Advice for Internet Subnetwork Designers
RFC 3819

Network Working Group                                       P. Karn, Ed.
Request for Comments: 3819                                      Qualcomm
BCP: 89                                                       C. Bormann
Category: Best Current Practice                  Universitaet Bremen TZI
                                                            G. Fairhurst
                                                  University of Aberdeen
                                                             D. Grossman
                                                          Motorola, Inc.
                                                               R. Ludwig
                                                       Ericsson Research
                                                              J. Mahdavi
                                                                  Novell
                                                           G. Montenegro
                                   Sun Microsystems Laboratories, Europe
                                                                J. Touch
                                                                 USC/ISI
                                                                 L. Wood
                                                           Cisco Systems
                                                               July 2004

                Advice for Internet Subnetwork Designers

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document provides advice to the designers of digital
   communication equipment, link-layer protocols, and packet-switched
   local networks (collectively referred to as subnetworks), who wish to
   support the Internet protocols but may be unfamiliar with the
   Internet architecture and the implications of their design choices on
   the performance and efficiency of the Internet.

Karn, et al.             Best Current Practice                  [Page 1]
RFC 3819        Advice for Internet Subnetwork Designers       July 2004

Table of Contents

   1.  Introduction and Overview. . . . . . . . . . . . . . . . . . .  2
   2.  Maximum Transmission Units (MTUs) and IP Fragmentation . . . .  4
       2.1.  Choosing the MTU in Slow Networks. . . . . . . . . . . .  6
   3.  Framing on Connection-Oriented Subnetworks . . . . . . . . . .  7
   4.  Connection-Oriented Subnetworks. . . . . . . . . . . . . . . .  9
   5.  Broadcasting and Discovery . . . . . . . . . . . . . . . . . . 10
   6.  Multicasting . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  Bandwidth on Demand (BoD) Subnets. . . . . . . . . . . . . . . 13
   8.  Reliability and Error Control. . . . . . . . . . . . . . . . . 14
       8.1.  TCP vs Link-Layer Retransmission . . . . . . . . . . . . 14
       8.2.  Recovery from Subnetwork Outages . . . . . . . . . . . . 17
       8.3.  CRCs, Checksums and Error Detection. . . . . . . . . . . 18
       8.4.  How TCP Works. . . . . . . . . . . . . . . . . . . . . . 20
       8.5.  TCP Performance Characteristics. . . . . . . . . . . . . 22
             8.5.1.  The Formulae . . . . . . . . . . . . . . . . . . 22
             8.5.2.  Assumptions. . . . . . . . . . . . . . . . . . . 23
             8.5.3.  Analysis of Link-Layer Effects on TCP
                     Performance. . . . . . . . . . . . . . . . . . . 24
   9.  Quality-of-Service (QoS) Considerations. . . . . . . . . . . . 26
   10. Fairness vs Performance. . . . . . . . . . . . . . . . . . . . 29
   11. Delay Characteristics. . . . . . . . . . . . . . . . . . . . . 30
   12. Bandwidth Asymmetries. . . . . . . . . . . . . . . . . . . . . 31
   13. Buffering, Flow and Congestion Control . . . . . . . . . . . . 31
   14. Compression. . . . . . . . . . . . . . . . . . . . . . . . . . 34
   15. Packet Reordering. . . . . . . . . . . . . . . . . . . . . . . 36
   16. Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
   17. Routing. . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
   18. Security Considerations. . . . . . . . . . . . . . . . . . . . 41
   19. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 44
   20. Informative References . . . . . . . . . . . . . . . . . . . . 45
   21. Contributors' Addresses. . . . . . . . . . . . . . . . . . . . 57
   22. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 58
   23. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 60

1.  Introduction and Overview

   IP, the Internet Protocol [RFC791] [RFC2460], is the core protocol of
   the Internet.  IP defines a simple "connectionless" packet-switched