Saving a Round Trip
A normal TCP connection spends one round trip on its handshake before any data flows. TCP Fast Open lets a client send application data inside the first handshake packet, saving that round trip on repeat visits.
How the Cookie Works
The trick uses a small token issued by the server.
- On the first visit the client completes a normal handshake and the server returns a fast open cookie.
- On later visits the client sends the cookie plus data in the opening packet.
- The server validates the cookie and processes the data immediately.
The cookie proves the client controls its address, which guards against blind spoofed requests.
Where It Helps and Hurts
The benefit is largest for short repeated requests to the same server. It is less useful when middleboxes strip unknown options or when most connections are first contacts, so adoption depends on the path and the workload.
Key idea
TCP Fast Open carries data in the opening handshake packet using a server issued cookie to prove the client address, saving a round trip on repeat connections to the same server.