http://www.rfc-editor.org/rfc/rfc2004.txt
RFC 2004: Minimal Encapsulation within IP, C. Perkins1996-10-01T23:00:00-00:00This document specifies a method by which an IP datagram may be encapsulated (carried as payload) within an IP datagram, with less overhead than "conventional" IP encapsulation that adds a second IP header to each encapsulated datagram. [STANDARDS-TRACK]