<P> The spread of TCP / IP was fueled further in June 1989, when AT&T agreed to place the TCP / IP code developed for UNIX into the public domain . Various vendors, including IBM, included this code in their own TCP / IP stacks . Many companies sold TCP / IP stacks for Windows until Microsoft released a native TCP / IP stack in Windows 95 . This event was a little late in the evolution of the Internet, but it cemented TCP / IP's dominance over other protocols, which began to lose ground . These protocols included IBM Systems Network Architecture (SNA), Digital Equipment Corporation's DECnet, Open Systems Interconnection (OSI), and Xerox Network Systems (XNS). </P> <P> An early architectural document, RFC 1122, emphasizes architectural principles over layering . </P> <P> The end - to - end principle has evolved over time . Its original expression put the maintenance of state and overall intelligence at the edges, and assumed the Internet that connected the edges retained no state and concentrated on speed and simplicity . Real - world needs for firewalls, network address translators, web content caches and the like have forced changes in this principle . </P> <P> The robustness principle states: "In general, an implementation must be conservative in its sending behavior, and liberal in its receiving behavior . That is, it must be careful to send well - formed datagrams, but must accept any datagram that it can interpret (e.g., not object to technical errors where the meaning is still clear)." "The second part of the principle is almost as important: software on other hosts may contain deficiencies that make it unwise to exploit legal but obscure protocol features ." Postel famously summarized the principle as, "Be conservative in what you do, be liberal in what you accept from others"--a saying that came to be known as "Postel's Law ." </P>

Who is credited with developing the tcp/ip protocol