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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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RTP header format<br />

The header of an RTP packet has the following format, illustrated in Figure 21-5.<br />

0 8 16 31<br />

V P X CSRC<br />

Count<br />

M<br />

Figure 21-5 RTP header format<br />

The first 12 octets are required in every RTP packet. The list of CSRC identifiers<br />

is present only when inserted by a mixer. The individual fields have the following<br />

interpretation:<br />

V Indicates the RTP version.<br />

P Contains the padding bit. If this field is set, the packet<br />

contains a set of padding octets that are not part of the<br />

payload. This function is used by certain encryption<br />

algorithms.<br />

X Contains the extension bit. If this field is set, a header<br />

extension follows the fixed header.<br />

CSRC count This field contains the number of contributing source<br />

identifiers that follow the fixed header.<br />

M This field allows significant events to be marked in the<br />

packet stream (that is, frame boundaries).<br />

Payload type Specifies the format of the payload in the RTP packet. An<br />

RTP sender emits a single RTP payload type at any given<br />

time. Refer to “Payload type identification” on page 759 for<br />

further information regarding the contents of the payload<br />

type field.<br />

758 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong><br />

Payload<br />

Type<br />

Timestamp<br />

Synchronization Source (SSRC) Identifier<br />

Contributing Source (CSRC) Identifiers<br />

Sequence<br />

Number

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