Maintaining Robust Protocols
RFC 9413



Internet Architecture Board (IAB)                             M. Thomson
Request for Comments: 9413                                              
Category: Informational                                      D. Schinazi
ISSN: 2070-1721                                                June 2023

                      Maintaining Robust Protocols

Abstract

   The main goal of the networking standards process is to enable the
   long-term interoperability of protocols.  This document describes
   active protocol maintenance, a means to accomplish that goal.  By
   evolving specifications and implementations, it is possible to reduce
   ambiguity over time and create a healthy ecosystem.

   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.  However, it has been
   interpreted in a variety of ways.  While some interpretations help
   ensure the health of the Internet, others can negatively affect
   interoperability over time.  When a protocol is actively maintained,
   protocol designers and implementers can avoid these pitfalls.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Architecture Board (IAB)
   and represents information that the IAB has deemed valuable to
   provide for permanent record.  It represents the consensus of the
   Internet Architecture Board (IAB).  Documents approved for
   publication by the IAB are not candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9413.

Copyright Notice

   Copyright (c) 2023 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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Table of Contents

   1.  Introduction
   2.  Protocol Robustness
     2.1.  Fallibility of Specifications
     2.2.  Extensibility
     2.3.  Flexible Protocols
   3.  Applicability
   4.  Harmful Consequences of Tolerating the Unexpected
     4.1.  Protocol Decay
     4.2.  Ecosystem Effects
   5.  Active Protocol Maintenance
     5.1.  Virtuous Intolerance
     5.2.  Exclusion
   6.  Security Considerations
   7.  IANA Considerations
   8.  Informative References
   IAB Members at the Time of Approval
   Acknowledgments
   Authors' Addresses

1.  Introduction

   There is good evidence to suggest that many important protocols are
   routinely maintained beyond their inception.  In particular, a
   sizable proportion of IETF activity is dedicated to the stewardship
   of existing protocols.  This document first discusses hazards in
   applying the robustness principle too broadly (see Section 2) and
   offers an alternative strategy for handling interoperability problems
   in deployments (see Section 5).

   Ideally, protocol implementations can be actively maintained so that
   unexpected conditions are proactively identified and resolved.  Some
   deployments might still need to apply short-term mitigations for
   deployments that cannot be easily updated, but such cases need not be
   permanent.  This is discussed further in Section 5.

2.  Protocol Robustness

   The robustness principle has been hugely influential in shaping the
   design of the Internet.  As stated in the IAB document "Architectural
   Principles of the Internet" [RFC1958], 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
   robustness principle to be instrumental in the success of the
   Internet as well as the design of interoperable protocols in general.

   There are three main aspects to the robustness principle:

   Robustness to software defects:  No software is perfect, and failures
      can lead to unexpected behavior.  Well-designed software strives
      to be resilient to such issues, whether they occur in the local
      software or in software that it communicates with.  In particular,
      it is critical for software to gracefully recover from these
      issues without aborting unrelated processing.

   Robustness to attacks:  Since not all actors on the Internet are
      benevolent, networking software needs to be resilient to input
      that is intentionally crafted to cause unexpected consequences.
      For example, software must ensure that invalid input doesn't allow
      the sender to access data, change data, or perform actions that it
      would otherwise not be allowed to.

   Robustness to the unexpected:  It can be possible for an
      implementation to receive inputs that the specification did not
      prepare it for.  This scenario excludes those cases where a the
      specification explicitly defines how a faulty message is handled.
      Instead, this refers to cases where handling is not defined or
      where there is some ambiguity in the specification.  In this case,
      some interpretations of the robustness principle advocate that the
      implementation tolerate the faulty input and silently discard it.
      Some interpretations even suggest that a faulty or ambiguous
      message be processed according to the inferred intent of the
      sender.

   The facets of the robustness principle that protect against defects
   or attacks are understood to be necessary guiding principles for the
   design and implementation of networked systems.  However, an
   interpretation that advocates for tolerating unexpected inputs is no
   longer considered best practice in all scenarios.

   Time and experience show that negative consequences to
   interoperability accumulate over time if implementations silently
   accept faulty input.  This problem originates from an implicit
   assumption that it is not possible to effect change in a system the
   size of the Internet.  When one assumes that changes to existing
   implementations are not presently feasible, tolerating flaws feels
   inevitable.

   Many problems that this third aspect of the robustness principle was
   intended to solve can instead be better addressed by 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 will no longer
   need to rely on the robustness principle to avoid interoperability
   issues.

2.1.  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 (the Internet Protocol
   specification [RFC0760]) is preceded by a sentence that reveals a
   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.

   Imperfect specifications are unavoidable, largely because it is more
   important to proceed to implementation and deployment than it is to
   perfect a specification.  A protocol benefits greatly from experience
   with its use.  A deployed protocol is immeasurably more useful than a
   perfect protocol specification.  This is particularly true in early
   phases of system design, to which the robustness principle is best
   suited.

   As demonstrated by the IAB document "What Makes for a Successful
   Protocol?" [RFC5218], success or failure of a protocol depends far
   more on factors like usefulness than on technical excellence.  Timely
   publication of protocol specifications, even with the potential for
   flaws, likely contributed significantly to the eventual success of
   the Internet.

   This premise that specifications will be imperfect is correct.
   However, ignoring faulty or ambiguous input is almost always the
   incorrect solution to the problem.

2.2.  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.

   The ability to extend a protocol 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 new types of input.

   A well-designed extensibility mechanism establishes clear rules for
   the handling of elements like new messages or parameters.  This
   depends on specifying the handling of malformed or illegal inputs so
   that implementations behave consistently in all cases that might
   affect interoperation.  New messages or parameters thereby become
   entirely expected.  If extension mechanisms and error handling are
   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 handle
   unexpected input 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].  It is
   better and easier to design a protocol for extensibility initially
   than to retrofit the capability (see also [EDNS0]).

2.3.  Flexible Protocols

   A protocol could be designed to permit a narrow set of valid inputs,
   or it could be designed to treat a wide range of inputs as valid.

   A more flexible protocol is more complex to specify and implement;
   variations, especially those that are not commonly used, can create
   potential interoperability hazards.  In the absence of strong reasons
   to be flexible, a simpler protocol is more likely to successfully
   interoperate.

   Where input is provided by users, allowing flexibility might serve to
   make the protocol more accessible, especially for non-expert users.
   HTML authoring [HTML] is an example of this sort of design.

   In protocols where there are many participants that might generate
   messages based on data from other participants, some flexibility
   might contribute to resilience of the system.  A routing protocol is
   a good example of where this might be necessary.

   In BGP [BGP], a peer generates UPDATE messages based on messages it
   receives from other peers.  Peers can copy attributes without
   validation, potentially propagating invalid values.  RFC 4271 [BGP]
   mandated a session reset for invalid UPDATE messages, a requirement
   that was not widely implemented.  In many deployments, peers would
   treat a malformed UPDATE in less stringent ways, such as by treating
   the affected route as having been withdrawn.  Ultimately, RFC 7606
   [BGP-REH] documented this practice and provided precise rules,
   including mandatory actions for different error conditions.

   A protocol can explicitly allow for a range of valid expressions of
   the same semantics, with precise definitions for error handling.
   This is distinct from a protocol that relies on the application of
   the robustness principle.  With the former, interoperation depends on
   specifications that capture all relevant details, whereas
   interoperation in the latter depends more extensively on
   implementations making compatible decisions, as noted in Section 4.2.

3.  Applicability

   The guidance in this document is intended for protocols that are
   deployed to the Internet.  There are some situations in which this
   guidance might not apply to a protocol due to conditions on its
   implementation or deployment.

   In particular, this guidance depends on an ability to update and
   deploy implementations.  Being able to rapidly update implementations
   that are deployed to the Internet helps manage security risks, but in
   reality, some software deployments have lifecycles that make software
   updates either rare or altogether impossible.

   Where implementations are not updated, there is no opportunity to
   apply the practices that this document recommends.  In particular,
   some practices -- such as those described in Section 5.1 -- only
   exist to support the development of protocol maintenance and
   evolution.  Employing this guidance is therefore only applicable
   where there is the possibility of improving deployments through
   timely updates of their implementations.

4.  Harmful Consequences of Tolerating the Unexpected

   Problems in other implementations can create an unavoidable need to
   temporarily tolerate unexpected inputs.  However, this course of
   action carries risks.

4.1.  Protocol Decay

   Tolerating unexpected input might be an expedient tool for systems in
   early phases of deployment, which was the case for the early
   Internet.  Being lenient in this way defers the effort of dealing
   with interoperability problems and prioritizes progress.  However,
   this deferral can amplify the ultimate cost of handling
   interoperability 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.  Using a protocol in
   ways that were not anticipated in the original design or ambiguities
   and errors in the specification are often contributing factors.
   Disagreements on the interpretation of specifications should be
   expected over the lifetime of a protocol.

   Even with the best intentions to maintain protocol correctness, 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 enters a pathological feedback cycle.
   Over time:

   *  Implementations progressively add logic to constrain how data is
      transmitted or to permit variations in what is received.

   *  Errors in implementations or confusion about semantics are
      permitted or ignored.

   *  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
   tolerating the unexpected 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 can be added to messages in any order.  However, some server
   implementations terminated connections if they encountered a TLS
   ClientHello message that ends with an empty extension.  To maintain
   interoperability with these servers, which were widely deployed,
   client implementations were required to be aware of this bug and
   ensure that a ClientHello message ends in a non-empty extension.

   Overapplication of the robustness principle therefore encourages a
   chain reaction that can create interoperability problems over time.
   In particular, tolerating unexpected behavior is particularly
   deleterious for early implementations of new protocols, as quirks in
   early implementations can affect all subsequent deployments.

4.2.  Ecosystem Effects

   From observing widely deployed protocols, it appears there are two
   stable points on the spectrum between being strict versus permissive
   in the presence of protocol errors:

   *  If implementations predominantly enforce strict compliance with
      specifications, newer implementations will experience failures if
      they do not comply with protocol requirements.  Newer
      implementations need to fix compliance issues in order to be
      successfully deployed.  This ensures that most deployments are
      compliant over time.

   *  Conversely, if non-compliance is tolerated by existing
      implementations, non-compliant implementations can be deployed
      successfully.  Newer implementations then have a strong incentive
      to tolerate any existing non-compliance in order to be
      successfully deployed.  This ensures that most deployments are
      tolerant of the same non-compliant behavior.

   This happens because interoperability requirements for protocol
   implementations are set by other deployments.  Specifications and
   test suites -- where they exist -- can guide the initial development
   of implementations.  Ultimately, the need to interoperate with
   deployed implementations is a de facto conformance test suite that
   can supersede any formal protocol definition.

   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.  In the worst case, a
   new implementer might have to choose between deployments that have
   diverged so far as to no longer be interoperable.

   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 key to the continued viability of a protocol.
   New protocol implementations are also more likely to be developed for
   new and diverse use cases and are often 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 the
   IAB document "Long-Term Viability of Protocol Extension Mechanisms"
   [RFC9170]).

   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
   specifications and then deploying those changes, makes the protocol
   better.  Tolerating unexpected input instead conceals problems,
   making it harder, if not 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 incentive to be tolerant of errors accumulates
   over time.  Indeed, when faced with divergent interpretations of an
   immutable specification, the only way for an implementation to remain
   interoperable is to be tolerant of differences in interpretation and
   implementation errors.  However, when official specifications fail to
   be updated, then deployed implementations -- including their quirks
   -- often become a substitute standard.

   Tolerating unexpected inputs from another implementation might seem
   logical, even necessary.  However, that conclusion relies on an
   assumption that existing specifications and implementations cannot
   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.
   For example, when an implementer discovers a scenario where a
   specification defines some input as faulty but does not define how to
   handle that input, the implementer can provide significant value to
   the ecosystem by reporting the issue and helping to evolve the
   specification.

   When a discrepancy is found between a specification and its
   implementation, a maintenance discussion inside the standards process
   allows reaching consensus on how best to evolve the specification.
   Subsequently, updating implementations to match evolved
   specifications 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
   [RFC5218].

   Protocol designers are strongly encouraged to continue to maintain
   and evolve protocol specifications beyond their initial inception and
   definition.  This might require the development of revised
   specifications, extensions, or other supporting material that evolves
   in concert with implementations.  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, as software defects can happen even when
   the specification is unambiguous.  For instance, the most effective
   means of dealing with a defective implementation in a peer could be
   to contact 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.  In addition to specifications, later
   implementations will be guided by what existing deployments accept.
   Tolerance of errors in early deployments is most likely to result in
   problems.  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 relevance 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
   workarounds following the spirit of the robustness principle might be
   necessary.  For this, improvements in software update mechanisms
   ensure that the cost of reacting to changes is much lower than it was
   in the past.  Alternatively, if specifications can be updated more
   readily than deployments, details of the workaround can be
   documented, including the desired form of the protocols once the need
   for workarounds no longer exists and plans for removing the
   workaround.

5.1.  Virtuous Intolerance

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

   Choosing to generate fatal errors for unspecified conditions instead
   of attempting error recovery can ensure that faults receive
   attention.  This intolerance can be harnessed to reduce occurrences
   of aberrant implementations.

   Intolerance toward violations of specification improves feedback for
   new implementations in particular.  When a new implementation
   encounters a peer that is intolerant of an error, it receives strong
   feedback that allows the problem to be discovered quickly.

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

   Interoperability problems also need to be made known to those in a
   position to address them.  In particular, systems with human
   operators, such as user-facing clients, are ideally suited to
   surfacing errors.  Other systems might need to use less direct means
   of making errors known.

   This does not mean that intolerance of errors in early deployments of
   protocols has the effect of preventing interoperability.  On the
   contrary, when existing implementations follow clearly specified
   error handling, new implementations or features can be introduced
   more readily, as the effect on existing implementations can be easily
   predicted; see also Section 2.2.

   Any intolerance also needs to be strongly supported by
   specifications; otherwise, they encourage fracturing of the protocol
   community or proliferation of workarounds.  See Section 5.2.

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

   A notification for a fatal error is best sent as explicit error
   messages to the entity that made the error.  Error messages benefit
   from being able to carry arbitrary information that might help the
   implementer of the sender of the faulty input understand and fix the
   issue in their software.  QUIC error frames [QUIC] are an example of
   a fatal error mechanism that helped implementers improve software
   quality throughout the protocol lifecycle.  Similarly, the use of
   Extended DNS Errors [EDE] has been effective in providing better
   descriptions of DNS resolution errors to clients.

   Stateless protocol endpoints might generate denial-of-service attacks
   if they send an error message in response to every message that is
   received from an unauthenticated sender.  These implementations might
   need to silently discard these messages.

5.2.  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 backward-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.

   Accidentally excluding unexpected participants is not usually a good
   outcome.  When developing and deploying changes, it is best to first
   understand the extent to which the change affects existing
   deployments.  This ensures that any exclusion that occurs is
   intentional.

   In some cases, existing deployments might need to change in order to
   avoid being excluded.  Though it might be preferable to avoid forcing
   deployments to change, this might be considered necessary.  To avoid
   unnecessarily excluding deployments that might take time to change,
   developing a migration plan can be prudent.

   Exclusion is a direct goal when choosing to be intolerant of errors
   (see Section 5.1).  Exclusionary actions are employed with the
   deliberate 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
   that might arise from tolerating the unexpected.  The IAB document
   "Uncoordinated Protocol Development Considered Harmful" [RFC5704]
   describes how conflict or competition in the maintenance of protocols
   can lead to similar problems.

6.  Security Considerations

   Careless 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, weak primitives
   [MD5] and obsolete mechanisms [SSL3] are good examples of the use of
   unsafe security practices where forcing exclusion (Section 5.2) can
   be desirable.

7.  IANA Considerations

   This document has no IANA actions.

8.  Informative References

   [BGP]      Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [BGP-REH]  Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
              Patel, "Revised Error Handling for BGP UPDATE Messages",
              RFC 7606, DOI 10.17487/RFC7606, August 2015,
              <https://www.rfc-editor.org/info/rfc7606>.

   [EDE]      Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
              Lawrence, "Extended DNS Errors", RFC 8914,
              DOI 10.17487/RFC8914, October 2020,
              <https://www.rfc-editor.org/info/rfc8914>.

   [EDNS0]    Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,
              <https://www.rfc-editor.org/info/rfc6891>.

   [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]     WHATWG, "HTML - Living Standard",
              <https://html.spec.whatwg.org/>.

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [HTTP/2]   Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
              DOI 10.17487/RFC9113, June 2022,
              <https://www.rfc-editor.org/info/rfc9113>.

   [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>.

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

   [RFC0760]  Postel, J., "DoD standard Internet Protocol", RFC 760,
              DOI 10.17487/RFC0760, January 1980,
              <https://www.rfc-editor.org/info/rfc760>.

   [RFC1958]  Carpenter, B., Ed., "Architectural Principles of the
              Internet", RFC 1958, DOI 10.17487/RFC1958, June 1996,
              <https://www.rfc-editor.org/info/rfc1958>.

   [RFC3117]  Rose, M., "On the Design of Application Protocols",
              RFC 3117, DOI 10.17487/RFC3117, November 2001,
              <https://www.rfc-editor.org/info/rfc3117>.

   [RFC5218]  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>.

   [RFC5704]  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>.

   [RFC9170]  Thomson, M. and T. Pauly, "Long-Term Viability of Protocol
              Extension Mechanisms", RFC 9170, DOI 10.17487/RFC9170,
              December 2021, <https://www.rfc-editor.org/info/rfc9170>.

   [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>.

   [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>.

IAB Members at the Time of Approval

   Internet Architecture Board members at the time this document was
   approved for publication were:

      Jari Arkko
      Deborah Brungard
      Lars Eggert
      Wes Hardaker
      Cullen Jennings
      Mallory Knodel
      Mirja Kühlewind
      Zhenbin Li
      Tommy Pauly
      David Schinazi
      Russ White
      Qin Wu
      Jiankang Yao

   The document had broad but not unanimous approval within the IAB,
   reflecting that while the guidance is valid, concerns were expressed
   in the IETF community about how broadly it applies in all situations.

Acknowledgments

   Constructive feedback on this document has been provided by a
   surprising number of people including, but not limited to, the
   following: Bernard Aboba, Brian Carpenter, Stuart Cheshire, Joel
   Halpern, Wes Hardaker, Russ Housley, Cullen Jennings, Mallory Knodel,
   Mirja Kühlewind, Mark Nottingham, Eric Rescorla, Henning Schulzrinne,
   Job Snijders, Robert Sparks, Dave Thaler, Brian Trammell, and Anne
   van Kesteren.  Some of the properties of protocols described in
   Section 4.1 were observed by Marshall Rose in Section 4.5 of
   [RFC3117].

Authors' Addresses

   Martin Thomson
   Email: mt@lowentropy.net

   David Schinazi
   Email: dschinazi.ietf@gmail.com