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Network Working Group W. NaylorRequest for Comment: 619 H. OpderbeckNIC 21990 UCLA-NMC March 7, 1974Mean Round-Trip Times in the ARPANETIn one of our current measurement projects we are interested in theaverage values of important network parameters. For this purpose wecollect data on the network activity over seven consecutive days. Thisdata collection is only interrupted by down-time or maintenance ofeither the net or our collecting facility (the "late" Sigma-7 or, infuture, the 360/91 at CCN).The insight gained from the analysis of this data has been reported inNetwork 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) thatwere observed during these week-long measurements since we think thesefigures are of general interest to the ARPA community. Let us firstdefine the term "round trip time" as it is used by the statisticsgathering program in the IMPs. When a message is sent from a sourceHOST to a destination HOST, the following events, among others, can bedistinguished (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 IMPNaylor & 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 HOSTThe time intervals T(i)-T(i-1) are mainly due to the following delaysand 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 IMPNaylor & 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 RFNMIMP processing delays are not included in this table since they areusually very small. Also, some of the abovementioned waiting timesreduce to zero in many cases, e.g. the waiting time for a message numberto become available and the waiting time for a buffer to become free.If the source and destination HOSTs are attached to the same IMP, thistable 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 beforeUp to now we have neglected the possibility that a single packet messageis rejected at the destination IMP because of lack of reassembly space.If this occurs, the single packet message is treated as a request forbuffer space allocation and the time interval T(3)-T(2) increased by thewaiting 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 waitingtime until the ALLOCATE is received. It does, however, not include thesource HOST processing delay (i.e. delays in the NCP), the HOST-IMPtransmission delay, and the waiting time until a message number becomesavailable. Note also, that the RFNM is sent after the first packet of amultiple packet message has been received by the destination HOST.Let us now turn to the presentation of the average round trip times asthey were measured during continuous seven-day periods in August andDecember '73. In August, an average number of 2935 messages/minute wereentering the ARPANET. The overall mean round trip delay for all thesemessages was 93 milliseconds (msec). The corresponding numbers forDecember were 2226 messages/minute and 200 msec. An obvious questionthat immediately arises is: why did the average round trip delay morethan double while the rate of incoming messages decreased? The answerto this question can be found in the large round trip delays for thestatus 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 thereNaylor & Opderbeck [Page 3]
RFC 619 Mean Round-Trip Times in the ARPANET March 1974were 45 sites connected to the net in December, a total of 103.05 statusreports per minute were sent to the NCC. Thus 4.63 percent of allmessages that entered the net were status reports.The average round trip delay for all these status reports in Decemberwas 1.66 sec. This number is five to ten times larger than the averageround-trip delay for status reports we observed in August. It is notyet clear what change in the collection of status reports caused thisincrease. One reason appears to be that the number of these reports wasdoubled between August and December. Since the large round-trip delaysof these status reports distort the overall picture somewhat, we aregoing to present the December data - wherever appropriate - with andwithout the effect of these delays. (We should point out here that thetraffic/delay picture is distorted by the accumulated statisticsmessages which were collected to produce this data. We have, however,ignored this effect since these measurement messages represent less than0.3% of the total traffic.) The overall mean round trip delay withoutthe status reports in December is 132 msec. This value is still morethan 35 msec larger than the corresponding value for August. However,before we shall attempt to explain this difference we will first presentthe measured data.Table 1 shows the mean round trip delay as a function of the number ofhops over the minimum-hop path. This minimum number of hops wascalculated from the (static) topology of the net as it existed in Augustand December of last year. The actual number of hops over which anygiven message travels may, of course, be larger due to networkcongestion, line failures or IMP failures. In fact, for August weobserved a minimum mean path length of 3.24 while the actual measuredmean path length was 3.30; in December we observed 4.02 and 4.40,respectively. (See Network Measurement Group Note #18 for anexplanation of the computation of actual mean path length.) As expectedwe observe a sharp increase of the mean round trip delay as the minimumnumber of hops is increased. Note, however, that the mean round tripdelay is not a strictly increasing function of the minimum number ofhops.Table 2 gives the mean round trip delay for messages from a given site.The December data is presented with and without the large delaysincurred by the sending of status reports to the NCC. Table 3 shows themean round trip delay for messages to a given site. The largest roundtrip delays, in December, were incurred by messages sent to the NCC-TIPsince these messages include all the status reports.Table 4, finally, gives for each site the mean round trip delays tothose three destination IMP/TIP's to which the most messages were sentduring the seven-day measurement period in December. Let us first sayfew words about the traffic distribution which is dealt with in moreNaylor & Opderbeck [Page 4]
RFC 619 Mean Round-Trip Times in the ARPANET March 1974detail in Network Measurement Group Note #18. There are several siteswhich like to use their IMP as a kind of local multiplexer (UTAH, MIT,HARV, CMU, USCT, CCAT, XROX, HAWT, MIT2). For these sites the mostfavorite destination site is the source IMP itself. For several othersites the most favorite destination site is just one hop away (BBN,AMES, AMST, NCCT, RUTT). Nobody will be surprised that for many sitesISI (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 wererather inactive in terms of generating traffic during the seven-daymeasurement period in December. Most of their messages were statusreports sent to the NCC. (Those IMPs, for which the frequency ofmessages to the NCC-TIP is less than 2.2 messages per minute, were downfor some time during the measurement period).Let us now attempt to give a few explanations for the overall increasein the mean round trip delay between August and December. Theseexplanations may also help to understand the differences in the meanround trip delays for any given source IMP-destination IMP pair asobserved 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 19744. 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 DELAYHOPS AUG DEC AUG DEC AUG DEC DEC WITH W/OUT STAT STAT RPTS RPTSO 646.9 378.3 39 45 27 44 411 487.6 288.7 86 100 25 65 502 191.0 143.1 118 138 70 119 803 380.7 226.9 148 168 95 131 1124 218.5 274.1 176 196 102 167 1195 276.3 185.6 204 228 109 217 1346 183.8 136.3 210 258 175 355 1677 333.6 212.7 218 256 178 301 2408 156.7 161.1 160 234 222 365 2419 59.0 160.3 102 208 270 308 21810 0.6 29.9 40 124 331 939 41011 1.0 18.9 20 46 344 998 69912 - 10.2 - 20 - 992 65513 - 0.01 - 4 - 809 809Naylor & 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 7210 LL 4.4 6.7 201 602 18711 STAN 49.7 39.7 173 300 19112 ILL 26.8 53.4 158 216 16513 CASE 57.6 2.5 138 1592 33514 CMU 61.1 59.5 153 220 17015 AMES 242.4 114.1 43 120 8116 AMST 304.0 163.0 39 94 6717 MTRT 89.5 60.0 126 199 14218 RADT 27.7 29.1 145 273 16019 NBST 98.4 48.2 118 213 15220 ETAT 24.1 20.6 119 280 11921 LLL - 6.8 - 721 16922 ISI 372.0 304.4 110 147 14223 USCT 298.1 210.3 60 92 7024 GWCT 10.5 14.1 144 381 10225 DOCT 5.5 7.0 236 791 17126 SDAT 14.7 22.9 164 322 17727 BELV 1.3 2.4 243 1469 46628 ARPT 57.9 64.3 84 150 9329 ABRD 1.3 2.4 183 1402 55430 BBNT 40.8 10.0 75 372 12431 CCAT 177.7 86.7 83 147 11532 XROX 56.8 71.7 79 136 7833 FNWT 2.3 3.5 347 1466 17434 LBL 1.2 2.7 384 1653 62135 UCSD 11.9 19.3 237 413 20536 HAWT 27.5 5.2 654 569 47637 RMLT 10.4 13.0 122 387 9740 NCCT - 59.3 - 110 9741 NSAT 0.6 3.4 1022 1870 105642 LONT - 20.8 - 998 84843 TYMT - 3.7 - 1352 157Naylor & Opderbeck [Page 8]
RFC 619 Mean Round-Trip Times in the ARPANET March 197444 MIT2 - 5.6 - 720 10045 MOFF - 2.4 - 1982 44746 RUTT - 22.4 - 271 15347 WPAT - 2.7 - 1399 380Naylor & 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 7010 LL 4.6 5.2 195 20211 STAN 65.3 40.6 135 16212 ILL 29.1 69.8 156 14913 CASE 52.6 4.0 127 26214 CMU 74.8 68.9 135 16515 AMES 210.3 117.2 40 7516 AMST 316.7 135.0 38 8617 MTRT 77.7 51.7 130 15118 RADT 23.4 23.9 142 20219 NBST 92.2 39.5 125 16920 ETAT 25.4 22.8 110 11121 LLL - 3.7 - 18522 ISI 361.9 299.2 107 13023 USCT 298.1 190.6 60 6824 GWCT 10.5 7.3 144 12225 DOCT 5.5 4.2 236 18726 SDAT 13.3 19.7 149 17727 BELV 0.9 0.9 196 28528 ARPT 55.4 58.3 78 9529 ABRD 1.3 0.7 183 27130 BBNT 40.8 6.4 75 15931 CCAT 177.7 76.3 83 11932 XROX 56.8 75.3 79 6933 FNWT 2.3 1.4 347 16534 LBL 1.2 0.9 384 30535 UCSD 11.9 24.0 237 15736 HAWT 27.5 5.0 654 45837 RMLT 10.4 11.0 122 9740 NCCT - 140.1 - 126341 NSAT 0.6 1.6 1022 91842 LONT - 17.3 - 85543 TYMT - 1.6 - 16044 MIT2 - 3.9 - 8345 MOFF - 0.2 - 21946 RUTT - 14.7 - 15347 WPAT - 0.5 - 282Naylor & 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 DELAYFROM 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 3710 LL 40 NCCT - 2.2 - 1420 10 LL 1.5 1.8 238 135 24 GWCT 0.04 0.6 146 8011 STAN 14 CMU 3.0 7.0 215 207 4 UTAH 0.2 5.5 117 117 6 MIT 6.5 5.0 186 225Naylor & Opderbeck [Page 11]
RFC 619 Mean Round-Trip Times in the ARPANET March 197412 ILL 22 ISI 13.3 20.3 146 142 15 AMES 0.8 14.6 109 135 35 UCSD 6.7 6.5 192 26913 CASE 40 NCCT - 2.2 - 1744 1 UCLA 0.2 0.2 296 400 2 SRI 7.1 0.01 163 31614 CMU 14 CMU 13.8 23.4 129 94 3 UCSB 13.8 9.2 153 166 11 STAN 3.2 5.1 193 20915 AMES 16 AMST 205.0 65.8 15 34 12 ILL 1.2 19.6 115 120 31 CCAT 3.2 4.6 174 23016 AMST 15 AMES 176.8 74.3 13 28 22 ISI 63.6 33.2 50 69 32 XROX 13.3 17.4 41 6017 MTRT 22 ISI 26.3 27.5 115 118 2 SRI 23.8 20.3 137 155 5 BBN 3.5 4.2 179 13318 RADT 2 SRI 17.7 21.7 139 156 1 UCLA 0.4 2.3 265 181 40 NCCT - 2.3 - 161819 NBST 2 SRI 14.1 12.1 132 163 22 ISI 29.6 11.8 100 117 5 BBN 21.6 9.6 71 9720 ETAT 22 ISI 11.9 11.3 106 107 24 GWCT 5.0 5.9 99 107 40 NCCT - 2.2 - 160221 LLL 5 BBN - 2.9 - 183 40 NCCT - 2.2 - 1847 4 UTAH - 0.5 - 7122 ISI 28 ARPT 26.0 38.3 106 104 23 USCT 69.0 32.7 80 92 16 AMST 62.0 28.5 53 8723 USCT 23 USCT 160.9 119.2 19 23 22 ISI 69.2 34.1 78 91 6 MIT 12.9 19.6 135 150Naylor & Opderbeck [Page 12]
RFC 619 Mean Round-Trip Times in the ARPANET March 197424 GWCT 20 ETAT 6.6 10.8 93 91 40 NCCT - 2.1 - 1978 10 LL 0.03 0.5 359 11525 DOCT 40 NCCT - 2.3 - 2091 22 ISI 1.0 1.6 220 118 15 AMES 1.9 1.2 167 19826 SDAT 22 ISI 2.9 8.7 154 138 1 UCLA 5.9 6.0 169 209 2 SRI 1.0 4.4 182 18427 BELV 40 NCCT - 2.2 - 1553 1 UCLA 0.1 0.2 405 517 22 ISI - 0.01 - 32528 ARPT 22 ISI 27.4 41.6 106 101 28 ARPT 19.2 13.7 20 35 2 SRI 3.3 3.3 139 15729 ABRD 40 NCCT - 2.2 - 1461 1 UCLA 0.2 0.2 439 562 9 HARV - 0.01 - 11230 BBNT 5 BBN 24.2 5.1 36 64 40 NCCT - 2.1 - 1327 22 ISI 4.2 1.1 170 21731 CCAT 31 CCAT 81.9 28.2 15 31 22 ISI 31.3 23.3 156 171 5 BBN 7.8 7.3 45 4232 XROX 32 XROX 20.2 36.4 19 15 16 AMST 10.5 13.3 69 93 14 CMU 2.5 3.0 193 25133 FNWT 40 NCCT - 2.2 - 2210 9 HARV 0.01 0.3 208 194 7 RAND 0.3 0.3 96 17134 LBL 40 NCCT - 2.4 - 1814 41 NSAT - 0.2 - 1674 1 UCLA 0.1 0.2 295 47835 UCSD 12 ILL 6.0 7.5 220 260 16 AMST 1.7 4.9 120 172 40 NCCT - 2.0 - 2183Naylor & Opderbeck [Page 13]
RFC 619 Mean Round-Trip Times in the ARPANET March 197436 HAWT 36 HAWT 0.04 1.6 17 26 22 ISI 5.1 1.0 600 623 15 AMES 2.5 0.8 551 59037 RMLT 22 ISI 7.5 9.0 68 67 40 NCCT - 2.2 - 1918 28 ARPT - 1.0 - 6340 NCCT 5 BBN - 41.2 - 33 40 NCCT - 6.6 - 433 22 ISI - 3.2 - 15141 NSAT 40 NCCT - 2.2 - 2308 2 SRI 0.01 0.4 1046 1002 3 UCSB 0.01 0.2 1169 101842 LONT 22 ISI - 6.1 - 837 2 SRI - 3.7 - 884 4 UTAH - 2.2 - 92143 TYMT 40 NCCT - 2.6 - 1859 2 SRI - 0.5 - 79 3 UCSB - 0.2 - 7444 MIT2 44 MIT2 - 2.8 - 18 40 NCCT - 2.3 - 1664 1 UCLA - 0.2 - 58946 MOFF 40 NCCT - 2.2 - 2091 1 UCLA - 0.2 - 44746 RUTT 9 HARV - 4.3 - 38 5 BBN - 3.5 - 93 22 ISI - 2.9 - 17247 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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