[vlc-devel] Re: Unicast RTP/RTCP - SDP RTP/AVP 33

Florian Lohoff flo at rfc822.org
Thu Apr 6 23:31:51 CEST 2006

On Thu, Apr 06, 2006 at 10:26:15PM +0200, Derk-Jan Hartman wrote:
> VLC doesn't support RTCP server-side, and client-side, only with  
> liveMedia.
> However with RTP MPEG-TS  RTCP isn't really useful either.

So how to do you tell the server side as a client about you willing to
receive an RTP stream. AFAIK you would need to send an ReceiverReport
to start out the stream.

> It's not that it's not allowed, it's just VERY rare case that you are  
> presenting here.
> The reason is that with unicast, a SAP announce just isn't that  
> useful, since you are sending to ONE machine. In that case sending a  
> multicasted announce of something like that is not logical.

I am announcing a stream source by multicast although the stream source
is only reachable by unicast. Id like SAP/SDP receiving clients to request
the stream by sending an RR to the in the SDP advertised server. 

As a Demo i set the RTP/AVP media type to 31 which whould H261. This
causes VLC to hand off the media connect to the liblivecom library. This
causes the lib to send out an RR and thus my stream app to receiven an
RR and start streaming to the client. I can verify this behaviour with
ethereal. Seeing the SAP/SDP, the RR from VLC to my little app, the
the RTP stream to VLC and the repeating RRs.

> All  in all, i don't understand why you are doing things so  
> difficult. Why use SAP at all, when you are simply unicasting?

I'd like to broadcast TV programs into our companys network
infrastructure. I tried doing so with simple MPEG-TS in UDP via Multicast
including SAP/SDP. This worked quite well except i killed the core network
infrastructure, some Cisco Cat5500 with an Layer 3 RouteSwitch Module.
Typically the RSM feeds the layer2 switch with mls (MultiLayerSwitching)
informations about the ip routing short-paths. This does not work reliably
with multicast traffic. I causes once in a while all multicast traffic
go through the control channel to the RSM. This interconnect within
the switch is only 100Mbit/s. For unicast traffic this does not present
any problem as the RSM and the switch correctly speak MLS and install
routing short-paths in the layer2 fabric.
So after killing the Cat5500 with about 150MBit/s multicast traffic in
~40 Groups and nobody beeing able to work, or even call anyone (VOIP)
i had to stop doing so ;)

My solution now would be to do SAP/SDP by Multicast, very few packets but
distributed in the correct domain. Then advertise the TV Streaming boxes
and letting the clients connect via RTCP/RTP. I'd like to use SAP/SDP as
it would allow people to simply open VLC and point-and-click the right
TV/Radio channel. As i dont expect many clients this would be okay.

I agree that with unicast with predefined destinations RTCP or SAP/SDP
does not make any sense. But i'd like to offer streams via SAP/SDP to
clients which then may decide to join by sending an RTCP and as a result
get the RTP stream which imho should be perfectly valid usage.

Florian Lohoff                  flo at rfc822.org             +49-171-2280134
                        Heisenberg may have been here.
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