![]() The M bit is intended to allow significant events such as frame boundaries to be marked in the packet stream. ■ Marker (M): 1 bit-The interpretation of the marker is defined by the RTP profile in use. CSRC is explained in much more detail later in this chapter in the section "Contributing Source Identifiers." ![]() ■ Contributing Source (CSRC) count (CC): 4 bits-The CSRC count contains the number of CSRC identifiers that follow the fixed header. ■ Extension (X): 1 bit-If the extension bit is set, exactly one header extension must follow the fixed header. Some encryption algorithms with fixed block sizes might need padding to carry several RTP packets in a lower-layer protocol data unit. The last octet of the padding contains a count of how many padding octets should be ignored, including itself. ■ Padding (P): 1 bit-If the padding bit is set, the packet contains one or more additional padding octets at the end, which are not part of the payload. The Version field is set to a value of 2 in most RTP implementations to denote the RTP profile defined in RFC 3551. ![]() ■ Version (V): 2 bits-This field identifies the version of RTP. The fields in this first octet of the RTP header are described as follows: The following sections describe the octets in the RTP header shown in Figure 4-2. Payload Header (Optional Depending on the Codec) Payload The header has the format illustrated in Figure 4-2. As stated in RFC 3550, the RTP header has a 12-octet mandatory part followed by an optional header extension.
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