Subject: Re: The argument for writing a general purpose NAT for IPv6

From: Magnus Westerlund <[email-masked]>
Date: Mon, 14 May 2007 16:53:35 +0200

With my individual hat on. Which is one that participated in the work to 
update RTSP for a few years now.

james woodyatt skrev:
> On Apr 19, 2007, at 12:47, Iljitsch van Beijnum wrote:
>> On 19-apr-2007, at 19:44, james woodyatt wrote:
>>>
>>> An RTSP server may *also* put any pair of UDP port numbers (starting 
>>> on an even number) in the client_port attribute of a Transport 
>>> header, so the ports that would have to be opened to the whole 
>>> Internet include a very wide range, potentially *all* of them.
>>
>> That seems strange. Wouldn't the client want to have some say in the 
>> port number it needs to open up?
> 
> I wasn't involved in the design of the RTSP protocol.

This is a misunderstanding of how RTSP works. What happens when one 
configures a media stream is that the client includes the ports and 
address on which it like to receive the media stream. The server do add 
an address and port(s), the one it is going to send from.

When it comes to RTSP proxies they can select if they relay the media 
streams or not. But unless they actually are implemented in the 
FW/NAT/NAP they do not really provide any benefit the application to 
traverse the middlebox with filtering behavior. And I have to agree 
strongly that ALG functions only should be deployed in managed boxes.

Regards

Magnus Westerlund

IETF Transport Area Director & TSVWG Chair
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