Layer Two Tunneling Protocol (L2TP) over Frame Relay
RFC 3070

Document Type RFC - Proposed Standard (February 2001; No errata)
Last updated 2013-03-02
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Network Working Group                                           V. Rawat
Request for Comments: 3070                             ONI Systems, Inc.
Category: Standards Track                                         R. Tio
                                                                S. Nanji
                                                  Redback Networks, Inc.
                                                                R. Verma
                                                     Deloitte Consulting
                                                           February 2001

          Layer Two Tunneling Protocol (L2TP) over Frame Relay

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 (2001).  All Rights Reserved.


   Layer Two Tunneling Protocol (L2TP) describes a mechanism to tunnel
   Point-to-Point (PPP) sessions.  The protocol has been designed to be
   independent of the media it runs over.  The base specification
   describes how it should be implemented to run over the User Datagram
   Protocol (UDP) and the Internet Protocol (IP).  This document
   describes how L2TP is implemented over Frame Relay Permanent Virtual
   Circuits (PVCs) and Switched Virtual Circuits (SVCs).


   This specification is intended for those implementations which desire
   to use facilities which are defined for L2TP and  applies only to the
   use of Frame Relay pont-to-point circuits.

1.0 Introduction

   L2TP [1] defines a general purpose mechanism for tunneling PPP over
   various media.  By design, it insulates L2TP operation from the
   details of the media over which it operates.  The base protocol
   specification illustrates how L2TP may be used in IP environments.
   This document specifies the encapsulation of L2TP over native Frame
   Relay and addresses relevant issues.

Rawat, et al.               Standards Track                     [Page 1]
RFC 3070                 L2TP over Frame Relay             February 2001

2.0 Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [2].

3.0 Problem Space Overview

   In this section we describe in high level terms the scope of the
   problem being addressed.  Topology:

         +------+           +---------------+          |
         | PSTN |           |  Frame Relay  |          |
   User--|      |----LAC ===|               |=== LNS --+ LANs
         | ISDN |           |     Cloud     |          |
         +------+           +---------------+          |

   An L2TP Access Concentrator (LAC) is a device attached to the
   switched network fabric (e.g., PSTN or ISDN) or co-located with a PPP
   end system capable of handling the L2TP protocol.  The LAC need only
   implement the media over which L2TP is to operate to pass traffic to
   one or more LNS's.  It may tunnel any protocol carried within PPP.

   L2TP Network Server (LNS) operates on any platform capable of PPP
   termination.  The LNS handles the server side of the L2TP protocol.
   L2TP is connection-oriented.  The LNS and LAC maintain state for each
   user that is attached to an LAC.  A session is created when an end-
   to-end PPP connection is attempted between a user and the LNS.  The
   datagrams related to a session are sent over the tunnel between the
   LAC and LNS.  A tunnel is defined by an LNS-LAC pair.  The tunnel
   carries PPP datagrams between the LAC and the LNS.

   L2TP protocol operates at a level above the particular media over
   which it is carried.  However, some details of its connection to
   media are required to permit interoperable implementations.  L2TP
   over IP/UDP is described in the base L2TP specification [1].  Issues
   related to L2TP over Frame Relay are addressed in later sections of
   this document.

4.0 Encapsulation and Packet Format

   L2TP MUST be able to share a Frame Relay virtual circuit (VC) with
   other protocols carried over the same VC.  The Frame Relay header
   format for data packet needs to be defined to identify the protocol
   being carried in the packets.  The Frame Relay network may not
   understand these formats.

Rawat, et al.               Standards Track                     [Page 2]
RFC 3070                 L2TP over Frame Relay             February 2001

   All protocols over this circuit MUST encapsulate their packets within
   a Q.922 frame.  Additionally, frames must contain information
   necessary to identify the protocol carried within the frame relay
   Protocol Data Unit (PDU), thus allowing the receiver to properly
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