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Dual Tone Multi Frequency and Key Press Markup Language

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

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  1. 12/1/2008 Dual Tone Multi FrequencyandKey Press Markup Language VoIP Security (COMS W4995) Prof. Henning Schulzrinne Aniruddha Niranjan Computer Science Department

  2. Overview KPML • Introduction • Operation • Key Concepts • DRegex • Security DTMF • Introduction • Architecture • Encoding • Transmission • Decoding • Security 2

  3. 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

  4. Architecture • Three modules • Generation at keypad • Transmission using RTP • Decoding Generation Transmission Decoding 4

  5. Keypad Frequencies Set of high frequencies Set of low frequencies Button click generates a combination 8 => S(852, 1336) 5

  6. 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

  7. 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

  8. Decoding • Firstly checks are performed based on • Frequency groups • Inter-digit interval • Signal strength • Goertzel algorithm for decoding • Uses Discrete Fourier Transforms 8

  9. Security • Telephone event payload highly compressed • Very high sensitivity to even small changes in bit values • Secure Real Time Protocol must be implemented 9

  10. 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

  11. 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

  12. Key Concepts Subscription Duration Timers Inter-Digit Extra Non-persistent Persistent Critical Single NOTIFY Continuous NOTIFY 12

  13. Key Concepts • Pattern matches • Digit suppression • User input buffer behavior 13

  14. DRegex • Digit Regular expression – mapping of POSIX extended regular expressions • POSIX not used directly Mapping used 14

  15. DRegex A few example expressions 15

  16. 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

  17. 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

  18. 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

  19. 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

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