An Update to Happy Eyeballs
draft-pauly-v6ops-happy-eyeballs-update-00

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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   This Internet-Draft will expire on September 9, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

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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