[tor-bugs] #27647 [Core Tor/Tor]: When randomly choosing IPv4 or IPv6, set IPv6 probability based on IPv6 weight

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#27647: When randomly choosing IPv4 or IPv6, set IPv6 probability based on IPv6
weight
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     Reporter:  teor          |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: unspecified
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-client ipv6
Actual Points:                |  Parent ID:  #17835
       Points:                |   Reviewer:
      Sponsor:                |
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 We can't make 50% of clients use IPv6 until most relays have IPv6.
 Otherwise, we would overload the IPv6 Guards. Right now, about 25% of
 Guard consensus weight has IPv6:
 https://metrics.torproject.org/advbw-ipv6.html

 When we are randomly choosing IPv4 or IPv6, we need to set the initial
 IPv6 probability based on the IPv6 Guard consensus weight. (Or the number
 of IPv6 bridges, if we're using bridges.)

 With IPv4-only, IPv6-only, and DualStack Entry nodes, the formulas are:
 {{{
 IPv4-capable-weight = IPv4-only + DualStack
 IPv6-capable-weight = IPv6-only + DualStack
 Total-weight = IPv4-only + IPv6-only + DualStack

 IPv4-capable-fraction = IPv4-capable-weight / Total-weight
 IPv6-capable-fraction = IPv6-capable-weight / Total-weight

 IPv4-probability = IPv4-capable-fraction / (IPv4-capable-fraction + IPv6
 -capable-fraction)
 IPv6-probability = IPv6-capable-fraction / (IPv4-capable-fraction + IPv6
 -capable-fraction)
 }}}

 We should update these probabilities whenever we get a new consensus, new
 bridge lines, or new bridge descriptors.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/27647>
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