RFC上面说"ICMP, uses the basic support of IP as if it were a higher level protocol, however, ICMP is actually an integral part of IP, and must be implemented by every IP module."(ICMP,使用IP的基本支持,好像它是一个更高的级协议,然而,ICMP 实际上是 IP 的一个组成部分,并且必须由每个 IP 模块实现。) 它也是网络层的协议之一,根本不是高层协议 :(