Subject: Re: port selection problem (was Re: RTSP and SDP with container files)

From: Rob Lanphier <[email-masked]>
Date: Mon, 23 Jun 1997 17:43:39 -0700

At 10:37 AM 6/23/97 -0700, Alagu Periyannan wrote:
>A better solution is to have a 0 in the port number field
>in the SDP file and negotiate the port number over RTSP.
>To do this we may need to add "cport=" and "sport="
>fields to the Transport header instead of the current
>"port=" field.

Agreed.  We (PN) quite frankly haven't put a lot of thought into the RTCP
problem yet, and so I don't have a quick answer for how the RTCP should be
handled (your suggestion sounds as good as any).  However, I'm definitely
in favor of getting transport issues out of the media description, since I
would hope that firewalls wouldn't have to parse different media
description formats to get the essential transport parameters.

>On a related subject, how does the server get the client's
>IP address in the current scheme? Again this is not a problem
>for the multicast case since a class D address is used.
>But what about for the unicast case?

In the unicast cast, the destination is assumed to be the initiator of the
RTSP connection most of the time.  In the Transport: header field, there is
a way of overriding this default using:

Transport: rtp/udp;port=2344;destination=123.123.123.123

This must be used with great care, though, because this may allow a client
to use a server as a UDP-denial-of-service tool (redirecting large streams
from several servers to an unsuspecting destination).

Rob
---
Rob Lanphier               Voice: (206)674-2322         Fax: (206)674-2699
Program Manager-Protocols                         Email: [email-masked]
Progressive Networks-Home of RealAudio            Web: http://www.real.com
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