Latency is the time it takes for a packet to get from source to destination. It’s also known as one-way delay.

To measure latency, APM sends out multiple packet trains every 1 minute, each with a specific packet size and number of packets per train. Among all of those test packets, the one with the fastest round-trip time is used to calculate latency. Latency is equal to one half the round-trip time.

Latency is important to TCP apps and time sensitive UDP apps like real-time voice and video streaming. Latency has a severe, compounding effect on TCP because of how its congestion control mechanism works. Basically, TCP throughput takes a nose dive in the presence of high latency. Modern video streaming services like Youtube and Netflix use TCP. VOIP is the primary example of a time sensitive UDP app. Large latencies here can introduce both conversational difficulty and packet loss.

There are several ways latency gets introduced into your data stream. Propagation delay is the time it takes for packets to travel on wire to the destination. The farther apart the source and destination, the greater the propagation delay. Queuing delay is introduced when a network hop is congested and can’t route incoming packets immediately upon ingress. There’s also handing delay which is the time it takes it takes to put a packet on the wire, but generally that is out of the user’s control and negligible compared to the other two.

High latency can be caused by any number of things, see the following table for guidance.

Cause Try this…
Network congestion Use Delivery monitoring to identify low capacity links. Use flow analysis to try and free up the network.
Geographic distance Consider using an overlay network to bring source and destination closer together.
Sub-optimal routes Check your routing config to ensure low latency routes are preferred. For routes over the public Internet, run a diagnostic to identify poor performing hops, then contact your ISP for help.
No QoS or misconfigured QoS Make sure you use traffic shaping techniques to prioritize traffic, and that your traffic shaping config has the intended effect.
Add-on features Turn off services like proxy and VPN, and smart features like DPI and ACLs to see if they’re introducing appreciable latency.