Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Informational                            March 23, 2018
Expires: September 24, 2018

          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 that deploying changes can
   be difficult and costly.

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   This Internet-Draft will expire on September 24, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Implementations of Protocols are Imperfect  . . . . . . . . .   2
     2.1.  Good Protocol Design is Not Sufficient  . . . . . . . . .   3
     2.2.  Multi-Party Interactions and Middleboxes  . . . . . . . .   4
   3.  Retaining Viable Protocol Evolution Mechanisms  . . . . . . .   5
     3.1.  Practice Can Ensure Viability . . . . . . . . . . . . . .   5
     3.2.  Dependency is Better  . . . . . . . . . . . . . . . . . .   6
     3.3.  Unused Extension Points Become Unusable . . . . . . . . .   7
   4.  Defensive Design Principles for Protocols . . . . . . . . . .   7
     4.1.  Active Use  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Grease  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Cryptography  . . . . . . . . . . . . . . . . . . . . . .   8
     4.4.  Visibility of Faults  . . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Informative References  . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   A successful protocol [SUCCESS] will 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.
   Protocols can react to these shifts in one of three ways: adjust
   usage patterns within the constraints of the protocol, extend the
   protocol, and replace the protocol.  These reactions are
   progressively more disruptive, but are also dictated by the nature of
   the change in requirements over longer periods.

   Experience with Internet-scale protocol deployment shows that
   changing protocols is not uniformly successful.  [TRANSITIONS]
   examines the problem more broadly.

   This document examines the specific conditions that determine whether
   protocol maintainers have the ability to design and deploy new or
   modified protocols.  Section 4 outlines several strategies that might
   aid in ensuring that protocol changes remain possible over time.

2.  Implementations of Protocols are Imperfect

   A change to a protocol can be made extremely difficult to deploy if
   there are bugs in implementations with which the new deployment needs
   to interoperate.  Bugs in the handling of new codepoints or

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   extensions can mean that instead of handling the mechanism as
   designed, endpoints react poorly.  This can manifest as abrupt
   termination of sessions, errors, crashes, or disappearances of
   endpoints and timeouts.

   Interoperability with other implementations is usually highly valued,
   so deploying mechanisms that trigger adverse reactions like these 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.

   Deploying a change to a protocol could require fixing 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, coordinating a bug
   fix and release plan, contacting the operator of affected services,
   and waiting for the fix to be deployed to those services.

   Given the effort involved in fixing these problems, the existence of
   these sorts of bugs can outright prevent the deployment of some types
   of protocol changes.  It could even be necessary to come up with a
   new protocol design that uses a different method to achieve the same

2.1.  Good Protocol Design is Not Sufficient

   It is often argued that the 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 these critical mechanisms.
   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

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      _after_ the base protocol has been deployed for a while, and its
      deficiencies have become evident.

   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

2.2.  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 in a protocol is
   ensuring compatibility with all actors that could influence the

   Protocols that deploy without active measures against intermediation
   can accrue middleboxes that depend on certain aspects of the protocol
   [PATH-SIGNALS].  In particular, one of the consequences of an
   unencrypted protocol is that any element on path can interact with

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   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

   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.

   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 acting badly.

   Thus, middleboxes can increase the difficulty of deploying changes to
   a protocol considerably.

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.  Only active use of those mechanisms can ensure that they
   remain available for new uses.

3.1.  Practice Can Ensure Viability

   The fact that the freedom to change depends on practice is evident in
   protocols that are known to have viable version negotiation or
   extension points.  The definition of mechanisms alone is
   insufficient; it's the active use of those mechanisms that determines
   the existence of freedom.

   For example, header fields in email [SMTP], HTTP [HTTP] and SIP [SIP]
   all derive from the same basic design.  There is no evidence of
   significant barriers to deploying header fields with new names and
   semantics in email and HTTP, though the widespread deployment of SIP
   B2BUAs means that new SIP header fields can be more difficult.

   In another example, the attribute-value pairs (AVPs) in Diameter
   [DIAMETER] are fundamental to the design of the protocol.  The
   definition of new uses of Diameter regularly exercise the ability to

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   add new AVPs and do so with no fear that the new feature might not be
   successfully deployed.

   These examples show extension points that are heavily used 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 is not a
   prerequisite for 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 using a protocol capability is able to ensure availability of
   that capability.  Protocols that fail to use a mechanism, or a
   protocol that only rarely uses a mechanism, 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 it critical to an endpoint participating in that protocol.  This
   means that implementations rely on both the existence of the protocol
   mechanism and its use.

   For example, the message format in SMTP relies on header fields for
   most of its functions, including the most basic 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 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 a new variation.

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3.3.  Unused Extension Points Become Unusable

   In contrast, 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

   HTTP has a number of very effective extension points in addition to
   the aforementioned header fields.  It also has some examples of
   extension point 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 [HTTP], the ability to use transfer codings other
   than the chunked coding, and the range unit in a range request

4.  Defensive Design Principles for Protocols

   There are several potential approaches that can provide some measure
   of protection against a protocol deployment becoming resistant to

4.1.  Active Use

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

4.2.  Grease

   "Grease" [GREASE] identifies lack of use as an issue (protocol
   mechanisms "rusting" shut) and proposes a system of use that
   exercises extension points by using dummy values.

   The primary feature of the grease design is aimed at the style of
   negotiation most used in TLS, where the client offers a set of
   options and the server chooses the one that it most prefers from
   those that it supports.  A client 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 the
   server will never have cause to genuinely select one of these values.

   The principle that grease operates on is that an implementation that
   is regularly exposed to unknown values is not likely to become
   intolerant of new values when they appear.  This depends largely on
   the assumption that the difficulty of implementing the protocol
   mechanism correctly is not significantly more effort than
   implementing code to specifically filter out the randomized grease

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   To avoid simple techniques for filtering greasing codepoints, grease
   values are not reserved from a single contiguous block of code
   points, but are distributed evenly across the entire space of code
   points.  Reserving a randomly selected set of code points has a
   greater chance of avoiding this problem, though it might be more
   difficult to specify and implement, especially over larger code point

   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 correctly identify these code
   points and ignore them.

   Grease is deployed with the intent of quickly detecting errors in
   implementing the mechanisms it safeguards.

   This form of defensive design has some limitations.  It does not
   necessarily create the need for an implementation to rely on the
   mechanism it safeguards; that is determined by the underlying
   protocol itself.  More critically, it does not easily translate to
   other forms of extension point.  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.

4.3.  Cryptography

   Cryptography can be used to reduce the number of entities that can
   participate in a protocol.  Using tools like TLS ensures that only
   authorized participants are able to influence whether a new protocol
   feature is used.

   Data that is exchanged under encryption cannot be seen by
   middleboxes, excluding them from participating in that part of the
   protocol.  Similarly, data that is exchanged with integrity
   protection cannot be modified without being detected and discarded.

   The QUIC protocol [QUIC] adopts both encryption and integrity
   protection.  Encryption is used to carefully control what information
   is exposed to middleboxes.  QUIC also uses integrity protection over
   all the data it exchanges to prevent modification.

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4.4.  Visibility of Faults

   Feedback is critical to the success of the grease technique (see
   Section 4.2).  The system only works if an protocol deployment has a
   means of detecting and analyzing errors.  Ignoring errors could allow
   those errors to become entrenched.  This process can be automated,
   but when operating at scale it might be difficult or impossible to
   collect details of specific errors.

   Feedback on errors is more important during the development and early
   deployment of a change.  Disabling automatic error recovery methods
   during development improves visibility of errors.

   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.

5.  Security Considerations

   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].

6.  IANA Considerations

   This document makes no request of IANA.

7.  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,

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

   [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,

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

              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 GREASE to TLS Extensibility",
              draft-ietf-tls-grease-00 (work in progress), January 2017.

   [HTTP]     Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

              Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
              draft-ietf-httpbis-header-structure-04 (work in progress),
              March 2018.

              Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
              RFC 7233, DOI 10.17487/RFC7233, June 2014,

              Kario, H., "Re: [TLS] Thoughts on Version Intolerance",
              July 2016, <

              Hardie, T., "Path signals", draft-hardie-path-signals-02
              (work in progress), November 2017.

   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-10 (work
              in progress), March 2018.

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   [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,

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

   [SUCCESS]  Thaler, D. and B. Aboba, "What Makes for a Successful
              Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,

   [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", draft-ietf-tls-tls13-28 (work in progress),
              March 2018.

              Thaler, D., Ed., "Planning for Protocol Adoption and
              Subsequent Transitions", RFC 8170, DOI 10.17487/RFC8170,
              May 2017, <>.

Author's Address

   Martin Thomson


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