Glossary · Networking

TCP (Transmission Control Protocol)

TCP is a connection-oriented transport protocol that delivers data reliably and in order between two hosts, retransmitting anything that is lost.

Updated October 6, 2026

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.

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