Multiple Encapsulation Methods Considered Harmful
RFC 4840

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Network Working Group                                      B. Aboba, Ed.
Request for Comments: 4840                                     E. Davies
Category: Informational                                        D. Thaler
                                             Internet Architecture Board
                                                              April 2007

           Multiple Encapsulation Methods Considered Harmful

Status of This Memo

   This memo provides information 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 architectural and operational issues that
   arise from link-layer protocols supporting multiple Internet Protocol
   encapsulation methods.

Aboba, et al.                Informational                      [Page 1]
RFC 4840         Multiple Encapsulation Methods Harmful       April 2007

Table of Contents

   1. Introduction ....................................................3
      1.1. Terminology ................................................3
      1.2. Ethernet Experience ........................................4
           1.2.1. IEEE 802.2/802.3 LLC Type 1 Encapsulation ...........6
           1.2.2. Trailer Encapsulation ...............................7
      1.3. PPP Experience ............................................10
      1.4. Potential Mitigations .....................................10
   2. Evaluation of Arguments for Multiple Encapsulations ............11
      2.1. Efficiency ................................................11
      2.2. Multicast/Broadcast .......................................12
      2.3. Multiple Uses .............................................13
   3. Additional Issues ..............................................15
      3.1. Generality ................................................15
      3.2. Layer Interdependence .....................................16
      3.3. Inspection of Payload Contents ............................17
      3.4. Interoperability Guidance .................................17
      3.5. Service Consistency .......................................19
      3.6. Implementation Complexity .................................19
      3.7. Negotiation ...............................................19
      3.8. Roaming ...................................................20
   4. Security Considerations ........................................20
   5. Conclusion .....................................................21
   6. References .....................................................22
      6.1. Normative Reference .......................................22
      6.2. Informative References ....................................22
   7. Acknowledgments ................................................25
   Appendix A. IAB Members at the Time of This Writing ...............26

Aboba, et al.                Informational                      [Page 2]
RFC 4840         Multiple Encapsulation Methods Harmful       April 2007

1.  Introduction

   This document describes architectural and operational issues arising
   from the use of multiple ways of encapsulating IP packets on the same
   link.

   While typically a link-layer protocol supports only a single Internet
   Protocol (IP) encapsulation method, this is not always the case.  For
   example, on the same cable it is possible to encapsulate an IPv4
   packet using Ethernet [DIX] encapsulation as defined in "A Standard
   for the Transmission of IP Datagrams over Ethernet Networks"
   [RFC894], the IEEE 802.2/802.3 LLC [IEEE-802.3.2002] Type 1
   encapsulation defined in "Two Methods For The Transmission of IP
   Datagrams over IEEE 802.3 Networks" [RFC948], or the IEEE 802
   [IEEE-802.1A.1990] encapsulation defined in "A Standard for the
   Transmission of IP Datagrams over IEEE 802 Networks" [RFC1042].
   Historically, a further encapsulation method was used on some
   Ethernet systems as specified in "Trailer Encapsulations" [RFC893].
   Similarly, ATM (e.g., see [RFC2684]), the Point-to-Point Protocol
   (PPP) [RFC1661], and IEEE 802.16 [IEEE-802.16e.2005] also support
   multiple encapsulation mechanisms.

1.1.  Terminology

   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 [RFC2119].

   Broadcast domain
        The set of all endpoints that receive broadcast frames sent by
        an endpoint in the set.

   Classification
        As defined in [IEEE-802.16e.2005], the process by which a Medium
        Access Control (MAC) Service Data Unit (SDU) is mapped into a
        particular transport connection for transmission between MAC
        peers.

   Connection Identifier (CID)
        In [IEEE-802.16e.2005] the connection identifier is a 16-bit