RTP port for sending media

Paul Long Plong at SMITHMICRO.COM
Thu Sep 14 13:15:29 EDT 2000


H.323 has never placed any requirements on the send port--it never even
mentions the send, or source, port. Many entities therefore justifiably use
"random" send ports, so there is no way we can now place requirements on

Paul Long
Smith Micro Software, Inc.

-----Original Message-----
From: Paul E. Jones [mailto:paulej at packetizer.com]
Sent: Tuesday, September 12, 2000 11:20 AM
To: Anatoli Levine
Cc: h323implementors at imtc.org; ITU-SG16 at mailbag.cps.intel.com
Subject: Re: RTP port for sending media

Anatoli, et al,

I agree that it would be very helpful for NAT devices or other equipment
to be able to safely rely on receiving media from the specified RTP port
that is one lower than the RTCP port advertised in the original OLC
message.  I thought that was the expected behavior when using RTP/RTCP,
but recent discussions with folks working on SIP is that "no, that's not
necessarily true."  Apparently, it is believed that a SIP device may
send RTP from any port.

So, if you have a line like this in the INVITE message:
to indicate a bi-directional media flow, the RTP address is interpreted
only as the "receive" address.  But then I questioned, "what about the
fact that the associated RTCP port (next odd port) is used for receiving
RR packets.. would that not suggest that the RTP port for sending is
also the same port specified in this 'm=' line?"

The answer was 'no'.  So this made me curious about H.323

Do we need to clarify that the RTP port used to send data associated
with the RTCP address in an OLC is the next lower port (even) number, or
do we have implementations out there that send RTP traffic from and
random port, irrespective of the RTCP port put in the
forwardLogicalChannelParameters of the OLC message?  If the latter is
the case, I think we need to state this clearly, too.


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