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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-00.txt>                            G. Zorn
                                                           cisco Systems
                                                               I. Goyret
                                                     Lucent Technologies
                                                                 G. Pall
                                                   Microsoft Corporation
                                                               A. Rubens
                                                                 Nexthop
                                                               B. Palter
                                                        Redback Networks
                                                               July 2001


                  Layer Two Tunneling Protocol "L2TP"


Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress''.

     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.


   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
   Directories on ftp.ietf.org (US East Coast), nic.nordu.net (Europe),
   ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).

   The distribution of this memo is unlimited.  It is filed as <draft-
   ietf-l2tpext-l2tp-base-00.txt> and expires January 2002.  Please send
   comments to the L2TP mailing list (l2tp@l2tp.net).

Copyright Notice







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

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.

   The L2TP rewrite team for splitting RFC2661 into the base and
   companion PPP specifications consisted of Ignacio Goyret, Jed Lau,
   Bill Palter, Mark Townsley, and Madhvi Verma.

   This document was based upon RFC2661, 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 RFC2661.

   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 Format..............   12
         3.2.2  L2TP Data Message Header Format.................   13
      3.3  Control Connection Management........................   15
         3.3.1  Control Connection Establishment................   15
         3.3.2  Control Connection Teardown.....................   15
      3.4  Call Management......................................   16
         3.4.1  Incoming Call Establishment.....................   16
         3.4.2  Outgoing Call Establishment.....................   16
         3.4.3  Session Teardown................................   17

   4.  Control Message Attribute Value Pairs....................   17
      4.1  AVP Format...........................................   17
      4.2  Mandatory AVPs.......................................   18
      4.3  Hiding of AVP Attribute Values.......................   19
      4.4  AVP Summary..........................................   21
         4.4.1  AVPs Applicable to All Control Messages.........   22
         4.4.2  Result and Error Codes..........................   23
         4.4.3  Control Connection Management AVPs..............   25
         4.4.4  Call Management AVPs............................   32
         4.4.5  Call Status AVPs................................   39

   5.  Protocol Operation.......................................   40
      5.1  Migration from L2TPv2 to L2TPv3......................   40
         5.1.1  L2TPv3-Only Implementations.....................   41
         5.1.2  L2TPv2/v3 Implementations.......................   41
      5.2  Reliable Delivery of Control Messages................   41
      5.3  LCCE Authentication..................................   44
      5.4  Keepalive (Hello)....................................   44
      5.5  Forwarding Session Frames............................   45
      5.7  In-Band Operation of the Control and Data Channels...   45

   6.  Control Connection Protocol Specification................   46
      6.1  Start-Control-Connection-Request (SCCRQ).............   46
      6.2  Start-Control-Connection-Reply (SCCRP)...............   46



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      6.3  Start-Control-Connection-Connected (SCCCN)...........   47
      6.4  Stop-Control-Connection-Notification (StopCCN).......   47
      6.5  Hello (HELLO)........................................   48
      6.6  Incoming-Call-Request (ICRQ).........................   48
      6.7  Incoming-Call-Reply (ICRP)...........................   48
      6.8  Incoming-Call-Connected (ICCN).......................   49
      6.9  Outgoing-Call-Request (OCRQ).........................   49
      6.10  Outgoing-Call-Reply (OCRP)..........................   50
      6.11  Outgoing-Call-Connected (OCCN)......................   51

   7.  Control Connection State Machines........................   51
      7.1  Malformed Control Messages...........................   51
      7.2  Timing Considerations................................   52
      7.3  Control Connection States............................   52
      7.4  Incoming Calls.......................................   54
         7.4.1  ICRQ Sender States..............................   55
         7.4.2  ICRQ Recipient States...........................   56
      7.5  Outgoing Calls.......................................   57
         7.5.1  OCRQ Sender States..............................   58
         7.5.2  OCRQ Recipient (LAC) States.....................   59
      7.6  Termination of a Control Connection..................   60

   8.  L2TP Over Specific Media.................................   60
      8.1  L2TP Control Connection over UDP/IP..................   61
      8.2  L2TP Data Channel over IP............................   61
      8.3  L2TP Data Channel over UDP...........................   61

   9.  Security Considerations..................................   62
      9.1  Control Connection Endpoint Security.................   62
      9.2  Packet Level Security................................   63
      9.3  End-to-End Security..................................   63
      9.4  L2TP and IPsec.......................................   63

   10.  IANA Considerations.....................................   64
      10.1  AVP Attributes......................................   64
      10.2  Message Type AVP Values.............................   64
      10.3  Result Code AVP Values..............................   64
         10.3.1  Result Code Field Values.......................   64
         10.3.2  Error Code Field Values........................   65
      10.4  AVP Header Bits.....................................   65

   11.  References..............................................   65

   12.  Editors' Addresses......................................   66

   Appendix A: Control Slow Start and Congestion Avoidance......   67

   Appendix B: Control Message Examples.........................   68



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   Appendix C: Intellectual Property Notice.....................   69

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 of a number of other L2 protocols.  In order to
   provide greater modularity, this document describes the base L2TP
   protocol, independent of the L2 encapsulation 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 protocol-independent portion of the L2TP encapsulation to
   multiplex and demultiplex arbitrary L2 packet streams.

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 reuse, interoperability
   experience with RFC 2661, and extension of L2TP into new application
   spaces.

   For the remainder of this document, 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.  (Recall that L2F was
   defined as version 1).  L2TP as defined in this document will be
   referred to as "L2TPv3".

   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 contructs 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.
   - Better separation of the data channel and control channel.

   Details of the these changes and a recommendation for transitioning
   to L2TPv3 may be found in Section 5.1.






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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

      The action of transitioning a circuit on an LAC to an "up" or
      "established" state.  A call may be dynamically established
      through signaling properties (e.g. an incoming or outgoing call
      through the PSTN) or statically established (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 method of in-band or out-of-band
      control channel governing the establishment, maintenance, and
      teardown of the circuit.  For the purposes of this document, a
      statically configured circuit is considered to be largely
      equivalent to a simple dynamic circuit.  (See also: Call, Remote
      System.)



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   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 4.4.3.)

   Incoming Call

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

   L2TP Access Concentrator (LAC)

      An LCCE that tunnels a circuit (either physically connected or
      logically connected, as via another L2TP session) to another



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      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.)

   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.

   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.)



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   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 ------------>|

   (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).



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   +-----+  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 5.2 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
   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.







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   +-------------------+
   | L2 Frames         |
   +-------------------+    +-----------------------+
   | L2TP Data Messages|    | L2TP Control Messages |
   +-------------------+    +-----------------------+
   | L2TP Data Channel |    | L2TP Control Channel  |
   | (unreliable)      |    | (reliable)            |
   +-------------------+    +-----------------------+
   | IP, UDP, ATM, etc.|    | UDP, ATM, etc.        |
   +-------------------+    +-----------------------+

                   Figure 3.0: L2TPv3 Structure

   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 then a packet transport such as UDP,
   Frame Relay, ATM, etc.  Control messages are sent over a reliable
   L2TP control channel, which may transmit packets either in-band (over
   the same packet network) or out-of-band (over a different packet
   network).

   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 4.4.1) defines the specific type of
   control message being sent.

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

      Control Connection Management

      0  (reserved)

      1  (SCCRQ)    Start-Control-Connection-Request
      2  (SCCRP)    Start-Control-Connection-Reply
      3  (SCCCN)    Start-Control-Connection-Connected
      4  (StopCCN)  Stop-Control-Connection-Notification



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      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

3.2  L2TP Header Formats

   This specification defines separate header formats for L2TP control
   messages and L2TP data messages.

   Where a field is marked as optional, its space does not exist in the
   message if the field is, indeed, not present.  All values are placed
   into their respective fields and sent in network order (high order
   octets first).

3.2.1  L2TP Control Message Header Format

   The L2TP control message header is formatted 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |T|L|x|x|S|x|x|x|x|x|x|x|  Ver  |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Control Connection ID                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Ns              |               Nr              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 3.2.1: L2TP Control Message Header

   The T bit MUST be set to 1, indicating that this is a control
   message.  This provides backwards compatibility with L2TPv2 control
   messages and enables the ability for data and control messages to
   operate in-band over the same channel.



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   The L and S bits MUST be set to 1 for compatibility with L2TPv2.

   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 (with one exception, see Section 5.1 for
   details).  Upon receipt, an implementation MUST accept an SCCRQ with
   the Ver field set to 2 or 3 (see Section 5.1).  All other messages
   MUST have the Ver field set to 3 to be accepted by an L2TPv3
   implementation.

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

   The Control Connection ID field indicates 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 different Control Connection IDs by
   each LCCE.  The Control Connection ID in each message is that of the
   intended recipient, not the sender.  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 Sections 5.4 and 5.8 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 5.2 for more
   information on using this field.

3.2.2  L2TP Data Message Header Format

   The L2TP data message header is formatted as follows:














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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Session ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Cookie                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | L2-Specific Sublayer (arbitrary length)...                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Tunneled L2 Frame...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 3.2.2: L2TP Data Message Header

   The Session ID field contains the identifier for a session.  L2TP
   sessions are named by identifiers that have local significance only.
   That is, the same session will be given different Session IDs by each
   end of the session.  The Session ID specified in each message is that
   of the intended recipient, not the sender.  Session IDs are selected
   and exchanged as Assigned Session ID AVPs during the creation of a
   session.  (See Section 5.7 for a discussion of in-band operation of
   the control connection and data channel, which affects the Session ID
   field.)

   The Cookie field contains a 32-bit value used to check the
   association of a received data packet with the session identified by
   the Session ID.  The cookie guards against the misrouting of data
   packets, which could result if the incorrect Session ID is specified
   in received packets (due to misconfiguration, header corruption, or
   otherwise).  Cookie values are selected and exchanged as Assigned
   Cookie AVPs during the creation of a session.

   The L2-Specific Sublayer is an intermediary layer between the fixed
   L2TP data header (consisting of the Session ID and Cookie fields) and
   the start of the inner L2 frame.  It may contain control fields that
   L2TP uses to facilitate the tunneling of the L2 frames (e.g. offset
   bytes or sequence numbers).  Because 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 AVP (see
   Section 4.4.4), which identifies the L2 payload.  Further details are
   defined in the appropriate L2 payload-specific companion documents.

   The Tunneled L2 Frame consists of the encapsulated L2 traffic,
   including any L2 framing that might be present.







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3.3  Control Connection Management

   Two peers that wish to tunnel L2 traffic to each other must first
   establish a reliable control connection between them.  The control
   connection handles the establishment and teardown of the L2TP
   sessions and of the control connection itself.  The reliable delivery
   of control messages is described in Section 5.2.

   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.  (See Section 5.2 for a description of the reliable
   control message delivery mechanism.)

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 5.2).  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  Call 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  Incoming Call Establishment

   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  Outgoing Call Establishment

   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.  Control Message Attribute Value Pairs

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

4.1  AVP Format

   Each AVP is encoded as:

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

   The first six bits are a bit mask, describing 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



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   unrecognized or malformed AVP in a 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 4.3 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.
   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.

4.2  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



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   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 is either not sent, or sent with the M
   bit not set.

   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.

4.3  Hiding of AVP Attribute Values

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

   The H bit MUST only be set if a shared secret exists between the
   LCCEs and LCCE authentication has completed.  The shared secret is
   the same secret that is used for LCCE authentication (see Section
   5.3).  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:




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

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

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

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

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

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

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

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

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



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

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

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

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

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

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

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

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

4.4  AVP Summary

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

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



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4.4.1  AVPs Applicable to All Control Messages

Message Type (All Messages)

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

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

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

   The Message Type is a 2-octet unsigned integer.

   The Message Type AVP MUST be the first AVP in a message, immediately
   following the control message header (defined in Section 3.2.1).  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 4.1).

Random Vector (All Messages)

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

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







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

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

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

   The M bit for this AVP 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.

4.4.2  Result and Error Codes

Result Code (CDN, StopCCN)

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

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

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

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

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



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   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

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

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

   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



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   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 4.2).  The Error
       Message SHOULD contain the attribute of the offending AVP in
       (human readable) text form.
   9 - Try another directed.  If an LAC or LNS is aware of other possible
       destinations, it should inform the initiator of the 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.
   Furthermore, a vendor-specific AVP MAY be sent to indicate the
   problem more precisely.

4.4.3  Control Connection Management AVPs

Protocol Version (SCCRP, SCCRQ)

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

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

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

   The Ver field is a 1-octet unsigned integer containing the value 1.
   Rev field is a 1-octet unsigned integer containing 0.  This pertains
   to L2TP 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)



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   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
   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:





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    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 (e.g. general
   purpose computers running L2TP software modules), the revision of the
   L2TP software module may be reported instead.

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

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

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

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

   This name should be as broadly unique as possible; for hosts
   participating in DNS [RFC1034], a hostname with fully qualified
   domain would be appropriate.  The Host Name MAY be used to identify
   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.




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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Vendor Name ...(arbitrary number of octets)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

   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 a value 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.

   If an LCCE needs to send a StopCCN to a peer but has not received an
   Assigned Control Connection ID AVP from the peer, there are two cases
   to consider.  If an Assigned Control Connection ID AVP has been sent
   to the peer in a previous message, its value MUST be sent as the



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   Assigned Control Connection ID AVP in the StopCCN to allow the peer
   to try to identify the appropriate control connection via a reverse
   lookup.  Alternatively, if an Assigned Control Connection ID has not
   been sent to the peer in a previous message, a Control Connection ID
   SHOULD be allocated and sent as the Assigned Control Connection ID
   AVP so that the StopCCN may be reliably delivered.  This is most
   important if the StopCCN carries an essential directive within (e.g.
   a Result Code of value 9 with an alternate address to which to
   attempt connection).  If an Assigned Control Connection ID AVP is not
   sent in the StopCCN or any previous message, the StopCCN MUST NOT be
   retransmitted.

   This AVP may be hidden (the H bit may be 0 or 1).  The M bit for this
   AVP SHOULD be set to 1 (see Section 5.1).  The Length (before hiding)
   of this AVP is 10.

Receive Window Size (SCCRQ, SCCRP)

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

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

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

   The Window Size is a 2-octet unsigned integer.

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

   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:






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

   The Challenge is one or more octets of random data.

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

   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, and 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.

Data Channel Capabilities List (SCCRP, SCCRQ)

   The Data Channel Capabilities List AVP, Attribute Type TBA, indicates
   the Data Channel Types that will be accepted by the sender.  The



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   sender of this AVP MUST be prepared to accept one or a combination of
   the Data Channel Types specified in this list for a given session.
   The specific Data Channel Types used for a session are identified by
   the Data Channel Type List AVP.

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Data Channel Type 0      |             ...               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              ...              |      Data Channel Type N      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Data Channel Types that may be included in the Data Channel
   Capabilities List are as follows:

   1 - IP
   2 - UDP/IP in-band
   3 - UDP/IP out-of-band

   See Section 8 for a discussion of L2TP over specific media.

   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 5.1).  The Length (before hiding)
   of this AVP is 8 octets with one Data Channel Type specified, plus 2
   octets for each additional Data Channel Type.

Pseudo Wire Capabilities List (SCCRP, SCCRQ)

   The Pseudo Wire Capabilities List AVP, Attribute Type TBA, indicates
   the L2 payload types that will be accepted by the sender.  The
   specific payload type of a given session is identified by the Pseudo
   Wire Type AVP.

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Pseudo Wire Type 0       |             ...               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              ...              |      Pseudo Wire Type N       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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



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   0 - PPP

   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 5.1).  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.

4.4.4  Call Management AVPs

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

   The Assigned Session ID AVP, 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Assigned Session ID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   The Assigned Session ID AVP establishes a value used to multiplex and
   demultiplex data sent over a control connection between two LCCEs.
   Once this AVP has been received, the LCCE MUST pass this value in the
   Session ID AVP of all session control messages that it subsequently
   transmits to its peer on behalf of this session.  Before the Assigned
   Session ID AVP is received from a peer, control messages MUST be sent
   to the peer with a Session ID AVP value of 0.  Because a Session ID
   value of 0 is used in this special manner, the zero value MUST NOT be
   sent as an Assigned Session ID value.

   If an LCCE needs to send a CDN to a peer but has not received an
   Assigned Session ID AVP from the peer, there are two cases to
   consider.  If an Assigned Session ID AVP has been sent to the peer in
   a previous message, its value MUST be sent as the Session ID AVP in
   the CDN to allow the peer to try to identify the appropriate session
   via a reverse lookup.  Alternatively, if an Assigned Session ID has
   not been sent to the peer in a previous message, the Session ID AVP
   MUST NOT be sent in the CDN.

   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 5.1).  The Length (before hiding)
   of this AVP is 10.

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



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   The 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Session ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   The Session ID AVP communicates the previously established value used
   to multiplex and demultiplex data sent over a control connection
   between two LCCEs.  For an outgoing control message, the value of the
   Session ID AVP is set to the Assigned Session ID AVP that had been
   received from the peer in an earlier control message exchange.
   Before the Assigned Session ID AVP is received from the peer, control
   messages MUST be sent to the peer with a Peer-Assigned Session ID AVP
   value of 0.

   If an LCCE needs to send a CDN to a peer but has not received an
   Assigned Session ID AVP from the peer, there are two cases.  If an
   Assigned Session ID AVP has been sent to the peer in a previous
   message, its value MUST be sent as the Session ID AVP in the CDN to
   allow the peer to attempt identify the appropriate session via a
   reverse lookup.  Alternatively, if an Assigned Session ID has not
   been sent to the peer in a previous message, the Session ID AVP MUST
   NOT be sent in the CDN.

   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 5.1).  The Length (before hiding)
   of this AVP is 10.

Call Serial Number (ICRQ, OCRQ)

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

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

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




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   The Call Serial Number is a 32-bit value.

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

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

End Identifier AVP (ICRQ, OCRQ)

   The End Identifier AVP, Attribute Type TBA, encodes an identifier
   assigned by the LAC or LNS to this call, used 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.  The
   field may be a simple ASCII string.  For example, a source interface
   serial 1/1 and DLCI 100, and a destination interface serial 1/1 with
   DLCI 200, could be represented as "serial 1/1 DLCI 100, serial 1/1
   DLCI 200".

   The format of the information contained in this AVP should be agreed
   on by the administrators at the two LCCEs.  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 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 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



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   AVP SHOULD be set to 1 (see Section 5.1).  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.

Data Channel Type List (ICRQ, OCRQ)

   The Data Channel Type List AVP, Attribute Type TBA, indicates the
   data channel configurations that will be used by the sender for
   outgoing data packets.



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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Data Channel Type 0      |             ...               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              ...              |      Data Channel Type N      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   See the definition for the Data Channel Capabilities List AVP for a
   list of defined Data Channel Types that may be included.  Identifying
   more than one Data Channel Type is an indication that packets may
   arrive on any or all of the listed data channels.  See Section 8 for
   a discussion of L2TP over specific media.

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

   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 5.1).  The Length (before hiding)
   of this AVP is 8 octets with one Data Channel Type specified, plus 2
   octets for each additional Data Channel Type.

Pseudo Wire Type (ICRQ, OCRQ)

   The Pseudo Wire Type AVP, Attribute Type TBA, indicates the L2
   payload type for the requested call.

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

   See the definition for the Pseudo Wire Capabilities List AVP for a
   list of defined Pseudo Wire Type values.

   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
   Capabilities List AVP it received during control connection
   establishment.  Attempts to do so will result in the call being
   rejected.



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   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 5.1).  The Length (before hiding)
   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



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   there is no physical point-to-point link.

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

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

Physical Channel ID (ICRQ, 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



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   the LNS is individually connected to several private networks using
   unregistered addresses, this AVP may be included by the LAC to
   indicate that a given call should be associated with one of the
   private networks.

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

   This AVP may be hidden (the H bit MAY be 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.

Assigned Cookie (ICRP, ICRQ, OCRQ, OCRP)

   The Assigned Cookie AVP, Attribute Type TBA, encodes the cookie value
   that the LCCE MUST include in the Cookie field of all outgoing data
   packets.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Cookie                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The cookie is a 4-octet unsigned integer.

   The cookie value MUST be random data generated by the sender.

   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 5.1).  The Length (before hiding)
   of this AVP is 10.

4.4.5  Call 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:







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

   The following fields are defined:

      Reserved: Not used, MUST be 0.
      Hardware Overruns: Number of receive buffer over-runs since call
         was established.
      Buffer Overruns: Number of buffer over-runs detected since call
         was established.
      Time-out Errors: Number of time-outs since call was established.
      Alignment Errors: Number of alignment errors since call was
         established.

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

5.  Protocol Operation

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

5.1  Migration from L2TPv2 to L2TPv3

   This section defines the methods that MUST be followed in order to
   provide a smooth transition from the installed base of L2TPv2 to
   L2TPv3.  The first section describes what is expected of an
   L2TPv3-only implementation that does not fall back to L2TPv2 mode to
   interoperate with the existing installed base.  The second section
   describes how an L2TPv2/v3-capable implementation may "autodetect"
   whether its peer is L2TPv3-capable, and how to fall back to L2TPv2
   mode if so desired.







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5.1.1  L2TPv3-Only Implementations

   As the recipient of an SCCRQ, a new L2TPv3 implementation MUST accept
   an SCCRQ with a Ver field of 2 in the header.  The AVPs in the
   message MUST be parsed to determine whether the message suggests a
   native L2TPv2 implementation or an L2TPv2/v3 implementation
   attempting version detection.  The presence of the 32-bit Assigned
   Control Connection ID AVP indicates that the sender is an
   L2TPv3-capable implementation, and an L2TPv3 message SHOULD be sent
   in response to the SCCRQ.  If the 32-bit Assigned Control Connection
   ID AVP is not present, then an L2TPv2 StopCCN that includes the
   16-bit Assigned Tunnel ID AVP (as defined in [RFC2661]) MUST be
   constructed and sent in response to the SCCRQ.  Note that this does
   not require additional state on either implementation.  Further, the
   StopCCN MAY be sent as a single message without waiting for an
   acknowledgement or providing retransmission on the message.

   An L2TPv3-only implementation SHOULD always send messages with Ver 3
   in the control message header, including the SCCRQ.  L2TPv2
   implementations are expected to drop such messages.

5.1.2  L2TPv2/v3 Implementations

   An L2TPv2/v3 implementation is one that can operate in either mode
   for the intended application.  L2TPv3 is always favored, but
   L2TPv2-only implementations will still operate within the
   specifications of L2TPv2.  Beyond the added benefits of L2TPv3,
   fallback to L2TPv2 should be seamless and occur automatically.

   In order to provide such fallback, an L2TPv2/v3 implementation MUST
   send an SCCRQ that looks enough like an L2TPv2 SCCRQ to be accepted
   by L2TPv2 implementations.  Thus, the SCCRQ is sent with a Ver field
   of 2 in the control message header, along with the other AVPs
   expected in an L2TPv2 SCCRQ as defined in [RFC2661].  All required
   L2TPv3 AVPs for an SCCRQ (e.g. the 32-bit Assigned Control Connection
   ID) MUST be sent as well, with their M bits set to 0.

   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 the L2TPv2/v3 implementation MUST fallback to an
   L2TPv2 mode of operation.

5.2  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 3.1) belong to this delivery mechanism.  The upper level



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   functions of L2TP are not concerned with retransmission or ordering
   of control messages.  The reliable control messaging mechanism is a
   sliding window mechanism that provides control message retransmission
   and congestion control.  Each peer maintains separate sequence number
   state for each control connection.

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



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

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



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   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.

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

5.3  LCCE Authentication

   L2TP incorporates a simple, optional, CHAP-like [RFC1994] LCCE
   authentication system during control connection establishment.  If a
   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 and 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 4.3).  See Section 4.4.3 for details on
   construction of the Challenge and Response AVPs.

5.4  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



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   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.

   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 running Hello timers across multiple control connections
   SHOULD employ a jittered timer mechanism.

5.5  Forwarding Session Frames

   Once session establishment is complete, L2 frames are received at the
   LAC or LNS, 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.  The sender of a message associated with a particular
   session places the Assigned Session ID (specified by its peer) in the
   Session ID field of the L2TP data header for every outgoing message.
   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).

5.7  In-Band Operation of the Control and Data Channels

   Whether the control connection and data channel operate in-band or
   out-of-band is determined by the Data Channel Capabilities List AVP
   and the Data Channel Type List AVP (see Section 4.4).

   To support in-band operation of the control connection and data
   channel, the high-order bit of the Assigned Session ID AVP MUST be
   set to 0.  Control messages can thus be distinguished from data
   messages, since all control messages are required to have this bit
   set to 1.  Drawbacks include a reduction in the number of total



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   sessions supported by an single LCCE to 2**31 sessions, and the
   additional operation of checking this bit for all packets received.
   If 2**32 sessions are needed, then the control and data channel MUST
   operate out-of-band.  It is recommended that implementations choose
   to operate in out-of-band mode unless specifically needed for NAT,
   firewall, or other requirements.  See Section 8 for details about
   L2TP over specific media.

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
      Data Channel Capabilities List
      Pseudo Wire 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.



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   The following AVPs MUST be present in the SCCRP:

      Message Type
      Protocol Version
      Host Name
      Assigned Control Connection ID
      Data Channel Capabilities List
      Pseudo Wire 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

   The Assigned Control Connection ID MUST be present in the StopCCN if



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   it has been sent in a previous message (see Section 4.4.3).

6.5  Hello (HELLO)

   The Hello (HELLO) message is an L2TP control message sent by either
   peer of a control connection.  This control message is used as a
   "keepalive" for the control connection.  See Section 5.4 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:

      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 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
      Assigned Session ID
      Call Serial Number
      Data Channel Type List
      Pseudo Wire Type
      Assigned Cookie

   The following AVP MAY be present in the ICRQ:

      End Identifier
      Physical Channel ID

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



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   used for establishing sessions within an L2TP control connection.

   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
      Session ID
      Assigned Session ID
      Assigned Cookie

   The following AVP MAY be present in the ICRP:

      End Identifier

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 third 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
      Session ID
      (Tx) Connect Speed

   The following AVPs MAY be present in the ICCN:

      Private Group ID
      Rx Connect Speed

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
   established based on specific destination information sent in this
   message.  It is the first in a three-message exchange used for



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   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
      Assigned Session ID
      Call Serial Number
      Minimum BPS
      Maximum BPS
      Data Channel Type List
      Pseudo Wire
      Assigned Cookie

   The following AVPs MAY be present in the OCRQ:

      End Identifier

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
      Session ID
      Assigned Session ID
      Assigned Cookie

   The following AVPs MAY be present in the OCRP:

      End Identifier
      Physical Channel ID




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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 t.sp
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
   5.2).

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.

   An invalid control message is defined as (1) a message that contains
   a Message Type marked as mandatory (see Section 4.4.1) 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



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   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 5.8) 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.

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 4.4.3 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



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   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

   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



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      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 5.8.

      When an SCCRP is received, the message is examined for a
      compatible protocol version, as specified by the Protocol Version
      AVP.  If the version of the reply is lower than the version sent
      in the request, the older (lower) version should be used, provided
      that it is supported.  If the version in the reply is earlier and
      supported, the originator moves to the established state.  If the
      version is earlier and not supported, a StopCCN MUST be sent to
      the peer and the originator cleans up and terminates the control
      connection.  (Note that this policy is independent of the
      versioning specified by the Ver field in the control message
      header, which is examined for L2TPv2/v3 interoperability.  See
      Section 5.1.)

   wait-ctl-conn
      Awaiting an SCCCN.  Upon receipt, the challenge response contained
      in the message is checked.  The control connection either 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
   response from the peer.  However, it may choose to postpone
   establishment of the call (e.g. answering the call, or 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



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   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

   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



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   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 don't 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.

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




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   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
      a CDN from the peer.  Clean up follows on both sides regardless of
      the initiator.

7.5  Outgoing Calls

   Outgoing calls are initiated by an LCCE and instruct an LAC to place
   a call.  There are three messages for outgoing calls: OCRQ, OCRP, and
   OCCN.  The 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.





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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

   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



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      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

   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



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   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.

   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.  L2TP Over Specific Media

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



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   following sections describe details needed to permit interoperability
   over specific media.

   Note that if the control connection and data channel are carried in-
   band over the same media, the policy described in Section 5.7 for
   distinguishing control and data messages still applies.  Whether the
   control connection and data channel are carried in-band or out-of-
   band is determined by the Data Channel Capabilities AVP and Data
   Channel Type AVP (see Section 4.4).

8.1  L2TP Control Connection over UDP/IP

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

   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 which can pass TFTP traffic
   should be able to pass L2TP UDP traffic as they employ similar
   policies with regard to UDP port selection.

8.2  L2TP Data Channel over IP

   L2TP data messages may be sent directly over IP or as UDP datagrams
   (see Section 8.3).  When operating directly over IP (Data Channel
   Type 1), the IP Protocol ID TBA MUST be used.  Note that while there
   are certain efficiencies gained by running directly over IP, there
   are possible side affects as well.  For instance, L2TP over IP is
   likely not as NAT or firewall friendly as L2TP over UDP.

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

8.3  L2TP Data Channel over UDP

   L2TP data messages may be sent as UDP datagrams, operating in-band or
   out-of-band with the control connection.




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   If operating in-band (Data Channel Type 2), all data messages MUST
   use the same UDP ports as the control connection (see Section 5.7).
   This method has some inefficencies 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.

   When operating over UDP out-of-band (Data Channel Type 3), UDP port
   TBA MUST be used as the initial port for the data channel.  As with
   the control channel, either side may then utilize a free port other
   than TBA.  All data messages MUST send UDP datagrams with a
   destination port equal to the source port of the last packet
   received.  It is recommended that an implementation always use the
   source port of TBA.

   UDP-encapsulated data packets MAY turn on UDP checksums.  It should
   be noted that enabling checksums may significantly increase the
   packet processing burden for tunneled packets.

9.  Security Considerations

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

9.1  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 4.4) SHOULD be selected in an unpredictable manner rather



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   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.

9.2  Packet Level Security

   Securing L2TP requires that the underlying transport make available
   encryption, integrity and authentication services for all L2TP
   traffic.  This secure transport operates on the entire L2TP packet
   and is functionally independent of the data being carried on an L2TP
   data session, between the remote system and an LCCE.  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.

9.3  End-to-End Security

   Protecting the L2TP packet stream via a secure transport does, in
   turn, also protect the data within the tunneled 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.

9.4  L2TP and IPsec

   When running over IP, IPsec provides packet-level security via ESP
   and/or AH.  All L2TP control and data packets for a particular
   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



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   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.

10.  IANA Considerations

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

10.1  AVP Attributes

   As defined in Section 4.1, 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 4.4.  The
   remaining values are available for assignment upon Expert Review [RFC
   2434].

10.2  Message Type AVP Values

   As defined in Section 4.4.1, Message Type AVPs (Attribute Type 0)
   have an associated value maintained by IANA.  Values 0-16 are defined
   in Section 3.1.  The remaining values are available for assignment
   upon Expert Review [RFC 2434].

10.3  Result Code AVP Values

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

10.3.1  Result Code Field Values

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



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10.3.2  Error Code Field Values

   Values 0-9 are defined in Section 4.4.2.  The remaining values are
   available for assignment upon Expert Review [RFC 2434].

10.4  AVP Header Bits

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

11.  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.

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



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   [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.

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

12.  Editors' Addresses

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

   Email: jedlau@cisco.com

   Gurdeep Singh Pall
   Microsoft Corporation
   Redmond, WA




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   Email: gurdeep@microsoft.com

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

   Email: palter@zev.net

   Allan Rubens

   Email: acr@del.com

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

   Email: mark@townsley.net

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

   Email: vandys@zendo.com

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

   Email: 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].

   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



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   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.

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



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                                   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

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



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   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementers or users of this specification can
   be obtained from the IETF Secretariat."

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

   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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