RFC 3817 (rfc3817) - Page 2 of 17

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

Alternative Format: Original Text Document
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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
   transit or upon receipt.

2.  Protocol Operation

   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 RFC 2119 [4].




Townsley & da Silva          Informational
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