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85 lines
3.5 KiB
Plaintext
Filename: 108-mtbf-based-stability.txt
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Title: Base "Stable" Flag on Mean Time Between Failures
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Version: $Revision$
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Last-Modified: $Date$
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Author: Nick Mathewson
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Created: 10-Mar-2007
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Status: Open
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Overview:
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This document proposes that we change how directory authorities set the
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stability flag from inspection of a router's declared Uptime to the
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authorities' perceived mean time between failure for the router.
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Motivation:
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Clients prefer nodes that the authorities call Stable. This flag is (as
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of 0.2.0.0-alpha-dev) set entirely based on the node's declared value for
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uptime. This creates an opportunity for malicious nodes to declare
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falsely high uptimes in order to get more traffic.
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Spec changes:
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Replace the current rule for setting the Stable flag with:
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"Stable" -- A router is 'Stable' if it is active and its observed Stability
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for the past month is at or above the median Stability for active routers.
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Routers are never called stable if they are running a version of Tor
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known to drop circuits stupidly. (0.1.1.10-alpha through 0.1.1.16-rc
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are stupid this way.)
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Stability shall be defined as the mean length of the runs observed by a
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given directory authority. A run begins when an authority decides
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that the server is Running, and ends when the authority decides that
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the server is not Running. In-progress runs are counted when
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measuring Stability.
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Issues:
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How do you define a clipped MTBF? If the current month begins with one
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day at the end of a one-year uptime, and then has 29 days of uptime, do we
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average one day and 29 days? Or do we average one year and 29 days? Or
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take 29 days on its own and discard the year?
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Surely somebody has done this kinds of thing before.
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Alternative:
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"A router's Stability shall be defined as the sum of $\alpha ^ d$ for every
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$d$ such that the router was not observed to be unavailable $d$ days ago."
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This allows a simpler implementation: every day, we multiply
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yesterday's Stability by alpha, and if the router was observed to be
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available every time we looked today, we add 1.
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Instead of "day", we could pick an arbitrary time unit. We should
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pick alpha to be high enough that long-term stability counts, but low
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enough that the distant past is eventually forgotten. Something
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between .8 and .95 seems right.
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(By requiring that routers be up for an entire day to get their
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stability increased, instead of counting fractions of a day, we
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capture the notion that stability is more like "probability of being
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staying up for the next hour" than it is like "probability of being
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up at some randomly chosen time over the next hour." The former
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notion of stability is far more relevant for long-lived circuits.)
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Limitations:
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Authorities can have false positives and false negatives when trying to
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tell whether a router is up or down. So long as these aren't terribly
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wrong, and so long as they aren't significantly biased, we should be able
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to use them to estimate stability pretty well.
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Probing approaches like the above could miss short incidents of
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downtime. If we use the router's declared uptime, we could detect
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these: but doing so would penalize routers who reported their uptime
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accurately.
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Implementation:
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For now, the easiest way to store this information at authorities
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would probably be in some kind of periodically flushed flat file.
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Later, we could move to Berkeley db or something if we really had to.
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