Subject: [L2tpext] wrong version of draft-ietf-l2tpext-l2tp-base-08.txt posted

From: Mark Townsley <[email-masked]>
Date: Mon, 14 Jul 2003 07:25:42 -0700 (PDT)

Please find a corrected version of draft-ietf-l2tpext-l2tp-base-08.txt
below. An early version was posted in error a few weeks ago. It did
not contain a number of small fixes. 

This version has been submitted to [email-masked] as -09, but
will not be available until after the IETF meeting this week.

Apologies for any confusion.

- Mark

Network Working Group                                             J. Lau
Internet-Draft                                               M. Townsley
Category: Standards Track                                  cisco Systems
<draft-ietf-l2tpext-l2tp-base-08.txt>                          I. Goyret
                                                     Lucent Technologies
                                                                 Editors
                                                               June 2003

                Layer Two Tunneling Protocol (Version 3)

Status of this Memo

   This document is an Internet-Draft and is subject to 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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/1id-abstracts.html.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document describes "version 3" of the Layer Two Tunneling
   Protocol (L2TPv3). L2TPv3 defines the base control protocol and
   encapsulation for tunneling multiple layer 2 connections between two
   IP connected nodes.  Additional documents detail the specifics for
   each link-type being emulated.

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   Contents

   Status of this Memo..........................................    1

   1.  Introduction.............................................    4
      1.1  Changes from RFC 2661................................    5
      1.2  Specification of Requirements........................    5
      1.3  Terminology..........................................    5

   2.  Topology.................................................    9

   3.  Protocol Overview........................................   10
      3.1  Control Message Types................................   11
      3.2  L2TP Header Formats..................................   12
         3.2.1  L2TP Control Message Header.....................   12
         3.2.2  L2TP Data Message...............................   13
      3.3  Control Connection Management........................   14
         3.3.1  Control Connection Establishment................   14
         3.3.2  Control Connection Teardown.....................   15
      3.4  Session Management...................................   15
         3.4.1  Session Establishment for an Incoming Call......   15
         3.4.2  Session Establishment for an Outgoing Call......   16
         3.4.3  Session Teardown................................   16

   4.  Protocol Operation.......................................   17
      4.1  L2TP Over Specific Packet-Switched Networks (PSN)....   17
         4.1.1  L2TPv3 over IP..................................   18
         4.1.2  L2TP over UDP...................................   19
         4.1.3  IP Fragmentation Issues.........................   21
      4.2  Reliable Delivery of Control Messages................   21
      4.3  Control Connection and Control Message Authentication   23
      4.4  Keepalive (Hello)....................................   24
      4.5  Forwarding Session Data Frames.......................   25
      4.6  Default L2-Specific Sublayer.........................   25
         4.6.1  Sequencing Data Packets.........................   26
      4.7  L2TPv2/v3 Interoperability and Migration.............   27
         4.7.1  L2TPv3 over IP..................................   27
         4.7.2  L2TPv3 over UDP.................................   27
         4.7.3  Automatic L2TPv2 Fallback.......................   28

   5.  Control Message Attribute Value Pairs....................   28
      5.1  AVP Format...........................................   28
      5.2  Mandatory AVPs.......................................   30
      5.3  Hiding of AVP Attribute Values.......................   31
      5.4  AVP Summary..........................................   33
         5.4.1  General Control Message AVPs....................   33
         5.4.2  Result and Error Codes..........................   36
         5.4.3  Control Connection Management AVPs..............   39

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         5.4.4  Session Management AVPs.........................   44
         5.4.5  Circuit Status AVPs.............................   53

   6.  Control Connection Protocol Specification................   55
      6.1  Start-Control-Connection-Request (SCCRQ).............   55
      6.2  Start-Control-Connection-Reply (SCCRP)...............   56
      6.3  Start-Control-Connection-Connected (SCCCN)...........   56
      6.4  Stop-Control-Connection-Notification (StopCCN).......   57
      6.5  Hello (HELLO)........................................   57
      6.6  Incoming-Call-Request (ICRQ).........................   58
      6.7  Incoming-Call-Reply (ICRP)...........................   58
      6.8  Incoming-Call-Connected (ICCN).......................   59
      6.9  Outgoing-Call-Request (OCRQ).........................   60
      6.10  Outgoing-Call-Reply (OCRP)..........................   60
      6.11  Outgoing-Call-Connected (OCCN)......................   61
      6.12  Call-Disconnect-Notify (CDN)........................   62
      6.13  WAN-Error-Notify (WEN)..............................   62
      6.14  Set-Link-Info (SLI).................................   62
      6.15  Explicit-Acknowledgement (ACK)......................   63

   7.  Control Connection State Machines........................   63
      7.1  Malformed Control Messages...........................   64
      7.2  Timing Considerations................................   65
      7.3  Control Connection States............................   65
      7.4  Incoming Calls.......................................   67
         7.4.1  ICRQ Sender States..............................   67
         7.4.2  ICRQ Recipient States...........................   69
      7.5  Outgoing Calls.......................................   70
         7.5.1  OCRQ Sender States..............................   70
         7.5.2  OCRQ Recipient (LAC) States.....................   71
      7.6  Termination of a Control Connection..................   73

   8.  Security Considerations..................................   73
      8.1  Control Connection Endpoint and Message Security.....   73
      8.2  Data Channel Security................................   73
      8.3  End-to-End Security..................................   74
      8.4  L2TP and IPsec.......................................   74
      8.5  Impact of L2TPv3 Features on RFC 3193................   75

   9.  Internationalization Considerations......................   75

   10.  IANA Considerations.....................................   75
      10.1  Control Message Attribute Value Pairs (AVPs)........   75
      10.2  Message Type AVP Values.............................   76
      10.3  Result Code AVP Values..............................   76
         10.3.2  Error Code Field Values........................   76
      10.4  AVP Header Bits.....................................   76
      10.5  L2TP Control Message Header Bits....................   76

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      10.6 Pseudowire Types.....................................   77
      10.7 Application Code.....................................   77
      10.8 Circuit Status Bits..................................   77
      10.9 Default L2-Specific Sublayer bits....................   77
      10.10 L2-Specific Sublayer Type...........................   78
      10.11 Data Sequencing Level...............................   78

   13.  Acknowledgments.........................................   78

   11.  References..............................................   79
      11.1  Normative References................................   79
      11.2  Informative References..............................   80

   12.  Editors' Addresses......................................   81

   Appendix A: Control Slow Start and Congestion Avoidance......   81

   Appendix B: Control Message Examples.........................   82

   Appendix C: Processing Sequence Numbers......................   84

   Appendix D: Intellectual Property Notice.....................   86

   Appendix E: Full Copyright Statement.........................   86

1.  Introduction

   The Layer Two Tunneling Protocol (L2TP) provides a dynamic mechanism
   for tunneling Layer 2 (L2) "circuits" across a packet-oriented data
   network (e.g., over IP). L2TP, as originally defined in RFC 2661, is
   a standard method for tunneling Point to Point Protocol (PPP)
   [RFC1661] sessions.  L2TP has since been adopted for tunneling a
   number of other L2 protocols.  In order to provide greater
   modularity, this document describes the base L2TP protocol,
   independent of the L2 payload that is being tunneled.

   The base L2TP protocol defined in this document consists of (1) the
   control protocol for dynamic creation, maintenance, and teardown of
   L2TP sessions, and (2) the L2TP data encapsulation to multiplex and
   demultiplex L2 data streams between two L2TP nodes across an IP
   network. Additional documents are expected to be published for each
   layer 2 data link emulation type (a.k.a. pseudowire-type) supported
   by L2TP (i.e., PPP, Ethernet, Frame Relay, etc.). These documents
   will contain any individual details that are outside the scope of
   this base specification.

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1.1  Changes from RFC 2661

   Many of the protocol constructs described in this document are
   carried over from RFC 2661.  Changes include clarifications based on
   years of interoperability and deployment experience as well as
   modifications to either improve protocol operation or provide a
   clearer separation from PPP.  The intent of these modifications is to
   achieve a healthy balance between code reuse, interoperability
   experience, and a directed evolution of L2TP as it is applied to new
   tasks.

   When the designation between L2TPv2 and L2TPv3 is necessary, L2TP as
   defined in RFC 2661 will be referred to as "L2TPv2", corresponding to
   the value in the Version field of an L2TP header.  (Layer 2
   Forwarding, L2F, [RFC2341] was defined as "version 1".)  At times,
   L2TP as defined in this document will be referred to as "L2TPv3".
   Otherwise, the acronym "L2TP" will refer to L2TPv3 or L2TP in
   general.

   Notable differences between L2TPv2 and L2TPv3 include the following:
   - Separation of all PPP-related AVPs, references, etc., including a
     portion of the L2TP data header that was specific to the needs of
     PPP.  The PPP-specific constructs are described in a companion
     document.
   - Transition from a 16-bit Session ID and Tunnel ID to a 32-bit
     Session ID and Control Connection ID, respectively.

   Details of these changes and a recommendation for transitioning to
   L2TPv3 may be found in Section 4.7.

1.2  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].

1.3  Terminology

   Attribute Value Pair (AVP)

      The variable-length concatenation of a unique Attribute
      (represented by an integer), a length field, and a Value
      containing the actual value identified by the attribute.  Zero or
      more AVPs make up the body of control messages, which are used in
      the establishment, maintenance, and teardown of control
      connections.  This basic construct is sometimes referred to as a
      Type-Length-Value (TLV) in some specifications.  (See also:
      Control Connection, Control Message.)

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   Call (Circuit Up)

      The action of transitioning a circuit on an L2TP Access
      Concentrator (LAC) to an "up" or "active" state.  A call may be
      dynamically established through signaling properties (e.g., an
      incoming or outgoing call through the Public Switched Telephone
      Network (PSTN)) or statically configured (e.g., provisioning a
      Virtual Circuit on an interface).  A call is defined by its
      properties (e.g., type of call, called number, etc.) and its data
      traffic.  (See also: Circuit, Session, Incoming Call, Outgoing
      Call, Outgoing Call Request.)

   Circuit

      A general term identifying any one of a wide range of L2
      connections.  A circuit may be virtual in nature (e.g., an ATM
      PVC, an ethernet VLAN, or an L2TP session), or it may have direct
      correlation to a physical layer (e.g., an RS-232 serial line).
      Circuits may be statically configured with a relatively long-lived
      uptime, or dynamically established with signaling to govern the
      establishment, maintenance, and teardown of the circuit.  For the
      purposes of this document, a statically configured circuit is
      considered to be essentially the same as a very simple, long-
      lived, dynamic circuit.  (See also: Call, Remote System.)

   Client

      (See Remote System.)

   Control Connection

      An L2TP control connection is a reliable control channel that is
      used to establish, maintain, and release individual L2TP sessions
      as well as the control connection itself.  (See also: Control
      Message, Data Channel.)

   Control Message

      An L2TP message used by the control connection.  (See also:
      Control Connection.)

   Data Message

      Message used by the data channel.  (a.k.a. Data Packet, See also:
      Data Channel.)

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   Data Channel

      The channel for L2TP-encapsulated data traffic that passes between
      two LCCEs over a Packet Switched Network (i.e. IP).  (See also:
      Control Connection, Data Message.)

   Incoming Call

      The action of receiving a call (circuit up event) on an LAC.  The
      call may have been placed by a remote system (e.g., a phone call
      over a PSTN), or it may have been triggered by a local event
      (e.g., interesting traffic routed to a virtual interface).  An
      incoming call that needs to be tunneled (as determined by the LAC)
      results in the generation of an L2TP ICRQ message.  (See also:
      Call, Outgoing Call, Outgoing Call Request.)

   L2TP Access Concentrator (LAC)

      If an L2TP Control Connection Endpoint (LCCE) is being used to
      cross-connect an L2TP session directly to a data link, we refer to
      it as an L2TP Access Concentrator (LAC). An LCCE may act as both
      an L2TP Network Server (LNS) for some sessions and an LAC for
      others, so these terms must only be used within the context of a
      given set of sessions unless the LCCE is in fact single purpose
      for a given topology.  (See also: LCCE, LNS.)

   L2TP Control Connection Endpoint (LCCE)

      An L2TP node which exists at either end of an L2TP control
      connection. May also be referred to as an LAC or LNS, depending on
      whether tunneled frames are processed at the data link (LAC) or
      network layer (LNS). (See also: LAC, LNS.)

   L2TP Network Server (LNS)

      If a given L2TP session is terminated at the L2TP node and the
      encapsulated network layer (L3) packet processed on a virtual
      interface, we refer to this L2TP node as an L2TP Network Server
      (LNS). A given LCCE may act as both an LNS for some sessions and
      an LAC for others, so these terms must only be used within the
      context of a given set of sessions unless the LCCE is in fact
      single purpose for a given topology. (See also: LCCE, LAC.)

   Outgoing Call

      The action of placing a call by an LAC, typically in response to
      policy directed by the peer in an Outgoing Call Request message.
      (See also: Call, Incoming Call, Outgoing Call Request.)

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   Outgoing Call Request

      A request sent to an LAC to place an outgoing call.  The request
      contains specific information not known a priori by the LAC (i.e.,
      a number to dial).  (See also: Call, Incoming Call, Outgoing
      Call.)

   Packet-Switched Network (PSN)

      A network that uses packet-switching technology for data delivery.
      For L2TPv3, this layer is principally IP.  Other examples include
      MPLS, Frame Relay, and ATM.

   Peer

      When used in context with L2TP, Peer refers to the far end of an
      L2TP control connection (i.e., the remote LCCE).  An LAC's peer
      may be either an LNS or another LAC.  Similarly, an LNS's peer may
      be either an LAC or another LNS.  (See also: LAC, LCCE, LNS.)

   Pseudowire (PW)

      An emulated circuit as it traverses a PSN.  There is one
      Pseudowire per L2TP Session.  (See also: Packet-Switched Network,
      Session.)

   Pseudowire Type

      The payload type being carried within an L2TP session.  Examples
      include PPP, Ethernet, and Frame Relay.  (See also: Session.)

   Remote System

      An end-system or router connected by a circuit to an LAC.

   Session

      An L2TP session is the entity which is created between two LCCEs
      in order to exchange parameters for and maintain an emulated L2
      connection. Multiple sessions may be associated with a single
      Control Connection.

   Zero-Length Body (ZLB) Message

      A control message with only an L2TP header.  ZLB messages are used
      only to acknowledge messages on the L2TP reliable control channel.
      (See also: Control Message.)

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2.  Topology

   L2TP operates between two L2TP Control Connection Endpoints (LCCEs),
   tunneling traffic across a packet network.  There are three
   predominant tunneling models in which L2TP operates: LAC-LNS (or vice
   versa), LAC-LAC, and LNS-LNS.  These models are diagrammed below.
   (Dotted lines designate network connections.  Solid lines designate
   circuit connections.)

                     Figure 2.0: L2TP Reference Models

   (a) LAC-LNS Reference Model: On one side, the LAC receives traffic
   from an L2 circuit, which it forwards via L2TP across an IP or other
   packet-based network.  On the other side, an LNS logically terminates
   the L2 circuit locally and routes network traffic to the home
   network.  The action of session establishment is driven by the LAC
   (as an incoming call) or the LNS (as an outgoing call).

   +-----+  L2  +-----+                        +-----+
   |     |------| LAC |.........[ IP ].........| LNS |...[home network]
   +-----+      +-----+                        +-----+
   remote
   system
                      |<-- emulated service -->|
         |<----------- L2 service ------------>|

   (b) LAC-LAC Reference Model: In this model, both LCCEs are LACs.
   Each LAC forwards circuit traffic from the remote system to the peer
   LAC using L2TP, and vice versa.  In its simplest form, an LAC acts as
   a simple cross-connect between a circuit to a remote system and an
   L2TP session.  This model typically involves symmetric establishment;
   that is, either side of the connection may initiate a session at any
   time (or simultaneously, in which a tie-breaking mechanism is
   utilized).

   +-----+  L2  +-----+                      +-----+  L2  +-----+
   |     |------| LAC |........[ IP ]........| LAC |------|     |
   +-----+      +-----+                      +-----+      +-----+
   remote                                                 remote
   system                                                 system
                      |<- emulated service ->|
         |<----------------- L2 service ----------------->|

   (c) LNS-LNS Reference Model: This model has two LNSs as the LCCEs.  A
   user-level, traffic-generated, or signaled event typically drives
   session establishment from one side of the tunnel. For example, a
   tunnel generated from a PC by a user, or automatically by customer
   premises equipment.

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                    +-----+                      +-----+
   [home network]...| LNS |........[ IP ]........| LNS |...[home network]
                    +-----+                      +-----+
                          |<- emulated service ->|
                          |<---- L2 service ---->|

   Note: In L2TPv2, user-driven tunneling of this type is often referred
   to as "voluntary tunneling" [RFC2809]. Further, an LNS acting as part
   of a software package on a host is sometimes referred to as an "LAC
   Client" [RFC2661].

3.  Protocol Overview

   L2TP utilizes two types of messages, control messages and data
   messages (sometimes referred to as "control packets" and "data
   packets", respectively).  Control messages are used in the
   establishment, maintenance, and clearing of control connections and
   sessions.  These messages utilize a reliable control channel within
   L2TP to guarantee delivery (see Section 4.2 for details).  Data
   messages are used to encapsulate the L2 traffic being carried over
   the L2TP session.  Unlike control messages, data messages are not
   retransmitted when packet loss occurs.

   The L2TPv3 control message format defined in this document borrows
   largely from L2TPv2. These control messages are used in conjunction
   with the associated protocol state machines that govern the dynamic
   setup, maintenance, and teardown for L2TP sessions. The data message
   format for tunneling data packets may be utilized with or without the
   L2TP control channel, either via manual configuration or other
   signaling methods to pre-configure or distribute L2TP session
   information. Utilization of the L2TP data message format with other
   signaling methods is outside the scope of this document.

                       Figure 3.0: L2TPv3 Structure

   +-------------------+    +-----------------------+
   | Tunneled Frame    |    | L2TP Control Message  |
   +-------------------+    +-----------------------+
   | L2TP Data Header  |    | L2TP Control Header   |
   +-------------------+    +-----------------------+
   | L2TP Data Channel |    | L2TP Control Channel  |
   | (unreliable)      |    | (reliable)            |
   +-------------------+----+-----------------------+
   | Packet-Switched Network (IP, FR, MPLS, etc.)   |
   +------------------------------------------------+

   Figure 3.0 depicts the relationship of control messages and data
   messages over the L2TP control and data channels, respectively.  Data

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   messages are passed over an unreliable data channel, encapsulated by
   an L2TP header, and sent over a Packet-Switched Network (PSN) such as
   IP, UDP, Frame Relay, ATM, MPLS, etc.  Control messages are sent over
   a reliable L2TP control channel, which operates over the same PSN.

   The necessary setup for tunneling a session with L2TP consists of two
   steps: (1) Establishing the control connection, and (2) establishing
   a session as triggered by an incoming call or outgoing call.  An L2TP
   session MUST be established before L2TP can begin to forward session
   frames.  Multiple sessions may be bound to a single control
   connection, and multiple control connections may exist between the
   same two LCCEs.

3.1  Control Message Types

   The Message Type AVP (see Section 5.4.1) defines the specific type of
   control message being sent.

   This document defines the following control message types (see
   Sections 6.1 through 6.13 for details on the construction and use of
   each message):

   Control Connection Management

      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
      TBA-M1 (ACK)  Explicit Acknowledgement

   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

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   Link Status Change Reporting

      16 (SLI)      Set-Link-Info

3.2  L2TP Header Formats

   This section defines header formats for L2TP control messages and
   L2TP data messages.  All values are placed into their respective
   fields and sent in network order (high-order octets first).

3.2.1  L2TP Control Message Header

   The L2TP control message header provides information for the reliable
   transport of messages that govern the establishment, maintenance, and
   teardown of L2TP sessions.  By default, control messages are sent
   over the underlying media in-band with L2TP data messages.

   The L2TP control message header is formatted as follows:

                 Figure 3.2.1: L2TP Control Message Header

    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|x|x|x|x|x|x|  Ver  |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Control Connection ID                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Ns              |               Nr              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The T bit MUST be set to 1, indicating that this is a control
   message.

   The L and S bits MUST be set to 1, indicating that the Length field
   and sequence numbers are present.

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

   The Ver field indicates the version of the L2TP control message
   header described in this document.  On sending, this field MUST be
   set to 3 for all messages (unless operating in an environment which
   includes L2TPv2 [RFC2661] and/or L2F [RFC2341] as well, see Section
   4.1 for details).

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

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   always calculated from the start of the control message header itself
   (beginning with the T bit).

   The Control Connection ID field contains the identifier for the
   control connection.  L2TP control connections are named by
   identifiers that have local significance only.  That is, the same
   control connection will be given unique Control Connection IDs by
   each LCCE from within each endpoint's own Control Connection ID
   number space.  As such, the Control Connection ID in each message is
   that of the intended recipient, not the sender.  Non-zero Control
   Connection IDs are selected and exchanged as Assigned Control
   Connection ID AVPs during the creation of a control connection.

   Ns indicates the sequence number for this control message, beginning
   at zero and incrementing by one (modulo 2**16) for each message sent.
   See Section 4.2 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).  See Section 4.2 for more
   information on using this field.

3.2.2  L2TP Data Message

   In general, an L2TP data message consists of a (1) Session Header,
   (2) an optional L2-Specific Sublayer, and (3) the Tunnel Payload, as
   depicted below.

                  Figure 3.2.2: L2TP Data Message Header

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      L2TP Session Header                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      L2-Specific Sublayer                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Tunnel Payload                      ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The L2TP Session Header is specific to the encapsulating PSN over
   which the L2TP traffic is delivered.  The Session Header MUST provide
   (1) a method of distinguishing traffic among multiple L2TP data
   sessions and (2) a method of distinguishing data messages from
   control messages.

   Each type of encapsulating PSN MUST define its own session header,
   clearly identifying the format of the header and parameters necessary
   to setup the session.  Section 4.1 defines two session headers, one

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   for transport over UDP and one for transport over IP.

   The L2 Specific Sublayer is an intermediary layer between the L2TP
   session header and the start of the tunneled frame.  It contains
   control fields that are used to facilitate the tunneling of each
   frame (e.g. sequence numbers or flags).  The default L2-Specific
   Sublayer for L2TPv3 is defined in Section 4.6.

   The Data Message Header is followed by the Tunnel Payload, including
   any necessary L2 framing as defined in the payload-specific companion
   documents.

3.3  Control Connection Management

   The L2TP Control Connection handles dynamic establishment, teardown,
   and maintenance of the L2TP sessions and of the control connection
   itself.  The reliable delivery of control messages is described in
   Section 4.2.

   This section describes typical control connection establishment and
   teardown exchanges.  It is important to note that, in the diagrams
   that follow, the reliable control message delivery mechanism exists
   independently of the L2TP state machine.  For instance, Explicit
   Acknowledgement (ACK) messages may be sent after any of the control
   messages indicated in the exchanges below if an acknowledgment is not
   piggybacked on a later control message.

   LCCEs are identified during control connection establishment either
   by the Host Name AVP, the Router ID AVP, or a combination of the two
   (see Section 5.4.3).  The identity of a peer LCCE is central to
   selecting proper configuration parameters (i.e. Hello interval,
   window size, etc.) for a control connection, as well as for
   determination of how to setup associated sessions within the control
   connection, password lookup for control connection authentication,
   control connection level tie-breaking, etc.

3.3.1  Control Connection Establishment

   Establishment of the control connection involves an exchange of AVPs
   that identifies the peer and its capabilities.

   A three-message exchange is used to establish the control connection.
   The following is a typical message exchange:

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      LCCE A      LCCE B
      ------      ------
      SCCRQ ->
                  <- SCCRP
      SCCCN ->

3.3.2  Control Connection Teardown

   Control connection teardown may be initiated by either LCCE and is
   accomplished by sending a single StopCCN control message.  As part of
   the reliable control message delivery mechanism, the recipient of a
   StopCCN MUST send an ACK message 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 ACK message is lost).  The recommended time for a
   full retransmission cycle is at least 31 seconds (see Section 4.2).
   The following is an example of a typical control message exchange:

      LCCE A      LCCE B
      ------      ------
      StopCCN ->
      (Clean up)

                  (Wait)
                  (Clean up)

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

3.4  Session Management

   After successful control connection establishment, individual
   sessions may be created.  Each session corresponds to a single data
   stream between the two LCCEs.  This section describes the typical
   call establishment and teardown exchanges.

3.4.1  Session Establishment for an Incoming Call

   A three-message exchange is used to establish the session.  The
   following is a typical sequence of events:

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      LCCE A      LCCE B
      ------      ------
      (Call
       Detected)

      ICRQ ->
                 <- ICRP
      (Call
       Accepted)

      ICCN ->

3.4.2  Session Establishment for an Outgoing Call

   A three-message exchange is used to set up the session.  The
   following is a typical sequence of events:

      LCCE A      LCCE B
      ------      ------
                 <- OCRQ
      OCRP ->

      (Perform
       Call
       Operation)

      OCCN ->

      (Call Operation
       Completed
       Successfully)

3.4.3  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).  The following is an example of a typical control
   message exchange:

      LCCE A      LCCE B
      ------      ------
      CDN ->
      (Clean up)

                  (Clean up)

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4.  Protocol Operation

4.1  L2TP Over Specific Packet-Switched Networks (PSN)

   If necessary, L2TP may operate over a variety of Packet Switched
   Networks. There are two modes described for operation over IPv4, L2TP
   over IP (Section 4.1.1) and L2TP over UDP (Section 4.1.2).  L2TPv3
   implementations MUST support L2TP over IP and SHOULD support L2TP
   over UDP for better NAT and FW traversal, and easier migration from
   L2TPv2.

   L2TP over other PSNs may be defined, but the specifics are outside
   the scope of this document.  Examples of L2TPv2 over other PSNs
   include [RFC3070] and [RFC3355].

   The following field definitions are defined for use in all L2TP
   Session Header encapsulations.

   Session ID

      A 32-bit field containing a non-zero identifier for a session.
      L2TP sessions are named by identifiers that have local
      significance only.  That is, the same logical session will be
      given different Session IDs by each end of the control connection
      for the life of the session.  When the L2TP control connection is
      used for session establishment, Session IDs are selected and
      exchanged as Local Session ID AVPs during the creation of a
      session.

   Cookie

      The optional Cookie field contains a variable length (maximum 64
      bits), value used to check the association of a received data
      message with the session identified by the Session ID.  The Cookie
      MUST be set to the configured or signaled random value for this
      session utilizing all bits in the field.  The Cookie provides an
      additional level of guarantee that a data message has been
      directed to the proper session by the Session ID.  A well-chosen
      Cookie may prevent inadvertent misdirection of stray packets with
      recently reused Session IDs, Session IDs subject to packet
      corruption, etc.

      When the L2TP control connection is used for session
      establishment, random Cookie values are selected and exchanged as
      Assigned Cookie AVPs during session creation.

      The Cookie may also provide protection against some specific
      malicious packet insertion attacks, as described in section 8.2.

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4.1.1  L2TPv3 over IP

   L2TPv3 over IP utilizes the IANA assigned IP protocol ID 115.

4.1.1.1  L2TPv3 Session Header Over IP

   Unlike L2TP over UDP, the L2TPv3 session header over IP is free of
   any restrictions imposed by coexistence with L2TPv2 and L2F.  As
   such, the header format has been redesigned to optimize packet
   processing.  The following session header format is utilized when
   operating L2TPv3 over IP:

               Figure 4.1.1.1: L2TPv3 Session Header Over IP

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Session ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Cookie (optional, maximum 64 bits)...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Session ID and Cookie fields are as defined in Section 4.1.  The
   Session ID of zero is reserved for use by L2TP control messages (see
   Section 4.1.1.2).

4.1.1.2  L2TP Control and Data Traffic over IP

   Unlike L2TP over UDP which uses the common T bit to distinguish
   between L2TP control and data packets, L2TP over IP uses the reserved
   Session ID of zero (0) when sending control messages.  It is presumed
   that checking for the zero Session ID is more efficient -- both in
   header size for data packets and in processing speed for
   distinguishing between control and data messages -- than checking for
   the presence of a given single bit.

   The entire control message header over IP, including the zero session
   ID, appears as follows:

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           Figure 4.1.1.2: L2TPv3 Control Message Header Over IP

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      (32 bits of zeros)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |T|L|x|x|S|x|x|x|x|x|x|x|  Ver  |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Control Connection ID                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Ns              |               Nr              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Named fields are as defined in Section 3.2.1.  Note that the Length
   field is still calculated from the beginning of the control message
   header, beginning with the T bit.  It does NOT include the "(32 bits
   of zeros)" depicted above.

   When operating directly over IP, L2TP packets lose the ability to
   take advantage of the UDP checksum as a simple packet integrity
   check.  This is of particular concern for L2TP control messages.
   Control Message Authentication (Section 4.3), even with an empty
   password field, provides for a sufficient packet integrity check and
   SHOULD always be enabled.

4.1.2  L2TP over UDP

   L2TPv3 over UDP must consider other L2 tunneling protocols that may
   be operating in the same environment, including L2TPv2 [RFC2661] and
   L2F [RFC2341].

   While there are efficiencies gained by running L2TP directly over IP,
   there are possible side effects as well.  For instance, L2TP over IP
   is not as NAT-friendly as L2TP over UDP.

4.1.2.1  L2TP Session Header Over UDP

   The following session header format is utilized when operating L2TPv3
   over UDP:

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              Figure 4.1.2.1: L2TPv3 Session Header over UDP

    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|x|x|x|x|x|x|x|x|x|x|x|  Ver  |          Reserved             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Session ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Cookie (optional, maximum 64 bits)...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The T bit MUST be set to 0, indicating that this is a data message.

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

   The Ver field MUST be set to 3, indicating an L2TPv3 message.

   The Session ID and Cookie fields are as defined in Section 4.1.

4.1.2.2  UDP Port Selection

   L2TPv3 utilizes the same UDP port selection method as defined in
   L2TPv2 [RFC2661].

   When negotiating a control connection over UDP, control messages must
   be sent as UDP datagrams using the registered UDP port 1701
   [RFC1700].  The initiator of an L2TP control connection picks an
   available source UDP port (which may or may not be 1701), and sends
   to the desired destination address at port 1701.  The recipient picks
   a free port on its own system (which may or may not be 1701) and
   sends its reply to the initiator's UDP port and address, setting its
   own source port to the free port it found.

   Any subsequent traffic associated with this control connection
   (either control traffic or data traffic from a session established
   through this control connection) must use these same UDP ports.

   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 control connection, i.e., 1701) may make it more
   difficult for L2TP to traverse some NAT devices.  Implementations
   should consider the potential implication of this capability before

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   choosing an arbitrary source port.  A NAT device that can pass TFTP
   traffic with variant UDP ports should be able to pass L2TP UDP
   traffic since both protocols employ similar policies with regard to
   UDP port selection.

4.1.2.3  UDP Checksum

   UDP checksums MUST be enabled for control messages and MAY be enabled
   for data messages.  It should be noted that enabling checksums on
   data packets may significantly increase the data packet processing
   burden.

4.1.3  IP Fragmentation Issues

   IP fragmentation may occur as the L2TP packet travels over the IP
   substrate.  L2TP makes no special efforts defined in this document to
   optimize this.

4.2  Reliable Delivery of Control Messages

   L2TP provides a lower level reliable delivery service for all control
   messages.  The Nr and Ns fields of the control message header (see
   Section 3.2.1) belong to this delivery mechanism.  The upper level
   functions of L2TP are not concerned with retransmission or ordering
   of control messages.  The reliable control messaging mechanism is a
   sliding window mechanism that provides control message retransmission
   and congestion control.  Each peer maintains separate sequence number
   state for each control connection.

   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 ACK message), receipt of duplicate messages MUST be
   acknowledged by the reliable delivery mechanism.  This acknowledgment
   may either piggybacked on a message in queue or sent explicitly via
   an ACK message.

   All control messages take up one slot in the control message sequence
   number space, except the ACK message.  Thus, Ns is not incremented

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   after an ACK 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-
   ACK message received plus 1, modulo 65536).  While the Nr in a
   received ACK message is used to flush messages from the local
   retransmit queue (see below), the Nr of the next message sent is not
   updated by the Ns of the ACK message.  Nr SHOULD be sanity-checked
   before flushing the retransmit queue.  For instance, if the Nr
   received in a control message is greater than the last Ns sent plus 1
   modulo 65536, the control message is clearly invalid.

   The reliable delivery mechanism 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.  Alternatively, they may be discarded, thus requiring a
   retransmission by the peer.  When dropping out of order control
   packets, Nr MAY be updated before the packet is discarded.

   Each control connection 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
   but SHOULD be configurable) passes without acknowledgment, 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 SHOULD be no less than 8 seconds per retransmission.  If no peer
   response is detected after several retransmissions (a recommended
   default is 10, but MUST be configurable), the control connection and
   all associated sessions MUST be cleared. As it is the first message
   to establish a control connection, the SCCRQ MAY employ a different
   retransmission maximum than other control messages in order to help
   facilitate failover to alternate LCCEs in a timely fashion.

   When a control connection 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 (e.g. 1 + 2 + 4 + 8 + 8...

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   seconds).

   A sliding window mechanism is used for control message transmission.
   Consider two peers, A and B.  Suppose A specifies a Receive Window
   Size AVP with a value of N in the SCCRQ or SCCRP message.  B is now
   allowed to have up to N outstanding control messages.  Once N
   messages have been sent, B must wait for an acknowledgment from A
   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 (i.e. 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.  A recommended procedure is described in Appendix
   A.

   A peer MUST NOT withhold acknowledgment of messages as a technique
   for flow control of 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 actions.

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

4.3  Control Connection and Control Message Authentication

   L2TP incorporates an optional authentication and integrity check for
   all control messages. This mechanism consists of a computed one-way
   hash over the entire contents of the L2TP control message, a pre-
   configured shared secret, and a local and remote nonce (random value)
   exchanged via the Random Vector AVP. This per-message authentication
   and integrity check is designed to perform a mutual authentication
   between L2TP nodes, integrity checking of all control messages, and
   guard against control message spoofing and replay attacks that would
   otherwise be trivial to mount.

   A shared secret (password) MUST exist between communicating L2TP
   nodes to obtain the benefit of message or peer authentication. If a
   shared secret is not configured on either node, the per-message
   integrity check may still be performed using an empty shared secret
   of zero length. See Section 5.4.3 for details on calculation of the
   Message Digest and construction of the Random Vector and Message
   Digest AVPs.

   L2TPv3 Control Connection and Control Message Authentication is
   similar to L2TPv2 [RFC2661] Tunnel Authentication in its use of a

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   shared secret for peer authentication, use of a one-way hash
   calculation, and exchange of a random value. The principal difference
   is that, instead of computing the hash over selected contents of a
   received control message (e.g. the Challenge AVP and Message Type) as
   in L2TPv2, the entire message is used in the hash in L2TPv3. In
   addition, instead of including the hash digest in just the SCCRP and
   SCCCN messages, it is now included in all L2TP messages.

   The Control Message Authentication mechanism is optional, and may be
   disabled if both peers agree. For example, if IPsec is already being
   used for security and integrity checking between the LCCEs, the L2TP
   mechanism defined here becomes redundant and may be disabled.
   Presence of the Message Digest AVP in an SCCRQ or SCCRP message
   serves as an indication to a peer that Control Message Authentication
   is enabled. If an SCCRQ or SCCRP contains a Message Digest AVP, the
   receiver of the message MUST respond with a Message Digest AVP in all
   subsequent messages sent. If an SCCRQ or SCCRP is received with a
   missing or incorrect Message Digest AVP value, a StopCCN MAY be sent
   with the Result Code set to 4 (see Section 5.4.2). Care should be
   taken to limit such responses as to not end up in a denial of service
   situation responding to rogue SCCRQ or SCCRP control messages. All
   other control messages with missing or incorrect Message Digest AVPs
   MUST be dropped.

4.4  Keepalive (Hello)

   A keepalive mechanism is employed by L2TP to detect loss of
   connectivity between a pair of LCCEs.  This is accomplished by
   injecting Hello control messages (see Section 6.5) after a period of
   time has elapsed since the last data message or control message was
   received on an L2TP session or control connection, respectively.  As
   with any other control message, if the Hello message is not reliably
   delivered, the sending LCCE declares that the control connection is
   down and resets its state for the control connection.  This behavior
   ensures that a connectivity failure between the LCCEs is detected
   independently by each end of a control connection.

   Since the control channel is operated in-band with data traffic over
   the PSN, this single mechanism can be used to infer basic data
   connectivity between a pair of LCCEs for all sessions associated with
   the control connection.

   Periodic keepalive for the control connection MUST be implemented by
   sending a Hello if a period of time (a recommended default is 60
   seconds, but MUST be configurable) has passed without receiving any
   message (data or control) from the peer.  An LCCE sending Hello
   messages across multiple control connections between the same LCCE
   endpoints MUST employ a jittered timer mechanism to prevent grouping

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   of Hello messages.

4.5  Forwarding Session Data Frames

   Once session establishment is complete, circuit frames are received
   at an LCCE, encapsulated in L2TP (with appropriate attention to
   framing as described in documents for the particular pseudowire
   type), and forwarded over the appropriate session.  For every
   outgoing data message, the sender places the identifier specified in
   the Local Session ID AVP (received from peer during session
   establishment) in the Session ID field of the L2TP data header.  In
   this manner, session frames are multiplexed and demultiplexed between
   a given pair of LCCEs.  Multiple control connections may exist
   between a given pair of LCCEs, and multiple sessions may be
   associated with a given control connection.

   The peer LCCE receiving the L2TP data packet identifies the session
   with which the packet is associated by the Session ID in the data
   packet's header.  The LCCE then checks the Cookie field in the data
   packet against the Cookie value received in the Assigned Cookie AVP
   during session establishment.  It is important for implementers to
   note that the Cookie field check occurs after looking up the session
   context by the Session ID, and as such consists merely of a value
   match of the Cookie field and that stored in the retrieved context.
   There is no need to perform a lookup across the Session ID and Cookie
   as a single value.  Any received data packets that contain invalid
   Session IDs or associated Cookie values MUST be dropped.  Finally,
   the LCCE either forwards the network packet within the tunneled frame
   (e.g., as an LNS) or switches the frame to a circuit (e.g., as an
   LAC).

4.6  Default L2-Specific Sublayer

   This document defines a default L2-Specific Sublayer (see Section
   3.2.2) format that a pseudowire may use for features such as basic
   sequencing support, marking of packets with a single high-priority
   bit, or other general per-data-packet control operations.  The
   default L2-Specific Sublayer SHOULD be used by a given PW type to
   support these features if it is adequate, and its presence is
   requested by a peer during session negotiation.  Alternative
   sublayers MAY be defined (e.g. an encapsulation with a larger
   Sequence Number field or timing information) and identified for use
   via the L2-Specific Sublayer Type AVP.

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              Figure 4.6: Default L2-Specific Sublayer 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |P|S|x|x|x|x|x|x|              Sequence Number                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The P (Priority) bit is used to identify a data packet that should be
   dropped only as a last resort after being received by an L2TP peer.
   This bit should be set to 1 for any traffic that should be given
   higher priority than other data traffic in a congested environment.
   For example, end-to-end L2 keepalive packets (e.g. PPP keepalives) or
   other control packets vital to the life of the circuit or network may
   need special handling by an LCCE upon receipt.  This is not a
   replacement for, or to be used as, a per-hop QoS method of any sort.
   It is only to be used by the L2TP receiving node to prioritize
   incoming traffic.

   The S (Sequence) bit is set to 1 when the Sequence Number contains a
   valid number for this sequenced frame.  If the S bit is set to zero,
   the Sequence Number contents are undefined and MUST be ignored by the
   receiver.

   The Sequence Number field contains a free-running counter of 2^24
   sequence numbers.  If the number in this field is valid, the S bit
   MUST be set to 1.  The Sequence Number begins at zero, which is a
   valid sequence number.  (In this way, implementations inserting
   sequence numbers do not have to "skip" zero when incrementing.)  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 (2^23-1)
   values, inclusive.

4.6.1  Sequencing Data Packets

   The Sequence Number field may be used to detect lost, duplicate, or
   out-of-order packets within a given session.

   When L2 frames are carried over an L2TP-over-IP or L2TP-over-UDP/IP
   data channel, this part of the link has the characteristic of being
   able to reorder, duplicate, or silently drop packets.  Reordering may
   break some non-IP protocols or L2 control traffic being carried by
   the link.  Silent dropping or duplication of packets may break
   protocols that assume per-packet indications of error, such as TCP
   header compression.  While a common mechanism for packet sequence
   detection is provided, the sequence dependency characteristics of
   individual protocols are outside the scope of this document.

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   If any protocol being transported by over L2TP data channels cannot
   tolerate misordering of data packets, packet duplication, or silent
   packet loss, sequencing may be enabled on some or all packets by
   using the S bit and Sequence Number field defined in the default
   L2-Specific Sublayer(see Section 4.6).  For a given L2TP session,
   each LCCE is responsible for communicating to its peer the level of
   sequencing support that it requires of data packets that it receives.
   Mechanisms to advertise this information during session negotiation
   are provided (see Data Sequencing AVP in Section 5.4.4).

   When determining whether a packet is in or out of sequence, an
   implementation SHOULD utilize a method that is resilient to temporary
   dropouts in connectivity coupled with high per-session packet rates.
   The recommended method is outlined in Appendix C.

4.7  L2TPv2/v3 Interoperability and Migration

   L2TPv2 and L2TPv3 environments should be able to coexist while a
   migration to L2TPv3 is made.  Migration issues are discussed for each
   media type in this section.  Most issues apply only to
   implementations that require both L2TPv2 and L2TPv3 operation.
   However, even L2TPv3-only implementations must at least be mindful of
   these issues in order to interoperate with implementations that
   support both versions.

4.7.1  L2TPv3 over IP

   L2TPv3 implementations running strictly over IP with no desire to
   interoperate with L2TPv2 implementations may safely disregard most
   migration issues from L2TPv2.  All control messages and data messages
   are sent as described in this document, without normative reference
   to RFC2661.

   If one wishes to tunnel PPP over L2TPv3, and fallback to L2TPv2 only
   if it is not available, then L2TPv3 over UDP with the automatic
   fallback as described in section 4.7.3 MUST be used. There is no
   deterministic method for automatic fallback from L2TPv3 over IP to
   either L2TPv2 or L2TPv3 over UDP. One could infer whether L2TPv3 over
   IP is supported by sending an SCCRQ and waiting for a response, but
   this could be problematic during periods of packet loss between L2TP
   nodes.

4.7.2  L2TPv3 over UDP

   The format of the L2TPv3 over UDP header is defined in Section
   4.1.2.1.

   When operating over UDP, L2TPv3 uses the same port (1701) as L2TPv2

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   and shares the first two octets of header format with L2TPv2. The Ver
   field is used to distinguish L2TPv2 packets from L2TPv3 packets. If
   an implementation is capable of operating in L2TPv2 or L2TPv3 modes,
   it is possible to automatically detect whether a peer can support
   L2TPv2 or L2TPv3 and operate accordingly. The details of this
   fallback capability is defined in the following section.

4.7.3  Automatic L2TPv2 Fallback

   When running over UDP, an implementation may detect whether a peer is
   L2TPv3-capable by sending an SCCRQ that is properly formatted for
   L2TPv2 and L2TPv3.  This is accomplished by sending the SCCRQ with
   the Ver field set to 2, and ensuring that any L2TPv3-specific AVPs
   within the message are sent with each M-bit set to 0 (so that they
   may be safely ignored by an L2TPv2 implementation).  Note that, in
   both L2TPv2 and L2TPv3, the value contained in the space of the
   control message header utilized by the 32-bit Control Connection ID
   in L2TPv3, and the 16-bit Tunnel ID and 16-bit Session ID in L2TPv2,
   is always 0 for an SCCRQ. This effectively hides the fact that there
   are a pair of 16-bit fields in L2TPv2, and a single 32-bit field in
   L2TPv3.

   If the peer implementation is L2TPv3-capable, a control message with
   Ver set to 3 and corresponding header and message format will be sent
   in response to the SCCRQ.  Operation may then continue as L2TPv3.  If
   a message is received with Ver set to 2, it must be assumed that the
   peer implementation is L2TPv2-only and fallback to L2TPv2 mode may
   occur if local policy and capability permit.

   Note Well: The L2TPv2/v3 auto-detection mode requires that all L2TPv3
   implementations over UDP be liberal in acceptance of an SCCRQ control
   message with the Ver field set to 2 or 3. This is the single
   consideration for an L2TPv3-only implementation to ensure conformity
   to.

5.  Control Message Attribute Value Pairs

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

5.1  AVP Format

   Each AVP is encoded as follows:

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                          Figure 5.1: AVP 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |M|H| rsvd  |      Length       |           Vendor ID           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Attribute Type        |        Attribute Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       (until Length is reached)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The first six bits comprise a bit mask that describes the general
   attributes of the AVP.  Two bits are defined in this document; the
   remaining bits 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 that receives an unrecognized or malformed AVP.  The M
   bit of a given AVP should only be checked if the AVP is unrecognized
   or malformed.  If the M bit is set on an unrecognized or malformed
   AVP in a control message associated with a particular session, the
   session MUST be terminated.  If the M bit is set on an unrecognized
   or malformed AVP within a control message associated with a control
   connection, the control connection (and all sessions bound to the
   control connection) 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 5.3 describes the procedure for performing AVP hiding.

   Length: Contains the number of octets (including the Overall Length
   and bit mask 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 its own L2TP
   extensions can use its own Vendor ID along with private Attribute
   values, guaranteeing that they will not collide with any other

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   vendor's extensions or 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.

5.2  Mandatory AVPs

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

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

   On the receiving side, the recipient of a control message should only
   check the M bit of an AVP when the AVP is determined to be
   unrecognized or malformed.  The M bit should not be checked for a
   recognized and well-formatted AVP.  This rule prevents the
   possibility of a valid AVP resulting in a session or control
   connection teardown simply because its M bit was set to a value that
   was unexpected by the receiving LCCE.

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5.3  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 (1) a shared secret exists between the
   LCCEs and (2) LCCE authentication has completed.  The shared secret
   is the same secret that is used for LCCE authentication (see Section
   4.3).  Hidden values MUST NOT be unhidden until after LCCE
   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:

                     Figure 5.3: Hidden AVP Subformat

    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 that 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

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   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 (in network byte order) on the
   concatenation of the following:

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

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

   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 [RFC2865], 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

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   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.

5.4  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.

5.4.1  General Control Message AVPs

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 3.2.1).  See

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   Section 3.1 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 or malformed, it is an indication
   as to whether the control message itself should be ignored.  If the M
   bit is set within the Message Type AVP and the Message Type is
   unknown to the implementation, the control connection 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 5.1).  The Message Type AVP MUST still be
   the first AVP in the control message.

   Message Digest (All Messages)

      The Message Digest AVP, Attribute Type AVP-TBA-1, is used as an
      integrity check and authentication of the L2TP Control Message
      header and body.

      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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Digest Type  | Message Digest ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

      Where Digest Type is a one octet integer indicating the Digest
      calculation algorithm:
         0 HMAC-MD5 [RFC1321]
         1 HMAC-SHA-1 [RFC2104]

      Digest type 0 (HMAC-MD5) MUST be supported, Digest Type 1 (HMAC-
      SHA-1) MAY be supported.

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      The Message Digest is of variable length and contains the result
      of the control message authenticity and integrity calculation. For
      Digest Type 0 (HMAC-MD5) the length of the digest MUST be 160
      bits. The nonces are advertised via the Random Vector AVP, also
      defined in this section.

      Calculation of the digest is as follows for all messages other
      than the SCCRQ:

         Digest = Hash (local_nonce | remote_nonce | password |
         control_message)

         Hash: Hashing algorithm identified by the Digest Type

         local_nonce: Nonce chosen locally and advertised to the remote
         LCCE.

         remote_nonce: Nonce received from the remote LCCE

         password: Configured shared secret for this Control Connection

         control_message: The entire contents of the L2TP Control
         Message, including the Control Message header and all AVPs.
         Note that the Control Message header in this case begins after
         the 0 Session ID when running over IP, and after the UDP header
         when running over UDP.

      When calculating the Message Digest, the Message Digest AVP MUST
      be present within the control message with the Digest Type set to
      its proper value, but the Message Digest itself set to zeros. It
      may be beneficial for an implementation to position the Message
      Digest AVP just after the Message Type AVP when building a control
      message so that the Digest may be inserted at a well-known offset
      within the message.

      When receiving a control message, the contents of the Message
      Digest AVP MUST be compared against the expected digest value
      based on local calculation. This is done by performing the same
      digest calculation above, with the local_nonce and remote_nonce
      reversed. This message authenticity and integrity checking MUST be
      performed before utilizing any information contained within the
      control message. If the calculation fails, the message MUST be
      dropped.

      The SCCRQ has special treatment as it is the initial message
      commencing a new Control Connection. As such, there is only one
      nonce available, and it is present within the message itself (so
      there is no need to use it in the calculation explicitly).

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      Calculation of the SCCRQ Digest is performed as follows:

         Digest = Hash (password | control_message)

      This AVP MUST NOT be hidden (the H bit MUST be 0).  The M bit for
      this AVP MUST be set to 1.  The Length is 28 for Digest Type 1
      (HMAC-MD5), and may vary for other digest types.

      Random Vector (All Messages)

         The Random Vector AVP, Attribute Type 36, MUST contain a
         cryptographically random value [RFC1750]. This value is used
         for AVP Hiding and as a nonce for Control Message
         Authentication calculations.

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

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

         The Random Octet String is of arbitrary length, though 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 5.3), and as a
         nonce for the Control Message Authentication calculation (see
         Message Digest AVP definition in this section).

         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. As such, at least one Random Vector AVP MUST
         precede the first AVP with the H bit set. The the first
         instance of the Random Vector AVP in a message is set to the
         Remote Control Message Authentication nonce after the message
         itself has been verified as authentic.

         The M bit for this AVP SHOULD 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.

5.4.2  Result and Error Codes

Result Code (StopCCN, CDN)

   The Result Code AVP, Attribute Type 1, indicates the reason for

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   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 (optional)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Error Message ... (optional, arbitrary number of octets)      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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
   is indicated by the AVP Length field.  The Error Message contains an
   arbitrary string providing further (human-readable) text associated
   with the condition.  Human-readable text in all error messages MUST
   be provided in the UTF-8 charset using the Default Language
   [RFC2277].

   This AVP MUST NOT be hidden (the H bit MUST be 0).  The M bit for
   this AVP SHOULD 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 plus the length of the Error Message if there is an Error Code
   and Message.

   Defined Result Code values for the StopCCN message are as follows:

      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.

   General Result Code values for the CDN message are as follows:

      0 - Reserved.
      1 - Session disconnected due to loss of carrier or circuit disconnect.
      2 - Session disconnected for the reason indicated in Error Code.
      3 - Session disconnected for administrative reasons.
      4 - Session establishment failed due to lack of appropriate
          facilities being available (temporary condition).
      5 - Session establishment failed due to lack of appropriate

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          facilities being available (permanent condition).
      6 - 11 Reserved (PPP-specific codes defined outside this document).
      RC-TBA-1 - Session not established due to losing tie breaker.
      RC-TBA-2 - Session not established due to unsupported PW type.
      RC-TBA-3 - Session not established, sequencing required without valid
                L2-Specific Sublayer.

   Additional service-specific Result Codes are defined outside this
   document.

   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 as follows:

   0 - No general error.
   1 - No control connection exists yet for this pair of LCCEs.
   2 - Length is wrong.
   3 - One of the field values was out of range.
   4 - Insufficient resources to handle this operation now.
   5 - Invalid Session ID.
   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.
   8 - The session or control connection was shut down due to receipt of
       an unknown AVP with the M bit set (see Section 5.2).  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 control
       connection or session.  The Error Message MUST contain a
       comma-separated list of addresses from which the initiator may
       choose.  If the L2TP data channel runs over IPv4, then this would
       be a comma-separated list of IP addresses in the canonical
       dotted-decimal format (e.g. "10.0.0.1, 10.0.0.2, 10.0.0.3") in the
       UTF-8 charset using the Default Language [RFC2277].  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.  A vendor-
   specific AVP MAY be sent to more precisely detail a vendor-specific
   problem.

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5.4.3  Control Connection Management AVPs

Control Connection Tie Breaker (SCCRQ)

   The Control Connection Tie Breaker AVP, Attribute Type 5, indicates
   that the sender desires a single control connection to exist between
   a given pair of LCCEs.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Control Connection Tie Breaker Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                              ... (64 bits)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Control Connection Tie Breaker Value is an 8-octet random value
   that is used to choose a single control connection when two LCCEs
   request a control connection concurrently.  The recipient of a SCCRQ
   must check to see if a SCCRQ has been sent to the peer; if so, a tie
   has been detected.  In this case, the LCCE must compare its Control
   Connection Tie Breaker value with the one received in the SCCRQ.  The
   lower value "wins", and the "loser" MUST discard its control
   connection, sending a StopCCN if the SCCRQ that it had sent was
   acknowledged by the receiving peer.  In the case in which a tie
   breaker is present on both sides and the value is equal, both sides
   MUST discard their control connections and restart control connection

   negotiation with a new, random tie breaker value.

   If a tie breaker is received and an outstanding SCCRQ has no tie
   breaker value, the initiator that included the Control Connection Tie
   Breaker AVP "wins".  If neither side issues a tie breaker, then two
   separate control connections are opened.

   Applications which employ a distinct and well-known initiator have no
   need for tie-breaking, and this AVP MAY be omitted and the tie-
   breaking functionality disabled. Applications which require tie-
   breaking also require that an LCCE be uniquely identifiable upon
   receipt of an SCCRQ. For L2TP over IP, this MUST be accomplished via
   the Router ID AVP.

   Note that in [RFC2661], this AVP was referred to as the "Tie-Breaker
   AVP".  Here, the AVP serves the same purpose and has the same
   attribute value and composition. The name was changed simply to
   distinguish between the Session and Control Connection Tie Breaker

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   AVP.

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

Host Name (SCCRQ, SCCRP)

   The Host Name AVP, Attribute Type 7, indicates the name of the
   issuing LAC or LNS, encoded in the US-ASCII charset.

   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 host name with fully qualified
   domain would be appropriate.  The Host Name AVP and/or Router ID AVP
   MUST be used to identify an LCCE as described in Section 3.3.

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

Router ID (SCCRQ, SCCRP)

   The Router ID AVP, Attribute Type AVP-TBA-2, is an identifier used to
   identify an LCCE for control connection setup, tie breaking, and/or
   tunnel 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Router Identifier                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Router Identifier is a 4-octet unsigned integer.  Its value is
   unique for a given LCCE, per Section 8.1 of [RFC2072].  The Host Name
   AVP and/or Router ID AVP MUST be used to identify an LCCE as
   described in Section 3.3.

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   This AVP MUST NOT be hidden (the H bit MUST be 0).  The M bit for
   this AVP SHOULD be set to 1.  The Length of this AVP is 10.

Vendor Name (SCCRQ, SCCRP)

   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 US-ASCII charset [RFC1958, RFC2277].

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

Assigned Control Connection ID (SCCRQ, SCCRP, StopCCN)

   The Assigned Control Connection ID AVP, Attribute Type AVP-TBA-3,
   contains the ID being assigned to this control connection 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Assigned Control Connection ID                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Assigned Control Connection ID is a 4-octet non-zero unsigned
   integer.

   The Assigned Control Connection ID AVP establishes the identifier
   used to multiplex and demultiplex multiple control connections
   between a pair of LCCEs.  Once the Assigned Control Connection ID AVP
   has been received by an LCCE, the Control Connection ID specified in
   the AVP MUST be included in the Control Connection ID field of all
   control packets sent to the peer for the lifetime of the control

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   connection.  Before the Assigned Control Connection ID AVP is
   received from a peer, all control messages MUST be sent to that peer
   with a Control Connection ID value of 0 in the header.  Because a
   Control Connection ID value of 0 is used in this special manner, the
   zero value MUST NOT be sent as an Assigned Control Connection ID
   value.

   Under certain circumstances, an LCCE may need to send a StopCCN to a
   peer without having yet received an Assigned Control Connection ID
   AVP from the peer (i.e. SCCRQ sent, no SCCRP received yet).  In this
   case, the Assigned Control Connection ID AVP that had been sent to
   the peer earlier (i.e. in the SCCRQ) MUST be sent as the Assigned
   Control Connection ID AVP in the StopCCN.  This policy allows the
   peer to try to identify the appropriate control connection via a
   reverse lookup.

   This AVP MAY be hidden (the H bit MAY be 0 or 1).  The M bit for this
   AVP SHOULD be set to 1 (see Section 4.7.3).  The Length (before
   hiding) of this AVP is 10.

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.  See Section 4.2 for more
   information on reliable control message delivery.

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

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Pseudowire Capabilities List (SCCRQ, SCCRP)

   The Pseudowire Capabilities List (PW Capabilities List) AVP,
   Attribute Type AVP-TBA-4, indicates the L2 payload types the sender
   can support.  The specific payload type of a given session is
   identified by the Pseudowire Type AVP.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           PW Type 0           |             ...               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              ...              |          PW Type N            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Defined PW types that may appear in this list are managed by IANA and
   MUST appear in associated pseudowire-specific documents for each PW
   type.

   If a sender includes a given PW type in the PW Capabilities List AVP,
   the sender assumes full responsibility for supporting that particular
   payload, such as any payload-specific AVPs, L2-Specific Sublayer, or
   control messages that may be defined in the appropriate companion
   document.

   This AVP MAY be hidden (the H bit MAY be 0 or 1).  The M bit for this
   AVP SHOULD be set to 1 (see Section 4.7.3).  The Length (before
   hiding) of this AVP is 8 octets with one PW type specified, plus 2
   octets for each additional PW type.

Preferred Language (SCCRQ, SCCRP)

   The Preferred Language AVP, Attribute Type AVP-TBD-14, provides a
   method for an LCCE to indicate to the peer the language in which
   human-readable messages it sends SHOULD be composed.  This AVP
   contains a single language tag or language range [RFC3066].

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Preferred Language... (arbitrary number of octets)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Preferred Language is the indicated number of octets representing

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   the language tag or language range, encoded in the US-ASCII charset.

   This AVP MAY be hidden (the H bit MAY be 0 or 1).  The M bit for this
   AVP SHOULD be set to 0.  The Length (before hiding) of this AVP is 6
   plus the length of the Preferred Language.

5.4.4  Session Management AVPs

Local Session ID (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, CDN, WEN, SLI)

   The Local Session ID AVP (analogous to the Assigned Session ID in
   L2TPv2), Attribute Type AVP-TBA-5, contains the identifier being
   assigned to this session 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Local Session ID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Local Session ID is a 4-octet non-zero unsigned integer.

   The Local Session ID AVP establishes the two identifiers used to
   multiplex and demultiplex sessions between two LCCEs.  Each LCCE
   chooses any free value it desires, and sends it to the remote LCCE
   using this AVP. The remote LCCE MUST then send all data packets
   associated with this session using this value.  Additionally, for all
   session-oriented control messages sent after this AVP is received
   (e.g. ICRP, ICCN, CDN, SLI, etc.), the remote LCCE MUST echo this
   value in the Remote Session ID AVP.

   Note that a Session ID value is unidirectional.  Because each LCCE
   chooses its Session ID independent of its peer LCCE, the value does
   not have to match in each direction for a given session."

   See Section 4.1 for additional information about the Session ID.

   This AVP MAY be hidden (the H bit MAY be 0 or 1).  The M bit for this
   AVP MUST be 1 for implementations that support only L2TPv3 (see
   Section 4.7 for L2TPv2 migration issues).  The Length (before hiding)
   of this AVP is 10.

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Remote Session ID (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, CDN, WEN, SLI)

   The Remote Session ID AVP, Attribute Type AVP-TBA-6, contains the
   identifier that was assigned to this session by the 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Remote Session ID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Remote Session ID is a 4-octet non-zero unsigned integer.

   The Remote Session ID AVP MUST be present in all session-level
   control messages.  The AVP's value echoes the session identifier
   advertised by the peer via the Local Session ID AVP.  It is the same
   value that will be used in all transmitted data messages by this side
   of the session.  In most cases, this identifier is sufficient for the
   peer to look up session-level context for this control message.

   When a session-level control message must be sent to the peer before
   the Local Session ID AVP has been received, the value of the Remote
   Sesson ID AVP MUST be set to zero.  Additionally, the Local Session
   ID AVP (sent in a previous control message for this session) MUST be
   included in the control message.  The peer must then use the Local
   Session ID AVP to perform a reverse lookup to find its session
   context.  Session-level control messages defined in this document
   that might be subject to a reverse lookup by a receiving peer include
   the CDN, WEN, and SLI.

   This AVP MAY be hidden (the H bit MAY be 0 or 1).  The M bit for this
   AVP MUST be 1 for implementations that support only L2TPv3 (see
   Section 4.7 for L2TPv2 migration issues).  The Length (before hiding)
   of this AVP is 10.

Assigned Cookie (ICRQ, ICRP, OCRQ, OCRP)

   The Assigned Cookie AVP, Attribute Type AVP-TBA-7, contains the
   Cookie value being assigned to this session by the sender.

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

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    0                   1                   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Assigned Cookie (32 or 64 bits) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Assigned Cookie is a 4-octet or 8-octet random value.

   The Assigned Cookie AVP contains the value used to check the
   association of a received data message with the session identified by
   the Session ID.  All data messages sent to a peer MUST use the
   Assigned Cookie sent by the peer in this AVP.  The value's length (0,
   32, or 64 bits) is obtained by the Length of the AVP.

   A missing Assigned Cookie AVP or Assigned Cookie Value of zero length
   indicates that the Cookie field should not be present in any data
   packets sent to the LCCE sending this AVP.

   See Section 4.1 for additional information about the Assigned Cookie.

   This AVP MAY be hidden (the H bit MAY be 0 or 1).  The M bit for this
   AVP MUST be 1 for implementations that support only L2TPv3 (see
   Section 4.7 for L2TPv2 migration issues).  The Length (before hiding)
   of this AVP may be 6, 10, or 14 octets.

Serial Number (ICRQ, OCRQ)

   The Serial Number AVP, Attribute Type 15, contains an identifier
   assigned by the LAC or LNS to this 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Serial Number                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Serial Number is a 32-bit value.

   The Serial Number is intended to be an easy reference for
   administrators on both ends of a control connection to use when
   investigating session failure problems.  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.

   Note that in RFC 2661, this value was referred to as the "Call Serial

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   Number AVP".  It serves the same purpose and has the same attribute
   value and composition.

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

Remote End ID (ICRQ, OCRQ)

   The Remote End ID AVP, Attribute Type AVP-TBA-8, contains an
   identifier used to bind L2TP sessions to a given circuit, interface,
   or bridging instance. It also may be used to detect session-level
   ties.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Remote End Identifier ... (arbitrary number of octets)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Remote End Identifier field is a variable-length field whose
   value is unique for a given LCCE peer, as described in Section 3.3.

   A session-level tie is detected if an LCCE receives an ICRQ or OCRQ
   with an End ID AVP whose value matches that which was just sent in an
   outgoing ICRQ or OCRQ to the same peer.  If the two values match, an
   LCCE recognizes that a tie exists (e.g. both LCCEs are attempting to
   establish sessions for the same circuit).  The tie is broken by the
   Session Tie Breaker AVP.

   By default, the LAC-LAC cross-connect application (see section
   2.0(b)) of L2TP over an IP network MUST utilize the Router ID AVP and
   Remote End ID AVP to associate a circuit to an L2TP session. Other
   AVPs MAY be used for LCCE or circuit identification as specified in
   companion documents.

   This AVP MAY be hidden (the H bit MAY be 0 or 1).  The M bit for this
   AVP SHOULD be set to 0.  The Length (before hiding) of this AVP is 6
   plus the length of the Remote End Identifier value.

Application Code (ICRQ, OCRQ)

   The Application Code AVP, Attribute Type AVP-TBA-9, is a 2 octet
   value for enumerating application types for a given L2TP session.

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

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

   The Application Code is a 2 octet value used to identify a specific
   application for an L2TP session, perhaps causing certain values
   within AVPs defined in this document to be interpreted or acted upon
   in a different manner dictated by the Application Code. For example,
   a given Application Code could instruct an LCCE to perform a specific
   directory lookup on the Hostname and/or Router ID AVP information
   associated with this session (perhaps even encoding the destination
   address of the given directory server).

   An Application Code of 0, or absence of this AVP in any control
   message, indicates that all AVPs should be interpreted as defined in
   this document.

   This AVP MAY be hidden (the H bit MAY be 0 or 1).  The M bit for this
   AVP SHOULD be set to 0.  The Length (before hiding) of this AVP is 8
   plus the length of the Application Info field.

Session Tie Breaker (ICRQ, OCRQ)

   The Session Tie Breaker AVP, Attribute Type TBD, is used to break
   ties when two peers concurrently attempt to establish a session for
   the same circuit.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Session Tie Breaker Value ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                              ... (64 bits)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Session Tie Breaker Value is an 8-octet random value that is used
   to choose a session when two LCCEs concurrently request a session for
   the same circuit.  A tie is detected by examining the peer's identity
   (described in Section 3.3) plus the per-session shared value
   communicated via the End ID AVP.  In the case of a tie, the recipient
   of an ICRQ or OCRQ must compare the received tie breaker value with
   the one that it sent earlier.  The LCCE with the lower value "wins",
   and the "loser" MUST send a CDN with result code set to RC-TBA-1 (as

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   defined in Section 5.4.2) to tear down the session it instigated.  In
   the case in which a tie is detected, tie breakers are sent by both
   sides, and the tie breaker values are equal, both sides MUST discard
   their sessions and restart session negotiation with new random tie
   breaker values.

   If a tie is detected but only one side sends a Session Tie Breaker
   AVP, the session initiator that included the Session Tie Breaker AVP
   "wins".  If neither side issues a tie breaker, then both sides MUST
   tear down the session.

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

Pseudowire Type (ICRQ, OCRQ)

   The Pseudowire Type (PW Type) AVP, Attribute Type AVP-TBA-10,
   indicates the L2 payload type of the packets that will be tunneled
   using this L2TP session.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           PW Type             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A peer MUST NOT request an incoming or outgoing call with a PW Type
   AVP specifying a value not advertised in the PW Capabilities List AVP
   it received during control connection establishment.  Attempts to do
   so MUST result in the call being rejected via a CDN with the Result
   Code set to RC-TBA-2 (see Section 5.4.2).

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

   MUST be 1 for implementations that support only L2TPv3 (see Section
   4.7 for L2TPv2 migration issues).  The Length (before hiding) of this
   AVP is 8.

L2-Specific Sublayer (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN)

   The L2-Specific Sublayer AVP, Attribute Type AVP-TBA-11, indicates
   the the presence and format of the L2-Specific Sublayer the sender of
   this AVP requires on all incoming data packets for this L2TP session.

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    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   L2-Specific Sublayer Type   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The L2-Specific Sublayer Type is a 2-octet unsigned integer with the
   following values defined in this document:

      0 - There is no L2-Specific Sublayer present.
      1 - The default L2-Specific Sublayer (defined in Section 4.6)
          is used.

   If this AVP is received and has a value other than zero, the
   receiving LCCE MUST include the identified L2-Specific Sublayer in
   its outgoing data messages.  If the AVP is not received, it is
   assumed that there is no sublayer present.

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

Data Sequencing (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN)

   The Data Sequencing AVP, Attribute Type AVP-TBA-12, indicates that
   the sender requires some or all of the data packets that it receives
   to be sequenced.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Data Sequencing Level     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Data Sequencing Level is a 2-octet unsigned integer indicating
   the degree of incoming data traffic that the sender of this AVP
   wishes to be marked with sequence numbers.

   The following values are valid data sequencing levels:

      0 - No incoming data packets require sequencing.
      1 - Only non-IP data packets require sequencing.
      2 - All incoming data packets require sequencing.

   If a data sequencing level of 0 is specified, there is no need to

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   send packets with sequence numbers.  If sequence numbers are sent,
   they will be ignored upon receipt.  If no Data Sequencing AVP is
   received, a data sequencing level of 0 is assumed.

   If a data sequencing level of 1 is specified, only non-IP traffic
   carried within the tunneled L2 frame should have sequence numbers
   applied. Non-IP traffic here refers to any packets that cannot be
   classified as an IP packet within their respective L2 framing (i.e.,
   a PPP control packet or NETBIOS frame encapsulated by Frame Relay
   before being tunneled). All traffic that can be classified as IP MUST
   be sent with no sequencing (e.g. the S bit in the L2-Specific
   Sublayer is set to zero). If a packet is unable to be classified at
   all (e.g. due to it being compressed or encrypted at layer 2) or if
   an implementation is unable to perform such classification within L2
   frames, all packets MUST be provided with sequence numbers
   (essentially falling back to a data sequencing level of 2).

   If a data sequencing level of 2 is specified, all traffic MUST be
   sequenced.

   Data sequencing may only be requested when there is an L2-Specific
   Sublayer present that can provide sequence numbers. If sequencing is
   requested without requesting a L2-Specific Sublayer AVP, the session
   MUST be disconnected with a Result Code of RC-TBA-3.

   This AVP MAY be hidden (the H bit MAY be 0 or 1).  The M bit for this
   AVP SHOULD be set to 0.  The Length (before hiding) of this AVP is 6.

Tx Connect Speed (ICRQ, ICRP, ICCN)

   The Tx Connect Speed BPS AVP, Attribute Type 24, contains 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Tx Connect Speed 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 that 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

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   optional Rx Connect Speed AVP is NOT present, the connection speed
   between the remote system and LAC is assumed to be symmetric and is
   represented by the single value in this AVP.

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

Rx Connect Speed (ICRQ, ICRP, ICCN)

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Rx Connect Speed BPS                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Rx 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 that 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 0 or 1).  The M bit for this
   AVP SHOULD be set to 0.  The Length (before hiding) of this AVP is
   10.

Physical Channel ID (ICRQ, ICRP, OCRP)

   The Physical Channel ID AVP, Attribute Type 25, contains 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                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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   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 SHOULD be set to 0.  The Length (before hiding) of this AVP is
   10.

5.4.5  Circuit Status AVPs

Circuit Status (ICRQ, ICRP, ICCN, OCRQ, OCRP, OCCN, SLI)

   The Circuit Status AVP, Attribute Type AVP-TBA-13, indicates the
   initial status of or a status change in the circuit to which the
   session is bound.

   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          |N|A|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The A (Active) bit indicates whether the circuit is up/active/ready
   (1) or down/inactive/not-ready (0).

   The N (New) bit indicates whether the circuit status indication is
   for a new circuit (1) or an existing circuit (0). Links which have a
   similar mechanism available (e.g. Frame Relay) MUST map the setting
   of this bit to the associated signaling for that link. Otherwise, the
   New bit SHOULD still be set the first time the L2TP session is
   established after provisioning.

   The remaining bits are reserved for future use.  Reserved bits MUST
   be set to 0 when sending and ignored upon receipt.

   The Circuit Status AVP is used to advertise whether a circuit or
   interface bound to an L2TP session is up and ready to send and/or
   receive traffic. Different circuit types have different names for
   status types.  For example, HDLC primary and secondary stations refer
   to a circuit as being "Receive Ready" or "Receive Not Ready", while
   Frame Relay refers to a circuit as "Active" or "Inactive".  This AVP
   adopts the latter terminology, though the concept remains the same
   regardless of the PW type for the L2TP session.

   In the simplest case, the circuit referred by this AVP is a single
   physical interface, port, or circuit depending on application and how

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   the session was setup. The status indication in this AVP may then be
   used to provide simple ILMI interworking for a variety of circuit
   types. For virtual or multipoint interfaces, the Circuit Status AVP
   is still utilized, but effectively refers to the state of an internal
   structure or a logical set of circuits. Each PW-specific companion
   document MUST then specify precisely how this AVP is translated for
   each circuit type.

   If this AVP is received with a Not Active notification for a given
   L2TP session, all data traffic for that session MUST cease (or not
   begin) in the direction of the sender of the Circuit Status AVP until
   the circuit is advertised as Active.

   The Circuit Status MUST be advertised by this AVP in ICRQ, ICRP,
   OCRQ, and OCRP messages.  Often, the circuit type will be marked
   Active when initiated, but MAY be advertised as Inactive, indicating
   that an L2TP session is to be created but that the interface or
   circuit is still not ready to pass traffic.  The ICCN, OCCN, and SLI
   control messages all MAY contain this AVP to update the status of the
   circuit after establishment of the L2TP session is requested.

   If additional circuit status information is needed for a given PW
   type, PW-specific AVPs MUST be defined in a separate document for
   that information.  This AVP is only for general circuit status
   information applicable to all circuit/interface types.

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

Circuit Errors (WEN)

   The Circuit Errors AVP, Attribute Type 34, conveys circuit error
   information to the peer.

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

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
                                  |             Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Hardware Overruns                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Buffer Overruns                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Timeout Errors                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Alignment Errors                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The following fields are defined:

      Reserved: 2 octets of Reserved data is present (providing longword
         alignment within the AVP of the following values).  Reserved
         data MUST be zero on sending and ignored upon receipt.
      Hardware Overruns: Number of receive buffer overruns since call
         was established.
      Buffer Overruns: Number of buffer overruns detected since call was
         established.
      Timeout Errors: Number of timeouts 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 SHOULD be set to 1.  The Length (before hiding) of this AVP is
   32.

6.  Control Connection Protocol Specification

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

   The exchanges in which these messages are involved are outlined in
   Section 3.3.

6.1  Start-Control-Connection-Request (SCCRQ)

   Start-Control-Connection-Request (SCCRQ) is a control message used to
   initiate a control connection between two LCCEs.  It is sent by
   either the LAC or the LNS to begin the control connection

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   establishment process.

   The following AVPs MUST be present in the SCCRQ:

      Message Type
      Host Name
      Router ID
      Assigned Control Connection ID
      Pseudowire Capabilities List

   The following AVPs MAY be present in the SCCRQ:

      Random Vector
      Message Digest
      Control Connection Tie Breaker
      Vendor Name
      Receive Window Size
      Preferred Language

6.2  Start-Control-Connection-Reply (SCCRP)

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

   The following AVPs MUST be present in the SCCRP:

      Message Type
      Host Name
      Router ID
      Assigned Control Connection ID
      Pseudowire Capabilities List

   The following AVPs MAY be present in the SCCRP:

      Random Vector
      Message Digest
      Vendor Name
      Receive Window Size
      Preferred Language

6.3  Start-Control-Connection-Connected (SCCCN)

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

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   The following AVP MUST be present in the SCCCN:

      Message Type

   The following AVP MAY be present in the SCCCN:

      Random Vector
      Message Digest

6.4  Stop-Control-Connection-Notification (StopCCN)

   Stop-Control-Connection-Notification (StopCCN) is the control message
   sent by either LCCE to inform its peer that the control connection is
   being shut down and that the control connection should be closed.  In
   addition, all active sessions are implicitly cleared (without sending
   any explicit session 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 delivery layer.

   The following AVPs MUST be present in the StopCCN:

      Message Type
      Result Code

   The following AVPs MAY be present in the StopCCN:

      Random Vector
      Message Digest
      Assigned Control Connection ID

   Note that the Assigned Control Connection ID MUST be present if the
   StopCCN is sent after an SCCRQ or SCCRP message has been sent.

6.5  Hello (HELLO)

   The Hello (HELLO) message is an L2TP control message sent by either
   peer of a control connection.  This control message is used as a
   "keepalive" for the control connection.  See Section 4.2 for a
   description of the keepalive mechanism.

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

   The following AVP MUST be present in the HELLO:

      Message Type

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   The following AVP MAY be present in the HELLO:

      Random Vector
      Message Digest

6.6  Incoming-Call-Request (ICRQ)

   Incoming-Call-Request (ICRQ) is the control message sent by an LCCE
   to a peer when an incoming call is detected (although the ICRQ may
   also be sent as a result of a local event).  It is the first in a
   three-message exchange used for establishing a session via an L2TP
   control connection.

   The ICRQ is used to indicate that a session is to be established
   between an LCCE and a peer.  The sender of an ICRQ provides the peer
   with parameter information for the session.  However, the sender
   makes no demands about how the session is terminated at the peer
   (i.e. whether the L2 traffic is processed locally, forwarded, etc.).

   The following AVPs MUST be present in the ICRQ:

      Message Type
      Local Session ID
      Remote Session ID
      Serial Number
      Pseudowire Type
      Circuit Status

   The following AVPs MAY be present in the ICRQ:

      Random Vector
      Message Digest
      Assigned Cookie
      End ID
      Application ID
      Session Tie Breaker
      L2-Specific Sublayer
      Data Sequencing
      Tx Connect Speed
      Rx Connect Speed
      Physical Channel ID

6.7  Incoming-Call-Reply (ICRP)

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

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   The ICRP is used to indicate that the ICRQ was successful and that
   the peer should establish (i.e. answer) the incoming call if it has
   not already done so.  It also allows the sender to indicate specific
   parameters about the L2TP session.

   The following AVPs MUST be present in the ICRP:

      Message Type
      Local Session ID
      Remote Session ID
      Circuit Status

   The following AVPs MAY be present in the ICRP:

      Random Vector
      Message Digest
      Assigned Cookie
      L2-Specific Sublayer
      Data Sequencing
      Tx Connect Speed
      Rx Connect Speed
      Physical Channel ID

6.8  Incoming-Call-Connected (ICCN)

   Incoming-Call-Connected (ICCN) is the control message sent by the
   LCCE that originally sent an ICRQ upon receiving an ICRP from its
   peer.  It is the final message in the three-message exchange used for
   establishing L2TP sessions.

   The ICCN is used to indicate that the ICRP was accepted, that the
   call has been established, and that the L2TP session should move to
   the established state.  It also allows the sender to indicate
   specific parameters about the established call (parameters that may
   not have been available at the time the ICRQ is issued).

   The following AVPs MUST be present in the ICCN:

      Message Type
      Local Session ID
      Remote Session ID

   The following AVPs MAY be present in the ICCN:

      Random Vector
      Message Digest
      L2-Specific Sublayer
      Data Sequencing

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      Tx Connect Speed
      Rx Connect Speed
      Circuit Status

6.9  Outgoing-Call-Request (OCRQ)

   Outgoing-Call-Request (OCRQ) is the control message sent by an LCCE
   to an LAC to indicate that an outbound call at the LAC is to be
   established based on specific destination information sent in this
   message.  It is the first in a three-message exchange used for
   establishing a session and placing a call on behalf of the initiating
   LCCE.

   Note that a call may be any L2 connection requiring well-known
   destination information to be sent from an LCCE to an LAC.  This call
   could be a dialup connection to the PSTN, an SVC connection, the IP
   address of another LCCE, or any other destination dictated by the
   sender of this message.

   The following AVPs MUST be present in the OCRQ:

      Message Type
      Local Session ID
      Remote Session ID
      Serial Number
      Pseudowire Type
      Circuit Status

   The following AVPs MAY be present in the OCRQ:

      Random Vector
      Message Digest
      Assigned Cookie
      End ID
      Application ID
      Session Tie Breaker
      L2-Specific Sublayer
      Data Sequencing

6.10  Outgoing-Call-Reply (OCRP)

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

   OCRP is used to indicate that the LAC has been able to attempt the
   outbound call.  The message returns any relevant parameters regarding

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   the call attempt.  Data MUST not be forwarded until the OCCN is
   received indicating that the call has been placed.

   The following AVPs MUST be present in the OCRP:

      Message Type
      Local Session ID
      Remote Session ID
      Circuit Status

   The following AVPs MAY be present in the OCRP:

      Random Vector
      Message Digest
      Assigned Cookie
      L2-Specific Sublayer
      Data Sequencing
      Physical Channel ID

6.11  Outgoing-Call-Connected (OCCN)

   Outgoing-Call-Connected (OCCN) is the control message sent by an LAC
   to another LCCE after 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.

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

   The following AVPs MUST be present in the OCCN:

      Message Type
      Local Session ID
      Remote Session ID

   The following AVPs MAY be present in the OCCN:

      Random Vector
      Message Digest
      L2-Specific Sublayer
      Data Sequencing
      Circuit Status

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6.12  Call-Disconnect-Notify (CDN)

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

   The following AVPs MUST be present in the CDN:

      Message Type
      Result Code
      Local Session ID
      Remote Session ID

   The following AVP MAY be present in the CDN:

      Random Vector
      Message Digest

6.13  WAN-Error-Notify (WEN)

   The WAN-Error-Notify (WEN) is a control message sent from an LAC to an
   LNS to indicate WAN error conditions.  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
      Local Session ID
      Remote Session ID
      Circuit Errors

   The following AVP MAY be present in the WEN:

      Random Vector
      Message Digest

6.14  Set-Link-Info (SLI)

   The Set-Link-Info control message is sent by an LCCE to convey link
   or circuit status change information regarding the circuit associated
   with this L2TP session.  For example, if PPP renegotiates LCP at an
   LNS or between an LAC and a Remote System, or if a forwarded Frame
   Relay VC transitions to Active or Inactive at an LAC, an SLI message
   SHOULD be sent to indicate this event.  Precise details of when the

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   SLI is sent, what PW type-specific AVPs must be present, and how
   those AVPs should be interpreted by the receiving peer are outside
   the scope of this document.  These details should be described in the
   associated pseudowire-specific documents that require use of this
   message.

   The following AVPs MUST be present in the SLI:

      Message Type
      Local Session ID
      Remote Session ID

   The following AVPs MAY be present in the SLI:

      Random Vector
      Message Digest
      Circuit Status

6.15  Explicit-Acknowledgement (ACK)

   The Explicit Acknowledgement (ACK) message is used only to
   acknowledge receipt of a message or messages on the Control
   Connection (e.g. for purposes of updating Ns and Nr values). Receipt
   of this message does not trigger an event for the L2TP protocol state
   machine.

   A message received without any AVPs (including the Message Type AVP),
   is referred to as a Zero Length Body (ZLB) message, and serves the
   same function as the Explicit Acknowledgement. ZLB messages are only
   permitted when the Control Message Authentication defined in Section
   4.3 is not enabled.

   The following AVPs MAY be present in the ACK message:

      Message Type
      Message Digest

7.  Control Connection State Machines

   The state tables defined in this section govern the exchange of
   control messages defined in Section 6.  Tables are defined for
   incoming call placement and outgoing call placement, as well as for
   initiation of the control connection itself.  The state tables do not
   encode timeout and retransmission behavior, as this is handled in the
   underlying reliable control message delivery mechanism (see Section
   4.2).

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7.1  Malformed Control Messages

   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 (1) a message that contains
   a Message Type marked as mandatory (see Section 5.4.1) but that is
   unknown to the implementation, or (2) a control message that is
   received in the wrong state.

   Examples of malformed control messages include (1) a message that has
   an invalid value in its header, (2) a message that contains an AVP
   that is formatted incorrectly or whose value is out of range, and (3)
   a message that is missing a required AVP.  A control message with a
   malformed header MUST be discarded.

   If a malformed AVP is received with the M bit set, the session or
   control connection MUST be terminated with a proper Result or Error
   Code sent.  A malformed yet non-mandatory (M bit is not set) AVP
   within a control message should be handled like an unrecognized non-
   mandatory AVP.  That is, the AVP MUST be ignored (with the exception
   of logging a local error message), and the message MUST be accepted.

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

   In several cases in the following tables, a protocol message is sent,
   and then a "clean up" occurs.  Note that, regardless of the initiator
   of the control connection destruction, the reliable delivery
   mechanism must be allowed to run (see Section 4.2) before destroying
   the control connection.  This permits the control connection
   management messages to be reliably delivered to the peer.

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

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7.2  Timing Considerations

   Due to the real-time nature of L2 circuit signaling, an LCCE should
   be implemented using a multi-threaded architecture 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.

7.3  Control Connection States

   The L2TP control connection protocol is not distinguishable between
   the two LCCEs but is distinguishable between the originator and
   receiver.  The originating peer is the one that first initiates
   establishment of the control connection.  (In a tie breaker
   situation, this is the winner of the tie.)  Since either the LAC or
   the LNS can be the originator, a collision can occur.  See the
   Control Connection Tie Breaker AVP in Section 5.4.3 for a description
   of this and its resolution.

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

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

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

   idle            Receive SCCRP     Send StopCCN,        idle
                                     clean up

   idle            Receive SCCCN     Clean up             idle

   wait-ctl-reply  Receive SCCRP,    Send SCCCN,          established
                   acceptable        send control-conn
                                     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

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   wait-ctl-reply  Receive SCCCN     Send StopCCN,        idle
                                     clean up

   wait-ctl-conn   Receive SCCCN,    Send control-conn    established
                   acceptable        open 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 open        Send control-conn    established
                   request           open event to
                   (new call)        waiting sessions

   established     Administrative    Send StopCCN,        idle
                   control-conn      clean up
                   close event

   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 an LCCE for control connection
   establishment are as follows:

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

   wait-ctl-reply
      The originator checks to see if another connection has been
      requested from the same peer, and if so, handles the collision
      situation described in Section 5.4.3.

   wait-ctl-conn
      Awaiting an SCCCN.  Upon receipt, the challenge response contained
      in the message is checked.  The control connection is established
      if authentication succeeds; otherwise, it is torn down.

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   established
      An established connection may be terminated by either a local
      condition or the receipt of a StopCCN.  In the event of a local
      termination, the originator MUST send a StopCCN and clean up the
      control connection.  If the originator receives a StopCCN, it MUST
      also clean up the control connection.

7.4  Incoming Calls

   An ICRQ is generated by an LCCE, typically in response to an incoming
   call or a local event.  Once the LCCE sends the ICRQ, it waits for a
   response from the peer.  However, it may choose to postpone
   establishment of the call (e.g. answering the call, bringing up the
   circuit) until the peer has indicated with an ICRP that it will
   accept the call.  The peer may choose not to accept the call if, for
   instance, there are insufficient resources to handle an additional
   session.

   If the peer chooses to accept the call, it responds with an ICRP.
   When the local LCCE receives the ICRP, it attempts to establish the
   call.  A final call connected message, the ICCN, is sent from the
   local LCCE to the peer to indicate that the call states for both
   LCCEs should enter the established state.  If the call is terminated
   before the peer can accept it, a CDN is sent by the local LCCE to
   indicate this condition.

   When a call transitions to a "disconnected" or "down" state, the call
   is cleared normally, and the local LCCE sends a CDN.  Similarly, if
   the peer wishes to clear a call, it sends a CDN and cleans up its
   session.

7.4.1  ICRQ Sender States

   State           Event              Action            New State
   -----           -----              ------            ---------
   idle            Call signal or     Initiate local    wait-control-conn
                   ready to receive   control-conn
                   incoming conn      open

   idle            Receive ICCN,      Clean up          idle
                   ICRP, CDN

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

   wait-control-   control-conn-open  Send ICRQ         wait-reply
   conn

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   wait-reply      Receive ICRP,      Send ICCN         established
                   acceptable

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

   wait-reply      Receive ICRQ       Send CDN,         idle
                                      clean up

   wait-reply      Receive CDN,       Clean up          idle
                   ICCN

   wait-reply      Local close        Send CDN,         idle
                   request            clean up

   established     Receive CDN        Clean up          idle

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

   established     Local close        Send CDN,         idle
                   request            clean up

   The states associated with the ICRQ sender are as follows:

   idle
      The LCCE detects an incoming call on one of its interfaces (e.g.
      an analog PSTN line rings, or an ATM PVC is provisioned), or a
      local event occurs.  The LCCE initiates its control connection
      establishment state machine and moves to a state waiting for
      confirmation of the existence of a control connection.

   wait-control-connection
      In this state, the session is waiting for either the control
      connection to be opened or for verification that the control
      connection is already open.  Once an indication that the control
      connection has been opened is received, session control messages
      may be exchanged.  The first of these messages is the ICRQ.

   wait-reply
      The ICRQ sender receives either (1) a CDN indicating the peer is
      not willing to accept the call (general error or do not accept)
      and moves back into the idle state, or (2) an ICRP indicating the
      call is accepted.  In the latter case, the LCCE sends an ICCN and
      enters the established state.

   established
      Data is exchanged over the session.  The call may be cleared by

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      any of the following:
         + An event on the connected interface: The LCCE sends a CDN.
         + Receipt of a CDN: The LCCE cleans up, disconnecting the call.
         + A local reason: The LCCE sends a CDN.

7.4.2  ICRQ Recipient 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 close        Send CDN,         idle
   established     request            clean up

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

   The states associated with the ICRQ recipient are as follows:

   idle
      An ICRQ is received.  If the request is not acceptable, a CDN is
      sent back to the peer LCCE, and the local LCCE remains in the idle
      state.  If the ICRQ is acceptable, an ICRP is sent.  The session
      moves to the wait-connect state.

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   wait-connect
      The local LCCE is waiting for an ICCN from the peer.  Upon receipt
      of the ICCN, the local LCCE moves to established state.

   established
      The session is terminated either by sending a CDN or by receiving
      a CDN from the peer.  Clean up follows on both sides regardless of
      the initiator.

7.5  Outgoing Calls

   Outgoing calls instruct an LAC to place a call.  There are three
   messages for outgoing calls: OCRQ, OCRP, and OCCN.  An LCCE first
   sends an OCRQ to an LAC to request an outgoing call.  The LAC MUST
   respond to the OCRQ with an OCRP once it determines that the proper
   facilities exist to place the call and that the call is
   administratively authorized.  Once the outbound call is connected,
   the LAC sends an OCCN to the peer indicating the final result of the
   call attempt.

7.5.1  OCRQ Sender States

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

   idle            Receive OCCN,      Clean up          idle
                   OCRP

   wait-control-   control-conn-open  Send OCRQ         wait-reply
   conn

   wait-reply      Receive OCRP,      none              wait-connect
                   acceptable

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

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

   wait-connect    Receive OCCN       none              established

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

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

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

   wait-reply,     Local close        Send CDN,         idle
   wait-connect,   request            clean up
   established

   wait-control-   Local close        Clean up          idle
   conn            request

   The states associated with the OCRQ sender are as follows:

   idle, wait-control-conn
      When an outgoing call request is initiated, a control connection
      is created as described above, if not already present.  Once the
      control connection is established, an OCRQ is sent to the LAC, and
      the session moves into the wait-reply state.

   wait-reply
      If a CDN is received, the session is cleaned up and returns to
      idle state.  If an OCRP is received, the call is in progress, and
      the session moves to the wait-connect state.

   wait-connect
      If a CDN is received, the session is cleaned up and returns to
      idle state.  If an OCCN is received, the call has succeeded, and
      the session may now exchange data.

   established
      If a CDN is received, the session is cleaned up and returns to
      idle state.  Alternatively, if the LCCE chooses to terminate the
      session, it sends a CDN to the LAC, cleans up the session, and
      moves the session to idle state.

7.5.2  OCRQ Recipient (LAC) States

   State           Event              Action            New State
   -----           -----              ------            ---------
   idle            Receive OCRQ,      Send OCRP,        wait-cs-answer
                   acceptable         Place call

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

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   idle            Receive OCRP       Send CDN,         idle
                                      clean up

   idle            Receive OCCN,      Clean up          idle
                   CDN

   wait-cs-answer  Call placement     Send OCCN         established
                   successful

   wait-cs-answer  Call placement     Send CDN,         idle
                   failed             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     Local close        Send CDN,         idle
                   request            clean up

   The states associated with the LAC for outgoing calls are as follows:

   idle
      If the OCRQ is received in error, respond with a CDN.  Otherwise,
      place the call, send an OCRP, and move to the wait-cs-answer
      state.

   wait-cs-answer
      If the call is not completed or a timer expires while waiting for
      the call to complete, send a CDN with the appropriate error
      condition set, and go to idle state.  If a circuit-switched
      connection is established, send an OCCN indicating success, and go
      to established state.

   established
      If the LAC receives a CDN from the peer, the call MUST be released
      via appropriate mechanisms, and the session cleaned up.  If the
      call is disconnected because the circuit transitions to a
      "disconnected" or "down" state, the LAC MUST send a CDN to the
      peer and return to idle state.

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7.6  Termination of a Control Connection

   The termination of a control connection consists of either peer
   issuing a StopCCN.  The sender of this message SHOULD wait a full
   control message retransmission cycle (e.g. 1 + 2 + 4 + 8 ... seconds)
   for the acknowledgment of this message before releasing the control
   information associated with the control connection.  The recipient of
   this message should send an acknowledgment of the message to the
   peer, then release the associated control information.

   When to release a control connection 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 control
   connection open for a period of time or perhaps indefinitely after
   the last session for that control connection is cleared.  Others may
   choose to disconnect the control connection immediately after the
   last call on the control connection disconnects.

8.  Security Considerations

   This section addresses some of the security issues that L2TP
   encounters in its operation.

8.1  Control Connection Endpoint and Message Security

   The LCCEs may configure a shared secret (password) in order to
   perform a mutual authentication of one another, and construct an
   authentication and integrity check of all arriving Control Messages.
   This mechanism is built-in to L2TPv3, and is described in section 4.3
   and in the definition of the Message Digest and Nonce AVPs in section
   5.4.3.

   This mechanism provides strong mutual peer authentication, and
   authentication and integrity checking for individual Control
   Messages.

8.2  Data Channel Security

   As described in section 4.1, the Assigned Cookie sent with each data
   packet MUST be selected in an unpredictable manner (with the added
   restriction that two same Cookie values not be selected within a
   short period of time for a given Session ID).

   A 64-bit Cookie provides effective protection against a blind packet
   insertion attack on a given PE. This is useful as a security feature
   only within networks where sniffing and correlating packets between

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   L2TP nodes is considered impossible, though inserting IP packets
   destined to an LCCE may be considered possible (and perhaps trivial
   by an individual armed with the proper hacking tools). In such cases,
   the Cookie provides an effective barrier against packet insertion
   into a VPN by enforcing that a given Session ID match the random 64
   bit Cookie.  A 32 bit Cookie is vulnerable to brute force guessing at
   high packet rates, and as such should not be considered an effective
   barrier to insertion attacks (it still provides an additional
   integrity check for the Session ID, as described in section 4.1).

   The L2TPv3 Cookie MUST not be regarded as a substitute for packet-
   level security such as that of IPsec when operating over an open or
   untrusted network where packets may be sniffed and values correlated
   to spoofed packets. L2TPv3 does not attempt to provide data packet
   encryption of any kind (without the aid of IPsec).

8.3  End-to-End Security

   Protecting the L2TP packet stream with IPsec does, in turn, also
   protect the data within the tunneled session packets while
   transported from one LCCE to the other.  Such protection MUST not be
   considered a substitution for end-to-end security between
   communicating hosts or applications.

8.4  L2TP and IPsec

   When running over IP, IPsec [RFC2401] provides packet-level security
   via ESP [RFC3193].  All L2TP control and data packets for a
   particular control connection appear as homogeneous UDP or IP data
   packets to the IPsec system.

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

   IPsec also defines access control features that are required of a
   compliant IPsec implementation.  These features allow filtering of
   packets based upon network and transport layer characteristics such
   as IP address, ports, etc.  In the L2TP tunneling model, analogous
   filtering is logically performed at the network layer above L2TP.
   These network layer access control features may be handled at an LCCE
   via vendor-specific authorization features based upon the
   authenticated 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.

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8.5  Impact of L2TPv3 Features on RFC 3193

   [RFC3193] defines the recommended method for securing L2TPv2.  L2TPv3
   possesses identical characteristics to IPsec as L2TPv2 when running
   on UDP/IP.  When operating over IP directly, the principles defined
   in [RFC3193] still apply, though references to UDP port selection (in
   particular Section 4 "IPsec Filtering details when protecting L2TP")
   become far simpler as there is only a single IP protocol ID to worry
   about when applying filters. Specific details for operating L2TPv3
   with IPsec will be specified in an update to [RFC3193].

9.  Internationalization Considerations

   The Host Name and Vendor Name AVPs are not internationalized.  The
   Vendor Name AVP, although intended to be human-readable, would seem
   to fit in the category of "globally visible names" [RFC3066] and so
   is represented in US-ASCII.

   The Preferred Language AVP is not mandatory.  If an LCCE does not
   signify a language preference by the inclusion of this AVP in the
   SCCRQ or SCCRP, the Preferred Language AVP is unrecognized, or the
   requested language is not supported by the peer LCCE, the default
   language [RFC2277] MUST be used for all internationalized strings
   sent by the peer.

10.  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  Control Message Attribute Value Pairs (AVPs)

   This number space is managed by IANA as per [RFC3438].

   New AVPs requiring assignment in this document are defined in the
   "AVP Summary," Section 5.4, with the encoding "AVP-TBA-x," where "x"
   is 1, 2, 3 ...

   A short summary of the new AVPs follows:

   AVP-TBA-1  Message Digest
   AVP-TBA-2  Router ID,
   AVP-TBA-3  Assigned Control Connection ID
   AVP-TBA-4  Pseudowire Capabilities List
   AVP-TBA-5  Local Session ID

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   AVP-TBA-6  Remote Session ID
   AVP-TBA-7  Assigned Cookie
   AVP-TBA-8  Remote End ID
   AVP-TBA-9  Application Code
   AVP-TBA-10 Pseudowire Type
   AVP-TBA-11 L2-Specific Sublayer
   AVP-TBA-12 Data Sequencing
   AVP-TBA-13 Circuit Status
   AVP-TBA-14 Preferred Language

10.2  Message Type AVP Values

   This number space is managed by IANA as per [RFC3438]. There is one
   new message type, defined in section 3.1, necessary to be allocated
   for this specification:
   TBA-M1 (ACK)  Explicit Acknowledgement

10.3  Result Code AVP Values

   This number space is managed by IANA as per [RFC3438].

   New Result Code values for the CDN message are defined in section
   5.4. Following is a summary:

   RC-TBA-1 - Session not established due to losing tie breaker.
   RC-TBA-2 - Session not established due to unsupported PW type.
   RC-TBA-3 - Session not established, sequencing required without valid
              L2-Specific Sublayer.

   There are a few cases in Section 5 where these values are referred to
   directly within the document text with the RC-TBA-x format. The
   assigned values should be inserted within the text for these cases.

10.3.2  Error Code Field Values

   This number space is managed by IANA as per [RFC3438].

10.4  AVP Header Bits

   There are four remaining reserved bits in the AVP header.  Additional
   bits should only be assigned via a Standards Action [RFC2434].

10.5  L2TP Control Message Header Bits

   There are nine remaining reserved bits in the control message header.
   Additional bits should only be assigned via a Standards Action
   [RFC2434].

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   Care should be taken before using reserved bits 6 and 7 in the L2TPv3
   control message header since these bits have meaning for L2TPv2 data
   messages.  Using these two bits in L2TPv3 MAY trigger an unforeseen
   interoperability problem with L2TPv3 implementations based on L2TPv2.
   Therefore, it is recommended that these two bits be utilized last,
   after the other reserved bits have been assigned roles.

10.6 Pseudowire Types

   The Pseudowire Type (PW Type, Section 5.4) is a two-octet value used
   in the Pseudowire Type AVP and Pseudowire Capabilities List AVP
   defined in Section 5.4.3.  0 to 32767 are assignable by Expert Review
   [RFC2434], 32768 to 65535 by a First Come First Served policy
   [RFC2434]. There are no specific pseudowire types assigned within
   this document. Each pseudowire-specific document MUST allocate its
   own PW types from IANA as necessary.

10.7 Application Code

   The Application Code (Section 5.4) is a two-octet value used in the
   Application Code AVP. Value 0 is assigned to the base application
   defined in this document. Additional Application Codes may be
   assigned by IETF Consensus [RFC2434].

10.8 Circuit Status Bits

   The Circuit Status (Section 5.4) field is a 16 bit mask, with the two
   high order bits assigned.

   Bit 15 - A (Active) bit
   Bit 16 - N (New) bit

   Additional bits may be assigned by IETF Consensus [RFC2434].

10.9 Default L2-Specific Sublayer bits

   The Default L2 Specific Sublayer defined in Section 4.6 contains 8
   bits in the low-order portion of the header, two of which have been
   assigned and 6 remain.

   Bit 0 - P (Priority) bit
   Bit 1 - S (Sequence) bit

   Additional values may be assigned by IETF Consensus [RFC2434].

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10.10 L2-Specific Sublayer Type

   The L2-Specific Sublayer Type is a 2 octet unsigned integer of which
   two values have been assigned.

   0 - No L2-Specific Sublayer
   1 - Default L2-Specific Sublayer present

   Additional values may be assigned by Expert Review [RFC2434].

10.11 Data Sequencing Level

   The Data Sequencing Level is a 2 octet unsigned integer of which
   three values have been assigned.

   0 - No incoming data packets require sequencing.
   1 - Only non-IP data packets require sequencing.
   2 - All incoming data packets require sequencing.

   Additional values may be assigned by Expert Review [RFC2434].

13.  Acknowledgments

   Many of the protocol constructs were originally defined in, and the
   text of this document began with, RFC 2661, "L2TPv2". RFC 2661
   authors are W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn and
   B. Palter.

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

   Danny Mcpherson and Suhail Nanji published the first "L2TP Service
   Type" draft which defined the use of L2TP for tunneling of various L2
   payload types (initially, Ethernet and Frame Relay).

   The team for splitting RFC 2661 into this base document and the
   companion PPP document consisted of Ignacio Goyret, Jed Lau, Bill
   Palter, Mark Townsley, and Madhvi Verma.  Skip Booth also provided
   very helpful review and comment.

   Some constructs of L2TPv3 were based in part on UTI (Universal
   Transport Interface), which was originally conceived by Peter
   Lothberg and Tony Bates.

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   Stewart Bryant and Simon Barber provided valuable input for the
   L2TPv3 over IP header.

   Juha Heinanen provided helpful review, and input for the Application
   ID AVP.

   A number of people provided valuable input and effort for RFC2661, on
   which this document was based:

   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 RFC 2661.

   Thomas Narten provided a great deal of critical review and
   formatting.  He originally wrote the IANA Considerations section.

   Dory Leifer made valuable refinements to the protocol definition of
   L2TP and contributed to the editing of early drafts leading to RFC
   2661.

   Steve Cobb and Evan Caves redesigned the state machine tables.

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

11.  References

11.1  Normative References

   [RFC1958] Carpenter, B., "Architectural Principles of the Internet",
             RFC 1958, June 1996.

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

   [RFC2072] Berkowitz, H., "Router Renumbering Guide", RFC 2072,
             January 1997.

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

   [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
             Languages", BCP 18, RFC 2277, January 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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   [RFC2661] Townsley, W., et al., "Layer Two Tunneling Layer Two Tunneling
             Protocol (L2TP)", RFC 2661, August 1999.

   [RFC2865] Rigney, C., Rubens, A., Simpson, W., and Willens, S.,
             "Remote Authentication Dial In User Service (RADIUS)",
             RFC 2865, June 2000.

   [RFC3066] Alvestrand, H., "Tags for the Identification of Languages",
             RFC 3066, January 2001.

   [RFC3193] Patel, B., Aboba, B., Dixon, W., Zorn, G., and Booth, S.,
             "Securing L2TP using IPsec", RFC 3193, November 2001.

11.2  Informative References

   [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.

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

   [RFC1321] R. Rivest, "The MD5 Message-Digest Algorithm", RFC 1321,
             04/16/1992

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

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

   [RFC1750] D. Eastlake III, S. Crocker, J. Schiller, "Randomness
             Recommendations for Security", RFC 1750, December 1994

   [RFC2104] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing for
             Message Authentication", RFC 2104, February 1997

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

   [RFC2341] Valencia, A., Littlewood, M., and Kolar, T.,
             "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.

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

   [RFC2809] Aboba, B., and Zorn, G., "Implementation of L2TP Compulsory
            Tunneling via RADIUS", RFC 2809, April 2000.

   [RFC3070] Rawat, V., Tio, R., Nanji, S., and Verma, R.,
             "Layer Two Tunneling Protocol (L2TP) over Frame Relay",
             RFC 3070, February 2001.

   [RFC3355] Singh, A., Turner, R., Tio, R., Nanji, S., "Layer Two
             Tunnelling Protocol (L2TP) Over ATM Adaptation
             Layer 5 (AAL5)", RFC 3355, August 2002

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

12.  Editors' Addresses

   Jed Lau
   cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   [email-masked]

   W. Mark Townsley
   cisco Systems
   [email-masked]

   Ignacio Goyret
   Lucent Technologies
   [email-masked]

Appendix A: Control 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

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   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 acknowledgment (either explicit or
   piggybacked).  When the acknowledgment 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 ACK message or
   piggybacked) is received.  Increasing CWND by one on each ACK has the
   effect of doubling CWND with each round trip, resulting in an
   exponential increase.  When the value of CWND reaches SSHTRESH, the
   slow start phase ends and the congestion avoidance phase begins.

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

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

Appendix B: Control Message Examples

B.1: Lock-Step Control Connection Establishment

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

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          LCCE A                   LCCE B
          ------                   ------
          SCCRQ     ->
          Nr: 0, Ns: 0
                                   <-     SCCRP
                                   Nr: 1, Ns: 0
          SCCCN     ->
          Nr: 1, Ns: 1
                                   <-       ACK
                                   Nr: 2, Ns: 1

B.2: Lost Packet with Retransmission

   An existing control connection has a new session requested by LCCE A.
   The ICRP is lost and must be retransmitted by LCCE B.  Note that loss
   of the ICRP has two effects: It not only keeps the upper level state
   machine from progressing, but also keeps LCCE A from seeing a timely
   lower level acknowledgment of its ICRQ.

           LCCE A                           LCCE B
           ------                           ------
           ICRQ      ->
           Nr: 1, Ns: 2
                            (packet lost)   <-      ICRP
                                            Nr: 3, Ns: 1

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

          ICRQ      ->
          Nr: 1, Ns: 2

         (Realizing that it has already seen this packet,
          LCCE B discards the packet and sends an ACK message)

                                            <-       ACK
                                            Nr: 3, Ns: 2

         (LCCE B's retransmit timer fires)

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

                                            <-       ACK
                                            Nr: 4, Ns: 2

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Appendix C: Processing Sequence Numbers

   The Default L2-Specific Sublayer, defined in Section 4.6, provides a
   24-bit field for sequencing of data packets within an L2TP session.
   L2TP data packets are never retransmitted, so this sequence is used
   only to detect packet order, duplicate packets, or lost packets.

   The 24-bit field of the Default L2-Specific Sublayer contains a free-
   running counter, including zero. Each sequenced data packet that is
   sent must contain the sequence number, incremented by one, of the
   previous sequenced packet sent on a given L2TP session. Upon receipt,
   any packet with a sequence number equal to or greater than the
   current expected packet (the last received in-order packet plus one)
   should be considered "new" and accepted. All other packets are
   considered "old" and discarded.

   Larger or smaller sequence number fields are possible with L2TP if an
   alternative format to the Default L2-Specific Sublayer defined in
   this document is used. While 24 bits may be adequate in a number of
   circumstances, a larger sequence number space will be less
   susceptible to sequence number wrapping problems for very high
   session data rates across long dropout periods. The sequence number
   processing recommendations below should hold for any size sequence
   number field.

   When detecting whether a packet sequence number is "greater" or
   "less" than a given sequence number value, wrapping of the sequence
   number must be considered. This is typically accomplished by keeping
   a window of sequence numbers beyond the current expected sequence
   number for determination of whether a packet is "new" or not. The
   window may be sized based on the link speed and sequence number space
   and SHOULD be configurable with a default equal to one half the size
   of the available number space (e.g. 2^(n-1), where n is the number of
   bits available in the sequence number).

   Upon receipt, packets which exactly match the expected sequence
   number are processed immediately and the next expected sequence
   number incremented. Packets that fall within the window for new
   packets may either be processed immediately and the next expected
   sequence number updated to one plus that received in the new packet,
   or held for a very short period of time in hopes of receiving the
   missing packet(s). This 'very short period' should be configurable,
   with a default corresponding to a time lapse which is at least an
   order of magnitude less than the retransmission timeout periods of
   higher layer protocols such as TCP.

   For typical transient packet mis-orderings, dropping out-of-order
   packets alone should suffice and generally requires far less

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   resources than actively reordering packets within L2TP. An exception
   is a case where a pair of packet fragments are persistently
   retransmitted and sent out-of-order. For example, if an IP packet has
   been fragmented into a very small packet followed by a very large
   packet before being tunneled by L2TP, it is possible (though
   admittedly wrong) that the two resulting L2TP packets may be
   consistently mis-ordered by the PSN in transit between L2TP nodes. If
   sequence numbers were being enforced at the receiving node without
   any buffering of out-of-order packets, then the fragmented IP packet
   may never reach its destination. It may be worth noting here that
   this condition is true for any tunneling mechanism of IP packets
   which include sequence number checking on receipt (i.e. GRE
   [RFC2890]).

   Utilization of a Data Sequencing Level (see Section 5.4.3) of 1 (only
   non-IP data packets require sequencing) allows IP data packets being
   tunneled by L2TP to not utilize sequence numbers, while utilizing
   sequence numbers and enforcing packet order for any remaining non-IP
   data packets. Depending on the requirements of the link-layer being
   tunneled, and the network data traversing the data-link, this is
   sufficient in many cases to enforce packet order on frames which
   require it (such as end-to-end data-link control messages), while not
   on IP packets which are known to be resilient to packet reordering.

   If a large number of packets (e.g. more than one new packet window)
   are dropped due to an extended outage, or loss of sequence number
   state on one side of the connection (perhaps as part of a forwarding
   plane reset or failover to a standby node), it is possible that a
   large number of packets will be sent in-order, but be wrongly
   detected by the peer as out-of-order. This can be generally
   characterized for a window size, w, sequence number space, s, and
   number of packets lost in transit between L2TP endpoints, p, as
   follows:

   If s > p > w, then an additional (s - p) packets that were otherwise
   received in-order, will be incorrectly classified as out-of-order and
   dropped. Thus, for a sequence number space, s = 128, window size, w =
   64, and number of lost packets, p = 70; 128 - 70 = 58 additional
   packets would be dropped after the outage until the sequence number
   wrapped back to the current expected next sequence number.

   To mitigate this additional packet loss, one MUST inspect the
   sequence numbers of packets dropped due to being classified as "old"
   and reset the expected sequence number accordingly. This may be
   accomplished by counting the number of "old" packets dropped that
   were in sequence among themselves and upon reaching a threshold,
   resetting the next expected sequence number to that seen in the
   arriving data packets. Packet timestamps may also be used as an

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   indicator to reset the expected sequence number by detecting a period
   of time over which "old" packets have been received in-sequence. The
   ideal thresholds will vary depending on link speed, sequence number
   space, and link tolerance to out-of-order packets, and MUST be
   configurable.

Appendix D: 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 that may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

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

Appendix E: Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be

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   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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