AlgoPlus//networking / tcp-handshake

TCP 3-Way Handshake

SYN, SYN-ACK, ACK to agree on sequence numbers — then data, then a clean teardown.

Client

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Server

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Legend

SYN / FIN

ACK

Data

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In a nutshell

TCP gives a reliable, ordered byte stream, so before any data moves both sides agree on starting sequence numbers. The client sends a SYN with its number, the server replies SYN-ACK (its own SYN plus an acknowledgement), and the client ACKs. This three-way handshake costs one round trip; closing takes a four-way FIN exchange.

Ready

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Before trusting a stranger with an ordered byte stream, both sides swap starting sequence numbers and confirm they heard each other — that's the SYN / SYN-ACK / ACK dance.

Key terms

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