Real-Time Messaging Protocol


Real-Time Messaging Protocol was initially a proprietary protocol developed by Macromedia for streaming audio, video and data over the Internet, between a Flash player and a server. Macromedia is now owned by Adobe, which has released an incomplete version of the specification of the protocol for public use.
The RTMP protocol has multiple variations:
  1. RTMP proper, the "plain" protocol which works on top of TCP and uses port number 1935 by default.
  2. RTMPS, which is RTMP over a TLS/SSL connection.
  3. RTMPE, which is RTMP encrypted using Adobe's own security mechanism. While the details of the implementation are proprietary, the mechanism uses industry standard cryptographic primitives.
  4. RTMPT, which is encapsulated within HTTP requests to traverse firewalls. RTMPT is frequently found utilizing cleartext requests on TCP ports 80 and 443 to bypass most corporate traffic filtering. The encapsulated session may carry plain RTMP, RTMPS, or RTMPE packets within.
  5. RTMFP, which is RTMP over UDP instead of TCP, replacing RTMP Chunk Stream. The Secure Real-Time Media Flow Protocol suite has been developed by Adobe Systems and enables end‐users to connect and communicate directly with each other.
While the primary motivation for RTMP was to be a protocol for playing Flash video, it is also used in some other applications, such as the Adobe LiveCycle Data Services ES.

Basic operation

RTMP is a TCP-based protocol which maintains persistent connections and allows low-latency communication. To deliver streams smoothly and transmit as much information as possible, it splits streams into fragments, and their size is negotiated dynamically between the client and server. Sometimes, it is kept unchanged; the default fragment sizes are 64 bytes for audio data, and 128 bytes for video data and most other data types. Fragments from different streams may then be interleaved, and multiplexed over a single connection. With longer data chunks, the protocol thus carries only a one-byte header per fragment, so incurring very little overhead. However, in practice, individual fragments are not typically interleaved. Instead, the interleaving and multiplexing is done at the packet level, with RTMP packets across several different active channels being interleaved in such a way as to ensure that each channel meets its bandwidth, latency, and other quality-of-service requirements. Packets interleaved in this fashion are treated as indivisible, and are not interleaved on the fragment level.
The RTMP defines several virtual channels on which packets may be sent and received, and which operate independently of each other. For example, there is a channel for handling RPC requests and responses, a channel for video stream data, a channel for audio stream data, a channel for out-of-band control messages, and so on. During a typical RTMP session, several channels may be active simultaneously at any given time. When RTMP data is encoded, a packet header is generated. The packet header specifies, amongst other matters, the ID of the channel on which it is to be sent, a timestamp of when it was generated, and the size of the packet's payload. This header is then followed by the actual payload content of the packet, which is fragmented according to the currently agreed-upon fragment size before it is sent over the connection. The packet header itself is never fragmented, and its size does not count towards the data in the packet's first fragment. In other words, only the actual packet payload is subject to fragmentation.
At a higher level, the RTMP encapsulates MP3 or AAC audio and FLV1 video multimedia streams, and can make remote procedure calls using the Action Message Format. Any RPC services required are made asynchronously, using a single client/server request/response model, such that real-time communication is not required.

Encryption

RTMP sessions may be encrypted using either of two methods:
In RTMP Tunneled, RTMP data is encapsulated and exchanged via HTTP, and messages from the client are addressed to port 80 on the server.
While the messages in RTMPT are larger than the equivalent non-tunneled RTMP messages due to HTTP headers, RTMPT may facilitate the use of RTMP in scenarios where the use of non-tunneled RTMP would otherwise not be possible, such as when the client is behind a firewall that blocks non-HTTP and non-HTTPS outbound traffic.
The protocol works by sending commands through the POST URL, and AMF messages through the POST body. An example is
POST /open/1 HTTP/1.1

for a connection to be opened.

Specification document and patent license

Adobe has released a specification for version 1.0 of the protocol dated December 21, 2012. The web landing page leading to that specification notes that "To benefit customers who want to protect their content, the open RTMP specification does not include Adobe's unique secure RTMP measures".
A document accompanying the Adobe specification grants "non-exclusive, royalty-free, nontransferable, non-sublicensable, personal, worldwide" patent license to all implementations of the protocol, with two restrictions: one forbids use for intercepting streaming data, and another prohibits circumvention of "technological measures for the protection of audio, video and/or data content, including any of Adobe’s secure RTMP measures".

Patents and related litigation

Stefan Richter, author of some books on Flash, noted in 2008 that while Adobe is vague as to which patents apply to RTMP, appears to be one of them.
In 2011, Adobe did sue Wowza Media Systems claiming among other things, infringement of their RTMP patents. In 2015, Adobe and Wowza announced that the lawsuits have been settled and dismissed with prejudice.

Packet structure

Packets are sent over a TCP connection which is established first between client and server. They contain a header and a body which, in the case of connection and control commands, is encoded using the Action Message Format. The header is split into the Basic Header and Chunk Message Header. The Basic Header is the only constant part of the packet and is usually composed of a single composite byte, where the 2 most significant bits are the Chunk Type and the rest form the Stream ID. Depending on the value of the former, some fields of the Message Header can be omitted and their value derived from previous packets while depending on the value of the latter, the Basic Header can be extended with 1 or 2 extra bytes. If the value of the remaining 6 bits of the Basic Header is 0 then the BH is 2 bytes and represents from Stream ID 64 to 319 ; if the value is 1, then the BH is 3 bytes and represents from Stream ID 64 to 65599 ; if the value is 2, then BH is 1 byte and is reserved for low-level protocol control messages and commands. The Chunk Message Header contains meta-data information such as the message size, the Timestamp Delta and Message Type. This last value is a single byte and defines whether the packet is an audio, video, command or "low level" RTMP packet such as an RTMP Ping.
An example is shown below as captured when a flash client executes the following code:

var stream:NetStream = new NetStream;

this will generate the following Chunk:
Hex CodeASCII
03 00 0B 68 00 00 19 14 00 00 00 00 02 00 0C 63 72 65 61 74 65 53 74 72 65 61 6D 00 40 00 00 00 00 00 00 00 05␃ ␀ @ I ␀ ␀ ␙ ␔ ␀ ␀ ␀ ␀ ␂ ␀ ␌ c r e a t e S t r e a m ␀ @ ␀ ␀ ␀ ␀ ␀ ␀ ␀ ␅

The packet starts with a Basic Header of a single byte where the 2 most significant bits define a chunk header type of 0 while the rest define a Chunk Stream ID of 3. The 4 possible values of the header type and their significance are:
The last type is always used in the case of aggregate messages where, in the example above, the second message will start with an id of 0xC3 and would mean that all Message Header fields should be derived from the message with a stream Id of 3. The 6 least significant bits that form the Stream ID can take values between 3 and 63. Some values have special meaning like 1 that stands for an extended ID format, in which case there will be 2 bytes following that. A value of 2 is for low level messages such as Ping and Set Client Bandwidth.
The next bytes of the RTMP Header are decoded as follows:
The Message Type ID byte defines whether the packet contains audio/video data, a remote object or a command. Some possible values for are:
Following the header, 0x02 denotes a string of size 0x000C and values 0x63 0x72... 0x6D. Following that we have a 0x00 which is the transaction id of value 2.0. The last byte is 0x05 which means there are no arguments.

Invoke Message Structure (0x14, 0x11)

Some of the message types shown above, such as Ping and Set Client/Server Bandwidth, are considered low level RTMP protocol messages which do not use the AMF encoding format. Command messages on the other hand, whether AMF0 or AMF3, use the format and have the general form shown below:

 
 
  ex. Null, String, Object:

The transaction id is used for commands that can have a reply. The value can be either a string like in the example above or one or more objects, each composed of a set of key/value pairs where the keys are always encoded as strings while the values can be any AMF data type, including complex types like arrays.

Control Message Structure (0x04)

Control messages are not AMF encoded. They start with a stream Id of 0x02 which implies a full header and have a message type of 0x04. The header is followed by 6 bytes which are interpreted as such:
The first two bytes of the message body define the Ping Type which can apparently take 6 possible values.
Pong is the name for a reply to a Ping with the values used as seen above.

ServerBw/ClientBw Message Structure (0x05, 0x06)

This relates to messages that have to do with the client up-stream and server down-stream bit-rate. The body is composed of 4 bytes showing the bandwidth value with a possible extension of one byte which sets the Limit Type. This can have one of 3 possible values which can be: hard, soft or dynamic.

Set Chunk Size (0x01)

The value received in the 4 bytes of the body. A default value of 128 bytes exists and the message is sent only when a change is wanted

The protocol

Handshake

After establishing a TCP connection, an RTMP connection is established first performing a handshake through the exchange of 3 packets from each side. These are referred in the official spec as C0-2 for the client sent packets and S0-2 for the server side respectively and are not to be confused with RTMP packets that can be exchanged only after the handshake is complete. These packets have a structure of their own and C1 contains a field setting the "epoch" timestamp but since this can be set to zero, as is done in third party implementations, the packet can be simplified. The client initialises the connection by sending the C0 packet with a constant value of 0x03 representing the current protocol version. It follows straight with C1 without waiting for S0 to be received first which contains 1536 bytes, with the first 4 representing the epoch timestamp, the second 4 all being 0, and the rest being random. C2 and S2 are an echo of S1 and C1 respectively, except with the second 4 bytes being the time the respective message was received. After C2 and S2 are received the handshake is considered complete.

Connect

At this point, the client and server can negotiate a connection by exchanging AMF encoded messages. These include key value pairs which relate to variables that are needed for a connection to be established. An example message from the client is:

"connect"
1.0


The Flash Media Server and other implementations uses the concept of an "app" to conceptually define a container for audio/video and other content, implemented as a folder on the server root which contains the media files to be streamed. The first variable contains the name of this app as "sample" which is the name provided by the Wowza Server for their testing. The flashVer string is the same as returned by the Action-script getversion function. The audioCodec and videoCodec are encoded as doubles and their meaning can be found in the original spec. The same is true for the videoFunction variable which in this case is the self-explanatory SUPPORT_VID_CLIENT_SEEK constant. Of special interest is the objectEncoding which will define whether the rest of the communication will make use of the extended AMF3 format or not. As version 3 is the current default, the flash client has to be told explicitly in Action-script code to use AMF0 if that is requested. The server then replies with a ServerBW, a ClientBW and a SetPacketSize message sequence, finally followed by an Invoke, with an example message.

"_result"
1.0



Some of the values above are serialised into properties of a generic Action-script Object which is then passed to the NetConnection event listener. The clientId will establish a number for the session to be started by the connection. Object encoding must match the value previously set.

Play video

To start a video stream, the client sends a "createStream" invocation followed by a ping message, followed by a "play" invocation with the file name as argument. The server will then reply with a series of "onStatus" commands followed by the video data as encapsulated within RTMP messages.
After a connection is established, media is sent by encapsulating the content of FLV tags into RTMP messages of type 8 and 9 for audio and video respectively.

HTTP tunneling (RTMPT)

This refers to the HTTP tunneled version of the protocol. It communicates over port 80 and passes the AMF data inside HTTP POST request and responses. The sequence for connection is as follows:

POST /fcs/ident2 HTTP/1.1
Content-Type: application/x-fcs\r\n
HTTP/1.0 404 Not Found


POST /open/1 HTTP/1.1
Content-Type: application/x-fcs\r\n
HTTP/1.1 200 OK
Content-Type: application/x-fcs\r\n
1728724019

The first request has an path and the correct reply is a 404 Not Found error. The client then sends an /open/1 request where the server must reply with a 200 ok appending a random number that will be used as the session identifier for the said communication. In this example 1728724019 is returned in the response body.

POST /idle/1728724019/0 HTTP/1.1
HTTP/1.1 200 OK
0x01

From now on the is a polling request where the session id has been generated and returned from the server and the sequence is just a number that increments by one for every request. The appropriate response is a 200 OK with an integer returned in the body signifying the interval time. AMF data is sent through

Software implementations

RTMP is implemented at these three stages:
The open-source RTMP client command-line tool rtmpdump is designed to play back or save to disk the full RTMP stream including the RTMPE protocol Adobe uses for encryption. RTMPdump runs on Linux, Android, Solaris,, and most other Unix-derived operating systems, as well as Microsoft Windows. Originally supporting all versions of 32-bit Windows including Windows 98, from version 2.2 the software will run only on Windows XP and above.
Packages of the rtmpdump suite of software are available in the major open-source repositories. These include the front-end apps "rtmpdump", "rtmpsrv" and "rtmpsuck."
Development of RTMPdump was restarted in October 2009, outside the United States, at the MPlayer site. The current version features greatly improved functionality, and has been rewritten to take advantage of the benefits of the C programming language. In particular, the main functionality was built into a library which can easily be used by other applications. The RTMPdump developers have also written support for librtmp for MPlayer, FFmpeg, XBMC, cURL, VLC and a number of other open source software projects. Use of librtmp provides these projects with full support of RTMP in all its variants without any additional development effort.

FLVstreamer

FLVstreamer is a fork of RTMPdump, without the code which Adobe claims violates the DMCA in the USA. This was developed as a response to Adobe's attempt in 2008 to suppress RTMPdump. FLVstreamer is an RTMP client that will save to disk a stream of audio or video content from any RTMP server, if encryption is not enabled on the stream.