I'll cover all three traceroute packets of interest in one clip since
this is similar to IPv4. First, the UDP probe. The destination port
numbers still start at 33434 as the server is not listening on these
UDP ports for any specific application services. Notice the
increasing hop limits too.
Sending traffic to a known closed port should trigger a porch
unreachable from the target, which we'll explore in a moment.
The probe carries no payload data as it is wholly unnecessary.
Overall,
not much new with his UDP probe for IPv6. Now let's explore the hop limit
exceeded message. I mentioned earlier how the concept of TTL still exists
for IPv6, but is now known as hop limit.
I personally prefer this new name as it is more self‑explanatory. When
the hop limit reaches 0 in transit, the message is called the hop limit
exceeded message, which has type 3 and code 0.
The hop limit exceeded message also encapsulates the UDP probe that
expired in transit to help the sender marry up the ICMPv6 message
with the UDP probe that caused it. Again, not much new here other
than the name and the type code changes. Last, the port unreachable
IPV6 message.
This message has type 1, which represents a generic destination
unreachable message. Code 4 specifies that it was a TCP or UCP port
unreachable specifically, which makes sense since the UDP probe reached
the target and was rejected. The final target returns this unreachable to
signal to the sender that it was not listening on the bogus destination
port used by traceroute.
And, as expected,
this unreachable encapsulates the original UDP probe that caused the
unreachable to be generated. This whole process and its component messages are very consistent with IPv4 so you can breathe easy.