[FFmpeg-devel] [PATCH] SAP muxer/demuxer

Luca Abeni lucabe72
Thu Oct 7 13:02:37 CEST 2010

Hi Martin,

On 10/07/2010 10:35 AM, Martin Storsj? wrote:
>> Quick question: by looking at the patches (patch 0005, in particular),
>> it seems to me that you send SAP announcements to the RTP destination
>> (at least, I get this impression when reading how sap->ann_fd is initialised).
>> What am I missing? As far as I remember, SAP used a well known multicast
>> address for announcements, and the RTP destination address could be a
>> different multicast address (this is what I've always seen) or an unicast
>> address...
> You're absolutely right. Changed it to send the announcement to the fixed
> destination IP now, unless a specific one is set on the url via
> ?announce_addr=<ip>.

Looks good, thanks.

>> So, you use connect() in order to obtain the source address of your RTP
>> packets? I am not sure if this is the best solution... Anyway, if other
>> people feel that this is ok I will not object.
> Do you have any other suggestion for finding out the address? Enumerating
> the local interface addresses and trying to guess which one the OS will
> use to send packets to this destination is way more error prone and
> clumsier IMO.

This (or asking the user to explicitly specify the output interface) is
the approach I generally used... As I said, I am not sure about what's
the best solution. If other people are ok with connect(), I do not object.

>> And it seems to me that your patch compiles it when the SDP demuxer is
>> enabled?
> Yes - this code is called from the codepaths that is shared between the
> SDP demuxer, RTSP demuxer and RTSP muxer. So even if it actually is used
> only by the RTSP muxer, it is needed to avoid linking failures when
> compiling any of the other, too.

I think some proper #ifdeffery would solve this problem, but #ifdefs do
not seem to be too trendy, so I leave this discussion to someone else ;-)

> And both the RTSP demuxer and muxer
> declare the SDP demuxer as one of its dependencies, so this way, there
> should be no linking error regardless of which of them is
> enabled/disabled.

Ok, this works... But having the SDP demuxer as a dependency for the RTSP
muxer looks quite strange to me...

> ffmpeg -re -i<input>  -vcodec mpeg4 -acodec libfaac -f sap
> sap://
> (Now the announcement is sent to the fixed address, but the
> packets are sent to
> You'll see that the stream pops up under Network streams (SAP) in VLC. If
> I try the same without same_port=1, I get this message in the VLC message
> console:
>> sap debug: Multiple media ports not supported ->  live555

Ok, so this is a known vlc/live555 issue... I think your muxer is ok, then.


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