IP#1    IP#2/2b   IP#3     IP#4   Section  Diff
                          ==============================================================
OK   (alternate route):   HOP1 --- HOP2  ----------- HOP3   (#1)
                          HOP1 --- HOP2b ----------- HOP3   (#2)  --> @hop2: IP#2-->IP#2b

EEK! (non-equidistant):   HOP1 --- HOP2  ----------- HOP3   (#1)
     a) shorter route     HOP1 --------------------- HOP2   (#2a) --> @hop2: IP#2-->IP#4 (err?)
     b) longer  route     HOP1 --- HOP2 --- HOP3 --- HOP4   (#2b) --> @hop3: IP#4-->IP#3 (err?)