Subject: Re: [AVT] RE: RTP/RTCP Port Sharing (Was: Comedia Issues.)

From: Ross Finlayson <[email-masked]>
Date: Mon, 16 Jul 2001 23:13:07 -0700

At 09:20 PM 7/16/01, Jonathan Rosenberg wrote:
>The specific problem I'm running into is the lifetime of NAT bindings. Right
>now, RTP and RTCP will both require a separate binding. NAT bindings are
>kept alive by activity. Unfortunately, if silence suppression is used, the
>RTP nat binding will timeout, and the media is not recovered.

Out of curiosity, what kind of timeouts do these NAT bindings typically have?

As is typical when NATs are involved, there's probably no 'clean' solution 
to this problem, but I wonder if it might be better to try to address the 
specific problem of RTP (in)activity, rather than trying to kludge around 
this by putting RTCP on the same port?  In particular, the RTP sender could 
be told (e.g., though yet-another-new-SDP-attribute) to ensure that it 
sends some data (at the very least, an 'empty' frame) every 'n' 
seconds.  This has its own difficulties, of course - e.g., the fact that 
'empty frame' might be codec-specific.  And like your proposed solution, it 
requires that existing RTP implementations be modified (unless they support 
only payload formats that always have continuous data).

>Now, you will immediately say that this is why you need an ALG

No, actually, I agree with your argument against requiring ALG support.

         Ross.

---

From: Henning Schulzrinne <[email-masked]>
Date: Tue, 17 Jul 2001 08:07:04 -0400

In reality, is this lack of RTP packets a real problem? Any decent audio
codec doesn't just stop sending packets, it sends infrequent 'silence'
packets updating the background noise shape, to avoid strange
transitions of the background noise when speech starts again. See G.729B
and others. The frequency of these packets is probably on the order of
at least once per second, although I haven't measured this.

I can see that this might be an issue when a stream is being put on
hold, but maybe some periodic zero-payload-length RTP packets would be
an easy solution for that. No need to define new packet types.

---

From: "Muhammad Mukarram Bin TARIQ" <[email-masked]>
Date: Tue, 17 Jul 2001 15:09:21 -0700

Hi all,

Just another thought regarding the NAT binding timeout issues.
What will happen if a stream is restarted after a "hold" (which can be a
fairly long period of inactivity) You can be almost sure that the NAT
binding will timeout. Under such circumstances we may even need to tell the
new port once "hold" is taken back.

Muhammad Mukarram Bin Tariq
------------------------------------------
DoCoMo Communications Laboratories USA, Inc.
Tel: 408-451-4739, Fax: +1-408-573-1090
Email: [email-masked]

----- Original Message -----
From: "Henning Schulzrinne" <[email-masked]>
To: "Ross Finlayson" <[email-masked]>
Cc: "Jonathan Rosenberg" <[email-masked]>; <[email-masked]>;
<[email-masked]>
Sent: Tuesday, July 17, 2001 5:07 AM
Subject: Re: [AVT] RE: RTP/RTCP Port Sharing (Was: Comedia Issues.)