I am not sure it would help NAT devices. Couldn't a sender "fake" the RTP sender port by inserting another value?
 
More commonly, I would believe that the port would typically be set to "0"?
RFC 768 (UDP) says:
Source Port is an optional field, when meaningful, it indicates the port
of the sending  process,  and may be assumed  to be the port  to which a
reply should  be addressed  in the absence of any other information.  If
not used, a value of zero is inserted.
-----Original Message-----
From: Paul E. Jones [mailto:paulej@PACKETIZER.COM]
Sent: Tuesday, September 12, 2000 9:20 AM
To: ITU-SG16@MAILBAG.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:
    m=
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 implementations.
 
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.
 
Thanks,
Paul
 
----- Original Message -----
From: Anatoli Levine
To: Paul E. Jones
Cc: h323implementors@imtc.org
Sent: Tuesday, September 12, 2000 9:37 AM
Subject: Re: RTP port for sending media

Paul,

when you are transmitting, you can probably can use any port, but the problem is that usually it make sense to use the same RTP port for both receive and transmit - in this case if you will use a random RTP port you will violate the rules. Another dangerous thing is that we don 't know what assumptions can firewall vendors make, so we can run into trouble with this also ( if assumption will be made from the 2501 port number). I would suggest that we will make it clear in the standard that two consequent port numbers would be used for RTP/RTCP in any case.

Anatoli

"Paul E. Jones" wrote:

Folks, I have a question... I thought this was clear, but perhaps it is not. Suppose I send an OLC proposing a channel to be opened and I provide my RTCP port in the mediaControlChannel field.  Say, 2501. H.245 states in B.3.1:
The mediaChannel indicates a transportAddress to be used for the logical channel. When the transport is unicast, mediaChannel is not present in the OpenLogicalChannel forwardLogicalChannelParameters, but may be present in the reverseLogicChannelParameters.
Is it safe to assume, then, that I will send my media to you from the port 2500, or am I at liberty to send my RTP data to you using any port I choose? My understanding was that I was obligated to transmit media from port 2500, since I indicated that the RTCP port was 2501. Thanks,Paul