For the time being, VPN.com offers OpenVPN as its primary communication protocol. OpenVPN is (as the name implies) an open source VPN protocol which offers (in our opinion) the best combination of features that users look for in their favorite VPN services. This includes fast speeds, increased functionality, and the utmost security, all utilized with the help of the SSL/TLS encryption protocol and the OpenSSL library for encryption.
This means OpenVPN allows for the use of several cryptographic algorithms depending on the specific application (Blowfish, AES, and so on).
At default OpenVPN runs in UDP mode and shows marked improvements over L2TP, however it can still be configured to run in either UDP or TCP depending on the users' preference. The main reason this is so desirable is because it adds another level of flexibility to your VPN experience. Say for example you were trying to get past a particularly restrictive firewall, TCP would be great for that, whereas UDP may not (depending on the application). You can find out more about the specific differences between UDP and TCP by reading our resource article on the subject linked here [EDIT: Link].
VPN.com has plans in the works to add L2TP functionality in the near future, so be sure to like our Facebook page for all the updates you need to keep yourself current with our product as it progresses through development!