$(10\%)$ Provide short answers for each question.

a$)$ Is it possible for an application to ensure reliable data transfer even when it runs over UDP? If so$,$ how? $(2\mathrm{.}5\%)$

b$)$ Consider two mobile nodes in a foreign network having a foreign agent. Is it possible for the two mobile nodes to use the same care$-$of$-$address in mobile IP$?$ Explain your answer. $(2\mathrm{.}5\%)$

c$)$ As a wireless $802\mathrm{.}11$ node moves away from a base station, give two actions that a base station could take to ensure that the loss probability of a transmitted frame does not increase. Discuss their impacts on signal to noise ratio of the base station. $($Hint: loss probability $=$ received frames $\frac{?}{?}$ sent frames$)(2\mathrm{.}5\%)$

d$)$ Suppose two users Alice and Bob are behind NATs of their networks. What problem does Skype has to solve in order for Alice and Bob successfully communicate through NATs? $(2\mathrm{.}5\%)$