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RFC 8421

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

Pages: 9
Best Current Practice: 217

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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
                                                               July 2018

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


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 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 ( 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.
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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.
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   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.
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   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 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.
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   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

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 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 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.
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   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 of [RFC8445]).
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   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 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

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.
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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, <>. [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, <>. [RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: Better Connectivity Using Concurrency", RFC 8305, DOI 10.17487/RFC8305, December 2017, <>. [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, <>.

8.2. Informative References

[ICE_dualstack_imp] "ICE Happy Eyeball Test Algorithms", commit 45083fb, January 2014, <>.


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.
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Authors' Addresses

Paal-Erik Martinsen Cisco Systems, Inc. Philip Pedersens Vei 22 Lysaker, Akershus 1325 Norway Email: Tirumaleswar Reddy McAfee, Inc. Embassy Golf Link Business Park Bangalore, Karnataka 560071 India Email: Prashanth Patil Cisco Systems, Inc. Bangalore India Email: