Discovering units of metadata reported by INT switches

[mhira1]

Certain metadata reported by INT switches, such as Timestamp and Queue occupancy may have different units at different hops. For example, some switches may report timestamp in nanoseconds, while others report in microseconds. Some switches may report queue occupancy in bytes while others may report in number of cells. Cell size in bytes may also vary from one switch to another.

The monitoring system needs to identify units reported by each switch.

[jafingerhut]

And some timestamps may be in neither of those units, at least as currently allowed in the latest draft PSA specification.

e.g. a device with a clock rate of 1.25 GHz might report timestamps in units of clock cycles that increment 1.25 billion times per second.

One way to specify the rate of timestamp advance in a very precise way, without resorting to floating point numbers, is to use integer ratios, e.g. something like this:

  // The timestamp value of this device increments
  // `increments_per_period` times every `period_in_seconds` seconds.
  uint64 increments_per_period = 2;
  uint64 period_in_seconds = 3;

That allows clock speeds like 1.333... GHz to be specified as increments_per_period = 4,000,000,000 and period_in_seconds = 3.