mirror of
https://gitlab.torproject.org/tpo/core/tor.git
synced 2024-11-11 05:33:47 +01:00
106 lines
4.3 KiB
Plaintext
106 lines
4.3 KiB
Plaintext
Filename: 160-bandwidth-offset.txt
|
|
Title: Authorities vote for bandwidth offsets in consensus
|
|
Author: Roger Dingledine
|
|
Created: 4-May-2009
|
|
Status: Open
|
|
Target: 0.2.2.x
|
|
|
|
1. Motivation
|
|
|
|
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.
|
|
|
|
2. Options
|
|
|
|
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 measurements: more accurate bandwidth values that
|
|
have actually been evaluated. In this way, authorities can vote on
|
|
the median measured value if sufficient measured votes exist for a router,
|
|
and otherwise fall back to the median value taken from the published router
|
|
descriptors.
|
|
|
|
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
|
|
find attractive.
|
|
|
|
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
|
|
authorities.
|
|
|
|
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.
|
|
|
|
4. Design
|
|
|
|
First, we need a new consensus method to support this new calculation.
|
|
|
|
Now v3 votes can have an additional value on the "w" line:
|
|
"w Bandwidth=X Measured=" INT.
|
|
|
|
Once we're using the new consensus method, the new way to compute the
|
|
Bandwidth weight is by checking if there are at least 3 "Measured"
|
|
votes. If so, the median of these is taken. Otherwise, the median
|
|
of the "Bandwidth=" values are taken, as described in Proposal 141.
|
|
|
|
Then the actual consensus looks just the same as it did before,
|
|
so clients never have to know that this additional calculation is
|
|
happening.
|
|
|
|
5. Implementation
|
|
|
|
The Measured values will be read from a file provided by the scanners
|
|
described in proposal 161. Files with a timestamp older than 3 days
|
|
will be ignored.
|
|
|
|
The file will be read in from dirserv_generate_networkstatus_vote_obj()
|
|
in a location specified by a new config option "V3MeasuredBandwidths".
|
|
A helper function will be called to populate new 'measured' and
|
|
'has_measured' fields of the routerstatus_t 'routerstatuses' list with
|
|
values read from this file.
|
|
|
|
An additional for_vote flag will be passed to
|
|
routerstatus_format_entry() from format_networkstatus_vote(), which will
|
|
indicate that the "Measured=" string should be appended to the "w Bandwith="
|
|
line with the measured value in the struct.
|
|
|
|
routerstatus_parse_entry_from_string() will be modified to parse the
|
|
"Measured=" lines into routerstatus_t struct fields.
|
|
|
|
Finally, networkstatus_compute_consensus() will set rs_out.bandwidth
|
|
to the median of the measured values if there are more than 3, otherwise
|
|
it will use the bandwidth value median as normal.
|
|
|
|
|
|
|