/**
   * Retransmits a packet to {@link #channel}. If the destination supports the RTX format, the
   * packet will be encapsulated in RTX, otherwise, the packet will be retransmitted as-is.
   *
   * @param pkt the packet to retransmit.
   * @param after the {@code TransformEngine} in the chain of {@code TransformEngine}s of the
   *     associated {@code MediaStream} after which the injection of {@code pkt} is to begin
   * @return {@code true} if the packet was successfully retransmitted, {@code false} otherwise.
   */
  public boolean retransmit(RawPacket pkt, TransformEngine after) {
    boolean destinationSupportsRtx = rtxPayloadType != -1;
    boolean retransmitPlain;

    if (destinationSupportsRtx) {
      long rtxSsrc = getRtxSsrc(pkt);

      if (rtxSsrc == -1) {
        logger.warn(
            "Cannot find SSRC for RTX, retransmitting plain. " + "SSRC=" + pkt.getSSRCAsLong());
        retransmitPlain = true;
      } else {
        retransmitPlain = !encapsulateInRtxAndTransmit(pkt, rtxSsrc, after);
      }
    } else {
      retransmitPlain = true;
    }

    if (retransmitPlain) {
      MediaStream mediaStream = channel.getStream();

      if (mediaStream != null) {
        try {
          mediaStream.injectPacket(pkt, /* data */ true, after);
        } catch (TransmissionFailedException tfe) {
          logger.warn("Failed to retransmit a packet.");
          return false;
        }
      }
    }

    return true;
  }
  /**
   * Encapsulates {@code pkt} in the RTX format, using {@code rtxSsrc} as its SSRC, and transmits it
   * to {@link #channel} by injecting it in the {@code MediaStream}.
   *
   * @param pkt the packet to transmit.
   * @param rtxSsrc the SSRC for the RTX stream.
   * @param after the {@code TransformEngine} in the chain of {@code TransformEngine}s of the
   *     associated {@code MediaStream} after which the injection of {@code pkt} is to begin
   * @return {@code true} if the packet was successfully retransmitted, {@code false} otherwise.
   */
  private boolean encapsulateInRtxAndTransmit(RawPacket pkt, long rtxSsrc, TransformEngine after) {
    byte[] buf = pkt.getBuffer();
    int len = pkt.getLength();
    int off = pkt.getOffset();

    byte[] newBuf = new byte[len + 2];
    RawPacket rtxPkt = new RawPacket(newBuf, 0, len + 2);

    int osn = pkt.getSequenceNumber();
    int headerLength = pkt.getHeaderLength();
    int payloadLength = pkt.getPayloadLength();

    // Copy the header.
    System.arraycopy(buf, off, newBuf, 0, headerLength);

    // Set the OSN field.
    newBuf[headerLength] = (byte) ((osn >> 8) & 0xff);
    newBuf[headerLength + 1] = (byte) (osn & 0xff);

    // Copy the payload.
    System.arraycopy(buf, off + headerLength, newBuf, headerLength + 2, payloadLength);

    MediaStream mediaStream = channel.getStream();
    if (mediaStream != null) {
      rtxPkt.setSSRC((int) rtxSsrc);
      rtxPkt.setPayloadType(rtxPayloadType);
      // Only call getNextRtxSequenceNumber() when we're sure we're going
      // to transmit a packet, because it consumes a sequence number.
      rtxPkt.setSequenceNumber(getNextRtxSequenceNumber(rtxSsrc));
      try {
        mediaStream.injectPacket(rtxPkt, /* data */ true, after);
      } catch (TransmissionFailedException tfe) {
        logger.warn("Failed to transmit an RTX packet.");
        return false;
      }
    }

    return true;
  }