yes you are right.
RTSP is support from 2.x but , sdp format is not supported by older android versions.
So what is the possible ways to support this?
If you have rights to access and modify RTSP server you can implement FFMPEG library that will convert the sdp format to mp3 which is widely accepted and also work with android 2.x devices.
If RTSP server is of third party and you do not have rights to modify it, you cant use FFMPEG library in you android project. But to use that you need to have enough knowledge of Android NDK.