$(1)(4$ pts$)$ "ping" is a well$-$known utility program to test the reachability of a destination machine.

The results of a ping message printed on the screen are the measured round trip delays between

the two machines. In the mobile IP network shown below, assume that the one$-$way delay

between any two routers in the figure is $15ms,$ and that the delay between two machines in $a$

broadcast domain can be ignored. $($a$)$ Suppose indirect routing is used in the mobile IP network

shown below. When user B sends a ping message to user A$,$ what would be the measured round

trip delay? $($b$)$ Repeat $($a$)$ if user B is also a mobile device and is now located in a foreign

network.