ANT : A System for Audio
Signaling Based NAT Traversal
Ashish Patro Yadi Ma Fatemah Panahi Jordan Walker
Suman Banerjee
Department of Computer Sciences
University of Wisconsin-Madison
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Outline
 Problem
 Overview of ANT
 Component Details
 Evaluation
 Summary
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Typical NAT scenario
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Typical NAT scenario
Problem : Setting up a direct connection between NATed
clients across different LANs.
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Scenario
Cellular Phones
Client B
Client A
Laptop
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Goal
 Create a direct end to end connection
between any 2 NATed clients.
 Use no third party intermediate node.
 Can be used as a building block by
applications.
 e.g. : Direct Transfer of Large Files, Chat
etc.
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Issues
 Identify internal and external IP/port
information.
 Common Solutions : Modify router
configuration (Port Mapping), use external
relay servers etc.
 Cumbersome for lay users to deal with
NAT traversal
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Solution??
 ANT….
 A system for Audio signaling based NAT
Traversal.
 Uses an ordinary phone connection as an
“out of band” mechanism for NAT
traversal.
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Outline
 Problem
 Overview of ANT
 Component Details
 Evaluation
 Summary
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ANT in action
Client B
Client A
Step 1 : Setup a phone connection.
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ANT in action
Client B
Client A
Step 1 : Setup a phone connection.
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ANT in action
Client B
Client A
Step 1 : Setup a phone connection.
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ANT in action
Internal IP:Port : 10.0.0.3:6000
NAT Router
Configuration Info :External IP:Mapped port : 128.32.12.3:54678
Client B
Client A
Step 2 : A obtains configuration information for NAT traversal
from its UPnP enabled router.
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ANT in action
NAT Router
10101001……
Client B
Client A
Step 3 : A encodes configuration data into audio signals.
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ANT in action
NAT Router
Client B
Client A
Step 4 : Sound is transferred from A’s laptop to A’s phone,
then from B’s phone to B’s laptop.
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ANT in action
NAT Router
Client B
Client A
Step 4 : Sound is transferred from A’s laptop to A’s phone,
then from B’s phone to B’s laptop.
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ANT in action
NAT Router
Client B
Client A
Step 4 : Sound is transferred from A’s laptop to A’s phone,
then from B’s phone to B’s laptop.
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ANT in action
NAT Router
Client B
Client A
10101001……
Step 5 : B decodes audio, corrects errors and gets
the data back.
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ANT in action
NAT Router
NAT Router
Client B
Client A
Step 6 : Setup a direct Peer to Peer (P2P) connection.
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Outline
 Problem
 Overview of ANT
 Component Details
 Evaluation
 Summary
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ANT Components
 Port Mapping at Client A.
 Encoding/decoding audio signals.
 Synchronizing encoder/decoder.
 Noise filtering.
 Error Correction.
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NAT Traversal
 NATed machines can only make outgoing
connections directly.
 NAT traversal requires internal-external
IP/Port Mapping.
 UPnP helps here.
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UPnP for NAT traversal
 Standard interface and widely available in
Internet Gateway Devices (IGD) for the
purpose of mapping ports to clients.
 Retrieves the external IP address device.
 Can add/remove port mappings.
 We use Java Library for UPnP to create a
port mapping for ANT.
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Data to Audio Conversion
 Modulate binary data to audio signals.
 Keying techniques



ASK : Amplitude Shift Keying
PSK : Pulse Shift Keying
FSK : Frequency Shift Keying
 We use a technique analogous to FSK.
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Audio Frequencies
 Human audible frequency range : 20Hz to
20,000Hz.
 Laptop soundcards work well between
800Hz to 9000Hz.
 Phone soundcards work well between
1000Hz to 4000Hz.
 We used the range 1200Hz – 3100Hz.
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Encoding Scheme
 We group consecutive bits into symbols.
 Use 1 unique frequency per symbol.
 Symbol Size = N bits.
 Unique frequencies required = 2N.
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Encoding Scheme
------- 1200Hz
------- 1230Hz
.
.
.
------- 8820Hz
------- 8850Hz
Symbol Size : 8 bits
e.g. : 1001011
256 frequencies
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Encoding Scheme
------- 1200Hz
------- 1230Hz
.
.
.
------- 8820Hz
------- 8850Hz
Symbol Size : 8 bits
e.g. : 1001011
256 frequencies
Requires a band of 7650 Hz for 256 unique frequencies
separated by 30 Hz.
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Encoding Scheme
------- 1200Hz
------- 1250Hz
.
.
.
------- 1900Hz
------- 1950Hz
Symbol Size : 4 bits
e.g. : 1001
16 frequencies
Easily fits within the frequency band available.
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Synchronization
Problem :
- Detect start and end of the audio transmission.
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Synchronization
Problem :
- Detect start and end of the audio transmission.
Solution :
- Use a preamble/postamble for each transmission.
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Synchronization
 Detect start of transmission.
 Similar to “preamble” in wireless networks.
 Also used to infer timing information.
 2 synchronization frequencies for start.
 1 synchronization frequency for end.
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Synchronization
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Synchronization
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Synchronization
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Synchronization
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Distinguish Consecutive Symbols
 Consecutive symbols can be same.
 Need to distinguish between them.
 One solution : Use a separation beep.
 Problem : Halves the data rate.
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Distinguish Consecutive Symbols
 Our Solution : Use two non-overlapping
frequency bands.
 Transmit consecutive symbols in
alternating bands.
 Much easier to decode symbols.
 ANT uses two separate bands of 16
frequencies (< 2000 Hz).
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Decoding Audio Symbols
 Synchronization to detect start of
transmission.
 Fast Fourier Transform (FFT) on captured
audio samples.
 Noise Filtering : Discard noise away from
relevant frequencies (F – 15Hz to F +
15Hz) and below certain amplitude.
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Error Correction
 Handle errors/erasures during
transmission/reception.
 ANT uses Forward Error Correction (FEC).
 FEC avoids a back-channel.
 ANT uses Reed-Solomon codes in GF(16).
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Putting the pieces together…
 Use UPnP to setup a port mapping at
Client A.
 Client A transmits mapping via the out-ofband audio channel to Client B.
 Client B decodes audio signals to obtain
mapping.
 Client B connects to Client A.
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Putting the pieces together…
NAT Router
NAT Router
Client B
Client A
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Other applications of ANT
 Password Transfer.
 Product Key exchange.
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Outline
 Problem
 Overview of ANT
 Component Details
 Evaluation
 Summary
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Evaluation
 Tested ANT with a variety of devices
(laptops, phones).
 Environments

No or Light Noise : Quiet Conference Room.

Medium Noise : Offices, homes.

Heavy Noise : Compute Lab (Background noise).
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Experiment 1
 Clients A and B are in quiet environments.
 Used different combinations of phones.
 Tested ANT 4-6 times for each
combination.
 Reported the overall success rate.
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Results : Experiment 1
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Results : Experiment 1
- ANT performed really well in the presence of little or no
noise.
- Failures occurred due to a bad speaker in the Sony phone.
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Experiment 2
 First client is a heavy noise environment.
 Second client in a light/medium noise
environment.
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Results : Experiment 2
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Results : Experiment 2
- Errors/erasures caused by noise cause a few failures.
- Success rate still above 66% in all cases.
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Experiment 3 (Speak Test)
 Human speakers present near laptop with
4 scenarios :
Scenario
Client A
Client B
Scenario 1 (S1)
Speaking with soft
voice
Quiet, no/light
background noise
Scenario 2 (S2)
Speaking with loud
voice
Quiet, no/light
background noise
Scenario 3 (S3)
Speaking with soft
voice
Quiet, heavy
background noise
Scenario 4 (S4)
Speaking with loud
voice
Quiet, heavy
background noise
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Results : Experiment 3
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Results : Experiment 3
- Human voice causes some problems for ANT.
- 2-3 attempts enough to establish a connection in most cases.
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Playing Background Music…
 Cause major problems to ANT.
 Music generates a vast range of
frequencies.
 Interferes with ANT by inserting spurious
symbols into audio transmissions.
 Causes problems for the decoder.
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Outline
 Problem
 Overview of ANT
 Component Details
 Evaluation
 Summary
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Summary
 UPnP is easily available and provides a
standard interface for port mapping.
 Short Audio transmission (few seconds).
 Out-of-band audio mechanism for NAT
traversal works well is most scenarios.
 Can also use audio mechanism for short,
robust and secure data transmissions.
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ANT Demo.
 A demo video demonstrating ANT is
available at :http://www.cs.wisc.edu/wings/projects/ant
 Link also available in the paper.
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Thank you.
Questions ?
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