Explain briefly, in general terms, the purpose of the each of the three exchanges:

$1.$ between a Kerberos user and a Kerberos Key Distribution Centre $($KDC$),$

$2.$ a Kerberos user and a Kerberos Ticket Granting Setvice $($TGS$),$ and

$3.$ a Kerberos user and a Network Service$).$