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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                        September 12, 2016
Expires: March 16, 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 March 16, 2017.

Copyright Notice

   Copyright (c) 2016 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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   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Transition vs. Co-existence . . . . . . . . . . . . . . . . .   4
   3.  Translation/Adaptation Location . . . . . . . . . . . . . . .   5
   4.  Translation Plans . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  IAB Members at the Time of This Writing . . . . . . . . . . .   6
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Case Studies . . . . . . . . . . . . . . . . . . . .   9
     A.1.  Explicit Congestion Notification  . . . . . . . . . . . .  10
     A.2.  Classless Inter-Domain Routing (CIDR) . . . . . . . . . .  10
     A.3.  Internationalized Domain Names  . . . . . . . . . . . . .  11
     A.4.  IPv6  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     A.5.  HTTP/2  . . . . . . . . . . . . . . . . . . . . . . . . .  13
       A.5.1.  Bundling of Features with New Versions  . . . . . . .  13
       A.5.2.  Planning for Replacement  . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

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 the IETF web site, or the
   RFC production center).  This document focuses solely on technical
   transitions, although some principles might apply to other types as

   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.

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   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.

   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: Don't underestimate the cost of things other than the
       hardware/software itself.  For example, operational tools and
       processes, personnel training, business model (accounting/
       billing) dependencies, and legal (regulation, patents, etc.)
       costs all add up.

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   4.  Extensibility: Design for extensibility 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.  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, and the difference between the two
   types of transition is not particularly significant.  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.  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 difficult, such as in the case of IDNA2008 [RFC5891]
   [RFC5895] and Unicode TR46 [TR46].

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3.  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 where such a translator is best placed.

   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].

   Any transition strategy for a non-backward-compatible mechanism
   should include a discussion of where it is placed and a rationale.

4.  Translation Plans

   The case studies described in Appendix A suggest that a good
   transition plan includes at least the following components:

   1.  An explanation of incentives for each entity involved

   2.  A description of transition phases.  For example, there might be
       pilot, co-existence, deprecation, and removal phases for a
       transition from one technology to another incompatible one.

   3.  A proposed timeline

   4.  A way to measure whether the transition is succeeding

   5.  A contingency plan, in case something goes wrong as a result of
       the transition

   6.  A way to effectively communicate the proposed plan to the
       entities affected, and incorporate their feedback

   We recommend that such criteria be considered when evaluating
   proposals to transition to new or updated protocols.

5.  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

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   target.  For further discussion of some of these issues, see
   Section 5 of [RFC7754].

6.  IANA Considerations

   This document requires no actions by the IANA.

7.  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

8.  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

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   [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>.

   [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>.

   [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>.

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   [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,

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

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, 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,

   [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>.

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

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   [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,

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

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   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.
   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.  Classless Inter-Domain Routing (CIDR)

   TODO To be filled in... [RFC4632]

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A.3.  Internationalized Domain Names

   TODO To be filled in...

A.4.  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.

   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

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   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
   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

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   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.5.  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
   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 request multiplexing, which is supported
   by request prioritization and flow control.  HTTP/2 includes
   efficiency improvements with header compression [RFC7541] and binary

A.5.1.  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 posture.  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.

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Internet-Draft           Planning for Transition          September 2016

A.5.2.  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

   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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