http://www.rfc-editor.org/rfc/rfc2784.txt
RFC 2784: Generic Routing Encapsulation (GRE), D. Farinacci, T. Li, S. Hanks, D. Meyer, P. Traina2000-03-01T23:00:00-00:00This document specifies a protocol for encapsulation of an arbitrary network layer protocol over another arbitrary network layer protocol. [STANDARDS-TRACK]