Network D. Schinazi
Internet-Draft T. Pauly
Obsoletes: 6555 (if approved) Apple Inc.
Intended status: Standards Track October 26, 2017
Expires: April 29, 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, referred to as "Happy Eyeballs". This document obsoletes the original algorithm description in [RFC6555].

Status of This Memo

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This Internet-Draft will expire on April 29, 2018.

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Table of Contents

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 defines the algorithm for "Happy Eyeballs", a technique of 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 generalization.

The Happy Eyeballs algorithm of racing resolved addresses has several stages of ordering and racing 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.

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. 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, 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 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 a positive AAAA response is received 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 [Section 4] and staggered connection attempts [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 [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, the client SHOULD 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 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 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 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.

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 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 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 which 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.

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:

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 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 (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:

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

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 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.
[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.
[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., 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.

13.2. Informative References

[DNS-PUSH] 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.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017.

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:

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.

Authors' Addresses

David Schinazi Apple Inc. 1 Infinite Loop Cupertino, California 95014 US EMail:
Tommy Pauly Apple Inc. 1 Infinite Loop Cupertino, California 95014 US EMail: