Long-term Viability of Protocol Extension Mechanisms

Document Type Active Internet-Draft (iab)
Authors Martin Thomson  , Tommy Pauly 
Last updated 2021-07-14
Replaces draft-thomson-use-it-or-lose-it
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Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Informational                                  T. Pauly
Expires: 15 January 2022                                           Apple
                                                            14 July 2021

          Long-term Viability of Protocol Extension Mechanisms


   The ability to change protocols depends on exercising the extension
   and version negotiation mechanisms that support change.  Protocols
   that don't use these mechanisms can find it difficult and costly to
   deploy changes.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the EDM Program mailing
   list (edm@iab.org), which is archived at

   Source for this draft and an issue tracker can be found at

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   This Internet-Draft will expire on 15 January 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Imperfect Implementations Limit Protocol Evolution  . . . . .   3
     2.1.  Good Protocol Design is Not Itself Sufficient . . . . . .   4
     2.2.  Disuse Can Hide Problems  . . . . . . . . . . . . . . . .   5
       2.2.1.  TLS . . . . . . . . . . . . . . . . . . . . . . . . .   5
       2.2.2.  DNS . . . . . . . . . . . . . . . . . . . . . . . . .   6
       2.2.3.  SNMP  . . . . . . . . . . . . . . . . . . . . . . . .   6
       2.2.4.  HTTP  . . . . . . . . . . . . . . . . . . . . . . . .   6
       2.2.5.  IPv4  . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.3.  Multi-Party Interactions and Middleboxes  . . . . . . . .   7
   3.  Retaining Viable Protocol Evolution Mechanisms  . . . . . . .   8
     3.1.  Examples of Active Use  . . . . . . . . . . . . . . . . .   9
     3.2.  Dependency is Better  . . . . . . . . . . . . . . . . . .   9
     3.3.  Restoring Active Use  . . . . . . . . . . . . . . . . . .  10
   4.  Active Use  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  Version Negotiation . . . . . . . . . . . . . . . . . . .  11
     4.2.  Falsifying Active Use . . . . . . . . . . . . . . . . . .  11
   5.  Complementary Techniques  . . . . . . . . . . . . . . . . . .  13
     5.1.  Cryptography  . . . . . . . . . . . . . . . . . . . . . .  13
     5.2.  Fewer Extension Points  . . . . . . . . . . . . . . . . .  13
       5.2.1.  Invariants  . . . . . . . . . . . . . . . . . . . . .  14
     5.3.  Effective Feedback  . . . . . . . . . . . . . . . . . . .  14
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  15
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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1.  Introduction

   A successful protocol [SUCCESS] needs to change in ways that allow it
   to continue to fulfill the needs of its users.  New use cases,
   conditions and constraints on the deployment of a protocol can render
   a protocol that does not change obsolete.

   Usage patterns and requirements for a protocol shift over time.  In
   response, implementations might adjust usage patterns within the
   constraints of the protocol, the protocol could be extended, or a
   replacement protocol might be developed.  Experience with Internet-
   scale protocol deployment shows that each option comes with different
   costs.  [TRANSITIONS] examines the problem of protocol evolution more

   This document examines the specific conditions that determine whether
   protocol maintainers have the ability to design and deploy new or
   modified protocols.  Section 2 highlights some historical examples of
   difficulties in transitions to new protocol features.  Section 3
   argues that ossified protocols are more difficult to update and
   successful protocols make frequent use of new extensions and code-
   points.  Section 4 and Section 5 outline several strategies that
   might aid in ensuring that protocol changes remain possible over

   The experience that informs this document is predominantly at
   "higher" layers of the network stack, in protocols that operate at
   very large scale and Internet-scale applications.  It is possible
   that these conclusions are less applicable to protocol deployments
   that have less scale and diversity, or operate under different

2.  Imperfect Implementations Limit Protocol Evolution

   It can be extremely difficult to deploy a change to a protocol if
   there are bugs in implementations with which the new deployment needs
   to interoperate.  Bugs in how new codepoints or extensions are
   handled often mean that endpoints will react poorly to the use of
   extension mechanisms.  This can manifest as abrupt termination of
   sessions, errors, crashes, or disappearances of endpoints and

   Interoperability with other implementations is usually highly valued,
   so deploying mechanisms that trigger adverse reactions can be
   untenable.  Where interoperability is a competitive advantage, this
   is true even if the negative reactions happen infrequently or only
   under relatively rare conditions.

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   Deploying a change to a protocol could require implementations fix a
   substantial proportion of the bugs that the change exposes.  This can
   involve a difficult process that includes identifying the cause of
   these errors, finding the responsible implementation(s), coordinating
   a bug fix and release plan, contacting users and/or the operator of
   affected services, and waiting for the fix to be deployed.

   Given the effort involved in fixing problems, the existence of these
   sorts of bugs can outright prevent the deployment of some types of
   protocol changes, especially for protocols involving multiple parties
   or that are considered critical infrastructure (e.g., IP, BGP, DNS,
   or TLS).  It could even be necessary to come up with a new protocol
   design that uses a different method to achieve the same result.

   The set of interoperable features in a protocol is often the subset
   of its features that have some value to those implementing and
   deploying the protocol.  It is not always the case that future
   extensibility is in that set.

2.1.  Good Protocol Design is Not Itself Sufficient

   It is often argued that the careful design of a protocol extension
   point or version negotiation capability is critical to the freedom
   that it ultimately offers.

   RFC 6709 [EXTENSIBILITY] contains a great deal of well-considered
   advice on designing for extension.  It includes the following advice:

      This means that, to be useful, a protocol version-negotiation
      mechanism should be simple enough that it can reasonably be
      assumed that all the implementers of the first protocol version at
      least managed to implement the version-negotiation mechanism

   This has proven to be insufficient in practice.  Many protocols have
   evidence of imperfect implementation of critical mechanisms of this
   sort.  Mechanisms that aren't used are the ones that fail most often.
   The same paragraph from RFC 6709 acknowledges the existence of this
   problem, but does not offer any remedy:

      The nature of protocol version-negotiation mechanisms is that, by
      definition, they don't get widespread real-world testing until
      _after_ the base protocol has been deployed for a while, and its
      deficiencies have become evident.

   Indeed, basic interoperability is considered critical early in the
   deployment of a protocol.  A desire to deploy can result in an
   engineering practice that values simplicity, which could result in

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   deferring implementation of version negotiation and extension
   mechanisms.  This leads to these mechanisms being particularly
   affected by this problem.

2.2.  Disuse Can Hide Problems

   There are many examples of extension points in protocols that have
   been either completely unused, or their use was so infrequent that
   they could no longer be relied upon to function correctly.

2.2.1.  TLS

   Transport Layer Security (TLS) [TLS12] provides examples of where a
   design that is objectively sound fails when incorrectly implemented.
   TLS provides examples of failures in protocol version negotiation and

   Version negotiation in TLS 1.2 and earlier uses the "Highest mutually
   supported version (HMSV)" scheme exactly as it is described in
   [EXTENSIBILITY].  However, clients are unable to advertise a new
   version without causing a non-trivial proportions of sessions to fail
   due to bugs in server and middlebox implementations.

   Intolerance to new TLS versions is so severe [INTOLERANCE] that TLS
   1.3 [TLS13] has abandoned HMSV version negotiation for a new

   The server name indication (SNI) [TLS-EXT] in TLS is another
   excellent example of the failure of a well-designed extensibility
   point.  SNI uses the same technique for extension that is used with
   considerable success in other parts of the TLS protocol.  The
   original design of SNI includes the ability to include multiple names
   of different types.

   What is telling in this case is that SNI was defined with just one
   type of name: a domain name.  No other type has ever been
   standardized, though several have been proposed.  Despite an
   otherwise exemplary design, SNI is so inconsistently implemented that
   any hope for using the extension point it defines has been abandoned

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   Even where extension points have multiple valid values, if the set of
   permitted values does not change over time, there is still a risk
   that new values are not tolerated by existing implementations.  If
   the set of values for a particular field remains fixed over a long
   period, some implementations might not correctly handle a new value
   when it is introduced.  For example, implementations of TLS broke
   when new values of the signature_algorithms extension were

2.2.2.  DNS

   Ossified DNS code bases and systems resulted in fears that new
   Resource Record Codes (RRCodes) would take years of software
   propagation before new RRCodes could be used.  The result for a long
   time was heavily overloaded use of the TXT record, such as in the
   Sender Policy Framework [SPF].  It wasn't until after the standard
   mechanism for dealing with new RRCodes [RRTYPE] was considered widely
   deployed that new RRCodes can be safely created and used.

2.2.3.  SNMP

   As a counter example, the first version of the Simple Network
   Management Protocol (SNMP) [SNMPv1] defines that unparseable or
   unauthenticated messages are simply discarded without response:

      It then verifies the version number of the SNMP message.  If there
      is a mismatch, it discards the datagram and performs no further

   When SNMP versions 2, 2c and 3 came along, older agents did exactly
   what the protocol specifies.  Deployment of new versions was likely
   successful because the handling of newer versions was both clear and

2.2.4.  HTTP

   HTTP has a number of very effective extension points in addition to
   the aforementioned header fields.  It also has some examples of
   extension points that are so rarely used that it is possible that
   they are not at all usable.

   Extension points in HTTP that might be unwise to use include the
   extension point on each chunk in the chunked transfer coding
   Section 7.1 of [HTTP11], the ability to use transfer codings other
   than the chunked coding, and the range unit in a range request
   Section 14 of [HTTP].

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2.2.5.  IPv4

   Codepoints that are reserved for future use can be especially
   problematic.  Reserving codepoints without attributing semantics to
   their use can result in diverse or conflicting semantics being
   attributed without any hope of interoperability.  An example of this
   is the "class E" address space in IPv4 [RFC0988], which was reserved
   without assigning any semantics.

   For protocols that can use negotiation to attribute semantics to
   codepoints, it is possible that unused codepoints can be reclaimed
   for active use, though this requires that the negotiation include all
   protocol participants.  For something as fundamental as addressing,
   negotiation is difficult or even impossible, as all nodes on the
   network path plus potential alternative paths would need to be

2.3.  Multi-Party Interactions and Middleboxes

   Even the most superficially simple protocols can often involve more
   actors than is immediately apparent.  A two-party protocol has two
   ends, but even at the endpoints of an interaction, protocol elements
   can be passed on to other entities in ways that can affect protocol

   One of the key challenges in deploying new features is ensuring
   compatibility with all actors that could be involved in the protocol.

   Protocols deployed without active measures against intermediation
   will tend to become intermediated over time, as network operators
   deploy middleboxes to perform some function on traffic
   [PATH-SIGNALS].  In particular, one of the consequences of an
   unencrypted protocol is that any element on path can interact with
   the protocol.  For example, HTTP was specifically designed with
   intermediation in mind, transparent proxies [HTTP] are not only
   possible but sometimes advantageous, despite some significant
   downsides.  Consequently, transparent proxies for cleartext HTTP are
   commonplace.  The DNS protocol was designed with intermediation in
   mind through its use of caching recursive resolvers [DNS].  What was
   less anticipated was the forced spoofing of DNS records by many
   middle-boxes such as those that inject authentication or pay-wall
   mechanisms as an authentication and authorization check, which are
   now prevalent in hotels, coffee shops and business networks.

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   Middleboxes are also protocol participants, to the degree that they
   are able to observe and act in ways that affect the protocol.  The
   degree to which a middlebox participates varies from the basic
   functions that a router performs to full participation.  For example,
   a SIP back-to-back user agent (B2BUA) [B2BUA] can be very deeply
   involved in the SIP protocol.

   This phenomenon appears at all layers of the protocol stack, even
   when protocols are not designed with middlebox participation in mind.
   TCP's [TCP] extension points have been rendered difficult to use,
   largely due to middlebox interactions, as experience with Multipath
   TCP [MPTCP] and Fast Open [TFO] has shown.  IP's version field was
   rendered useless when encapsulated over Ethernet, requring a new
   ethertype with IPv6 [RFC2464], due in part to layer 2 devices making
   version-independent assumptions about the structure of the IPv4
   header.  The announcements of new optional transitive attributes in
   BGP caused significant routing instability [RIPE-99].

   By increasing the number of different actors involved in any single
   protocol exchange, the number of potential implementation bugs that a
   deployment needs to contend with also increases.  In particular,
   incompatible changes to a protocol that might be negotiated between
   endpoints in ignorance of the presence of a middlebox can result in a
   middlebox interfering in negative and unexpected ways.

   Unfortunately, middleboxes can considerably increase the difficulty
   of deploying new versions or other changes to a protocol.

3.  Retaining Viable Protocol Evolution Mechanisms

   The design of a protocol for extensibility and eventual replacement
   [EXTENSIBILITY] does not guarantee the ability to exercise those
   options.  The set of features that enable future evolution need to be
   interoperable in the first implementations and deployments of the
   protocol.  Implementations of mechanisms that support evolution is
   necessary to ensure that they remain available for new uses, and
   history has shown this occurs almost exclusively through active
   mechanism use.

   The conditions for retaining the ability to evolve a design is most
   clearly evident in the protocols that are known to have viable
   version negotiation or extension points.  The definition of
   mechanisms alone is insufficient; it's the assured implementation
   through active use of those mechanisms that determines the existence
   of freedom.  Protocols that routinely add new extensions and code
   points rarely have trouble adding additional ones, especially when
   the handling of new versions or extension is well defined.

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3.1.  Examples of Active Use

   For example, header fields in email [SMTP], HTTP [HTTP] and SIP [SIP]
   all derive from the same basic design, which amounts to a list name/
   value pairs.  There is no evidence of significant barriers to
   deploying header fields with new names and semantics in email and
   HTTP as clients and servers can ignore headers they do not understand
   or need.  The widespread deployment of SIP B2BUAs means that new SIP
   header fields do not reliably reach peers, however, which doesn't
   necessarily cause interoperability issues but rather causes feature
   deployment issues due to the lack of option passing Section 2.3.

   As another example, the attribute-value pairs (AVPs) in Diameter
   [DIAMETER] are fundamental to the design of the protocol.  Any use of
   Diameter requires exercising the ability to add new AVPs.  This is
   routinely done without fear that the new feature might not be
   successfully deployed.

   These examples show extension points that are heavily used are also
   being relatively unaffected by deployment issues preventing addition
   of new values for new use cases.

   These examples also confirm the case that good design does not
   guarantee success.  On the contrary, success is often despite
   shortcomings in the design.  For instance, the shortcomings of HTTP
   header fields are significant enough that there are ongoing efforts
   to improve the syntax [HTTP-HEADERS].

   Only by using a protocol's extension capabilities does it ensure the
   availability of that capability.  Protocols that fail to use a
   mechanism, or a protocol that only rarely uses a mechanism, may
   suffer an inability to rely on that mechanism.

3.2.  Dependency is Better

   The best way to guarantee that a protocol mechanism is used is to
   make the handling of it critical to an endpoint participating in that
   protocol.  This means that implementations must rely on both the
   existence of extension mechanisms and their continued, repeated
   expansion over time.

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   For example, the message format in SMTP relies on header fields for
   most of its functions, including the most basic delivery functions.
   A deployment of SMTP cannot avoid including an implementation of
   header field handling.  In addition to this, the regularity with
   which new header fields are defined and used ensures that deployments
   frequently encounter header fields that it does not yet (and may
   never) understand.  An SMTP implementation therefore needs to be able
   to both process header fields that it understands and ignore those
   that it does not.

   In this way, implementing the extensibility mechanism is not merely
   mandated by the specification, it is crucial to the functioning of a
   protocol deployment.  Should an implementation fail to correctly
   implement the mechanism, that failure would quickly become apparent.

   Caution is advised to avoid assuming that building a dependency on an
   extension mechanism is sufficient to ensure availability of that
   mechanism in the long term.  If the set of possible uses is narrowly
   constrained and deployments do not change over time, implementations
   might not see new variations or assume a narrower interpretation of
   what is possible.  Those implementations might still exhibit errors
   when presented with new variations.

3.3.  Restoring Active Use

   With enough effort, active use can be used to restore capabililities.

   EDNS [EDNS] was defined to provide extensibility in DNS.  Intolerance
   of the extension in DNS servers resulted in a fallback method being
   widely deployed (see Section 6.2.2 of [EDNS]), This fallback resulted
   in EDNS being disabled for affected servers.  Over time, greater
   support for EDNS and increased reliance on it for different features
   motivated a flag day [DNSFLAGDAY] where the workaround was removed.

   The EDNS example shows that effort can be used to restore
   capabilities.  This is in part because EDNS was actively used with
   most resolvers and servers.  It was therefore possible to force a
   change to ensure that extension capabilities would always be
   available.  However, this required an enormous coordination effort.
   A small number of incompatible servers and the names they serve also
   become inaccessible to most clients.

4.  Active Use

   As discussed in Section 3, the most effective defense against
   ossification of protocol extension points is active use.

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   Implementations are most likely to be tolerant of new values if they
   depend on being able to frequently use new values.  Failing that,
   implementations that routinely see new values are more likely to
   correctly handle new values.  More frequent changes will improve the
   likelihood that incorrect handling or intolerance is discovered and
   rectified.  The longer an intolerant implementation is deployed, the
   more difficult it is to correct.

   What constitutes "active use" can depend greatly on the environment
   in which a protocol is deployed.  The frequency of changes necessary
   to safeguard some mechanisms might be slow enough to attract
   ossification in another protocol deployment, while being excessive in

4.1.  Version Negotiation

   As noted in Section 2.1, protocols that provide version negotiation
   mechanisms might not be able to test that feature until a new version
   is deployed.  One relatively successful design approach has been to
   use the protocol selection mechanisms built into a lower-layer
   protocol to select the protocol.  This could allow a version
   negotiation mechanism to benefit from active use of the extension
   point by other protocols.

   For instance, all published versions of IP contain a version number
   as the four high bits of the first header byte.  However, version
   selection using this field proved to be unsuccessful.  Ultimately,
   successful deployment of IPv6 over Ethernet [RFC2464] required a
   different EtherType from IPv4.  This change took advantage of the
   already-diverse usage of EtherType.

   Other examples of this style of design include Application-Layer
   Protocol Negotiation ([ALPN]) and HTTP content negotiation
   (Section 12 of [HTTP]).

   This technique relies on the codepoint being usable.  For instance,
   the IP protocol number is known to be unreliable and therefore not
   suitable [NEW-PROTOCOLS].

4.2.  Falsifying Active Use

   "Grease" was originally defined for TLS [GREASE], but has been
   adopted by other protocols, such as QUIC [QUIC].  Grease identifies
   lack of use as an issue (protocol mechanisms "rusting" shut) and
   proposes reserving values for extensions that have no semantic value

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   The design in [GREASE] is aimed at the style of negotiation most used
   in TLS, where one endpoint offers a set of options and the other
   chooses the one that it most prefers from those that it supports.  An
   endpoint that uses grease randomly offers options - usually just one
   - from a set of reserved values.  These values are guaranteed to
   never be assigned real meaning, so its peer will never have cause to
   genuinely select one of these values.

   More generally, greasing is used to refer to any attempt to exercise
   extension points without changing endpoint behavior, other than to
   encourage participants to tolerate new or varying values of protocol

   The principle that grease operates on is that an implementation that
   is regularly exposed to unknown values is less likely to be
   intolerant of new values when they appear.  This depends largely on
   the assumption that the difficulty of implementing the extension
   mechanism correctly is as easy or easier than implementing code to
   identify and filter out reserved values.  Reserving random or
   unevenly distributed values for this purpose is thought to further
   discourage special treatment.

   Without reserved greasing codepoints, an implementation can use code
   points from spaces used for private or experimental use if such a
   range exists.  In addition to the risk of triggering participation in
   an unwanted experiment, this can be less effective.  Incorrect
   implementations might still be able to identify these code points and
   ignore them.

   In addition to advertising bogus capabilities, an endpoint might also
   selectively disable non-critical protocol elements to test the
   ability of peers to handle the absence of certain capabilities.

   This style of defensive design is limited because it is only
   superficial.  As greasing only mimics active use of an extension
   point, it only exercises a small part of the mechanisms that support
   extensibility.  More critically, it does not easily translate to all
   forms of extension points.  For instance, HMSV negotiation cannot be
   greased in this fashion.  Other techniques might be necessary for
   protocols that don't rely on the particular style of exchange that is
   predominant in TLS.

   Grease is deployed with the intent of quickly revealing errors in
   implementing the mechanisms it safeguards.  Though it has been
   effective at revealing problems in some cases with TLS, the efficacy
   of greasing isn't proven more generally.  Where implementations are
   able to tolerate a non-zero error rate in their operation, greasing
   offers a potential option for safeguarding future extensibility.

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   However, this relies on there being a sufficient proportion of
   participants that are willing to invest the effort and tolerate the
   risk of interoperability failures.

5.  Complementary Techniques

   The protections to protocol evolution that come from active use
   (Section 4) can be improved through the use of other defensive
   techniques.  The techniques listed here might not prevent
   ossification on their own, but can make active use more effective.

5.1.  Cryptography

   Cryptography can be used to reduce the number of middlebox entities
   that can participate in a protocol or limit the extent of
   participation.  Using TLS or other cryptographic tools can therefore
   reduce the number of entities that can influence whether new features
   are usable.

   [PATH-SIGNALS] recommends the use of encryption and integrity
   protection to limit participation.  For example, encryption is used
   by the QUIC protocol [QUIC] to limit the information that is
   available to middleboxes and integrity protection prevents

5.2.  Fewer Extension Points

   A successful protocol will include many potential types of extension.
   Designing multiple types of extension mechanism, each suited to a
   specific purpose, might leave some extension points less heavily used
   than others.

   Disuse of a specialized extension point might render it unusable.  In
   contrast, having a smaller number of extension points with wide
   applicability could improve the use of those extension points.  Use
   of a shared extension point for any purpose can protect rarer or more
   specialized uses.

   Both extensions and core protocol elements use the same extension
   points in protocols like HTTP [HTTP] and DIAMETER [DIAMETER]; see
   Section 3.1.

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5.2.1.  Invariants

   Documenting aspects of the protocol that cannot or will not change as
   extensions or new versions are added can be a useful exercise.
   Understanding what aspects of a protocol are invariant can help guide
   the process of identifying those parts of the protocol that might

   As a means of protecting extensibility, a declaration of protocol
   invariants is useful only to the extent that protocol participants
   are willing to allow new uses for the protocol.  Like with greasing,
   protocol participants could still purposefully block the deployment
   of new features.  A protocol that declares protocol invariants relies
   on implementations understanding and respecting those invariants.

   Protocol invariants need to be clearly and concisely documented.
   Including examples of aspects of the protocol that are not invariant,
   such as the appendix of [QUIC-INVARIANTS], can be used to clarify

5.3.  Effective Feedback

   While not a direct means of protecting extensibility mechanisms,
   feedback systems can be important to discovering problems.

   Visibility of errors is critical to the success of techniques like
   grease (see Section 4.2).  The grease design is most effective if a
   deployment has a means of detecting and reporting errors.  Ignoring
   errors could allow problems to become entrenched.

   Feedback on errors is more important during the development and early
   deployment of a change.  It might also be helpful to disable
   automatic error recovery methods during development.

   Automated feedback systems are important for automated systems, or
   where error recovery is also automated.  For instance, connection
   failures with HTTP alternative services [ALT-SVC] are not permitted
   to affect the outcome of transactions.  An automated feedback system
   for capturing failures in alternative services is therefore necessary
   for failures to be detected.

   How errors are gathered and reported will depend greatly on the
   nature of the protocol deployment and the entity that receives the
   report.  For instance, end users, developers, and network operations
   each have different requirements for how error reports are created,
   managed, and acted upon.

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   Automated delivery of error reports can be critical for rectifying
   deployment errors as early as possible, such as seen in [DMARC] and

6.  Security Considerations

   Many of the problems identified in this document are not the result
   of deliberate actions by an adversary, but more the result of
   mistakes, decisions made without sufficient context, or simple
   neglect.  Problems therefore not the result of opposition by an
   adversary.  In response, the recommended measures generally assume
   that other protocol participants will not take deliberate action to
   prevent protocol evolution.

   The use of cryptographic techniques to exclude potential participants
   is the only strong measure that the document recommends.  However,
   authorized protocol peers are most often responsible for the
   identified problems, which can mean that cryptography is insufficient
   to exclude them.

   The ability to design, implement, and deploy new protocol mechanisms
   can be critical to security.  In particular, it is important to be
   able to replace cryptographic algorithms over time [AGILITY].  For
   example, preparing for replacement of weak hash algorithms was made
   more difficult through misuse [HASH].

7.  IANA Considerations

   This document makes no request of IANA.

8.  Informative References

   [AGILITY]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,

   [ALPN]     Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/rfc/rfc7301>.

   [ALT-SVC]  Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <https://www.rfc-editor.org/rfc/rfc7838>.

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   [B2BUA]    Kaplan, H. and V. Pascual, "A Taxonomy of Session
              Initiation Protocol (SIP) Back-to-Back User Agents",
              RFC 7092, DOI 10.17487/RFC7092, December 2013,

   [DIAMETER] Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
              Ed., "Diameter Base Protocol", RFC 6733,
              DOI 10.17487/RFC6733, October 2012,

   [DMARC]    Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,

   [DNS]      Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

              "DNS Flag Day 2019", May 2019,

   [EDNS]     Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,

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

   [GREASE]   Benjamin, D., "Applying Generate Random Extensions And
              Sustain Extensibility (GREASE) to TLS Extensibility",
              RFC 8701, DOI 10.17487/RFC8701, January 2020,

   [HASH]     Bellovin, S. and E. Rescorla, "Deploying a New Hash
              Algorithm", Proceedings of NDSS '06 , 2006,

   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-16, 27 May 2021,

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              Nottingham, M. and P-H. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,

   [HTTP11]   Fielding, R. T., Nottingham, M., and J. Reschke,
              "HTTP/1.1", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-messaging-16, 27 May 2021,

              Kario, H., "Re: [TLS] Thoughts on Version Intolerance", 20
              July 2016, <https://mailarchive.ietf.org/arch/msg/tls/

   [MPTCP]    Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,

              Barik, R., Welzl, M., Fairhurst, G., Elmokashfi, A.,
              Dreibholz, T., and S. Gjessing, "On the usability of
              transport protocols other than TCP: A home gateway and
              internet path traversal study", Computer Networks Vol.
              173, pp. 107211, DOI 10.1016/j.comnet.2020.107211, May
              2020, <https://doi.org/10.1016/j.comnet.2020.107211>.

              Wendt, C. and M. Barnes, "Personal Assertion Token
              (PaSSporT) Extension for Signature-based Handling of
              Asserted information using toKENs (SHAKEN)", RFC 8588,
              DOI 10.17487/RFC8588, May 2019,

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,

              Thomson, M., "Version-Independent Properties of QUIC",
              RFC 8999, DOI 10.17487/RFC8999, May 2021,

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   [RFC0988]  Deering, S., "Host extensions for IP multicasting",
              RFC 988, DOI 10.17487/RFC0988, July 1986,

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,

   [RIPE-99]  Romijn, E., "RIPE NCC and Duke University BGP Experiment",
              27 August 2010, <https://labs.ripe.net/Members/erik/ripe-

   [RRTYPE]   Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September
              2003, <https://www.rfc-editor.org/rfc/rfc3597>.

   [SIP]      Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,

   [SMTP]     Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,

              Margolis, D., Brotman, A., Ramakrishnan, B., Jones, J.,
              and M. Risher, "SMTP TLS Reporting", RFC 8460,
              DOI 10.17487/RFC8460, September 2018,

   [SNI]      Langley, A., "Accepting that other SNI name types will
              never work", 3 March 2016,

   [SNMPv1]   Case, J., Fedor, M., Schoffstall, M., and J. Davin,
              "Simple Network Management Protocol (SNMP)", RFC 1157,
              DOI 10.17487/RFC1157, May 1990,

   [SPF]      Kitterman, S., "Sender Policy Framework (SPF) for
              Authorizing Use of Domains in Email, Version 1", RFC 7208,
              DOI 10.17487/RFC7208, April 2014,

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   [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,

   [TFO]      Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

   [TLS-EXT]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [TLS12]    Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

              Thaler, D., Ed., "Planning for Protocol Adoption and
              Subsequent Transitions", RFC 8170, DOI 10.17487/RFC8170,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8170>.


   Wes Hardaker, Mirja Kuehlewind, Mark Nottingham, and Brian Trammell
   made significant contributions to this document.

Authors' Addresses

   Martin Thomson

   Email: mt@lowentropy.net

   Tommy Pauly

   Email: tpauly@apple.com

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