http://www.rfc-editor.org/rfc/rfc4366.txt
RFC 4366: Transport Layer Security (TLS) Extensions, S. Blake-Wilson, M. Nystrom, D. Hopwood, J. Mikkelsen, T. Wright2006-04-01T23:00:00-00:00The extensions may be used by TLS clients and servers. The extensions are backwards compatible: communication is possible between TLS clients that support the extensions and TLS servers that do not support the extensions, and vice versa. [STANDARDS-TRACK]