Internet Delay Experiments
RFC 889

Document Type RFC - Informational (December 1983; No errata)
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Network Working Group                                   D.L.  Mills
Request for Comments:  889                              December 1983

                          Internet Delay Experiments

This memo reports on some measurement experiments and suggests some possible
improvements to the TCP retransmission timeout calculation.  This memo is
both a status report on the measurements and advice to implementers of TCP.

1.  Introduction

     This memorandum describes two series of experiments designed to explore
the transmission characteristics of the Internet system.  One series of
experiments was designed to determine the network delays with respect to
packet length, while the other was designed to assess the effectiveness of the
TCP retransmission-timeout algorithm specified in the standards documents.
Both sets of experiments were conducted during the October - November 1983
time frame and used many hosts distributed throughout the Internet system.

     The objectives of these experiments were first to accumulate experimental
data on actual network paths that could be used as a benchmark of Internet
system performance, and second to apply these data to refine individual TCP
implementations and improve their performance.

     The experiments were done using a specially instrumented measurement host
called a Fuzzball, which consists of an LSI-11 running IP/TCP and various
application-layer protocols including TELNET, FTP and SMTP mail.  Among the
various measurement packages is the original PING (Packet InterNet Groper)
program used over the last six years for numerous tests and measurements of
the Internet system and its client nets.  This program contains facilities to
send various kinds of probe packets, including ICMP Echo messages, process the
reply and record elapsed times and other information in a data file, as well
as produce real-time snapshot histograms and traces.

     Following an experiment run, the data collected in the file were reduced
by another set of programs and plotted on a Peritek bit-map display with color
monitor.  The plots have been found invaluable in the indentification and
understanding of the causes of netword glitches and other "zoo" phenomena.
Finally, summary data were extracted and presented in this memorandum.  The
raw data files, including bit-map image files of the various plots, are
available to other experimenters upon request.

     The Fuzzballs and their local-net architecture, called DCN, have about
two-dozen clones scattered worldwide, including one (DCN1) at the Linkabit
Corporation offices in McLean, Virginia, and another at the Norwegian
Telecommunications Adminstration (NTA) near Oslo, Norway.  The DCN1 Fuzzball
is connected to the ARPANET at the Mitre IMP by means of 1822 Error Control
Units operating over a 56-Kbps line.  The NTA Fuzzball is connected to the
NTARE Gateway by an 1822 interface and then via VDH/HAP operating over a
9.6-Kbps line to SATNET at the Tanum (Sweden) SIMP.  For most experiments
described below, these details of the local connectivity can be ignored, since
only relatively small delays are involved.



Internet Delay Experiments                                              Page 2
D.L. Mills

     The remote test hosts were selected to represent canonical paths in the
Internet system and were scattered all over the world.  They included some on
the ARPANET, MILNET, MINET, SATNET, TELENET and numerous local nets reachable
via these long-haul nets.  As an example of the richness of the Internet
system connectivity and the experimental data base, data are included for
three different paths from the ARPANET-based measurement host to London hosts,
two via different satellite links and one via an undersea cable.

2.  Packet Length Versus Delay

     This set of experiments was designed to determine whether delays across
the Internet are significantly influenced by packet length.  In cases where
the intrinsic propagation delays are high relative to the time to transmit an
individual packet, one would expect that delays would not be strongly affected
by packet length.  This is the case with satellite nets, including SATNET and
WIDEBAND, but also with terrestrial nets where the degree of traffic
aggregation is high, so that the measured traffic is a small proportion of the
total traffic on the path.  However, in cases where the intrinsic propagation
delays are low and the measured traffic represents the bulk of the traffic on
the path, quite the opposite would be expected.

     The objective of the experiments was to assess the degree to which TCP
performance could be improved by refining the retransmission-timeout algorithm
to include a dependency on packet length.  Another objective was to determine
the nature of the delay characteristic versus packet length on tandem paths
spanning networks of widely varying architectures, including local-nets,
terrestrial long-haul nets and satellite nets.

2.1.  Experiment Design

     There were two sets of experiments to measure delays as a function of
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