/**
   * Returns the SSRC paired with <tt>ssrc</tt> in an FID source-group, if any. If none is found,
   * returns -1.
   *
   * @return the SSRC paired with <tt>ssrc</tt> in an FID source-group, if any. If none is found,
   *     returns -1.
   */
  private long getPrimarySsrc(RawPacket pkt) {
    MediaStreamTrackReceiver receiver = channel.getStream().getMediaStreamTrackReceiver();

    if (receiver == null) {
      if (logger.isDebugEnabled()) {
        logger.debug("Dropping an incoming RTX packet from an unknown source.");
      }
      return -1;
    }

    RTPEncoding encoding = receiver.resolveRTPEncoding(pkt);
    if (encoding == null) {
      if (logger.isDebugEnabled()) {
        logger.debug("Dropping an incoming RTX packet from an unknown source.");
      }
      return -1;
    }

    return encoding.getPrimarySSRC();
  }
  /**
   * Removes the RTX encapsulation from a packet.
   *
   * @param pkt the packet to remove the RTX encapsulation from.
   * @return the original media packet represented by {@code pkt}, or null if we couldn't
   *     reconstruct the original packet.
   */
  private RawPacket deRtx(RawPacket pkt) {
    boolean success = false;

    if (pkt.getPayloadLength() - pkt.getPaddingSize() < 2) {
      // We need at least 2 bytes to read the OSN field.
      if (logger.isDebugEnabled()) {
        logger.debug("Dropping an incoming RTX packet with padding only: " + pkt);
      }
      return null;
    }

    long mediaSsrc = getPrimarySsrc(pkt);
    if (mediaSsrc != -1) {
      if (rtxAssociatedPayloadType != -1) {
        int osn = pkt.getOriginalSequenceNumber();
        // Remove the RTX header by moving the RTP header two bytes
        // right.
        byte[] buf = pkt.getBuffer();
        int off = pkt.getOffset();
        System.arraycopy(buf, off, buf, off + 2, pkt.getHeaderLength());

        pkt.setOffset(off + 2);
        pkt.setLength(pkt.getLength() - 2);

        pkt.setSSRC((int) mediaSsrc);
        pkt.setSequenceNumber(osn);
        pkt.setPayloadType(rtxAssociatedPayloadType);
        success = true;
      } else {
        logger.warn(
            "RTX packet received, but no APT is defined. Packet "
                + "SSRC "
                + pkt.getSSRCAsLong()
                + ", associated media"
                + " SSRC "
                + mediaSsrc);
      }
    }

    // If we failed to handle the RTX packet, drop it.
    return success ? pkt : null;
  }