The Real-time Transport Protocol (RTP) provides end-to-end delivery services for real-time data like audio and video. It identifies payload types, numbers sequences, timestamps packets, and monitors delivery. RTP is designed to be lightweight, flexible, scalable, and works with various transport protocols like UDP. It is used with the RTP Control Protocol (RTCP) for quality of service feedback.

1. Real-time Transport Protocol
(RTP)
Emil Diaz
VoIP Security COMS 4995 – 03
Dept. of Computer Science
Henning Schulzrinne
Columbia University
Fall 2008

2. Overview
 Protocol goals & background
 Real-time Transport Protocol (RTP)
 RTP Control Protocol (RTCP)
 Use Scenarios
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3. Real-Time Transport Protocol
 End-to-end delivery services for applications
transmitting real-time data, such as audio and video
 Payload type identification
 Sequence numbering
 Time stamping
 Delivery monitoring
 Goals
 lightweight
 flexible
 scalable
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4. Origin of Protocol
 Developed by the Audio-Video Transport Working
Group of the IETF:
 H. Schulzrinne (Columbia University)
 S. Casner (Packet Design)
 R. Frederick (Blue Coat Systems Inc.)
 V. Jacobson (Packet Design)
 First published as RFC 1889 in the year 1996
 Superseded by RFC 3550 in the year 2003
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5. Protocol Structure
 Application layer protocol
 Typically used on top of IP and UDP
 Applications that use RTP are:
 Less sensitive to packet loss
 Very sensitive to packet delays
 UDP provides key services:
 Multiplexing
 Checksum
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6. RTP Does NOT
 Define media data formats or encodings
 Need media specific profiles
 Handle connection setups or tear-downs
 Need other protocols like SIP or H.323
 Handle resource reservation
 Need other protocols like RSVP
 Guarantee timely delivery or Quality of Service
 However, it does provide necessary data to application to
order packets and adjust signal quality
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7. RTP Sample Packet
http://www.codeproject.com/KB/IP/Video_Voice_Conferencing.aspx
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8. RTCP Sample Packet
5 main types:
 Sender Report (SR)
 Receiver Report (RR)
 Source Description (SDES)
 Bye Message (BYE)
 Application-defined (APP)
http://developer.apple.com/documentation/QuickTime/QTSS/Concepts/chapter_2_section_13.html
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9. Use Scenarios
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10. Simple Multicast Audio Conference
1. Call connection established
2. Audio sampled at 20ms durations
3. Each data chunk is packaged with an RTP header
4. RTP packet is wrapped around UDP packet
5. Sent through network
6. Receiver receives and parses RTP header
7. Based on payload type, application reconstructs
audio stream in 20ms chunks
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11. Audio and Video Conference
 Audio and Video has separate RTP/RTCP sessions
 Receiver can choose media type to use
 Encodings can change independently
 Network resource allocations per media basis
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12. Mixers and Translators
 Accommodate participant network resources
 Mixer – Low Bandwidth
 Mixer – Combining media streams
 Translator – Forward RTP packets to private networks
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13. Quality & Data Distribution Feedback
 Packet loss, congestion, jitter, delivery times
 Directly useful for control of adaptive encodings
 Identify if problems are local or global
 Short-term and long-term statistical analysis
 Self-adjusting network
 Each participant eventually knows about the other members
 Source description dynamically identifies who is sending
 Active senders get more bandwidth
 Session bandwidth kept constant by adjusting transmission
rate based on the number of participants
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14. Source Description Information
 CNAME – Conical name (joe@example.com)
 NAME – Display Name (Joe Smith)
 EMAIL – Email (different than CNAME)
 PHONE – International phone number
 LOC – Location information (City, Building, Room?)
 TOOL – Application (VideoView 2.0)
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15. References
 RFC 3550 - http://tools.ietf.org/html/rfc1889
 RFC 3551 - http://tools.ietf.org/html/rfc3551
 RTP News - http://www.cs.columbia.edu/~hgs/rtp/
 Wikepedia:
 RTP - http://en.wikipedia.org/wiki/Real-time_Transport_Protocol
 RTCP - http://en.wikipedia.org/wiki/RTCP
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16. Summary
 Services:
 payload type identification
 sequence numbering
 time stamping
 delivery monitoring
 mixers & translators
 Goals :
 lightweight
 flexible
 protocol-neutral
 scalable
 separate control/data
 secure
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