In general, an implementation must be conservative in its sending behavior, and liberal in its receiving behavior.
Jon PostelTCP implementations will follow a general principle of robustness: be conservative in what you do, be liberal in what you accept from others.
Jon PostelI also administer the Internet Assigned Names Authority, which is the central coordinator for the Internet address space, domain names and Internet protocol conventions essential to the use and operation of the Internet.
Jon Postel