Post by fish on Mar 17, 2023 8:33:59 GMT 12
RTT (lets call it ping) is the length of time it takes a packet to reach it's destination PLUS the length of time it takes for an acknowledgement to come back.
latency is the length of time that ONE side of the RTT took. SO you have a latency there and a latency back.
Generally speaking you can divide the ping time in half and that will give you the latency, but strictly speaking that isn't legitimate as you could have an asymmetric path and one path could be congested, or one path could just be physically longer so the speed of light becomes the limiting factor.
delay is the length of time it takes for a message which consist of lots of packets to arrive at the destination
jitter is technically the average of the absolute difference between latency samples - so if you have 1 packet that takes 10ms and the next takes 15ms and the next takes 12ms then it's (|(10-15)| + |(15-12)|)/2 = (5 + 3)/2 = 4ms for a sample size of 3 packets.
it's impossible to measure jitter against latency unless you have an synchronized timing source and that timing is encoded into the packet.
So on a device like that jitter is actually measured against the RTT/ping time. So the device is just counting the # of ms it took to receive a response and doing the maths locally.
Packet loss is another important metric.
Packet loss of less than 1%, avg latency of max 150ms and jitter of max 30ms is acceptable for reasonable Internet performance - once you get outside of these metrics - stuff goes to shit really quickly.
You will find some sites that claim latency is the RTT, but it wasn't defined as that in the beginning. Ultimately it comes down to the determination by the engineer at the time of what they want to call latency. But if you see PING it is safe to assume that they are talking RTT and if you see jitter measured against PING then it is safe to assume that that jitter is the RTT jitter.
It's only when you see latency reported that you might want to did into the technical docs to understand what they are actually measuring.
(I also happen to know that Ookla Speedtest uses RTT, the server is dumb, to make it as fast as possible, all the smarts and calculations are done on the phone.)
