TCP and UDP: differences and testing limits
TCP
TCP establishes a connection, orders bytes and retransmits losses. Its handshake allows observation of connection acceptance or refusal. This is useful for web, SSH and many email services, but does not prove login, correct content or application health. An open TCP port does not mean the same UDP port number is available.
UDP
UDP sends datagrams without a handshake or delivery and ordering guarantees. DNS, voice, games and QUIC use it, adding reliability when implemented by the application. No response can mean filtering, loss, invalid data ignored by the service or no reply required by the protocol. This is why the site does not offer generic UDP port checks.
Choose the right test
Read the service documentation before opening rules. DNS uses UDP and TCP on port 53; HTTPS may use TCP 443 or QUIC over UDP 443. Ports are conventions, not definitive software identification. For UDP, use a protocol-specific client and server logs. For TCP, combine the checker with a real application test.