Layer Two Tunnelling Protocol (L2TP) Over ATM Adaptation Layer 5 (AAL5)
RFC 3355

Document Type RFC - Proposed Standard (September 2002; No errata)
Authors Rene Tio  , Suhail Nanji  , Ajoy Singh  , Rollins Turner 
Last updated 2013-03-02
Stream IETF
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IESG IESG state RFC 3355 (Proposed Standard)
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Responsible AD Thomas Narten
IESG note Published as RFC 3301
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Network Working Group                                           A. Singh
Request for Comments: 3355                                      Motorola
Category: Standards Track                                      R. Turner
                                                                  R. Tio
                                                                S. Nanji
                                                        Redback Networks
                                                             August 2002

                  Layer Two Tunnelling Protocol (L2TP)
                   Over ATM Adaptation Layer 5 (AAL5)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   The Layer Two Tunneling Protocol (L2TP) provides a standard method
   for transporting the link layer of the Point-to-Point Protocol (PPP)
   between a dial-up server and a Network Access Server, using a network
   connection in lieu of a physical point-to-point connection.  This
   document describes the use of an Asynchronous Transfer Mode (ATM)
   network for the underlying network connection.  ATM User-Network
   Interface (UNI) Signaling Specification Version 4.0 or Version 3.1
   with ATM Adaptation Layer 5 (AAL5) are supported as interfaces to the
   ATM network.


   This specification is intended for implementations of L2TP that use
   ATM to provide the communications link between the L2TP Access
   Concentrator and the L2TP Network Server.

Singh, et. al.              Standards Track                     [Page 1]
RFC 3355                     L2TP Over AAL5                  August 2002

1. Introduction

   The Point-to-Point Protocol (PPP) [RFC1661], is frequently used on
   the link between a personal computer with a dial modem and a network
   service provider, or NSP.  The Layer Two Tunneling Protocol (L2TP)
   [RFC2661] enables a dial-up server to provide access to a remote NSP
   by extending the PPP connection through a tunnel in a network to
   which both it and the NSP are directly connected.  A "tunnel" is a
   network layer connection between two nodes, used in the role of a
   data link layer connection between those nodes, possibly as part of a
   different network.  In [RFC2661] the dial-up server is called an L2TP
   Access Concentrator, or LAC.  The remote device that provides access
   to a network is called an L2TP Network Server, or LNS.  L2TP uses a
   packet delivery service to create a tunnel between the LAC and the
   LNS.  "L2TP is designed to be largely insulated from the details of
   the media over which the tunnel is established; L2TP requires only
   that the tunnel media provide packet oriented point-to-point
   connectivity" [RFC2661].  An ATM network with AAL5 offers a suitable
   form of packet oriented connection.  This standard supplements
   [RFC2661] by providing details specific to the use of AAL5 for a
   point-to-point connection between LAC and LNS.

2. Conventions

   Requirements keywords The key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" in this document are to be interpreted as described in

   A list of acronyms used in this document is given at the end of the
   document as Appendix A.

3. AAL5 Layer Service Interface

   L2TP treats the underlying ATM AAL5 layer service as a bit-
   synchronous point-to-point link.  In this context, the L2TP link
   corresponds to an ATM AAL5 virtual circuit (VC).  The VC MUST be
   full-duplex, point to point, and it MAY be either dedicated (i.e.,
   permanent, set up by provisioning) or switched (set up on demand.)

   The AAL5 message mode service, in the non-assured mode of operation,
   without the corrupted delivery option MUST be used.

   Interface Format - The L2TP/AAL5 layer boundary presents an octet
   service interface to the AAL5 layer.  There is no provision for sub-
   octets to be supplied or accepted.

Singh, et. al.              Standards Track                     [Page 2]
RFC 3355                     L2TP Over AAL5                  August 2002

3.1 Maximum Transfer Unit

   Each L2TP PDU MUST be transported within a single AAL5 PDU.
   Therefore the maximum transfer unit (MTU) of the AAL5 connection
   constrains the MTU of the L2TP tunnel that uses the connection and
   the MTU of all PPP connections that use the tunnel.  ([RFC1661]
   refers to this as Maximum Receive Unit, or MRU.  In [SIG31], it is
   the Forward and Backward Maximum CPCS-SDU Size.)
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