[tor-bugs] #26805 [Core Tor/Tor]: Work out why Tor networks with all-zero bandwidths are unstable

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#26805: Work out why Tor networks with all-zero bandwidths are unstable
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 Reporter:  teor          |          Owner:  (none)
     Type:  defect        |         Status:  new
 Priority:  Medium        |      Milestone:  Tor: unspecified
Component:  Core Tor/Tor  |        Version:
 Severity:  Normal        |     Resolution:
 Keywords:  tor-bwauth    |  Actual Points:
Parent ID:  #26803        |         Points:
 Reviewer:                |        Sponsor:
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Comment (by teor):

 Replying to [comment:2 neel]:
 > Does "bandwidth" mean consensus weight? Is so, I believe all-zero
 bandwidth networks are unstable because it's hard for a client to
 distinguish the network between different relays as no fraction of a
 relay's bandwidth can be made. Keep in mind that I'm not an expert on Tor
 design and this could be completely inaccurate.

 Yes, clients have trouble when the total is zero, because they try
 dividing by zero  (or similar).
 And clients also have trouble when a relay they need is weighted at zero.

 > I think the solution is that if there's a bandwidth of 0, we
 automatically make it 1 either on the client or on the consensus. I
 haven't looked fully into a solution so this may be an imperfect one.

 That's one solution.

 arma suggested that we fix all the little specific bugs that cause these
 issues, which is a good idea. We should also add unit tests with zero
 bandwidths so they don't happen again.

 I also suggest that we decide what a zero consensus weight means:
 * if it means "never use this relay", we could drop zero-bandwidth relays
 from the consensus
 * if it means "try not to use this relay", we could:
   * act as if zero-bandwidth relays have a very small bandwidth, like 1
   * avoid using the relay in our first pass, and only choose it if we
 can't find any other relays (like we do with the existing ExitNodes option
 when StrictNodes is 0)

 I think that zero consensus weights generally happen when relays are slow
 or unmeasured, so "try not to use this relay" seems like a good response.
 (And a better response for test networks, too.)

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26805#comment:3>
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