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87 lines
3.4 KiB
Plaintext
87 lines
3.4 KiB
Plaintext
Filename: 146-long-term-stability.txt
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Title: Add new flag to reflect long-term stability
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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: 19-Jun-2008
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Status: Open
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Target: 0.2.1.x
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Overview
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This document proposes a new flag to indicate that a router has
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existed at the same address for a long time, describes how to
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implement it, and explains what it's good for.
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Motivation
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Tor has had three notions of "stability" for servers. Older
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directory protocols based a server's stability on its
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(self-reported) uptime: a server that had been running for a day was
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more stable than a server that had been running for five minutes,
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regardless of their past history. Current directory protocols track
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weighted mean time between failure (WMTBF) and weighted fractional
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uptime (WFU). WFU is computed as the fraction of time for which the
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server is running, with measurements weighted to exponentially
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decay such that old days count less. WMTBF is computed as the
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average length of intervals for which the server runs between
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downtime, with old intervals weighted to count less.
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WMTBF is useful in answering the question: "If a server is running
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now, how long is it likely to stay running?" This makes it a good
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choice for picking servers for streams that need to be long-lived.
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WFU is useful in answering the question: "If I try connecting to
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this server at an arbitrary time, is it likely to be running?" This
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makes it an important factor for picking guard nodes, since we want
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guard nodes to be usually-up.
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There are other questions that clients want to answer, however, for
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which the current flags aren't very useful. The one that this
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proposal addresses is,
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"If I found this server in an old consensus, is it likely to
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still be running at the same address?"
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This one is useful when we're trying to find directory mirrors in a
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fallback-consensus file. This property is equivalent to,
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"If I find this server in a current consensus, how long is it
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likely to exist on the network?"
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This one is useful if we're trying to pick introduction points or
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something and care more about churn rate than about whether every IP
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will be up all the time.
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Implementation:
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I propose we add a new flag, called "Longterm." Authorities should
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set this flag for routers if their Longevity is in the upper
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quartile of all routers. A router's Longevity is computed as the
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total amount of days in the last year or so[*] for which the router has
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been Running at least once at its current IP:orport pair.
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Clients should use directory servers from a fallback-consensus only
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if they have the Longterm flag set.
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Authority ops should be able to mark particular routers as not
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Longterm, regardless of history. (For instance, it makes sense to
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remove the Longterm flag from a router whose op says that it will
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need to shutdown in a month.)
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[*] This is deliberately vague, to permit efficient implementations.
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Compatibility and migration issues:
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The voting protocol already acts gracefully when new flags are
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added, so no change to the voting protocol is needed.
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Tor won't have collected this data, however. It might be desirable
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to bootstrap it from historical consensuses. Alternatively, we can
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just let the algorithm run for a month or two.
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Issues and future possibilities:
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Longterm is a really awkward name.
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