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Internet Engineering Task Force (IETF)                           D. WingRequest for Comments: 6555                                A. YourtchenkoCategory: Standards Track                                          CiscoISSN: 2070-1721                                               April 2012Happy Eyeballs: Success with Dual-Stack HostsAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6555.Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Wing & Yourtchenko           Standards Track                    [Page 1]

RFC 6555                Happy Eyeballs Dual Stack             April 2012Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .31.1.  Additional Network and Host Traffic  . . . . . . . . . . .32.  Notational Conventions . . . . . . . . . . . . . . . . . . . .33.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .43.1.  Hostnames  . . . . . . . . . . . . . . . . . . . . . . . .43.2.  Delay When IPv6 Is Not Accessible  . . . . . . . . . . . .54.  Algorithm Requirements . . . . . . . . . . . . . . . . . . . .64.1.  Delay IPv4 . . . . . . . . . . . . . . . . . . . . . . . .74.2.  Stateful Behavior When IPv6 Fails  . . . . . . . . . . . .84.3.  Reset on Network (Re-)Initialization . . . . . . . . . . .94.4.  Abandon Non-Winning Connections  . . . . . . . . . . . . .95.  Additional Considerations  . . . . . . . . . . . . . . . . . .105.1.  Determining Address Type . . . . . . . . . . . . . . . . .105.2.  Debugging and Troubleshooting  . . . . . . . . . . . . . .105.3.  Three or More Interfaces . . . . . . . . . . . . . . . . .105.4.  A and AAAA Resource Records  . . . . . . . . . . . . . . .105.5.  Connection Timeout . . . . . . . . . . . . . . . . . . . .115.6.  Interaction with Same-Origin Policy  . . . . . . . . . . .115.7.  Implementation Strategies  . . . . . . . . . . . . . . . .126.  Example Algorithm  . . . . . . . . . . . . . . . . . . . . . .127.  Security Considerations  . . . . . . . . . . . . . . . . . . .128.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .139.  References . . . . . . . . . . . . . . . . . . . . . . . . . .139.1.  Normative References . . . . . . . . . . . . . . . . . . .139.2.  Informative References . . . . . . . . . . . . . . . . . .13Wing & Yourtchenko           Standards Track                    [Page 2]

RFC 6555                Happy Eyeballs Dual Stack             April 20121.  Introduction   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 (Section 3).  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., [WHITELIST]).   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., [RFC3484]).1.1.  Additional Network and Host Traffic   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.2.  Notational Conventions   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 [RFC2119].Wing & Yourtchenko           Standards Track                    [Page 3]

RFC 6555                Happy Eyeballs Dual Stack             April 20123.  Problem Statement   The basis of the IPv6/IPv4 selection problem was first described in   1994 in [RFC1671]:      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 inSection 3.1, it is important that the   same hostname be used for IPv4 and IPv6.   As discussed in more detail inSection 3.2, 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   [RFC6157]).3.1.  Hostnames   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.Wing & Yourtchenko           Standards Track                    [Page 4]

RFC 6555                Happy Eyeballs Dual Stack             April 20123.2.  Delay When IPv6 Is Not Accessible   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 [Experiences].Wing & Yourtchenko           Standards Track                    [Page 5]

RFC 6555                Happy Eyeballs Dual Stack             April 20124.  Algorithm Requirements   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.Wing & Yourtchenko           Standards Track                    [Page 6]

RFC 6555                Happy Eyeballs Dual Stack             April 2012           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.4.1.  Delay IPv4   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:Wing & Yourtchenko           Standards Track                    [Page 7]

RFC 6555                Happy Eyeballs Dual Stack             April 2012   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 [RFC6269] 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 inSection 4.2.  This usually means giving preference to IPv6 over IPv4,   although that preference can be overridden by user configuration or   by network configuration [ADDR-SELECT].  If the host's policy is   unknown or not attainable, implementations MUST prefer IPv6 over   IPv4.4.2.  Stateful Behavior When IPv6 Fails   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.Wing & Yourtchenko           Standards Track                    [Page 8]

RFC 6555                Happy Eyeballs Dual Stack             April 2012   After making a connection attempt on the preferred address family   (e.g., IPv6) and failing to establish a connection within a certain   time period (seeSection 5.5), 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).4.3.  Reset on Network (Re-)Initialization   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)   [RFC4436], DNAv6 [RFC6059]).   If the client application is a web browser, see alsoSection 5.6.4.4.  Abandon Non-Winning Connections   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 fromWing & Yourtchenko           Standards Track                    [Page 9]

RFC 6555                Happy Eyeballs Dual Stack             April 2012      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 (seeSection 5.6).5.  Additional Considerations   This section discusses considerations related to Happy Eyeballs.5.1.  Determining Address Type   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.5.2.  Debugging and Troubleshooting   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).5.3.  Three or More Interfaces   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.5.4.  A and AAAA Resource Records   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 firstWing & Yourtchenko           Standards Track                   [Page 10]

RFC 6555                Happy Eyeballs Dual Stack             April 2012   address.  If that fails after a certain time (seeSection 5.5), the   next address SHOULD be the IPv4 address.   If that fails to connect after a certain time (seeSection 5.5), 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.5.5.  Connection Timeout   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.5.6.  Interaction with Same-Origin Policy   Web browsers implement a same-origin policy [RFC6454] 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.Wing & Yourtchenko           Standards Track                   [Page 11]

RFC 6555                Happy Eyeballs Dual Stack             April 20125.7.  Implementation Strategies   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 [RFC3484] based on configuration received from the   network, or observing connection failures to IPv6 and IPV4   destinations.6.  Example Algorithm   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 [Perreault] and [Andrews].7.  Security Considerations   See Sections4.4 and5.6.Wing & Yourtchenko           Standards Track                   [Page 12]

RFC 6555                Happy Eyeballs Dual Stack             April 20128.  Acknowledgements   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 [RFC5245],   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.9.  References9.1.  Normative References   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate                 Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3484]     Draves, R., "Default Address Selection for Internet                 Protocol version 6 (IPv6)",RFC 3484, February 2003.9.2.  Informative References   [ADDR-SELECT] Matsumoto, A., Fujisaki, T., Kato, J., and T. Chown,                 "Distributing Address Selection Policy using DHCPv6",                 Work in Progress, February 2012.   [Andrews]     Andrews, M., "How to connect to a multi-homed server                 over TCP", January 2011, <http://www.isc.org/community/blog/201101/how-to-connect-to-a-multi-homed-server-over-tcp>.Wing & Yourtchenko           Standards Track                   [Page 13]

RFC 6555                Happy Eyeballs Dual Stack             April 2012   [Experiences] Savolainen, T., Miettinen, N., Veikkolainen, S., Chown,                 T., and J. Morse, "Experiences of host behavior in                 broken IPv6 networks", March 2011,                 <http://www.ietf.org/proceedings/80/slides/v6ops-12.pdf>.   [Perreault]   Perreault, S., "Happy Eyeballs in Erlang", February                 2011, <http://www.viagenie.ca/news/index.html#happy_eyeballs_erlang>.   [RFC1671]     Carpenter, B., "IPng White Paper on Transition and                 Other Considerations",RFC 1671, August 1994.   [RFC4436]     Aboba, B., Carlson, J., and S. Cheshire, "Detecting                 Network Attachment in IPv4 (DNAv4)",RFC 4436, March                 2006.   [RFC5245]     Rosenberg, J., "Interactive Connectivity Establishment                 (ICE): A Protocol for Network Address Translator (NAT)                 Traversal for Offer/Answer Protocols",RFC 5245, April                 2010.   [RFC6059]     Krishnan, S. and G. Daley, "Simple Procedures for                 Detecting Network Attachment in IPv6",RFC 6059,                 November 2010.   [RFC6157]     Camarillo, G., El Malki, K., and V. Gurbani, "IPv6                 Transition in the Session Initiation Protocol (SIP)",RFC 6157, April 2011.   [RFC6269]     Ford, M., Boucadair, M., Durand, A., Levis, P., and P.                 Roberts, "Issues with IP Address Sharing",RFC 6269,                 June 2011.   [RFC6454]     Barth, A., "The Web Origin Concept",RFC 6454, December                 2011.   [WHITELIST]   Google, "Google over IPv6",                 <http://www.google.com/intl/en/ipv6>.Wing & Yourtchenko           Standards Track                   [Page 14]

RFC 6555                Happy Eyeballs Dual Stack             April 2012Authors' 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.comWing & Yourtchenko           Standards Track                   [Page 15]