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

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.

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.

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.

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 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.

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]).

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 [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.

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 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.

Authors' Addresses

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


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


Prashanth Patil
Cisco Systems, Inc.