Network Working Group M. Thomson
Internet-Draft Mozilla
Intended status: Informational March 11, 2019
Expires: September 12, 2019
The Harmful Consequences of the Robustness Principle
draft-iab-protocol-maintenance-02
Abstract
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. For a protocol
that is actively maintained, the Postel's robustness principle can,
and should, be avoided.
Status of This Memo
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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. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Informative References . . . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
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
[PRINCIPLES].
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.
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The flaw in Postel's logic originates from the presumption of an
inability to affect change in a system the size of the Internet.
That is, once a protocol specification is published, changes that
might be different to the practice of existing implementations are
not feasible.
Many of the shortcomings that 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 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.
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 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, the same statement 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.
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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.
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
received.
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".
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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
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
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
discover.
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.
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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.
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 it harder, or even
impossible, to fix 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.
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 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 protocol implementers is a critical part of this
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process, as they provide input on their experience with
implementation and deployment of the protocol.
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. 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
deployed.
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
inputs.
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 can confidently predict the effect that
feature will have on existing implementations.
Relying on implementations consistently applying 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 is preferable in
the absence of strong reasons to be flexible.
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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 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 intolerant implementation,
it receives strong feedback that allows problems to be discovered
quickly.
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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 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 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. 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.
9. IANA Considerations
This document has no IANA actions.
10. Informative References
[ECMA262] "ECMAScript(R) 2018 Language Specification", ECMA-262 9th
Edition, June 2018, <https://www.ecma-
international.org/publications/standards/Ecma-262.htm>.
[EXT] Carpenter, B., Aboba, B., Ed., and S. Cheshire, "Design
Considerations for Protocol Extensions", RFC 6709,
DOI 10.17487/RFC6709, September 2012,
<https://www.rfc-editor.org/info/rfc6709>.
[HOSTS] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[HTML] "HTML", WHATWG Living Standard, March 2019,
<https://html.spec.whatwg.org/>.
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[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,
<https://www.rfc-editor.org/info/rfc7230>.
[HTTP2] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[I-JSON] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/info/rfc7493>.
[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,
<https://www.rfc-editor.org/info/rfc8240>.
[IP] Postel, J., "DoD standard Internet Protocol", RFC 760,
DOI 10.17487/RFC0760, January 1980,
<https://www.rfc-editor.org/info/rfc760>.
[JSON] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627,
DOI 10.17487/RFC4627, July 2006,
<https://www.rfc-editor.org/info/rfc4627>.
[JSON-BIS]
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>.
[PRINCIPLES]
Carpenter, B., Ed., "Architectural Principles of the
Internet", RFC 1958, DOI 10.17487/RFC1958, June 1996,
<https://www.rfc-editor.org/info/rfc1958>.
[SUCCESS] Thaler, D. and B. Aboba, "What Makes for a Successful
Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,
<https://www.rfc-editor.org/info/rfc5218>.
[TLS] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
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[UNCOORDINATED]
Bryant, S., Ed., Morrow, M., Ed., and IAB, "Uncoordinated
Protocol Development Considered Harmful", RFC 5704,
DOI 10.17487/RFC5704, November 2009,
<https://www.rfc-editor.org/info/rfc5704>.
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
Mozilla
Email: mt@lowentropy.net
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