



Internet Engineering Task Force (IETF)                       G. Fioccola
Request for Comments: 9386                                    P. Volpato
Obsoletes: 6036                                      Huawei Technologies
Category: Informational                                J. Palet Martinez
ISSN: 2070-1721                                         The IPv6 Company
                                                               G. Mishra
                                                            Verizon Inc.
                                                                  C. Xie
                                                           China Telecom
                                                              April 2023


                         IPv6 Deployment Status

Abstract

   This document provides an overview of the status of IPv6 deployment
   in 2022.  Specifically, it looks at the degree of adoption of IPv6 in
   the industry, analyzes the remaining challenges, and proposes further
   investigations in areas where the industry has not yet taken a clear
   and unified approach in the transition to IPv6.  It obsoletes RFC
   6036.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9386.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
   2.  IPv6: The Global Picture
     2.1.  IPv4 Address Exhaustion
       2.1.1.  IPv4 Addresses per Capita and IPv6 Status
     2.2.  IPv6 Users
     2.3.  IPv6 Web Content
     2.4.  IPv6 Public Actions and Policies
   3.  A Survey on IPv6 Deployments
     3.1.  IPv6 Allocations
     3.2.  IPv6 among Internet Service Providers
     3.3.  IPv6 among Enterprises
       3.3.1.  Government and Universities
   4.  IPv6 Deployment Scenarios
     4.1.  Dual-Stack
     4.2.  IPv6-Only Overlay
     4.3.  IPv6-Only Underlay
     4.4.  IPv4-as-a-Service
     4.5.  IPv6-Only
   5.  Common IPv6 Challenges
     5.1.  Transition Choices
       5.1.1.  Service Providers: Fixed and Mobile Operators
       5.1.2.  Enterprises
       5.1.3.  Industrial Internet
       5.1.4.  Content and Cloud Service Providers
       5.1.5.  CPEs and User Devices
       5.1.6.  Software Applications
     5.2.  Network Management and Operations
     5.3.  Performance
       5.3.1.  IPv6 Packet Loss and Latency
       5.3.2.  Customer Experience
     5.4.  IPv6 Security and Privacy
       5.4.1.  Protocols' Security Issues
   6.  IANA Considerations
   7.  Security Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Appendix A.  Summary of Questionnaire and Replies for Network
           Operators
   Appendix B.  Summary of Questionnaire and Replies for Enterprises
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   [RFC6036] describes IPv6 deployment scenarios that were adopted or
   foreseen by a number of Internet Service Providers (ISPs) who
   responded to a technical questionnaire in early 2010, and [RFC6036]
   also provides practices and plans that were expected to take place in
   the following years.  Since the publication of [RFC6036], several
   other documents have contributed to the IPv6 transition discussion in
   operational environments.  To name a few:

   *  [RFC6180] discusses IPv6 deployment models and transition
      mechanisms, recommending those proven to be effective in
      operational networks.

   *  [RFC6883] provides guidance and suggestions for Internet content
      providers and Application Service Providers (ASPs).

   *  [RFC7381] introduces the guidelines of IPv6 deployment for
      enterprises.

   [RFC6540] recommends the support of IPv6 to all IP-capable nodes.  It
   was referenced in the IAB statement on IPv6 [IAB], which represented
   a major step in driving the IETF and other Standards Development
   Organizations (SDOs) towards using IPv6 in their works.

   In more recent times, organizations, such as ETSI, provided more
   contributions to the use of IPv6 in operational environments,
   targeting IPv6 in different industry segments.  As a result,
   [ETSI-IP6-WhitePaper] was published to provide an updated view on the
   IPv6 best practices adopted so far, in particular, in the ISP domain.

   Considering all of the above, and after more than ten years since the
   publication of [RFC6036], it is useful to assess the status of the
   transition to IPv6.  Some reasons include:

   *  In some areas, the lack of IPv4 addresses forced both carriers and
      content providers to shift to IPv6 to support the introduction of
      new applications, in particular, in wireless networks.

   *  Some governmental actions took place to encourage or even enforce
      the adoption of IPv6 in certain countries.

   *  Looking at the global adoption of IPv6, this seems to have reached
      a threshold that justifies speaking of end-to-end IPv6
      connectivity, at least at the IPv6 service layer.

   This document aims to provide a survey of the status of IPv6
   deployment and highlight both the achievements and remaining
   obstacles in the transition to IPv6 networks (and its coexistence
   with continued IPv4 services).  The target is to give an updated view
   of the practices and plans already described in [RFC6036] to
   encourage further actions and more investigations in those areas that
   are still under discussion and to present the main incentives for the
   adoption of IPv6.

   This document is intended for a general audience interested in
   understanding the status of IPv6 in different industries and network
   domains.  People who provide or use network services may find it
   useful for the transition to IPv6.  Also, people developing plans for
   IPv6 adoption in an organization or in an industry may find
   information and references for their analysis.  Attention is given to
   the different stages of the transition to IPv6 networks and services.
   In particular, terminology on the use of "IPv6-only" is provided,
   considering IPv6-only networks and services as the final stage of
   such transition.

   The topics discussed in this document are organized into four main
   chapters.

   *  Section 2 reports data and analytics about the status of IPv6.

   *  Section 3 provides a survey of IPv6 deployments in different
      environments, including ISPs, enterprises, and universities.

   *  Section 4 describes the IPv6 deployment approaches for Mobile
      Broadband (MBB), Fixed Broadband (FBB), and enterprises.

   *  Section 5 analyzes the general challenges to be solved in the IPv6
      transition.  Specific attention is given to operations,
      performance, and security issues.

1.1.  Terminology

   This section defines the terminology regarding the usage of IPv6-only
   expressions within this document.  The term IPv6-only is defined in
   relation to the specific scope it is referring to.  In this regard,
   it may happen that only part of a service, a network, or even a node
   is in an IPv6-only scope, and the rest is not.  The most used terms
   in relation to the different scopes are listed below:

   IPv6-only interface:
      The interface of a node is configured to forward only IPv6.  This
      denotes that just part of the node can be IPv6-only since the rest
      of the interfaces of the same node may work with IPv4 as well.  A
      Dual-Stack interface is not an IPv6-only interface.

   IPv6-only node:
      The node uses only IPv6.  All interfaces of the host only have
      IPv6 addresses.

   IPv6-only service:
      It is used if, between the host's interface and the interface of
      the content server, all packet headers of the service session are
      IPv6.

   IPv6-only overlay:
      It is used if, between the end points of the tunnels, all inner
      packet headers of the tunnels are IPv6.  For example, IPv6-only
      overlay in a fixed network means that the encapsulation is only
      IPv6 between the interfaces of the Provider Edge (PE) nodes or
      between the Customer Provider Edge (CPE) node and the Broadband
      Network Gateway (BNG).

   IPv6-only underlay:
      It is used if the data plane and control plane are IPv6, but this
      is not necessarily true for the management plane.  For example,
      IPv6-only underlay in a fixed network means that the underlay
      network protocol is only IPv6 between any PE nodes, but they can
      be Dual-Stack in overlay.  Segment Routing over IPv6 (SRv6) is an
      example of IPv6-only underlay.

   IPv6-only network:
      It is used if every node in this network is IPv6-only.  IPv4
      should not exist in an IPv6-only network.  In particular, an
      IPv6-only network's data plane, control plane, and management
      plane must be IPv6.  All PEs must be IPv6-only.  Therefore, if
      tunnels exist among PEs, both inner and outer headers must be
      IPv6.  For example, an IPv6-only access network means that every
      node in this access network must be IPv6-only, and similarly, an
      IPv6-only backbone network means that every node in this backbone
      network must be IPv6-only.

   IPv4-as-a-Service (IPv4aaS):
      IPv4 service support is provided by means of a transition
      mechanism; therefore, there is a combination of encapsulation/
      translation + IPv6-only underlay + decapsulation/translation.  For
      an IPv6-only network, connectivity to legacy IPv4 is either non-
      existent or provided by IPv4aaS mechanisms.

   Note that IPv6-only definitions are also discussed in
   [IPv6-ONLY-DEF].

2.  IPv6: The Global Picture

   This section deals with some key questions related to IPv6, namely:
   (1) the status of IPv4 exhaustion, often considered as one of the
   triggers to switch to IPv6, (2) the number of IPv6 end users, a
   primary measure to sense IPv6 adoption, (3) the percentage of
   websites reachable over IPv6, and (4) a report on IPv6 public actions
   and policies.

   These parameters are monitored by the Regional Internet Registries
   (RIRs) and other institutions worldwide, as they provide a first-
   order indication on the adoption of IPv6.

2.1.  IPv4 Address Exhaustion

   According to [CAIR], there will be 29.3 billion networked devices by
   2023, up from 18.4 billion in 2018.  This poses the question about
   whether the IPv4 address space can sustain such a number of
   allocations and, consequently, if this may affect the process of its
   exhaustion.  The answer is not straightforward, as many aspects have
   to be considered.

   On one hand, the RIRs are reporting scarcity of available and still-
   reserved addresses.  Table 3 of [POTAROO1] (January 2022) shows that
   the available pool of the five RIRs at the end of 2021 counted 5.2
   million IPv4 addresses, while the reserved pool included another 12.1
   million, for a total of 17.3 million IPv4 addresses (-5.5% year over
   year, comparing 2021 against 2020).  Table 1 of [POTAROO1] shows that
   the total IPv4 allocated pool equaled 3.685 billion addresses
   (+0.027% year over year).  The ratio between the available addresses
   and the total allocated was brought to 0.469% of the remaining IPv4
   address space (from 0.474% at the end of 2020).

   On the other hand, [POTAROO1] again highlights the role of both
   address transfer and Network Address Translation (NAT) to counter the
   IPv4 exhaustion.  The transfer of IPv4 addresses can be done under
   the control or registration of an RIR or on the so-called grey
   market, where third parties operate to enable the buying/selling of
   IPv4 addresses.  In all cases, a set of IPv4 addresses is
   "transferred" to a different holder that has the need to expand their
   address range.  As an example, [IGP-GT] and [NRO] show the amount of
   transfers to recipient organizations in the different regions.  Cloud
   Service Providers (CSPs) appear to be the most active in buying IPv4
   addresses to satisfy their need of providing IPv4 connectivity to
   their tenants.  NAT systems provide a means to absorb at least a
   portion of the demand of public IPv4 addresses, as they enable the
   use of private addressing in internal networks while limiting the use
   of public addresses on their WAN-facing side.  In the case of NAT,
   architectural and operational issues remain.  Private address space
   cannot provide an adequate address span, especially for large
   organizations, and the reuse of addresses may make the network more
   complex.  In addition, multiple levels of address translation may
   coexist in a network, e.g., Carrier-Grade NAT (CGN) [RFC6264], based
   on two stages of translation.  This comes with an economic and
   operational burden, as discussed later in this document.

2.1.1.  IPv4 Addresses per Capita and IPv6 Status

   The IPv4 addresses per capita ratio measures the quantity of IPv4
   addresses allocated to a given country divided by the population of
   that country.  It provides an indication of the imbalanced
   distribution of the IPv4 addresses worldwide.  It clearly derives
   from the allocation of addresses made in the early days of the
   Internet.

   The sources for measuring the IPv4 addresses per capita ratio are the
   allocations done by the RIRs and the statistics about the world
   population.  In this regard, [POTAROO2] provides distribution files.
   The next tables compare the number of IPv4 addresses available per
   person in a certain country (IPv4 address per capita) against the
   relative adoption of IPv6 in the same country (expressed as the
   number of IPv6-capable users in the considered country).  The table
   shows just a subset of the data available from [POTAROO2].  In
   particular, the following table provides the data for the 25 most
   populated countries in the world.  The table is ordered based on the
   IPv4 addresses per capita ratio, and the data refer to 1 January
   2022.

   +==============================+=================+=================+
   | Country                      | IPv4 per Capita | IPv6 Deployment |
   +==============================+=================+=================+
   | United States of America     |            4.89 |           47.1% |
   +------------------------------+-----------------+-----------------+
   | United Kingdom               |            1.65 |           33.2% |
   +------------------------------+-----------------+-----------------+
   | Japan                        |            1.50 |           36.7% |
   +------------------------------+-----------------+-----------------+
   | Germany                      |            1.48 |           53.0% |
   +------------------------------+-----------------+-----------------+
   | France                       |            1.27 |           42.1% |
   +------------------------------+-----------------+-----------------+
   | Italy                        |            0.91 |            4.7% |
   +------------------------------+-----------------+-----------------+
   | South Africa                 |            0.46 |            2.4% |
   +------------------------------+-----------------+-----------------+
   | Brazil                       |            0.41 |           38.7% |
   +------------------------------+-----------------+-----------------+
   | Russian Federation           |            0.31 |            9.7% |
   +------------------------------+-----------------+-----------------+
   | China                        |            0.24 |        60.1%(*) |
   +------------------------------+-----------------+-----------------+
   | Egypt                        |            0.24 |            4.3% |
   +------------------------------+-----------------+-----------------+
   | Mexico                       |            0.23 |           41.8% |
   +------------------------------+-----------------+-----------------+
   | Turkey                       |            0.20 |            0.2% |
   +------------------------------+-----------------+-----------------+
   | Vietnam                      |            0.17 |           48.0% |
   +------------------------------+-----------------+-----------------+
   | Iran (Islamic Republic)      |            0.15 |            0.1% |
   +------------------------------+-----------------+-----------------+
   | Thailand                     |            0.13 |           40.8% |
   +------------------------------+-----------------+-----------------+
   | Indonesia                    |            0.07 |            5.0% |
   +------------------------------+-----------------+-----------------+
   | Philippines                  |            0.05 |           13.8% |
   +------------------------------+-----------------+-----------------+
   | India                        |            0.03 |           76.9% |
   +------------------------------+-----------------+-----------------+
   | Pakistan                     |            0.03 |            2.1% |
   +------------------------------+-----------------+-----------------+
   | United Republic of Tanzania  |            0.02 |            0.0% |
   +------------------------------+-----------------+-----------------+
   | Nigeria                      |            0.02 |            0.2% |
   +------------------------------+-----------------+-----------------+
   | Bangladesh                   |            0.01 |            0.3% |
   +------------------------------+-----------------+-----------------+
   | Ethiopia                     |            0.00 |            0.0% |
   +------------------------------+-----------------+-----------------+
   | Democratic Republic of Congo |            0.00 |            0.1% |
   +------------------------------+-----------------+-----------------+

     Table 1: IPv4 per Capita and IPv6 Deployment for the Top 25 Most
          Populated Countries in the World (as of January 2022)

   (*) The IPv6 deployment information in China is derived from
   [CN-IPv6].

   A direct correlation between low IPv4 per capita and high IPv6
   adoption is not immediate, yet some indications emerge.  For example,
   some countries, such as Brazil, China, and India, have clearly moved
   towards IPv6 adoption.  As discussed later, this appears related to
   several factors in addition to the lack of IPv4 addresses, including
   local regulation and market-driven actions.  The 5 countries at the
   top of the table, with relative high availability of IPv4 addresses,
   have also shown a good level of IPv6 adoption.  In other cases, a
   relative scarcity of IPv4 addresses has not meant a clear move
   towards IPv6, as several countries listed in the table still have low
   or very low IPv6 adoption.

2.2.  IPv6 Users

   The count of the IPv6 users is the key parameter to get an immediate
   understanding of the adoption of IPv6.  Some organizations constantly
   track the usage of IPv6 by aggregating data from several sources.  As
   an example, the Internet Society constantly monitors the volume of
   IPv6 traffic for the networks that joined the World IPv6 Launch
   initiative [WIPv6L].  The measurement aggregates statistics from
   organizations, such as [Akm-stats], that provide data down to the
   single network level, measuring the number of hits to their content
   delivery platform.  For the scope of this document, the approach used
   by APNIC to quantify the adoption of IPv6 by means of a script that
   runs on a user's device [CAIDA] is considered.  To give a rough
   estimation of the relative growth of IPv6, the next table aggregates
   the total number of estimated IPv6-capable users as of 1 January 2022
   and compares it against the total Internet users, as measured by
   [POTAROO2].

   +=====+==========+==========+==========+==========+==========+=====+
   |     | Jan 2018 | Jan 2019 | Jan 2020 | Jan 2021 | Jan 2022 |CAGR |
   +=====+==========+==========+==========+==========+==========+=====+
   |IPv6 |   513.07 |   574.02 |   989.25 | 1,136.28 | 1,207.61 |23.9%|
   +-----+----------+----------+----------+----------+----------+-----+
   |World| 3,410.27 | 3,470.36 | 4,065.00 | 4,091.62 | 4,093.69 | 4.7%|
   +-----+----------+----------+----------+----------+----------+-----+
   |Ratio|    15.0% |    16.5% |    24.3% |    27.8% |    29.5% |18.4%|
   +-----+----------+----------+----------+----------+----------+-----+

     Table 2: IPv6-Capable Users against Total Users (in Millions) as
                             of January 2022

   Two figures appear: first, the IPv6 Internet population is growing
   with a two-digit Compound Annual Growth Rate (CAGR), and second, the
   ratio IPv6 over total is also growing steadily.

2.3.  IPv6 Web Content

   [W3Techs] keeps track of the use of several technical components of
   websites worldwide through different analytical engines.  The
   utilization of IPv6 for websites is shown in the next table, where
   the percentages refer to the websites that are accessible over IPv6.

       +===========+=======+=======+=======+=======+=======+=======+
       | Worldwide | Jan   | Jan   | Jan   | Jan   | Jan   | CAGR  |
       | Websites  | 2018  | 2019  | 2020  | 2021  | 2022  |       |
       +===========+=======+=======+=======+=======+=======+=======+
       | % of IPv6 | 11.4% | 13.3% | 15.0% | 17.5% | 20.6% | 15.9% |
       +-----------+-------+-------+-------+-------+-------+-------+

          Table 3: Usage of IPv6 in Websites (as of January 2022)

   Looking at the growth rate, it may not appear particularly high.  It
   has to be noted, though, that not all websites are equal.  The
   largest content providers, which already support IPv6, generate a lot
   more content than small websites.  At the beginning of January 2022,
   [Csc6lab] measured that out of the world's top 500 sites, 203 are
   IPv6 enabled (+3.6% from January 2021 to January 2022).  If we
   consider that the big content providers (such as Google, Facebook,
   and Netflix) generate more than 50% of the total mobile traffic
   [SNDVN], and in some cases even more up to 65% [ISOC1] [HxBld], the
   percentage of content accessible over IPv6 is clearly more relevant
   than the number of enabled IPv6 websites.  Of that 50% of all mobile
   traffic, it would be interesting to know what percentage is IPv6.
   Unfortunately, this information is not available.

   Related to that, a question that sometimes arises is whether the
   content stored by content providers would be all accessible on IPv6
   in the hypothetical case of a sudden IPv4 switch off.  Even if this
   is pure speculation, the numbers above may bring to state that this
   is likely the case.  This would reinforce the common thought that, in
   quantitative terms, most of the content is accessible via IPv6.

2.4.  IPv6 Public Actions and Policies

   As previously noted, the worldwide deployment of IPv6 is not uniform
   [G_stats] [APNIC1].  It is worth noticing that, in some cases, higher
   IPv6 adoption in certain countries has been achieved as a consequence
   of actions taken by the local governments through regulation or
   incentive to the market.  Looking at the European Union area, some
   countries, such as Belgium, France, and Germany, are well ahead in
   terms of IPv6 adoption.

   In the case of Belgium, the Belgian Institute for Postal services and
   Telecommunications (BIPT) acted to mediate an agreement between the
   local ISPs and the government to limit the use of Carrier-Grade NAT
   (CGN) systems and of public IPv4 addresses for lawful investigations
   in 2012 [BIPT].  The agreement limited the use of one IPv4 address
   per 16 customers behind NAT.  The economic burden sustained by the
   ISPs for the unoptimized use of NAT induced the shift to IPv6 and its
   increased adoption in the latest years.

   In France, the National Regulator (Autorite de regulation des
   communications electroniques, or Arcep) introduced an obligation for
   the mobile carriers awarded with a license to use 5G frequencies
   (3.4-3.8 GHz) in Metropolitan France to be IPv6 compatible [ARCEP].
   As stated in [ARCEP] (translated from French),

   |  The goal is to ensure that services are interoperable and to
   |  remove obstacles to using services that are only available in
   |  IPv6, as the number of devices in use continues to soar, and
   |  because the RIPE NCC has run out of IPv4 addresses.

   A slow adoption of IPv6 could prevent new Internet services from
   spreading widely or create a barrier to entry for newcomers to the
   market.  Per [ARCEP] (translated from French), "IPv6 can help to
   increase competition in the telecom industry, and help to
   industrialize a country for specific vertical sectors".

   Increased IPv6 adoption in Germany depended on a mix of industry and
   public actions.  Specifically, the Federal Office for Information
   Technology (under the Federal Ministry of the Interior) issued over
   the years a few recommendations on the use of IPv6 in the German
   public administration.  The latest guideline in 2019 constitutes a
   high-level road map for mandatory IPv6 introduction in the federal
   administration networks [GFA].

   In the United States, the Office of Management and Budget is also
   calling for IPv6 adoption [US-FR] [US-CIO].  These documents define a
   plan to have 80% of the US federal IP-capable networks based on
   IPv6-only by the year 2025.  China is another example of a government
   that is supporting a country-wide IPv6 adoption [CN].  In India, the
   high adoption of IPv6 took origin from the decision of Reliance Jio
   to move to IPv6 in their networks [RelJio].  In addition, the
   Department of Telecommunications (under the Ministry of
   Communications and Information Technology) issued guidelines for the
   progressive adoption of IPv6 in public and private networks.  The
   latest one dates 2021 [IDT] and fosters further moves to IPv6
   connection services.

3.  A Survey on IPv6 Deployments

   This section discusses the status of IPv6 adoption in service
   provider and enterprise networks.

3.1.  IPv6 Allocations

   RIRs are responsible for allocating IPv6 address blocks to ISPs,
   Local Internet Registries (LIRs), and enterprises or other
   organizations.  An ISP/LIR will use the allocated block to assign
   addresses to their end users.  The following table shows the amount
   of individual allocations, per RIR, in the time period from 2017-2021
   [APNIC2].

    +==========+=====+=======+=======+=======+=======+===========+====+
    | Registry |Dec  | Dec   | Dec   | Dec   | Dec   | Cumulated |CAGR|
    |          |2017 | 2018  | 2019  | 2020  | 2021  |           |    |
    +==========+=====+=======+=======+=======+=======+===========+====+
    | AFRINIC  |  112|   110 |   115 |   109 |   136 |       582 | 51%|
    +----------+-----+-------+-------+-------+-------+-----------+----+
    | APNIC    |1,369| 1,474 | 1,484 | 1,498 | 1,392 |     7,217 | 52%|
    +----------+-----+-------+-------+-------+-------+-----------+----+
    | ARIN     |  684|   659 |   605 |   644 |   671 |     3,263 | 48%|
    +----------+-----+-------+-------+-------+-------+-----------+----+
    | LACNIC   |1,549| 1,448 | 1,614 | 1,801 |   730 |     7,142 | 47%|
    +----------+-----+-------+-------+-------+-------+-----------+----+
    | RIPE NCC |2,051| 2,620 | 3,104 | 1,403 | 2,542 |    11,720 | 55%|
    +----------+-----+-------+-------+-------+-------+-----------+----+
    | Total    |5,765| 6,311 | 6,922 | 5,455 | 5,471 |    29,924 | 51%|
    +----------+-----+-------+-------+-------+-------+-----------+----+

          Table 4: IPv6 Allocations Worldwide (as of January 2022)

   The trend shows the steady progress of IPv6.  The decline of IPv6
   allocations in 2020 and 2021 may be due to the COVID-19 pandemic.  It
   also happened to IPv4 allocations.

   [APNIC2] also compares the number of allocations for both address
   families.  The CAGR looks quite similar in 2021 but a little higher
   for the IPv4 allocations in comparison to the IPv6 allocations (53.6%
   versus 50.9%).

   +=========+=====+=====+========+=======+=======+===========+=======+
   | Address |Dec  |Dec  | Dec    | Dec   | Dec   | Cumulated | CAGR  |
   | family  |2017 |2018 | 2019   | 2020  | 2021  |           |       |
   +=========+=====+=====+========+=======+=======+===========+=======+
   | IPv6    |5,765|6,311|  6,922 | 5,455 | 5,471 |    29,924 | 50.9% |
   +---------+-----+-----+--------+-------+-------+-----------+-------+
   | IPv4    |8,091|9,707| 13,112 | 6,263 | 7,829 |    45,002 | 53.6% |
   +---------+-----+-----+--------+-------+-------+-----------+-------+

       Table 5: Allocations per Address Family (as of January 2022)

   The reason may be that the IPv4 allocations in 2021 included many
   allocations of small address ranges (e.g., /24) [APNIC2].  On the
   contrary, a single IPv6 allocation is large enough to cope with the
   need of an operator for long period.  After an operator receives an
   IPv6 /30 or /32 allocation, it is unlikely that a new request of
   addresses is repeated in the short term.

   The next table is based on [APNIC3] and [APNIC4] and shows the
   percentage of Autonomous Systems (ASes) supporting IPv6 compared to
   the total ASes worldwide.  The number of IPv6-capable ASes increased
   from 24.3% in January 2018 to 38.7% in January 2022.  This equals to
   18% of the CAGR for IPv6-enabled networks.  In comparison, the CAGR
   for the total of IPv6 and IPv4 networks is just 5%.

   +==============+========+========+========+========+========+======+
   | Advertised   | Jan    | Jan    | Jan    | Jan    | Jan    | CAGR |
   | ASN          | 2018   | 2019   | 2020   | 2021   | 2022   |      |
   +==============+========+========+========+========+========+======+
   | IPv6-capable | 14,500 | 16,470 | 18,650 | 21,400 | 28,140 |  18% |
   +--------------+--------+--------+--------+--------+--------+------+
   | Total ASN    | 59,700 | 63,100 | 66,800 | 70,400 | 72,800 |   5% |
   +--------------+--------+--------+--------+--------+--------+------+
   | Ratio        | 24.3%  | 26.1%  | 27.9%  | 30.4%  | 38.7%  |      |
   +--------------+--------+--------+--------+--------+--------+------+

      Table 6: Percentage of IPv6-Capable ASes (as of January 2022)

   The tables above provide an aggregated view of the allocations'
   dynamic.  The next subsections will zoom into each specific domain to
   highlight its relative status.

3.2.  IPv6 among Internet Service Providers

   A survey was submitted to a group of service providers in Europe
   during the third quarter of 2020 (see Appendix A for the complete
   poll) to understand their plans about IPv6 and their technical
   preferences regarding its adoption.  Although this poll does not give
   an exhaustive view on the IPv6 status, it provides some insights that
   are relevant to the discussion.

   The poll revealed that the majority of ISPs interviewed had plans
   concerning IPv6 (79%).  Of them, 60% had ongoing activities already,
   while 33% were expected to start activities in a 12-month timeframe.
   The transition to IPv6 involved all business segments: mobile (63%),
   fixed (63%), and enterprise (50%).

   The reasons to move to IPv6 varied.  Global IPv4 address depletion
   and the run out of private address space recommended in [RFC1918]
   were reported as the important drivers for IPv6 deployment (48%).  In
   a few cases, respondents cited the requirement of national IPv6
   policies and the launch of 5G as the reasons (13%).  Enterprise
   customer demand was also a reason to introduce IPv6 (13%).

   From a technical preference standpoint, Dual-Stack [RFC4213] was the
   most adopted solution in both wireline (59%) and cellular networks
   (39%).  In wireline, the second most adopted mechanism was Dual-Stack
   Lite (DS-Lite) [RFC6333] (19%).  In cellular networks, the second
   preference was 464XLAT [RFC6877] (21%).

   More details about the answers received can be found in Appendix A.

3.3.  IPv6 among Enterprises

   As described in [RFC7381], enterprises face different challenges than
   ISPs.  Publicly available reports show how the enterprise deployment
   of IPv6 lags behind ISP deployment [cmpwr].

   [NST_1] provides estimations on the deployment status of IPv6 for
   domains such as example.com, example.net, or example.org in the
   United States.  The measurement encompasses many industries,
   including telecommunications, so the term "enterprises" is a bit
   loose in this context.  In any case, it provides a first indication
   of IPv6 adoption in several US industry sectors.  The analysis tries
   to infer whether IPv6 is supported by looking from "outside" a
   company's network.  It takes into consideration the support of IPv6
   to external services, such as Domain Name System (DNS), mail, and
   websites.  [BGR_1] has similar data for China, while [CNLABS_1]
   provides the status in India.

      +===============+==================+=======+=======+=========+
      | Country       | Domains analyzed | DNS   | Mail  | Website |
      +===============+==================+=======+=======+=========+
      | China         |              478 | 74.7% |  0.0% |   19.7% |
      +---------------+------------------+-------+-------+---------+
      | India         |              104 | 51.9% | 15.4% |   16.3% |
      +---------------+------------------+-------+-------+---------+
      | United States |             1070 | 66.8% | 21.2% |    6.3% |
      | of America    |                  |       |       |         |
      +---------------+------------------+-------+-------+---------+

        Table 7: IPv6 Support for External-Facing Services across
                     Enterprises (as of January 2022)

   A poll submitted to a group of large enterprises in North America in
   early 2021 (see Appendix B) shows that the operational issues are
   even more critical than for ISPs.

   Looking at current implementations, almost one third has dual-stacked
   networks, while 20% declares that portions of their networks are
   IPv6-only.  Additionally, 35% of the enterprises did not implement
   IPv6 at all or are stuck at the training phase.  In no case is the
   network fully based on IPv6.

   Speaking of training, the most critical needs are in the field of
   IPv6 security and IPv6 troubleshooting (both highlighted by the two
   thirds of respondents), followed by address planning / network
   configurations (57.41%).

   Coming to implementation, the three areas of concern are IPv6
   security (31.48%), training (27.78%), and application conversion
   (25.93%), and 33.33% of respondents think that all three areas are
   all simultaneously of concern.

   The full poll is reported in Appendix B.

3.3.1.  Government and Universities

   This section focuses specifically on the adoption of IPv6 in
   governments and academia.

   As far as governmental agencies are concerned, [NST_2] provides
   analytics on the degree of IPv6 support for DNS, mail, and websites
   across second-level domains associated with US federal agencies.
   These domains are in the form of example.gov or example.fed.  The
   script used by [NST_2] has also been employed to measure the same
   analytics in other countries, e.g., China [BGR_2], India [CNLABS_2],
   and the European Union [IPv6Forum].  For this latter analytic, some
   post-processing is necessary to filter out the non-European domains.

    +====================+==================+=======+=======+=========+
    | Country            | Domains analyzed | DNS   | Mail  | Website |
    +====================+==================+=======+=======+=========+
    | China              |               52 |  0.0% |  0.0% |   98.1% |
    +--------------------+------------------+-------+-------+---------+
    | European Union (*) |               19 | 47.4% |  0.0% |   21.1% |
    +--------------------+------------------+-------+-------+---------+
    | India              |              618 |  7.6% |  6.5% |    7.1% |
    +--------------------+------------------+-------+-------+---------+
    | United States of   |             1283 | 87.1% | 14.0% |   51.7% |
    | America            |                  |       |       |         |
    +--------------------+------------------+-------+-------+---------+

         Table 8: IPv6 Support for External-Facing Services across
               Governmental Institutions (as of January 2022)

   (*) Both EU and country-specific domains are considered.

   IPv6 support in the US is higher than other countries.  This is
   likely due to the IPv6 mandate set by [US-CIO].  In the case of
   India, the degree of support seems still quite low.  This is also
   true for China, with the notable exception of a high percentage of
   IPv6-enabled websites for government-related organizations.

   Similar statistics are also available for higher education.  [NST_3]
   measures the data from second-level domains of universities in the
   US, such as example.edu.  [BGR_3] looks at Chinese education-related
   domains.  [CNLABS_1] analyzes domains in India (mostly third level),
   while [IPv6Forum] lists universities in the European Union (second
   level), again after filtering the non-European domains.

      +================+==================+=======+=======+=========+
      | Country        | Domains analyzed | DNS   | Mail  | Website |
      +================+==================+=======+=======+=========+
      | China          |              111 | 36.9% |  0.0% |   77.5% |
      +----------------+------------------+-------+-------+---------+
      | European Union |              118 | 83.9% | 43.2% |   35.6% |
      +----------------+------------------+-------+-------+---------+
      | India          |              100 | 31.0% | 54.0% |    5.0% |
      +----------------+------------------+-------+-------+---------+
      | United States  |              346 | 49.1% | 19.4% |   21.7% |
      | of America     |                  |       |       |         |
      +----------------+------------------+-------+-------+---------+

         Table 9: IPv6 Support for External-Facing Services across
                     Universities (as of January 2022)

   Overall, the universities have wider support of IPv6-based services
   compared to the other sectors.  Apart from a couple of exceptions
   (e.g., the support of IPv6 mail in China and IPv6 websites in India),
   the numbers shown in the table above indicate good support of IPv6 in
   academia.

4.  IPv6 Deployment Scenarios

   The scope of this section is to discuss the network and service
   scenarios applicable for the transition to IPv6.  Most of the related
   definitions have been provided in Section 1.1.  This clause is
   intended to focus on the technical and operational characteristics.
   The sequence of scenarios described here does not necessarily have to
   be intended as a road map for the IPv6 transition.  Depending on
   their specific plans and requirements, service providers may either
   adopt the scenarios proposed in a sequence or jump directly to a
   specific one.

4.1.  Dual-Stack

   Based on the poll answers provided by network operators (Appendix A),
   Dual-Stack [RFC4213] appears to be currently the most widely deployed
   IPv6 solution (about 50%; see both Appendix A and the statistics
   reported in [ETSI-IP6-WhitePaper]).

   With Dual-Stack, IPv6 can be introduced together with other network
   upgrades, and many parts of network management and IT systems can
   still work in IPv4.  This avoids a major upgrade of such systems to
   support IPv6, which is possibly the most difficult task in the IPv6
   transition.  The cost and effort on the network management and IT
   systems upgrade are moderate.  The benefits are to start using IPv6
   and save NAT costs.

   Although Dual-Stack may provide advantages in the introductory stage,
   it does have a few disadvantages in the long run, like the
   duplication of the network resources and states.  It also requires
   more IPv4 addresses, thus increasing both Capital Expenses (CAPEX)
   and Operating Expenses (OPEX).  For example, even if private
   addresses are used with Carrier-Grade NAT (CGN), there is extra
   investment in the CGN devices, logs storage, and help desk to track
   CGN-related issues.

   For this reason, when IPv6 usage exceeds a certain threshold, it may
   be advantageous to start a transition to the next scenario.  For
   example, the process may start with the IPv4aaS stage, as described
   hereinafter.  It is difficult to establish the criterion for
   switching (e.g., to properly identify the upper bound of the IPv4
   decrease or the lower bound of the IPv6 increase).  In addition to
   the technical factors, the switch to the next scenarios may also
   cause a loss of customers.  Based on the feedback of network
   operators participating in the World IPv6 Launch [WIPv6L] in June
   2021, 108 out of 346 operators exceed 50% of IPv6 traffic volume
   (31.2%), 72 exceed 60% (20.8%), and 37 exceed 75% (10.7%).  The
   consensus to move to IPv6-only might be reasonable when IPv6 traffic
   volume is between 50% and 60%.

4.2.  IPv6-Only Overlay

   As defined in Section 1.1, IPv6-only is generally associated with a
   scope, e.g., IPv6-only overlay or IPv6-only underlay.

   The IPv6-only overlay denotes that the overlay tunnel between the end
   points of a network is based only on IPv6.  Tunneling provides a way
   to use an existing IPv4 infrastructure to carry IPv6 traffic.  IPv6
   or IPv4 hosts and routers can tunnel IPv6 packets over IPv4 regions
   by encapsulating them within IPv4 packets.  The approach with
   IPv6-only overlay helps to maintain compatibility with the existing
   base of IPv4, but it is not a long-term solution.

   As a matter of fact, IPv4 reachability must be provided for a long
   time to come over IPv6 for IPv6-only hosts.  Most ISPs are leveraging
   CGN to extend the life of IPv4 instead of going with IPv6-only
   solutions.

4.3.  IPv6-Only Underlay

   The IPv6-only underlay network uses IPv6 as the network protocol for
   all traffic delivery.  Both the control and data planes are based on
   IPv6.  The definition of IPv6-only underlay needs to be associated
   with a scope in order to identify the domain where it is applicable,
   such as the IPv6-only access network or IPv6-only backbone network.

   When both enterprises and service providers begin to transition from
   an IPv4/MPLS backbone to introduce IPv6 in the underlay, they do not
   necessarily need to Dual-Stack the underlay.  Forwarding plane
   complexity on the Provider (P) nodes of the ISP core should be kept
   simple as a backbone with a single protocol.  Hence, when operators
   decide to transition to an IPv6 underlay, the ISP backbone should be
   IPv6-only because Dual-Stack is not the best choice.  The underlay
   could be IPv6-only and allow IPv4 packets to be tunneled using a VPN
   over an IPv6-only backbone while leveraging [RFC8950], which
   specifies the extensions necessary to allow advertising IPv4 Network
   Layer Reachability Information (NLRI) with an IPv6 next hop.

   IPv6-only underlay network deployment for access and backbone
   networks seems to not be the first option, and the current trend is
   to keep the IPv4/MPLS data plane and run IPv4/IPv6 Dual-Stack to edge
   nodes.

   As ISPs do the transition in the future to an IPv6-only access
   network or backbone network, e.g., Segment Routing over IPv6 (SRv6)
   data plane, they start the elimination of IPv4 from the underlay
   transport network while continuing to provide IPv4 services.
   Basically, as also shown by the poll among network operators, from a
   network architecture perspective, it is not recommended to apply
   Dual-Stack to the transport network per reasons mentioned above
   related to the forwarding plane complexities.

4.4.  IPv4-as-a-Service

   IPv4aaS can be used to ensure IPv4 support, and it can be a complex
   decision that depends on several factors, such as economic aspects,
   policy, and government regulation.

   [RFC9313] compares the merits of the most common transition solutions
   for IPv4aaS, i.e., 464XLAT [RFC6877], DS-Lite [RFC6333], Lightweight
   4over6 (lw4o6) [RFC7596], Mapping of Address and Port with
   Encapsulation (MAP-E) [RFC7597], and Mapping of Address and Port
   using Translation (MAP-T) [RFC7599], but does not provide an explicit
   recommendation.  However, the poll in Appendix A indicates that the
   most widely deployed IPv6 transition solution in the Mobile Broadband
   (MBB) domain is 464XLAT, while in the Fixed Broadband (FBB) domain,
   it is DS-Lite.

   Both are IPv4aaS solutions that leverage IPv6-only underlay.  IPv4aaS
   offers Dual-Stack service to users and allows an ISP to run IPv6-only
   in the network, typically the access network.

   While it may not always be the case, IPv6-only transition
   technologies, such as 464XLAT, require far fewer IPv4 addresses
   [RFC9313], because they are more efficient and do not restrict the
   number of ports per subscriber.  This helps to reduce troubleshooting
   costs and to remove some operational issues related to permanent
   block listing of IPv4 address blocks when used via CGN in some
   services.

   IPv4aaS may be facilitated by the natural upgrade or replacement of
   CPEs because of newer technologies (triple-play, higher bandwidth WAN
   links, better Wi-Fi technologies, etc.).  The CAPEX and OPEX of other
   parts of the network may be lowered (for example, CGN and associated
   logs) due to the operational simplification of the network.

   For deployments with a large number of users (e.g., large mobile
   operators) or a large number of hosts (e.g., large Data Centers
   (DCs)), even the full private address space [RFC1918] is not enough.
   Also, Dual-Stack will likely lead to duplication of network resources
   and operations to support both IPv6 and IPv4, which increases the
   amount of state information in the network.  This suggests that, for
   scenarios such as MBB or large DCs, IPv4aaS could be more efficient
   from the start of the IPv6 introduction.

   So, in general, when the Dual-Stack disadvantages outweigh the
   IPv6-only complexity, it makes sense to transition to IPv4aaS.  Some
   network operators have already started this process, as in the case
   of [TMus], [RelJio], and [EE].

4.5.  IPv6-Only

   IPv6-only is the final stage of the IPv6 transition, and it happens
   when a complete network, end to end, no longer has IPv4.  No IPv4
   address is configured for network management or anything else.

   Since IPv6-only means that both underlay networks and overlay
   services are only IPv6, it will take longer to happen.

5.  Common IPv6 Challenges

   This section lists common IPv6 challenges, which have been validated
   and discussed during several meetings and public events.  The scope
   is to encourage more investigations.  Despite that IPv6 has already
   been well proven in production, there are some challenges to
   consider.  In this regard, it is worth noting that [ETSI-GR-IPE-001]
   also discusses gaps that still exist in IPv6-related use cases.

5.1.  Transition Choices

   A service provider, an enterprise, or a CSP may perceive quite a
   complex task with the transition to IPv6 due to the many technical
   alternatives available and the changes required in management and
   operations.  Moreover, the choice of the method to support the
   transition is an important challenge and may depend on factors
   specific to the context, such as the IPv6 network design that fits
   the service requirements, the network operations, and the deployment
   strategy.

   The subsections below briefly highlight the approaches that the
   different parties may take and the related challenges.

5.1.1.  Service Providers: Fixed and Mobile Operators

   For fixed operators, the massive software upgrade of CPEs to support
   Dual-Stack already started in most of the service provider networks.
   On average, looking at the global statistics, the IPv6 traffic
   percentage is currently around 40% [G_stats].  As highlighted in
   Section 3.2, all major content providers have already implemented
   Dual-Stack access to their services, and most of them have
   implemented IPv6-only in their Data Centers.  This aspect could
   affect the decision on the IPv6 adoption for an operator, but there
   are also other factors, like the current IPv4 address shortage, CPE
   costs, CGN costs, and so on.

   *  Fixed operators with a Dual-Stack architecture can start defining
      and applying a new strategy when reaching the limit in terms of
      the number of IPv4 addresses available.  This may be done through
      CGN or with an IPv4aaS approach.

   *  Most of the fixed operators remain attached to a Dual-Stack
      architecture, and many have already employed CGN.  In this case,
      it is likely that CGN boosts their ability to supply IPv4
      connectivity to CPEs for more years to come.  Indeed, only few
      fixed operators have chosen to move to an IPv6-only scenario.

   For mobile operators, the situation is quite different, since in some
   cases, mobile operators are already stretching their IPv4 address
   space.  The reason is that CGN translation limits have been reached
   and no more IPv4 public pool addresses are available.

   *  Some mobile operators choose to implement Dual-Stack as a first
      and immediate mitigation solution.

   *  Other mobile operators prefer to move to IPv4aaS solutions (e.g.,
      464XLAT) since Dual-Stack only mitigates and does not solve the
      IPv4 address scarcity issue completely.

   For both fixed and mobile operators, the approach for the transition
   is not unique, and this brings different challenges in relation to
   the network architecture and related costs; therefore, each operator
   needs to do their own evaluations for the transition based on the
   specific situation.

5.1.2.  Enterprises

   At present, the usage of IPv6 for enterprises often relies on
   upstream service providers, since the enterprise connectivity depends
   on the services provided by their upstream provider.  Regarding the
   enterprises' internal infrastructures, IPv6 shows its advantages in
   the case of a merger and acquisition, because it can be avoided by
   the overlapping of the two address spaces, which is common in case of
   IPv4 private addresses.  In addition, since several governments are
   introducing IPv6 policies, all the enterprises providing consulting
   services to governments are also required to support IPv6.

   However, enterprises face some challenges.  They are shielded from
   IPv4 address depletion issues due to their prevalent use of proxy and
   private addressing [RFC1918]; thus, they do not have the business
   requirement or technical justification to transition to IPv6.
   Enterprises need to find a business case and a strong motivation to
   transition to IPv6 to justify additional CAPEX and OPEX.  Also, since
   Information and Communication Technologies (ICTs) are not the core
   business for most of the enterprises, the ICT budget is often
   constrained and cannot expand considerably.  However, there are
   examples of big enterprises that are considering IPv6 to achieve
   business targets through a more efficient IPv6 network and to
   introduce newer services that require IPv6 network architecture.

   Enterprises worldwide, in particular small- and medium-sized
   enterprises, are quite late to adopt IPv6, especially on internal
   networks.  In most cases, the enterprise engineers and technicians do
   not have a great experience with IPv6, and the problem of application
   porting to IPv6 looks quite difficult.  As highlighted in the
   relevant poll, the technicians may need to be trained, but the
   management does not see a business need for adoption.  This creates
   an unfortunate cycle where the perceived complexity of the IPv6
   protocol and concerns about security and manageability combine with
   the lack of urgent business needs to prevent adoption of IPv6.  In
   2019 and 2020, there has been a concerted effort by some ARIN and
   APNIC initiatives to provide training [ARIN-CG] [ISIF-ASIA-G].

5.1.3.  Industrial Internet

   In an industrial environment, Operational Technology (OT) refers to
   the systems used to monitor and control processes within a factory or
   production environment, while Information Technology (IT) refers to
   anything related to computer technology and networking connectivity.
   IPv6 is frequently mentioned in relation to Industry 4.0 and the
   Internet of Things (IoT), affecting the evolution of both OT and IT.

   There are potential advantages for using IPv6 for the Industrial
   Internet of Things (IIoT), in particular, the large IPv6 address
   space, the automatic IPv6 address configuration, and resource
   discovery.  However, its industrial adoption, in particular, in smart
   manufacturing systems, has been much slower than expected.  There are
   still many obstacles and challenges that prevent its pervasive use.
   The key problems identified are the incomplete or underdeveloped tool
   support, the dependency on manual configuration, and the poor
   knowledge of the IPv6 protocols.  To promote the use of IPv6 for
   smart manufacturing systems and IIoT applications, a generic approach
   to remove these pain points is highly desirable.  Indeed, as for
   enterprises, it is important to provide an easy way to familiarize
   system architects and software developers with the IPv6 protocol.

   Advances in cloud-based platforms and developments in artificial
   intelligence (AI) and machine learning (ML) allow OT and IT systems
   to integrate and migrate to a centralized analytical, processing, and
   integrated platform, which must act in real time.  The limitation is
   that manufacturing companies have diverse corporate cultures, and the
   adoption of new technologies may lag as a result.

   For Industrial Internet and related IIoT applications, it would be
   desirable to leverage the configurationless characteristic of IPv6 to
   automatically manage and control the IoT devices.  In addition, it
   could be interesting to have the ability to use IP-based
   communication and standard application protocols at every point in
   the production process and further reduce the use of specialized
   communication systems.

5.1.4.  Content and Cloud Service Providers

   The high number of addresses required to connect the virtual and
   physical elements in a Data Center and the necessity to overcome the
   limitation posed by [RFC1918] have been the drivers to the adoption
   of IPv6 in several CSP networks.

   Most CSPs have adopted IPv6 in their internal infrastructure but are
   also active in gathering IPv4 addresses on the transfer market to
   serve the current business needs of IPv4 connectivity.  As noted in
   the previous section, most enterprises do not consider the transition
   to IPv6 as a priority.  To this extent, the use of IPv4-based network
   services by the CSPs will last.

   Several public references, as reported hereinafter, discuss how most
   of the major players find themselves at different stages in the
   transition to IPv6-only in their Data Center (DC) infrastructure.  In
   some cases, the transition already happened and the DC infrastructure
   of these hyperscalers is completely based on IPv6.

   It is interesting to look at how much traffic in a network is going
   to Caches and Content Delivery Networks (CDNs).  The response is
   expected to be a high percentage, at least higher than 50% in most of
   the cases, since all the key Caches and CDNs are ready for IPv6
   [Cldflr] [Ggl] [Ntflx] [Amzn] [Mcrsft].  So the percentage of traffic
   going to the key Caches/CDNs is a good approximation of the potential
   IPv6 traffic in a network.

   The challenges for CSPs are mainly related to the continuous support
   of IPv4 to be guaranteed, since most CSPs already completed the
   transition to IPv6-only.  If, in the next years, the scarcity of IPv4
   addresses becomes more evident, it is likely that the cost of buying
   an IPv4 address by a CSP could be charged to their customers.

5.1.5.  CPEs and User Devices

   It can be noted that most of the user devices (e.g., smartphones)
   have been IPv6 enabled for many years.  But there are exceptions, for
   example, for the past few years, smart TVs have typically had IPv6
   support; however, not all the economies replace them at the same
   pace.

   As already mentioned, ISPs who historically provided public IPv4
   addresses to their customers generally still have those IPv4
   addresses (unless they chose to transfer them).  Some have chosen to
   put new customers on CGN but without touching existing customers.
   Because of the extremely small number of customers who notice that
   IPv4 is done via NAT444 (i.e., the preferred CGN solution for
   carriers), it could be less likely to run out of IPv4 addresses and
   private IPv4 space.  But as IPv4-only devices and traffic reduce, the
   need to support private and public IPv4 lessens.  So to have CPEs
   completely support IPv6 serves as an important challenge and
   incentive to choose IPv4aaS solutions [ANSI] over Dual-Stack.

5.1.6.  Software Applications

   The transition to IPv6 requires that the application software is
   adapted for use in IPv6-based networks ([ARIN-SW] provides an
   example).  The use of transition mechanisms like 464XLAT is essential
   to support IPv4-only applications while they evolve to IPv6.
   Depending on the transition mechanism employed, some issues may
   remain.  For example, in the case of NAT64/DNS64, the use of literal
   IPv4 addresses, instead of DNS names, will fail unless mechanisms
   such as Application Level Gateways (ALGs) are used.  This issue is
   not present in 464XLAT (see [RFC8683]).

   It is worth mentioning Happy Eyeballs [RFC8305] as a relevant aspect
   of application transition to IPv6.

5.2.  Network Management and Operations

   There are important IPv6 complementary solutions related to
   Operations, Administration, and Maintenance (OAM) that look less
   mature compared to IPv4.  A Network Management System (NMS) has a
   central role in the modern networks for both network operators and
   enterprises, and its transition is a fundamental issue.  This is
   because some IPv6 products are not as field proven as IPv4 products,
   even if conventional protocols (e.g., SNMP and RADIUS) already
   support IPv6.  In addition, an incompatible vendor road map for the
   development of new NMS features affects the confidence of network
   operators or enterprises.

   An important factor is represented by the need for training the
   network operations workforce.  Deploying IPv6 requires that policies
   and procedures have to be adjusted in order to successfully plan and
   complete an IPv6 transition.  Staff has to be aware of the best
   practices for managing IPv4 and IPv6 assets.  In addition to network
   nodes, network management applications and equipment need to be
   properly configured and, in some cases, also replaced.  This may
   introduce more complexity and costs for the transition.

   Availability of both systems and training is necessary in areas such
   as IPv6 addressing.  IPv6 addresses can be assigned to an interface
   through different means, such as Stateless Auto-Configuration (SLAAC)
   [RFC4862], or by using the stateful Dynamic Host Configuration
   Protocol (DHCP) [RFC8415].  IP Address Management (IPAM) systems may
   contribute by handling the technical differences and automating some
   of the configuration tasks, such as the address assignment or the
   management of DHCP services.

5.3.  Performance

   People tend to compare the performance of IPv6 versus IPv4 to argue
   or motivate the IPv6 transition.  In some cases, IPv6 behaving
   "worse" than IPv4 may be used as an argument for avoiding the full
   adoption of IPv6.  However, there are some aspects where IPv6 has
   already filled (or is filling) the gap to IPv4.  This position is
   supported when looking at available analytics on two critical
   parameters: packet loss and latency.  These parameters have been
   constantly monitored over time, but only a few comprehensive
   measurement campaigns are providing up-to-date information.  While
   performance is undoubtedly an important issue to consider and worth
   further investigation, the reality is that a definitive answer cannot
   be found on what IP version performs better.  Depending on the
   specific use case and application, IPv6 is better; in others, the
   same applies to IPv4.

5.3.1.  IPv6 Packet Loss and Latency

   [APNIC5] provides a measurement of both the failure rate and Round-
   Trip Time (RTT) of IPv6 compared against IPv4.  Both measures are
   based on scripts that employ the three-way handshake of TCP.  As
   such, the measurement of the failure rate does not provide a direct
   measurement of packet loss (which would need an Internet-wide
   measurement campaign).  That said, despite that IPv4 is still
   performing better, the difference seems to have decreased in recent
   years.  Two reports, namely [RIPE1] and [APRICOT], discussed the
   associated trend, showing how the average worldwide failure rate of
   IPv6 is still a bit worse than IPv4.  Reasons for this effect may be
   found in endpoints with an unreachable IPv6 address, routing
   instability, or firewall behavior.  Yet, this worsening effect may
   appear as disturbing for a plain transition to IPv6.

   [APNIC5] also compares the latency of both address families.
   Currently, the worldwide average is slightly in favor of IPv6.
   Zooming at the country or even at the operator level, it is possible
   to get more detailed information and appreciate that cases exist
   where IPv6 is faster than IPv4.  Regions (e.g., Western Europe,
   Northern America, and Southern Asia) and countries (e.g., US, India,
   and Germany) with an advanced deployment of IPv6 (e.g., greater than
   45%) are showing that IPv6 has better performance than IPv4.
   [APRICOT] highlights how when a difference in performance exists, it
   is often related to asymmetric routing issues.  Other possible
   explanations for a relative latency difference relate to the
   specificity of the IPv6 header, which allows packet fragmentation.
   In turn, this means that hardware needs to spend cycles to analyze
   all of the header sections, and when it is not capable of handling
   one of them, it drops the packet.  A few measurement campaigns on the
   behavior of IPv6 in CDNs are also available [MAPRG] [INFOCOM].  The
   TCP connection time is still higher for IPv6 in both cases, even if
   the gap has reduced over the analysis time window.

5.3.2.  Customer Experience

   It is not totally clear if the customer experience is in some way
   perceived as better when IPv6 is used instead of IPv4.  In some
   cases, it has been publicly reported by IPv6 content providers that
   users have a better experience when using IPv6-only compared to IPv4
   [ISOC2].  This could be explained because, in the case of an IPv6
   user connecting to an application hosted in an IPv6-only Data Center,
   the connection is end to end, without translations.  Instead, when
   using IPv4, there is a NAT translation either in the CPE or in the
   service provider's network, in addition to IPv4 to IPv6 (and back to
   IPv4) translation in the IPv6-only content provider Data Center.
   [ISOC2] and [FB] provide reasons in favor of IPv6.  In other cases,
   the result seems to be still slightly in favor of IPv4 [INFOCOM]
   [MAPRG], even if the difference between IPv4 and IPv6 tends to vanish
   over time.

5.4.  IPv6 Security and Privacy

   An important point that is sometimes considered as a challenge when
   discussing the transition to IPv6 is related to the security and
   privacy.  [RFC9099] analyzes the operational security issues in
   several places of a network (enterprises, service providers, and
   residential users).  It is also worth considering the additional
   security issues brought by the applied IPv6 transition technologies
   used to implement IPv4aaS (e.g., 464XLAT and DS-Lite) [ComputSecur].

   The security aspects have to be considered to keep at least the same,
   or even a better, level of security as it exists nowadays in an IPv4
   network environment.  The autoconfiguration features of IPv6 will
   require some more attention.  Router discovery and address
   autoconfiguration may produce unexpected results and security holes.
   IPsec protects IPv6 traffic at least as well as it does IPv4, and the
   security protocols for constrained devices (IoT) are designed for
   IPv6 operation.

   IPv6 was designed to restore the end-to-end model of communications
   with all nodes on networks using globally unique addresses.  But
   considering this, IPv6 may imply privacy concerns due to greater
   visibility on the Internet.  IPv6 nodes can (and typically do) use
   privacy extensions [RFC8981] to prevent any tracking of their burned-
   in Media Access Control (MAC) address(es), which are easily readable
   in the original modified 64-bit Extended Unique Identifier (EUI-64)
   interface identifier format.  On the other hand, stable IPv6
   interface identifiers [RFC8064] were developed, and this can also
   affect privacy.

   As reported in [ISOC3], in comparing IPv6 and IPv4 at the protocol
   level, one may probably conclude that the increased complexity of
   IPv6 will result in an increased number of attack vectors that imply
   more possible ways to perform different types of attacks.  However, a
   more interesting and practical question is how IPv6 deployments
   compare to IPv4 deployments in terms of security.  In that sense,
   there are a number of aspects to consider.

   Most security vulnerabilities related to network protocols are based
   on implementation flaws.  Typically, security researchers find
   vulnerabilities in protocol implementations, which eventually are
   "patched" to mitigate such vulnerabilities.  Over time, this process
   of finding and patching vulnerabilities results in more robust
   implementations.  For obvious reasons, the IPv4 protocols have
   benefited from the work of security researchers for much longer, and
   thus IPv4 implementations are generally more robust than IPv6.
   However, with more IPv6 deployment, IPv6 will also benefit from this
   process in the long run.  It is also worth mentioning that most
   vulnerabilities nowadays are caused by human beings and are in the
   application layer, not the IP layer.

   Besides the intrinsic properties of the protocols, the security level
   of the resulting deployments is closely related to the level of
   expertise of network and security engineers.  In that sense, there is
   obviously much more experience and confidence with deploying and
   operating IPv4 networks than with deploying and operating IPv6
   networks.

5.4.1.  Protocols' Security Issues

   In general, there are security concerns related to IPv6 that can be
   classified as follows:

   *  Basic IPv6 protocol (basic header, extension headers, addressing)

   *  IPv6-associated protocols (ICMPv6, NDP, MLD, DNS, DHCPv6)

   *  Internet-wide IPv6 security (filtering, DDoS, transition
      mechanisms)

   ICMPv6 is an integral part of IPv6 and performs error reporting and
   diagnostic functions.  The Neighbor Discovery Protocol (NDP) is a
   node discovery protocol in IPv6, which replaces and enhances
   functions of ARP.  Multicast Listener Discovery (MLD) is used by IPv6
   routers for discovering multicast listeners on a directly attached
   link, much like how the Internet Group Management Protocol (IGMP) is
   used in IPv4.

   These IPv6-associated protocols, like ICMPv6, NDP, and MLD, are
   something new compared to IPv4, so they add new security threats and
   the related solutions are still under discussion today.  NDP has
   vulnerabilities [RFC3756] [RFC6583].  [RFC3756] says to use IPsec,
   but it is impractical and not used; on the other hand, SEcure
   Neighbor Discovery (SEND) [RFC3971] is not widely available.  It is
   worth mentioning that applying host isolation may address many of
   these concerns, as described in [ND-CONSIDERATIONS].

   [RIPE2] describes the most important threats and solutions regarding
   IPv6 security.

5.4.1.1.  IPv6 Extension Headers and Fragmentation

   IPv6 extension headers provide a hook for interesting new features to
   be added and are more flexible than IPv4 options.  This does add some
   complexity.  In particular, some security mechanisms may require
   processing the full chain of headers, and some firewalls may require
   filtering packets based on their extension headers.  Additionally,
   packets with IPv6 extension headers may be dropped in the public
   Internet [RFC7872].  Some documents, e.g., [HBH-PROCESSING],
   [HBH-OPT-HDR], and [IPv6-EXT-HDR], analyze and provide guidance
   regarding the processing procedures of IPv6 extension headers.

   Defense against possible attacks through extension headers is
   necessary.  For example, the original IPv6 Routing Header type 0
   (RH0) was deprecated because of possible remote traffic amplification
   [RFC5095].  In addition, it is worth mentioning that the unrecognized
   Hop-by-Hop Options Header and Destination Options Header will not be
   considered by the nodes if they are not configured to deal with it
   [RFC8200].  Other attacks based on extension headers may be based on
   IPv6 header chains and fragmentation that could be used to bypass
   filtering.  To mitigate this effect, the initial IPv6 header, the
   extension headers, and the upper-layer header must all be in the
   first fragment [RFC8200].  Also, the use of the IPv6 fragment header
   is forbidden in all Neighbor Discovery messages [RFC6980].

   The fragment header is used by the IPv6 source node to send a packet
   bigger than the path MTU, and the destination host processes fragment
   headers.  There are several threats related to fragmentation to pay
   attention to, e.g., overlapping fragments (not allowed), resource
   consumption while waiting for the last fragment (to discard), and
   atomic fragments (to be isolated).

   The operational implications of IPv6 packets with extension headers
   are further discussed in [RFC9098].

6.  IANA Considerations

   This document has no IANA actions.

7.  Security Considerations

   This document has no impact on the security properties of specific
   IPv6 protocols or transition tools.  In addition to the discussion
   above in Section 5.4, the security considerations relating to the
   protocols and transition tools are described in the relevant
   documents.

8.  References

8.1.  Normative References

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <https://www.rfc-editor.org/info/rfc1918>.

   [RFC3756]  Nikander, P., Ed., Kempf, J., and E. Nordmark, "IPv6
              Neighbor Discovery (ND) Trust Models and Threats",
              RFC 3756, DOI 10.17487/RFC3756, May 2004,
              <https://www.rfc-editor.org/info/rfc3756>.

   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971,
              DOI 10.17487/RFC3971, March 2005,
              <https://www.rfc-editor.org/info/rfc3971>.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213,
              DOI 10.17487/RFC4213, October 2005,
              <https://www.rfc-editor.org/info/rfc4213>.

   [RFC6036]  Carpenter, B. and S. Jiang, "Emerging Service Provider
              Scenarios for IPv6 Deployment", RFC 6036,
              DOI 10.17487/RFC6036, October 2010,
              <https://www.rfc-editor.org/info/rfc6036>.

   [RFC6180]  Arkko, J. and F. Baker, "Guidelines for Using IPv6
              Transition Mechanisms during IPv6 Deployment", RFC 6180,
              DOI 10.17487/RFC6180, May 2011,
              <https://www.rfc-editor.org/info/rfc6180>.

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,
              <https://www.rfc-editor.org/info/rfc6333>.

   [RFC6540]  George, W., Donley, C., Liljenstolpe, C., and L. Howard,
              "IPv6 Support Required for All IP-Capable Nodes", BCP 177,
              RFC 6540, DOI 10.17487/RFC6540, April 2012,
              <https://www.rfc-editor.org/info/rfc6540>.

   [RFC6583]  Gashinsky, I., Jaeggli, J., and W. Kumari, "Operational
              Neighbor Discovery Problems", RFC 6583,
              DOI 10.17487/RFC6583, March 2012,
              <https://www.rfc-editor.org/info/rfc6583>.

   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              RFC 6877, DOI 10.17487/RFC6877, April 2013,
              <https://www.rfc-editor.org/info/rfc6877>.

   [RFC6883]  Carpenter, B. and S. Jiang, "IPv6 Guidance for Internet
              Content Providers and Application Service Providers",
              RFC 6883, DOI 10.17487/RFC6883, March 2013,
              <https://www.rfc-editor.org/info/rfc6883>.

   [RFC7381]  Chittimaneni, K., Chown, T., Howard, L., Kuarsingh, V.,
              Pouffary, Y., and E. Vyncke, "Enterprise IPv6 Deployment
              Guidelines", RFC 7381, DOI 10.17487/RFC7381, October 2014,
              <https://www.rfc-editor.org/info/rfc7381>.

   [RFC7596]  Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and I.
              Farrer, "Lightweight 4over6: An Extension to the Dual-
              Stack Lite Architecture", RFC 7596, DOI 10.17487/RFC7596,
              July 2015, <https://www.rfc-editor.org/info/rfc7596>.

   [RFC7597]  Troan, O., Ed., Dec, W., Li, X., Bao, C., Matsushima, S.,
              Murakami, T., and T. Taylor, Ed., "Mapping of Address and
              Port with Encapsulation (MAP-E)", RFC 7597,
              DOI 10.17487/RFC7597, July 2015,
              <https://www.rfc-editor.org/info/rfc7597>.

   [RFC7599]  Li, X., Bao, C., Dec, W., Ed., Troan, O., Matsushima, S.,
              and T. Murakami, "Mapping of Address and Port using
              Translation (MAP-T)", RFC 7599, DOI 10.17487/RFC7599, July
              2015, <https://www.rfc-editor.org/info/rfc7599>.

   [RFC8950]  Litkowski, S., Agrawal, S., Ananthamurthy, K., and K.
              Patel, "Advertising IPv4 Network Layer Reachability
              Information (NLRI) with an IPv6 Next Hop", RFC 8950,
              DOI 10.17487/RFC8950, November 2020,
              <https://www.rfc-editor.org/info/rfc8950>.

   [RFC9099]  Vyncke, É., Chittimaneni, K., Kaeo, M., and E. Rey,
              "Operational Security Considerations for IPv6 Networks",
              RFC 9099, DOI 10.17487/RFC9099, August 2021,
              <https://www.rfc-editor.org/info/rfc9099>.

   [RFC9313]  Lencse, G., Palet Martinez, J., Howard, L., Patterson, R.,
              and I. Farrer, "Pros and Cons of IPv6 Transition
              Technologies for IPv4-as-a-Service (IPv4aaS)", RFC 9313,
              DOI 10.17487/RFC9313, October 2022,
              <https://www.rfc-editor.org/info/rfc9313>.

8.2.  Informative References

   [Akm-stats]
              Akamai, "IPv6 Adoption Visualization", 2023,
              <https://www.akamai.com/uk/en/resources/our-thinking/
              state-of-the-internet-report/state-of-the-internet-ipv6-
              adoption-visualization>.

   [Amzn]     Amazon Web Services, "Announcing Internet Protocol Version
              6 (IPv6) support for Amazon CloudFront, AWS WAF, and
              Amazon S3 Transfer Acceleration", October 2016,
              <https://aws.amazon.com/es/about-aws/whats-new/2016/10/
              ipv6-support-for-cloudfront-waf-and-s3-transfer-
              acceleration/>.

   [ANSI]     ANSI, "Host and Router Profiles for IPv6", ANSI/
              CTA 2048-A, October 2020, <https://shop.cta.tech/products/
              host-and-router-profiles-for-ipv6>.

   [APNIC1]   APNIC Labs, "IPv6 Capable Rate by country (%)",
              <https://stats.labs.apnic.net/ipv6>.

   [APNIC2]   Huston, G., "IP addressing in 2021", January 2022,
              <https://blog.apnic.net/2022/01/19/ip-addressing-in-
              2021/>.

   [APNIC3]   Huston, G., "BGP in 2020 - The BGP Table", January 2021,
              <https://blog.apnic.net/2021/01/05/bgp-in-2020-the-bgp-
              table/>.

   [APNIC4]   Huston, G., "BGP in 2021 - The BGP Table", January 2022,
              <https://blog.apnic.net/2022/01/06/bgp-in-2021-the-bgp-
              table/>.

   [APNIC5]   APNIC Labs, "Average RTT Difference (ms) (V6 - V4) for
              World (XA)", <https://stats.labs.apnic.net/v6perf/XA>.

   [APRICOT]  Huston, G., "IPv6 Performance Measurement", February 2020,
              <https://2020.apricot.net/assets/files/APAE432/ipv6-
              performance-measurement.pdf>.

   [ARCEP]    ARCEP, "Proposant au ministre chargé des communications
              électroniques les modalités et les conditions
              d'attribution d'autorisations d'utilisation de fréquences
              dans la bande 3,4 - 3,8 GHz", [Decision on the terms and
              conditions for awarding licenses to use frequencies in the
              3.4 – 3.8 GHz band], Décision n° [Decision No.] 2019-1386,
              November 2019,
              <https://www.arcep.fr/uploads/tx_gsavis/19-1386.pdf>.

   [ARIN-CG]  ARIN, "2020 ARIN Community Grant Program Recipients: IPv6
              Security, Applications, and Training for Enterprises",
              2020, <https://www.arin.net/about/community_grants/
              recipients/2020>.

   [ARIN-SW]  ARIN, "Preparing Applications for IPv6",
              <https://www.arin.net/resources/guide/ipv6/
              preparing_apps_for_v6.pdf>.

   [BGR_1]    BIIGROUP, "China Commercial IPv6 and DNSSEC Deployment
              Monitor", December 2021,
              <http://218.2.231.237:5001/cgi-bin/generate>.

   [BGR_2]    BIIGROUP, "China Government IPv6 and DNSSEC Deployment
              Monitor", December 2021,
              <http://218.2.231.237:5001/cgi-bin/generate_gov>.

   [BGR_3]    BIIGROUP, "China Education IPv6 and DNSSEC Deployment
              Monitor", December 2021,
              <http://218.2.231.237:5001/cgi-bin/generate_edu>.

   [BIPT]     Vannieuwenhuyse, J., "IPv6 in Belgium", September 2017,
              <https://www.ripe.net/participate/meetings/roundtable/
              september-2017/government-roundtable-meeting-bahrain-26-
              september-2017/presentations/belgium-experience-bahrain-
              roundtable.pdf>.

   [CAIDA]    Huston, G., "Client-Side IPv6 Measurement", June 2020,
              <https://www.cmand.org/workshops/202006-v6/
              slides/2020-06-16-client-side.pdf>.

   [CAIR]     Cisco, "Cisco Annual Internet Report (2018-2023) White
              Paper", March 2020,
              <https://www.cisco.com/c/en/us/solutions/collateral/
              executive-perspectives/annual-internet-report/white-paper-
              c11-741490.html>.

   [Cldflr]   Cloudflare, "Understanding and configuring Cloudflare's
              IPv6 support", <https://support.cloudflare.com/hc/en-us/
              articles/229666767-Understanding-and-configuring-
              Cloudflare-s-IPv6-support>.

   [cmpwr]    Elkins, N., "Impact on Enterprises of the IPv6-Only
              Direction for the U.S. Federal Government",
              <https://mydigitalpublication.com/article/
              Impact+on+Enterprises+of+the+IPv6-Only+Direction+for+the+U
              .S.+Federal+Government/3702828/664279/article.html>.

   [CN]       China.org.cn, "China to speed up IPv6-based Internet
              development", November 2017, <http://www.china.org.cn/
              business/2017-11/27/content_41948814.htm>.

   [CN-IPv6]  National IPv6 Deployment and Monitoring Platform, "Active
              IPv6 Internet Users", (in Chinese), 2022,
              <https://www.china-ipv6.cn/#/activeconnect/simpleInfo>.

   [CNLABS_1] CNLABS, "Industry IPv6 and DNSSEC Statistics", 2022,
              <https://cnlabs.in/IPv6_Mon/generate_industry.html>.

   [CNLABS_2] CNLABS, "Government IPv6 and DNSSEC Statistics", 2022,
              <https://cnlabs.in/IPv6_Mon/generate_gov.html>.

   [ComputSecur]
              Lencse, G. and Y. Kadobayashi, "Methodology for the
              identification of potential security issues of different
              IPv6 transition technologies: Threat analysis of DNS64 and
              stateful NAT64", Computers and Security, Volume 77, Issue
              C, pp. 397-411, DOI 10.1016/j.cose.2018.04.012, August
              2018, <https://doi.org/10.1016/j.cose.2018.04.012>.

   [Csc6lab]  Cisco, "Display global data", 2023,
              <https://6lab.cisco.com/stats/>.

   [EE]       Heatley, N., "IPv6-only Devices on EE Mobile", January
              2017,
              <https://indico.uknof.org.uk/event/38/contributions/489/
              attachments/612/736/
              Nick_Heatley_EE_IPv6_UKNOF_20170119.pdf>.

   [ETSI-GR-IPE-001]
              ETSI, "IPv6 Enhanced Innovation (IPE) Gap Analysis", ETSI
              GR IPE 001, V1.1.1, August 2021,
              <https://www.etsi.org/deliver/etsi_gr/
              IPE/001_099/001/01.01.01_60/gr_IPE001v010101p.pdf>.

   [ETSI-IP6-WhitePaper]
              ETSI, "IPv6 Best Practices, Benefits, Transition
              Challenges and the Way Forward", ETSI White Paper No. 35,
              ISBN 979-10-92620-31-1, August 2020.

   [FB]       "Paul Saab Facebook V6 World Congress 2015", YouTube
              video, 25:32, posted by Upperside Conferences, March 2015,
              <https://youtu.be/An7s25FSK0U>.

   [GFA]      German Federal Government Commissioner for Information
              Technology, "IPv6-Masterplan für die Bundesverwaltung",
              [IPv6 Master Plan for the Federal Administration],
              November 2019, <https://media.frag-den-
              staat.de/files/foi/531501/de-government-ipv6-masterplan-
              v100-entwurf_konvertiert.pdf>.

   [Ggl]      Google, "Introduction to GGC",
              <https://support.google.com/interconnect/
              answer/9058809?hl=en>.

   [G_stats]  Google, "Google IPv6 Statistics",
              <https://www.google.com/intl/en/ipv6/statistics.html>.

   [HBH-OPT-HDR]
              Peng, S., Li, Z., Xie, C., Qin, Z., and G. Mishra,
              "Operational Issues with Processing of the Hop-by-Hop
              Options Header", Work in Progress, Internet-Draft, draft-
              ietf-v6ops-hbh-04, 10 March 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-v6ops-
              hbh-04>.

   [HBH-PROCESSING]
              Hinden, R. and G. Fairhurst, "IPv6 Hop-by-Hop Options
              Processing Procedures", Work in Progress, Internet-Draft,
              draft-ietf-6man-hbh-processing-07, 6 April 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-6man-
              hbh-processing-07>.

   [HxBld]    HexaBuild, "IPv6 Adoption Report 2020: The IPv6 Internet
              is the Corporate Network", November 2020,
              <https://hexabuild.io/assets/files/HexaBuild-IPv6-
              Adoption-Report-2020.pdf>.

   [IAB]      IAB, "IAB Statement on IPv6", November 2016,
              <https://www.iab.org/2016/11/07/iab-statement-on-ipv6/>.

   [IDT]      Government of India: Department of Telecommunications,
              "Revision of IPv6 Transition Timelines", February 2021,
              <https://dot.gov.in/revision-ipv6-transition-timelines-
              2021>.

   [IGP-GT]   Kuerbis, B. and M. Mueller, "The hidden standards war:
              economic factors affecting IPv6 deployment", DOI 
              10.1108/DPRG-10-2019-0085, February 2019,
              <https://www.emerald.com/insight/content/doi/10.1108/DPRG-
              10-2019-0085/full/html>.

   [INFOCOM]  Doan, T., Bajpai, V., and S. Crawford, "A Longitudinal
              View of Netflix: Content Delivery over IPv6 and Content
              Cache Deployments", IEEE INFOCOM 2020, IEEE Conference on
              Computer Communications, pp. 1073-1082,
              DOI 10.1109/INFOCOM41043.2020.9155367, July 2020,
              <https://dl.acm.org/doi/abs/10.1109/
              INFOCOM41043.2020.9155367>.

   [IPv6-EXT-HDR]
              Bonica, R. and T. Jinmei, "Inserting, Processing And
              Deleting IPv6 Extension Headers", Work in Progress,
              Internet-Draft, draft-bonica-6man-ext-hdr-update-07, 24
              February 2022, <https://datatracker.ietf.org/doc/html/
              draft-bonica-6man-ext-hdr-update-07>.

   [IPv6-ONLY-DEF]
              Palet Martinez, J., "IPv6-only Terminology Definition",
              Work in Progress, Internet-Draft, draft-palet-v6ops-ipv6-
              only-05, 9 March 2020,
              <https://datatracker.ietf.org/doc/html/draft-palet-v6ops-
              ipv6-only-05>.

   [IPv6Forum]
              IPv6Forum, "Estimating IPv6 & DNSSEC External Service
              Deployment Status", 2023,
              <https://www.ipv6forum.com/IPv6-Monitoring/index.html>.

   [ISIF-ASIA-G]
              India Internet Engineering Society (IIESoc), "IPv6
              Deployment at Enterprises", March 2022,
              <https://isif.asia/ipv6-deployment-at-enterprises/>.

   [ISOC1]    Internet Society, "State of IPv6 Deployment 2018", June
              2018, <https://www.internetsociety.org/resources/2018/
              state-of-ipv6-deployment-2018/>.

   [ISOC2]    York, D., "Facebook News Feeds Load 20-40% Faster Over
              IPv6", April 2015,
              <https://www.internetsociety.org/blog/2015/04/facebook-
              news-feeds-load-20-40-faster-over-ipv6/>.

   [ISOC3]    Gont, F., "IPv6 Security Frequently Asked Questions
              (FAQ)", January 2019, <https://www.internetsociety.org/wp-
              content/uploads/2019/02/Deploy360-IPv6-Security-FAQ.pdf>.

   [MAPRG]    Bajpai, V., "Measuring YouTube Content Delivery over
              IPv6", IETF 99 Proceedings, July 2017,
              <https://datatracker.ietf.org/meeting/99/materials/slides-
              99-maprg-measuring-youtube-content-delivery-over-ipv6-00>.

   [Mcrsft]   Microsoft, "IPv6 for Azure VMs available in most regions",
              September 2016, <https://azure.microsoft.com/en-
              us/updates/ipv6-for-azure-vms/>.

   [ND-CONSIDERATIONS]
              Xiao, X., Vasilenko, E., Metz, E., Mishra, G., and N.
              Buraglio, "Selectively Applying Host Isolation to Simplify
              IPv6 First-hop Deployment", Work in Progress, Internet-
              Draft, draft-ietf-v6ops-nd-considerations-00, 24 October
              2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
              v6ops-nd-considerations-00>.

   [NRO]      NRO, "Internet Number Resource Status Report", September
              2021, <https://www.nro.net/wp-content/uploads/NRO-
              Statistics-2021-Q3-FINAL.pdf>.

   [NST_1]    NIST, "Estimating Industry IPv6 & DNSSEC External Service
              Deployment Status", 2023, <https://fedv6-
              deployment.antd.nist.gov/cgi-bin/generate-com>.

   [NST_2]    NIST, "Estimating USG IPv6 & DNSSEC External Service
              Deployment Status", 2023, <https://fedv6-
              deployment.antd.nist.gov/cgi-bin/generate-gov>.

   [NST_3]    NIST, "Estimating University IPv6 & DNSSEC External
              Service Deployment Status", 2023, <https://fedv6-
              deployment.antd.nist.gov/cgi-bin/generate-edu>.

   [Ntflx]    Aggarwal, R. and D. Temkin, "Enabling Support for IPv6",
              July 2012, <https://netflixtechblog.com/enabling-support-
              for-ipv6-48a495d5196f>.

   [POTAROO1] Huston, G., "IP Addressing through 2021", January 2022,
              <https://www.potaroo.net/ispcol/2022-01/addr2021.html>.

   [POTAROO2] POTAROO, "IPv6 Resource Allocations", March 2023,
              <https://www.potaroo.net/bgp/iso3166/v6cc.html>.

   [RelJio]   Chandra, R., "IPv6-only adoption challenges and
              standardization requirements", IETF 109 Proceedings,
              November 2020,
              <https://datatracker.ietf.org/meeting/109/materials/
              slides-109-v6ops-ipv6-only-adoption-challenges-and-
              standardization-requirements-03>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [RFC5095]  Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
              of Type 0 Routing Headers in IPv6", RFC 5095,
              DOI 10.17487/RFC5095, December 2007,
              <https://www.rfc-editor.org/info/rfc5095>.

   [RFC6264]  Jiang, S., Guo, D., and B. Carpenter, "An Incremental
              Carrier-Grade NAT (CGN) for IPv6 Transition", RFC 6264,
              DOI 10.17487/RFC6264, June 2011,
              <https://www.rfc-editor.org/info/rfc6264>.

   [RFC6980]  Gont, F., "Security Implications of IPv6 Fragmentation
              with IPv6 Neighbor Discovery", RFC 6980,
              DOI 10.17487/RFC6980, August 2013,
              <https://www.rfc-editor.org/info/rfc6980>.

   [RFC7872]  Gont, F., Linkova, J., Chown, T., and W. Liu,
              "Observations on the Dropping of Packets with IPv6
              Extension Headers in the Real World", RFC 7872,
              DOI 10.17487/RFC7872, June 2016,
              <https://www.rfc-editor.org/info/rfc7872>.

   [RFC8064]  Gont, F., Cooper, A., Thaler, D., and W. Liu,
              "Recommendation on Stable IPv6 Interface Identifiers",
              RFC 8064, DOI 10.17487/RFC8064, February 2017,
              <https://www.rfc-editor.org/info/rfc8064>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

   [RFC8683]  Palet Martinez, J., "Additional Deployment Guidelines for
              NAT64/464XLAT in Operator and Enterprise Networks",
              RFC 8683, DOI 10.17487/RFC8683, November 2019,
              <https://www.rfc-editor.org/info/rfc8683>.

   [RFC8981]  Gont, F., Krishnan, S., Narten, T., and R. Draves,
              "Temporary Address Extensions for Stateless Address
              Autoconfiguration in IPv6", RFC 8981,
              DOI 10.17487/RFC8981, February 2021,
              <https://www.rfc-editor.org/info/rfc8981>.

   [RFC9098]  Gont, F., Hilliard, N., Doering, G., Kumari, W., Huston,
              G., and W. Liu, "Operational Implications of IPv6 Packets
              with Extension Headers", RFC 9098, DOI 10.17487/RFC9098,
              September 2021, <https://www.rfc-editor.org/info/rfc9098>.

   [RIPE1]    Huston, G., "Measuring IPv6 Performance", October 2016,
              <https://ripe73.ripe.net/wp-content/uploads/
              presentations/35-2016-10-24-v6-performance.pdf>.

   [RIPE2]    RIPE, "IPv6 Security", January 2023,
              <https://www.ripe.net/support/training/material/ipv6-
              security/ipv6security-slides.pdf>.

   [SNDVN]    Cullen, C., "Sandvine releases 2020 Mobile Internet
              Phenomena Report: YouTube is over 25% of all mobile
              traffic", February 2020, <https://www.sandvine.com/press-
              releases/sandvine-releases-2020-mobile-internet-phenomena-
              report-youtube-is-over-25-of-all-mobile-traffic>.

   [TMus]     Lagerholm, S., "Going IPv6 Only", June 2018,
              <https://pc.nanog.org/static/published/meetings/
              NANOG73/1645/20180625_Lagerholm_T-
              Mobile_S_Journey_To_v1.pdf>.

   [US-CIO]   Vought, R., "Memorandum for Heads of Executive Departments
              and Agencies: Completing the Transition to Internet
              Protocol Version 6 (IPv6)", 2020,
              <https://www.cio.gov/assets/resources/internet-protocol-
              version6-draft.pdf>.

   [US-FR]    Federal Register, "Request for Comments on Updated
              Guidance for Completing the Transition to the Next
              Generation Internet Protocol, Internet Protocol Version 6
              (IPv6)", March 2020, <https://www.federalregister.gov/
              documents/2020/03/02/2020-04202/request-for-comments-on-
              updated-guidance-for-completing-the-transition-to-the-
              next-generation>.

   [W3Techs]  W3Techs, "Historical yearly trends in the usage statistics
              of site elements for websites", 2023,
              <https://w3techs.com/technologies/history_overview/
              site_element/all/y>.

   [WIPv6L]   World IPv6 Launch, "Measurements", June 2022,
              <https://www.worldipv6launch.org/measurements/>.

Appendix A.  Summary of Questionnaire and Replies for Network Operators

   A survey was proposed to more than 50 service providers in the
   European region during the third quarter of 2020 to ask for their
   plans on IPv6 and the status of IPv6 deployment.

   In this survey, 40 people, representing 38 organizations, provided
   responses.  This appendix summarizes the results obtained.

   Respondents' business:

                      +============+========+=======+
                      | Convergent | Mobile | Fixed |
                      +============+========+=======+
                      | 82%        | 8%     | 11%   |
                      +------------+--------+-------+

                        Table 10: Type of Operators

   Question 1.  Do you have plans to move more fixed, mobile, or
   enterprise users to IPv6 in the next 2 years?

   A.  If so, fixed, mobile, or enterprise?

   B.  What are the reasons to do so?

   C.  When to start: already ongoing, in 12 months, or after 12 months?

   D.  Which transition solution will you use: Dual-Stack, DS-Lite,
       464XLAT, or MAP-T/E?

   Answers for 1.A (38 respondents)

                               +=====+=====+
                               | Yes | No  |
                               +=====+=====+
                               | 79% | 21% |
                               +-----+-----+

                           Table 11: Plan to Move
                           to IPv6 within 2 Years

               +========+=======+============+=============+
               | Mobile | Fixed | Enterprise | No Response |
               +========+=======+============+=============+
               | 63%    | 63%   | 50%        | 3%          |
               +--------+-------+------------+-------------+

                         Table 12: Business Segment

   Answers for 1.B (29 respondents)

   Even though this was an open question, some common answers can be
   found.

   *  14 respondents (48%) highlighted issues related to IPv4 depletion.
      The reason to move to IPv6 is to avoid private and/or overlapping
      addresses.

   *  6 respondents (20%) stated that 5G/IoT is a business incentive to
      introduce IPv6.

   *  4 respondents (13%) highlighted that there is a national
      regulation request to associate and enable IPv6 with the launch of
      5G.

   *  4 respondents (13%) considered IPv6 as a part of their innovation
      strategy or an enabler for new services.

   *  4 respondents (13%) introduced IPv6 because of enterprise customer
      demand.

   Answers for 1.C (30 respondents)

        +=========+==============+=================+=============+
        | Ongoing | In 12 months | After 12 months | No Response |
        +=========+==============+=================+=============+
        | 60%     | 33%          | 0%              | 7%          |
        +---------+--------------+-----------------+-------------+

                           Table 13: Timeframe

   Answers for 1.D (28 respondents for cellular, 27 for wireline)

              +============+=========+=======+=============+
              | Dual-Stack | 464XLAT | MAP-T | No Response |
              +============+=========+=======+=============+
              | 39%        | 21%     | 4%    | 36%         |
              +------------+---------+-------+-------------+

                  Table 14: Transition in Use: Cellular

             +============+=========+==========+=============+
             | Dual-Stack | DS-Lite | 6RD/6VPE | No Response |
             +============+=========+==========+=============+
             | 59%        | 19%     | 4%       | 19%         |
             +------------+---------+----------+-------------+

                   Table 15: Transition in Use: Wireline

   Question 2.  Do you need to change network devices for the above
   goal?

   A.  If yes, what kind of devices: CPE, BNG/mobile core, or NAT?

   B.  Will you start the transition of your metro, backbone, or
       backhaul network to support IPv6?

   Answers for 2.A (30 respondents)

                        +=====+=====+=============+
                        | Yes | No  | No Response |
                        +=====+=====+=============+
                        | 43% | 33% | 23%         |
                        +-----+-----+-------------+

                          Table 16: Need to Change

               +======+=========+=====+=====+=============+
               | CPEs | Routers | BNG | CGN | Mobile core |
               +======+=========+=====+=====+=============+
               | 47%  | 27%     | 20% | 33% | 27%         |
               +------+---------+-----+-----+-------------+

                         Table 17: What to Change

   Answers for 2.B (22 respondents)

                           +=====+========+=====+
                           | Yes | Future | No  |
                           +=====+========+=====+
                           | 9%  | 9%     | 82% |
                           +-----+--------+-----+

                            Table 18: Plans for
                                 Transition

Appendix B.  Summary of Questionnaire and Replies for Enterprises

   The Industry Network Technology Council (INTC) developed the
   following poll to verify the need or willingness of medium-to-large
   US-based enterprises for training and consultancy on IPv6
   <https://industrynetcouncil.org/> in early 2021.

   54 organizations provided answers.

   Question 1.  How much IPv6 implementation have you done at your
   organization?  (54 respondents)

       +-------------------------------------------------+--------+
       | None                                            | 16.67% |
       +-------------------------------------------------+--------+
       | Some people have gotten some training           | 16.67% |
       +-------------------------------------------------+--------+
       | Many people have gotten some training           |  1.85% |
       +-------------------------------------------------+--------+
       | Website is IPv6 enabled                         |  7.41% |
       +-------------------------------------------------+--------+
       | Most equipment is dual-stacked                  | 31.48% |
       +-------------------------------------------------+--------+
       | Have an IPv6 transition plan for entire network |  5.56% |
       +-------------------------------------------------+--------+
       | Running IPv6-only in many places                | 20.37% |
       +-------------------------------------------------+--------+
       | Entire network is IPv6-only                     |  0.00% |
       +-------------------------------------------------+--------+

                      Table 19: IPv6 Implementation

   Question 2.  What kind of help or classes would you like to see INTC
   do? (54 respondents)

        +------------------------------------------------+--------+
        | Classes/labs on IPv6 security                  | 66.67% |
        +------------------------------------------------+--------+
        | Classes/labs on IPv6 fundamentals              | 55.56% |
        +------------------------------------------------+--------+
        | Classes/labs on address planning/network conf. | 57.41% |
        +------------------------------------------------+--------+
        | Classes/labs on IPv6 troubleshooting           | 66.67% |
        +------------------------------------------------+--------+
        | Classes/labs on application conversion         | 35.19% |
        +------------------------------------------------+--------+
        | Other                                          | 14.81% |
        +------------------------------------------------+--------+

                      Table 20: Help/Classes from INTC

   Question 3.  As you begin to think about the implementation of IPv6
   at your organization, what areas do you feel are of concern?  (54
   respondents)

                 +-----------------------------+--------+
                 | Security                    | 31.48% |
                 +-----------------------------+--------+
                 | Application conversion      | 25.93% |
                 +-----------------------------+--------+
                 | Training                    | 27.78% |
                 +-----------------------------+--------+
                 | All the above               | 33.33% |
                 +-----------------------------+--------+
                 | Don't know enough to answer | 14.81% |
                 +-----------------------------+--------+
                 | Other                       |  9.26% |
                 +-----------------------------+--------+

                   Table 21: Areas of Concern for IPv6
                              Implementation

Acknowledgements

   The authors of this document would like to thank Brian Carpenter,
   Fred Baker, Alexandre Petrescu, Fernando Gont, Barbara Stark,
   Haisheng Yu (Johnson), Dhruv Dhody, Gábor Lencse, Shuping Peng,
   Daniel Voyer, Daniel Bernier, Hariharan Ananthakrishnan, Donavan
   Fritz, Igor Lubashev, Erik Nygren, Eduard Vasilenko, and Xipeng Xiao
   for their comments and review of this document.

Contributors

   Nalini Elkins
   Inside Products
   Email: nalini.elkins@insidethestack.com


   Sébastien Lourdez
   Post Luxembourg
   Email: sebastien.lourdez@post.lu


Authors' Addresses

   Giuseppe Fioccola
   Huawei Technologies
   Riesstrasse, 25
   80992 Munich
   Germany
   Email: giuseppe.fioccola@huawei.com


   Paolo Volpato
   Huawei Technologies
   Via Lorenteggio, 240
   20147 Milan
   Italy
   Email: paolo.volpato@huawei.com


   Jordi Palet Martinez
   The IPv6 Company
   Molino de la Navata, 75
   28420 La Navata - Galapagar, Madrid
   Spain
   Email: jordi.palet@theipv6company.com


   Gyan S. Mishra
   Verizon Inc.
   Email: gyan.s.mishra@verizon.com


   Chongfeng Xie
   China Telecom
   Email: xiechf@chinatelecom.cn
