Hemant,
I thought the same thing you did, but I was corrected on this issue: the 'm=' line corresponds to the "receive" side only. It tell you nothing about what port numbers will be used for sending an RTP stream.
For example, if I send an INVITE message with this line: m=audio 53000 RTP/AVP 4
If the called endpoint wishes to send media to the caller, it would do so to port 53000. However, if the called endpoint wants to also receive media, it would return an SDP 'm=' line in a message like 200. Suppose it was: m=audio 26000 RTP/AVP 4
The caller may send media to the called party to port 26000, but there is nothing in SIP or SDP that says that the caller shall use port 53000 to send media to the callee's port 26000-- this is a different RTP stream. So, in effect, there are two RTP streams:
caller callee port port ??/?? -----------> 26000/26001 53000/53001 <------ ?? / ??
The question marks indicate that, until media starts flowing, the other endpoint does not know what port number will be utilized.
Paul
----- Original Message ----- From: "Hemant Agrawal" hemantag@globespan.net To: ITU-SG16@mailbag.cps.intel.com Sent: Thursday, September 14, 2000 7:08 AM Subject: Re: SIP and H.323 Interworking question
Hi Paul SDP used by SIP contains the connection address in the 'c='
line,
while it contains the RTP port information in 'm = ' line. theRTCP port used will be RTP port +1
c=<network type> <address type> <connection address> m=<media> <port> <transport> <fmt list> The call originator will sends it's RTP port information &connection address in the INVITE message, and it will get the other end
RTP
port info & connection address in the response ( e.g.200 OK).
Thanks & Regards, Hemant Agrawal
Folks,
I was recently pondering issues related to H.323/SIP interworking-- particularly the handling of media streams.
If I understand things correctly, the 'm=' line in the SDP used by SIP indicates only the media address to which RTP packets will be sent. It
says
nothing about the RTP/RTCP port used for sending media. Those can be another pair of ports, apparently.
How, then, can an H.323 IWF send an OLC to an H.323 endpoint? (Let's
assume
that the IWF does not want to proxy the media-- only handle signaling.)
Assume that the IWF has received the INVITE message, containing the RTP address of the reverse channel. It then goes ahead and establishes the
call
with the H.323 endpoint, starts H.245 and exchanges capability
information.
Suppose the IWF now wishes to open a channel. Part of the forwardLogicalChannel parameters of the OLC is a required field called "mediaControlChannel". However, this information is not passed in the
SDP.
Are people assuming, as I had done, that the 'm=' line contained the RTP address for both sending and receiving media?
Am I misreading something or is there a real interworking issue here?
Thanks, Paul
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