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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Sequence numbering<br />

Sequence numbers are used by the receiving RTP host to restore the original<br />

packet order. The receiver is able to detect packet loss using the information in<br />

this field.<br />

The sequence number increments by one for each RTP data packet sent. The<br />

initial value of the sequence number is r<strong>and</strong>omly determined. This makes<br />

hacking attacks on encryption more difficult. A r<strong>and</strong>om number is used even if<br />

the source device does not encrypt the RTP packet. The packets can flow<br />

through a translator that does provide encryption services.<br />

Time stamps<br />

Time stamps are used in RTP to synchronize packets from different sources. The<br />

time stamp represents the sampling (creation) time of the first octet in the RTP<br />

data packet. It is derived from a clock that increments monotonically <strong>and</strong> linearly.<br />

The resolution of the timer depends on the desired synchronization accuracy<br />

required by the application.<br />

It is possible that several consecutive RTP packets have the same time stamp.<br />

For example, this can occur when a single video frame is transmitted in multiple<br />

RTP packets. Because the payloads of these packets were logically generated at<br />

the same instant, the time stamps remain constant. The initial value of the time<br />

stamp is r<strong>and</strong>om. Figure 21-6 on page 762 shows an example of time stamp<br />

generation in a video application.<br />

Chapter 21. Internet Protocol Television 761

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