说明：  LWIP是TCP/IP协议栈的一种实现。LWIP的主要目的是减少存储器利用量和代码尺寸，使LWIP适合应用于小的、资源有限的处理器如嵌入式系统。为了减少处理器和存储器要求，lwIP可以通过不需任何数据拷贝的API进行裁减。
(LWIP is an implementation of the TCP/IP protocol stack. LWIP main purpose is to reduce the amount of memory utilization and code size, make LWIP suitable for small processors with limited resources, such as embedded systems. In order to reduce the processor and memory requirements lwIP cuts through the API without any data copies.)