RTP

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RTP

• Dr. Abhay Karandikar
• Department of Electrical Engineering
• Indian Institute of Technology, Bombay.

Dr. Abhay Karandikar Department of Electrical
Engineering Indian Institute of Technology,
Bombay.
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RTP as Transport Layer
Application Layer
RTP
Transport Layer
UDP
IP
Data Link layer
Physical layer
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RTP as Application Layer
Application Layer
RTP
UDP Socket Interface
UDP
IP
Data Link layer
Physical layer
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RTP Packet Header
Payload Type 7 Bits
Sequence Number 16 Bits
Timestamp 32 bits
Synchronization Source Identifier 32 bits
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continued..

• Payload type indicates encoding used for audio
  and video
• Sender can change encoding on the fly
• RTP itself does not provide any mechanism to
  ensure timely delivery of packets
• Does not guarantee delivery of packets or
  prevent out-of-order delivery
• RTP encapsulation is only seen at the end systems

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Continued..

• Payload Type Field

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Continued.

• Sequence Number
• Increments by one for each RTP packet
• Used to detect packet losses
• Time Stamp
• Denotes the sampling instant of the first byte in
  the RTP data
• To remove packet jitter
• Derived from sampling clock at the sender
• SSRI
• Identifies the source of the RTP stream
• Distinct from IP address

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RTP Control Protocol ( RTCP )

• RTCP is used during multicast audio/video
  transmission
• RTCP packets are distributed to all the
  participants using IP multicast
• Distinguished from RTP through the use of
  distinct port numbers
• RTCP packets contain sender/receiver reports
• Number of packets sent
• Number of packets lost
• Inter-arrival time jitter
• Upto the application to make use of RTCP packets

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RTCP Packet Types

• Receiver Reception Packets
• SSRI of the RTP stream
• Fraction of packets lost
• Last sequence number received in the stream of
  RTP packets
• Inter-arrival jitter
• Sender Report Packets
• SSRI of the RTP stream
• Timestamp and wall clock time of the most
  recently generated RTP packet in the stream.
• Number of packets sent in the stream
• Number of bytes sent in the stream
• (Used to synchronize different media streams
  within a RTP session.)

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Continued...

• Source Description Packets
• Email address of the sender
• Senders Name
• SSRI of the associated RTP stream

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H.323 and RTP
GateKeeper
Telephone network
Gateway
Internet
H.323 End Hosts
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H.323 Specifications

• How endpoints negotiate common Audio/Video
  encoding
• Encapsulation of audio and video channels
• RTP
• How end points communicate with gate-keepers
• How H.323 endpoints communicate with PSTN phones

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H.323 Protocol Architecture
G.711 G.722 G.729
H.261
RAS H.225
Q.391
H.245
RTP
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H.323 Protocols

• RTP
• H.245
• For controlling media between H.323 end points
• For negotiating common audio/video compression
  standard
• Q.931
• For establishing and terminating calls
• RAS
• For allowing H.323 end points to communicate with
  gate-keeper

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H.323 Channels
H.323 End Points
Media channel1 RTP
Media channel2 RTP
Call Control Channel H.245
Call signal of Channel Q.931
Media control channel RTCP
Internet