Which tunnels support authentication of clients using a username and password?

The correct answer is PPPoE (Point-to-Point Protocol over Ethernet). PPPoE is designed to encapsulate PPP frames in Ethernet frames and is commonly used by Internet Service Providers to manage connections between customers and the internet. One of the key features of PPPoE is its support for authentication methods such as PAP (Password Authentication Protocol) and CHAP (Challenge Handshake Authentication Protocol). This allows clients to authenticate themselves to the server using a username and a password, ensuring that only authorized users can establish an internet connection.

OpenVPN also allows for client authentication via username and password, but it is primarily based on SSL/TLS protocols, making it distinct from the traditional PPPoE structure. Meanwhile, IPIP (IP in IP) is simply a tunneling protocol without built-in authentication mechanisms, and PPTP/L2TP offers some form of authentication but is less widely used in modern implementations compared to PPPoE for straightforward username and password authentication.