Encapsulation Methods for Transport of Layer 2 Frames over MPLS Networks
RFC 4905

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Network Working Group                                    L. Martini, Ed.
Request for Comments: 4905                                 E. Rosen, Ed.
Category: Historic                                   Cisco Systems, Inc.
                                                        N. El-Aawar, Ed.
                                             Level 3 Communications, LLC
                                                               June 2007

                Encapsulation Methods for Transport of
                   Layer 2 Frames over MPLS Networks

Status of This Memo

   This memo defines a Historic Document 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 IETF Trust (2007).

Abstract

   This document describes methods for encapsulating the Protocol Data
   Units (PDUs) of layer 2 protocols such as Frame Relay, Asynchronous
   Transfer Mode (ATM), or Ethernet for transport across an MPLS
   network.  This document describes the so-called "draft-martini"
   protocol, which has since been superseded by the Pseudowire Emulation
   Edge to Edge Working Group specifications described in RFC 4447 and
   related documents.

Martini, et al.                 Historic                        [Page 1]
RFC 4905         Encapsulation for L2 Frames over MPLS         June 2007

Table of Contents

   1. Introduction ....................................................3
   2. Specification of Requirements ...................................3
   3. Special Note ....................................................4
   4. General Encapsulation Method ....................................4
      4.1. The Control Word ...........................................4
           4.1.1. Setting the Sequence Number .........................5
           4.1.2. Processing the Sequence Number ......................6
      4.2. MTU Requirements ...........................................6
   5. Protocol-Specific Details .......................................7
      5.1. Frame Relay ................................................7
      5.2. ATM ........................................................8
           5.2.1. ATM AAL5 CPCS-SDU Mode ..............................9
           5.2.2. ATM Cell Mode ......................................10
           5.2.3. OAM Cell Support ...................................12
           5.2.4. CLP bit to Quality of Service Mapping ..............12
      5.3. Ethernet VLAN .............................................12
      5.4. Ethernet ..................................................12
      5.5. High-Level Data Link Control (HDLC) .......................13
      5.6. PPP .......................................................13
   6. Using an MPLS Label as the Demultiplexer Field .................13
      6.1. MPLS Shim EXP Bit Values ..................................14
      6.2. MPLS Shim S Bit Value .....................................14
      6.3. MPLS Shim TTL Values ......................................14
   7. Security Considerations ........................................14
   8. Normative References ...........................................14
   9. Informative References .........................................16
   10. Co-Authors ....................................................16

Martini, et al.                 Historic                        [Page 2]
RFC 4905         Encapsulation for L2 Frames over MPLS         June 2007

1.  Introduction

   In an MPLS network, it is possible to use control protocols such as
   those specified in [RFC4906] to set up "emulated virtual circuits"
   that carry the Protocol Data Units of layer 2 protocols across the
   network.  A number of these emulated virtual circuits (VCs) may be
   carried in a single tunnel.  This requires, of course, that the layer
   2 PDUs be encapsulated.  We can distinguish three layers of this
   encapsulation:

      - the "tunnel header", which contains the information needed to
        transport the PDU across the MPLS network; this header belongs
        to the tunneling protocol, e.g., MPLS, Generic Routing
        Encapsulation (GRE), and Layer 2 Tunneling Protocol (L2TP).

      - the "demultiplexer field", which is used to distinguish
        individual emulated virtual circuits within a single tunnel;
        this field must be understood by the tunneling protocol as well;
        it may be, e.g., an MPLS label or a GRE key field.

      - the "emulated VC encapsulation", which contains the information
        about the enclosed layer 2 PDU that is necessary in order to
        properly emulate the corresponding layer 2 protocol.

   This document specifies the emulated VC encapsulation for a number of
   layer 2 protocols.  Although different layer 2 protocols require
   different information to be carried in this encapsulation, an attempt