[juancb]

And on the active networking component side of things it doesn't touch on MPLS which also doesn't modify the IP headers. You can enter a network in New York and get MPLS switched across the country via active network devices all the way to California and have it show up as a single hop on traceroute.

The explanation is great for a toy network bu in today's Internet the vast majority of routes are going to be asymmetrical and that requires running traceroutes from both ends and interpreting the results to find the faulty hop.

The author also doesn't cover equal cost multipath (ECMP) which is everywhere. With ECMP you have multiple ports that lead to the same. Next hop and packets are hashed based on some part of the fourtuple, sometimes five tuple including the input Port. In order to track down the faulty link, you need to pro each and every one of the ports which requires that you use a higher level protocol like UDP. Using icmp in this case will not show you an issue some percent of time, providing false negatives which makes it less useful.