ICMP

Internet Control Message Protocol - Wikipedia, the free encyclopedia

It is chiefly used by networked computers' operating systems to send error messages—indicating, for instance, that a requested service is not available or that a host or router could not be reached.

ICMP differs in purpose from TCP and UDP in that it is usually not used directly by user network applications. One exception is the ping tool, which sends ICMP Echo Request messages (and receives Echo Response messages) to determine whether a host is reachable and how long packets take to get to and from that host.

Ping Tunnel - Send TCP traffic over ICMP - 應用場合像是pulic需要登入才能使用的無線網路，如果沒有檔ping的話，更甚者還有利用ptt的chatroom來達成。