Telnet

In the computing world of the early 1970s, ARPANET had just been born, but its promises of universal interconnection were running up against technical reality. Each manufacturer developed its own protocols and standards, creating so many isolated technological islands. A terminal operating with the EBCDIC character set simply could not communicate with a PDP-10 computer configured for ASCII. Machines from different manufacturers needed to communicate with each other.

The Telnet protocol emerged to address this problem. Its designers understood that they needed to invent a common computer language that would allow any terminal to connect to any remote computer. This insight gave birth to the Network Virtual Terminal (NVT), a concept of remarkable elegance in its simplicity. It functions as a universal translator. Rather than forcing each machine to understand all the possible dialects of its peers, the system imposes a single standardized format. Each computer needs to master only one translation: that between its native language and this virtual lingua franca. An EBCDIC terminal translates to the NVT, which then retranslates to the ASCII expected by the destination machine. The efficiency of this approach immediately struck ARPANET engineers.

But Telnet’s history contains a delightful episode. In 1972, Bernard Cosell and David Walden, both BBN employees working on the Terminal Interface Message Processor, found themselves facing a recurring puzzle. Telnet specifications were constantly evolving, and each modification required simultaneously updating all existing instances across the network. A monumental task that considerably slowed development. It was on a plane en route to UCLA that Cosell had his epiphany. Why not let the two ends of a connection negotiate between themselves the functionalities they wished to use? This idea, which he developed with Walden during the flight, revolutionized the approach to network protocols. Back on the ground, they designed a mechanism based on four terse commands: DO, DON’T, WILL, and WON’T. These four magic words transformed Telnet into a living protocol, capable of evolving without breaking what already existed. When a machine proposed a new functionality via WILL, its counterpart could accept it with DO or politely decline with DON’T. If it did not understand the proposal, a simple refusal sufficed, and communication resumed on the common ground of the NVT. This ongoing negotiation allowed the protocol to gradually enrich itself, like a language that naturally integrates new terms without losing its coherence. Ray Tomlinson, already famous for inventing email, was then working on the Telnet implementation for the TENEX system. As soon as he discovered the mechanism imagined by Cosell and Walden, he immediately grasped its significance. David Walden formalized the idea in RFC 435, published in January 1973, laying the groundwork for what would become a lasting standard.

Final standardization came ten years later, with RFC 854 in 1983. Telnet gained its modern form: bidirectional TCP connection on port 23, 7-bit ASCII encoding, and sophisticated data flow management. Engineers added an ingenious synchronization mechanism, designed for emergency situations where a user must regain control of a runaway remote program. The “Synch” signal uses TCP’s urgent capabilities to bypass normal flow control and guarantee immediate transmission of critical commands. This elegant architecture made Telnet far more than a simple terminal emulator. The protocol became the backbone of remote access to computer systems. In the 1980s and 1990s, it fueled the explosion of the first online services and BBSes, those electronic bulletin boards that preceded today’s forums. Its simplicity appealed to developers and users alike.

Designed in the 1970s, when computing existed in a world of relative trust, Telnet encrypted nothing. Passwords, sensitive data—everything traveled in clear text across networks. This weakness, trivial at first, became prohibitive with the democratization of the Internet and the emergence of security concerns. The arrival of graphical interfaces dealt an additional blow to the venerable protocol. Users gradually abandoned text-mode terminals for colored windows and clickable icons. SSH, which appeared in the 1990s, inherited Telnet’s strengths while adding the encryption it so sorely lacked.

Though banished by security experts, Telnet has not disappeared. In server rooms, it remains the diagnostic tool par excellence. Network administrators have long used it—perhaps still do—to test connectivity on a given port or configure recalcitrant old equipment. This persistence testifies to the robustness of its original design.

The virtual terminal concept spread to countless subsequent protocols. The option negotiation mechanism, born on that plane to UCLA, inspired engineers facing the challenge of extensibility. How do you evolve a protocol without breaking what exists? Cosell and Walden had found a disarmingly simple answer that continues to serve as a model. Faced with the growing complexity of systems, the temptation was strong to develop sophisticated solutions with abundant features. Telnet chose to favor interoperability and flexibility over sophistication.