TCP: an ordered byte stream
TCP establishes a connection and provides applications with an ordered byte stream. Lost segments can be retransmitted; flow and congestion control regulate transmission. This does not guarantee that a connection never fails or that the receiving application stores data permanently. TCP does not preserve original message boundaries.
UDP: individual datagrams
UDP sends datagrams without its own connection setup, delivery acknowledgements or automatic retransmission. Messages can be lost, duplicated or reordered. Applications can build reliability mechanisms above it. UDP is therefore not universally faster or inherently unsuitable for reliable transfer.
Typical applications
SSH and many email protocols use TCP. Traditional DNS uses both UDP and TCP. Real-time communication often uses UDP-based protocols. HTTP/1.1 and HTTP/2 normally run over TCP, while HTTP/3 uses QUIC over UDP. QUIC provides features including reliable streams and encryption. Video streaming is therefore not exclusively a UDP example.
What do the tests show?
Our TCP ping and port checker test TCP connections from the server in Düsseldorf. They do not demonstrate UDP reachability. UDP applications require suitable protocol requests because silence has several possible causes. When troubleshooting, distinguish the application, transport and measurement location.