RTT (Round-Trip Time) (RTT)

Performance

The time it takes for a packet to travel from client to server and back. The fundamental unit of network latency. Every TCP handshake costs 1 RTT, every TLS handshake costs 1-2 RTT.

Updated Mar 9, 2026

Full Explanation

RTT is the speed of light tax. You can't make packets travel faster than physics allows, so reducing RTT means reducing distance. That's the whole point of CDN PoPs—cut the RTT from 200ms (cross-continent) to 5-20ms (same city).

Why RTT matters so much: a typical HTTPS request needs 3-4 round trips before any content is delivered (DNS + TCP + TLS + HTTP). At 100ms RTT, that's 300-400ms of pure waiting. At 10ms RTT from a CDN edge, it's 30-40ms. TCP's congestion control also starts slow and ramps up over multiple RTTs, so high RTT means slower large downloads too.

HTTP/2 and HTTP/3 reduce the number of round trips needed. HTTP/3 with 0-RTT can send data on the very first packet. But nothing beats just being closer—which is why CDNs deploy in 200+ cities.

Examples 

# Measure RTT with ping
$ ping -c 5 cdn.example.com
round-trip min/avg/max/stddev = 4.2/5.1/6.8/0.9 ms

# Measure RTT as part of HTTP request
$ curl -w 'TCP connect: %{time_connect}s\n' -o /dev/null -s https://cdn.example.com/
TCP connect: 0.005s  # ~5ms RTT

# Compare origin vs CDN RTT
$ ping origin.example.com   # 85ms (cross-Atlantic)
$ ping cdn.example.com      # 5ms  (local PoP)
# 17x improvement

Video Explanation

Frequently Asked Questions

The time it takes for a packet to travel from client to server and back. The fundamental unit of network latency. Every TCP handshake costs 1 RTT, every TLS handshake costs 1-2 RTT.

# Measure RTT with ping
$ ping -c 5 cdn.example.com
round-trip min/avg/max/stddev = 4.2/5.1/6.8/0.9 ms

# Measure RTT as part of HTTP request
$ curl -w 'TCP connect: %{time_connect}s\n' -o /dev/null -s https://cdn.example.com/
TCP connect: 0.005s  # ~5ms RTT

# Compare origin vs CDN RTT
$ ping origin.example.com   # 85ms (cross-Atlantic)
$ ping cdn.example.com      # 5ms  (local PoP)
# 17x improvement

Related CDN concepts include:

  • Anycast — A routing technique where the same IP address is announced from multiple locations worldwide. The …
  • Latency — The time delay between a request and the start of its response. For CDNs, it's …
  • TTFB (Time To First Byte) (TTFB) — The time from the start of a request to receiving the first byte of the …
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