ICE IPv4/IPv6 Dual Stack Fairness
draft-martinsen-mmusic-ice-dualstack-fairness-01

Abstract

This document provides guidelines on how to make Interactive Connectivity Establishment (ICE) conclude faster in IPv4/IPv6 dual-stack scenarios where broken paths exist. The provided guidelines are backwards compatible with the original ICE specification.

Status of This Memo

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

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This Internet-Draft will expire on July 9, 2015.

Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Notational Conventions
• 3. Improving ICE Dual Stack Fairness
• 4. Compatibility
• 5. Example Algorithm for Choosing the Local Preference
• 6. IANA Considerations
• 7. Security Considerations
• 8. Acknowledgements
• 9. Normative References
• Authors' Addresses

1. Introduction

There is a need to introduce more fairness in the handling of connectivity checks for different IP address families in dual-stack IPv4/IPv6 ICE scenarios. Section 4.1.2.1 of ICE [RFC5245] points to [RFC3484] for prioritizing among the different IP families. [RFC3484] is obsoleted by [RFC6724] but following the recommendations from the updated RFC 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 setup delays if the path for the prioritized address family is broken.

To avoid such user noticeable delays when either IPv6 or IPv4 path is broken or excessive slow, this specification encourages intermingling the different address families when connectivity checks are performed. Introducing IP address family fairness into ICE connectivity checks 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.

Applications should also take special care to prioritize network interfaces know to be unreliable. For example certain tunnel services might provide unreliable connectivity. This specification does not provide any guidelines on how to prioritize different network interfaces. This is best solved by the application itself and the best solution would vary from use-case to use-case.

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] are still in effect. The introduced fairness might be better, but not worse than what exists today.

2. Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

This document uses terminology defined in [RFC5245].

3. Improving ICE Dual Stack Fairness

Candidates SHOULD be prioritized such that a long 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 modify the formula for calculating the candidate priority value described in ICE [RFC5245] section 4.1.2.1.

Implementations are encouraged to prioritize IPv6 candidates by putting some of them first in the the intermingled checklist. This increases the chance of a IPv6 connectivity checks to complete first and be ready for nomination or usage.

4. Compatibility

ICE [RFC5245] section 4.1.2 states that the formula in section 4.1.2.1 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 [RFC5245] section 4.1.2.2 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 ends up intermingled within the same candidate type.

It is also possible to dynamically change the type preference 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.

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. 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 same foundation will be unfrozen ([RFC5245] section 5.7.4. 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 by candidate pairs. A candidate pair is two candidates paired up and given a candidate pair priority as described in [RFC5245] section 5.7.2. 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. The introduced fairness might be better, but not worse than what exists today

5. Example Algorithm for Choosing the Local Preference

The algorithm described in this section MAY be used by an implementation to introduce IPv4/IPv6 dual stack fairness. Implementations implementing their own algorithm must take care not to break any ICE compatibility. See Section Section 4 for details.

The value space for the local preference is from 0 to 65535 inclusive. This value space can be divided up in chunks for each IP address family.

An IPv6 and IPv4 start priority must be given. In this example IPv6 starts at 60000 and IPv4 at 59000. This leaves enough address space to further play with the values if different interface priorities needs to be added. This is especially useful if tunneled interfaces with bad connectivity are known to exist. The highest value should be given to the address family that should be prioritized.

       IPv6    IPv4 
       Start   Start 
65535  60k     59k    58k    57k    56k    55k                    0
+--------+------+------+------+------+------+---------------------+
|        | IPv6 | IPv4 | IPv6 | IPv4 | IPv6 |                     |
|        | (1)  |  (1) |  (2) |  (2) |  (3) |                     |
+--------+------+------+------+------+------+---------------------+
          <- N->
          

The local preference can be calculated by the given formula:

          local_preference = S - N*2*(Cn/Cmax)
          

S:

Address Type specific start value (IPv4 or IPv6 Start)

N:

Absolute value of IPv6_start-IPv4_start. This ensures a positive number even if IPv4 is the highest priority.

Cn:

Number of current candidates of a specific IP address type and candidate type (host, server reflexive or relay).

Cmax:

Number of allowed consecutive candidates of the same IP address type.

 (1)  HOST  IPv6 (1) Priority: 2129289471
 (2)  HOST  IPv6 (2) Priority: 2129289470
 (3)  HOST  IPv4 (1) Priority: 2129033471
 (4)  HOST  IPv4 (2) Priority: 2129033470
 (5)  HOST  IPv6 (1) Priority: 2128777471
 (6)  HOST  IPv6 (2) Priority: 2128777470
 (7)  HOST  IPv4 (1) Priority: 2128521471
 (8)  HOST  IPv4 (2) Priority: 2128521470
 (9)  HOST  IPv6 (1) Priority: 2128265471
 (10) HOST  IPv6 (2) Priority: 2128265470
 (11) SRFLX IPv6 (1) Priority: 1693081855
 (12) SRFLX IPv6 (2) Priority: 1693081854
 (13) SRFLX IPv4 (1) Priority: 1692825855
 (14) SRFLX IPv4 (2) Priority: 1692825854
 (15) RELAY IPv6 (1) Priority: 15360255
 (16) RELAY IPv6 (2) Priority: 15360254
 (17) RELAY IPv4 (1) Priority: 15104255
 (18) RELAY IPv4 (2) Priority: 15104254
          

Using the values N=abs(60000-59000) and Cmax = 2 yields the following sorted local candidate list:

6. IANA Considerations

None.

7. Security Considerations

STUN connectivity check using MAC computed during key exchanged in the signaling channel provides message integrity and data origin authentication as described in section 2.5 of [RFC5245] apply to this use.

8. Acknowledgements

Authors would like to thank Dan Wing, Ari Keranen, Bernard Aboba, Martin Thomson, Jonathan Lennox, Balint Menyhart and Simon Perreault for their comments and review.

9. Normative References

Authors' Addresses