TCP three-way handshake.
also: three-way handshake · TCP handshake · SYN · SYN-ACKSYN, SYN-ACK, ACK: the exchange that opens a TCP connection, costing one round trip before the first request can go.
the two-minute refresher
- Client sends SYN, server answers SYN-ACK, client sends ACK: then data can flow.
- Cost: 1 RTT before the first request; NYC to London is about 80 ms.
- TLS adds its own round trip(s) on top, which is why new connections are expensive.
- Reusing connections (keep-alive, pools) avoids paying this on every request.