Subject: Please post draft-ietf-l2tpext-l2tpbis-01.txt

From: William Mark Townsley <[email-masked]>
Date: Thu, 23 Nov 2000 22:24:37 -0800 (PST)

Network Working Group                                        W. Townsley
Internet-Draft                                               A. Valencia
Category: Standards Track                                        G. Zorn
<draft-ietf-l2tpext-l2tpbis-01.txt>                        cisco Systems
                                                               A. Rubens
                                                             Tut Systems
                                                                 G. Pall
                                                   Microsoft Corporation
                                                               B. Palter
                                                        Redback Networks
                                                           November 2000

                  Layer Two Tunneling Protocol "L2TP"

Status of this Memo

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

   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
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``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
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   ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).

   The distribution of this memo is unlimited.  It is filed as <draft-
   ietf-l2tpext-l2tpbis-01.txt> and expires May 31, 2001.  Please send
   comments to the L2TP mailing list ([email-masked]).

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

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

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

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   Contents

   Status of this Memo..........................................    1
      1.0 Introduction..........................................    4
      1.1 Specification of Requirements.........................    5
      1.2 Terminology...........................................    5
      2.0 Topology..............................................    8
      3.0 Protocol Overview.....................................    9
      3.1 L2TP Header Format....................................   10
      3.2 Control Message Types.................................   12
      4.0 Control Message Attribute Value Pairs.................   13
      4.1 AVP Format............................................   13
      4.2 Mandatory AVPs........................................   15
      4.3 Hiding of AVP Attribute Values........................   15
      4.4 AVP Summary...........................................   17
      5.0 Protocol Operation....................................   42
      5.1 Control Connection Establishment......................   42
      5.2 Session Establishment.................................   43
      5.3 Forwarding PPP Frames.................................   44
      5.4 Using Sequence Numbers on the Data Channel............   45
      5.5 Keepalive (Hello).....................................   45
      5.6 Session Teardown......................................   46
      5.7 Control Connection Teardown...........................   46
      5.8 Reliable Delivery of Control Messages.................   47
      6.0 Control Connection Protocol Specification.............   49
      6.1 Start-Control-Connection-Request (SCCRQ)..............   49
      6.2 Start-Control-Connection-Reply (SCCRP)................   49
      6.3 Start-Control-Connection-Connected (SCCCN)............   50
      6.4 Stop-Control-Connection-Notification (StopCCN)........   50
      6.5 Hello (HELLO).........................................   50
      6.6 Incoming-Call-Request (ICRQ)..........................   51
      6.7 Incoming-Call-Reply (ICRP)............................   52
      6.8 Incoming-Call-Connected (ICCN)........................   52
      6.9 Outgoing-Call-Request (OCRQ)..........................   53
      6.10 Outgoing-Call-Reply (OCRP)...........................   54
      6.11 Outgoing-Call-Connected (OCCN).......................   54
      6.12 Call-Disconnect-Notify (CDN).........................   54
      6.13 WAN-Error-Notify (WEN)...............................   55
      6.14 Set-Link-Info (SLI)..................................   55
      7.0 Control Connection State Machines.....................   56
      7.1 Control Connection Protocol Operation.................   56
      7.2 Control Connection States.............................   57
         7.2.1 Control Connection Establishment.................   57
      7.3 Timing considerations.................................   59
      7.4 Incoming calls........................................   59
         7.4.1 LAC Incoming Call States.........................   60
         7.4.2 LNS Incoming Call States.........................   62
      7.5 Outgoing calls........................................   63

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

   Appendix A: Control Channel Slow Start and Congestion Avoidance   74

   Appendix B: Control Message Examples.........................   75

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

1.0 Introduction

   The Layer Two Tunneling Protocol (L2TP) provides a mechanism for
   aggregation of multiple layer two connections across packet oriented
   data networks. These layer two connections may be PPP [RFC1661] or
   other layer two connections such as Frame Relay, ATM, etc. This
   document defines the specific mechanisms for tunneling of PPP,
   including a control protocol for on-demand creation of tunnels
   between nodes and the accompanying encapsulation for multiplexing
   multiple, tunneled, PPP sessions.  The specifics of tunneling layer 2
   links other than PPP with L2TP will be defined separately.

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

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   termination point and PPP session endpoint reside on the same
   physical device (i.e., the NAS).

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

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

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

1.1 Specification of Requirements

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

1.2 Terminology

   Analog Channel

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

   Attribute Value Pair (AVP)

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

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      teardown of tunnels.

   Call

      A connection (or attempted connection) between a Remote System and
      LAC. For example, a telephone call through the PSTN. A Call
      (Incoming or Outgoing) which is successfully established between a
      Remote System and LAC results in a corresponding L2TP Session
      within a previously established Tunnel between the LAC and LNS.
      (See also: Session, Incoming Call, Outgoing Call).

   Called Number

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

   Calling Number

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

   CHAP

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

   Control Connection

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

   Control Messages

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

   Digital Channel

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

   DSLAM

      Digital Subscriber Line (DSL) Access Module. A network device used
      in the deployment of DSL service. This is typically a concentrator

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      of individual DSL lines located in a central office (CO) or local
      exchange.

   Incoming Call

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

   L2TP Access Concentrator (LAC)

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

   L2TP Network Server (LNS)

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

   Management Domain (MD)

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

   Network Access Server (NAS)

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

   Outgoing Call

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

   Peer

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

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      with PPP, a peer is either side of the PPP connection.

   POTS

      Plain Old Telephone Service.

   Remote System

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

   Session

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

   Tunnel

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

   Zero-Length Body (ZLB) Message

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

2.0 Topology

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

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

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

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

3.0 Protocol Overview

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

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   +-------------------+
   | PPP Frames        |
   +-------------------+    +-----------------------+
   | L2TP Data Messages|    | L2TP Control Messages |
   +-------------------+    +-----------------------+
   | L2TP Data Channel |    | L2TP Control Channel  |
   | (unreliable)      |    | (reliable)            |
   +------------------------------------------------+
   |      Packet Transport (UDP, FR, ATM, etc.)     |
   +------------------------------------------------+

   Figure 3.0 L2TP Protocol Structure

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

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

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

3.1 L2TP Header Format

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

   This header is formatted:

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |T|L|x|x|S|x|O|P|x|x|x|x|  Ver  |          Length (opt)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tunnel ID           |           Session ID          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Ns (opt)          |             Nr (opt)          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Offset Size (opt)        |    Offset pad... (opt)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 3.1 L2TP Message Header

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

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

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

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

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

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

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

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

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

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

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

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

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

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

3.2 Control Message Types

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

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

      Control Connection Management

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      0  (reserved)

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

   Call Management

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

   Error Reporting

      15 (WEN)      WAN-Error-Notify

   PPP Session Control

      16 (SLI)      Set-Link-Info

4.0 Control Message Attribute Value Pairs

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

4.1 AVP Format

   Each AVP is encoded as:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |M|H| rsvd  |      Length       |           Vendor ID           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Attribute Type        |        Attribute Value...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       [until Length is reached]...                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

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

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

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

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

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

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4.2 Mandatory AVPs

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

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

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

4.3 Hiding of AVP Attribute Values

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

   The H bit MUST only be set if a shared secret exists between the LAC
   and LNS and tunnel authentication has completed. The shared secret is
   the same secret that is used for tunnel authentication (see section
   5.1.1).  Hidden values MUST NOT be unhidden until after tunnel
   authentication has completed successfully (perhaps requiring the
   hidden value to be stored until after receipt of additional setup
   messages). To do otherwise runs the risk of AVP data being utilized
   without verifying the integrity of the shared secret. If the H bit is
   set in any AVP(s) in a given control message, a Random Vector AVP
   must also be present in the message and MUST precede the first AVP
   having an H bit of 1.

   Hiding an AVP value is done in several steps. The first step is to
   take the length and value fields of the original (cleartext) AVP and
   encode them into a Hidden AVP Subformat as follows:

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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Length of Original Value    |   Original Attribute Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ...                          |             Padding ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

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

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

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

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

   The MD5 hash value is then XORed with the first 16 octet (or less)
   segment of the Hidden AVP Subformat and placed in the Attribute Value
   field of the Hidden AVP.  If the Hidden AVP Subformat is less than 16
   octets, the Subformat is transformed as if the Attribute Value field
   had been padded to 16 octets before the XOR, but only the actual

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   octets present in the Subformat are modified, and the length of the
   AVP is not altered.

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

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

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

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

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

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

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

4.4 AVP Summary

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

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

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   4.4.1 AVPs Applicable To All Control Messages

   Message Type (All Messages)

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

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

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

      The Message Type is a 2 octet unsigned integer.

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

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

      A Vendor Specific control message may be defined by setting the
      Vendor ID of the Message Type AVP to a value other than the IETF
      Vendor ID of 0 (see section 4.1).

   Random Vector (All Messages)

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

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

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

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

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

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

   4.4.2 Result and Error Codes

   Result Code (CDN, StopCCN)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Result Code          |        Error Code (opt)       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Error Message (opt) ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

      This AVP MUST NOT be hidden (the H-bit MUST be 0). The M-bit for

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      this AVP MUST be set to 1.  The Length is 8 if there is no Error
      Code or Message, 10 if there is an Error Code and no Error Message
      or 10 + the length of the Error Message if there is an Error Code
      and Message.

      Defined Result Code values for the StopCCN message are:

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

      Defined Result Code values for the CDN message are:

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

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

      0 - No general error
      1 - No control connection exists yet for this LAC-LNS pair
      2 - Length is wrong
      3 - One of the field values was out of range or
          reserved field was non-zero
      4 - Insufficient resources to handle this operation now

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      5 - The Session ID is invalid in this context
      6 - A generic vendor-specific error occurred
      7 - Try another.  If initiator is aware of other possible responder
          destinations, it should try one of them.  This can be
          used to guide an LAC or LNS based on policy, for instance,
          the existence of multilink PPP bundles at an LNS.
      8 - The session or tunnel was shutdown due to receipt of an unknown
          AVP with the M-bit set (see section 4.2). The Error Message
          SHOULD contain the attribute of the offending AVP in (human
          readable) text form.
      9 - Try another directed. If an LAC or LNS is aware of other possible
          destinations, it should inform the initiator of the tunnel or
          session. The Error Message MUST contain a comma separated list
          of addresses for the initiator to choose from. If the L2TP
          transport is IPv4, then this would be a comma separated list
          of IP addresses in the canonical dotted-decimal format. i.e.
          "10.0.0.1, 10.0.0.2, 10.0.0.3" If there are no servers for
          the LAC or LNS to suggest, then Error Code 7 should be used.
          The delimiter between addresses MUST be precisely a single
          comma and a single space.

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

   4.4.3 Control Connection Management AVPs

   Protocol Version (SCCRP, SCCRQ)

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

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

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

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

      This AVP MUST NOT be hidden (the H-bit MUST be 0). The M-bit for

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      this AVP MUST be set to 1.  The Length of this AVP is 8.

   Framing Capabilities (SCCRP, SCCRQ)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Reserved for future framing type definitions          |A|S|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

   Bearer Capabilities (SCCRP, SCCRQ)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Reserved for future bearer type definitions         |V|A|D|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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      link.

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

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

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

   Tie Breaker (SCCRQ)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Tie Break Value...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                 ...(64 bits)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

   Firmware Revision (SCCRP, SCCRQ)

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

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

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

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

      For devices which do not have a firmware revision (general purpose
      computers running L2TP software modules, for instance), the
      revision of the L2TP software module may be reported instead.

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

   Host Name (SCCRP, SCCRQ)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Host Name ... (arbitrary number of octets)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

      This name should be as broadly unique as possible; for hosts
      participating in DNS [RFC1034], a hostname with fully qualified
      domain would be appropriate. The Host Name MAY be used to identify
      tunnel configuration, including the shared secret for tunnel
      authentication (if enabled) and any other options defined for the
      tunnel.

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

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   Vendor Name (SCCRP, SCCRQ)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Vendor Name ...(arbitrary number of octets)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

   Assigned Tunnel ID (SCCRP, SCCRQ, StopCCN)

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

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

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

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

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

      In the StopCCN control message, the Assigned Tunnel ID AVP MUST be
      the same as the Assigned Tunnel ID AVP first sent to the receiving
      peer, permitting the peer to identify the appropriate tunnel even

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      if a StopCCN is sent before an Assigned Tunnel ID AVP is received.
      If an Assigned Tunnel ID AVP has not been sent in a previous
      message, a tunnel ID SHOULD be allocated and sent via the Assigned
      Tunnel ID AVP so that the StopCCN may be reliably delivered.  This
      is most important if the StopCCN carries an essential directive
      within, for instance, a Result Code of value 9 with an alternate
      address to attempt connecting to. If an Assigned Tunnel ID AVP is
      not sent in the StopCCN or any previous message, the StopCCN MUST
      NOT be retransmitted.

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

   Receive Window Size (SCCRQ, SCCRP)

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

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

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

      The Window Size is a 2 octet unsigned integer.

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

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

   Challenge (SCCRP, SCCRQ)

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

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

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

      The Challenge is one or more octets of random data.

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

   Challenge Response (SCCCN, SCCRP)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Response ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

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

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

   4.4.4 Call Management AVPs

   Q.931 Cause Code (CDN)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Cause Code           |   Cause Msg   | Advisory Msg...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

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

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

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

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

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

      If an Assigned Session ID AVP has been sent in a previous message,
      it MUST be sent in a CDN as well to permit a peer to identify the

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      appropriate session even if CDN is sent before an Assigned Session
      ID is received. If the CDN is sent before an Assigned Session ID
      is communicated (e.g. in response to an ICRQ), it MUST NOT be sent
      in the CDN message.

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

   Call Serial Number (ICRQ, OCRQ)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Call Serial Number                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The Call Serial Number is a 32 bit value.

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

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

   Minimum BPS (OCRQ)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Minimum BPS                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

   Maximum BPS (OCRQ)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Maximum BPS                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

   Bearer Type (ICRQ, OCRQ)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Reserved for future Bearer Types              |V|A|D|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

      Bits set in the Bearer Type AVP in an OCRQ message indicate which
      bearer type(s) an outgoing call may be placed on.  If more than

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      one bit is set, the LAC may choose which bearer type to place the
      call on. If no bits are set, any type of available channel may be
      used.

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

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

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

   Framing Type (ICCN, OCCN, OCRQ)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Reserved for future Framing Types               |A|S|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

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

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

   Called Number (ICRQ, OCRQ)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Called Number... (arbitrary number of octets)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

   Calling Number (ICRQ)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Calling Number... (arbitrary number of octets)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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      this AVP MUST be set to 1.  The Length (before hiding) of this AVP
      is 6 plus the length of the Calling Number.

   Sub-Address (ICRQ, OCRQ)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Sub-Address ... (arbitrary number of octets)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

   (Tx) Connect Speed (ICCN, OCCN)

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

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

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

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

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

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

   Rx Connect Speed (ICCN, OCCN)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           BPS (H)             |            BPS (L)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

   Physical Channel ID (ICRQ, OCRP)

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

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

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

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

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

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      is 10.

   Private Group ID (ICCN)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Private Group ID ... (arbitrary number of octets)           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

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

   Sequencing Required (ICCN, OCCN)

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

      This AVP has no Attribute Value field.

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

   4.4.5 Proxy LCP and Authentication AVPs

      The LAC may have answered the call and negotiated LCP with the

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

   Initial Received LCP CONFREQ (ICCN)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | LCP CONFREQ... (arbitrary number of octets)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

   Last Sent LCP CONFREQ (ICCN)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | LCP CONFREQ... (arbitrary number of octets)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

   Last Received LCP CONFREQ (ICCN)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | LCP CONFREQ... (arbitrary number of octets)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

   Proxy Authen Type (ICCN)

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

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

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

      Authen Type is a 2 octet unsigned integer, holding:

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

      Defined Authen Type values are:
         0 - Reserved
         1 - Textual username/password exchange
         2 - PPP CHAP

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         3 - PPP PAP
         4 - No Authentication
         5 - Microsoft CHAP Version 1 (MSCHAPv1)

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

      Associated AVPs for each type of authentication follow.

   Proxy Authen Name (ICCN)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Authen Name... (arbitrary number of octets)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

   Proxy Authen Challenge (ICCN)

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

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

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

      The Challenge is a string of one or more octets.

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

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

   Proxy Authen ID (ICCN)

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

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

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

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

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

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

   Proxy Authen Response (ICCN)

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

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

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

      The Response is a string of octets.

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

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

   4.4.6 Call Status AVPs

   Call Errors (WEN)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Reserved              |        CRC Errors (H)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         CRC Errors (L)        |        Framing Errors (H)     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Framing Errors (L)    |        Hardware Overruns (H)  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Hardware Overruns (L) |        Buffer Overruns (H)    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Buffer Overruns  (L)  |        Time-out Errors (H)    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Time-out Errors (L)   |        Alignment Errors (H)   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Alignment Errors (L)  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The following fields are defined:

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

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

   ACCM (SLI)

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

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

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Reserved             |    Send ACCM (H)              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Send ACCM   (L)      |    Receive ACCM (H)           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Receive ACCM  (L)    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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5.0 Protocol Operation

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

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

   Figure 5.1 Tunneling PPP

5.1 Control Connection Establishment

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

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

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

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

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   5.1.1 Tunnel Authentication

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

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

5.2 Session Establishment

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

   5.2.1 Incoming Call Establishment

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

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

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

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

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   5.2.2 Outgoing Call Establishment

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

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

         (Perform
          Call
          Operation)

         OCCN ->
                  <- ZLB ACK

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

5.3 Forwarding PPP Frames

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

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

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

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5.4 Using Sequence Numbers on the Data Channel

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

   Unlike the L2TP control channel, the L2TP data channel does not use
   sequence numbers to retransmit lost data messages. Rather, data
   messages may use sequence numbers to detect lost packets and/or
   restore the original sequence of packets that may have been reordered
   during transport.  The LAC may request that sequence numbers be
   present in data messages via the Sequencing Required AVP (see section
   4.4.6). If this AVP is present during session setup, sequence numbers
   MUST be present at all times at the LNS and LAC.

   If the Sequencing Required AVP is not present, sequence number
   presence is under control of the LNS. The LNS controls enabling and
   disabling of sequence numbers by sending a data message with or
   without sequence numbers present at any time during the life of a
   session. Thus, if the LAC receives a data message without sequence
   numbers present, it MUST stop sending sequence numbers in future data
   messages. If the LAC receives a data message with sequence numbers
   present, it MUST begin sending sequence numbers in future outgoing
   data messages. If the LNS enables sequencing after disabling it
   earlier in the session, the sequence number count picks up where it
   left off before.

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

5.5 Keepalive (Hello)

   A keepalive mechanism is employed by L2TP in order to differentiate
   tunnel outages from extended periods of no control or data activity
   on a tunnel. This is accomplished by injecting Hello control messages
   (see section 6.5) after a specified period of time has elapsed since
   the last data or control message was received on a tunnel. As for any

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   other control message, if the Hello message is not reliably delivered
   then the tunnel is declared down and is reset. The transport reset
   mechanism along with the injection of Hello messages ensures that a
   connectivity failure between the LNS and the LAC will be detected at
   both ends of a tunnel.

5.6 Session Teardown

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

      LAC or LNS  LAC or LNS

      CDN ->
      (Clean up)

                  <- ZLB ACK
                     (Clean up)

5.7 Control Connection Teardown

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

      LAC or LNS  LAC or LNS

      StopCCN ->
      (Clean up)

                  <- ZLB ACK
                     (Wait)
                     (Clean up)

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

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5.8 Reliable Delivery of Control Messages

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

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

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

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

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

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   dropping out of order control packets, Nr MAY be updated before the
   packet is discarded.

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

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

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

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

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

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

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

6.0 Control Connection Protocol Specification

   The following control connection messages are used to establish,
   clear and maintain L2TP tunnels. All data is sent in network order
   (high order octets first). Any "reserved" or "empty" fields MUST be
   sent as 0 values to allow for protocol extensibility.

6.1 Start-Control-Connection-Request (SCCRQ)

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

   The following AVPs MUST be present in the SCCRQ:

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

   The Following AVPs MAY be present in the SCCRQ:

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

6.2 Start-Control-Connection-Reply (SCCRP)

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

   The following AVPs MUST be present in the SCCRP:

      Message Type
      Protocol Version
      Framing Capabilities
      Host Name
      Assigned Tunnel ID

   The following AVPs MAY be present in the SCCRP:

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

6.3 Start-Control-Connection-Connected (SCCCN)

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

   The following AVP MUST be present in the SCCCN:

      Message Type

   The following AVP MAY be present in the SCCCN:

      Challenge Response

6.4 Stop-Control-Connection-Notification (StopCCN)

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

   The following AVPs MUST be present in the StopCCN:

      Message Type
      Result Code

   The Assigned Tunnel ID MUST be present in the StopCCN if it has been
   sent in a previous message (see section 4.4.3).

6.5 Hello (HELLO)

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

   The sending of HELLO messages and the policy for sending them are
   left up to the implementation. A peer MUST NOT expect HELLO messages

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   at any time or interval. As with all messages sent on the control
   connection, the receiver will return either a ZLB ACK or an
   (unrelated) message piggybacking the necessary acknowledgement
   information.

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

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

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

   The Following AVP MUST be present in the HELLO message:

      Message Type

6.6 Incoming-Call-Request (ICRQ)

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

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

   The following AVPs MUST be present in the ICRQ:

      Message Type
      Assigned Session ID
      Call Serial Number

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   The following AVPs MAY be present in the ICRQ:

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

6.7 Incoming-Call-Reply (ICRP)

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

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

   The following AVPs MUST be present in the ICRP:

      Message Type
      Assigned Session ID

6.8 Incoming-Call-Connected (ICCN)

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

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

   If the ICCN is carrying crucial PPP initialization information (such
   as Proxy LCP or Auth), the LAC SHOULD wait until the ICCN has been
   acknowledged before sending data messages. This is recommended
   because (1) there is no guarantee that the ICCN and first data
   messages will not be reordered during transit, and (2) the LNS may
   take some time to initialize its PPP session with the proxied
   information.

   The following AVPs MUST be present in the ICCN:

      Message Type

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      (Tx) Connect Speed
      Framing Type

   The following AVPs MAY be present in the ICCN:

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

6.9 Outgoing-Call-Request (OCRQ)

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

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

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

   The following AVPs MUST be present in the OCRQ:

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

   The following AVPs MAY be present in the OCRQ:

      Sub-Address

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6.10 Outgoing-Call-Reply (OCRP)

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

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

   The following AVPs MUST be present in the OCRP:

      Message Type
      Assigned Session ID

   The following AVPs MAY be present in the OCRP:

      Physical Channel ID

6.11 Outgoing-Call-Connected (OCCN)

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

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

   The following AVPs MUST be present in the OCCN:

      Message Type
      (Tx) Connect Speed
      Framing Type

   The following AVPs MAY be present in the OCCN:

      Rx Connect Speed
      Sequencing Required

6.12 Call-Disconnect-Notify (CDN)

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

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   success or failure for such cleanup.

   The following AVPs MUST be present in the CDN:

      Message Type
      Result Code

   The Assigned Session ID MUST be present in the CDN if it has been
   sent in a previous message (see section 4.4.3).

   The following AVPs MAY be present in the CDN:

      Q.931 Cause Code

6.13 WAN-Error-Notify (WEN)

   The WAN-Error-Notify message is an L2TP control message sent by the
   LAC to the LNS to indicate WAN error conditions (conditions that
   occur on the interface supporting PPP). The counters in this message
   are cumulative. This message should only be sent when an error
   occurs, and not more than once every 60 seconds. The counters are
   reset when a new call is established.

   The following AVPs MUST be present in the WEN:

      Message Type
      Call Errors

6.14 Set-Link-Info (SLI)

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

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

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   The following AVPs MUST be present in the SLI:

      Message Type
      ACCM

7.0 Control Connection State Machines

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

7.1 Control Connection Protocol Operation

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

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

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

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

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

   This MUST NOT be considered a license to send malformed AVPs, but
   simply a guide towards how to handle an improperly formatted message

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

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

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

7.2 Control Connection States

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

7.2.1 Control Connection Establishment

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

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

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

   idle            Receive SCCRP     Send StopCCN         idle
                                     Clean up

   idle            Receive SCCCN     Clean up             idle

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

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

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

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

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

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

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

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

   established     Admin             Send StopCCN         idle
                   Tunnel Close      Clean up

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

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

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

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

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

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

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

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

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

7.3 Timing considerations

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

7.4 Incoming calls

   An Incoming-Call-Request message is generated by the LAC when an
   incoming call is detected (for example, an associated telephone line
   rings). The LAC selects a Session ID and serial number and indicates
   the call bearer type. Modems should always indicate analog call type.
   ISDN calls should indicate digital when unrestricted digital service
   or rate adaptation is used and analog if digital modems are involved.
   Calling Number, Called Number, and Subaddress may be included in the
   message if they are available from the telephone network.

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

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

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

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

7.4.1 LAC Incoming Call States

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

   idle            Receive ICCN,      Clean up          idle
                   ICRP, CDN

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

   wait-tunnel     tunnel-open        Send ICRQ         wait-reply

   wait-reply      Receive ICRP,      Send ICCN         established
                   acceptable

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

   wait-reply      Receive ICRQ       Send CDN          idle
                                      Clean up

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   wait-reply      Receive CDN        Clean up          idle
                   ICCN

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

   established     Receive CDN        Clean up          idle

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

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

   The states associated with the LAC for incoming calls are:

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

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

   wait-reply
      The LAC receives either a CDN message indicating the LNS is not
      willing to accept the call (general error or don't accept) and
      moves back into the idle state, or an Incoming-Call-Reply message
      indicating the call is accepted, the LAC sends an Incoming-Call-
      Connected message and enters the established state.

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

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            message.

7.4.2 LNS Incoming Call States

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

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

   idle            Receive ICRP       Send CDN          idle
                                      Clean up

   idle            Receive ICCN       Clean up          idle

   wait-connect    Receive ICCN       Prepare for       established
                   acceptable         data

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

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

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

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

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

   The states associated with the LNS for incoming calls are:

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

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

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

7.5 Outgoing calls

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

7.5.1 LAC Outgoing Call States

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

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

   idle            Receive OCRP       Send CDN          idle
                                      Clean up

   idle            Receive OCCN,      Clean up          idle
                   CDN

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

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

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

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

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

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

   The states associated with the LAC for outgoing calls are:

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

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

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

7.5.2 LNS Outgoing Call States

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

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   idle            Receive OCCN,      Clean up          idle
                   OCRP, CDN

   wait-tunnel     tunnel-open        Send OCRQ         wait-reply

   wait-reply      Receive OCRP,      none              wait-connect
                   acceptable

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

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

   wait-connect    Receive OCCN       none              established

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

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

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

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

   wait-tunnel     Local              Clean up          idle
                   Close request

   The states associated with the LNS for outgoing calls are:

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

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

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

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

7.6 Tunnel Disconnection

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

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

8.0 L2TP Over Specific Media

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

8.1 L2TP over UDP/IP

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

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   the free port it found. Once the source and destination ports and
   addresses are established, they MUST remain static for the life of
   the tunnel.

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

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

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

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

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

   Allowing packets to be dropped silently is perhaps more problematic
   with some protocols. If PPP reliable delivery [RFC1663] is enabled,
   no upper PPP protocol will encounter lost packets. If L2TP sequence
   numbers are enabled, L2TP can detect the packet loss. In the case of
   an LNS, the PPP and L2TP stacks are both present within the LNS, and
   packet loss signaling may occur precisely as if a packet was received

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   with a CRC error. Where the LAC and PPP stack are co-resident, this
   technique also applies. Where the LAC and PPP client are physically
   distinct, the analogous signaling MAY be accomplished by sending a
   packet with a CRC error to the PPP client. Note that this would
   greatly increase the complexity of debugging client line problems,
   since the client statistics could not distinguish between true media
   errors and LAC-initiated ones. Further, this technique is not
   possible on all hardware.

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

   In general, it is wise to remember that the L2TP/UDP/IP transport is
   an unreliable transport. As with any PPP media that is subject to
   loss, care should be taken when using protocols that are particularly
   loss-sensitive. Such protocols include compression and encryption
   protocols that employ history.

8.2 IP

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

9.0 Security Considerations

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

9.1 Tunnel Endpoint Security

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

   For authentication to occur, the LAC and LNS MUST share a single

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

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

9.2 Packet Level Security

   Securing L2TP requires that the underlying transport make available
   encryption, integrity and authentication services for all L2TP
   traffic.  This secure transport operates on the entire L2TP packet
   and is functionally independent of PPP and the protocol being carried
   by PPP. As such, L2TP is only concerned with confidentiality,
   authenticity, and integrity of the L2TP packets between its tunnel
   endpoints (the LAC and LNS), not unlike link-layer encryption being
   concerned only about protecting the confidentiality of traffic
   between its physical endpoints.

9.3 End to End Security

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

9.4 L2TP and IPsec

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

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

   IPsec also defines access control features that are required of a
   compliant IPsec implementation. These features allow filtering of
   packets based upon network and transport layer characteristics such
   as IP address, ports, etc. In the L2TP tunneling model, analogous

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   filtering is logically performed at the PPP layer or network layer
   above L2TP.  These network layer access control features may be
   handled at the LNS via vendor-specific authorization features based
   upon the authenticated PPP user, or at the network layer itself by
   using IPsec transport mode end-to-end between the communicating
   hosts. The requirements for access control mechanisms are not a part
   of the L2TP specification and as such are outside the scope of this
   document.

9.5 Proxy PPP Authentication

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

10.0 IANA Considerations

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

10.1 AVP Attributes

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

10.2 Message Type AVP Values

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

10.3 Result Code AVP Values

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

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10.3.1 Result Code Field Values

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

10.3.2 Error Code Field Values

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

10.4 Framing Capabilities & Bearer Capabilities

   The Framing Capabilities AVP and Bearer Capabilities AVPs (defined in
   section 4.4.3) both contain 32-bit bitmasks. Additional bits should
   only be defined via a Standards Action [RFC 2434].

10.5 Proxy Authen Type AVP Values

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

10.6 AVP Header Bits

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

11.0 References

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

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

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

   [RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",

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             STD 13, RFC 1034, November 1987.

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

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

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

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

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

   [RFC1990] Sklower, K., Lloyd, B., McGregor, G., Carr, D. and T.
             Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990,
             August 1996.

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

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

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

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

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

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

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

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

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

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

12.0 Acknowledgments

   The basic concept for L2TP and many of its protocol constructs were
   adopted from L2F [RFC2341] and PPTP [RFC2637]. Authors of these are
   A. Valencia, M. Littlewood, T. Kolar, K. Hamzeh, G. Pall, W.
   Verthein, J. Taarud, W. Little, and G. Zorn.

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

   Steve Cobb and Evan Caves redesigned the state machine tables.

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

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

13.0 Authors' Addresses

   Gurdeep Singh Pall
   Microsoft Corporation
   Redmond, WA

   Email: [email-masked]

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

   Email: [email-masked]

   Allan Rubens

   Email: [email-masked]

   W. Mark Townsley

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   cisco Systems
   7025 Kit Creek Road
   PO Box 14987
   Research Triangle Park, NC 27709

   Email: [email-masked]

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

   Email: [email-masked]

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

   Email: [email-masked]

Appendix A: Control Channel Slow Start and Congestion Avoidance

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

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

   A sender starts out in the slow start phase. CWND is initialized to
   one packet, and SSHTRESH is initialized to the advertised window
   (obtained from the Receive Window AVP).  The sender then transmits
   one packet and waits for its acknowledgement (either explicit or
   piggybacked). When the acknowledgement is received, the congestion
   window is incremented from one to two.  During slow start, CWND is
   increased by one packet each time an ACK (explicit ZLB or
   piggybacked) is received. Increasing CWND by one on each ACK has the

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   effect of doubling CWND with each round trip, resulting in an
   exponential increase. When the value of CWND reaches SSHTRESH, the
   slow start phase ends and the congestion avoidance phase begins.

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

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

Appendix B: Control Message Examples

B.1: Lock-step tunnel establishment

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

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

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

B.2: Lost packet with retransmission

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

               LAC                               LNS
               ---                               ---

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           ICRQ      ->
           Nr: 1, Ns: 2

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

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

          ICRQ      ->
          Nr: 1, Ns: 2

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

                                            <-       ZLB
                                            Nr: 3, Ns: 2

                          (LNS's retransmit timer fires)

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

                                            <-       ZLB
                                            Nr: 4, Ns: 2

Appendix C: Intellectual Property Notice

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

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

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INTERNET DRAFT                    L2TP                     November 2000

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

Townsley, et al.            Standards Track                    [Page 77]

---

From: Jhilmil Kochar <[email-masked]>
Date: Wed, 29 Nov 2000 18:07:11 +0530

Hi,
    I had two questions regarding the latest L2TP draft.

1.section 4.3. AVP Hiding.
"The H bit MUST only be set if a shared secret exists between the LAC
   having an H bit of 1."

I am a little confused with this paragraph. I will mention my interpretation of
this. Please correct me if I go wrong.
a) If peer is sending SCCRQ and does not use any authentication, then it
may  hide  AVPs in the messsage.
b) If peer is sending SCCRQ and uses some tunnel authentication procedure (CHAP
or hostname), then it MUST not hide any AVPs in the messsage (SCCRQ).
c) On receiving side, for SCCRQ, the peer will need to have a 2 step procedure
for decoding, first it should decode Hostname and use that to identify the
tunnel (or some other means to identify the tunnel), get the shared secret,
once it has been identified, then decode the hidden AVPs of that message.
OR, does the peer need to wait for further messages (that may contain CHAP
response) before it can decode hidden AVPs? Under what condition will
additional setup messages be awaited for? When we say authentication, is it
local , or at the remote end, ie if the local peer has received CHAP resp and
has authenticated, it talks of that , or after sending a message and getting a
SCCCN indicating that peer authentication is complete.

Please clarify the various scenarios here for both peers identifying who can
enable AVP hiding and when based on whether they are using CHAP, or
authentication using hostname or no authentication.

2. section 4.4.4.
"If the CDN is sent before an Assigned Session ID
      is communicated (e.g. in response to an ICRQ), it MUST NOT be sent
      in the CDN message."
Does this also imply that this SHOULD not be retransmitted as in the case of
Stop CCN in this case?

Thanks in advance for the response.

regards
Jhilmil

------------------------------
Jhilmil Kochar
Globespan India
E-mail:mailto:[email-masked]
Web: http://www.globespan.net

---

From: Jhilmil Kochar <[email-masked]>
Date: Wed, 29 Nov 2000 18:07:11 +0530

Hi,

2. section 4.4.4.

regards
Jhilmil

---

From: "W. Mark Townsley" <[email-masked]>
Date: Wed, 29 Nov 2000 08:54:38 -0500

Rereading this, I believe the first sentence is confusing. It should
read "The H bit MUST only be set if a shared secret exists between the
LAC and LNS and tunnel authentication is enabled."

Completion of tunnel authentication has nothing to do with *setting* the
H bit. The most important point I was trying to make is covered in the
third sentence, "Hidden values MUST NOT be unhidden until..."

Answers to your questions below:

> may  hide  AVPs in the messsage.

No, it may NOT hide AVPs if authentication is turned off.

> or hostname), then it MUST not hide any AVPs in the messsage (SCCRQ).

Hostname is not a tunnel authentication procedure, only CHAP is.
You have it backwards, you may only hide if tunnel auth is configured
and unhide AVPs if tunnel auth occurred.

> SCCCN indicating that peer authentication is complete.

The idea is that if you do receive any hidden AVPs in the SCCxxx
messages, you should wait until your peer has been authenticated before
unhiding the values. To do otherwise risks unhiding with an incorrect
password and sending bogus values into your AVP processing logic. If
there were a checksum of sorts on each AVP being unhidden, this would
not be a problem. As it stands, if tunnel auth is not configured and a
shared secret is used for hiding/unhiding that does not match between
the LAC and LNS, there is no way to verify the integrity of the unhidden
value.

> Does this also imply that this SHOULD not be retransmitted as in the case of

Additional text was included for the Assigned Tunnel ID AVP in the
StopCCN as well, if that is what you mean. The StopCCN never contains an
Assigned Session ID AVP.