Why Was There No IPv5? Uncovering the Forgotten Internet Stream Protocol

Early Internet Development

In the late 1960s the U.S. Department of Defense’s Advanced Research Projects Agency (ARPA) launched a nationwide project to interconnect computers. The first technical goal was to define a suite of protocols that could reliably transport ordered, error‑checked byte streams across the network. This effort produced the Transmission Control Protocol (TCP) and the Internet Protocol (IP). Initially TCP performed both end‑to‑end communication and packet routing, but the designers soon split the responsibilities: IP handled packet encapsulation and routing, while TCP remained a pure transport layer. Because three earlier versions of the protocol suite had already been labeled IPv1‑IPv3, the new routing protocol was named IPv4 and used a 32‑bit address space (four octets, each ranging 0‑255).

Internet Stream Protocol (IPv5)

The protocol that later became known as IPv5 was originally called the Internet Stream Protocol (ST). It was jointly developed by Apple, NeXT, and Sun Microsystems in the early 1980s as an experimental transport for real‑time voice and video. IPv5 was assigned the IP protocol number 5 and introduced a packet header that included a type‑of‑service field and a timestamp field to support synchronized streaming at a fixed frequency. The design allowed a host to maintain an ongoing communication session while transmitting data packets at regular intervals, which was essential for early multimedia applications.

Why IPv5 Was Not Adopted as a Standard

IPv5 never became an official Internet Standard for two main reasons:

Addressing limitation : IPv5 relied on the same 32‑bit IPv4 address format (four decimal octets). This limited the total address pool to roughly 4.3 billion unique addresses, which proved insufficient as the Internet grew rapidly in the 1990s.

Scope of the protocol : IPv5 was designed as a specialized streaming transport, not as a general‑purpose routing protocol. The IETF therefore chose to focus on extending the existing IP architecture rather than standardising a niche protocol.

By the early 1990s the IPv4 address space was approaching exhaustion, culminating in the global depletion of free IPv4 addresses in 2011. This pressure led to the development of the Internet Protocol next generation (IPng) project, which produced IPv6 with a 128‑bit address space (eight groups of four hexadecimal digits), providing an astronomically larger pool of addresses.

Legacy and Influence of IPv5

Although IPv5 never entered the standards track, its concepts influenced later real‑time transport protocols. The idea of a dedicated streaming layer paved the way for the Real‑time Transport Protocol (RTP) and the Session Initiation Protocol (SIP), which underpin modern Voice over IP (VoIP) and video‑conferencing services. In this sense, the experimental work on IPv5 contributed to the evolution of IP‑based multimedia communication that is ubiquitous today.