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Internet Engineering Task Force (IETF)                      P. MartinsenRequest for Comments: 8421                                         CiscoBCP: 217                                                        T. ReddyCategory: Best Current Practice                             McAfee, Inc.ISSN: 2070-1721                                                 P. Patil                                                                   Cisco                                                               July 2018Guidelines for Multihomed and IPv4/IPv6 Dual-StackInteractive 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 (seeRFC 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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://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 toBCP 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 2018Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Notational Conventions  . . . . . . . . . . . . . . . . . . .33.  ICE Multihomed Recommendations  . . . . . . . . . . . . . . .34.  ICE Dual-Stack Recommendations  . . . . . . . . . . . . . . .45.  Compatibility . . . . . . . . . . . . . . . . . . . . . . . .56.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .77.  Security Considerations . . . . . . . . . . . . . . . . . . .78.  References  . . . . . . . . . . . . . . . . . . . . . . . . .88.1.  Normative References  . . . . . . . . . . . . . . . . . .88.2.  Informative References  . . . . . . . . . . . . . . . . .8   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .8   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .91.  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 fromRFC 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 inSection 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 thatSection 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" inSection 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   inSection 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.  InSection 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 20188.  References8.1.  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 2018Authors' 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.comMartinsen, et al.         Best Current Practice                 [Page 9]