Network T. Pauly
Internet-Draft D. Schinazi
Intended status: Standards Track Apple Inc.
Expires: September 9, 2017 March 8, 2017
An Update to Happy Eyeballs
draft-pauly-v6ops-happy-eyeballs-update-00
Abstract
"Happy Eyeballs" (RFC6555) is the name for a technique of reducing
user-visible delays on dual-stack hosts. Since one address family
(IPv4 or IPv6) may be blocked, broken, or sub-optimal on a network,
clients that attempt connections for both address families in
parallel have a higher chance of establishing a connection sooner.
Now that this approach has been deployed at scale and measured for
several years, the algorithm specification can be refined to improve
its reliability and generalization.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Hostname Resolution Query Handling . . . . . . . . . . . . . 3
2.1. Handling Multiple DNS Server Addresses . . . . . . . . . 3
3. Sorting Addresses . . . . . . . . . . . . . . . . . . . . . . 3
4. Connection Attempt Delay . . . . . . . . . . . . . . . . . . 4
5. Handling DNS Answer Changes . . . . . . . . . . . . . . . . . 5
6. Summary of Configurable Values . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
10.1. Normative References . . . . . . . . . . . . . . . . . . 6
10.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
"Happy Eyeballs" [RFC6555] is the name for a technique of reducing
user-visible delays on dual-stack hosts. Since one address family
(IPv4 or IPv6) may be blocked, broken, or sub-optimal on a network,
clients that attempt connections for both address families in
parallel have a higher chance of establishing a connection sooner.
Now that this approach has been deployed at scale and measured for
several years, the algorithm specification can be refined to improve
its reliability and generalization.
This document recommends an algorithm of racing resolved addresses
that has several stages of ordering and racing to avoid delays to the
user whenever possible, while preferring the use of IPv6.
Specifically, it discusses how to handle DNS queries when starting a
connection on a dual-stack client, how to create an ordered list of
addresses to which to attempt connections, and how to race the
connection attempts.
1.1. Requirements Language
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 "Key words for use in
RFCs to Indicate Requirement Levels" RFC 2119 [RFC2119].
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2. Hostname Resolution Query Handling
When a client has both IPv4 and IPv6 connectivity, and is trying to
establish a connection with a named host, it needs to send out both A
and AAAA DNS queries. Both queries SHOULD be sent on the wire as
close together as possible, with the ordering being dictated by the
system's host address preference policy. For example, if the system
has a preference for IPv6, it will send the AAAA query first,
immediately followed by the A query.
Implementations MUST NOT wait for both families of answers to return
before attempting connection establishment. If one query fails to
return, or takes significantly longer to return, waiting for the
second address family can significantly delay the connection
establishment of the first one. If the system's host address
preference policy is set to prefer IPv6, if the AAAA query returns
first, the first IPv6 connection attempt MUST be immediately started.
If the A query returns first, the client SHOULD wait for a short time
for the AAAA response. This delay will be referred to as the
"Resolution Delay". A recommended delay is 50 milliseconds. If the
AAAA response is received within the delay period, the client MUST
start the IPv6 connection attempt. If the AAAA response has not been
received at the end of the delay period, the client SHOULD start the
IPv4 connection attempt.
2.1. Handling Multiple DNS Server Addresses
If multiple DNS server addresses are configured for the current
network, the client may have the option of sending its DNS queries
over IPv4 or IPv6. In keeping with the Happy Eyeballs approach,
queries SHOULD be sent over IPv6 first. If DNS queries sent to the
IPv6 address do not receive responses, that address may be marked as
penalized, and queries can be sent to other DNS server addresses.
As native IPv6 deployments become more prevalent, and IPv4 addresses
are exhausted, it is expected that IPv6 connectivity will have
preferential treatment within networks. If a DNS server is
configured to be accessible over IPv6, IPv6 should be assumed to be
the preferred address family.
3. Sorting Addresses
Before attempting to connect to any of the resolved addresses, the
client should define the order in which to start the attempts. Once
the order has been defined, the client can use a simple algorithm for
racing each option after a short delay [Section 4]. It is important
that the ordered list involves all addresses from both families, as
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this allows the client to get the racing effect of Happy Eyeballs for
the entire list, not just the first IPv4 and first IPv6 addresses.
First, the client MUST sort the addresses using Destination Address
Selection ([RFC6724], Section 6).
If the client is stateful and has history of expected round-trip
times (RTT) for the routes to access each address, it SHOULD add a
Destination Address Selection rule between rules 8 and 9 that prefers
addresses with lower RTTs. If the client keeps track of which
addresses it has used in the past, it SHOULD add another destination
address selection rule between the RTT rule and rule 9, which prefers
used addresses over unused ones. This helps servers that use the
client's IP address for authentication, as is the case for TCP Fast
Open ([RFC7413]) and some HTTP cookies. This historical data MUST
NOT be used across networks, and SHOULD be flushed on network
changes.
Next, the client SHOULD modify the ordered list to interleave address
families. Whichever address family is first in the list should be
followed by an address of the other address family; that is, if the
first address in the sorted list is IPv6, then the first IPv4 address
should be moved up in the list to be second in the list. An
implementation MAY want to favor one address family more by allowing
multiple addresses of that family to be attempted before trying the
other family. The number of contiguous addresses of the first
address family will be referred to as the "First Address Family
Count", and can be a configurable value.
4. Connection Attempt Delay
Once the list of addresses has been constructed, the client will
attempt to make connections. In order to minimize network load,
connection attempts SHOULD NOT be made simultaneously. Instead, one
connection attempt to a single address is started first, followed by
the others in the list, one at a time. Starting a new connection
attempt does not affect previous attempts, as multiple connection
attempts may occur in parallel. Once one of the connection attempts
succeeds (generally when the TCP handshake completes), all other
connections attempts that have not yet succeeded SHOULD be cancelled.
Any address that was not yet attempted as a connection SHOULD be
ignored.
A simple implementation can have a fixed delay for how long to wait
before starting the next connection attempt. This delay is referred
to as the "Connection Attempt Delay". One recommended value for this
delay is 250 milliseconds. If the client has historical RTT data, it
can also use the expected RTT to choose a more nuanced delay value.
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The recommended formula for calculating the delay after starting a
connection attempt is: MAX( 1.25 * RTT_MEAN + 4 * RTT_VARIANCE, 2 *
RTT_MEAN ), where the RTT values are based on the statistics for
previous address used. If the TCP implementation leverages
historical RTT data to compute SYN timeout, these algorithms should
match so that a new attempt will be started at the same time as the
previous is sending its second TCP SYN.
5. Handling DNS Answer Changes
If, during the course of connection establishment, the DNS answers
change either by adding resolved addresses, or removing previously
resolved addresses, the client should react based on its current
progress.
If an address is removed from the list that already had a connection
attempt started, the connection attempt SHOULD NOT be cancelled, but
rather be allowed to continue. If the removed address had not yet
had a connection attempt started, it SHOULD be removed from the list
of addresses to try.
If an address is added to the list, it should be sorted into the list
of addresses not yet attempted according to the rules above
(Section 3).
6. Summary of Configurable Values
The values that may be configured as defaults on a client for use in
Happy Eyeballs are as follows:
o Resolution Delay (Section 2): The time to wait for a AAAA response
after receiving an A response. Recommended at 50 milliseconds.
o First Address Family Count (Section 3): The number of addresses
belonging to the first address family (such as IPv6) that should
be attempted before attempting another address family.
Recommended as 1, or 2 to more aggressively favor one address
family.
o Connection Attempt Delay (Section 4): The time to wait between
connection attempts in the absence of RTT data. Recommended at
250 milliseconds.
7. Security Considerations
This memo has no direct security considerations.
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8. IANA Considerations
This memo includes no request to IANA.
9. Acknowledgments
The authors thank Dan Wing, Andrew Yourtchenko, and everyone else who
worked on the original Happy Eyeballs design ([RFC6555]), Josh
Graessley, Stuart Cheshire, and the rest of team at Apple that helped
implement and instrument this algorithm, and Jason Fesler and Paul
Saab who helped measure and refine this algorithm.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
2012, <http://www.rfc-editor.org/info/rfc6555>.
[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<http://www.rfc-editor.org/info/rfc6724>.
10.2. Informative References
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<http://www.rfc-editor.org/info/rfc7413>.
Authors' Addresses
Tommy Pauly
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
US
Email: tpauly@apple.com
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David Schinazi
Apple Inc.
1 Infinite Loop
Cupertino, California 95014
US
Email: dschinazi@apple.com
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