Internet Delay Measurements using Test Traffic

Our project focuses on obtaining information about the networks between Internet Service Providers (ISP's), although the experimental techniques can also be used for measuring the properties of the network of a single ISP. At the moment, the project is still in a development phase and some experimental effects still have to be understood. However, once the development phase is over, we will start to provide ISP's with (almost) real-time information about the performance of their networks.

For our project, we developed test-boxes consisting of a PC running the FreeBSD operating system and a GPS-based clock module. The clock module is used to synchronize the local clock with an accuracy of down to 100 ns.

The software on the boxes can send data packets to other test-boxes. The packets are time-stamped at the application layer by the sending test-box, as well as at the data-link layer of the receiving test-box. From this, one can calculate the time that the packet needed to travel from one box to another. A test on an unloaded network showed that this setup can measure delays with accuracys of the order of 0.01 ms. Not all packets will arrive on the receiving host and by comparing the number of packets sent against the number of packets that arrive, one can determine the packet-loss rate.

The software on the test-boxes also determines the path that a data packet follows to travel from one test-box to another. This information is interesting in itself and has already been used to explain changes in the network performance.

At the moment, we are in the process of installing test-boxes at ISP's participating in this project. The first 4 test-boxes were installed at various sites in Holland and Germany in January 1998. Another 20 test-boxes will be installed during the spring of 1998.

The test-boxes that were in place in January have been used to do delay measurements on networks that are being used by the general public. Our measurements show that, although there can be differences of the order of 10 ms between 2 consecutive measurements, the distribution of the delays between two hosts over a period of several hours is stable. This observation can be used by network operators to detect both short- and long-term performance changes in their networks.

In this talk, we will describe the hardware and software of the project in more detail as well as present more results obtained with the test-boxes that have been installed so-far.