rfc889.txt

RFC 的详细文档！

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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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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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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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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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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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