Internet Info 1997 December/Internet_Info_CD-ROM_Walnut_Creek_December_1997.iso/drafts/draft_ietf_j_p/draft-ietf-pppext-l2tp-06.txt

Text File | 1997-08-28 | 216KB | 5,211 lines

PPP Working Group                                             Kory Hamzeh
INTERNET DRAFT                                      Ascend Communications
Category: Internet Draft                                        Tim Kolar
Title: draft-ietf-pppext-l2tp-06.txt                        Cisco Systems
Date: August 1997                                       Morgan Littlewood
                                                            Cisco Systems
                                                       Gurdeep Singh Pall
                                                    Microsoft Corporation
                                                              Bill Palter
                                                            Cisco Systems
                                                              Jeff Taarud
                                                 Copper Mountain Networks
                                                         W. Mark Townsley
                                                                      IBM
                                                       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.





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Table of Contents

   1.0 Introduction
   1.1 Conventions
   1.2 Terminology
   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-Notification
   6.5 (reserved)
   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 (reserved)
   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 Establishment
   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



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   8.1 IP/UDP
   8.2 IP
   9.0 Security Considerations
   9.1 Tunnel Endpoint Security
   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:




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      o   MUST, SHALL, or MANDATORY -- This item is an absolute
         requirement of the specification.

      o   SHOULD or RECOMMEND -- This item should generally be followed
         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



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      Extensible Authentication Protocol, a framework for a family of
      PPP authentication protocols, including cleartext,
      challenge/response, and arbitrary dialog sequences.

   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



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      connectivity requirements.

   Tunnel

      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.
   [e-mail]).  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.

   Upon receipt of a non-zero-length message, the receiving peer must
   acknowledge the message by sending back the latest Nr value.  This
   updated Nr value can be piggybacked in any non-zero-length outgoing
   messages that the peer may happen to send back.  However, if the peer
   does not have a non-zero-length message to transmit for a period of
   1/4 of the timeout interval after receiving a non-zero-length message
   then it should send a zero-length message containing the latest
   sequence number state, as described above.

   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 call.



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      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
      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. If both ends of the tunnel have the ability to act as an
      LAC and LNS concurrently, then nothing prohibits establishing
      incoming or outgoing calls from both sides of the same tunnel.
      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.

      A mechanism for detection of tunnel connectivity problems is
      provided by the reliable transport layer of L2TP.  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.

      A keepalive mechanism is employed by the L2TP higher layer in
      order to differentiate tunnel outages from extended periods of no
      control or data activity on a tunnel. This is accomplished by the
      higher injecting Hello control messages into the control stream
      after a specified period of time elapses since the last data or
      control was received on the tunnel.  As for any other control
      message, if the transport does not receive an ACK for the Hello
      message after the normal number of retransmissions the tunnel is
      declared down and is reset.  The transport layer reset mechanism
      along with the injection of Hello messages ensures that a
      connectivity failure between the LNS and the LAC will be detected
      at both ends of a tunnel 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




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      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
      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 and
      flow 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.

      The receiving peer indicates whether flow-control is to be
      performed for payload packets sent to it.  If a peer issues a
      Receive Window Size AVP with a non-zero value during session
      establishment then the sending peer must abide by the indicated
      window size value.  If a receiving peer does not wish to flow
      control the payload messages sent to it, it should not issue the
      Receive Window Size AVP with a non-zero value.  Issuing a Receive
      Window Size AVP with a zero value has special significance.  It
      indicates that the receiver does not want to perform flow-control
      but it does want the sending peer to provide Ns values in payload
      packets so that it can detect (and perhaps reorder) packets
      received out of order.

      In the case where neither peer issues a Receive Window Size AVP
      during session establishment, the optional Nr and Ns fields are
      absent in all payload messages for that session.  In the case
      where either peer wishes to perform flow-control or to detect
      out-of-order receive messages (as indicated by the sending of the
      Receive Window Size AVP with non-zero or zero value, respectively)
      the Nr and Ns fields MUST be present in payload messages sent by
      both peers.  Both MUST provide proper Nr and Ns values in all
      messages transmitted.  A proper Ns value starts at 0 and
      increments by one for each transmitted payload message and a
      proper Nr value is the current value of the receive sequence
      number state variable.

      If a receiving peer offers a non-zero receive window size to a
      sending peer then the sending peer SHOULD abide by this window
      size value.  The sending peer should stop sending payload messages
      when the window is full; i.e., x consecutive messages have been



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      sent but have not been acknowledged, where x is the value of the
      Receive Window Size AVP.  Implementors should take care to avoid
      the situation where loss of an ACK by a sending peer with a full
      transmit window causes a session to hang forever, due to the fact
      there are no retransmissions of payload messages.  Steps must be
      taken to reopen the transmit window (at least to a value of 1)
      upon expiration of an ACK wait timeout.  See Appendix B for more
      details.

      L2TP does not specify the particular algorithms to use for
      congestion 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 on an AVP associated call management, 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



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      followed by zero or more AVP's.  This header is formatted:

       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|0|0|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 following
      seven bits MUST be set to 1001000, making the header more
      compatible in encoding with the payload message (defined in the
      next section).

      Ver MUST be the value 2, indicating a version 1 L2TP message
      (values 0 and 1 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-Notification 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
      LNS is expected to be able to process user data packets of 1500
      octets, not including L2TP and media encapsulation.  The smallest
      L2TP encapsulation is 6 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|0|0|0|0|0| Ver |          Length (opt)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Tunnel ID           |             Call ID           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Ns (opt)          |               Nr (opt)        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Offset Size (opt)        |    Offset pad... (opt)        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The T bit MUST be 0, indicating payload.

      Ver MUST be 2, 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 are reserved and MUST be set to 0.  These bit
      positions represent options no longer present in L2TP.

      If the F bit is set, both the Nr and Ns fields are present.  Ns
      indicates indicates the sequence number of the packet being sent.
      The Ns value of a message being transmitted is copied from the
      current value of the send sequence number state variable.  The
      send sequence number state variable is incremented by one after
      copying to the Ns field only if the payload size of the message
      being sent is non-zero.  Nr indicates the next packet sequence
      number to be received (if the last non-zero-length 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 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 is reserved MUST be set to 0.  This bit position
      represents a function no longer present in L2TP. The bits
      following the K bit MUST all be 0.

      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.

      A packet received with a reserved bit set to 1 MUST be silently
      discarded, unless the bit is defined for an extension that is



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      known to the implementation.

      If the P bit is set, this packet should receive preferential
      treatment at the LAC in its queueing for transmission to the
      client.  LCP echo requests used as a keepalive for the link, for
      instance, should generally be sent from the LNS 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 indicates: the integer
      value assigned to the message type; the message type abbreviation
      used in the AVP Summary Table of Sec. 5.5; and the full name of
      the message type.

      The integer value assigned to each message type 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  (StopCCN)  Stop-Control-Connection-Notification
         5  (reserved)
         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 (reserved)
         14 (CDN)      Call-Disconnect-Notify

      Error Reporting

         15 (WEN)      WAN-Error-Notify

      PPP Session Control




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         16 (SLI)      Set-Link-Info

   5.5 AVP Summary

      The following table lists all standard L2TP attributes currently
      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.

      The "(usage)" list for each entry indicates the message types that
      utilize each AVP (See command table of Sec. 5.4).  An abbreviation
      shown in mixed or upper case letters indicates that the
      corresponding AVP MUST be present in this message type; All lower
      case indicates that the AVP may optionally appear in this message
      type.

      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, StopCCN)
         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  1 6+  Vendor Name (SCCRP, SCCRQ)
         ASCII string describing issuing device

        9  1 8   Assigned Tunnel ID (SCCRP, SCCRQ, StopCCN)
         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)
         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 in Sec. 5.7).

       37  1 6+ Private Group ID (iccrq, iccn, ocrq)
         Variable length octet value used by the LAC or LNS to indicate
         that this call is to be associated with a particular customer
         group.



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   5.6 Result and Error Code Summary

   The StopCCN and CDN message types contain a Result Code AVP which
   indicates the result of the previously requested operation.  The
   Result Code can 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 Optional Message
   field that follows the Error Code field in the Result Code AVP.

   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



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   message, a Random Vector AVP must also be present in the message and
   MUST precede the first AVP having an H-bit of 1.

   The following mechanism is applied to the contents of the value field
   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.

   A value to be hidden MAY be padded with additional octets to obscure
   its  actual length. The subformat of the AVP's value, the entire
   contents of which is XORed with the MD5 hash, is indicated below.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Length             |             Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ...                          |             Padding ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length
      This is length of the Original AVP Value to be obscured in octets.

   Original AVP Value
      Value to be obscured.

   Padding
      Random additional bytes used to obscure length of Value.

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

   If the subformat 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 subformat value and
   placed in the corresponding octets of the Value field of the AVP.




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   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 precede any AVP
   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 preceding 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 (Sec. 6.1).

   6.0.2 Reliable Delivery of Control Messages



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      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.

      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, possibly 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
      messages to be outstanding on a given tunnel.  If a peer specifies
      a  Receive Window Size AVP 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. Unlike payload sessions, a value of 0 for the
      Receive Window Size AVP is invalid for a control session.

      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.



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   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 (SCCRQ)

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



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   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               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               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



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      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
      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 sliently discard its tunnel.  In the case where a tie breaker
      is present on both sides, and the value is equal, both sides MUST
      discard their  tunnels.

      If a tie breaker is received, and 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.



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      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.

   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



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      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

       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 using a CHAP-style authentication mechanism.  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 (SCCRP)

   The Start-Control-Connection-Reply is an L2TP control message sent in
   reply to a received Start-Control-Connection-Request message. Sending
   this message indicates that the request was successful.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    L2TP Control Message Header      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Start-Control-Connection-Reply     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Protocol Version      |
   | Framing Capabilities  |
   | Bearer Capabilities   |



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   | 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

       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



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      bit A is set, asynchronous framing is supported.  If bit S is set,
      synchronous framing is supported.  This AVP MUST be present.

   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.

   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



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      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
       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.

   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.

   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-Reply
      indicates that the peer wishes to authenticate the tunnel
      initiator using a CHAP-style authentication mechanism.  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
      value 2 (corresponding to the Value field of the Start-Control-
      Connection-Reply AVP) MUST be used.

6.3 Start-Control-Connection-Connected (SCCCN)

   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.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    L2TP Control Message Header      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Start-Control-Connection-Connected |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Challenge Response    |
   +-+-+-+-+-+-+-+-+-+-+-+-+




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   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 value 3
      (corresponding to the Value field of the Start-Control-
      Connection-Connected AVP) MUST be used.

6.4 Stop-Control-Connection-Notification (StopCCN)

   The Stop-Control-Connection-Notification 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      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Stop-Control-Connection-Notification|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Assigned Tunnel ID    |
   | Result Code           |
   +-+-+-+-+-+-+-+-+-+-+-+-+




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   Stop-Control-Connection-Notification 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-Notification.  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-Notification 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-Notification
      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
      the Error Code value, which for a Stop-Control-Connection-
      Notification 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



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         2 - General error--Error Code indicates the problem
         3 - Control channel already exists
         4 - Requester is not authorized to establish a control channel
         5 - The protocol version of the requester is not supported
             Error Code indicates highest version supported
         6 - Requester is being shut down
         7 - Finite State Machine error

6.5 (reserved)

   The function previously described here has been deleted from L2TP.

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 tunnel.

   Keepalives should be implemented by sending a Hello once every 60
   seconds if 60 seconds have passed without receiving any message
   (payload or control, including zero-length messages) from 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 (OCRQ)



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   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
   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             |
   | Private Group ID        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+

   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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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      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
      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               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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      |              17               |           BPS (H)             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            BPS (L)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      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



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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1|0|0|0|           8           |               0               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              10               |             Size              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      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               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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      |              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.

      Private Group 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| 6 + Private Group ID  |               0               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              37               | Private Group ID   ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The Private Group ID AVP is sent by the LNS to the LAC to indicate
      that a particular call is to be treated specially, for example, to
      select a particular port group or exchange carrier. The Attribute
      is 37, Private Group ID, and is marked optional.  The presence of
      this AVP is optional.  The Private Group ID is a string
      corresponding to a table in the LAC that defines the particular
      characteristics of the selected group.


6.8 Outgoing-Call-Reply (OCRP)

   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 that 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          |
   | 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-



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   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.  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.

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



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   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 (OCCN)

   Outgoing-Call-Connected is an L2TP control message sent by the LAC to
   the LNS to indicate that the result of a requested outgoing call was
   successful.  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          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 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 9, indicating Outgoing-
   Call-Connected.  The Outgoing-Call-Connected message encodes the
   final result of an outgoing call request.  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 final negotiated speed for the
   connection.  The Attribute value is 24, indicating Connect Speed, and



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   is marked mandatory.  This AVP MUST be present.  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.  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 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



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   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 (ICRQ)

   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               |
   | Private Group ID          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Incoming-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               |              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



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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               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              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              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              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



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

   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 ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   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.

   Private Group 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| 6 + Private Group ID  |               0               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              37               | Private Group ID   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The PrivateGroup ID AVP is used by the LAC to indicate that this call
   is to be associated with a particular customer group. The Attribute
   is 37, Private Group ID, and is marked optional.  The presence of
   this AVP is optional.  The LNS MAY treat the PPP session as well as
   network traffic through this session specially in a manner determined
   by the peer. For example, if the LNS is individually connected to
   several private networks using unregistered addresses, this AVP may
   be included by the LAC to indicate that a given call should be
   associated with one of the private networks.

   The Private Group ID is a string corresponding to a table in the LNS
   that defines the particular characteristics of the selected group. A
   LAC MAY determine the Private Group ID from a RADIUS response
   containing the PrivateGroupID attribute.

   The Private Group ID AVP MAY be included in either incoming call
   request or incoming call connected messages. This AVP SHOULD NOT be
   included in both messages.

6.11 Incoming-Call-Reply (ICRP)

   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 that the request was successful.  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 that the
   call should be answered.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          L2TP Control Message Header        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Incoming-Call-Reply             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Assigned Call ID              |
   | Receive Window Size           |
   | Packet Processing Delay       |



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

   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 11, indicating Incoming-
   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.  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.

   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



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   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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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 (ICCN)

   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.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          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     |
   | Private Group ID          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   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 12, 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |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



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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               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              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
    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




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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |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               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              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



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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 + 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               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   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.

   Private Group 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| 6 + Private Group ID  |               0               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              37               | Private Group ID   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   PrivateGroup ID AVP is used by the LAC to indicate that this call is
   to be associated with a particular customer group. The Attribute is
   37, Private Group ID, and is marked optional.  The presence of this
   AVP is optional.  The LNS MAY treat the PPP session as well as
   network traffic through this session specially in a manner determined
   by the peer. For example, if the LNS is individually connected to
   several private networks using unregistered addresses, this AVP may
   be included by the LAC to indicate that a given call should be
   associated with one of the private networks.

   The Private Group ID is a string corresponding to a table in the LNS
   that defines the particular characteristics of the selected group. A
   LAC MAY determine the Private Group ID from a RADIUS response
   containing the PrivateGroupID attribute.

   The Private Group ID AVP MAY be included in either incoming call
   request or incoming call connected messages. This AVP SHOULD NOT be
   included in both messages.

6.13 (reserved)

   The function previously described here has been deleted from L2TP.

6.14 Call-Disconnect-Notify (CDN)

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

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          L2TP Control Message Header        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Call-Disconnect-Notify          |



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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | 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 14, indicating Call-
   Disconnect-Notify.  The Call-Disconnect-Notify message encodes a
   disconnect indication for a specific call within the tunnel.  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 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 Error Code is not meaningful it
      MUST be set to 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 disconnected due to loss of carrier
         2 - Call disconnected for the reason indicated in Error Code.
         3 - Call disconnected for administrative reasons
         4 - Call failed due to no appropriate facilities being
            available (temporary condition)
         5 - Call failed due to no appropriate facilities being
            available (permanent condition)
         6 - Invalid destination
         7 - Call failed due to no carrier detected
         8 - Call failed due to detection of a busy signal



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         9 - Call failed due to lack of a dial tone
         10 - Call was not established within time allotted by LAC
         11 - 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
      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
      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 (WEN)

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



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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          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 15, 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

    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



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      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 (SLI)

   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

    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 16, 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                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   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
   be logged appropriately, and the control connection should be closed
   and restarted to ensure recovery into a known state.

   In several cases in the following tables, a protocol message is sent,
   and then a "clean up" occurs.  Note that regardless of the initiator
   of the tunnel destruction, the reliable delivery mechanism must be
   allowed to run (see 6.0.2) before destroying the tunnel.  This
   permits the tunnel management messages to be reliably delivered to
   the peer.

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.

7.2.1 Control Connection Establishment

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



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

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

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

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

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

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

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

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

   established     Admin             Send StopCCN         idle
                   Tunnel Close      Clean up

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

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

   wait-ctl-reply
      The originator checks to see if another connection has been
      requested from the same peer, and if so, handles the collision
      situation described in Section 6.0.1.

      When a SCCRP is received, it is examined for a compatible version.
      If the version of the reply is lower than the version sent in the
      request, the older (lower) version should be used provided it is
      supported.  If the version in the reply is earlier and supported,
      the originator moves to the established state.  If the version is
      earlier and not supported, a StopCCN MUST be sent to the peer and
      the originator cleans up and terminates the tunnel.




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   wait-ctl-conn
      This is where an SCCCN is awaited; upon receipt, protocol version
      compatibility is checked.  The tunnel is either established, or is
      torn down if a protocol mismatch is detected.

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

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

7.3 Timing considerations

   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 do not correspond to
         an authorized user
      -  The bearer service is not authorized or supported

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

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



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

7.4.1 LAC Incoming Call States

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

   wait-tunnel     tunnel-open        Send ICRQ         wait-reply

   wait-reply      Receive ICRP,      Answer call,      established
                   acceptable         Send ICCN

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

   wait-reply      Receive CDN        Clean up          idle

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

   established     Receive CDN        Hang up bearer,   idle
                                      Clean up

   established     Local              Hang up bearer,   idle
                   Close request      Send CDN,
                                      Clean up

   established     Bearer             Send CDN,         idle
                   Line drop          Clean up

The states associated with the LAC for incoming calls are:

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

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

wait-reply
   The LAC receives 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



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   the call is accepted, the LAC sends an Incoming-Call-Connected
   message and enters the established state.

established
   Data is exchanged over the tunnel.  The call may be cleared
   following:
      An event on the connected interface.  The LAC sends a Call-
         Disconnect-Notify message
      Receipt of a Call-disconnect-Notify message.  The LAC cleans up,
         disconnecting the call.
      A local reason.  The LAC sends a Call-Disconnect-Notify message.


7.4.2 LNS Incoming Call States

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

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

   wait-connect    Receive ICCN       Prepare for       established
                   acceptable         data

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

   wait-connect,
   established     Receive CDN        Clean up          idle

   established     Local              Send CDN,         idle
                   Close request      Clean up

   The states associated with the LNS for incoming calls are:

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

   wait-connect
      If the session is still connected on the LAC, the LAC sends an
      Incoming-Call-Connected message to the LNS which then moves into
      established state.  The LAC may send a Call-Disconnect-Notify to
      indicate that the incoming caller could not be connected.  This
      could happen, for example, if a telephone user accidentally places
      a standard voice call to an LAC resulting in a handshake failure
      on the called modem.

   established



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      The session is terminated either by receipt of a Call-Disconnect-
      Notify message from the LAC or by sending a Call-Disconnect-
      Notify.  Clean up follows on both sides regardless of the
      initiator.

7.5 Outgoing calls

   Outgoing calls are initiated by an LNS and instruct an LAC to place a
   call.  There are three messages for outgoing calls:  Outgoing-Call-
   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 OCRQ,      Send OCRP,        wait-cs-answer
                   acceptable         Open bearer

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

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

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

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

   The states associated with the LAC for outgoing calls are:

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

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



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      go to established state.

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

7.5.2 LNS Outgoing Call States

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

   wait-tunnel     tunnel-open        Send OCRQ         wait-reply

   wait-reply      Receive OCRP,      none              wait-connect
                   acceptable

   wait-reply      Receive OCRP,      Send CDN          idle
                   not acceptable

   wait-connect    Receive OCCN       none              established

   established,
   wait-connect    Receive CDN        Clean up          idle

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

   The states associated with the LNS for outgoing calls are:

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

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

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




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

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
   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.

   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 offer the UDP
   encapsulation described in 8.1 as its default configuration for IP
   operation.  Other configurations (perhaps corresponding to a



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   compressed header format) may be defined, and made available as a
   configurable option.

















9.0 Security Considerations

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

   9.1 Tunnel Endpoint Security

      The tunnel endpoints may authenticate each other during tunnel
      establishment.  This authentication has the same security
      attributes as CHAP, and has reasonable protection against replay
      and snooping.  In environments where some L2TP peers will be
      authenticated, but others not, a mechanism should be provided to
      control when tunnel endpoint authentication will be active.

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

      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.

      Because the shared secret used for endpoint authentication is the
      same shared secret used to obscure AVP contents using the H
      (Hidden) bit, tunnel authentication must be used in all cases
      where the H bit will be used.  Proxy authentication involving PAP



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      may be usable without the H bit if PAP is only carrying one-time
      passwords; if clear text passwords are carried in the proxy
      authentication, the H bit (and, by implication, tunnel
      authentication) should be used.

      To protect against exhaustive attack during tunnel authentication,
      no tunnel authentication response should ever be accepted if it
      corresponds to a challenge more than one minute old.

   9.2 Client Security

      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.

      All implementations MUST accept proxy authentication AVP's, but
      are free to silently discard them and initiate an entirely new
      authentication with the PPP client.

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.




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   Steve Cobb and Evan Caves redesigned the state machine tables.

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

11.0 Contacts

   Kory Hamzeh
   Ascend Communications
   1275 Harbor Bay Parkway
   Alameda, CA 94502
   [e-mail]

   Tim Kolar, Morgan Littlewood, Bill Palter, Andrew J. Valencia
   Cisco Systems
   170 West Tasman Drive
   San Jose CA 95134-1706
   [e-mail]
   [e-mail]
   [e-mail]
   [e-mail]

   Gurdeep Singh Pall
   Microsoft Corporation
   Redmond, WA
   [e-mail]

   Jeff Taarud
   Copper Mountain Networks
   [e-mail]

   W. Mark Townsley
   IBM Corporation
   700 Park Office Dr.
   Raleigh, NC 27709
   [e-mail]

   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



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   [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)
      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 its
      offered receive packet window.  The PPD is the value exchanged
      between the LAC and LNS when a call is established.  For the LNS,



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      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
   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.



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   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
      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



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   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.

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 maximum 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 maximum window size.  As the receiver successfully
   digests each window, the maximum window size on the transmitter is
   bumped up by one packet until the maximum specified by the receiver
   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 maximum value when it
   failed.  Fractions are rounded up, and the minimum window size is
   one.



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B.3 Opening the Window

   With every successful transmission of a window's worth of packets
   without a time-out, the maximum transmit window size is increased by
   one packet until it reaches the maximum window size that was sent by
   its peer when the call was connected.  As stated earlier, no
   retransmission is done on a time-out.  After a time-out, transmission
   resumes with the maximum transmit window starting at one half the
   size of the maximum transmit window when the time-out occurred (with
   a minimum window size of 1) and adjusting upward by one each time the
   current maximum 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,
   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,



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   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
   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 interpreted.

   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 acknowledgment in the example is
      explicitly sent within a zero-length message, although most
      typically the acknowledgment 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.



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            LAC                                           LNS
             ->    Start-Control-Connection-Request
                   Nr: 0, Ns: 0

                        Start-Control-Connection-Reply    <-
                                          Nr: 1, Ns: 0

             ->    Start-Control-Connection-Connected
                   Nr: 1, Ns: 1

            (delay)

                                         (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
                                             (payload)    <-
                                    Nr: 1000, Ns: 7000
                                             (payload)    <-
                                    Nr: 1000, Ns: 7001
                                             (payload)    <-
                                    Nr: 1000, Ns: 7002

      (LAC's timer indicates it should acknowledge pending traffic)

             ->    (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 acknowledgment of its -Request
      packet.

            LAC                                           LNS
             ->    Incoming-Call-Request
                   Nr: 1, Ns: 2




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                                   Incoming-Call-Reply    <-
                                          Nr: 3, Ns: 1

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

             ->    Incoming-Call-Request
                   Nr: 1, Ns: 2

            (LNS realizes it has already seen this packet)
            (LNS might use this as a cue to retransmit, as in this example)

                                   Incoming-Call-Reply    <-
                                          Nr: 3, Ns: 1
             ->    Incoming-Call-Connected
                   Nr: 2, Ns: 3

            (delay)

                                         (zero-length)    <-
                                          Nr: 4, Ns: 2





































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