Packet Latency

Packet Latency #

TRex Traffic Generator (TG) is used for measuring one-way latency in 2-Node and 3-Node physical testbed topologies. TRex integrates High Dynamic Range Histogram (HDRH) functionality and reports per packet latency distribution for latency streams sent in parallel to the main load packet streams.

Following methodology is used:

  • Only NDRPDR test type measures latency and only after NDR and PDR values are determined. Other test types do not involve latency streams.

  • Latency is measured at different background load packet rates:

    • No-Load: latency streams only.
    • Low-Load: at 10% PDR.
    • Mid-Load: at 50% PDR.
    • High-Load: at 90% PDR.
  • Latency is measured for all tested packet sizes except IMIX due to TRex TG restriction.

  • TG sends dedicated latency streams, one per direction, each at the rate of 9 kpps at the prescribed packet size; these are sent in addition to the main load streams.

  • TG reports Min/Avg/Max and HDRH latency values distribution per stream direction, hence two sets of latency values are reported per test case (marked as E-W and W-E).

  • +/- 1 usec is the measurement accuracy of TRex TG and the data in HDRH latency values distribution is rounded to microseconds.

  • TRex TG introduces a (background) always-on Tx + Rx latency bias of 4 usec on average per direction resulting from TRex software writing and reading packet timestamps on CPU cores. Quoted values are based on TG back-to-back latency measurements.

  • Latency graphs are not smoothed, each latency value has its own horizontal line across corresponding packet percentiles.

  • Percentiles are shown on X-axis using a logarithmic scale, so the maximal latency value (ending at 100% percentile) would be in infinity. The graphs are cut at 99.9999% (hover information still lists 100%).