TCP (Transmission Control Protocol) is a transport-layer protocol that sets up a connection with a three-way handshake, numbers every byte, acknowledges receipt and retransmits lost segments so applications get a reliable, ordered byte stream.
Why it matters
HTTP/1.1, HTTP/2, SMTP, SSH and most databases run over TCP. Its congestion control prevents senders from overwhelming the network, but the handshake costs one round trip and a lost packet can stall everything behind it, which is called head-of-line blocking. HTTP/3 moved to QUIC over UDP partly to avoid that.
Example
Client -> Server SYN
Server -> Client SYN-ACK
Client -> Server ACK (connection established)If you connect to a server 80 ms away, the handshake alone takes about 80 ms before any HTTP data is sent. A closed port answers with RST; a filtered one gives no answer.
Common mistakes
- Mixing up "connection refused" (RST received, nothing listening) and "timed out" (no reply, often a firewall).
- Assuming TCP guarantees delivery in bounded time. It guarantees order, not speed.
- Ignoring TIME_WAIT buildup on busy servers.
Related
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