Problem $4[9$ points$]$ Short answer questions on rdt protocols:

$($a$)$ In our rdt protocols, why did we need to introduce sequence numbers?

$($b$)$ In our rdt protocols, why did we need to introduce timers?

$($c$)$ Suppose that the roundtrip delay between sender and receiver is constant and known to the sender$.$ Would a timer still be necessary in protocol rdt $3\mathrm{.}0,$ assuming that packets can be lost? Explain.