What ever happened to IPv5?

In all of the debate about the transition from IPv4 to IPv6, few people ever seem to ask about whatever happened to IPv5. Like Formula 408 and Special J cereal, some wonder whether an IPv5 ever existed. If so, what happened to it?

Indeed, IPv5 did exist. To understand why there is a jump from Version 4 to Version 6, we must review some Internet history.

The timeline stretches back to the early 1970s when the Advanced Research Projects Agency — now the Defense Advanced Research Projects Agency — began fleshing out its fledgling ARPAnet.

That eventually turned into NSFnet, a network the National Science Foundation operated primarily for government scientists. That, in turn, grew into the modern Internet.

IP was first developed as a counterweight to TCP, which was the first complete set of protocols developed for ARPAnet. TCP is the transport layer of the Internet, layer four in the Open System Interconnection’s seven-layer reference model. It manages network connections and data transport. IP is layer three, the component that enables addressing and routing, among other activities.

For several years, there was only TCP, which scientists were developing as a host-level, end-to-end protocol and a packaging and routing protocol. By the late 1970s, however, people realized they were trying to do too much with a single protocol. IP was created to handle packaging and routing functions.

The engineering world rarely discards anything, however. TCP development alone included two versions of the protocol. So by the time developers decided to split the work and create IP, the TCP line had already reached its third version. When the first complete set of TCP/IP protocols were announced in 1980, it was the fourth iteration, hence IPv4.

So IPv4 was actually the first standardized version of IP.

But as early as the 1970s, people realized the network would not be able to handle future requirements, so engineers created the Internet Stream (ST) Protocol to experiment with voice, video and distributed simulation via the network. Separate development of ST eventually led to ST2 in the 1990s. IBM, NeXT, Apple Computer and Sun Microsystems used that version in their commercial networking products.

ST2, which offered connection-based communications and guaranteed quality of service, was considered a great advance over IP and was formally designated IPv5.

By the time that happened, however, the idea of the next generation of the Internet, or IPng, had already started to percolate. IPng work began in 1994. Instead of moving smoothly through the ST2-based IPv5 to this next-generation Internet, people working on the upgrades decided to improve IPv4 and add everything they thought would be needed for the future Internet. That meant skipping from IPv4 to IPv6.