Which tunnel protocol is a Cisco-proprietary protocol?

Generic Routing Encapsulation (GRE) is a Cisco-proprietary standard for encapsulating layer 3 protocols over an IP network, such as the Internet. Point-to-Point Protocol (PPP) is an IEEE standardized protocol for point-to-point links. IP Security (IPsec) is an IEEE standardized protocol for encryption of IP packets. Secure Sockets Layer (SSL) is an IEEE standardized protocol for mutual authentication and encryption between two hosts using certificates.