PPP Working Group Kory Hamzeh
INTERNET DRAFT Ascend Communications
Category: Internet Draft Tim Kolar
Title: draft-ietf-pppext-l2tp-04.txt Cisco Systems
Date: June 1997 Morgan Littlewood
Cisco Systems
Gurdeep Singh Pall
Microsoft Corporation
Jeff Taarud
Copper Mountain Networks
Andrew J. Valencia
Cisco Systems
William Verthein
U.S. Robotics
Layer Two Tunneling Protocol "L2TP"
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months. Internet-Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet-
Drafts as reference material or to cite them other than as a
``working draft'' or ``work in progress.''
To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net, nic.nordu.net, ftp.nisc.sri.com, or
munnari.oz.au.
Abstract
Virtual dial-up allows many separate and autonomous protocol domains
to share common access infrastructure including modems, Access
Servers, and ISDN routers. RFC1661 specifies multiprotocol dial-up
via PPP [1]. This document describes the Layer Two Tunneling
Protocol (L2TP) which permits the tunneling of the link layer (i.e.,
HDLC, async HDLC) of PPP. Using such tunnels, it is possible to
divorce the location of the initial dial-up server from the location
at which the dial-up protocol connection is terminated and access to
the network provided.
Table of Contents
1.0 Introduction
1.1 Conventions
1.2 Terminology
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2.0 Problem Space Overview
2.1 Initial Assumptions
2.2 Topology
2.3 Providing Virtual Dial-up Services--a walk-through
3.0 Service Model Issues
3.1 Security
3.2 Address Allocation
3.3 Authentication
3.4 Accounting
4.0 Protocol Overview
4.1 Control Message Overview
4.2 Payload Packet Overview
5.0 Message Format and Protocol Extensibility
5.1 AVP
5.2 Control Message Format
5.3 Payload Message Format
5.4 Control Message Types
5.5 AVP Summary
5.6 Result and Error Code Summary
5.7 Hiding of AVP values
6.0 Control Connection Protocol Specification
6.1 Start-Control-Connection-Request
6.2 Start-Control-Connection-Reply
6.3 Start-Control-Connection-Connected
6.4 Stop-Control-Connection-Request
6.5 Stop-Control-Connection-Reply
6.6 Hello
6.7 Outgoing-Call-Request
6.8 Outgoing-Call-Reply
6.9 Outgoing-Call-Connected
6.10 Incoming-Call-Request
6.11 Incoming-Call-Reply
6.12 Incoming-Call-Connected
6.13 Call-Clear-Request
6.14 Call-Disconnect-Notify
6.15 WAN-Error-Notify
6.16 Set-Link-Info
7.0 Control Connection State Machines
7.1 Control Connection Protocol Operation
7.2 Control Connection States
7.2.1 Control Connection Originator
7.2.2 Control connection Receiver
7.3 Timing considerations
7.4 Incoming Calls
7.4.1 LAC Incoming Call States
7.4.2 LNS Incoming Call States
7.5 Outgoing calls
7.5.1 LAC Outgoing Call States
7.5.2 LNS Outgoing Call States
8.0 L2TP Over Specific Media
8.1 IP/UDP
8.2 IP
9.0 Security Considerations
9.1 Tunnel Endpoint Security
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9.2 Client Security
10.0 Acknowledgments
11.0 Contacts
12.0 References
Appendix A: Acknowledgment Time-Outs
Appendix B: Acknowledgment Time-Out and Window Adjustment
Appendix C: Handling of out-of-order packets
Appendix D: Transport Layer Adaptive Time-Outs and Window Adjustment
Appendix E: Examples of L2TP sequence numbering
E.1 Lock-step tunnel establishment
E.2 Multiple packets acknowledged
E.3 Lost packet with retransmission
1.0 Introduction
The traditional dial-up network service on the Internet is for
registered IP addresses only. A new class of virtual dial-up
application which allows multiple protocols and unregistered IP
addresses is also desired on the Internet. Examples of this class of
network application are support for privately addressed IP, IPX, and
AppleTalk dial-up via PPP across existing Internet infrastructure.
The support of these multiprotocol virtual dial-up applications is of
significant benefit to end users, enterprises, and Internet Service
providers as it allows the sharing of very large investments in
access and core infrastructure and allows local calls to be used. It
also allows existing investments in non-IP protocol applications to
be supported in a secure manner while still leveraging the access
infrastructure of the Internet.
It is the purpose of this draft to identify the issues encountered in
integrating multiprotocol dial-up services into an existing Internet
Service Provider's Point of Presence (hereafter referred to as ISP
and POP, respectively), and to describe the L2TP protocol which
permits the leveraging of existing access protocols.
This protocol may also be used to solve the "multilink hunt-group
splitting" problem. Multilink PPP, often used to aggregate ISDN B
channels, requires that all channels composing a multilink bundle be
grouped at a single NAS. Because L2TP makes a PPP session appear at
a location other than the physical point at which the session was
physically received, it can be used to make all channels appear at a
single NAS, allowing multilink operation even when the physical calls
are spread across distinct physical NAS's.
1.1 Conventions
The following language conventions are used in the items of
specification in this document:
o MUST, SHALL, or MANDATORY -- This item is an absolute
requirement of the specification.
o SHOULD or RECOMMEND -- This item should generally be followed
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for all but exceptional circumstances.
o MAY or OPTIONAL -- This item is truly optional and may be
followed or ignored according to the needs of the implementor.
1.2 Terminology
Analog Channel
A circuit-switched communication path which is intended to carry
3.1 Khz audio in each direction.
Digital Channel
A circuit-switched communication path which is intended to carry
digital information in each direction.
Call
A connection or attempted connection between two terminal
endpoints on a PSTN or ISDN--for example, a telephone call between
two modems.
CHAP
Challenge Authentication Protocol, a PPP cryptographic
challenge/response authentication protocol in which the cleartext
password is not passed in the clear over the line.
CLID
Calling Line ID, an indication to the receiver of a call as to the
phone number of the caller.
Control Messages
Control messages are exchanged between LAC, LNS pairs, and operate
in-band within the tunnel protocol. Control messages govern
aspects of the tunnel and sessions within the tunnel.
Dial User
An end-system or router attached to an on-demand PSTN or ISDN
which is either the initiator or recipient of a call.
DNIS
Dialed Number Information String, an indication to the receiver of
a call as to what phone number the caller used to reach it.
EAP
Extensible Authentication Protocol, a framework for a family of
PPP authentication protocols, including cleartext,
challenge/response, and arbitrary dialog sequences.
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L2TP Access Concentrator (LAC)
A device attached to one or more PSTN or ISDN lines capable of PPP
operation and of handling the L2TP protocol. The LAC needs only
implement the media over which L2TP is to operate to pass traffic
to one or more LNS's. It may tunnel any protocol carried within
PPP.
L2TP Network Server (LNS)
An LNS operates on any platform capable of PPP termination. The
LNS handles the server side of the L2TP protocol. Since L2TP
relies only on the single media over which L2TP tunnels arrive,
the LNS may have only a single LAN or WAN interface, yet still be
able to terminate calls arriving at any LAC's full range of PPP
interfaces (async, synchronous ISDN, V.120, etc.).
Network Access Server (NAS)
A device providing temporary, on-demand network access to users.
This access is point-to-point using PSTN or ISDN lines.
PAP
Password Authentication Protocol, a simple PPP authentication
mechanism in which a cleartext username and password are
transmitted to prove identity.
Session
L2TP is connection-oriented. The LNS and LAC maintain state for
each user that is attached to an LAC. A session is created when
an end-to-end PPP connection is attempted between a dial user and
the LNS, or when a outbound call is initiated. The datagrams
related to a session are sent over the tunnel between the LAC and
LNS.
Quality of Service (QOS)
A given Quality of Service level is sometimes required for a given
user being tunneled between an LNS-LAC pair. For this scenario, a
unique L2TP tunnel is created (generally on top of a new SVC) and
encapsulated directly on top of the media providing the indicated
QOS.
Switched Virtual Circuit (SVC)
An L2TP-compatible media on top of which L2TP is directly
encapsulated. SVC's are dynamically created, permitting tunnel
media to be created dynamically in response to desired LNS-LAC
connectivity requirements.
Tunnel
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A tunnel is defined by an LNS-LAC pair. The tunnel carries PPP
datagrams between the LAC and the LNS; many sessions can be
multiplexed over a single tunnel. A control connection operating
in-band over the same tunnel controls the establishment, release,
and maintenance of sessions and of the tunnel itself.
2.0 Problem Space Overview
In this section we describe in high level terms the scope of the
problem that will be explored in more detail in later sections.
2.1 Initial Assumptions
We begin by assuming that Internet access is provided by an ISP and
that the ISP wishes to offer services other than traditional
registered IP address based services to dial-up users of the network.
We also assume that the user of such a service wants all of the
security facilities that are available to him in a dedicated dial-up
configuration. In particular, the end user requires:
+ End System transparency: Neither the remote end system nor his
home site hosts should require any special software to use this
service in a secure manner.
+ Authentication as provided via dial-up PPP CHAP, PAP, EAP, or
through other dialogs, for instance, a textual exchange on V.120
before starting PPP. This will include TACACS+ [7] and RADIUS [8]
solutions as well as support for smart cards and one-time passwords.
The authentication should be manageable by the user independently of
the ISP.
+ Addressing should be as manageable as dedicated dial-up solutions.
The address should be assigned by the home site and not the ISP.
+ Authorization should be managed by the home site as it would in a
direct dial-up solution.
+ Accounting should be performed both by the ISP (for billing
purposes) and by the user (for charge-back and auditing).
2.2 Topology
Shown below is a generic Internet with Public switched Telephone
Network (PSTN) access (i.e., async PPP via modems) and Integrated
Services Digital Network (ISDN) access (i.e., synchronous PPP
access). Remote users (either async or ISDN PPP) will access the
Home LAN as if they were dialed into the L2TP Network Server (LNS),
although their physical dial-up is via the ISP Network Access Server.
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...----[L]----+---[L]-----...
|
|
[H]
|
________|________________________
| |
________|__ ______|________
| | | |
| PSTN [R] [R] ISDN |
| Cloud | | Cloud [N]__[U]
| | Internet | |
| | [R] |
[N]______[R] |_____________|
| | |
| | |
[U] |________________________________|
[H] = LNS
[L] = Home LAN(s)
[R] = Router
[U] = Remote User
[N] = ISP Network Access Server
2.3 Providing Virtual dial-up Services--a walk-through
To motivate the following discussion, this section walks through an
example of what might happen when a Virtual dial-up client initiates
access.
The remote user initiates a PPP connection to an ISP via either the
PSTN or ISDN. The Network Access Server (NAS) accepts the connection
and the PPP link is established (L2TP also permits the NAS to check
with an LNS after call indication prior to accepting the call--this
is useful where DNIS or CLID information is available in the incoming
call notification).
The ISP may now undertake a partial authentication of the end
system/user. Only the username field would be interpreted to
determine whether the user requires a Virtual dial-up service. It is
expected--but not required--that usernames will be structured (e.g.
username@company.com). Alternatively, the ISP may maintain a
database mapping users to services. In the case of Virtual dial-up,
the mapping will name a specific endpoint, the LNS.
Alternatively, the ISP may have already determined the target LNS
from DNIS. If the LNS is willing to accept tunnel creation without
any authentication of the caller, the NAS may tunnel the PPP
connection without ever having communicated with the remote user.
If a virtual dial-up service is not required, standard access to the
Internet may be provided.
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If no tunnel connection currently exists to the desired LNS, one is
initiated. L2TP is designed to be largely insulated from the details
of the media over which the tunnel is established; L2TP requires only
that the tunnel media provide packet oriented point-to-point
connectivity. Obvious examples of such media are UDP, Frame Relay
PVC's, or X.25 VC's.
Once the tunnel exists, an unused slot within the tunnel, a "Call
ID", is allocated, and a connect indication is sent to notify the LNS
of this new dial-up session. The LNS either accepts the connection,
or rejects it. Rejection may include a reason indication, which may
be displayed to the dial-up user, after which the call should be
disconnected.
The initial setup notification may include the authentication
information required to allow the LNS to authenticate the user and
decide to accept or decline the connection. In the case of CHAP, the
set-up packet includes the challenge, username and raw response. For
PAP or text dialog, it includes username and clear text password.
The LNS may choose to use this information to complete its
authentication, avoiding an additional cycle of authentication.
If the LAC negotiated PPP LCP before initiating the tunnel, the
initial setup notification may also include a copy of the LCP
CONFREQ's sent in each direction which completed LCP negotiation.
The LNS may use this information to initialize its own PPP state
(thus avoiding an additional LCP negotiation), or it may choose to
initiate a new LCP CONFREQ exchange.
If the LNS accepts the connection, it creates a "virtual interface"
for PPP in a manner analogous to what it would use for a direct-
dialed connection. With this "virtual interface" in place, link
layer frames may now pass over this tunnel in both directions.
Frames from the remote user are received at the POP, stripped of CRC,
link framing, and transparency bytes, encapsulated in L2TP, and
forwarded over the appropriate tunnel.
The LNS accepts these frames, strips L2TP, and processes them as
normal incoming frames for the appropriate interface and protocol.
The "virtual interface" behaves very much like a hardware interface,
with the exception that the hardware in this case is physically
located at the ISP POP. The other direction behaves analogously,
with the LNS encapsulating the packet in L2TP, and the NAS stripping
L2TP before transmitting it out the physical interface to the remote
user. For the remainder of this document, a NAS operating as a peer
to an LNS will be referred to as an L2TP Access Concentrator, or
"LAC".
At this point, the connectivity is a point-to-point PPP session whose
endpoints are the remote user's networking application on one end and
the termination of this connectivity into the LNS's PPP support on
the other. Because the remote user has become simply another dial-up
client of the LNS, client connectivity can now be managed using
traditional mechanisms with respect to further authorization,
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protocol access, and packet filtering.
Accounting can be performed at both the LAC as well as the LNS. This
document illustrates some Accounting techniques which are possible
using L2TP, but the policies surrounding such Accounting are outside
the scope of this specification.
L2TP offers optional facilities which maximize compatibility with
legacy client requirements; L2TP connect notifications for PPP
clients can contain sufficient information for an LNS to authenticate
and initialize its LCP state machine. With these facilities, the
remote user need not be queried a second time for PPP authentication,
nor undergo multiple rounds of LCP negotiation and convergence.
These techniques are intended to optimize connection setup, and are
not intended to deprecate any functions required by the PPP
specification.
3.0 Service Model Issues
There are several significant differences between the standard
Internet access service and the Virtual dial-up service with respect
to authentication, address allocation, authorization and accounting.
The details of the differences between these services and the
problems presented by these differences are described below. The
mechanisms used for Virtual Dial-up service are intended to coexist
with more traditional mechanisms; it is intended that an ISP's POP
can simultaneously service ISP clients as well as Virtual dial-up
clients.
3.1 Security
For the Virtual dial-up service, the ISP pursues authentication only
to the extent required to discover the user's apparent identity (and
by implication, their desired LNS). This may involve no more than
detecting DNIS information when a call arrives, or may involve full
LCP negotiation and initiation of PPP authentication. As soon as the
apparent identity is determined, a connection to the LNS is initiated
with any authentication information gathered by the ISP. The LNS
completes the authentication by either accepting the connection, or
rejecting it.
The LNS may need to protect against attempts by third parties to
establish tunnels to the LNS. Tunnel establishment can include
authentication to protect against such attacks.
3.2 Address Allocation
For an Internet service, the user accepts that the IP address may be
allocated dynamically from a pool of ISP addresses. This model often
means that the remote user has little or no access to their home
network's resources, due to firewalls and other security policies
applied by the home network to accesses from external IP addresses.
For the Virtual dial-up service, the LNS can exist behind the home
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firewall, allocating addresses which are internal (and, in fact, can
be RFC1918 addresses, or non-IP addresses). Because L2TP tunnels
exclusively at the frame layer, the actual policies of such address
management are irrelevant to correct Virtual dial-up service; for all
purposes of PPP protocol handling, the dial-in user appears to have
connected at the LNS.
3.3 Authentication
The authentication of the user occurs in three phases; the first at
the ISP, and the second and optional third at the LNS.
The ISP uses DNIS, CLID, or username to determine that a Virtual
dial-up service is required and initiates the tunnel connection to
the appropriate LNS. Once a tunnel is established, The ISP NAS
allocates a new Call ID and initiates a session by forwarding the
gathered authentication information.
The LNS undertakes the second phase by deciding whether or not to
accept the connection. The connection indication may include CHAP,
PAP, EAP, or textual authentication information. Based on this
information, the LNS may accept the connection, or may reject it (for
instance, it was a PAP request and the username/password are found to
be incorrect).
Once the connection is accepted, the LNS is free to pursue a third
phase of authentication at the PPP layer. These activities are
outside the scope of this specification, but might include an
additional cycle of LCP authentication, proprietary PPP extensions,
or textual challenges carried via a TCP/IP telnet session.
3.4 Accounting
It is a requirement that both the LAC and the LNS be capable of
providing accounting data and hence both may count packets, octets
and connection start and stop times.
Since Virtual dial-up is an access service, accounting of connection
attempts (in particular, failed connection attempts) is of
significant interest. The LNS can reject new connections based on
the authentication information gathered by the LAC, with
corresponding logging. For cases where the LNS accepts the
connection and then continues with further authentication, the LNS
might subsequently disconnect the client. For such scenarios, the
disconnection indication back to the LAC may also include a reason.
Because the LNS can decline a connection based on the authentication
information collected by the LAC, accounting can easily draw a
distinction between a series of failed connection attempts and a
series of brief successful connections. Lacking this facility, the
LNS must always accept connection requests, and would need to
exchange a number of PPP packets with the remote system. Note that
the LNS could use this information to decide to accept the connection
(which protects against most invalid connection attempts) while still
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insisting on running its own CHAP authentication (for instance, to
protect against CHAP replay attacks).
4.0 Protocol Overview
There are two parallel components of L2TP operating over a given
tunnel: control messages between each LAC-LNS pair, and payload
packets between the same LAC-LNS pair. The latter are used to
transport L2TP encapsulated PPP packets for user sessions between the
pair.
The Nr (Next Received) and Ns (Next Sent) fields are always present
in control messages, and are optionally present in payload messages.
A single sequence number state is maintained for all control
messages, and a distinct state is maintained for the payload of each
user session within the tunnel. Each state is initialized so the
first packet is sent with an Ns of 0. Nr is sent reflecting one more
than the last in-order received packet; if sent before any packet is
received it would be 0, indicating that it expects the next new Ns
value received to be 0.
The sequence number state is maintained and updated as packets are
sent. A message (control or payload) with a zero-length body
indicates that the packet is only used to communicate Nr and Ns
fields. The Nr and Ns fields are filled in as above, but the
sequence number state remains the same. For non-zero-length body
messages, the sequence number state is incremented (modulo 2**16)
after it is copied to the packet's Ns field. Thus a zero-length
message's Ns field will reflect one more than the Ns of the last
non-zero-length message sent.
If a new Ns value has been received by the peer, and no packet has
been sent with an Nr value to indicate this reception within 1/4 of
the timeout interval, such a zero-length message is sent.
See Appendix E for some examples of how sequence numbers progress.
4.1 Control Message Overview
Before PPP tunneling can occur between an LAC and LNS, control
messages must be exchanged between them. Control messages are
exchanged over the same tunnel which will be used to forward
payload data once L2TP call control and management information
have been passed. The control messages are responsible for
establishment, management, and release of sessions carried through
the tunnel, as well as status on the tunnel itself. It is the
means by which an LNS is notified of an incoming call at an
associated LAC, as well as the means by which an LAC is instructed
to place an outgoing dial call.
A tunnel may be established by either an LAC (for incoming calls)
or an LNS (for outgoing calls). Following the establishment of
the tunnel, the LNS and LAC configure the tunnel by exchanging
Start-Control-Connection-Request and -Reply messages. These
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messages are also used to exchange information about basic
operating capabilities of the LAC and LNS. Once the control
message exchange is complete, the LAC may initiate sessions by
indicating inbound requests, or the LNS by requesting outbound
calls. Control messages may indicate changes in operating
characteristics of an individual user session with a Set-Link-Info
message. Individual sessions may be released by either the LAC or
LNS, also through control messages.
Independent Call ID values are established for each end of a user
session. The sender of a packet associated with a particular
session places the Call ID established by its peer in the Call ID
header field of all outgoing packets. For the cases where a Call
ID has not yet been assigned from the peer (i.e., during call
establishment of a new session), the Call ID field is sent as 0,
and further fields within the message are used to identify the
session. The Call ID value of 0 is thus special and MUST NOT be
used as an Assigned Call ID.
Two mechanisms provide for detection of tunnel connectivity
problems, one by the reliable transport layer of L2TP and another
by the higher layer. The transport layer of L2TP performs control
message retransmission. If the number of retransmission attempts
for a given control message exceeds a configured maximum value,
the tunnel is reset. This retransmission mechanism exists in both
the LNS and LAC ends of the tunnel. In addition, keepalive
control echo messages are injected into the control stream by the
higher L2TP layer after a certain duration of inactivity on a
given tunnel is detected. A response to the sent keepalive is
expected within a configured time interval. If not received
within the allowed time interval, the tunnel is reset. These two
mechanisms ensure that a connectivity failure between the LNS and
the LAC can be detected at either end of a tunnel in a timely
manner.
It is intended that control messages will also carry management
related information in the future, such as a message allowing the
LNS to request the status of a given LAC; these message types are
not defined in this document.
4.2 Payload Packet Overview
Once a tunnel is established and control messages have completed
tunnel setup, the tunnel can be used to carry user session PPP
packets for sessions involving a given LNS-LAC pair. The "Call
ID" field in the L2TP header indicates to which session a
particular PPP packet belongs. In this manner, PPP packets are
multiplexed and demultiplexed over a single tunnel between a given
LNS-LAC pair. The "Call ID" field value is established during the
exchange of call setup control messages.
It is legal for multiple tunnels to exist between a given LNS-LAC
pair. This is useful where each tunnel is used for a single user
session, and the tunnel media (an SVC, for instance) has specific
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QOS attributes dedicated to a given user. L2TP provides a tunnel
identifier so that individual tunnels can be identified, even when
arriving from a single source LAC or LNS.
The L2TP header also contains optional acknowledgment and
sequencing information that can be used to perform congestion
control over the tunnel. Control messages are used to determine
rate and buffering parameters that are used to regulate the flow
of PPP packets for a particular session over the tunnel. All
implementations MUST implement flow control, but may indicate that
flow control is not desired by omitting the Packet Window Size and
Packet Processing Delay AVP's during call setup. L2TP does not
specify the particular algorithms to use for congestion and flow
control. Suggested algorithms for the determination of adaptive
time-outs to recover from dropped data or acknowledgments on the
tunnel are included in Appendix A of this document.
L2TP does not include a "Receive-Not-Ready" function. It is
expected that the flow-control mechanism used will provide an
adequate "pacing" mechanism so that the sender does not overflow
the receiver's allotted receive window and receive buffers. It is
permissible for the receiving peer to withhold Acks if it is
unable to accept more data for a connection. Thus, unlike for the
Control Message session, the sending peer MUST NOT clear a session
(or the whole tunnel) as a result of not receiving timely
acknowledgments for transmitted packets. The job of detecting a
non-functioning tunnel lies solely with the Control Message
functions of L2TP.
5. Message Format and Protocol Extensibility
L2TP defines a set of control messages sent in packets over the
tunnel between an LNS and a given LAC. The exact technique for
initiating a tunnel between an LNS-LAC pair is specific to the tunnel
media, and specific media are described in section 8.0.
Once media-level connectivity is reached, L2TP message formats define
the protocol for an LAC and LNS to manage the tunnel and its
associated sessions.
In each case where a field is optional, if the field is not present,
its space does not exist in the packet. Existing fields are placed
back-to-back to form the packet.
5.1 AVP
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 an AVP (Attribute-
Value Pair) in the remainder of this document. Each AVP is
encoded as:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|M|H|0|0|0|0| Overall Length | Vendor ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute | Value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [until Overall Length is reached]... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first six bits are a bit mask, describing the general
attributes of the AVP. The M bit, known as the "mandatory" bit,
controls the behavior required of an implementation which receives
an AVP which it does not recognize. If M is set, any session
associated with this AVP MUST be terminated. If the AVP is
associated with the overall tunnel, the entire tunnel (and all
sessions within) MUST be terminated. If M is not set, an
unrecognized AVP should be ignored.
The H bit, known as the "hidden" bit, controls the hiding of the
data in the 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 5.7 describes the procedure for
performing AVP value hiding.
Overall Length encodes the number of octets (including the Overall
Length field itself) contained in this AVP. It is 10 bits,
permitting a maximum of 1024 bytes of data in a single AVP.
Vendor ID is the IANA assigned "SMI Network Management Private
Enterprise Codes" value, encoded in network byte order. The value
0, reserved in this table, corresponds to IETF adopted Attribute
values, defined within this document. Any vendor wishing to
implement 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.
Attribute is the actual attribute, a 16-bit value with a unique
interpretation across all AVP's defined under a given Vendor ID.
Value follows immediately after the Attribute field, and runs for
the remaining octets indicated in the Overall Length (i.e.,
Overall Length minus six octets of header).
AVP's should be kept compact; the combined AVP's within a control
message MUST NOT ever cause a control message's total length to
exceed 1500 bytes in length.
5.2 Control Message Format
Each L2TP control message begins with a 16 octet header portion
followed by zero or more AVP's. 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|1|1|1|1|0|0|0| | Ver | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tunnel ID | Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ns | Nr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type AVP... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... (8 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The T bit MUST be 1, indicating a control message. The next four
bits MUST be set to 1, making the header more compatible in
encoding with the payload message (defined in the next section).
The K bit is optional, documented below. The bit following the K
bit MUST be 0.
Ver MUST be the value 002, indicating a version 1 L2TP message
(values 000 and 001 are reserved to permit detection of L2F [2]
and PPTP [3] packets if they arrive intermixed).
Length is the overall length of the message, including header,
message type AVP, plus any additional AVP's associated with a
given control message type.
Tunnel ID and Call ID identify the tunnel and user session within
the tunnel to which a control message applies. If a control
message does not apply to a single user session within the tunnel
(for instance, a Stop-Control-Connection-Request message), Call ID
MUST be set to 0. If an Assigned Tunnel ID has not yet been
received from the peer, Tunnel ID MUST be set to 0. Once an
Assigned Tunnel ID is received, all further packets MUST be sent
with Tunnel ID set to the indicated value.
Nr and Ns reflect the currently transmitted packet and latest
received packet respectively. See section 4.0.
5.3 Payload Message Format
PPP payload packets tunneled within L2TP have a smaller
encapsulation than the L2TP control message header, reducing
overhead of L2TP during the life of a tunneled PPP session. The
MTU for the user data packets encapsulated in L2TP is expected to
be 1500 octets, not including L2TP and media encapsulation. The
smallest L2TP encapsulation is 2 octets; the largest is 14 octets
(plus padding bytes, if present). See section 8.1 for further MTU
considerations.
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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|I|C|F|K|O|P| | Ver | Length (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tunnel ID (opt) | Call ID (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ns (opt) | Nr (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset Size (opt) | Offset pad... (opt) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The T bit MUST be 0, indicating payload.
Ver MUST be 002, indicating version 1 of the L2TP protocol.
If the L bit is set, the Length field is present, indicating the
total length of the received packet.
The I and C bits indicate the presence of Tunnel ID and Call ID,
respectively. The interpretation of these fields, if present, is
described in section 5.2.
If the F bit is set, both the Nr and Ns fields are present. Ns
indicates the sequence number of the packet being sent. The
current packet will be this sequence number if the payload size is
non-zero, otherwise this packet is only an acknowledgment
(sequence number does not advance). Nr indicates the next packet
sequence number to be received (if the last data packet had Ns set
to 1, the Nr sent back would be 2). Together, these fields can be
used to handle out-of-order packets, and to provide flow control
for the connection.
An L2TP peer setting the F bit, and placing Nr and Ns fields in
its messages, MUST have previously received or sent a Receive
Window Size AVP from its peer during establishment of the session.
The Nr and Ns fields are present and updated as described in
section 4.0 if either side has specified an intention to do
payload flow control.
The K bit MUST be set to 0. This bit position represents a
function no longer present in L2TP.
The Offset Size field is present if the O bit is set in the header
flags. This field specifies the number of bytes past the L2TP
header at which the payload data is expected to start. It is
recommended that data thus skipped be initialized to 0's. If
Offset Size is 0, or the O bit is not set, the first byte
following the last byte of L2TP header is the first byte of
payload data.
If the P bit is set, this packet should receive preferential
treatment in its queueing for transmission. LCP echo requests
used as a keepalive for the link, for instance, should generally
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be sent with this bit set. Without it, a temporary interval of
congestion of the transmission queues could result in the
interference with keepalive messages and unnecessary loss of the
link.
5.4 Control Message Types
Control message and AVP types defined in this specification exist
under Vendor ID 0, indicating IETF defined behavior. The actual
message and AVP semantics are defined in the next section. This
section includes tables that summarize all currently defined
message and AVP types.
Each message type entry in the table below consists of 3 columns.
The "Num." column indicates the integer value assigned to this
message type. i The "(Abbrev)" column lists the abbreviation for
each message type used in the AVP table that follows.
The number in the "Value" column is placed in the Value field of
the Message Type AVP. This AVP MUST be the first AVP in a
message. The currently defined control message types, grouped by
function, are:
Control Connection Management
1 (SCCRQ) Start-Control-Connection-Request
2 (SCCRP) Start-Control-Connection-Reply
3 (SCCCN) Start-Control-Connection-Connected
4 (StopCCRQ) Stop-Control-Connection-Request
5 (StopCCRP) Stop-Control-Connection-Reply
6 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 (CCRQ) Call-Clear-Request
14 (CDN) Call-Disconnect-Notify
Error Reporting
15 (WEN) WAN-Error-Notify
PPP Session Control
16 (SLI) Set-Link-Info
5.5 AVP Summary
The following table lists all standard L2TP attributes currently
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defined. The "Attr" column indicates the integer value assigned to
this attribute. The "M" column indicates the setting of the
"Mandatory" bit of the AVP header for each attribute. The "Len"
field indicates the size of the AVP including the AVP header. A
"+" in this column indicates that the length varies depending upon
the length of the actual contents of the value field.
Under the Name column, the parenthesized lists of message type
abbreviations indicate the message types that utilize each AVP
(See command table above). An abbreviation shown in mixed or
uppercase letters indicates that the corresponding AVP MUST be
present in this message type; All lowercase indicates that the AVP
may optionally appear in this message type. A "+" appended to a
message type abbreviation indicates that the AVP is only mandatory
in a "positive" (non-error) condition -- The AVP is optional in a
message indicating an error condition.
A brief summary of the type and contents of the value field for
each attribute is also given for each entry. Refer to the
individual message type descriptions that appear in Section 6 for
further details about the use of a particular AVP in a particular
message type.
Attr M Len Attribute Name (usage)
0 1 8 Message Type (ALL MESSAGES)
16 bit integer value indicating the message type, as defined in
table above. MUST be the first AVP in each message
1 1 10+ Result Code (CDN, ICRP, OCCN, OCRP, SCCRP, StopCCRP,
StopCCRQ)
16 bit Integer value indicating result of corresponding request
or reason for issuing a request, 16 bit integer General Error
code and an optional ASCII string error message. See Result
and General Error code tables below.
2 1 8 Protocol Version (SCCRP, SCCRQ)
8 bit L2TP Protocol and Revision numbers
3 1 10 Framing Capabilities (SCCRP, SCCRQ)
32 bit bitmask indicating supported framing types (e.g.,
synchronous and asynchronous)
4 1 10 Bearer Capabilities (SCCRP, SCCRQ)
32 bit bitmask indicating supported bearer types (e.g., analog
and digital)
5 0 14 Tie Breaker (sccrq)
8 byte value used to break control connection establishment
collisions
6 0 8 Firmware Revision (sccrp, sccrq)
16 bit integer representing vendor's firmware revision
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7 0 6+ Host Name (sccrp, sccrq)
ASCII string name (e.g., DNS name) of issuer
8 0 6+ Vendor Name (sccrp, sccrq)
ASCII string describing issuing device
9 1 8 Assigned Tunnel ID (SCCRP+, SCCRQ, StopCCRQ)
16 bit integer tunnel ID assigned by sender
10 1 8 Receive Window Size (iccn, icrp, occn, ocrq, sccrp,
sccrq)
16 bit integer receive window size offered by sender for a
given call or control session
11 1 6+ Challenge (sccrp, sccrq)
1 or more octet value issued by sender wishing to authenticate
control session peer
12 0 9+ Q.931 Cause Code (cdn, occn)
16 bit cause code, 1 octet cause message, and optional ASCII
advisory message
13 1 22 Challenge Response (scccn, sccrp)
16 octet CHAP type response to peer's Challenge
14 1 8 Assigned Call ID (CCRQ, CDN, ICRP+, ICRQ, OCRP+, OCRQ)
16 bit integer ID assigned to a call by sender
15 1 6+ Call Serial Number (ICRQ, OCRQ)
1 or more octet identifier assigned to a call
16 1 10 Minimum BPS (OCRQ)
32 bit integer indicating lowest acceptable line speed for call
17 1 10 Maximum BPS (OCRQ)
32 bit integer indicating highest acceptable line speed for
call
18 1 10 Bearer Type (ICRQ, OCRQ)
Indicates bearer type (i.e., analog or digital) for call
19 1 10 Framing Type (ICCN, OCCN+, OCRQ)
Indicates framing type (i.e., synchronous or asynchronous) for
call
20 1 8 Packet Processing Delay (iccn, icrp, occn, ocrq)
16 bit integer estimate of processing time of full window of
received packets by sender
21 1 6+ Dialed Number (icrq, OCRQ)
ASCII string phone number called or to be called
22 1 6+ Dialing Number (icrq)
ASCII string phone number of caller
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23 1 6+ Sub-Address (icrq, ocrq)
ASCII string containing additional dialing information
24 1 10 Connect Speed (ICCN, OCCN+, OCRP+)
16 bit integer actual line speed of connection
25 1 10 Physical Channel ID (icrq, ocrp)
16 bit vendor specific physical device identifier used for call
26 0 6+ Initial LCP Confreq (iccn)
Octet string containing initial CONFREQ received from client
27 0 6+ Last Sent LCP Confreq (iccn)
Octet string containing final CONFREQ sent to client
28 0 6+ Last Received LCP Confreq (iccn)
Octet string containing final CONFREQ received from client
29 1 8 Proxy Authen Type (ICCN)
16 bit integer code indicating client authentication type
negotiated (e.g., PAP, CHAP)
30 0 6+ Proxy Authen Name (iccn)
ASCII string containing name returned by client in
authentication response
31 0 6+ Proxy Authen Challenge (iccn)
Octet string Challenge presented by LAC to client
32 0 8 Proxy Authen ID (iccn)
16 bit integer of which low order octet is ID presented to
client with Challenge. High order octet must be 0.
33 1 6+ Proxy Authen Response (iccn)
Octet string CHAP response or ASCII string password depending
on authentication type used
34 1 32 Call Errors (WEN)
A reserved 16 bit word set to 0 followed by 6 32 bit integer
connection error counters
35 1 16 ACCM (SLI)
A reserved 16 bit word set to 0 followed by 2 32 bit bitmasks
containing Send and Receive ACCM values respectively
36 1 6+ Random Vector (all messages)
Variable length octet string containing a random sequence of
values used to accomplish the optional "hiding" of other AVP
values (See "H" bit description)
5.6 Result and Error Code Summary
In general, all Reply Message types contain a Result Code AVP which
indicates the result of the requested operation. The Result Code can
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indicate that additional information pertaining to an error situation
can be found in the Error Code field of the Result Code AVP. The
meaning of the result code is tabulated under the specific type of
message containing the result. Each 16-bit Result Code is
immediately followed (in the same AVP) by a 16-bit General Error code
value.
General error codes pertain to types of errors which 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 Call ID is invalid in this context
6 - A generic vendor-specific error occurred in the LAC
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.
If the length of the Result Code AVP specifies that the Value field
is more than four octets in length, the remaining bytes after the
General Error Code field are an arbitrary string providing further
(possibly human readable) text associated with the condition.
Generally, when a General Error Code of 6 is used, additional
information about the error will be included in the Result Code AVP
in the Optional Message field that follows the Error Code field.
5.7 Hiding of AVP values
The H ("Hidden") bit in the header of each AVP in a control message
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 ID's. The H bit MUST NOT be set in the Random
Vector AVP.
The H bit MUST only be set if tunnel authentication was used and,
therefore, a shared secret exists between the peers on either end of
the tunnel. Therefore, the H bit MUST NOT be set in AVP's contained
within the Start-Control-Connection-Request, -Reply, and -Connected
messages. If the H bit is set in any AVP(s) in a given command
message, a Random Vector AVP must also be present in the message and
MUST preceed the first AVP having an H-bit of 1.
The following mechanism is applied to the contents of the value field
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of each AVP to which hiding is to be applied. An MD5 hash is
performed on the concatenation of:
- the 2 octet Attribute number of the AVP
- the shared authentication secret
- and 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 can 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 Value and placed in the Value field of the AVP. If
the Value is less than 16 octets, the Value is transformed as if the
Value field had been padded to 16 octets before the XOR, but only the
actual bytes present in the Value are modified, and the length of the
AVP is not altered.
If the value 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 value and placed in the
corresponding octets of the Value field of the AVP.
If necessary, this operation is repeated, with each XOR result being
used along with the shared secret to generate the next hash to XOR
the next segment of the value with. This technique results in the
content of the AVP being obscured, although the length of the AVP is
still known.
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. For more
details on this hiding method, consult the RADIUS [8] RFC.
The Random Vector AVP has the following format:
Random Vector
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 6 + String length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 36 | Random Octet String ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Random Vector AVP may be used in any message type. The Attribute
value is 36 and it is marked mandatory. It is used to enable the
hiding of the values of arbitrary AVPs. It MUST preceed any AVP
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containing an AVP with the H-bit set but it MUST NOT itself have the
H-bit set. More than one Random Vector AVP may appear in a message,
in which case the one most closely preceeding an AVP with the H-bit
set pertains to that AVP. The Random Octet String is the random
vector value to use in computing the MD5 hash to retrieve the
original value of a hidden AVP. This string can be of arbitrary
length, although a random vector of at least 16 octets is
recommended.
6.0 Control Connection Protocol Specification
Control Connection messages are used to establish and clear user
sessions. The first set of Control Connection messages are used to
maintain the control connection itself. The control connection is
initiated by an LAC or LNS after establishing the underlying tunnel-
over-media connection.
6.0.1 Control Connection Collision
For the case where an LAC and LNS both initiate tunnels to each
other concurrently, and where the LAC and LNS both determine that
a single tunnel suffices (generally because of media
characteristic considerations, for instance, whether individual
tunnels are needed to gain QOS guarantees for each tunnel), a "tie
breaker" may be undertaken. The details of breaking a tie are
documented with the tunnel establishment messages.
6.0.2 Reliable Delivery of Control Messages
Since L2TP may run across media where packets may be lost, an L2TP
peer sending a control message will retransmit the control message
after deciding that its remote peer has not received it. The
reliable transport mechanism built into L2TP is essentially a
lower layer transport service; the Nr and Ns fields of the control
message header belong to this transport layer. The higher layer
functions of L2TP are not concerned with retransmission or
ordering of control messages.
Each tunnel maintains a queue of control messages to be
transmitted to the 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 fixed (recommended default is 1 second) or
adaptive (see Appendix D) timeout interval expires without
receiving such an acknowledgment, the control message packet is
retransmitted. The retransmitted packet contains the same Ns
value, but the Nr value MUST be updated to reflect any packets
received in the interim.
If no peer response is detected after several retransmissions (a
recommended default is 5, but may be altered due to media
considerations), the tunnel and all sessions within MUST be
cleared.
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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. This permits reliable
delivery of closing messages in environments where these closing
messages might be dropped.
Unlike payload traffic, a peer MUST NOT withhold acknowledgment of
packets as a technique for flow controlling control messages. An
L2TP implementation is expected to be able to keep up with
incoming control messages, possible responding to some with errors
reflecting an inability to honor the requested action.
A sliding window mechanism is used, by default, for control
message transmission. The default is to permit four control
message to be outstanding on a given tunnel. If a peer specifies
a control message window in the Start-Control-Connection-Request
and -Reply packets, up to the indicated number of control messages
may be sent and held outstanding. An implementation may only
support a receive window of 1, but MUST accept at least a window
of 4 from its peer.
The transport layer at a receiving peer is responsible for making
sure that control messages are delivered in order to the higher
layer and that duplicate messages are not delivered to the higher
layer. 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.
6.0.3 Control Message Format
The following Control Connection messages are all sent as packets
on the established tunnel connection between a given LNS-LAC pair.
All data is sent in network order (high order octets first). Any
"reserved" or "empty" fields MUST be sent as 0 values to allow for
protocol extensibility.
Each control message has a header, specified in section 5.2,
including an AVP indicating the type of control message, followed
by zero or more AVP's appropriate for the given type of control
message. Each control message is described first at a block
level, and then with details of each AVP.
6.1 Start-Control-Connection-Request
The Start-Control-Connection-Request is an L2TP control message used
to initialize the tunnel between an LNS and an LAC. The tunnel must
be initialized through the exchange of these control messages before
any other L2TP messages can be issued. The establishment of the
control connection is started by the initiator of the underlying
tunnel.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Control Message Header |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Start-Control-Connection-Request |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol Version |
| Framing Capabilities |
| Bearer Capabilities |
| Tie Breaker |
| Firmware Revision |
| Host Name |
| Vendor Name |
| Assigned Tunnel ID |
| Receive Window Size |
| Challenge |
+-+-+-+-+-+-+-+-+-+-+-+-+
Start-Control-Connection-Request
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 1, indicating Start-
Control-Connection-Request. The Flags indicate a mandatory
option. Details associated with this tunneled session follow in
additional AVP's within the packet.
Protocol Version
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 0x01 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Protocol Version AVP within a Start-Control-Connection-Request
packet indicates the L2TP protocol version available. The
Attribute value is 2, indicating Protocol Version, and is marked
mandatory. This AVP MUST be present. The Value field is a 16-bit
hexadecimal value 0x100, indicating L2TP protocol version 1,
revision 0.
Framing Capabilities
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 3 | 0x00 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 |0|0|0|0|0|0|A|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Framing Capabilities AVP within a Start-Control-Connection-
Request indicates the type of framing that the sender of this
message can provide. The Attribute value is 3, indicating Framing
Capabilities, and is marked mandatory. This AVP MUST be present.
The Value field is a 32-bit quantity, with two bits defined. If
bit A is set, asynchronous framing is supported. If bit S is set,
synchronous framing is supported.
Bearer Capabilities
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | 0x00 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 |0|0|0|0|0|0|A|D|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Bearer Capabilities AVP within a Start-Control-Connection-
Request indicates the bearer capabilities that the sender of this
message can provide. The Attribute value is 4, indicating Bearer
Capabilities, and is marked mandatory. This AVP MUST be present.
The Value field is a 32-bit quantity with two bits defined. If
bit A is set, analog access is supported. If bit D is set,
digital access is supported.
Tie Breaker
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0| 14 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | Tie Break Value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...(64 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Tie Breaker AVP within a Start-Control-Connection-Request
contains a 64-bit Value used to break ties in tunnel establishment
between an LAC-LNS pair. The Attribute value is 5, indicating Tie
Breaker, and is marked optional. This AVP itself is optional.
The 8 byte Value is used as a 64-bit tie breaker value.
If present, it indicates the sender wishes a single tunnel to
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exist between the given LAC-LNS pair, and this value will be used
to choose a single tunnel where both LAC and LNS initiate a tunnel
concurrently. The recipient of a Start-Control-Connection-Request
must check to see if a Start-Control-Connection-Request 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 initiate a tunnel disconnect for their outstanding tunnel.
In the case where a tie breaker is present on both sides, and the
value is equal, both sides MUST initiate tunnel disconnects.
If a tie breaker is received, and the outstanding Start-Control-
Connection-Request had no tie breaker value, the initiator which
included the Tie Breaker AVP "wins".
It is recommended that the Value be set to the MAC address of a
LAN interface on the sender. If no MAC address is available, a
64-bit random number should be used instead.
Firmware Revision
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 6 | Firmware Revision |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Firmware Revision AVP within a Start-Control-Connection-
Request indicates the firmware revision of the issuing device.
The Attribute value is 6, indicating Firmware Revision, and is
marked optional. This AVP itself is optional. The Value field is
a 16-bit 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 revision of the
L2TP software module may be reported instead.
Host Name
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 6 + Host name length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 7 | Host name...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Host Name AVP within a Start-Control-Connection-Request
indicates the name of the issuing LAC or LNS. The Attribute value
is 7, indicating Host Name, and is marked mandatory. This AVP
itself MUST be present. This name should be as broadly unique as
possible; for hosts participating in DNS [4], a hostname with
fully qualified domain would be appropriate.
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Vendor Name
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|6 + vendor name length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 | Vendor name...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Vendor Name AVP within a Start-Control-Connection-Request
contains a vendor specific string describing the type of LAC or
LNS being used. The Attribute value is 8, indicating Vendor Name,
and is marked optional. This AVP itself is optional. The Value
is the indicated number of bytes representing the vendor string.
Assigned Tunnel ID
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 9 | Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Assigned Tunnel ID AVP within a Start-Control-Connection-
Request specifies the Tunnel ID which the receiving peer MUST use
in the Tunnel ID field of all subsequent packets. The Attribute
value is 9, indicating Assigned Tunnel ID, and is marked
mandatory. This AVP MUST be present. Before the Assigned Tunnel
ID AVP is received, packets MUST be sent with a Tunnel ID value of
0. The Value is a 16-bit non-zero Tunnel ID value.
Receive Window Size
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 10 | Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Receive Window Size AVP within a Start-Control-Connection-
Request specifies the receive window size being offered to the
remote peer. The Attribute value is 10, indicating Receive Window
Size, and is mandatory. This AVP itself is optional. Value is a
16-bit word indicating the offered window size. If absent, the
peer must assume a value of 4 for its transmit window. The remote
peer may send the specified number of control messages before it
must wait for an acknowledgment.
Challenge
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 6 + Challenge length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 11 | Challenge...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Challenge AVP within a Start-Control-Connection-Request
indicates that the issuing peer wishes to authenticate the tunnel
endpoints. The Attribute value is 11, indicating Challenge, and
is marked mandatory. This AVP is optional. The Value is one or
more octets of challenge value.
6.2 Start-Control-Connection-Reply
The Start-Control-Connection-Reply is an L2TP control message sent in
reply to a received Start-Control-Connection-Request message. This
message contains a result code indicating the result of the control
connection establishment attempt.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Start-Control-Connection-Reply |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol Version |
| Result Code |
| Framing Capabilities |
| Bearer Capabilities |
| Firmware Revision |
| Host Name |
| Vendor Name |
| Assigned Tunnel ID |
| Receive Window Size |
| Challenge |
| Challenge Response |
+-+-+-+-+-+-+-+-+-+-+-+-+
Start-Control-Connection-Reply
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 2, indicating Start-
Control-Connection-Reply. The Flags indicate a mandatory option.
Protocol Version
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 0x01 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Protocol Version AVP within a Start-Control-Connection-Reply
packet indicates the L2TP protocol version available. The
Attribute value is 2, indicating Protocol Version, and the Value
field is a 16-bit hexadecimal value 0x100, indicating L2TP
protocol version 1, revision 0. This AVP MUST be present.
Framing Capabilities
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | 0x00 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 |0|0|0|0|0|0|A|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Framing Capabilities AVP within a Start-Control-Connection-
Reply indicates the type of framing that the sender of this
message can provide. The Attribute is 3, it is a mandatory AVP,
the Value field is a 32-bit quantity, with two bits defined. If
bit A is set, asynchronous framing is supported. If bit S is set,
synchronous framing is supported. This AVP MUST be present if the
Result AVP indicates success.
Bearer Capabilities
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | 0x00 | 0x00 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x00 |0|0|0|0|0|0|A|D|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Bearer Capabilities AVP within a Start-Control-Connection-
Reply indicates the bearer capabilities that the sender of this
message can provide. The Attribute is 4, it is a mandatory AVP,
the Value field is a 32-bit quantity with two bits defined. If
bit A is set, analog access is supported. If bit D is set,
digital access is supported. This AVP MUST be present if the
Result AVP indicates success.
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Firmware Revision
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 6 | Firmware Revision |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Firmware Revision AVP within a Start-Control-Connection-Reply
indicates the firmware revision of the issuing device. The
Attribute is 6, it is not a mandatory AVP, the Value field is a
16-bit integer encoded in a vendor specific format. For devices
which do not have a firmware revision (general purposes computers
running L2TP software modules, for instance), the revision of the
L2TP software module may be reported instead. This AVP is
optional.
Host Name
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 6 + Host name length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 7 | Host name...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Host Name AVP within a Start-Control-Connection-Reply
indicates the name of the issuing LAC or LNS. See the notes in
section 6.1 concerning Host Name contents. It is encoded as the
Attribute 7, mandatory, with the indicated number of bytes
representing the host name string. This AVP MUST be present.
Vendor Name
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|6 + Vendor name length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 |Vendor name...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Vendor Name AVP within a Start-Control-Connection-Reply
contains a vendor specific string describing the type of LAC or
LNS being used. It is encoded as the Attribute 8, not mandatory,
with the indicated number of bytes representing the vendor string.
This AVP is optional.
Assigned Tunnel ID
0 1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 9 | Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Assigned Tunnel ID AVP within a Start-Control-Connection-Reply
specifies the Tunnel ID which the receiving peer MUST use in all
subsequent packets. It is encoded as the Attribute 9, mandatory,
with a 16-bit non-zero Tunnel ID value. This AVP MUST be present
if the Result Code indicates "Successful channel establishment".
Receive Window Size
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 10 | size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Receive Window Size AVP within a Start-Control-Connection-
Reply specifies the receive window size being offered to the
remote peer. The Attribute value is 10, indicating Receive Window
Size, and is mandatory. This AVP itself is optional. Value is a
16-bit word indicating the offered window size. If absent, the
peer must assume a value of 4 for its transmit window. The remote
peer may send the specified number of control messages before it
must wait for an acknowledgment.
Result Code
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 + Message length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | Optional Message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Result Code AVP within a Start-Control-Connection-Reply packet
indicates the result of the control channel establishment attempt.
It is encoded as Attribute 1, indicating a Result Code AVP. This
AVP is mandatory and MUST be present. The Result Code is a 16-bit
word. The 16-bit word following the Result Code field contains
the Error Code value. The Result Code value indicates whether the
Error Code value is meaningful or not. If it is not meaningful it
MUST be set to a value of 0. An optional error message can follow
the Error Code field. Its presence and length is indicated by the
value of the AVP length field.
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Result code values are:
1 - Successful channel establishment
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
Challenge
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 6 + Challenge length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 11 | Challenge...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Challenge AVP within a Start-Control-Connection-Reply
indicates that the peer wishes to authenticate the tunnel
initiator. It is encoded as the Attribute 11, mandatory, with at
least one byte of challenge value embedded. If this AVP is not
present, it indicates to the receiving peer that the sender does
not wish to authenticate that peer.
Challenge Response
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 22 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 13 | Response... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Response... (128 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Response AVP within a Start-Control-Connection-Reply packet
provides a response to a challenge received. The Attribute value
is 13, indicating Response, and the Value field is a 128-bit value
reflecting the CHAP-style response to the challenge. This AVP
marked mandatory, and MUST be present if a challenge was received
and this Start-Control-Connection-Reply indicates success. For
purposes of the ID value in the CHAP response calculation, the
fixed value 0 MUST be used.
6.3 Start-Control-Connection-Connected
The Start-Control-Connection-Connected message is an L2TP control
message sent in reply to a received Start-Control-Connection-Reply
message. This message provides closure to the tunnel establishment
process, and includes a challenge response if the peer sent a
challenge in the Start-Control-Connection-Reply message.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Start-Control-Connection-Connected |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Challenge Response |
+-+-+-+-+-+-+-+-+-+-+-+-+
Start-Control-Connection-Connected
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 3, indicating Start-
Control-Connection-Connected. The Flags indicate a mandatory
option.
Challenge Response
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 22 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 13 | Response... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Response... (128 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Challenge Response AVP within a Start-Control-Connection-
Connected packet provides a response to a challenge received. The
Attribute value is 13, indicating Response, and the Value field is
a 128-bit value reflecting the CHAP-style response to the
challenge. This AVP is marked mandatory, and MUST be present if a
challenge was received, otherwise MUST be omitted. For purposes
of the ID value in the CHAP response calculation, the fixed value
0 MUST be used.
6.4 Stop-Control-Connection-Request
The Stop-Control-Connection-Request is an L2TP control message sent
by one peer of an LAC-LNS control connection to inform the other peer
that the control connection should be closed. In addition to closing
the control connection, all active user calls are implicitly cleared.
The reason for issuing this request is indicated in the Result Code
AVP.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Stop-Control-Connection-Request |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assigned Tunnel ID |
| Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+
Stop-Control-Connection-Request AVP
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 4, indicating Stop-
Control-Connection-Request. The Flags indicate a mandatory
option.
Assigned Tunnel ID
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 9 | Tunnel ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Attribute value is 9, indicating Assigned Tunnel ID, and is
marked mandatory. This AVP MUST be present. The Value MUST be
the same Assigned Tunnel ID first sent to the receiving peer.
This AVP permits the peer to identify the appropriate tunnel even
if Stop-Control-Connection-Request must be sent before an Assigned
Tunnel ID is received.
Result Code
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 + Message length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | Optional Message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Result Code AVP within a Stop-Control-Connection-Request
packet indicates the reason for terminating the control channel.
It is encoded as Attribute 1, indicating a Result Code AVP. This
AVP is mandatory and MUST be present. The Result Code is a 16-bit
word. The 16-bit word following the Result Code field contains
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the Error Code value, which for a Stop-Control-Connection-Request
is always 0. An optional message can follow the Error Code field.
Its presence and length is indicated by the value of the AVP
length field. Defined Result Code values are:
1 - General request to clear control connection
2 - Can't support peer's version of the protocol
3 - Requester is being shut down
6.5 Stop-Control-Connection-Reply
The Stop-Control-Connection-Reply is an L2TP control message sent by
one peer of an LAC-LNS control connection upon receipt of a Stop-
Control-Connection-Request from the other peer.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stop-Control-Connection-Reply |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+
Stop-Control-Connection-Reply
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 5, indicating Stop-
Control-Connection-Reply. The Flags indicate a mandatory option.
Result Code
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 + Message length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | Optional Message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Result Code AVP within a Stop-Control-Connection-Reply packet
indicates the result of the control channel close attempt. It is
encoded as Attribute 1, indicating a Result Code AVP. This AVP is
mandatory and MUST be present. The Result Code is a 16-bit word.
The 16-bit word following the Result Code field contains the Error
Code value. The Result Code value indicates whether the Error
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Code value is meaningful or not. If it is not meaningful it
should be set to a value of 0. An optional error message can
follow the Error Code field. Defined Values are:
1 - Control connection closed
2 - Control connection not closed for reason indicated in Error Code
6.6 Hello
The 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 control connection.
Keepalives should be implemented by sending a Hello once every 60
seconds if 60 seconds have passed without sending a message to the
peer. When a Hello is received, it MUST be silently discarded (after
updating any effects of the indicated Nr/Ns values).
Because 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 unable to deliver the Hello across, and will clean
up the tunnel.
Hello messages are global to the tunnel; the Call ID field of these
control messages MUST be 0.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hello |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Hello
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 6 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 6, indicating Hello The
Flags indicate a mandatory option.
6.7 Outgoing-Call-Request
The Outgoing-Call-Request is an L2TP control message sent by the LNS
to the LAC to indicate that an outbound call from the LNS is to be
established. This request provides the LAC with information required
to make the call. It also provides information to the LAC that is
used to regulate the transmission of data to the LNS for this session
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once it is established.
This message is the first in the "three-way handshake" used by L2TP
for establishing outgoing calls. The first message requests the
call; the second indicates that the call was successfully initiated.
The third and final message indicates that the call was established.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outgoing-Call-Request |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assigned Call ID |
| Call Serial Number |
| Minimum BPS |
| Maximum BPS |
| Bearer Type |
| Framing Type |
| Receive Window Size |
| Packet Processing Delay |
| Dialed Number |
| Sub-Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+
Outgoing-Call-Request
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 7 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 7, indicating Outgoing-
Call-Request. The Outgoing-Call-Request encodes a request to an
LAC to establish an outgoing call. The flags indicate a mandatory
option.
Assigned Call ID
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 14 | Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Assigned Call ID AVP encodes the ID being assigned to this
call by the LNS. The Attribute value is 14, indicating Assigned
Call ID, and is marked mandatory. This AVP MUST be present. The
LAC places this value in the Call ID header field of all command
and payload packets that it subsequently transmits over the tunnel
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that belong to this call. The Call ID value is an identifier
assigned by the LNS to this session. It is used to multiplex and
demultiplex data sent over that tunnel between the LNS and LAC.
Call Serial Number
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 6 + Number length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 15 | Number...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Call Serial Number AVP encodes an identifier assigned by the LNS to
this call.
Attribute is 15, indicating Call Serial Number, and is marked mandatory.
This AVP MUST be present.
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 LNS and LACs. Other
identification information may also be prepended.
Minimum BPS
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 16 | BPS (H) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BPS (L) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Minimum BPS AVP encodes the lowest acceptable line speed for this
call. Attribute is 16, Minimum BPS, and is marked mandatory.
This AVP MUST be present. The BPS value indicates the speed in
bits/second.
Maximum BPS
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 17 | BPS (H) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BPS (L) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Maximum BPS AVP encodes the highest acceptable line speed for this
call. Attribute is 17, indicating Maximum BPS, and is marked
mandatory. This AVP MUST be present. The BPS value indicates the
speed in bits/second.
Bearer Type
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 18 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0|A|D|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Bearer Type AVP encodes the bearer type for the requested call.
The value bit field Attribute is 18, indicating Bearer Type, and
is marked mandatory. This AVP MUST be present. The Value is a
32-bit quantity indicating the bearer capability required for this
outgoing call. If set, bit A indicates that the call should be on
an analog channel. If set, bit D indicates that the call should
be on a digital channel. Both may be set, indicating that the
call can be of either type.
Framing Type
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 19 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0|A|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Framing Type AVP encodes the framing type for the requested call.
Attribute is 19, indicating Framing Type, and is marked mandatory.
This AVP MUST be present. The 32-bit field indicates the type of
PPP framing to be used for the outgoing call. Bit A if set
indicates that asynchronous framing should be used. Bit S is set
indicates that synchronous framing should be used.
Receive Window Size
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 10 | Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Receive Window Size AVP encodes the window size being advertised
by the LNS for this call. Attribute is 10, indicating Receive
Window Size, and is marked mandatory. This AVP is optional. The
Size value indicates the number of received data packets the LNS
will buffer for this call, which is also the maximum number of
data packets the LAC should send before waiting for an
acknowledgment. The absence of this AVP indicates that Sequence
and Acknowledgment Numbers are not to be used in the payload
session for this call.
Packet Processing Delay
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 20 | Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Packet Processing Delay AVP encodes the delay LNS has for
processing a window full of packets sent by the LAC. Attribute is
20, indicating Packet Processing Delay, and is marked mandatory.
This AVP is optional. The Delay value is specified in units of
1/10 seconds. Refer to Appendix A for a description of how this
value is determined and used.
Dialed Number
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0|6 + Phone Number length| 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 21 | Phone Number..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Phone Number AVP encodes the phone number to be called. Attribute
is 21, indicating Phone Number, and is marked mandatory. This AVP
MUST be present. The Phone Number value is an ASCII string.
Sub-Address
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0|6 + Sub-Address length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 23 |Sub-Address ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Address AVP encodes additional dialing information. Attribute
is 23, indicating Sub-Address, and is marked mandatory. This AVP
is optional. The Sub-Address value is an ASCII string.
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6.8 Outgoing-Call-Reply
The Outgoing-Call-Reply is an L2TP control message sent by the LAC to
the LNS in response to a received Outgoing-Call-Request message. The
reply indicates whether or not the LAC is able to attempt the
outbound call and also returns certain parameters regarding the call
attempt.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outgoing-Call-Reply |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assigned Call ID |
| Result Code |
| Connect Speed |
| Physical Channel Id |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Outgoing-Call-Reply
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 8, indicating Outgoing-
Call-Reply. The Outgoing-Call-Reply message encodes the immediate
result of attempting an outgoing call request. The flags indicate a
mandatory option.
Assigned Call ID
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 14 | Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Assigned Call ID AVP encodes the ID being assigned to this call
by the LAC. Attribute is 14, indicating Assigned Call ID, and is
marked mandatory. This AVP MUST be present if the Result Code
indicates a call is in progress. Call ID value is an identifier,
unique within the tunnel, assigned by the sender to this session.
The remote peer MUST place this Call ID in the Call ID portion of all
future packets it sends associated with this session. It is used to
multiplex and demultiplex data sent over that tunnel between the LNS
and LAC.
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Result Code
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 + Message length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | Optional Message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Result Code AVP within an Outgoing-Call-Request indicates the
result of the outgoing call establishment attempt. It is encoded
as Attribute 1, indicating a Result Code AVP. This AVP is
mandatory and MUST be present. The Result Code is a 16-bit word.
The 16-bit word following the Result Code field contains the Error
Code value. The Result Code value indicates whether the Error
Code value is meaningful or not. If it is not meaningful it
should be set to a value of 0. An optional error message can
follow the Error Code field. Its presence and length is indicated
by the value of the AVP length field. Defined Result Code values
are:
1 - Call attempt in progress
2 - Outgoing Call not attempted for the reason indicated in Error Code
3 - No appropriate facilities are available (temporary condition)
4 - No appropriate facilities are available (permanent condition)
5 - Invalid destination
6 - Outgoing Call administratively prohibited
Connect Speed
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 24 | BPS (H) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BPS (L) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Connect Speed BPS AVP encodes the speed of the facility chosen for
the connection attempt. The Attribute value is 24, indicating
Connect Speed, and is marked mandatory. This AVP MUST be present
if the Result indicates a call is in progress. The BPS is a 32-
bit value indicating the speed in bits/second.
Physical Channel ID
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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 25 | ID (H) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID (L) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Physical Channel ID AVP encodes the vendor specific physical
channel number used for the call. The Attribute value is 25,
indicating Physical Channel ID, and is marked optional. This AVP
itself is optional. ID is a 32-bit value in network byte order.
The value is used for logging purposes only.
6.9 Outgoing-Call-Connected
Outgoing-Call-Connected is an L2TP control message sent by the LAC to
the LNS to indicate the result of a requested outgoing call. The LAC
MUST send the corresponding Outgoing-Call-Reply to the LNS before
sending this message. This message provides information to the LNS
about the particular parameters used for the call. It provides
information to allow the LNS to regulate the transmission of data to
the LAC for this session.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outgoing-Call-Connected |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Result Code |
| Q.931 Cause Code |
| Connect Speed |
| Framing Type |
| Receive Window Size |
| Packet Processing Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Outgoing-Call-Connected
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 16, indicating Outgoing-
Call-Connected. The Outgoing-Call-Connected message encodes the
final result of an outgoing call request. The flags indicate a
mandatory option.
Result Code
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 + Message length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | Optional Message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Result Code AVP within an Outgoing-Call-Connected message
indicates the final result of the outgoing call establishment
attempt. It is encoded as Attribute 1, indicating a Result Code
AVP. This AVP is mandatory and MUST be present. The Result Code
is a 16-bit word. The 16-bit word following the Result Code field
contains the Error Code value. The Result Code value indicates
whether the Error Code value is meaningful or not. If it is not
meaningful it should be set to a value of 0. An optional error
message can follow the Error Code field. Its presence and length
is indicated by the value of the AVP length field. Defined Result
Code values are:
1 - Call established with no errors
2 - Outgoing Call not established for the reason indicated in Error Code
3 - Outgoing Call failed due to no carrier detected
4 - Outgoing Call failed due to detection of a busy signal
5 - Outgoing Call failed due to lack of a dial tone
6 - Outgoing Call was not established within time allotted by LAC
7 - Outgoing Call was connected but no appropriate framing was detected
Q.931 Cause Code
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|9 + Advisory Msg length| 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 12 | Cause Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Msg |Advisory Msg...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Q.931 Cause Code AVP is used to give additional information in
cases of call failure. The Attribute value is 12, indicating
Cause Code, and is marked mandatory. This AVP is optional. It is
only relevant when the LAC uses Q.931/DSS1 for the outbound call
attempt. 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. An additional ASCII text Advisory Message
may also be included (presence indicated by the AVP length) to
further explain the call failure.
Connect Speed
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 24 | BPS (H) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BPS (L) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Connect Speed BPS AVP encodes the final negotiated speed for the
connection. The Attribute value is 24, indicating Connect Speed,
and is marked mandatory. This AVP MUST be present if the call
attempt is successful. The BPS value indicates the speed in
bits/second.
Framing Type
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 19 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0|A|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Framing Type AVP encodes the framing type for the call. The
Attribute value is 19, indicating Framing Type, and is marked
mandatory. This AVP MUST be present if the call attempt is
successful. The value bit field indicates the type of PPP framing
is used for the call. If set, bit A indicates that asynchronous
framing is being used. If set, bit S indicates that synchronous
framing is being used.
Receive Window Size
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 10 | Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Receive Window Size AVP encodes the window size being offered by
the LNS for this call. The Attribute value is 10, indicating
Receive Window Size, and is marked mandatory. The Size is a 16-
bit value indicating the number of received data packets the LAC
will buffer for this call, which is also the maximum number of
data packets the LNS should send before waiting for an
acknowledgment. This AVP MUST be present if and only if Sequence
and Acknowledgment Numbers are to be used in the payload session
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for this call.
Packet Processing Delay
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 20 | Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Packet Processing Delay AVP encodes the delay the LAC expects for
processing a window full of packets sent by the LNS. The
Attribute value is 20, indicating Packet Processing Delay, and is
marked mandatory. This AVP is optional. The Delay value is
specified in units of 1/10 seconds. Refer to Appendix A to see a
description of how this value is determined and used.
6.10 Incoming-Call-Request
Incoming-Call-Request is an L2TP control message sent by the LAC to
the LNS to indicate that an inbound call is to be established from
the LAC. This request provides the LNS with parameter information
for the incoming call.
This message is the first in the "three-way handshake" used by L2TP
for establishing incoming calls. The LAC may defer answering the
call until it has received an Incoming-Call-Reply from the LNS
indicating that the call should be established. This mechanism
allows the LNS to obtain sufficient information about the call before
it is answered to determine whether the call 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
Incoming-Call-Reply message is received; the LAC simply spoofs the
"call indication/answer call" phase in this case.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Incoming-Call-Request |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assigned Call ID |
| Call Serial Number |
| Bearer Type |
| Physical Channel ID |
| Dialed Number |
| Dialing Number |
| Sub-Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Incoming-Call-Request
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 10 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 10, indicating Incoming-
Call-Request. The Incoming-Call-Request message encodes an incoming
call being indicated by the LAC. The flags indicate a mandatory
option.
Assigned Call ID
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 14 | Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Assigned Call ID AVP encodes the Call ID being assigned to call
by the LAC. The Attribute value is 14, indicating Call ID, and is
marked mandatory. This AVP MUST be present. The LNS places this
value in the Call ID header field of all command and payload packets
that it subsequently transmits over the tunnel that belong to this
call. The Call ID value is an identifier assigned by the LAC to this
session. It is used to multiplex and demultiplex data sent over that
tunnel between the LNS and LAC.
Call Serial Number
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 6 + Number length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 15 | Number...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Call Serial Number AVP encodes an identifier assigned by the LAC to
this call. The Attribute value is 15, Call Serial Number, and is
marked mandatory. This AVP MUST be present. Please refer to the
description of this field from section 6.8.
Bearer Type
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 18 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0|A|D|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Bearer Type AVP encodes the bearer type for the incoming call. The
Attribute value is 18, Bearer Type, and is marked mandatory. This
AVP MUST be present. The Value is a 32-bit field indicating the
bearer capability being used by the incoming call. If set, bit A
indicates that the call is on an analog channel. If set, bit D
indicates that the call is on a digital channel.
Physical Channel ID
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 25 | ID (H) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID (L) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Physical Channel ID AVP encodes the vendor specific physical channel
number used for the call. The Attribute value is 25, Physical
Channel ID, and is marked mandatory. The presence of this AVP is
optional. ID is a 32-bit value in network byte order. The value is
used for logging purposes only.
Dialed Number
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0|6 + Phone Number length| 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 21 | Phone Number..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Dialed Number AVP encodes the dialed number for the incoming call,
that is, DNIS. The Attribute value is 21, Dialed Number, and is
marked mandatory. The presence of this AVP is optional. The value
is an ASCII string.
Dialing Number
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0|6 + Phone Number length| 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 22 |Phone Number...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Dialing Number AVP encodes the originating number for the incoming
call, that is, CLID. The Attribute value is 22, Dialing Number, and
is marked mandatory. The presence of this AVP is optional. The
value is an ASCII string.
Sub-Address
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0|6 + Sub-Address length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 23 |Sub-Address ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sub-Address AVP encodes additional dialing information. The
Attribute value is 23, Sub-Address, and is marked mandatory. The
presence of this AVP is optional. The Sub-Address value is an ASCII
string.
6.11 Incoming-Call-Reply
The Incoming-Call-Reply is an L2TP control message sent by the LNS to
the LAC in response to a received Incoming-Call-Request message. The
reply indicates the result of the incoming call attempt. It also
provides information to allow the LAC to regulate the transmission of
data to the LNS for this session.
This message is the second in the three-way handshake used by L2TP
for establishing incoming calls. It indicates to the LAC whether the
call should be answered or not.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Incoming-Call-Reply |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assigned Call ID |
| Result Code |
| Receive Window Size |
| Packet Processing Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Incoming-Call-Reply
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 11 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 10, indicating Incoming-
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Call-Reply. The Incoming-Call-Reply message encodes a response by
the LNS to the incoming call indication given by the LAC. The flags
indicate a mandatory option.
Assigned Call ID
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 14 | Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Assigned Call ID AVP encodes the ID being assigned to call by the
LNS. The Attribute value is 14, Assigned Call ID, and is marked
mandatory. This AVP MUST be present if the Result Code indicates the
call was successful. The LAC places this value in the Call ID header
field of all command and payload packets that it subsequently
transmits over the tunnel that belong to this call. The Call ID
value is an identifier assigned by the LNS to this session. It is
used to multiplex and demultiplex data sent over that tunnel between
the LNS and LAC.
Result Code
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 + Message length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | Optional Message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Result Code AVP within an Incoming-Call-Reply message
indicates the result of the incoming call establishment attempt.
It is encoded as Attribute 1, indicating a Result Code AVP. This
AVP is mandatory and MUST be present. The Result Code is a 16-bit
word. The 16-bit word following the Result Code field contains
the Error Code value. The Result Code value indicates whether the
Error Code value is meaningful or not. If it is not meaningful it
should be set to a value of 0. An optional error message can
follow the Error Code field. Its presence and length is indicated
by the value of the AVP length field. Defined Result Code values
are:
1 - The LAC should answer the incoming call
2 - The Incoming Call should not be established due to the
reason indicated in Error Code
3 - The LAC should not accept the incoming call. It should
hang up or issue a busy indication
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Receive Window Size
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 10 | Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Message... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Receive Window Size AVP encodes the receive window size being
offered by the LNS for this call. The Attribute value is 10,
Receive Window Size, and is marked mandatory. The Size value
indicates the number of received data packets the LNS will buffer
for this call, which is also the maximum number of data packets
the LAC should send before waiting for an acknowledgment. This
AVP is optional if Sequence and Acknowledgment Numbers are not to
be used in the payload session for this call, or if the Result AVP
indicates failure.
Packet Processing Delay
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 20 | Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Packet Processing Delay AVP encodes the delay the LNS expects for
processing a window full of packets sent by the LAC. The
Attribute value is 20, Packet Processing Delay AVP, and is marked
mandatory. The presence of this AVP is optional. The Delay value
is specified in units of 1/10 seconds. Refer to Appendix A to see
a description of how this value is determined and used.
6.12 Incoming-Call-Connected
The Incoming-Call-Connected message is an L2TP control message sent
by the LAC to the LNS in response to a received Incoming-Call-Reply.
It provides information to the LNS about particular parameters used
for the call. It also provides information to allow the LNS to
regulate the transmission of data to the LAC for this session.
This message is the third in the three-way handshake used by L2TP for
establishing incoming calls. It provides a mechanism for providing
the LNS with additional information about the call that cannot, in
general, be obtained at the time the Incoming-Call-Request is issued
by the LAC.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Incoming-Call-Connected |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Connect Speed |
| Framing Type |
| Receive Window Size |
| Packet Processing Delay |
| Initial 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Incoming-Call-Connected
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 11, indicating Incoming-
Call-Connected. The Incoming-Call-Connected message encodes a
response by the LAC to the Incoming-Call-Reply message sent by the
LAC. The flags indicate a mandatory option.
Connect Speed
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 24 | BPS (H) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BPS (L) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Connect Speed BPS AVP encodes the speed for the connection. The
Attribute value is 24, Connect Speed, and is marked mandatory. This
AVP MUST be present. The value is a 32-bit quantity indicating the
speed in bits/second.
Framing Type
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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 19 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0|A|S|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Framing Type AVP encodes the framing type for the call. The
Attribute value is 19, Framing Type, and is marked mandatory. This
AVP MUST be present. The value is a 32-bit bit field indicating the
type of PPP framing used for the call. If set, bit A indicates that
asynchronous framing is being used. If set, bit S indicates that
synchronous framing is being used.
Receive Window Size
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 10 | Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Receive Window Size AVP encodes the window size being offered by the
LAC for this call. The Attribute value is 10, Receive Window Size,
and is marked mandatory. This AVP is optional if Sequence and
Acknowledgment Numbers are not to be used in the payload session for
this call. The 16-bit Size value indicates the number of received
data packets the LAC will buffer for this call, which is also the
maximum number of data packets the LNS should send before waiting for
an acknowledgment.
Packet Processing Delay
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 20 | Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Packet Processing Delay AVP encodes the delay the LAC expects for
processing a window full of packets sent by the LNS. The Attribute
value is 20, Packet Processing Delay, and is marked mandatory. The
presence of this AVP is optional. The 16-bit Delay value is
specified in units of 1/10 seconds. Refer to Appendix A to see a
description of how this value is determined and used.
Initial LCP Confreq
0 1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|6 + LCP confreq length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 26 | LCP confreq...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The LAC may have answered the phone call and negotiated LCP with the
dial-in client in order to establish the client's apparent identity.
In this case, this option may be included to indicate the link
properties the client requested in its initial LCP CONFREQ request.
The Attribute value is 26, Initial LCP Confreq, and is marked
optional. The presence of this AVP is optional. The Value field is
a copy of the body of the initial CONFREQ received, starting at the
first option within this packet's body.
Last Sent LCP Confreq
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|6 + LCP confreq length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 27 | LCP confreq...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
See Initial LCP Confreq above for rationale. The Attribute value is
27, Last Sent LCP Confreq, and is marked optional. The presence of
this AVP is optional. The Value field is a copy of the body of the
final CONFREQ sent to the client to complete LCP negotiation,
starting at the first option within this packet's body.
Last Received LCP Confreq
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|6 + LCP confreq length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 28 | LCP confreq...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
See Initial LCP Confreq above for rationale. The Attribute value is
28, Last Received LCP Confreq, and is marked optional. The presence
of this AVP is optional. The Value field is a copy of the body of
the final CONFREQ received from the client to complete LCP
negotiation, starting at the first option within this packet's body.
Proxy Authen Type
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 29 | Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Attribute value is 29, Proxy Authen Type, and is marked
mandatory. This AVP MUST be present. The value Type is a 16-bit
word, holding a value:
1 - Textual username/password exchange
2 - PPP CHAP
3 - PPP PAP
4 - None
Associated AVP's for each type of authentication follow.
Proxy Authen Name
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0| 6 + Name length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 30 | Name...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Attribute value is 30, Proxy Authen Name, and is marked
mandatory. This AVP MUST be present for Proxy Authen Type values 1,
2, and 3. The Name field contains the name specified in the client's
authentication response. Note that the AVP H bit may be desirable
for obscuring the cleartext name.
Proxy Authen Challenge
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0| 6 + Challenge length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 31 | Challenge...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Attribute value is 31, Proxy Authen Challenge, and is marked
mandatory. The AVP itself MUST be present for Proxy authen type 2.
The Challenge field contains the value presented to the client by the
LAC.
Proxy Authen ID
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 32 | ID |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Attribute value is 32, Proxy Authen ID, and is marked mandatory.
The AVP itself MUST be present for Proxy authen types 2 and 3. For
CHAP, the ID field contains the byte ID value presented to the client
by the LAC in its Challenge. For PAP, it is the Identifier value of
the Authenticate-Request. The most significant 8 bits of ID MUST be
0, and are reserved.
Proxy Authen Response
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 6 + Response length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 33 | Response...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Attribute value is 33, Proxy Authen Response, and is marked
mandatory. The AVP itself MUST be present for Proxy authen types 1,
2, and 3. The Response field contains the client's response to the
challenge. For Proxy authen type 2, 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, use of the AVP H bit is recommended.
6.13 Call-Clear-Request
The Call-Clear-Request is an L2TP control message sent by the LNS to
the LAC indicating that a particular call is to be disconnected. The
call being cleared can be either an incoming or outgoing call, in any
state. The LAC responds to this message with a Call-Disconnect-
Notify message.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Call-Clear-Request |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assigned Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+
Call-Clear-Request
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 13 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 12, indicating Call-Clear-
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Request. The Call-Clear-Request message encodes a request by the LNS
to the LAC to disconnect the call. The flags indicate a mandatory
option.
Assigned Call ID
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 14 | Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This attribute is used to provide the LAC with the Call ID assigned
by the LNS for the call to be cleared in the case where the LNS has
not yet learned the LAC's Call ID for the call. The Attribute value
is 14, Assigned Call ID, and is marked mandatory. This AVP MUST be
present. The value Call ID MUST be the same value sent from the LNS
to the LAC in the initial call setup exchange.
6.14 Call-Disconnect-Notify
The Call-Disconnect-Notify message is an L2TP control message sent by
the LAC to the LNS. It is issued whenever a call is disconnected,
due to the receipt by the LAC of a Call-Clear-Request or for any
other reason. Its purpose is to inform the LNS of the disconnection
and the reason why the disconnection occurred.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Call-Disconnect-Notify |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Result Code |
| Q.931 Cause Code |
| Assigned Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+
Call-Disconnect-Notify
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 14 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 13, indicating Call-
Disconnect-Notify. The Call-Disconnect-Notify message encodes a
disconnect indication from the LAC to the LNS. The flags indicate a
mandatory option.
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Result Code
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 10 + Message length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | Optional Message ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Result Code AVP within a Call-Disconnect-Notify message
indicates the reason for the call disconnect. It is encoded as
Attribute 1, indicating a Result Code AVP. This AVP is mandatory
and MUST be present. The Result Code is a 16-bit word. The 16-
bit word following the Result Code field contains the Error Code
value. The Result Code value indicates whether the Error Code
value is meaningful or not. If it is not meaningful it should be
set to a value of 0. An optional error message can follow the
Error Code field. Its presence and length is indicated by the
value of the AVP length field. Defined Result Code values are:
1 (Lost Carrier) - Call disconnected due to loss of carrier
2 (General Error) - Call disconnected for the reason indicated
in Error Code.
3 (Admin Shutdown) - Call disconnected for administrative
reasons
4 (Request) - Call disconnected due to received Call-Clear-
Request
Q.931 Cause Code
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|9 + Advisory Msg length| 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 12 | Cause Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Msg |Advisory Msg...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Q.931 Cause Code AVP is used to give additional information in
case of unsolicited call disconnection. The Attribute value is
12, Cause Code, and is marked mandatory. The presence of this AVP
is optional. The Cause Code AVP is used to give additional
information about the reason for disconnecting. It is only
relevant when the LAC is using Q.931/DSS1 for the call. This AVP
is optional. 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. An additional Ascii text Advisory Message
may also be included (presence indicated by the AVP length) to
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further explain the reason for disconnecting.
Assigned Call ID
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 14 | Call ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Assigned Call ID which was provided to the LNS from this LAC
is included in the Call-Disconnect-Notify message. This permits a
connection to be terminated even before the LNS has provided its
own Assigned Call ID to this LAC (the Call ID field in the control
message header is 0). The Attribute value is 14, Assigned Call
ID, and is marked mandatory. This AVP MUST be present.
6.15 WAN-Error-Notify
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
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.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| WAN-Error-Notify |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Call Errors |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
WAN-Error-Notify
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 15 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 14, indicating WAN-Error-
Notify. The WAN-Error-Notify message encodes information about line
and other errors detected on the LAC's physical interface to the
client. This message is sent by the LAC to the LNS. The flags
indicate a mandatory option.
Call Errors
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 32 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 34 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CRC Errors |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Framing Errors |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hardware Overruns |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Buffer Overruns |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time-out Errors |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Alignment Errors |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Call Errors AVP is used by the LAC to send error information to
the LNS. The Attribute value is 34, WAN-Error-Notify, and is marked
mandatory. This AVP MUST be present. The value contains the
following fields:
Reserved - Not used, MUST be 0
CRC Errors - Number of PPP frames received with CRC errors since
session was established
Framing Errors - Number of improperly framed PPP packets received
Hardware Overruns - Number of receive buffer over-runs since
session was established
Buffer Overruns - Number of buffer over-runs detected since
session was established
Time-out Errors - Number of time-outs since call was established
Alignment Errors - Number of alignment errors since call was
established
6.16 Set-Link-Info
The Set-Link-Info message is an L2TP control message sent by the LNS
to the LAC to set PPP-negotiated options. Because these options can
change at any time during the life of the call, the LAC MUST be able
to update its internal call information dynamically and update its
behavior on an active PPP session.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| L2TP Control Message Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Set-Link-Info |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACCM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+
Set-Link-Info
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 8 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | 16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Message Type AVP contains a Value of 15, indicating Set-Link-
Info. The Set-Link-Info message encodes ACCM information sent from
the LNS to the LAC after it is negotiated in LCP. The flags indicate
a mandatory option.
ACCM
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0|0|0| 32 | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 35 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Send ACCM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receive ACCM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The ACCM AVP is used by the LNS to inform LAC of the ACCM to the LNS.
The Attribute value is 35, ACCM, and is marked mandatory. This
attribute MUST be present. The value contains Send ACCM and Receive
ACCM fields. The send ACCM value should be used by the LAC to
process packets it is 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.
7.0 Control Connection State Machines
The control messages defined in section 6 are exchanged by way of state
tables defined in this section. 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 semantics
defined in 6.0.2.
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.
Receipt of an invalid or malformed Control Connection message should
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be logged appropriately, and the control connection should be closed
and restarted to ensure recovery into a known state.
7.2 Control Connection States
Control messages are carried over the same media as the payload
messages which are carried following successful connection
establishment. 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 establishes the tunnel. Since either LAC or LNS can be
the originator, a collision can occur. See Section 6.0.1 for a
description of this and its resolution situation.
7.2.1 Control Connection Originator
State Event Action New State
----- ----- ------ ---------
idle Open request Send wait-ctl-reply
Start-Control-
Connection-Request
wait-ctl-reply Collision If terminating, idle
clean-up.
wait-ctl-reply Collision If not terminating, wait-stop-reply
Send Stop-Control-
Connection-Request
wait-ctl-reply Receive If version OK established
Start-Control- Send Start-Control-
Connection-Reply Connection-Connected
wait-ctl-reply Receive If version not OK wait-stop-reply
Start-Control- or bad auth, Send
Connection-Reply Stop-Control-
Connection-Request
established Local terminate Send wait-stop-reply
Stop-Control-
Connection-Request
established Receive Acknowledge via idle
Stop-Control- reliable transport
Connection-
Request
wait-stop-reply Receive Clean-up idle
Acknowledgement
via reliable
transport
idle
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The control connection originator attempts to open a connection to
the peer during idle state. When the connection is open, the
originator transmits a send Start-Control-Connection-Request and
then enters the wait-ctl-reply state.
wait-ctl-reply
The originator checks to see if another connection has been
requested from the same peer, and if so, handles the collision
situation described in Section 6.0.1.
When a Start-Control-Connection-Reply is received, it is examined
for a compatible version. If the version of the reply is lower
than the version sent in the request, the older (lower) version
should be used provided it is supported. If the version in the
reply is earlier and supported, the originator moves to the
established state. If the version is earlier and not supported, a
Stop-Control-Connection-Request SHOULD be sent to the peer and the
originator moves into the wait-stop-reply state.
established
An established connection may be terminated by either a local
condition or the receipt of a Stop-Control-Connection-Request. In
the event of a local termination, the originator MUST send a
Stop-Control-Connection-Request and enter the wait-stop-reply
state.
If the originator receives a Stop-Control-Connection-Request it
MUST run its reliable delivery mechanism to acknowledge the stop
request (see 6.0.2) before destroying the tunnel.
wait-stop-reply
Once the reliable transport has verified that the peer has
received the stop request, the tunnel may be cleared.
7.2.2 Control connection Receiver
State Event Action New State
----- ----- ------ ---------
idle Receive If version not OK idle
Start-Control- send
Connection-Request Start-Control-
Connection-Reply
with error
idle Receive Version OK, send wait-ctl-reply
Start-Control- Start-Control-
Connection-Request Connection-Reply
wait-ctl-reply Receive Clean-up, send idle
Stop-Control- Stop-Control-
Connection-Request Connection-Reply
wait-ctl-reply Receive If auth OK established
Start-Control-
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Connection-Connected
wait-ctl-reply Receive If auth not OK wait-stop-reply
Start-Control- Send Stop-Control-
Connection- Connection-Request
Connected
established Receive Acknowledge via idle
Stop-Control- reliable transport
Connection-
Request
established Local terminate Send wait-stop-reply
Stop-Control-
Connection-Request
wait-stop-reply Receive Clean-up idle
Acknowledgement
via reliable
transport
idle
The control connection receiver waits for an incoming connection
attempt. When notified of a new connection, it should prepare to
receive L2TP messages. When a Start-Control-Connection-Request is
received its version field MUST be examined. If the version is
earlier than the receiver's version and the earlier version can be
supported by the receiver, the receiver SHOULD send a Start-
Control-Connection-Reply.
If the version is earlier than the receiver's version and the
version cannot be supported, the receiver SHOULD send a Start-
Connection-Reply message indicating this error and remain in the
idle state. If the receiver's version is the same as or earlier
than the peer's, the receiver SHOULD send a Start-Control-
Connection-Reply with the receiver's version and enter the wait-
ctl-reply state.
wait-ctl-reply
The peer waits in this state after sending a Start-Control-
Connection-Reply. If it receives a Start-Control-Connection-
Reply, it checks to see if the message is properly authenticated
and, if so, it enters the established state. If authentication
fails, a Stop-Control-Connection-Request with the reason code set
appropriately is sent and wait-stop-reply state is entered. if a
Stop-Control-Connection-Request is received, a Stop-Control-
Connection-Reply is issued and idle state is entered.
established
An established connection may be terminated by either a local
condition or the reception of a Stop-Control-Connection-Request.
In the event of a local termination, the originator MUST send a
Stop-Control-Connection-Request and enter the wait-stop-reply
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state.
If the originator receives a Stop-Control-Connection-Request it
MUST run its reliable delivery mechanism to acknowledge the stop
request (see 6.0.2) before destroying the tunnel.
wait-stop-reply
Once the reliable transport has verified that the peer has
received the stop request, the tunnel may be cleared.
7.3 Timing considerations
Because of 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 6.0.2.
7.4 Incoming calls
An Incoming-Call-Request message is generated by the LAC when an
associated telephone line rings. The LAC selects a Call ID and serial
number and indicates the call bearer type. Modems should always
indicate analog call type. ISDN calls should indicate digital when
unrestricted digital service or rate adaption is used and analog if
digital modems are involved. CLID, DNIS, 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 are not indicative of 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 which also indicates window sizes (see Appendix B). When the
LAC receives the Incoming-Call-Reply, it attempts to connect the call,
assuming the calling party has not hung up. 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.
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 clear
a call, it sends a Call-Clear-Request message and then waits for a
Call-Disconnect-Notify.
7.4.1 LAC Incoming Call States
State Event Action New State
----- ----- ------ ---------
idle Ring OR Send wait-reply
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Ready to indicate Incoming-Call-Request
incoming conn.
wait-reply Receive Clean-up idle
Incoming-Call-Reply
Not Accepting
wait-reply Receive Answer call established
Incoming-Call-Reply Send
Accepting Incoming-Call-Connected
wait-reply Abort Clean-up idle
Send Call-Disconnect-
Notify
established Receive Hang-up and send idle
Call-Clear-Request Call-Disconnect-Notify
established telco line drop Send idle
Call-Disconnect-Notify
established local disconnect Send idle
Call-Disconnect-Notify
The states associated with the LAC for incoming calls are:
idle
The LAC detects an incoming call on one of its telco interfaces.
Typically this means an analog line is ringing or an ISDN TE has
detected an incoming Q.931 SETUP message. The LAC sends an
Incoming-Call-Request message and moves to the wait-reply state.
wait-reply
The LAC receives an Incoming-Call-Reply message indicating non-
willingness to accept the call (general error or don't accept) and
moves back into the idle state. If the reply message indicates that
the call is accepted, the 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 telco connection. The LAC sends a Call-
Disconnect-Notify message
Receipt of a Call-Clear-Request. The LAC sends a Call-
Disconnect-Notify message
A local reason. The LAC sends a Call-Disconnect-Notify message.
7.4.2 LNS Incoming Call States
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State Event Action New State
----- ----- ------ ---------
idle Receive If not accepting idle
Incoming-Call-Request Send
Incoming-Call-Reply
with Error
idle Receive If accepting wait-connect
Incoming-Call-Request Send
Incoming-Call-Reply
wait-connect Receive Clean-up idle
Call-Disconnect-Notify
wait-connect Receive Get ready for data established
Incoming-Call-Connect
established Receive Clean-up idle
Call-Disconnect-Notify
established Local terminate Send wait-
Call-Clear-Request disconnect
wait- Receive Clean-up idle
disconnect Call-Disconnect-Notify
The states associated with the LNS for incoming calls are:
idle
An Incoming-Call-Request message is received. If the request is
not acceptable, an Incoming-Call-Reply is sent back to the LAC and
the LNS remains in the idle state. If the Incoming-Call-Request
message is acceptable, an Incoming-Call-Reply is sent indicating
accept in the result code. The session moves to the wait-connect
state.
wait-connect
If the session is still connected on the LAC, the LAC sends an
incoming call connect 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-Clear-Request.
Once a Call-Clear-Request has been sent, the session enters the
wait-disconnect state.
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wait-disconnect
Once a Call-Disconnect-Notify is received the session moves back
to the idle state.
7.5 Outgoing calls
Outgoing calls are initiated by an LNS and instruct an LAC to place a
call on a telco interface. There are three messages for outgoing calls:
Outgoing-Call-Request, Outgoing-Call-Reply, and Outgoing-Call-Connected.
The LNS sends an Outgoing-Call-Request specifying the dialed party phone
number and subaddress as well as speed and window 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 If cannot service, idle
Outgoing-Call- send Outgoing-Call-Reply
Request with Error
idle Receive If can service, wait-cs-ans
Outgoing-Call- send
Request Outgoing-Call-Reply
wait-cs-ans Telco answer Send established
and framing Outgoing-Call-Connected
detected
wait-cs-ans Call failure Send
Outgoing-Call-Connected idle
with Error
wait-cs-ans Receive Hang-up, send idle
Call-Clear-Request Call-Disconnect-Notify
established Receive Hang-up, send idle
Call-Clear-Request Call-Disconnect-Notify
or detect call
disconnected
The states associated with the LAC for outgoing calls are:
idle
Received Outgoing-Call-Request. If this is received in error,
respond with an Outgoing-Call-Reply with error condition set.
Otherwise, allocate physical channel to dial on and send an
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Outgoing-Call-Reply. Place the outbound call and move to the
wait-cs-ans state.
wait-cs-ans
If the call is not completed or a timer expires waiting for the
call to complete, send an Outgoing-Call-Connected 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-Clear-Request is received, the telco call SHOULD be
released via appropriate mechanisms and a Call-Disconnect-Notify
message SHOULD BE sent to the LNS. If the call is disconnected by
the client or by the telco interface, a Call-Disconnect-Notify
message MUST be sent to the LNS. Return to idle state after
sending the Call-Disconnect-Notify.
7.5.2 LNS Outgoing Call States
State Event Action New State
----- ----- ------ ---------
idle Open request Send wait-reply
Outgoing-Call-Request
wait-reply Receive Clean-up idle
Outgoing-Call-Reply
with Error
wait-reply Receive Null wait-connect
Outgoing-Call-Reply
wait-reply Abort request Send wait-disconnect
Call-Clear-Request
wait-connect Abort request Send
Call-Clear-Request wait-disconnect
wait-connect Receive Get ready for data established
Outgoing-Call-Connected
no Error
wait-connect Receive Clean-up idle
Outgoing-Call-Connected
with Error
established Receive Clean-up idle
Call-Disconnect-Notify
established Local terminate Send wait-disconnect
Call-Clear-Request
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wait-disconnect Receive Clean-up idle
Call-Disconnect-
Notify
The states associated with the LNS for outgoing calls are:
idle
An Outgoing-Call-Request message is sent to the LAC and the
session moves into the wait-reply state.
wait-reply
An Outgoing-Call-Reply is received which indicates an error. The
session returns to idle state. If the Outgoing-Call-Reply does
not indicate an error, the telco call is in progress and the
session moves to the wait-connect state.
wait-connect
An Outgoing-Call-Connected is received which indicates an error.
The session returns to idle state. No telco call is active. If
the Outgoing-Call-Connected does not indicate an error, the telco
call is
established
If a Call-Disconnect-Notify is received, the telco call has been
terminated for the reason indicated in the Result and Cause Codes.
The session moves back to the idle state. If the LNS chooses to
terminate the session, it sends a Call-Clear-Request to the LAC
and then enters the wait-disconnect state.
wait-disconnect
A session disconnection is waiting to be confirmed by the LAC.
Once the LNS receives the Call-Disconnect-Notify message, the
session enters idle state.
8.0 L2TP Over Specific Media
L2TP tries to be self-describing, operating at a level above the
particular 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 IP/UDP
L2TP uses the well-known UDP port 1701 [3]. 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,
and sends to the desired destination at port 1701. The recipient
picks a free port on its own system, and sends its reply to the
initiator's UDP port, setting its own UDP source port set to the free
port it found. All subsequent packets exchanged will use these UDP
ports. It is legal for a peer's IP address used for a given tunnel
to change over the life of a connection; this may correspond to a
peer with multiple IP interfaces responding to a network topology
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change. Responses should reflect the last source IP address for that
Tunnel ID.
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 MTUs of the
path over which the L2TP packets are likely to travel have a
consistent value.
When operating over UDP, both the I and C bits MUST be present, and
are used to permit correct demultiplexing and tunnel identification.
The default for any L2TP implementation is that UDP checksums MUST be
enabled for both control and payload messages. An L2TP
implementation MAY provide an option to disable UDP checksums for
payload packets. It is recommended that UDP checksums always be
enabled on control packets.
Port 1701 is used for both L2F [5] and L2TP packets. The two types
of packets may be detected by their headers; L2TP has a Vers field of
2, L2F has a 1 in this field instead. An L2TP implementation running
on a system which does not support L2F MUST silently discard all
packets whose Vers field is set to 1.
8.2 IP
When operating in IP environments, L2TP MUST use the UDP
encapsulation described in 8.1.
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 authenticate each other during tunnel
establishment. This authentication has the same security
attributes as CHAP, and has reasonable protection against reply
and snooping.
For L2TP tunnels over IP, IP-level packet security provides very
strong protection of the tunnel. This requires no modification to
the L2TP protocol, and leverages extensive IETF work in this area.
For L2TP tunnels over Frame Relay or other switched networks,
current practice indicates that these media are much less likely
to experience attacks of in-transit data. If these attacks became
prevalent, either the media or the L2TP packets would have to be
encrypted.
9.2 Client Security
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A more systematic method of protection in tunneled PPP
environments may be achieved through client security. PPP layer
encryption would provide end-to-end security for both direct
dial-in clients as well as PPP clients carried within a tunnel.
With this level of client security, sessions are protected against
attacks against the carrying tunnel, as well as attacks on the LAC
itself. Because both encryption and compression can occur at the
PPP layer, the two can be coordinated, permitting compression to
precede encryption.
9.3 Proxy Authentication
The optional proxy CHAP function of L2TP can permit an entirely
transparent PPP tunnel, with a single LCP and CHAP sequence being
seen by the client. For cases where the LAC and the entire path
to the LNS are operated by a single entity, this function may
provide acceptable security. For cases where LNS-initiated
authentication is required, proxy CHAP still permits an initial
access decision to be made before accepting the tunnel, permitting
the LNS in most cases to reject tunnel initiations rather than
accept them and later disconnect.
The optional proxy PAP may result in the cleartext password
traversing the tunnel. Where PAP is being used in conjunction
with static passwords, this may pose a significant security issue.
Where PAP is only used to transport one-time passwords, such
issues may be greatly mitigated. The H bit of the carrying AVP
may be used to protect against this.
10.0 Acknowledgments
The AVP construct comes from Glen Zorn, who thought up the framework
for permitting multiple vendors to contribute to a common attribute
space in a relatively orderly fashion.
Dory Leifer and Allan Rubens of Ascend Communications made valuable
refinements to the protocol definition of L2TP and contributed to the
editing of this document.
11.0 Contacts
Kory Hamzeh
Ascend Communications
1275 Harbor Bay Parkway
Alameda, CA 94502
kory@ascend.com
Tim Kolar, Morgan Littlewood, Andrew J. Valencia
Cisco Systems
170 West Tasman Drive
San Jose CA 95134-1706
tkolar@cisco.com
littlewo@cisco.com
vandys@cisco.com
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Gurdeep Singh Pall
Microsoft Corporation
Redmond, WA
gurdeep@microsoft.com
Jeff Taarud
Copper Mountain Networks
jtaarud@coppermountain.com
William Verthein
U.S. Robotics
12.0 References
[1] W. Simpson, "The Point-to-Point Protocol (PPP)", RFC 1661,
07/21/1994
[2] A. Valencia, M. Littlewood, T. Kolar, "Layer 2 Forwarding", Internet
draft, April 1996
[3] K. Hamzeh, G. Pall, W. Verthein, J. Taarud, W. Little, "Point-to-Point
Tunneling Protocol", Internet draft, June 1996
[4] P. Mockapetris, "Domain Names - Concepts and Facilities", RFC1034,
November 1987
[5] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
USC/Information Sciences Institute, July 1992.
[6] Sally Floyd, Van Jacobson, "Random Early Detection Gateways for
Congestion Avoidance", IEEE/ACM Transactions on Networking,
August 1993
[7] D. Carrel, L. Grant, "The TACACS+ Protocol", draft-grant-tacacs-00.txt,
October 1996
[8] C. Rigney, A. Rubens, W. A. Simpson, S. Willens. "Remote Authentication
Dial In User Service (RADIUS)." draft-ietf-radius-radius-05.txt,
Livingston, Merit, Daydreamer, July 1996.
Appendix A: Acknowledgment Time-Outs
L2TP uses sliding windows and time-outs to provide both user session
flow-control across the underlying medium (which may be an
internetwork) and to perform efficient data buffering to keep the
LAC-LNS data channels full without causing receive buffer overflow.
L2TP requires that a time-out be used to recover from dropped data or
acknowledgment packets. The exact implementation of the time-out is
vendor-specific. It is suggested that an adaptive time-out be
implemented with backoff for congestion control. The time-out
mechanism proposed here has the following properties:
Independent time-outs for each session. A device (LAC or LNS)
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will have to maintain and calculate time-outs for every active
session.
An administrator-adjustable maximum time-out, MaxTimeOut, unique
to each device.
An adaptive time-out mechanism that compensates for changing
throughput. To reduce packet processing overhead, vendors may
choose not to recompute the adaptive time-out for every received
acknowledgment. The result of this overhead reduction is that the
time-out will not respond as quickly to rapid network changes.
Timer backoff on time-out to reduce congestion. The backed-off
timer value is limited by the configurable maximum time-out value.
Timer backoff is done every time an acknowledgment time-out
occurs.
In general, this mechanism has the desirable behavior of quickly
backing off upon a time-out and of slowly decreasing the time-out
value as packets are delivered without time-outs.
Some definitions:
Packet Processing Delay, "PPD", is the amount of time required for
each peer to process the maximum amount of data buffered in their
offered receive packet window. The PPD is the value exchanged
between the LAC and LNS when a call is established. For the LNS,
this number should be small. For an LAC supporting modem
connections, this number could be significant.
"Sample" is the actual amount of time incurred receiving an
acknowledgment for a packet. The Sample is measured, not
calculated.
Round-Trip Time, "RTT", is the estimated round-trip time for an
Acknowledgment to be received for a given transmitted packet.
When the network link is a local network, this delay will be
minimal (if not zero). When the network link is the Internet,
this delay could be substantial and vary widely. RTT is adaptive:
it will adjust to include the PPD and whatever shifting network
delays contribute to the time between a packet being transmitted
and receiving its acknowledgment.
Adaptive Time-Out, "ATO", is the time that must elapse before an
acknowledgment is considered lost. After a time-out, the sliding
window is partially closed and the ATO is backed off.
Packet Processing Delay (PPD)
The PPD parameter is a 16-bit time value exchanged during the Call
Control phase expressed in units of tenths of a second (64 means 6.4
seconds). The protocol only specifies that the parameter is
exchanged, it does not specify how it is calculated. The way values
for ATO are calculated is implementation-dependent and need not be
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variable (static time-outs are allowed). IF adaptive time-outs are
to be used then the PPD should be exchanged in the call connect
sequences. A possible way to calculate the PPD is:
PPD = ((PPP_MAX_DATA_MTU - Header) * WindowSize * 8) / ConnectRate
+ LACFudge (for an LAC)
or
PPD = ((PPP_MAX_DATA_MTU - Header) * WindowSize * 8) / AvePathRate
+ LNSFudge (for an LNS)
Header is the total size of the L2TP and media dependent headers.
MTU is the overall MTU for the link between the LAC and LNS.
WindowSize represents the number of packets in the sliding window,
and is implementation-dependent. The latency of the underlying
connection path between the LAC and LNS could be used to pick a
window size sufficient to keep the current session's pipe full. The
constant 8 converts octets to bits (assuming ConnectRate is in bits
per second). If ConnectRate is in bytes per second, omit the 8.
LACFudge and LNSFudge are not required but can be used to take
overall processing overhead of the LAC or LNS into account.
In the case of the computed PPD for an LNS, AvePathRate is the
average bit rate of the path between the LNS and LAC. Given that
this number is probably very large and WindowSize is relatively
small, LNSFudge will be the dominant factor in the computation of
PPD. It is recommended that the minimum value of PPD be on the order
of 0.5 second.
The value of PPD is used to seed the adaptive algorithm with the
initial RTT[n-1] value.
A.1 Calculating Adaptive Acknowledgment Time-Out
We still must decide how much time to allow for acknowledgments to
return. If the time-out is set too high, we may wait an
unnecessarily long time for dropped packets. If the time-out is too
short, we may time out just before the acknowledgment arrives. The
acknowledgment time-out should also be reasonable and responsive to
changing network conditions.
The suggested adaptive algorithm detailed below is based on the TCP
1989 implementation and is explained in Richard Steven's book TCP/IP
Illustrated, Volume 1 (page 300). 'n' means this iteration of the
calculation, and 'n-1' refers to values from the last calculation.
DIFF[n] = SAMPLE[n] - RTT[n-1] DEV[n] = DEV[n-1] + (beta *
(|DIFF[n]| - DEV[n-1])) RTT[n] = RTT[n-1] + (alpha * DIFF[n])
ATO[n] = MIN (RTT[n] + (chi * DEV[n]), MaxTimeOut)
DIFF represents the error between the last estimated round-trip
time and the measured time. DIFF is calculated on each iteration.
DEV is the estimated mean deviation. This approximates the
standard deviation. DEV is calculated on each iteration and
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stored for use in the next iteration. Initially, it is set to 0.
RTT is the estimated round-trip time of an average packet. RTT is
calculated on each iteration and stored for use in the next
iteration. Initially, it is set to PPD.
ATO is the adaptive time-out for the next transmitted packet. ATO
is calculated on each iteration. Its value is limited, by the MIN
function, to be a maximum of the configured MaxTimeOut value.
Alpha is the gain for the average and is typically 1/8 (0.125).
Beta is the gain for the deviation and is typically 1/4 (0.250).
Chi is the gain for the time-out and is typically set to 4.
To eliminate division operations for fractional gain elements, the
entire set of equations can be scaled. With the suggested gain
constants, they should be scaled by 8 to eliminate all division. To
simplify calculations, all gain values are kept to powers of two so
that shift operations can be used in place of multiplication or
division. The above calculations are carried out each time an
acknowledgment is received for a packet that was not retransmitted
(no time-out occurs).
A.2 Congestion Control: Adjusting for Time-Out
This section describes how the calculation of ATO is modified in the
case where a time-out does occur. When a time-out occurs, the time-
out value should be adjusted rapidly upward. Although L2TP payload
packets are not retransmitted when a time-out occurs, the time-out
should be adjusted up toward a maximum limit. To compensate for
shifting internetwork time delays, a strategy must be employed to
increase the time-out when it expires. A simple formula called
Karn's Algorithm is used in TCP implementations and may be used in
implementing the backoff timers for the LNS or the LAC. Notice that
in addition to increasing the time-out, we also shrink the size of
the window as described in the next section.
Karn's timer backoff algorithm, as used in TCP, is:
NewTIMEOUT = delta * TIMEOUT
Adapted to our time-out calculations, for an interval in which a
time-out occurs, the new time-out interval ATO is calculated as:
RTT[n] = delta * RTT[n-1] DEV[n] = DEV[n-1] ATO[n] = MIN (RTT[n] +
(chi * DEV[n]), MaxTimeOut)
In this modified calculation of ATO, only the two values that
contribute to ATO and that are stored for the next iteration are
calculated. RTT is scaled by delta, and DEV is unmodified. DIFF is
not carried forward and is not used in this scenario. A value of 2
for Delta, the time-out gain factor for RTT, is suggested.
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Appendix B: Acknowledgment Time-Out and Window Adjustment
B.1 Initial Window Size
Although each side has indicated the maximum size of its receive
window, it is recommended that a slow start method be used to begin
transmitting data. The initial window size on the transmitter is set
to half the maximum size the receiver requested, with a minimum size
of one packet. The transmitter stops sending packets when the number
of packets awaiting acknowledgment is equal to the current window
size. As the receiver successfully digests each window, the window
size on the transmitter is bumped up by one packet until the maximum
is reached. This method prevents a system from flooding an already
congested network because no history has been established.
When for any reason an LAC or LNS receives more data than it can
queue for the tunnel, a packet must be discarded. In this case, it
is recommended that a "random early discard" algorithm [6] be used
rather than the obvious "drop last" algorithm.
B.2 Closing the Window
When a time-out does occur on a packet, the sender adjusts the size
of the transmit window down to one half its value when it failed.
Fractions are rounded up, and the minimum window size is one.
B.3 Opening the Window
With every successful transmission of a window's worth of packets
without a time-out, the transmit window size is increased by one
packet until it reaches the maximum window size that was sent by the
other side when the call was connected. As stated earlier, no
retransmission is done on a time-out. After a time-out, the
transmission resumes with the window starting at one half the size of
the transmit window when the time-out occurred and adjusting upward
by one each time the transmit window is filled with packets that are
all acknowledged without time-outs.
B.4 Window Overflow
When a receiver's window overflows with too many incoming packets,
excess packets are thrown away. This situation should not arise if
the sliding window procedures are being properly followed by the
transmitter and receiver. It is assumed that, on the transmit side,
packets are buffered for transmission and are no longer accepted from
the packet source when the transmit buffer fills.
Appendix C: Handling of out-of-order packets
When the Sequence Number and Acknowledgment Number fields are present
in payload packets, they are used to manage packet rate. In
addition, they may be used to handle out-of-order arrival of packets.
A simple L2TP client would simply discard any packet other than a
packet with a sequence greater than that last received; if packets 1,
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2, 3 arrived as 1, 3, 2, this would result in packet 2 being
discarded.
Such behavior does not affect the L2TP protocol itself, but
significantly improved throughput in such environments may be
attained by queueing and reordering packets when they arrive out of
order. The number of packets to be queued is a function of memory
resources on the L2TP implementation, but should never be more than
1/4 of the total sequence number space (i.e., 16384 packets), to
avoid aliasing.
An implementation which queues packets in this way must also employ
an algorithm for deciding that a given sequence number corresponds to
a packet which is lost, rather than one which is out of order but
still in transit. Such a decision would likely be based upon timing,
buffering conditions, and packet arrival characteristics. The
details of such a tradeoff are outside the scope of this
specification, but it is recommended a packet should be afforded an
interval at least twice the estimated RTT from the L2TP peer.
Appendix D: Transport Layer Adaptive Time-outs and Window Adjustment
Appendixes A, B, and C dealt with operation of the payload packet
sessions of L2TP. This Appendix explains how these three algorithms
pertain to the transport layer for L2TP control sessions. Appendix B,
Time-out Window Management, directly applies to the Transport Layer
except in the case where a window size of 1 is being used. Appendix
C, does not really apply because, for the Control Session, control
messages MUST always be processed by the receiver in order. Also,
there are no lost control packets because they are retransmitted by
the L2TP Transport Layer. Thus, the main topic of this Appendix is
how to adapt the example adaptive time-out algorithm of Appendix A to
the Control Session Transport Layer.
There are two main differences between the Control Session and
payload sessions: 1) Unlike lost payload packets, lost control
messages are retransmitted and 2) There is no Packet Processing Delay
value provided in the control session setup messages. The latter
affects the manner in which the initial value of the RTT estimate is
determined. The former really doesn't affect the algorithm at all,
except that upon a time-out, retransmission of unacknowledged control
messages should be attempted, up to the number that fit in the
sliding window with size computed as in Appendix B.
Using the symbol definitions of Appendix A, the calculation of the
value for the PPD of the remote peer can be estimated as:
PPD = ((PPP_MAX_DATA_MTU - Header) * WindowSize * 8) / AvePathRate
+ Fudge
This is simply the number of bits in a full control session window
divided by the average speed of the path between the LAC and LNS with
a fudge factor added on to make it work. In cases where the average
rate of the connection between LAC and LNS isn't known, it is
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suggested that some value be configured that is associated with each
possible peer. Because Control Session windows will most likely be
small and the connection speed will be quite high, fudge will be the
dominant factor in this calculation. For this reason, just
configuring a single fixed initial PPD estimate to be used for all
possible peers will probably provide adequate results. This fudge
factor should probably be at least 0.5 second.
Appendix E: Examples of L2TP sequence numbering
Although sequence numbers serve distinct purposes for control and
data messages, both types of messages use identical techniques for
assigning sequence numbers. This appendix shows several common
scenarios, and illustrates how sequence number state progresses and
is intepreted.
E.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
is contrived, in that the final acknowledgement in the example is
explicitly sent within a zero-length message, although most
typically the acknowledgement would have been included in the
processing of the Incoming-Call-Request which had prompted the LAC
to initiate the tunnel in the first place.
LAC -> LNS: Start-Control-Connection-Request
Nr: 0, Ns: 0
LNS -> LAC: Start-Control-Connection-Reply
Nr: 1, Ns: 0
LAC -> LNS: Start-Control-Connection-Connected
Nr: 1, Ns: 1
(delay)
LNS -> LAC: (zero-length)
Nr: 2, Ns: 1
E.2: Multiple packets acknowledged
This example shows a flow of payload packets from the LNS to the
LAC, with the LAC having no traffic of its own. The LAC is
waiting 1/4 of its timeout interval, and then acknowledging all
packets seen since the last interval.
(previous packet flow precedes this)
LAC -> LNS: (zero-length)
Nr: 7000, Ns: 1000
LNS -> LAC: (payload)
Nr: 1000, Ns: 7000
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LNS -> LAC: (payload)
Nr: 1000, Ns: 7001
LNS -> LAC: (payload)
Nr: 1000, Ns: 7002
(LAC's timer indicates it should acknowledge pending traffic)
LAC -> LNS: (zero-length)
Nr: 7003, Ns: 1000
E.3: Lost packet with retransmission
As a final example, an existing tunnel has a new session requested
by the LAC. The Incoming-Call-Reply is lost and must be
retransmitted by the LNS. This example continues from the
sequence state reached at the end of example E.1. Note that loss
of the -Reply has two impacts: not only does it keep the upper
level state machine from progressive, but it also keeps the LAC
from seeing a timely lower level acknowledgement of its -Request
packet.
LAC -> LNS: Incoming-Call-Request
Nr: 1, Ns: 2
LNS -> LAC: Incoming-Call-Reply
Nr: 3, Ns: 1
(pause; LAC's timer started first, so fires first)
LAC -> LNS: Incoming-Call-Request
Nr: 1, Ns: 2
(LNS realizes he has already seen this packet)
(LNS might use this as a cue to retransmit, as in this example)
LNS -> LAC: Incoming-Call-Reply
Nr: 3, Ns: 1
LAC -> LNS: Incoming-Call-Connected
Nr: 2, Ns: 3
(delay)
LNS -> LAC: (zero-length)
Nr: 4, Ns: 2
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