Internet DRAFT - draft-pauly-v6ops-happy-eyeballs-update
draft-pauly-v6ops-happy-eyeballs-update
Network T. Pauly
Internet-Draft D. Schinazi
Intended status: Standards Track Apple Inc.
Expires: September 13, 2017 March 12, 2017
An Update to Happy Eyeballs
draft-pauly-v6ops-happy-eyeballs-update-01
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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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 13, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Hostname Resolution Query Handling . . . . . . . . . . . . . 3
3.1. Handling Multiple DNS Server Addresses . . . . . . . . . 4
4. Sorting Addresses . . . . . . . . . . . . . . . . . . . . . . 4
5. Connection Attempts . . . . . . . . . . . . . . . . . . . . . 5
6. DNS Answer Changes during Happy Eyeballs Connection Setup . . 5
7. Summary of Configurable Values . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
11.1. Normative References . . . . . . . . . . . . . . . . . . 7
11.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. Differences from RFC6555 . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
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"Key words for use in RFCs to Indicate Requirement Levels" RFC 2119
[RFC2119].
2. Overview
This document defines a method of connection establishment, defined
as "Happy Eyeballs Connection Setup". This approach has several
distinct phases:
1. Initiation of asynchronous DNS queries [Section 3]
2. Sorting of resolved addresses [Section 4]
3. Initiation of asynchronous connection attempts [Section 5]
4. Establishment of one connection, which cancels all other
attempts
Note that this document assumes that the host address preference
policy favors IPv6 over IPv4. If the host is configured differently,
the recommendations in this document can be easily adapted.
3. 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
AAAA and A DNS queries. Both queries SHOULD be made as soon after
one another as possible, with the AAAA query made 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. Therefore, the client MUST treat DNS
resolution as asynchronous. Note that if the platforms does not
offer an asynchronous DNS API, this behavior can be simulated by
making two separate synchronous queries on different threads, one per
address family. 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".
The RECOMMENDED value for the Resolution Delay is 50 milliseconds.
If the AAAA response is received within the Resolution Delay period,
the client MUST immediately start the IPv6 connection attempt. If,
at the end of the Resolution Delay period, the AAAA response has not
been received but the A response has been received, the client SHOULD
proceed to Sorting Addresses [Section 4] and staggered connection
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attempts [Section 5] using only the IPv4 addresses returned so far.
If the AAAA response arrives while these connection attempts are in
progress, but before any connection has been established, then the
newly received IPv6 addresses are incorporated into the list of
available candidate addresses [Section 6] and the process of
connection attempts will continue with the IPv6 addresses added,
until one connection is established.
3.1. 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 (note that this is not
referring to the sending of AAAA or A queries, but rather the address
of the DNS server itself). 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.
4. 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 5]. It is important
that the ordered list involves all addresses from both families, as
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
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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.
5. Connection Attempts
Once the list of addresses has been constructed, the client will
attempt to make connections. In order to avoid unreasonable 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.
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.
6. DNS Answer Changes during Happy Eyeballs Connection Setup
If, during the course of connection establishment, the DNS answers
change either by adding resolved addresses, or removing previously
resolved addresses (for example, due to expiry of the TTL on that DNS
record), the client should react based on its current progress.
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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 4).
7. 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 3): The time to wait for a AAAA response
after receiving an A response. RECOMMENDED at 50 milliseconds.
o First Address Family Count (Section 4): 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 5): The time to wait between
connection attempts in the absence of RTT data. RECOMMENDED at
250 milliseconds.
8. Security Considerations
This memo has no direct security considerations.
9. IANA Considerations
This memo includes no request to IANA.
10. 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. The authors would
also like to thank Nick Chettle, Paul Hoffman, Philip Homburg, Joe
Touch and James Woodyatt for their input and contributions.
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11. References
11.1. 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>.
11.2. 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>.
Appendix A. Differences from RFC6555
"Happy Eyeballs: Success with Dual-Stack Hosts" [RFC6555] mostly
concentrates on how to stagger connections to a hostname that has an
AAAA and an A record. This document additionally discusses:
o how to perform DNS queries to obtain these addresses
o how to handle multiple addresses from each address family
o how to handle DNS updates while connections are being raced
o how to leverage historical information
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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