Estimate TLS setup latency from round-trip time, protocol version and resumption mode, split into network and crypto cost.
Handshake latency is dominated by round trips, not by cryptography: on a 40 ms link, dropping from two TLS round trips to one saves 40 ms while the asymmetric operations cost only a couple of milliseconds. Resumption skips certificate verification, which is why its crypto cost collapses to a symmetric key derivation. TLS setup is on the critical path of the first request, so the version and resumption strategy you enable decide whether a mobile user waits 120 ms or 40 ms before any application byte moves.
TLS Handshake Time
handshake time = (TCP round trips + TLS round trips) × RTT + crypto time, where TLS 1.3 needs 1 round trip, TLS 1.2 needs 2, and 0-RTT needs none.
handshake time = (TCP round trips + TLS round trips) × RTT + crypto time, where TLS 1.3 needs 1 round trip, TLS 1.2 needs 2, and 0-RTT needs none. Handshake latency is dominated by round trips, not by cryptography: on a 40 ms link, dropping from two TLS round trips to one saves 40 ms while the asymmetric operations cost only a couple of milliseconds. Resumption skips certificate verification, which is why its crypto cost collapses to a symmetric key derivation.
TLS setup is on the critical path of the first request, so the version and resumption strategy you enable decide whether a mobile user waits 120 ms or 40 ms before any application byte moves.
This calculator takes 6 inputs: TLS version, Handshake type, Round-trip time, Include the TCP three-way handshake, Server asymmetric operation time, Client verification time. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Early data is replayable, because the server has no way to confirm freshness before processing it. Restrict 0-RTT to idempotent requests such as GETs, and never let it carry a state-changing operation.