Subject: [Ietf-behave] draft-ietf-behave-nat-00: RTP behavior description

From: Magnus Westerlund <[email-masked]>
Date: Thu, 28 Oct 2004 07:27:17 -0700 (PDT)

Hi,

In Section 3.2, there is a talk about port parity preserving.

- First I think the description on what is required by RTP is a bit 
unclear. I think it should be clarified that for RTP sessions that do 
not have explicit signalling of RTCP ports, the RTP ports needs to use 
an even port and RTCP the next port number, which is odd. Thus simple 
preserving the parity is not sufficient as this could result in RTP 
port: 2040 -> 4126 RTCP port: 2041 -> 4789 which would still not help 
the RTP based application to get working RTCP traffic.

- I am lacking experience from deployed scenarios, do the parity 
preservation actually help? Also is there any other UDP user that 
benefit from this? I trying to understand if REQ-4 actually provide any 
benefit.

However I do agree with the previous discussion that trying to do 
something smarter would be difficult and probably create worse problems. 
The main argument is that transmission of RTP and RTCP are not closely 
dependent, and the either could proceed the other with at least seconds, 
up to minutes or never in cases when a receiver do only later or not at 
all transmit RTP packets.

Cheers

Magnus Westerlund
AVT Chair
Multimedia Technologies, Ericsson Research EAB/TVA/A
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