Two-way metrics give a partial information.
One-way metrics are difficult to measure.
One-way metrics are often based on the measurement of a one-way delay: the accuracy of its measurement depends on the accuracy of the local clocks.
When clocks are synchronized using NTP-like protocols, their accuracy is in its turn dependent on the roundtrip delay: the granularity of the one-way delay measurement in this case is of the same order of the measure (e.g., a measured delay of 1msec corresponds to an estimate in the range [0-2]msecs)!
An alternative consists in using specialized hardware, or use a proxy NTP server. But this is either expensive or impractical.
We show that there are one-way metrics that do not depend on the one
way delay: they can therefore be computed with acceptable accuracy
using the same link under test, and without additional hardware.
Next: One-way bandwidth