http://www.rfc-editor.org/rfc/rfc4636.txt
RFC 4636: Foreign Agent Error Extension for Mobile IPv4, C. Perkins2006-10-01T23:00:00-00:00This document specifies a new extension for use by Foreign Agents operating Mobile IP for IPv4. Currently, a foreign agent cannot supply status information without destroying the ability for a mobile node to verify authentication data supplied by the home agent. The new extension solves this problem by making a better place for the foreign agent to provide its status information to the mobile node. [STANDARDS-TRACK]