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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.
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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
packet length. One of these was based at DCN1, while the other was based at
NTA. All experiments used ICMP Echo/Reply messages with embedded timestamps.
A cycle consisted of sending an ICMP Echo message of specified length, waiting
for the corresponding ICMP Reply message to come back and recording the
elapsed time (normalized to one-way delay). An experiment run, resulting in
one line of the table below, consisted of 512 of these volleys.
The length of each ICMP message was determined by a random-number
generator uniformly distributed between zero and 256. Lengths less than 40
were rounded up to 40, which is the minimum datagram size for an ICMP message
containing timestamps and just happens to also be the minimum TCP segment
size. The maximum length was chosen to avoid complications due to
fragmentation and reassembly, since ICMP messages are not ordinarily
fragmented or reassembled by the gateways.
The data collected were first plotted as a scatter diagram on a color
bit-map display. For all paths involving the ARPANET, this immediately
revealed two distinct characteristics, one for short (single-packet) messages
less than 126 octets in length and the other for long (multi-packet) messages
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D.L. Mills
longer than this. Linear regression lines were then fitted to each
characteristic with the results shown in the following table. (Only one
characteristic was assumed for ARPANET-exclusive paths.) The table shows for
each host the delays, in milliseconds, for each type of message along with a
rate computed on the basis of these delays. The "Host ID" column designates
the host at the remote end of the path, with a letter suffix used when
necessary to identify a particular run.
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D.L. Mills
Host Single-packet Rate Multi-packet Rate Comments
ID 40 125 (bps) 125 256 (bps)
---------------------------------------------------------------------------
DCN1 to nearby local-net hosts (calibration)
DCN5 9 13 366422 DMA 1822
DCN8 14 20 268017 Ethernet
IMP17 22 60 45228 56K 1822/ECU
FORD1 93 274 9540 9600 DDCMP base
UMD1 102 473 4663 4800 synch
DCN6 188 550 4782 4800 DDCMP
FACC 243 770 3282 9600/4800 DDCMP
FOE 608 1917 1320 9600/14.4K stat mux
DCN1 to ARPANET hosts and local nets
MILARP 61 105 15358 133 171 27769 MILNET gateway
ISID-L 166 263 6989 403 472 15029 low-traffic period
SCORE 184 318 5088 541 608 15745 low-traffic period
RVAX 231 398 4061 651 740 11781 Purdue local net
AJAX 322 578 2664 944 1081 7681 MIT local net
ISID-H 333 520 3643 715 889 6029 high-traffic period
BERK 336 967 1078 1188 1403 4879 UC Berkeley
WASH 498 776 2441 1256 1348 11379 U Washington
DCN1 to MILNET/MINET hosts and local nets
ISIA-L 460 563 6633 1049 1140 11489 low-traffic period
ISIA-H 564 841 2447 1275 1635 2910 high-traffic period
BRL 560 973 1645 1605 1825 4768 BRL local net
LON 585 835 2724 1775 1998 4696 MINET host (London)
HAWAII 679 980 2257 1817 1931 9238 a long way off
OFFICE3 762 1249 1396 2283 2414 8004 heavily loaded host
KOREA 897 1294 1712 2717 2770 19652 a long, long way off
DCN1 to TELENET hosts via ARPANET
RICE 1456 2358 754 3086 3543 2297 via VAN gateway
DCN1 to SATNET hosts and local nets via ARPANET
UCL 1089 1240 4514 1426 1548 8558 UCL zoo
NTA-L 1132 1417 2382 1524 1838 3339 low-traffic period
NTA-H 1247 1504 2640 1681 1811 8078 high-traffic period
NTA to SATNET hosts
TANUM 107 368 6625 9600 bps Tanum line
ETAM 964 1274 5576 Etam channel echo
GOONY 972 1256 6082 Goonhilly channel echo
Internet Delay Experiments Page 5
D.L. Mills
2.2 Analysis of Results
The data clearly show a strong correlation between delay and length, with
the longest packets showing delays two to three times the shortest. On paths
via ARPANET clones the delay characteristic shows a stonger correlation with
length for single-packet messages than for multi-packet messages, which is
consistent with a design which favors low delays for short messages and high
throughputs for longer ones.
Most of the runs were made during off-peak hours. In the few cases where
runs were made for a particular host during both on-peak and off-peak hours,
comparison shows a greater dependency on packet length than on traffic shift.
TCP implementors should be advised that some dependency on packet length
may have to be built into the retransmission-timeout estimation algorithm to
insure good performance over lossy nets like SATNET. They should also be
advised that some Internet paths may require stupendous timeout intervals
ranging to many seconds for the net alone, not to mention additional delays on
host-system queues.
I call to your attention the fact that the delays (at least for the
larger packets) from ARPANET hosts (e.g. DCN1) to MILNET hosts (e.g. ISIA)
are in the same ballpark as the delays to SATNET hosts (e.g. UCL)! I have
also observed that the packet-loss rates on the MILNET path are at present not
neglible (18 in 512 for ISIA-2). Presumably, the loss is in the gateways;
however, there may well be a host or two out there swamping the gateways with
retransmitted data and which have a funny idea of the "normal" timeout
interval. The recent discovery of a bug in the TOPS-20 TCP implementation,
where spurious ACKs were generated at an alarming rate, would seem to confirm
that suspicion.
3. Retransmission-Timeout Algorithm
One of the basic features of TCP which allow it to be used on paths
spanning many nets of widely varying delay and packet-loss characteristics is
the retranansmission-timeout algorithm, sometimes known as the "RSRE
Algorithm" for the original designers. The algorithm operates by recording
the time and initial sequence number when a segment is transmitted, then
computing the elapsed time for that sequence number to be acknowledged. There
are various degrees of sophistication in the implementation of the algorithm,
ranging from allowing only one such computation to be in progress at a time to
allowing one for each segment outstanding at a time on the connection.
The retransmission-timeout algorithm is basically an estimation process.
It maintains an extimate of the current roundtrip delay time and updates it as
new delay samples are computed. The algorithm smooths these samples and then
establishes a timeout, which if exceeded causes a retransmission. The
selection of the parameters of this algorithm are vitally important in order
to provide effective data transmission and avoid abuse of the Internet system
by excessive retransmissions. I have long been suspicious of the parameters
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D.L. Mills
suggested in the specification and used in some implementations, especially in
cases involving long-delay paths involving lossy nets. The experiment was
designed to simulate the operation of the algorithm using data collected from
real paths involving some pretty leaky Internet plumbing.
3.1. Experiment Design
The experiment data base was constructed of well over a hundred runs
using ICMP Echo/Reply messages bounced off hosts scattered all over the world.
Most runs, including all those summarized here, consisted of 512 echo/reply
cycles lasting from several seconds to twenty minutes or so. Other runs
designed to detect network glitches lasted several hours. Some runs used
packets of constant length, while others used different lengths distributed
from 40 to 256 octets. The maximum length was chosen to avoid complications
fragmented or reassembled by the gateways.
The object of the experiment was to simulate the packet delay
distribution seen by TCP over the paths measured. Only the network delay is
of interest here, not the queueing delays within the hosts themselves, which
can be considerable. Also, only a single packet was allowed in flight, so
that stress on the network itself was minimal. Some tests were conducted
during busy periods of network activity, while others were conducted during
quiet hours.
The 512 data points collected during each run were processed by a program
which plotted on a color bit-map display each data point (x,y), where x
represents the time since initiation of the experiment the and y the measured
delay, normalized to the one-way delay. Then, the simulated
retransmission-timeout algorithm was run on these data and its computed
timeout plotted in the same way. The display immediately reveals how the
algorithm behaves in the face of varying traffic loads, network glitches, lost
packets and superfluous retransmissions.
Each experiment run also produced summary statistics, which are
summarized in the table below. Each line includes the Host ID, which
identifies the run. The suffix -1 indicates 40-octet packets, -2 indicates
256-octet packets and no suffix indicates uniformly distributed lengths
between 40 and 256. The Lost Packets columns refer to instances when no ICMP
Reply message was received for thirty seconds after transmission of the ICMP
Echo message, indicating probable loss of one or both messages. The RTX
Packets columns refer to instances when the computed timeout is less than the
measured delay, which would result in a superfluous retransmission. For each
of these two types of packets the column indicates the number of instances
and the Time column indicates the total accumulated time required for the
recovery action.
For reference purposes, the Mean column indicates the computed mean delay
of the echo/reply cycles, excluding those cycles involving packet loss, while
the CoV column indicates the coefficient of variation. Finally, the Eff
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D.L. Mills
column indicates the efficiency, computed as the ratio of the total time
accumulated while sending good data to this time plus the lost-packet and
rtx-packet time.
Complete sets of runs were made for each of the hosts in the table below
for each of several selections of algorithm parameters. The table itself
reflects values, selected as described later, believed to be a good compromise
for use on existing paths in the Internet system.
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D.L. Mills
Host Total Lost Packets RTX Packets Mean CoV Eff
ID Time Time Time
-------------------------------------------------------------------
DCN1 to nearby local-net hosts (calibration)
DCN5 5 0 0 0 0 11 .15 1
DCN8 8 0 0 0 0 16 .13 1
IMP17 19 0 0 0 0 38 .33 1
FORD1 86 0 0 1 .2 167 .33 .99
UMD1 135 0 0 2 .5 263 .45 .99
DCN6 177 0 0 0 0 347 .34 1
FACC 368 196 222.1 6 9.2 267 1.1 .37
FOE 670 3 7.5 21 73.3 1150 .69 .87
FOE-1 374 0 0 26 61.9 610 .75 .83
FOE-2 1016 3 16.7 10 47.2 1859 .41 .93
DCN1 to ARPANET hosts and local nets
MILARP 59 0 0 2 .5 115 .39 .99
ISID 163 0 0 1 1.8 316 .47 .98
ISID-1 84 0 0 2 1 163 .18 .98
ISID-2 281 0 0 3 17 516 .91 .93
ISID * 329 0 0 5 12.9 619 .81 .96
SCORE 208 0 0 1 .8 405 .46 .99
RVAX 256 1 1.3 0 0 499 .42 .99
AJAX 365 0 0 0 0 713 .44 1
WASH 494 0 0 2 2.8 960 .39 .99
WASH-1 271 0 0 5 8 514 .34 .97
WASH-2 749 1 9.8 2 17.5 1411 .4 .96
BERK 528 20 50.1 4 35 865 1.13 .83
DCN1 to MILNET/MINET hosts and local nets
ISIA 436 4 7.4 2 15.7 807 .68 .94
ISIA-1 197 0 0 0 0 385 .27 1
ISIA-2 615 0 0 2 15 1172 .36 .97
ISIA * 595 18 54.1 6 33.3 992 .77 .85
BRL 644 1 3 1 1.9 1249 .43 .99
BRL-1 318 0 0 4 13.6 596 .68 .95
BRL-2 962 2 8.4 0 0 1864 .12 .99
LON 677 0 0 3 11.7 1300 .51 .98
LON-1 302 0 0 0 0 589 .06 1
LON-2 1047 0 0 0 0 2044 .03 1
HAWAII 709 4 12.9 3 18.5 1325 .55 .95
OFFICE3 856 3 12.9 3 10.3 1627 .54 .97
OFF3-1 432 2 4.2 2 6.9 823 .31 .97
OFF3-2 1277 7 39 3 41.5 2336 .44 .93
KOREA 1048 3 14.5 2 18.7 1982 .48 .96
KOREA-1 506 4 8.6 1 2.2 967 .18 .97
KOREA-2 1493 6 35.5 2 19.3 2810 .19 .96
DCN1 to TELENET hosts via ARPANET
RICE 677 2 6.8 3 12.1 1286 .41 .97
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D.L. Mills
RICE-1 368 1 .1 3 2.3 715 .11 .99
RICE-2 1002 1 4.4 1 9.5 1930 .19 .98
DCN1 to SATNET hosts and local nets via ARPANET
UCL 689 9 26.8 0 0 1294 .21 .96
UCL-1 623 39 92.8 2 5.3 1025 .32 .84
UCL-2 818 4 13.5 0 0 1571 .15 .98
NTA 779 12 38.7 1 3.7 1438 .24 .94
NTA-1 616 24 56.6 2 5.3 1083 .25 .89
NTA-2 971 19 71.1 0 0 1757 .2 .92
NTA to SATNET hosts and local nets
TANUM 110 3 1.6 0 0 213 .41 .98
GOONY 587 19 44.2 1 2.9 1056 .23 .91
ETAM 608 32 76.3 1 3.1 1032 .29 .86
UCL 612 5 12.6 2 8.5 1154 .24 .96
Note: * indicates randomly distributed packets during periods of high ARPANET
activity. The same entry without the * indicates randomly distributed packets
during periods of low ARPANET activity.
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D.L. Mills
3.2 Discussion of Results
It is immediately obvious from visual inspection of the bit-map display
that the delay distribution is more-or-less Poissonly distributed about a
relatively narrow range with important exceptions. The exceptions are
characterized by occasional spasms where one or more packets can be delayed
many times the typical value. Such glitches have been commonly noted before
on paths involving ARPANET and SATNET, but the true impact of their occurance
on the timeout algorithm is much greater than I expected. What commonly
happens is that the algorithm, when confronted with a short burst of
long-delay packets after a relatively long interval of well-mannered behavior,
takes much too long to adapt to the spasm, thus inviting many superfluous
retransmissions and leading to congestion.
The incidence of long-delay bursts, or glitches, varied widely during the
experiments. Some of them were glitch-free, but most had at least one glitch
in 512 echo/reply volleys. Glitches did not seem to correlate well with
increases in baseline delay, which occurs as the result of traffic surges, nor
did they correlate well with instances of packet loss. I did not notice any
particular periodicity, such as might be expected with regular pinging, for
example; however, I did not process the data specially for that.
There was no correction for packet length used in any of these
experiments, in spite of the results of the first set of experiments described
previously. This may be done in a future set of experiments. The algorithm
does cope well in the case of constant-length packets and in the case of
randomly distributed packet lengths between 40 and 256 octets, as indicated in
the table. Future experiments may involve bursts of short packets followed by
bursts of longer ones, so that the speed of adaptation of the algorithm can be
directly deterimend.
One particularily interesting experiment involved the FOE host
(FORD-FOE), which is located in London and reached via a 14.4-Kbps undersea
cable and statistical multiplexor. The multiplexor introduces a moderate mean
delay, but with an extremely large delay dispersion. The specified
retransmission-timeout algorithm had a hard time with this circuit, as might
be expected; however, with the improvments described below, TCP performance
was acceptable. It is unlikely that many instances of such ornery circuits
will occur in the Internet system, but it is comforting to know that the
algorithm can deal effectively with them.
3.3. Improvments to the Algorithm
The specified retransmission-timeout algorithm, really a first-order
linear recursive filter, is characterized by two parameters, a weighting
factor F and a threshold factor G. For each measured delay sample R the delay
estimator E is updated:
E = F*E + (1 - F)*R .
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D.L. Mills
Then, if an interval equal to G*E expires after transmitting a packet, the
packet is retransmitted. The current TCP specification suggests values in the
range 0.8 to 0.9 for F and 1.5 to 2.0 for G. These values have been believed
reasonable up to now over ARPANET and SATNET paths.
I found that a simple change to the algorithm made a worthwhile change in
the efficiency. The change amounts to using two values of F, one (F1) when R
< E in the expression above and the other (F2) when R >= E, with F1 > F2. The
effect is to make the algorithm more responsive to upward-going trends in
delay and less respnsive to downward-going trends. After a number of trials I
concluded that values of F1 = 15/16 and F2 = 3/4 (with G = 2) gave the best
all-around performance. The results on some paths (FOE, ISID, ISIA) were
better by some ten percent in efficiency, as compared to the values now used
in typical implementations where F = 7/8 and G = 2. The results on most paths
were better by five percent, while on a couple (FACC, UCL) the results were
worse by a few percent.
There was no clear-cut gain in fiddling with G. The value G = 2 seemed
to represent the best overall compromise. Note that increasing G makes
superfluous retransmissions less likely, but increases the total delay when
packets are lost. Also, note that increasing F2 too much tends to cause
overshoot in the case of network glitches and leads to the same result. The
table above was constructed using F1 = 15/16, F2 = 3/4 and G = 2.
Readers familiar with signal-detection theory will recognize my
suggestion as analogous to an ordinary peak-detector circuit. F1 represents
the discharge time-constant, while F2 represents the charge time-constant. G
represents a "squelch" threshold, as used in voice-operated switches, for
example. Some wag may be even go on to suggest a network glitch should be
called a netspurt.
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D.L. Mills
Appendix. Index of Test Hosts
Name Address NIC Host Name
-------------------------------------
DCN1 to nearby local-net hosts (calibration)
DCN5 128.4.0.5 DCN5
DCN8 128.4.0.8 DCN8
IMP17 10.3.0.17 DCN-GATEWAY
FORD1 128.5.0.1 FORD1
UMD1 128.8.0.1 UMD1
DCN6 128.4.0.6 DCN6
FACC 128.5.32.1 FORD-WDL1
FOE 128.5.0.15 FORD-FOE
DCN1 to ARPANET hosts and local nets
MILARP 10.2.0.28 ARPA-MILNET-GW
ISID 10.0.0.27 USC-ISID
SCORE 10.3.0.11 SU-SCORE
RVAX 128.10.0.2 PURDUE-MORDRED
AJAX 18.10.0.64 MIT-AJAX
WASH 10.0.0.91 WASHINGTON
BERK 10.2.0.78 UCB-VAX
DCN1 to MILNET/MINET hosts and local nets
ISIA 26.3.0.103 USC-ISIA
BRL 192.5.21.6 BRL-VGR
LON 24.0.0.7 MINET-LON-EM
HAWAII 26.1.0.36 HAWAII-EMH
OFFICE3 26.2.0.43 OFFICE-3
KOREA 26.0.0.117 KOREA-EMH
DCN1 to TELENET hosts via ARPANET
RICE 14.0.0.12 RICE
DCN1 to SATNET hosts and local nets via ARPANET
UCL 128.16.9.0 UCL-SAM
NTA 128.39.0.2 NTARE1
NTA to SATNET hosts and local nets
TANUM 4.0.0.64 TANUM-ECHO
GOONY 4.0.0.63 GOONHILLY-ECHO
ETAM 4.0.0.62 ETAM-ECHO