Definition
UDP is one of the two main transport protocols of the internet. Unlike TCP, it is connectionless: each packet (datagram) is sent independently with no guarantee of delivery, ordering, or duplication detection. This minimal design makes UDP fast and lightweight, at the cost of reliability that the application must provide itself if needed. UDP is the foundation of DNS, video streaming, online gaming, and most modern VPN protocols.
UDP and VPNs
UDP is the preferred transport for VPN tunnels because it avoids the TCP-over-TCP problem — a performance pathology in which two layers of retransmission interfere with each other on lossy links. WireGuard runs only over UDP. OpenVPN supports both but defaults to UDP. IKEv2 runs over UDP. The VPN protocols that need reliability handle it themselves at the inner layer rather than relying on the outer transport.
When UDP Is Blocked
Some restrictive networks block all UDP traffic except DNS, forcing VPN clients to fall back to TCP. SSL VPNs and OpenVPN over TCP 443 are the standard answer in those environments. The trade-off is reduced performance — sometimes substantial — but at least connectivity is preserved. Modern VPN clients often try UDP first and silently fall back to TCP if UDP fails to establish.