Source: https://uptimeify.io/resources/platform-and-support/glossary/tcp

# TCP

> TCP (Transmission Control Protocol) is the connection-oriented protocol that opens a reliable link between a client and a specific port on a server before any application data is exchanged.

Every TCP connection targets a numbered port, for example port 22 for [SSH](/resources/platform-and-support/glossary/ssh) or port 443 for HTTPS, and only succeeds if something is listening there and completes the three-way handshake.

A successful TCP connection only proves the port is reachable; it says nothing about the protocol running on top of it, which is why a [TLS handshake](/resources/platform-and-support/glossary/tls-handshake) or an SSH login can still fail even after the underlying TCP connection succeeds.