The author is grateful to the members of the Internet Engineering Task Force, chaired by Phill Gross, who provided encouragement and thoughtful review of this work. Several patient beta-testers, particularly Sam Leffler and Craig Leres, tracked down and fixed problems in the initial implementation. Cynthia Livingston and Craig Partridge carefully read and greatly improved an unending sequence of partial drafts of this document. And last but not least, Telebit modem corporation, particularly Mike Ballard, encouraged this work from its inception and has been an ongoing champion of serial line and dial-up IP.