Network Working Group M. Thomson
Internet-Draft Mozilla
Intended status: Informational November 04, 2019
Expires: May 7, 2020
The Harmful Consequences of the Robustness Principle
draft-iab-protocol-maintenance-04
Abstract
The robustness principle, often phrased as "be conservative in what
you send, and liberal in what you accept", 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 over time. For a protocol
that is actively maintained, the robustness principle can, and
should, be avoided.
Note to Readers
Discussion of this document takes place on the Architecture-Discuss
mailing list (architecture-discuss@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/architecture-discuss/ [1].
Source for this draft and an issue tracker can be found at
https://github.com/intarchboard/protocol-maintenance [2].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on May 7, 2020.
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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 . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Virtuous Intolerance . . . . . . . . . . . . . . . . . . . . 8
8. Exclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Informative References . . . . . . . . . . . . . . . . . 10
11.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The robustness principle has been hugely influential in shaping the
design of the Internet. As stated in IAB RFC 1958 [PRINCIPLES], the
robustness 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 simple statement captures a significant concept in the design of
interoperable systems. Many consider the application of the
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robustness principle to be instrumental in the success of the
Internet as well as the design of interoperable protocols in general.
Time and experience shows that negative consequences to
interoperability accumulate over time if implementations apply the
robustness principle. This problem originates from an 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 require
existing implementations to change are not feasible.
Many 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 protocol specifications alongside implementations and
deployments of those protocols. A community that takes an active
role in the maintenance of protocols can greatly reduce and even
eliminate opportunities to apply the robustness principle.
There is good evidence to suggest that many important protocols are
routinely maintained beyond their inception. In particular, a
sizeable proportion of IETF activity is dedicated to the stewardship
of existing protocols. 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, 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.
This formulation of the principle expressly recognizes the
possibility that the specification could be imperfect. This
contextualizes the principle in an important way.
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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.
Timely publication of protocol specifications, even with the
potential for flaws, likely 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 uses that were not anticipated in the original design, or
ambiguities and errors in the specification are often confounding
factors. 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 recommended
in the robustness principle sets up a feedback cycle. Over time:
o Implementations progressively add logic to constrain how data is
transmitted, or to permit variations in what is received.
o Errors in implementations or confusion about semantics are
permitted or ignored.
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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 the robustness principle, and a product of a natural
reluctance to avoid fatal error conditions. Ensuring
interoperability in this environment is often 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
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 can
create interoperability problems. In particular, the application of
the robustness principle is particularly deleterious for early
implementations of new protocols as quirks in early implementations
can affect all subsequent deployments.
4. Ecosystem Effects
Once deviations become entrenched, it can be extremely difficult - if
not impossible - to rectify the situation.
Interoperability requirements for protocol implementations are set by
other deployments. Specifications and - where they exist -
conformance test suites might guide the initial development of
implementations, but implementations ultimately need to interoperate
with deployed implementations.
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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 using
reverse engineering 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 might be forced into niche uses,
where the problems arising from interoperability issues can be more
closely managed. 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. An accumulation of
mitigations for interoperability issues makes implementations more
difficult to maintain and can constrain extensibility (see also
[USE-IT]).
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
instead conceals problems, making it harder, or even impossible, to
fix them later.
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
implementation errors.
From this perspective, application of 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 existing specifications and implementations are
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unable to change. Applying the robustness principle in this way
disproportionately values short-term gains over the negative effects
on future implementations and the protocol as a whole.
For a protocol to have sustained viability, 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.
Maintaining specifications so that they closely match deployments
ensures that implementations are consistently interoperable and
removes needless barriers for new implementations. Maintenance also
enables 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 protocol specificationss beyond their initial inception
and definition. This might require the development of revised
specifications, extensions, or other supporting material that
documents the current state of the protocol. Involvement of those
who implement and deploy the protocol is a critical part of this
process, as they provide input on their experience with how the
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 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.
Early implementations of protocols have a stronger obligation to
closely follow specifications as their behavior will affect all
subsequent implementations. Protocol specifications might need more
frequent revision during early deployments to capture feedback from
early rounds of deployment.
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.
Maintenance is most effective if it is responsive, which is greatly
affected by how rapidly protocol changes can be deployed. For
protocol deployments that operate on longer time scales, temporary
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workarounds following the spirit of the robustness principle might be
necessary. If specifications can be updated more readily than
deployments, details of the workaround can be document, including the
desired form of the protocols once the need for workarounds no longer
exists and plans for removing the workaround.
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, new
protocol features can be deployed with confidence in the
understanding of the effect they have on existing implementations.
In contrast, relying on implementations to consistently 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 variability is preferable
in the absence of strong reasons to be flexible.
7. Virtuous Intolerance
A well-specified protocol includes rules for consistent handling of
aberrant conditions. This increases the chances that implementations
will have interoperable handling of unusual conditions.
Intolerance of any deviation from specification, where
implementations generate fatal errors in response to observing
undefined or unusual 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.
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This improves feedback for new implementations in particular. When a
new implementation encounters an intolerant implementation, it
receives strong feedback that allows problems to be discovered
quickly.
To be effective, intolerant implementations need to be sufficiently
widely deployed that they are encountered by new implementations with
high probability. This could depend on multiple implementations
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 (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. 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.
Deliberate exclusion of problematic implementations is an important
tool that can ensure that the interoperability of a protocol remains
viable. While compatible changes are always preferable to
incompatible ones, it is not always possible to produce a design that
protects the ability of all current and future protocol participants
to interoperate. Developing and deploying changes that risk
exclusion of previously interoperating implementations requires some
care, but changes to a protocol should not be blocked on the grounds
of the risk of exclusion alone.
Exclusion is a direct goal when choosing to be intolerant of errors
(see Section 7), which is deployed with the intent of protecting
future interoperability.
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. RFC 5704
[UNCOORDINATED] describes how conflict or competition in the
maintenance of protocols can lead to similar problems.
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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. For instance, use of unsafe
security mechanisms, such as weak primitives [MD5] or obsolete
mechanisms [SSL3], are good examples of where forcing exclusion
(Section 8) can be desirable.
10. IANA Considerations
This document has no IANA actions.
11. References
11.1. 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>.
[HTML] "HTML", WHATWG Living Standard, March 2019,
<https://html.spec.whatwg.org/>.
[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>.
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[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>.
[MD5] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/info/rfc6151>.
[PRINCIPLES]
Carpenter, B., Ed., "Architectural Principles of the
Internet", RFC 1958, DOI 10.17487/RFC1958, June 1996,
<https://www.rfc-editor.org/info/rfc1958>.
[SSL3] Barnes, R., Thomson, M., Pironti, A., and A. Langley,
"Deprecating Secure Sockets Layer Version 3.0", RFC 7568,
DOI 10.17487/RFC7568, June 2015,
<https://www.rfc-editor.org/info/rfc7568>.
[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] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[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>.
[USE-IT] Thomson, M., "Long-term Viability of Protocol Extension
Mechanisms", draft-thomson-use-it-or-lose-it-04 (work in
progress), July 2019.
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11.2. URIs
[1] https://mailarchive.ietf.org/arch/browse/architecture-discuss/
[2] https://github.com/intarchboard/protocol-maintenance
Appendix A. Acknowledgments
Constructive feedback on this document has been provided by a
surprising number of people including Bernard Aboba, Brian Carpenter,
Stuart Cheshire, 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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