blob: 4d583968fcd3c56729d3510b1d44772c12ab0611 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
|
Filename: 151-path-selection-improvements.txt
Title: Improving Tor Path Selection
Version:
Last-Modified:
Author: Fallon Chen, Mike Perry
Created: 5-Jul-2008
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.
Motivation
Tor's performance can be improved by excluding those circuits that
have long buildtimes (and by extension, high latency). For those Tor
users who require better performance and have lower requirements for
anonymity, this would be a very useful option to have.
Implementation
Learning the CircuitBuildTimeout
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.
Calculating the preferred CircuitBuildTimeout
Circuits that have longer buildtimes than some x% of the estimated
CDF of the Pareto distribution will be excluded. x will be tunable
as well.
Circuit timeouts
In the event of a timeout, backoff values should include the 100-x%
of expected CDF of timeouts. Also, in the event of network failure,
the observation mechanism should stop collecting timeout data.
Other notes
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
Impact on anonymity
|
