Question 1373

Three Round Trips Deep

A browser in London calls an API served only from New York, 5,585 km away. The connection is cold: a TCP handshake, then a TLS 1.3 handshake, then the request itself. Light in fibre travels about 2×1082 \times 10^8 m/s.

TCP, then TLS, then the request, each costing one round trip London New York SYN ClientHello GET /v1/quote

Measured p50 for the whole call is 190 ms and the target is 60 ms. What is the honest read?

A

60 ms is below the speed of light for this distance, so the target is physically impossible and should be renegotiated.

B

About 168 ms of the 190 is round trip time. Only cutting the number or the length of those round trips, by reusing connections and terminating TLS at a London edge, reaches 60 ms.

C

The floor is about 56 ms, so roughly 134 ms is server work and the handler is where the tuning effort belongs.

D

The path is bandwidth limited. Upgrading the transatlantic link removes most of the 190 ms.