Subject: N-1-4-040.92 What's the response time ? by Frode Greisen and Greg Lloyd Most network providers give a very good response time and availability, both for interactive work, file transfer and electronic mail. That is, on the days when everything is working well. Users develop an intuitive feeling of the quality of the network which may be somewhat dependent on temper. Some people tend to remember the normal days when things went fine while others concentrate on that mail which stayed in a postmaster's mailbox for a week or two before it was manually forwarded or the mail that never got through, either because it really was lost or because the recipient cancelled it by mistake. In order to get an objective picture, EARN has been monitoring its network's performance along three scales for the last five months. These are link availability, link file-queues and round trip times (RTTs) for both files and interactive messages. These measurements are designed to provide not only technical performance data for the network, but also to evaluate the quality of the network as perceived by a typical end-user. EARN monitors its network links, file-queues and message RTTs down to its international level. That is, each member country subscribed to the EARN Association has designated one international node that acts as that country's gateway into the international network. A subset of these international nodes have been selected as the EARN backbone and make up the EARN Core nodes. The remaining international nodes are allocated into regions, each region being serviced by a specific EARN Core node. In addition to collecting figures relating solely to its own international backbone, round trip time figures are also recorded for EARN's transatlantic links with the BITNET network. These files traverse a section of the BITNET backbone, cross the Atlantic and enter the EARN backbone and are subsequently returned to the USA. File RTTs are measured down to an inter-regional level (across the EARN backbone). They are measured for two different file sizes; the first is 50 records files (representative of a typical piece of electronic mail) and the second, 1001 records files (representative of a medium sized data file). Every hour around the clock, seven days a week, these files are transmitted and returned from each EARN core site to every other EARN core site over the backbone. The time in seconds taken for this round trip is recorded and used to calculate the averages displayed below. Avg RTTs Avg RTTs (1001 recs) (50 recs) (seconds) (seconds) June '92 525 423 July '92 564 459 August '92 521 502 September '92 464 307 October '92 410 362 The above figures show the round trip times for differently sized files to traverse and return over the EARN backbone, EARN's transatlantic links to the USA and a section of the BITNET backbone. In ideal conditions, these figures range from 12 to 19 seconds for 1001 files and 6 to 8 seconds for 50 records files. But as can be seen, the cases where a node is down over the week-end or when any of the intermediary links between sender and receiver has a failure seriously affects the average. The numbers above are for two hops over the backbone. Now, the average number of hops between any two of the 3389 EARN/BITNET nodes in 48 countries is 6.7. So if we divide the measurements by two and multiply by 6.7 we get the likely average transit time for an e-mail of around 20 minutes. This is not perfect, and we are working to improve it, but it is better than postal services.