BUFFER_READY

Type: 32 (0x20)

This user control type is not specified in any official documentation, but is sent by Flash Media Server 3.5. Thanks to the rtmpdump devs for their explanation:

Buffer Ready (unofficial name): After the server has sent a complete buffer, and sends a Buffer Empty message, it will wait until the play duration of that buffer has passed before sending a new buffer. The Buffer Ready message will be sent when the new buffer starts. (There is no BufferReady message for the very first buffer; presumably the Stream Begin message is sufficient for that purpose.)

(see also: http://repo.or.cz/w/rtmpdump.git/blob/8880d1456b282ee79979adbe7b6a6eb8ad371081:/librtmp/rtmp.c#l2787)