1.1.d (iv) IPv4 TTL and IPv6 Hop Limit

This section will be super short will just discuss the IPv4 Time to live field, and the IPv6 Hop Limit field. Both are very similar.

The TTL field as described in 1.1.d (ii) starts off at 255. It decrements once every time a router receives an IP packet to forward.
Routers may choose not to decrement the TTL if configured to do so. This field is critical for preventing routing loops as no other mechanism
exists for stopping such scenarios. Once a packet’s TTL hits 1, a routing receiving that packet will immediately drop it and generate an
ICMP Type 11 Code 0, time to live exceeded in transit. Regardless if the original IP source receives this message or not the routing loop has been
blocked.

This same behavior is seen in IPv6, except the headers and messages have changed. Instead of the Time To Live field, we now have a Hop Limit field
in the IPv6 header.

We also have the ICMPv6 time exceeded message, which is an ICMPv6 Type 3 Code 0 message.
I find that they are reusing Type and Codes quite annoying as it makes remembering these more difficult. We have an infinite list of numbers to choose from
and they had to re-use the same numbers.