The Problem

When the Internet is bad you want to know where its broken.  

If there are high latencies you want to ping to each hop along the way.

Current protocols have limitations.

multiple packets
things change
false routes

can't ICMP to points along the path
ICMP is handled by the central CPU on many routers
path from the router back to the source may be different
want to measure both forward and reverse paths

matching interfaces to routers