[tor-bugs] #27336 [Core Tor/sbws]: Does sbws need a node cap?

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#27336: Does sbws need a node cap?
---------------------------+-------------------------------------
 Reporter:  teor           |          Owner:  (none)
     Type:  enhancement    |         Status:  new
 Priority:  Medium         |      Milestone:  sbws 1.0 (MVP must)
Component:  Core Tor/sbws  |        Version:
 Severity:  Normal         |     Resolution:
 Keywords:                 |  Actual Points:
Parent ID:  #27107         |         Points:
 Reviewer:                 |        Sponsor:
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Comment (by teor):

 Replying to [comment:1 teor]:
 > Since we're using observed bandwidths, we must limit the maximum node
 bandwidth.
 >
 > 5% is ok, but 1% might be better, because the largest relay is only
 0.5%.

 sbws needs to cap, so that relays can't gain too much bandwidth by
 changing their observed bandwidths.
 This is a security requirement. If torflow doesn't do it, that's a bug in
 torflow.

 Replying to [comment:3 juga]:
 > I think (but i'm not sure), that ``tot_net_bw`` [0] is not being
 applied.

 It looks like the NODE_CAP is applied unconditionally in torflow:
 https://gitweb.torproject.org/torflow.git/tree/NetworkScanners/BwAuthority/aggregate.py#n778

 But from the graph, there doesn't seem to be a cap on torflow's
 bandwidths.

 > Also, since ``new_bw`` it's used in the non-pid part as the final value
 to write in the file, i thought ``tot_net_bw`` it just the sum of all the
 final relays' bandwidth that are going to be written in the file.
 >
 > In previous experiments, i capped to 0.05 * sum final bandwidths, and
 the result makes sense, see the attchemnt.

 Great!

 > [0]
 https://gitweb.torproject.org/torflow.git/tree/NetworkScanners/BwAuthority/aggregate.py#n758

 The cap must be configurable, so that we can turn it off in test networks,
 or adjust it if it is a problem.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/27336#comment:5>
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