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Add guard node failure plans to proposal.

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svn:r15706
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Mike Perry 2008-07-06 23:36:33 +00:00
parent 0f8761f9fa
commit 272165e659
1 changed files with 47 additions and 14 deletions
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doc/spec/proposals/151-path-selection-improvements.txt
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@ -9,9 +9,9 @@ Status: Draft
Overview
The performance of paths selected can be improved by adjusting the
CircuitBuildTimeout and the number of guards. This proposal describes
a method of tracking buildtime statistics, and using those statistics
to adjust the CircuitBuildTimeout and the number of guards.
CircuitBuildTimeout and avoiding failing guard nodes. This proposal
describes a method of tracking buildtime statistics, and using those
statistics to adjust the CircuitBuildTimeout and the number of guards.
Motivation
@ -26,14 +26,17 @@ Implementation
Based on studies of build times, we found that the distribution of
circuit buildtimes appears to be a Pareto distribution. The number
of circuits to observe (ncircuits_to_observe) before changing the
CircuitBuildTimeout will be tunable. From our preliminary
measurements, it is likely that ncircuits_to_observe will be
somewhere on the order of 1000. The values can be represented
compactly in Tor in milliseconds as a circular array of 16 bit
integers. More compact long-term storage representations can be
implemented by simply storing a histogram with 50 millisecond
buckets when writing out the statistics to disk.
of circuits to observe (ncircuits_to_cutoff) before changing the
CircuitBuildTimeout will be tunable. From out measurements,
ncircuits_to_cuttoff appears to be on the order of 100.
In addition, the total number of circuits gathered
(ncircuits_to_observe) will also be tunable. It is likely that
ncircuits_to_observe will be somewhere on the order of 1000. The values
can be represented compactly in Tor in milliseconds as a circular array
of 16 bit integers. More compact long-term storage representations can
be implemented by simply storing a histogram with 50 millisecond buckets
when writing out the statistics to disk.
Calculating the preferred CircuitBuildTimeout
@ -47,13 +50,43 @@ Implementation
of expected CDF of timeouts. Also, in the event of network failure,
the observation mechanism should stop collecting timeout data.
Other notes
Dropping Failed Guards
In addition, we have noticed that some entry guards are much more
failure prone than others. In particular, the circuit failure rates for
the fastest entry guards was approximately 20-25%, where as slower
guards exhibit failure rates as high as 45-50%. In [1], it was
demonstrated that failing guard nodes can deliberately bias path
selection to improve their success at capturing traffic. For both these
reasons, failing guards should be avoided.
We propose increasing the number of entry guards to five, and gathering
circuit failure statistics on each entry guard. Any guards that exceed
the average failure rate of all guards by 10% after we have
gathered ncircuits_to_observe circuits will be replaced.
Issues
Impact on anonymity
Since this follows a Pareto distribution, large reductions on the
timeout can be achieved without cutting off a great number of the
total paths. However, hard statistics on which cutoff percentage
gives optimal performance have not yet been gathered.
Issues
Guard Turnover
We contend that the risk from failing guards biasing path selection
outweighs the risk of exposure to larger portions of the network
for the first hop. Furthermore, from our observations, it appears
that circuit failure is strongly correlated to node load. Allowing
clients to migrate away from failing guards should naturally
rebalance the network, and eventually clients should converge on
a stable set of reliable guards. It is also likely that once clients
begin to migrate away from failing guards, their load should go
down, causing their failure rates to drop as well.
[1] http://www.crhc.uiuc.edu/~nikita/papers/relmix-ccs07.pdf
Impact on anonymity