As I know, a rtp (outgoing) packet generated by ccRTP sets padding bit following code.
===============================
  uint8 padding = total & 0x03;
  if ( padding ) {
    padding = 4 - padding;
    total += padding;
  }
  // now we know the actual total length of the packet
  buffer = new unsigned char[total];
  (reinterpret_cast<uint32>(getHeader())) = 0;
  getHeader()->version = CCRTP_VERSION;
  if ( padding ) {
    memset(buffer + total - padding,0,padding - 1);
    buffer[total - 1] = padding;
    getHeader()->padding = 1;
  } else {
    getHeader()->padding = 0;
  }
===============================

It's seems to be that some Multimedia player, such as "freeamp", can't accept/play padded rtp packet stream.
I hope to be added a function to set padding flag by USER.