RFC 3437 (rfc3437) - Page 2 of 10

Layer-Two Tunneling Protocol Extensions for PPP Link Control Protocol Negotiation

RFC 3437        L2TP Extensions for PPP LCP Negotiation    December 2002


Table of Contents

   1. Introduction...............................................  2
     1.1 Specification of Requirements...........................  3
   2. LCP Options From LAC to LNS................................  3
     2.1 LCP Want Options (iccn, occn)...........................  4
     2.2 LCP Allow Options (iccn, occn)..........................  6
     2.3 LCP Options From LNS to LAC.............................  7
   3. Security Considerations....................................  8
   4. IANA Considerations........................................  8
   5. Normative References.......................................  8
   6. Author's Addresses.........................................  9
   7. Full Copyright Statement................................... 10

1. Introduction

   L2TP [RFC 2661] provides a very limited amount of guidance to the LNS
   as to what type of interface a tunneled PPP session arrived on at an
   LAC.  Such information is limited to whether the interface was
   "synchronous" or "asynchronous", "digital" or "analog."  These
   indications provide some guidance when negotiating PPP LCP at the
   LNS, but they are not as robust as they could be.

   This document defines a more robust way to inform the LAC of LCP
   negotiated options, and provides guidance to the LNS on the limits
   and values that the LAC requires during LCP negotiation.  Deep
   knowledge of PPP [RFC 1661] and L2TP [RFC 2661] are expected for the
   remainder of this document.

   L2TP Proxy LCP allows options to be negotiated where the native PPP
   link resides, thus circumventing issues with ACCM, Alternate FCS, and
   other LCP Options that the LNS would not necessarily know how to
   properly negotiate without access to the physical media for the
   native PPP connection, interface type, or configuration.  However,
   use of Proxy LCP introduces other problems as well as there are
   options within LCP PPP negotiation which should be set or adjusted by
   the LNS, such as the PPP Authentication Type and MRU.  Finally, the
   PPP Client may reinitiate LCP negotiation at any time, and unless the
   LAC is sniffing every PPP data packet it forwards, it would not be
   aware that this is even occurring.

   LCP options may be classified into roughly three different categories
   with respect to their affect on L2TP; (1) options which affect
   framing in a way that the LAC may need to know about or handle
   specifically (e.g., ALT-FCS, ACCM, MRU), (2) options that are mostly
   transparent to the LAC (e.g., AUTH-TYPE), and (3) options that the





Palter & Townsley           Standards Track