Tor design proposals

Filename: 160-bandwidth-offset.txt
Title: Authorities vote for bandwidth offsets in consensus
Author: Roger Dingledine
Created: 4-May-2009
Status: Closed
Target: 0.2.1.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.