如何查找数据包类型

我正在使用toyvpn示例在android中实现一个vpn客户端。 我要检查通过隧道发送和接收的数据包的详细信息。我正在使用jpcap库来读取数据包,但我不知道数据包的协议是什么。我的问题是如何找到数据包的类型。

        // Packets to be sent are queued in this input stream.
        FileInputStream in = new FileInputStream(mInterface.getFileDescriptor());

        // Packets received need to be written to this output stream.
        FileOutputStream out = new FileOutputStream(mInterface.getFileDescriptor());

        // Allocate the buffer for a single packet.
        ByteBuffer packet = ByteBuffer.allocate(32767);
        // Read the outgoing packet from the input stream.
        int length = in.read(packet.array());
        if (length > 0) {
                // Write the outgoing packet to the tunnel.
                packet.limit(length);
                tunnel.write(packet);

                // i use ip packet of jpcap but i think it is wrong
                IPPacket ipPacket = new IPPacket(length,packet.array());
        }