Monitoring > Tunnel Health > Latency > Summary
The Latency tab shows summary statistics for latency (transmission delay) on an in-band, end-to-end tunnel basis for the selected time/date range. Either overlay or underlay tunnels can be displayed, and anywhere between the top 10 to top 1000 tunnels are displayed by round-trip time (RTT).
On this tab, latency is a measure of the RTT within a tunnel in milliseconds. Values on the left display RTT as measured by the local appliance. Values on the right display RTT as measured by the appliance at the remote end of the tunnel.
Some column descriptions follow:
Std. Latency – Standard deviation (in milliseconds) of latency values for the tunnel within the specified period.
Standard deviation is a measure of the amount of variation in a set of values. Low standard deviation indicates that the values tend to be close to the mean or expected value while a high standard deviation indicates that the values are spread over a wider range.
Max Latency (ms) – Maximum RTT value (in milliseconds) for the tunnel within the specified range.
Avg Latency (ms) – Average RTT value (in milliseconds) for the tunnel within the specified range.
High latency can negatively affect throughput in the network, most noticeably for TCP traffic. Physical distance has the most significant impact on latency. For example:
If data is crossing the United States, you can expect delays from 60 to 120 milliseconds.
International transmissions can normally experience delays up to 200 milliseconds.
Satellite transmissions often have delays of about 1/2 second, and up to several seconds are possible.
High latency can also be caused by equipment (hop-by-hop delays), or by loss or congestion resulting from lost packets, lost acknowledgments, and necessary retransmissions.
TCP Acceleration (a function of Boost) can mitigate the impact of latency on throughput. In addition, path conditioning and packet re-ordering (a function of Business Intent Overlay link bonding) can mitigate the impact of loss and out-of-order packets on TCP throughput by reducing the number of retransmissions.