Filename: xxx-automatic-node-promotion.txt
Title: Automatically promoting Tor clients to nodes
Author: Steven Murdoch
Created: 12-Mar-2010
Status: Draft
Target:

1. Overview

   This proposal describes how Tor clients could determine when they
   have sufficient bandwidth capacity and are sufficiently reliable to
   become either bridges or Tor relays. When they meet this
   criteria, they will automatically promote themselves, based on user
   preferences. The proposal also defines the new controller messages
   and options which will control this process.

   Note that for the moment, only transitions between client and
   bridge are being considered. Transitions to public relay will
   be considered at a future date, but will use the same
   infrastructure for measuring capacity and reliability.

2. Motivation and history

   Tor has a growing user-base and one of the major impediments to the
   quality of service offered is the lack of network capacity. This is
   particularly the case for bridges, because these are gradually
   being blocked, and thus no longer of use to people within some
   countries. By automatically promoting Tor clients to bridges, and
   perhaps also to full public relays, this proposal aims to solve
   these problems.
 
   Only Tor clients which are sufficiently useful should be promoted,
   and the process of determining usefulness should be performed
   without reporting the existence of the client to the central
   authorities. The criteria used for determining usefulness will be
   in terms of bandwidth capacity and uptime, but parameters should be
   specified in the directory consensus. State stored at the client
   should be in no more detail than necessary, to prevent sensitive
   information being recorded.

3. Design

3.x Opt-in state model

   Tor can be in one of five node-promotion states:

   - off (O): Currently a client, and will stay as such
   - auto (A): Currently a client, but will consider promotion
   - bridge (B): Currently a bridge, and will stay as such
   - auto-bridge (AB): Currently a bridge, but will consider promotion
   - relay (R): Currently a public relay, and will stay as such

   The state can be fully controlled from the configuration file or
   controller, but the normal state transitions are as follows:

   Any state -> off: User has opted out of node promotion
   Off -> any state: Only permitted with user consent

   Auto -> auto-bridge: Tor has detected that it is sufficiently
    reliable to be a *bridge*
   Auto -> bridge: Tor has detected that it is sufficiently reliable
    to be a *relay*, but the user has chosen to remain a *bridge*
   Auto -> relay: Tor has detected that it is sufficiently reliable
    to be *relay*, and will skip being a *bridge*
   Auto-bridge -> relay: Tor has detected that it is sufficiently
    reliable to be a *relay*

   Note that this model does not support automatic demotion. If this
   is desirable, there should be some memory as to whether the
   previous state was relay, bridge, or auto-bridge. Otherwise the
   user may be prompted to become a relay, although he has opted to
   only be a bridge.

3.x User interaction policy

   There are a variety of options in how to involve the user into the
   decision as to whether and when to perform node promotion. The
   choice also may be different when Tor is running from Vidalia (and
   thus can readily prompt the user for information), and standalone
   (where Tor can only log messages, which may or may not be read).

   The option requiring minimal user interaction is to automatically
   promote nodes according to reliability, and allow the user to opt
   out, by changing settings in the configuration file or Vidalia user
   interface.

   Alternatively, if a user interface is available, Tor could prompt
   the user when it detects that a transition is available, and allow
   the user to choose which of the available options to select. If
   Vidalia is not available, it still may be possible to solicit an
   email address on install, and contact the operator to ask whether
   a transition to bridge or relay is permitted.

   Finally, Tor could by default not make any transition, and the user
   would need to opt in by stating the maximum level (bridge or
   relay) to which the node may automatically promote itself.

3.x New options

3.x New controller message

4. Migration plan

   We should start by setting a high bandwidth and uptime requirement
   in the consensus, so as to avoid overloading the bridge authority
   with too many bridges. Once we are confident our systems can scale,
   the criteria can be gradually shifted down to gain more bridges.

5. Related proposals

6. Open questions:

   - What user interaction policy should we take?

   - When (if ever) should we turn a relay into an exit relay?

   - What should the rate limits be for auto-promoted bridges/relays?
     Should we prompt the user for this?

   - Perhaps the bridge authority should tell potential bridges
     whether to enable themselves, by taking into account whether
     their IP address is blocked

   - How do we explain the possible risks of running a bridge/relay
     * Use of bandwidth/congestion
     * Publication of IP address
     * Blocking from IRC (even for non-exit relays)

   - What feedback should we give to bridge relays, to encourage then
     e.g. number of recent users (what about reserve bridges)?