docs: function getSsrcAttribute (mLine, ssrc, attributeName) {
      return mLine
          .ssrcs
          .filter(function(ssrcInfo) { return ssrcInfo.id === ssrc; })
          .filter(function(ssrcInfo) { return ssrcInfo.attribute === attributeName; })
          .map(function(ssrcInfo) { return ssrcInfo.value; })[0];
  }