Proposal: remove down routers from consensus

[Peter Palfrader]

This is a pretty low hanging fruit and part of the job to make tor
bootstrap faster even if your bandwidth is really small.


Filename: xxx-remove-down-routers-from-consensus
Title: Remove routers that are not Running from consensus documents
Version: $Revision$
Last-Modified: $Date$
Author: Peter Palfrader
Created: 11-Jun-2008
Status: Open

1. Overview.

  Tor directory authorities hourly vote and agree on a consensus document
  which lists all the routers on the network together with some of their
  basic properties, like if a router is an exit node, whether it is
  stable or whether it is a version 2 directory mirror.

  One of the properties given with each router is the 'Running' flag.
  Clients do not use routers that are not listed as running.

  This proposal suggests that routers without the Running flag are not
  listed at all.

2. Current status

  At a typical bootstrap a client downloads a 140KB consensus, about
  10KB of certificates to verify that consensus, and about 1.6MB of
  server descriptors, about half of which it requires before it will
  start building circuits.

  Another proposal deals with how to get that huge 1.6MB fraction to
  effectively zero (by downloading only individual descriptors, on
  demand).  Should that get successfully implemented that will leave the
  140KB compressed consensus as a large fraction of what a client needs
  to get in order to work.

  About one third of the routers listed in a consensus are not running
  and will therefor never be used by clients who use this consensus.
  Not listing those routers will safe about 30% to 40% in size.

3. Proposed change

  Authority directory servers produce vote documents that include all
  the servers they know about, running or not, like they currently
  do.  In addition these vote documents also state that the authority
  supports a new consensus forming method (method number 4).

  If more than two thirds of votes that an authority has received claim
  they support method 4 then this new method will be used:  The
  consensus document is formed like before but a new last step removes
  all routers from the listing that are not marked as Running.