/**
   * 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 = channel.getRtxPayloadType() != -1;
    boolean retransmitPlain;

    if (destinationSupportsRtx) {
      long rtxSsrc = getPairedSsrc(pkt.getSSRC());

      if (rtxSsrc == -1) {
        logger.warn("Cannot find SSRC for RTX, retransmitting plain.");
        retransmitPlain = true;
      } else {
        retransmitPlain = !encapsulateInRtxAndTransmit(pkt, rtxSsrc);
      }
    } 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;
  }
 /**
  * Tries to find an SSRC paired with {@code ssrc} in an FID group in one of the channels from
  * {@link #channel}'s {@code Content}. Returns -1 on failure.
  *
  * @param ssrc the SSRC for which to find a paired SSRC.
  * @return An SSRC paired with {@code ssrc} in an FID group, or -1.
  */
 private long getPairedSsrc(long ssrc) {
   RtpChannel sourceChannel = channel.getContent().findChannelByFidSsrc(ssrc);
   if (sourceChannel != null) {
     return sourceChannel.getFidPairedSsrc(ssrc);
   }
   return -1;
 }
  /** Implements {@link PacketTransformer#transform(RawPacket[])}. {@inheritDoc} */
  @Override
  public RawPacket transform(RawPacket pkt) {
    byte rtxPt;
    if (pkt != null
        && (rtxPt = channel.getRtxPayloadType()) != -1
        && pkt.getPayloadType() == rtxPt) {
      pkt = handleRtxPacket(pkt);
    }

    return pkt;
  }
  /**
   * Handles an RTX packet and returns it.
   *
   * @param pkt the packet to handle.
   * @return the packet
   */
  private RawPacket handleRtxPacket(RawPacket pkt) {
    boolean destinationSupportsRtx = channel.getRtxPayloadType() != -1;
    RawPacket mediaPacket = createMediaPacket(pkt);

    if (mediaPacket != null) {
      RawPacketCache cache = channel.getStream().getPacketCache();
      if (cache != null) {
        cache.cachePacket(mediaPacket);
      }
    }

    if (destinationSupportsRtx) {
      pkt.setSequenceNumber(
          getNextRtxSequenceNumber(pkt.getSSRC() & 0xffffffffL, pkt.getSequenceNumber()));
    } else {
      // If the media packet was not reconstructed, drop the RTX packet
      // (by returning null).
      return mediaPacket;
    }

    return pkt;
  }
  /**
   * Creates a {@code RawPacket} which represents the original packet encapsulated in {@code pkt}
   * using the RTX format.
   *
   * @param pkt the packet from which to extract a media packet.
   * @return the extracted media packet.
   */
  private RawPacket createMediaPacket(RawPacket pkt) {
    RawPacket mediaPacket = null;
    long rtxSsrc = pkt.getSSRC() & 0xffffffffL;

    // We need to know the SSRC paired with rtxSsrc *as seen by the
    // receiver (i.e. this.channel)*. However, we only store SSRCs
    // that endpoints *send* with.
    // We therefore assume that SSRC re-writing has not introduced any
    // new SSRCs and therefor the FID mappings known to the senders
    // also apply to receivers.
    RtpChannel sourceChannel = channel.getContent().findChannelByFidSsrc(rtxSsrc);
    if (sourceChannel != null) {
      long mediaSsrc = sourceChannel.getFidPairedSsrc(rtxSsrc);
      if (mediaSsrc != -1) {
        byte apt = sourceChannel.getRtxAssociatedPayloadType();
        if (apt != -1) {
          mediaPacket = new RawPacket(pkt.getBuffer().clone(), pkt.getOffset(), pkt.getLength());

          // Remove the RTX header by moving the RTP header two bytes
          // right.
          byte[] buf = mediaPacket.getBuffer();
          int off = mediaPacket.getOffset();
          System.arraycopy(buf, off, buf, off + 2, mediaPacket.getHeaderLength());

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

          mediaPacket.setSSRC((int) mediaSsrc);
          mediaPacket.setSequenceNumber(pkt.getOriginalSequenceNumber());
          mediaPacket.setPayloadType(apt);
        }
      }
    }

    return mediaPacket;
  }
  /**
   * 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.
   * @return {@code true} if the packet was successfully retransmitted, {@code false} otherwise.
   */
  private boolean encapsulateInRtxAndTransmit(RawPacket pkt, long rtxSsrc) {
    byte[] buf = pkt.getBuffer();
    int len = pkt.getLength();
    int off = pkt.getOffset();
    byte[] newBuf = buf;
    if (buf.length < len + 2) {
      // FIXME The byte array newly allocated and assigned to newBuf must
      // be made known to pkt eventually.
      newBuf = new byte[len + 2];
    }

    int osn = pkt.getSequenceNumber();
    int headerLength = pkt.getHeaderLength();
    int payloadLength = len - headerLength;
    System.arraycopy(buf, off, newBuf, 0, headerLength);
    // FIXME If newBuf is actually buf, then we will override the first two
    // bytes of the payload bellow.
    newBuf[headerLength] = (byte) ((osn >> 8) & 0xff);
    newBuf[headerLength + 1] = (byte) (osn & 0xff);
    System.arraycopy(buf, off + headerLength, newBuf, headerLength + 2, payloadLength);
    // FIXME We tried to extend the payload of pkt by two bytes above but
    // we never told pkt that its length has increased by these two bytes.

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

    return true;
  }