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uberj committed Mar 3, 2012
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Expand Up @@ -3,6 +3,8 @@ Vocabulary:
Over lay network
Connection - A node to node connection is defined as a TCP/TLS connection between two nodes on the network.
Circuit - A circuit exists on a connection. Multiple circuits can exists over a single connection.
Cell - One 512K data unit in the Tor network.
Socket Buffer -

Primary Research Question
-------------------------
Expand Down Expand Up @@ -142,6 +144,66 @@ ACM conference on Computer and communications security. CCS ’10. New York, NY,
339.
URL http://doi.acm.org/10.1145/1866307.1866345

Primary Research Question
-------------------------
How can priority be given to circuits that contain latency sensitive traffic?

Scope
-----
The majority of users experience delay when using Tor. Users that experience
too much delay will choose not to use Tor which consequently will make Tor
less effective tool. By prioritizing latency sensitive traffic like HTTP, the
authors tried to make interactive content appear to load faster while not hurting
the overall performance of bulk transfer protocols.

EWMA (Estimated Weighted Moving Average), sometimes referred to as the "rolling
average", is an algorithm to remove the influence of data collected in the
past and reflects recent trends in data set. TCP uses a similar weighted
average when calculating RTT (Round Trip Times) when calibrating it's sliding
window. EWMA was used to estimate how active a circuit was (cells per second).
Circuits that were on average less active were given higher priority over
circuits that had a higher cells-per-second count.

Three experiments were conducted to test the effectiveness of using the EWMA
method. The first experiment consisted of isolating three Onion Routers and
three clients and having the clients build circuits through the same
connection. Two of the clients preformed bulk transfer over their circuit and
the third client attempted to download a small 300KB file (similar to the size of
webpage). Timing data was collected with the stock Tor and then all three ORs
were enabled with EWMA and data was again collected. Results showed an
preformance boost of 2.17 seconds when downloading the small file.

The second experiment was set up exactly like the first and explored whether
enabling EWMA on some routers and not others had any affect on performance.
The results found that the effectivness of EWMA depends on severity of
congestion within the network.

The third experiment asked, where does a cell spends most of it's time being queued <-- TODO
when traveling through an Onion Router? This experiment was run locally on a
single router. Timing data was collected at three points: when a cell entered
a circuit queue, when the cell moved from the circuit queue to the socket
output buffer, and when the cell finially leaves the socket buffer. The result
proved that a cell spends most of it's time waiting to be flushed out of the
circuit queue into the socket buffer. When the EWMA algorithm was enabled the
observed time a cell spent in the circuit buffer was dramatically reduced from
653 milliseconds to 115 milliseconds.

Secondary Research Questions
----------------------------


Findings
--------
The negative effect felt by bulk transfer protocols when the EWMA algorithm
was enabled was neglagable. Also, overhead caused by the calculations required
to cary out EWMA were also neglagable. It is fair to assume that enabling an
EWMA algorithm to schedule Tor traffic improves the preformance of latency
sensative protocols on the Tor network.

Recommendations
---------------


+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Tschorsch, Florian and Scheuermann, Bjorn. “Tor is unfair: And what to do about it.” Local Computer
Networks (LCN), 2011 IEEE 36th Conference on. 2011, 432 –440.
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