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Versions: 00 draft-thomson-use-it-or-lose-it

Network Working Group                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Standards Track                          March 13, 2017
Expires: September 14, 2017

          Factors Influencing the Freedom to Change Protocols


   The ability to change protocols depends on exercising that ability.
   Protocols that don't change can find that the mechanisms that support
   change become unusable.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 14, 2017.

Copyright Notice

   Copyright (c) 2017 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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   described in the Simplified BSD License.

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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.  Protocol Freedom  . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Use of Protocol Freedom . . . . . . . . . . . . . . . . .   5
     3.2.  Reliance on Protocol Freedom  . . . . . . . . . . . . . .   6
     3.3.  Unused Extension Points Become Unusable . . . . . . . . .   7
   4.  Defensive Design Principles for Protocol Freedom  . . . . . .   7
     4.1.  Grease  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Cryptography  . . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  Visibility  . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Informative References  . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   A successful protocol [RFC5218] 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.
   [I-D.iab-protocol-transitions] examines the problem more broadly.

   This document examines the specific conditions that determine whether
   protocol maintainers have the ability - the freedom - to design and
   deploy new or modified protocols.

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
   extensions can mean that instead of handling the mechanism as
   designed, endpoints react poorly.  This can manifest as abrupt

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   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 [RFC6709] 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
      _after_ the base protocol has been deployed for a while, and its
      deficiencies have become evident.

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   Transport Layer Security (TLS) [RFC5246] 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)" exactly as described in [RFC6709].
   However, clients are unable to advertise a new version without
   causing a non-trivial proportions of sessions to fail due to bugs in
   server (or middlebox) implementations.

   Note:  It is possible that some middleboxes prevent negotiation of
      protocol versions and features they do not understand.  It is hard
      to imagine what those middleboxes hope to gain from doing so for a
      protocol like TLS.

   Intolerance to new TLS versions is so severe [INTOLERANCE] that TLS
   1.3 [I-D.ietf-tls-tls13] has abandoned HMSV version negotiation for a
   different mechanism.

   The server name indication (SNI) [RFC6066] 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 defined,
   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 [SNI].

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 still has
   two ends, and even at the endpoints of an interaction, protocol
   elements can be passed on to other entities in ways that can affect
   protcol operation.

   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.  In particular, one of the consequences of an unencrypted

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   protocol is that any element on path can interact with the protocol.
   For example, HTTP recognizes the role of a transparent proxy
   [RFC7230].  Because HTTP was specifically designed with
   intermediation in mind, transparent proxies are not only possible,
   but sometimes advantageous.  Consequently, transparent proxies for
   HTTP are commonplace.

   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) [RFC7092] 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.  Protocol Freedom

   If design is insufficient, what then would give protocol designers
   the freedom to later change a deployed protocol?

   Michel Foucault defines freedom as a practice rather than a state
   that is bestowed or attained:

      Freedom is practice; [...] the freedom of men is never assured by
      the laws and the institutions that are intended to guarantee them.
      [...] I think it can never be inherent in the structure of things
      to guarantee the exercise of freedom.  The guarantee of freedom is
      freedom. -[FOUCAULT1]

   In the same way, the design of a protocol for extensibility and
   eventual replacement [RFC6709] does not guarantee the ability to
   exercise those options.

3.1.  Use of Protocol Freedom

   Though planning and specifying these options is a necessarily
   precondition for their availability, whether they are available
   depends on more than what is written in a specification.  The nature

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   a protocol deployment has a significant effect on whether that
   protocol can be changed.

      This is why I emphasize practices of freedom over processes of
      liberation; again, the latter indeed have their place, but they do
      not seem to me, to be capable by themselves of defining all the
      practical forms of freedom.  -[FOUCAULT2]

   The fact that freedom 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

   For example, header fields in email [RFC5322], HTTP [RFC7230] and SIP
   [RFC3261] all derive from the same basic design.  There is no
   evidence of any barriers to deploying header fields with new names
   and semantics.

   In another example, the attribute-value pairs (AVPs) in Diameter
   [RFC6733] are fundamental to the design of the protocol.  The
   definition of new uses of Diameter regularly exercise the ability to
   add new AVPs and do so with no fear that the definition might be

   These examples show extension points that are heavily used also being
   relatively unaffected by deployment issues.  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

   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.  Reliance on Protocol Freedom

   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 a mechanism
   and it being used.

   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

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   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 critical to the functioning of a
   protocol deployment.  Should an implementation fail to correctly
   implement the mechanism, that failure would become immediately

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 [RFC7230], the ability to use transfer codings other
   than the chunked coding, and the range unit in a range request

4.  Defensive Design Principles for Protocol Freedom

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

4.1.  Grease

   "Grease" [I-D.ietf-tls-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 grease design aims 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.  In that
   design, the client 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.

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   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 somewhat on
   the fact that the difficulty of implementing the protocol mechanism
   correctly is not significantly more effort than implementing code to
   specifically filter out the randomized "grease" values.  To that end,
   the values that are reserved are not taken from a single contiguous
   block of code points, but are distributed across the entire space of
   code points.

   The hope with grease is that errors in implementing the mechanisms it
   safeguards are quickly detected.  If many implementations send these
   "grease" values as part of regular operation, then any failure to
   properly handle these apparently new values will be detected.

   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.  Other techniques might be necessary
   for protocols that don't rely on the particular style of exchange
   that is predominant in TLS.

4.2.  Cryptography

   A method of defensive design is that of using cryptography (such as
   TLS) to forcibly reduce the number of entities that can participate
   in the protocol.

   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 by middleboxes.

   The QUIC protocol [I-D.ietf-quic-transport], adopts both encryption
   to carefully control what information is exposed to middleboxes.
   QUIC also uses integrity protection over all the data it exchanges to
   prevent modification.

4.3.  Visibility

   Modern software engineering practice includes a strong emphasis on
   measuring the effects of changes and correcting based on that
   feedback.  Runtime monitoring of system health is an important part
   of that, which relies on systems of logging and synthetic health
   indicators, such as aggregate transaction failure rates.

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   Feedback is critical to the success of the grease technique (see
   Section 4.1).  The system only works if an implementer creates a way
   to ensure that errors are detected and analyzed.  This process can be
   automated, but when operating at scale it might be difficult or
   impossible to collect details of specific errors.

   Treating errors in protocol implementation as fatal can greatly
   improve visibility.  Disabling automatic recovery from protocol
   errors can be disruptive to users when those errors occur, but it
   also ensures that errors are made visible.  Where users are part of
   the feedback system, visibility of error conditions is especially

   New protocol designs are encouraged to define conditions that result
   in fatal errors.  Competitive pressures often force implementations
   to favor strategies that mask or hide errors.  Standardizing on error
   handling that ensures visibility of flaws avoids handling that hides

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

   Automated feedback systems are important for automated systems, or
   where error recovery is also automated.  For instance, failures to
   connect to HTTP alternative services [RFC7838] are not permitted to
   affect the outcome of transactions.  A feedback system for capturing
   failures in alternative services is therefore crucial to ensuring the
   mechanism remains viable.

5.  Security Considerations

   The ability to design, implement, and deploy new protocol mechanisms
   can be critical to security.  In particular, the need to be able to
   replace cryptographic algorithms over time has been well established

6.  IANA Considerations

   This document makes no request of IANA.

7.  Informative References

              Foucault, M. and P. Rabinow, Ed., "The Foucault Reader",
              ISBN 0394713400, November 1984.

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              Foucault, M. and P. Rabinow, Ed., "Ethics: Subjectivity
              and Truth", ISBN 1565844343, May 1998.

              Thaler, D., "Planning for Protocol Adoption and Subsequent
              Transitions", draft-iab-protocol-transitions-08 (work in
              progress), March 2017.

              Kamp, P., "HTTP Header Common Structure", draft-ietf-
              httpbis-header-structure-00 (work in progress), December

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-01 (work
              in progress), January 2017.

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

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-19 (work in progress),
              March 2017.

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

   [RFC3261]  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,

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

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

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

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

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

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

   [RFC7092]  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,

   [RFC7230]  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,

   [RFC7233]  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,

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

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

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

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Author's Address

   Martin Thomson

   Email: martin.thomson@gmail.com

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