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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt">We recently ran into something regarding RTCP – Generic NACK Feedback,</span><span lang="EN-US"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt">which was introduced with
<a href="https://issues.asterisk.org/jira/browse/ASTERISK-27810">https://issues.asterisk.org/jira/browse/ASTERISK-27810</a></span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt">Me and my colleague read through RFC 4585 Section 6.2.1 “Generic NACK” but we didn’t really find an answer to our question,</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt">which is the following:</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt">Asterisk sends Receiver Reports on the RTCP port (RTP+1), whereas</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt">the NACK is sent on the RTP port which confuses the other end of our system (rtpengine).</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt">Is Generic NACK supposed to be sent to the RTP port or isn’t it supposed to be sent on the RTCP port (which would make this a bug in the current implementation)?</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt">Maybe you guys can enlighten us somehow.</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt">Since this is not configurable we had to patch this out on the Asterisk side for now, but this is just a very dirty workaround.</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt">I have copied the relevant infos from the wireshark capture below</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt">RTCP Receiver report to RTCP port (41971):</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> </span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">Frame 19635: 124 bytes on wire (992 bits), 124 bytes captured (992 bits)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">Linux cooked capture</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">Internet Protocol Version 4, Src: 172.17.217.9, Dst: 172.17.217.10</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">User Datagram Protocol, Src Port: 11729 (11729), Dst Port: 41971 (41971)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">Real-time Transport Control Protocol (Receiver Report)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> [Stream setup by SDP (frame 1)]</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> 10.. .... = Version: RFC 1889 Version (2)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> ..0. .... = Padding: False</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> ...0 0001 = Reception report count: 1</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Packet type: Receiver Report (201)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Length: 7 (32 bytes)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Sender SSRC: 0x4ef37590 (1324578192)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Source 1</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">Real-time Transport Control Protocol (Source description)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> [Stream setup by SDP (frame 1)]</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> 10.. .... = Version: RFC 1889 Version (2)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> ..0. .... = Padding: False</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> ...0 0001 = Source count: 1</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Packet type: Source description (202)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Length: 11 (48 bytes)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Chunk 1, SSRC/CSRC 0x4EF37590</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> [RTCP frame length check: OK - 80 bytes]</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt">RTCP Receiver Report with Generic NACK to RTP port (41970):<br>
<br>
<br>
</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">Frame 20013: 92 bytes on wire (736 bits), 92 bytes captured (736 bits)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">Linux cooked capture</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">Internet Protocol Version 4, Src: 172.17.217.9, Dst: 172.17.217.10</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">User Datagram Protocol, Src Port: 11729 (11729), Dst Port: 41970 (41970)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">Real-time Transport Control Protocol (Receiver Report)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> 10.. .... = Version: RFC 1889 Version (2)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> ..0. .... = Padding: False</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> ...0 0001 = Reception report count: 1</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Packet type: Receiver Report (201)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Length: 7 (32 bytes)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Sender SSRC: 0x4ef37590 (1324578192)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Source 1</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">Real-time Transport Control Protocol (Generic RTP Feedback): NACK: 1 frames lost</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> 10.. .... = Version: RFC 1889 Version (2)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> ..0. .... = Padding: False</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> ...0 0001 = RTCP Feedback message type (FMT): Generic negative acknowledgement (NACK) (1)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Packet type: Generic RTP Feedback (205)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Length: 3 (16 bytes)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Sender SSRC: 0x4ef37590 (1324578192)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> Media source SSRC: 0x42d0de62 (1120984674)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> RTCP Transport Feedback NACK PID: 8089</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> RTCP Transport Feedback NACK BLP: 0x0000 (No additional frames lost)</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> [RTCP frame length check: OK - 48 bytes]</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="font-family:"Times New Roman",serif;mso-fareast-language:DE"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-US" style="font-family:"Times New Roman",serif;mso-fareast-language:DE"> </span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE">With best regards</span><o:p></o:p></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black;mso-fareast-language:DE"><br>
<b>Florian Floimair<br>
</b>Innovation - Software-Development<br>
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<b>COMMEND INTERNATIONAL GMBH<br>
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