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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Consider an application using both video <strong>and</strong> audio components. RTP can mark<br />

the packets associated with the individual video <strong>and</strong> audio streams. This allows<br />

the streams to be synchronized at the receiving host. Figure 21-4 shows the<br />

operation of RTP in a multimedia transmission. Audio <strong>and</strong> video data are<br />

encapsulated in RTP packets prior to transmission from the sender to the<br />

receiver.<br />

Video<br />

(V)<br />

Audio<br />

Video<br />

RTP RTP<br />

(A)<br />

RTP Application Layer Framing (V)<br />

UDP<br />

<strong>IP</strong><br />

Figure 21-4 RTP operation<br />

<strong>IP</strong><br />

UDP<br />

RTP Stream<br />

Payload<br />

If the multimedia application does not use RTP services, the receiver might not<br />

be able to associate the corresponding audio <strong>and</strong> video packets. This can be<br />

attributed to the varying levels of network performance provided during a<br />

multimedia session. Congestion or other transient conditions within the<br />

environment can cause packets to be lost or reordered during transit. This can<br />

delay delivery of packets by varying amounts of time. This behavior causes<br />

quality problems with typical multimedia applications.<br />

The st<strong>and</strong>ards specify that RTP can be used with any appropriate network or<br />

transport protocol. In practice, multimedia applications typically use RTP in<br />

conjunction with UDP. This allows the application to use the multiplexing <strong>and</strong><br />

checksum services provided by UDP. RTP is often implemented to support<br />

multicast applications.<br />

RTP<br />

A V A V<br />

The RTP protocol alone does not include any mechanism to provide guaranteed<br />

delivery or other quality of service functions. The st<strong>and</strong>ard does not prevent<br />

out-of-sequence packet delivery, nor does it assume that the underlying network<br />

is reliable <strong>and</strong> delivers packets in-sequence. RTP also does not prevent the<br />

occurrence of network congestion. Designers of each application must determine<br />

if these levels of service are acceptable.<br />

UDP<br />

Chapter 21. Internet Protocol Television 757<br />

<strong>IP</strong><br />

Audio<br />

(A)

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