Question 5.0 on the handout

Question

Has anyone figured out how it is impossible for S1 to send the pack from A directly to S2 but then to be flooded on the unspecified part connected to S2?

Answer 1

Suppose S1 knows the location of B.
This implies that B has sent a message to A, because B->S2->S1->A is the only path from A to B
Since S1 knows the location of B, it will not flood, however, by the above implication, we know that S2 will also not flood.

Therefore whenever S1 doesn't flood, S2 will not flood.
Therefore the given situation is impossible

Answer 2

In my opinion, we do not need to suppose that S1 know B because S1 send direct to B, so it knows where are B. And we also know that S2 do not know where are B. The only way for S1 to know where are B is B sent something through S1, and that way through S2. Therefore, S2 know where is B, which contract with the thing we know.
Therefore, this situation is impossible.