Layer Two Tunneling Protocol - Version 3 (L2TPv3)
RFC 3931

Network Working Group                                        J. Lau, Ed.
Request for Comments: 3931                              M. Townsley, Ed.
Category: Standards Track                                  Cisco Systems
                                                          I. Goyret, Ed.
                                                     Lucent Technologies
                                                              March 2005

           Layer Two Tunneling Protocol - Version 3 (L2TPv3)

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 (2005).

Abstract

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
       1.1.  Changes from RFC 2661. . . . . . . . . . . . . . . . . .  4
       1.2.  Specification of Requirements. . . . . . . . . . . . . .  4
       1.3.  Terminology. . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Topology . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  Protocol Overview. . . . . . . . . . . . . . . . . . . . . . .  9
       3.1.  Control Message Types. . . . . . . . . . . . . . . . . . 10
       3.2.  L2TP Header Formats. . . . . . . . . . . . . . . . . . . 11
             3.2.1.  L2TP Control Message Header. . . . . . . . . . . 11
             3.2.2.  L2TP Data Message. . . . . . . . . . . . . . . . 12
       3.3.  Control Connection Management. . . . . . . . . . . . . . 13
             3.3.1.  Control Connection Establishment . . . . . . . . 14
             3.3.2.  Control Connection Teardown. . . . . . . . . . . 14
       3.4.  Session Management . . . . . . . . . . . . . . . . . . . 15
             3.4.1.  Session Establishment for an Incoming Call . . . 15
             3.4.2.  Session Establishment for an Outgoing Call . . . 15

Lau, et al.                 Standards Track                     [Page 1]
RFC 3931                         L2TPv3                       March 2005

             3.4.3.  Session Teardown . . . . . . . . . . . . . . . . 16
   4.  Protocol Operation . . . . . . . . . . . . . . . . . . . . . . 16
       4.1.  L2TP Over Specific Packet-Switched Networks (PSNs) . . . 16
             4.1.1.  L2TPv3 over IP . . . . . . . . . . . . . . . . . 17
             4.1.2.  L2TP over UDP. . . . . . . . . . . . . . . . . . 18
             4.1.3.  L2TP and IPsec . . . . . . . . . . . . . . . . . 20
             4.1.4.  IP Fragmentation Issues. . . . . . . . . . . . . 21
       4.2.  Reliable Delivery of Control Messages. . . . . . . . . . 23
       4.3.  Control Message Authentication . . . . . . . . . . . . . 25
       4.4.  Keepalive (Hello). . . . . . . . . . . . . . . . . . . . 26
       4.5.  Forwarding Session Data Frames . . . . . . . . . . . . . 26
       4.6.  Default L2-Specific Sublayer . . . . . . . . . . . . . . 27
             4.6.1.  Sequencing Data Packets. . . . . . . . . . . . . 28
       4.7.  L2TPv2/v3 Interoperability and Migration . . . . . . . . 28
             4.7.1.  L2TPv3 over IP . . . . . . . . . . . . . . . . . 29
             4.7.2.  L2TPv3 over UDP. . . . . . . . . . . . . . . . . 29
             4.7.3.  Automatic L2TPv2 Fallback. . . . . . . . . . . . 29
   5.  Control Message Attribute Value Pairs. . . . . . . . . . . . . 30
       5.1.  AVP Format . . . . . . . . . . . . . . . . . . . . . . . 30
       5.2.  Mandatory AVPs and Setting the M Bit . . . . . . . . . . 32
       5.3.  Hiding of AVP Attribute Values . . . . . . . . . . . . . 33
       5.4.  AVP Summary. . . . . . . . . . . . . . . . . . . . . . . 36
             5.4.1.  General Control Message AVPs . . . . . . . . . . 36
             5.4.2.  Result and Error Codes . . . . . . . . . . . . . 40
             5.4.3.  Control Connection Management AVPs . . . . . . . 43
             5.4.4.  Session Management AVPs. . . . . . . . . . . . . 48
             5.4.5.  Circuit Status AVPs. . . . . . . . . . . . . . . 57
   6.  Control Connection Protocol Specification. . . . . . . . . . . 59
       6.1.  Start-Control-Connection-Request (SCCRQ) . . . . . . . . 60
       6.2.  Start-Control-Connection-Reply (SCCRP) . . . . . . . . . 60
       6.3.  Start-Control-Connection-Connected (SCCCN) . . . . . . . 61