Real Time Messaging Protocol

Real Time Messaging Protocol (RTMP) is a proprietary protocol developed by Adobe Systems for streaming audio, video and data over the Internet, between a Flash player and a server.

The RTMP protocol has three variations:

1. The "plain" protocol which works on top of TCP and uses port number 1935
2. RTMPT which is encapsulated within HTTP requests to traverse firewalls
3. RTMPS which works just like RTMPT, but over a secure HTTPS connection.

While the primary motivation for RTMP was a persistent protocol for Flash, it is also used in some other applications, such as the Adobe LiveCycle Data Services ES.

On 20 January 2009 Adobe announced it will publish the RTMP specification.^[1] On 21 May 2009 Adobe filed a DMCA removal request on a Sourceforge-hosted implementation of the protocol.^[2][3]

On Monday, June 15, 2009 Adobe released the RTMP Specification

Operation

The raw TCP-based RTMP maintains a single persistent connection and allows real-time communication. To guarantee smooth delivery of video and audio streams, while still maintaining the ability to transmit bigger chunks of information, the protocol may split video and data into fragments. The size of the fragments used can be negotiated dynamically between the client and server, and even disabled completely if desired, although the default fragment sizes are 128 bytes for video and most other data types, and 64-bytes for audio data. Fragments from different streams may then be interleaved and multiplexed over a single connection. With longer data chunks, the protocol would then only carry a one-byte header per fragment, thus incurring very little overhead. In practice however, 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 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 channels on which packets may be sent/received, and which operate independently of each other. For example, there is a channel dedicated 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 (fragment size negotiation, etc.), and so on. During a typical RTMP session, several channels may be active simultaneously at any given time. When RTMP data is packetized, a packet header is generated. The packet header specifies, among other things, the id of the channel that it is to be sent on, the timestamp at which is was generated (if necessary), and the size of the packet payload. This is then followed by the packet payload, which is fragmented according to the currently agreed-upon fragment size before it is serialized 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 data is subject to fragmentation.

At a higher level, the RTMP encapsulates MP3 and Flash Video multimedia streams, and can make remote procedure calls (RPCs) using the Action Message Format.

Other RPC services are made asynchronously with a single client/server request/response model, so real-time communication is not necessary.^{[clarification needed][4]}

HTTP tunneling

RTMP packets can be exchanged via two HTTP tunneling protocols:

• In RTMP Tunneled (RTMPT), RTMP data is encapsulated and exchanged via HTTP, and messages from the client (the media player, in this case) are addressed to port 80 (the default for HTTP) on the server.

• In RTMP Secure (RTMPS), RTMP data is encapsulated and exchanged via HTTPS, and messages from the client are addressed to port 443 (the default for HTTPS) on the server.

While the messages in RTMPT and RTMPS are larger than the equivalent non-tunneled RTMP messages due to HTTP and HTTPS headers, RTMPT and RTMPS 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.

RTMP client software

This article may require copy-editing for grammar, style, cohesion, tone or spelling. You can assist by editing it. (November 2009)

The most well-known RTMP client is Adobe's Flash player which has full support for streaming video and audio from RTMP servers.

Other third-party players have partial support for RTMP, such as the open source media player XBMC which has acquired preliminary support for playing RTMP streams in its SVN version,Youtube Video Download Tool which supports capturing rtmp stream to a flv file and also version 2.0 of Moyea Web Player with the RTMP Stream Plugin which supports the playback of live streaming videos, SMIL presentations, and streaming videos from RTMP servers.

rtmpdump is a open-source command-line tool that was designed to dump the full RTMP stream and foil any encryption present, however it has been removed from it's original SourceForge page due to violations of the DMCA. flvstreamer is a fork of rtmpdump which removes all violating code (due to encryption) but still allows users to download a stream of audio or video content from all RTMP servers.

RTMP server software

Currently, the only full implementation RTMP servers are:

• Adobe Flash Media Server,
• Onlinelib VCS Video Communication Server (including iPhone Support)
• Wowza Media Server
• WebORB Integration Server (free and available for .NET, Java and ColdFusion) both commercial and closed source implementations.

There is a reverse engineered open source project called Red5 which aims to produce a feature-complete implementation written in Java. As of October 2007, the majority of the functionality is implemented, although the project is still in the beta stage. OneTeam Media Server has also been announced by ProcessOne^[5]. C++ implementation is also available. It is called crtmpserver and is located here.

RTMP Specification

Adobe has opened the Specification for the RTMP Protocol on June 15 of 2009 http://www.adobe.com/devnet/rtmp/

RTMFP Specification

• People at crtmpserver are reverse engineering the rtmfp protocol. Work is in progress.
• RTMFP adobe specification

See also

• Real Time Media Flow Protocol (RTMFP), based on UDP
• Protected Streaming Info about RTMPS and RTMPE

References

This article's citation style may be unclear. The references used may be made clearer with a different or consistent style of citation, footnoting, or external linking. (September 2009)

• Using RPC services in Flex Data Services 2 - Adobe Flex Article
• HTTP Tunneling protocols - Adobe TechNote

• RTMP Specification - Adobe Developer Connection Site
• RTMFP unofficial specification - www.rtmpd.com

External links

• OSFlash - RTMP Protocol
• crtmpserver - unofficial RTMFP specification
• OSFlash - RTMP OS