| v6ops | D. Wing |
| Internet-Draft | A. Yourtchenko |
| Intended status: Standards Track | Cisco |
| Expires: September 15, 2011 | March 14, 2011 |
Happy Eyeballs: Trending Towards Success with Dual-Stack Hosts
draft-ietf-v6ops-happy-eyeballs-01
This document describes how a dual-stack client can determine the functioning path to a dual-stack server. This provides a seamless user experience during initial deployment of dual-stack networks and during outages of IPv4 or outages of IPv6.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 15, 2011.
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In order to use HTTP successfully over IPv6, it is necessary that the user enjoys nearly identical performance as compared to IPv4. A combination of today's applications, IPv6 tunneling and IPv6 service providers, and some of today's content providers all cause the user experience to suffer (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 is not scalable to all service providers worldwide, nor is it scalable for other content providers to operate their own DNS white list.
Instead, this document suggests a mechanism for applications to quickly determine if IPv6 or IPv4 is the most optimal to connect to a server. The suggestions in this document provide a user experience which is superior to connecting to ordered IP addresses which is helpful during the IPv6/IPv4 transition with dual stack hosts.
This problem is described also in [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.)"
Following the procedures in this document, once a certain address family is successful, the application trends towards preferring that address family. Thus, repeated use of the application DOES NOT cause repeated probes over both address families.
While the application recommendations in this document are described in the context of HTTP clients ("web browsers"), it is also useful and applicable to other interactive applications.
Code which implements some of the ideas described in this document has been made available [Perreault] [Andrews].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
As discussed in more detail in Section 3.1, it is important that the same URI and hostname be used for IPv4 and IPv6. Using separate namespaces causes namespace fragmentation and reduces the ability for users to share URIs and hostnames, and complicates printed material that includes the URI or hostname.
As discussed in more detail in Section 3.2, IPv6 connectivity is sometimes broken entirely or slower than native IPv4 connectivity.
URIs are often used between users to exchange pointers to content -- such as on social networks, email, instant messaging, or other systems. Thus, production URIs and production hostnames containing references to IPv4 or IPv6 will only function if the other party is also using an application, OS, and a network that can access the URI or the hostname.
When IPv6 connectivity is impaired, today's IPv6-capable web browsers incur many seconds of delay before falling back to IPv4. This harms the user's experience with IPv6, which will slow the acceptance of IPv6, because IPv6 is frequently disabled in its entirety on the end systems to improve the user experience.
Reasons for such failure include no connection to the IPv6 Internet, broken 6to4 or Teredo tunnels, and broken IPv6 peering.
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------->|
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.
To provide fast connections for users, clients should make connections quickly over various technologies, automatically tune itself to avoid flooding the network with unnecessary connections (i.e., for technologies that have not made successful connections), and occasionally flush its self-tuning.
If a TCP client supports IPv6 and IPv4 and is connected to IPv4 and IPv6 networks, it can perform the procedures described in this section.
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 |
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).
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------->|
The diagram above shows a case where both IPv6 and IPv4 are working, and IPv4 is abandoned (12).
This section details how to provide robust dual stack service for both IPv6 and IPv4, so that the user perceives very fast application response.
The TCP client application is configured with one application-wide value of P. A positive value indicates a preference for IPv6 and a negative value indicates a preference for IPv4. A value of 0 indicates equal weight, which means the A and AAAA queries and associated connection attempts will be sent as quickly as possible. The absolute value of P is the measure of a delay before initiating a DNS lookup and a connection attempt on the other address family. There are two P values maintained: one is application-wide and the other is specific per each destination (hostname and port).
The algorithm attempts to delay the DNS query until it expects that address family will be necessary; that is, if the preference is towards IPv6, then AAAA will be queried immediately and the A query will be delayed.
The TCP client application starts two concurrent execution flows (they will be referred to as "threads" but this reference does not imply the implementation detail of using the threading library, merely the property of mutual concurrency) in order to minimize the user-noticeable delay ("dead time") during the connection attempts:
The first thread that succeeds returns the completed connection to the parent code and aborts the other thread (Section 5.2).
After a connection is successful, we want to adjust the application-wide preference and the per-destination preference. The value of P is incremented (decremented) each time an IPv6 (IPv4) connection wins the race.. When a connection using the less-preferred address family is successful, it indicates the wrong address family was used and the value of P is halved:
After adjusting P, the resulting delay should never be larger than 4 seconds -- which is similar to the value used by many IPv6-capable TCP client applications to switch to an alternate A or AAAA record.
For the purposes of this section, "client" is defined as the entity initiating the connection.
For protocols which support DNS SRV [RFC2782], the client performs the IN SRV query (e.g. IN SRV _xmpp-client._tcp.example.com) as normal. The client MUST perform the following steps:
It is RECOMMENDED, but not required, for the client to cache the winning connection's address information and reuse it on subsequent connections. If a significant network event occurs (e.g. network interface is activated/deactivated, IP address changes), the client MUST forget the cached address information and perform all of the steps from above. The definition of "significant network event" is intentionally vague.
This section discusses considerations and requirements that are common to new technology deployment.
Additional network traffic and additional server load is created due to these recommendations and mitigated by application-wide and per-destination timer adjustments. The procedures described in this document retain a quality user experience while transitioning from IPv4-only to dual stack. The quality user experience benefits the user but to the detriment of the network and server that are serving the user.
It is RECOMMENDED that the non-winning connections be abandoned, even though they could be used to download content. This is because some web sites provide HTTP clients with cookies (after logging in) that incorporate the client's IP address, or use IP addresses to identify users. If some connections from the same HTTP client are arriving from different IP addresses, such HTTP applications will break. It's also important to abandon connections to avoid consuming server or middlebox (e.g., NAT) resources (file descriptors, memory, TCP control blocks) and avoid sending TCP or application-level keepalives on otherwise unused connections.
Because every network has different characteristics (e.g., working or broken IPv6 connectivity) the IPv6/IPv4 preference value (P) SHOULD be reset to its default whenever the host is connected to a new network ([cx-osx], [cx-win]). However, in some instances the application and the host are unaware the network connectivity has changed so it is RECOMMENDED that per-destination values expire after 10 minutes of inactivity.
For some transitional technologies such as a dual-stack host, it is easy for the application to recognize the native IPv6 address (learned via a AAAA query) and the native IPv4 address (learned via an A query). For other transitional technologies [RFC2766] it is impossible for the host to differentiate a transitional technology IPv6 address from a native IPv6 address (see Section 4.1 of [RFC4966]). Replacement transitional technologies are attempting to bridge this gap. It is necessary for applications to distinguish between native and transitional addresses in order to provide the most seamless user experience.
Application awareness of transitional technologies, if implemented, SHOULD provide a facility to give the preference only to native IPv6 addresses.
This mechanism is aimed at ensuring a reliable user experience regardless of connectivity problems affecting any single transport. However, this naturally means that applications employing these techniques are by default less useful for diagnosing issues with any particular transport. To assist in that regard, the applications implementing the proposal in this document SHOULD also provide a mechanism to revert the behavior to that of a default provided by the operating system - the [RFC3484].
Unique to DNS AAAA queries are the problems described in [RFC4074] which, if they still persist, require applications to perform an A query before the AAAA query.
Some devices are known to exhibit what amounts to a bug, when the A and AAAA requests are sent back-to-back over the same 4-tuple, and drop one of the requests or replies [DNS-middlebox]. However, in some cases fixing this behaviour may not be possible either due to the architectural limitations or due to the administrative constraints (location of the faulty device is unknown to the end hosts or not controlled by the end hosts). The algorithm described in this draft, in the case of this erroneous behaviour will eventually pace the queries such that this issue is will be avoided. The algorithm described in this draft also avoids calling the operating system's getaddrinfo() with "any", which should prevent the operating system from sending the A and AAAA queries on the same port.
For the large part, these issues are believed to be fixed, in which case the getaddrinfo() with AF_UNSPEC as the address family in its hints.
Interaction of the suggestions in this document with multiple interfaces, and interaction with the MIF working group, is for further study ([I-D.chen-mif-happy-eyeballs-extension] is devoted to this).
Content providers SHOULD provide both AAAA and A records for servers using the same DNS name for both IPv4 and IPv6.
[[Placeholder.]]
See Section 5.2.
The mechanism described in this paper was inspired by Stuart Cheshire's discussion at the IAB Plenary at IETF72, the author's understanding of Safari's operation with SRV records, Interactive Connectivity Establishment (ICE [RFC5245]), and the current IPv4/IPv6 behavior of SMTP mail transfer agents.
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 for providing feedback on the document.
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, Cameron Byrne, Mark Smith, Gert Doering, Martin Millnert, Tim Durack, Matthew Palmer.
This document has no IANA actions.
| [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. |
| [RFC2782] | Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000. |