Internet Engineering Task Force (IETF) D. Wing Request for Comments: 6555 A. Yourtchenko Category: Standards Track Cisco ISSN: 2070-1721 April 2012 Happy Eyeballs: Success with Dual-Stack Hosts
AbstractWhen a server's IPv4 path and protocol are working, but the server's IPv6 path and protocol are not working, a dual-stack client application experiences significant connection delay compared to an IPv4-only client. This is undesirable because it causes the dual- stack client to have a worse user experience. This document specifies requirements for algorithms that reduce this user-visible delay and provides an algorithm. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6555. Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Additional Network and Host Traffic . . . . . . . . . . . 3 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3 3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Hostnames . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Delay When IPv6 Is Not Accessible . . . . . . . . . . . . 5 4. Algorithm Requirements . . . . . . . . . . . . . . . . . . . . 6 4.1. Delay IPv4 . . . . . . . . . . . . . . . . . . . . . . . . 7 4.2. Stateful Behavior When IPv6 Fails . . . . . . . . . . . . 8 4.3. Reset on Network (Re-)Initialization . . . . . . . . . . . 9 4.4. Abandon Non-Winning Connections . . . . . . . . . . . . . 9 5. Additional Considerations . . . . . . . . . . . . . . . . . . 10 5.1. Determining Address Type . . . . . . . . . . . . . . . . . 10 5.2. Debugging and Troubleshooting . . . . . . . . . . . . . . 10 5.3. Three or More Interfaces . . . . . . . . . . . . . . . . . 10 5.4. A and AAAA Resource Records . . . . . . . . . . . . . . . 10 5.5. Connection Timeout . . . . . . . . . . . . . . . . . . . . 11 5.6. Interaction with Same-Origin Policy . . . . . . . . . . . 11 5.7. Implementation Strategies . . . . . . . . . . . . . . . . 12 6. Example Algorithm . . . . . . . . . . . . . . . . . . . . . . 12 7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 9.1. Normative References . . . . . . . . . . . . . . . . . . . 13 9.2. Informative References . . . . . . . . . . . . . . . . . . 13
Section 3). For IPv6, a content provider may ensure a positive user experience by using a DNS white list of IPv6 service providers who peer directly with them (e.g., [WHITELIST]). However, this does not scale well (to the number of DNS servers worldwide or the number of content providers worldwide) and does react to intermittent network path outages. Instead, applications reduce connection setup delays themselves, by more aggressively making connections on IPv6 and IPv4. There are a variety of algorithms that can be envisioned. This document specifies requirements for any such algorithm, with the goals that the network and servers not be inordinately harmed with a simple doubling of traffic on IPv6 and IPv4 and the host's address preference be honored (e.g., [RFC3484]). RFC2119].
RFC1671]: The dual-stack code may get two addresses back from DNS; which does it use? During the many years of transition the Internet will contain black holes. For example, somewhere on the way from IPng host A to IPng host B there will sometimes (unpredictably) be IPv4-only routers which discard IPng packets. Also, the state of the DNS does not necessarily correspond to reality. A host for which DNS claims to know an IPng address may in fact not be running IPng at a particular moment; thus an IPng packet to that host will be discarded on delivery. Knowing that a host has both IPv4 and IPng addresses gives no information about black holes. A solution to this must be proposed and it must not depend on manually maintained information. (If this is not solved, the dual-stack approach is no better than the packet translation approach.) As discussed in more detail in Section 3.1, it is important that the same hostname be used for IPv4 and IPv6. As discussed in more detail in Section 3.2, IPv6 connectivity is broken to specific prefixes or specific hosts or is slower than native IPv4 connectivity. The mechanism described in this document is directly applicable to connection-oriented transports (e.g., TCP, SCTP), which is the scope of this document. For connectionless transport protocols (e.g., UDP), a similar mechanism can be used if the application has request/ response semantics (e.g., as done by Interactive Connectivity Establishment (ICE) to select a working IPv6 or IPv4 media path [RFC6157]).
DNS Server Client Server | | | 1. |<--www.example.com A?-----| | 2. |<--www.example.com AAAA?--| | 3. |---192.0.2.1------------->| | 4. |---2001:db8::1----------->| | 5. | | | 6. | |==TCP SYN, IPv6===>X | 7. | |==TCP SYN, IPv6===>X | 8. | |==TCP SYN, IPv6===>X | 9. | | | 10. | |--TCP SYN, IPv4------->| 11. | |<-TCP SYN+ACK, IPv4----| 12. | |--TCP ACK, IPv4------->| Figure 1: Existing Behavior Message Flow The client obtains the IPv4 and IPv6 records for the server (1-4). The client attempts to connect using IPv6 to the server, but the IPv6 path is broken (6-8), which consumes several seconds of time. Eventually, the client attempts to connect using IPv4 (10), which succeeds. Delays experienced by users of various browser and operating system combinations have been studied [Experiences].
DNS Server Client Server | | | 1. |<--www.example.com A?-----| | 2. |<--www.example.com AAAA?--| | 3. |---192.0.2.1------------->| | 4. |---2001:db8::1----------->| | 5. | | | 6. | |==TCP SYN, IPv6===>X | 7. | |--TCP SYN, IPv4------->| 8. | |<-TCP SYN+ACK, IPv4----| 9. | |--TCP ACK, IPv4------->| 10. | |==TCP SYN, IPv6===>X | Figure 2: Happy Eyeballs Flow 1, IPv6 Broken In the diagram above, the client sends two TCP SYNs at the same time over IPv6 (6) and IPv4 (7). In the diagram, the IPv6 path is broken but has little impact to the user because there is no long delay before using IPv4. The IPv6 path is retried until the application gives up (10). After performing the above procedure, the client learns whether connections to the host's IPv6 or IPv4 address were successful. The client MUST cache information regarding the outcome of each connection attempt, and it uses that information to avoid thrashing the network with subsequent attempts. In the example above, the cache indicates that the IPv6 connection attempt failed, and therefore the system will prefer IPv4 instead. Cache entries should be flushed when their age exceeds a system-defined maximum on the order of 10 minutes.
DNS Server Client Server | | | 1. |<--www.example.com A?-----| | 2. |<--www.example.com AAAA?--| | 3. |---192.0.2.1------------->| | 4. |---2001:db8::1----------->| | 5. | | | 6. | |==TCP SYN, IPv6=======>| 7. | |--TCP SYN, IPv4------->| 8. | |<=TCP SYN+ACK, IPv6====| 9. | |<-TCP SYN+ACK, IPv4----| 10. | |==TCP ACK, IPv6=======>| 11. | |--TCP ACK, IPv4------->| 12. | |--TCP RST, IPv4------->| Figure 3: Happy Eyeballs Flow 2, IPv6 Working The diagram above shows a case where both IPv6 and IPv4 are working, and IPv4 is abandoned (12). Any Happy Eyeballs algorithm will persist in products for as long as the client host is dual-stacked, which will persist as long as there are IPv4-only servers on the Internet -- the so-called "long tail". Over time, as most content is available via IPv6, the amount of IPv4 traffic will decrease. This means that the IPv4 infrastructure will, over time, be sized to accommodate that decreased (and decreasing) amount of traffic. It is critical that a Happy Eyeballs algorithm not cause a surge of unnecessary traffic on that IPv4 infrastructure. To meet that goal, compliant Happy Eyeballs algorithms must adhere to the requirements in this section.
they can only utilize IPv4. If all hosts (dual-stack and IPv4-only) are using IPv4, there is additional contention for the shared IPv4 address. The IPv4-only hosts cannot avoid that contention (as they can only use IPv4), while the dual-stack hosts can avoid it by using IPv6. As dual-stack hosts proliferate and content becomes available over IPv6, there will be proportionally less IPv4 traffic. This is true especially for dual-stack hosts that do not implement Happy Eyeballs, because those dual-stack hosts have a very strong preference to use IPv6 (with timeouts in the tens of seconds before they will attempt to use IPv4). When deploying IPv6, both content providers and Internet Service Providers (who supply mechanisms for IPv4 address sharing such as Carrier-Grade NAT (CGN)) will want to reduce their investment in IPv4 equipment -- load-balancers, peering links, and address sharing devices. If a Happy Eyeballs implementation treats IPv6 and IPv4 equally by connecting to whichever address family is fastest, it will contribute to load on IPv4. This load impacts IPv4-only devices (by increasing contention of IPv4 address sharing and increasing load on IPv4 load-balancers). Because of this, ISPs and content providers will find it impossible to reduce their investment in IPv4 equipment. This means that costs to migrate to IPv6 are increased because the investment in IPv4 cannot be reduced. Furthermore, using only a metric that measures the connection speed ignores the benefits that IPv6 brings when compared with IPv4 address sharing, such as improved geo-location [RFC6269] and the lack of fate-sharing due to traversing a large translator. Thus, to avoid harming IPv4-only hosts, implementations MUST prefer the first IP address family returned by the host's address preference policy, unless implementing a stateful algorithm described in Section 4.2. This usually means giving preference to IPv6 over IPv4, although that preference can be overridden by user configuration or by network configuration [ADDR-SELECT]. If the host's policy is unknown or not attainable, implementations MUST prefer IPv6 over IPv4.
After making a connection attempt on the preferred address family (e.g., IPv6) and failing to establish a connection within a certain time period (see Section 5.5), a Happy Eyeballs implementation will decide to initiate a second connection attempt using the same address family or the other address family. Such an implementation MAY make subsequent connection attempts (to the same host or to other hosts) on the successful address family (e.g., IPv4). So long as new connections are being attempted by the host, such an implementation MUST occasionally make connection attempts using the host's preferred address family, as it may have become functional again, and it SHOULD do so every 10 minutes. The 10-minute delay before retrying a failed address family avoids the simple doubling of connection attempts on both IPv6 and IPv4. Implementation note: this can be achieved by flushing Happy Eyeballs state every 10 minutes, which does not significantly harm the application's subsequent connection setup time. If connections using the preferred address family are again successful, the preferred address family SHOULD be used for subsequent connections. Because this implementation is stateful, it MAY track connection success (or failure) based on IPv6 or IPv4 prefix (e.g., connections to the same prefix assigned to the interface are successful whereas connections to other prefixes are failing). RFC4436], DNAv6 [RFC6059]). If the client application is a web browser, see also Section 5.6.
the same client are arriving from different IP addresses (or worse, different IP address families), such applications will break. Additionally, for HTTP, using the non-winning connection can interfere with the browser's same-origin policy (see Section 5.6).
address. If that fails after a certain time (see Section 5.5), the next address SHOULD be the IPv4 address. If that fails to connect after a certain time (see Section 5.5), a Happy Eyeballs implementation SHOULD try the other addresses returned; the order of these connection attempts is not important. On the Internet today, servers commonly have multiple A records to provide load-balancing across their servers. This same technique would be useful for AAAA records, as well. However, if multiple AAAA records are returned to a client that is not using Happy Eyeballs and that has broken IPv6 connectivity, it will further increase the delay to fall back to IPv4. Thus, web site operators with native IPv6 connectivity SHOULD NOT offer multiple AAAA records. If Happy Eyeballs is widely deployed in the future, this recommendation might be revisited. RFC6454] that causes subsequent connections to the same hostname to go to the same IPv4 (or IPv6) address as the previous successful connection. This is done to prevent certain types of attacks. The same-origin policy harms user-visible responsiveness if a new connection fails (e.g., due to a transient event such as router failure or load-balancer failure). While it is tempting to use Happy Eyeballs to maintain responsiveness, web browsers MUST NOT change their same-origin policy because of Happy Eyeballs, as that would create an additional security exposure.
RFC5245], the current IPv4/IPv6 behavior of SMTP mail transfer agents, and the implementation of Happy Eyeballs in Google Chrome and Mozilla Firefox. Thanks to Fred Baker, Jeff Kinzli, Christian Kuhtz, and Iljitsch van Beijnum for fostering the creation of this document. Thanks to Scott Brim, Rick Jones, Stig Venaas, Erik Kline, Bjoern Zeeb, Matt Miller, Dave Thaler, Dmitry Anipko, Brian Carpenter, and David Crocker for their feedback. Thanks to Javier Ubillos, Simon Perreault, and Mark Andrews for the active feedback and the experimental work on the independent practical implementations that they created. Also the authors would like to thank the following individuals who participated in various email discussions on this topic: Mohacsi Janos, Pekka Savola, Ted Lemon, Carlos Martinez-Cagnazzo, Simon Perreault, Jack Bates, Jeroen Massar, Fred Baker, Javier Ubillos, Teemu Savolainen, Scott Brim, Erik Kline, Cameron Byrne, Daniel Roesen, Guillaume Leclanche, Mark Smith, Gert Doering, Martin Millnert, Tim Durack, and Matthew Palmer. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3484] Draves, R., "Default Address Selection for Internet Protocol version 6 (IPv6)", RFC 3484, February 2003. [ADDR-SELECT] Matsumoto, A., Fujisaki, T., Kato, J., and T. Chown, "Distributing Address Selection Policy using DHCPv6", Work in Progress, February 2012. [Andrews] Andrews, M., "How to connect to a multi-homed server over TCP", January 2011, <http://www.isc.org/community/ blog/201101/how-to-connect-to-a-multi-homed-server- over-tcp>.
[Experiences] Savolainen, T., Miettinen, N., Veikkolainen, S., Chown, T., and J. Morse, "Experiences of host behavior in broken IPv6 networks", March 2011, <http://www.ietf.org/proceedings/80/slides/ v6ops-12.pdf>. [Perreault] Perreault, S., "Happy Eyeballs in Erlang", February 2011, <http://www.viagenie.ca/news/ index.html#happy_eyeballs_erlang>. [RFC1671] Carpenter, B., "IPng White Paper on Transition and Other Considerations", RFC 1671, August 1994. [RFC4436] Aboba, B., Carlson, J., and S. Cheshire, "Detecting Network Attachment in IPv4 (DNAv4)", RFC 4436, March 2006. [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", RFC 5245, April 2010. [RFC6059] Krishnan, S. and G. Daley, "Simple Procedures for Detecting Network Attachment in IPv6", RFC 6059, November 2010. [RFC6157] Camarillo, G., El Malki, K., and V. Gurbani, "IPv6 Transition in the Session Initiation Protocol (SIP)", RFC 6157, April 2011. [RFC6269] Ford, M., Boucadair, M., Durand, A., Levis, P., and P. Roberts, "Issues with IP Address Sharing", RFC 6269, June 2011. [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, December 2011. [WHITELIST] Google, "Google over IPv6", <http://www.google.com/intl/en/ipv6>.