draft-ietf-v6ops-rfc6555bis-07.txt   rfc8305.txt 
Network D. Schinazi Internet Engineering Task Force (IETF) D. Schinazi
Internet-Draft T. Pauly Request for Comments: 8305 T. Pauly
Obsoletes: 6555 (if approved) Apple Inc. Obsoletes: 6555 Apple Inc.
Intended status: Standards Track October 25, 2017 Category: Standards Track December 2017
Expires: April 28, 2018 ISSN: 2070-1721
Happy Eyeballs Version 2: Better Connectivity Using Concurrency Happy Eyeballs Version 2: Better Connectivity Using Concurrency
draft-ietf-v6ops-rfc6555bis-07
Abstract Abstract
Many communication protocols operated over the modern Internet use Many communication protocols operating over the modern Internet use
host names. These often resolve to multiple IP addresses, each of hostnames. These often resolve to multiple IP addresses, each of
which may have different performance and connectivity which may have different performance and connectivity
characteristics. Since specific addresses or address families (IPv4 characteristics. Since specific addresses or address families (IPv4
or IPv6) may be blocked, broken, or sub-optimal on a network, clients or IPv6) may be blocked, broken, or sub-optimal on a network, clients
that attempt multiple connections in parallel have a higher chance of that attempt multiple connections in parallel have a chance of
establishing a connection sooner. This document specifies establishing a connection more quickly. This document specifies
requirements for algorithms that reduce this user-visible delay and requirements for algorithms that reduce this user-visible delay and
provides an example algorithm, referred to as "Happy Eyeballs". This provides an example algorithm, referred to as "Happy Eyeballs". This
document obsoletes the original algorithm description in [RFC6555]. document obsoletes the original algorithm description in RFC 6555.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
This Internet-Draft will expire on April 28, 2018. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8305.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 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
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to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
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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
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Hostname Resolution Query Handling . . . . . . . . . . . . . 4 3. Hostname Resolution Query Handling . . . . . . . . . . . . . 4
3.1. Handling Multiple DNS Server Addresses . . . . . . . . . 5 3.1. Handling Multiple DNS Server Addresses . . . . . . . . . 5
4. Sorting Addresses . . . . . . . . . . . . . . . . . . . . . . 5 4. Sorting Addresses . . . . . . . . . . . . . . . . . . . . . . 6
5. Connection Attempts . . . . . . . . . . . . . . . . . . . . . 6 5. Connection Attempts . . . . . . . . . . . . . . . . . . . . . 7
6. DNS Answer Changes during Happy Eyeballs Connection Setup . . 7 6. DNS Answer Changes during Happy Eyeballs Connection Setup . . 8
7. Supporting IPv6-only Networks with NAT64 and DNS64 . . . . . 8 7. Supporting IPv6-Only Networks with NAT64 and DNS64 . . . . . 8
7.1. IPv4 Address Literals . . . . . . . . . . . . . . . . . . 8 7.1. IPv4 Address Literals . . . . . . . . . . . . . . . . . . 8
7.2. Host Names with Broken AAAA Records . . . . . . . . . . . 9 7.2. Hostnames with Broken AAAA Records . . . . . . . . . . . 9
7.3. Virtual Private Networks . . . . . . . . . . . . . . . . 10 7.3. Virtual Private Networks . . . . . . . . . . . . . . . . 10
8. Summary of Configurable Values . . . . . . . . . . . . . . . 11 8. Summary of Configurable Values . . . . . . . . . . . . . . . 10
9. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 11 9. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Path Maximum Transmission Unit Discovery . . . . . . . . 12 9.1. Path Maximum Transmission Unit Discovery . . . . . . . . 11
9.2. Application Layer . . . . . . . . . . . . . . . . . . . . 12 9.2. Application Layer . . . . . . . . . . . . . . . . . . . . 11
9.3. Hiding Operational Issues . . . . . . . . . . . . . . . . 12 9.3. Hiding Operational Issues . . . . . . . . . . . . . . . . 11
10. Security Considerations . . . . . . . . . . . . . . . . . . . 12 10. Security Considerations . . . . . . . . . . . . . . . . . . . 12
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 12.1. Normative References . . . . . . . . . . . . . . . . . . 12
13.1. Normative References . . . . . . . . . . . . . . . . . . 13 12.2. Informative References . . . . . . . . . . . . . . . . . 13
13.2. Informative References . . . . . . . . . . . . . . . . . 14 Appendix A. Differences from RFC 6555 . . . . . . . . . . . . . 14
Appendix A. Differences from RFC6555 . . . . . . . . . . . . . . 14 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction 1. Introduction
Many communication protocols operated over the modern Internet use Many communication protocols operating over the modern Internet use
host names. These often resolve to multiple IP addresses, each of hostnames. These often resolve to multiple IP addresses, each of
which may have different performance and connectivity which may have different performance and connectivity
characteristics. Since specific addresses or address families (IPv4 characteristics. Since specific addresses or address families (IPv4
or IPv6) may be blocked, broken, or sub-optimal on a network, clients or IPv6) may be blocked, broken, or sub-optimal on a network, clients
that attempt multiple connections in parallel have a higher chance of that attempt multiple connections in parallel have a chance of
establishing a connection sooner. This document specifies establishing a connection more quickly. This document specifies
requirements for algorithms that reduce this user-visible delay and requirements for algorithms that reduce this user-visible delay and
provides an example algorithm. provides an example algorithm.
This document defines the algorithm for "Happy Eyeballs", a technique This document defines the algorithm for "Happy Eyeballs", a technique
of reducing user-visible delays on dual-stack hosts. This definition for reducing user-visible delays on dual-stack hosts. This
obsoletes the original description in [RFC6555]. Now that this definition obsoletes the original description in [RFC6555]. Now that
approach has been deployed at scale and measured for several years, this approach has been deployed at scale and measured for several
the algorithm specification can be refined to improve its reliability years, the algorithm specification can be refined to improve its
and generalization. reliability and general applicability.
The Happy Eyeballs algorithm of racing resolved addresses has several The Happy Eyeballs algorithm of racing connections to resolved
stages of ordering and racing to avoid delays to the user whenever addresses has several stages to avoid delays to the user whenever
possible, while preferring the use of IPv6. This document discusses possible, while preferring the use of IPv6. This document discusses
how to handle DNS queries when starting a connection on a dual-stack how to handle DNS queries when starting a connection on a dual-stack
client, how to create an ordered list of destination addresses to client, how to create an ordered list of destination addresses to
which to attempt connections, and how to race the connection which to attempt connections, and how to race the connection
attempts. attempts.
1.1. Requirements Language 1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
[RFC2119] [RFC8174] when, and only when, they appear in all capitals, [RFC2119] [RFC8174] when, and only when, they appear in all capitals,
as shown here. as shown here.
2. Overview 2. Overview
This document defines a method of connection establishment, named This document defines a method of connection establishment, named the
"Happy Eyeballs Connection Setup". This approach has several "Happy Eyeballs Connection Setup". This approach has several
distinct phases: distinct phases:
1. Initiation of asynchronous DNS queries [Section 3] 1. Initiation of asynchronous DNS queries [Section 3]
2. Sorting of resolved destination addresses [Section 4] 2. Sorting of resolved destination addresses [Section 4]
3. Initiation of asynchronous connection attempts [Section 5] 3. Initiation of asynchronous connection attempts [Section 5]
4. Establishment of one connection, which cancels all other attempts 4. Establishment of one connection, which cancels all other attempts
Note that this document assumes that the host destination address [Section 5]
preference policy favors IPv6 over IPv4. IPv6 has many desirable
properties designed to be improvements over IPv4 [RFC8200]. If the Note that this document assumes that the preference policy for the
host is configured to have a different preference, the host destination address favors IPv6 over IPv4. IPv6 has many
desirable properties designed to be improvements over IPv4 [RFC8200].
If the host is configured to have a different preference, the
recommendations in this document can be easily adapted. recommendations in this document can be easily adapted.
3. Hostname Resolution Query Handling 3. Hostname Resolution Query Handling
When a client has both IPv4 and IPv6 connectivity, and is trying to 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 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 AAAA and A DNS queries. Both queries SHOULD be made as soon after
one another as possible, with the AAAA query made first, immediately one another as possible, with the AAAA query made first and
followed by the A query. immediately followed by the A query.
Implementations SHOULD NOT wait for both families of answers to Implementations SHOULD NOT wait for both families of answers to
return before attempting connection establishment. If one query return before attempting connection establishment. If one query
fails to return, or takes significantly longer to return, waiting for fails to return or takes significantly longer to return, waiting for
the second address family can significantly delay the connection the second address family can significantly delay the connection
establishment of the first one. Therefore, the client SHOULD treat establishment of the first one. Therefore, the client SHOULD treat
DNS resolution as asynchronous. Note that if the platform does not DNS resolution as asynchronous. Note that if the platform does not
offer an asynchronous DNS API, this behavior can be simulated by offer an asynchronous DNS API, this behavior can be simulated by
making two separate synchronous queries on different threads, one per making two separate synchronous queries on different threads, one per
address family. address family.
The algorithm proceeds as follows: if a positive AAAA response (a The algorithm proceeds as follows: if a positive AAAA response (a
response with at least one valid AAAA record) is received first, the response with at least one valid AAAA record) is received first, the
first IPv6 connection attempt is immediately started. If a positive first IPv6 connection attempt is immediately started. If a positive
A response is received first due to reordering, the client SHOULD A response is received first due to reordering, the client SHOULD
wait for a short time for the AAAA response to ensure preference is wait a short time for the AAAA response to ensure that preference is
given to IPv6 (it is common for the AAAA response to follow the A given to IPv6 (it is common for the AAAA response to follow the A
response by a few milliseconds). This delay will be referred to as response by a few milliseconds). This delay will be referred to as
the "Resolution Delay". The recommended value for the Resolution the "Resolution Delay". The recommended value for the Resolution
Delay is 50 milliseconds. If a positive AAAA response is received Delay is 50 milliseconds. If a positive AAAA response is received
within the Resolution Delay period, the client immediately starts the within the Resolution Delay period, the client immediately starts the
IPv6 connection attempt. If a negative AAAA response (no error, no IPv6 connection attempt. If a negative AAAA response (no error, no
data) is received within the Resolution Delay period or the AAAA data) is received within the Resolution Delay period or the AAAA
response has not been received by the end of the Resolution Delay response has not been received by the end of the Resolution Delay
period, the client SHOULD proceed to Sorting Addresses [Section 4] period, the client SHOULD proceed to sorting addresses (see
and staggered connection attempts [Section 5] using any IPv4 Section 4) and staggered connection attempts (see Section 5) using
addresses returned so far. If the AAAA response arrives while these any IPv4 addresses returned so far. If the AAAA response arrives
connection attempts are in progress, but before any connection has while these connection attempts are in progress but before any
been established, then the newly received IPv6 addresses are connection has been established, then the newly received IPv6
incorporated into the list of available candidate addresses addresses are incorporated into the list of available candidate
[Section 6] and the process of connection attempts will continue with addresses (see Section 6) and the process of connection attempts will
the IPv6 addresses added, until one connection is established. continue with the IPv6 addresses added, until one connection is
established.
3.1. Handling Multiple DNS Server Addresses 3.1. Handling Multiple DNS Server Addresses
If multiple DNS server addresses are configured for the current If multiple DNS server addresses are configured for the current
network, the client may have the option of sending its DNS queries network, the client may have the option of sending its DNS queries
over IPv4 or IPv6. In keeping with the Happy Eyeballs approach, over IPv4 or IPv6. In keeping with the Happy Eyeballs approach,
queries SHOULD be sent over IPv6 first (note that this is not 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 referring to the sending of AAAA or A queries, but rather the address
of the DNS server itself and IP version used to transport DNS of the DNS server itself and IP version used to transport DNS
messages). If DNS queries sent to the IPv6 address do not receive messages). If DNS queries sent to the IPv6 address do not receive
responses, that address may be marked as penalized, and queries can responses, that address may be marked as penalized and queries can be
be sent to other DNS server addresses. sent to other DNS server addresses.
As native IPv6 deployments become more prevalent, and IPv4 addresses As native IPv6 deployments become more prevalent and IPv4 addresses
are exhausted, it is expected that IPv6 connectivity will have are exhausted, it is expected that IPv6 connectivity will have
preferential treatment within networks. If a DNS server is preferential treatment within networks. If a DNS server is
configured to be accessible over IPv6, IPv6 should be assumed to be configured to be accessible over IPv6, IPv6 should be assumed to be
the preferred address family. the preferred address family.
Client systems SHOULD NOT have an explicit limit to the number of DNS Client systems SHOULD NOT have an explicit limit to the number of DNS
servers that can be configured, either manually or by the network. servers that can be configured, either manually or by the network.
If such a limit is required by hardware limitations, the client If such a limit is required by hardware limitations, the client
SHOULD use at least one address from each address family from the SHOULD use at least one address from each address family from the
available list. available list.
4. Sorting Addresses 4. Sorting Addresses
Before attempting to connect to any of the resolved destination Before attempting to connect to any of the resolved destination
addresses, the client should define the order in which to start the addresses, the client should define the order in which to start the
attempts. Once the order has been defined, the client can use a attempts. Once the order has been defined, the client can use a
simple algorithm for racing each option after a short delay simple algorithm for racing each option after a short delay (see
[Section 5]. It is important that the ordered list involves all Section 5). It is important that the ordered list involve all
addresses from both families that have been received by this point, addresses from both families that have been received by this point,
as this allows the client to get the racing effect of Happy Eyeballs 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 for the entire list, not just the first IPv4 and first IPv6
addresses. addresses.
First, the client MUST sort the addresses received up to this point First, the client MUST sort the addresses received up to this point
using Destination Address Selection ([RFC6724], Section 6). using Destination Address Selection ([RFC6724], Section 6).
If the client is stateful and has history of expected round-trip If the client is stateful and has a history of expected round-trip
times (RTT) for the routes to access each address, it SHOULD add a times (RTTs) for the routes to access each address, it SHOULD add a
Destination Address Selection rule between rules 8 and 9 that prefers Destination Address Selection rule between rules 8 and 9 that prefers
addresses with lower RTTs. If the client keeps track of which addresses with lower RTTs. If the client keeps track of which
addresses it has used in the past, it SHOULD add another destination addresses it used in the past, it SHOULD add another Destination
address selection rule between the RTT rule and rule 9, which prefers Address Selection rule between the RTT rule and rule 9, which prefers
used addresses over unused ones. This helps servers that use the used addresses over unused ones. This helps servers that use the
client's IP address during authentication, as is the case for TCP client's IP address during authentication, as is the case for TCP
Fast Open [RFC7413] and some HTTP cookies. This historical data MUST Fast Open [RFC7413] and some Hypertext Transport Protocol (HTTP)
NOT be used across different network interfaces, and SHOULD be cookies. This historical data MUST NOT be used across different
flushed whenever a device changes the network to which it is network interfaces and SHOULD be flushed whenever a device changes
attached. the network to which it is attached.
Next, the client SHOULD modify the ordered list to interleave address Next, the client SHOULD modify the ordered list to interleave address
families. Whichever address family is first in the list should be families. Whichever address family is first in the list should be
followed by an address of the other address family; that is, if the 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 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 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 implementation MAY want to favor one address family more by allowing
multiple addresses of that family to be attempted before trying the multiple addresses of that family to be attempted before trying the
other family. The number of contiguous addresses of the first other family. The number of contiguous addresses of the first
address family will be referred to as the "First Address Family address family will be referred to as the "First Address Family
Count", and can be a configurable value. This is performed to avoid Count" and can be a configurable value. This is performed to avoid
waiting through a long list of addresses from a given address family waiting through a long list of addresses from a given address family
if connectivity over that address family is impaired. if connectivity over that address family is impaired.
Note that the address selection described in this section only Note that the address selection described in this section only
applies to destination addresses; Source Address Selection applies to destination addresses; Source Address Selection
([RFC6724], Section 5) is performed once per destination address and ([RFC6724], Section 5) is performed once per destination address and
is out of scope of this document. is out of scope of this document.
5. Connection Attempts 5. Connection Attempts
Once the list of addresses received up to this point has been Once the list of addresses received up to this point has been
constructed, the client will attempt to make connections. In order constructed, the client will attempt to make connections. In order
to avoid unreasonable network load, connection attempts SHOULD NOT be to avoid unreasonable network load, connection attempts SHOULD NOT be
made simultaneously. Instead, one connection attempt to a single made simultaneously. Instead, one connection attempt to a single
address is started first, followed by the others in the list, one at address is started first, followed by the others in the list, one at
a time. Starting a new connection attempt does not affect previous a time. Starting a new connection attempt does not affect previous
attempts, as multiple connection attempts may occur in parallel. attempts, as multiple connection attempts may occur in parallel.
Once one of the connection attempts succeeds (generally when the TCP Once one of the connection attempts succeeds (generally when the TCP
handshake completes), all other connections attempts that have not handshake completes), all other connections attempts that have not
yet succeeded SHOULD be cancelled. Any address that was not yet yet succeeded SHOULD be canceled. Any address that was not yet
attempted as a connection SHOULD be ignored. At that time, the attempted as a connection SHOULD be ignored. At that time, the
asynchronous DNS query MAY be cancelled as new addresses will not be asynchronous DNS query MAY be canceled as new addresses will not be
used for this connection. However, the DNS client resolver SHOULD used for this connection. However, the DNS client resolver SHOULD
still process DNS replies from the network for a short period of time still process DNS replies from the network for a short period of time
(recommended to be 1 second), as they will populate the DNS cache and (recommended to be 1 second), as they will populate the DNS cache and
can be used for subsequent connections. can be used for subsequent connections.
A simple implementation can have a fixed delay for how long to wait A simple implementation can have a fixed delay for how long to wait
before starting the next connection attempt. This delay is referred before starting the next connection attempt. This delay is referred
to as the "Connection Attempt Delay". One recommended value for a to as the "Connection Attempt Delay". One recommended value for a
default delay is 250 milliseconds. A more nuanced implementation's default delay is 250 milliseconds. A more nuanced implementation's
delay should correspond to the time when the previous attempt is delay should correspond to the time when the previous attempt is
sending its second TCP SYN, based on TCP's retransmission timer sending its second TCP SYN, based on the TCP's retransmission timer
[RFC6298]. If the client has historical RTT data gathered from other [RFC6298]. If the client has historical RTT data gathered from other
connections to the same host or prefix, it can use this information connections to the same host or prefix, it can use this information
to influence its delay. Note that this algorithm should only try to to influence its delay. Note that this algorithm should only try to
approximate the time of the first SYN retransmission, and not any approximate the time of the first SYN retransmission, and not any
further retransmissions which may be influenced by exponential timer further retransmissions that may be influenced by exponential timer
back off. back off.
The Connection Attempt Delay MUST have a lower bound, especially if The Connection Attempt Delay MUST have a lower bound, especially if
it is computed using historical data. More specifically, a it is computed using historical data. More specifically, a
subsequent connection MUST NOT be started within 10 milliseconds of subsequent connection MUST NOT be started within 10 milliseconds of
the previous attempt. The recommended minimum value is 100 the previous attempt. The recommended minimum value is 100
milliseconds, which is referred to as the "Minimum Connection Attempt milliseconds, which is referred to as the "Minimum Connection Attempt
Delay". This minimum value is required to avoid congestion collapse Delay". This minimum value is required to avoid congestion collapse
in the presence of high packet loss rates. The Connection Attempt in the presence of high packet-loss rates. The Connection Attempt
Delay SHOULD have an upper bound, referred to as the "Maximum Delay SHOULD have an upper bound, referred to as the "Maximum
Connection Attempt Delay". The current recommended value is 2 Connection Attempt Delay". The current recommended value is 2
seconds. seconds.
6. DNS Answer Changes during Happy Eyeballs Connection Setup 6. DNS Answer Changes during Happy Eyeballs Connection Setup
If, during the course of connection establishment, the DNS answers If, during the course of connection establishment, the DNS answers
change either by adding resolved addresses (for example, due to DNS change by either adding resolved addresses (for example due to DNS
push notifications [DNS-PUSH]), or removing previously resolved push notifications [DNS-PUSH]) or removing previously resolved
addresses (for example, due to expiry of the TTL on that DNS record), addresses (for example, due to expiry of the TTL on that DNS record),
the client should react based on its current progress. the client should react based on its current progress.
If an address is removed from the list that already had a connection If an address is removed from the list that already had a connection
attempt started, the connection attempt SHOULD NOT be cancelled, but attempt started, the connection attempt SHOULD NOT be canceled, but
rather be allowed to continue. If the removed address had not yet rather be allowed to continue. If the removed address had not yet
had a connection attempt started, it SHOULD be removed from the list had a connection attempt started, it SHOULD be removed from the list
of addresses to try. of addresses to try.
If an address is added to the list, it should be sorted into the list 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 of addresses not yet attempted according to the rules above (see
(Section 4). Section 4).
7. Supporting IPv6-only Networks with NAT64 and DNS64 7. Supporting IPv6-Only Networks with NAT64 and DNS64
While many IPv6 transition protocols have been standardized and While many IPv6 transition protocols have been standardized and
deployed, most are transparent to client devices. The combined use deployed, most are transparent to client devices. The combined use
of NAT64 [RFC6146] and DNS64 [RFC6147] is a popular solution that is of NAT64 [RFC6146] and DNS64 [RFC6147] is a popular solution that is
being deployed and requires changes in client devices. One possible being deployed and requires changes in client devices. One possible
way to handle these networks is for the client device networking way to handle these networks is for the client device networking
stack to implement 464XLAT [RFC6877]. 464XLAT has the advantage of stack to implement 464XLAT [RFC6877]. 464XLAT has the advantage of
not requiring changes to user space software, however it requires not requiring changes to user space software; however, it requires
per-packet translation if the application is using IPv4 literals and per-packet translation if the application is using IPv4 literals and
does not encourage client application software to support native does not encourage client application software to support native
IPv6. On platforms that do not support 464XLAT, the Happy Eyeballs IPv6. On platforms that do not support 464XLAT, the Happy Eyeballs
engine SHOULD follow the recommendations in this section to properly engine SHOULD follow the recommendations in this section to properly
support IPv6-only networks with NAT64 and DNS64. support IPv6-only networks with NAT64 and DNS64.
The features described in this section SHOULD only be enabled when The features described in this section SHOULD only be enabled when
the host detects one of these networks. A simple heuristic to the host detects one of these networks. A simple heuristic to
achieve that is to check if the network offers routable IPv6 achieve that is to check if the network offers routable IPv6
addressing, does not offer routable IPv4 addressing, and offers a DNS addressing, does not offer routable IPv4 addressing, and offers a DNS
resolver address. resolver address.
7.1. IPv4 Address Literals 7.1. IPv4 Address Literals
If client applications or users wish to connect to IPv4 address If client applications or users wish to connect to IPv4 address
literals, the Happy Eyeballs engine will need to perform NAT64 literals, the Happy Eyeballs engine will need to perform NAT64
address synthesis for them. The solution is similar to "Bump-in-the- address synthesis for them. The solution is similar to "Bump-in-the-
Host" [RFC6535] but is implemented inside the Happy Eyeballs library. Host" [RFC6535] but is implemented inside the Happy Eyeballs library.
When an IPv4 address is passed in to the library instead of a host When an IPv4 address is passed into the library instead of a
name, the device queries the network for the NAT64 prefix using hostname, the device queries the network for the NAT64 prefix using
"Discovery of the IPv6 Prefix Used for IPv6 Address Synthesis" "Discovery of the IPv6 Prefix Used for IPv6 Address Synthesis"
[RFC7050] then synthesizes an appropriate IPv6 address (or several) [RFC7050] and then synthesizes an appropriate IPv6 address (or
using the encoding described in "IPv6 Addressing of IPv4/IPv6 several) using the encoding described in "IPv6 Addressing of IPv4/
Translators" [RFC6052]. The synthesized addresses are then inserted IPv6 Translators" [RFC6052]. The synthesized addresses are then
into the list of addresses as if they were results from DNS queries; inserted into the list of addresses as if they were results from DNS
connection attempts follow the algorithm described above (Section 5). queries; connection attempts follow the algorithm described above
(see Section 5).
7.2. Host Names with Broken AAAA Records 7.2. Hostnames with Broken AAAA Records
At the time of writing, there exist a small but non negligible number At the time of writing, there exist a small but non-negligible number
of host names that resolve to valid A records and broken AAAA of hostnames that resolve to valid A records and broken AAAA records,
records, which we define as AAAA records that contain seemingly valid which we define as AAAA records that contain seemingly valid IPv6
IPv6 addresses but those addresses never reply when contacted on the addresses but those addresses never reply when contacted on the usual
usual ports. These can be for example caused by: ports. These can be, for example, caused by:
o Mistyping of the IPv6 address in the DNS zone configuration o Mistyping of the IPv6 address in the DNS zone configuration
o Routing black holes o Routing black holes
o Service outages o Service outages
While an algorithm complying with the other sections of this document While an algorithm complying with the other sections of this document
would correctly handle such host names on a dual-stack network, they would correctly handle such hostnames on a dual-stack network, they
will not necessarily function correctly on IPv6-only networks with will not necessarily function correctly on IPv6-only networks with
NAT64 and DNS64. Since DNS64 recursive resolvers rely on the NAT64 and DNS64. Since DNS64 recursive resolvers rely on the
authoritative name servers sending negative ("no error no answer") authoritative name servers sending negative ("no error no answer")
responses for AAAA records in order to synthesize, they will not responses for AAAA records in order to synthesize, they will not
synthesize records for these particular host names, and will instead synthesize records for these particular hostnames and will instead
pass through the broken AAAA record. pass through the broken AAAA record.
In order to support these scenarios, the client device needs to query In order to support these scenarios, the client device needs to query
the DNS for the A record then perform local synthesis. Since these the DNS for the A record and then perform local synthesis. Since
types of host names are rare, and in order to minimize load on DNS these types of hostnames are rare and, in order to minimize load on
servers, this A query should only be performed when the client has DNS servers, this A query should only be performed when the client
given up on the AAAA records it initially received. This can be has given up on the AAAA records it initially received. This can be
achieved by using a longer timeout, referred to as the "Last Resort achieved by using a longer timeout, referred to as the "Last Resort
Local Synthesis Delay" and recommended to be 2 seconds. The timer is Local Synthesis Delay"; the delay is recommended to be 2 seconds.
started when the last connection attempt is fired. If no connection The timer is started when the last connection attempt is fired. If
attempt has succeeded when this timer fires, the device queries the no connection attempt has succeeded when this timer fires, the device
DNS for the IPv4 address and on reception of a valid A record, treats queries the DNS for the IPv4 address and, on reception of a valid A
it as if it were provided by the application (Section 7.1). record, treats it as if it were provided by the application (see
Section 7.1).
7.3. Virtual Private Networks 7.3. Virtual Private Networks
Some Virtual Private Networks (VPN) may be configured to handle DNS Some Virtual Private Networks (VPNs) may be configured to handle DNS
queries from the device. The configuration could encompass all queries from the device. The configuration could encompass all
queries, or a subset such as "*.internal.example.com". These VPNs queries or a subset such as "*.internal.example.com". These VPNs can
can also be configured to only route part of the IPv4 address space, also be configured to only route part of the IPv4 address space, such
such as 192.0.2.0/24. However, if an internal hostname resolves to as 192.0.2.0/24. However, if an internal hostname resolves to an
an external IPv4 address, these can cause issues if the underlying external IPv4 address, these can cause issues if the underlying
network is IPv6-only. As an example, let's assume that network is IPv6-only. As an example, let's assume that
"www.internal.example.com" has exactly one A record, 198.51.100.42, "www.internal.example.com" has exactly one A record, 198.51.100.42,
and no AAAA records. The client will send the DNS query to the and no AAAA records. The client will send the DNS query to the
company's recursive resolver and that resolver will reply with these company's recursive resolver and that resolver will reply with these
records. The device now only has an IPv4 address to connect to, and records. The device now only has an IPv4 address to connect to and
no route to that address. Since the company's resolver does not know no route to that address. Since the company's resolver does not know
the NAT64 prefix of the underlying network, it cannot synthesize the the NAT64 prefix of the underlying network, it cannot synthesize the
address. Similarly, the underlying network's DNS64 recursive address. Similarly, the underlying network's DNS64 recursive
resolver does not know the company's internal addresses, so it cannot resolver does not know the company's internal addresses, so it cannot
resolve the hostname. Because of this, the client device needs to resolve the hostname. Because of this, the client device needs to
resolve the A record using the company's resolver then locally resolve the A record using the company's resolver and then locally
synthesize an IPv6 address, as if the resolved IPv4 address were synthesize an IPv6 address, as if the resolved IPv4 address were
provided by the application (Section 7.1). provided by the application (Section 7.1).
8. Summary of Configurable Values 8. Summary of Configurable Values
The values that may be configured as defaults on a client for use in The values that may be configured as defaults on a client for use in
Happy Eyeballs are as follows: Happy Eyeballs are as follows:
o Resolution Delay (Section 3): The time to wait for a AAAA response o Resolution Delay (Section 3): The time to wait for a AAAA response
after receiving an A response. Recommended to be 50 milliseconds. after receiving an A response. Recommended to be 50 milliseconds.
o First Address Family Count (Section 4): The number of addresses o First Address Family Count (Section 4): The number of addresses
belonging to the first address family (such as IPv6) that should belonging to the first address family (such as IPv6) that should
be attempted before attempting another address family. be attempted before attempting another address family.
Recommended to be 1, or 2 to more aggressively favor one address Recommended to be 1; 2 may be used to more aggressively favor a
family. particular address family.
o Connection Attempt Delay (Section 5): The time to wait between o Connection Attempt Delay (Section 5): The time to wait between
connection attempts in the absence of RTT data. Recommended to be connection attempts in the absence of RTT data. Recommended to be
250 milliseconds. 250 milliseconds.
o Minimum Connection Attempt Delay (Section 5): The minimum time to o Minimum Connection Attempt Delay (Section 5): The minimum time to
wait between connection attempts. Recommended to be 100 wait between connection attempts. Recommended to be 100
milliseconds. MUST NOT be less than 10 milliseconds. milliseconds. MUST NOT be less than 10 milliseconds.
o Maximum Connection Attempt Delay (Section 5): The maximum time to o Maximum Connection Attempt Delay (Section 5): The maximum time to
skipping to change at page 11, line 48 skipping to change at page 11, line 23
expected that the properties of networks will evolve. For that expected that the properties of networks will evolve. For that
reason, it is expected that these values will change over time. reason, it is expected that these values will change over time.
Implementors should feel welcome to use different values without Implementors should feel welcome to use different values without
changing this specification. Since IPv6 issues are expected to be changing this specification. Since IPv6 issues are expected to be
less common, the delays SHOULD be increased with time as client less common, the delays SHOULD be increased with time as client
software is updated. software is updated.
9. Limitations 9. Limitations
Happy Eyeballs will handle initial connection failures at the TCP/IP Happy Eyeballs will handle initial connection failures at the TCP/IP
layer, however other failures or performance issues may still affect layer; however, other failures or performance issues may still affect
the chosen connection. the chosen connection.
9.1. Path Maximum Transmission Unit Discovery 9.1. Path Maximum Transmission Unit Discovery
Since Happy Eyeballs is only active during the initial handshake and Since Happy Eyeballs is only active during the initial handshake and
TCP does not pass the initial handshake, issues related to MTU can be TCP does not pass the initial handshake, issues related to MTU can be
masked and go unnoticed during Happy Eyeballs. Solving this issue is masked and go unnoticed during Happy Eyeballs. Solving this issue is
out of scope of this document. One solution is to use Packetization out of scope of this document. One solution is to use "Packetization
Layer Path MTU Discovery [RFC4821]. Layer Path MTU Discovery" [RFC4821].
9.2. Application Layer 9.2. Application Layer
If the DNS returns multiple addresses for different application If the DNS returns multiple addresses for different application
servers, the application itself may not be operational and functional servers, the application itself may not be operational and functional
on all of them. Common examples include Transport Layer Security on all of them. Common examples include Transport Layer Security
(TLS) and the Hypertext Transport Protocol (HTTP). (TLS) and HTTP.
9.3. Hiding Operational Issues 9.3. Hiding Operational Issues
It has been observed in practice that Happy Eyeballs can hide issues It has been observed in practice that Happy Eyeballs can hide issues
in networks. For example, if a misconfiguration causes IPv6 to in networks. For example, if a misconfiguration causes IPv6 to
consistently fail on a given network while IPv4 is still functional, consistently fail on a given network while IPv4 is still functional,
Happy Eyeballs may impair the operator's ability to notice the issue. Happy Eyeballs may impair the operator's ability to notice the issue.
It is recommended that network operators deploy external means of It is recommended that network operators deploy external means of
monitoring to ensure functionality of all address families. monitoring to ensure functionality of all address families.
10. Security Considerations 10. Security Considerations
Note that applications should not rely upon a stable hostname-to- Note that applications should not rely upon a stable hostname-to-
address mapping to ensure any security properties, since DNS results address mapping to ensure any security properties, since DNS results
may change between queries. Happy Eyeballs may make it more likely may change between queries. Happy Eyeballs may make it more likely
that subsequent connections to a single hostname use different IP that subsequent connections to a single hostname use different IP
addresses. addresses.
11. IANA Considerations 11. IANA Considerations
This memo includes no request to IANA. This document does not require any IANA actions.
12. 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 Fred Baker, Nick Chettle, Lorenzo Colitti, Igor
Gashinsky, Geoff Huston, Jen Linkova, Paul Hoffman, Philip Homburg,
Warren Kumari, Erik Nygren, Jordi Palet Martinez, Rui Paulo, Stephen
Strowes, Jinmei Tatuya, Dave Thaler, Joe Touch and James Woodyatt for
their input and contributions.
13. References 12. References
13.1. 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, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>. <https://www.rfc-editor.org/info/rfc4821>.
skipping to change at page 14, line 14 skipping to change at page 13, line 28
[RFC7050] Savolainen, T., Korhonen, J., and D. Wing, "Discovery of [RFC7050] Savolainen, T., Korhonen, J., and D. Wing, "Discovery of
the IPv6 Prefix Used for IPv6 Address Synthesis", the IPv6 Prefix Used for IPv6 Address Synthesis",
RFC 7050, DOI 10.17487/RFC7050, November 2013, RFC 7050, DOI 10.17487/RFC7050, November 2013,
<https://www.rfc-editor.org/info/rfc7050>. <https://www.rfc-editor.org/info/rfc7050>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
13.2. Informative References 12.2. Informative References
[DNS-PUSH] [DNS-PUSH] Pusateri, T. and S. Cheshire, "DNS Push Notifications",
Pusateri, T. and S. Cheshire, "DNS Push Notifications", Work in Progress, draft-ietf-dnssd-push-13, October 2017.
Work in Progress, draft-ietf-dnssd-push, March 2017.
[RFC6877] Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT: [RFC6877] Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
Combination of Stateful and Stateless Translation", Combination of Stateful and Stateless Translation",
RFC 6877, DOI 10.17487/RFC6877, April 2013, RFC 6877, DOI 10.17487/RFC6877, April 2013,
<https://www.rfc-editor.org/info/rfc6877>. <https://www.rfc-editor.org/info/rfc6877>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>. <https://www.rfc-editor.org/info/rfc7413>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200, (IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017, DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>. <https://www.rfc-editor.org/info/rfc8200>.
Appendix A. Differences from RFC6555 Appendix A. Differences from RFC 6555
"Happy Eyeballs: Success with Dual-Stack Hosts" [RFC6555] mostly "Happy Eyeballs: Success with Dual-Stack Hosts" [RFC6555] mostly
concentrates on how to stagger connections to a hostname that has an concentrates on how to stagger connections to a hostname that has a
AAAA and an A record. This document additionally discusses: AAAA and an A record. This document additionally discusses:
o how to perform DNS queries to obtain these addresses o how to perform DNS queries to obtain these addresses
o how to handle multiple addresses from each address family o how to handle multiple addresses from each address family
o how to handle DNS updates while connections are being raced o how to handle DNS updates while connections are being raced
o how to leverage historical information o how to leverage historical information
skipping to change at page 15, line 4 skipping to change at page 14, line 23
o how to handle multiple addresses from each address family o how to handle multiple addresses from each address family
o how to handle DNS updates while connections are being raced o how to handle DNS updates while connections are being raced
o how to leverage historical information o how to leverage historical information
o how to support IPv6-only networks with NAT64 and DNS64 o how to support IPv6-only networks with NAT64 and DNS64
Note that a simple implementation of the algorithm described in this Note that a simple implementation of the algorithm described in this
document is still compliant with the previous specification document is still compliant with the previous specification
[RFC6555]. Implementations should take the new considerations into [RFC6555]. Implementations should take the new considerations into
account when applicable to optimize their behavior. account when applicable to optimize their behavior.
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 Fred Baker, Nick Chettle, Lorenzo Colitti, Igor
Gashinsky, Geoff Huston, Jen Linkova, Paul Hoffman, Philip Homburg,
Warren Kumari, Erik Nygren, Jordi Palet Martinez, Rui Paulo, Stephen
Strowes, Jinmei Tatuya, Dave Thaler, Joe Touch, and James Woodyatt
for their input and contributions.
Authors' Addresses Authors' Addresses
David Schinazi David Schinazi
Apple Inc. Apple Inc.
1 Infinite Loop 1 Infinite Loop
Cupertino, California 95014 Cupertino, California 95014
US United States of America
Email: dschinazi@apple.com Email: dschinazi@apple.com
Tommy Pauly Tommy Pauly
Apple Inc. Apple Inc.
1 Infinite Loop 1 Infinite Loop
Cupertino, California 95014 Cupertino, California 95014
US United States of America
Email: tpauly@apple.com Email: tpauly@apple.com
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