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RFC 8305

Happy Eyeballs Version 2: Better Connectivity Using Concurrency

Pages: 15
Proposed Standard
Obsoletes:  6555

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Internet Engineering Task Force (IETF)                       D. Schinazi
Request for Comments: 8305                                      T. Pauly
Obsoletes: 6555                                               Apple Inc.
Category: Standards Track                                  December 2017
ISSN: 2070-1721

    Happy Eyeballs Version 2: Better Connectivity Using Concurrency


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 RFC 6555. 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 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at
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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 BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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

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

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 [RFC6555]. 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.

1.1. Requirements Language

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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
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2. Overview

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 [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 [Section 5] 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 [RFC8200]. If the host is configured to have a different preference, the recommendations in this document can be easily adapted.

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 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
   Section 4) and staggered connection attempts (see Section 5) 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 Section 6) and the process of connection attempts will
   continue with the IPv6 addresses added, until one connection is

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, the client SHOULD use at least one address from each address family from the available list.
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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 (see Section 5). 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 ([RFC6724], Section 6). 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 [RFC7413] 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 ([RFC6724], Section 5) is performed once per destination address and is out of scope of this document.
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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 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 [RFC6298]. 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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6. DNS Answer Changes during Happy Eyeballs Connection Setup

If, during the course of connection establishment, the DNS answers change by either 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 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 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.
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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"
   [RFC7050] and 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
   (see Section 5).

7.2. Hostnames with Broken AAAA Records

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 Section 7.1).
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7.3. Virtual Private Networks

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 "*". 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 and 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 to be 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 to be 1; 2 may be used to more aggressively favor a particular address family. o Connection Attempt Delay (Section 5): The time to wait between connection attempts in the absence of RTT data. Recommended to be 250 milliseconds. o Minimum Connection Attempt Delay (Section 5): 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 (Section 5): The maximum time to wait between connection attempts. Recommended to be 2 seconds.
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   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 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.

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 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.
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10. Security Considerations

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.

11. IANA Considerations

This document does not require any IANA actions.

12. References

12.1. 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, <>. [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, <>. [RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent, "Computing TCP's Retransmission Timer", RFC 6298, DOI 10.17487/RFC6298, June 2011, <>.
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   [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,

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

12.2. Informative References

[DNS-PUSH] Pusateri, T. and S. Cheshire, "DNS Push Notifications", Work in Progress, draft-ietf-dnssd-push-13, October 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, <>. [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017, <>.
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Appendix A. Differences from RFC 6555

"Happy Eyeballs: Success with Dual-Stack Hosts" [RFC6555] 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 [RFC6555]. Implementations should take the new considerations into account when applicable to optimize their behavior.
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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.

Authors' Addresses

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