OpenVPNはUDPのみを使用するけどTLSで認証できる。
UDPの上に独自にバーチャルサーキットを作っている。
以下manpageより。

In TLS mode, OpenVPN uses a dual control-channel/data-channel mode of
operation. The control channel runs a TLS session which is used to ex-
change randomly generated cipher and HMAC keys. Those keys are then
used to encrypt and authenticate the data channel. Both channels are
multiplexed over a single UDP port. A reliability layer is placed over
the control channel since TLS expects a reliable connection.