Subject: Protocol Action: Layer Two Tunneling Protocol 'L2TP' to Proposed Standard

From: The IESG <[email-masked]>
Date: Tue, 15 Jun 1999 16:50:08 -0400

The IESG has approved the Internet-Draft Layer Two Tunneling Protocol
'L2TP' <draft-ietf-pppext-l2tp-16.txt> as a Proposed Standard.  This
document is the product of the Point-to-Point Protocol Extensions
Working Group.  The IESG contact persons are Thomas Narten and Erik
Nordmark.

Technical Summary

PPP [RFC1661] defines an encapsulation mechanism for transporting
multiprotocol packets across layer 2 (L2) point-to-point links.
Typically, a user obtains a L2 connection to a Network Access Server
(NAS) using one of a number of techniques (e.g., dialup POTS, ISDN,
ADSL, etc.)  and then runs PPP over that connection. In such a
configuration, the L2 termination point and PPP session endpoint reside
on the same physical device (i.e., the NAS).

The Layer Two Tunneling Protocol (L2TP) extends the PPP model by
allowing the L2 and PPP endpoints to reside on different devices
interconnected by a packet-switched network.  With L2TP, a user has an
L2 connection to an access concentrator (e.g., modem bank, ADSL DSLAM,
etc.), and the concentrator then tunnels individual PPP frames to the
NAS. This allows the actual processing of PPP packets to be divorced
from the termination of the L2 circuit.  From the client's perspective,
L2TP is invisible. No client changes are needed.

This document defines the L2TP protocol and its operation.

Working Group Summary

This document has a long history. L2TP is a merging of two earlier
protocols, Cisco's L2F and PPTP (the product of a vendor consortium).
Implementations of L2TP were made as the document was being written.
Consequently, at the time L2TP was forwarded to the IESG (Spring 1998),
multiple implementations already existed. From the begining, however,
the IESG had concerns about certain aspects of the protocol (e.g., an
aggressive retransmission strategy) as well as the overall readability
of the document. After much back-and-forth (and several iterations of
the draft) the IESG remanded the document back to the Working Group
with specific instructions to rewrite the document (November 1998). The
IESG's overall concern was that the protocol was not implementable from
the document itself.

At the Orlando IETF, a set of interested parties had "reading party"
and started reworking the draft. After a concerted effort that lasted
almost two months, the document was redone, with much of the text
completely rewritten from scratch. In addition, aspects related to flow
control (optional to implement in the original documunt) were removed
from the document. The current document is the result of the rewrite.

There was strong consensus in the WG for the final version of this
document. During the IETF Last Call, three issues were raised. The
first had to do with design choices that were made early on, but the WG
did not want to revisit those decisions and indeed argued that they had
made the right decision. That is, the specific issues raised did not
point to incorrectness in the protocol, so much as perhaps less
efficient operations in some circumstances.

The second issue had to do with the use of IPSec and the role of the
L2TP NAS as a security gateway. It was pointed out that in IPSec, a
security gateway is required to have number of filters with fine
granualarity to control which packets are let through once they are
authenticated. However, L2TP carries PPP traffic, which can carry any
arbitrary higher layer protocol, including IP. No such filter rules are
specified.  The WG felt that the L2TP wasn't attempting to be a
security gateway in the same sense as IPSec. Rather, IPSec is used to
protect the basic integrity of the L2TP packets themselves. A revised
Security Considerations section captures this issue.

The final issue was the disclosure that one of the co-authors of the
document had filed a patent that may cover aspects of L2TP. After much
discussion on the mailing list and at the Minneapolis meeting, the
overwhelming consensus of the WG was to advance the document.

Protocol Quality

This spec has been reviewed for the IESG by Thomas Narten.