Cisco Layer Two Forwarding (Protocol) "L2F"
RFC 2341

Document Type RFC - Historic (May 1998; No errata)
Was draft-valencia-l2f (individual)
Last updated 2013-03-02
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Network Working Group                                         A. Valencia
Request for Comments: 2341                                  M. Littlewood
Category: Historic                                               T. Kolar
                                                            Cisco Systems
                                                                 May 1998

              Cisco Layer Two Forwarding (Protocol) "L2F"

Status of Memo

   This memo describes a historic protocol for the Internet community.
   It does not specify an Internet standard of any kind.  Distribution
   of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

   Virtual dial-up allows many separate and autonomous protocol domains
   to share common access infrastructure including modems, Access
   Servers, and ISDN routers.  Previous RFCs have specified protocols
   for supporting IP dial-up via SLIP [1] and multiprotocol dial-up via
   PPP [2].  This document describes the Layer Two Forwarding protocol
   (L2F) which permits the tunneling of the link layer (i.e., HDLC,
   async HDLC, or SLIP frames) of higher level protocols.  Using such
   tunnels, it is possible to divorce the location of the initial dial-
   up server from the location at which the dial-up protocol connection
   is terminated and access to the network provided.

Table of Contents

   1.0 Introduction                                                3
   1.1 Conventions                                                 3
   2.0 Problem Space Overview                                      3
   2.1 Initial Assumptions                                         3
   2.2 Topology                                                    4
   2.3 Virtual dial-up Service - a walk-though                     5
   3.0 Service Model Issues                                        7
   3.1 Security                                                    7
   3.2 Address allocation                                          8
   3.3 Authentication                                              8
   3.4 Accounting                                                  8
   4.0 Protocol Definition                                         9
   4.1 Encapsulation within L2F                                   10
   4.1.1 Encapsulation of PPP within L2F                          10

Valencia, et. al.               Historic                        [Page 1]
RFC 2341                       Cisco L2F                        May 1998

   4.1.2 Encapsulation of SLIP within L2F                         10
   4.2 L2F Packet Format                                          10
   4.2.1 Overall Packet Format                                    10
   4.2.2 Packet Header                                            11
   4.2.3 Version field                                            11
   4.2.4 Protocol field                                           11
   4.2.5 Sequence Number                                          12
   4.2.6 Packet Multiplex ID                                      12
   4.2.7 Client ID                                                13
   4.2.8 Length                                                   13
   4.2.9 Packet Checksum                                          13
   4.2.10 Payload Offset                                          14
   4.2.11 Packet Key                                              14
   4.2.12 Packet priority                                         14
   4.3 L2F Tunnel Establishment                                   14
   4.3.1 Normal Tunnel Negotiation Sequence                       15
   4.3.2 Normal Client Negotiation Sequence                       17
   4.4 L2F management message types                               18
   4.4.1 L2F message type: Invalid                                18
   4.4.2 L2F_CONF                                                 19
   4.4.3 L2F_OPEN, tunnel establishment                           20
   4.4.4 L2F_OPEN, client establishment                           20
   4.4.5 L2F_CLOSE                                                22
   4.4.6 L2F_ECHO                                                 22
   4.4.7 L2F_ECHO_RESP                                            23
   4.5 L2F Message Delivery                                       23
   4.5.1 Sequenced Delivery                                       23
   4.5.2 Flow Control                                             23
   4.5.3 Tunnel State Table                                       24
   4.5.4 Client State Table                                       25
   5.0 Protocol Considerations                                    26
   5.1 PPP Features                                               26
   5.2 Termination                                                26
   5.3 Extended Authentication                                    26
   5.4 MNP4 and Apple Remote Access Protocol                      27
   5.5 Operation over IP/UDP                                      27
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