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rfc:bcp:bcp217

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]

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