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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


   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
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   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
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of

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   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

   Of the great many contributions Jon Postel made to the Internet, his
   remarkable technical achievements are often ignored in favor of the
   design and implementation philosophy that he first captured in the
   original IPv4 specification [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.

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   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

   Divergent implementations of a specification emerge over time.  When
   variations occur in the interpretation or expression of semantic
   components, implementations cease to be perfectly interoperable.

   Implementation bugs are often identified as the cause of variation,
   though it is often a combination of factors.  Application of a
   protocol to new and unanticipated uses, and ambiguities or errors in
   the specification are often confounding factors.

   Of course, situations where two peers disagree are common, and should
   be expected over the lifetime of a protocol.  Even with the best
   intentions, the pressure to interoperate can be significant.  No
   implementation can hope to avoid having to trade correctness for
   interoperability indefinitely.

   An implementation that reacts to variations in the manner advised by
   Postel sets up a feedback cycle:

   o  Over time, implementations progressively add new code to constrain
      how data is transmitted, or to permit variations what is received.

   o  Errors in implementations, or confusion about semantics can
      thereby be masked.

   o  As a result, errors can become entrenched, forcing other
      implementations to be tolerant of those errors.

   An entrenched flaw can become 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 Postel's advice, and a product of a natural reluctance to
   avoid fatal error conditions.  This is colloquially referred to as
   being "bug for bug compatible".

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   It is debatable as to whether such a process can be completely
   avoided, but Postel's maxim encourages a reaction that compounds this

3.  The Long Term Costs

   Once deviations become entrenched, there is little that can be done
   to rectify the situation.

   For widely used protocols, the massive scale of the Internet makes
   large scale interoperability testing infeasible for all a privileged
   few.  Without good maintenance, new implementations can be restricted
   to niche uses, where the prolems arising from interoperability issues
   can be more closely managed.

   This has a negative impact on the ecosystem of a protocol.  New
   implementations of a protocol are important in ensuring the continued
   viability of a protocol.  New protocol implementations are also more
   likely to be developed for new and diverse use cases and often are
   the origin of features and capabilities that can be of benefit to
   existing users.  These problems also reduce the ability of
   established implementations to change.

   Protocol maintenance can help by carefully documenting divergence and
   recommending limits on what is both acceptable and interoperable.
   The time-consuming process of documenting the actual protocol -
   rather than the protocol as it was originally conceived - can restore
   the ability to create and maintain interoperable implementations.

   Such a process was undertaken for HTTP/1.1 [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

   The following principle applies not just to the implementation of a
   protocol, but to the design and specification of the protocol.

      Protocol designs and implementations should be maximally strict.

   Though less pithy than Postel's formulation, this principle is based
   on the lessons of protocol deployment.  The principle is also based
   on valuing early feedback, a practice central to modern engineering

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4.1.  Fail Fast and Hard

   Protocols need to include error reporting mechanisms that ensure
   errors are surfaced in a visible and expedient fashion.

   Generating fatal errors for what would otherwise be a minor or
   recoverable error is preferred, especially if there is any risk that
   the error represents an implementation flaw.  A fatal error provides
   excellent motivation for addressing problems.

   On the whole, implementations already have ample motivation to prefer
   interoperability over correctness.  The primary function of a
   specification is to proscribe behavior in the interest of

4.2.  Implementations Are Ultimately Responsible

   Implementers are encouraged to expose errors immediately and
   prominently in addition to what a specification mandates.

   Exposing errors is particularly important for early implementations
   of a protocol.  If preexisting implementations generate errors in
   response to divergent behaviour, then new implementations will be
   able to detect and correct flaws quickly.

   An implementer that discovers a scenario that is not covered by the
   specification does the community a greater service by generating a
   fatal error than by attempted to interpret and adapt.  Hiding errors
   can cause long-term problems.  Ideally, specification shortcomings
   are taken to protocol maintainers.

   Unreasoning strictness can be detrimental.  Protocol designers and
   implementers expected to exercise judgment in determining what level
   of strictness is ultimately appropriate.  In every case, documenting
   the decision to deviate from what is specified can avoid later

4.3.  Protocol Maintenance is Important

   Protocol designers are strongly encouraged to continue to maintain
   and evolve protocols beyond their initial inception and definition.
   If protocol implementations are less tolerant of variation, protocol
   maintenance becomes critical.  Good extensibility [RFC6709] can
   relieve some of the pressure on maintenance.

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5.  IANA Considerations

   This document has no IANA actions.

6.  Security Considerations

   Sloppy implementations, lax interpretations of specifications, and
   uncoordinated extrapolation of requirements to cover gaps in
   specification can result in security problems.  Hiding the
   consequences of protocol variations encourages the hiding of issues,
   which can conceal bugs and make them difficult to discover.

   Designers and implementers of security protocols generally understand
   these concerns.  However, general-purpose protocols are not exempt
   from careful consideration of security issues.  Furthermore, because
   general-purpose protocols tend to deal with flaws or obsolescence in
   a less urgent fashion than security protocols, there can be fewer
   opportunities to correct problems in protocols that develop
   interoperability problems.

7.  Informative References

   [RFC0760]  Postel, J., "DoD standard Internet Protocol", RFC 760,
              January 1980.

   [RFC6709]  Carpenter, B., Aboba, B., and S. Cheshire, "Design
              Considerations for Protocol Extensions", RFC 6709,
              September 2012.

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230, June

Author's Address

   Martin Thomson

   Email: martin.thomson@gmail.com

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