LASTor: A Low-Latency AS-Aware Tor Client
Tor
Stands for The Onion Router
 Goals:

 Anonymity
○ Each hop only knows previous and next hop
on a path
 Low latency communication
○ 90% of Tor traffic is interactive
Problems on Tor

High latency in practice
 Over 90% of connections in Tor network
incurs latencies over 5x than on the direct
Internet path.

Anonymity can break down
 Path selection to establish a circuit in Tor is
oblivious to Internet routing
 Autonomous System(AS) correlate traffic
cross entry and exit segments of a circuit
Potential Solution
Measure latencies and routes from each
relay to all end-hosts
 Choose a path with low latency and no
common Ases on both entry and exit
segment


However, none of them is deployed yet
LASTor
A low-latency AS-aware Tor client
 Improve poor latency for interactive
communications

 Modify path selection to reduce latency

Prevent anonymity from breaking down
 AS-aware path selection
Improve poor latency

Factors of causing latency
 Propagation delay
 Queuing delay
 Transmission delay
Weighted Shortest Path
Based on the expected latency
 Path with lower latency has high
probability to be chosen
 Clustering of relays

Effection on WSP
Shorter paths can greatly reduce latency
 Path can not be derministic

Tunable Path Selection


User could balance anonymity and
latency
Modified weight w to w(1-α) where 0 ≤α≤ 1
AS-aware Path Selection

Goal
 Detect common Ases on entry and exit
segments

Manner
 AS set prediction algorithm
AS Set Prediction Algorithm
Modified version of Dijkstra’s algorithm
 Predict ASes on all paths compliant with
routing policies
 Return a set of Ases on policy-compliant
routes

Latency Comparison
Conclusion
LASTor client reduce latency and protect
anonymity of Tor
 LASTor can deliver a 25% reduction in
median path latency
