A client makes a new HTTPS connection (TLS 1.3 over TCP) to a web server with a round-trip time (RTT) of 100 ms. Roughly how long does it take until the response to the first request starts to arrive? Assume DNS resolution is already done, and ignore server processing time and data transfer time.

Answer: B. About 300 ms

First, the TCP three-way handshake takes one round trip (the client can send its first TLS message together with the final ACK). Next, the TLS 1.3 handshake takes one round trip, and then the HTTP request and response take one more, for three round trips in total: 3 × 100 = 300 ms. With TLS 1.2 the key exchange takes two round trips, giving four round trips, or 400 ms. This waiting time depends on the number of RTTs, which is set by distance, so adding link bandwidth does not shorten it. QUIC, used by HTTP/3, combines the transport and cryptographic handshakes into one round trip and also offers a way to send data in the first flight when reconnecting (0-RTT).

• A. This counts only the HTTP request-response round trip, not the TCP and TLS handshakes.
• B. Correct. TCP, TLS 1.3 and HTTP each take one round trip, three in total.
• C. This leaves out either the TCP or the TLS handshake.