handshake
termnetworkingcryptography
The exchange that establishes a conversation before data flows - agreeing that both sides are present, and on what terms.
Every handshake costs at least one round trip, and that is why latency dominates the experience of a distant service more than bandwidth does: a page needing several connections pays the distance repeatedly before a byte of content moves. It is also the point where capability is negotiated, which makes it the place where downgrade attacks live and the reason modern protocols authenticate the negotiation itself rather than trusting it.
Also known as: three-way handshake