v6ops                                                            D. Wing
Internet-Draft                                            A. Yourtchenko
Intended status:  Standards Track                                  Cisco
Expires:  September 4, 2011                                March 3, 2011

     Happy Eyeballs: Trending Towards Success with Dual-Stack Hosts


   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.

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
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   This Internet-Draft will expire on September 4, 2011.

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   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  4
   3.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  URIs and hostnames . . . . . . . . . . . . . . . . . . . .  4
     3.2.  IPv6 connectivity  . . . . . . . . . . . . . . . . . . . .  4
   4.  Client Recommendations . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Dualstack behavior . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Implementation details . . . . . . . . . . . . . . . . . .  6
     4.3.  Additional Considerations  . . . . . . . . . . . . . . . .  8
       4.3.1.  Additional Network and Host Traffic  . . . . . . . . .  8
       4.3.2.  Abandon Non-Winning Connections  . . . . . . . . . . .  9
       4.3.3.  Flush or Expire Cache  . . . . . . . . . . . . . . . .  9
       4.3.4.  Determining Address Type . . . . . . . . . . . . . . .  9
       4.3.5.  Debugging and Troubleshooting  . . . . . . . . . . . .  9
       4.3.6.  DNS Behavior . . . . . . . . . . . . . . . . . . . . . 10
       4.3.7.  Middlebox Issues . . . . . . . . . . . . . . . . . . . 10
       4.3.8.  Multiple Interfaces  . . . . . . . . . . . . . . . . . 11
     4.4.  Content Provider Recommendations . . . . . . . . . . . . . 11
     4.5.  Security Considerations  . . . . . . . . . . . . . . . . . 11
     4.6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . 11
     4.7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . 12
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     5.2.  Informational References . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

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1.  Introduction

   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].

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2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Problem Statement

   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.

3.1.  URIs and hostnames

   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.

3.2.  IPv6 connectivity

   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.

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           DNS Server                  Client                  Server
               |                          |                       |
         1.    |<--www.example.com A?-----|                       |
         2.    |<--www.example.com AAAA?--|                       |
         3.    |--->|                       |
         4.    |---2001:dba::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

4.  Client Recommendations

   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.

4.1.  Dualstack behavior

   If a TCP client supports IPv6 and IPv4 and is connected to IPv4 and
   IPv6 networks, it can perform the procedures described in this

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           DNS Server                  Client                  Server
               |                          |                       |
         1.    |<--www.example.com A?-----|                       |
         2.    |<--www.example.com AAAA?--|                       |
         3.    |--->|                       |
         4.    |---2001:dba::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).

           DNS Server                  Client                  Server
               |                          |                       |
         1.    |<--www.example.com A?-----|                       |
         2.    |<--www.example.com AAAA?--|                       |
         3.    |--->|                       |
         4.    |---2001:dba::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).

4.2.  Implementation details

   This section details how to provide robust dual stack service for
   both IPv6 and IPv4, so that the user perceives very fast application

   The TCP client application is configured with one application-wide

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   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:

   thread 1: (IPv6)

      *  If P<0, wait for absolute value of p*10 milliseconds

      *  send DNS query for AAAA

      *  wait until DNS response is received

      *  Attempt to connect over IPv6 using TCP

   thread 2: (IPv4)

      *  if P>0, wait for p*10 milliseconds

      *  send DNS query for A

      *  wait until DNS response is received

      *  Attempt to connect over IPv4 using TCP

   The first thread that succeeds returns the completed connection to
   the parent code and aborts the other thread (Section 4.3.2).

   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

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   value of P is halved:

   o  If P>0 (indicating IPv6 is preferred over IPv4) and the first
      thread to finish was the IPv6 thread it indicates the IPv6
      preference is correct and we need to re-enforce this by increasing
      the application-wide P value by 1.  However, if the first thread
      to finish was the IPv4 thread it indicates an IPv6 connection
      problem occurred and we need to aggressively prefer IPv4 more by
      halving P and rounding towards 0.

   o  If P<0 (indicating IPv4 is preferred over IPv6) and the first
      thread to finish was the IPv4 thread it indicates the preference
      is correct and we need to re-enforce this gently by decreasing the
      application-wide P value by 1.  However, if the first thread to
      finish was the IPv6 thread it indicates an IPv4 connection problem
      and we need to aggressively avoid IPv4 by halving P and rounding
      towards 0.

   o  If P=0 (indicating equal preference), P is incremented by one if
      the first thread to complete was the IPv6 thread, or decremented
      by one if the first thread to complete was the IPv4 thread.

   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.

      Editor's Note 01:  Proof of concept tests on fast networks show
      that even smaller value (around 0.5 seconds) may be practical.
      More extensive testing would be useful to find the best upper
      boundary that still ensures a good user experience.

      Editor's Note 02:  A strict implementation of the above steps
      results in "P" being adjusted if there are no AAAA records or are
      no A records.  This is undesirable.  Thus, a future version of
      this specification is expected to recommend that "P" only be
      adjusted if there was both an A and AAAA record.

4.3.  Additional Considerations

   This section discusses considerations and requirements that are
   common to new technology deployment.

4.3.1.  Additional Network and Host Traffic

   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

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   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.

4.3.2.  Abandon Non-Winning Connections

   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.

4.3.3.  Flush or Expire Cache

   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.

4.3.4.  Determining Address Type

   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

4.3.5.  Debugging and Troubleshooting

   This mechanism is aimed at ensuring a reliable user experience
   regardless of connectivity problems affecting any single transport.

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   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].

      [[[ To be discussed.

      Some sites may wish to be informed when the the hosts adjust their
      "P" value, in order to troubleshoot the underlying cause.  To help
      these sites, a strawman proposal is to send a syslog message or
      other notification to an address that may be configured by a site
      administrator in a centralized fashion.  (The exact method TBD -
      DHCP option, domain name, etc.)  This syslog message should be
      sent only first N times that the host expects to prefer IPv6 but
      has to use IPv4.  I.e. the first N times it decreases the value of
      P. N - TBD.


4.3.6.  DNS Behavior

   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.

      [[Editor's Note 03:  It is believed these defective DNS servers
      have long since been upgraded.  If so, we can remove this

4.3.7.  Middlebox Issues

   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

   For the large part, these issues are believed to be fixed, in which

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   case the getaddrinfo() with AF_UNSPEC as the address family in its

4.3.8.  Multiple Interfaces

   Interaction of the suggestions in this document with multiple
   interfaces, and interaction with the MIF working group, is for
   further study.

4.4.  Content Provider Recommendations

   Content providers SHOULD provide both AAAA and A records for servers
   using the same DNS name for both IPv4 and IPv6.

4.5.  Security Considerations


   See Section 4.3.2.

4.6.  Acknowledgements

   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 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.

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4.7.  IANA Considerations

   This document has no IANA actions.

5.  References

5.1.  Normative References

   [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.

5.2.  Informational References

   [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-h omed-server-over-tcp>.

              Various, "DNS middlebox behavior with multiple queries
              over same source port", June 2009,

              Perreault, S., "Happy Eyeballs in Erlang", February 2011,

   [RFC1671]  Carpenter, B., "IPng White Paper on Transition and Other
              Considerations", RFC 1671, August 1994.

   [RFC2766]  Tsirtsis, G. and P. Srisuresh, "Network Address
              Translation - Protocol Translation (NAT-PT)", RFC 2766,
              February 2000.

   [RFC4074]  Morishita, Y. and T. Jinmei, "Common Misbehavior Against
              DNS Queries for IPv6 Addresses", RFC 4074, May 2005.

   [RFC4966]  Aoun, C. and E. Davies, "Reasons to Move the Network
              Address Translator - Protocol Translator (NAT-PT) to
              Historic Status", RFC 4966, July 2007.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,

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              April 2010.

   [cx-osx]   Adium, "AIHostReachabilityMonitor", June 2009,

   [cx-win]   Microsoft, "NetworkChange.NetworkAvailabilityChanged
              Event", June 2009, <http://msdn.microsoft.com/en-us/

              Google, "Google IPv6 DNS Whitelist", January 2009,

Authors' Addresses

   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134

   Email:  dwing@cisco.com

   Andrew Yourtchenko
   Cisco Systems, Inc.
   De Kleetlaan, 7
   San Jose, Diegem  B-1831

   Email:  ayourtch@cisco.com

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