Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Informational                            March 11,                              May 07, 2019
Expires: September 12, November 8, 2019

          The Harmful Consequences of the Robustness Principle


   Jon Postel's famous statement of "Be liberal in what you accept, and
   conservative in what you send" is a principle that has long guided
   the design and implementation of Internet protocols.  The posture
   this statement advocates promotes interoperability in the short term,
   but can negatively affect the protocol ecosystem. ecosystem over time.  For a
   protocol that is actively maintained, the Postel's robustness principle can,
   and should, be avoided.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Fallibility of Specifications . . . . . . . . . . . . . . . .   3
   3.  Protocol Decay  . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Ecosystem Effects . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Active Protocol Maintenance . . . . . . . . . . . . . . . . .   6
   6.  Extensibility . . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  The Role of Feedback  . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Feedback from Implementations . . . . . . . . . . . . . .   8
     7.2.  Virtuous Intolerance  . . . . . . . . . . . . . . . . . .   8
   8.  Risk of Exclusion . . . . . . . . . . . . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10.  10
   11. Informative References  . . . . . . . . . . . . . . . . . . .   9  10
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Of the great many contributions

   Jon Postel made to Postel's robustness principle has been hugely influential in
   shaping the Internet, his
   remarkable technical achievements are often shadowed by his
   contribution of a design and implementation philosophy known as of the Internet.  As stated in IAB RFC 1958
   [PRINCIPLES], the robustness principle: principle advises to:

      Be strict when sending and tolerant when receiving.
      Implementations must follow specifications precisely when sending
      to the network, and tolerate faulty input from the network.  When
      in doubt, discard faulty input silently, without returning an
      error message unless this is required by the specification.

   This being the version of the text that appears in IAB RFC 1958

   Postel's robustness principle has been hugely influential simple statement captures a significant concept in shaping the Internet and the systems that use Internet protocols. design of
   interoperable systems.  Many consider the application of the
   robustness principle to be instrumental in the success of the
   Internet as well as the design of interoperable protocols in general.

   Over time, considerable experience has been accumulated with
   protocols that were designed by the application of Postel's maxim.

   Time and experience shows that there are negative long-term consequences to
   interoperability accumulate over time if an implementation applies Postel's advice.

   The flaw in Postel's logic implementations apply the
   robustness principle.  This problem originates from the presumption of an
   inability assumption
   implicit in the principle that it is not possible to affect change in
   a system the size of the Internet.  That is, the idea that once a
   protocol specification is published, changes that might be different to the practice of require
   existing implementations to change are not feasible.

   Many of the shortcomings problems that might lead to applications of the robustness
   principle are avoided for protocols under active maintenance.  Active
   protocol maintenance is where a community of protocol designers,
   implementers, and deployers work together to continuously improve and
   evolve protocols.  A community that takes an active role in the
   maintenance of protocols can greatly reduce and even eliminate
   opportunities to apply Postel's guidance. the robustness principle.

   There is good evidence to suggest that many important protocols are
   routinely maintained beyond their inception.  This document serves
   primarily as a record of the hazards inherent in applying the
   robustness principle and to offer an alternative strategy for
   handling interoperability problems in deployments.

   Ideally, protocol implementations never have to apply the robustness
   principle.  Or, where it is unavoidable, any application can use of the robustness
   principle is viewed as a short term workaround that needs to be
   quickly reverted.

2.  Fallibility of Specifications

   The context from which the robustness principle was developed
   provides valuable insights into its intent and purpose.  The earliest
   form of the principle in the RFC series (in RFC 760 [IP]) is preceded
   by a sentence that reveals the motivation for the principle:

      While the goal of this specification is to be explicit about the
      protocol there is the possibility of differing interpretations.
      In general, an implementation should be conservative in its
      sending behavior, and liberal in its receiving behavior.

   Here Postel recognizes the possibility that the specification could
   be imperfect.  As a frank admission of fallibility it is a
   significant statement.  However,  This contextualizes the same statement principle in an important way.
   However, that context is inexplicably absent from the later versions
   in [HOSTS] and [PRINCIPLES].

   An imperfect specification is natural, largely because it is more
   important to proceed to implementation and deployment than it is to
   perfect a specification.  A protocol, like any complex system,
   benefits greatly from experience with its use.  A deployed protocol
   is immeasurably more useful than a perfect protocol.  The robustness
   principle is a tool that is suited to early phases of system design.

   As [SUCCESS] demonstrates, success or failure of a protocol depends
   far more on factors like usefulness than on on technical excellence.
   Postel's timely
   Timely publication of protocol specifications, even with the
   potential for flaws, likely had a significant effect in contributed significantly to the eventual
   success of the Internet.

   The problem is therefore not with the premise, but with its
   conclusion: the robustness principle itself.

3.  Protocol Decay

   The application of the robustness principle to the early Internet, or
   any system that is in early phases of deployment, is expedient.  The
   consequence of applying the principle is deferring the effort of
   dealing with interoperability problems, which can amplify the
   ultimate cost of handling those problems.

   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 uses that were not anticipated in the original design, or
   ambiguities and errors in the specification are often confounding
   factors.  Situations where
   two peers disagree  Disagreements on the interpretation of specifications
   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 recommended
   in the robustness principle sets up a feedback cycle:

   o cycle.  Over time, implementations time:

   o  Implementations progressively add new code logic to constrain how data is
      transmitted, or to permit variations in what is received.

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

   o  These errors can become entrenched, forcing other implementations
      to be tolerant of those errors.

   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 Postel's advice, the robustness principle, and a product of a natural
   reluctance to avoid fatal error conditions.  Ensuring
   interoperability in this environment is often colloquially referred to as aiming
   to be "bug for bug compatible".

   For example, in TLS [TLS] extensions use a tag-length-value format,
   and they can be added to messages in any order.  However, some server
   implementations terminate connections if they encounter a TLS
   ClientHello message that ends with an empty extension.  To maintain
   interoperability, client implementations are required to be aware of
   this bug and ensure that a ClientHello message ends in a non-empty

   The original JSON specification [JSON] demonstrates the effect of
   specification shortcomings.  RFC 4627 omitted critical details on a
   range of key details including Unicode handling, ordering and
   duplication of object members, and number encoding.  Consequently, a
   range of interpretations were used by implementations.  An updated
   specification [JSON-BIS] did not correct these errors, concentrating
   instead on identifying the interoperable subset of JSON.  I-JSON
   [I-JSON] takes that subset and defines a new format that prohibits
   the problematic parts of JSON.  Of course, that means that I-JSON is
   not fully interoperable with JSON.  Consequently, I-JSON is not
   widely implemented in parsers.  Many JSON parsers now implement the
   more precise algorithm specified in [ECMA262].

   The robustness principle therefore encourages a reaction that
   compounds and entrenches interoperability problems.

4.  Ecosystem Effects

   Once deviations become entrenched, it can be extremely difficult - if
   not impossible - to rectify the situation.

   For widely used protocols, the massive scale of the Internet makes
   large-scale interoperability testing infeasible for all but a
   privileged few.  The cost of building a new implementation increases
   as the number of implementations and bugs increases.  Worse, the set
   of tweaks necessary for wide interoperability can be difficult to

   Consequently, new implementations can be restricted to niche uses,
   where the problems arising from interoperability issues can be more
   closely managed.  Restricting new implementations to narrow contexts
   also  However, restricting new implementations into
   limited deployments risks causing forks in the protocol.  If
   implementations do not interoperate, little prevents those
   implementations from diverging more over time.

   This has a negative impact on the ecosystem of a protocol.  New
   implementations 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.

   The need to work around interoperability problems also reduces the
   ability of established implementations to change.  For instance, an  An accumulation of
   mitigations for interoperability issues makes implementations more
   difficult to maintain. maintain and can constrain extensibility (see also

   Sometimes what appear to be interoperability problems are symptomatic
   of issues in protocol design.  A community that is willing to make
   changes to the protocol, by revising or extending it, makes the
   protocol better in the process.  Applying the robustness principle
   might conceal the problem.  That can make
   instead conceals problems, making it harder, or even impossible, to
   fix them later.

   A similar class of problems is described in RFC 5704 [UNCOORDINATED],
   which addresses conflict or competition in the maintenance of
   protocols.  This document concerns itself primarily with the absence
   of maintenance, though the problems are similar.

5.  Active Protocol Maintenance

   The robustness principle can be highly effective in safeguarding
   against flaws in the implementation of a protocol by peers.
   Especially when a specification remains unchanged for an extended
   period of time, the inclination to be tolerant accumulates over time.
   Indeed, when faced with divergent interpretations of an immutable
   specification, the best way for an implementation to remain
   interoperable is to be tolerant of differences in interpretation and
   an occasional outright
   implementation error. errors.

   From this perspective, application of Postel's advice the robustness principle to the
   implementation of a protocol specification that does not change is
   logical, even necessary.  But that suggests that the problem is with
   the assumption that the situation - existing specifications and
   implementations - are unable to change.

   As already established, this is not sustainable.  For a protocol to be
   viable, it is necessary for both specifications and implementations
   to be responsive to changes, in addition to handling new and old
   problems that might arise over time.

   Active maintenance of a protocol is critical in ensuring that
   specifications correctly reflect the requirements for
   interoperability.  Maintenance enables both new implementations and
   the continued improvement of the protocol.  New use cases are an
   indicator that the protocol could be successful [SUCCESS].

   Protocol designers are strongly encouraged to continue to maintain
   and evolve protocols beyond their initial inception and definition.
   Involvement of those who implement and deploy the protocol implementers is a
   critical part of this process, as they provide input on their
   experience with
   implementation and deployment of how the protocol. protocol is used.

   Most interoperability problems do not require revision of protocols
   or protocol specifications.  For instance, the most effective means
   of dealing with a defective implementation in a peer could be to
   email the developer of the stack. responsible.  It is far more efficient in the
   long term to fix one isolated bug than it is to deal with the
   consequences of workarounds.

   Neglect can quickly produce the negative consequences this document
   describes.  Restoring the protocol to a state where it can be
   maintained involves first discovering the properties of the protocol
   as it is deployed, rather than the protocol as it was originally
   documented.  This can be difficult and time-consuming, particularly
   if the protocol has a diverse set of implementations.  Such a process
   was undertaken for HTTP [HTTP] after a period of minimal maintenance.
   Restoring HTTP specifications to currency took significant effort.

6.  Extensibility

   Good extensibility [EXT] can make it easier to respond to new use
   cases or changes in the environment in which the protocol is

   Extensibility is sometimes mistaken for an application of the
   robustness principle.  After all, if one party wants to start using a
   new feature before another party is prepared to receive it, it might
   be assumed that the receiving party is being tolerant of unexpected

   A well-designed extensibility mechanism establishes clear rules for
   the handling of things like new messages or parameters.  If an
   extension mechanism is designed and implemented correctly, the user
   of a new
   protocol feature features can confidently predict be deployed with confidence in the
   understanding of the effect that
   feature will they have on existing implementations.


   In contrast, relying on implementations to consistently applying apply the
   robustness principle is not a good strategy for extensibility.  Using
   undocumented or accidental features of a protocol as the basis of an
   extensibility mechanism can be extremely difficult, as is
   demonstrated by the case study in Appendix A.3 of [EXT].

   A protocol could be designed to permit a narrow set of valid inputs,
   or it could allow a wide range of inputs as a core feature (see for
   example [HTML]).  Specifying and implementing a more flexible
   protocol is more difficult; allowing less variation variability is preferable
   in the absence of strong reasons to be flexible.

7.  The Role of Feedback

   Protocol maintenance is only possible if there is sufficient
   information about the deployment of the protocol.  Feedback from
   deployment is critical to effective protocol maintenance.

   For a protocol specification, the primary and most effective form of
   feedback comes from people who implement and deploy the protocol.
   This comes in the form of new requirements, or in experience with the
   protocol as it is deployed.

   Managing and deploying changes to implementations can be expensive.
   However, it is widely recognized that regular updates are a vital
   part of the deployment of computer systems for security reasons (see
   for example [IOTSU]).

7.1.  Feedback from Implementations

   Automated error reporting mechanisms in protocol implementations
   allows for better feedback from deployments.  Exposing faults through
   operations and management systems interfaces is highly valuable, but it might
   be necessary to ensure that the information is propagated further.

   Building telemetry and error logging systems that report faults to
   the developers of the implementation is superior in many respects.
   However, this is only possible in deployments that are conducive to
   the collection of this type of information.  Giving due consideration
   to protection of the privacy of protocol participants is critical
   prior to deploying any such system.

7.2.  Virtuous Intolerance

   A well-specified protocol includes rules for consistent handling of
   aberrant conditions.  This increases the changes that implementations
   have interoperable handling of unusual conditions.

   Intolerance of any deviation from specification, where
   implementations generate fatal errors in response to observing
   undefined or unusal behaviour, can be harnessed to reduce occurrences
   of aberrant implementations.  Choosing to generate fatal errors for
   unspecified conditions instead of attempting error recovery can
   ensure that faults receive attention.

   This improves feedback for new implementations in particular.  When a
   new implementation encounters a virtuously an intolerant implementation, it
   receives strong feedback that allows problems to be discovered

   To be effective, virtuously intolerant implementations need to be sufficiently
   widely deployed that they are encountered by new implementations with
   high probability.  This could depend on multiple implementations of
   deploying strict checks.

   Any intolerance also needs to be strongly supported by
   specifications, otherwise they encourage fracturing of the protocol
   community or proliferation of workarounds.

   Virtuous intolerance workarounds (see Section 8).

   Intolerance can be used to motivate compliance with any protocol
   requirement.  For instance, the INADEQUATE_SECURITY error code and
   associated requirements in HTTP/2 [HTTP2] resulted in improvements in
   the security of the deployed base.

8.  Risk of Exclusion

   Any protocol participant that is affected by changes arising from
   maintenance might be excluded if they are unwilling or unable to
   implement or deploy changes that are made to the protocol.  RFC 5704
   [UNCOORDINATED] describes how conflict or competition in the
   maintenance of protocols can lead to the same sorts of problems.

   The effect on existing systems is an important design criterion when
   considering changes to a protocol.  While compatible changes are
   always preferable to incompatible ones, it is not always possible to
   produce a design that allow all current protocol participants to
   continue to participate.

   Excluding implementations or deployments can lead to a fracturing of
   the protocol system that could be more harmful than any divergence
   resulting from following the robustness principle.  Any change to a
   protocol carries a risk of exclusion, but exclusion is a direct goal
   when choosing to be intolerant of errors (see Section 7.2).  Any
   change that excludes implementations needs extraordinary care to
   ensure that the effect on existing deployments is understood and

9.  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.

   The consequences of the problems described in this document are
   especially acute for any protocol where security depends on agreement
   about semantics of protocol elements.


10.  IANA Considerations

   This document has no IANA actions.


11.  Informative References

   [ECMA262]  "ECMAScript(R) 2018 Language Specification", ECMA-262 9th
              Edition, June 2018, <https://www.ecma-

   [EXT]      Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
              Considerations for Protocol Extensions", RFC 6709,
              DOI 10.17487/RFC6709, September 2012,

   [HOSTS]    Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,

   [HTML]     "HTML", WHATWG Living Standard, March 2019,

   [HTTP]     Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [HTTP2]    Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [I-JSON]   Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
              DOI 10.17487/RFC7493, March 2015,

   [IOTSU]    Tschofenig, H. and S. Farrell, "Report from the Internet
              of Things Software Update (IoTSU) Workshop 2016",
              RFC 8240, DOI 10.17487/RFC8240, September 2017,

   [IP]       Postel, J., "DoD standard Internet Protocol", RFC 760,
              DOI 10.17487/RFC0760, January 1980,

   [JSON]     Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627,
              DOI 10.17487/RFC4627, July 2006,

              Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <>.

              Carpenter, B., Ed., "Architectural Principles of the
              Internet", RFC 1958, DOI 10.17487/RFC1958, June 1996,

   [SUCCESS]  Thaler, D. and B. Aboba, "What Makes for a Successful
              Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,

   [TLS]      Dierks, T. and E.      Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.2", 1.3", RFC 5246, 8446, DOI 10.17487/RFC5246, 10.17487/RFC8446, August 2008,
              <>. 2018,

              Bryant, S., Ed., Morrow, M., Ed., and IAB, "Uncoordinated
              Protocol Development Considered Harmful", RFC 5704,
              DOI 10.17487/RFC5704, November 2009,

   [USE-IT]   Thomson, M., "Long-term Viability of Protocol Extension
              Mechanisms", draft-thomson-use-it-or-lose-it-03 (work in
              progress), January 2019.

Appendix A.  Acknowledgments

   Constructive feedback on this document has been provided by a
   surprising number of people including Bernard Aboba, Brian Carpenter,
   Mark Nottingham, Russ Housley, Henning Schulzrinne, Robert Sparks,
   Brian Trammell, and Anne Van Kesteren.  Please excuse any omission.

Author's Address

   Martin Thomson