The Internet Stream Protocol (ST and later ST-II) is a family of experimental protocols first defined in Internet Engineering Note IEN-119 (1979), and later substantially revised in RFC 1190 (ST-II) and RFC 1819 (ST2+).
The Internet Stream Protocol family was never introduced for public use, but many of the concepts available in ST are similar to later Asynchronous Transfer Mode protocols and can be found in Multiprotocol Label Switching (MPLS). They also presaged Voice over IP.
ST arose as the transport protocol of the Network Voice Protocol, a pioneering computer network protocol for transporting human speech over packetized communications networks, first implemented in December 1973 by Internet researcher Danny Cohen of the Information Sciences Institute (ISI) as part of ARPA's Network Secure Communications (NSC) project; see RFC 741 (published 1977).
First specified in 1979, ST was envisioned to be a connection-oriented complement to IPv4, operating on the same level as IP but using a different header format than that used for IP datagrams. According to IEN-119, its concepts came from Danny Cohen, Estil Hoversten, and James W. Forgie. The protocol was notable for introducing the concepts of packetized voice (now Voice over IP), a talkspurt (a continuous segment of speech between silent intervals), and specified delay and drop-rate requirements for packet services. It was implemented in the Voice Funnel.
Its second version, known variously as ST-II or ST2, was drafted by Topolcic and others in 1987 and specified in 1990. In RFC 1819, ST2 distinguishes its own packets with an Internet Protocol version number 5, although it was never known as IPv5.
IP and ST packets can be distinguished by the IP Version Number field, i.e., the first four (4) bits of the packet; ST has been assigned the value 5 (see [RFC1700]). There is no requirement for compatibility between IP and ST packet headers beyond the first four bits. (IP uses value 4.)
ST uses the same IP address structure and the same Link Layer protocol number (ethertype 0x800) as IP.
According to http://www.iana.org/assignments/protocol-numbers/, there is a "datagram mode" in which ST packets can be encapsulated over IP headers using protocol number 5.