RTMP Chunks Specification

NOTE: Here is a part of the documentation to understand how the Chunks' headers work.

To have a complete documentation, YOU HAVE TO READ rtmp_specification_1.0.pdf (from page 13)

This is the format of a chunk. Here, we store all except the chunk data:

+-------------+----------------+-------------------+--------------+
| Basic header|Chunk Msg Header|Extended Time Stamp|  Chunk Data  |
+-------------+----------------+-------------------+--------------+

This are the formats of the basic header:

 0 1 2 3 4 5 6 7      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|fmt|   cs id   |    |fmt|     0     |   cs id - 64  |    |fmt|     1     |        cs id - 64             |
+-+-+-+-+-+-+-+-+    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  (cs id < 64)            (64 <= cs id < 320)                           (320 <= cs id)

fmt store the format of the chunk message header. There are four different formats.

Type 0 (fmt=00):

This type MUST be used at the start of a chunk stream, and whenever the stream timestamp goes backward (e.g., because of a backward seek).

 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 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 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      timestamp                |                message length                 |message type id|                message stream id              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type 1 (fmt=01):

Streams with variable-sized messages (for example, many video formats) SHOULD use this format for the first chunk of each new message after the first.

 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 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                timestamp delta                |                message length                 |message type id|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type 2 (fmt=10):

Streams with constant-sized messages (for example, some audio and data formats) SHOULD use this format for the first chunk of each message after the first.

 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                timestamp delta                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type 3 (fmt=11):

Chunks of Type 3 have no header. Stream ID, message length and timestamp delta are not present; chunks of this type take values from the preceding chunk. When a single message is split into chunks, all chunks of a message except the first one, SHOULD use this type.

Extended Timestamp:

This field is transmitted only when the normal time stamp in the chunk message header is set to 0x00ffffff. If normal time stamp is set to any value less than 0x00ffffff, this field MUST NOT be present. This field MUST NOT be present if the timestamp field is not present. Type 3 chunks MUST NOT have this field. This field if transmitted is located immediately after the chunk message header and before the chunk data.