The Robustness Principle (Postel’s Law)

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In a digital world increasingly defined by interconnectivity, ensuring robust communication between different software components is crucial. The Robustness Principle, also known as Postel's Law, offers a guiding principle for designing software that effectively handles this communication. Understanding and applying this principle can significantly enhance the quality and reliability of digital software products.

Understanding the Robustness Principle

Formulated by Jon Postel, an early pioneer of the Internet, the Robustness Principle states: "Be liberal in what you accept, and conservative in what you send." In practical terms, this means that while software should strictly adhere to specifications when sending messages, it should also be designed to handle and accept non-conformant input as long as the meaning is clear.

The Role of the Robustness Principle in Digital Software Products

The Robustness Principle plays a pivotal role in digital software product development, impacting how software components communicate and interact. Here are three ways this principle comes into play:

  1. Interoperability: By being liberal in what a software accepts and conservative in what it sends, developers can enhance the interoperability of their software, allowing it to interact effectively with other software components, even those that may not strictly adhere to expected specifications.
  2. Robustness: This principle also enhances the robustness of software, helping it to handle unexpected or improperly formatted inputs without breaking down or producing errors.
  3. User Experience: Lastly, by enhancing interoperability and robustness, the Robustness Principle can significantly improve user experience. Users can use software products without worrying about compatibility issues or unexpected errors.

Real-World Examples of the Robustness Principle in Action

To illustrate the application of the Robustness Principle, let's consider three real-world examples:

  1. Web Browsers: Web browsers are a prime example of the Robustness Principle in action. They are designed to render web pages accurately (being conservative in what they send), but can also handle and display web pages that may not conform strictly to HTML standards (being liberal in what they accept).
  2. Email Clients: Email clients also embody the Robustness Principle. They strictly adhere to email protocols when sending messages, but can accept and display emails that may not strictly conform to these protocols.
  3. APIs: APIs (Application Programming Interfaces) often follow the Robustness Principle. While they adhere strictly to their defined interfaces when sending responses, they are often designed to handle a wide range of inputs, enhancing their flexibility and robustness.

In conclusion, the Robustness Principle offers an invaluable guideline for developing digital software products that are robust, interoperable, and user-friendly. By adhering to specifications when sending messages and accepting a wide range of inputs, software developers can create products that effectively communicate and interact with other software components, enhancing their functionality and reliability.


Postel’s Law (also known as the Robustness Principle) was formulated by Jon Postel, an early pioneer of the Internet. The Law is a design guideline for software, specifically in regards to TCP and networks, and states “TCP implementations should follow a general principle of robustness: be conservative in what you do, be liberal in what you accept from others”. In other words, programs that send messages to other machines (or to other programs on the same machine) should conform completely to the specifications, but programs that receive messages should accept non-conformant input as long as the meaning is clear.


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