End-to-end Performance Implications of Slow Links
RFC 3150

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Network Working Group                                         S. Dawkins
Request for Comments: 3150                                 G. Montenegro
BCP: 48                                                         M . Kojo
Category: Best Current Practice                                V. Magret
                                                               July 2001

           End-to-end Performance Implications of Slow Links

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 (2001).  All Rights Reserved.

Abstract

   This document makes performance-related recommendations for users of
   network paths that traverse "very low bit-rate" links.

   "Very low bit-rate" implies "slower than we would like".  This
   recommendation may be useful in any network where hosts can saturate
   available bandwidth, but the design space for this recommendation
   explicitly includes connections that traverse 56 Kb/second modem
   links or 4.8 Kb/second wireless access links - both of which are
   widely deployed.

   This document discusses general-purpose mechanisms.  Where
   application-specific mechanisms can outperform the relevant general-
   purpose mechanism, we point this out and explain why.

   This document has some recommendations in common with RFC 2689,
   "Providing integrated services over low-bitrate links", especially in
   areas like header compression.  This document focuses more on
   traditional data applications for which "best-effort delivery" is
   appropriate.

Dawkins, et al.          Best Current Practice                  [Page 1]
RFC 3150                   PILC - Slow Links                   July 2001

Table of Contents

   1.0 Introduction .................................................  2
   2.0 Description of Optimizations .................................  3
           2.1 Header Compression Alternatives ......................  3
           2.2 Payload Compression Alternatives .....................  5
           2.3 Choosing MTU sizes ...................................  5
           2.4 Interactions with TCP Congestion Control [RFC2581] ...  6
           2.5 TCP Buffer Auto-tuning ...............................  9
           2.6 Small Window Effects ................................. 10
   3.0 Summary of Recommended Optimizations ......................... 10
   4.0 Topics For Further Work ...................................... 12
   5.0 Security Considerations ...................................... 12
   6.0 IANA Considerations .......................................... 13
   7.0 Acknowledgements ............................................. 13
   8.0 References ................................................... 13
   Authors' Addresses ............................................... 16
   Full Copyright Statement ......................................... 17

1.0 Introduction

   The Internet protocol stack was designed to operate in a wide range
   of link speeds, and has met this design goal with only a limited
   number of enhancements (for example, the use of TCP window scaling as
   described in "TCP Extensions for High Performance" [RFC1323] for
   very-high-bandwidth connections).

   Pre-World Wide Web application protocols tended to be either
   interactive applications sending very little data (e.g., Telnet) or
   bulk transfer applications that did not require interactive response
   (e.g., File Transfer Protocol, Network News).  The World Wide Web has
   given us traffic that is both interactive and often "bulky",
   including images, sound, and video.

   The World Wide Web has also popularized the Internet, so that there
   is significant interest in accessing the Internet over link speeds
   that are much "slower" than typical office network speeds.  In fact,
   a significant proportion of the current Internet users is connected
   to the Internet over a relatively slow last-hop link.  In future, the
   number of such users is likely to increase rapidly as various mobile
   devices are foreseen to to be attached to the Internet over slow
   wireless links.

   In order to provide the best interactive response for these "bulky"
   transfers, implementors may wish to minimize the number of bits
   actually transmitted over these "slow" connections.  There are two

Dawkins, et al.          Best Current Practice                  [Page 2]
RFC 3150                   PILC - Slow Links                   July 2001

   areas that can be considered - compressing the bits that make up the
   overhead associated with the connection, and compressing the bits
   that make up the payload being transported over the connection.

   In addition, implementors may wish to consider TCP receive window
   settings and queuing mechanisms as techniques to improve performance