RTP標頭前12個固定位元組機構圖：
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
| timestamp |
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| …. |
V – Version. Identifies the RTP version.
RTP版本号
P – Padding. When set, the packet contains one or more additional padding octets at the end which are not part of the payload.
是否填充，如果設定為允許填充的話，在包的末尾填充一個或多個位元組，這些填充的位元組不是有效負載的一部分。
X – Extension bit. When set, the fixed header is followed by exactly one header extension, with a defined format.
擴充位，如果設定為允許的話，固定頭結構後面（即包的12個位元組後面，有效負載的前面）緊跟着一個擴充頭結構，該結構是已定義的一種格式
CSRC count （CC） -Contains the number of CSRC identifiers that follow the fixed header.
資料源的個數（即源的個數），如果隻有一個源那麼此時的值為0。
M – Marker. The interpretation of the marker is defined by a profile. It is intended to allow significant events such as frame boundaries to be marked in the packet stream.
辨別，在文檔中是這樣定義的，它有意辨別重要的事物比如：在流媒體中辨別一幀資料的邊界（結束或開始）。
Payload type – Identifies the format of the RTP payload and determines its interpretation by the application. A profile specifies a default static mapping of payload type codes to payload formats. Additional payload type codes may be defined dynamically through non-RTP means.
有效負載，RTP資料的有效負載（不包括頭12個位元組），由具體的應用程式來确定負載的格式和意義。官方文檔裡有表格說明，該表格顯示了格式代碼和具體格式的對應關系，附加的格式代碼可能不在RTP協定裡定義。
Sequence number – Increments by one for each RTP data packet sent, and may be used by the receiver to detect packet loss and to restore packet sequence.
資料包序号，發送的RTP資料包序号，接收端可用它來檢查丢失的資料包和确定儲存資料包次序。
Timestamp – Reflects the sampling instant of the first octet in the RTP data packet. The sampling instant must be derived from a clock that increments monotonically and linearly in time to allow synchronization and jitter calculations.
時間戳， 紀錄了RTP資料包中第一個位元組的采樣時間，采樣時間必須源自一個時間增量且允許同步和計算。
SSRC – Synchronization source. This identifier is chosen randomly, with the intent that no two synchronization sources within the same RTP session will have the same SSRC identifier.
同步辨別， 是一個随機數，在同一個RTP會話中隻有一個同步辨別。
CSRC – Contributing source identifiers list. Identifies the contributing sources for the payload contained in this packet.