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12/1/2008. Dual Tone Multi Frequency and Key Press Markup Language. VoIP Security (COMS W4995) Prof. Henning Schulzrinne Aniruddha Niranjan Computer Science Department. Overview. KPML Introduction Operation Key Concepts DRegex Security. DTMF Introduction Architecture Encoding

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dual tone multi frequency and key press markup language

12/1/2008

Dual Tone Multi FrequencyandKey Press Markup Language

VoIP Security (COMS W4995)

Prof. Henning Schulzrinne

Aniruddha Niranjan

Computer Science Department

overview
Overview

KPML

  • Introduction
  • Operation
  • Key Concepts
  • DRegex
  • Security

DTMF

  • Introduction
  • Architecture
  • Encoding
  • Transmission
  • Decoding
  • Security

2

slide3
DTMF
  • Method to instruct the telephone switching system of number to be dialed
  • To issue commands to switching system or related telephony systems
  • Numbers and commands issued from a keypad
  • Used mostly in interactive response systems

3

architecture
Architecture
  • Three modules
    • Generation at keypad
    • Transmission using RTP
    • Decoding

Generation

Transmission

Decoding

4

keypad frequencies
Keypad Frequencies

Set of high frequencies

Set of low frequencies

Button click generates a combination

8 => S(852, 1336)

5

transmission using rtp
Transmission using RTP
  • Payload format designated as “telephone-event”
  • Media type is “audio/telephone-event”
  • Carried as part of audio stream
  • Considered a very highly compressed audio codec

6

transmission using rtp7
Transmission using RTP
  • Timestamp and marker bit of RTP header used
  • Payload format

Number between 0 and 255 identifies specific telephony event

End of event

Reserved, set to 0

Power level of the tone

Duration of event or segment

7

decoding
Decoding
  • Firstly checks are performed based on
    • Frequency groups
    • Inter-digit interval
    • Signal strength
  • Goertzel algorithm for decoding
    • Uses Discrete Fourier Transforms

8

security
Security
  • Telephone event payload highly compressed
  • Very high sensitivity to even small changes in bit values
  • Secure Real Time Protocol must be implemented

9

slide10
KPML
  • Capturing DTMF tones that are supplemental or mid-call key presses
  • SIP event notification package
  • SUBSCRIBE and NOTIFY methods used
  • Goal is to optimize memory usage and processing time

10

sip based operation
SIP Based Operation

Caller

Application Server

Callee

(1) INVITE

(2) 200 OK

(3) ACK

(4) SUBSCRIBE

(5) 200 OK

(6) NOTIFY

(7) 200 OK

(8)

(9) NOTIFY (Digits)

(10) 200 OK

11

key concepts
Key Concepts

Subscription

Duration

Timers

Inter-Digit

Extra

Non-persistent

Persistent

Critical

Single NOTIFY

Continuous NOTIFY

12

key concepts13
Key Concepts
  • Pattern matches
  • Digit suppression
  • User input buffer behavior

13

dregex
DRegex
  • Digit Regular expression – mapping of POSIX extended regular expressions
  • POSIX not used directly

Mapping used

14

dregex15
DRegex

A few example expressions

15

dregex formal syntax
DRegex Formal Syntax

DRegex = 1*( DRegexPosition [ RepeatCount ] )

DRegexPosition = DRegexSymbol / DRegexSet

DRegexSymbol = [ "L" ] DRegexCharacter

DRegexSet = "[" 1*DRegexSetList "]"

DRegexSetList = DRegexCharacter [ "-" DRegexCharacter ]

DRegexCharacter = DIGIT / "A" / "B" / "C" / "D" / “R” / "*" / "#" /

"a" / "b" / "c" / "d" / “r”

RepeatCount = "." / "{" RepeatRange "}"

RepeatRange = Count / ( Count "," Count ) /

( Count "," ) / ( "," Count )

Count = 1*DIGIT

Augmented Backus-Naur Form

16

kpml security
KPML Security
  • Sensitive information may be typed in through keypad
  • Unauthenticated subscriptions can create problems too
  • Use of mechanisms such as TLS, sips, S/MIME recommended

17

conclusion
Conclusion
  • DTMF maps keypad characters to a combination of predefined frequencies
  • Transported as an RTP payload and decoded using Goertzel algorithm
  • KPML enables transport of mid-call key press information
  • A SIP-centric model that makes use of DRegex

18

references
References
  • http://nemesis.lonestar.org/reference/telecom/signaling/dtmf.html
  • http://www.corp.att.com/history/milestone_1963.html
  • http://ltl13.exp.sis.pitt.edu/WebSite/Glossary/Letters/D.htm
  • RFC 4733; H. Schulzrinne, M.Taylor
  • RFC 4730; E. Burger, M. Dolly

19