Name: Cisco Catalyst 2960 Series
Brand: Cisco
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while waiting to be processed. In this situation, the response times would not accurately represent true network

delays. IP SLAs minimize these processing delays on the source device as well as on the target device (if the

responder is being used) to determine true round-trip times. IP SLA test packets use time stamping to minimize

the processing delays.

When the IP SLA responder is enabled, it allows the target device to take time stamps when the packet arrives

on the interface at interrupt level and again just as it is leaving, eliminating the processing time. This time

stamping is made with a granularity of sub-milliseconds (ms).

The following figure demonstrates how the responder works. Four time stamps are taken to make the calculation

for round-trip time. At the target router, with the responder functionality enabled, time stamp 2 (TS2) is

subtracted from time stamp 3 (TS3) to produce the time spent processing the test packet as represented by

delta. This delta value is then subtracted from the overall round-trip time. Notice that the same principle is

applied by IP SLAs on the source router where the incoming time stamp 4 (TS4) is also taken at the interrupt

level to allow for greater accuracy.

Figure 115: Cisco IOS IP SLA Responder Time Stamping

An additional benefit of the two time stamps at the target device is the ability to track one-way delay, jitter,

and directional packet loss. Because much network behavior is asynchronous, it is critical to have these

statistics. However, to capture one-way delay measurements, you must configure both the source router and

target router with Network Time Protocol (NTP) so that the source and target are synchronized to the same

clock source. One-way jitter measurements do not require clock synchronization.

Cisco Catalyst 2960 Series User Manual

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