title: WebRTC Chrome video is black, how to debug?
body: <p>I am trying to build a test app to test server SRTP decode-encode, in client, it create one PeerConnection, it provide the OFFER(actpass mode), server gives the ANSWER(passive mode).
  The test app will decrypt the received RTP data and re-encrypt and send back to client.
  I expected to see my local video echoed back, but the video is black.</p>
  
  <p>I want through the negotiation part, the remote SDP has been set correctly, the remote video is added and connected. I also captured the network traffic, I checked the STUN/DTLS parts are completed successfully. In the test app, I found the srtp decrypt has all success, and re-encrypt without error.</p>
  
  <p>How can I debug why it is still black? decode failure? I guess the SRTP decrypt is successful because if I give wrong SRTP data, it will print srtp failed in chrome_debug.log</p>