http://www.rfc-editor.org/rfc/rfc4840.txt
RFC 4840: Multiple Encapsulation Methods Considered Harmful, B. Aboba, E. Davies, D. Thaler2007-04-01T23:00:00-00:00This document describes architectural and operational issues that arise from link-layer protocols supporting multiple Internet Protocol encapsulation methods. This memo provides information for the Internet community.