Re: [rtcweb] Multiplexing using the same port number for multiple media descritions

Magnus Westerlund <magnus.westerlund@ericsson.com> Wed, 31 August 2011 11:13 UTC

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Date: Wed, 31 Aug 2011 13:15:09 +0200
From: Magnus Westerlund <magnus.westerlund@ericsson.com>
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To: Harald Alvestrand <harald@alvestrand.no>
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Cc: "rtcweb@ietf.org" <rtcweb@ietf.org>
Subject: Re: [rtcweb] Multiplexing using the same port number for multiple media descritions
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On 2011-08-30 17:15, Harald Alvestrand wrote:
> On 08/30/11 13:21, Christer Holmberg wrote:

> In that case, we have an RTP spec problem, not just an SDP problem: RTP
> "straight" claims to identify sessions by destination address + port
> (RFC 3550 section 5.2, for instance). In normal unicast / point-to-point
> usage, we expect all packets to come from the same address + port too,
> but RFC 3550 doesn't say that.
> 
> I don't know if this is an issue. If it is an issue, sender might have
> to start over.
> 

Well, that is one way to id the session a flow is supposed to end up in.
 If I understand the situation you are proposing would still allow the
inviting party to use 5-tuples to identify flows coming into each
session. This as you will learn from the ICE processing each peers
visible source address for each session through the ICE plus signaling.

And I don't think we have a spec problem if we like to use a session
transport setup that looks like this.

Session 1
A:10000 <---> B:20000

Session 2
A:10000 <---> B:30000

As this provides different 5-tuples it does fulfill for example the
following from section 5.2 of RFC3550:

   For example, in a teleconference
   composed of audio and video media encoded separately, each medium
   SHOULD be carried in a separate RTP session with its own destination
   transport address.

At least if you interpret "own destination transport address" as the
full five tuple rather than a 3-tuple of destiantion address, port and
protocol.

Cheers

Magnus Westerlund

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