Okay, this is our final ICMP unreachable packet analysis for a little while,
so let's get after it.
This traceroute iteration is the final step in the traceroute process.
Of course, traceroute doesn't really know this at the
beginning, so it sends its usual UDP probes.
The ports used here are 33440 and 33442.
We extrapolated this in a previous clip knowing that we could
simply count by threes to figure this out.
Let's look into the packet details a bit more.
The ICMP type of this packet is 3, which is a generic destination
unreachable. Packets of this type have a variety of codes, which help
answer the question why is it unreachable. In this case, we see code
3, meaning port unreachable.
This unreachable is sent specifically when a TCP or UDP
Layer 4 port is closed on the target host,
meaning that the host has no processes listening on that port.
It's like sending back a gift from your friend because you didn't like it, to
use our postal service example once more. Like the TTL exceeded message, the
port unreachable encapsulates the UDP traceroute probe that caused the
unreachable to be generated in the first place.
Again, this is how the traceroute client marries up the probe to the reply,
creating a useful transit time measurement.
Note that the port unreachable always indicates the end of a traceroute.
Yes, it is true that routers in the path are not listening on port 33440 either.
However, since TTL expired in transit,
they don't actually look at the packet's contents, but instead just
say you didn't put enough postage on the package.
I'm not going to waste my time looking inside.
The end host doesn't have a postage problem per se since the
packet was received and processed. This message terminates the
traceroute process gracefully.
If traceroute never receives port unreachables,
then traceroute can only terminate by timing out, similar to
ICMP echo requests that go unanswered.
We will explore this phenomenon in our security discussions. Now that we've
broken traceroute down as much as we could, let's apply it to some
real‑life network problems to help Globomantics troubleshoot a performance
problem in Wired Brain Coffee's network. Check out the next video to learn this critical skill.