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---
title: "Packet Latency"
weight: 2
---
# 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)](http://hdrhistogram.org/)
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%).
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