clean up proposal 108

Overview:

   This document proposes that we change how directory authorities set the

   stability flag from inspection of routers declared Uptime to the

   stability flag from inspection of a router's declared Uptime to the

   authorities' perceived mean time between failure for the router.

Motivation:

   Clients prefer nodes that the authorities call Stable.  This flags are (as

   of 0.2.0.0-alpha-dev) set entirely based on the nodes' declared values for

   Clients prefer nodes that the authorities call Stable.  This flag is (as

   of 0.2.0.0-alpha-dev) set entirely based on the node's declared value for

   uptime.  This creates an opportunity for malicious nodes to declare

   falsely high uptimes in order to get more traffic.

Spec changes:

   Instead of setting the current rule for setting the Stable flag:

   Replace the current rule for setting the Stable flag with:

   "An authority should call a server Stable if its observed MTBF for

   the past month is at or above the median MTBF for Valid servers.

   "Stable" -- A router is 'Stable' if it is active and its observed MTBF

   for the past month is at or above the median MTBF for active routers.

   Routers are never called stable if they are running a version of Tor

   known to drop circuits stupidly. (0.1.1.10-alpha through 0.1.1.16-rc

   are stupid this way.)

   MTBF shall be defined as the mean length of the runs observed by a

   given directory authority.  A run begins when an authority decides

   that the server is Running, and ends when the authority decides that

   the server is not Running.  In-progress runs are counted when

   measuring MTBF."

   measuring MTBF.

Issues:

   take 29 days on its own and discard the year?

   Surely somebody has done this kinds of thing before.