[Live-devel] Fwd: Why is RTSP needed apart from SDP?

[Ross Finlayson]

>  >>>  Ross Finlayson finlayson at live555.com
>>>>   Wed Oct 1 03:04:23 PDT 2008
>>>>   You can't (in general) play a unicast stream just by reading a
>>>>   SDP description, because the SDP description does not, by
>>>>   itself, contain enough information about the stream endpoints.
>>>
>>>  What's that missing information?
>>
>>  The client's port numbers.
>
>I'm sorry for my misunderstanding. What's the difference between those
>client port numbers and the port numbers that appear in the sdp m=
>description?

Generally speaking, the SDP description describes the session 
information that is known to the server.  The port numbers, 
therefore, are *server* port numbers (that the client would use when 
sending RTCP reports back to the server, for example).  The client 
port numbers are not in the SDP description.

For multicast sessions, this is not a problem, because  - for such 
sessions - the client and server port numbers are always the same (as 
is the IP (multicast) address).

For unicast sessions, however, the port numbers in the SDP 
description are typically specified as 0, and the RTSP protocol is 
used to determine the client and server port numbers (and the client 
IP address).  Once again, our RTSP server implementation does all of 
this automatically.
-- 

Ross Finlayson
Live Networks, Inc.
http://www.live555.com/