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TCP Fast Open (TFO)

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The cost of TCP's handshake

Every TCP connection starts with a 3-way handshake: SYN → SYN-ACK → ACK. Only after the ACK can the client send data. On a 100ms round-trip path, this adds 200ms of latency before the first byte of application data arrives.

TCP Fast Open (RFC 7413) allows sending application data in the very first SYN packet — saving one full RTT for repeat connections.

Packets in motion — click to inspect:

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