Network Working Group M. Thomson Internet-Draft Mozilla Intended status: Informational March 9, 2015 Expires: September 10, 2015 The Harmful Consequences of Postel's Maxim draft-thomson-postel-was-wrong-00 Abstract Jon Postel's famous statement in RFC 1122 of "Be liberal in what you accept, and conservative in what you send" - is a principle that has long guided the design of Internet protocols and implementations of those protocols. The posture this statement advocates might promote interoperability in the short term, but that short term advantage is outweighed by negative consequences that affect the long term maintenance of a protocol and its ecosystem. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on September 10, 2015. Copyright Notice Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of Thomson Expires September 10, 2015 [Page 1]

Internet-Draft Elephants Out, Donkeys In March 2015 the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. The Protocol Decay Hypothesis . . . . . . . . . . . . . . . . 3 3. The Long Term Costs . . . . . . . . . . . . . . . . . . . . . 4 4. A New Design Principle . . . . . . . . . . . . . . . . . . . 4 4.1. Fail Fast and Hard . . . . . . . . . . . . . . . . . . . 5 4.2. Implementations Are Ultimately Responsible . . . . . . . 5 4.3. Protocol Maintenance is Important . . . . . . . . . . . . 5 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 7. Informative References . . . . . . . . . . . . . . . . . . . 6 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6 1 . Introduction RFC0760]: In general, an implementation should be conservative in its sending behavior, and liberal in its receiving behavior. In comparison, his contributions to the underpinnings of the Internet, which are in many respects more significant, enjoy less conscious recognition. Postel's principle has been hugely influential in shaping the Internet and the systems that use Internet protocols. Many consider this principle to be instrumental in the success of the Internet as well as the design of interoperable protocols in general. Over time, considerable changes have occurred in both the scale of the Internet and the level of skill and experience available to protocol and software designers. Part of that experience is with protocols that were designed, informed by Postel's maxim, in the early phases of the Internet. That experience shows that there are negative long-term consequences to interoperability if an implementation applies Postel's advice. Correcting the problems caused by divergent behavior in implementations can be difficult or impossible. Thomson Expires September 10, 2015 [Page 2]

Internet-Draft Elephants Out, Donkeys In March 2015 It might be suggested that the posture Postel advocates was indeed necessary during the formative years of the Internet, and even key to its success. This document takes no position on that claim. This document instead describes the negative consequences of the application of Postel's principle to the modern Internet. A replacement design principle is suggested. There is good evidence to suggest that designers of protocols in the IETF widely understand the limitations of Postel's principle. This document serves primarily as a record of the shortcomings of His principle for the wider community. 2 . The Protocol Decay Hypothesis Thomson Expires September 10, 2015 [Page 3]

Internet-Draft Elephants Out, Donkeys In March 2015 It is debatable as to whether such a process can be completely avoided, but Postel's maxim encourages a reaction that compounds this issue. 3 . The Long Term Costs RFC7230]. This this effort took more than 6 years, it has been successful in documenting protocol variations and describing what has over time become a far more complex protocol. 4 . A New Design Principle Thomson Expires September 10, 2015 [Page 4]

Internet-Draft Elephants Out, Donkeys In March 2015 4.1 . Fail Fast and Hard 4.2 . Implementations Are Ultimately Responsible 4.3 . Protocol Maintenance is Important RFC6709] can relieve some of the pressure on maintenance. Thomson Expires September 10, 2015 [Page 5]