Realtime Transport Protocol

1
Real-timeTransport Protocol
11 Oct 2004

• 960129 Changil Kim

2
Overview

• Real-time Transport Protocol
• RFC 1889
• Transporting various formats for audio/video
• Carry data that has real-time properties
• Sequence number, timestamp
• Unreliable, no error recovery, no flow/congestion
  control
• Neither timely delivery nor other
  quality-of-service guarantees
• Why is it called a real-time protocol?
• It does not ensure real-time delivery, but
• It provides functionality suited for carrying
  real-time content

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Protocol Stack

• RTP Protocol Stack
• Runs on the top of UDP
• RTP Header
• Encoding type, sequence number, timestamp
• Normally 12 bytes

Application
Media Chunk
Media Chunk
RTP
RTP Packet
RTP Packet
UDP
UDP Segment
UDP Segment
IP
IP Datagram
IP Datagram
Data Link
Frame
Frame
Physical
1-PDU
1-PDU
4
Protocol Stack

• Transport Layer or Application Layer
  ?
• It can be viewed as either

Application
Application
Application Layer
RTP
RTP
Transport Layer
UDP
UDP
IP
IP
Data Link
Data Link
Physical
Physical
5
Protocol Stack

• As a transport protocol
• It runs on end systems
• It provides demultiplexing

Application
RTP
Transport Layer
UDP
IP
Data Link
Physical
6
Protocol Stack

• As an application protocol
• It is mostly implemented within applications
• It passes RTP packets to the UDP socket

Application
Application Layer
RTP
Socket
UDP
IP
Data Link
Physical
7
Packet Header Fields

• Packet Header 12 bytes

Sequence number
Payload Type
Miscellaneous
Timestamp
Synchronization source identifier (SSRC)
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Packet Header Fields

• Packet Header 12 bytes
• Payload Type
• 7 bits
• Indicate the type of audio/video encoding

Sequence number
Payload Type
Miscellaneous
Timestamp
Synchronization source identifier (SSRC)
9
Packet Header Fields

• Packet Header 12 bytes
• Sequence number
• 16 bits
• Used to detect packet loss and to restore packet
  sequence
• Increase by one for each RTP packet transmitted

Sequence number
Payload Type
Miscellaneous
Timestamp
Synchronization source identifier (SSRC)
10
Packet Header Fields

• Packet Header 12 bytes
• Timestamp
• 32 bits
• Used to place the incoming packets in the correct
  timing order
• Remove jitter and provide synchronous playout
• Increase by the time covered by a packet

Sequence number
Payload Type
Miscellaneous
Timestamp
Synchronization source identifier (SSRC)
11
Packet Header Fields

• Packet Header 12 bytes
• Synchronization source identifier (SSRC)
• 32 bits
• Identify the source of the RTP stream
• Each stream in an RTP session can have a distinct
  SSRC

Sequence number
Payload Type
Miscellaneous
Timestamp
Synchronization source identifier (SSRC)
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Thank You