newbie's question about rtsp

hi,
i would say sorry if here is not the place ask such a
question.
When using RTSP to play an unicast stream,how is the
receiving ports 
determined? I suppose using
SDP for presentation description.
Here is my understanding about RTSP.
A client sends 'Describe' message to the server.
The server response 'describe' with SDP. I am not sure if
the rtp port 
numbers given in the SDP are for client's receiving ports
or server's 
sending ports. On my opinion. server should not designate
the client's receving port number on unicast case.
The Client then sends 'setup' message to the server.Here
it can give its 
receiving port range.But it never clearly clarify what is
audio port number 
and what is video port number.
  Maybe i misunderstand the rtsp standard.
  Thanks in advance!

Regards,

woo



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