Layer Two Tunneling Protocol (L2TP), itself an augmentation of the Point-to-Point Tunneling Protocol (PPTP), is a VPN protocol that leverages User Datagram Protocol (UDP) port 1701.
Unfortunately, it does not provide any confidentiality nor meaningful identity authentication.
The reason why you would still want to send some [emphasis on some] data over this or any other UDP port over one belonging to a Transmission Control Protocol (TCP) is that the latter can be such a bad fit for certain scenarios that critical failures stemming thereof are a full-fledged phenomenon with a powerful name – “TCL meltdowns.”
Back to the point, L2TP has been in use for almost the entirety of this century, having evolved not just into a useful support protocol for VPNs, but also into a frequently encountered component of ISP infrastructure. While it ensures neither user authentication nor user confidentiality, the lack of both ironically makes it not a complete privacy disaster.
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