draft-ietf-v6ops-happy-eyeballs-01.txt   draft-ietf-v6ops-happy-eyeballs-02.txt 
v6ops D. Wing v6ops D. Wing
Internet-Draft A. Yourtchenko Internet-Draft A. Yourtchenko
Intended status: Standards Track Cisco Intended status: Standards Track Cisco
Expires: September 15, 2011 March 14, 2011 Expires: November 25, 2011 May 24, 2011
Happy Eyeballs: Trending Towards Success with Dual-Stack Hosts Happy Eyeballs: Trending Towards Success with Dual-Stack Hosts
draft-ietf-v6ops-happy-eyeballs-01 draft-ietf-v6ops-happy-eyeballs-02
Abstract Abstract
This document describes how a dual-stack client can determine the This document describes an algorithm for a dual-stack client to
functioning path to a dual-stack server. This provides a seamless quickly determine the functioning address family to a dual-stack
user experience during initial deployment of dual-stack networks and server, and trend towards using that same address family for
during outages of IPv4 or outages of IPv6. subsequent connections. This improves the dual-stack user experience
during IPv6 or IPv4 server or network outages.
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 15, 2011. This Internet-Draft will expire on November 25, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 11 skipping to change at page 2, line 11
include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 4 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4 3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3.1. URIs and hostnames . . . . . . . . . . . . . . . . . . . . 4 3.1. URIs and hostnames . . . . . . . . . . . . . . . . . . . . 4
3.2. IPv6 connectivity . . . . . . . . . . . . . . . . . . . . 4 3.2. IPv6 connectivity . . . . . . . . . . . . . . . . . . . . 5
4. Client Recommendations . . . . . . . . . . . . . . . . . . . . 5 4. Client Recommendations . . . . . . . . . . . . . . . . . . . . 5
4.1. Dualstack behavior . . . . . . . . . . . . . . . . . . . . 5 5. Implementation details: A and AAAA . . . . . . . . . . . . . . 7
4.2. Implementation details . . . . . . . . . . . . . . . . . . 6 5.1. Description of State Variables . . . . . . . . . . . . . . 7
4.2.1. Applications that use address records . . . . . . . . 6 5.2. Initialization, Cache Flush, and Resetting Smoothed P . . 9
4.2.2. Applications that use the SRV records . . . . . . . . 8 5.3. Connecting to a Server . . . . . . . . . . . . . . . . . . 9
5. Additional Considerations . . . . . . . . . . . . . . . . . . 9 5.4. Adjusting Address Family Preferences . . . . . . . . . . . 10
5.1. Additional Network and Host Traffic . . . . . . . . . . . 9 5.5. Exception Cache . . . . . . . . . . . . . . . . . . . . . 11
5.2. Abandon Non-Winning Connections . . . . . . . . . . . . . 10 6. Implementation Details: SRV . . . . . . . . . . . . . . . . . 12
5.3. Flush or Expire Cache . . . . . . . . . . . . . . . . . . 10 7. Additional Considerations . . . . . . . . . . . . . . . . . . 13
5.4. Determining Address Type . . . . . . . . . . . . . . . . . 10 7.1. Additional Network and Host Traffic . . . . . . . . . . . 13
5.5. Debugging and Troubleshooting . . . . . . . . . . . . . . 10 7.2. Abandon Non-Winning Connections . . . . . . . . . . . . . 13
5.6. DNS Behavior . . . . . . . . . . . . . . . . . . . . . . . 11 7.3. Determining Address Type . . . . . . . . . . . . . . . . . 13
5.7. Middlebox Issues . . . . . . . . . . . . . . . . . . . . . 11 7.4. Debugging and Troubleshooting . . . . . . . . . . . . . . 13
5.8. Multiple Interfaces . . . . . . . . . . . . . . . . . . . 12 7.5. DNS Behavior . . . . . . . . . . . . . . . . . . . . . . . 14
6. Content Provider Recommendations . . . . . . . . . . . . . . . 12 7.6. Middlebox Issues . . . . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 7.7. Multiple Interfaces . . . . . . . . . . . . . . . . . . . 14
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 7.8. Interaction with Same Origin Policy . . . . . . . . . . . 14
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 8. Content Provider Recommendations . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 9. Security Considerations . . . . . . . . . . . . . . . . . . . 15
10.1. Normative References . . . . . . . . . . . . . . . . . . . 13 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
10.2. Informational References . . . . . . . . . . . . . . . . . 13 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . . 16
12.2. Informational References . . . . . . . . . . . . . . . . . 16
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . . 17
A.1. changes from -01 to -02 . . . . . . . . . . . . . . . . . 18
A.2. changes from -00 to -01 . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction 1. Introduction
In order to use HTTP successfully over IPv6, it is necessary that the In order to use HTTP successfully over IPv6, it is necessary that the
user enjoys nearly identical performance as compared to IPv4. A user enjoys nearly identical performance as compared to IPv4. A
combination of today's applications, IPv6 tunneling and IPv6 service combination of today's applications, IPv6 tunneling and IPv6 service
providers, and some of today's content providers all cause the user providers, and some of today's content providers all cause the user
experience to suffer (Section 3). For IPv6, a content provider may experience to suffer (Section 3). For IPv6, a content provider may
ensure a positive user experience by using a DNS white list of IPv6 ensure a positive user experience by using a DNS white list of IPv6
service providers who peer directly with them, e.g. [whitelist]. service providers who peer directly with them, e.g. [whitelist].
However, this is not scalable to all service providers worldwide, nor However, this is not scalable to all service providers worldwide, nor
is it scalable for other content providers to operate their own DNS is it scalable for other content providers to operate their own DNS
white list. white list.
Instead, this document suggests a mechanism for applications to Instead, this document suggests a mechanism for applications to
quickly determine if IPv6 or IPv4 is the most optimal to connect to a quickly determine if IPv6 or IPv4 is the most optimal to connect to a
server. The suggestions in this document provide a user experience server. The suggestions in this document provide a user experience
which is superior to connecting to ordered IP addresses which is which is superior to connecting to ordered IP addresses which is
helpful during the IPv6/IPv4 transition with dual stack hosts. helpful during the IPv6/IPv4 transition with dual stack hosts.
This problem is described also in [RFC1671]: "The dual-stack code This problem is also described in [RFC1671], published in 1994:
may get two addresses back from DNS; which does it use? During the
many years of transition the Internet will contain black holes. For "The dual-stack code may get two addresses back from DNS; which
example, somewhere on the way from IPng host A to IPng host B there does it use? During the many years of transition the Internet
will sometimes (unpredictably) be IPv4-only routers which discard will contain black holes. For example, somewhere on the way from
IPng packets. Also, the state of the DNS does not necessarily IPng host A to IPng host B there will sometimes (unpredictably) be
correspond to reality. A host for which DNS claims to know an IPng IPv4-only routers which discard IPng packets. Also, the state of
address may in fact not be running IPng at a particular moment; thus the DNS does not necessarily correspond to reality. A host for
an IPng packet to that host will be discarded on delivery. Knowing which DNS claims to know an IPng address may in fact not be
that a host has both IPv4 and IPng addresses gives no information running IPng at a particular moment; thus an IPng packet to that
about black holes. A solution to this must be proposed and it must host will be discarded on delivery. Knowing that a host has both
not depend on manually maintained information. (If this is not IPv4 and IPng addresses gives no information about black holes. A
solved, the dual stack approach is no better than the packet solution to this must be proposed and it must not depend on
translation approach.)" 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 Following the procedures in this document, once a certain address
family is successful, the application trends towards preferring that family is successful, the application trends towards preferring that
address family. Thus, repeated use of the application DOES NOT cause address family. Thus, repeated use of the application DOES NOT cause
repeated probes over both address families. 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 While the application recommendations in this document are described
in the context of HTTP clients ("web browsers"), it is also useful in the context of HTTP clients ("web browsers") and SRV clients
and applicable to other interactive applications. (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 Code which implements some of the ideas described in this document
has been made available [Perreault] [Andrews]. has been made available [Perreault] [Andrews].
2. Notational Conventions 2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Problem Statement 3. Problem Statement
As discussed in more detail in Section 3.1, it is important that the 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 same URI and hostname be used for IPv4 and IPv6. Using separate
namespaces causes namespace fragmentation and reduces the ability for namespaces causes namespace fragmentation and reduces the ability for
users to share URIs and hostnames, and complicates printed material users to share URIs and hostnames, and complicates printed material
that includes the URI or hostname. that includes the URI or hostname.
As discussed in more detail in Section 3.2, IPv6 connectivity is As discussed in more detail in Section 3.2, IPv6 connectivity is
sometimes broken entirely or slower than native IPv4 connectivity. broken to specific prefixes or specific hosts, or slower than native
IPv4 connectivity.
3.1. URIs and hostnames 3.1. URIs and hostnames
URIs are often used between users to exchange pointers to content -- URIs are often used between users to exchange pointers to content --
such as on social networks, email, instant messaging, or other such as on social networks, email, instant messaging, or other
systems. Thus, production URIs and production hostnames containing systems. Thus, production URIs and production hostnames containing
references to IPv4 or IPv6 will only function if the other party is 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 also using an application, OS, and a network that can access the URI
or the hostname. or the hostname.
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12. | |--TCP ACK, IPv4------->| 12. | |--TCP ACK, IPv4------->|
Figure 1: Existing behavior message flow Figure 1: Existing behavior message flow
The client obtains the IPv4 and IPv6 records for the server (1-4). 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 The client attempts to connect using IPv6 to the server, but the IPv6
path is broken (6-8), which consumes several seconds of time. path is broken (6-8), which consumes several seconds of time.
Eventually, the client attempts to connect using IPv4 (10) which Eventually, the client attempts to connect using IPv4 (10) which
succeeds. succeeds.
Delays experienced by users of various browser and operating system
combinations have been studied [Experiences].
4. Client Recommendations 4. Client Recommendations
To provide fast connections for users, clients should make Happy Eyeballs does two things:
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 1. Provides fast connection for users. 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 it trended towards IPv4 Section 5.2.
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 IPv6 and IPv4 and is connected to IPv4 and If a TCP client supports IPv6 and IPv4 and is connected to IPv4 and
IPv6 networks, it can perform the procedures described in this IPv6 networks, it can perform the procedures described in this
section. section.
DNS Server Client Server DNS Server Client Server
| | | | | |
1. |<--www.example.com A?-----| | 1. |<--www.example.com A?-----| |
2. |<--www.example.com AAAA?--| | 2. |<--www.example.com AAAA?--| |
3. |---192.0.2.1------------->| | 3. |---192.0.2.1------------->| |
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10. | |==TCP SYN, IPv6===>X | 10. | |==TCP SYN, IPv6===>X |
Figure 2: Happy Eyeballs flow 1, IPv6 broken Figure 2: Happy Eyeballs flow 1, IPv6 broken
In the diagram above, the client sends two TCP SYNs at the same time 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 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 but has little impact to the user because there is no long delay
before using IPv4. The IPv6 path is retried until the application before using IPv4. The IPv6 path is retried until the application
gives up (10). 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 DNS Server Client Server
| | | | | |
1. |<--www.example.com A?-----| | 1. |<--www.example.com A?-----| |
2. |<--www.example.com AAAA?--| | 2. |<--www.example.com AAAA?--| |
3. |---192.0.2.1------------->| | 3. |---192.0.2.1------------->| |
4. |---2001:db8::1----------->| | 4. |---2001:db8::1----------->| |
5. | | | 5. | | |
6. | |==TCP SYN, IPv6=======>| 6. | |==TCP SYN, IPv6=======>|
7. | |--TCP SYN, IPv4------->| 7. | |--TCP SYN, IPv4------->|
8. | |<=TCP SYN+ACK, IPv6====| 8. | |<=TCP SYN+ACK, IPv6====|
9. | |<-TCP SYN+ACK, IPv4----| 9. | |<-TCP SYN+ACK, IPv4----|
10. | |==TCP ACK, IPv6=======>| 10. | |==TCP ACK, IPv6=======>|
11. | |--TCP ACK, IPv4------->| 11. | |--TCP ACK, IPv4------->|
12. | |--TCP RST, IPv4------->| 12. | |--TCP RST, IPv4------->|
Figure 3: Happy Eyeballs flow 2, IPv6 working Figure 3: Happy Eyeballs flow 2, IPv6 working
The diagram above shows a case where both IPv6 and IPv4 are working, The diagram above shows a case where both IPv6 and IPv4 are working,
and IPv4 is abandoned (12). and IPv4 is abandoned (12).
4.2. Implementation details 5. Implementation details: A and AAAA
4.2.1. Applications that use address records
This section details how to provide robust dual stack service for This section details how to provide robust dual stack service for
both IPv6 and IPv4, so that the user perceives very fast application both IPv6 and IPv4, so that the user perceives very fast application
response. response.
The TCP client application is configured with one application-wide Depending on implementation, the variables and procedures described
value of P. A positive value indicates a preference for IPv6 and a below might be implemented or maintained within a specific
negative value indicates a preference for IPv4. A value of 0 application (e.g., web browser), library, framework, or by the
indicates equal weight, which means the A and AAAA queries and operating system itself. An API call such as "connect_by_name()" is
associated connection attempts will be sent as quickly as possible. envisioned which would call the Happy Eyeballs routine and implement
The absolute value of P is the measure of a delay before initiating a the functions described in this section.
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 5.1. Description of State Variables
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 The system maintains a Smoothed P (which provides the overall
(they will be referred to as "threads" but this reference does not preference to IPv6 or IPv4), and an exception cache. Both of these
imply the implementation detail of using the threading library, change over time and are described below:
merely the property of mutual concurrency) in order to minimize the
user-noticeable delay ("dead time") during the connection attempts:
thread 1: (IPv6) Exception Cache: This is a cache, indexed by IP prefixes, contains
a "P" value for each prefix. Entries are added to this cache if a
connection to the expected address family failed and a connection
to the other address family succeeded. That is, these are
exceptions to the Smoothed P variable. See Section 5.5 for
description of how these prefixes are defined.
* If P<0, wait for absolute value of p*10 milliseconds (Note: In previous versions of this document, this was the
"per-destination P (preference) value".)
* send DNS query for AAAA P: Address family preference. This is computed for this connection
attempt. A positive value is a preference to start the IPv6
connection first, a negative value to start the IPv4 connection
first, and zero indicates both IPv6 and IPv4 connections are
started simultaneously. The absolute value is the number of
milliseconds between the connection attempts on two address
families.
* wait until DNS response is received Smoothed P: Smoothed address family preference. This is the address
family preference for destinations that are not in the exception
cache. This variable can be positive or negative, with values
having the same meaning as "P". In the absence of more specific
configuration information, it is RECOMMENDED that implementations
enforce a maximum value of 8000 (8 seconds) for this variable.
* Attempt to connect over IPv6 using TCP (Note: In previous versions of this document, this was the
"application-wide P (preference) value".)
thread 2: (IPv4) The following values are configured and constant:
* if P>0, wait for p*10 milliseconds TI: Tolerance Interval, in milliseconds. This is the allowance in
the time a connection is expected to complete and its actual
completion, 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.
* send DNS query for A 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.
* wait until DNS response is received MAXWAIT: Maximum wait time for a connection to complete, before
trying additional IP addresses. This is RECOMMENDED to be 10
seconds.
* Attempt to connect over IPv4 using TCP 5.2. Initialization, Cache Flush, and Resetting Smoothed P
The first thread that succeeds returns the completed connection to Because every network has different characteristics (e.g., working or
the parent code and aborts the other thread (Section 5.2). broken IPv6 or IPv4 connectivity) the Smoothed P variable SHOULD be
set to its default value (Smoothed P = Initial Headstart) and the
exception cache SHOULD be emptied whenever the host is connected to a
new network (e.g., DNAv4 [RFC4436], DNAv6 [RFC6059], [cx-osx],
[cx-win]).
After a connection is successful, we want to adjust the application- If there are IPv6 failures to specific hosts or prefixes, the
wide preference and the per-destination preference. The value of P exception cache will build up exception entries preferring IPv4, and
is incremented (decremented) each time an IPv6 (IPv4) connection wins if there are significant IPv6 failures to many hosts or prefixes,
the race.. When a connection using the less-preferred address family Smoothed P will become negative. When this occurs, IPv6 will not be
is successful, it indicates the wrong address family was used and the attempted at all. To avoid this problem, it is strongly RECOMMENDED
value of P is halved: to occasionally flush 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 the IPv6 is
working again, it will quickly be preferred again.
o If P>0 (indicating IPv6 is preferred over IPv4) and the first 5.3. Connecting to a Server
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 The steps when connecting to a server are as follows:
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 1. query DNS using getaddrinfo(). This will return addresses sorted
the first thread to complete was the IPv6 thread, or decremented by the host's default address selection ordering [RFC3484], its
by one if the first thread to complete was the IPv4 thread. updates, or the address selection as chosen by the network
administrator [I-D.ietf-6man-addr-select-opt].
After adjusting P, the resulting delay should never be larger than 4 2. If this returns both an IPv6 and IPv4 address, continue
seconds -- which is similar to the value used by many IPv6-capable processing to the next stop. Otherwise, Happy Eyeballs
TCP client applications to switch to an alternate A or AAAA record. processing stops here.
Editor's Note 01: Proof of concept tests on fast networks show 3. Of the addresses returned in step (1), look up the first IPv6
that even smaller value (around 0.5 seconds) may be practical. address and first IPv4 address in the Happy Eyeballs exception
More extensive testing would be useful to find the best upper cache. Matching entries in the exception cache influence the P
boundary that still ensures a good user experience. 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).
Editor's Note 02: A strict implementation of the above steps 4. If P>=0, initiate a connection attempt using the first IPv6
results in "P" being adjusted if there are no AAAA records or are address returned by step (1). If that connection has not
no A records. This is undesirable. Thus, a future version of completed after P milliseconds, initiate a connection attempt
this specification is expected to recommend that "P" only be using IPv4.
adjusted if there was both an A and AAAA record.
4.2.2. Applications that use the SRV records 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 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 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 be
done in a future update to this document.]]
For the purposes of this section, "client" is defined as the entity For the purposes of this section, "client" is defined as the entity
initiating the connection. initiating the connection.
For protocols which support DNS SRV [RFC2782], the client performs For protocols which support DNS SRV [RFC2782], the client performs
the IN SRV query (e.g. IN SRV _xmpp-client._tcp.example.com) as the IN SRV query (e.g. IN SRV _xmpp-client._tcp.example.com) as
normal. The client MUST perform the following steps: normal. The client MUST perform the following steps:
1. Sort all SRV records according to priority (lowest priority 1. Sort all SRV records according to priority (lowest priority
first) first)
2. Process all of the SRV targets of the same priority with a weight 2. Process all of the SRV targets of the same priority with a weight
greater than 0: greater than 0:
A. Perform A/AAAA queries for each SRV target in parallel, as A. Perform A/AAAA queries for each SRV target in parallel, as
described in Section 4.2.1 described in the A/AAAA processing section
B. Connect to the IPv4/IPv6 addresses B. Connect to the IPv4/IPv6 addresses
C. If at least one connection succeeds, stop processing SRV C. If at least one connection succeeds, stop processing SRV
records records
3. If there is no connection, process all of the SRV targets of the 3. If there is no connection, process all of the SRV targets of the
same priority with a weight of 0, as per steps 2.1 through 2.3 same priority with a weight of 0, as per steps 2.1 through 2.3
above above
4. Repeat steps 2.1 through 2.3 for the next priority, until a 4. Repeat steps 2.1 through 2.3 for the next priority, until a
connection is established or all SRV records have been exhausted connection is established or all SRV records have been exhausted
5. If there is still no connection, fallback to using the domain 5. If there is still no connection, fallback to using the domain
(e.g. example.com), following steps 2.1 through 2.3 above (e.g., example.com), following steps 2.1 through 2.3 above
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.
5. Additional Considerations 7. Additional Considerations
This section discusses considerations and requirements that are This section discusses considerations and requirements that are
common to new technology deployment. common to new technology deployment.
5.1. Additional Network and Host Traffic 7.1. Additional Network and Host Traffic
Additional network traffic and additional server load is created due Additional network traffic and additional server load is created due
to these recommendations and mitigated by application-wide and per- to the recommendations in this document. This additional load is
destination timer adjustments. The procedures described in this mitigated by the P value, especially the exception cache P value.
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.
5.2. Abandon Non-Winning Connections
It is RECOMMENDED that the non-winning connections be abandoned, even The procedures described in this document retain a quality user
though they could be used to download content. This is because some experience while transitioning from IPv4-only to dual stack, while
web sites provide HTTP clients with cookies (after logging in) that still giving IPv6 a slight preference over IPv4 (in order to remove
incorporate the client's IP address, or use IP addresses to identify load from IPv4 networks, most importantly to reduce the load on IPv4
users. If some connections from the same HTTP client are arriving network address translators). The improvement in the user experience
from different IP addresses, such HTTP applications will break. It's benefits the user to only a small detriment of the network, DNS
also important to abandon connections to avoid consuming server or server, and server that are serving the user.
middlebox (e.g., NAT) resources (file descriptors, memory, TCP
control blocks) and avoid sending TCP or application-level keepalives
on otherwise unused connections.
5.3. Flush or Expire Cache 7.2. Abandon Non-Winning Connections
Because every network has different characteristics (e.g., working or It is RECOMMENDED that the non-winning connections be abandoned, even
broken IPv6 connectivity) the IPv6/IPv4 preference value (P) SHOULD though they could -- in some cases -- be put to reasonable use. To
be reset to its default whenever the host is connected to a new take HTTP as an example, the design of some sites can break because
network ([cx-osx], [cx-win]). However, in some instances the of HTTP cookies that incorporate the client's IP address, require all
application and the host are unaware the network connectivity has connections be from the same IP address. If some connections from
changed so it is RECOMMENDED that per-destination values expire after the same client are arriving from different IP addresses, such
10 minutes of inactivity. applications will break. It is also important to abandon connections
to avoid consuming server resources (file descriptors, TCP control
blocks) or middlebox resources (e.g., NAPT). Using the non-winning
connection can also interfere with the browser's Same Origin Policy
(see Section 7.8).
5.4. Determining Address Type 7.3. Determining Address Type
For some transitional technologies such as a dual-stack host, it is For some transitional technologies such as a dual-stack host, it is
easy for the application to recognize the native IPv6 address easy for the application to recognize the native IPv6 address
(learned via a AAAA query) and the native IPv4 address (learned via (learned via a AAAA query) and the native IPv4 address (learned via
an A query). For other transitional technologies [RFC2766] it is an A query). While IPv6/IPv4 translation makes that difficult,
impossible for the host to differentiate a transitional technology fortunately IPv6/IPv4 translators are not deployed on networks with
IPv6 address from a native IPv6 address (see Section 4.1 of dual stack clients, which is the scope of this document.
[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.
5.5. Debugging and Troubleshooting 7.4. Debugging and Troubleshooting
This mechanism is aimed at ensuring a reliable user experience This mechanism is aimed at ensuring a reliable user experience
regardless of connectivity problems affecting any single transport. regardless of connectivity problems affecting any single transport.
However, this naturally means that applications employing these However, this naturally means that applications employing these
techniques are by default less useful for diagnosing issues with any techniques are by default less useful for diagnosing issues with any
particular transport. To assist in that regard, the applications particular transport. To assist in that regard, the applications
implementing the proposal in this document SHOULD also provide a implementing the proposal in this document SHOULD also provide a
mechanism to revert the behavior to that of a default provided by the mechanism to revert the behavior to that of a default provided by the
operating system - the [RFC3484]. operating system - the [RFC3484].
[[[ To be discussed. 7.5. DNS Behavior
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.
]]]
5.6. DNS Behavior
Unique to DNS AAAA queries are the problems described in [RFC4074] Unique to DNS AAAA queries are the problems described in [RFC4074]
which, if they still persist, require applications to perform an A which, if they still persist, require applications to perform an A
query before the AAAA query. query before the AAAA query.
[[Editor's Note 03: It is believed these defective DNS servers [[Editor's Note 03: It is believed these defective DNS servers
have long since been upgraded. If so, we can remove this have long since been upgraded. If so, we can remove this
section.]] section.]]
5.7. Middlebox Issues 7.6. Middlebox Issues
Some devices are known to exhibit what amounts to a bug, when the A 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 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 drop one of the requests or replies [DNS-middlebox]. However, in
some cases fixing this behaviour may not be possible either due to some cases fixing this behaviour may not be possible either due to
the architectural limitations or due to the administrative the architectural limitations or due to the administrative
constraints (location of the faulty device is unknown to the end constraints (location of the faulty device is unknown to the end
hosts or not controlled by the end hosts). The algorithm described hosts or not controlled by the end hosts). The algorithm described
in this draft, in the case of this erroneous behaviour will in this draft, in the case of this erroneous behaviour will
eventually pace the queries such that this issue is will be avoided. eventually pace the queries such that this middlebox issue is
The algorithm described in this draft also avoids calling the avoided. The algorithm described in this draft also avoids calling
operating system's getaddrinfo() with "any", which should prevent the the operating system's getaddrinfo() with "any", which should prevent
operating system from sending the A and AAAA queries on the same the operating system from sending the A and AAAA queries from the
port. same port.
For the large part, these issues are believed to be fixed, in which For the large part, these issues with simultaneous DNS requests are
case the getaddrinfo() with AF_UNSPEC as the address family in its believed to be fixed.
hints.
5.8. Multiple Interfaces 7.7. Multiple Interfaces
Interaction of the suggestions in this document with multiple Interaction of the suggestions in this document with multiple
interfaces, and interaction with the MIF working group, is for interfaces, and interaction with the MIF working group, is for
further study ([I-D.chen-mif-happy-eyeballs-extension] is devoted to further study ([I-D.chen-mif-happy-eyeballs-extension] is devoted to
this). this).
6. Content Provider Recommendations 7.8. 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 Content providers SHOULD provide both AAAA and A records for servers
using the same DNS name for both IPv4 and IPv6. using the same DNS name for both IPv4 and IPv6.
7. Security Considerations 9. Security Considerations
[[Placeholder.]] [[Placeholder.]]
See Section 5.2. See Section 7.2 and Section 7.8.
8. Acknowledgements 10. Acknowledgements
The mechanism described in this paper was inspired by Stuart The mechanism described in this paper was inspired by Stuart
Cheshire's discussion at the IAB Plenary at IETF72, the author's Cheshire's discussion at the IAB Plenary at IETF72, the author's
understanding of Safari's operation with SRV records, Interactive understanding of Safari's operation with SRV records, Interactive
Connectivity Establishment (ICE [RFC5245]), and the current IPv4/IPv6 Connectivity Establishment (ICE [RFC5245]), and the current IPv4/IPv6
behavior of SMTP mail transfer agents. behavior of SMTP mail transfer agents.
Thanks to Fred Baker, Jeff Kinzli, Christian Kuhtz, and Iljitsch van Thanks to Fred Baker, Jeff Kinzli, Christian Kuhtz, and Iljitsch van
Beijnum for fostering the creation of this document. Beijnum for fostering the creation of this document.
Thanks to Scott Brim, Rick Jones, Stig Venaas, Erik Kline, Bjoern Thanks to Scott Brim, Rick Jones, Stig Venaas, Erik Kline, Bjoern
Zeeb, Matt Miller for providing feedback on the document. 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 Thanks to Javier Ubillos, Simon Perreault and Mark Andrews for the
active feedback and the experimental work on the independent active feedback and the experimental work on the independent
practical implementations that they created. practical implementations that they created.
Also the authors would like to thank the following individuals who Also the authors would like to thank the following individuals who
participated in various email discussions on this topic: Mohacsi participated in various email discussions on this topic: Mohacsi
Janos, Pekka Savola, Ted Lemon, Carlos Martinez-Cagnazzo, Simon Janos, Pekka Savola, Ted Lemon, Carlos Martinez-Cagnazzo, Simon
Perreault, Jack Bates, Jeroen Massar, Fred Baker, Javier Ubillos, Perreault, Jack Bates, Jeroen Massar, Fred Baker, Javier Ubillos,
Teemu Savolainen, Scott Brim, Erik Kline, Cameron Byrne, Daniel Teemu Savolainen, Scott Brim, Erik Kline, Cameron Byrne, Daniel
Roesen, Guillaume Leclanche, Cameron Byrne, Mark Smith, Gert Doering, Roesen, Guillaume Leclanche, Mark Smith, Gert Doering, Martin
Martin Millnert, Tim Durack, Matthew Palmer. Millnert, Tim Durack, Matthew Palmer.
9. IANA Considerations 11. IANA Considerations
This document has no IANA actions. This document has no IANA actions.
10. References 12. References
10.1. Normative References 12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782, specifying the location of services (DNS SRV)", RFC 2782,
February 2000. February 2000.
[RFC3484] Draves, R., "Default Address Selection for Internet [RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003. Protocol version 6 (IPv6)", RFC 3484, February 2003.
10.2. Informational References 12.2. Informational References
[Andrews] Andrews, M., "How to connect to a multi-homed server over [Andrews] Andrews, M., "How to connect to a multi-homed server over
TCP", January 2011, <http://www.isc.org/community/blog/ TCP", January 2011, <http://www.isc.org/community/blog/
201101/how-to-connect-to-a-multi-h omed-server-over-tcp>. 201101/how-to-connect-to-a-multi-h omed-server-over-tcp>.
[DNS-middlebox] [DNS-middlebox]
Various, "DNS middlebox behavior with multiple queries Various, "DNS middlebox behavior with multiple queries
over same source port", June 2009, over same source port", June 2009,
<https://bugzilla.redhat.com/show_bug.cgi?id=505105>. <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] [I-D.chen-mif-happy-eyeballs-extension]
Chen, G., "Happy Eyeballs Extension for Multiple Chen, G. and C. Williams, "Happy Eyeballs Extension for
Interfaces", draft-chen-mif-happy-eyeballs-extension-00 Multiple Interfaces",
(work in progress), March 2011. 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, "Distributing
Address Selection Policy using DHCPv6",
draft-ietf-6man-addr-select-opt-00 (work in progress),
December 2010.
[Perreault] [Perreault]
Perreault, S., "Happy Eyeballs in Erlang", February 2011, Perreault, S., "Happy Eyeballs in Erlang", February 2011,
<http://www.viagenie.ca/news/ <http://www.viagenie.ca/news/
index.html#happy_eyeballs_erlang>. index.html#happy_eyeballs_erlang>.
[RFC1671] Carpenter, B., "IPng White Paper on Transition and Other [RFC1671] Carpenter, B., "IPng White Paper on Transition and Other
Considerations", RFC 1671, August 1994. 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 [RFC4074] Morishita, Y. and T. Jinmei, "Common Misbehavior Against
DNS Queries for IPv6 Addresses", RFC 4074, May 2005. DNS Queries for IPv6 Addresses", RFC 4074, May 2005.
[RFC4966] Aoun, C. and E. Davies, "Reasons to Move the Network [RFC4436] Aboba, B., Carlson, J., and S. Cheshire, "Detecting
Address Translator - Protocol Translator (NAT-PT) to Network Attachment in IPv4 (DNAv4)", RFC 4436, March 2006.
Historic Status", RFC 4966, July 2007.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT) (ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, Traversal for Offer/Answer Protocols", RFC 5245,
April 2010. 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, [cx-osx] Adium, "AIHostReachabilityMonitor", June 2009,
<https://bugzilla.redhat.com/show_bug.cgi?id=505105>. <https://bugzilla.redhat.com/show_bug.cgi?id=505105>.
[cx-win] Microsoft, "NetworkChange.NetworkAvailabilityChanged [cx-win] Microsoft, "NetworkChange.NetworkAvailabilityChanged
Event", June 2009, <http://msdn.microsoft.com/en-us/ Event", June 2009, <http://msdn.microsoft.com/en-us/
library/ library/
system.net.networkinformation.networkchange.networkavailab system.net.networkinformation.networkchange.networkavailab
ilitychanged.aspx>. ilitychanged.aspx>.
[sop] W3C, "Same Origin Policy", January 2010,
<http://www.w3.org/Security/wiki/Same_Origin_Policy>.
[whitelist] [whitelist]
Google, "Google IPv6 DNS Whitelist", January 2009, Google, "Google IPv6 DNS Whitelist", January 2009,
<http://www.google.com/intl/en/ipv6>. <http://www.google.com/intl/en/ipv6>.
Appendix A. Changes
A.1. 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. changes from -00 to -01
o added SRV section (thanks to Matt Miller)
Authors' Addresses Authors' Addresses
Dan Wing Dan Wing
Cisco Systems, Inc. Cisco Systems, Inc.
170 West Tasman Drive 170 West Tasman Drive
San Jose, CA 95134 San Jose, CA 95134
USA USA
Email: dwing@cisco.com Email: dwing@cisco.com
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