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
Abstract
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
http://www.rfc-editor.org/info/rfc6589.
Livingood Informational [Page 1]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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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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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
domain.
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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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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
stack.
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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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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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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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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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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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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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
actions.
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 4.3.1.1 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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4.3.1.1. 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
exist.
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
Balancing
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
domains.
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
tactic.
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
DNSSEC.
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
Livingood Informational [Page 23]RFC 6589 Transitioning Content to IPv6 April 2012
- 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
(BITAG)
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.
Livingood Informational [Page 24]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,
<http://www.h-online.com/features/
The-big-IPv6-experiment-1165042.html>.
[IETF-77-DNSOP]
Gashinsky, I., "IPv6 & recursive resolvers: How do we make
the transition less painful?", IETF 77 DNS Operations
Working Group, March 2010,
<http://www.ietf.org/proceedings/77/slides/dnsop-7.pdf>.
Livingood Informational [Page 25]RFC 6589 Transitioning Content to IPv6 April 2012
[IPv6-Brokenness]
Anderson, T., "Measuring and Combating IPv6 Brokenness",
Reseaux IP Europeens (RIPE) 61st Meeting, November 2010,
<http://ripe61.ripe.net/presentations/162-ripe61.pdf>.
[IPv6-Growth]
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.
[NW-Article-DNS-WL]
Marsan, C., "Google, Microsoft, Netflix in talks to create
shared list of IPv6 users", Network World, March 2010,
<http://www.networkworld.com/news/2010/
032610-dns-ipv6-whitelist.html>.
[NW-Article-DNSOP]
Marsan, C., "Yahoo proposes 'really ugly hack' to DNS",
Network World, March 2010, <http://www.networkworld.com/
news/2010/032610-yahoo-dns.html>.
[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.
[Rethinking]
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,
<http://groups.csail.mit.edu/ana/Publications/PubPDFs/
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,
<http://groups.csail.mit.edu/ana/Publications/PubPDFs/
Tussle2002.pdf>.
[W6D] The Internet Society, "World IPv6 Day - June 8, 2011",
Internet Society Website http://www.isoc.org,
January 2011, <http://isoc.org/wp/worldipv6day/>.
Livingood Informational [Page 26]RFC 6589 Transitioning Content to IPv6 April 2012
[WL-Concerns]
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,
<http://www.comcast6.net/
IPv6_DNS_Whitelisting_Concerns_20100416.pdf>.
[WL-Ops] Kline, E., "IPv6 Whitelist Operations", Google IPv6
Implementors Conference, June 2010,
<http://sites.google.com/site/ipv6implementors/2010/
agenda/IPv6_Whitelist_Operations.pdf>.
[Wild-Resolvers]
Ager, B., Smaragdakis, G., Muhlbauer, W., and S. Uhlig,
"Comparing DNS Resolvers in the Wild", ACM Sigcomm
Internet Measurement Conference 2010, November 2010,
<http://conferences.sigcomm.org/imc/2010/papers/p15.pdf>.
[World-IPv6-Launch]
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
US
EMail: jason_livingood@cable.comcast.com
URI: http://www.comcast.com
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