rtmp_specification_1.0

M. Thornburgh, Ed.
Adobe
December 21, 2012

Adobe’s Real Time Messaging Protocol

Abstract

This memo describes Adobe’s Real Time Messaging Protocol (RTMP), an
application-level protocol designed for multiplexing and packetizing
multimedia transport streams (such as audio, video, and interactive
content) over a suitable transport protocol (such as TCP).

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3

2. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 3

3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3

4. Byte Order, Alignment, and Time Format . . . . . . . . . . . . 5

5. RTMP Chunk Stream . . . . . . . . . . . . . . . . . . . . . . 5

5.1. Message Format . . . . . . . . . . . . . . . . . . . . . . 6

5.2. Handshake . . . . . . . . . . . . . . . . . . . . . . . . 7

5.2.1. Handshake Sequence . . . . . . . . . . . . . . . . . . 7

5.2.2. C0 and S0 Format . . . . . . . . . . . . . . . . . . . 7

5.2.3. C1 and S1 Format . . . . . . . . . . . . . . . . . . . 8

5.2.4. C2 and S2 Format . . . . . . . . . . . . . . . . . . . 8

5.2.5. Handshake Diagram . . . . . . . . . . . . . . . . . . 10

5.3. Chunking . . . . . . . . . . . . . . . . . . . . . . . . . 11

5.3.1. Chunk Format . . . . . . . . . . . . . . . . . . . . . 11

5.3.1.1. Chunk Basic Header . . . . . . . . . . . . . . . . 12

5.3.1.2. Chunk Message Header . . . . . . . . . . . . . . . 13

5.3.1.2.1. Type 0 . . . . . . . . . . . . . . . . . . . . 14

5.3.1.2.2. Type 1 . . . . . . . . . . . . . . . . . . . . 14

5.3.1.2.3. Type 2 . . . . . . . . . . . . . . . . . . . . 15

5.3.1.2.4. Type 3 . . . . . . . . . . . . . . . . . . . . 15

5.3.1.2.5. Common Header Fields . . . . . . . . . . . . . 15

5.3.1.3. Extended Timestamp . . . . . . . . . . . . . . . . 16

5.3.2. Examples . . . . . . . . . . . . . . . . . . . . . . . 16

5.3.2.1. Example 1 . . . . . . . . . . . . . . . . . . . . 16

5.3.2.2. Example 2 . . . . . . . . . . . . . . . . . . . . 17

5.4. Protocol Control Messages . . . . . . . . . . . . . . . . 18

5.4.1. Set Chunk Size (1) . . . . . . . . . . . . . . . . . . 19

5.4.2. Abort Message (2) . . . . . . . . . . . . . . . . . . 19

5.4.3. Acknowledgement (3) . . . . . . . . . . . . . . . . . 20

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5.4.4. Window Acknowledgement Size (5) . . . . . . . . . . . 20

5.4.5. Set Peer Bandwidth (6) . . . . . . . . . . . . . . . . 21

6. RTMP Message Formats . . . . . . . . . . . . . . . . . . . . . 21

6.1. RTMP Message Format . . . . . . . . . . . . . . . . . . . 22

6.1.1. Message Header . . . . . . . . . . . . . . . . . . . . 22

6.1.2. Message Payload . . . . . . . . . . . . . . . . . . . 22

6.2. User Control Messages (4) . . . . . . . . . . . . . . . . 23

7. RTMP Command Messages . . . . . . . . . . . . . . . . . . . . 23

7.1. Types of Messages . . . . . . . . . . . . . . . . . . . . 24

7.1.1. Command Message (20, 17) . . . . . . . . . . . . . . . 24

7.1.2. Data Message (18, 15) . . . . . . . . . . . . . . . . 24

7.1.3. Shared Object Message (19, 16) . . . . . . . . . . . . 24

7.1.4. Audio Message (8) . . . . . . . . . . . . . . . . . . 26

7.1.5. Video Message (9) . . . . . . . . . . . . . . . . . . 26

7.1.6. Aggregate Message (22) . . . . . . . . . . . . . . . . 26

7.1.7. User Control Message Events . . . . . . . . . . . . . 27

7.2. Types of Commands . . . . . . . . . . . . . . . . . . . . 28

7.2.1. NetConnection Commands . . . . . . . . . . . . . . . . 29

7.2.1.1. connect . . . . . . . . . . . . . . . . . . . . . 29

7.2.1.2. Call . . . . . . . . . . . . . . . . . . . . . . . 35

7.2.1.3. createStream . . . . . . . . . . . . . . . . . . . 36

7.2.2. NetStream Commands . . . . . . . . . . . . . . . . . . 37

7.2.2.1. play . . . . . . . . . . . . . . . . . . . . . . . 38

7.2.2.2. play2 . . . . . . . . . . . . . . . . . . . . . . 42

7.2.2.3. deleteStream . . . . . . . . . . . . . . . . . . . 43

7.2.2.4. receiveAudio . . . . . . . . . . . . . . . . . . . 44

7.2.2.5. receiveVideo . . . . . . . . . . . . . . . . . . . 45

7.2.2.6. publish . . . . . . . . . . . . . . . . . . . . . 45

7.2.2.7. seek . . . . . . . . . . . . . . . . . . . . . . . 46

7.2.2.8. pause . . . . . . . . . . . . . . . . . . . . . . 47

7.3. Message Exchange Examples . . . . . . . . . . . . . . . . 48

7.3.1. Publish Recorded Video . . . . . . . . . . . . . . . . 48

7.3.2. Broadcast a Shared Object Message . . . . . . . . . . 50

7.3.3. Publish Metadata from Recorded Stream . . . . . . . . 50

8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 51
Authors’ Addresses . . . . . . . . . . . . . . . . . . . . . . . . 52

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1. Introduction
Adobe’s Real Time Messaging Protocol (RTMP) provides a bidirectional
message multiplex service over a reliable stream transport, such as
TCP [RFC0793], intended to carry parallel streams of video, audio,
and data messages, with associated timing information, between a pair
of communicating peers. Implementations typically assign different
priorities to different classes of messages, which can effect the
order in which messages are enqueued to the underlying stream
transport when transport capacity is constrained.

This memo describes the syntax and operation of the Real Time
Messaging Protocol.

1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this memo are to be interpreted as described in
[RFC2119].

2. Contributors
Rajesh Mallipeddi, formerly of Adobe Systems, was the original editor
of this specification, and provided most of its original text.

Mohit Srivastava of Adobe Systems contributed to the development of
this specification.

3. Definitions
Payload: The data contained in a packet, for example audio samples
or compressed video data. The payload format and interpretation
are beyond the scope of this document.

Packet: A data packet consists of fixed header and payload data.
Some underlying protocols may require an encapsulation of the
packet to be defined.

Port: The "abstraction that transport protocols use to distinguish
among multiple destinations within a given host computer. TCP/IP
protocols identify ports using small positive integers." The
transport selectors (TSEL) used by the OSI transport layer are
equivalent to ports.

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Transport address: The combination of a network address and port
that identifies a transport-level endpoint, for example an IP
address and a TCP port. Packets are transmitted from a source
transport address to a destination transport address.

Message stream: A logical channel of communication in which messages
flow.

Message stream ID: Each message has an ID associated with it to
identify the message stream in which it is flowing.

Chunk: A fragment of a message. The messages are broken into
smaller parts and interleaved before they are sent over the
network. The chunks ensure timestamp-ordered end-to-end delivery
of all messages, across multiple streams.

Chunk stream: A logical channel of communication that allows flow of
chunks in a particular direction. The chunk stream can travel
from the client to the server and reverse.

Chunk stream ID: Every chunk has an ID associated with it to
identify the chunk stream in which it is flowing.

Multiplexing: Process of making separate audio/video data into one
coherent audio/video stream, making it possible to transmit
several video and audio simultaneously.

DeMultiplexing: Reverse process of multiplexing, in which
interleaved audio and video data are assembled to form the
original audio and video data.

Remote Procedure Call (RPC): A request that allows a client or a
server to call a subroutine or procedure at the peer end.

Metadata: A description about the data. The metadata of a movie
includes the movie title, duration, date of creation, and so on.

Application Instance: The instance of the application at the server
with which the clients connect by sending the connect request.

Action Message Format (AMF): A compact binary format that is used to
serialize ActionScript object graphs. AMF has two versions: AMF 0
[AMF0] and AMF 3 [AMF3].

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4. Byte Order, Alignment, and Time Format
All integer fields are carried in network byte order, byte zero is
the first byte shown, and bit zero is the most significant bit in a
word or field. This byte order is commonly known as big-endian. The
transmission order is described in detail in Internet Protocol
[RFC0791]. Unless otherwise noted, numeric constants in this
document are in decimal (base 10).

Except as otherwise specified, all data in RTMP is byte-aligned; for
example, a 16-bit field may be at an odd byte offset. Where padding
is indicated, padding bytes SHOULD have the value zero.

Timestamps in RTMP are given as an integer number of milliseconds
relative to an unspecified epoch. Typically, each stream will start
with a timestamp of 0, but this is not required, as long as the two
endpoints agree on the epoch. Note that this means that any
synchronization across multiple streams (especially from separate
hosts) requires some additional mechanism outside of RTMP.

Because timestamps are 32 bits long, they roll over every 49 days, 17
hours, 2 minutes and 47.296 seconds. Because streams are allowed to
run continuously, potentially for years on end, an RTMP application
SHOULD use serial number arithmetic [RFC1982] when processing
timestamps, and SHOULD be capable of handling wraparound. For
example, an application assumes that all adjacent timestamps are
within 2^31 - 1 milliseconds of each other, so 10000 comes after
4000000000, and 3000000000 comes before 4000000000.

Timestamp deltas are also specified as an unsigned integer number of
milliseconds, relative to the previous timestamp. Timestamp deltas
may be either 24 or 32 bits long.

5. RTMP Chunk Stream
This section specifies the Real Time Messaging Protocol Chunk Stream
(RTMP Chunk Stream). It provides multiplexing and packetizing
services for a higher-level multimedia stream protocol.

While RTMP Chunk Stream was designed to work with the Real Time
Messaging Protocol (Section 6), it can handle any protocol that sends
a stream of messages. Each message contains timestamp and payload
type identification. RTMP Chunk Stream and RTMP together are
suitable for a wide variety of audio-video applications, from one-toone
and one-to-many live broadcasting to video-on-demand services to
interactive conferencing applications.

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When used with a reliable transport protocol such as TCP [RFC0793],
RTMP Chunk Stream provides guaranteed timestamp-ordered end-to-end
delivery of all messages, across multiple streams. RTMP Chunk Stream
does not provide any prioritization or similar forms of control, but
can be used by higher-level protocols to provide such prioritization.
For example, a live video server might choose to drop video messages
for a slow client to ensure that audio messages are received in a
timely fashion, based on either the time to send or the time to
acknowledge each message.

RTMP Chunk Stream includes its own in-band protocol control messages,
and also offers a mechanism for the higher-level protocol to embed
user control messages.

5.1. Message Format
The format of a message that can be split into chunks to support
multiplexing depends on a higher level protocol. The message format
SHOULD however contain the following fields which are necessary for
creating the chunks.

Timestamp: Timestamp of the message. This field can transport 4
bytes.

Length: Length of the message payload. If the message header cannot
be elided, it should be included in the length. This field
occupies 3 bytes in the chunk header.

Type Id: A range of type IDs are reserved for protocol control
messages. These messages which propagate information are handled
by both RTMP Chunk Stream protocol and the higher-level protocol.
All other type IDs are available for use by the higher-level
protocol, and treated as opaque values by RTMP Chunk Stream. In
fact, nothing in RTMP Chunk Stream requires these values to be
used as a type; all (non-protocol) messages could be of the same
type, or the application could use this field to distinguish
simultaneous tracks rather than types. This field occupies 1 byte
in the chunk header.

Message Stream ID: The message stream ID can be any arbitrary value.
Different message streams multiplexed onto the same chunk stream
are demultiplexed based on their message stream IDs. Beyond that,
as far as RTMP Chunk Stream is concerned, this is an opaque value.
This field occupies 4 bytes in the chunk header in little endian
format.

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5.2. Handshake
An RTMP connection begins with a handshake. The handshake is unlike
the rest of the protocol; it consists of three static-sized chunks
rather than consisting of variable-sized chunks with headers.

The client (the endpoint that has initiated the connection) and the
server each send the same three chunks. For exposition, these chunks
will be designated C0, C1, and C2 when sent by the client; S0, S1,
and S2 when sent by the server.

5.2.1. Handshake Sequence
The handshake begins with the client sending the C0 and C1 chunks.

The client MUST wait until S1 has been received before sending C2.
The client MUST wait until S2 has been received before sending any
other data.

The server MUST wait until C0 has been received before sending S0 and
S1, and MAY wait until after C1 as well. The server MUST wait until
C1 has been received before sending S2. The server MUST wait until
C2 has been received before sending any other data.

5.2.2. C0 and S0 Format

The C0 and S0 packets are a single octet, treated as a single 8-bit

integer field:

0 1 2 3 4 5 6 7

+-+-+-+-+-+-+-+-+

| version |

+-+-+-+-+-+-+-+-+

C0 and S0 bits

Following are the fields in the C0/S0 packets:

Version (8 bits): In C0, this field identifies the RTMP version
requested by the client. In S0, this field identifies the RTMP
version selected by the server. The version defined by this
specification is 3. Values 0-2 are deprecated values used by
earlier proprietary products; 4-31 are reserved for future
implementations; and 32-255 are not allowed (to allow
distinguishing RTMP from text-based protocols, which always start
with a printable character). A server that does not recognize the
client’s requested version SHOULD respond with 3. The client MAY
choose to degrade to version 3, or to abandon the handshake.

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5.2.3. C1 and S1 Format

The C1 and S1 packets are 1536 octets long, consisting of the
following fields:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| time (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| zero (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| random bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

C1 and S1 bits

Time (4 bytes): This field contains a timestamp, which SHOULD be
used as the epoch for all future chunks sent from this endpoint.
This may be 0, or some arbitrary value. To synchronize multiple
chunkstreams, the endpoint may wish to send the current value of
the other chunkstream’s timestamp.

Zero (4 bytes): This field MUST be all 0s.

Random data (1528 bytes): This field can contain any arbitrary
values. Since each endpoint has to distinguish between the
response to the handshake it has initiated and the handshake
initiated by its peer,this data SHOULD send something sufficiently
random. But there is no need for cryptographically-secure
randomness, or even dynamic values.

5.2.4. C2 and S2 Format

The C2 and S2 packets are 1536 octets long, and nearly an echo of S1
and C1 (respectively), consisting of the following fields:

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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| time (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| time2 (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| random echo |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

C2 and S2 bits

Time (4 bytes): This field MUST contain the timestamp sent by the
peer in S1 (for C2) or C1 (for S2).

Time2 (4 bytes): This field MUST contain the timestamp at which the
previous packet(s1 or c1) sent by the peer was read.

Random echo (1528 bytes): This field MUST contain the random data
field sent by the peer in S1 (for C2) or S2 (for C1). Either peer
can use the time and time2 fields together with the current
timestamp as a quick estimate of the bandwidth and/or latency of
the connection, but this is unlikely to be useful.

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5.2.5. Handshake Diagram
+-------------+ +-------------+
| Client | TCP/IP Network | Server |
+-------------+ | +-------------+
| | |
Uninitialized | Uninitialized
| C0 | |
|------------------->| C0 |
| |-------------------->|
| C1 | |
|------------------->| S0 |
| |<--------------------|
| | S1 |

Version sent |<--------------------|
| S0 | |
|<-------------------| |
| S1 | |
|<-------------------| Version sent
| | C1 |
| |-------------------->|
| C2 | |
|------------------->| S2 |
| |<--------------------|

Ack sent | Ack Sent
| S2 | |
|<-------------------| |
| | C2 |
| |-------------------->|

Handshake Done | Handshake Done
| | |

Pictorial Representation of Handshake

The following describes the states mentioned in the handshake
diagram:

Uninitialized: The protocol version is sent during this stage. Both
the client and server are uninitialized. The The client sends the
protocol version in packet C0. If the server supports the
version, it sends S0 and S1 in response. If not, the server
responds by taking the appropriate action. In RTMP, this action
is terminating the connection.

Version Sent: Both client and server are in the Version Sent state
after the Uninitialized state. The client is waiting for the
packet S1 and the server is waiting for the packet C1. On
receiving the awaited packets, the client sends the packet C2 and

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the server sends the packet S2. The state then becomes Ack Sent.

Ack Sent The client and the server wait for S2 and C2 respectively.

Handshake Done: The client and the server exchange messages.

5.3. Chunking
After handshaking, the connection multiplexes one or more chunk
streams. Each chunk stream carries messages of one type from one
message stream. Each chunk that is created has a unique ID
associated with it called chunk stream ID. The chunks are
transmitted over the network. While transmitting, each chunk must be
sent in full before the next chunk. At the receiver end, the chunks
are assembled into messages based on the chunk stream ID.

Chunking allows large messages at the higher-level protocol to be
broken into smaller messages, for example to prevent large lowpriority
messages (such as video) from blocking smaller high-priority
messages (such as audio or control).

Chunking also allows small messages to be sent with less overhead, as
the chunk header contains a compressed representation of information
that would otherwise have to be included in the message itself.

The chunk size is configurable. It can be set using a Set Chunk Size
control message (Section 5.4.1). Larger chunk sizes reduce CPU
usage, but also commit to larger writes that can delay other content
on lower bandwidth connections. Smaller chunks are not good for high
bit rate streaming. Chunk size is maintained independently for each
direction.

5.3.1. Chunk Format
Each chunk consists of a header and data. The header itself has
three parts:

Chunk Format

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Basic Header (1 to 3 bytes): This field encodes the chunk stream ID
and the chunk type. Chunk type determines the format of the
encoded message header. The length depends entirely on the chunk
stream ID, which is a variable-length field.

Message Header (0, 3, 7, or 11 bytes): This field encodes
information about the message being sent (whether in whole or in
part). The length can be determined using the chunk type
specified in the chunk header.

Extended Timestamp (0 or 4 bytes): This field is present in certain
circumstances depending on the encoded timestamp or timestamp
delta field in the Chunk Message header. See Section 5.3.1.3 for
more information.

Chunk Data (variable size): The payload of this chunk, up to the
configured maximum chunk size.

5.3.1.1. Chunk Basic Header
The Chunk Basic Header encodes the chunk stream ID and the chunk type
(represented by fmt field in the figure below). Chunk type
determines the format of the encoded message header. Chunk Basic
Header field may be 1, 2, or 3 bytes, depending on the chunk stream
ID.

An implementation SHOULD use the smallest representation that can
hold the ID.

The protocol supports up to 65597 streams with IDs 3-65599. The IDs
0, 1, and 2 are reserved. Value 0 indicates the 2 byte form and an
ID in the range of 64-319 (the second byte + 64). Value 1 indicates
the 3 byte form and an ID in the range of 64-65599 ((the third
byte)*256 + the second byte + 64). Values in the range of 3-63
represent the complete stream ID. Chunk Stream ID with value 2 is
reserved for low-level protocol control messages and commands.

The bits 0-5 (least significant) in the chunk basic header represent
the chunk stream ID.

Chunk stream IDs 2-63 can be encoded in the 1-byte version of this
field.

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

Chunk basic header 1

Chunk stream IDs 64-319 can be encoded in the 2-byte form of the

header. ID is computed as (the second byte + 64).

0 1

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

|fmt| 0 | cs id - 64 |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Chunk basic header 2

Chunk stream IDs 64-65599 can be encoded in the 3-byte version of
this field. ID is computed as ((the third byte)*256 + (the second
byte) + 64).

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| 1 | cs id - 64 |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Chunk basic header 3

cs id (6 bits): This field contains the chunk stream ID, for values
from 2-63. Values 0 and 1 are used to indicate the 2- or 3-byte
versions of this field.

fmt (2 bits): This field identifies one of four format used by the
’chunk message header’. The ’chunk message header’ for each of
the chunk types is explained in the next section.

cs id - 64 (8 or 16 bits): This field contains the chunk stream ID
minus 64. For example, ID 365 would be represented by a 1 in cs
id, and a 16-bit 301 here.

Chunk stream IDs with values 64-319 could be represented by either

the 2-byte or 3-byte form of the header.

5.3.1.2. Chunk Message Header
There are four different formats for the chunk message header,
selected by the "fmt" field in the chunk basic header.

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An implementation SHOULD use the most compact representation possible
for each chunk message header.

5.3.1.2.1. Type 0

Type 0 chunk headers are 11 bytes long. 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

Chunk Message Header - Type 0

timestamp (3 bytes): For a type-0 chunk, the absolute timestamp of
the message is sent here. If the timestamp is greater than or
equal to 16777215 (hexadecimal 0xFFFFFF), this field MUST be
16777215, indicating the presence of the Extended Timestamp field

to encode the full 32 bit timestamp. Otherwise, this field SHOULD
be the entire timestamp.
5.3.1.2.2. Type 1
Type 1 chunk headers are 7 bytes long. The message stream ID is not

included; this chunk takes the same stream ID as the preceding chunk.
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

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp delta |message length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| message length (cont) |message type id|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Chunk Message Header - Type 1

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5.3.1.2.3. Type 2

Type 2 chunk headers are 3 bytes long. Neither the stream ID nor the
message length is included; this chunk has the same stream ID and
message length as the preceding chunk. 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

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 |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Chunk Message Header - Type 2

5.3.1.2.4. Type 3

Type 3 chunks have no message header. The stream ID, message length
and timestamp delta fields are not present; chunks of this type take
values from the preceding chunk for the same Chunk Stream ID. When a
single message is split into chunks, all chunks of a message except
the first one SHOULD use this type. Refer to Example 2
(Section 5.3.2.2). A stream consisting of messages of exactly the
same size, stream ID and spacing in time SHOULD use this type for all
chunks after a chunk of Type 2. Refer to Example 1
(Section 5.3.2.1). If the delta between the first message and the
second message is same as the timestamp of the first message, then a
chunk of Type 3 could immediately follow the chunk of Type 0 as there
is no need for a chunk of Type 2 to register the delta. If a Type 3
chunk follows a Type 0 chunk, then the timestamp delta for this Type
3 chunk is the same as the timestamp of the Type 0 chunk.

5.3.1.2.5. Common Header Fields
Description of each field in the chunk message header:

timestamp delta (3 bytes): For a type-1 or type-2 chunk, the
difference between the previous chunk’s timestamp and the current
chunk’s timestamp is sent here. If the delta is greater than or
equal to 16777215 (hexadecimal 0xFFFFFF), this field MUST be
16777215, indicating the presence of the Extended Timestamp field
to encode the full 32 bit delta. Otherwise, this field SHOULD be
the actual delta.

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message length (3 bytes): For a type-0 or type-1 chunk, the length
of the message is sent here. Note that this is generally not the
same as the length of the chunk payload. The chunk payload length
is the maximum chunk size for all but the last chunk, and the
remainder (which may be the entire length, for small messages) for
the last chunk.

message type id (1 byte): For a type-0 or type-1 chunk, type of the
message is sent here.

message stream id (4 bytes): For a type-0 chunk, the message stream
ID is stored. Message stream ID is stored in little-endian
format. Typically, all messages in the same chunk stream will
come from the same message stream. While it is possible to
multiplex separate message streams into the same chunk stream,
this defeats the benefits of the header compression. However, if
one message stream is closed and another one subsequently opened,
there is no reason an existing chunk stream cannot be reused by
sending a new type-0 chunk.

5.3.1.3. Extended Timestamp
The Extended Timestamp field is used to encode timestamps or
timestamp deltas that are greater than 16777215 (0xFFFFFF); that is,
for timestamps or timestamp deltas that don’t fit in the 24 bit
fields of Type 0, 1, or 2 chunks. This field encodes the complete
32-bit timestamp or timestamp delta. The presence of this field is
indicated by setting the timestamp field of a Type 0 chunk, or the
timestamp delta field of a Type 1 or 2 chunk, to 16777215 (0xFFFFFF).
This field is present in Type 3 chunks when the most recent Type 0,
1, or 2 chunk for the same chunk stream ID indicated the presence of
an extended timestamp field.

5.3.2. Examples
5.3.2.1. Example 1

This example shows a simple stream of audio messages. This example
demonstrates the redundancy of information.

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+---------+-----------------+-----------------+-----------------+
| |Message Stream ID| Message TYpe ID | Time | Length |
+---------+-----------------+-----------------+-------+---------+
| Msg # 1 | 12345 | 8 | 1000 | 32 |
+---------+-----------------+-----------------+-------+---------+
| Msg # 2 | 12345 | 8 | 1020 | 32 |
+---------+-----------------+-----------------+-------+---------+
| Msg # 3 | 12345 | 8 | 1040 | 32 |
+---------+-----------------+-----------------+-------+---------+
| Msg # 4 | 12345 | 8 | 1060 | 32 |
+---------+-----------------+-----------------+-------+---------+

Sample audio messages to be made into chunks

The next table shows chunks produced in this stream. From message 3
onward, data transmission is optimized. There is only 1 byte of
overhead per message beyond this point.

+--------+---------+-----+------------+------- ---+------------+
| | Chunk |Chunk|Header Data |No.of Bytes|Total No.of |
| |Stream ID|Type | | After |Bytes in the|
| | | | |Header |Chunk |
+--------+---------+-----+------------+-----------+------------+
|Chunk#1 | 3 | 0 | delta: 1000| 32 | 44 |
| | | | length: 32,| | |
| | | | type: 8, | | |
| | | | stream ID: | | |
| | | | 12345 (11 | | |
| | | | bytes) | | |
+--------+---------+-----+------------+-----------+------------+
|Chunk#2 | 3 | 2 | 20 (3 | 32 | 36 |
| | | | bytes) | | |
+--------+---------+-----+----+-------+-----------+------------+
|Chunk#3 | 3 | 3 | none (0 | 32 | 33 |
| | | | bytes) | | |
+--------+---------+-----+------------+-----------+------------+
|Chunk#4 | 3 | 3 | none (0 | 32 | 33 |
| | | | bytes) | | |
+--------+---------+-----+------------+-----------+------------+

Format of each of the chunks of audio messages

5.3.2.2. Example 2

This example illustrates a message that is too long to fit in a 128byte
chunk and is broken into several chunks.

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+-----------+-------------------+-----------------+-----------------+
| | Message Stream ID | Message TYpe ID | Time | Length |
+-----------+-------------------+-----------------+-----------------+
| Msg # 1 | 12346 | 9 (video) | 1000 | 307 |
+-----------+-------------------+-----------------+-----------------+

Sample Message to be broken to chunks

Here are the chunks that are produced:

+-------+------+-----+-------------+-----------+------------+
| |Chunk |Chunk|Header |No. of |Total No. of|
| |Stream| Type|Data |Bytes after| bytes in |
| | ID | | | Header | the chunk |
+-------+------+-----+-------------+-----------+------------+
|Chunk#1| 4 | 0 | delta: 1000 | 128 | 140 |
| | | | length: 307 | | |
| | | | type: 9, | | |
| | | | stream ID: | | |
| | | | 12346 (11 | | |
| | | | bytes) | | |
+-------+------+-----+-------------+-----------+------------+
|Chunk#2| 4 | 3 | none (0 | 128 | 129 |
| | | | bytes) | | |
+-------+------+-----+-------------+-----------+------------+
|Chunk#3| 4 | 3 | none (0 | 51 | 52 |
| | | | bytes) | | |
+-------+------+-----+-------------+-----------+------------+

Format of each of the chunks

The header data of chunk 1 specifies that the overall message is 307

bytes.

Notice from the two examples, that chunk type 3 can be used in two
different ways. The first is to specify the continuation of a
message. The second is to specify the beginning of a new message
whose header can be derived from the existing state data.

5.4. Protocol Control Messages
RTMP Chunk Stream uses message type IDs 1, 2, 3, 5, and 6 for
protocol control messages. These messages contain information needed
by the RTMP Chunk Stream protocol.

These protocol control messages MUST have message stream ID 0 (known
as the control stream) and be sent in chunk stream ID 2. Protocol
control messages take effect as soon as they are received; their

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timestamps are ignored.

5.4.1. Set Chunk Size (1)
Protocol control message 1, Set Chunk Size, is used to notify the
peer of a new maximum chunk size.

The maximum chunk size defaults to 128 bytes, but the client or the
server can change this value, and updates its peer using this
message. For example, suppose a client wants to send 131 bytes of
audio data and the chunk size is 128. In this case, the client can
send this message to the server to notify it that the chunk size is
now 131 bytes. The client can then send the audio data in a single
chunk.

The maximum chunk size SHOULD be at least 128 bytes, and MUST be at
least 1 byte. The maximum chunk size is maintained independently for
each direction.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| chunk size (31 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Payload for the ‘Set Chunk Size’ protocol message

0: This bit MUST be zero.
chunk size (31 bits): This field holds the new maximum chunk size,
in bytes, which will be used for all of the sender’s subsequent
chunks until further notice. Valid sizes are 1 to 2147483647
(0x7FFFFFFF) inclusive; however, all sizes greater than 16777215
(0xFFFFFF) are equivalent since no chunk is larger than one
message, and no message is larger than 16777215 bytes.

5.4.2. Abort Message (2)
Protocol control message 2, Abort Message, is used to notify the peer
if it is waiting for chunks to complete a message, then to discard
the partially received message over a chunk stream. The peer
receives the chunk stream ID as this protocol message’s payload. An
application may send this message when closing in order to indicate
that further processing of the messages is not required.

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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| chunk stream id (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Payload for the ‘Abort Message’ protocol message

chunk stream ID (32 bits): This field holds the chunk stream ID,
whose current message is to be discarded.

5.4.3. Acknowledgement (3)

The client or the server MUST send an acknowledgment to the peer
after receiving bytes equal to the window size. The window size is
the maximum number of bytes that the sender sends without receiving
acknowledgment from the receiver. This message specifies the
sequence number, which is the number of the bytes received so far.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sequence number (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Payload for the ‘Acknowledgement’ protocol message

sequence number (32 bits): This field holds the number of bytes
received so far.

5.4.4. Window Acknowledgement Size (5)
The client or the server sends this message to inform the peer of the
window size to use between sending acknowledgments. The sender
expects acknowledgment from its peer after the sender sends window
size bytes. The receiving peer MUST send an Acknowledgement
(Section 5.4.3) after receiving the indicated number of bytes since
the last Acknowledgement was sent, or from the beginning of the
session if no Acknowledgement has yet been sent.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgement Window size (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Payload for the ‘Window Acknowledgement Size’ protocol message

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5.4.5. Set Peer Bandwidth (6)
The client or the server sends this message to limit the output
bandwidth of its peer. The peer receiving this message limits its
output bandwidth by limiting the amount of sent but unacknowledged
data to the window size indicated in this message. The peer
receiving this message SHOULD respond with a Window Acknowledgement
Size message if the window size is different from the last one sent
to the sender of this message.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgement Window size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Limit Type |
+-+-+-+-+-+-+-+-+

Payload for the ‘Set Peer Bandwidth’ protocol message

The Limit Type is one of the following values:

0 - Hard: The peer SHOULD limit its output bandwidth to the
indicated window size.

1 - Soft: The peer SHOULD limit its output bandwidth to the the
window indicated in this message or the limit already in effect,
whichever is smaller.

2 - Dynamic: If the previous Limit Type was Hard, treat this message
as though it was marked Hard, otherwise ignore this message.

6. RTMP Message Formats
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 audiovideo
applications, from one-to-one and one-to-many live broadcasting
to video-on-demand services to interactive conferencing applications.

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6.1. RTMP Message Format
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.

6.1.1. Message Header
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

Message Header

6.1.2. Message Payload
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.

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6.2. User Control Messages (4)
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.

7. RTMP Command Messages
This section describes the different types of messages and commands
that are exchanged between the server and the client to communicate
with each other.

The different types of messages that are exchanged between the server
and the client include audio messages for sending the audio data,
video messages for sending video data, data messages for sending any
user data, shared object messages, and command messages. Shared
object messages provide a general purpose way to manage distributed
data among multiple clients and a server. Command messages carry the
AMF encoded commands between the client and the server. A client or
a server can request Remote Procedure Calls (RPC) over streams that
are communicated using the command messages to the peer.

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7.1. Types of Messages
The server and the client send messages over the network to
communicate with each other. The messages can be of any type which
includes audio messages, video messages, command messages, shared
object messages, data messages, and user control messages.

7.1.1. Command Message (20, 17)

Command messages carry the AMF-encoded commands between the client
and the server. These messages have been assigned message type value
of 20 for AMF0 encoding and message type value of 17 for AMF3
encoding. These messages are sent to perform some operations like
connect, createStream, publish, play, pause on the peer. Command
messages like onstatus, result etc. are used to inform the sender
about the status of the requested commands. A command message
consists of command name, transaction ID, and command object that
contains related parameters. A client or a server can request Remote
Procedure Calls (RPC) over streams that are communicated using the
command messages to the peer.

7.1.2. Data Message (18, 15)

The client or the server sends this message to send Metadata or any
user data to the peer. Metadata includes details about the
data(audio, video etc.) like creation time, duration, theme and so
on. These messages have been assigned message type value of 18 for
AMF0 and message type value of 15 for AMF3.

7.1.3. Shared Object Message (19, 16)
A shared object is a Flash object (a collection of name value pairs)
that are in synchronization across multiple clients, instances, and
so on. The message types 19 for AMF0 and 16 for AMF3 are reserved
for shared object events. Each message can contain multiple events.

+------+------+-------+-----+-----+------+-----+ +-----+------+-----+

+------+------+-------+-----+-----+------+-----+ +-----+------+-----+
| |
|<- - - - - - - - - - - - - - - - - - - - - - - - - - - - - >|
| AMF Shared Object Message body |

The shared object message format

The following event types are supported:

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7.1.4. Audio Message (8)
The client or the server sends this message to send audio data to the
peer. The message type value of 8 is reserved for audio messages.

7.1.5. Video Message (9)
The client or the server sends this message to send video data to the
peer. The message type value of 9 is reserved for video messages.

7.1.6. Aggregate Message (22)
An aggregate message is a single message that contains a series of
RTMP sub-messages using the format described in Section 6.1. Message
type 22 is used for aggregate messages.

+---------+-------------------------+
| Header | Aggregate Message body |
+---------+-------------------------+

The Aggregate Message format

The Aggregate Message body format

The message stream ID of the aggregate message overrides the message

stream IDs of the sub-messages inside the aggregate.

The difference between the timestamps of the aggregate message and
the first sub-message is the offset used to renormalize the
timestamps of the sub-messages to the stream timescale. The offset
is added to each sub-message’s timestamp to arrive at the normalized
stream time. The timestamp of the first sub-message SHOULD be the
same as the timestamp of the aggregate message, so the offset SHOULD
be zero.

The back pointer contains the size of the previous message including
its header. It is included to match the format of FLV file and is
used for backward seek.

Using aggregate messages has several performance benefits:

o The chunk stream can send at most a single complete message within
a chunk. Therefore, increasing the chunk size and using the
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aggregate message reduces the number of chunks sent.

o The sub-messages can be stored contiguously in memory. It is more
efficient when making system calls to send the data on the
network.
7.1.7. User Control Message Events
The client or the server sends this message to notify the peer about
the user control events. For information about the message format,
see Section 6.2.

The following user control event types are supported:

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7.2. Types of Commands
The client and the server exchange commands which are AMF encoded.
The sender sends a command message that consists of command name,
transaction ID, and command object that contains related parameters.
For example, the connect command contains ’app’ parameter, which
tells the server application name the client is connected to. The
receiver processes the command and sends back the response with the
same transaction ID. The response string is either _result, _error,
or a method name, for example, verifyClient or contactExternalServer.

A command string of _result or _error signals a response. The
transaction ID indicates the outstanding command to which the
response refers. It is identical to the tag in IMAP and many other
protocols. The method name in the command string indicates that the
sender is trying to run a method on the receiver end.

The following class objects are used to send various commands:

NetConnection An object that is a higher-level representation of
connection between the server and the client.

NetStream An object that represents the channel over which audio
streams, video streams and other related data are sent. We also
send commands like play , pause etc. which control the flow of the
data.

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7.2.1. NetConnection Commands
The NetConnection manages a two-way connection between a client
application and the server. In addition, it provides support for
asynchronous remote method calls.

The following commands can be sent on the NetConnection :

o connect
o call
o close
o createStream
7.2.1.1. connect
The client sends the connect command to the server to request
connection to a server application instance.

The command structure from the client to the server is as follows:

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Following is the description of the name-value pairs used in Command
Object of the connect command.

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Flag values for the audioCodecs property:

+----------------------+----------------------------+--------------+
| Codec Flag | Usage | Value |
+----------------------+----------------------------+--------------+
| SUPPORT_SND_NONE | Raw sound, no compression | 0x0001 |
+----------------------+----------------------------+--------------+
| SUPPORT_SND_ADPCM | ADPCM compression | 0x0002 |
+----------------------+----------------------------+--------------+
| SUPPORT_SND_MP3 | mp3 compression | 0x0004 |
+----------------------+----------------------------+--------------+
| SUPPORT_SND_INTEL | Not used | 0x0008 |
+----------------------+----------------------------+--------------+
| SUPPORT_SND_UNUSED | Not used | 0x0010 |
+----------------------+----------------------------+--------------+
| SUPPORT_SND_NELLY8 | NellyMoser at 8-kHz | 0x0020 |
| | compression | |
+----------------------+----------------------------+--------------+
| SUPPORT_SND_NELLY | NellyMoser compression | 0x0040 |
| | (5, 11, 22, and 44 kHz) | |
+----------------------+----------------------------+--------------+
| SUPPORT_SND_G711A | G711A sound compression | 0x0080 |
| | (Flash Media Server only) | |
+----------------------+----------------------------+--------------+
| SUPPORT_SND_G711U | G711U sound compression | 0x0100 |
| | (Flash Media Server only) | |
+----------------------+----------------------------+--------------+
| SUPPORT_SND_NELLY16 | NellyMouser at 16-kHz | 0x0200 |
| | compression | |
+----------------------+----------------------------+--------------+
| SUPPORT_SND_AAC | Advanced audio coding | 0x0400 |
| | (AAC) codec | |
+----------------------+----------------------------+--------------+
| SUPPORT_SND_SPEEX | Speex Audio | 0x0800 |
+----------------------+----------------------------+--------------+
| SUPPORT_SND_ALL | All RTMP-supported audio | 0x0FFF |
| | codecs | |
+----------------------+----------------------------+--------------+

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Flag values for the videoCodecs Property:

+----------------------+----------------------------+--------------+
| Codec Flag | Usage | Value |
+----------------------+----------------------------+--------------+
| SUPPORT_VID_UNUSED | Obsolete value | 0x0001 |
+----------------------+----------------------------+--------------+
| SUPPORT_VID_JPEG | Obsolete value | 0x0002 |
+----------------------+----------------------------+--------------+
| SUPPORT_VID_SORENSON | Sorenson Flash video | 0x0004 |
+----------------------+----------------------------+--------------+
| SUPPORT_VID_HOMEBREW | V1 screen sharing | 0x0008 |
+----------------------+----------------------------+--------------+
| SUPPORT_VID_VP6 (On2)| On2 video (Flash 8+) | 0x0010 |
+----------------------+----------------------------+--------------+
| SUPPORT_VID_VP6ALPHA | On2 video with alpha | 0x0020 |
| (On2 with alpha | channel | |
| channel) | | |
+----------------------+----------------------------+--------------+
| SUPPORT_VID_HOMEBREWV| Screen sharing version 2 | 0x0040 |
| (screensharing v2) | (Flash 8+) | |
+----------------------+----------------------------+--------------+
| SUPPORT_VID_H264 | H264 video | 0x0080 |
+----------------------+----------------------------+--------------+
| SUPPORT_VID_ALL | All RTMP-supported video | 0x00FF |
| | codecs | |
+----------------------+----------------------------+--------------+

Flag values for the videoFunction property:

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Values for the object encoding property:

+----------------------+----------------------------+--------------+
| Encoding Type | Usage | Value |
+----------------------+----------------------------+--------------+
| AMF0 | AMF0 object encoding | 0 |
| | supported by Flash 6 and | |
| | later | |
+----------------------+----------------------------+--------------+
| AMF3 | AMF3 encoding from | 3 |
| | Flash 9 (AS3) | |
+----------------------+----------------------------+--------------+

The command structure from server to client is as follows:

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+--------------+ +-------------+
| Client | | | Server |
+------+-------+ | +------+------+
| Handshaking done |
| | |
| | |
| | |
| | |
|----------- Command Message(connect) ------->|
| |
|<------- Window Acknowledgement Size --------|
| |
|<----------- Set Peer Bandwidth -------------|
| |
|-------- Window Acknowledgement Size ------->|
| |
|<------ User Control Message(StreamBegin) ---|
| |
|<------------ Command Message ---------------|
| (_result- connect response) |
| |

Message flow in the connect command

The message flow during the execution of the command is:

1. Client sends the connect command to the server to request to
connect with the server application instance.
2. After receiving the connect command, the server sends the
protocol message ’Window Acknowledgement Size’ to the client.
The server also connects to the application mentioned in the
connect command.
3. The server sends the protocol message ’Set Peer Bandwidth’ to the
client.
4. The client sends the protocol message ’Window Acknowledgement
Size’ to the server after processing the protocol message ’Set
Peer Bandwidth’.
5. The server sends an another protocol message of type User Control
Message(StreamBegin) to the client.
6. The server sends the result command message informing the client
of the connection status (success/fail). The command specifies
the transaction ID (always equal to 1 for the connect command).
The message also specifies the properties, such as Flash Media
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Server version (string). In addition it specificies other
connection response related information like level (string), code
(string), description (string), objectencoding (number), etc.

7.2.1.2. Call
The call method of the NetConnection object runs remote procedure
calls (RPC) at the receiving end. The called RPC name is passed as a
parameter to the call command.

The command structure from the sender to the receiver is as follows:

The command structure of the response is as follows:

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7.2.1.3. createStream
The client sends this command to the server to create a logical
channel for message communication The publishing of audio, video, and
metadata is carried out over stream channel created using the
createStream command.

NetConnection is the default communication channel, which has a
stream ID 0. Protocol and a few command messages, including
createStream, use the default communication channel.

The command structure from the client to the server is as follows:

The command structure from server to client is as follows:

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7.2.2. NetStream Commands
The NetStream defines the channel through which the streaming audio,
video, and data messages can flow over the NetConnection that
connects the client to the server. A NetConnection object can
support multiple NetStreams for multiple data streams.

The following commands can be sent on the NetStream by the client to
the server:

o play
o play2
o deleteStream
o closeStream
o receiveAudio
o receiveVideo
o publish
o seek
o pause
The server sends NetStream status updates to the client using the
"onStatus" command:

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+--------------+----------+----------------------------------------+

Format of NetStream status message commands.

7.2.2.1. play
The client sends this command to the server to play a stream. A

playlist can also be created using this command multiple times.

If you want to create a dynamic playlist that switches among
different live or recorded streams, call play more than once and pass
false for reset each time. Conversely, if you want to play the
specified stream immediately, clearing any other streams that are
queued for play, pass true for reset.

The command structure from the client to the server is as follows:

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+-------------+ +----------+
| Play Client | | | Server |
+------+------+ | +-----+----+

| Handshaking and Application |
| connect done |
| | |
| | |
| | |
| | |
---+---- |------Command Message(createStream) ----->|
Create| | |
Stream| | |
---+---- |<---------- Command Message --------------|
| (_result- createStream response) |
| |
---+---- |------ Command Message (play) ----------->|
| | |
| |<------------SetChunkSize --------------|
| | |
| |<---- User Control (StreamIsRecorded) ----|
Play | | |
| |<---- UserControl (StreamBegin) ----------|
| | |
| |<--Command Message(onStatus-play reset) --|
| | |
| |<--Command Message(onStatus-play start) --|
| | |
| |<-------------Audio Message---------------|
| | |
| |<-------------Video Message---------------|
| | | |

|
Keep receiving audio and video stream till finishes

Message flow in the play command

The message flow during the execution of the command is:

1. The client sends the play command after receiving result of the
createStream command as success from the server.
2. On receiving the play command, the server sends a protocol
message to set the chunk size.
3. The server sends another protocol message (user control)
specifying the event ’StreamIsRecorded’ and the stream ID in that
message. The message carries the event type in the first 2 bytes
and the stream ID in the last 4 bytes.
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4. The server sends another protocol message (user control)
specifying the event ’StreamBegin’, to indicate beginning of the
streaming to the client.
5. The server sends an onStatus command messages
NetStream.Play.Start & NetStream.Play.Reset if the play command
sent by the client is successful. NetStream.Play.Reset is sent
by the server only if the play command sent by the client has set
the reset flag. If the stream to be played is not found, the
Server sends the onStatus message NetStream.Play.StreamNotFound.
After this, the server sends audio and video data, which the client
plays.

7.2.2.2. play2

Unlike the play command, play2 can switch to a different bit rate
stream without changing the timeline of the content played. The
server maintains multiple files for all supported bitrates that the
client can request in play2.

The command structure from the client to the server is as follows:

The public properties for the NetStreamPlayOptions object are
described in the ActionScript 3 Language Reference [AS3].

The message flow for the command is shown in the following
illustration.

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+--------------+ +-------------+
| Play2 Client | | | Server |
+--------+-----+ | +------+------+

| Handshaking and Application |
| connect done |
| | |
| | |
| | |
| | |
---+---- |---- Command Message(createStream) --->|
Create | | |
Stream | | |
---+---- |<---- Command Message (_result) -------|
| |
---+---- |------ Command Message (play) -------->|
| | |
| |<------------ SetChunkSize ------------|
| | |
| |<--- UserControl (StreamIsRecorded)----|
Play | | |
| |<------- UserControl (StreamBegin)-----|
| | |
| |<--Command Message(onStatus-playstart)-|
| | |
| |<---------- Audio Message -------------|
| | |
| |<---------- Video Message -------------|
| | |
| |
---+---- |-------- Command Message(play2) ------>|
| | |
| |<------- Audio Message (new rate) -----|

Play2 | | |
| |<------- Video Message (new rate) -----|
| | | |
| | | |
| Keep receiving audio and video stream till finishes

Message flow in the play2 command

7.2.2.3. deleteStream
NetStream sends the deleteStream command when the NetStream object is
getting destroyed.

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The command structure from the client to the server is as follows:

The server does not send any response.

7.2.2.4. receiveAudio
NetStream sends the receiveAudio message to inform the server whether
to send or not to send the audio to the client.

The command structure from the client to the server is as follows:

The server does not send any response, if the receiveAudio command is
sent with the bool flag set as false. If this flag is set to true,
server responds with status messages NetStream.Seek.Notify and
NetStream.Play.Start

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7.2.2.5. receiveVideo
NetStream sends the receiveVideo message to inform the server whether
to send the video to the client or not.

The command structure from the client to the server is as follows:

The server does not send any response, if the receiveVideo command is
sent with the bool flag set as false. If this flag is set to true,
server responds with status messages NetStream.Seek.Notify and
NetStream.Play.Start

7.2.2.6. publish
The client sends the publish command to publish a named stream to the
server. Using this name, any client can play this stream and receive
the published audio, video, and data messages.

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The command structure from the client to the server is as follows:

+--------------+----------+----------------------------------------+

The server responds with the onStatus command to mark the beginning
of publish.

7.2.2.7. seek
The client sends the seek command to seek the offset (in
milliseconds) within a media file or playlist.

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The command structure from the client to the server is as follows:

The server sends a status message NetStream.Seek.Notify when seek is
successful. In failure, it returns an _error message.

7.2.2.8. pause
The client sends the pause command to tell the server to pause or
start playing.

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The command structure from the client to the server is as follows:

+--------------+----------+----------------------------------------+

The server sends a status message NetStream.Pause.Notify when the
stream is paused. NetStream.Unpause.Notify is sent when a stream in
un-paused. In failure, it returns an _error message.

7.3. Message Exchange Examples
Here are a few examples to explain message exchange using RTMP.
7.3.1. Publish Recorded Video
This example illustrates how a publisher can publish a stream and
then stream the video to the server. Other clients can subscribe to
this published stream and play the video.

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+--------------------+ +-----------+
| Publisher Client | | | Server |
+----------+---------+ | +-----+-----+

| Handshaking Done |
| | |
| | |
---+---- |----- Command Message(connect) ----->|
| | |
| |<----- Window Acknowledge Size ------|
Connect | | |
| |<-------Set Peer BandWidth ----------|
| | |
| |------ Window Acknowledge Size ----->|
| | |
| |<------User Control(StreamBegin)-----|
| | |
---+---- |<---------Command Message -----------|
| (_result- connect response) |
| |
---+---- |--- Command Message(createStream)--->|
Create | | |
Stream | | |
---+---- |<------- Command Message ------------|
| (_result- createStream response) |
| |
---+---- |---- Command Message(publish) ------>|
| | |
| |<------User Control(StreamBegin)-----|
| | |
| |-----Data Message (Metadata)-------->|
| | |
Publishing| |------------ Audio Data ------------>|
Content | | |
| |------------ SetChunkSize ---------->|
| | |
| |<----------Command Message ----------|
| | (_result- publish result) |
| | |
| |------------- Video Data ----------->|

Message flow in publishing a video stream

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7.3.2. Broadcast a Shared Object Message
This example illustrates the messages that are exchanged during the
creation and changing of a shared object. It also illustrates the
process of shared object message broadcasting.

+----------+ +----------+
| Client | | | Server |
+-----+----+ | +-----+----+
| Handshaking and Application |
| connect done |
| | |
| | |
| | |
| | |

Create and ---+---- |---- Shared Object Event(Use)---->|
connect | | |
Shared Object | | |
---+---- |<---- Shared Object Event---------|
| (UseSuccess,Clear) |
| |
---+---- |------ Shared Object Event ------>|
Shared object | | (RequestChange) |
Set Property | | |
---+---- |<------ Shared Object Event ------|
| (Success) |
| |
---+---- |------- Shared Object Event ----->|
Shared object| | (SendMessage) |
Message | | |
Broadcast ---+---- |<------- Shared Object Event -----|
| (SendMessage) |
| |
| |

Shared object message broadcast

7.3.3. Publish Metadata from Recorded Stream
This example describes the message exchange for publishing metadata.

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+------------------+ +---------+
| Publisher Client | | | FMS |
+---------+--------+ | +----+----+

| Handshaking and Application |
| connect done |
| | |
| | |
---+--- |---Command Messsage(createStream) -->|
Create | | |
Stream | | |
---+--- |<---------Command Message------------|
| (_result - command response) |
| |
---+--- |---- Command Message(publish) ------>|
Publishing | | |
metadata | |<------ UserControl(StreamBegin)-----|
from file | | |
| |-----Data Message (Metadata) ------->|
| |

Publishing metadata

8. References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.

[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.

[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
August 1996.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[AS3] Adobe Systems, Inc., "ActionScript 3.0 Reference for the
Adobe Flash Platform", 2011, <http://www.adobe.com/devnet/
actionscript/documentation.html>.

[AMF0] Adobe Systems, Inc., "Action Message Format -- AMF 0",
December 2007, <http://opensource.adobe.com/wiki/download/
attachments/1114283/amf0_spec_121207.pdf>.

[AMF3] Adobe Systems, Inc., "Action Message Format -- AMF 3",
May 2008, <http://opensource.adobe.com/wiki/download/
attachments/1114283/amf3_spec_05_05_08.pdf>.

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Authors’ Addresses

Hardeep Singh Parmar (editor)
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110-2704
US

Phone: +1 408 536 6000
Email: hparmar@adobe.com
URI: http://www.adobe.com/

Michael C. Thornburgh (editor)
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110-2704
US

Phone: +1 408 536 6000
Email: mthornbu@adobe.com
URI: http://www.adobe.com/

Parmar & Thornburgh [Page 52]

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