TechNote (Archived)

Information priority within the RTMP protocol

Information in RTMP
Due to the streaming nature of RTMP (Real Time Messaging Protocol), performance will be affected depending on the connection established between the server and the client. RTMP uses the TCP/IP protocol for packet transmission, so the RTMP server will have information on which packets have successfully been transmitted and which have not. If the packets start to get backed up at the server, it may be necessary to start to "drop" some packets, which means they will not be sent, and the server will move on to newer information. The packets sent through the protocol are broken down into three different types, so that certain types of data can be dropped if needed. The three types of information are Data, Audio, and Video.

Information priority
When the server determines that packets need to be dropped to accommodate for a slow connection, it will always begin dropping the Video packets first (lowest priority), then the Audio packets as necessary. The Data packets are the first priority and are never dropped The packet priority cannot be changed, although there are methods to control this to some degree. The reason that packets are dropped in this order, is based on how critical information is for a communications application to function properly. If Data were to be dropped, the client is likely not going to have all the information to perform necessary logic or operations. Audio is deemed more important than Video, because it is generally more difficult to understand broken or choppy audio than video. Video tends to be a smooth progression of changes, so lost frames may not be too noticeable.

Controlling priority
It is not possible to change the order in which packets are dropped. It is possible, however, to design a movie in such a way where the user can choose what they want to receive. For instance, giving the user the ability to mute the application (and therefore tell the Flash Communication Server to not send audio information) will then change the priority to Data first, then Video (since Audio is no longer present).