Network                                                      D. Schinazi
Internet-Draft                                                  T. Pauly
Obsoletes: 6555 (if approved)                                 Apple Inc.
Intended status: Standards Track                         August 17,                      September 10, 2017
Expires: February 18, March 14, 2018

    Happy Eyeballs Version 2: Better Connectivity Using Concurrency


   Many communication protocols operated over the modern Internet use
   host names.  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 higher chance of
   establishing a connection sooner.  This document specifies
   requirements for algorithms that reduce this user-visible delay and
   provides an example algorithm.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 18, March 14, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Hostname Resolution Query Handling  . . . . . . . . . . . . .   4
     3.1.  Handling Multiple DNS Server Addresses  . . . . . . . . .   5
   4.  Sorting Addresses . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Connection Attempts . . . . . . . . . . . . . . . . . . . . .   6
   6.  DNS Answer Changes during Happy Eyeballs Connection Setup . .   7
   7.  Supporting IPv6-only Networks with NAT64 and DNS64  . . . . .   8
     7.1.  IPv4 Address Literals . . . . . . . . . . . . . . . . . .   8
     7.2.  Host Names with Broken AAAA Records . . . . . . . . . . .   9
     7.3.  Virtual Private Networks  . . . . . . . . . . . . . . . .  10
   8.  Summary of Configurable Values  . . . . . . . . . . . . . . .  11
   9.  Limitations . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Path Maximum Transmission Unit Discovery  . . . . . . . .  12
     9.2.  Application Layer . . . . . . . . . . . . . . . . . . . .  12
     9.3.  Hiding Operational Issues . . . . . . . . . . . . . . . .  12
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  12
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     13.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Appendix A.  Differences from RFC6555 . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Many communication protocols operated over the modern Internet use
   host names.  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 higher chance of
   establishing a connection sooner.  This document specifies
   requirements for algorithms that reduce this user-visible delay and
   provides an example algorithm.

   This document expands on "Happy Eyeballs" [RFC6555], a technique of
   reducing user-visible delays on dual-stack hosts.  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 generalization.  This document recommends an algorithm of racing
   resolved addresses that has several stages of ordering and racing to
   avoid delays to the user whenever possible, while preferring the use
   of IPv6.  Specifically, it 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.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].

2.  Overview

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

   1.  Initiation of asynchronous DNS queries [Section 3]

   2.  Sorting of resolved destination addresses [Section 4]

   3.  Initiation of asynchronous connection attempts [Section 5]

   4.  Establishment of one connection, which cancels all other attempts
   Note that this document assumes that the host destination address
   preference policy favors IPv6 over IPv4.  If the host is configured
   differently, the recommendations in this document can be easily

3.  Hostname Resolution Query Handling

   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, 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 RECOMMENDED algorithm proceeds as follows: if the a positive AAAA query
   response (a response with at least one valid AAAA record) is received
   first, the first IPv6 connection attempt is immediately started.  If the
   a positive A query returns response is received first due to reordering, the client
   SHOULD wait for a short time for the AAAA response to ensure
   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 the a positive AAAA response is
   received within the Resolution Delay period, the client immediately
   starts the IPv6 connection attempt.  If, at the end of  If a negative AAAA response (no
   error, no data) is received within the Resolution Delay period, period or the
   AAAA response has not been received but by the A
   response has been received, end of the Resolution
   Delay period, the client SHOULD proceed to Sorting Addresses
   [Section 4] and staggered connection attempts [Section 5] using only the 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
   [Section 6] and the process of connection attempts will continue with
   the IPv6 addresses added, until one connection is established.

3.1.  Handling Multiple DNS Server Addresses

   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, it is
   RECOMMENDED to use at least one address from each address family from
   the available list.

4.  Sorting Addresses

   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
   [Section 5].  It is important that the ordered list involves 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

   First, the client MUST sort the addresses received up to this point
   using Destination Address Selection ([RFC6724], Section 6).

   If the client is stateful and has history of expected round-trip
   times (RTT) 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 has 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 [RFC7413] and some HTTP cookies.  This historical data MUST
   NOT be used across networks, and SHOULD be flushed on network

   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
   ([RFC6724], Section 5) is performed once per destination address and
   is out of scope of this document.

5.  Connection Attempts

   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 cancelled.  Any address that was not yet
   attempted as a connection SHOULD be ignored.  At that time, the
   asynchronous DNS query MAY be cancelled 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 at 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 this
   delay is 250 milliseconds.  If the client has historical RTT data, it
   can also use the expected RTT to choose a more nuanced delay value.
   The recommended formula for calculating the delay after starting a
   connection attempt is: MAX( 1.25 * RTT_MEAN + 4 * RTT_VARIANCE, 2 *
   RTT_MEAN ), where the RTT values are based on the statistics for
   previous address used.  If the TCP implementation leverages
   historical RTT data to compute SYN timeout, these algorithms should
   match so that a new attempt will be started at the same time as the
   previous is sending its second TCP SYN.  While TCP implementations
   often leverage an exponential backoff when they detect packet loss,
   the "Connection Attempt Delay" SHOULD NOT perform such an aggressive
   backoff, as it would harm user experience.

   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

6.  DNS Answer Changes during Happy Eyeballs Connection Setup

   If, during the course of connection establishment, the DNS answers
   change either by adding resolved addresses (for example, due to DNS
   push notifications [DNS-PUSH]), 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 cancelled, 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
   (Section 4).

7.  Supporting IPv6-only Networks with NAT64 and DNS64

   While many IPv6 transition protocols have been standardized and
   deployed, most are transparent to client devices.  The combined use
   of NAT64 [RFC6146] and DNS64 [RFC6147] 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 [RFC6877]. 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.

7.1.  IPv4 Address Literals

   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" [RFC6535] but is implemented inside the Happy Eyeballs library.

   When an IPv4 address is passed in to the library instead of a host
   name, the device queries the network for the NAT64 prefix using
   "Discovery of the IPv6 Prefix Used for IPv6 Address Synthesis"
   [RFC7050] then synthesizes an appropriate IPv6 address (or several)
   using the encoding described in "IPv6 Addressing of IPv4/IPv6
   Translators" [RFC6052].  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 (Section 5).

7.2.  Host Names with Broken AAAA Records

   At the time of writing, there exist a small but non negligible number
   of host names 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 host names 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 host names, 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 then perform local synthesis.  Since these
   types of host names 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" and RECOMMENDED at 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 (Section 7.1).

7.3.  Virtual Private Networks

   Some Virtual Private Networks (VPN) may be configured to handle DNS
   queries from the device.  The configuration could encompass all
   queries, or a subset such as "*".  These VPNs
   can also be configured to only route part of the IPv4 address space,
   such as  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
   "" has exactly one A record,,
   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 then locally
   synthesize an IPv6 address, as if the resolved IPv4 address were
   provided by the application (Section 7.1).

8.  Summary of Configurable Values

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

   o  Resolution Delay (Section 3): The time to wait for a AAAA response
      after receiving an A response.  RECOMMENDED at 50 milliseconds.

   o  First Address Family Count (Section 4): The number of addresses
      belonging to the first address family (such as IPv6) that should
      be attempted before attempting another address family.
      RECOMMENDED as 1, or 2 to more aggressively favor one address

   o  Connection Attempt Delay (Section 5): The time to wait between
      connection attempts in the absence of RTT data.  RECOMMENDED at
      250 milliseconds.

   o  Minimum Connection Attempt Delay (Section 5): The minimum time to
      wait between connection attempts.  RECOMMENDED at 100
      milliseconds.  MUST NOT be less than 10 milliseconds.

   o  Maximum Connection Attempt Delay (Section 5): The maximum time to
      wait between connection attempts.  RECOMMENDED at 2 seconds.

   o  Last Resort Local Synthesis Delay (Section 7.2): The time to wait
      after starting the last IPv6 attempt and before sending the A
      query.  RECOMMENDED at 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.

9.  Limitations

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

9.1.  Path Maximum Transmission Unit Discovery

   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 [RFC4821].

9.2.  Application Layer

   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 the Hypertext Transport Protocol (HTTP).

9.3.  Hiding Operational Issues

   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.

10.  Security Considerations

   This memo has no direct security considerations.

11.  IANA Considerations

   This memo includes no request to IANA.

12.  Acknowledgments

   The authors thank Dan Wing, Andrew Yourtchenko, and everyone else who
   worked on the original Happy Eyeballs design [RFC6555], 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.

13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              DOI 10.17487/RFC6052, October 2010, <https://www.rfc-

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <>.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              DOI 10.17487/RFC6147, April 2011, <https://www.rfc-

   [RFC6535]  Huang, B., Deng, H., and T. Savolainen, "Dual-Stack Hosts
              Using "Bump-in-the-Host" (BIH)", RFC 6535,
              DOI 10.17487/RFC6535, February 2012, <https://www.rfc-

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
              2012, <>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,

   [RFC7050]  Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
              the IPv6 Prefix Used for IPv6 Address Synthesis",
              RFC 7050, DOI 10.17487/RFC7050, November 2013,

13.2.  Informative References

              Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              Work in Progress, draft-ietf-dnssd-push, March 2017.

   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              RFC 6877, DOI 10.17487/RFC6877, April 2013,

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

Appendix A.  Differences from RFC6555

   "Happy Eyeballs: Success with Dual-Stack Hosts" [RFC6555] mostly
   concentrates on how to stagger connections to a hostname that has an
   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

Authors' Addresses

   David Schinazi
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014


   Tommy Pauly
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014