Point-to-Point Tunneling Protocol is another obsolete method to encapsulate PPP packets in a GRE tunnel. PPTP uses TCP (port 1723) for control channel. As L2TP, PPTP does not define encryption or authentication. PPP traffic in MS implementation of PPTP is authenticated by various mechanisms such as MS-CHAP-v1 or v2. All of them are considered insecure these days. EAP-TLS is a better choice.