TCP is connection-oriented: both ends agree on initial sequence numbers (ISN) and window sizes before data flows. The three-way handshake (SYN → SYN-ACK → ACK) also allows each side to learn the other's receive buffer and avoid half-open states.

Three-way handshake
SYN
seq=x, choose ISN
SYN-ACK
seq=y, ack=x+1
ACK
ack=y+1 — connection ESTABLISHED
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