rfc2661.txt

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Network Working Group                                        W. Townsley
Request for Comments: 2661                                   A. Valencia
Category: Standards Track                                  cisco Systems
                                                               A. Rubens
                                                   Ascend Communications
                                                                 G. Pall
                                                                 G. Zorn
                                                   Microsoft Corporation
                                                               B. Palter
                                                        Redback Networks
                                                             August 1999


                  Layer Two Tunneling Protocol "L2TP"

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

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

Abstract

   This document describes the Layer Two Tunneling Protocol (L2TP).  STD
   51, RFC 1661 specifies multi-protocol access via PPP [RFC1661].  L2TP
   facilitates the tunneling of PPP packets across an intervening
   network in a way that is as transparent as possible to both end-users
   and applications.

Table of Contents

   1.0 Introduction..........................................    3
   1.1 Specification of Requirements.........................    4
   1.2 Terminology...........................................    4
   2.0 Topology..............................................    8
   3.0 Protocol Overview.....................................    9
   3.1 L2TP Header Format....................................    9
   3.2 Control Message Types.................................   11
   4.0 Control Message Attribute Value Pairs.................   12
   4.1 AVP Format............................................   13
   4.2 Mandatory AVPs........................................   14
   4.3 Hiding of AVP Attribute Values........................   14



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RFC 2661                          L2TP                       August 1999


   4.4 AVP Summary...........................................   17
      4.4.1 AVPs Applicable To All Control Messages..........   17
      4.4.2 Result and Error Codes...........................   18
      4.4.3 Control Connection Management AVPs...............   20
      4.4.4 Call Management AVPs.............................   27
      4.4.5 Proxy LCP and Authentication AVPs................   34
      4.4.6 Call Status AVPs.................................   39
   5.0 Protocol Operation....................................   41
   5.1 Control Connection Establishment......................   41
      5.1.1 Tunnel Authentication............................   42
   5.2 Session Establishment.................................   42
      5.2.1 Incoming Call Establishment......................   42
      5.2.2 Outgoing Call Establishment......................   43
   5.3 Forwarding PPP Frames.................................   43
   5.4 Using Sequence Numbers on the Data Channel............   44
   5.5 Keepalive (Hello).....................................   44
   5.6 Session Teardown......................................   45
   5.7 Control Connection Teardown...........................   45
   5.8 Reliable Delivery of Control Messages.................   46
   6.0 Control Connection Protocol Specification.............   48
   6.1 Start-Control-Connection-Request (SCCRQ)..............   48
   6.2 Start-Control-Connection-Reply (SCCRP)................   48
   6.3 Start-Control-Connection-Connected (SCCCN)............   49
   6.4 Stop-Control-Connection-Notification (StopCCN)........   49
   6.5 Hello (HELLO).........................................   49
   6.6 Incoming-Call-Request (ICRQ)..........................   50
   6.7 Incoming-Call-Reply (ICRP)............................   51
   6.8 Incoming-Call-Connected (ICCN)........................   51
   6.9 Outgoing-Call-Request (OCRQ)..........................   52
   6.10 Outgoing-Call-Reply (OCRP)...........................   53
   6.11 Outgoing-Call-Connected (OCCN).......................   53
   6.12 Call-Disconnect-Notify (CDN).........................   53
   6.13 WAN-Error-Notify (WEN)...............................   54
   6.14 Set-Link-Info (SLI)..................................   54
   7.0 Control Connection State Machines.....................   54
   7.1 Control Connection Protocol Operation.................   55
   7.2 Control Connection States.............................   56
      7.2.1 Control Connection Establishment.................   56
   7.3 Timing considerations.................................   58
   7.4 Incoming calls........................................   58
      7.4.1 LAC Incoming Call States.........................   60
      7.4.2 LNS Incoming Call States.........................   62
   7.5 Outgoing calls........................................   63
      7.5.1 LAC Outgoing Call States.........................   64
      7.5.2 LNS Outgoing Call States.........................   66
   7.6 Tunnel Disconnection..................................   67
   8.0 L2TP Over Specific Media..............................   67
   8.1 L2TP over UDP/IP......................................   68



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RFC 2661                          L2TP                       August 1999


   8.2 IP....................................................   69
   9.0 Security Considerations...............................   69
   9.1 Tunnel Endpoint Security..............................   70
   9.2 Packet Level Security.................................   70
   9.3 End to End Security...................................   70
   9.4 L2TP and IPsec........................................   71
   9.5 Proxy PPP Authentication..............................   71
   10.0 IANA Considerations..................................   71
   10.1 AVP Attributes.......................................   71
   10.2 Message Type AVP Values..............................   72
   10.3 Result Code AVP Values...............................   72
      10.3.1 Result Code Field Values........................   72
      10.3.2 Error Code Field Values.........................   72
   10.4 Framing Capabilities & Bearer Capabilities...........   72
   10.5 Proxy Authen Type AVP Values.........................   72
   10.6 AVP Header Bits......................................   73
   11.0 References...........................................   73
   12.0 Acknowledgments......................................   74
   13.0 Authors' Addresses...................................   75
   Appendix A: Control Channel Slow Start and Congestion
               Avoidance.....................................   76
   Appendix B: Control Message Examples......................   77
   Appendix C: Intellectual Property Notice..................   79
   Full Copyright Statement..................................   80

1.0 Introduction

   PPP [RFC1661] defines an encapsulation mechanism for transporting
   multiprotocol packets across layer 2 (L2) point-to-point links.
   Typically, a user obtains a L2 connection to a Network Access Server
   (NAS) using one of a number of techniques (e.g., dialup POTS, ISDN,
   ADSL, etc.)  and then runs PPP over that connection. In such a
   configuration, the L2 termination point and PPP session endpoint
   reside on the same physical device (i.e., the NAS).

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

   One obvious benefit of such a separation is that instead of requiring
   the L2 connection terminate at the NAS (which may require a
   long-distance toll charge), the connection may terminate at a (local)
   circuit concentrator, which then extends the logical PPP session over




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RFC 2661                          L2TP                       August 1999


   a shared infrastructure such as frame relay circuit or the Internet.
   From the user's perspective, there is no functional difference between
   having the L2 circuit terminate in a NAS directly or using L2TP.

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

   This document defines the necessary control protocol for on-demand
   creation of tunnels between two nodes and the accompanying
   encapsulation for multiplexing multiple, tunneled PPP sessions.

1.1 Specification of Requirements

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

1.2 Terminology

   Analog Channel

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

   Attribute Value Pair (AVP)

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

   Call

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





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RFC 2661                          L2TP                       August 1999


   Called Number

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

   Calling Number

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

   CHAP

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

   Control Connection

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

   Control Messages

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

   Digital Channel

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

   DSLAM

      Digital Subscriber Line (DSL) Access Module. A network device used
      in the deployment of DSL service. This is typically a concentrator
      of individual DSL lines located in a central office (CO) or local
      exchange.

   Incoming Call

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







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RFC 2661                          L2TP                       August 1999


   L2TP Access Concentrator (LAC)

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

   L2TP Network Server (LNS)

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

   Management Domain (MD)

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

   Network Access Server (NAS)

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

   Outgoing Call

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