    Internet Engineering Task Force (IETF)                      P. Martinsen
    Request for Comments: 8421                                         Cisco
    BCP: 217                                                        T. Reddy
    Category: Best Current Practice                             McAfee, Inc.
    ISSN: 2070-1721                                                 P. Patil
                                                                       Cisco
                                                                   July 2018

               Guidelines for Multihomed and IPv4/IPv6 Dual-Stack
                  Interactive Connectivity Establishment (ICE)

    Abstract

       This document provides guidelines on how to make Interactive
       Connectivity Establishment (ICE) conclude faster in multihomed and
       IPv4/IPv6 dual-stack scenarios where broken paths exist.  The
       provided guidelines are backward compatible with the original ICE
       specification (see RFC 5245).

    Status of This Memo

       This memo documents an Internet Best Current Practice.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       BCPs is available in Section 2 of RFC 7841.

       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/rfc8421.

    Copyright Notice

       Copyright (c) 2018 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
       (https://trustee.ietf.org/license-info) in effect on the date of
       publication of this document.  Please review these documents
       carefully, as they describe your rights and restrictions with respect
       to this document.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

    Martinsen, et al.         Best Current Practice                 [Page 1]

    RFC 8421        ICE Multihomed and Dual-Stack Guidelines       July 2018

    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
       2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
       3.  ICE Multihomed Recommendations  . . . . . . . . . . . . . . .   3
       4.  ICE Dual-Stack Recommendations  . . . . . . . . . . . . . . .   4
       5.  Compatibility . . . . . . . . . . . . . . . . . . . . . . . .   5
       6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
       7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
       8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
         8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
         8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
       Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   8
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

    1.  Introduction

       In multihomed and IPv4/IPv6 dual-stack environments, ICE [RFC8445]
       would benefit by a fair distribution of its connectivity checks
       across available interfaces or IP address types.  With a fair
       distribution of the connectivity checks, excessive delays are avoided
       if a particular network path is broken or slow.  Arguably, it would
       be better to put the interfaces or address types known to the
       application last in the checklist.  However, the main motivation by
       ICE is to make no assumptions regarding network topology; hence, a
       fair distribution of the connectivity checks is more appropriate.  If
       an application operates in a well-known environment, it can safely
       override the recommendation given in this document.

       Applications should take special care to deprioritize network
       interfaces known to provide unreliable connectivity when operating in
       a multihomed environment.  For example, certain tunnel services might
       provide unreliable connectivity.  Doing so will ensure a more fair
       distribution of the connectivity checks across available network
       interfaces on the device.  The simple guidelines presented here
       describe how to deprioritize interfaces known by the application to
       provide unreliable connectivity.

       There is also a need to introduce better handling of connectivity
       checks for different IP address families in dual-stack IPv4/IPv6 ICE
       scenarios.  Following the recommendations from RFC 6724 [RFC6724]
       will lead to prioritization of IPv6 over IPv4 for the same candidate
       type.  Due to this, connectivity checks for candidates of the same
       type (host, reflexive, or relay) are sent such that an IP address
       family is completely depleted before checks from the other address
       family are started.  This results in user-noticeable delays with
       setup if the path for the prioritized address family is broken.

    Martinsen, et al.         Best Current Practice                 [Page 2]

    RFC 8421        ICE Multihomed and Dual-Stack Guidelines       July 2018

       To avoid user-noticeable delays when either the IPv6 or IPv4 path is
       broken or excessively slow, this specification encourages
       intermingling the different address families when connectivity checks
       are performed.  This will lead to more sustained dual-stack IPv4/IPv6
       deployment as users will no longer have an incentive to disable IPv6.
       The cost is a small penalty to the address type that otherwise would
       have been prioritized.  Further, this document recommends keeping
       track of previous known connectivity problems and assigning a lower
       priority to those addresses.  Specific mechanisms and rules for
       tracking connectivity issues are out of scope for this document.

       This document describes what parameters an agent can safely alter to
       fairly order the checklist candidate pairs in multihomed and dual-
       stack environments, thus affecting the sending order of the
       connectivity checks.  The actual values of those parameters are an
       implementation detail.  Dependent on the nomination method in use,
       this might have an effect on what candidate pair ends up as the
       active one.  Ultimately, it should be up to the agent to decide what
       candidate pair is best suited for transporting media.

       The guidelines outlined in this specification are backward compatible
       with the original ICE implementation.  This specification only alters
       the values used to create the resulting checklists in such a way that
       the core mechanisms from the original ICE specification [RFC5245] and
       its replacement [RFC8445] are still in effect.

    2.  Notational Conventions

       This document uses terminology defined in [RFC8445].

    3.  ICE Multihomed Recommendations

       A multihomed ICE agent can potentially send and receive connectivity
       checks on all available interfaces and IP addresses.  It is possible
       for an interface to have several IP addresses associated with it.  To
       avoid unnecessary delay when performing connectivity checks, it would
       be beneficial to prioritize interfaces and IP addresses known by the
       agent to provide stable connectivity.

       The application knowledge regarding the reliability of an interface
       can also be based on simple metrics like previous connection success/
       failure rates, or it can be a more static model based on interface
       types like wired, wireless, cellular, virtual, and tunneled in
       conjunction with other operational metrics.  This would require the
       application to have the right permissions to obtain such operational
       metrics.

    Martinsen, et al.         Best Current Practice                 [Page 3]

    RFC 8421        ICE Multihomed and Dual-Stack Guidelines       July 2018

       Candidates from an interface known to the application to provide
       unreliable connectivity should get a low candidate priority.  When to
       consider connectivity as unreliable is implementation specific.
       Usage of ICE is not limited to Voice over IP (VoIP) applications.
       What an application sees as unreliability might be determined by a
       mix of how long lived the connection is, how often setup is required,
       and other, for now unknown, requirements.  This is purely an
       optimization to speed up the ICE connectivity check phase.

       If the application is unable to get any interface information
       regarding type or is unable to store any relevant metrics, it should
       treat all interfaces as if they have reliable connectivity.  This
       ensures that all interfaces get a fair chance to perform their
       connectivity checks.

    4.  ICE Dual-Stack Recommendations

       Candidates should be prioritized such that a sequence of candidates
       belonging to the same address family will be intermingled with
       candidates from an alternate IP family, for example, promote IPv4
       candidates in the presence of many IPv6 candidates such that an IPv4
       address candidate is always present after a small sequence of IPv6
       candidates (i.e., reorder candidates such that both IPv6 and IPv4
       candidates get a fair chance during the connectivity check phase).
       This makes ICE connectivity checks more responsive to broken-path
       failures of an address family.

       An ICE agent can select an algorithm or a technique of its choice to
       ensure that the resulting checklists have a fair intermingled mix of
       IPv4 and IPv6 address families.  However, modifying the checklist
       directly can lead to uncoordinated local and remote checklists that
       result in ICE taking longer to complete or, in the worst case
       scenario, fail.  The best approach is to set the appropriate value
       for local preference in the formula for calculating the candidate
       priority value as described in the "Recommended Formula" section
       (Section 5.1.2.1) of [RFC8445].

       Implementations should prioritize IPv6 candidates by putting some of
       them first in the intermingled checklist.  This increases the chance
       of IPv6 connectivity checks to complete first and be ready for
       nomination or usage.  This enables implementations to follow the
       intent of "Happy Eyeballs: Success with Dual-Stack Hosts" [RFC8305].
       It is worth noting that the timing recommendations in [RFC8305] will
       be overruled by how ICE paces out its connectivity checks.

    Martinsen, et al.         Best Current Practice                 [Page 4]

    RFC 8421        ICE Multihomed and Dual-Stack Guidelines       July 2018

       A simple formula to calculate how many IPv6 addresses to put before
       any IPv4 addresses could look like:

                    Hi = (N_4 + N_6) / N_4

                    Where Hi  = Head start before intermingling starts
                          N_4 = Number of IPv4 addresses
                          N_6 = Number of IPv6 addresses

       If a host has two IPv4 addresses and six IPv6 addresses, it will
       insert an IPv4 address after four IPv6 addresses by choosing the
       appropriate local preference values when calculating the pair
       priorities.

    5.  Compatibility

       The formula in Section 5.1.2 of [RFC8445] should be used to calculate
       the candidate priority.  The formula is as follows:

                    priority = (2^24)*(type preference) +
                               (2^8)*(local preference) +
                               (2^0)*(256 - component ID)

       "Guidelines for Choosing Type and Local Preferences" (Section 5.1.2.2
       of [RFC8445]) has guidelines for how the type preference and local
       preference value should be chosen.  Instead of having a static local
       preference value for IPv4 and IPv6 addresses, it is possible to
       choose this value dynamically in such a way that IPv4 and IPv6
       address candidate priorities end up intermingled within the same
       candidate type.  It is also possible to assign lower priorities to IP
       addresses derived from unreliable interfaces using the local
       preference value.

       It is worth mentioning that Section 5.1.2.1 of [RFC8445] states that
       "if there are multiple candidates for a particular component for a
       particular data stream that have the same type, the local preference
       MUST be unique for each one".

       The local type preference can be dynamically changed in such a way
       that IPv4 and IPv6 address candidates end up intermingled regardless
       of candidate type.  This is useful if there are a lot of IPv6 host
       candidates effectively blocking connectivity checks for IPv4 server
       reflexive candidates.

       Candidates with IP addresses from an unreliable interface should be
       ordered at the end of the checklist, i.e., not intermingled as the
       dual-stack candidates.

    Martinsen, et al.         Best Current Practice                 [Page 5]

    RFC 8421        ICE Multihomed and Dual-Stack Guidelines       July 2018

       The list below shows a sorted local candidate list where the priority
       is calculated in such a way that the IPv4 and IPv6 candidates are
       intermingled (no multihomed candidates).  To allow for earlier
       connectivity checks for the IPv4 server reflexive candidates, some of
       the IPv6 host candidates are demoted.  This is just an example of how
       candidate priorities can be calculated to provide better fairness
       between IPv4 and IPv6 candidates without breaking any of the ICE
       connectivity checks.

                         Candidate   Address Component
                           Type       Type      ID     Priority
                      -------------------------------------------
                      (1)  HOST       IPv6      (1)    2129289471
                      (2)  HOST       IPv6      (2)    2129289470
                      (3)  HOST       IPv4      (1)    2129033471
                      (4)  HOST       IPv4      (2)    2129033470
                      (5)  HOST       IPv6      (1)    2128777471
                      (6)  HOST       IPv6      (2)    2128777470
                      (7)  HOST       IPv4      (1)    2128521471
                      (8)  HOST       IPv4      (2)    2128521470
                      (9)  HOST       IPv6      (1)    2127753471
                      (10) HOST       IPv6      (2)    2127753470
                      (11) SRFLX      IPv6      (1)    1693081855
                      (12) SRFLX      IPv6      (2)    1693081854
                      (13) SRFLX      IPv4      (1)    1692825855
                      (14) SRFLX      IPv4      (2)    1692825854
                      (15) HOST       IPv6      (1)    1692057855
                      (16) HOST       IPv6      (2)    1692057854
                      (17) RELAY      IPv6      (1)    15360255
                      (18) RELAY      IPv6      (2)    15360254
                      (19) RELAY      IPv4      (1)    15104255
                      (20) RELAY      IPv4      (2)    15104254

                       SRFLX = server reflexive

       Note that the list does not alter the component ID part of the
       formula.  This keeps the different components (RTP and the Real-time
       Transport Control Protocol (RTCP)) close in the list.  What matters
       is the ordering of the candidates with component ID 1.  Once the
       checklist is formed for a media stream, the candidate pair with
       component ID 1 will be tested first.  If the ICE connectivity check
       is successful, then other candidate pairs with the same foundation
       will be unfrozen (see "Computing Candidate Pair States" in
       Section 6.1.2.6 of [RFC8445]).

    Martinsen, et al.         Best Current Practice                 [Page 6]

    RFC 8421        ICE Multihomed and Dual-Stack Guidelines       July 2018

       The local and remote agent can have different algorithms for choosing
       the local preference and type preference values without impacting the
       synchronization between the local and remote checklists.

       The checklist is made up of candidate pairs.  A candidate pair is two
       candidates paired up and given a candidate pair priority as described
       in Section 6.1.2.3 of [RFC8445].  Using the pair priority formula:

            pair priority = 2^32*MIN(G,D) + 2*MAX(G,D) + (G>D?1:0)

       Where G is the candidate priority provided by the controlling agent,
       and D is the candidate priority provided by the controlled agent.
       This ensures that the local and remote checklists are coordinated.

       Even if the two agents have different algorithms for choosing the
       candidate priority value to get an intermingled set of IPv4 and IPv6
       candidates, the resulting checklist, that is a list sorted by the
       pair priority value, will be identical on the two agents.

       The agent that has promoted IPv4 cautiously, i.e., lower IPv4
       candidate priority values compared to the other agent, will influence
       the checklist the most due to (2^32*MIN(G,D)) in the formula.

       These recommendations are backward compatible with the original ICE
       implementation.  The resulting local and remote checklist will still
       be synchronized.

       Dependent of the nomination method in use, the procedures described
       in this document might change what candidate pair ends up as the
       active one.

       A test implementation of an example algorithm is available at
       [ICE_dualstack_imp].

    6.  IANA Considerations

       This document has no IANA actions.

    7.  Security Considerations

       The security considerations described in [RFC8445] are valid.  It
       changes recommended values and describes how an agent could choose
       those values in a safe way.  In Section 3, the agent can prioritize
       the network interface based on previous network knowledge.  This can
       potentially be unwanted information leakage towards the remote agent.

    Martinsen, et al.         Best Current Practice                 [Page 7]

    RFC 8421        ICE Multihomed and Dual-Stack Guidelines       July 2018

    8.  References

    8.1.  Normative References

       [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
                  (ICE): A Protocol for Network Address Translator (NAT)
                  Traversal for Offer/Answer Protocols", RFC 5245,
                  DOI 10.17487/RFC5245, April 2010,
                  <https://www.rfc-editor.org/info/rfc5245>.

       [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,
                  <https://www.rfc-editor.org/info/rfc6724>.

       [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
                  Better Connectivity Using Concurrency", RFC 8305,
                  DOI 10.17487/RFC8305, December 2017,
                  <https://www.rfc-editor.org/info/rfc8305>.

       [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
                  Connectivity Establishment (ICE): A Protocol for Network
                  Address Translator (NAT) Traversal", RFC 8445,
                  DOI 10.17487/RFC8445, July 2018,
                  <https://www.rfc-editor.org/info/rfc8445>.

    8.2.  Informative References

       [ICE_dualstack_imp]
                  "ICE Happy Eyeball Test Algorithms", commit 45083fb,
                  January 2014,
                  <https://github.com/palerikm/ICE-DualStackFairness>.

    Acknowledgements

       The authors would like to thank Dan Wing, Ari Keranen, Bernard Aboba,
       Martin Thomson, Jonathan Lennox, Balint Menyhart, Ole Troan, Simon
       Perreault, Ben Campbell, and Mirja Kuehlewind for their comments and
       review.

    Martinsen, et al.         Best Current Practice                 [Page 8]

    RFC 8421        ICE Multihomed and Dual-Stack Guidelines       July 2018

    Authors' Addresses

       Paal-Erik Martinsen
       Cisco Systems, Inc.
       Philip Pedersens Vei 22
       Lysaker, Akershus  1325
       Norway

       Email: palmarti@cisco.com

       Tirumaleswar Reddy
       McAfee, Inc.
       Embassy Golf Link Business Park
       Bangalore, Karnataka  560071
       India

       Email: TirumaleswarReddy_Konda@McAfee.com

       Prashanth Patil
       Cisco Systems, Inc.
       Bangalore
       India

       Email: praspati@cisco.com

    Martinsen, et al.         Best Current Practice                 [Page 9]