[tor-dev] Proposal: Expose raw bwauth votes

[Tom Ritter]

I'm not sure, but I think
https://trac.torproject.org/projects/tor/ticket/21377 needed a
proposal so I tried to write one up.

-tom
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Filename: xxx-expose-bwauth_votes.txt
Title: Have Directory Authorities expose raw bwauth vote documents
Author: Tom Ritter
Created: 11-December-2017
Status: Open

1. Introduction

Bandwidth Authorities (bwauths) perform scanning of the Tor Network
and calculate observed speeds for each relay. They produce a 'bwauth
vote file' that is given to a Directory Authority. The Directory
Authority uses the speed value from this file in its vote file
denoting its view of the speed of the relay.

After collecting all of the votes from other Authorities, a consensus
is calculated, and the consensus's view of a relay's speed is
determined by choosing the low-median value [or is it high-median?]
of all the authorities' values for each relay.

Only a single metric from the bwauth vote file is exposed by a 
Directory Authority's vote, however the original file contains
considerably more diagnostic information about how the bwauth arrives
at that measurement for that relay.

2. Motivation

The bwauth vote file contains more information that is exposed in the
overall vote file. This information is useful to debug anomalies in
relays' utilization and suspected bugs in the (decrepit) bwauth code.

Currently, all bwauths expose the raw vote file through various (non-
standard) means, and that file is downloaded (hourly) by a single person
(as long as his home internet connection and home server is working)
and archived (with a small amount of robustness.)  

It would be preferable to have this exposed in a standard manner.
Doing so would no longer require bwauths to run HTTP servers to expose
the file, no longer require them to take additional manual steps to
provide it, and would enable public consumption by any interested
parties.  We hope that Collector will begin archiving the files.

3. Specification

An authority SHOULD publish the bwauth vote used to calculate its
current vote. It should make the bwauth vote file available at the
same time as its normal vote file. It should make the file available
at
  http://<hostname>/tor/status-vote/next/bwauth.z

It MUST NOT attempt to send its bwauth vote file in a HTTP POST to
other authorities and it SHOULD NOT make bwauth vote files from other
authorities available.

4. Security Implications

The raw bwauth vote file does not [really: is not believed to] expose
any sensitive information.  All authorities currently make this
document public already, an example is at
  https://bwauth.ritter.vg/bwauth/bwscan.V3BandwidthsFile

5. Compatibility

Exposing the document presents no compatibility concerns.

The compatibility concern is with applications that want to consume
the document. The bwauth vote file has no specification, and has been
extended in ad-hoc ways. Applications that merely wish to archive the
document (e.g. Collector) won't have a problems. Applications that
want to parse it may encounter errors if a new (unexpected) field is
added, or assumptions are made about the text encoding or formatting
of the document.