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Versions: (draft-thaler-transition-principles) 00 01 02 03 04 05 06 07 08 RFC 8170

Internet Architecture Board                               D. Thaler, Ed.
Internet-Draft                                                 Microsoft
Intended status: Informational                         February 21, 2017
Expires: August 25, 2017

Out With the Old and In With the New: Planning for Protocol Transitions


   Over the many years since the introduction of the Internet Protocol,
   we have seen a number of transitions from one protocol or technology
   to another, throughout the protocol stack.  Many protocols and
   technologies were not designed to enable smooth transition to
   alternatives or to easily deploy extensions, and thus some
   transitions, such as the introduction of IPv6, have been difficult.
   This document attempts to summarize some basic principles to enable
   future transitions, and also summarizes what makes for a good
   transition plan.

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 August 25, 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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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Extensibility . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Transition vs. Co-existence . . . . . . . . . . . . . . . . .   5
   4.  Translation/Adaptation Location . . . . . . . . . . . . . . .   5
   5.  Transition Plans  . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Understanding of Existing Deployment  . . . . . . . . . .   7
     5.2.  Explanation of Incentives . . . . . . . . . . . . . . . .   7
     5.3.  Description of Phases and Proposed Criteria . . . . . . .   7
     5.4.  Measurement of Success  . . . . . . . . . . . . . . . . .   8
     5.5.  Contingency Planning  . . . . . . . . . . . . . . . . . .   8
     5.6.  Communicating the Plan  . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   10. IAB Members at the Time of This Writing . . . . . . . . . . .  10
   11. Informative References  . . . . . . . . . . . . . . . . . . .  10
   Appendix A.  Case Studies . . . . . . . . . . . . . . . . . . . .  14
     A.1.  Explicit Congestion Notification  . . . . . . . . . . . .  14
     A.2.  Internationalized Domain Names  . . . . . . . . . . . . .  15
     A.3.  IPv6  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     A.4.  HTTP/2  . . . . . . . . . . . . . . . . . . . . . . . . .  18
       A.4.1.  Bundling of Features with New Versions  . . . . . . .  19
       A.4.2.  Planning for Replacement  . . . . . . . . . . . . . .  19
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   A "transition" is the process or period of changing from one state or
   condition to another.  There are several types of such transitions,
   including both technical transitions (e.g., changing protocols or
   deploying an extension) and organizational transitions (e.g.,
   changing what organization manages a web site).  This document
   focuses solely on technical transitions, although some principles
   might apply to other types as well.

   In this document we use the term transition generically to apply to
   any of:

   o  adoption of a new protocol where none existed before,

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   o  deployment of a new protocol that obsoletes a previous protocol,

   o  deployment of a updated version of an existing protocol, or

   o  decommissioning of an obsolete protocol.

   There have been many IETF and IAB RFCs and IAB statements discussing
   transitions of various sorts.  Most are protocol-specific documents
   about specific transitions.  For example, some relevant ones in which
   the IAB has been involved include:

   o  IAB RFC 3424 [RFC3424] recommended that any technology for so-
      called "unilateral self-address fixing (UNSAF)" across NATs
      include an exit strategy to transition away from such a mechanism.
      Since the IESG, not the IAB, approves IETF documents, the IESG
      thus became the body to enforce (or not) such a requirement.

   o  IAB RFC 4690 [RFC4690] gave recommendations around
      internationalized domain names.  It discussed issues around the
      process of transitioning to new versions of Unicode, and this
      resulted in the creation of the IETF Precis WG to address this

   o  The IAB statement on "Follow-up-work on NAT-PT"
      [IabIpv6TransitionStatement] pointed out gaps at the time in
      transitioning to IPv6, and this resulted in the rechartering of
      the IETF Behave WG to solve this problem.

   More recently, the IAB has done work on more generally applicable
   principles, including two RFCs.

   IAB RFC 5218 [RFC5218] on "What Makes for a Successful Protocol?"
   studied specifically what factors contribute to, and detract from,
   the success of a protocol and it made a number of recommendations.
   It discussed two types of transitions: "initial success" (the
   transition to the technology) and extensibility (the transition to
   updated versions of it).  The principles and recommendations in that
   document are generally applicable to all technical transitions.  Some
   important principles included:

   1.  Incentive: Transition is easiest when the benefits come to those
       bearing the costs.  That is, the benefits should outweigh the
       costs at *each* entity.  Some successful cases did this by
       providing incentives (e.g., tax breaks), or by reducing costs
       (e.g., freely available source), or by imposing costs of not
       transitioning (e.g., regulation), or even by narrowing the
       scenarios of applicability to just the cases where benefits do
       outweigh costs at all relevant entities.

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   2.  Incremental Deployability: Backwards compatibility makes
       transition easier.  Furthermore, transition is easiest when
       changing only one entity still benefits that entity.  In the
       easiest case, the benefit immediately outweighs the cost and so
       entities are naturally incented to transition.  More commonly,
       the benefits only outweigh the costs once a significant number of
       other entities also transition.  Unfortunately, in such cases,
       the natural incentive is often to delay transitioning.

   3.  Total Cost: It is import to consider costs that go beyond the
       core hardware and software, such as operational tools and
       processes, personnel training, business model (accounting/
       billing) dependencies, and legal (regulation, patents, etc.)

   4.  Extensibility: Design for extensibility [RFC6709] so that things
       can be fixed up later.

   IAB RFC 7305 [RFC7305] reported on a IAB workshop on Internet
   Technology Adoption and Transition (ITAT).  Like RFC 5218, this
   workshop also discussed economic aspects of transition, not just
   technical aspects.  Some important observations included:

   1.  Early-Adopter Incentives: Part of Bitcoin's strategy was extra
       incentives for early adopters compared to late adopters.  That
       is, providing a long-term advantage to early adopters can help
       stimulate transition even when the initial costs outweigh the
       initial benefit.

   2.  Policy Partners: Policy-making organizations of various sorts
       (RIRs, ICANN, etc.) can be important partners in enabling and
       facilitating transition.

   The remainder of this document continues the discussion started in
   those two RFCs and provides some additional thoughts on the topic of
   transition strategies and plans.

2.  Extensibility

   Many protocols are designed to be extensible, using mechanisms such
   as options, version negotiation, etc., to ease the transition to new
   features.  However, implementations often succumb to commercial
   pressures to ignore this flexibility in favor of performance or
   economy, and as a result such extension mechanisms (e.g., IPv6 Hop-
   by-Hop Options) often experience problems in practice once they begin
   to be used.  In other cases, a mechanism might be put into a protocol
   for future use without having an adequate sense of how it will be
   used, which causes problems later (e.g., SNMP's original 'security'

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   field, or the IPv6 Flow Label).  Thus, designers need to consider
   whether it would be easier to transition to a new protocol than it
   would be to ensure that an extension point is correctly specified and
   implemented such that it would be available when needed.

   When there is interest in translation between a new mechanism and an
   old one, complexity of such translation must also be considered.  The
   major challenge in translation is for semantic differences.  Often,
   syntactic differences can be translated seamlessly; semantic ones
   almost never.  Hence when designing for translatability, syntactic
   and semantic differences should be clearly documented.

   See RFC 3692 [RFC3692] and RFC 6709 [RFC6709] for more discussion of
   design considerations for protocol extensions.

3.  Transition vs. Co-existence

   There is an important distinction between a strict "flag-day" style
   transition where an old mechanism is immediately replaced with a new
   mechanism, vs. a looser co-existence based approach where transition
   proceeds in stages where a new mechanism is first added alongside an
   existing one for some overlap period, and then the old mechanism is
   removed at a later stage.

   When a new mechanism is backwards compatible with an existing
   mechanism, transition is easiest because different parties can
   transition at different times.  However, when no backwards
   compatibility exists such as in the IPv4 to IPv6 transition, a
   transition plan must choose either a "flag day" or a period of co-
   existence.  When a large number of entities are involved, a flag day
   becomes impractical or even impossible.  Coexistence, on the other
   hand, involves additional costs of maintaining two separate
   mechanisms during the overlap period which could be quite long.
   Furthermore, the longer the overlap period, the more the old
   mechanism might get further deployment and thus increase the overall
   pain of transition.

   Often the decision between a "flag day" and a sustained co-existence
   period may be complicated when differing incentives are involved
   (e.g., see the case studies in the Appendix).

4.  Translation/Adaptation Location

   A translation or adaptation mechanism is often required if the old
   and new mechanisms are not interoperable.  Care must be taken when
   determining whether one will work and where such a translator is best

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   A translation mechanism may not work for every use case.  For
   example, if a translation from one protocol (or protocol version) to
   another produces indeterminate results, translation will not work
   reliably.  In addition, if translation always produces a downgraded
   protocol result, the incentive considerations in Section 5.2 will be

   Requiring a translator in the middle of the path can hamper end-to-
   end security and reliability.  For example, see the discussion of
   network-based filtering in [RFC7754].

   On the other hand, requiring a translation layer within an endpoint
   can be a resource issue in some cases, such as if the endpoint could
   be a constrained node [RFC7228].

   In addition, when a translator is within an endpoint, it can can
   attempt to hide the difference between an older protocol and a newer
   protocol, either by exposing one of the two sets of behavior to
   applications and internally mapping it to the other set of behavior,
   or by exposing a higher level of abstraction which is then
   alternatively mapped to either one depending on detecting which is
   needed.  In contrast, when a translator is in the middle of the path,
   typically only the first approach can be done since the middle of the
   path is typically unable to provide a higher level of abstraction.

   Any transition strategy for a non-backward-compatible mechanism
   should include a discussion of where it is placed and a rationale.
   The transition plan should also consider the transition away from the
   use of translation and adaptation technologies.

5.  Transition Plans

   A review of the case studies described in Appendix A suggests that a
   good transition plan includes at least the following components: an
   understanding of what is already deployed and in use, an explanation
   of incentives for each entity involved, a description of the phases
   of the transition along with a proposed criteria for each phase, a
   method for measuring the transition's success, a contingency plan for
   failure of the transition, and an effective method for communicating
   the plan to the entities involved and incorporating their feedback
   thereon.  We recommend that such criteria be considered when
   evaluating proposals to transition to new or updated protocols.  Each
   of these components is discussed in the subsections below.

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5.1.  Understanding of Existing Deployment

   Often an existing mechanism has variations in implementations and
   operational deployments.  For example, a specification might include
   optional behaviors that may or may not be implemented or deployed.
   In addition, there may also be implementations or deployments that
   deviate from, or include vendor-specific extensions to, various
   aspects of a specification.  It is important when considering a
   transition to understand what variations one is intending to
   transition from or co-exist with, since the technical and non-
   technical issues may vary greatly as a result.

5.2.  Explanation of Incentives

   A transition plan should explain the incentives to each involved
   entity to support the transition.  Note here that many entities other
   than the endpoint applications and their users may be affected, and
   the barriers to transition may be nontechnical as well as technical.
   When considering these incentives, also consider network operations
   tools, practices, and processes, personnel training, accounting and
   billing dependencies, and legal and regulatory incentives.

   If there is opposition to a particular new protocol (e.g., from
   another standards organization, or a government, or some other
   affected entity), various non-technical issues arise that should be
   part of what is planned and dealt with.  Similarly, if there are
   significant costs or other disincentives, the plan needs to consider
   how to overcome them.

   It's worth noting that an analysis of incentives can be difficult and
   at times led astray by wishful thinking, as opposed to adequately
   considering economic realities.  Thus, honestly considerating any
   barriers to adoption, and justifying one's conclusions about others'
   incentives, are key to a successful analysis.

5.3.  Description of Phases and Proposed Criteria

   Transition phases might include pilot/experimental deployment,
   coexistence, deprecation, and removal phases for a transition from
   one technology to another incompatible one.

   Timelines are notoriously difficult to predict and impossible to
   impose on uncoordinated transitions at the scale of the Internet, but
   rough estimates can sometimes help all involved entities to
   understand the intended duration of each phase.  More often, it is
   useful to provide criteria for what must be met in order to move to
   the next phrase.  For example, is removal scheduled for a particular
   date (e.g., FCC regulation to discontinue analog TV broadcasts in the

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   U.S. by June 12, 2009)?  Or is removal to be based on the use of the
   old mechanism falling below a specified level?  Or some other

   As one example, RFC 5211 [RFC5211] proposed a transition plan for
   IPv6 that included a proposed timeline, and criteria specific to each
   phase.  While the timeline was not accurately followed, the phases
   and timeline did serve as inputs to the World IPv6 Day and World IPv6
   Launch events.

5.4.  Measurement of Success

   The degree of deployment of a given protocol or feature at a given
   phase in its transition can be measured differently, depending on its
   design.  For example, server-side protocols and options which
   identify themselves through a versioning or negotiation mechanism can
   be discovered through active Internet measurement studies.

5.5.  Contingency Planning

   A contingency plan can be as simple as providing for indefinite
   coexistence between an old and new protocol, or for reverting to the
   old protocol until an updated version of the new protocol is
   available.  Such a plan is useful in the event that unforeseen
   problems are discovered during deployment, so that such problems can
   be quickly mitigated.  For example, World IPv6 Day was designed to
   help discover such problems and fall back to the original state at
   the end of the day regardless of whether any problems were found,
   though some participants chose to leave the new state in place when
   they did not encounter problems and so for such participants the
   contingency plan was to fall back at the end of the day like other
   participants did.

5.6.  Communicating the Plan

   Many of the entities involved in a protocol transition may not be
   aware of the IETF or the RFC series, so dissemination through other
   channels is key for sufficiently broad communication of the
   transition plan.  While flag days are impractical at Internet scale,
   coordinated "events" such as World IPv6 Launch may improve general
   awareness of an ongoing transition.

   Also, there is often a need for an entity facilitating the transition
   through advocacy and focus.  Such an entity, independent of the IETF,
   can be key in communicating the plan and its progress.

   Some transitions have a risk of breaking backwards compatibility for
   some fraction of users.  In such a case, when a transition affects

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   competing entities facing the risk of losing customers to each other,
   there is an economic disincentive to transition.  Thus, one role for
   a facilitating entity is to get competitors to transition during the
   same timeframe, so as to mitigate this fear.  For example, the
   success of World IPv6 Launch was largely due to ISOC playing this

6.  Security Considerations

   This document discusses attributes of protocol transitions.  Some
   types of transition can adversely affect security or privacy.  For
   example, requiring a translator in the middle of the path may hamper
   end-to-end security and privacy, since it creates an attractive
   target.  For further discussion of some of these issues, see
   Section 5 of [RFC7754].

   In addition, coexistence of two protocols in general increases risk
   in the sense that it doubles the attack surface and allows attacks
   that exploit the weaker of the two protocols by claiming not to
   understand the stronger one.

7.  IANA Considerations

   This document requires no actions by the IANA.

8.  Conclusion

   This document summarized the set of issues that should be considered
   by protocol designers and deployers to facilitate such transition and
   provides pointers to previous work (e.g., [RFC3692], [RFC6709]) that
   provided detailed design guidelines.  This document also covered what
   makes for a good transition plan, and includes several case studies
   that provide examples.  As more experience is gained over time on how
   to successfully apply these principles and design effective
   transition plans, we encourage the community to share such learnings
   with the IETF community and on the architecture-discuss@ietf.org
   mailing list so that any future document on this topic can leverage
   such experience.

9.  Acknowledgements

   This document is a product of the IAB Stack Evolution Program, with
   input from many others.  In particular, Dave Crocker, Eliot Lear, Joe
   Touch, Cameron Byrne, the IETF Applications Area WG, and others
   provided helpful input on this document.

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10.  IAB Members at the Time of This Writing

   Jari Arkko
   Ralph Droms
   Ted Hardie
   Joe Hildebrand
   Russ Housley
   Lee Howard
   Erik Nordmark
   Robert Sparks
   Andrew Sullivan
   Dave Thaler
   Martin Thomson
   Brian Trammell
   Suzanne Woolf

11.  Informative References

   [HTTP0.9]  Tim Berners-Lee, "The Original HTTP as defined in 1991",
              1991, <https://www.w3.org/Protocols/HTTP/

              IAB, "Follow-up work on NAT-PT", October 2007,

              Botterman, M., "IPv6 Deployment Survey", 2011,

              British Telecommunications, "IPv6 Industry Survey Report",
              August 2015, <http://www.globalservices.bt.com/static/asse

   [PAM2015]  Trammell, B., Kuehlewind, M., Boppart, D., Learmonth, I.,
              Fairhurst, G., and R. Scheffenegger, "Enabling Internet-
              Wide Deployment of Explicit Congestion Notification",
              Proceedings of PAM 2015, 2015, <target:

   [RFC1883]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 1883, DOI 10.17487/RFC1883,
              December 1995, <http://www.rfc-editor.org/info/rfc1883>.

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   [RFC1933]  Gilligan, R. and E. Nordmark, "Transition Mechanisms for
              IPv6 Hosts and Routers", RFC 1933, DOI 10.17487/RFC1933,
              April 1996, <http://www.rfc-editor.org/info/rfc1933>.

   [RFC1945]  Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945,
              DOI 10.17487/RFC1945, May 1996,

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616,
              DOI 10.17487/RFC2616, June 1999,

   [RFC2893]  Gilligan, R. and E. Nordmark, "Transition Mechanisms for
              IPv6 Hosts and Routers", RFC 2893, DOI 10.17487/RFC2893,
              August 2000, <http://www.rfc-editor.org/info/rfc2893>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,

   [RFC3424]  Daigle, L., Ed. and IAB, "IAB Considerations for
              UNilateral Self-Address Fixing (UNSAF) Across Network
              Address Translation", RFC 3424, DOI 10.17487/RFC3424,
              November 2002, <http://www.rfc-editor.org/info/rfc3424>.

   [RFC3692]  Narten, T., "Assigning Experimental and Testing Numbers
              Considered Useful", BCP 82, RFC 3692,
              DOI 10.17487/RFC3692, January 2004,

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              DOI 10.17487/RFC4380, February 2006,

   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", BCP 122, RFC 4632, DOI 10.17487/RFC4632, August
              2006, <http://www.rfc-editor.org/info/rfc4632>.

   [RFC4690]  Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
              Recommendations for Internationalized Domain Names
              (IDNs)", RFC 4690, DOI 10.17487/RFC4690, September 2006,

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   [RFC5211]  Curran, J., "An Internet Transition Plan", RFC 5211,
              DOI 10.17487/RFC5211, July 2008,

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

   [RFC5894]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Background, Explanation, and
              Rationale", RFC 5894, DOI 10.17487/RFC5894, August 2010,

   [RFC5895]  Resnick, P. and P. Hoffman, "Mapping Characters for
              Internationalized Domain Names in Applications (IDNA)
              2008", RFC 5895, DOI 10.17487/RFC5895, September 2010,

   [RFC6055]  Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
              Encodings for Internationalized Domain Names", RFC 6055,
              DOI 10.17487/RFC6055, February 2011,

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <http://www.rfc-editor.org/info/rfc6146>.

   [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
              P. Roberts, "Issues with IP Address Sharing", RFC 6269,
              DOI 10.17487/RFC6269, June 2011,

   [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol",
              RFC 6455, DOI 10.17487/RFC6455, December 2011,

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

   [RFC7021]  Donley, C., Ed., Howard, L., Kuarsingh, V., Berg, J., and
              J. Doshi, "Assessing the Impact of Carrier-Grade NAT on
              Network Applications", RFC 7021, DOI 10.17487/RFC7021,
              September 2013, <http://www.rfc-editor.org/info/rfc7021>.

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   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,

   [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,

   [RFC7301]  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, <http://www.rfc-editor.org/info/rfc7301>.

   [RFC7305]  Lear, E., Ed., "Report from the IAB Workshop on Internet
              Technology Adoption and Transition (ITAT)", RFC 7305,
              DOI 10.17487/RFC7305, July 2014,

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [RFC7541]  Peon, R. and H. Ruellan, "HPACK: Header Compression for
              HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,

   [RFC7754]  Barnes, R., Cooper, A., Kolkman, O., Thaler, D., and E.
              Nordmark, "Technical Considerations for Internet Service
              Blocking and Filtering", RFC 7754, DOI 10.17487/RFC7754,
              March 2016, <http://www.rfc-editor.org/info/rfc7754>.

   [TR46]     Unicode Consortium, "Unicode IDNA Compatibility
              Processing", June 2015,

   [TSV2007]  Sridharan, M., Bansal, D., and D. Thaler, "Implementation
              Report on Experiences with Various TCP RFCs", Proceedings
              of IETF 68, March 2007,

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Appendix A.  Case Studies

   Appendix A of [RFC5218] describes a number of case studies that are
   relevant to this document and highlight various transition problems
   and strategies (see for instance the Inter-Domain Multicast case
   study in Section A.4 of [RFC5218]).  We now include several
   additional case studies that focus on transition problems and
   strategies.  Many other equally good case studies could have been
   included, but, in the interests of brevity, only a sampling is
   included here that is sufficient to justify the conclusions in the
   body of this document.

A.1.  Explicit Congestion Notification

   Explicit Congestion Notification (ECN) is a mechanism to replace loss
   as the only signal for the detection of congestion, with an explicit
   signal sent from a router to the recipient of a packet, then
   reflected back to the sender.  It was standardized in 2000 in
   [RFC3168], and the mechanism consists of two parts: congestion
   detection in the IP layer, reusing two bits of the old IP Type of
   Service (TOS) field, and congestion feedback in the transport layer.
   Feedback in TCP uses two TCP flags, ECN Echo and Congestion Window
   Reduced.  Together with a suitably configured active queue management
   (AQM), ECN can improve TCP performance on congested links.

   The deployment of ECN is a case study in failed transition followed
   by possible redemption.  Initial deployment of ECN in the early and
   mid 2000s led to severe problems with some network equipment,
   including home router crashes and reboots when packets with ECN IP or
   TCP flags was received [TSV2007].  This led to firewalls stripping
   ECN IP and TCP flags, or even dropping packets with these flags set.
   This stalled deployment.  The need for both endpoints (to negotiate
   and support ECN) and on-path devices (to mark traffic when congestion
   occurs) to cooperate in order to see any benefits from ECN deployment
   was a further issue.  The deployment of ECN across the Interent had

   In the late 2000s, Linux and Windows servers began defaulting to
   "passive ECN support", meaning they would negotiate ECN if asked by
   the client, but would not ask to negotiate ECN by default.  This
   decision was regarded as without risk: only if a client were
   explicitly configured to negotiate ECN would any possible
   connectivity problems surface.  Gradually, this has increased server
   support in the Internet from near zero in 2008, to 11% of the top
   million Alexa webservers in 2011, to 30% in 2012, to 65% in late
   2014.  In the meantime, the risk to connectivity of ECN negotiation
   has reduced dramatically [PAM2015], leading to ongoing work to make
   Windows, Apple iOS, OSX, and Linux clients negotiate ECN by default.

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   It is hoped that a critical mass of clients and servers negotiating
   ECN will provide an incentive to mark congestion on ECN-enabled
   traffic, thus breaking the logjam.

A.2.  Internationalized Domain Names

   The deployment of Internationalized Domain Names (IDN) has a long and
   complicated history.  This should not be surprising, since
   internationalization deals with language and cultural issues
   regarding differing expectations of users around the world, thus
   making it inherently difficult to agree on common rules.
   Furthermore, because human languages evolve and change over time,
   even if common rules can be established, there is likely to be a need
   to review and update them regularly.

   There have been multiple technical transitions related to IDNs,
   including the introduction of non-ASCII in DNS, the transition to
   each new version of Unicode, and the transition from IDNA 2003 to
   IDNA 2008.  A brief history of the introduction of non-ASCII in DNS
   and the various complications that arose therein, can be found in
   section 3 of [RFC6055].  While IDNA 2003 was limited to Unicode
   version 3.2 only, one of the IDNA 2008 changes was to decouple its
   rules from any particular version of Unicode (see [RFC5894],
   especially section 1.4, for more discussion of this point, and see
   [RFC4690] for a list of other issues with IDNA 2003 that motivated
   IDNA 2008).  However, the transition from IDNA 2003 to IDNA 2008
   itself presented a problem since IDNA 2008 did not preserve backwards
   compatibility with IDNA 2003 for a couple of codepoints.
   Investigations and discussions with affected parties led to the IETF
   ultimately choosing IDNA 2008 because the overall gain by moving to
   IDNA 2008 to fix the problems with IDNA 2003 was seen to be much
   greater than the problems due to the few incompatibilities at the
   time of the change, as not many IDNs were in use, and even fewer that
   might see incompatibilities.

   A couple of browser vendors in particular were concerned about the
   differences between IDNA 2003 and IDNA 2008, and the fact that if a
   browser stopped being able to get to some site, or unknowingly sent a
   user to a different (e.g., phishing) site instead, the browser would
   be blamed.  As such, any user-perceivable change from IDNA 2003
   behavior would be painful to the vendor to deal with, and hence they
   could not depend on solutions that would need action by other

   Thus, to deal with issues like such incompatibilities, some
   applications and client-side frameworks wanted to map one string into
   another (namely, a string that would give the same result as when
   IDNA 2003 was used) before invoking DNS.

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   To provide such mapping (and some other functioanlity), the Unicode
   Consortium published [TR46] that continued down the path of IDNA 2003
   with a code point by code point selection mechanism.  This was
   implemented by some, but never adopted by the IETF.

   Meanwhile, the IETF did not publish any mapping mechanism, but
   [RFC5895] was published on the Independent Submission stream.  In
   discussions around mapping, one of the key topics was about how long
   the transition should last.  At one end of the duration spectrum is a
   flag day where some entities would be broken initially but the change
   would happen before IDN usage became even more ubiquitous.  At the
   other end of the spectrum is the need to maintain mappings
   indefinitely.  Local incentives at each entity who needed to change,
   however, meant that a short timeframe was impractical.

   There are many affected types of entities with very different
   incentives.  For example, the incentives affecting browser vendors,
   registries, domain name marketers and applicants, app developers, and
   protocol designers are each quite different, and the various
   solutions require changes by multiple types of entities, where the
   benefits do not always align with the costs.  If there is some group
   (or even an individual) that is opposed to a change/transition and
   able to put significant resources behind their opposition,
   transitions get a lot harder.

   Finally, there are multiple naming contexts, and the protocol
   behavior (including how internationalized domain names are handled)
   within each naming context can be different.  Hence applications and
   frameworks often encounter a variety of behaviors and may or may not
   be designed to deal with them.  See sections 2 and 3 of [RFC6055] for
   more discussion.

   In summary, all this diversity can cause problems for each affected
   entity, especially if a competitor does not have such a problem,
   e.g., for browser vendors if competing browsers do not have the same
   problems, or for an email server provider if competing server
   providers do not have the same problems.

A.3.  IPv6

   Twenty-one years after publication of [RFC1883], the transition to
   IPv6 is still in progress.  The first document to describe a
   transition plan ([RFC1933], later obsoleted by [RFC2893]) was
   published less than a year after the protocol itself.  It recommended
   co-existence (dual-stack or tunneling technology) with the
   expectation that over time, all hosts would have IPv6, and IPv4 could
   be quietly retired.

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   In the early stages, deployment was limited to peer-to-peer uses,
   tunneled over IPv4 networks.  For example, Teredo [RFC4380] aligned
   the cost of fixing the problem with the benefit, and allowed for
   incremental benefits to those who used it.

   Operating System vendors had incentives because with such tunneling
   protocols, they could get peer-to-peer apps working without depending
   on any infrastructure changes.  That resulted in the main apps using
   IPv6 being in the peer-to-peer category (BitTorrent, XBox gaming,

   Router vendors had some incentive because IPv6 could be used within
   an intra-domain network more efficiently than tunneling, once the OS
   vendors already had IPv6 support and some special-purpose apps

   For content providers and ISPs, on the other hand, there was little
   incentive for deployment: there was no incremental benefit to
   deploying locally.  Since everyone already had IPv4, there was no
   network effect benefit to deploying IPv6.  Even as proponents argued
   that workarounds to extend the life of IPv4--such as CIDR, NAT, and
   stingy allocations--made it more complex, IPv4 continued to work well
   enough for most applications.

   Workarounds to NAT problems documented in [RFC6269] and [RFC7021]
   included ICE, STUN, and TURN, technologies that allowed those
   experiencing the problems to deploy technologies to resolve them.  As
   with end-to-end IPv6 tunneling (e.g., Teredo), the incentives there
   aligned the cost of fixing the problem with the benefit, and allowed
   for incremental benefits to those who used them.  The IAB discussed
   NAT technology proposals [RFC3424] and recommended they be considered
   short-term fixes, and said that proposals must include an exit plan,
   such that they would decline over time.  In particular, the IAB
   warned against generalizing NAT solutions, which would lead to
   greater dependence on them.  In some ways, these solutions, along
   with other IPv4 development (e.g., the workarounds above, and
   retrofitting IPsec into IPv4) continued to reduce the incentive to
   deploy IPv6.

   Indeed, not until a few years after IPv4 runout in various Regional
   Address Registry (RIR) regions did IPv6 deployment significantly
   increase.  The RIRs and others conducted surveys of different
   industries and industry segments to learn why people did not deploy
   IPv6 [IPv6Survey2011] [IPv6Survey2015], which commonly listed lack of
   a business case, lack of training, and lack of vendor support as
   primary hurdles.  Arguably forward-looking companies collaborated
   with ISOC on World IPv6 Day and World IPv6 Launch to jump start
   global IPv6 deployment, and arguably their work gave vendors

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   incentives to support IPv6 well.  Key aspects of World IPv6 Day and
   World IPv6 Launch that contributed to their successes were the
   communication mechanism, and the measurement metrics and contingency
   plans that were announced in advance.

   Several efforts have been made to mitigate the lack of a business
   case.  Some governments (South Korea, Japan) provided tax incentives
   to include IPv6.  Other governments (Belgium, Singapore) mandated
   IPv6 support by private companies.  Few of these had enough value to
   drive significant IPv6 deployment.

   The concern about lack of training is often a common issue in
   transitions.  Because IPv4 is so ubiquitous, its use is routine and
   simplified with common tools, and it is taught in network training
   everywhere.  While IPv6 deployment was low, ignorance of it was no
   obstacle to being hired as a network administrator or developer.

   Organizations with the greatest incentives to deploy IPv6 are those
   which continue to grow quickly, even after IPv4 free pool exhaustion.
   Thus, ISPs have had varying levels of commitment, based on the growth
   of their user base, services being added (especially video over IP),
   and the number of IPv4 addresses they had available.  Cloud-based
   providers, including CDN and hosting companies, have been major
   buyers of IPv4 addresses, and several have been strong deployers and
   advocates of IPv6.

   Different organizations will use different transition models for
   their networks, based on their needs.  Some are electing to use
   IPv6-only hosts in the network with IPv6-IPv4 translation at the
   edge.  Others are using dual-stack hosts with IPv6-only routers in
   the core of the network, and IPv4 tunneled or translated through them
   to dual-stack edge routers.  Still others are using native dual-stack
   throughout the network, but that generally persists as an interim
   measure: adoption of two technologies is not the same as
   transitioning from one technology to another.  Finally, some walled
   gardens or isolated networks, such as management networks, use
   IPv6-only end-to-end.

   It is impossible to predict with certainty the path IPv6 deployment
   will have taken when it is complete.  Lessons learned so far include
   aligning costs and benefits (incentive), and ensuring incremental
   benefit (network effect, or backward compatibility).

A.4.  HTTP/2

   HTTP/2 [RFC7540] is a new version of the popular HTTP protocol
   [RFC7230].  The original versions of HTTP (0.9 [HTTP0.9], 1.0
   [RFC1945], and 1.1 [RFC2616]) have only small differences; each

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   iteration made small improvements over the previous version without
   making major changes.

   The changes in HTTP/2 are largely aimed at improving performance.
   The primary improvement is for request multiplexing, which is
   supported by request prioritization and flow control.  HTTP/2
   includes efficiency improvements with header compression [RFC7541]
   and binary framing.

A.4.1.  Bundling of Features with New Versions

   The bundling of additional constraints on a new version of a protocol
   could affect adoption by making the transition more costly.  However,
   the transition to a new version also represents an opportunity to
   improve multiple aspects of a protocol at the same time.

   The HTTP working group decided that a new version of the protocol
   represented an opportunity to improve security.  HTTP/2 is much
   stricter about its use of TLS.  In particular, a long list of TLS
   cipher suites are prohibited, constraints are placed on the key
   exchange method, and renegotiation is prohibited.  These changes did
   cause deployment problems.  Though most were minor and transitory,
   disabling renegotiation caused problems for deployments that relied
   on the feature to authenticate clients and prompted new work to
   replace the feature.

   A number of other features or characteristics of HTTP were identified
   as potentially undesirable.  Several such features were considered
   for removal during the design process.  This included trailers, the
   1xx series of responses, certain modes of request forms, and the
   unsecured (http://) variant of the protocol.  For each of these, the
   risk to the successful deployment of the new version was considered
   to be too great to justify removing the feature.  However, deployment
   of the unsecured variant of HTTP/2 remains extremely limited.

A.4.2.  Planning for Replacement

   HTTP/1.1 provides a mechanism, Upgrade, to transition to an entirely
   different protocol.  That same facility was little used other than to
   enable the use of WebSockets [RFC6455].  However, with performance
   being a primary motivation for HTTP/2, a new mechanism was needed to
   avoid spending an additional round trip on this negotiation.  A new
   mechanism was added to TLS to permit the negotiation of the new
   version of HTTP: Application Layer Protocol Negotiation (ALPN)
   [RFC7301].  Upgrade was used only for the unsecured variant of the

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   ALPN was identified as the way in which future protocol versions
   would be negotiated.  The mechanism was well-tested during
   development of the specification, which proved that new versions
   could be deployed safely and easily using ALPN.  Several draft
   versions of the protocol were successfully deployed during protocol
   development, and version negotiation was never shown to be an issue.

   Confidence that new versions would be easy to deploy if necessary
   lead to a particular design stance that might be considered unusual
   in light of the advice in RFC 5218 [RFC5218], though is completely
   consistent with RFC 6709 [RFC6709]: many of the ways in which the
   protocol might be extended were removed unless an immediate need was
   understood.  This decision was made on the basis that it would be
   easier to revise the entire protocol than it would be to ensure that
   an extension point was correctly specified and implemented such that
   it would be available when needed.

Author's Address

   Dave Thaler (editor)
   One Microsoft Way
   Redmond, WA  98052

   Email: dthaler@microsoft.com

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