RFC 3355 (rfc3355) - Page 2 of 13


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



Alternative Format: Original Text Document



RFC 3355                     L2TP Over AAL5                  August 2002


1. Introduction

   The Point-to-Point Protocol (PPP) [RFC 1661], 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)
   [RFC 2661] 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 [RFC 2661] 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" [RFC 2661].  An ATM network with AAL5 offers a suitable
   form of packet oriented connection.  This standard supplements
   [RFC 2661] 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",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" in this document are to be interpreted as described in
   [RFC 2119].

   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