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

Derk-Jan Hartman hartman at videolan.org
Thu Apr 6 22:26:15 CEST 2006

On 6-apr-2006, at 21:19, Florian Lohoff wrote:
> On Thu, Apr 06, 2006 at 07:51:12PM +0200, Derk-Jan Hartman wrote:
>> On 6-apr-2006, at 17:51, Florian Lohoff wrote:
>>> Hi,
>>> i wrote a little streaming server myself which is capable of doing
>>> UDP or UDP/RTP Unicast/Multicast. Now as we have some Multicast
>>> difficulties concerning bandwidth i tried to go the path of sending
>>> SAP/SDP multicast and the datastream unicast via RTP/RTCP. I am
>>> streaming MPEG2-TS directly from an DVB source.
>> Did you read the RFCs ?
> When you mean reading like reading in a book then No. By looking up
> the relations between the protocols and protocol numbers Yes. RFC2327
> RFC3550 come to my mind having open most of the time. But you seem to
> have something more specific on your mind - probably willing to  
> share ?

Sorry, i started out a certain way in my mail, but discovered half  
way trough it that something else was actually going on.

>>> My first guess from looking at the source seems to be that VLC is  
>>> not
>>> capable of doing RTP/RTCP for RTP/AVP 33.
>> It was built for it.
> Okay - i cant find any reference to RTCP in the code except the  
> live.com
> module. As module/access/udp.c is the one trying to bind to the  
> port/ip
> address in the SAP/SDP and complaining about the servers address not
> beeing a local address and beeing able to bind to i would expect to  
> see
> RTCP mentioned there. It isnt but RTP is. So my assumption was that  
> tries to handle RTP/AVP 33 itself but failing to see the connection
> point not beeing a multicast address and even missing RTCP code for
> handling this.

VLC doesn't support RTCP server-side, and client-side, only with  
However with RTP MPEG-TS  RTCP isn't really useful either.

>> VLC assumes that any SAP originated SDP announce of a MPEG-TS stream
>> is multicast, yes. it being unicast is also very uncommon.
> It beeing uncommon i see. It beeing impossible or against any  
> standard/rfc
> i couldnt find. It would solve my problem with old Cat5500 not beeing
> able to handle multicast traffic as good (Not beeing able to handle
> multilayer multiprotocol switching concerning multicast, forwarding  
> all
> mcast traffic the route-switch engine and overflowing the 100MBit/s  
> link
> between layer 2 and layer 3 engine).

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.

You can simply enter rtp://:portnumber on the client and it should work.

Besides with unicast, the c= line should always be:
c=IN IP4
Not that VLC will figure it out in that case, but it is more correct.

>> Use --no-sap-preparse (or something) to avoid this behaviour.
> Could you specify the "or something" ?
> vlc --help --advanced --longhelp | egrep "sap|pre|parse"
> VLC media player 0.8.4 Janus
>             and that overrides previous settings.
>       --sap-flow-control, --no-sap-flow-control
>       --sap-interval <integer>   SAP announcement interval
>       --key-prev <integer>       Previous
>       --key-title-prev <integer> Select previous DVD title
>       --key-chapter-prev <integer>
>                                  Select prev DVD chapter
>                                  Select prev DVD chapter
> I see a ChangeLog entry mentioning "--sap-parse" but giving that
> doesnt change anything.

Seems like an old version of VLC. --no-sap-parse is the option in  
All  in all, i don't understand why you are doing things so  
difficult. Why use SAP at all, when you are simply unicasting?


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