Ethereal-dev: Re: [Ethereal-dev] RTP or UDP?

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From: Guy Harris <gharris@xxxxxxxxxxxx>
Date: Sun, 31 Dec 2000 11:01:10 -0800
On Mon, Jan 01, 2001 at 01:35:18AM +0800, jonathan wrote:
>    I just captured some packets that are a conversation between 2 IP
> phones and was surprised to find that Ethereal showed these to be UDP
> rather than RTP. (I do realize that RTP rides on UDP). But isn't the main
> protocol RTP ?

It probably is, but Ethereal has no way of knowing that; RTP doesn't run
on a specific port number - some other protocol will be used to set up
the conversation, and some packet in another protocol, e.g.  RTSP, will
indicate that port X will be used.

Ethereal *will*, if it sees an RTSP packet that specifies that a given
port will be used for RTP, recognize that conversation as being RTP.

However, there's no such "conversation setup" traffic in your capture,
so Ethereal has no way of knowing that the traffic in question is RTP.