Dual-stack ICE: Guidelines for Multihomed and IPv4/IPv6 Dual-Stack Interactive Connectivity Establishment (ICE)
How to order ICE connectivity checks so a broken IPv6, IPv4 or interface path does not stall call setup.
Martinsen, P., Reddy, T., and P. Patil, "Guidelines for Multihomed and IPv4/IPv6 Dual-Stack Interactive Connectivity Establishment (ICE)", BCP 217, RFC 8421, DOI 10.17487/RFC8421, July 2018, <https://www.rfc-editor.org/info/rfc8421>.
@misc{rfc8421,
series = {Request for Comments},
number = 8421,
howpublished = {RFC 8421},
publisher = {RFC Editor},
doi = {10.17487/RFC8421},
url = {https://www.rfc-editor.org/info/rfc8421},
author = {P. Martinsen and T. Reddy and P. Patil},
title = {{Guidelines for Multihomed and IPv4/IPv6 Dual-Stack Interactive Connectivity Establishment (ICE)}},
pagetotal = 9,
year = 2018,
month = jul,
} RFC text
Contents
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.
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 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.
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.
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]).
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.
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.
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
AI ELI5
On this page
§1Why Check Order Matters
In a multihomed host (one with several interfaces) or an IPv4/IPv6 dual-stack host, ICE (Interactive Connectivity Establishment, RFC 8445) benefits from a fair spread of its connectivity checks across the available interfaces and address types. A fair spread avoids long delays when one network path is broken or slow.
Following the address selection rules of RFC 6724 puts IPv6 ahead of IPv4 for the same candidate type. Checks for candidates of the same type (host, reflexive or relay) are therefore sent so that one address family is completely depleted before the other starts. If the preferred family's path is broken, setup is delayed in a way the user notices.
The RFC says this approach also supports dual-stack deployment, because users no longer have an incentive to disable IPv6. The cost is a small penalty for the address type that would otherwise have been first. Specific rules for tracking connectivity problems are out of scope.
If you always try all the red doors before any blue door, and the red doors are all locked, you waste a lot of time. This document says to mix red and blue doors.
§3Multihomed Recommendations
A multihomed ICE agent can send and receive checks on every interface and IP address, and one interface can have several addresses. It is useful to prioritize interfaces and addresses that the agent knows give stable connectivity.
The knowledge can come from simple metrics such as earlier connection success and failure rates. It can also come from a static model based on interface types such as wired, wireless, cellular, virtual and tunneled. Gathering such metrics may need permissions from the operating system.
Candidates from an interface known to be unreliable should get a low candidate priority. What counts as unreliable is implementation specific, because ICE is not limited to Voice over IP. If the application cannot learn interface types or store metrics, it should treat every interface as reliable so all of them get a fair chance. The RFC calls all of this purely an optimization.
If you know one bike path is bumpy, try it last. If you do not know anything about the paths, treat them all the same and give each a turn.
§4Dual-Stack Recommendations
Candidates should be ordered so that a run of candidates from one address family is broken up by candidates from the other. For example, an IPv4 candidate should always appear after a small run of IPv6 candidates. That makes ICE checks respond better when one address family's path is broken.
An agent may use any algorithm that gives a fair mix. It should not edit the checklist directly, because that can leave the local and remote checklists uncoordinated and make ICE slower or even fail. The recommended way is to set the local preference in the candidate priority formula of RFC 8445.
Implementations should still put some IPv6 candidates first, so IPv6 checks are likely to finish first. This follows the intent of Happy Eyeballs (RFC 8305), although ICE's own pacing of checks overrules the timing recommendations in that RFC.
A simple head-start formula is Hi = (N_4 + N_6) / N_4, where N_4 is the number of IPv4 addresses and N_6 the number of IPv6 addresses. With two IPv4 and six IPv6 addresses, an IPv4 address is inserted after four IPv6 addresses.
Imagine two lines of kids waiting for a slide. Let a few from one line go, then one from the other, so neither line waits forever.
§5Compatibility and Priority Values
The candidate priority formula is priority = (2^24)*(type preference) + (2^8)*(local preference) + (2^0)*(256 - component ID). Instead of a fixed local preference for IPv4 and IPv6, the value can be chosen dynamically so the two families interleave within a candidate type. It can also be lowered for addresses from unreliable interfaces. Because RFC 8445 requires the local preference to be unique among same-type candidates for a component and stream, each value must stay distinct.
The preference can also be changed so the families interleave regardless of candidate type. This helps when many IPv6 host candidates would otherwise block checks for IPv4 server reflexive candidates. Candidates from unreliable interfaces go at the end of the checklist and are not intermingled.
The component ID part of the formula is not changed, which keeps the RTP and RTCP components close together. What matters is the order of candidates with component ID 1, which are tested first. A success unfreezes other pairs with the same foundation.
Each agent may use a different algorithm. The pair priority is 2^32*MIN(G,D) + 2*MAX(G,D) + (G>D?1:0), where G comes from the controlling agent and D from the controlled agent. Sorting by this value gives an identical checklist on both sides, so the lists stay synchronized. The agent that promoted IPv4 more cautiously, with lower IPv4 priorities, influences the list most. Depending on the nomination method, the chosen pair may change.
Both players follow the same scoring rule for each pair, so even if they like different orders, they end up with the same list to work through.
| Entry | Type | Address | Component | 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 |
§7Security Considerations
The security considerations of RFC 8445 still apply. This document only changes recommended values and shows how an agent can choose them safely.
There is one new concern. When an agent prioritizes interfaces using what it knows about the network, that choice can leak information to the remote agent. The RFC has no IANA actions.
If you pick your order based on secrets about your roads, the other side might guess those secrets. Be careful what your choices give away.