Skip to main content

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

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8421.
Authors Paal-Erik Martinsen , Tirumaleswar Reddy.K , Prashanth Patil
Last updated 2018-07-19 (Latest revision 2016-11-13)
Replaces draft-ietf-mmusic-ice-dualstack-fairness
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Best Current Practice
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Ari Keränen
Shepherd write-up Show Last changed 2016-04-07
IESG IESG state Became RFC 8421 (Best Current Practice)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Ben Campbell
Send notices to "Ari Keranen" <>
IANA IANA review state IANA OK - No Actions Needed
IANA action state No IANA Actions
ICE                                                         P. Martinsen
Internet-Draft                                                  T. Reddy
Intended status: Best Current Practice                          P. Patil
Expires: May 18, 2017                                              Cisco
                                                       November 14, 2016

           ICE Multihomed and IPv4/IPv6 Dual Stack Guidelines


   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 backwards compatible with the original ICE

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 18, 2017.

Copyright Notice

   Copyright (c) 2016 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

Martinsen, et al.         Expires May 18, 2017                  [Page 1]
Internet-Draft   ICE Multihomed and DualStack Guidelines   November 2016

   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 Recomendations . . . . . . . . . . . . . . . .   3
   4.  ICE Dual Stack Recomendations . . . . . . . . . . . . . . . .   4
   5.  Compatibility . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .   7
     7.1.  ICE-Dual Stack Fairness Test code . . . . . . . . . . . .   8
     7.2.  ICE-Dual Stack Fairness Test code . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   In multihomed and IPv4/IPv6 dual-stack environments ICE
   [I-D.ietf-ice-rfc5245bis] 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.  It would
   arguably 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 is 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 RFC6724 [RFC6724] will
   lead to prioritization of IPv6 over IPv4 for the same candidate type.

Martinsen, et al.         Expires May 18, 2017                  [Page 2]
Internet-Draft   ICE Multihomed and DualStack Guidelines   November 2016

   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 setup delays if the path
   for the prioritized address family is broken.

   To avoid user noticeable delays when either 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 to keep
   track of previous known connectivity problem and assign 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.  Actual values of those parameters is an
   implementation detail.  Dependant 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 a standard ICE implementation.  This specification only alters
   the values used to create the resulting checklists in such a way that
   the core mechanisms from ICE [RFC5245] and ICEbis
   [I-D.ietf-ice-rfc5245bis] are still in effect.

2.  Notational Conventions

   This document uses terminology defined in [I-D.ietf-ice-rfc5245bis].

3.  ICE Multihomed Recomendations

   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 a more static model based on interface types like

Martinsen, et al.         Expires May 18, 2017                  [Page 3]
Internet-Draft   ICE Multihomed and DualStack Guidelines   November 2016

   wired, wireless, cellular, virtual, 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 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 others,
   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 unable to store any relevant metrics, it should
   treat all interfaces as if they have reliable connectivity.  This
   ensures all interfaces gets their fair chance to perform their
   connectivity checks.

4.  ICE Dual Stack Recomendations

   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, promoting 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., reordering 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 choose an algorithm or a technique of its choice to
   ensure that the resulting check lists have a fair intermingled mix of
   IPv4 and IPv6 address families.  However, modifying the check list
   directly can lead to uncoordinated local and remote check lists 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 described in ICE [I-D.ietf-ice-rfc5245bis] section "
   Recommended Formula".

   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 [RFC6555] "Happy Eyeballs: Success with Dual-Stack Hosts".
   It is worth noting that the timing recommendations in [RFC6555] will
   be overruled by how ICE paces out its connectivity checks.

Martinsen, et al.         Expires May 18, 2017                  [Page 4]
Internet-Draft   ICE Multihomed and DualStack Guidelines   November 2016

   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 have 2 IPv4 addresses and 6 IPv6 addresses, it will insert
   an IP4 address after 4 IPv6 addresses by choosing the appropriate
   local preference values when calculating the pair priorities.

5.  Compatibility

   ICE [I-D.ietf-ice-rfc5245bis] section "4.1.2 Prioritizing Candidates"
   states that the formula in section " Recommended Formula"
   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)

   ICE [I-D.ietf-ice-rfc5245bis] section " Guidelines for
   Choosing Type and Local Preferences" 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 [I-D.ietf-ice-rfc5245bis] section
   " Recommended Formula" says that; "if there are multiple
   candidates for a particular component for a particular media stream
   that has the same type, the local preference MUST be unique for each

   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.

Martinsen, et al.         Expires May 18, 2017                  [Page 5]
Internet-Draft   ICE Multihomed and DualStack Guidelines   November 2016

   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
   a 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 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 ICE
   connectivity check is successful then other candidate pairs with the

Martinsen, et al.         Expires May 18, 2017                  [Page 6]
Internet-Draft   ICE Multihomed and DualStack Guidelines   November 2016

   same foundation will be unfrozen ([I-D.ietf-ice-rfc5245bis] section  Computing States).

   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 check lists.

   The check list is made up of candidate pairs.  A candidate pair is
   two candidates paired up and given a candidate pair priority as
   described in [I-D.ietf-ice-rfc5245bis] section  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 the candidate priority provided by the controlled agent.  This
   ensures that the local and remote check lists 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 check
   list the most due to (2^32*MIN(G,D)) in the formula.

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

   Dependant of the nomination method in use the procedures described in
   this document might change what candidate pair ends up as the active

   A test implementation of an example algorithm is available at

6.  IANA Considerations


7.  Implementation Status

   [Note to RFC Editor: Please remove this section and reference to
   [RFC6982] prior to publication.]

Martinsen, et al.         Expires May 18, 2017                  [Page 7]
Internet-Draft   ICE Multihomed and DualStack Guidelines   November 2016

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC6982].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may

   According to [RFC6982], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

7.1.  ICE-Dual Stack Fairness Test code

   Organization:   Cisco

   Description:   Open-Source ICE, TURN and STUN implementation.


   Level of maturity:   Code is stable.

   Coverage:   Follows the recommendations in this document

   Licensing:   BSD

   Implementation experience:   Straightforward as there are no
      compatibility issues.

   Contact:   Paal-Erik Martinsen

7.2.  ICE-Dual Stack Fairness Test code

   Organization:   Others

   Description:   Major ICE implementations, browser based and stand-
      alone ICE, TURN and STUN implementations.

   Implementation:   Product specific.

Martinsen, et al.         Expires May 18, 2017                  [Page 8]
Internet-Draft   ICE Multihomed and DualStack Guidelines   November 2016

   Level of maturity:   Code is stable and available in the wild.

   Coverage:   Implements the recommendations in this document.

   Licensing:   Some open source, some close source

   Implementation experience:   Already implemented in some of the
      implementations.  This document describes what needs to be done to
      achieve the desired fairness.

8.  Security Considerations

   The security considerations described in [I-D.ietf-ice-rfc5245bis]
   are still valid.  It changes recommended values and describes how an
   agent could choose those values in a safe way.  In section 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.

9.  Acknowledgements

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

10.  References

10.1.  Normative References

              Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", draft-ietf-ice-
              rfc5245bis-05 (work in progress), October 2016.

   [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,

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April
              2012, <>.

Martinsen, et al.         Expires May 18, 2017                  [Page 9]
Internet-Draft   ICE Multihomed and DualStack Guidelines   November 2016

   [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,

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982, DOI
              10.17487/RFC6982, July 2013,

10.2.  Informative References

              Martinsen, P., "ICE DualStack Test Implementation github
              repo", <

Authors' Addresses

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


   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103


   Prashanth Patil
   Cisco Systems, Inc.


Martinsen, et al.         Expires May 18, 2017                 [Page 10]