Robustness principle: Difference between revisions

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{{Short description|Design guideline for software}}

In [[computing]], the '''robustness principle''' is a design guideline for software that states: "be conservative in what you do, be liberal in what you accept from others". It is often reworded as: "be conservative in what you send, be liberal in what you accept". The principle is also known as '''Postel's law''', after [[Jon Postel]], who used the wording in an early specification of [[Transmission Control Protocol|TCP]].<ref>{{cite IETF |title=Transmission Control Protocol |rfc=761 |editor1-last=Postel |editor1-first=Jon |editor1-link=Jon Postel |date=January 1980 |publisher=[[Internet Engineering Task Force|IETF]] |access-date=June 9, 2014}}</ref>

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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== Interpretation ==

The principle was first written down by Jon Postel in the 1979 [[IPv4]] specification.<ref>{{Cite book |last1=Council |first1=National Research |url=https://books.google.com/books?id=lm2C8PJY-NYC&pg=PA39 |title=The Internet's Coming of Age |last2=Sciences |first2=Division on Engineering and Physical |last3=Applications |first3=Commission on Physical Sciences, Mathematics, and |last4=Board |first4=Computer Science and Telecommunications |last5=Infrastructure |first5=Committee on the Internet in the Evolving Information |date=2001-01-22 |publisher=National Academies Press |isbn=978-0-309-17205-9 |language=en}}</ref> RFC 1122 (1989) expanded on Postel's principle by recommending that programmers "assume that the network is filled with malevolent entities that will send in packets designed to have the worst possible effect".<ref>{{cite IETF |title=Requirements for Internet Hosts: Communication Layers |rfc=1122 |editor1-last=Braden |editor1-first=R. |editor1-link=Bob Braden |date=October 1989 |publisher=[[Internet Engineering Task Force|IETF]] |access-date=June 9, 2014}}</ref> Protocols should allow for the addition of new codes for existing fields in future versions of protocols by accepting messages with unknown codes (possibly logging them). Programmers should avoid sending messages with "legal but obscure protocol features" that might expose deficiencies in receivers, and design their code "not just to survive other misbehaving hosts, but also to cooperate to limit the amount of disruption such hosts can cause to the shared communication facility".<ref name="Wilde2012">{{cite book |last=Wilde |first=Erik |title=Wilde's WWW: Technical Foundations of the World Wide Web |url=https://archive.org/details/springer_10.1007-978-3-642-95855-7 |date=2012 |orig-year=1999 |publisher=Springer‑Verlag |doi=10.1007/978-3-642-95855-7 |isbn=978-3-642-95855-7| page=[https://archive.org/details/springer_10.1007-978-3-642-95855-7/page/n48 26]|s2cid=19897299 }}</ref>

== Criticism ==

In 2001, [[Marshall Rose]] characterized several deployment problems when applying Postel's principle in the design of a new application protocol.<ref>{{cite IETF |title=On the Design of Application Protocols |rfc=3117 |last=Rose |first=M. |author-link=Marshall Rose |date=November 2001 |publisher=[[Internet Engineering Task Force|IETF]] |access-date=June 9, 2014}}</ref> For example, a defective implementation that sends non-conforming messages might be used only with implementations that tolerate those deviations from the specification until, possibly several years later, it is connected with a less tolerant application that rejects its messages. In such a situation, identifying the problem is often difficult, and deploying a solution can be costly. Rose therefore recommended "explicit consistency checks in a protocol&nbsp;... even if they impose implementation overhead".

From 2015 to 2018, in a series of [[Internet Draft]]s, Martin Thomson argues that Postel's robustness principle actually leads to a ''lack'' of robustness, including security:<ref>{{cite IETF |title=The Harmful Consequences of the Robustness Principle |last=Thomson |first=Martin |date=May 2019 |url=https://tools.ietf.org/html/draft-iab-protocol-maintenance |publisher=[[Internet Engineering Task Force|IETF]] |access-date=October 4, 2019}}</ref>{{Quote|A flaw can become entrenched as a de facto standard. Any implementation of the protocol is required to replicate the aberrant behavior, or it is not interoperable. This is both a consequence of applying the robustness principle, and a product of a natural reluctance to avoid fatal error conditions. Ensuring interoperability in this environment is often referred to as aiming to be "[[Bug compatibility|bug for bug compatible]]".}}

In 2018, a paper on [[privacy-enhancing technologies]] by Florentin Rochet and Olivier Pereira showed how to exploit Postel's robustness principle inside the [[Tor (anonymity network)|Tor]] [[Onion routing|routing protocol]] to compromise the anonymity of onion services and Tor clients.<ref>{{cite journal | url = https://petsymposium.org/2018/files/papers/issue2/popets-2018-0011.pdf | title = Dropping on the Edge: Flexibility and Traffic Confirmation in Onion Routing Protocols | first1 = Florentin | last1 = Rochet | first2= Olivier | last2 = Pereira | journal = Proceedings of the Privacy Enhancing Technologies Symposium | issn = 2299-0984 | publisher = De Gruyter Open | year = 2018 | volume = 2018 | issue = 2 | pages = 27–46 | doi = 10.1515/popets-2018-0011 }}</ref>

FromIn 2015 to 20182023, inMartin aThomson seriesand ofDavid [[Internet Draft]]s, Martin ThomsonSchinazi arguesargued that Postel's robustness principle actually leads to a ''lack'' of robustness, including security:<ref>{{cite IETFRef RFC|title=The Harmful Consequences of the Robustness Principle |last=Thomson |first=Martin |date=May 2019 |url=https://tools.ietf.org/html/draft-iab-protocol-maintenance |publisher=[[Internet Engineering Task Force|IETF]] |access-date=October 4, 20199413}}</ref>{{Quote|A flaw can become entrenched as a de facto standard. Any implementation of the protocol is required to replicate the aberrant behavior, or it is not interoperable. This is both a consequence of applyingtolerating the robustness principle,unexpected and a product of a natural reluctance to avoid fatal error conditions. Ensuring interoperability in this environment is often referred to as aiming to be "[[Bug compatibility|bug -for -bug compatible]]".}}

== See also ==

{{Portal|Internet}}

* [[Normalization of deviance]]

* [[Open–closed principle]]

* [[Protocol ossification]]

* [[Static discipline]]

* [[Unix philosophy]]