Middleboxes: Taxonomy and Issues
RFC 3234

Document Type RFC - Informational (February 2002; No errata)
Was draft-carpenter-midtax (individual)
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Network Working Group                                       B. Carpenter
Request for Comments: 3234                IBM Zurich Research Laboratory
Category: Informational                                          S. Brim
                                                           February 2002

                    Middleboxes: Taxonomy and Issues

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 Internet Society (2002).  All Rights Reserved.

Abstract

   This document is intended as part of an IETF discussion about
   "middleboxes" - defined as any intermediary box performing functions
   apart from normal, standard functions of an IP router on the data
   path between a source host and destination host.  This document
   establishes a catalogue or taxonomy of middleboxes, cites previous
   and current IETF work concerning middleboxes, and attempts to
   identify some preliminary conclusions.  It does not, however, claim
   to be definitive.

Carpenter & Brim             Informational                      [Page 1]
RFC 3234            Middleboxes: Taxonomy and Issues       February 2002

Table of Contents

   1. Introduction and Goals.........................................  3
   1.1. Terminology..................................................  3
   1.2. The Hourglass Model, Past and Future.........................  3
   1.4. Goals of this Document.......................................  4
   2. A catalogue of middleboxes.....................................  5
   2.1 NAT...........................................................  6
   2.2 NAT-PT........................................................  7
   2.3 SOCKS gateway.................................................  7
   2.4 IP Tunnel Endpoints...........................................  8
   2.5. Packet classifiers, markers and schedulers...................  8
   2.6 Transport relay...............................................  9
   2.7. TCP performance enhancing proxies............................ 10
   2.8. Load balancers that divert/munge packets..................... 10
   2.9. IP Firewalls................................................. 11
   2.10. Application Firewalls....................................... 11
   2.11. Application-level gateways.................................. 12
   2.12. Gatekeepers/ session control boxes.......................... 12
   2.13. Transcoders................................................. 12
   2.14. Proxies..................................................... 13
   2.15. Caches...................................................... 14
   2.16. Modified DNS servers........................................ 14
   2.17. Content and applications distribution boxes................. 15
   2.18. Load balancers that divert/munge URLs....................... 16
   2.19. Application-level interceptors.............................. 16
   2.20. Application-level multicast................................. 16
   2.21. Involuntary packet redirection.............................. 16
   2.22. Anonymisers................................................. 17
   2.23. Not included................................................ 17
   2.24. Summary of facets........................................... 17
   3. Ongoing work in the IETF and elsewhere......................... 18
   4. Comments and Issues............................................ 19
   4.1. The end to end principle under challenge..................... 19
   4.2. Failure handling............................................. 20
   4.3. Failures at multiple layers.................................. 21
   4.4. Multihop application protocols............................... 21
   4.5. Common features.............................................. 22
   5. Security Considerations........................................ 22
   6. Acknowledgements............................................... 23
   7. References..................................................... 23
   Authors' Addresses................................................ 26
   Acknowledgement................................................... 26
   Full Copyright Statement.......................................... 27

Carpenter & Brim             Informational                      [Page 2]
RFC 3234            Middleboxes: Taxonomy and Issues       February 2002

1. Introduction and Goals

1.1. Terminology

   The phrase "middlebox" was coined by Lixia Zhang as a graphic
   description of a recent phenomenon in the Internet.  A middlebox is
   defined as any intermediary device performing functions other than
   the normal, standard functions of an IP router on the datagram path
   between a source host and destination host.

   In some discussions, especially those concentrating on HTTP traffic,
   the word "intermediary" is used.  For the present document, we prefer
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