BUFFER_EMPTY

Type: 31 (0x1F)

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 Empty (unofficial name): After the server has sent a complete buffer, and sends this 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.

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