Layer 2 Tunneling Protocol (L2TP) Active Discovery Relay for PPP over Ethernet (PPPoE)
RFC 3817

Network Working Group                                        W. Townsley
Request for Comments: 3817                                 cisco Systems
Category: Informational                                      R. da Silva
                                                         AOL Time Warner
                                                               June 2004

        Layer 2 Tunneling Protocol (L2TP) Active Discovery Relay
                     for PPP over Ethernet (PPPoE)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   The Point-to-Point Protocol (PPP) provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.
   Layer Two Tunneling Protocol (L2TP), facilitates the tunneling of PPP
   packets across an intervening packet-switched network.  And yet a
   third protocol, PPP over Ethernet (PPPoE) describes how to build PPP
   sessions and to encapsulate PPP packets over Ethernet.

   L2TP Active Discovery Relay for PPPoE describes a method to relay
   Active Discovery and Service Selection functionality from PPPoE over
   the reliable control channel within L2TP.  Two new L2TP control
   message types and associated PPPoE-specific Attribute Value Pairs
   (AVPs) for L2TP are defined.  This relay mechanism provides enhanced
   integration of a specific feature in the PPPoE tunneling protocol
   with L2TP.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Protocol Operation . . . . . . . . . . . . . . . . . . . . . .  2
       2.1.  PPPoE Active Discovery Stage . . . . . . . . . . . . . .  3
       2.2.  Session Establishment and Teardown . . . . . . . . . . .  4
       2.3.  PPPoE PAD Message Exchange Coherency . . . . . . . . . .  6
       2.4.  PPPoE Service Relay Capabilities Negotiation . . . . . .  8
             2.4.1.  PPPoE Service Relay Response Capability AVP. . .  8
             2.4.2.  PPPoE Service Relay Forward Capability AVP . . .  9
   3.  L2TP Service Relay Messages. . . . . . . . . . . . . . . . . .  9

Townsley & da Silva          Informational                      [Page 1]
RFC 3817                  L2TP Relay for PPPoE                 June 2004

       3.1.  Service Relay Request Message (SRRQ) . . . . . . . . . .  9
       3.2.  Service Relay Reply Message (SRRP) . . . . . . . . . . . 10
   4.  PPPoE Relay AVP. . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  Security Considerations. . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 11
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
       8.1.  Normative References . . . . . . . . . . . . . . . . . . 12
       8.2.  Informative References . . . . . . . . . . . . . . . . . 12
   Appendix A: PPPoE Relay in Point to Multipoint Environments. . . . 13
   Appendix B: PAD Message Exchange Coherency Examples. . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 17

1.  Introduction

   PPPoE [1] is often deployed in conjunction with L2TP [2] to carry PPP
   [3] frames over a network beyond the reach of the local Ethernet
   network to which a PPPoE Host is connected.  For example, PPP frames
   tunneled within PPPoE may be received by an L2TP Access Concentrator
   (LAC) and then tunneled to any L2TP Network Server (LNS) reachable
   via an IP network.

   In addition to tunneling PPP over Ethernet, PPPoE defines a simple
   method for discovering services offered by PPPoE Access Concentrators
   (PPPoE AC) reachable via Ethernet from the PPPoE Host.  Since the
   packets used in this exchange are not carried over PPP, they are not
   tunneled with the PPP packets over L2TP, thus the discovery
   negotiation cannot extend past the LAC without adding functionality.

   This document describes a simple method for relaying PPPoE Active
   Discovery (PAD) messages over L2TP by extracting the PAD messages and
   sending them over the L2TP control channel.  After the completion of
   setup through the processing of PAD messages, PPP packets arriving
   via PPPoE are then tunneled over L2TP in the usual manner as defined
   in L2TP [2].  Thus, there are no data plane changes required at the
   LAC or LNS to support this feature.  Also, by utilizing the L2TP
   control channel, the PPPoE discovery mechanism is transported to the
   LNS reliably, before creation of any L2TP sessions, and may take
   advantage of any special treatment applied to control messages in