Realtime Transport Protocol RTP

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

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

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• Superseded by RFC 3550 in the year 2003

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

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

7
RTP Sample Packet
http//www.codeproject.com/KB/IP/Video_Voice_Confe
rencing.aspx
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
9
Use Scenarios
10
Simple Multicast Audio Conference

• Call connection established
• Audio sampled at 20ms durations
• Each data chunk is packaged with an RTP header
• RTP packet is wrapped around UDP packet
• Sent through network
• Receiver receives and parses RTP header
• Based on payload type, application reconstructs
  audio stream in 20ms chunks

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

12
Mixers and Translators

• Accommodate participant network resources
• Mixer Low Bandwidth
• Mixer Combining media streams
• Translator Forward RTP packets to private
  networks

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

14
Source Description Information

• CNAME Conical name (joe_at_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)

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_Trans
  port_Protocol
• RTCP - http//en.wikipedia.org/wiki/RTCP

16
Summary

• Services
• payload type identification
• sequence numbering
• time stamping
• delivery monitoring
• mixers translators
• Goals
• lightweight
• flexible
• protocol-neutral
• scalable
• separate control/data
• secure