Dell PowerConnect J-EX4200-24T Software Manual page 3478

Dell PowerConnect J-Series Ethernet Switch Complete Software Guide for Junos OS
3406
Figure 87: RPM Timestamps
T1 is the time the packet leaves the requester port.
T2 is the time the responder receives the packet.
T3 is the time the responder sends the response.
T4 is the time the requester receives the response.
The round-trip time is (T2 – T1) + (T4 – T3). If the responder does not support hardware
timestamps, then the round-trip time is (T4 – T1) / 2, and thus includes the processing
time of the responder.
You can use RPM probes to find the following time measurements:
Minimum round-trip time
Maximum round-trip time
Average round-trip time
Standard deviation of the round-trip time
Jitter of the round-trip time—Difference between the minimum and maximum round-trip
time
NOTE: Configure hardware timestamps on the requester by including the
hardware-timestamp
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test test-name
The RPM feature provides a configuration option to set one-way hardware timestamps.
Use one-way timestamps when you want information about one-way time, rather than
round-trip times, for packets to traverse the network between the requester and the
responder. As shown in Figure 87 on page 3406, one-way timestamps represent the time
T2 – T1 and the time from T4 – T3. Use one-way timestamps when you want to gather
information about delay in each direction and to find egress and ingress jitter values.
NOTE: For correct one-way measurement, the clocks of the requester and
responder must be synchronized. If the clocks are not synchronized, one-way
jitter measurements and calculations can include significant variations, in
some cases orders of magnitude greater than the round-trip times.
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