1. Introduction

   This document specifies IP-SFS, a method for the encapsulation and
   transmission of IPv4/IPv6 packets over the Semaphore Flag Signaling
   System (SFSS).  The SFSS is an internationally recognized alphabetic
   communication system based upon the waving of a pair of hand-held
   flags [JCroft,Wikipedia].  Under the SFSS, each alphabetic character
   or control signal is indicated by a particular flag pattern, called a
   Semaphore Flag Signal (SFS).

   IP-SFS provides reliable transmission of IP datagrams over a half-
   duplex channel between two interfaces.  At the physical layer, SFSS
   uses optical transmission, normally through the atmosphere using
   solar illumination and line-of-sight photonics.  A control protocol
   (Section 4) allows each interface to contend for transmission on the
   common channel.

   This specification defines only unicast transmission.  Broadcast is
   theoretically possible, but there are some physical restrictions on
   channel direction dispersion.  This is a topic for future study.

   The diagram in Figure 1 illustrates the place of the SFSS in the
   Internet protocol hierarchy.

             +-----+     +-----+       +-----+
             | TCP |     | UDP |  ...  |     |  Host Layer
             +-----+     +-----+       +-----+
                |           |             |
             +-------------------------------+
             |    Internet Protocol & ICMP   |  Internet Layer
             +-------------------------------+
                            |
             +-------------------------------+
             |             SFSS              |  Link Layer
             +-------------------------------+

                     Figure 1: Protocol Relationships