CEG 4183

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CEG 4183
RTP
By, Saied Nourian Taranmeet Singh Kindra

• "This report was prepared for Professor L.
  Orozco-Barbosa in partial
• fulfillment of the requirements for the course
  ELG/CEG 4183"

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Table of Content

• RTP Overview
• Bit deeper into RTP
• Designed for multicasting
• RTP Header
• RTP Packet Size
• RTP Header Compression
• What is really the difference
• Audio/Video Transmission RTP vs. TCP
• RTP Use Scenario
• Audio and Video Conference

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Overview

• What is RTP?

• Internet-standard protocol for the transport of
  real-time data
• RFC 1889 and an ITU standard H.225.0
• Difference between RTP and TCP?
• Does RTP ensure real time delivery of data?

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Bit deeper into RTP

• RTP consists of a data and a control part
• The data part of RTP
• including timing reconstruction,
• loss detection,
• security and content identification.
• The Control part- RTCP
• Source identification
• Support for gateways like audio and video bridges
  as well as multicast-to-unicast translators

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Designed for multicasting

• Work well with small session and large ones
• Scalability problem?
• RTCP packet transmissions from a host.
• increases linearly with the group size.
• group size estimate, L, computed locally at each
  host.
• The interval between packet transmissions is set
  to scale linearly with L.
• effect of giving each group member (independent
  of group size) a fair share of some fixed RTCP
  packet rate to the multicast group.
• Problems when applied to larger groups,
  particularly ones whose group membership is
  dynamic.

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RTP Header
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RTP Header cont

• Version (V) 2 bits
• Padding (P) 1 bit
• More octets
• Extension (X) 1 bit
• Fixed header is followed by header extension
• CSRC count (CC) 4 bits
• Number of CSRC (Contributing resources)
  identifiers
• Marker (M) 1 bit
• Marking significant events, ie. Frame boundaries

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RTP Header cont

• Payload type (PT) 7 bits
• Packet encoding format to be interpreted by
  Application
• Could be used for Mappings for Audio/Video
• ie. Payload type 32, MPEG2 video
• Receiver ignores packets with unfamiliar type
• Sequence number 16 bits
• Timestamp 32 bits
• Of first octet in data field

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RTP Header cont

• SSRC 32 bits
• Identifies synchronization source
• Chosen by random
• CSRC list 0 to 15 items, 32 bits each
• List of contributing sources to payload type
• Size specified in CC field

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RTP Packet Size

• IPUDPRTP headers 40 bytes
• At 64 Kbps PCM
• 20 ms 160 Bytes
• overall rate 80 Kbps

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RTP Packet Size cont

• At 8 Kbps (e.g. over modem lines)
• 20 ms 20 Bytes
• overall rate 10 Kbps

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RTP Header Compression

• Fixed fields removal
• parts of the headers remain unchanged between
  pkts
• Differential encoding
• some fields vary in a predictive, monotonic way
• Re-coding combinations of fields
• some fields may be combined and hash coded

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What is really the difference?

• RTP is a protocol framework that is deliberately
  not complete.
• Conventional protocols, additional functionality
  added by
• making the protocol more general
• Adding an option mechanism that would require
  parsing
• RTP is intended to be tailored through
  modifications and/or additions to the headers as
  needed.

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Audio/Video TransmissionRTP vs. TCP

• The RTP is preferred over TCP because of
  following advantages
• Dealing with unreliable connections
• Multicast capability
• Lack of congestion control (ie. no slow start)

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RTP Use Scenarios

• Simple Multicast Audio Conference
• Using the IP multicast services of the Internet
  for voice communications.
• One port is used for audio data, and other for
  control (RTCP)
• Each conference participant sends audio data in
  small chunks of, say, 20 ms duration
• Each chunk of audio data is preceded by an RTP
  header RTP header and data are in turn contained
  in a UDP packet. The RTP header indicates what
  type of audio encoding is contained in each
  packet.
• senders can change the encoding during a
  conference

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RTP Use Scenarios cont

• The Internet, occasionally loses and reorders
  packets and delays them by variable amounts of
  time
• RTP header
• Timing info
• Sequence number
• members of the working group join and leave
  during the conference
• useful to know who is participating
• how well they are receiving
• each instance of the audio application in the
  conference periodically multicasts a reception
  report plus the name of its user on the RTCP
  (control) port.

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Audio and Video Conference

• If both audio and video media are used in a
  conference,
• transmitted as separate RTP sessions
• RTCP packets are transmitted for each medium
  using two different UDP port pairs and/or
  multicast addresses.
• There is no direct coupling at the RTP level
  between the audio and video sessions
• allow some participants in the conference to
  receive only one medium if they choose
• Synchronized playback of a source's audio and
  video can be achieved using timing information
  carried in the RTCP packets for both sessions..

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Questions

• What is RPT and RTCP?
• The data part -- RTP
• including timing reconstruction,
• loss detection,
• security and content identification.
• The Control part -- RTCP
• Source identification
• Support for gateways like audio and video bridges
  as well as multicast-to-unicast translators

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Questions continued

• 2) Draw the layered architecture of RPT relative
  to IP and Application Layers.
• 3) Name and describe two of the RTP Header
  fields.
• Payload type Identifies the format of the RTP
  payload and determines its interpretation by the
  application. A profile specifies a default static
  mapping of payload type codes to payload formats.
  Additional payload type codes may be defined
  dynamically through non-RTP means.

Application
RTP
IP
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Questions continued

• Timestamp Reflects the sampling instant of the
  first octet in the RTP data packet. The sampling
  instnt must be derived from a clock that
  increments monotonically and lineraly in time to
  allow synchronization and mitter calculations.
• 4) Name and describe one of the RTP advantages
  over TCP in audio/video transferring.
• For real-time delivery of audio and video, TCP
  (reliable transport protocols) is inappropriate,
  retransmission of packet would cause unacceptable
  delay.
• Lack of congestion control (ie. no slow start)
• Low overhead

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Questions continued

• 5) What makes RTP suitable for multicasting?
• The RTP specification requires that end systems
  utilizing RTP listen to the multicast group, and
  count the number of distinct RTP end systems
  which have sent an RTCP packet. This results in a
  group size estimate, L, computed locally at each
  host.

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References

• Dr. Henning Schulzrinne, RTP, March 10, 2002
• http//www.cs.columbia.edu/hgs/rtp/
• Schulzrinne, Casner, Frederick, Jacobson, RTP A
  transport Protocol for Real-Time Applications,
  26/02/1999
• http//www.cs.columbia.edu/hgs/rtp/drafts/draft-i
  etf-avt-rtp-new-03.txt
• Multicasting, by Jonathan Angel, 02/01/99
• http//www.networkmagazine.com/article/NMG20000727
  S0026/2
• CENTRE DE PHYSIQUE DES PARTICULES DE MARSEILLE
  ,Transporting Audio-video over the Internet ,
  05/03/2002 http//marwww.in2p3.fr/Jinfo/Fluckiger_
  IN2P3_Informatics_Days_Part3.pdf
• RTP Header, by protocols.com, 05/03/2002
• http//www.protocols.com/voip/rtp_header.htm
• Professor Keith W. Ross ,RTP header, 05/03/2002
  
• http//www.eurecom.fr/ross/streaming/sld025.htm