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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 3931

Network Working Group                                             J. Lau
Internet-Draft                                               M. Townsley
Category: Standards Track                                    A. Valencia
<draft-ietf-l2tpext-l2tp-base-02.txt>                            G. Zorn
                                                           cisco Systems
                                                               I. Goyret
                                                     Lucent Technologies
                                                                 G. Pall
                                                   Microsoft Corporation
                                                               A. Rubens
                                                                 Nexthop
                                                               B. Palter
                                                        Redback Networks
                                                              March 2002


           Layer Two Tunneling Protocol (Version 3) "L2TPv3"

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 (2002).  All Rights Reserved.









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Abstract

   This document describes the Layer Two Tunneling Protocol (L2TP).
   L2TP tunnels Layer 2 packets across an intervening network in a way
   that is as transparent as possible to both end-users and
   applications.

Acknowledgments

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

   Danny Mcpherson and Suhail Nanji published the first "L2TP Service
   Type" draft which defined the use of L2TP for tunneling of multiple
   L2 payload types. This step led to the eventual creation of this
   document and the modularization of L2TP and PPP tunneling with L2TP.

   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.

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

   This document was based upon RFC 2661, for which a number of people
   provided valuable input and effort:

   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, formatting,
   and 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
   RFC2661.

   Steve Cobb and Evan Caves redesigned the state machine tables.

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





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   Contents

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

   1.  Introduction.............................................    5
      1.1  Changes from RFC 2661................................    5
      1.2  Specification of Requirements........................    6
      1.3  Terminology..........................................    6

   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...............................   14
      3.3  Control Connection Management........................   15
         3.3.1  Control Connection Establishment................   15
         3.3.2  Control Connection Teardown.....................   15
      3.4  Session Management...................................   16
         3.4.1  Session Establishment for an Incoming Call......   16
         3.4.2  Session Establishment for an Outgoing Call......   16
         3.4.3  Session Teardown................................   17

   4.  Protocol Operation.......................................   17
      4.1  L2TP Over Specific Packet Switched Networks (PSNs)...   17
         4.1.1  L2TP 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  Tunnel Endpoint Authentication.......................   24
      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..........................   27
      4.7  L2TPv2/v3 Interoperability and Migration.............   27
         4.7.1  L2TPv3 over IP..................................   27
         4.7.2  L2TPv3 over UDP.................................   28
         4.7.3  Automatic L2TPv2 Fallback.......................   28

   5.  Control Message Attribute Value Pairs....................   29
      5.1  AVP Format...........................................   29
      5.2  Mandatory AVPs.......................................   30
      5.3  Hiding of AVP Attribute Values.......................   31
      5.4  AVP Summary..........................................   33
         5.4.1  AVPs Applicable to All Control Messages.........   33
         5.4.2  Result and Error Codes..........................   35
         5.4.3  Control Connection Management AVPs..............   37



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         5.4.4  Session Management AVPs.........................   43
         5.4.5  Circuit Status AVPs.............................   52

   6.  Control Connection Protocol Specification................   53
      6.1  Start-Control-Connection-Request (SCCRQ).............   53
      6.2  Start-Control-Connection-Reply (SCCRP)...............   53
      6.3  Start-Control-Connection-Connected (SCCCN)...........   54
      6.4  Stop-Control-Connection-Notification (StopCCN).......   54
      6.5  Hello (HELLO)........................................   55
      6.6  Incoming-Call-Request (ICRQ).........................   55
      6.7  Incoming-Call-Reply (ICRP)...........................   55
      6.8  Incoming-Call-Connected (ICCN).......................   56
      6.9  Outgoing-Call-Request (OCRQ).........................   56
      6.10  Outgoing-Call-Reply (OCRP)..........................   57
      6.11  Outgoing-Call-Connected (OCCN)......................   58
      6.12  Call-Disconnect-Notify (CDN)........................   58
      6.13  WAN-Error-Notify (WEN)..............................   59
      6.14 Set-Link-Info (SLI)..................................   59

   7.  Control Connection State Machines........................   59
      7.1  Malformed Control Messages...........................   59
      7.2  Timing Considerations................................   60
      7.3  Control Connection States............................   61
      7.4  Incoming Calls.......................................   62
         7.4.1  ICRQ Sender States..............................   63
         7.4.2  ICRQ Recipient States...........................   65
      7.5  Outgoing Calls.......................................   66
         7.5.1  OCRQ Sender States..............................   66
         7.5.2  OCRQ Recipient (LAC) States.....................   67
      7.6  Termination of a Control Connection..................   68

   8.  Security Considerations..................................   69
      8.1  Control Connection Endpoint Security.................   69
      8.2  Packet Level Security................................   70
      8.3  End-to-End Security..................................   70
      8.4  L2TP and IPsec.......................................   70
      8.5  Impact of L2TPv3 Features on RFC 3193................   71

   9.  IANA Considerations......................................   71
      9.1  AVP Attributes.......................................   71
      9.2  Message Type AVP Values..............................   71
      9.3  Result Code AVP Values...............................   71
         9.3.1  Result Code Field Values........................   71
         9.3.2  Error Code Field Values.........................   72
      9.4  AVP Header Bits......................................   72

   10.  References..............................................   72




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   11.  Editors' Addresses......................................   74

   Appendix A: Control Slow Start and Congestion Avoidance......   74

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

   Appendix C: Intellectual Property Notice.....................   77

1.  Introduction

   The Layer Two Tunneling Protocol (L2TP) provides a dynamic tunneling
   mechanism for multiple Layer 2 (L2) circuits across a packet-oriented
   data network.  L2TP, as originally defined in RFC 2661, describes a
   standard method for tunneling PPP 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 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 IP-connected L2TP nodes.

1.1  Changes from RFC 2661

   Most 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, interoperability experience
   with RFC 2661, and a thoughtful and directed evolution of the
   protocol 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 control message header.
   (L2F is 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:
   - 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.



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   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) and a Value containing the actual
      value identified by the attribute.  Multiple AVPs make up control
      messages, which are used in the establishment, maintenance, and
      teardown of control connections.  This construct is known as the
      Type-Length-Value (TLV) in some specifications.  (See also:
      Control Connection, Control Message.)

   Call (Circuit Up)

      The action of transitioning a circuit on an 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 PSTN) or statically configured (e.g. provisioning a VC 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.)

   CHAP

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

   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
      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 some type of control channel
      governing the establishment, maintenance, and teardown of the
      circuit.  For the purposes of this document, a statically



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      configured circuit is considered to be largely equivalent to a
      simple 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 channel 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.  (See also: Data Channel.)

   Data Channel

      The channel of L2TP-encapsulated L2 traffic that passes between
      two LCCEs, utilizing a specific data encapsulation method.  L2TP
      defines one base encapsulation method for L2 traffic, although
      others may be used as well.  (See also: Control Connection, Data
      Message.)

   Dominant LCCE

      The LCCE that either solely initiated establishment of a control
      connection or won the tie breaker during control connection
      establishment.  (See also: LCCE, Section 5.4.3.)

   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.)





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   L2TP Access Concentrator (LAC)

      An LCCE that tunnels a circuit (either physically connected or
      logically connected, as via another L2TP session) to another
      location using L2TP, without performing any native L2 packet
      processing on the circuit.  The LAC may tunnel to either an LNS or
      another LAC.  (See also: LCCE, LNS.)

   L2TP Control Connection Endpoint (LCCE)

      One end of an L2TP control connection, either an LAC or an LNS.
      (See also: LAC, LNS.)

   L2TP Network Server (LNS)

      An LCCE that logically terminates a tunneled circuit locally and
      that processes the tunneled traffic as though the circuit were
      physically connected to the device.  The LNS may tunnel to either
      an LAC or another LNS.  (See also: LCCE, LAC.)

   Outgoing Call

      The action of placing a call on 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.)

   Outgoing Call Request

      A request sent to an LAC to place an outgoing call.  The request
      contains specific information for the LAC in placing the call,
      information that is typically not known a priori by the LAC.  (See
      also: Call, Incoming Call, Outgoing Call.)

   Packet-Switched Network (PSN)

      A network layer that uses packet-switching technology for data
      delivery (e.g. an IP network).

   Peer

      When used in context with L2TP, Peer refers to the far end of an
      L2TP control connection (i.e. the far 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.)

   Remote System

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



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   Session

      An L2TP session is created by a particular L2TP control connection
      between two LCCEs when a circuit is successfully established.  The
      circuit may either pass through (LAC) or terminate locally (LNS)
      on the LCCEs, which maintain state for the circuit.  There is a
      one-to-one relationship between established L2TP sessions and
      their associated circuits.  (See also: Circuit, LAC, LCCE, LNS.)

   Zero-Length Body (ZLB) Message

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

2.  Topology

   L2TP operates between two L2TP Control Connection Endpoints (LCCEs),
   tunneling circuit traffic across a packet network.  An L2TP Network
   Server (LNS) is an LCCE that decapsulates tunneled L2 traffic and
   directs it as incoming data towards a virtual L2 interface.  In
   contrast, an L2TP Access Concentrator (LAC) is an LCCE that merely
   forwards tunneled traffic directly to a circuit (which may even be
   another L2TP session).

   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 traffic (at Layer 3) to the home
   network.  The action of session establishment may be driven by the
   LAC (perhaps as an incoming call) or the LNS (perhaps as an outgoing
   call).  This model typically has, but does not require, a clear
   initiator and responder.

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



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   (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.  A LAC does not perform any native
   handling of the tunneled L2 frame, and thus, does not utilize a
   virtual L2 interface.  Rather, a LAC acts as a simple cross-connect
   between a circuit 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 perhaps simultaneously).

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

   (c) LNS-LNS Reference Model: This model has two LNSs as the LCCEs.
   Each LNS logically terminates the L2TP session locally, requiring
   virtual L2 interfaces for each L2TP session on each side of the L2TP
   session.  A user-level or traffic-generated event typically drives
   session establishment from one side of the control connection.  Also
   known as "voluntary tunneling" [RFC2809].

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

   Note: If an LNS initiates session establishment due to an event
   (generally user-driven), the LNS is sometimes referred to as a "LAC
   Client" as defined in [RFC2661].

3.  Protocol Overview

   L2TP utilizes two types of messages: control messages and data
   messages.  Control messages are used in the establishment,
   maintenance, and clearing of control connections and calls.  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.

   While both the L2TP control channel and the L2TP data channel are
   defined strictly in this document, the L2TP data channel MAY be
   substituted with a different L2 tunneling encapsulation whose format



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   can negotiated by the L2TP control connection.  Furthermore, the L2TP
   data channel MAY be used without the control channel, if so desired.
   However, it is strongly recommended that such practice be limited to
   relatively small-scale deployments, or deployments in which some
   other form of automatic control information distribution is employed.

                       Figure 3.0: L2TPv3 Structure

   +-------------------+
   | L2 Frames         |
   +-------------------+    +-----------------------+
   | L2TP Data Messages|    | L2TP Control Messages |
   +-------------------+    +-----------------------+
   | 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
   messages are passed over an unreliable data channel, encapsulated
   first 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 in-band
   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.  The
   control connection MUST be established before an incoming or outgoing
   call is initiated.  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)



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

   Call Management

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

   Error Reporting

      15 (WEN)      WAN-Error-Notify

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.  As such,
   L2TP also includes a default method (borrowing from RFC 2661 by
   utilizing the high bit of the first octet in the L2TP header) that
   may be used to distinguish L2TP control messages from L2TP data
   messages.  Other methods for distinguishing between control and data
   MAY be utilized for specific media (an example is L2TP over IP as
   defined in 4.1).

   The L2TP control message header is formatted as follows:





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                 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 with
   L2TPv2 [RFC2661] and/or L2F [RFC2341], see Section 4.1 for details).

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



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   information on using this field.

3.2.2  L2TP Data Message

   In general, an L2TP data message consists of a (1) Tunnel Header, (2)
   an L2-Specific Sublayer (when needed), and (3) the Tunneled L2 Frame,
   as depicted below.

                      Figure 3.2.2: L2TP Data Message


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Packet Switched Network (PSN) Delivery Header         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       L2TP Tunnel Header                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      L2-Specific Sublayer                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Tunneled L2 Frame                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Packet Switched Network is any network layer that uses packet-
   switching technology for data delivery. This is principally IP, but
   may be MPLS, FR, ATM, or any other packet-switched network.

   The L2TP Tunnel Header is specific to the PSN over which the L2TP
   traffic is delivered.  The tunnel header MUST, at a minimum, provide
   (1) a 32-bit longword-aligned Session ID field to uniquely identify a
   tunneled stream of data, and (2) a method of distinguishing data
   messages from control messages.  Each type of PSN MUST define its own
   tunnel header, clearly identifying the format of the header and
   parameters necessary to setup the session.  Section 4.1 defines two
   tunnel headers, one for transport over UDP and one for transport over
   IP.  Either of these formats MAY be used for other PSNs, but the
   actual definition of such remains outside the scope of this document.

   The L2-specific sublayer is an intermediary layer between the fixed
   L2TP tunnel header and the start of the inner L2 frame.  It may
   contain control fields that the are used to facilitate the tunneling
   of the L2 frames (e.g. offset bytes or sequence numbers).  Since the
   sublayer is specific to each L2 payload that may be tunneled using
   the L2TP data encapsulation, the format of the sublayer is determined
   by the Pseudo Wire Type AVP (see Section 5.4.4), which identifies the
   L2 payload.  Specific sublayer formats are defined in the appropriate
   L2 payload-specific companion documents. A default L2-Sublayer is
   defined in Section 4.6.

   The tunneled L2 frame consists of the encapsulated L2 traffic,



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   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 the 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, ZLB
   ACKs may be sent after any of the control messages indicated in the
   exchanges below if an acknowledgement is not piggybacked on a later
   control message.

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:

      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 a ZLB ACK to acknowledge receipt of the message and
   maintain enough control connection state to properly accept StopCCN
   retransmissions over at least a full retransmission cycle (in case
   the ZLB ACK is lost).  The recommended time for a full retransmission
   cycle is 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)



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                  (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:

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

      ICRQ ->
               <- ICRP
      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 ->







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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)

4.  Protocol Operation

   This section addresses various operational issues in both the control
   connection and data channel of L2TP.

4.1  L2TP Over Specific Packet Switched Networks (PSNs)

   L2TP is designed to allow operation over a variety of Packet Switched
   Networks. In consideration of any specific characteristics of an
   underlying PSN, the actual L2TP Tunnel Header encapsulation may vary.
   For instance, a payload length field for data packets traversing a
   UDP or IP network is unnecessary as this is readily available from
   the underlying layer.

   This document describes the standard method for operation of L2TP
   over IPv4 networks. There are two modes described, 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, integration with IPsec
   [RFC3193], and easier migration from L2TPv2.

   L2TP over other PSNs may be defined, but the specifics are outside
   the scope of this document. Whenever possible, the field definitions
   in this section should be used as they are described here. Examples
   of L2TPv2 over other PSNs include [RFC3070], and [L2TPAAL5].

   The following field definitions are defined for use in all L2TP
   Tunnel 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.



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   That is, the same logical session will be given different Session IDs
   by each end of the tunnel 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), longword-aligned value used to check the association of a
   received data message with the session identified by the Session ID.
   The Cookie MUST be configured with a random value utilizing all bits
   in the field.  The Cookie provides an additional level of guarantee,
   beyond the Session ID lookup, that a data message has been directed
   to the proper session.  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 the creation of a session.  The maximum size of the
   Cookie field is 64 bits.

4.1.1  L2TP over IP

   L2TP over IP utilizes the IANA assigned IP protocol ID 115.

4.1.1.1  L2TP over IP Tunnel Header

   Unlike L2TP over UDP, the L2TPv3 tunnel 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 tunnel header format is utilized when operating L2TPv3
   over IP:

               Figure 4.1.1.1: L2TPv3 over IP Tunnel 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Session ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Cookie (optional, maximum 64 bits)...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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



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   Session ID of zero is reserved for use by L2TP control messages (see
   Section 4.1.1.2).

   It should be noted that the absence of the Version and Flags fields,
   which are present in L2TP over UDP, prevents straightforward version
   extension flexibility for data messages.  However, given the freedom
   of setting the first 32 bits in the data message header here, this
   limitation can be alleviated with an acceptable workaround if an
   extension to the demultiplexing capabilities of L2TP is ever in need
   of further revision.  In contrast, the control message header still
   retains all version checking ability.

4.1.1.2  L2TP Control and Data Traffic over IP

   As shown in Section 4.1.1.1, there are no Version and Flags fields in
   the L2TP Tunnel Header over IP.  Specifically, the T bit does not
   exist to distinguish control packets and data packets.  Instead, all
   control packets are sent over the reserved session ID of 0.  It is
   presumed that this method is more efficient -- both in header size
   for data packets and in processing speed for distinguishing control
   messages -- than checking for the presence of certain bits.

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

           Figure 4.1.1.2: L2TPv3 over IP 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      (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.  The length calculation does NOT
   include the "(32 bits of zeros)" depicted above.

4.1.2  L2TP over UDP

   L2TPv3 over UDP must take into careful consideration other L2
   tunneling protocols that may be operating in the same environment,



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   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, in some
   circumstances, L2TP over IP may not be as NAT-friendly as L2TP over
   UDP.  Also, control messages transmitted over IP are not protected by
   a network-layer checksum as they are with UDP. As such, and for
   easier migration from L2TPv2, this mode over operation is provided.

4.1.2.1  L2TP over UDP Tunnel Header

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

               Figure 4.1.2.1: L2TPv3 over UDP Tunnel 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|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  L2TP over 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
   first 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



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   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.  This
   method has some inefficiencies with regard to packet processing.
   However, it is the most NAT-friendly method since there is only one
   entry in the NAT table to be kept valid, and the control connection
   can provide a keepalive to ensure that the NAT entry remains valid.
   Also, firewalls can be configured to pass all control and data
   traffic with a single entry rather than separate entries for control
   and for data.

   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 before choosing an
   arbitrary source port.  Any NAT device that can pass TFTP traffic
   should be able to pass L2TP UDP traffic as they 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, however, that enabling
   checksums on data packets may significantly increase 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.




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

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

   The last received message number, Nr, is used to acknowledge messages
   received by an L2TP peer.  It contains the sequence number of the
   message the peer expects to receive next (e.g. the last Ns of a non-
   ZLB message received plus 1, modulo 65536).  While the Nr in a
   received ZLB is used to flush messages from the local retransmit
   queue (see below), the Nr of the next message sent is not updated by
   the Ns of the ZLB.  As a precaution, 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, it 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)
   passes without acknowledgement, the message is retransmitted.  The
   retransmitted message contains the same Ns value, but the Nr value
   MUST be updated with the sequence number of the next expected



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   message.

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

   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.

   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 controlling control messages.  An L2TP implementation is
   expected to be able to keep up with incoming control messages,
   possibly responding to some with errors reflecting an inability to
   honor the requested action.

   In addition, a peer MUST NOT withhold acknowledgement of messages in
   order to maintain state in the L2TP state machine.  Conversely, the
   L2TP state machine MUST be capable of maintaining state if a ZLB ACK
   is received in response to a control message.  However, determining
   when a state should no longer be maintained (e.g. how long to wait in
   wait-reply state for an ICRP from the peer) before destroying a
   session or control connection is an issue that is left to each
   implementation.




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

4.3  Tunnel Endpoint Authentication

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

   To participate in LCCE authentication, a single shared secret MUST
   exist between the two LCCEs.  This is the same shared secret used for
   AVP hiding (see Section 5.3).  See Section 5.4.3 for details on
   construction of the Challenge and Response AVPs.

4.4  Keepalive (Hello)

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

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

   Since a Hello is a control message, and since control messages are
   reliably sent by the lower level delivery mechanism, this keepalive
   function operates by causing the reliable delivery of a message.  If
   a media interruption has occurred, the delivery mechanism will be
   unable to deliver the Hello across and will clean up the control
   connection. Since the control channel is operated in-band with all
   data traffic over the PSN, this single mechanism can be used to infer
   basic connectivity between tunnel endpoints for all sessions



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   associated with a control connection.  Thus, per-session keepalives
   are considered redundant unless they are sent end-to-end from or to a
   remote system beyond the L2TP tunnel.

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

4.5  Forwarding Session Data Frames

   Once session establishment is complete, L2 frames are received at an
   LCCE, encapsulated in L2TP (with appropriate attention to framing and
   L2 dependencies as described in documents for the particular Pseudo
   Wire 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 the same 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.  Any received data packets that contain
   invalid Session IDs or associated Cookie values MUST be dropped.
   Finally, the LCCE either processes the encapsulated session frame
   locally (i.e. as an LNS) or forwards the frame to a circuit (i.e. as
   an LAC).

4.6 Default L2-Specific Sublayer

   L2TP provides a default mechanism that a specific Pseudo Wire Type
   MAY use for basic sequencing support, offset of the Tunneled L2
   Frame, and marking of packets with a single high-priority bit. As
   each PW-Type is different, each has different needs regarding these
   features. This format is being provided as a suggestion for PW-
   specific documents, and SHOULD be used as a common method for support
   of these features if it is adequate for the given PW-Type. If this
   mechanism is not well-suited for a particular Pseudo Wire Type, or
   where there may be overlapping functionality at another layer (such
   as sequencing for a PW which is using RTP) the mechanism defined here
   may be omitted and another L2-specific layer identified for that



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   particular Pseudo Wire Type.

   This header is not a part of the base L2TP Tunnel Header (see Section
   3.2.2), and its presence or lack of presence for a given PW-Type is
   defined within the scope of the PW-Specific companion documents. PW-
   Specific documents for L2TP may refer to this document and section as
   a default method for PW support of some or all of these features.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 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| OffSz |              Sequence Number                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Offset padding... (optional, up to 15 octets)
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The P (Priority) bit is used to identify a data packet which should
   be dropped only as a last resort after being received by an L2TP
   peer. This bit should be set to 1 for any layer-2 traffic which
   should be given higher priority in a congested environment. For
   example end-to-end keepalive packets, or other control packets vital
   to the life of the circuit may need special handling by a tunnel
   endpoint 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 OffSz (Offset Size) field defines the number of bytes of padding
   that exist after the Sequence Number, and before the beginning of the
   L2 frame. This may be used to ensure alignment of an inner L3 packet
   in cases where the L2 framing itself may not be word-aligned. This is
   generally of most use when sending packets to an LNS which is going
   to route the framed L3 packet locally rather than sending it out
   another data link.

   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 (thus, 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.



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   See Section 4.6.1 for more information on sequencing layer 2 frames.

4.6.1 Sequencing Data Packets

   The Sequence Number field may be used to detect lost packets and/or
   restore the original sequence of packets that may have been reordered
   during traversal of the packet network.

   When Layer 2 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 or silently drop packets.  Reordering may break
   some non-IP protocols or layer 2 control traffic being carried by the
   link.  Silent dropping of packets may break protocols that assume
   per-packet indication of error, such as TCP header compression.

   If any protocol being transported by over L2TP data channels cannot
   tolerate misordering, sequencing may be turned on some or all packets
   by using the sequence number field and S bit defined in section 4.6.
   The sequence dependency characteristics of individual protocols are
   outside the scope of this document. L2TP takes the very basic and
   simple approach that by default it is always up to the sender as to
   which packets it will try and apply sequence numbers to, and up to
   the receiver as to how much attention will be paid to any sequenced
   packets being processed. L2TP provides mechanisms to advertise this
   information to both sides of the connection (see Section 5.4.4) to
   help with debugging or to adjust sequencing policy according to the
   advertised policy of one's peer.  PW-specific documents MAY place
   greater constraints on sequence number enforcement than those defined
   here.

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 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.

   An L2TP implementation may first attempt to operate in L2TPv3 over IP



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   mode, then fall back to L2TPv2 (over UDP) if L2TPv3 over IP is
   unavailable.  It does so by first sending an L2TPv3-formatted SCCRQ
   over IP to try to initiate an L2TPv3 control connection.  If the
   SCCRQ elicits no response, the implementation may fall back to L2TPv2
   operation, as defined in [RFC2661].  Fallback to L2TPv2 should be
   seamless and occur automatically.  (See Section 4.7.3 for further
   details.)

4.7.2  L2TPv3 over UDP

   In order to allow simultaneous operation with L2TPv2, L2TPv3 uses the
   same UDP port (port 1701) as L2TPv2 and shares the first two octets
   of header format (via the tunnel header) with L2TPv2.  Furthermore,
   though the control message and data message headers have changed, an
   LCCE sends an SCCRQ that looks enough like an L2TPv2 SCCRQ to be
   accepted by both L2TPv2 and L2TPv3 implementations.  If the response
   to the SCCRQ is a properly formatted L2TPv3 message, then operation
   can continue as described in this document for an L2TPv3
   implementation.  If the response is a properly formatted L2TPv2
   message, then an L2TPv2 mode of operation must be adopted.

4.7.3  Automatic L2TPv2 Fallback

   When running over UDP, an implementation may detect whether a peer is
   L2TPv3-capable by sending an L2TPv3-formatted SCCRQ.  The SCCRQ is
   sent with the Ver field set to 2, and any L2TPv3-specific AVPs within
   the message are sent without setting the M bit on each AVP (so that
   they may be 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 (16-bit
   Tunnel ID and 16-bit Session ID in L2TPv2) is always 0 for an SCCRQ,
   a key feature for this capability.

   If the peer implementation is an L2TPv3-capable implementation, 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,
   one may assume that the peer implementation is L2TPv2-only and fall
   back to L2TPv2 mode if local policy and capability permits.

   The auto-detection mode requires that an L2TPv3-only implementation
   be liberal in its acceptance of SCCRQ control messages with the Ver
   field set to 2.  Thus, an L2TPv3 over UDP implementation MUST allow
   receipt of an SCCRQ with Ver field of 2 or Ver field of 3.







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5.  Control Message Attribute Value Pairs

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

5.1  AVP Format

   Each AVP is encoded as:

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

   The first six bits comprise a bit mask that describe 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 AVP that is unrecognized or
   malformed.  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: Encodes 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.



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   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
   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 is the
   intended consequence.

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

   Use of the M bit with new AVPs (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.




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   On the other 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.

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:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 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.



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

   Next, an MD5 hash is performed (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 [RFC2138], which was taken from
   the "Mixing in the Plaintext" section in the book "Network Security"
   by Kaufman, Perlman and Speciner [KPS].  A detailed explanation of
   the method follows:



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

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

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

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

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  AVPs Applicable to All Control Messages

Message Type (All Messages)

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

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

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

   The Message Type is a 2-octet unsigned integer.



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   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
   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.

Random Vector (All Messages)

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

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Random Octet String...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Random Octet String may be of arbitrary length, although a random
   vector of at least 16 octets is recommended.  The string contains the
   random vector for use in computing the MD5 hash to retrieve or hide
   the Attribute Value of a hidden AVP (see Section 5.3).

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

   The M bit for this AVP 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.





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5.4.2  Result and Error Codes

Result Code (CDN, StopCCN)

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

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

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

   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 + 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.
   Additional service-specific error codes are defined outside this
   document.




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       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
           facilities being available (permanent condition)
       6 - 11 Reserved (PPP-specific codes defined outside this document)
      TBA - Session was not established due to losing tie breaker
      TBA - Session was not established due to unsupported PW-Type combination

   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 shutdown 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 (i.e. "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.



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   Furthermore, a vendor-specific AVP MAY be sent to indicate the
   problem more precisely.

5.4.3  Control Connection Management AVPs

Protocol Version (SCCRP, SCCRQ)

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

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

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

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

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

Tie Breaker (SCCRQ)

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

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

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

   The Tie Breaker Value is an 8-octet 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, and if so, must compare its Tie Breaker
   value with the received one.  The lower value "wins", and the "loser"
   MUST silently discard its control connection.  In the case in which a



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   tie breaker is present on both sides and the value is equal, both
   sides MUST discard their control connections and restart control
   connection negotiation.

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

   In the case of a tie, the "winner" of the tie is declared the
   "dominant LCCE".  Session-level ties, as detected by End Identifier
   AVP, are always won by the dominant LCCE.  If there is no tie, the
   dominant LCCE is always the initiator of the control connection (the
   sender of the SCCRQ).

   Tie breaker values MUST be random values.

   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.

Firmware Revision (SCCRP, SCCRQ)

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

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

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

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

   For devices that do not have a firmware revision, the revision of the
   L2TP software module or system software module may be reported
   instead.

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

Host Name (SCCRP, SCCRQ)

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




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

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

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

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

   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.

Vendor Name (SCCRP, SCCRQ)

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

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

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

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

   This AVP may be hidden (the H bit may be 0 or 1).  The M bit for this
   AVP 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 (SCCRP, SCCRQ, StopCCN)

   The Assigned Control Connection ID AVP, Attribute Type TBA, encodes
   the ID being assigned to this control connection by the sender.




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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                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
   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 (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 (SCCRP, SCCRQ)

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

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

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



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   The Window Size is a 2-octet unsigned integer.

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

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

Challenge (SCCRP, SCCRQ)

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

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

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

   The Challenge is one or more octets of random data.

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

Challenge Response (SCCCN, SCCRP)

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

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

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

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

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




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

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

   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
   22.

Pseudo Wire Transmit Capabilities List (SCCRP, SCCRQ)

   The Pseudo Wire Transmit Capabilities List AVP, Attribute Type TBA,
   indicates the L2 payload types the sender of this AVP can transmit.
   The specific payload type of a given session is identified by the
   Pseudo Wire Type AVP.

   Often, the Pseudo Wire Transmit Capabilities List will be the same as
   the Pseudo Wire Receive Capabilities List. The case where it is not
   is limited to where one might be able to support receiving data of
   one pseudowire type, but not transmission of that same pseudowire
   type. This could be due to data plane limitations, or other factors.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Pseudo Wire Type 0       |             ...               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              ...              |      Pseudo Wire Type N       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Defined Pseudo Wire Types that may be included in this list may be
   found in section 5.4.4, Psuedo Wire Type 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 (see Section 4.7.3).  The Length (before
   hiding) of this AVP is 8 octets with one Pseudo Wire Type specified,
   plus 2 octets for each additional Pseudo Wire Type.

Pseudo Wire Receive Capabilities List (SCCRP, SCCRQ)

   The Pseudo Wire Receive Capabilities List AVP, Attribute Type TBA,
   indicates the L2 payload types that will be accepted by the sender of



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   this AVP.  The specific payload type of a given session is identified
   by the Pseudo Wire Type AVP.

   Often, the Pseudo Wire Receive Capabilities List will be the same as
   the Pseudo Wire Transmit Capabilities List. The case where it is not
   is limited to where one might be able to support receiving data of
   one pseudowire type, but not transmission of that same pseudowire
   type. This could be due to data plane limitations, or other factors.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Pseudo Wire Type 0       |             ...               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              ...              |      Pseudo Wire Type N       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Defined Pseudo Wire Types that may be included in this list may be found in
   section 5.4.4, Psuedo Wire Type 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 (see Section 4.7.3).  The Length (before
   hiding) of this AVP is 8 octets with one Pseudo Wire Type specified,
   plus 2 octets for each additional Pseudo Wire Type.

5.4.4  Session Management AVPs

Local Session ID (CDN, ICRP, ICRQ, OCRP, OCRQ, SLI, WEN, occn, iccn)

   The Local Session ID AVP (analogous to the Assigned Session ID in
   L2TPv2), Attribute Type TBA, encodes the ID 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 identifier used to multiplex
   and demultiplex both data and control connection traffic for a given
   session between two LCCEs.  The local session lookup mechanism



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   chooses a free value that it expects to see in all received data
   messages for this session, as well as an AVP in any subsequent
   session-level control messages.  The receiving LCCE MUST use this
   value as the Session ID in the header of all data messages sent to
   this peer.  In addition, the receiving LCCE MUST echo this value back
   as the Remote Session ID AVP in all session-related control messages,
   allowing efficient session context lookup when processing these
   control messages.

   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.

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

   The Remote Session ID AVP, Attribute Type TBA, encodes the ID 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 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 is sufficient for our peer to lookup session-
   level context to apply this control message to. The cases where this
   is not sufficient involve sending a session-level message before a
   Local Session ID AVP is received from a peer.  In these cases, the
   Local Session ID AVP will have to be used, and a "reverse lookup" on
   session context performed.

   The Remote Session ID MUST be present in all session-level control
   messages. In cases where a Local Session ID AVP has not been received
   from our peer, its value is set to zero to indicate this. If the
   Remote Session ID is set to zero, the Local Session ID AVP containing
   our previously advertised Session ID MUST be included in the control
   messages being sent.




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   Examples of valid messages defined in this document that might be
   subject to a reverse lookup due to the Local Session ID AVP not being
   received from our 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 (ICRP, ICRQ, OCRP, OCRQ)

   The Assigned Cookie AVP, Attribute Type TBA, encodes the Cookie value
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               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 Cookie value
   of zero length serves as positive acknowledgement that the Cookie
   field should not be present in any data packets sent to this LCCE.
   The Assigned Cookie AVP MAY not be sent, which has the same effect as
   sending the AVP to designate a Cookie value of zero length.

   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.

Session Serial Number (ICRQ, OCRQ)

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

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Session Serial Number                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Session Serial Number is a 32-bit value.

   The Session 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.  Session Serial Numbers
   should be set to progressively increasing values, which are likely to
   be unique for a significant period of time across all interconnected
   LNSs and LACs.

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

   Note that in [RFC2661] this value was referred to as the Call Serial
   Number. It serves the same purpose and has the same attribute value
   and composition.

End Identifier AVP (ICRQ, OCRQ)

   The End Identifier AVP, Attribute Type TBA, encodes an identifier
   assigned by the LAC or LNS to detect ties in session establishment
   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | End Identifier ... (arbitrary number of octets)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The End Identifier contains interface, circuit, and other
   information, depending on the circuit that is being tunneled.  For
   example, the field may be a simple 4 octet binary value, or an ASCII
   string. Specification of this format is outside the scope of this
   document.

   A session-level tie is detected if an LCCE receives an ICRQ or OCRQ
   with an End Identifier AVP whose value and length matches the End
   Identifier AVP that was just sent in an outgoing ICRQ or OCRQ to the
   same peer.  If the two End Identifier values match, an LCCE
   recognizes that a tie exists (i.e. both LCCEs are attempting to



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   establish sessions for the same circuit).  The tie is broken by the
   dominant LCCE.  The "losing" LCCE must send a CDN to its peer to
   cancel the ICRQ or OCRQ that it had sent to the peer.

   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 End Identifier value.

Minimum BPS (OCRQ)

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

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

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

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

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

Maximum BPS (OCRQ)

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

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

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

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

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




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Pseudo Wire Type (ICRQ, OCRQ)

   The Pseudo Wire Type (PW-Type) AVP, Attribute Type TBA, indicates the
   L2 payload type for packets being transmitted by the sender of this
   AVP into the L2TP tunnel.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Pseudo Wire Type        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Defined Pseudo Wire Types that may be included in the Pseudo Wire
   Capabilities List are as follows:

     0 - PPP
     1 - Frame Relay
     2 - Ethernet

   Additional Types are to be managed by IANA. Values 0 - 32767 are
   assignable by IETF Consensus [RFC2434]. The remaining values may be
   assigned on a First Come First Served basis [RFC2434].

   A peer MUST NOT request an incoming or outgoing call with a Pseudo
   Wire Type AVP specifying a value not advertised in the Pseudo Wire
   Receive Capabilities List AVP it received during control connection
   establishment.  Attempts to do so will result in the call being
   rejected.

   While it may be possible to transmit and receive different pseudowire
   types in either direction across a single L2TP session, it is not
   required nor recommended as common practice. Thus, if the Pseudo Wire
   Type AVP in an ICRQ and ICRP do not match, the session MAY be
   disconnected via a CDN with a "session not established due to
   unsupported PW-Type combination" Result Code defined in Section
   5.4.2. If asymmetric PW-Types are attempted, it should be understood
   in advance that the combination is supported by both vendors. In an
   ideal implementation, any PW Type identified in the Pseudo Wire
   Receive/Transmit Capabilities List would be usable in all possible
   combinations, but it is understood that this might be an unreasonable
   goal for some equipment and even some PW-Type Combinations in
   general.

   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)



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   of this AVP is 8.

Tx Connect Speed (ICCN, OCCN)

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

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

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

   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
   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 (ICCN, OCCN)

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

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

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

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




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   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, OCRP)

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

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

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

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

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

Private Group ID (ICCN)

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

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

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

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

   The LNS MAY treat the session as well as network traffic through this
   session in a special manner determined by the peer.  For example, if
   the LNS is individually connected to several private networks using
   unregistered addresses, this AVP may be included by the LAC to



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   indicate that a given call should be associated with one of the
   private networks.

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

   This AVP may be hidden (the H bit MAY be 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 Private Group ID.

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

   The Data Sequencing AVP, Attribute TBA, identifies the sequencing
   that will be provided by the sender of this AVP (Seq Send) as well as
   the level to which received data sequence numbers will be honored
   (Seq Receive).

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Seq Send   |  Seq Receive  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Seq Send is one octet field identifying what type of sequencing will
   be provided for data packets on this session. The following values
   and sequencing modes are defined:

     0 - No data traffic will have sequence numbers
     1 - Selected data traffic will have sequence numbers
     2 - All data traffic will have sequence numbers

   Seq Receive is a one octet field identifying what sequence numbers
   will be honored (by dropping out of order packets or actively
   reordering packets) for this session when they are sent by the peer.
   The following values and sequencing modes are defined:

     0 - All sequence numbers on data traffic will be ignored
     1 - Sequence numbers on selected data traffic will be honored
     2 - All sequence numbers will be honored

   It is always up to the sender of a each individual data packet as to
   what packets will include sequence numbers and which will not.
   However, based on the information provided in these AVPs, the sender
   may wish to alter its policy. For example, if one side of a session



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   sends a Seq Receive of zero, indicating that all sequence numbers on
   data traffic would be ignored, its peer may decide to disable sending
   of sequence numbers for that session. Note that this AVP may be
   present in an ICRQ or ICCN. If it is present in both, the ICCN always
   takes precedence. If this AVP is never received in any control
   message before establishment of a session, the default of 0 for both
   values is assumed (no sequence numbers sent, and all received
   sequence numbers will be ignored). For more information on data
   sequencing, please see Section 4.6.

   This AVP may be used for any basic sequencing field for any PW-type,
   even if the format of the default L2-Specific sublayer defined in
   section 4.6 is not utilized.

5.4.5  Circuit Status AVPs

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:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 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



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         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 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
   establishment process.

   The following AVPs MUST be present in the SCCRQ:

      Message Type AVP
      Protocol Version
      Host Name
      Assigned Control Connection ID
      Pseudo Wire Transmit Capabilities List
      Pseudo Wire Receive Capabilities List

   The following AVPs MAY be present in the SCCRQ:

      Receive Window Size
      Challenge
      Tie Breaker
      Firmware Revision
      Vendor Name

6.2  Start-Control-Connection-Reply (SCCRP)

   Start-Control-Connection-Reply (SCCRP) is a control message sent in
   reply to a received SCCRQ message.  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:



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      Message Type
      Protocol Version
      Host Name
      Assigned Control Connection ID
      Pseudo Wire Transmit Capabilities List
      Pseudo Wire Receive Capabilities List

   The following AVPs MAY be present in the SCCRP:

      Firmware Revision
      Vendor Name
      Receive Window Size
      Challenge
      Challenge Response

6.3  Start-Control-Connection-Connected (SCCCN)

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

   The following AVP MUST be present in the SCCCN:

      Message Type

   The following AVP MAY be present in the SCCCN:

      Challenge Response

6.4  Stop-Control-Connection-Notification (StopCCN)

   Stop-Control-Connection-Notification (StopCCN) is a control message
   sent by either 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

   Additionally, the Assigned Control Connection ID AVP MUST be present
   in the StopCCN if it has been sent in a previous message (see Section
   5.4.3).



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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

6.6  Incoming-Call-Request (ICRQ)

   Incoming-Call-Request (ICRQ) is a 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
      Call Serial Number
      Pseudo Wire Type

   The following AVP MAY be present in the ICRQ:

      Assigned Cookie
      End Identifier
      Physical Channel ID
      Data Sequencing

6.7  Incoming-Call-Reply (ICRP)

   Incoming-Call-Reply (ICRP) is a control message sent by an LCCE in
   response to an 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 (e.g. 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
      Pseudo Wire Type

   The following AVP MAY be present in the ICRP:

      Assigned Cookie
      End Identifier
      Data Sequencing

6.8  Incoming-Call-Connected (ICCN)

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

   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
      Remote Session ID
      Tx Connect Speed

   The following AVPs MAY be present in the ICCN:

      Local Session ID
      Private Group ID
      Rx Connect Speed
      Data Sequencing

6.9  Outgoing-Call-Request (OCRQ)

   Outgoing-Call-Request (OCRQ) is a control message sent by an LCCE to
   an LAC to indicate that an outbound call at the LAC is to be



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   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
   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
      Call Serial Number
      Minimum BPS
      Maximum BPS
      Pseudo Wire

   The following AVPs MAY be present in the OCRQ:

      Assigned Cookie
      End Identifier
      Data Sequencing

6.10  Outgoing-Call-Reply (OCRP)

   Outgoing-Call-Reply (OCRP) is a control message sent by an LAC to an
   LCCE in response to an 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
   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

   The following AVPs MAY be present in the OCRP:

      Assigned Cookie



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      End Identifier
      Physical Channel ID

6.11  Outgoing-Call-Connected (OCCN)

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

   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
      Remote Session ID
      Tx Connect Speed

   The following AVPs MAY be present in the OCCN:

      Local Session ID
      Rx Connect Speed
      Data Sequencing

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
      Local Session ID
      Remote Session ID
      Result Code

      Additionally, the Local Session ID AVP MUST be present in the CDN if
      it has been sent in a previous message (see Section 5.4.4).

   The following AVPs MAY be present in the CDN:

      Q.931 Cause Code



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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
      Circuit Errors
      Local Session ID
      Remote Session ID

6.14 Set-Link-Info (SLI)

   The Set-Link-Info control message is sent by an LAC to indicate a link
   status change has taken place for the circuit associated with this L2TP
   session. For example, if PPP renegotiates LCP or a Frame Relay VC
   transitions to Active or Inactive, an SLI message should be sent to
   indicate this event. Precise details of when the SLI is sent, the
   PW-specific AVPs that must be present and their interpretation should be described in the
   associated PW-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

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).

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.




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   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.

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.



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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 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

   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




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   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.

   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



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   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

   wait-reply      Receive ICRP,      Send ICCN         established
                   acceptable

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

   wait-reply      Receive ICRQ       Send CDN,         idle
                                      clean up





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   wait-reply      Receive CDN,       Clean up          idle
                   ICCN

   wait-reply      Local 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 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
      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.








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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.

   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



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      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

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

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



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   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

   idle            Receive OCRP       Send CDN,         idle
                                      clean up

   idle            Receive OCCN,      Clean up          idle
                   CDN





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   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.

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 finite
   period of time 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.



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   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 Security

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

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

   The Assigned Control Connection ID and Assigned Session ID (see
   Section 5.4) SHOULD be selected in an unpredictable manner rather
   than sequentially or otherwise.  Doing so will help deter hijacking
   of a session by a malicious user who does not have access to packet
   traces between the LCCEs.

   The Assigned Cookie value MUST be selected in an unpredictable
   manner.  However, the Cookie MUST not be regarded as packet-level
   authentication or security of any kind.  It should be used for
   nothing more than simple configuration error detection and
   identification of misrouted packets.  Since the Cookie is sent and
   advertised in the clear, it is by no means a true packet-level
   security measure, such as that offered by IPsec.





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8.2  Packet Level Security

   Securing L2TP requires that the underlying transport make available
   encryption, integrity, and authentication services for all L2TP
   traffic.  This secure transport operates on the entire L2TP packet
   and is functionally independent of the data being carried on an L2TP
   data session.  As such, L2TP is only concerned with confidentiality,
   authenticity, and integrity of the L2TP packets between two LCCEs,
   not unlike link-layer encryption being concerned only about
   protecting the confidentiality of traffic between its physical
   endpoints.

8.3  End-to-End Security

   Protecting the L2TP packet stream via a secure transport does, in
   turn, also protect the data within the tunneled session packets while
   transported from one LCCE to the other.  Such protection should 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 provides packet-level security via ESP
   [RFC3193]. All L2TP control and data packets for a particular control
   connection appear as homogeneous UDP/IP data packets to the IPsec
   system.

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

   IPsec also defines access control features that are required of a
   compliant IPsec implementation.  These features allow filtering of
   packets based upon network and transport layer characteristics such
   as IP address, ports, etc.  In the L2TP tunneling model, analogous
   filtering is logically performed at the 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 RFC2661 L2TP.
   L2TP as defined in this document should posses the same interface to
   IPsec as RFC2661 when running on UDP/IP. UDP has the added advantage
   of being able to provide a native method for IPsec to distinguish
   multiple Security Associations (presumably with different policies)
   between the same tunnel endpoints without having to extend the
   definitions of IPsec or allocate additional IP addresses between
   endpoints. Therefore, when securing L2TP with IPsec via RFC3193,
   L2TPv3 MUST operate over UDP/IP as described in section 4.1.2.

9.  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.

9.1  AVP Attributes

   As defined in Section 5.1, AVPs contain Vendor ID, Attribute, and
   Value fields.  For a Vendor ID value of 0, IANA will maintain a
   registry of assigned Attributes and, in some cases, Values.
   Attributes 0-39 are assigned as defined in Section 5.4.  The
   remaining values are available for assignment upon Expert Review
   [RFC2434].

9.2  Message Type AVP Values

   As defined in Section 5.4.1, Message Type AVPs (Attribute Type 0)
   have an associated value maintained by IANA.  Values 0-16 are defined
   in Section 3.1.  The remaining values are available for assignment
   upon Expert Review [RFC2434].

9.3  Result Code AVP Values

   As defined in Section 5.4.2, Result Code AVPs (Attribute Type 1)
   contain three fields.  Two of these fields (the Result Code and Error
   Code fields) have associated values maintained by IANA.

9.3.1  Result Code Field Values

   The Result Code AVP may be included in CDN and StopCCN messages.  The
   allowable values for the Result Code field of the AVP differ
   depending upon the value of the Message Type AVP.  For the StopCCN
   message, values 0-7 are defined in Section 5.4.2; for the CDN



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   message, values 0-11 are defined in the same section.  The remaining
   values of the Result Code field for both messages are available for
   assignment upon Expert Review [RFC2434].

9.3.2  Error Code Field Values

   Values 0-9 are defined in Section 5.4.2.  The remaining values are
   available for assignment upon Expert Review [RFC2434].

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

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

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

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

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

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

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

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

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

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



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

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

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

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

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

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

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

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

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

   [RFC2661] Townsley W., et al., "Layer Two Tunneling Layer Two Tunneling
             Protocol (L2TP)", RFC 2661, August 1999.

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

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

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

   [L2TPAAL5] M. Davison, A. Lin, A. Singh, J. Stephens, R. Turner, R. Tio, S. Nanji,
              "L2TP Over AAL5," Internet Draft, August 2001,
              draft-ietf-l2tpext-l2tp-atm-02.txt.




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11.  Editors' Addresses

   Jed Lau
   cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   jedlau@cisco.com

   Gurdeep Singh Pall
   Microsoft Corporation
   Redmond, WA
   gurdeep@microsoft.com

   Bill Palter
   RedBack Networks, Inc
   1389 Moffett Park Drive
   Sunnyvale, CA 94089
   palter@zev.net

   Allan Rubens
   acr@del.com

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

   Andrew J. Valencia
   P.O. Box 2928
   Vashon, WA 98070
   vandys@zendo.com

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

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].



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

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

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

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

Appendix B: Control Message Examples

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 a ZLB
   ACK message.  An alternative would be to piggyback the acknowledgement
   within a message sent as a reply to the ICRQ or OCRQ that will likely
   follow from the side that initiated the 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
                                   <-       ZLB
                                   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 impacts: 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 a ZLB)

                                            <-       ZLB
                                            Nr: 3, Ns: 2

         (LCCE B's retransmit timer fires)

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

                                            <-       ZLB
                                            Nr: 4, Ns: 2





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Appendix C: Intellectual Property Notice

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

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights 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.

























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