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rfc:rfc619

Network Working Group W. Naylor Request for Comment: 619 H. Opderbeck NIC 21990 UCLA-NMC

                                                         March 7, 1974
                Mean Round-Trip Times in the ARPANET

In one of our current measurement projects we are interested in the average values of important network parameters. For this purpose we collect data on the network activity over seven consecutive days. This data collection is only interrupted by down-time or maintenance of either the net or our collecting facility (the "late" Sigma-7 or, in future, the 360/91 at CCN).

The insight gained from the analysis of this data has been reported in Network Measurement Group Note 18 (NIC 20793):

  L.  Kleinrock and W. Naylor
  "On Measured Behavior of the ARPA Network"

This paper will be presented at the NCC '74 in Chicago.

In this RFC we want to report the mean round-trip times (or delays) that were observed during these week-long measurements since we think these figures are of general interest to the ARPA community. Let us first define the term "round trip time" as it is used by the statistics gathering program in the IMPs. When a message is sent from a source HOST to a destination HOST, the following events, among others, can be distinguished (T(i) is the time of event i):

T(1): The message is passed from the user program to the NCP in the
      source HOST
T(2): The proper entry is made in the pending packet table (PPT) for
      single packet messages or the pending leader table (PLT) for
      multiple packet messages after the first packet is received by
      the source IMP
T(3): The first packet of the message is put on the proper output
      queue in the source IMP (at this time the input of the second
      packet is initiated)
T(4): The message is put on the HOST-output queue in the destination
      IMP (at this time the reassembly of the message is complete)
T(5): The RFNM is sent from the destination IMP to the source IMP

Naylor & Opderbeck [Page 1] RFC 619 Mean Round-Trip Times in the ARPANET March 1974

T(6): The RFNM arrives at the source IMP
T(7): The RFNM is accepted by the source HOST

The time intervals T(i)-T(i-1) are mainly due to the following delays and waiting times:

T(2)-T(1): -HOST processing delay
           -HOST-IMP transmission delay for the 32-bit leader
           -Waiting time for a message number to become free (only
            four messages can simultaneously be transmitted between
            any pair of source IMP - destination IMP)
           -Waiting time for a buffer to become free (there must be
            more than three buffers on the "free buffer list")
           -HOST-IMP transmission delay for the first packet
           -Waiting time for an entry in the PPT or PLT to become
            available (there are eight entries in the PPT and twelve
            in the PLT table)
T(3)-T(2): -Waiting time for a store-and-forward (S/F) buffer to
            become free (the maximum number of S/F-buffers is 20).
           -Waiting time for a logical ACK-channel to become free
            (there are 8 logical ACK-channels for each physical
            channel).
           -For multiple packet messages, waiting time until the
            ALLOCATE is received (unless an allocation from a previous
            multiple-packet message still exists; such an allocation
            is returned in the RFNM and expires after 125 msec)
T(4)-T(3): -Queuing delay, transmission delay, and propagation delay
            in all the IMPs and lines in the path from source IMP to
            destination IMP
           -Possibly retransmission delay due to transmission errors
            or lack of buffer space (for multiple packet messages the
            delays for the individual packets overlap)
T(5)-T(4): -Queuing delay in the destination IMP
           -IMP-HOST transmission delay for the first packet
           -For multiple-packet messages, waiting time for reassembly
            buffers to become free to piggy-back an ALLOCATE on the
            RFNM (if this waiting time exceeds one second then the
            RFNM is sent without the ALLOCATE)
T(6)-T(5): -Queuing delay, transmission delay, and propagation delay
            for the RFNM in all the IMPs and lines in the path from
            destination IMP to source IMP

Naylor & Opderbeck [Page 2] RFC 619 Mean Round-Trip Times in the ARPANET March 1974

T(7)-T(6): -Queuing delay for the RFNM in the source IMP
           -IMP-HOST transmission delay for the RFNM

IMP processing delays are not included in this table since they are usually very small. Also, some of the abovementioned waiting times reduce to zero in many cases, e.g. the waiting time for a message number to become available and the waiting time for a buffer to become free.

If the source and destination HOSTs are attached to the same IMP, this table can be simplified as follows:

T(2)-T(1):  as before
T(3)-T(2):  for multiple packet messages: waiting time until
            reassembly space becomes available (there are up to 66
            reassembly buffers)
T(4)-T(3):  for multiple packet messages: HOST-IMP transmission delay
            for packets 2,3,...
T(5)-T(4):  as before
T(6)-T(5):  0
T(7)-T(6):  as before

Up to now we have neglected the possibility that a single packet message is rejected at the destination IMP because of lack of reassembly space. If this occurs, the single packet message is treated as a request for buffer space allocation and the time interval T(3)-T(2) increased by the waiting time until the corresponding "ALLOCATE" is received.

The round trip time (RTT) is now defined as the time interval T(6)-T(2). Note that the RTT for multiple packet messages does include the waiting time until the ALLOCATE is received. It does, however, not include the source HOST processing delay (i.e. delays in the NCP), the HOST-IMP transmission delay, and the waiting time until a message number becomes available. Note also, that the RFNM is sent after the first packet of a multiple packet message has been received by the destination HOST.

Let us now turn to the presentation of the average round trip times as they were measured during continuous seven-day periods in August and December '73. In August, an average number of 2935 messages/minute were entering the ARPANET. The overall mean round trip delay for all these messages was 93 milliseconds (msec). The corresponding numbers for December were 2226 messages/minute and 200 msec. An obvious question that immediately arises is: why did the average round trip delay more than double while the rate of incoming messages decreased? The answer to this question can be found in the large round trip delays for the status reports that are sent from each IMP to the NCC. Each IMP sends, on the average, 2.29 status reports per minute to the NCC. Since there

Naylor & Opderbeck [Page 3] RFC 619 Mean Round-Trip Times in the ARPANET March 1974

were 45 sites connected to the net in December, a total of 103.05 status reports per minute were sent to the NCC. Thus 4.63 percent of all messages that entered the net were status reports.

The average round trip delay for all these status reports in December was 1.66 sec. This number is five to ten times larger than the average round-trip delay for status reports we observed in August. It is not yet clear what change in the collection of status reports caused this increase. One reason appears to be that the number of these reports was doubled between August and December. Since the large round-trip delays of these status reports distort the overall picture somewhat, we are going to present the December data - wherever appropriate - with and without the effect of these delays. (We should point out here that the traffic/delay picture is distorted by the accumulated statistics messages which were collected to produce this data. We have, however, ignored this effect since these measurement messages represent less than 0.3% of the total traffic.) The overall mean round trip delay without the status reports in December is 132 msec. This value is still more than 35 msec larger than the corresponding value for August. However, before we shall attempt to explain this difference we will first present the measured data.

Table 1 shows the mean round trip delay as a function of the number of hops over the minimum-hop path. This minimum number of hops was calculated from the (static) topology of the net as it existed in August and December of last year. The actual number of hops over which any given message travels may, of course, be larger due to network congestion, line failures or IMP failures. In fact, for August we observed a minimum mean path length of 3.24 while the actual measured mean path length was 3.30; in December we observed 4.02 and 4.40, respectively. (See Network Measurement Group Note #18 for an explanation of the computation of actual mean path length.) As expected we observe a sharp increase of the mean round trip delay as the minimum number of hops is increased. Note, however, that the mean round trip delay is not a strictly increasing function of the minimum number of hops.

Table 2 gives the mean round trip delay for messages from a given site. The December data is presented with and without the large delays incurred by the sending of status reports to the NCC. Table 3 shows the mean round trip delay for messages to a given site. The largest round trip delays, in December, were incurred by messages sent to the NCC-TIP since these messages include all the status reports.

Table 4, finally, gives for each site the mean round trip delays to those three destination IMP/TIP's to which the most messages were sent during the seven-day measurement period in December. Let us first say few words about the traffic distribution which is dealt with in more

Naylor & Opderbeck [Page 4] RFC 619 Mean Round-Trip Times in the ARPANET March 1974

detail in Network Measurement Group Note #18. There are several sites which like to use their IMP as a kind of local multiplexer (UTAH, MIT, HARV, CMU, USCT, CCAT, XROX, HAWT, MIT2). For these sites the most favorite destination site is the source IMP itself. For several other sites the most favorite destination site is just one hop away (BBN, AMES, AMST, NCCT, RUTT). Nobody will be surprised that for many sites ISI (ILL, MTRT, ETAT, SDAT, ARPT, RMLT, LONT) or SRI (UCSB, RADT, NBST) is the most favorite site. There are several other sites (SDC, LL, CASE, DOCT, BELV, ABRD, FNWT, LBL, NSAT, TYMT, MOFF, WPAT) which were rather inactive in terms of generating traffic during the seven-day measurement period in December. Most of their messages were status reports sent to the NCC. (Those IMPs, for which the frequency of messages to the NCC-TIP is less than 2.2 messages per minute, were down for some time during the measurement period).

Let us now attempt to give a few explanations for the overall increase in the mean round trip delay between August and December. These explanations may also help to understand the differences in the mean round trip delays for any given source IMP-destination IMP pair as observed in Table 4.

1. Frequency of routing messages. Routing messages are the major

  source of queuing delay in a very lightly loaded net.  In August, a
  routing message was sent every 640 msec.  Since a routing message is
  1160 bits long, 3.625 percent of the bandwidth of a 50 kbs circuit
  was used for the sending of routing messages.  For randomly arriving
  packets this corresponds to a mean queuing delay of 0.42 msec per
  hop.  Between August and December the frequency of sending routing
  messages was made dependent on line speed and line utilization.  As
  a result, routing messages are now sent on a 50 kbs circuit with
  zero load every 128 msec.  This corresponds to a line utilization of
  18.125 percent and a mean queuing delay of 2.10 msec.  The queuing
  delay due to routing messages in a very lightly loaded net in
  December was therefore five times as large as it was in August.

2. Traffic matrix. The overall mean round trip delay depends on the

  traffic matrix.  If most of the messages are sent over distances of
  0 or 1 hop the overall round trip delay will be small.  The heavy
  traffic between AMES and AMST over a high-speed circuit in August
  contributed to the small overall mean round trip delay.

3. Network topology. The mean round trip delay depends on the number

  of hops between source-IMP and destination-IMP and therefore on the
  network topology.  Disregarding line or IMP failures, the mean
  number of hops for a message in August and December was,
  respectively, 3.24 and 4.02.

Naylor & Opderbeck [Page 5] RFC 619 Mean Round-Trip Times in the ARPANET March 1974

4. Averaging. The network load, given in number or messages per

  minute, represents an average over a seven-day period.  Even though
  this number may be small, considerable queuing delays could have
  been incurred during bursts of traffic.

5. Host delays. The round trip delay includes the transmission delay

  of the first packet from the destination-IMP to the destination-
  HOST; therefore, the mean round trip delay may be influenced by HOST
  delays that are independent of the network load.

Naylor & Opderbeck [Page 6] RFC 619 Mean Round-Trip Times in the ARPANET March 1974

                 Table 1 Mean Round Trip Delay as a
                   Function of the Number of Hops
    #MESSAGES/MINUTE  #SITE PAIRS     MEAN ROUND TRIP DELAY

HOPS AUG DEC AUG DEC AUG DEC DEC

                                              WITH   W/OUT
                                              STAT   STAT
                                              RPTS   RPTS

O 646.9 378.3 39 45 27 44 41

1 487.6 288.7 86 100 25 65 50

2 191.0 143.1 118 138 70 119 80

3 380.7 226.9 148 168 95 131 112

4 218.5 274.1 176 196 102 167 119

5 276.3 185.6 204 228 109 217 134

6 183.8 136.3 210 258 175 355 167

7 333.6 212.7 218 256 178 301 240

8 156.7 161.1 160 234 222 365 241

9 59.0 160.3 102 208 270 308 218

10 0.6 29.9 40 124 331 939 410

11 1.0 18.9 20 46 344 998 699

12 - 10.2 - 20 - 992 655

13 - 0.01 - 4 - 809 809

Naylor & Opderbeck [Page 7] RFC 619 Mean Round-Trip Times in the ARPANET March 1974

   Table 2  Mean Round Trip Delays for Messages from a Given Site
             #MESSAGES/MINUTE       MEAN ROUND TRIP DELAY
  SITE      AUGUST    DECEMBER   AUGUST   DECEMBER  DECEMBER
                                              WITH   WITHOUT
                                            STATUS    STATUS
                                           REPORTS   REPORTS

1 UCLA 50.7 40.3 130 282 165 2 SRI 377.3 147.9 45 189 174 3 UCSB 80.2 70.3 120 221 161 4 UTAH 27.0 46.2 136 247 169 5 BBN 120.4 128.3 110 133 133 6 MIT 120.6 96.9 126 160 150 7 RAND 29.3 34.2 127 323 208 8 SDC 1.7 2.4 521 2068 131 9 HARV 50.3 96.0 105 88 72 10 LL 4.4 6.7 201 602 187 11 STAN 49.7 39.7 173 300 191 12 ILL 26.8 53.4 158 216 165 13 CASE 57.6 2.5 138 1592 335 14 CMU 61.1 59.5 153 220 170 15 AMES 242.4 114.1 43 120 81 16 AMST 304.0 163.0 39 94 67 17 MTRT 89.5 60.0 126 199 142 18 RADT 27.7 29.1 145 273 160 19 NBST 98.4 48.2 118 213 152 20 ETAT 24.1 20.6 119 280 119 21 LLL - 6.8 - 721 169 22 ISI 372.0 304.4 110 147 142 23 USCT 298.1 210.3 60 92 70 24 GWCT 10.5 14.1 144 381 102 25 DOCT 5.5 7.0 236 791 171 26 SDAT 14.7 22.9 164 322 177 27 BELV 1.3 2.4 243 1469 466 28 ARPT 57.9 64.3 84 150 93 29 ABRD 1.3 2.4 183 1402 554 30 BBNT 40.8 10.0 75 372 124 31 CCAT 177.7 86.7 83 147 115 32 XROX 56.8 71.7 79 136 78 33 FNWT 2.3 3.5 347 1466 174 34 LBL 1.2 2.7 384 1653 621 35 UCSD 11.9 19.3 237 413 205 36 HAWT 27.5 5.2 654 569 476 37 RMLT 10.4 13.0 122 387 97 40 NCCT - 59.3 - 110 97 41 NSAT 0.6 3.4 1022 1870 1056 42 LONT - 20.8 - 998 848 43 TYMT - 3.7 - 1352 157

Naylor & Opderbeck [Page 8] RFC 619 Mean Round-Trip Times in the ARPANET March 1974

44 MIT2 - 5.6 - 720 100 45 MOFF - 2.4 - 1982 447 46 RUTT - 22.4 - 271 153 47 WPAT - 2.7 - 1399 380

Naylor & Opderbeck [Page 9] RFC 619 Mean Round-Trip Times in the ARPANET March 1974

    Table 3  Mean Round Trip Delay for Messages to a Given Site
             #MESSAGES/MINUTE    MEAN ROUND TRIP DELAY
  SITE      AUGUST    DECEMBER    AUGUST    DECEMBER

1 UCLA 57.1 43.5 134 209 2 SRI 382.3 149.4 45 158 3 UCSB 61.1 59.1 117 138 4 UTAH 28.1 50.4 128 159 5 BBN 160.8 149.2 185 110 6 MIT 150.4 107.1 116 130 7 RAND 22.6 25.0 95 161 8 SDC 1.7 0.8 149 174 9 HARV 59.3 98.3 101 70 10 LL 4.6 5.2 195 202 11 STAN 65.3 40.6 135 162 12 ILL 29.1 69.8 156 149 13 CASE 52.6 4.0 127 262 14 CMU 74.8 68.9 135 165 15 AMES 210.3 117.2 40 75 16 AMST 316.7 135.0 38 86 17 MTRT 77.7 51.7 130 151 18 RADT 23.4 23.9 142 202 19 NBST 92.2 39.5 125 169 20 ETAT 25.4 22.8 110 111 21 LLL - 3.7 - 185 22 ISI 361.9 299.2 107 130 23 USCT 298.1 190.6 60 68 24 GWCT 10.5 7.3 144 122 25 DOCT 5.5 4.2 236 187 26 SDAT 13.3 19.7 149 177 27 BELV 0.9 0.9 196 285 28 ARPT 55.4 58.3 78 95 29 ABRD 1.3 0.7 183 271 30 BBNT 40.8 6.4 75 159 31 CCAT 177.7 76.3 83 119 32 XROX 56.8 75.3 79 69 33 FNWT 2.3 1.4 347 165 34 LBL 1.2 0.9 384 305 35 UCSD 11.9 24.0 237 157 36 HAWT 27.5 5.0 654 458 37 RMLT 10.4 11.0 122 97 40 NCCT - 140.1 - 1263 41 NSAT 0.6 1.6 1022 918 42 LONT - 17.3 - 855 43 TYMT - 1.6 - 160 44 MIT2 - 3.9 - 83 45 MOFF - 0.2 - 219 46 RUTT - 14.7 - 153 47 WPAT - 0.5 - 282

Naylor & Opderbeck [Page 10] RFC 619 Mean Round-Trip Times in the ARPANET March 1974

  Table 4  Mean Round Trip Delay to the Three Most Favorite Sites
                          #MESSAGES/MINUTE  MEAN ROUND TRIP DELAY

FROM SITE TO SITE AUGUST DECEMBER AUGUST DECEMBER

1 UCLA 1 RAND 10.8 9.4 57 92

              26 SDAT      5.6         5.9    157     191
              22 ISI       3.1         3.1     99     146

2 SRI 12 RADT 16.6 19.5 142 163

              17 MTRT     21.9        18.7    140     161
               2 SRI     266.1        17.5     14      69

3 UCSB 2 SRI 8.1 17.8 72 68

              22 ISI      18.1        17.0     75      86
              14 CMU      16.6        11.8    140     152

4 UTAH 4 UTAH 3.5 13.5 136 27

              22 ISI       3.7         4.8    131     165
               5 BBN       4.2         4.1    168     204

5 BBN 40 NCCT - 81.4 - 105

               5 BBN      12.5        19.7    102      37
               9 HARV      0.5         9.2     22      37

6 MIT 6 MIT 40.6 24.0 81 85

              23 USCT      9.8        13.9    150     173
               9 HARV      1.7        12.0     63      88

7 RAND 1 UCLA 12.5 10.4 54 96

              16 AMST      0.8         2.6     99     190
              40 NCCT      -           2.5      -    1941

8 SDC 40 NCCT - 2.2 - 2217

               1 UCLA      0.2         0.2    110     136
               8 SDC       0.01        0.01    93      13

9 HARV 9 HARV 7.6 50.5 49 21

               2 MIT       1.6        11.9     62      85
               5 BBN       1.6         9.5     56      37

10 LL 40 NCCT - 2.2 - 1420

              10 LL        1.5         1.8    238     135
              24 GWCT      0.04        0.6    146      80

11 STAN 14 CMU 3.0 7.0 215 207

               4 UTAH      0.2         5.5    117     117
               6 MIT       6.5         5.0    186     225

Naylor & Opderbeck [Page 11] RFC 619 Mean Round-Trip Times in the ARPANET March 1974

12 ILL 22 ISI 13.3 20.3 146 142

              15 AMES      0.8        14.6    109     135
              35 UCSD      6.7         6.5    192     269

13 CASE 40 NCCT - 2.2 - 1744

               1 UCLA      0.2         0.2    296     400
               2 SRI       7.1         0.01   163     316

14 CMU 14 CMU 13.8 23.4 129 94

               3 UCSB     13.8         9.2    153     166
              11 STAN      3.2         5.1    193     209

15 AMES 16 AMST 205.0 65.8 15 34

              12 ILL       1.2        19.6    115     120
              31 CCAT      3.2         4.6    174     230

16 AMST 15 AMES 176.8 74.3 13 28

              22 ISI      63.6        33.2     50      69
              32 XROX     13.3        17.4     41      60

17 MTRT 22 ISI 26.3 27.5 115 118

               2 SRI      23.8        20.3    137     155
               5 BBN       3.5         4.2    179     133

18 RADT 2 SRI 17.7 21.7 139 156

               1 UCLA      0.4         2.3    265     181
              40 NCCT      -           2.3      -    1618

19 NBST 2 SRI 14.1 12.1 132 163

              22 ISI      29.6        11.8    100     117
               5 BBN      21.6         9.6     71      97

20 ETAT 22 ISI 11.9 11.3 106 107

              24 GWCT      5.0         5.9     99     107
              40 NCCT      -           2.2      -    1602

21 LLL 5 BBN - 2.9 - 183

              40 NCCT      -           2.2      -    1847
               4 UTAH      -           0.5      -      71

22 ISI 28 ARPT 26.0 38.3 106 104

              23 USCT     69.0        32.7     80      92
              16 AMST     62.0        28.5     53      87

23 USCT 23 USCT 160.9 119.2 19 23

              22 ISI      69.2        34.1     78      91
               6 MIT      12.9        19.6    135     150

Naylor & Opderbeck [Page 12] RFC 619 Mean Round-Trip Times in the ARPANET March 1974

24 GWCT 20 ETAT 6.6 10.8 93 91

              40 NCCT      -           2.1      -    1978
              10 LL        0.03        0.5    359     115

25 DOCT 40 NCCT - 2.3 - 2091

              22 ISI       1.0         1.6    220     118
              15 AMES      1.9         1.2    167     198

26 SDAT 22 ISI 2.9 8.7 154 138

               1 UCLA      5.9         6.0    169     209
               2 SRI       1.0         4.4    182     184

27 BELV 40 NCCT - 2.2 - 1553

               1 UCLA      0.1         0.2    405     517
              22 ISI       -           0.01     -     325

28 ARPT 22 ISI 27.4 41.6 106 101

              28 ARPT     19.2        13.7     20      35
               2 SRI       3.3         3.3    139     157

29 ABRD 40 NCCT - 2.2 - 1461

               1 UCLA      0.2         0.2    439     562
               9 HARV      -           0.01     -     112

30 BBNT 5 BBN 24.2 5.1 36 64

              40 NCCT      -           2.1      -    1327
              22 ISI       4.2         1.1    170     217

31 CCAT 31 CCAT 81.9 28.2 15 31

              22 ISI      31.3        23.3    156     171
               5 BBN       7.8         7.3     45      42

32 XROX 32 XROX 20.2 36.4 19 15

              16 AMST     10.5        13.3     69      93
              14 CMU       2.5         3.0    193     251

33 FNWT 40 NCCT - 2.2 - 2210

               9 HARV      0.01        0.3    208     194
               7 RAND      0.3         0.3     96     171

34 LBL 40 NCCT - 2.4 - 1814

              41 NSAT      -           0.2      -    1674
               1 UCLA      0.1         0.2    295     478

35 UCSD 12 ILL 6.0 7.5 220 260

              16 AMST      1.7         4.9    120     172
              40 NCCT      -           2.0      -    2183

Naylor & Opderbeck [Page 13] RFC 619 Mean Round-Trip Times in the ARPANET March 1974

36 HAWT 36 HAWT 0.04 1.6 17 26

              22 ISI       5.1         1.0    600     623
              15 AMES      2.5         0.8    551     590

37 RMLT 22 ISI 7.5 9.0 68 67

              40 NCCT      -           2.2      -    1918
              28 ARPT      -           1.0      -      63

40 NCCT 5 BBN - 41.2 - 33

              40 NCCT      -           6.6      -     433
              22 ISI       -           3.2      -     151

41 NSAT 40 NCCT - 2.2 - 2308

               2 SRI       0.01        0.4   1046    1002
               3 UCSB      0.01        0.2   1169    1018

42 LONT 22 ISI - 6.1 - 837

               2 SRI       -           3.7      -     884
               4 UTAH      -           2.2      -     921

43 TYMT 40 NCCT - 2.6 - 1859

               2 SRI       -           0.5      -      79
               3 UCSB      -           0.2      -      74

44 MIT2 44 MIT2 - 2.8 - 18

              40 NCCT      -           2.3      -    1664
               1 UCLA      -           0.2      -     589

46 MOFF 40 NCCT - 2.2 - 2091

               1 UCLA      -           0.2      -     447

46 RUTT 9 HARV - 4.3 - 38

               5 BBN       -           3.5      -      93
              22 ISI       -           2.9      -     172

47 WPAT 40 NCCT - 2.2 - 1643

               3 UCSB      -           0.2      -     301
               1 UCLA      -           0.2      -     671
     [ This RFC was put into machine readable form for entry ]
     [ into the online RFC archives by Alex McKenzie with    ]
     [ support from GTE, formerly BBN Corp.            12/99 ]

Naylor & Opderbeck [Page 14]

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