The Internet Stream Protocol (ST), also known as IPv5, was an experimental protocol defined in 1979 in IEN 119 (Internet Engineering Note), and was later revised in RFC 1190 (ST2) and RFC 1819 (ST2+). ST was experimental packet carrying non-IP real-time stream protocol. ST was envisioned to be the connection oriented complement to IPv4, but it has never been introduced for public usage. Many of the concepts available in ST can be found today in MPLS. In datagram mode, ST was assigned Internet Protocol version number 5. The version number "5" in the IP header was assigned to ST.
