hidden AVP Subformat 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of Original Value | Original Attribute Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... | Padding ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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Length of Original Attribute Value: This is length of the Original
Attribute Value to be obscured in octets. This is necessary to
determine the original length of the Attribute Value which is lost
when the additional Padding is added.

Original Attribute Value: Attribute Value that is to be obscured.

Padding: Random additional octets used to obscure length of the
Attribute Value that is being hidden.

To mask the size of the data being hidden, the resulting subformat
MAY be padded as shown above. Padding does NOT alter the value placed
in the Length of Original Attribute Value field, but does alter the
length of the resultant AVP that is being created. For example, If an
Attribute Value to be hidden is 4 octets in length, the unhidden AVP
length would be 10 octets (6 + Attribute Value length). After hiding,
the length of the AVP will become 6 + Attribute Value length + size
of the Length of Original Attribute Value field + Padding. Thus, if
Padding is 12 octets, the AVP length will be 6 + 4 + 2 + 12 = 24
octets.

Next, An MD5 hash is performed on the concatenation of:

+ the 2 octet Attribute number of the AVP
+ the shared secret
+ an arbitrary length random vector

The value of the random vector used in this hash is passed in the
value field of a Random Vector AVP. This Random Vector AVP must be
placed in the message by the sender before any hidden AVPs. The same
random vector may be used for more than one hidden AVP in the same
message. If a different random vector is used for the hiding of
subsequent AVPs then a new Random Vector AVP must be placed in the
command message before the first AVP to which it applies.

The MD5 hash value is then XORed with the first 16 octet (or less)
segment of the Hidden AVP Subformat and placed in the Attribute Value
field of the Hidden AVP. If the Hidden AVP Subformat is less than 16
octets, the Subformat is transformed as if the Attribute Value field
had been padded to 16 octets before the XOR, but only the actual
octets present in the Subformat are modified, and the length of the
AVP is not altered.

If the Subformat is longer than 16 octets, a second one-way MD5 hash
is calculated over a stream of octets consisting of the shared secret
followed by the result of the first XOR. That hash is XORed with the
second 16 octet (or less) segment of the Subformat and placed in the
corresponding octets of the Value field of the Hidden AVP.

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If necessary, this operation is repeated, with the shared secret used
along with each XOR result to generate the next hash to XOR the next
segment of the value with.

The hiding method was adapted from RFC 2138 [RFC2138] which was taken
from the "Mixing in the Plaintext" section in the book "Network
Security" by Kaufman, Perlman and Speciner [KPS]. A detailed
explanation of the method follows:

Call the shared secret S, the Random Vector RV, and the Attribute
Value AV. Break the value field into 16-octet chunks p1, p2, etc.
with the last one padded at the end with random data to a 16-octet
boundary. Call the ciphertext blocks c(1), c(2), etc. We will also
define intermediate values b1, b2, etc.

b1 = MD5(AV + S + RV) c(1) = p1 xor b1
b2 = MD5(S + c(1)) c(2) = p2 xor b2
. .
. .
. .
bi = MD5(S + c(i-1)) c(i) = pi xor bi

The String will contain c(1)+c(2)+...+c(i) where + denotes
concatenation.

On receipt, the random vector is taken from the last Random Vector
AVP encountered in the message prior to the AVP to be unhidden. The
above process is then reversed to yield the original value.

4.4 AVP Summary

The following sections contain a list of all L2TP AVPs defined in
this document.

Following the name of the AVP is a list indicating the message types
that utilize each AVP. After each AVP title follows a short
description of the purpose of the AVP, a detail (including a graphic)
of the format for the Attribute Value, and any additional information
needed for proper use of the avp.

4.4.1 AVPs Applicable To All Control Messages

Message Type (All Messages)

The Message Type AVP, Attribute Type 0, identifies the control
message herein and defines the context in which the exact meaning
of the following AVPs will be determined.

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The Attribute Value field for this AVP has the following format:

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Message Type is a 2 octet unsigned integer.

The Message Type AVP MUST be the first AVP in a message,
immediately following the control message header (defined in
section 3.1). See section 3.2 for the list of defined control
message types and their identifiers.

The Mandatory (M) bit within the Message Type AVP has special
meaning. Rather than an indication as to whether the AVP itself
should be ignored if not recognized, it is an indication as to
whether the control message itself should be ignored. Thus, if the
M-bit is set within the Message Type AVP and the Message Type is
unknown to the implementation, the tunnel MUST be cleared. If the
M-bit is not set, then the implementation may ignore an unknown
message type. The M-bit MUST be set to 1 for all message types
defined in this document. This AVP may not be hidden (the H-bit
MUST be 0). The Length of this AVP is 8.

Random Vector (All Messages)

The Random Vector AVP, Attribute Type 36, is used to enable the
hiding of the Attribute Value of arbitrary AVPs.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Random Octet String ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Random Octet String may be of arbitrary length, although a
random vector of at least 16 octets is recommended. The string
contains the random vector for use in computing the MD5 hash to
retrieve or hide the Attribute Value of a hidden AVP (see section
4.3).

More than one Random Vector AVP may appear in a message, in which
case a hidden AVP uses the Random Vector AVP most closely
preceding it. This AVP MUST precede the first AVP with the H bit

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set.

The M-bit for this AVP MUST be set to 1. This AVP MUST NOT be
hidden (the H-bit MUST be 0). The Length of this AVP is 6 plus the
length of the Random Octet String.

4.4.2 Result and Error Codes

Result Code (CDN, StopCCN)

The Result Code AVP, Attribute Type 1, indicates the reason for
terminating the control channel or session.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Result Code | Error Code (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Message (opt) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Result Code is a 2 octet unsigned integer. The optional Error
Code is a 2 octet unsigned integer. An optional Error Message can
follow the Error Code field. Presence of the Error Code and
Message are indicated by the AVP Length field. The Error Message
contains an arbitrary string providing further (human readable)
text associated with the condition. Human readable text in all
error messages MUST be provided in the UTF-8 charset using the
Default Language [RFC2277].

This AVP MUST NOT be hidden (the H-bit MUST be 0). The M-bit for
this AVP MUST be set to 1. The Length is 8 if there is no Error
Code or Message, 10 if there is an Error Code and no Error Message
or 10 + the length of the Error Message if there is an Error Code
and Message.

Defined Result Code values for the StopCCN message are:

0 - Reserved
1 - General request to clear control connection
2 - General error, Error Code indicates the problem
3 - Control channel already exists
4 - Requester is not authorized to establish a control channel
5 - The protocol version of the requester is not supported,
Error Code indicates highest version supported
6 - Requester is being shut down

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7 - Finite State Machine error

Defined Result Code values for the CDN message are:

0 - Reserved
1 - Call disconnected due to loss of carrier
2 - Call disconnected for the reason indicated
in Error Code
3 - Call disconnected for administrative reasons
4 - Call failed due to lack of appropriate facilities being
available (temporary condition)
5 - Call failed due to lack of appropriate facilities being
available (permanent condition)
6 - Invalid destination
7 - Call failed due to no carrier detected
8 - Call failed due to detection of a busy signal
9 - Call failed due to lack of a dial tone
10 - Call was not established within time allotted
11 - Call was connected but no appropriate framing was detected

The Error Codes defined below pertain to types of errors that are
not specific to any particular L2TP request, but rather to
protocol or message format errors. If an L2TP reply indicates in
its Result Code that a general error occurred, the General Error
value should be examined to determine what the error was. The
currently defined General Error codes and their meanings are:

0 - No general error
1 - No control connection exists yet for this LAC-LNS pair
2 - Length is wrong
3 - One of the field values was out of range or
reserved field was non-zero
4 - Insufficient resources to handle this operation now
5 - The Session ID is invalid in this context
6 - A generic vendor-specific error occurred
7 - Try another. If LAC is aware of other possible LNS
destinations, it should try one of them. This can be
used to guide an LAC based on LNS policy, for instance,
the existence of multilink PPP bundles.
8 - Session or tunnel was shutdown due to receipt of an unknown
AVP with the M-bit set (see section 4.2). The Error Message
SHOULD contain the attribute of the offending AVP in (human
readable) text form.
9 - Try another directed. If the LNS is aware of other possible
destinations, it should inform the LAC. The Error Message
MUST contain an comma separated list of addresses for the LAC
to choose from. If the L2TP transport is IPv4, then this would
be a comma separated list of IP addresses in the canonical

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dotted-decimal format. i.e. "10.0.0.1, 10.0.0.2, 10.0.0.3"
If there are no servers for the LNS to suggest, then Error
Code 7 should be used. The delimiter between addresses MUST be
precisely a single comma and a single space.

When a General Error Code of 6 is used, additional information
about the error SHOULD be included in the Error Message field.
Further, a vendor specific AVP MAY be sent to indicate the problem
more precisely.

4.4.3 Control Connection Management AVPs

Protocol Version (SCCRP, SCCRQ)

The Protocol Version AVP, Attribute Type 2, indicates the L2TP
protocol version of the sender.

The Attribute Value field for this AVP has the following format:

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ver | Rev |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Ver field is a 1 octet unsigned integer containing the value
1. Rev field is a 1 octet unsigned integer containing 0. This
pertains to L2TP protocol version 1, revision 0. Note this is not
the same version number that is included in the header of each
message.

This AVP MUST NOT be hidden (the H-bit MUST be 0). The M-bit for
this AVP MUST be set to 1. The Length of this AVP is 8.

Framing Capabilities (SCCRP, SCCRQ)

The Framing Capabilities AVP, Attribute Type 3, provides the peer
with an indication of the types of framing that will be accepted
or requested by the sender.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved for future framing type definitions |A|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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The Attribute Value field is a 32-bit mask, with two bits defined.
If bit A is set, asynchronous framing is supported. If bit S is
set, synchronous framing is supported.

The framing capabilitiies defined in this AVP refer only to the
physical interfaces available for dialout usage on an LAC. An LNS
MUST not send an OCRQ with a Framing Type AVP specifying a value
not advertised in this AVP. Presence of this message is not a
guarantee that a given outgoing call will be placed by the sender
if requested, just that the physical capability exists.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) is 10.

Bearer Capabilities (SCCRP, SCCRQ)

The Bearer Capabilities AVP, Attribute Type 4, provides the peer
with an indication of the bearer device types supported by the
hardware interfaces of the sender for outgoing calls.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved for future bearer type definitions |V|A|D|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

This is a 32-bit mask, with two bits defined. If bit A is set,
analog access is supported. If bit D is set, digital access is
supported. Bit V is set, virtual acces is supported. Virtual
access refers to access where there is no physical point to point
link.

The framing capabilitiies defined in this AVP refer only to the
physical interfaces available for dialout usage on an LAC. An LNS
MUST not send an OCRQ with a Bearer Type AVP specifying a value
not advertised in this AVP.

This AVP MUST be present if the sender can place outgoing calls
when requested. Presence of this message is not a guarantee that a
given outgoing call will be placed by the sender if requested,
just that the physical capability exists.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) is 10.

Tie Breaker (SCCRQ)

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The Tie Breaker AVP, Attribute Type 5, indicates that the sender
wishes a single tunnel to exist between the given LAC-LNS pair.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tie Break Value...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...(64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Tie Breaker Value is an 8 octet value that is used to choose a
single tunnel where both LAC and LNS request a tunnel
concurrently. The recipient of a SCCRQ must check to see if a
SCCRQ has been sent to the peer, and if so, must compare its Tie
Breaker value with the received one. The lower value "wins", and
the "loser" MUST silently discard its tunnel. In the case where a
tie breaker is present on both sides, and the value is equal, both
sides MUST discard their tunnels.

If a tie breaker is received, and an outstanding SCCRQ had no tie
breaker value, the initiator which included the Tie Breaker AVP
"wins". If neither side issues a tie breaker, then two separate
tunnels are opened.

This AVP MUST NOT be hidden (the H-bit MUST be 0). The M-bit for
this AVP MUST be set to 0. The Length of this AVP is 14.

Firmware Revision (SCCRP, SCCRQ)

The Firmware Revision AVP, Attribute Type 6, indicates the
firmware revision of the issuing device.

The Attribute Value field for this AVP has the following format:

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Firmware Revision |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Firmware Revision is a 2 octet unsigned integer encoded in a
vendor specific format.

For devices which do not have a firmware revision (general purpose
computers running L2TP software modules, for instance), the

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revision of the L2TP software module may be reported instead.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) is 8.

Host Name (SCCRP, SCCRQ)

The Host Name AVP, Attribute Type 7, indicates the name of the
issuing LAC or LNS.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Host Name ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Host Name is of arbitrary length, but MUST be at least 1
octet.

This name should be as broadly unique as possible; for hosts
participating in DNS [RFC1034], a hostname with fully qualified
domain would be appropriate.

This AVP MUST NOT be hidden (the H-bit MUST be 0). The M-bit for
this AVP MUST be set to 1. The Length of this AVP is 6 plus the
length of the Host Name.

Vendor Name (SCCRP, SCCRQ)

The Vendor Name AVP, Attribute Type 8, contains a vendor specific
(possibly human readable) string describing the type of LAC or LNS
being used.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor Name ...(arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Vendor Name is the indicated number of octets representing the
vendor string. Human readable text for this AVP MUST be provided
in the UTF-8 charset using the Default Language [RFC2277].

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for

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this AVP MUST be set to 0. The Length (before hiding) of this AVP
is 6 plus the length of the Vendor Name.

Assigned Tunnel ID (SCCRP, SCCRQ, StopCCN)

The Assigned Tunnel ID AVP, Attribute Type 9, encodes the ID being
assigned to this tunnel by the sender.

The Attribute Value field for this AVP has the following format:

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assigned Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Assigned Tunnel ID is a 2 octet non-zero unsigned integer.

The Assigned Tunnel ID AVP establishes a value used to multiplex
and demultiplex multiple tunnels between the LNS and LAC. The L2TP
peer MUST place this value in the Tunnel ID header field of all
control and data messages that it subsequently transmits over the
associated tunnel. Before the Assigned Tunnel ID AVP is received
from a peer, messages MUST be sent to that peer with a Tunnel ID
value of 0 in the header of all control messages.

In the StopCCN control message, the Assigned Tunnel ID AVP MUST be
the same as the Assigned Tunnel ID AVP first sent to the receiving
peer, permitting the peer to identify the appropriate tunnel even
if a StopCCN is sent before an Assigned Tunnel ID AVP is received.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) of this AVP
is 8.

Receive Window Size (SCCRQ, SCCRP)

The Receive Window Size AVP, Attribute Type 10, specifies the
receive window size being offered to the remote peer.

The Attribute Value field for this AVP has the following format:

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Window Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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The Window Size is a 2 octet unsigned integer.

If absent, the peer must assume a Window Size of 4 for its
transmit window. The remote peer may send the specified number of
control messages before it must wait for an acknowledgment.

This AVP MUST NOT be hidden (the H-bit MUST be 0). The M-bit for
this AVP MUST be set to 1. The Length of this AVP is 8.

Challenge (SCCRP, SCCRQ)

The Challenge AVP, Attribute Type 11, indicates that the issuing
peer wishes to authenticate the tunnel endpoints using a CHAP-
style authentication mechanism.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Challenge ... (arbitrary number of octets)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Challenge is one or more octets of random data.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) of this AVP
is 6 plus the length of the Challenge.

Challenge Response (SCCCN, SCCRP)

The Response AVP, Attribute Type 13, provides a response to a
challenge received.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Response ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... (16 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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The Response is a 16 octet value reflecting the CHAP-style
[RFC1994] response to the challenge.

This AVP MUST be present in an SCCRP or SCCCN if a challenge was
received in the preceding SCCRQ or SCCRP. For purposes of the ID
value in the CHAP response calculation, the value of the Message
Type AVP for this message is used (e.g. 2 for an SCCRP, and 3 for
an SCCCN).

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) of this AVP
is 22.

4.4.4 Call Management AVPs

Q.931 Cause Code (CDN)

The Q.931 Cause Code AVP, Attribute Type 12, is used to give
additional information in case of unsolicited call disconnection.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code | Cause Msg | Advisory Msg...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Cause Code is the returned Q.931 Cause code, and Cause Msg is the
returned Q.931 message code (e.g., DISCONNECT) associated with the
Cause Code. Both values are returned in their native ITU
encodings [DSS1]. An additional ASCII text Advisory Message may
also be included (presence indicated by the AVP Length) to further
explain the reason for disconnecting.

This AVP MUST NOT be hidden (the H-bit MUST be 0). The M-bit for
this AVP MUST be set to 1. The Length of this AVP is 9, plus the
size of the Advisory Message.

Assigned Session ID (CDN, ICRP, ICRQ, OCRP, OCRQ)

The Assigned Session ID AVP, Attribute Type 14, encodes the ID
being assigned to this session by the sender.

The Attribute Value field for this AVP has the following format:

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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assigned Session ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Assigned Session ID is a 2 octet non-zero unsigned integer.

The Assigned Session ID AVP is establishes a value used to
multiplex and demultiplex data sent over a tunnel between the LNS
and LAC. The L2TP peer MUST place this value in the Session ID
header field of all control and data messages that it subsequently
transmits over the tunnel that belong to this session. Before the
Assigned Session ID AVP is received from a peer, messages MUST be
sent to that peer with a Session ID of 0 in the header of all
control messages.

In the CDN control message, the same Assigned Session ID AVP first
sent to the receiving peer is used, permitting the peer to
identify the appropriate tunnel even if CDN is sent before an
Assigned Session ID is received.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) of this AVP
is 8.

Call Serial Number (ICRQ, OCRQ)

The Call Serial Number AVP, Attribute Type 15, encodes an
identifier assigned by the LAC or LNS to this call.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Call Serial Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Call Serial Number is a 32 bit value.

The Call Serial Number is intended to be an easy reference for
administrators on both ends of a tunnel to use when investigating
call failure problems. Call Serial Numbers should be set to
progressively increasing values, which are likely to be unique for
a significant period of time across all interconnected LNSs and
LACs.

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This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) of this AVP
is 10.

Minimum BPS (OCRQ)

The Minimum BPS AVP, Attribute Type 16, encodes the lowest
acceptable line speed for this call.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Minimum BPS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Minimum BPS is a 32 bit value indicates the speed in bits per
second.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) of this AVP
is 10.

Maximum BPS (OCRQ)

The Maximum BPS AVP, Attribute Type 17, encodes the highest
acceptable line speed for this call.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum BPS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Maximum BPS is a 32 bit value indicates the speed in bits per
second.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) of this AVP
is 10.

Bearer Type (ICRQ, OCRQ)

The Bearer Type AVP, Attribute Type 18, encodes the bearer type
for the incoming or outgoing call.

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The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved for future Bearer Types |V|A|D|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Bearer Type is a 32-bit bit mask indicating the bearer
capability of the call (ICRQ) or required for the call (OCRQ). Bit
A refers to an analog channel. Bit D refers to a digital channel.
Bit V (virtual) refers to a channel for which there is no physical
point to point link.

Bits set in the Bearer Type AVP in an OCRQ message indicate which
bearer type(s) an outgoing call may be placed on. If more than
one bit is set, the LAC may choose which bearer type to place the
call on. If no bits are set, any type of available channel may be
used.

Bits in the Value field of this AVP MUST only be set by the LNS
for an OCRQ if the same bit was set in the Bearer Capabilities AVP
received from the LAC during control connection establishment.

Bits set in the Bearer Type AVP in an ICRQ message indicate what
bearer type an incoming call was received on at the LAC. If no
bits are set in an ICRQ, then it is assumed that the bearer type
was indeterminable.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) of this AVP
is 10.

Framing Type (ICCN, OCCN, OCRQ)

The Framing Type AVP, Attribute Type 19, encodes the framing type
for the incoming or outgoing call.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved for future Framing Types |A|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Framing Type is a 32-bit mask, which indicates the type of PPP
framing requested for an OCRQ, or the type of PPP framing

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negotiated for an OCCN or ICCN.

Bit A indicates asynchronous framing. Bit S indicates synchronous
framing. For an OCRQ, both may be set, indicating that the LAC may
decide the type of framing to be used.

For an ICRQ, only one framing type bit may be set. The framing
type SHOULD be used as an indication to PPP on the LNS as to what
link options to use for LCP negotiation [RFC1662]. For example, if
the A bit is not set in the Framing Type AVP in an ICRQ message
and an ACCM LCP option is requested by the PPP client, then the
LNS should try to respond with no bits set in the ACCM mask as the
LAC will likely not perform async mapping on a non-async
interface. Similarly, if the S bit is set, PPP may wish to reject
address field compression and protocol field compression options.

Bits in the Value field of this AVP MUST only be set by the LNS
for an OCRQ if it was set in the Framing Capabilities AVP received
from the LAC during control connection establishment.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) of this AVP
is 10.

Called Number (ICRQ, OCRQ)

The Called Number AVP, Attribute Type 21, encodes the telephone
number to be called for an OCRQ, and the Called number for an
ICRQ.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Called Number... (arbitrary number of octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Called Number is an ASCII string. Contact between the
administrator of the LAC and the LNS may be necessary to
coordinate interpretation of the value needed in this AVP.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) of this AVP
is 6 plus the length of the Called Number.

Calling Number (ICRQ)

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The Calling Number AVP, Attribute Type 22, encodes the originating
number for the incoming call.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Calling Number... (arbitrary number of octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Calling Number is an ASCII string. Contact between the
administrator of the LAC and the LNS may be necessary to
coordinate interpretation of the value in this AVP.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) of this AVP
is 6 plus the length of the Calling Number.

Sub-Address (ICRQ, OCRQ)

The Sub-Address AVP, Attribute Type 23, encodes additional dialing
information.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Address ... (arbitrary number of octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Sub-Address is an ASCII string. Contact between the
administrator of the LAC and the LNS may be necessary to
coordinate interpretation of the value in this AVP.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) of this AVP
is 6 plus the length of the Sub-Address.

(Tx) Connect Speed (ICCN, OCCN)

The (Tx) Connect Speed BPS AVP, Attribute Type 24, encodes the
speed of the facility chosen for the connection attempt.

The Attribute Value field for this AVP has the following format:

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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BPS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The (Tx) Connect Speed BPS is a 4 octet value indicating the speed
in bits per second. A value of zero indicates that the speed is
indeterminable, or there is no physical point to point link.

When the optional Rx Connect Speed AVP is present, the value in
this AVP represents the transmit connect speed, from the
perspective of the LAC (e.g. data flowing from the LAC to the
remote system). When the optional Rx Connect Speed AVP is NOT
present, the connection speed between the remote system and LAC is
assumed to be symmetric and is represented by the single value in
this AVP.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) of this AVP
is 10.

Rx Connect Speed (ICCN, OCCN)

The Rx Connect Speed AVP, Attribute Type 38, represents the speed
of the connection from the perspective of the LAC (e.g. data
flowing from the remote system to the LAC).

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BPS (H) | BPS (L) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

BPS is a 4 octet value indicating the speed in bits per second. A
value of zero indicates that the speed is indeterminable, or there
is no physical point to point link.

Presence of this AVP implies that the connection speed may be
asymmetric with respect to the transmit connect speed given in the
(Tx) Connect Speed AVP.

This AVP may be hidden (the H-bit MAY be 1 or 0). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) of this AVP
is 10.

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Physical Channel ID (ICRQ, OCRP)

The Physical Channel ID AVP, Attribute Type 25, encodes the vendor
specific physical channel number used for a call.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Physical Channel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Physical Channel ID is a 4 octet value intended to be used for
logging purposes only.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) of this AVP
is 10.

Private Group ID (ICCN)

The Private Group ID AVP, Attribute Type 37, is used by the LAC to
indicate that this call is to be associated with a particular
customer group.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Private Group ID ... (arbitrary number of octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Private Group ID is a string of octets of arbitrary length.

The LNS MAY treat the PPP session as well as network traffic
through this session in a special manner determined by the peer.
For example, if the LNS is individually connected to several
private networks using unregistered addresses, this AVP may be
included by the LAC to indicate that a given call should be
associated with one of the private networks.

The Private Group ID is a string corresponding to a table in the
LNS that defines the particular characteristics of the selected
group. A LAC MAY determine the Private Group ID from a RADIUS
response, local configuration, or some other source.

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This AVP may be hidden (the H-bit MAY be 1 or 0). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) of this AVP
is 6 plus the length of the Private Group ID.

Sequencing Required (ICCN, OCCN)

The Sequencing Required AVP, Attribute Type 39, indicates to the
LNS that Sequence Numbers MUST always be present on the data
channel.

This AVP has no Attribute Value field.

This AVP MUST NOT be hidden (the H-bit MUST be 0). The M-bit for
this AVP MUST be set to 1. The Length of this AVP is 6.

4.4.5 Proxy LCP and Authentication AVPs

The LAC may have answered the call and negotiated LCP with the
remote system, perhaps in order to establish the system's apparent
identity. In this case, these AVPs may be included to indicate the
link properties the remote system initially requested, properties
the remote system and LAC ultimately negotiated, as well as PPP
authentication information sent and received by the LAC. This
information may be used to initiate the PPP LCP and authentication
systems on the LNS, allowing PPP to continue without renegotiation
of LCP. Note that the LNS policy may be to enter an additional
round of LCP negotiation and/or authentication if the LAC is not
trusted.

Initial Received LCP CONFREQ (ICCN)

In the Initial Received LCP CONFREQ AVP, Attribute Type 26,
provides the LNS with the Initial CONFREQ received by the LAC from
the PPP Peer.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LCP CONFREQ... (arbitrary number of octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LCP CONFREQ is a copy of the body of the initial CONFREQ received,
starting at the first option within the body of the LCP message.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) of this AVP

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is 6 plus the length of the CONFREQ.

Last Sent LCP CONFREQ (ICCN)

In the Last Sent LCP CONFREQ AVP, Attribute Type 27, provides the
LNS with the Last CONFREQ sent by the LAC to the PPP Peer.

The Attribute Value field for this AVP has the following format:

The LCP CONFREQ is a copy of the body of the final CONFREQ sent to
the client to complete LCP negotiation, starting at the first
option within the body of the LCP message.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) of this AVP
is 6 plus the length of the CONFREQ.

Last Received LCP CONFREQ (ICCN)

The Last Received LCP CONFREQ AVP, Attribute Type 28, provides the
LNS with the Last CONFREQ received by the LAC from the PPP Peer.

The Attribute Value field for this AVP has the following format:

The LCP CONFREQ is a copy of the body of the final CONFREQ
received from the client to complete LCP negotiation, starting at
the first option within the body of the LCP message.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) of this AVP
is 6 plus the length of the CONFREQ.

Proxy Authen Type (ICCN)

The Proxy Authen Type AVP, Attribute Type 29, determines if proxy
authentication should be used.

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The Attribute Value field for this AVP has the following format:

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authen Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authen Type is a 2 octet unsigned integer, holding:

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) of this AVP
is 8.

Defined Authen Type values are:
0 - Reserved
1 - Textual username/password exchange
2 - PPP CHAP
3 - PPP PAP
4 - No Authentication
5 - Microsoft CHAP Version 1 (MSCHAPv1)

This AVP MUST be present if proxy authentication is to be
utilized. If it is not present, then it is assumed that this
peer cannot perform proxy authentication, requiring
a restart of the authentication phase at the LNS if the client
has already entered this phase with the
LAC (which may be determined by the Proxy LCP AVP if present).

Associated AVPs for each type of authentication follow.

Proxy Authen Name (ICCN)

The Proxy Authen Name AVP, Attribute Type 30, specifies the name
of the authenticating client when using proxy authentication.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authen Name... (arbitrary number of octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Authen Name is a string of octets of arbitrary length. It
contains the name specified in the client's authentication
response.

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This AVP MUST be present in messages containing a Proxy Authen
Type AVP with an Authen Type of 1, 2, 3 or 5. It may be desirable
to employ AVP hiding for obscuring the cleartext name.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) is 6 plus
the length of the cleartext name.

Proxy Authen Challenge (ICCN)

The Proxy Authen Challenge AVP, Attribute Type 31, specifies the
challenge sent by the LAC to the PPP Peer, when using proxy
authentication.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Challenge... (arbitrary number of octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Challenge is a string of one or more octets.

This AVP MUST be present for Proxy Authen Types 2 and 5. The
Challenge field contains the CHAP challenge presented to the
client by the LAC.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) of this AVP
is 6, plus the length of the Challenge.

Proxy Authen ID (ICCN)

The Proxy Authen ID AVP, Attribute Type 32, specifies the ID value
of the PPP Authentication that was started between the LAC and the
PPP Peer, when proxy authentication is being used.

The Attribute Value field for this AVP has the following format:

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

ID is a 2 octet unsigned integer, the most significant octet MUST
be 0.

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The Proxy Authen ID AVP MUST be present for Proxy authen types 2,
3 and 5. For 2 and 5, the ID field contains the byte ID value
presented to the client by the LAC in its Challenge. For 3, it is
the Identifier value of the Authenticate-Request.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 0.

Proxy Authen Response (ICCN)

The Proxy Authen Response AVP, Attribute Type 33, specifies the
PPP Authentication response received by the LAC from the PPP Peer,
when proxy authentication is used.

The Attribute Value field for this AVP has the following format:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Response... (arbitrary number of octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Response is a string of octets.

This AVP MUST be present for Proxy authen types 1, 2, 3 and 5. The
Response field contains the client's response to the challenge.
For Proxy authen types 2 and 5, this field contains the response
value received by the LAC. For types 1 or 3, it contains the clear
text password received from the client by the LAC. In the case of
cleartext passwords, AVP hiding is recommended.

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 0. The Length (before hiding) of this AVP
is 6 plus the length of the Response.

4.4.6 Call Status AVPs

Call Errors (WEN)

The Call Errors AVP, Attribute Type 34, is used by the LAC to send
error information to the LNS.

The Attribute Value field for this AVP has the following format:

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The following fields are defined:

Reserved - Not used, MUST be 0
CRC Errors - Number of PPP frames received with CRC errors
since call was established
Framing Errors - Number of improperly framed PPP packets
received
Hardware Overruns - Number of receive buffer over-runs since
call was established
Buffer Overruns - Number of buffer over-runs detected since
call was established
Time-out Errors - Number of time-outs since call was
established
Alignment Errors - Number of alignment errors since call was
established

This AVP may be hidden (the H-bit may be 0 or 1). The M-bit for
this AVP MUST be set to 1. The Length (before hiding) of this AVP
is 32.

ACCM (SLI)

The ACCM AVP, Attribute Type 35, is used by the LNS to inform LAC
of the ACCM negotiated with the PPP Peer by the LNS.

The Attribute Value field for this AVP has the following format:

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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Send ACCM (H) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Send ACCM (L) | Receive ACCM (H) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receive ACCM (L) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Send ACCM and Receive ACCM are each 4 octet values preceded by a 2
octet reserved quantity. The send ACCM value should be used by the
LAC to process packets it sends on the connection. The receive
ACCM value should be used by the LAC to process incoming packets
on the connection. The default values used by the LAC for both
these fields are 0xFFFFFFFF. The LAC should honor these fields
unless it has specific configuration information to indicate that
the requested mask must be modified to permit operation.

This AVP may be hidden (the H-bit MAY be 1 or 0). The M-bit for
this AVP MUST be set to 1. The Length of this AVP is 16.

5.0 Protocol Operation

The necessary setup for tunneling a PPP session with L2TP consists of
two steps, (1) establishing the Control Connection for a Tunnel, and
(2) establishing a Session as triggered by an incoming or outgoing
call request. The Tunnel and corresponding Control Connection MUST be
established before an incoming or outgoing call is initiated. An L2TP
Session MUST be established before L2TP can begin to tunnel PPP
frames. Multiple Sessions may exist across a single Tunnel and
multiple Tunnels may exist between the same LAC and LNS.

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+-----+ +-----+
| |~~~~~~~~~~L2TP Tunnel~~~~~~~~~~| |
| LAC | | LNS |
| #######Control Connection######## |
[Remote] | | | |
[System]------Call----------*============L2TP Session=============* |
PPP +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ |
| | | |
[Remote] | | | |
[System]------Call----------*============L2TP Session=============* |
PPP +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ |
| | | |
| |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| |
+-----+ +-----+

Figure 5.1 Tunneling PPP

5.1 Control Connection Establishment

The Control Connection is the initial connection that must be
achieved between an LAC and LNS before sessions may be brought up.
Establishment of the control connection includes securing the
identity of the peer, as well as identifying the peer's L2TP version,
framing, and bearer capabilities, etc.

A three message exchange is utilized to setup the control connection.
Following is a typical message exchange:

LAC or LNS LAC or LNS
---------- ----------
SCCRQ ->
<- SCCRP
SCCCN ->
<- ZLB ACK

The ZLB ACK is sent if there are no further messages waiting in queue
for that peer.

5.1.1 Tunnel Authentication

L2TP incorporates a simple, optional, CHAP-like [RFC1994] tunnel
authentication system during control connection establishment. If
an LAC or LNS wishes to authenticate the identity of the peer it
is contacting or being contacted by, a Challenge AVP is included
in the SCCRQ or SCCRP message. If a Challenge AVP is received in
an SCCRQ or SCCRP, a Challenge Response AVP MUST be sent in the
following SCCRP or SCCCN, respectively. If the expected response
and response received from a peer does not match, establishment of

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the tunnel MUST be disallowed.

To participate in tunnel authentication, a single shared secret
MUST exist between the LAC and LNS. This is the same shared secret
used for AVP hiding (see section 4.3). See section 4.4.3 for
details on construction of the Challenge and Response AVPs.

5.2 Session Establishment

After successful control connection establishment, individual
sessions may be created. Each session corresponds to single PPP
stream between the LAC and LNS. Unlike control connection
establishment, session establishment is directional with respect to
the LAC and LNS. The LAC requests the LNS to accept a session for an
incoming call, and the LNS requests the LAC to accept a session for
placing an outgoing call.

5.2.1 Incoming Call Establishment

A three message exchange is employed to setup the session.
Following is a typical sequence of events:

LAC LNS
--- ---
(Call
Detected)

ICRQ ->
<- ICRP
ICCN ->
<- ZLB ACK

The ZLB ACK is sent if there are no further messages waiting in
queue for that peer.

5.2.2 Outgoing Call Establishment

A three message exchange is employed to setup the session.
Following is a typical sequence of events:

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LAC LNS
--- ---
<- OCRQ
OCRP ->

(Perform
Call
Operation)

OCCN ->
<- ZLB ACK

The ZLB ACK is sent if there are no further messages waiting in
queue for that peer.

5.3 Forwarding PPP Frames

Once tunnel establishment is complete, PPP frames from the remote
system are received at the LAC, stripped of CRC, link framing, and
transparency bytes, encapsulated in L2TP, and forwarded over the
appropriate tunnel. The LNS receives the L2TP packet, and processes
the encapsulated PPP frame as if it were received on a local PPP
interface.

The sender of a message associated with a particular session and
tunnel places the Session ID and Tunnel ID (specified by its peer) in
the Session ID and Tunnel ID header for all outgoing messages. In
this manner, PPP frames are multiplexed and demultiplexed over a
single tunnel between a given LNS-LAC pair. Multiple tunnels may
exist between a given LNS-LAC pair, and multiple sessions may exist
within a tunnel.

The value of 0 for Session ID and Tunnel ID is special and MUST NOT
be used as an Assigned Session ID or Assigned Tunnel ID. For the
cases where a Session ID has not yet been assigned by the peer (i.e.,
during establishment of a new session or tunnel), the Session ID
field MUST be sent as 0, and the Assigned Session ID AVP within the
message MUST be used to identify the session. Similarly, for cases
where the Tunnel ID has not yet been assigned from the peer, the
Tunnel ID MUST be sent as 0 and Assigned Tunnel ID AVP used to
identify the tunnel.

5.4 Using Sequence Numbers on the Data Channel

Sequence numbers are defined in the L2TP header for control messages
and optionally for data messages (see section 3.1). These are used to
provide a reliable control message transport (see section 5.8) and
optional data message sequencing. Each peer maintains separate

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sequence numbers for the control connection and each individual data
session within a tunnel.

Unlike the L2TP control channel, the L2TP data channel does not use
sequence numbers to retransmit lost data messages. Rather, data
messages may use sequence numbers to detect lost packets and/or
restore the original sequence of packets that may have been reordered
during transport. The LAC may request that sequence numbers be
present in data messages via the Sequencing Required AVP (see section
4.4.6). If this AVP is present during session setup, sequence numbers
MUST be present at all times. If this AVP is not present, sequencing
presence is under control of the LNS. The LNS controls enabling and
disabling of sequence numbers by sending a data message with or
without sequence numbers present at any time during the life of a
session. Thus, if the LAC receives a data message without sequence
numbers present, it MUST stop sending sequence numbers in future data
messages. If the LAC receives a data message with sequence numbers
present, it MUST begin sending sequence numbers in future outgoing
data messages. If the LNS enables sequencing after disabling it
earlier in the session, the sequence number state picks up where it
left off before.

The LNS may initiate disabling of sequencing at any time during the
session (including the first data message sent). It is recommended
that for connections where reordering or packet loss may occur,
sequence numbers always be enabled during the initial negotiation
stages of PPP and disabled only when and if the risk is considered
acceptable. For example, if the PPP session being tunneled is not
utilizing any stateful compression or encryption protocols and is
only carrying IP (as determined by the PPP NCPs that are
established), then the LNS might decide to disable sequencing as IP
is tolerant to datagram loss and reordering.

5.5 Keepalive (Hello)

A keepalive mechanism is employed by L2TP in order to differentiate
tunnel outages from extended periods of no control or data activity
on a tunnel. This is accomplished by injecting Hello control messages
(see section 6.5) after a specified period of time has elapsed since
the last data or control message was received on a tunnel. As for any
other control message, if the Hello message is not reliably delivered
then the tunnel is declared down and is reset. The transport reset
mechanism along with the injection of Hello messages ensures that a
connectivity failure between the LNS and the LAC will be detected at
both ends of a tunnel.

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5.6 Session Teardown

Session teardown may be initiated by either the LAC or LNS and is
accomplished by sending a CDN control message. After the last session
is cleared, the control connection MAY be torn down as well (and
typically is). Following is an example of a typical control message
exchange:

LAC or LNS LAC or LNS

CDN ->
(Clean up)

<- ZLB ACK
(Clean up)

5.7 Control Connection Teardown

Control connection teardown may be initiated by either the LAC or LNS
and is accomplished by sending a single StopCCN control message. The
receiver of a StopCCN MUST send a ZLB ACK to acknowledge receipt of
the message and maintain enough control connection state to properly
accept StopCCN retransmissions over at least a full retransmission
cycle (in case the ZLB ACK is lost). The recommended time for a full
retransmission cycle is 31 seconds (see section 5.8). Following is an
example of a typical control message exchange:

LAC or LNS LAC or LNS

StopCCN ->
(Clean up)

<- ZLB ACK
(Wait)
(Clean up)

An implementation may shut down an entire tunnel and all sessions on
the tunnel by sending the StopCCN. Thus, it is not necessary to clear
each session individually when tearing down the whole tunnel.

5.8 Reliable Delivery of Control Messages

L2TP provides a lower level reliable transport service for all
control messages. The Nr and Ns fields of the control message header
(see section 3.1) belong to this transport. The upper level
functions of L2TP are not concerned with retransmission or ordering
of control messages. The reliable control message is a sliding window
transport that provides control message retransmission and congestion

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control. Each peer maintains separate sequence number state for the
control connection within a tunnel.

The message sequence number, Ns, begins at 0. Each subsequent message
is sent with the next increment of the sequence number. The sequence
number is thus a free running counter represented modulo 65536. The
sequence number in the header of a received message is considered
less than or equal to the last received number if its value lies in
the range of the last received number and the preceding 32767 values,
inclusive. For example, if the last received sequence number was 15,
then messages with sequence numbers 0 through 15, as well as 32784
through 65535, would be considered less than or equal. Such a message
would be considered a duplicate of a message already received and
ignored from processing. However, in order to ensure that all
messages are acknowledged properly (particularly in the case of a
lost ZLB ACK message), receipt of duplicate messages MUST be
acknowledged by the reliable transport. This acknowledgement may
either piggybacked on a message in queue, or explicitly via a ZLB
ACK.

All control messages take up one slot in the control message sequence
number space, except the ZLB acknowledgement. Thus, Ns is not
incremented after a ZLB message is sent.

The last received message number, Nr, is used to acknowledge messages
received by an L2TP peer. It contains the sequence number of the
message the peer expects to receive next (e.g. the last Ns of a non-
ZLB message received plus 1, modulo 65536). While the Nr in a
received ZLB is used to flush messages from the local retransmit
queue (see below), Nr of the next message sent is not be updated by
the Ns of the ZLB. As a precaution, Nr should be sanity checked
before flushing the retransmit queue. e.g. if the Nr received in a
control message is greater than the last Ns sent plus 1 modulo 65536,
it is clearly invalid.

The reliable transport at a receiving peer is responsible for making
sure that control messages are delivered in order and without
duplication to the upper level. Messages arriving out of order may be
queued for in-order delivery when the missing messages are received,
or they may be discarded requiring a retransmission by the peer. When
dropping out of order control packets, Nr MAY be updated before the
packet is discarded.

Each tunnel maintains a queue of control messages to be transmitted
to its peer. The message at the front of the queue is sent with a
given Ns value, and is held until a control message arrives from the
peer in which the Nr field indicates receipt of this message. After a
period of time (a recommended default is 1 second) passes without

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acknowledgement, the message is retransmitted. The retransmitted
message contains the same Ns value, but the Nr value MUST be updated
with the sequence number of the next expected message.

Each subsequent retransmission of a message MUST employ an
exponential backoff interval. Thus, if the first retransmission
occurred after 1 second, the next retransmission should occur after 2
seconds has elapsed, then 4 seconds, etc. An implementation MAY place
a cap upon the maximum interval between retransmissions. This cap
MUST be no less than 8 seconds per retransmission. If no peer
response is detected after several retransmissions, (a recommended
default is 5, but SHOULD be configurable), the tunnel and all
sessions within MUST be cleared.

When a tunnel is being shut down for reasons other than loss of
connectivity, the state and reliable delivery mechanisms MUST be
maintained and operated for the full retransmission interval after
the final message exchange has occurred.

A sliding window mechanism is used for control message transmission.
Consider two peers A & B. Suppose A specifies a Receive Window Size
AVP with a value of N in the SCCRQ or SCCRP messages. B is now
allowed to have up to N outstanding control messages. Once N have
been sent, it must wait for an acknowledgment that advances the
window before sending new control messages. An implementation may
support a receive window of only 1 (i.e., by sending out a Receive
Window Size AVP with a value of 1), but MUST accept a window of up to
4 from its peer (e.g. have the ability to send 4 messages before
backing off). A value of 0 for the Receive Window Size AVP is
invalid.

When retransmitting control messages, a slow start and congestion
avoidance window adjustment procedure SHOULD be utilized. The
recommended procedure for this is described in Appendix A.

A peer MUST NOT withhold acknowledgment of messages as a technique
for flow controlling control messages. An L2TP implementation is
expected to be able to keep up with incoming control messages,
possibly responding to some with errors reflecting an inability to
honor the requested action.

Appendix B contains examples of control message transmission,
acknowledgement, and retransmission.

6.0 Control Connection Protocol Specification

The following control connection messages are used to establish,
clear and maintain L2TP tunnels. All data is sent in network order

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(high order octets first). Any "reserved" or "empty" fields MUST be
sent as 0 values to allow for protocol extensibility.

6.1 Start-Control-Connection-Request (SCCRQ)

Start-Control-Connection-Request (SCCRQ) is a control message used to
initialize a tunnel between an LNS and an LAC. It is sent by either
the LAC or the LNS to being the tunnel establishment process.

The following AVPs MUST be present in the SCCRQ:

Message Type AVP
Protocol Version
Host Name
Framing Capabilities
Assigned Tunnel ID

The Following AVPs MAY be present in the SCCRQ:

Bearer Capabilities
Receive Window Size
Challenge
Tie Breaker
Firmware Revision
Vendor Name

6.2 Start-Control-Connection-Reply (SCCRP)

Start-Control-Connection-Reply (SCCRP) is a control message sent in
reply to a received SCCRQ message. SCCRP is used to indicate that the
SCCRQ was accepted and establishment of the tunnel should continue.

The following AVPs MUST be present in the SCCRP:

Message Type
Protocol Version
Framing Capabilities
Host Name
Assigned Tunnel ID

The following AVPs MAY be present in the SCCRP:

Bearer Capabilities
Firmware Revision
Vendor Name
Receive Window Size
Challenge
Challenge Response

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6.3 Start-Control-Connection-Connected (SCCCN)

Start-Control-Connection-Connected (SCCCN) is a control message sent
in reply to an SCCRP. SCCCN completes the tunnel establishment
process.

The following AVP MUST be present in the SCCCN:

Message Type

The following AVP MAY be present in the SCCCN:

Challenge Response

6.4 Stop-Control-Connection-Notification (StopCCN)

Stop-Control-Connection-Notification (StopCCN) is a control message
sent by either the LAC or LNS to inform its peer that the tunnel is
being shutdown and the control connection should be closed. In
addition, all active sessions are implicitly cleared (without sending
any explicit call control messages). The reason for issuing this
request is indicated in the Result Code AVP. There is no explicit
reply to the message, only the implicit ACK that is received by the
reliable control message transport layer.

The following AVPs MUST be present in the StopCCN:

Message Type
Assigned Tunnel ID
Result Code

6.5 Hello (HELLO)

The Hello (HELLO) message is an L2TP control message sent by either
peer of a LAC-LNS control connection. This control message is used as
a "keepalive" for the tunnel.

The sending of HELLO messages and the policy for sending them are
left up to the implementation. A peer MUST NOT expect HELLO messages
at any time or interval. As with all messages sent on the control
connection, the receiver will return either a ZLB ACK or an
(unrelated) message piggybacking the necessary acknowledgement
information.

Since a HELLO is a control message, and control messages are reliably
sent by the lower level transport, this keepalive function operates
by causing the transport level to reliably deliver a message. If a
media interruption has occurred, the reliable transport will be

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unable to deliver the HELLO across, and will clean up the tunnel.

Keepalives for the tunnel MAY be implemented by sending a HELLO if a
period of time (a recommended default is 60 seconds, but SHOULD be
configurable) has passed without receiving any message (data or
control) from the peer.

HELLO messages are global to the tunnel. The Session ID in a HELLO
message MUST be 0.

The Following AVP MUST be present in the HELLO message:

Message Type

6.6 Incoming-Call-Request (ICRQ)

Incoming-Call-Request (ICRQ) is a control message sent by the LAC to
the LNS when an incoming call is detected. It is the first in a three
message exchange used for establishing a session within an L2TP
tunnel.

ICRQ is used to indicate that a session is to be established between
the LAC and LNS for this call and provides the LNS with parameter
information for the session. The LAC may defer answering the call
until it has received an ICRP from the LNS indicating that the
session should be established. This mechanism allows the LNS to
obtain sufficient information about the call before determining
whether it should be answered or not. Alternatively, the LAC may
answer the call, negotiate LCP and PPP authentication, and use the
information gained to choose the LNS. In this case, the call has
already been answered by the time the ICRP message is received; the
LAC simply spoofs the "call indication" and "call answer" steps in
this case.

The following AVPs MUST be present in the ICRQ:

Message Type
Assigned Session ID
Call Serial Number

The following AVPs MAY be present in the ICRQ:

Bearer Type
Physical Channel ID
Calling Number
Called Number
Sub-Address

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6.7 Incoming-Call-Reply (ICRP)

Incoming-Call-Reply (ICRP) is a control message sent by the LNS to
the LAC in response to a received ICRQ message. It is the second in
the three message exchange used for establishing sessions within an
L2TP tunnel.

ICRP is used to indicate that the ICRQ was successful and for the LAC
to answer the call if it has not already done so. It also allows the
LNS to indicate necessary parameters for the L2TP session.

The following AVPs MUST be present in the ICRP:

Message Type
Assigned Session ID

6.8 Incoming-Call-Connected (ICCN)

Incoming-Call-Connected (ICCN) is a control message sent by the LAC
to the LNS in response to a received ICRP message. It is the third
message in the three message exchange used for establishing sessions
within an L2TP tunnel.

ICCN is used to indicate that the ICRP was accepted, the call has
been answered, and that the L2TP session should move to the
established state. It also provides additional information to the
LNS about parameters used for the answered call (parameters that may
not always available at the time the ICRQ is issued).

The following AVPs MUST be present in the ICCN:

Message Type
(Tx) Connect Speed
Framing Type

The following AVPs MAY be present in the ICCN:

Initial Received LCP CONFREQ
Last Sent LCP CONFREQ
Last Received LCP CONFREQ
Proxy Authen Type
Proxy Authen Name
Proxy Authen Challenge
Proxy Authen ID
Proxy Authen Response
Private Group ID
Rx Connect Speed
Sequencing Required

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6.9 Outgoing-Call-Request (OCRQ)

Outgoing-Call-Request (OCRQ) is a control message sent by the LNS to
the LAC to indicate that an outbound call from the LAC is to be
established. It is the first in a three message exchange used for
establishing a session within an L2TP tunnel.

OCRQ is used to indicate that a session is to be established between
the LNS and LAC for this call and provides the LAC with parameter
information for both the L2TP session, and the call that is to be
placed

An LNS MUST have received a Bearer Capabilities AVP during tunnel
establishment from an LAC in order to request an outgoing call to
that LAC.

The following AVPs MUST be present in the OCRQ:

Message Type
Assigned Session ID
Call Serial Number
Minimum BPS
Maximum BPS
Bearer Type
Framing Type
Called Number

The following AVPs MAY be present in the OCRQ:

Sub-Address

6.10 Outgoing-Call-Reply (OCRP)

Outgoing-Call-Reply (OCRP) is a control message sent by the LAC to
the LNS in response to a received OCRQ message. It is the second in a
three message exchange used for establishing a session within an L2TP
tunnel.

OCRP is used to indicate that the LAC is able to attempt the outbound
call and returns certain parameters regarding the call attempt.

The following AVPs MUST be present in the OCRP:

Message Type
Assigned Session ID

The following AVPs MAY be present in the OCRP:

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Physical Channel ID

6.11 Outgoing-Call-Connected (OCCN)

Outgoing-Call-Connected (OCCN) is a control message sent by the LAC
to the LNS following the OCRP and after the outgoing call has been
completed. It is the final message in a three message exchange used
for establishing a session within an L2TP tunnel.

OCCN is used to indicate that the result of a requested outgoing call
was successful. It also provides information to the LNS about the
particular parameters obtained after the call was established.

The following AVPs MUST be present in the OCCN:

Message Type
(Tx) Connect Speed
Framing Type

The following AVPs MAY be present in the OCCN:

Rx Connect Speed
Sequencing Required

6.12 Call-Disconnect-Notify (CDN)

The Call-Disconnect-Notify (CDN) message is an L2TP control message
sent by either the LAC or LNS to request disconnection of a specific
call within the tunnel. Its purpose is to inform the peer of the
disconnection and the reason why the disconnection occurred. The peer
MUST clean up any resources, and does not send back any indication of
success or failure for such cleanup.

The following AVPs MUST be present in the CDN:

Message Type
Result Code
Assigned Session ID

The following AVPs MAY be present in the CDN:

Q.931 Cause Code

6.13 WAN-Error-Notify (WEN)

The WAN-Error-Notify message is an L2TP control message sent by the
LAC to the LNS to indicate WAN error conditions (conditions that
occur on the interface supporting PPP). The counters in this message

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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.

The following AVPs MUST be present in the WEN:

Message Type
Call Errors

6.14 Set-Link-Info (SLI)

The Set-Link-Info message is an L2TP control message sent by the LNS
to the LAC after the last LCP Conf ACK is received during PPP LCP
negotation. This AVP contains any relevant link level parameters that
the LAC may need to be aware of (for instance, ACCM map info). If
there is no relevant information to be sent in the SLI, then it MAY
be omitted. Since LCP may be renegotiated at any time, an SLI may be
sent at any time during the life of the call, thus the LAC MUST be
able to update its internal call information and behavior on an
active session. Further, if there are packets in queue at the LAC
when an SLI is received, these must be flushed before applying the
SLI information to the link.

If the PPP session at the LNS renegotiates LCP during the call, an
SLI MUST be sent to the LAC to return link level information to the
initial default values while the negotiation occurs. However, if the
last SLI sent was already set to default values or no SLI was sent at
all, this step MAY be omitted.

The following AVPs MUST be present in the SLI:

Message Type
ACCM

7.0 Control Connection State Machines

State tables defined in this section govern the exchange of control
messages defined in section 6. Tables are defined for incoming call
placement, outgoing call placement, as well as for initiation of the
tunnel itself. The state tables do not encode timeout and
retransmission behavior, as this is handled in the underlying
transport defined in section 5.8.

7.1 Control Connection Protocol Operation

This section describes the operation of various L2TP control
connection functions and the Control Connection messages which are
used to support them.

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Receipt of an invalid or unrecoverable malformed control message
should be logged appropriately and the control connection cleared to
ensure recovery to a known state. The control connection may then be
restarted by the initiator.

An invalid control message is defined as a message which contains a
Message Type that is marked mandatory (see section 4.4.1) and is
unknown to the implementation, or a control message that is received
in an improper sequence (e.g. an SCCCN sent in reply to an SCCRQ).

Examples of a malformed control message include one that has an
invalid value in its header, contains an AVP that is formatted
incorrectly or whose value is out of range, or a message that is
missing a required AVP. A control message with a malformed header
should be discarded. A control message with an invalid AVP should
look to the M-bit for that AVP to determine whether the error is
recoverable or not.

A malformed yet recoverable non-mandatory (M-bit is not set) AVP
within a control message should be treated in a similar manner as an
unrecognized non-mandatory AVP. Thus, if a malformed AVP is received
with the M-bit set, the session or tunnel should be terminated with a
proper Result or Error Code sent. If the M-bit is not set, the AVP
should be ignored (with the exception of logging a local error
message) and the message accepted.

This MUST NOT be considered a license to send malformed AVPs, but
simply a guide towards how to handle an improperly formatted message
if one is received. It is impossible to list all potential
malformations of a given message and give advice for each. That said,
one example of a recoverable, malformed AVP might be if the Rx
Connect Speed AVP, attribute 38, is received with a length of 8
rather than 10 and the BPS given in 2 octets rather than 4. Since the
Rx Connect Speed is non-mandatory, this condition should not be
considered catastrophic. As such, the control message should be
accepted as if the AVP had not been received (with the exception of a
local error message being logged).

In several cases in the following tables, a protocol message is sent,
and then a "clean up" occurs. Note that regardless of the initiator
of the tunnel destruction, the reliable delivery mechanism must be
allowed to run (see section 5.8) before destroying the tunnel. This
permits the tunnel management messages to be reliably delivered to
the peer.

Appendix B.1 contains an example of lock-step tunnel establishment.

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7.2 Control Connection States

The L2TP control connection protocol is not distinguishable between
the LNS and LAC, but is distinguishable between the originator and
receiver. The originating peer is the one which first initiates
establishment of the tunnel (in a tie breaker situation, this is the
winner of the tie). Since either LAC or LNS can be the originator, a
collision can occur. See the Tie Breaker AVP in section 4.4.3 for a
description of this and its resolution.

7.2.1 Control Connection Establishment

State Event Action New State
----- ----- ------ ---------
idle Local Send SCCRQ wait-ctl-reply
Open request

idle Receive SCCRQ, Send SCCRP wait-ctl-conn
acceptable

idle Receive SCCRQ, Send StopCCN, idle
not acceptable Clean up

idle Receive SCCRP Send StopCCN idle
Clean up

idle Receive SCCCN Clean up idle

wait-ctl-reply Receive SCCRP, Send SCCCN, established
acceptable Send tunnel-open
event to waiting
sessions

wait-ctl-reply Receive SCCRP, Send StopCCN, idle
not acceptable Clean up

wait-ctl-reply Receive SCCRQ, Clean up, idle
lose tie-breaker Re-queue SCCRQ
for idle state

wait-ctl-reply Receive SCCCN Send StopCCN idle
Clean up

wait-ctl-conn Receive SCCCN, Send tunnel-open established
acceptable event to waiting
sessions

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wait-ctl-conn Receive SCCCN, Send StopCCN, idle
not acceptable Clean up

wait-ctl-conn Receive SCCRP, Send StopCCN, idle
SCCRQ Clean up

established Local Send tunnel-open established
Open request event to waiting
(new call) sessions

established Admin Send StopCCN idle
Tunnel Close Clean up

established Receive SCCRQ, Send StopCCN idle
SCCRP, SCCCN Clean up

idle Receive StopCCN Clean up idle
wait-ctl-reply,
wait-ctl-conn,
established

The states associated with the LNS or LAC for control connection
establishment are:

idle
Both initiator and recipient start from this state. An initiator
transmits an SCCRQ, while a recipient remains in the idle state
until receiving an SCCRQ.

wait-ctl-reply
The originator checks to see if another connection has been
requested from the same peer, and if so, handles the collision
situation described in section 5.8.

When an SCCRP 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 StopCCN MUST be sent
to the peer and the originator cleans up and terminates the
tunnel.

wait-ctl-conn
This is where an SCCCN is awaited; upon receipt, the challenge
response is checked. The tunnel either is established, or is torn
down if an authorization failure is detected.

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established
An established connection may be terminated by either a local
condition or the receipt of a Stop-Control-Connection-
Notification. In the event of a local termination, the originator
MUST send a Stop-Control-Connection-Notification and clean up the
tunnel.

If the originator receives a Stop-Control-Connection-Notification
it MUST also clean up the tunnel.

7.3 Timing considerations

Due to the real-time nature of telephone signaling, both the LNS and
LAC should be implemented with multi-threaded architectures such that
messages related to multiple calls are not serialized and blocked.
The call and connection state figures do not specify exceptions
caused by timers. These are addressed in section 5.8.

7.4 Incoming calls

An Incoming-Call-Request message is generated by the LAC when an
incoming call is detected (for example, an associated telephone line
rings). The LAC selects a Session 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.
Calling Number, Called Number, and Subaddress may be included in the
message if they are available from the telephone network.

Once the LAC sends the Incoming-Call-Request, it waits for a response
from the LNS but it does not necessarily answer the call from the
telephone network yet. The LNS may choose not to accept the call if:

- No resources are available to handle more sessions
- The dialed, dialing, or subaddress fields do not correspond to
an authorized user
- The bearer service is not authorized or supported

If the LNS chooses to accept the call, it responds with an Incoming-
Call-Reply. When the LAC receives the Incoming-Call-Reply, it
attempts to connect the call. A final call connected message from
the LAC to the LNS indicates that the call states for both the LAC
and the LNS should enter the established state. If the call
terminated before the LNS could accept it, a Call-Disconnect-Notify
is sent by the LAC to indicate this condition.

When the dialed-in client hangs up, the call is cleared normally and
the LAC sends a Call-Disconnect-Notify message. If the LNS wishes to

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clear a call, it sends a Call-Disconnect-Notify message and cleans up
its session.

7.4.1 LAC Incoming Call States

State Event Action New State
----- ----- ------ ---------
idle Bearer Ring or Initiate local wait-tunnel
Ready to indicate tunnel open
incoming conn.

idle Receive ICCN, Clean up idle
ICRP, CDN

wait-tunnel Bearer line drop Clean up idle
or local close
request

wait-tunnel tunnel-open Send ICRQ wait-reply

wait-reply Receive ICRP, Send ICCN established
acceptable

wait-reply Receive ICRP, Send CDN, idle
Not acceptable Clean up

wait-reply Receive ICRQ Send CDN idle
Clean up

wait-reply Receive CDN Clean up idle
ICCN

wait-reply Local Send CDN, idle
close request or Clean up
Bearer line drop

established Receive CDN Clean up idle

established Receive ICRQ, Send CDN, idle
ICRP, ICCN Clean up

established Bearer line Send CDN, idle
drop or local Clean up
close request

The states associated with the LAC for incoming calls are:

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idle
The LAC detects an incoming call on one of its interfaces.
Typically this means an analog line is ringing or an ISDN TE has
detected an incoming Q.931 SETUP message. The LAC initiates its
tunnel establishment state machine, and moves to a state waiting
for confirmation of the existence of a tunnel.

wait-tunnel
In this state the session is waiting for either the control
connection to be opened or for verification that the tunnel is
already open. Once an indication that the tunnel has/was opened,
session control messages may be exchanged. The first of these is
the Incoming-Call-Request.

wait-reply
The LAC receives either a CDN message indicating the LNS is not
willing to accept the call (general error or don't accept) and
moves back into the idle state, or an Incoming-Call-Reply message
indicating the call is accepted, the LAC 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 connected interface: The LAC sends a Call-
Disconnect-Notify message
+ Receipt of a Call-Disconnect-Notify message: The LAC cleans
up, disconnecting the call.
+ A local reason: The LAC sends a Call-Disconnect-Notify
message.

7.4.2 LNS Incoming Call States

State Event Action New State
----- ----- ------ ---------
idle Receive ICRQ, Send ICRP wait-connect
acceptable

idle Receive ICRQ, Send CDN, idle
not acceptable Clean up

idle Receive ICRP Send CDN idle
Clean up

idle Receive ICCN Clean up idle

wait-connect Receive ICCN Prepare for established

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acceptable data

wait-connect Receive ICCN Send CDN, idle
not acceptable Clean up

wait-connect Receive ICRQ, Send CDN idle
ICRP Clean up

idle, Receive CDN Clean up idle
wait-connect,
established

wait-connect Local Send CDN, idle
established Close request Clean up

established Receive ICRQ, Send CDN idle
ICRP, ICCN Clean up

The states associated with the LNS for incoming calls are:

idle
An Incoming-Call-Request message is received. If the request is
not acceptable, a Call-Disconnect-Notify is sent back to the LAC
and the LNS remains in the idle state. If the Incoming-Call-
Request message is acceptable, an Incoming-Call-Reply is sent. The
session moves to the wait-connect state.

wait-connect
If the session is still connected on the LAC, the LAC sends an
Incoming-Call-Connected message to the LNS which then moves into
established state. The LAC 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 accidentally places
a standard voice call to an LAC 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 LAC or by sending a Call-Disconnect-
Notify. Clean up follows on both sides regardless of the
initiator.

7.5 Outgoing calls

Outgoing calls are initiated by an LNS and instruct an LAC to place a
call. There are three messages for outgoing calls: Outgoing-Call-

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Request, Outgoing-Call-Reply, and Outgoing-Call-Connected. The LNS
sends an Outgoing-Call-Request specifying the dialed party phone
number, subaddress and other parameters. The LAC MUST respond to the
Outgoing-Call-Request message with an Outgoing-Call-Reply message
once the LAC determines that the proper facilities exist to place the
call and the call is administratively authorized. For example, is
this LNS allowed to dial an international call? Once the outbound
call is connected, the LAC sends an Outgoing-Call-Connected message
to the LNS indicating the final result of the call attempt:

7.5.1 LAC Outgoing Call States

State Event Action New State
----- ----- ------ ---------
idle Receive OCRQ, Send OCRP, wait-cs-answer
acceptable Open bearer

idle Receive OCRQ, Send CDN, idle
not acceptable Clean up

idle Receive OCRP Send CDN idle
Clean up

idle Receive OCCN, Clean up idle
CDN

wait-cs-answer Bearer answer, Send OCCN established
framing detected

wait-cs-answer Bearer failure Send CDN, idle
Clean up

wait-cs-answer Receive OCRQ, Send CDN idle
OCRP, OCCN Clean up

established Receive OCRQ, Send CDN idle
OCRP, OCCN Clean up

wait-cs-answer, Receive CDN Clean up idle
established

established Bearer line drop, Send CDN, idle
Local close Clean up
request

The states associated with the LAC for outgoing calls are:

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idle
If Outgoing-Call-Request is received in error, respond with a
Call-Disconnect-Notify. Otherwise, allocate a physical channel and
send an Outgoing-Call-Reply. Place the outbound call and move to
the wait-cs-answer state.

wait-cs-answer
If the call is not completed or a timer expires waiting for the
call to complete, send a Call-Disconnect-Notify with the
appropriate error condition set and go to idle state. If a circuit
switched connection is established and framing is detected, send
an Outgoing-Call-Connected indicating success and go to
established state.

established
If a Call-Disconnect-Notify is received by the LAC, the telco call
MUST be released via appropriate mechanisms and the session
cleaned up. If the call is disconnected by the client or the
called interface, a Call-Disconnect-Notify message MUST be sent to
the LNS. The sender of the Call-Disconnect-Notify message returns
to the idle state after sending of the message is complete.

7.5.2 LNS Outgoing Call States

State Event Action New State
----- ----- ------ ---------
idle Local Initiate local wait-tunnel
open request tunnel-open

idle Receive OCCN, Clean up idle
OCRP, CDN

wait-tunnel tunnel-open Send OCRQ wait-reply

wait-reply Receive OCRP, none wait-connect
acceptable

wait-reply Receive OCRP, Send CDN idle
not acceptable Clean up

wait-reply Receive OCCN, Send CDN idle
OCRQ Clean up

wait-connect Receive OCCN none established

wait-connect Receive OCRQ, Send CDN idle
OCRP Clean up

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idle, Receive CDN, Clean up idle
wait-reply,
wait-connect,
established

established Receive OCRQ, Send CDN idle
OCRP, OCCN Clean up

wait-reply, Local Send CDN idle
wait-connect, Close request Clean up
established

wait-tunnel Local Clean up idle
Close request

The states associated with the LNS for outgoing calls are:

idle, wait-tunnel
When an outgoing call is initiated, a tunnel is first created,
much as the idle and wait-tunnel states for an LAC incoming call.
Once a tunnel is established, an Outgoing-Call-Request message is
sent to the LAC and the session moves into the wait-reply state.

wait-reply
If a Call-Disconnect-Notify is received, an error occurred, and
the session is cleaned up and returns to idle. If an Outgoing-
Call-Reply is received, the call is in progress and the session
moves to the wait-connect state.

wait-connect
If a Call-Disconnect-Notify is received, the call failed; the
session is cleaned up and returns to idle. If an Outgoing-Call-
Connected is received, the call has succeeded and the session may
now exchange data.

established
If a Call-Disconnect-Notify is received, the call has been
terminated for the reason indicated in the Result and Cause Codes;
the session moves back to the idle state. If the LNS chooses to
terminate the session, it sends a Call-Disconnect-Notify to the
LAC and then cleans up and idles its session.

7.6 Tunnel Disconnection

The disconnection of a tunnel consists of either peer issuing a
Stop-Control-Connection-Notification. The sender of this Notification
should wait a finite period of time for the acknowledgment of this
message before releasing the control information associated with the

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tunnel. The recipient of this Notification should send an
acknowledgment of the Notification and then release the associated
control information.

When to release a tunnel is an implementation issue and is not
specified in this document. A particular implementation may use
whatever policy is appropriate for determining when to release a
control connection. Some implementations may leave a tunnel open for
a period of time or perhaps indefinitely after the last session for
that tunnel is cleared. Others may choose to disconnect the tunnel
immediately after the last user connection on the tunnel disconnects.

8.0 L2TP Over Specific Media

L2TP is self-describing, operating at a level above the media over
which it is carried. However, some details of its connection to media
are required to permit interoperable implementations. The following
sections describe details needed to permit interoperability over
specific media.

8.1 L2TP over UDP/IP

L2TP uses the registered UDP port 1701 [RFC1700]. The entire L2TP
packet, including payload and L2TP header, is sent within a UDP
datagram. The initiator of an L2TP tunnel picks an available source
UDP port (which may or may not be 1701), and sends to the desired
destination address at port 1701. The recipient picks a free port on
its own system (which may or may not be 1701), and sends its reply to
the initiator's UDP port and address, setting its own source port to
the free port it found. Once the source and destination ports and
addresses are established, they MUST remain static for the life of
the tunnel.

It has been suggested that having the recipient choose an arbitrary
source port (as opposed to using the destination port in the packet
initiating the tunnel, i.e., 1701) may make it more difficult for
L2TP to traverse some NAT devices. Implementors should consider the
potential implication of this before choosing an arbitrary source
port.

IP fragmentation may occur as the L2TP packet travels over the IP
substrate. L2TP makes no special efforts to optimize this. A LAC
implementation MAY cause its LCP to negotiate for a specific MRU,
which could optimize for LAC environments in which the MTU's of the
path over which the L2TP packets are likely to travel have a
consistent value.

The default for any L2TP implementation is that UDP checksums MUST be

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enabled for both control and data messages. An L2TP implementation
MAY provide an option to disable UDP checksums for data messages. It
is recommended that UDP checksums always be enabled on control
packets.

Port 1701 is used for both L2F [RFC2341] and L2TP packets. The
Version field in each header may be used to discriminate between the
two packet types (L2F uses a value of 1, and the L2TP version
described in this document uses a value of 2). An L2TP implementation
running on a system which does not support L2F MUST silently discard
all L2F packets.

To the PPP clients using an L2TP-over-UDP/IP tunnel, the PPP link has
the characteristic of being able to reorder or silently drop packets.
The former may break non-IP protocols being carried by PPP,
especially LAN-centric ones such as bridging. The latter may break
protocols which assume per-packet indication of error, such as TCP
header compression. Sequencing may be handled by using L2TP data
message sequence numbers if any protocol being transported by the PPP
tunnel cannot tolerate reordering. The sequence dependency
characteristics of individual protocols are outside the scope of this
document.

Allowing packets to be dropped silently is perhaps more problematic
with some protocols. If PPP reliable delivery [RFC1663] is enabled,
no upper PPP protocol will encounter lost packets. If L2TP sequence
numbers are enabled, L2TP can detect the packet loss. In the case of
an LNS, the PPP and L2TP stacks are both present within the LNS, and
packet loss signaling may occur precisely as if a packet was received
with a CRC error. Where the LAC and PPP stack are co-resident, this
technique also applies. Where the LAC and PPP client are physically
distinct, the analogous signaling MAY be accomplished by sending a
packet with a CRC error to the PPP client. Note that this would
greatly increase the complexity of debugging client line problems,
since the client statistics could not distinguish between true media
errors and LAC-initiated ones. Further, this technique is not
possible on all hardware.

If VJ compression is used, and neither PPP reliable delivery nor
sequence numbers are enabled, each lost packet results in a 1 in
2**16 chance of a TCP segment being forwarded with incorrect contents
[RFC1144]. Where the combination of the packet loss rate with this
statistical exposure is unacceptable, TCP header compression SHOULD
NOT be used.

In general, it is wise to remember that the L2TP/UDP/IP transport is
an unreliable transport. As with any PPP media that is subject to
loss, care should be taken when using protocols that are particularly

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loss-sensitive. Such protocols include compression and encryption
protocols that employ history.

8.2 IP

When operating in IP environments, L2TP MUST offer the UDP
encapsulation described in 8.1 as its default configuration for IP
operation. Other configurations (perhaps corresponding to a
compressed header format) MAY be defined and made available as a
configurable option.

9.0 Security Considerations

L2TP encounters several security issues in its operation. The
general approach of L2TP to these issues is documented here.

9.1 Tunnel Endpoint Security

The tunnel endpoints may optionally perform an authentication
procedure of one another during tunnel establishment. This
authentication has the same security attributes as CHAP, and has
reasonable protection against replay and snooping during the tunnel
establishment process. This mechanism is not designed to provide any
authentication beyond tunnel establishment; it is fairly simple for a
malicious user who can snoop the tunnel stream to inject packets once
an authenticated tunnel establishment has been completed
successfully.

For authentication to occur, the LAC and LNS MUST share a single
secret. Each side uses this same secret when acting as authenticatee
as well as authenticator. Since a single secret is used, the tunnel
authentication AVPs include differentiating values in the CHAP ID
fields for each message digest calculation to guard against replay
attacks.

The Assigned Tunnel ID and Assigned Session ID (See section 4.4.3)
SHOULD be selected in an unpredictable manner rather than
sequentially or otherwise. Doing so will help deter hijacking of a
session by a malicious user who does not have access to packet traces
between the LAC and LNS.

9.2 Packet Level Security

Securing L2TP requires that the underlying transport make available
encryption, integrity and authentication services for all L2TP
traffic. This secure transport operates on the entire L2TP packet
and is functionally independent of PPP and the protocol being carried
by PPP. As such, L2TP is only concerned with confidentiality,

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authenticity, and integrity of the L2TP packets between its tunnel
endpoints (the LAC and LNS), not unlike link-layer encryption being
concerned only about protecting the confidentiality of traffic
between its physical endpoints.

9.3 End to End Security

Protecting the L2TP packet stream via a secure transport does, in
turn, also protect the data within the tunneled PPP packets while
transported from the LAC to the LNS. Such protection should not be
considered a substitution for end-to-end security between
communicating hosts or applications.

9.4 L2TP and IPsec

When running over IP, IPsec provides packet-level security via ESP
and/or AH. All L2TP control and data packets for a particular tunnel
appear as homogeneous UDP/IP data packets to the IPsec system.

In addition to IP transport security, IPsec defines a mode of
operation that allows tunneling of IP packets. The packet level
encryption and authentication provided by IPsec tunnel mode and that
provided by L2TP secured with IPsec provide an equivalent level of
security for these requirements.

IPsec also defines access control features that are required of a
compliant IPsec implementation. These features allow filtering of
packets based upon network and transport layer characteristics such
as IP address, ports, etc. In the L2TP tunneling model, analogous
filtering is logically performed at the PPP layer or network layer
above L2TP. These network layer access control features may be
handled at the LNS via vendor-specific authorization features based
upon the authenticated PPP user, or at the network layer itself by
using IPsec transport mode end-to-end between the communicating
hosts. The requirements for access control mechanisms are not a part
of the L2TP specification and as such are outside the scope of this
document.

9.5 Proxy PPP Authentication

L2TP defines AVPs that MAY be exchanged during session establishment
to provide forwarding of PPP authentication information obtained at
the LAC to the LNS for validation (see section 4.4.5). This implies a
direct trust relationship of the LAC on behalf of the LNS. If the LNS
chooses to implement proxy authentication, it MUST be able to be
configured off, requiring a new round a PPP authentication initiated
by the LNS (which may or may not include a new round of LCP
negotiation).

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10.0 IANA Considerations

This document defines a number of "magic" numbers to be maintained by
the IANA. This section explains the criteria to be used by the IANA
to assign additional numbers in each of these lists. The following
subsections describe the assignment policy for the namespaces defined
elsewhere in this document.

10.1 AVP Attributes

As defined in section 4.1, AVPs contain vendor ID, Attribute and
Value fields. For vendor ID value of 0, IANA will maintain a registry
of assigned Attributes and in some case also values. Attributes 0-39
are assigned as defined in section 4.4. The remaining values are
available for assignment upon Expert Review [RFC 2434].

10.2 Message Type AVP Values

As defined in section 4.4.1, Message Type AVPs (Attribute Type 0)
have an associated value maintained by IANA. Values 0-16 are defined
in section 3.2, the remaining values are available for assignment
upon Expert Review [RFC 2434]

10.3 Result Code AVP Values

As defined in section 4.4.2, Result Code AVPs (Attribute Type 1)
contain three fields. Two of these fields (the Result Code and Error
Code fields) have associated values maintained by IANA.

10.3.1 Result Code Field Values

The Result Code AVP may be included in CDN and StopCCN messages. The
allowable values for the Result Code field of the AVP differ
depending upon the value of the Message Type AVP. For the StopCCN
message, values 0-7 are defined in section 4.4.2; for the CDN
message, values 0-11 are defined in the same section. The remaining
values of the Result Code field for both messages are available for
assignment upon Expert Review [RFC 2434].

10.3.2 Error Code Field Values

Values 0-7 are defined in section 4.4.2. Remaining values are
available for assignment upon Expert Review [RFC 2434].

10.4 Framing Capabilities & Bearer Capabilities

The Framing Capabilities AVP and Bearer Capabilities AVPs (defined in
section 4.4.3) both contain 32-bit bitmasks. Additional bits should

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only be defined via a Standards Action [RFC 2434].

10.5 Proxy Authen Type AVP Values

The Proxy Authen Type AVP (Attribute Type 29) has an associated value
maintained by IANA. Values 0-5 are defined in section 4.4.5, the
remaining values are available for assignment upon Expert Review [RFC
2434].

10.6 AVP Header Bits

There are four remaining reserved bits in the AVP header. Additional
bits should only be assigned via a Standards Action [RFC 2434].

11.0 References

[DSS1] ITU-T Recommendation, "Digital subscriber Signaling System
No. 1 (DSS 1) - ISDN user-network interface layer 3
specification for basic call control", Rec. Q.931(I.451),
May 1998

[KPS] Kaufman, C., Perlman, R., and Speciner, M., "Network
Security: Private Communications in a Public World",
Prentice Hall, March 1995, ISBN 0-13-061466-1

[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.

[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC 1034, November 1987.

[RFC1144] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed
Serial Links", RFC 1144, February 1990.

[RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.

[RFC1662] Simpson, W., "PPP in HDLC-like Framing", STD 51, RFC 1662,
July 1994.

[RFC1663] Rand, D., "PPP Reliable Transmission", RFC 1663, July 1994.

[RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
1700, October 1994. See also:
http://www.iana.org/numbers.html

[RFC1990] Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T.
Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990,

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INTERNET DRAFT L2TP July 2000

August 1996.

[RFC1994] Simpson, W., "PPP Challenge Handshake Authentication
Protocol (CHAP)", RFC 1994, August 1996.

[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2138] Rigney, C., Rubens, A., Simpson, W. and S. Willens, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2138,
April 1997.

[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.

[RFC2341] Valencia, A., Littlewood, M. and T. Kolar, "Cisco Layer Two
Forwarding (Protocol) L2F", RFC 2341, May 1998.

[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.

[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations section in RFCs", BCP 26, RFC 2434,
October 1998.

[RFC2637] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W.
and G. Zorn, "Point-to-Point Tunneling Protocol (PPTP)",
RFC 2637, July 1999.

[STEVENS] Stevens, W. Richard, "TCP/IP Illustrated, Volume I The
Protocols", Addison-Wesley Publishing Company, Inc., March
1996, ISBN 0-201-63346-9

12.0 Acknowledgments

The basic concept for L2TP and many of its protocol constructs were
adopted from L2F [RFC2341] and PPTP [PPTP]. Authors of these are A.
Valencia, M. Littlewood, T. Kolar, K. Hamzeh, G. Pall, W. Verthein,
J. Taarud, W. Little, and G. Zorn.

Dory Leifer made valuable refinements to the protocol definition of
L2TP and contributed to the editing of this document.

Steve Cobb and Evan Caves redesigned the state machine tables.

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Barney Wolff provided a great deal of design input on the endpoint
authentication mechanism.

John Bray, Greg Burns, Rich Garrett, Don Grosser, Matt Holdrege,
Terry Johnson, Dory Leifer, and Rich Shea provided valuable input and
review at the 43rd IETF in Orlando, FL., which led to improvement of
the overall readability and clarity of this document.

13.0 Authors' Addresses

Gurdeep Singh Pall
Microsoft Corporation
Redmond, WA

Email: gurdeep@microsoft.com

Bill Palter
RedBack Networks, Inc
1389 Moffett Park Drive
Sunnyvale, CA 94089

Email: palter@zev.net

Allan Rubens
Ascend Communications
1701 Harbor Bay Parkway
Alameda, CA 94502

Email: acr@del.com

W. Mark Townsley
cisco Systems
7025 Kit Creek Road
PO Box 14987
Research Triangle Park, NC 27709

Email: mark@townsley.net

Andrew J. Valencia
P.O. Box 2928
Vashon, WA 98070

Email: vandys@zendo.com

Glen Zorn
cisco Systems
500 108th Avenue N.E., Suite 500
Bellevue, WA 98004

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Phone: +1 425 438 8218
FAX: +1 425 438 1848
Email: gwz@cisco.com

Appendix A: Control Channel Slow Start and Congestion Avoidance

Although each side has indicated the maximum size of its receive
window, it is recommended that a slow start and congestion avoidance
method be used to transmit control packets. The methods described
here are based upon the TCP congestion avoidance algorithm as
described in section 21.6 of TCP/IP Illustrated, Volume I, by W.
Richard Stevens [STEVENS].

Slow start and congestion avoidance make use of several variables.
The congestion window (CWND) defines the number of packets a sender
may send before waiting for an acknowledgment. The size of CWND
expands and contracts as described below. Note however, that CWND is
never allowed to exceed the size of the advertised window obtained
from the Receive Window AVP (in the text below, it is assumed any
increase will be limited by the Receive Window Size). The variable
SSTHRESH determines when the sender switches from slow start to
congestion avoidance. Slow start is used while CWND is less than
SSHTRESH.

A sender starts out in the slow start phase. CWND is initialized to
one packet, and SSHTRESH is initialized to the advertised window
(obtained from the Receive Window AVP). The sender then transmits
one packet and waits for its acknowledgement (either explicit or
piggybacked). When the acknowledgement is received, the congestion
window is incremented from one to two. During slow start, CWND is
increased by one packet each time an ACK (explicit ZLB or
piggybacked) is received. Increasing CWND by one on each ACK has the
effect of doubling CWND with each round trip, resulting in an
exponential increase. When the value of CWND reaches SSHTRESH, the
slow start phase ends and the congestion avoidance phase begins.

During congestion avoidance, CWND expands more slowly. Specifically,
it increases by 1/CWND for every new ACK received. That is, CWND is
increased by one packet after CWND new ACKs have been received.
Window expansion during the congestion avoidance phase is effectively
linear, with CWND increasing by one packet each round trip.

When congestion occurs (indicated by the triggering of a
retransmission) one half of the CWND is saved in SSTHRESH, and CWND
is set to one. The sender then reenters the slow start phase.

Appendix B: Control Message Examples

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B.1: Lock-step tunnel establishment

In this example, an LAC establishes a tunnel, with the exchange
involving each side alternating in sending messages. This example
shows the final acknowledgment explicitly sent within a ZLB ACK
message. An alternative would be to piggyback the acknowledgement
within a message sent as a reply to the ICRQ or OCRQ that will
likely follow from the side that initiated the tunnel.

LAC or LNS LNS or LAC
---------- ----------

SCCRQ ->
Nr: 0, Ns: 0
<- SCCRP
Nr: 1, Ns: 0
SCCCN ->
Nr: 1, Ns: 1
<- ZLB
Nr: 2, Ns: 1

B.2: Lost packet with retransmission

An existing tunnel has a new session requested by the LAC. The ICRP
is lost and must be retransmitted by the LNS. Note that loss of the
ICRP has two impacts: not only does it keep the upper level state
machine from progressing, but it also keeps the LAC from seeing a
timely lower level acknowledgment of its ICRQ.

LAC LNS
--- ---

ICRQ ->
Nr: 1, Ns: 2

(packet lost) <- ICRP
Nr: 3, Ns: 1

(pause; LAC's timer started first, so fires first)

ICRQ ->
Nr: 1, Ns: 2

(Realizing that it has already seen this packet,
the LNS discards the packet and sends a ZLB)

<- ZLB
Nr: 3, Ns: 2

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(LNS's retransmit timer fires)

<- ICRP
Nr: 3, Ns: 1
ICCN ->
Nr: 2, Ns: 3

<- ZLB
Nr: 4, Ns: 2

Appendix C: Intellectual Property Notice

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification can
be obtained from the IETF Secretariat."

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.

The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.

Townsley, et al. Standards Track [Page 76]