So the major workhorse protocols, as we’ve talked about, is ICMP, which is, you know, people know it. We talked about how that is used, for example, in the traceroute command, and how that works to allow you to trace the route that packets take from going from you to some designated remote. And, for example, you know, so an ICMP time-exceeded message is sent back when a packet expires on the Internet, back to its apparent sender, to notify that sender that for whatever reason it didn’t get there. Similarly, the ping command is another example of ICMP. The ping is sort of a human version of the ICMP called “echo request.” And so what that does is, when you ping a remote IP, basically you’re sending an ICMP echo-request packet to that IP. Any routers along the way just forward it on, as long as it hasn’t expired yet. When it gets to the router that it’s addressed to, or the computer, the computer, if it’s configured normally, will send back in response to an echo request, sends an echo reply, which is just a ping reply. And this is used, again, by network engineers and just, you know, regular users who know about this, just to see whether the machine is up at the other end or connected. Is there a path between me and that other IP? So it’s sort of another sort of fundamental plumbing layer which is very useful for making sure that, you know, the Internet and networking stuff is working.