OpenVPN is an open-source VPN protocol and program, first released in 2001, that builds its encrypted tunnel with TLS and can run over either UDP or TCP.
Explanation
OpenVPN is the long-standing workhorse of VPNs: free, open source, available on almost every platform, and supported by most commercial VPN apps and many company networks. It uses TLS, the same family of protocols behind HTTPS, to authenticate and agree keys, then encrypts the tunnel with ciphers chosen in its configuration. That configurability is both its strength and its weakness. Administrators can tune certificates, ciphers, ports and transport, but there are more settings to get wrong, and the program runs in user space, which usually makes it slower than WireGuard on the same hardware. Its registered port is UDP 1194. Running it over TCP on port 443 makes it pass simple port-based firewalls, because that port normally carries HTTPS and is rarely blocked. That trick does not defeat deep packet inspection: OpenVPN's handshake has a recognisable pattern, so national filters can identify it unless an obfuscation layer is added. TCP mode can also slow down on lossy networks, when both the outer and inner connections retry the same lost data. A web proxy avoids these problems differently, by being ordinary HTTPS from the start.
In practice
OpenVPN dates from 2001 and uses its own protocol on top of TLS. Its registered port is 1194, but it can also run over TCP on port 443 so the connection looks more like ordinary HTTPS — a common trick on restrictive networks. That flexibility comes at a cost: OpenVPN runs in user space and is usually slower than WireGuard, and its handshake can still be fingerprinted by deep packet inspection.
Sources: OpenVPN community resources · Wikipedia - OpenVPN (pointer)
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