RFC 3437 (rfc3437) - Page 2 of 10
Layer-Two Tunneling Protocol Extensions for PPP Link Control Protocol Negotiation
Alternative Format: Original Text Document
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