Hello,
In the 2021 exam, problem 1.4, it is said that the two nats must use different port numbers (p' != p). Why so, and how do they synchronise ?
Thank you in advance,
Jérôme
Hi Jérôme,
It is possible for N1 and N3 to use the same source port (p=p'). However, it is very unlikely.
As we have discussed in another Q&A about STP, the solution PDF does not always list all acceptable solutions and sometimes use implicit assumptions when they are straightforward. Here the straightforward assumption, the most likely one, is that N1 and N3 select two different source ports.
If you deliberately decide to describe an unlikely case or to rely on a non-straightforward assumption, then you should write the assumption explicitly. You will obtain the points, provided that the assumption is possible and explicit ("Here I assume that by chance N1 and N3 select the same source port"), and that the justification and the conclusion are coherent
with the assumption.
To conclude, no, there is no mechanism to synchronize N1 and N3 in order to prevent them from using the same source port.
The solution PDF simply tells that, in the absence of any explicit assumption in the student's answer that differs from the straightforward or most-likely case, a port number p' with p =/= p' is expected.
Ludovic
Hello Ludovic,
Thank you for your answer. I do agree p is unlikely to be the same as p', or as q, or as q', or any given number. There is, however, a specific mechanism preventing q and q' to be equal, so as both pairs were presented similarly I wasn't sure whether I had missed something or not. Thank you for having clarified that point. :-)
Yes you are right.
The port numbers selected for q and q' cannot be equal. The opposite assumption is not unlikely here (as opposed to p and p'), but impossible.
Ludovic
The port numbers selected for q and q' cannot be equal. The opposite assumption is not unlikely here (as opposed to p and p'), but impossible.
Ludovic
