Question

Traceroute is an application to trace the path from A to B. (A is your location and B is the server you want to trace). In Windows, you can type tracert. The main algorithm is:

send UDP with TTL = 1
Server A1 received, and return ICMP packet to A because TTL is expired.
--> know first machine between. For example A1.

send UDP with TTL = 2
Server A1 received, and send this UDP to server A2.
Server A2 received, and return ICMP packet to A because TTL is expired
--> know second machine between. In this example is A2.

Do it until to B. we can track down: A -> A1 -> A2 -> ... ->B

Does this algorithm work correctly? Because at different time, an intermediate server can send a message to different server. For example, at first time, UDP message is sent to A1, but at a later time, it can send to another server, for example, B1. So, trace route will not work properly.

Did I misunderstand something?

Answer 1

From the man page :

traceroute tracks the route packets take from an IP network on
their
way to a given host

So if you are trying to find one of the possible paths your packet may take, you’ll find a friend in traceroute .

Now because routing tables do not change every minute, the packets that you send will most probably take the same path as traced by traceroute.

Another important point that cannot be missed is the record route option in the IP v4 header.
Once you specify that you want to use this option, every router in the path will add it’s ip address to the options in the header. You can read more about it here. The catch being that the destination gets to know about the intermediate hops , not the source.

I see that you missed the role of icmp echo request and reply messages in the description of traceroute. In case this was not intentional , take a look.

Update : You can see the record route option in action by doing a ping -R

ping -R Turns on route recording for the Echo Request packets, and
display the route buffer on returned packets (ignored by many
routers).