<pre>Internet Engineering Task Force (IETF)                       D. Schinazi
Request for Comments: 8305                                      T. Pauly
Obsoletes: <a href="./rfc6555">6555</a>                                               Apple Inc.
Category: Standards Track                                  December 2017
ISSN: 2070-1721


    <span class="h1">Happy Eyeballs Version 2: Better Connectivity Using Concurrency</span>

Abstract

   Many communication protocols operating over the modern Internet use
   hostnames.  These often resolve to multiple IP addresses, each of
   which may have different performance and connectivity
   characteristics.  Since specific addresses or address families (IPv4
   or IPv6) may be blocked, broken, or sub-optimal on a network, clients
   that attempt multiple connections in parallel have a chance of
   establishing a connection more quickly.  This document specifies
   requirements for algorithms that reduce this user-visible delay and
   provides an example algorithm, referred to as "Happy Eyeballs".  This
   document obsoletes the original algorithm description in <a href="./rfc6555">RFC 6555</a>.

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="./rfc7841#section-2">Section&nbsp;2 of RFC 7841</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="https://www.rfc-editor.org/info/rfc8305">https://www.rfc-editor.org/info/rfc8305</a>.
















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Copyright Notice

   Copyright (c) 2017 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="https://trustee.ietf.org/license-info">https://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.

Table of Contents

   <a href="#section-1">1</a>.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-3">3</a>
     <a href="#section-1.1">1.1</a>.  Requirements Language . . . . . . . . . . . . . . . . . .   <a href="#page-3">3</a>
   <a href="#section-2">2</a>.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-4">4</a>
   <a href="#section-3">3</a>.  Hostname Resolution Query Handling  . . . . . . . . . . . . .   <a href="#page-4">4</a>
     <a href="#section-3.1">3.1</a>.  Handling Multiple DNS Server Addresses  . . . . . . . . .   <a href="#page-5">5</a>
   <a href="#section-4">4</a>.  Sorting Addresses . . . . . . . . . . . . . . . . . . . . . .   <a href="#page-6">6</a>
   <a href="#section-5">5</a>.  Connection Attempts . . . . . . . . . . . . . . . . . . . . .   <a href="#page-7">7</a>
   <a href="#section-6">6</a>.  DNS Answer Changes during Happy Eyeballs Connection Setup . .   <a href="#page-8">8</a>
   <a href="#section-7">7</a>.  Supporting IPv6-Only Networks with NAT64 and DNS64  . . . . .   <a href="#page-8">8</a>
     <a href="#section-7.1">7.1</a>.  IPv4 Address Literals . . . . . . . . . . . . . . . . . .   <a href="#page-8">8</a>
     <a href="#section-7.2">7.2</a>.  Hostnames with Broken AAAA Records  . . . . . . . . . . .   <a href="#page-9">9</a>
     <a href="#section-7.3">7.3</a>.  Virtual Private Networks  . . . . . . . . . . . . . . . .  <a href="#page-10">10</a>
   <a href="#section-8">8</a>.  Summary of Configurable Values  . . . . . . . . . . . . . . .  <a href="#page-10">10</a>
   <a href="#section-9">9</a>.  Limitations . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
     <a href="#section-9.1">9.1</a>.  Path Maximum Transmission Unit Discovery  . . . . . . . .  <a href="#page-11">11</a>
     <a href="#section-9.2">9.2</a>.  Application Layer . . . . . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
     <a href="#section-9.3">9.3</a>.  Hiding Operational Issues . . . . . . . . . . . . . . . .  <a href="#page-11">11</a>
   <a href="#section-10">10</a>. Security Considerations . . . . . . . . . . . . . . . . . . .  <a href="#page-12">12</a>
   <a href="#section-11">11</a>. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  <a href="#page-12">12</a>
   <a href="#section-12">12</a>. References  . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-12">12</a>
     <a href="#section-12.1">12.1</a>.  Normative References . . . . . . . . . . . . . . . . . .  <a href="#page-12">12</a>
     <a href="#section-12.2">12.2</a>.  Informative References . . . . . . . . . . . . . . . . .  <a href="#page-13">13</a>
   <a href="#appendix-A">Appendix A</a>.  Differences from <a href="./rfc6555">RFC 6555</a>  . . . . . . . . . . . . .  <a href="#page-14">14</a>
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-15">15</a>
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-15">15</a>









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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   Many communication protocols operating over the modern Internet use
   hostnames.  These often resolve to multiple IP addresses, each of
   which may have different performance and connectivity
   characteristics.  Since specific addresses or address families (IPv4
   or IPv6) may be blocked, broken, or sub-optimal on a network, clients
   that attempt multiple connections in parallel have a chance of
   establishing a connection more quickly.  This document specifies
   requirements for algorithms that reduce this user-visible delay and
   provides an example algorithm.

   This document defines the algorithm for "Happy Eyeballs", a technique
   for reducing user-visible delays on dual-stack hosts.  This
   definition obsoletes the original description in [<a href="./rfc6555" title="&quot;Happy Eyeballs: Success with Dual-Stack Hosts&quot;">RFC6555</a>].  Now that
   this approach has been deployed at scale and measured for several
   years, the algorithm specification can be refined to improve its
   reliability and general applicability.

   The Happy Eyeballs algorithm of racing connections to resolved
   addresses has several stages to avoid delays to the user whenever
   possible, while preferring the use of IPv6.  This document discusses
   how to handle DNS queries when starting a connection on a dual-stack
   client, how to create an ordered list of destination addresses to
   which to attempt connections, and how to race the connection
   attempts.

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Requirements Language</span>

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>] [<a href="./rfc8174" title="&quot;Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words&quot;">RFC8174</a>] when, and only when, they appear in all capitals,
   as shown here.

















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<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Overview</span>

   This document defines a method of connection establishment, named the
   "Happy Eyeballs Connection Setup".  This approach has several
   distinct phases:

   1.  Initiation of asynchronous DNS queries [<a href="#section-3">Section 3</a>]

   2.  Sorting of resolved destination addresses [<a href="#section-4">Section 4</a>]

   3.  Initiation of asynchronous connection attempts [<a href="#section-5">Section 5</a>]

   4.  Establishment of one connection, which cancels all other attempts
       [<a href="#section-5">Section 5</a>]

   Note that this document assumes that the preference policy for the
   host destination address favors IPv6 over IPv4.  IPv6 has many
   desirable properties designed to be improvements over IPv4 [<a href="./rfc8200" title="&quot;Internet Protocol, Version 6 (IPv6) Specification&quot;">RFC8200</a>].
   If the host is configured to have a different preference, the
   recommendations in this document can be easily adapted.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Hostname Resolution Query Handling</span>

   When a client has both IPv4 and IPv6 connectivity and is trying to
   establish a connection with a named host, it needs to send out both
   AAAA and A DNS queries.  Both queries SHOULD be made as soon after
   one another as possible, with the AAAA query made first and
   immediately followed by the A query.

   Implementations SHOULD NOT wait for both families of answers to
   return before attempting connection establishment.  If one query
   fails to return or takes significantly longer to return, waiting for
   the second address family can significantly delay the connection
   establishment of the first one.  Therefore, the client SHOULD treat
   DNS resolution as asynchronous.  Note that if the platform does not
   offer an asynchronous DNS API, this behavior can be simulated by
   making two separate synchronous queries on different threads, one per
   address family.

   The algorithm proceeds as follows: if a positive AAAA response (a
   response with at least one valid AAAA record) is received first, the
   first IPv6 connection attempt is immediately started.  If a positive
   A response is received first due to reordering, the client SHOULD
   wait a short time for the AAAA response to ensure that preference is
   given to IPv6 (it is common for the AAAA response to follow the A
   response by a few milliseconds).  This delay will be referred to as
   the "Resolution Delay".  The recommended value for the Resolution
   Delay is 50 milliseconds.  If a positive AAAA response is received



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   within the Resolution Delay period, the client immediately starts the
   IPv6 connection attempt.  If a negative AAAA response (no error, no
   data) is received within the Resolution Delay period or the AAAA
   response has not been received by the end of the Resolution Delay
   period, the client SHOULD proceed to sorting addresses (see
   <a href="#section-4">Section 4</a>) and staggered connection attempts (see <a href="#section-5">Section 5</a>) using
   any IPv4 addresses returned so far.  If the AAAA response arrives
   while these connection attempts are in progress but before any
   connection has been established, then the newly received IPv6
   addresses are incorporated into the list of available candidate
   addresses (see <a href="#section-6">Section 6</a>) and the process of connection attempts will
   continue with the IPv6 addresses added, until one connection is
   established.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Handling Multiple DNS Server Addresses</span>

   If multiple DNS server addresses are configured for the current
   network, the client may have the option of sending its DNS queries
   over IPv4 or IPv6.  In keeping with the Happy Eyeballs approach,
   queries SHOULD be sent over IPv6 first (note that this is not
   referring to the sending of AAAA or A queries, but rather the address
   of the DNS server itself and IP version used to transport DNS
   messages).  If DNS queries sent to the IPv6 address do not receive
   responses, that address may be marked as penalized and queries can be
   sent to other DNS server addresses.

   As native IPv6 deployments become more prevalent and IPv4 addresses
   are exhausted, it is expected that IPv6 connectivity will have
   preferential treatment within networks.  If a DNS server is
   configured to be accessible over IPv6, IPv6 should be assumed to be
   the preferred address family.

   Client systems SHOULD NOT have an explicit limit to the number of DNS
   servers that can be configured, either manually or by the network.
   If such a limit is required by hardware limitations, the client
   SHOULD use at least one address from each address family from the
   available list.














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<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Sorting Addresses</span>

   Before attempting to connect to any of the resolved destination
   addresses, the client should define the order in which to start the
   attempts.  Once the order has been defined, the client can use a
   simple algorithm for racing each option after a short delay (see
   <a href="#section-5">Section 5</a>).  It is important that the ordered list involve all
   addresses from both families that have been received by this point,
   as this allows the client to get the racing effect of Happy Eyeballs
   for the entire list, not just the first IPv4 and first IPv6
   addresses.

   First, the client MUST sort the addresses received up to this point
   using Destination Address Selection (<a href="./rfc6724#section-6">[RFC6724], Section&nbsp;6</a>).

   If the client is stateful and has a history of expected round-trip
   times (RTTs) for the routes to access each address, it SHOULD add a
   Destination Address Selection rule between rules 8 and 9 that prefers
   addresses with lower RTTs.  If the client keeps track of which
   addresses it used in the past, it SHOULD add another Destination
   Address Selection rule between the RTT rule and rule 9, which prefers
   used addresses over unused ones.  This helps servers that use the
   client's IP address during authentication, as is the case for TCP
   Fast Open [<a href="./rfc7413" title="&quot;TCP Fast Open&quot;">RFC7413</a>] and some Hypertext Transport Protocol (HTTP)
   cookies.  This historical data MUST NOT be used across different
   network interfaces and SHOULD be flushed whenever a device changes
   the network to which it is attached.

   Next, the client SHOULD modify the ordered list to interleave address
   families.  Whichever address family is first in the list should be
   followed by an address of the other address family; that is, if the
   first address in the sorted list is IPv6, then the first IPv4 address
   should be moved up in the list to be second in the list.  An
   implementation MAY want to favor one address family more by allowing
   multiple addresses of that family to be attempted before trying the
   other family.  The number of contiguous addresses of the first
   address family will be referred to as the "First Address Family
   Count" and can be a configurable value.  This is performed to avoid
   waiting through a long list of addresses from a given address family
   if connectivity over that address family is impaired.

   Note that the address selection described in this section only
   applies to destination addresses; Source Address Selection
   (<a href="./rfc6724#section-5">[RFC6724], Section&nbsp;5</a>) is performed once per destination address and
   is out of scope of this document.






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<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Connection Attempts</span>

   Once the list of addresses received up to this point has been
   constructed, the client will attempt to make connections.  In order
   to avoid unreasonable network load, connection attempts SHOULD NOT be
   made simultaneously.  Instead, one connection attempt to a single
   address is started first, followed by the others in the list, one at
   a time.  Starting a new connection attempt does not affect previous
   attempts, as multiple connection attempts may occur in parallel.
   Once one of the connection attempts succeeds (generally when the TCP
   handshake completes), all other connections attempts that have not
   yet succeeded SHOULD be canceled.  Any address that was not yet
   attempted as a connection SHOULD be ignored.  At that time, the
   asynchronous DNS query MAY be canceled as new addresses will not be
   used for this connection.  However, the DNS client resolver SHOULD
   still process DNS replies from the network for a short period of time
   (recommended to be 1 second), as they will populate the DNS cache and
   can be used for subsequent connections.

   A simple implementation can have a fixed delay for how long to wait
   before starting the next connection attempt.  This delay is referred
   to as the "Connection Attempt Delay".  One recommended value for a
   default delay is 250 milliseconds.  A more nuanced implementation's
   delay should correspond to the time when the previous attempt is
   sending its second TCP SYN, based on the TCP's retransmission timer
   [<a href="./rfc6298" title="&quot;Computing TCP's Retransmission Timer&quot;">RFC6298</a>].  If the client has historical RTT data gathered from other
   connections to the same host or prefix, it can use this information
   to influence its delay.  Note that this algorithm should only try to
   approximate the time of the first SYN retransmission, and not any
   further retransmissions that may be influenced by exponential timer
   back off.

   The Connection Attempt Delay MUST have a lower bound, especially if
   it is computed using historical data.  More specifically, a
   subsequent connection MUST NOT be started within 10 milliseconds of
   the previous attempt.  The recommended minimum value is 100
   milliseconds, which is referred to as the "Minimum Connection Attempt
   Delay".  This minimum value is required to avoid congestion collapse
   in the presence of high packet-loss rates.  The Connection Attempt
   Delay SHOULD have an upper bound, referred to as the "Maximum
   Connection Attempt Delay".  The current recommended value is 2
   seconds.









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<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  DNS Answer Changes during Happy Eyeballs Connection Setup</span>

   If, during the course of connection establishment, the DNS answers
   change by either adding resolved addresses (for example due to DNS
   push notifications [<a href="#ref-DNS-PUSH" title="&quot;DNS Push Notifications&quot;">DNS-PUSH</a>]) or removing previously resolved
   addresses (for example, due to expiry of the TTL on that DNS record),
   the client should react based on its current progress.

   If an address is removed from the list that already had a connection
   attempt started, the connection attempt SHOULD NOT be canceled, but
   rather be allowed to continue.  If the removed address had not yet
   had a connection attempt started, it SHOULD be removed from the list
   of addresses to try.

   If an address is added to the list, it should be sorted into the list
   of addresses not yet attempted according to the rules above (see
   <a href="#section-4">Section 4</a>).

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Supporting IPv6-Only Networks with NAT64 and DNS64</span>

   While many IPv6 transition protocols have been standardized and
   deployed, most are transparent to client devices.  The combined use
   of NAT64 [<a href="./rfc6146" title="&quot;Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers&quot;">RFC6146</a>] and DNS64 [<a href="./rfc6147" title="&quot;DNS64: DNS Extensions for Network Address Translation from IPv6 Clients to IPv4 Servers&quot;">RFC6147</a>] is a popular solution that is
   being deployed and requires changes in client devices.  One possible
   way to handle these networks is for the client device networking
   stack to implement 464XLAT [<a href="./rfc6877" title="&quot;464XLAT: Combination of Stateful and Stateless Translation&quot;">RFC6877</a>]. 464XLAT has the advantage of
   not requiring changes to user space software; however, it requires
   per-packet translation if the application is using IPv4 literals and
   does not encourage client application software to support native
   IPv6.  On platforms that do not support 464XLAT, the Happy Eyeballs
   engine SHOULD follow the recommendations in this section to properly
   support IPv6-only networks with NAT64 and DNS64.

   The features described in this section SHOULD only be enabled when
   the host detects one of these networks.  A simple heuristic to
   achieve that is to check if the network offers routable IPv6
   addressing, does not offer routable IPv4 addressing, and offers a DNS
   resolver address.

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  IPv4 Address Literals</span>

   If client applications or users wish to connect to IPv4 address
   literals, the Happy Eyeballs engine will need to perform NAT64
   address synthesis for them.  The solution is similar to "Bump-in-the-
   Host" [<a href="./rfc6535" title="&quot;Dual-Stack Hosts Using &quot;">RFC6535</a>] but is implemented inside the Happy Eyeballs library.






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   When an IPv4 address is passed into the library instead of a
   hostname, the device queries the network for the NAT64 prefix using
   "Discovery of the IPv6 Prefix Used for IPv6 Address Synthesis"
   [<a href="./rfc7050" title="&quot;Discovery of the IPv6 Prefix Used for IPv6 Address Synthesis&quot;">RFC7050</a>] and then synthesizes an appropriate IPv6 address (or
   several) using the encoding described in "IPv6 Addressing of IPv4/
   IPv6 Translators" [<a href="./rfc6052" title="&quot;IPv6 Addressing of IPv4/IPv6 Translators&quot;">RFC6052</a>].  The synthesized addresses are then
   inserted into the list of addresses as if they were results from DNS
   queries; connection attempts follow the algorithm described above
   (see <a href="#section-5">Section 5</a>).

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  Hostnames with Broken AAAA Records</span>

   At the time of writing, there exist a small but non-negligible number
   of hostnames that resolve to valid A records and broken AAAA records,
   which we define as AAAA records that contain seemingly valid IPv6
   addresses but those addresses never reply when contacted on the usual
   ports.  These can be, for example, caused by:

   o  Mistyping of the IPv6 address in the DNS zone configuration

   o  Routing black holes

   o  Service outages

   While an algorithm complying with the other sections of this document
   would correctly handle such hostnames on a dual-stack network, they
   will not necessarily function correctly on IPv6-only networks with
   NAT64 and DNS64.  Since DNS64 recursive resolvers rely on the
   authoritative name servers sending negative ("no error no answer")
   responses for AAAA records in order to synthesize, they will not
   synthesize records for these particular hostnames and will instead
   pass through the broken AAAA record.

   In order to support these scenarios, the client device needs to query
   the DNS for the A record and then perform local synthesis.  Since
   these types of hostnames are rare and, in order to minimize load on
   DNS servers, this A query should only be performed when the client
   has given up on the AAAA records it initially received.  This can be
   achieved by using a longer timeout, referred to as the "Last Resort
   Local Synthesis Delay"; the delay is recommended to be 2 seconds.
   The timer is started when the last connection attempt is fired.  If
   no connection attempt has succeeded when this timer fires, the device
   queries the DNS for the IPv4 address and, on reception of a valid A
   record, treats it as if it were provided by the application (see
   <a href="#section-7.1">Section 7.1</a>).






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<span class="h3"><a class="selflink" id="section-7.3" href="#section-7.3">7.3</a>.  Virtual Private Networks</span>

   Some Virtual Private Networks (VPNs) may be configured to handle DNS
   queries from the device.  The configuration could encompass all
   queries or a subset such as "*.internal.example.com".  These VPNs can
   also be configured to only route part of the IPv4 address space, such
   as 192.0.2.0/24.  However, if an internal hostname resolves to an
   external IPv4 address, these can cause issues if the underlying
   network is IPv6-only.  As an example, let's assume that
   "www.internal.example.com" has exactly one A record, 198.51.100.42,
   and no AAAA records.  The client will send the DNS query to the
   company's recursive resolver and that resolver will reply with these
   records.  The device now only has an IPv4 address to connect to and
   no route to that address.  Since the company's resolver does not know
   the NAT64 prefix of the underlying network, it cannot synthesize the
   address.  Similarly, the underlying network's DNS64 recursive
   resolver does not know the company's internal addresses, so it cannot
   resolve the hostname.  Because of this, the client device needs to
   resolve the A record using the company's resolver and then locally
   synthesize an IPv6 address, as if the resolved IPv4 address were
   provided by the application (<a href="#section-7.1">Section 7.1</a>).

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Summary of Configurable Values</span>

   The values that may be configured as defaults on a client for use in
   Happy Eyeballs are as follows:

   o  Resolution Delay (<a href="#section-3">Section 3</a>): The time to wait for a AAAA response
      after receiving an A response.  Recommended to be 50 milliseconds.

   o  First Address Family Count (<a href="#section-4">Section 4</a>): The number of addresses
      belonging to the first address family (such as IPv6) that should
      be attempted before attempting another address family.
      Recommended to be 1; 2 may be used to more aggressively favor a
      particular address family.

   o  Connection Attempt Delay (<a href="#section-5">Section 5</a>): The time to wait between
      connection attempts in the absence of RTT data.  Recommended to be
      250 milliseconds.

   o  Minimum Connection Attempt Delay (<a href="#section-5">Section 5</a>): The minimum time to
      wait between connection attempts.  Recommended to be 100
      milliseconds.  MUST NOT be less than 10 milliseconds.

   o  Maximum Connection Attempt Delay (<a href="#section-5">Section 5</a>): The maximum time to
      wait between connection attempts.  Recommended to be 2 seconds.





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   o  Last Resort Local Synthesis Delay (<a href="#section-7.2">Section 7.2</a>): The time to wait
      after starting the last IPv6 attempt and before sending the A
      query.  Recommended to be 2 seconds.

   The delay values described in this section were determined
   empirically by measuring the timing of connections on a very wide set
   of production devices.  They were picked to reduce wait times noticed
   by users while minimizing load on the network.  As time passes, it is
   expected that the properties of networks will evolve.  For that
   reason, it is expected that these values will change over time.
   Implementors should feel welcome to use different values without
   changing this specification.  Since IPv6 issues are expected to be
   less common, the delays SHOULD be increased with time as client
   software is updated.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Limitations</span>

   Happy Eyeballs will handle initial connection failures at the TCP/IP
   layer; however, other failures or performance issues may still affect
   the chosen connection.

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Path Maximum Transmission Unit Discovery</span>

   Since Happy Eyeballs is only active during the initial handshake and
   TCP does not pass the initial handshake, issues related to MTU can be
   masked and go unnoticed during Happy Eyeballs.  Solving this issue is
   out of scope of this document.  One solution is to use "Packetization
   Layer Path MTU Discovery" [<a href="./rfc4821" title="&quot;Packetization Layer Path MTU Discovery&quot;">RFC4821</a>].

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  Application Layer</span>

   If the DNS returns multiple addresses for different application
   servers, the application itself may not be operational and functional
   on all of them.  Common examples include Transport Layer Security
   (TLS) and HTTP.

<span class="h3"><a class="selflink" id="section-9.3" href="#section-9.3">9.3</a>.  Hiding Operational Issues</span>

   It has been observed in practice that Happy Eyeballs can hide issues
   in networks.  For example, if a misconfiguration causes IPv6 to
   consistently fail on a given network while IPv4 is still functional,
   Happy Eyeballs may impair the operator's ability to notice the issue.
   It is recommended that network operators deploy external means of
   monitoring to ensure functionality of all address families.







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<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Security Considerations</span>

   Note that applications should not rely upon a stable hostname-to-
   address mapping to ensure any security properties, since DNS results
   may change between queries.  Happy Eyeballs may make it more likely
   that subsequent connections to a single hostname use different IP
   addresses.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  IANA Considerations</span>

   This document does not require any IANA actions.

<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>.  References</span>

<span class="h3"><a class="selflink" id="section-12.1" href="#section-12.1">12.1</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>,
              DOI 10.17487/RFC2119, March 1997,
              &lt;<a href="https://www.rfc-editor.org/info/rfc2119">https://www.rfc-editor.org/info/rfc2119</a>&gt;.

   [<a id="ref-RFC4821">RFC4821</a>]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", <a href="./rfc4821">RFC 4821</a>, DOI 10.17487/RFC4821, March 2007,
              &lt;<a href="https://www.rfc-editor.org/info/rfc4821">https://www.rfc-editor.org/info/rfc4821</a>&gt;.

   [<a id="ref-RFC6052">RFC6052</a>]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", <a href="./rfc6052">RFC 6052</a>,
              DOI 10.17487/RFC6052, October 2010,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6052">https://www.rfc-editor.org/info/rfc6052</a>&gt;.

   [<a id="ref-RFC6146">RFC6146</a>]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", <a href="./rfc6146">RFC 6146</a>, DOI 10.17487/RFC6146,
              April 2011, &lt;<a href="https://www.rfc-editor.org/info/rfc6146">https://www.rfc-editor.org/info/rfc6146</a>&gt;.

   [<a id="ref-RFC6147">RFC6147</a>]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", <a href="./rfc6147">RFC 6147</a>,
              DOI 10.17487/RFC6147, April 2011,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6147">https://www.rfc-editor.org/info/rfc6147</a>&gt;.

   [<a id="ref-RFC6298">RFC6298</a>]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", <a href="./rfc6298">RFC 6298</a>,
              DOI 10.17487/RFC6298, June 2011,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6298">https://www.rfc-editor.org/info/rfc6298</a>&gt;.






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   [<a id="ref-RFC6535">RFC6535</a>]  Huang, B., Deng, H., and T. Savolainen, "Dual-Stack Hosts
              Using "Bump-in-the-Host" (BIH)", <a href="./rfc6535">RFC 6535</a>,
              DOI 10.17487/RFC6535, February 2012,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6535">https://www.rfc-editor.org/info/rfc6535</a>&gt;.

   [<a id="ref-RFC6555">RFC6555</a>]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", <a href="./rfc6555">RFC 6555</a>, DOI 10.17487/RFC6555, April
              2012, &lt;<a href="https://www.rfc-editor.org/info/rfc6555">https://www.rfc-editor.org/info/rfc6555</a>&gt;.

   [<a id="ref-RFC6724">RFC6724</a>]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", <a href="./rfc6724">RFC 6724</a>, DOI 10.17487/RFC6724, September 2012,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6724">https://www.rfc-editor.org/info/rfc6724</a>&gt;.

   [<a id="ref-RFC7050">RFC7050</a>]  Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
              the IPv6 Prefix Used for IPv6 Address Synthesis",
              <a href="./rfc7050">RFC 7050</a>, DOI 10.17487/RFC7050, November 2013,
              &lt;<a href="https://www.rfc-editor.org/info/rfc7050">https://www.rfc-editor.org/info/rfc7050</a>&gt;.

   [<a id="ref-RFC8174">RFC8174</a>]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in <a href="./rfc2119">RFC</a>
              <a href="./rfc2119">2119</a> Key Words", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc8174">RFC 8174</a>, DOI 10.17487/RFC8174,
              May 2017, &lt;<a href="https://www.rfc-editor.org/info/rfc8174">https://www.rfc-editor.org/info/rfc8174</a>&gt;.

<span class="h3"><a class="selflink" id="section-12.2" href="#section-12.2">12.2</a>.  Informative References</span>

   [<a id="ref-DNS-PUSH">DNS-PUSH</a>] Pusateri, T. and S. Cheshire, <a style="text-decoration: none" href='https://www.google.com/search?sitesearch=datatracker.ietf.org%2Fdoc%2Fhtml%2F&amp;q=inurl:draft-+%22DNS+Push+Notifications%22'>"DNS Push Notifications"</a>,
              Work in Progress, <a href="./draft-ietf-dnssd-push-13">draft-ietf-dnssd-push-13</a>, October 2017.

   [<a id="ref-RFC6877">RFC6877</a>]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              <a href="./rfc6877">RFC 6877</a>, DOI 10.17487/RFC6877, April 2013,
              &lt;<a href="https://www.rfc-editor.org/info/rfc6877">https://www.rfc-editor.org/info/rfc6877</a>&gt;.

   [<a id="ref-RFC7413">RFC7413</a>]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", <a href="./rfc7413">RFC 7413</a>, DOI 10.17487/RFC7413, December 2014,
              &lt;<a href="https://www.rfc-editor.org/info/rfc7413">https://www.rfc-editor.org/info/rfc7413</a>&gt;.

   [<a id="ref-RFC8200">RFC8200</a>]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, <a href="./rfc8200">RFC 8200</a>,
              DOI 10.17487/RFC8200, July 2017,
              &lt;<a href="https://www.rfc-editor.org/info/rfc8200">https://www.rfc-editor.org/info/rfc8200</a>&gt;.










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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Differences from <a href="./rfc6555">RFC 6555</a></span>

   "Happy Eyeballs: Success with Dual-Stack Hosts" [<a href="./rfc6555" title="&quot;Happy Eyeballs: Success with Dual-Stack Hosts&quot;">RFC6555</a>] mostly
   concentrates on how to stagger connections to a hostname that has a
   AAAA and an A record.  This document additionally discusses:

   o  how to perform DNS queries to obtain these addresses

   o  how to handle multiple addresses from each address family

   o  how to handle DNS updates while connections are being raced

   o  how to leverage historical information

   o  how to support IPv6-only networks with NAT64 and DNS64

   Note that a simple implementation of the algorithm described in this
   document is still compliant with the previous specification
   [<a href="./rfc6555" title="&quot;Happy Eyeballs: Success with Dual-Stack Hosts&quot;">RFC6555</a>].  Implementations should take the new considerations into
   account when applicable to optimize their behavior.































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Acknowledgments

   The authors thank Dan Wing, Andrew Yourtchenko, and everyone else who
   worked on the original Happy Eyeballs design [<a href="./rfc6555" title="&quot;Happy Eyeballs: Success with Dual-Stack Hosts&quot;">RFC6555</a>], Josh
   Graessley, Stuart Cheshire, and the rest of team at Apple that helped
   implement and instrument this algorithm, and Jason Fesler and Paul
   Saab who helped measure and refine this algorithm.  The authors would
   also like to thank Fred Baker, Nick Chettle, Lorenzo Colitti, Igor
   Gashinsky, Geoff Huston, Jen Linkova, Paul Hoffman, Philip Homburg,
   Warren Kumari, Erik Nygren, Jordi Palet Martinez, Rui Paulo, Stephen
   Strowes, Jinmei Tatuya, Dave Thaler, Joe Touch, and James Woodyatt
   for their input and contributions.

Authors' Addresses

   David Schinazi
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014
   United States of America

   Email: dschinazi@apple.com


   Tommy Pauly
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014
   United States of America

   Email: tpauly@apple.com




















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