PrivCount proof of concept: implement consumed bandwidth counters
We want to add a copy of the current consumed bandwidth statistic to PrivCount:
- consumed bandwidth by relay flags https://metrics.torproject.org/bandwidth-flags.html
We can defer these statistics, because they are not as sensitive to manipulation by hostile clients or services:
- Bandwidth spent on answering directory requests
- Fraction of connections used uni-/bidirectionally
We will need counters for:
- read-history
- write-history
Split each counter into 4 (G, E, G and E, not G and not E) using these flags:
- Guard
- Exit and not BadExit We can do the G/E split on the Data Collector (DC). We can also do a smaller check aggregation for each group on the Tally Reporter (TR).
Check that relays are Running. We can do the Running check on the DC. We should also check Running on the TR, and exclude DCs that weren't Running at all during the period.
How often should we update the bandwidth counters?
If we update counters at the end of the day, we can match the current statistics exactly: "we only include bandwidth histories for a given day if a relay was listed as running in a consensus at least once on that day. We attribute bandwidth to guards and/or exits if a relay was a guard and/or exit at least in one consensus on a day." But we would be storing some sensitive data in memory for the whole day.
Instead, we could update the counters whenever we queue data, based on the flags in the current consensus we have. (The difference will likely be minimal.)
If updating the counters multiple times per second is too CPU-intensive, we can update them every few seconds. If that's too often, we can update them just before we delete an old consensus.
Sources: https://metrics.torproject.org/reproducible-metrics.html#traffic https://trac.torproject.org/projects/tor/wiki/org/meetings/2018MexicoCity/Notes/PrivCount