[tor-bugs] #19610 [Core Tor/Tor]: IPv6-only clients fetch microdescriptors from 15/25 IPv6 fallbacks

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#19610: IPv6-only clients fetch microdescriptors from 15/25 IPv6 fallbacks
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     Reporter:  teor          |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:
    Component:  Core Tor/Tor  |    Version:  Tor: 0.2.8.4-rc
     Severity:  Normal        |   Keywords:  ipv6, microdesc, fallbacks
Actual Points:                |  Parent ID:
       Points:  1             |   Reviewer:
      Sponsor:                |
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 When an IPv6-only client bootstraps using microdescriptors (#19608), it
 fetches the microdescriptor consensus from an IPv6 fallback, but the
 microdescriptor consensus has no IPv6 addresses.

 So it falls back to the fallback directories, fetching ~7500/500 = 15 sets
 of descriptors from 15 of the 25 IPv6 fallbacks.

 We should improve this behaviour somehow, to avoid overloading the
 fallbacks. One simple way of doing this is selecting 200 fallbacks for
 0.2.9 in #18828.

 It's worth noting that this extra load only happens on bootstrap, when
 there are no cached microdescriptors. If an IPv6-only client has any IPv6
 microdescriptors that match the current consensus, it will use those
 relays instead.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19610>
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