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

Derk-Jan Hartman hartman at videolan.org
Fri Apr 7 03:46:45 CEST 2006

On 6-apr-2006, at 23:31, Florian Lohoff wrote:
> 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.

Seems to me that what you are looking for is actually RTSP..

>> 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

Apparently liveMedia is tolerant here.

>> 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 ;)

or buy a beter switch of course :D

> 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.

Then you should use RTSP.
It's what it was designed to do.


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