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Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Informational                              May 03, 2018
Expires: November 4, 2018

          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, but can produce
   negative effects in the protocol ecosystem in the long term.  Those
   effects can be avoided by maintaining protocols.

Status of This Memo

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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
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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.  Fallibility of Specifications . . . . . . . . . . . . . . . .   3
   3.  Protocol Decay  . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Ecosystem Effects . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Active Protocol Maintenance . . . . . . . . . . . . . . . . .   5
   6.  The Role of Feedback  . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Error Handling  . . . . . . . . . . . . . . . . . . . . .   7
     6.2.  Feedback from Implementations . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Of the great many contributions Jon Postel made to the Internet, his
   remarkable technical achievements are often shadowed by his
   contribution of a design and implementation philosophy known as the
   robustness principle:

      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 in shaping
   the Internet and the systems that use Internet protocols.  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.
   That experience shows that there are negative long-term consequences
   to interoperability if an implementation applies Postel's advice.

   This document shows that flaw in Postel's logic originates from the
   presumption of immutability of protocol specifications.  Thus rather

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   than apply the robustness principle, this document recommends
   continuing maintenance for protocols beyond their initial design and
   deployment.  Active maintenance of protocols reduces or eliminates
   the opportunities to apply Postel's guidance.

   There is good evidence to suggest that protocols are routinely
   maintained beyond their inception.  This document serves primarily as
   a record of the shortcomings of the robustness principle.

2.  Fallibility of Specifications

   What is often missed in discussions of the robustness principle is
   the context in which it appears.  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.

   This motivating statement is a frank admission of fallibility and
   remarkable for it.  Here Postel recognizes the possibility that the
   specification could be imperfect.  This is an important statement,
   but inexplicably absent from the later versions in [HOSTS] and

   Indeed, 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.

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

   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.

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   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.  Situations where
   two peers disagree on interpretation 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 in what is

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

   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, 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, TLS demonstrates the effect of bugs.  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 extension.

   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

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   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 interoperability can be difficult to learn.

   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 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
   accumulation of mitigations for interoperability issues makes
   implementations more difficult to maintain.

5.  Active Protocol Maintenance

   The robustness principle is best suited to safeguarding against flaws
   in a specification that is intended to remain unchanged for an
   extended period of time.  Indeed, in the face of divergent
   interpretations of an immutable specification, the only hope for an
   implementation to remain interoperable is to be tolerant of
   differences in interpretation and an occasional outright
   implementation error.

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   From this perspective, application of Postel's advice to the
   implementation of a protocol specification that does not change is
   logical, even necessary.  But that suggests that the problem is with
   the presumption of immutability of specifications.

   Active maintenance of a protocol can ensure that specifications
   remain accurate and that new implementations are possible.  Protocol
   designers are strongly encouraged to continue to maintain and evolve
   protocols beyond their initial inception and definition.

   Maintenance is needed in response to the discovery of errors in
   specification that might cause interoperability issues.  Maintenance
   is also critical for ensuring that the protocol is viable for
   application to use cases that might not have been envisaged during
   its original design.  New use cases are an indicator that the
   protocol could be successful [SUCCESS].

   Maintenance does not necessarily involve the development of new
   versions of protocols or protocol specifications.  For instance, RFC
   793 [TCP] remains the canonical TCP reference, but a large number of
   update and extension RFCs together document the protocol as deployed.

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

   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.  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.

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   Managing and deploying changes to implementations can be expensive.
   However, it is widely recognized that maintenance is a critical part
   of the deployment of computer systems for security reasons [IOTSU].

6.1.  Error Handling

   Ideally, specifications include rules for consistent handling of
   aberrant conditions as well as expected.  This increases the changes
   that implementations have interoperable handling of unusual

   Choosing to generate fatal error for unspecified conditions instead
   of attempting error recovery can ensure that faults receive
   attention.  Fatal errors can provide excellent motivation to address
   a problem if they are sufficiently rare.

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

6.2.  Feedback from Implementations

   Automated error reporting mechanisms in protocol implementations
   allows for better feedback from deployments.  Exposing faults through
   operations and management systems 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 consideration to
   protection of the privacy of protocol participants is critical prior
   to deploying any such system.

7.  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.

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8.  IANA Considerations

   This document has no IANA actions.

9.  Informative References

   [ECMA262]  "ECMAScript(R) 2017 Language Specification", ECMA-262 8th
              Edition, June 2017, <http://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, October 2017,

   [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,

   [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,

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              Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://www.rfc-editor.org/info/rfc7159>.

              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,

   [TCP]      Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,

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

Appendix A.  Acknowledgments

   Constructive feedback on this document has been provided by a
   surprising number of people including Mark Nottingham, Brian
   Trammell, and Anne Van Kesteren.  Please excuse any omission.

Author's Address

   Martin Thomson

   Email: martin.thomson@gmail.com

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