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Layer Two Tunneling Protocol "L2TP"
RFC 2661

Document type: RFC - Proposed Standard (August 1999; Errata)
Document stream: IETF
Last updated: 2013-03-02
Other versions: plain text, pdf, html

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IESG State: RFC 2661 (Proposed Standard)
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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

Townsley, et al.            Standards Track                     [Page 1]
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

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