RFC 3070 (rfc3070) - Page 2 of 7

Layer Two Tunneling Protocol (L2TP) over Frame Relay

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RFC 3070                 L2TP over Frame Relay             February 2001


2.0 Conventions

   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 [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
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