RFC 6589

Internet Engineering Task Force (IETF)                      J. Livingood
Request for Comments: 6589                                       Comcast
Category: Informational                                       April 2012
ISSN: 2070-1721

            Considerations for Transitioning Content to IPv6


   This document describes considerations for the transition of end-user
   content on the Internet to IPv6.  While this is tailored to address
   end-user content, which is typically web-based, many aspects of this
   document may be more broadly applicable to the transition to IPv6 of
   other applications and services.  This document explores the
   challenges involved in the transition to IPv6, potential migration
   tactics, possible migration phases, and other considerations.  The
   audience for this document is the Internet community generally,
   particularly IPv6 implementers.

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 a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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RFC 6589              Transitioning Content to IPv6           April 2012

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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RFC 6589              Transitioning Content to IPv6           April 2012

Table of Contents

   1. Introduction ....................................................4
   2. Challenges When Transitioning Content to IPv6 ...................4
      2.1. IPv6-Related Impairment ....................................5
      2.2. Operational Maturity Concerns ..............................5
      2.3. Volume-Based Concerns ......................................5
   3. IPv6 Adoption Implications ......................................6
   4. Potential Migration Tactics .....................................6
      4.1. Solving Current End-User IPv6 Impairments ..................7
      4.2. Using IPv6-Specific Names ..................................8
      4.3. Implementing DNS Resolver Whitelisting .....................8
           4.3.1. How DNS Resolver Whitelisting Works ................11
           4.3.2. Similarities to Content Delivery Networks
                  and Global Server Load Balancing ...................15
           4.3.3. Similarities to DNS Load Balancing .................15
           4.3.4. Similarities to Split DNS ..........................15
           4.3.5. Related Considerations .............................16
      4.4. Implementing DNS Blacklisting .............................17
      4.5. Transitioning Directly to Native Dual Stack ...............18
   5. Potential Implementation Phases ................................19
      5.1. No Access to IPv6 Content .................................19
      5.2. Using IPv6-Specific Names .................................19
      5.3. Deploying DNS Resolver Whitelisting Using Manual
           Processes .................................................19
      5.4. Deploying DNS Resolver Whitelisting Using
           Automated Processes .......................................19
      5.5. Turning Off DNS Resolver Whitelisting .....................20
      5.6. Deploying DNS Blacklisting ................................20
      5.7. Fully Dual-Stack Content ..................................20
   6. Other Considerations ...........................................20
      6.1. Security Considerations ...................................20
      6.2. Privacy Considerations ....................................21
      6.3. Considerations with Poor IPv4 and Good IPv6 Transport .....22
   7. Contributors ...................................................23
   8. Acknowledgements ...............................................23
   9. References .....................................................24
      9.1. Normative References ......................................24
      9.2. Informative References ....................................25

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RFC 6589              Transitioning Content to IPv6           April 2012

1.  Introduction

   This document describes considerations for the transition of end-user
   content on the Internet to IPv6.  While this is tailored to address
   end-user content, which is typically web-based, many aspects of this
   document may be more broadly applicable to the transition to IPv6 of
   other applications and services.  The issues explored herein will be
   of particular interest to major web content sites (sometimes
   described hereinafter as "high-service-level domains"), which have
   specific and unique concerns related to maintaining a high-quality
   user experience for all of their users during their transition to
   IPv6.  This document explores the challenges involved in the
   transition to IPv6, potential migration tactics, possible migration
   phases, and other considerations.  Some sections of this document
   also include information about the potential implications of various
   migration tactics or phased approaches to the transition to IPv6.

2.  Challenges When Transitioning Content to IPv6

   The goal in transitioning content to IPv6 is to make that content
   natively dual-stack enabled, which provides native access to all end
   users via both IPv4 and IPv6.  However, there are technical and
   operational challenges in being able to transition smoothly for all
   end users, which has led to the development of a variety of migration
   tactics.  A first step in understanding various migration tactics is
   to first outline the challenges involved in moving content to IPv6.

   Implementers of these various migration tactics are attempting to
   protect users of their services from having a negative experience
   (poor performance) when they receive DNS responses containing AAAA
   resource records or when attempting to use IPv6 transport.  There are
   two main concerns that pertain to this practice: one is IPv6-related
   impairment, and the other is the maturity or stability of IPv6
   transport (and associated network operations) for high-service-level
   domains.  Both can negatively affect the experience of end users.

   Not all domains may face the same challenges in transitioning content
   to IPv6, since the user base of each domain, traffic sources, traffic
   volumes, and other factors obviously will vary between domains.  As a
   result, while some domains have used an IPv6 migration tactic, others
   have run brief IPv6 experiments and then decided to simply turn on
   IPv6 for the domain without further delay and without using any
   specialized IPv6 migration tactics [Heise].  Each domain should
   therefore consider its specific situation when formulating a plan to
   move to IPv6; there is not one approach that will work for every

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RFC 6589              Transitioning Content to IPv6           April 2012

2.1.  IPv6-Related Impairment

   Some implementers have observed that when they added AAAA resource
   records to their authoritative DNS servers in order to support IPv6
   access to their content, a small fraction of end users had slow or
   otherwise impaired access to a given website with both AAAA and A
   resource records.  The fraction of users with such impaired access
   has been estimated to be as high as 0.078% of total Internet users
   [IETF-77-DNSOP] [NW-Article-DNSOP] [IPv6-Growth] [IPv6-Brokenness].

   While it is outside the scope of this document to explore the various
   reasons why a particular user's system (host) may have impaired IPv6
   access, and the potential solutions [RFC6555] [RFC6343], for the
   users who experience this impairment, it has a very real performance
   impact.  It would impact access to all or most dual-stack services to
   which the user attempts to connect.  This negative end-user
   experience can range from access that is somewhat slower than usual
   (as compared to native IPv4-based access), to extremely slow access,
   to no access to the domain's resources whatsoever.  In essence,
   whether the end user even has an IPv6 address or not, merely by
   receiving a AAAA record response, the user either cannot access a
   Fully Qualified Domain Name (FQDN, representing a service or resource
   sought) or it is so slow that the user gives up and assumes the
   destination is unreachable.

2.2.  Operational Maturity Concerns

   Some implementers have discovered that network operations, operations
   support and business support systems, and other operational processes
   and procedures are less mature for IPv6 as compared to IPv4, since
   IPv6 has not heretofore been pervasively deployed.  This operational
   immaturity may be observed not just within the network of a given
   domain but also in any directly or indirectly interconnected
   networks.  As a result, many domains consider it prudent to undertake
   any network changes that will cause traffic to shift to IPv6
   gradually, in order to provide time and experience for IPv6
   operations and network practices to mature.

2.3.  Volume-Based Concerns

   While Section 2.2 pertains to risks due to immaturity in operations,
   a related concern is that some technical issues may not become
   apparent until some moderate to high volume of traffic is sent via
   IPv6 to and from a domain.  As above, this may be the case not just
   within the network of that domain but also for any directly or
   indirectly interconnected networks.  Furthermore, compared to domains
   with small to moderate traffic volumes, whether by the count of end
   users or count of bytes transferred, high-traffic domains receive

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RFC 6589              Transitioning Content to IPv6           April 2012

   such a level of usage that it is prudent to undertake any network
   changes gradually and in a manner that minimizes the risk of
   disruption.  One can imagine that for one of the top ten sites
   globally, for example, the idea of suddenly turning on a significant
   amount of IPv6 traffic is quite daunting and would carry a relatively
   high risk of network and/or other disruptions.

3.  IPv6 Adoption Implications

   It is important that the challenges in transitioning content to IPv6
   as noted in Section 2 are addressed, especially for high-service-
   level domains.  Some high-service-level domains may find the prospect
   of transitioning to IPv6 extremely daunting without having some
   ability to address these challenges and to incrementally control
   their transition to IPv6.  Lacking such controls, some domains may
   choose to substantially delay their transition to IPv6.  A
   substantial delay in moving content to IPv6 could certainly mean
   there are somewhat fewer motivating factors for network operators to
   deploy IPv6 to end-user hosts (though they have many significant
   motivating factors that are largely independent of content).  At the
   same time, unless network operators transition to IPv6, there are of
   course fewer motivations for domain owners to transition content to
   IPv6.  Without progress in each part of the Internet ecosystem,
   networks and/or content sites may delay, postpone, or cease adoption
   of IPv6, or to actively seek alternatives to it.  Such alternatives
   may include the use of multi-layer or large-scale network address
   translation (NAT), which is not preferred relative to native dual

   Obviously, transitioning content to IPv6 is important to IPv6
   adoption overall.  While challenges do exist, such a transition is
   not an impossible task for a domain to undertake.  A range of
   potential migration tactics, as noted below in Section 4, can help
   meet these challenges and enable a domain to successfully transition
   content and other services to IPv6.

4.  Potential Migration Tactics

   Domains have a wide range of potential tactics at their disposal that
   may be used to facilitate the migration to IPv6.  This section
   includes many of the key tactics that could be used by a domain but
   by no means provides an exhaustive or exclusive list.  Only a
   specific domain can judge whether or not a given (or any) migration
   tactic applies to it and meets its needs.  A domain may also decide
   to pursue several of these tactics in parallel.  Thus, the usefulness
   of each tactic and the associated pros and cons will vary from domain
   to domain.

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RFC 6589              Transitioning Content to IPv6           April 2012

4.1.  Solving Current End-User IPv6 Impairments

   Domains can endeavor to fix the underlying technical problems
   experienced by their end users during the transition to IPv6, as
   noted in Section 2.1.  One challenge with this option is that a
   domain may have little or no control over the network connectivity,
   operating system, client software (such as a web browser), and/or
   other capabilities of the end users of that domain.  In most cases, a
   domain is only in a position to influence and guide its end users.
   While this is not the same sort of direct control that may exist, for
   example, in an enterprise network, major domains are likely to be
   trusted by their end users and may therefore be able to influence and
   guide these users in solving any IPv6-related impairments.

   Another challenge is that end-user impairments are something that one
   domain on its own cannot solve.  This means that domains may find it
   more effective to coordinate with many others in the Internet
   community to solve what is really a collective problem that affects
   the entire Internet.  Of course, it can sometimes be difficult to
   motivate members of the Internet community to work collectively
   towards such a goal, sharing the labor, time, and costs related to
   such an effort.  However, World IPv6 Day [W6D] shows that such
   community efforts are possible, and despite any potential challenges,
   the Internet community continues to work together in order to solve
   end-user IPv6 impairments.

   One potential tactic may be to identify which users have such
   impairments and then to communicate this information to affected
   users.  Such end-user communication is likely to be most helpful if
   the end users are not only alerted to a potential problem but are
   given careful and detailed advice on how to resolve this on their
   own, or are guided to where they can seek help in doing so.  Another
   potential tactic is for a domain to collect, track over time, and
   periodically share with the Internet community the rate of impairment
   observed for that domain.  In any such end-user IPv6-related analysis
   and communication, Section 6.2 is worth taking into account.

   However, while these tactics can help reduce IPv6-related impairments
   (Section 2.1), they do not address either operational maturity
   concerns (noted in Section 2.2) or volume-based concerns (noted in
   Section 2.3), which should be considered and addressed separately.

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RFC 6589              Transitioning Content to IPv6           April 2012

4.2.  Using IPv6-Specific Names

   Another potential migration tactic is for a domain to gain experience
   using a special FQDN.  This has become typical for domains beginning
   the transition to IPv6, whereby an address-family-specific name such
   as ipv6.example.com or www.ipv6.example.com is used.  An end user
   would have to know to use this special IPv6-specific name; it is not
   the same name used for regular traffic.

   This special IPv6-specific name directs traffic to a host or hosts
   that have been enabled for native IPv6 access.  In some cases, this
   name may point to hosts that are separate from those used for IPv4
   traffic (via www.example.com), while in other cases it may point to
   the same hosts used for IPv4 traffic.  A subsequent phase, if
   separate hosts are used to support special IPv6-specific names, is to
   move to the same hosts used for regular traffic in order to utilize
   and exercise production infrastructure more fully.  Regardless of
   whether or not dedicated hosts are used, the use of the special name
   is a way to incrementally control traffic as a tool for a domain to
   gain IPv6 experience and increase IPv6 use on a relatively controlled
   basis.  Any lessons learned can then inform plans for a full
   transition to IPv6.  This also provides an opportunity for a domain
   to develop any necessary training for staff, to develop IPv6-related
   testing procedures for its production network and lab, to deploy IPv6
   functionality into its production network, and to develop and deploy
   IPv6-related network and service monitors.  It is also an opportunity
   to add a relatively small amount of IPv6 traffic to ensure that
   network gear, network interconnects, and IPv6 routing in general are
   working as expected.

   While using a special IPv6-specific name is a good initial step to
   functionally test and prepare a domain for IPv6 -- including
   developing and maturing IPv6 operations, as noted in Section 2.2 --
   the utility of the tactic is limited, since users must know the IPv6-
   specific name, the traffic volume will be low, and the traffic is
   unlikely to be representative of the general population of end users
   (they are likely to be self-selecting early adopters and more
   technically advanced than average), among other reasons.  As a
   result, any concerns and risks related to traffic volume, as noted in
   Section 2.3, should still be considered and addressed separately.

4.3.  Implementing DNS Resolver Whitelisting

   Another potential tactic -- especially when a high-service-level
   domain is ready to move beyond an IPv6-specific name, as described in
   Section 4.2 -- is to selectively return AAAA resource records (RRs),
   which contain IPv6 addresses.  This selective response of DNS records
   is performed by an authoritative DNS server for a domain in response

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RFC 6589              Transitioning Content to IPv6           April 2012

   to DNS queries sent by DNS recursive resolvers [RFC1035].  This is
   commonly referred to in the Internet community as "DNS Resolver
   Whitelisting", and will be referred to as such hereafter, though in
   essence it is simply a tactic enabling the selective return of DNS
   records based upon various technical factors.  An end user is seeking
   a resource by name, and this selective response mechanism enables
   what is perceived to be the most reliable and best performing IP
   address family to be used (IPv4 or IPv6).  It shares similarities
   with Content Delivery Networks (CDNs), Global Server Load Balancing
   (GSLB), DNS Load Balancing, and Split DNS, as described below in
   Sections 4.3.2, 4.3.3, and 4.3.4.  A few high-service-level domains
   have either implemented DNS Resolver Whitelisting (one of many
   migration tactics they have used or are using) or are considering
   doing so [NW-Article-DNS-WL] [WL-Ops].

   This is a migration tactic used by domains as a method for
   incrementally transitioning inbound traffic to a domain to IPv6.  If
   an incremental tactic like this is not used, a domain might return
   AAAA resource records to any relevant DNS query, meaning the domain
   could go quickly from no IPv6 traffic to a potentially significant
   amount as soon as the AAAA resource records are published.  When DNS
   Resolver Whitelisting is implemented, a domain's authoritative DNS
   will selectively return a AAAA resource record to DNS recursive
   resolvers on a whitelist maintained by the domain, while returning no
   AAAA resource records to DNS recursive resolvers that are not on that
   whitelist.  This tactic will not have a direct impact on reducing
   IPv6-related impairments (Section 2.1), though it can help a domain
   address operational maturity concerns (Section 2.2) as well as
   concerns and risks related to traffic volume (Section 2.3).  While
   DNS Resolver Whitelisting does not solve IPv6-related impairments, it
   can help a domain to avoid users that have them.  As a result, the
   tactic removes their impact in all but the few networks that are
   whitelisted.  DNS Resolver Whitelisting also allows website operators
   to protect non-IPv6 networks (i.e., networks that do not support IPv6
   and/or do not have plans to do so in the future) from IPv6-related
   impairments in their networks.  Finally, domains using this tactic
   should understand that the onus is on them to ensure that the servers
   being whitelisted represent a network that has proven to their
   satisfaction that they are IPv6-ready and that this will not create a
   poor end-user experience for users of the whitelisted server.

   There are of course challenges and concerns related to DNS Resolver
   Whitelisting.  Some of the concerns with a whitelist of DNS recursive
   resolvers may be held by parties other than the implementing domain,
   such as network operators or end users that may not have had their
   DNS recursive resolvers added to a whitelist.  Additionally, the IP
   address of a DNS recursive resolver is not a precise indicator of the
   IPv6 preparedness, or lack of IPv6-related impairment, of end-user

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RFC 6589              Transitioning Content to IPv6           April 2012

   hosts that query (use) a particular DNS recursive resolver.  While
   the IP addresses of DNS recursive resolvers on networks known to have
   deployed IPv6 may be an imperfect proxy for judging IPv6
   preparedness, or lack of IPv6-related impairment, this method is one
   of the better available methods at the current time.  For example,
   implementers have found that it is possible to measure the level of
   IPv6 preparedness of the end users behind any given DNS recursive
   resolver by conducting ongoing measurement of the IPv6 preparedness
   of end users querying for one-time-use hostnames and then correlating
   the domain's authoritative DNS server logs with their web server
   logs.  This can help implementers form a good picture of which DNS
   recursive resolvers have working IPv6 users behind them and which do
   not, what the latency impact of turning on IPv6 for any given DNS
   recursive resolver is, etc.  In addition, given the current state of
   global IPv6 deployment, this migration tactic allows content
   providers to selectively expose the availability of their IPv6
   services.  While opinions in the Internet community concerning DNS
   Resolver Whitelisting are understandably quite varied, there is clear
   consensus that DNS Resolver Whitelisting can be a useful tactic for
   use during the transition of a domain to IPv6.  In particular, some
   high-service-level domains view DNS Resolver Whitelisting as one of
   the few practical and low-risk approaches enabling them to transition
   to IPv6, without which their transition may not take place for some
   time.  However, there is also consensus that this practice is
   workable on a manual basis (see below) only in the short term and
   that it will not scale over the long term.  Thus, some domains may
   find DNS Resolver Whitelisting a beneficial temporary tactic in their
   transition to IPv6.

   At the current time, generally speaking, a domain that implements DNS
   Resolver Whitelisting does so manually.  This means that a domain
   manually maintains a list of networks that are permitted to receive
   IPv6 records (via their DNS resolver IP addresses) and that these
   networks typically submit applications, or follow some other process
   established by the domain, in order to be added to the DNS Whitelist.
   However, implementers foresee that a subsequent phase of DNS Resolver
   Whitelisting is likely to emerge in the future, possibly in the near
   future.  In this new phase, a domain would return IPv6 and/or IPv4
   records dynamically based on automatically detected technical
   capabilities, location, or other factors.  It would then function
   much like (or indeed as part of) GSLB, a common practice already in
   use today, as described in Section 4.3.2.  Furthermore, in this
   future phase, networks would be added to and removed from a DNS
   Whitelist automatically, and possibly on a near-real-time basis.
   This means, crucially, that networks would no longer need to apply to
   be added to a whitelist, which may alleviate many of the key concerns
   that network operators may have with this tactic when it is
   implemented on a manual basis.

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RFC 6589              Transitioning Content to IPv6           April 2012

4.3.1.  How DNS Resolver Whitelisting Works

   Using a "whitelist" in a generic sense means that no traffic (or
   traffic of a certain type) is permitted to the destination host
   unless the originating host's IP address is contained in the
   whitelist.  In contrast, using a "blacklist" means that all traffic
   is permitted to the destination host unless the originating host's IP
   address is contained in the blacklist.  In the case of DNS Resolver
   Whitelisting, the resource that an end user seeks is a name, not an
   IP address or IP address family.  Thus, an end user is seeking a name
   such as www.example.com, without regard to the underlying IP address
   family (IPv4 or IPv6) that may be used to access that resource.

   DNS Resolver Whitelisting is implemented in authoritative DNS
   servers, not in DNS recursive resolvers.  These authoritative DNS
   servers selectively return AAAA resource records using the IP address
   of the DNS recursive resolver that has sent them a query.  Thus, for
   a given operator of a website, such as www.example.com, the domain
   operator implements whitelisting on the authoritative DNS servers for
   the domain example.com.  The whitelist is populated with the IPv4
   and/or IPv6 addresses or prefix ranges of DNS recursive resolvers on
   the Internet, which have been authorized to receive AAAA resource
   record responses.  These DNS recursive resolvers are operated by
   third parties, such as Internet Service Providers (ISPs),
   universities, governments, businesses, and individual end users.  If
   a DNS recursive resolver is not matched in the whitelist, then AAAA
   resource records WILL NOT be sent in response to a query for a
   hostname in the example.com domain (and an A record would be sent).
   However, if a DNS recursive resolver is matched in the whitelist,
   then AAAA resource records WILL be sent.  As a result, while
   Section 2.2 of [RFC4213] notes that a stub resolver can make a choice
   between whether to use a AAAA record or A record response, with DNS
   Resolver Whitelisting the authoritative DNS server can also decide
   whether to return a AAAA record, an A record, or both record types.

   When implemented on a manual basis, DNS Resolver Whitelisting
   generally means that a very small fraction of the DNS recursive
   resolvers on the Internet (those in the whitelist) will receive AAAA
   responses.  The large majority of DNS recursive resolvers on the
   Internet will therefore receive only A resource records containing
   IPv4 addresses.  Domains may find the practice imposes some
   incremental operational burdens insofar as it can consume staff time
   to maintain a whitelist (such as additions and deletions to the
   list), respond to and review applications to be added to a whitelist,
   maintain good performance levels on authoritative DNS servers as the
   whitelist grows, create new network monitors to check the health of a
   whitelist function, perform new types of troubleshooting related to
   whitelisting, etc.  In addition, manually based whitelisting imposes

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RFC 6589              Transitioning Content to IPv6           April 2012

   some incremental burdens on operators of DNS recursive resolvers
   (such as network operators), since they will need to apply to be
   whitelisted with any implementing domains, and will subsequently need
   processes and systems to track the status of whitelisting
   applications, respond to requests for additional information
   pertaining to these applications, and track any de-whitelisting

   When implemented on an automated basis in the future, DNS recursive
   resolvers listed in the whitelist could expand and contract
   dynamically, and possibly in near-real time, based on a wide range of
   factors.  As a result, it is likely that the number of DNS recursive
   resolvers on the whitelist will be substantially larger than when
   such a list is maintained manually, and it is also likely that the
   whitelist will grow at a rapid rate.  This automation can eliminate
   most of the significant incremental operational burdens on
   implementing domains as well as operators of DNS recursive resolvers,
   which is clearly a factor that is motivating implementers to work to
   automate this function.

   Section and Figure 1 provide more details on DNS Resolver
   Whitelisting in general.  In addition, the potential deployment
   models of DNS Resolver Whitelisting (manual and automated) are
   described in Section 5.  It is also important to note that DNS
   Resolver Whitelisting also works independently of whether an
   authoritative DNS server, DNS recursive resolver, or end-user host
   uses IPv4 transport, IPv6, or both.  So, for example, whitelisting
   may not result in the return of AAAA responses even in those cases
   where the DNS recursive resolver has queried the authoritative server
   over an IPv6 transport.  This may also be the case in some situations
   when the end-user host's original query to its DNS recursive resolver
   was over IPv6 transport, if that DNS recursive resolver is not on a
   given whitelist.  One important reason for this is that even though
   the DNS recursive resolver may have no IPv6-related impairments, this
   is not a reliable predictor of whether the same is true of the end-
   user host.  This also means that a DNS Whitelist can contain both
   IPv4 and IPv6 addresses.

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RFC 6589              Transitioning Content to IPv6           April 2012  Description of the Operation of DNS Resolver Whitelisting

   Specific implementations will vary from domain to domain, based on a
   range of factors such as the technical capabilities of a given
   domain.  As such, any examples listed herein should be considered
   general examples and are not intended to be exhaustive.

   The system logic of DNS Resolver Whitelisting is as follows:

   1.  The authoritative DNS server for example.com receives DNS queries
       for the A (IPv4) and/or AAAA (IPv6) address resource records for
       the FQDN www.example.com, for which AAAA (IPv6) resource records

   2.  The authoritative DNS server checks the IP address (IPv4, IPv6,
       or both) of the DNS recursive resolver sending the AAAA (IPv6)
       query against the whitelist (i.e., the DNS Whitelist).

   3.  If the DNS recursive resolver's IP address IS matched in the
       whitelist, then the response to that specific DNS recursive
       resolver can contain AAAA (IPv6) address resource records.

   4.  If the DNS recursive resolver's IP address IS NOT matched in the
       whitelist, then the response to that specific DNS recursive
       resolver cannot contain AAAA (IPv6) address resource records.  In
       this case, the server will likely return a response with the
       response code (RCODE) being set to 0 (No Error) with an empty
       answer section for the AAAA record query.

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 | Caching Server 1 - IS NOT ON the DNS Whitelist                     |
 | Caching Server 2 - IS ON the DNS Whitelist                         |
 | Note: Transport between each host can be IPv4 or IPv6.             |
 +----------+          +---------------+         +---------------+
 |   Stub   |          |  DNS Caching  |         |      DNS      |
 | Resolver |          |   Server 1    |         |     Server    |
 +----------+          +---------------+         +---------------+
    | DNS Query:            |                         |
    | example.com A, AAAA   |                         |
    |---------------------->|                         |
    |                       |                         |
    |                       | DNS Query:              |
    |                       | example.com A, AAAA     |
    |                       |------------------------>|
    |                       |                         |
    |                       |                         | NOT on Whitelist
    |                       |           DNS Response: |
    |                       |           example.com A |
    |                       |<------------------------|
    |                       |                         |
    |         DNS Response: |                         |
    |         example.com A |                         |
    |<----------------------|                         |

 +----------+          +---------------+         +---------------+
 |   Stub   |          |  DNS Caching  |         |      DNS      |
 | Resolver |          |   Server 2    |         |     Server    |
 +----------+          +---------------+         +---------------+
    | DNS Query:            |                         |
    | example.com A, AAAA   |                         |
    |---------------------->|                         |
    |                       |                         |
    |                       | DNS Query:              |
    |                       | example.com A, AAAA     |
    |                       |------------------------>|
    |                       |                         |
    |                       |                         | IS on Whitelist
    |                       |           DNS Response: |
    |                       |     example.com A, AAAA |
    |                       |<------------------------|
    |                       |                         |
    |         DNS Response: |                         |
    |   example.com A, AAAA |                         |
    |<----------------------|                         |

                Figure 1: DNS Resolver Whitelisting Diagram

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4.3.2.  Similarities to Content Delivery Networks and Global Server Load

   DNS Resolver Whitelisting is functionally similar to CDNs and GSLB.
   When using a CDN or GSLB, a geographically aware authoritative DNS
   server function is usually part of that overall system.  As a result,
   the use of a CDN or GSLB with an authoritative DNS server function
   enables the IP address resource records returned to a resolver in
   response to a query to vary, based on the estimated geographic
   location of the resolver [Wild-Resolvers] or a range of other
   technical factors.  This CDN or GSLB DNS function is performed in
   order to attempt to direct hosts to a) connect either to the nearest
   host (as measured in round-trip time) or to the host that has the
   best connectivity to an end user, b) route around failures, c) avoid
   sites where maintenance work has taken down hosts, and/or d) connect
   to the host that will otherwise provide the best service experience
   for an end user at a given point in time.  As a result, one can see a
   direct similarity to DNS Resolver Whitelisting insofar as different
   IP address resource records are selectively returned to resolvers
   based on the IP address of each resolver and/or other imputed factors
   related to that IP address.

4.3.3.  Similarities to DNS Load Balancing

   DNS Resolver Whitelisting has some similarities to DNS Load
   Balancing.  There are of course many ways that DNS Load Balancing can
   be performed.  In one example, multiple IP address resource records
   (A and/or AAAA) can be added to the DNS for a given FQDN.  This
   approach is referred to as DNS round robin [RFC1794].  DNS round
   robin may also be employed where SRV resource records are used
   [RFC2782].  In another example, one or more of the IP address
   resource records in the DNS will direct traffic to a load balancer.
   That load balancer, in turn, may be application-aware, and pass the
   traffic on to one or more hosts that are connected to the load
   balancer and that have different IP addresses.  In cases where
   private IPv4 addresses are used [RFC1918], as well as when public IP
   addresses are used, those end hosts may not necessarily be directly
   reachable without passing through the load balancer first.  So,
   similar to DNS Resolver Whitelisting, a load balancer will control
   what server host an end-user's host communicates with when using
   an FQDN.

4.3.4.  Similarities to Split DNS

   DNS Resolver Whitelisting has some similarities to so-called Split
   DNS, briefly described in Section 3.8 of [RFC2775].  When Split DNS
   is used, the authoritative DNS server selectively returns different
   responses, depending upon what host has sent the query.  While

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   [RFC2775] notes that the typical use of Split DNS is to provide one
   answer to hosts on an Intranet (internal network) and a different
   answer to hosts on the Internet (external or public network), the
   basic idea is that different answers are provided to hosts on
   different networks.  This is similar to the way that DNS Resolver
   Whitelisting works, whereby hosts on different networks that use
   different DNS recursive resolvers receive different answers if one
   DNS recursive resolver is on the whitelist and the other is not.
   However, Internet transparency and Internet fragmentation concerns
   regarding Split DNS are detailed in Section 2.1 of [RFC2956].
   Section 2.7 of [RFC2956] notes concerns regarding Split DNS,
   including the concern that the deployment of Split DNS "makes the use
   of Fully Qualified Domain Names (FQDNs) as endpoint identifiers more
   complex".  Section 3.5 of [RFC2956] further recommends that
   maintaining a stable approach to DNS operations is key during
   transitions, such as the one to IPv6 that is underway now, and states
   that "Operational stability of DNS is paramount, especially during a
   transition of the network layer, and both IPv6 and some network
   address translation techniques place a heavier burden on DNS".

4.3.5.  Related Considerations

   While techniques such as GSLB and DNS Load Balancing -- which share
   much in common with DNS Resolver Whitelisting -- are widespread, some
   in the community have raised a range of concerns about all of these
   practices.  Some concerns are specific to DNS Resolver Whitelisting
   [WL-Concerns].  Other concerns are not as specific and pertain to the
   general practice of implementing content location or other network
   policy controls in the "middle" of the network, in a so-called
   "middlebox" function.  Whether such DNS-related functions are really
   part of a middlebox is debatable.  Nevertheless, implementers should
   at least be aware of some of the risks of middleboxes, as noted in
   [RFC3724].  A related document, [RFC1958], explains that configured
   state, policies, and other functions needed in the middle of the
   network should be minimized as a design goal.  In addition,
   Section 2.16 of [RFC3234] makes specific statements concerning
   modified DNS servers.  Section 1.2 of [RFC3234] also outlines more
   general concerns about the introduction of new failure modes when
   configuration is no longer limited to two ends of a session, so that
   diagnosis of failures and misconfigurations could become more
   complex.  Two additional sources worth considering are [Tussle] and
   [Rethinking], in which the authors note concerns regarding the
   introduction of new control points (e.g., in middleboxes or in
   the DNS).

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   However, state, policies, and other functions have always been
   necessary to enable effective, reliable, and high-quality end-to-end
   communications on the Internet.  In addition, the use of GSLB, CDNs,
   DNS Load Balancing, and Split DNS are not only widely deployed but
   are almost uniformly viewed as essential to the functioning of the
   Internet and highly beneficial to the quality of the end-user
   experience on the Internet.  These techniques have had, and continue
   to have, a beneficial effect on the experience of a wide range of
   Internet applications and protocols.  So, while there are valid
   concerns about implementing policy controls in the "middle" of the
   network, or anywhere away from edge hosts, the definition of what
   constitutes the middle and edge of the network is debatable in this
   case.  This is particularly so given that GSLBs and CDNs facilitate
   connections from end hosts and the optimal content hosts, and could
   therefore be considered a modest and, in many cases, essential
   network policy extension of a network's edge, especially in the case
   of high-service-level domains.

   There may be additional implications for end users that have
   configured their hosts to use a third party as their DNS recursive
   resolver, rather than the one(s) provided by their network operator.
   In such cases, it will be more challenging for a domain using
   whitelisting to determine the level of IPv6-related impairment when
   such third-party DNS recursive resolvers are used, given the wide
   variety of end-user access networks that may be used and given that
   this mix may change in unpredictable ways over time.

4.4.  Implementing DNS Blacklisting

   With DNS Resolver Whitelisting, DNS recursive resolvers can receive
   AAAA resource records only if they are on the whitelist.  DNS
   Blacklisting is by contrast the opposite of that, whereby all DNS
   recursive resolvers can receive AAAA resource records unless they are
   on the blacklist.  Some implementers of DNS Resolver Whitelisting may
   choose to subsequently transition to DNS Blacklisting.  It is not
   clear when and if it may be appropriate for a domain to change from
   whitelisting to blacklisting, nor is it clear how implementers will
   judge that network conditions have changed sufficiently to justify
   disabling such controls.

   When a domain uses blacklisting, it is enabling all DNS recursive
   resolvers to receive AAAA record responses, except for what is
   presumed to be a relatively small number that are on the blacklist.
   Over time, it is likely that the blacklist will become smaller as the
   networks associated with the blacklisted DNS recursive resolvers are
   able to meaningfully reduce IPv6-related impairments to some
   acceptable level, though it is possible that some networks may never
   achieve that.  DNS Blacklisting is also likely less labor intensive

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   for a domain than performing DNS Resolver Whitelisting on a manual
   basis.  This is simply because the domain would presumably be focused
   on a smaller number of DNS recursive resolvers with well-known
   IPv6-related problems.

   It is also worth noting that the email industry has a long experience
   with blacklists and, very generally speaking, blacklists tend to be
   effective and well received when it is easy to discover if an IP
   address is on a blacklist, if there is a transparent and easily
   understood process for requesting removal from a blacklist, and if
   the decision-making criteria for placing a server on a blacklist are
   transparently disclosed and perceived as fair.  However, in contrast
   to an email blacklist where a blacklisted host cannot send email to a
   domain at all, with DNS Resolver Whitelisting, communications will
   still occur over IPv4 transport.

4.5.  Transitioning Directly to Native Dual Stack

   As an alternative to adopting any of the aforementioned migration
   tactics, domains can choose to transition to native dual stack
   directly by adding native IPv6 capabilities to their network and
   hosts and by publishing AAAA resource records in the DNS for their
   named resources.  Of course, a domain can still control this
   transition gradually, on a name-by-name basis, by adding native IPv6
   to one name at a time, such as mail.example.com first and
   www.example.com later.  So, even a "direct" transition can be
   performed gradually.

   It is then up to end users with IPv6-related impairments to discover
   and fix any applicable impairments.  However, the concerns and risks
   related to traffic volume (Section 2.3) should still be considered
   and managed, since those are not directly related to such
   impairments.  Not all content providers (or other domains) may face
   the challenges detailed herein or face them to the same degree, since
   the user base of each domain, traffic sources, traffic volumes, and
   other factors obviously vary between domains.

   For example, while some content providers have implemented DNS
   Resolver Whitelisting (one migration tactic), others have run IPv6
   experiments whereby they added AAAA resource records and observed and
   measured errors, and then decided not to implement DNS Resolver
   Whitelisting [Heise].  A more widespread example of such an
   experiment was World IPv6 Day [W6D], sponsored by the Internet
   Society, on June 8, 2011.  This was a unique opportunity for hundreds
   of domains to add AAAA resource records to the DNS without using DNS
   Resolver Whitelisting, all at the same time.  Some of the
   participating domains chose to leave AAAA resource records in place
   following the experiment based on their experiences.

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   Content providers can run their own independent experiments in the
   future, adding AAAA resource records for a brief period of time
   (minutes, hours, or days), and then analyzing any impacts or effects
   on traffic and the experience of end users.  They can also simply
   turn on IPv6 for their domain, which may be easier when the
   transition does not involve a high-service-level domain.

5.  Potential Implementation Phases

   The usefulness of each tactic in Section 4, and the associated pros
   and cons associated with each tactic, are relative to each potential
   implementer and will therefore vary from one implementer to another.
   As a result, it is not possible to say that the potential phases
   below make sense for every implementer.  This also means that the
   duration of each phase will vary between implementers, and even that
   different implementers may skip some of these phases entirely.
   Finally, the tactics listed in Section 4 are by no means exclusive.

5.1.  No Access to IPv6 Content

   In this phase, a site is accessible only via IPv4 transport.  At the
   time of this writing, the majority of content on the Internet is in
   this state and is not accessible natively over IPv6.

5.2.  Using IPv6-Specific Names

   One possible first step for a domain is to gain experience using a
   specialized new FQDN, such as ipv6.example.com or
   www.ipv6.example.com, as explained in Section 4.2.

5.3.  Deploying DNS Resolver Whitelisting Using Manual Processes

   As noted in Section 4.3, a domain could begin using DNS Resolver
   Whitelisting as a way to incrementally enable IPv6 access to content.
   This tactic may be especially interesting to high-service-level

5.4.  Deploying DNS Resolver Whitelisting Using Automated Processes

   For a domain that decides to undertake DNS Resolver Whitelisting on a
   manual basis, the domain may subsequently move to perform DNS
   Resolver Whitelisting on an automated basis.  This is explained in
   Section 4.3, and can significantly ease any operational burdens
   related to a manually maintained whitelist.

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5.5.  Turning Off DNS Resolver Whitelisting

   Domains that choose to implement DNS Resolver Whitelisting generally
   consider it to be a temporary measure.  Many implementers have
   announced that they plan to permanently turn off DNS Resolver
   Whitelisting beginning on the date of the World IPv6 Launch, on
   June 6, 2012 [World-IPv6-Launch].  For any implementers that do not
   turn off DNS Resolver Whitelisting at that time, it may be unclear
   how each and every one will judge the point in time that network
   conditions have changed sufficiently to justify turning off DNS
   Resolver Whitelisting.  That being said, it is clear that the extent
   of IPv6 deployment to end users in networks, the state of IPv6-
   related impairment, and the maturity of IPv6 operations are all
   important factors.  Any such implementers may wish to take into
   consideration that, as a practical matter, it will be impossible to
   get to a point where there are no longer any IPv6-related
   impairments; some reasonably small number of hosts will inevitably be
   left behind as end users elect not to upgrade them or because some
   hosts are incapable of being upgraded.

5.6.  Deploying DNS Blacklisting

   Regardless of whether a domain has first implemented DNS Resolver
   Whitelisting or has never done so, DNS Blacklisting, as described in
   Section 4.4, may be of interest.  This may be at the point in time
   when domains wish to make their content widely available over IPv6
   but still wish to protect end users of a few networks with well-known
   IPv6 limitations from having a bad end-user experience.

5.7.  Fully Dual-Stack Content

   A domain can arrive at this phase by either following the use of a
   previous IPv6 migration tactic or going directly to this point, as
   noted in Section 4.5.  In this phase, the site's content has been
   made natively accessible via both IPv4 and IPv6 for all end users on
   the Internet, or at least without the use of any other IPv6 migration

6.  Other Considerations

6.1.  Security Considerations

   If DNS Resolver Whitelisting is adopted, as noted in Section 4.3,
   then organizations that apply DNS Resolver Whitelisting policies in
   their authoritative servers should have procedures and systems that
   do not allow unauthorized parties to modify the whitelist (or
   blacklist), just as all configuration settings for name servers
   should be protected by appropriate procedures and systems.  Such

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   unauthorized additions or removals from the whitelist (or blacklist)
   can be damaging, causing content providers and/or network operators
   to incur support costs resulting from end-user and/or customer
   contacts, as well as causing potential dramatic and disruptive swings
   in traffic from IPv6 to IPv4 or vice versa.

   DNS Security Extensions (DNSSEC) as defined in [RFC4033], [RFC4034],
   and [RFC4035] use cryptographic digital signatures to provide origin
   authentication and integrity assurance for DNS data.  This is done by
   creating signatures for DNS data on a Security-Aware Authoritative
   Name Server that can be used by Security-Aware Resolvers to verify
   the answers.  Since DNS Resolver Whitelisting is implemented on an
   authoritative DNS server, which provides different answers, depending
   upon which DNS resolver has sent a query, the DNSSEC chain of trust
   is not altered.  So, even though an authoritative DNS server will
   selectively return AAAA resource records or a non-existence response,
   both types of responses will be signed and will validate.  In
   practical terms, this means that two separate views or zones are
   used, each of which is signed, so that whether or not particular
   resource records exist, the existence or non-existence of the record
   can still be validated using DNSSEC.  As a result, there should not
   be any negative impact on DNSSEC for those domains that have
   implemented DNSSEC on their Security-Aware Authoritative Name Servers
   and also implemented DNS Resolver Whitelisting.  As for any party
   implementing DNSSEC, such domains should of course ensure that
   resource records are being appropriately and reliably signed and are
   consistent with the response being returned.

   However, network operators that run DNS recursive resolvers should be
   careful not to modify the responses received from authoritative DNS
   servers.  It is possible that some networks may attempt to do so in
   order to prevent AAAA record responses from going to end-user hosts,
   due to some IPv6-related impairment or other lack of IPv6 readiness
   within that network.  But when a network operates a Security-Aware
   Resolver, modifying or suppressing AAAA resource records for a
   DNSSEC-signed domain could break the chain of trust established with

6.2.  Privacy Considerations

   As noted in Section 4.1, there is a benefit in sharing IPv6-related
   impairment statistics within the Internet community over time.  Any
   statistics that are shared or disclosed publicly should be aggregate
   statistics, such as "the domain example.com has observed an average
   daily impairment rate of 0.05% in September 2011, down from 0.15% in
   January 2011".  They should not include information that can directly

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   or indirectly identify individuals, such as names or email addresses.
   Sharing only aggregate data can help protect end-user privacy and any
   information that may be proprietary to a domain.

   In addition, there are often methods to detect IPv6-related
   impairments for a specific end user, such as running an IPv6 test
   when an end user visits the website of a particular domain.  Should a
   domain then choose to automatically communicate the facts of an
   impairment to an affected user, there are likely no direct privacy
   considerations.  However, if the domain then decides to share
   information concerning that particular end user with that user's
   network operator or another third party, then the domain may wish to
   consider advising the end user of this and seeking to obtain the end-
   user's consent to share such information.

   Appropriate guidelines for any information-sharing likely varies by
   country and/or legal jurisdiction.  Domains should consider any
   potential privacy issues when considering what information can be
   shared.  If a domain does publish or share detailed impairment
   statistics, it would be well advised to avoid identifying individual
   hosts or users.

   Finally, if a domain chooses to contact end users directly concerning
   their IPv6 impairments, that domain should ensure that such
   communication is permissible under any applicable privacy policies of
   the domain or its websites.

6.3.  Considerations with Poor IPv4 and Good IPv6 Transport

   There are situations where the differing quality of the IPv4 and IPv6
   connectivity of an end user could cause complications in accessing
   content when a domain is using an IPv6 migration tactic.  While today
   most end users' IPv4 connectivity is typically superior to IPv6
   connectivity (if such connectivity exists at all), there could be
   implications when the reverse is true and an end user has markedly
   superior IPv6 connectivity as compared to IPv4.  This is not a
   theoretical situation; it has been observed by at least one major
   content provider.

   For example, in one possible scenario, a user is issued IPv6
   addresses by their ISP and has a home network and devices or
   operating systems that fully support native IPv6.  As a result, this
   theoretical user has very good IPv6 connectivity.  However, this end-
   user's ISP has exhausted their available pool of unique IPv4
   addresses, and uses NAT in order to share IPv4 addresses among end
   users.  So, for IPv4 content, the end user must send their IPv4
   traffic through some additional network element (e.g., large-scale
   NAT, proxy server, tunnel server).  Use of this additional network

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   element might cause an end user to experience sub-optimal IPv4
   connectivity when certain protocols or applications are used.  This
   user then has good IPv6 connectivity but impaired IPv4 connectivity.
   As a result, the user's poor IPv4 connectivity situation could
   potentially be exacerbated when accessing a domain that is using a
   migration tactic that causes this user to only be able to access
   content over IPv4 transport for whatever reason.

   Should this sort of situation become widespread in the future, a
   domain may wish to take it into account when deciding how and when to
   transition content to IPv6.

7.  Contributors

   The following people made significant textual contributions to this
   document and/or played an important role in the development and
   evolution of this document:

      - John Brzozowski
      - Chris Griffiths
      - Tom Klieber
      - Yiu Lee
      - Rich Woundy

8.  Acknowledgements

   The author and contributors also wish to acknowledge the assistance
   of the following individuals or groups.  Some of these people
   provided helpful and important guidance in the development of this
   document and/or in the development of the concepts covered in this
   document.  Other people assisted by performing a detailed review of
   this document and then providing feedback and constructive criticism
   for revisions to this document, or engaged in a healthy debate over
   the subject of the document.  All of this was helpful, and therefore
   the following individuals merit acknowledgement:

      - Bernard Aboba
      - Mark Andrews
      - Jari Arkko
      - Fred Baker
      - Ron Bonica
      - Frank Bulk
      - Brian Carpenter
      - Lorenzo Colitti
      - Alissa Cooper
      - Dave Crocker
      - Ralph Droms
      - Wesley Eddy

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      - J.D. Falk
      - Adrian Farrel
      - Stephen Farrell
      - Tony Finch
      - Karsten Fleischhauer
      - Igor Gashinsky
      - Wesley George
      - Philip Homburg
      - Jerry Huang
      - Ray Hunter
      - Joel Jaeggli
      - Erik Kline
      - Suresh Krishnan
      - Victor Kuarsingh
      - Marc Lampo
      - Donn Lee
      - John Leslie
      - John Mann
      - Danny McPherson
      - Milo Medin
      - Martin Millnert
      - Russ Mundy
      - Thomas Narten
      - Pekka Savola
      - Robert Sparks
      - Barbara Stark
      - Joe Touch
      - Hannes Tschofenig
      - Tina Tsou
      - Members of the Broadband Internet Technical Advisory Group

9.  References

9.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1794]  Brisco, T., "DNS Support for Load Balancing", RFC 1794,
              April 1995.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC1958]  Carpenter, B., Ed., "Architectural Principles of the
              Internet", RFC 1958, June 1996.

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RFC 6589              Transitioning Content to IPv6           April 2012

   [RFC2775]  Carpenter, B., "Internet Transparency", RFC 2775,
              February 2000.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC2956]  Kaat, M., "Overview of 1999 IAB Network Layer Workshop",
              RFC 2956, October 2000.

   [RFC3234]  Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
              Issues", RFC 3234, February 2002.

   [RFC3724]  Kempf, J., Ed., Austein, R., Ed., and IAB, "The Rise of
              the Middle and the Future of End-to-End: Reflections on
              the Evolution of the Internet Architecture", RFC 3724,
              March 2004.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

9.2.  Informative References

   [Heise]    Heise.de, "The Big IPv6 Experiment", Heise.de
              Website http://www.h-online.com, January 2011,

              Gashinsky, I., "IPv6 & recursive resolvers: How do we make
              the transition less painful?", IETF 77 DNS Operations
              Working Group, March 2010,

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RFC 6589              Transitioning Content to IPv6           April 2012

              Anderson, T., "Measuring and Combating IPv6 Brokenness",
              Reseaux IP Europeens (RIPE) 61st Meeting, November 2010,

              Colitti, L., Gunderson, S., Kline, E., and T. Refice,
              "Evaluating IPv6 adoption in the Internet", Proceedings of
              the 11th International Conference on Passive and Active
              Measurement (PAM 2010), Springer, Lecture Notes in
              Computer Science, 2010, Volume 6032, Passive and Active
              Measurement, Pages 141-150.

              Marsan, C., "Google, Microsoft, Netflix in talks to create
              shared list of IPv6 users", Network World, March 2010,

              Marsan, C., "Yahoo proposes 'really ugly hack' to DNS",
              Network World, March 2010, <http://www.networkworld.com/

   [RFC6343]  Carpenter, B., "Advisory Guidelines for 6to4 Deployment",
              RFC 6343, August 2011.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, April 2012.

              Blumenthal, M. and D. Clark, "Rethinking the Design of the
              Internet: The End-to-End Arguments vs. the Brave New
              World", ACM Transactions on Internet Technology, Volume 1,
              Number 1, Pages 70-109, August 2001,
              Rethinking the design of the internet2001.pdf>.

   [Tussle]   Clark, D., Wroclawski, J., Sollins, K., and R. Braden,
              "Tussle in Cyberspace: Defining Tomorrow's Internet",
              Proceedings of ACM Sigcomm 2002, August 2002,

   [W6D]      The Internet Society, "World IPv6 Day - June 8, 2011",
              Internet Society Website http://www.isoc.org,
              January 2011, <http://isoc.org/wp/worldipv6day/>.

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RFC 6589              Transitioning Content to IPv6           April 2012

              Brzozowski, J., Griffiths, C., Klieber, T., Lee, Y.,
              Livingood, J., and R. Woundy, "IPv6 DNS Whitelisting -
              Could It Hinder IPv6 Adoption?", ISOC (Internet Society)
              IPv6 Deployment Workshop, April 2010,

   [WL-Ops]   Kline, E., "IPv6 Whitelist Operations", Google IPv6
              Implementors Conference, June 2010,

              Ager, B., Smaragdakis, G., Muhlbauer, W., and S. Uhlig,
              "Comparing DNS Resolvers in the Wild", ACM Sigcomm
              Internet Measurement Conference 2010, November 2010,

              The Internet Society, "World IPv6 Launch Website",
              June 2012, <http://www.worldipv6launch.org/>.

Author's Address

   Jason Livingood
   Comcast Cable Communications
   One Comcast Center
   1701 John F. Kennedy Boulevard
   Philadelphia, PA  19103

   EMail: jason_livingood@cable.comcast.com
   URI:   http://www.comcast.com

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