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PORTCP / RTP communication issue
 

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    <p>Unfortunately I'm still stuck with a little implementation of a RTP / RTCP communication to access my IP camera properly.</p> <p><em><strong>What do I want to do</em></strong></p> <p>The camera has an internal buffer which I want to read. So I communicate with the camera via RTSP and tell it to stream the data. When the camera went through the whole buffer, streaming will stop.</p> <p><em><strong>What do I have so far</em></strong></p> <ol> <li><p>A TCP connection which communicates via RTSP for the <code>DESCRIBE</code> / <code>SETUP</code> / <code>PLAY</code> Request ( RTSP ) to get the stream started. This connection <strong>must</strong> remain open while the Camera streams it's data. </p></li> <li><p>A Port on which I receive the data sent via RTP ( based on UDP ) - handling this is none of my concern, I even have absolutely no access to it, I just wanted to mention it for the sake of completeness.</p></li> <li><p>A UDP Socket which receives the RTCP <code>Sender Reports</code> / <code>Source Descriptions</code>. This is important since I don't know when the stream stops ( as mentioned in point 2, I can't just look when the streaming stopped ). On this Socket I read until the RTCP <code>Sender Report Goodbye</code> comes, which means streaming has finished. Then I can close the TCP Socket (from RTSP communication).</p></li> </ol> <p><em><strong>What is going wrong</em></strong></p> <p>It is working for small buffer sizes like 2MB or 4MB. I receive some Source Descriptions and then a <code>Goodbye</code>. But in my particular test case I wanted to use 16MB ( which is still less than half of what the camera is capable of ). I receive the Sender Reports but at some point ( always at around 8MB +/-300KB ) the camera just stops sending. </p> <p>Noteworthy is the fact that I can access the buffer via VLC without problems. I even looked at the communication ( RTSP and RTCP ) which is almost completely the same as in my application...the one thing missing I'm gonna mention below:</p> <p><em><strong>Possible Reasons</em></strong></p> <p>This is the part where I need your advise. </p> <p><em>Possibility: Lack of Receiver Reports</em> </p> <p>When streaming via VLC I noticed that there were some RTCP <code>Receiver Reports</code> sent from VLC to the camera ( cyclic like the <code>Sender Reports</code> ). So could it be that the camere expects at least one <code>Receiver Report</code> in a specific time ( or after a particular amount of bytes sent ) ? At the moment I can't think of any other reason.</p> <p><em><strong>Solution?</em></strong></p> <ul> <li><p>If we assume that the camera stops streaming because there are no <code>Receiver Reports</code> I'd like to know if there is a way to implement them without carrying to much information. I have already read some of the <a href="http://www.ietf.org/rfc/rfc3550.txt">RFC 3550</a> and I guess there is still a bunch of logic behind those Report messages. Now I actually don't <em>need</em> and so I also don't <em>want</em> to implement a complete RTCP protocol here. Would it be enough to send some <code>Receiver Report</code> Dummy frames so the camera just keeps on streaming? If so, how do they look?</p></li> <li><p>If it is not related to the lack of <code>Receiver Reports</code> and I just don't need them, what could be the reason then for the camera to stop streaming? Any suggestions? </p></li> </ul> <p><strong>Edit:</strong></p> <p>Okay I just managed to create some kind of Dummy <code>Receiver Report</code> and it seems to work ( I just could receive 12MB then I got the desired Goodbye )</p> <p>I just filled a 28Byte buffer and just used some values in the Header fields, meaning:</p> <pre><code>buffer[0] = 0x80; // Version 2 , Padding = false, Reception Count = 0 buffer[1] = 0xC9; // Packet Type 201 Receiver Report buffer[2] = 0x00; // Higher byte for total length buffer[3] = 0x06; // Lower byte for total length ( in 32 Bit words -&gt; 28 Byte ) </code></pre> <p>The rest of the buffer I just filled with zeros. Yeah I know it's just a hack and you should not tell your kids to program like this.</p> <p>Now my question changes a bit: Is this okay with this hack? It seems to work, but I'm still a bit concerned that my camera will use those dummy data and change configuration cause it interpets some weird stuff in it?</p>
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    1. COHave you considered using an existing RTP library that'd take care of this ?
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    2. CO@nos Of course this would be an alternative but I actually wanted to avoid the use of any library because I just want the stream to be kept alive - There is not even a need to parse anything from the RTCP ( expect if it's an Goodbye Message of course ). Second the whole application needs to be completely async so I'd like to manage all sockets etc. by myself. Afaik most libs will control their own sockets and connections. But if I really need to fill the Receiver Reports properly ( with some "complex" data ) I maybe really consider a lib
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    3. COHave you done a wireshark packet capture while the app is running? This will tell you exactly what is or isn't happening with your data.
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      1. USJonathan Henson
 

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