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rfc:rfc6555

Internet Engineering Task Force (IETF) D. Wing Request for Comments: 6555 A. Yourtchenko Category: Standards Track Cisco ISSN: 2070-1721 April 2012

           Happy Eyeballs: Success with Dual-Stack Hosts

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 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/rfc6555.

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.

Wing & Yourtchenko Standards Track [Page 1] RFC 6555 Happy Eyeballs Dual Stack April 2012

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  Additional Network and Host Traffic  . . . . . . . . . . .  3
 2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
 3.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  4
   3.1.  Hostnames  . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.2.  Delay When IPv6 Is Not Accessible  . . . . . . . . . . . .  5
 4.  Algorithm Requirements . . . . . . . . . . . . . . . . . . . .  6
   4.1.  Delay IPv4 . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.2.  Stateful Behavior When IPv6 Fails  . . . . . . . . . . . .  8
   4.3.  Reset on Network (Re-)Initialization . . . . . . . . . . .  9
   4.4.  Abandon Non-Winning Connections  . . . . . . . . . . . . .  9
 5.  Additional Considerations  . . . . . . . . . . . . . . . . . . 10
   5.1.  Determining Address Type . . . . . . . . . . . . . . . . . 10
   5.2.  Debugging and Troubleshooting  . . . . . . . . . . . . . . 10
   5.3.  Three or More Interfaces . . . . . . . . . . . . . . . . . 10
   5.4.  A and AAAA Resource Records  . . . . . . . . . . . . . . . 10
   5.5.  Connection Timeout . . . . . . . . . . . . . . . . . . . . 11
   5.6.  Interaction with Same-Origin Policy  . . . . . . . . . . . 11
   5.7.  Implementation Strategies  . . . . . . . . . . . . . . . . 12
 6.  Example Algorithm  . . . . . . . . . . . . . . . . . . . . . . 12
 7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
 8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
 9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   9.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
   9.2.  Informative References . . . . . . . . . . . . . . . . . . 13

Wing & Yourtchenko Standards Track [Page 2] RFC 6555 Happy Eyeballs Dual Stack April 2012

1. 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 2012

3. 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 in Section 3.1, it is important that the
 same hostname be used for IPv4 and IPv6.
 As discussed in more detail in Section 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 2012

3.2. 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 2012

4. 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 in
 Section 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 (see Section 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 also Section 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 from

Wing & 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 (see
    Section 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 first

Wing & Yourtchenko Standards Track [Page 10] RFC 6555 Happy Eyeballs Dual Stack April 2012

 address.  If that fails after a certain time (see Section 5.5), the
 next address SHOULD be the IPv4 address.
 If that fails to connect after a certain time (see Section 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 2012

5.7. 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 Sections 4.4 and 5.6.

Wing & Yourtchenko Standards Track [Page 12] RFC 6555 Happy Eyeballs Dual Stack April 2012

8. 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. References

9.1. 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 2012

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

Wing & Yourtchenko Standards Track [Page 15]

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