v6ops                                                            D. Wing
Internet-Draft                                            A. Yourtchenko
Intended status:  Standards Track                                  Cisco
Expires:  November 25, 2011                                 May 24,  January 9, 2012                                   July 8, 2011

             Happy Eyeballs: Trending Towards  Success with Dual-Stack Hosts
                   draft-ietf-v6ops-happy-eyeballs-02
                   draft-ietf-v6ops-happy-eyeballs-03

Abstract

   This document describes an algorithm for

   When the IPv4 server and path is working but the IPv6 server or IPv6
   path is down, a dual-stack client application experiences significant
   connection delay compared to
   quickly determine the functioning address family to a dual-stack
   server, and trend towards using that same address family for
   subsequent connections. an IPv4-only client.  This improves is
   undesirable because it causes the dual-stack client to have a worse
   user experience
   during IPv6 or IPv4 server or network outages. experience.  This document specifies requirements for algorithms
   that reduce this delay, and provides an example algorithm.

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 25, 2011. January 9, 2012.

Copyright Notice

   Copyright (c) 2011 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
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   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
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  4  3
   3.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  4  3
     3.1.  URIs and hostnames . . . . . . . . . . . . . . . . . . . .  4
     3.2.  IPv6 connectivity  . . . . . . . . . . . . . . . . . . . .  5  4
   4.  Client Recommendations  Algorithm Requirements . . . . . . . . . . . . . . . . . . . .  5
   5.  Implementation details: A and AAAA
     4.1.  Adhere to Address Preference Policy  . . . . . . . . . . .  6
     4.2.  Behavior when Preferred Address Family has Failed  . . . .  7
     5.1.  Description of State Variables
     4.3.  Reset on Network (re-)Initialization . . . . . . . . . . .  7
     4.4.  Abandon Non-Winning Connections  . . .  7
     5.2.  Initialization, Cache Flush, and Resetting Smoothed P . .  9
     5.3.  Connecting to a Server . . . . . . . .  7
   5.  Additional Considerations  . . . . . . . . . .  9
     5.4.  Adjusting Address Family Preferences . . . . . . . .  8
     5.1.  Additional Network and Host Traffic  . . . 10
     5.5.  Exception Cache . . . . . . . .  8
     5.2.  Determining Address Type . . . . . . . . . . . . . 11
   6.  Implementation Details: SRV . . . .  8
     5.3.  Debugging and Troubleshooting  . . . . . . . . . . . . . 12
   7.  Additional Considerations .  8
     5.4.  Multiple Interfaces  . . . . . . . . . . . . . . . . . 13
     7.1.  Additional Network and Host Traffic . .  9
     5.5.  Interaction with Same Origin Policy  . . . . . . . . . 13
     7.2.  Abandon Non-Winning Connections . .  9
     5.6.  Happy Eyeballs in an Operating System  . . . . . . . . . .  9
   6.  Example Algorithm  . 13
     7.3.  Determining Address Type . . . . . . . . . . . . . . . . . 13
     7.4.  Debugging and Troubleshooting . . . .  9
   7.  Security Considerations  . . . . . . . . . . 13
     7.5.  DNS Behavior . . . . . . . . . 10
   8.  Acknowledgements . . . . . . . . . . . . . . 14
     7.6.  Middlebox Issues . . . . . . . . . 10
   9.  IANA Considerations  . . . . . . . . . . . . 14
     7.7.  Multiple Interfaces . . . . . . . . . 10
   10. References . . . . . . . . . . 14
     7.8.  Interaction with Same Origin Policy . . . . . . . . . . . 14
   8.  Content Provider Recommendations . . . . . 11
     10.1. Normative References . . . . . . . . . . 15
   9.  Security Considerations . . . . . . . . . 11
     10.2. Informational References . . . . . . . . . . 15
   10. Acknowledgements . . . . . . . 11
   Appendix A.  Changes . . . . . . . . . . . . . . . . 15
   11. IANA Considerations . . . . . . . 12
     A.1.  changes from -02 to -03  . . . . . . . . . . . . . . 16
   12. References . . . 12
     A.2.  changes from -01 to -02  . . . . . . . . . . . . . . . . . 12
     A.3.  changes from -00 to -01  . . . . . . 16
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 16
     12.2. Informational References . . . . . . . . . . . . . . . . . 16
   Appendix A.  Changes . . . . . . . . . . . . . . . . . . . . . . . 17
     A.1.  changes from -01 to -02  . . . . . . 13
   Authors' Addresses . . . . . . . . . . . 18
     A.2.  changes from -00 to -01  . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18 13

1.  Introduction

   In order to use HTTP successfully applications over IPv6, it is necessary that the
   user enjoys users
   enjoy nearly identical performance as compared to IPv4.  A
   combination of today's applications, IPv6 tunneling and tunneling, 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]. them (e.g., [whitelist]).
   However, this is does not scalable to all service providers worldwide, nor
   is it scalable for other scale well (to the number of DNS servers
   worldwide or the number of content providers worldwide), and does not
   react to operate their own DNS
   white list. intermittent network path outages.

   Instead, this document suggests a mechanism for applications to
   quickly determine if can improve the user experience themselves, by
   more aggressively making connections on IPv6 or IPv4 is and IPv4.  There are a
   variety of algorithms that can be envisioned.  This document
   specifies requirements for any such algorithm, with the most optimal to connect to goals that
   the network and servers are not inordinately harmed with a
   server. simple
   doubling of traffic on IPv6 and IPv4, and the host's address
   preference is honored (e.g., [RFC3484]).

2.  Notational Conventions

   The suggestions key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document provide a user experience
   which is superior to connecting are to ordered IP addresses which is
   helpful during be interpreted as described in [RFC2119].

3.  Problem Statement

   The basis of the IPv6/IPv4 transition with dual stack hosts.

   This selection problem is also was first described in [RFC1671], published
   1994 in 1994: [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.)"

   Even after the transition, the procedure described in this document
   allows applications to strongly prefer IPv6 -- yet when an IPv6
   outage occurs the application will quickly start using IPv4 and
   continue using IPv4.  It will quietly continue trying to use IPv6
   until IPv6 becomes available again, and then trend again towards
   using IPv6.

   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.

   Applications would have to change in order to use the mechanism
   described in this document, by either implementing the mechanism
   directly, or by calling APIs made available to them.  To improve IPv6
   connectivity experience for legacy applications (e.g., applications
   which simply rely on the operating system's address preference
   order), operating systems may use other approaches.  These can
   include changing address sorting based on configuration received from
   the network, other configuration, or dynamic detection of the host
   connectivity to IPv6 and IPV4 destinations.

   While the application recommendations in this document are described
   in the context of HTTP clients ("web browsers") and SRV clients
   (e.g., XMPP clients) the procedure 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].

2.  Notational Conventions

   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].

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 (e.g., "ipv6.example.com") 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
   broken to specific prefixes or specific hosts, 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.

           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].

4.  Client Recommendations  Algorithm Requirements

   A Happy Eyeballs does algorithm has two things: primary goals:

   1.  Provides fast connection for users.  To provide fast connections
       for users, clients should make connections by quickly over various
       technologies, automatically tune itself attempting to avoid flooding the
       network with unnecessary connections (i.e., for technologies that
       have not made successful connections),
       connect using IPv6 and occasionally flush its
       self-tuning if it trended towards IPv4 Section 5.2. IPv4.

   2.  Avoids thrashing the network.  Clients need to avoid flooding the
       network or servers with excessive connection initiation traffic.
       One way to accomplish this, without significant impairment to the
       user experience, is to cache which address family has been
       unsuccessful and successful, and use that address family for
       subsequent connections to the same host.

   If a TCP client supports network, by not always making simultaneous
       IPv6 and IPv4 and connection attempts.

   The basic idea is connected to IPv4 and
   IPv6 networks, it can perform the procedures described depicted in this
   section. the following diagram:

           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 if
   connections to the host's IPv6 or IPv4 address were successful.  The
   client MUST cache that information to avoid thrashing the network
   with excessive subsequent connection attempts.  For example, in the
   diagram above, the client has noticed that IPv6 to that address
   failed, and it should provide a greater preference to using IPv4
   instead.

           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).

5.  Implementation details: A and AAAA

   This section details how to provide robust dual stack service

   Any Happy Eyeballs algorithm will persist in products for
   both IPv6 and IPv4, so that as long as
   the user perceives very fast application
   response.

   Depending client host is dual-stacked, which will persist as long as there
   are IPv4-only servers on implementation, the variables and procedures described
   below might be implemented or maintained within a specific
   application (e.g., web browser), library, framework, or by Internet -- the
   operating system itself.  An API call such so-called "long tail".
   Over time, as "connect_by_name()" most content is
   envisioned which would call the Happy Eyeballs routine and implement available via IPv6, the functions described in this section.

5.1.  Description amount of State Variables

   The system maintains a Smoothed P (which provides IPv4
   traffic will decrease.  This means that the overall
   preference IPv4 infrastructure will,
   over time, be sized to IPv6 or IPv4), and an exception cache.  Both accomodate that decreased (and decreasing)
   amount of these
   change over time and are described below:

   Exception Cache:    This traffic.  It is critical that a cache, indexed by IP prefixes, contains Happy Eyeballs algorithm
   not cause a "P" value for each prefix.  Entries are added surge of unnecessary traffic on that IPv4 infrastructure.
   To meet that goal, compliant Happy Eyeballs algorithms must adhere to
   the requirements in this cache if a
      connection section.

4.1.  Adhere to the expected Address Preference Policy

   All hosts have an address family failed selection policy.  IPv6-capable hosts
   usually implement [RFC3484] and a connection may allow the user (via configuration
   commands) or the network to modify that address selection policy
   (e.g., [I-D.ietf-6man-addr-select-opt]).  In most cases, the other
   preferred address family succeeded.  That is, these are
      exceptions to is IPv6.

   Happy Eyeballs implementations MUST follow the Smoothed P variable.  See Section 5.5 for
      description of how these prefixes are defined.

         (Note:  In previous versions of this document, this was the
         "per-destination P (preference) value".)

   P: Address family preference.  This is computed for this connection
      attempt.  A positive value is a host's address
   preference to start the policy or, if that policy is unknown, implementations MUST
   prefer IPv6
      connection first, a negative value to start the over IPv4.

      Justification:  This reduces load on stateful IPv4 connection
      first, middleboxes
      (NAT and zero indicates both IPv6 firewalls) and reduces IPv4 connections are
      started simultaneously.  The absolute value is the number of
      milliseconds between the address sharing contention.

4.2.  Behavior when Preferred Address Family has Failed

   After making a connection attempts attempt on two address
      families.

   Smoothed P:  Smoothed a certain address family preference.  This is (e.g.,
   IPv6), a Happy Eyeballs implementation will decide to initiate a
   second connection attempt using the other address family preference for destinations (e.g.,
   IPv4).

   After doing so and noticing that connections using the other address
   family (e.g., IPv4) are not in successful, a Happy Eyeballs implementation
   MAY make subsequent connection attempts on the exception
      cache.  This variable can be positive or negative, with values
      having successful address
   family (e.g., IPv4).  Such an implementationMUST occasionally make
   connection attempts using the same meaning host's preferred address family, as "P".  In the absence of more specific
      configuration information, it
   may have become functional.  It is RECOMMENDED that implementations
      enforce a maximum value of 8000 (8 seconds) for this variable.

         (Note:  In previous versions of this document,
   try the preferred address family at least every 10 minutes.  Note:
   this was can be achieved by connecting to both address families at the
         "application-wide P (preference) value".)

   The following values
   same time, which does not significantly harm the application's
   connection setup time for the successful address family.  If
   connections using the preferred address family are configured and constant:

   TI:  Tolerance Interval, in milliseconds.  This is successful, the allowance in
   preferred address family SHOULD be used for subsequent connections.

      Justification:  Once the time a connection is expected to complete IPv6 path becomes usable again, this
      reduces load on stateful IPv4 middleboxes (NAT and its actual
      completion, firewalls) and is provided to accommodate slight differences in
      network and server responsiveness.  In the absence of dynamic
      configuration information from the network (e.g., DHCP) or other
      configuration information, it is RECOMMENDED to use 20ms.

   Initial Headstart (IH):  The initial headstart ("preference") for
      IPv6, in milliseconds.  This value provides a preference towards
      IPv6 (if positive) or IPv4 (if negative) when the host joins a new
      network or otherwise flushes its cached information (see
      Section 5.2), and the distance to move P away from zero when P was
      zero.  In the absence of dynamic configuration information from
      the network (e.g., [I-D.ietf-6man-addr-select-opt]) or other
      configuration information (e.g., the node's address selection
      policy has been modified to prefer IPv4 over IPv6), the value
      100ms is recommended, which causes the initial IPv6 connection to
      be attempted 100ms before the IPv4 connection.

   MAXWAIT:  Maximum wait time for a connection to complete, before
      trying additional IP addresses.  This is RECOMMENDED to be 10
      seconds.

5.2.  Initialization, Cache Flush, and Resetting Smoothed P

   Because every
      reduces IPv4 address sharing contention.

4.3.  Reset on Network (re-)Initialization

   Because every network has different characteristics (e.g., working or
   broken IPv6 or IPv4 connectivity) the Smoothed P variable SHOULD be
   set to its default value (Smoothed P = Initial Headstart) and the
   exception cache connectivity), a Happy Eyeballs algorithm SHOULD be emptied whenever
   re-initialize when the host is connected to a new network.  Hosts can
   determine network (e.g., (re-)initialization by a variety of mechanisms
   including DNAv4 [RFC4436], DNAv6 [RFC6059], [cx-osx],
   [cx-win]).

   If there are IPv6 failures to specific hosts or prefixes, [cx-win].

      Justification:  This provides the
   exception cache will build up exception entries preferring IPv4, and
   if there are significant IPv6 failures to many hosts or prefixes,
   Smoothed P will become negative.  When this occurs, best chance that IPv6 will not be
      attempted at all.  To avoid this problem, it is strongly RECOMMENDED
   to occasionally flush over the exception cache of all entries and reset
   Smoothed P to Initial Offset.  This SHOULD be done every 10 minutes.
   In so doing, IPv6 and IPv4 are tried again so that if new interface.

   If the IPv6 client application is
   working again, it will quickly be preferred again.

5.3.  Connecting to a Server

   The steps when connecting to a server are as follows:

   1.  query DNS using getaddrinfo().  This will return addresses sorted
       by the host's default address selection ordering [RFC3484], its
       updates, or the address selection as chosen by web browser, see also Section 5.5.

4.4.  Abandon Non-Winning Connections

   It is RECOMMENDED that the network
       administrator [I-D.ietf-6man-addr-select-opt].

   2.  If this returns both an IPv6 and IPv4 address, continue
       processing to the next stop.  Otherwise, Happy Eyeballs
       processing stops here.

   3.  Of the addresses returned in step (1), look up the first IPv6
       address and first IPv4 address in the Happy Eyeballs exception
       cache.  Matching entries in the exception cache influence the P
       value for this connection attempt by setting P to the sum of
       Smoothed_P and of the P values from the matching IPv6 entry (if
       it exists) and the matching IPv4 entry (if it exists).

   4.  If P>=0, initiate a connection attempt using the first IPv6
       address returned by step (1).  If that connection has not
       completed after P milliseconds, initiate a connection attempt
       using IPv4.

   5.  If P<=0, initiate a connection attempt using the first IPv4
       address returned by getaddrinfo.  If that connection has not
       completed after absolute value(P) milliseconds, initiate a
       connection attempt using IPv6.

   6.  If neither connection has completed after MAXWAIT seconds, repeat
       the procedure at step (3) until the addresses are exhausted.

   After performing the above steps, there will be no connection at all
   or one connection will complete first.  If no connection was
   successful, it should be treated as a failure for both IPv6 and IPv4.

5.4.  Adjusting Address Family Preferences

   If the preferred address family completed first, Smoothed P is
   adjusted towards that address family.  If the non-preferred address
   family completed, we wait an additional Tolerance Interval
   milliseconds for the preferred address family to complete.  If the
   expected address family succeeded, we increment the absolute value of
   the Smoothed P; if the expected address family failed - we create an
   exception entry that will make an adjustment to the future value of P
   for the attempt on this pair in the direction opposite to the current
   sign of Smoothed P.

   The table below summarizes the adjustments:
               |   Connection completed within Tolerance Interval   |
      +--------+--------------|------------------|------------------+
      |        | v6 and v4 ok | v6 ok, v4 failed | v6 failed, v4 ok |
      +--------+--------------|------------------|------------------+
      | P > 0  |  SP=SP+10    |    SP=SP+10      | SP=SP/2 or cache |
      | P < 0  |  SP=SP+10    | SP=SP/2 or cache |     SP=SP-10     |
      | P = 0  |SP=big(10,IH) |    SP=IH         |     SP=(-IH)     |
      |--------+--------------|------------------|------------------+

                 Figure 4: Table summarizing P adjustments

   The the above table is described in textual form:

   o  If P > 0 (indicating IPv6 is preferred over IPv4):

      *  and both the IPv6 and IPv4 connection attempts completed within
         the Tolerance Interval, it means the IPv6 preference was
         accurate or we should gently prefer IPv6, so Smoothed P is
         increased by 10 milliseconds (Smoothed P = Smoothed P + 10).

      *  If the IPv6 connection completed but the IPv4 connection failed
         within the tolerance interval, it means future connections
         should prefer IPv6, so Smoothed P is increased by 10
         milliseconds (Smoothed_P = Smoothed_P + 10).

      *  If the IPv6 connection failed but the IPv4 connection completed
         within the tolerance interval, it means the IPv6 preference is
         inaccurate.  If no exception cache entry exists for the IPv6
         and IPv4 prefixes, the entries are created and their P value
         set to to the connection setup time * -1, and Smoothed P is
         halved and rounded towards zero (Smoothed_P = Smoothed_P *
         0.5).  If an exception cache entry already existed, its P value
         is doubled and Smoothed_P is not adjusted.

   o  If P < 0 (indicating IPv4 is preferred over IPv6):

      *  and both the IPv6 and IPv4 connection attempts completed within
         the tolerance interval, we should gently prefer IPv6, so
         Smoothed P is increased by 10 milliseconds (Smoothed_P =
         Smoothed_P + 10).

      *  If the IPv6 connection completed but the IPv4 connection failed
         within within the tolerance interval, it means the IPv4
         preference is inaccurate.  If no exception cache entry exists
         for the IPv6 and IPv4 prefixes, they are created and their P
         values set to the connection setup time and Smoothed P is
         halved and rounded towards 0 (Smoothed_P = Smoothed_P * 0.5).
         If an exception cahe entry already existed, its P value is
         doubled and Smoothed_P is not adjusted.

      *  If the IPv4 connection completed but the IPv6 connection failed
         within the tolerance interval, it means future non-winning connections
         should prefer IPv4, so Smoothed P is decreased by 10
         milliseconds (Smoothed_P = Smoothed_P - 10).

   o  If P = 0 (indicating IPv4 and IPv6 are equally preferred):

      *  and both the IPv6 and IPv4 connection attempts completed within
         the tolerance interval, we should prefer IPv6 significantly, so
         Smoothed P is set to the larger of Initial Headstart or 10
         (Smoothed_P = larger(Initial Headstart, 10)).

      *  if the IPv6 connection completed but the IPv4 connection failed
         within the Tolerance Interval, it means we need to prefer IPv6,
         so Smoothed P is increased by 10 (Smoothed_P = Smoothed_P +
         10).

      *  if the IPv4 connection completed but the IPv6 connection failed
         within the Tolerance Interval, it means we need to prefer IPv4,
         so P is decreased by 10 (Smoothed_P = Smoothed_P - 10).

5.5.  Exception Cache

   An exception cache is maintained of IPv6 prefixes and IPv4 prefixes,
   which are exceptions to the Smoothed P value at the time a connection
   was made.  For IPv6 prefixes, the default prefix length is 64.  For
   IPv4, the default prefix length is /32.

   The exception cache MAY be a fixed size, removing entires using a
   least-frequently used algorithm.  This works because the network path
   is likely to change over time (thus old entries aren't valuable
   anyway), and if an entry does not exist the Smoothed P value will
   still provide abandoned, even
   though they could -- in some avoidance of user-noticable connection setup
   delay.

6.  Implementation Details: SRV

      [[Editor's Note:  SRV processing needs to be incorporated into the
      above section, rather than described separately.  This will cases -- be
      done in a future update to this document.]]

   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:

   1.  Sort all SRV records according put to priority (lowest priority
       first)

   2.  Process all of the SRV targets of reasonable use.

      Justification:  This reduces the same priority with a weight
       greater than 0:

       A.  Perform A/AAAA queries for each SRV target in parallel, as
           described in load on the A/AAAA processing section

       B.  Connect to server (file
      descriptors, TCP control blocks), stateful middleboxes (NAT and
      firewalls) and, if the IPv4/IPv6 addresses

       C.  If at least one abandoned connection succeeds, stop processing SRV
           records

   3.  If there is no connection, process all IPv4, reduces IPv4
      address sharing contention.

      HTTP:  The design of the SRV targets some sites can break because of HTTP cookies
      that incorporate the client's IP address and require all
      connections be from the same priority with a weight of 0, as per steps 2.1 through 2.3
       above

   4.  Repeat steps 2.1 through 2.3 IP address.  If some connections from
      the same client are arriving from different IP addresses (or
      worse, different IP address families), such applications will
      break.  Additionally for HTTP, using the next priority, until a non-winning connection is established or all SRV records have been exhausted

   5.  If there is still no connection, fallback to using
      can interfere with the domain
       (e.g., example.com), following steps 2.1 through 2.3 above

7. browser's Same Origin Policy (see
      Section 5.5).

5.  Additional Considerations

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

7.1.

5.1.  Additional Network and Host Traffic

   Additional network traffic and additional server load is created due
   to the recommendations in this document.  This additional load is
   mitigated by the P value, document, especially when connections
   to the exception cache P value. perferred address family (usually IPv6) are not completing
   quickly.

   The procedures described in this document retain a quality user
   experience while transitioning from IPv4-only to dual stack, while
   still giving IPv6 a slight preference over IPv4 (in order to remove
   load from IPv4 networks, most importantly to reduce the load on IPv4
   network address translators).  The improvement in the user experience
   benefits the user to only a small detriment of the network, DNS
   server, and server that are serving the user.

7.2.  Abandon Non-Winning Connections

   It is RECOMMENDED that the non-winning connections be abandoned, even
   though they could -- in some cases -- be put to reasonable use.  To
   take HTTP as an example, the design of some sites can break because
   of HTTP cookies that incorporate the client's IP address, require all
   connections be (in order to remove
   load from IPv4 networks, most importantly to reduce the same IP address.  If some connections from load on IPv4
   network address translators).  The improvement in the same client are arriving from different IP addresses, such
   applications will break.  It is also important to abandon connections user experience
   benefits the user to avoid consuming server resources (file descriptors, TCP control
   blocks) or middlebox resources (e.g., NAPT).  Using only a small detriment of the non-winning
   connection can also interfere with network, DNS
   server, and server that are serving the browser's Same Origin Policy
   (see Section 7.8).

7.3. user.

5.2.  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).  While IPv6/IPv4 translation makes that difficult,
   fortunately IPv6/IPv4 translators are not deployed on networks with
   dual stack clients, which is the scope of this document.

7.4. clients.

5.3.  Debugging and Troubleshooting

   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 a
   particular transport. address family.  To assist in that regard, the applications
   implementing the proposal in this document SHOULD
   implementions MAY also provide a mechanism to revert the disable their Happy
   Eyeballs behavior to that of a default provided by the
   operating system - the [RFC3484].

7.5.  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
      section.]]

7.6.  Middlebox Issues

   Some devices are known to exhibit what amounts to via 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 middlebox issue is
   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 from the
   same port.

   For the large part, these issues with simultaneous DNS requests are
   believed to be fixed.

7.7. user setting.

5.4.  Multiple Interfaces

   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).

7.8. study.

5.5.  Interaction with Same Origin Policy

   Web browsers implement same origin policy (SOP, [sop],
   [I-D.abarth-origin]), which 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

8.  Content Provider Recommendations

   Content providers SHOULD provide both AAAA and A records for servers
   using

5.6.  Happy Eyeballs in an Operating System

   Applications would have to change in order to use the same DNS name mechanism
   described in this document, by either implementing the mechanism
   directly, or by calling APIs made available to them.  To improve IPv6
   connectivity experience for both IPv4 legacy applications (e.g., applications
   which simply rely on the operating system's address preference
   order), operating systems may consider more sophisticated approaches.
   These can include changing address sorting based on configuration
   received from the network, or observing connection failures to IPv6
   and IPv6.

9. IPV4 destinations.

6.  Example Algorithm

   What follows is the algorithm implemented in Google Chrome and
   Mozilla Firefox.

   1.  Call getaddinfo(), which returns a list of IP addresses sorted by
       the host's address preference policy.

   2.  Initiate a connection attempt with the first address in that list
       (e.g., IPv6).

   3.  If that connection does not complete within a short period of
       time (e.g., 200-300ms), initiate a connection attempt with the
       first address belonging to the other address family (e.g., IPv4)

   4.  The first connection that is established is used.  The other
       connection is discarded.

   Other example algorithms include [Perreault] and [Andrews].

7.  Security Considerations

   [[Placeholder.]]

   See Section 7.2 4.4 and Section 7.8.

10. 5.5.

8.  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. 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, and Dmitry Anipko 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, Mark Smith, Gert Doering, Martin
   Millnert, Tim Durack, Matthew Palmer.

11.

9.  IANA Considerations

   This document has no IANA actions.

12.

10.  References

12.1.
10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

12.2.

10.2.  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>.

   [DNS-middlebox]
              Various, "DNS middlebox behavior with multiple queries
              over same source port", June 2009,
              <https://bugzilla.redhat.com/show_bug.cgi?id=505105>.

   [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>.

   [I-D.abarth-origin]
              Barth, A., "The Web Origin Concept",
              draft-abarth-origin-09 (work in progress), November 2010.

   [I-D.chen-mif-happy-eyeballs-extension]
              Chen, G. and C. Williams, "Happy Eyeballs Extension for
              Multiple Interfaces",
              draft-chen-mif-happy-eyeballs-extension-01 (work in
              progress), March 2011.

   [I-D.ietf-6man-addr-select-opt]
              Matsumoto, A., Fujisaki, T., and J. Kato, J., and T. Chown,
              "Distributing Address Selection Policy using DHCPv6",
              draft-ietf-6man-addr-select-opt-00
              draft-ietf-6man-addr-select-opt-01 (work in progress),
              December 2010.
              June 2011.

   [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.

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

   [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.

   [cx-osx]   Adium, "AIHostReachabilityMonitor", June 2009,
              <https://bugzilla.redhat.com/show_bug.cgi?id=505105>.

   [cx-win]   Microsoft, "NetworkChange.NetworkAvailabilityChanged
              Event", June 2009, <http://msdn.microsoft.com/en-us/
              library/
              system.net.networkinformation.networkchange.networkavailab
              ilitychanged.aspx>.

   [sop]      W3C, "Same Origin Policy", January 2010,
              <http://www.w3.org/Security/wiki/Same_Origin_Policy>.

   [whitelist]
              Google, "Google IPv6 DNS Whitelist", January 2009,
              <http://www.google.com/intl/en/ipv6>.

Appendix A.  Changes

A.1.  changes from -02 to -03

   o  Re-casted this specification as a list of requirements for a
      compliant algorithm, rather than trying to dictate a One True
      algorithm.

A.2.  changes from -01 to -02

   o  Now honors host's address preference (RFC3484 and friends)

   o  No longer requires thread-safe DNS library.  It uses getaddrinfo()

   o  No longer describes threading.

   o  IPv6 is given a 200ms head start (Initial Headstart variable).

   o  If the IPv6 and IPv4 connection attempts were made at nearly the
      same time, wait Tolerance Interval milliseconds for both to
      complete before deciding which one wins.

   o  Renamed "global P" to "Smoothed P", and better described how it is
      calculated.

   o  introduced the exception cache.  This contains the set of networks
      that only work with IPv4 (or only with IPv6), so that subsequent
      connection attempts use that address family without them causing
      serious affect to Smoothed P.

   o  encourages that every 10 minutes the exception cache and Smoothed
      P be reset.  This allows IPv6 to be attempted again, so we don't
      get 'stuck' on IPv4.

   o  If we didn't get both A and AAAA, abandon all Happy Eyeballs
      processing (thanks to Simon Perreault).

   o  added discussion of Same Origin Policy

   o  Removed discussion of NAT-PT and address learning; those are only
      used with IPv6-only hosts whereas this document is about dual-
      stack hosts contacting dual-stack servers.

A.2.

A.3.  changes from -00 to -01

   o  added SRV section (thanks to Matt Miller)

Authors' Addresses

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

   Email:  dwing@cisco.com

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

   Email:  ayourtch@cisco.com