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					Real-Time Transport Protocol

What is RTP?
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Real-Time Transport Protocol (RTP) defines a standardized packet for delivering audio and video over the internet

What is RTP?
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Provides end-to-end delivery for data with real-time characteristics It was primarily designed for multiparty multimedia conferences The protocols themselves do not provide mechanisms to ensure timely delivery. They also do not give any Quality of Service (QoS) guarantees.

What is RTP?
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RTP does not provide any mechanism for timely delivery RTP relies on lower layer services (eg. UDP) to do so RTP can carry any data with real-time characteristics, such as interactive audio and video. Call setup and tear-down is usually performed by the SIP(Session Inititaiation Protocol).

What is RTP?
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RTP defined in RFC 3550 (formerly defined in RFC 1889)SERVICES include
• Payload-type identification - Indication of what kind of content is being carried • Sequence numbering - PDU sequence number • Time stamping - presentation time of the content being carried in the PDU • Delivery monitoring

What is RTP?
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Header Structure
+ Bits 0 32 64 96 96+(CC×32) 0-1 Ver. 2 P 3 X 4-7 CC 8 M 9-15 PT 16-31 Sequence Number

Timestamp SSRC identifier ... CSRC identifiers ... Additional header (optional), indicates length "AHL"

96+(CC×32) + (X×(AHL+16))

Data

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Ver. (2 bits) indicates the version of the protocol. P (one bit) is used to indicate if there are extra padding bytes at the end of the RTP packet. X (one bit) indicates if the extensions to the protocol are being used in the packet. CC (four bits) contains the number of CSRC identifiers that follow the fixed header. M (one bit) is used at the application level and is defined by a profile. If it's set, it means that the current data has some special relevance for the application.

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PT (7 bits) indicates the format of the payload and determines its interpretation by the application. SSRC indicates the synchronization source.

What is RTCP?
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RTP control protocol (RTCP) RTCP provides information about reception quality which the application can use to make local adjustments. It carries a persistent transport-level identifier for an RTP source called the canonical name or CNAME Each participant sends its control packets to all the others

What is RTCP?
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Types of RTCP Packets
• SR: Sender report • RR: Receiver report • SDES: Source description items, including CNAME • BYE: Indicates end of participation • APP: Application specific functions

What is RTP used for?
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Synchronization
• Synchronization Source (SSRC) • Sequence Number • Timestamp

Application Level Framing
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RTP is intended to be malleable RTP will often be integrated into the application processing rather than being implemented as a separate layer.

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Required Documents
A complete specification of RTP for a particular application will require
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Profile specification document Payload format specification document

Applications of RTP
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Simple Multicast Audio Conference

Applications of RTP
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Audio and Video Conference

What is RTP used for?
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Mixers in RTP
SSRC 1
Lower BW side

SSRC 2

MIXER

RECEIVER

SSRC 3

Higher BW side [3]

TCP
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Transmission Control Protocol Using TCP, applications can create connections to one another, over which they can exchange streams of data TCP guarantees reliable and in-order delivery of data from sender to receiver

TCP
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Reliable transmission is inappropriate for delay-sensitive data such as realtime audio and video TCP cannot support multicast. The TCP congestion control mechanisms decreases the congestion window when packet losses are detected ("slow start")


				
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posted:5/24/2009
language:English
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