The section specifies the format of RTMP messages that are transferred between entities on a network using a lower level transport layer, such as RTMP Chunk Stream.
While RTMP was designed to work with the RTMP Chunk Stream, it can send the messages using any other transport protocol. RTMP Chunk Stream and RTMP together are suitable for a wide variety of audio-video applications, from one-to-one and one-to-many live broadcasting to video-on-demand services to interactive conferencing applications.
The server and the client send RTMP messages over the network to communicate with each other. The messages could include audio, video, data, or any other messages.
The RTMP message has two parts, a header and its payload.
The message header contains the following:
Message Type: One byte field to represent the message type. A range of type IDs (1-6) are reserved for protocol control messages.
Length: Three-byte field that represents the size of the payload in bytes. It is set in big-endian format.
Timestamp: Four-byte field that contains a timestamp of the message. The 4 bytes are packed in the big-endian order.
Message Stream Id: Three-byte field that identifies the stream of the message. These bytes are set in big-endian format.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type | Payload length |
| (1 byte) | (3 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp |
| (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream ID |
| (3 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message header
The other part of the message is the payload, which is the actual data contained in the message. For example, it could be some audio samples or compressed video data. The payload format and interpretation are beyond the scope of this document.
RTMP uses message type ID 4 for User Control messages. These messages contain information used by the RTMP streaming layer. Protocol messages with IDs 1, 2, 3, 5, and 6 are used by the RTMP Chunk Stream protocol (Section 5.4).
User Control messages SHOULD use message stream ID 0 (known as the control stream) and, when sent over RTMP Chunk Stream, be sent on chunk stream ID 2. User Control messages are effective at the point they are received in the stream; their timestamps are ignored.
The client or the server sends this message to notify the peer about the user control events. This message carries Event type and Event data.
+------------------------------+-------------------------
| Event Type (16 bits) | Event Data
+------------------------------+-------------------------
Payload for the ‘User Control’ protocol message
The first 2 bytes of the message data are used to identify the Event type. Event type is followed by Event data. The size of Event Data field is variable. However, in cases where the message has to pass through the RTMP Chunk Stream layer, the maximum chunk size (Section 5.4.1) SHOULD be large enough to allow these messages to fit in a single chunk.
Event Types are and their Event Data formats are enumerated in Section 7.1.7.
