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Happy Eyeballs: Success with Dual-Stack Hosts
RFC 6555

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

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