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.

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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",
   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.

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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
   process the incoming packet.

   The frame format for L2TP MUST be SNAP encapsulation as defined in
   RFC 1490 [6] and FRF3.1 [3].  SNAP format uses NLPID followed by
   Organizationally Unique Identifier and a PID.


   The single octet identifier provides a mechanism to allow easy
   protocol identification.  For L2TP NLPID value 0x80 is used which
   indicates the presence of SNAP header.

   OUI & PID

   The three-octet Organizationally Unique Identifier (OUI) 0x00-00-5E
   identifies IANA who administers the meaning of the Protocol
   Identifier (PID) 0x0007.  Together they identify a distinct protocol.

   Format of L2TP frames encapsulated in Frame Relay is given in Figure

          Octet                      1
                 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
            1   |         Q.922 Address         |
            3   | Control  0x03 | pad   0       |
            5   | NLPID 0x80    |  OUI  0x00    |
                +-+-+-+-+-+-+-+-+               +
            7   | OUI     0x00-5E               |
            9   | PID     0x0007                |
                |                               |
                |          L2TP packet          |
                |                               |
                |              FCS              |

           Figure 1  Format for L2TP frames encapsulated in
                     Frame Relay

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RFC 3070                 L2TP over Frame Relay             February 2001

5.0 MTU Considerations

   FRF.12 [5] is the Frame Relay Fragmentation Implementation Agreement.
   If fragmentation is not supported, the two Frame Relay endpoints MUST
   support an MTU size of at least 1526 which is based on adding the PPP
   Max-Receive-Unit size with the PPP header size with the Max L2TP
   Header Size with the Frame Relay header size (PPP header size is the
   protocol field size plus HDLC framing bytes, which is required by
   L2TP).  To avoid packet discards on the Frame Relay interface, the
   RECOMMENDED default Frame Relay MTU is 1564 based on a PPP default
   MRU of 1500.  The means to ensure these MTU settings are left to

6.0 QOS Issues

   In general, QoS mechanisms can be roughly provided for with
   proprietary mechanisms localized within the LAC or LNS.  QoS
   considerations are beyond the scope of this document.

7.0 Frame Relay and L2TP Interaction

   In case of Frame Relay SVCs, connection setup will be triggered when
   L2TP tries to create a tunnel.  Details of triggering mechanism are
   left to implementation.  There SHALL NOT be any change in Frame Relay
   SVC signaling due to L2TP.  The endpoints of the L2TP tunnel MUST be
   identified by X.121/E.164 addresses in case of Frame Relay SVC.
   These addresses MAY be obtained as tunnel endpoints for a user as
   defined in [4].  In case of PVCs, the Virtual Circuit to carry L2TP
   traffic MAY be configured administratively.  The endpoints of the
   tunnel MUST be identified by DLCI, assigned to the PVC at
   configuration time.  This DLCI MAY be obtained as tunnel endpoints
   for a user as defined in [4].

   There SHALL be no framing issues between PPP and Frame Relay.  PPP
   frames received by LAC from remote user are stripped of CRC, link
   framing, and transparency bytes, encapsulated in L2TP, and forwarded
   over Frame Relay tunnel.

8.0 Security Considerations

   Currently there is no standard specification for Frame Relay security
   although the Frame Relay Forum is working on a Frame Relay Privacy
   Agreement.  In light of this work, the issue of security will be re-
   examined at a later date to see if L2TP over Frame Relay specific
   protection mechanisms are still required.  In the interim, basic
   security issues are discussed in the base L2TP specification [1].

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RFC 3070                 L2TP over Frame Relay             February 2001

9.0 Acknowledgments

   Ken Pierce (3Com Corporation) and (Rick Dynarski 3Com Corporation)
   contributed to the editing of this document.

10.0 References

   [1]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G. and
        B. Palter "Layer Two Tunneling Protocol 'L2TP'", RFC 2661,
        August 1999.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [3]  Multiprotocol Encapsulation Implementation Agreement, FRF.3.1 ,
        Frame Relay Forum Technical Committee, June 1995.

   [4]  Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M. and
        I. Goyret, "RADIUS Attributes for Tunnel Protocol Support", RFC
        2868, June 2000.

   [5]  Frame Relay Fragmentation Implementation Agreement, FRF.12,
        Frame Relay Forum Technical Committee, December 1997.

   [6]  Bradley, T., Brown, C. and A. Malis, "Multiprotocol Interconnect
        over Frame Relay", RFC 1490, July 1993.

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RFC 3070                 L2TP over Frame Relay             February 2001

11.0 Authors' Addresses

   Vipin Rawat
   ONI Systems, Inc.
   166 Baypointe Parkway
   San Jose CA 95134


   Rene Tio
   Redback Networks, Inc.
   300 Holger Way
   San Jose, CA 95134


   Rohit Verma
   Deloitte Consulting
   180 N. Stetson Avenue
   Chicago Illinois 60601


   Suhail Nanji
   Redback Networks, Inc.
   300 Holger Way
   San Jose, CA 95134


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RFC 3070                 L2TP over Frame Relay             February 2001

12.0  Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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