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<pre>Internet Engineering Task Force (IETF) D. Wing
Request for Comments: 6555 A. Yourtchenko
Category: Standards Track Cisco
ISSN: 2070-1721 April 2012
<span class="h1">Happy Eyeballs: Success with Dual-Stack Hosts</span>
Abstract
When a server's IPv4 path and protocol are working, but the server's
IPv6 path and protocol are not working, a dual-stack client
application experiences significant connection delay compared to an
IPv4-only client. This is undesirable because it causes the dual-
stack client to have a worse user experience. This document
specifies requirements for algorithms that reduce this user-visible
delay and provides an algorithm.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="http://www.rfc-editor.org/info/rfc6555">http://www.rfc-editor.org/info/rfc6555</a>.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) 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.
<span class="grey">Wing & Yourtchenko Standards Track [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc6555">RFC 6555</a> Happy Eyeballs Dual Stack April 2012</span>
Table of Contents
<a href="#section-1">1</a>. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-1.1">1.1</a>. Additional Network and Host Traffic . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-2">2</a>. Notational Conventions . . . . . . . . . . . . . . . . . . . . <a href="#page-3">3</a>
<a href="#section-3">3</a>. Problem Statement . . . . . . . . . . . . . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-3.1">3.1</a>. Hostnames . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-4">4</a>
<a href="#section-3.2">3.2</a>. Delay When IPv6 Is Not Accessible . . . . . . . . . . . . <a href="#page-5">5</a>
<a href="#section-4">4</a>. Algorithm Requirements . . . . . . . . . . . . . . . . . . . . <a href="#page-6">6</a>
<a href="#section-4.1">4.1</a>. Delay IPv4 . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-7">7</a>
<a href="#section-4.2">4.2</a>. Stateful Behavior When IPv6 Fails . . . . . . . . . . . . <a href="#page-8">8</a>
<a href="#section-4.3">4.3</a>. Reset on Network (Re-)Initialization . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-4.4">4.4</a>. Abandon Non-Winning Connections . . . . . . . . . . . . . <a href="#page-9">9</a>
<a href="#section-5">5</a>. Additional Considerations . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-5.1">5.1</a>. Determining Address Type . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-5.2">5.2</a>. Debugging and Troubleshooting . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-5.3">5.3</a>. Three or More Interfaces . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-5.4">5.4</a>. A and AAAA Resource Records . . . . . . . . . . . . . . . <a href="#page-10">10</a>
<a href="#section-5.5">5.5</a>. Connection Timeout . . . . . . . . . . . . . . . . . . . . <a href="#page-11">11</a>
<a href="#section-5.6">5.6</a>. Interaction with Same-Origin Policy . . . . . . . . . . . <a href="#page-11">11</a>
<a href="#section-5.7">5.7</a>. Implementation Strategies . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
<a href="#section-6">6</a>. Example Algorithm . . . . . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
<a href="#section-7">7</a>. Security Considerations . . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
<a href="#section-8">8</a>. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
<a href="#section-9">9</a>. References . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
<a href="#section-9.1">9.1</a>. Normative References . . . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
<a href="#section-9.2">9.2</a>. Informative References . . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
<span class="grey">Wing & Yourtchenko Standards Track [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc6555">RFC 6555</a> Happy Eyeballs Dual Stack April 2012</span>
<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
In order to use applications over IPv6, it is necessary that users
enjoy nearly identical performance as compared to IPv4. A
combination of today's applications, IPv6 tunneling, IPv6 service
providers, and some of today's content providers all cause the user
experience to suffer (<a href="#section-3">Section 3</a>). For IPv6, a content provider may
ensure a positive user experience by using a DNS white list of IPv6
service providers who peer directly with them (e.g., [<a href="#ref-WHITELIST" title=""Google over IPv6"">WHITELIST</a>]).
However, this does not scale well (to the number of DNS servers
worldwide or the number of content providers worldwide) and does
react to intermittent network path outages.
Instead, applications reduce connection setup delays themselves, by
more aggressively making connections on IPv6 and IPv4. There are a
variety of algorithms that can be envisioned. This document
specifies requirements for any such algorithm, with the goals that
the network and servers not be inordinately harmed with a simple
doubling of traffic on IPv6 and IPv4 and the host's address
preference be honored (e.g., [<a href="./rfc3484" title=""Default Address Selection for Internet Protocol version 6 (IPv6)"">RFC3484</a>]).
<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>. Additional Network and Host Traffic</span>
Additional network traffic and additional server load is created due
to the recommendations in this document, especially when connections
to the preferred address family (usually IPv6) are not completing
quickly.
The procedures described in this document retain a quality user
experience while transitioning from IPv4-only to dual stack, while
still giving IPv6 a slight preference over IPv4 (in order to remove
load from IPv4 networks and, most importantly, to reduce the load on
IPv4 network address translators). The user experience is improved
to the slight detriment of the network, DNS server, and server that
are serving the user.
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Notational Conventions</span>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].
<span class="grey">Wing & Yourtchenko Standards Track [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc6555">RFC 6555</a> Happy Eyeballs Dual Stack April 2012</span>
<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Problem Statement</span>
The basis of the IPv6/IPv4 selection problem was first described in
1994 in [<a href="./rfc1671" title=""IPng White Paper on Transition and Other Considerations"">RFC1671</a>]:
The dual-stack code may get two addresses back from DNS; which
does it use? During the many years of transition the Internet
will contain black holes. For example, somewhere on the way from
IPng host A to IPng host B there will sometimes (unpredictably) be
IPv4-only routers which discard IPng packets. Also, the state of
the DNS does not necessarily correspond to reality. A host for
which DNS claims to know an IPng address may in fact not be
running IPng at a particular moment; thus an IPng packet to that
host will be discarded on delivery. Knowing that a host has both
IPv4 and IPng addresses gives no information about black holes. A
solution to this must be proposed and it must not depend on
manually maintained information. (If this is not solved, the
dual-stack approach is no better than the packet translation
approach.)
As discussed in more detail in <a href="#section-3.1">Section 3.1</a>, it is important that the
same hostname be used for IPv4 and IPv6.
As discussed in more detail in <a href="#section-3.2">Section 3.2</a>, IPv6 connectivity is
broken to specific prefixes or specific hosts or is slower than
native IPv4 connectivity.
The mechanism described in this document is directly applicable to
connection-oriented transports (e.g., TCP, SCTP), which is the scope
of this document. For connectionless transport protocols (e.g.,
UDP), a similar mechanism can be used if the application has request/
response semantics (e.g., as done by Interactive Connectivity
Establishment (ICE) to select a working IPv6 or IPv4 media path
[<a href="./rfc6157" title=""IPv6 Transition in the Session Initiation Protocol (SIP)"">RFC6157</a>]).
<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. Hostnames</span>
Hostnames are often used between users to exchange pointers to
content -- such as on social networks, email, instant messaging, or
other systems. Using separate namespaces (e.g., "ipv6.example.com"),
which are only accessible with certain client technology (e.g., an
IPv6 client) and dependencies (e.g., a working IPv6 path), causes
namespace fragmentation and reduces the ability for users to share
hostnames. It also complicates printed material that includes the
hostname.
The algorithm described in this document allows production hostnames
to avoid these problematic references to IPv4 or IPv6.
<span class="grey">Wing & Yourtchenko Standards Track [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc6555">RFC 6555</a> Happy Eyeballs Dual Stack April 2012</span>
<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. Delay When IPv6 Is Not Accessible</span>
When IPv6 connectivity is impaired, today's IPv6-capable applications
(e.g., web browsers, email clients, instant messaging clients) incur
many seconds of delay before falling back to IPv4. This delays
overall application operation, including harming the user's
experience with IPv6, which will slow the acceptance of IPv6, because
IPv6 is frequently disabled in its entirety on the end systems to
improve the user experience.
Reasons for such failure include no connection to the IPv6 Internet,
broken 6to4 or Teredo tunnels, and broken IPv6 peering. The
following diagram shows this behavior.
The algorithm described in this document allows clients to connect to
servers without significant delay, even if a path or the server is
slow or down.
DNS Server Client Server
| | |
1. |<--www.example.com A?-----| |
2. |<--www.example.com AAAA?--| |
3. |---192.0.2.1------------->| |
4. |---2001:db8::1----------->| |
5. | | |
6. | |==TCP SYN, IPv6===>X |
7. | |==TCP SYN, IPv6===>X |
8. | |==TCP SYN, IPv6===>X |
9. | | |
10. | |--TCP SYN, IPv4------->|
11. | |<-TCP SYN+ACK, IPv4----|
12. | |--TCP ACK, IPv4------->|
Figure 1: Existing Behavior Message Flow
The client obtains the IPv4 and IPv6 records for the server (1-4).
The client attempts to connect using IPv6 to the server, but the IPv6
path is broken (6-8), which consumes several seconds of time.
Eventually, the client attempts to connect using IPv4 (10), which
succeeds.
Delays experienced by users of various browser and operating system
combinations have been studied [<a href="#ref-Experiences" title=""Experiences of host behavior in broken IPv6 networks"">Experiences</a>].
<span class="grey">Wing & Yourtchenko Standards Track [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc6555">RFC 6555</a> Happy Eyeballs Dual Stack April 2012</span>
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Algorithm Requirements</span>
A "Happy Eyeballs" algorithm has two primary goals:
1. Provides fast connection for users, by quickly attempting to
connect using IPv6 and (if that connection attempt is not quickly
successful) to connect using IPv4.
2. Avoids thrashing the network, by not (always) making simultaneous
connection attempts on both IPv6 and IPv4.
The basic idea is depicted in the following diagram:
DNS Server Client Server
| | |
1. |<--www.example.com A?-----| |
2. |<--www.example.com AAAA?--| |
3. |---192.0.2.1------------->| |
4. |---2001:db8::1----------->| |
5. | | |
6. | |==TCP SYN, IPv6===>X |
7. | |--TCP SYN, IPv4------->|
8. | |<-TCP SYN+ACK, IPv4----|
9. | |--TCP ACK, IPv4------->|
10. | |==TCP SYN, IPv6===>X |
Figure 2: Happy Eyeballs Flow 1, IPv6 Broken
In the diagram above, the client sends two TCP SYNs at the same time
over IPv6 (6) and IPv4 (7). In the diagram, the IPv6 path is broken
but has little impact to the user because there is no long delay
before using IPv4. The IPv6 path is retried until the application
gives up (10).
After performing the above procedure, the client learns whether
connections to the host's IPv6 or IPv4 address were successful. The
client MUST cache information regarding the outcome of each
connection attempt, and it uses that information to avoid thrashing
the network with subsequent attempts. In the example above, the
cache indicates that the IPv6 connection attempt failed, and
therefore the system will prefer IPv4 instead. Cache entries should
be flushed when their age exceeds a system-defined maximum on the
order of 10 minutes.
<span class="grey">Wing & Yourtchenko Standards Track [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
<span class="grey"><a href="./rfc6555">RFC 6555</a> Happy Eyeballs Dual Stack April 2012</span>
DNS Server Client Server
| | |
1. |<--www.example.com A?-----| |
2. |<--www.example.com AAAA?--| |
3. |---192.0.2.1------------->| |
4. |---2001:db8::1----------->| |
5. | | |
6. | |==TCP SYN, IPv6=======>|
7. | |--TCP SYN, IPv4------->|
8. | |<=TCP SYN+ACK, IPv6====|
9. | |<-TCP SYN+ACK, IPv4----|
10. | |==TCP ACK, IPv6=======>|
11. | |--TCP ACK, IPv4------->|
12. | |--TCP RST, IPv4------->|
Figure 3: Happy Eyeballs Flow 2, IPv6 Working
The diagram above shows a case where both IPv6 and IPv4 are working,
and IPv4 is abandoned (12).
Any Happy Eyeballs algorithm will persist in products for as long as
the client host is dual-stacked, which will persist as long as there
are IPv4-only servers on the Internet -- the so-called "long tail".
Over time, as most content is available via IPv6, the amount of IPv4
traffic will decrease. This means that the IPv4 infrastructure will,
over time, be sized to accommodate that decreased (and decreasing)
amount of traffic. It is critical that a Happy Eyeballs algorithm
not cause a surge of unnecessary traffic on that IPv4 infrastructure.
To meet that goal, compliant Happy Eyeballs algorithms must adhere to
the requirements in this section.
<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>. Delay IPv4</span>
The transition to IPv6 is likely to produce a mix of different hosts
within a subnetwork -- hosts that are IPv4-only, hosts that are IPv6-
only (e.g., sensors), and dual-stack hosts. This mix of hosts will
exist both within an administrative domain (a single home,
enterprise, hotel, or coffee shop) and between administrative
domains. For example, a single home might have an IPv4-only
television in one room and a dual-stack television in another room.
As another example, another subscriber might have hosts that are all
capable of dual-stack operation.
Due to IPv4 exhaustion, it is likely that a subscriber's hosts (both
IPv4-only hosts and dual-stack hosts) will be sharing an IPv4 address
with other subscribers. The dual-stack hosts have an advantage: they
can utilize IPv6 or IPv4, which means they can utilize the technique
described in this document. The IPv4-only hosts have a disadvantage:
<span class="grey">Wing & Yourtchenko Standards Track [Page 7]</span></pre>
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they can only utilize IPv4. If all hosts (dual-stack and IPv4-only)
are using IPv4, there is additional contention for the shared IPv4
address. The IPv4-only hosts cannot avoid that contention (as they
can only use IPv4), while the dual-stack hosts can avoid it by using
IPv6.
As dual-stack hosts proliferate and content becomes available over
IPv6, there will be proportionally less IPv4 traffic. This is true
especially for dual-stack hosts that do not implement Happy Eyeballs,
because those dual-stack hosts have a very strong preference to use
IPv6 (with timeouts in the tens of seconds before they will attempt
to use IPv4).
When deploying IPv6, both content providers and Internet Service
Providers (who supply mechanisms for IPv4 address sharing such as
Carrier-Grade NAT (CGN)) will want to reduce their investment in IPv4
equipment -- load-balancers, peering links, and address sharing
devices. If a Happy Eyeballs implementation treats IPv6 and IPv4
equally by connecting to whichever address family is fastest, it will
contribute to load on IPv4. This load impacts IPv4-only devices (by
increasing contention of IPv4 address sharing and increasing load on
IPv4 load-balancers). Because of this, ISPs and content providers
will find it impossible to reduce their investment in IPv4 equipment.
This means that costs to migrate to IPv6 are increased because the
investment in IPv4 cannot be reduced. Furthermore, using only a
metric that measures the connection speed ignores the benefits that
IPv6 brings when compared with IPv4 address sharing, such as improved
geo-location [<a href="./rfc6269" title=""Issues with IP Address Sharing"">RFC6269</a>] and the lack of fate-sharing due to traversing
a large translator.
Thus, to avoid harming IPv4-only hosts, implementations MUST prefer
the first IP address family returned by the host's address preference
policy, unless implementing a stateful algorithm described in
<a href="#section-4.2">Section 4.2</a>. This usually means giving preference to IPv6 over IPv4,
although that preference can be overridden by user configuration or
by network configuration [<a href="#ref-ADDR-SELECT" title=""Distributing Address Selection Policy using DHCPv6"">ADDR-SELECT</a>]. If the host's policy is
unknown or not attainable, implementations MUST prefer IPv6 over
IPv4.
<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>. Stateful Behavior When IPv6 Fails</span>
Some Happy Eyeballs algorithms are stateful -- that is, the algorithm
will remember that IPv6 always fails, or that IPv6 to certain
prefixes always fails, and so on. This section describes such
algorithms. Stateless algorithms, which do not remember the success/
failure of previous connections, are not discussed in this section.
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After making a connection attempt on the preferred address family
(e.g., IPv6) and failing to establish a connection within a certain
time period (see <a href="#section-5.5">Section 5.5</a>), a Happy Eyeballs implementation will
decide to initiate a second connection attempt using the same address
family or the other address family.
Such an implementation MAY make subsequent connection attempts (to
the same host or to other hosts) on the successful address family
(e.g., IPv4). So long as new connections are being attempted by the
host, such an implementation MUST occasionally make connection
attempts using the host's preferred address family, as it may have
become functional again, and it SHOULD do so every 10 minutes. The
10-minute delay before retrying a failed address family avoids the
simple doubling of connection attempts on both IPv6 and IPv4.
Implementation note: this can be achieved by flushing Happy Eyeballs
state every 10 minutes, which does not significantly harm the
application's subsequent connection setup time. If connections using
the preferred address family are again successful, the preferred
address family SHOULD be used for subsequent connections. Because
this implementation is stateful, it MAY track connection success (or
failure) based on IPv6 or IPv4 prefix (e.g., connections to the same
prefix assigned to the interface are successful whereas connections
to other prefixes are failing).
<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>. Reset on Network (Re-)Initialization</span>
Because every network has different characteristics (e.g., working or
broken IPv6 or IPv4 connectivity), a Happy Eyeballs algorithm SHOULD
re-initialize when the interface is connected to a new network.
Interfaces can determine network (re-)initialization by a variety of
mechanisms (e.g., Detecting Network Attachment in IPv4 (DNAv4)
[<a href="./rfc4436" title=""Detecting Network Attachment in IPv4 (DNAv4)"">RFC4436</a>], DNAv6 [<a href="./rfc6059" title=""Simple Procedures for Detecting Network Attachment in IPv6"">RFC6059</a>]).
If the client application is a web browser, see also <a href="#section-5.6">Section 5.6</a>.
<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>. Abandon Non-Winning Connections</span>
It is RECOMMENDED that the non-winning connections be abandoned, even
though they could -- in some cases -- be put to reasonable use.
Justification: This reduces the load on the server (file
descriptors, TCP control blocks) and stateful middleboxes (NAT and
firewalls). Also, if the abandoned connection is IPv4, this
reduces IPv4 address sharing contention.
HTTP: The design of some sites can break because of HTTP cookies
that incorporate the client's IP address and require all
connections be from the same IP address. If some connections from
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the same client are arriving from different IP addresses (or
worse, different IP address families), such applications will
break. Additionally, for HTTP, using the non-winning connection
can interfere with the browser's same-origin policy (see
<a href="#section-5.6">Section 5.6</a>).
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Additional Considerations</span>
This section discusses considerations related to Happy Eyeballs.
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. Determining Address Type</span>
For some transitional technologies such as a dual-stack host, it is
easy for the application to recognize the native IPv6 address
(learned via a AAAA query) and the native IPv4 address (learned via
an A query). While IPv6/IPv4 translation makes that difficult, IPv6/
IPv4 translators do not need to be deployed on networks with dual-
stack clients because dual-stack clients can use their native IP
address family.
<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. Debugging and Troubleshooting</span>
This mechanism is aimed at ensuring a reliable user experience
regardless of connectivity problems affecting any single transport.
However, this naturally means that applications employing these
techniques are by default less useful for diagnosing issues with a
particular address family. To assist in that regard, the
implementations MAY also provide a mechanism to disable their Happy
Eyeballs behavior via a user setting, and to provide data useful for
debugging (e.g., a log or way to review current preferences).
<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>. Three or More Interfaces</span>
A dual-stack host normally has two logical interfaces: an IPv6
interface and an IPv4 interface. However, a dual-stack host might
have more than two logical interfaces because of a VPN (where a third
interface is the tunnel address, often assigned by the remote
corporate network), because of multiple physical interfaces such as
wired and wireless Ethernet, because the host belongs to multiple
VLANs, or other reasons. The interaction of Happy Eyeballs with more
than two logical interfaces is for further study.
<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>. A and AAAA Resource Records</span>
It is possible that a DNS query for an A or AAAA resource record will
return more than one A or AAAA address. When this occurs, it is
RECOMMENDED that a Happy Eyeballs implementation order the responses
following the host's address preference policy and then try the first
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address. If that fails after a certain time (see <a href="#section-5.5">Section 5.5</a>), the
next address SHOULD be the IPv4 address.
If that fails to connect after a certain time (see <a href="#section-5.5">Section 5.5</a>), a
Happy Eyeballs implementation SHOULD try the other addresses
returned; the order of these connection attempts is not important.
On the Internet today, servers commonly have multiple A records to
provide load-balancing across their servers. This same technique
would be useful for AAAA records, as well. However, if multiple AAAA
records are returned to a client that is not using Happy Eyeballs and
that has broken IPv6 connectivity, it will further increase the delay
to fall back to IPv4. Thus, web site operators with native IPv6
connectivity SHOULD NOT offer multiple AAAA records. If Happy
Eyeballs is widely deployed in the future, this recommendation might
be revisited.
<span class="h3"><a class="selflink" id="section-5.5" href="#section-5.5">5.5</a>. Connection Timeout</span>
The primary purpose of Happy Eyeballs is to reduce the wait time for
a dual-stack connection to complete, especially when the IPv6 path is
broken and IPv6 is preferred. Aggressive timeouts (on the order of
tens of milliseconds) achieve this goal, but at the cost of network
traffic. This network traffic may be billable on certain networks,
will create state on some middleboxes (e.g., firewalls, intrusion
detection systems, NATs), and will consume ports if IPv4 addresses
are shared. For these reasons, it is RECOMMENDED that connection
attempts be paced to give connections a chance to complete. It is
RECOMMENDED that connection attempts be paced 150-250 ms apart to
balance human factors against network load. Stateful algorithms are
expected to be more aggressive (that is, make connection attempts
closer together), as stateful algorithms maintain an estimate of the
expected connection completion time.
<span class="h3"><a class="selflink" id="section-5.6" href="#section-5.6">5.6</a>. Interaction with Same-Origin Policy</span>
Web browsers implement a same-origin policy [<a href="./rfc6454" title=""The Web Origin Concept"">RFC6454</a>] that causes
subsequent connections to the same hostname to go to the same IPv4
(or IPv6) address as the previous successful connection. This is
done to prevent certain types of attacks.
The same-origin policy harms user-visible responsiveness if a new
connection fails (e.g., due to a transient event such as router
failure or load-balancer failure). While it is tempting to use Happy
Eyeballs to maintain responsiveness, web browsers MUST NOT change
their same-origin policy because of Happy Eyeballs, as that would
create an additional security exposure.
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<span class="h3"><a class="selflink" id="section-5.7" href="#section-5.7">5.7</a>. Implementation Strategies</span>
The simplest venue for the implementation of Happy Eyeballs is within
the application itself. The algorithm specified in this document is
relatively simple to implement and would require no specific support
from the operating system beyond the commonly available APIs that
provide transport service. It could also be added to applications by
way of a specific Happy Eyeballs API, replacing or augmenting the
transport service APIs.
To improve the IPv6 connectivity experience for legacy applications
(e.g., applications that simply rely on the operating system's
address preference order), operating systems may consider more
sophisticated approaches. These can include changing default address
selection sorting [<a href="./rfc3484" title=""Default Address Selection for Internet Protocol version 6 (IPv6)"">RFC3484</a>] based on configuration received from the
network, or observing connection failures to IPv6 and IPV4
destinations.
<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Example Algorithm</span>
What follows is the algorithm implemented in Google Chrome and
Mozilla Firefox.
1. Call getaddinfo(), which returns a list of IP addresses sorted by
the host's address preference policy.
2. Initiate a connection attempt with the first address in that list
(e.g., IPv6).
3. If that connection does not complete within a short period of
time (Firefox and Chrome use 300 ms), initiate a connection
attempt with the first address belonging to the other address
family (e.g., IPv4).
4. The first connection that is established is used. The other
connection is discarded.
If an algorithm were to cache connection success/failure, the caching
would occur after step 4 determined which connection was successful.
Other example algorithms include [<a href="#ref-Perreault" title=""Happy Eyeballs in Erlang"">Perreault</a>] and [<a href="#ref-Andrews" title=""How to connect to a multi-homed server over TCP"">Andrews</a>].
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. Security Considerations</span>
See Sections <a href="#section-4.4">4.4</a> and <a href="#section-5.6">5.6</a>.
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<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. Acknowledgements</span>
The mechanism described in this paper was inspired by Stuart
Cheshire's discussion at the IAB Plenary at IETF 72, the author's
understanding of Safari's operation with SRV records, ICE [<a href="./rfc5245" title=""Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols"">RFC5245</a>],
the current IPv4/IPv6 behavior of SMTP mail transfer agents, and the
implementation of Happy Eyeballs in Google Chrome and Mozilla
Firefox.
Thanks to Fred Baker, Jeff Kinzli, Christian Kuhtz, and Iljitsch van
Beijnum for fostering the creation of this document.
Thanks to Scott Brim, Rick Jones, Stig Venaas, Erik Kline, Bjoern
Zeeb, Matt Miller, Dave Thaler, Dmitry Anipko, Brian Carpenter, and
David Crocker for their feedback.
Thanks to Javier Ubillos, Simon Perreault, and Mark Andrews for the
active feedback and the experimental work on the independent
practical implementations that they created.
Also the authors would like to thank the following individuals who
participated in various email discussions on this topic: Mohacsi
Janos, Pekka Savola, Ted Lemon, Carlos Martinez-Cagnazzo, Simon
Perreault, Jack Bates, Jeroen Massar, Fred Baker, Javier Ubillos,
Teemu Savolainen, Scott Brim, Erik Kline, Cameron Byrne, Daniel
Roesen, Guillaume Leclanche, Mark Smith, Gert Doering, Martin
Millnert, Tim Durack, and Matthew Palmer.
<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. References</span>
<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>. Normative References</span>
[<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-RFC3484">RFC3484</a>] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", <a href="./rfc3484">RFC 3484</a>, February 2003.
<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>. Informative References</span>
[<a id="ref-ADDR-SELECT">ADDR-SELECT</a>] Matsumoto, A., Fujisaki, T., Kato, J., and T. Chown,
"Distributing Address Selection Policy using DHCPv6",
Work in Progress, February 2012.
[<a id="ref-Andrews">Andrews</a>] Andrews, M., "How to connect to a multi-homed server
over TCP", January 2011, <<a href="http://www.isc.org/community/blog/201101/how-to-connect-to-a-multi-homed-server-over-tcp">http://www.isc.org/community/</a>
<a href="http://www.isc.org/community/blog/201101/how-to-connect-to-a-multi-homed-server-over-tcp">blog/201101/how-to-connect-to-a-multi-homed-server-</a>
<a href="http://www.isc.org/community/blog/201101/how-to-connect-to-a-multi-homed-server-over-tcp">over-tcp</a>>.
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<span class="grey"><a href="./rfc6555">RFC 6555</a> Happy Eyeballs Dual Stack April 2012</span>
[<a id="ref-Experiences">Experiences</a>] Savolainen, T., Miettinen, N., Veikkolainen, S., Chown,
T., and J. Morse, "Experiences of host behavior in
broken IPv6 networks", March 2011,
<<a href="http://www.ietf.org/proceedings/80/slides/v6ops-12.pdf">http://www.ietf.org/proceedings/80/slides/</a>
<a href="http://www.ietf.org/proceedings/80/slides/v6ops-12.pdf">v6ops-12.pdf</a>>.
[<a id="ref-Perreault">Perreault</a>] Perreault, S., "Happy Eyeballs in Erlang", February
2011, <<a href="http://www.viagenie.ca/news/index.html#happy_eyeballs_erlang">http://www.viagenie.ca/news/</a>
<a href="http://www.viagenie.ca/news/index.html#happy_eyeballs_erlang">index.html#happy_eyeballs_erlang</a>>.
[<a id="ref-RFC1671">RFC1671</a>] Carpenter, B., "IPng White Paper on Transition and
Other Considerations", <a href="./rfc1671">RFC 1671</a>, August 1994.
[<a id="ref-RFC4436">RFC4436</a>] Aboba, B., Carlson, J., and S. Cheshire, "Detecting
Network Attachment in IPv4 (DNAv4)", <a href="./rfc4436">RFC 4436</a>, March
2006.
[<a id="ref-RFC5245">RFC5245</a>] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", <a href="./rfc5245">RFC 5245</a>, April
2010.
[<a id="ref-RFC6059">RFC6059</a>] Krishnan, S. and G. Daley, "Simple Procedures for
Detecting Network Attachment in IPv6", <a href="./rfc6059">RFC 6059</a>,
November 2010.
[<a id="ref-RFC6157">RFC6157</a>] Camarillo, G., El Malki, K., and V. Gurbani, "IPv6
Transition in the Session Initiation Protocol (SIP)",
<a href="./rfc6157">RFC 6157</a>, April 2011.
[<a id="ref-RFC6269">RFC6269</a>] Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
Roberts, "Issues with IP Address Sharing", <a href="./rfc6269">RFC 6269</a>,
June 2011.
[<a id="ref-RFC6454">RFC6454</a>] Barth, A., "The Web Origin Concept", <a href="./rfc6454">RFC 6454</a>, December
2011.
[<a id="ref-WHITELIST">WHITELIST</a>] Google, "Google over IPv6",
<<a href="http://www.google.com/intl/en/ipv6">http://www.google.com/intl/en/ipv6</a>>.
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Authors' Addresses
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: dwing@cisco.com
Andrew Yourtchenko
Cisco Systems, Inc.
De Kleetlaan, 7
Diegem B-1831
Belgium
EMail: ayourtch@cisco.com
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