[AG-TECH] Mapping IP addresses to Venues

Jay Beavers jbeavers at microsoft.com
Thu Mar 21 18:50:40 CST 2002

Thanks for the explanation Bob.

I was under the impression from reading the RFC that they strongly
discouraged multiplexing audio and video within one stream / SSRC but
that running audio and video streams, each with a separate SSRC, on one
IP / Port was just fine.

I, too, await the blessed day of SSM.  Until then, multicast IP
availability looks like a problem to me.  You do have an estimate on
when SSM will be commonly available across the routers and nodes of the

-----Original Message-----
From: Robert Olson [mailto:olson at mcs.anl.gov] 
Sent: Thursday, March 21, 2002 2:30 PM
To: Jay Beavers; ag-tech at mcs.anl.gov
Subject: Re: [AG-TECH] Mapping IP addresses to Venues

>Why are audio and video sent on different IP address / port pairs for a

>single Virtual Venue?  Why do they not share a single IP/Port 
>address?  Doesn t the payloadtype field in RTP allow you to effectively

>discern between audio and video streams?

they don't share a single IP address because the network routes traffic 
based on IP address, not port. If we want to separate audio and video 
traffic as presented to different computers (which I typically do), the 
media need different ports.

Given that, they need different ports so that an application does not
to sort through the data on the RTP session to demultiplex the media
RTP RFC makes the point that it is not recommended to combine streams in

that manner).

>How are the IP/port allocated for the Virtual Venues?

The initial rooms had addresses allocated randomly (via the code from
the newer rooms are allocated sequentially out of ANL's GLOP space.

>For instance, we requested a static allocation of a block of multicast
>addresses from Internet2.  Since multicast traffic is scoped purely on
>address and since the amount of traffic associated with a Virtual Venue

>would generally degrade any other multicast application sharing the
>IP address but using a different port, it seems to me that each venue 
>should have a dedicated single IP address.

At this point we're not viewing multicast addresses as a limited
If need be in future, we can either dynamically allocate them on demand
of a pool, or use SSM to effectively partition the space.

>If you have a dedicated single IP address per venue, is there anything 
>wrong with following the RTP default ports of 5004 for RTP traffic and 
>5005 for RTCP traffic?

Yes, if you wanted to subscribe to traffic from multiple rooms at a time

(say in a Voyager server, or in a monitoring application). You would not
able to distinguish between the streams (based on the address anyway).


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