Proposal 160: Authorities vote for bandwidth offsets in consensus
arma at mit.edu
Mon May 4 23:12:10 UTC 2009
Title: Authorities vote for bandwidth offsets in consensus
Author: Roger Dingledine
As part of proposal 141, we moved the bandwidth value for each relay
into the consensus. Now clients can know how they should load balance
even before they've fetched the corresponding relay descriptors.
Putting the bandwidth in the consensus also lets the directory
authorities choose more accurate numbers to advertise, if we come up
with a better algorithm for deciding weightings.
Our original plan was to teach directory authorities how to measure
bandwidth themselves; then every authority would vote for the bandwidth
it prefers, and we'd take the median of votes as usual.
The problem comes when we have 7 authorities, and only a few of them
have smarter bandwidth allocation algorithms. So long as the majority
of them are voting for the number in the relay descriptor, the minority
that have better numbers will be ignored.
One fix would be to demand that every authority also run the
new bandwidth measurement algorithms: in that case, part of the
responsibility of being an authority operator is that you need to run
this code too. But in practice we can't really require all current
authority operators to do that; and if we want to expand the set of
authority operators even further, it will become even more impractical.
Also, bandwidth testing adds load to the network, so we don't really
want to require that the number of concurrent bandwidth tests match
the number of authorities we have.
The better fix is to allow certain authorities to specify that they are
voting on bandwidth "offsets": how much they think the weight should
be changed for the relay in question. We should put the offset vote in
the stanza for the relay in question, so a given authority can choose
which relays to express preferences for and which not.
3. Security implications
If only some authorities choose to vote on an offset, then a majority of
those voting authorities can arbitrarily change the bandwidth weighting
for the relay. At the extreme, if there's only one offset-voting
authority, then that authority can dictate which relays clients will
This problem isn't entirely new: we already have the worry wrt
the subset of authorities that vote for BadExit.
To make it not so bad, we should deploy at least three offset-voting
Also, authorities that know how to vote for offsets should vote for
an offset of zero for new nodes, rather than choosing not to vote on
any offset in those cases.
First, we need a new consensus method to support this new calculation.
Now v3 votes can have a new weight on the "w" line:
Once we're using the new consensus method, the new way to compute the
Bandwidth weight is by taking the old vote (explained in proposal 141:
median, then choose the lower number in the case of ties), and adding
or subtracting the median offset (using the offset closer to 0 in the
case of ties, and with a sum of 0 if the sum is negative).
Then the actual consensus looks just the same as it did before,
so clients never have to know that this additional calculation is
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