Round-trip time (RTT) is the time between sending a packet and receiving the response to it from the other end. It combines the travel time in both directions, queueing in routers, and processing at the destination.
Why it matters
RTT is the practical measure of network latency. Many protocols wait for acknowledgments, so RTT limits how fast they can ramp up: a TCP connection setup costs one RTT, and a TLS handshake adds more, which is why distant servers feel slow to start. Light in fiber covers roughly 200 km per millisecond, so physical distance sets a floor: a round trip of about 10,000 km cannot be under 100 ms regardless of bandwidth.
Example
$ ping -c 3 example.com
64 bytes from 203.0.113.10: icmp_seq=1 ttl=57 time=21.4 ms
64 bytes from 203.0.113.10: icmp_seq=2 ttl=57 time=22.1 ms
64 bytes from 203.0.113.10: icmp_seq=3 ttl=57 time=21.0 msThe time values are RTT samples, about 21 ms here. Variation between samples is jitter.
Common mistakes
- Equating RTT with one-way delay; half of RTT is only an estimate because paths can be asymmetric.
- Judging a link by one ping sample instead of several.
- Assuming a ping reply proves a web service works, since ICMP and HTTP may be handled differently.
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