MMUSIC                                                      P. Martinsen
Internet-Draft                                                  T. Reddy
Intended status: Standards Track                                P. Patil
Expires: December 24, 2015                                         Cisco
                                                           June 22, 2015

            ICE Multihomed and IPv4/IPv6 Dual Stack Fairness


   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

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   This Internet-Draft will expire on December 24, 2015.

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   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.  Improving ICE Multihomed Fairness . . . . . . . . . . . . . .   3
   4.  Improving ICE Dual Stack Fairness . . . . . . . . . . . . . .   4
   5.  Compatibility . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   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
   describes how to deprioritize interfaces known by the application to
   provide unreliable connectivity.

   There is a also a need to introduce more fairness when handling of
   connectivity checks for different IP address families in dual-stack
   IPv4/IPv6 ICE scenarios.  Section 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.

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   The cost is a small penalty to the address type that otherwise would
   have been prioritized.

   This document describes how to fairly order the candidates in
   multihomed and dual-stack environments, thus affecting the sending
   order of the connectivity checks.  Ultimately it is 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] are still in effect.  The
   introduced fairness might be better, but not worse than what exists

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   This document uses terminology defined in [RFC5245].

3.  Improving ICE Multihomed Fairness

   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.  If the agent have access to
   information about the physical network it is connected to (Like SSID
   in a WiFi Network) this can be used as information regarding how that
   network interface should be prioritized at this point in time.

   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
   wired, wireless, cellular, virtual, tunneled and so on.

   Candidates from a interface known to the application to provide
   unreliable connectivity SHOULD get a low candidate priority.  This
   ensures they appear near the end of the candidate list, and would be
   the last to be tested during the connectivity check phase.  This
   allows candidate pairs more likely to succeed to be tested first.

   If the application is unable to get any interface information
   regarding type or unable to store any relevant metrics, it SHOULD

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   treat all interfaces as if they have reliable connectivity.  This
   ensures all interfaces gets their fair chance to perform their
   connectivity checks.

4.  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

   Implementations SHOULD 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.  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] are
   to excessive for ICE usage.

5.  Compatibility

   ICE [RFC5245] section 4.1.2 states that the formula in section 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 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 assign lower

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   priorities to IP addresses derived from unreliable interfaces using
   the local preference value.

   It is worth mentioning that [RFC5245] section 4.1.2 say that; "if
   there are multiple candidates for a particular component for a
   particular media 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 a unreliable interface SHOULD be
   ordered at the end of the checklist.  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.

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                     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

   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

        pair priority = 2^32*MIN(G,D) + 2*MAX(G,D) + (G>D?1:0)

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   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

   A test implementation with an example algorithm is available

6.  IANA Considerations


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

8.  Acknowledgements

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

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

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   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April

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

   [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, September 2012.

9.2.  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.


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