This document is organized in a top down manner from the point of view of the PXE client “boot behavior”. 这份是网卡启动 PXE 启动的Spec，详细讲解了网卡legacy 启动（PXE boot）的流程和协议。

This document specifies the Preboot Execution Environment (PXE). PXE embodies three technologies that will establish a common and consistent set of pre-boot services within the boot firmware of Intel Architecture systems.