a$)$ Suppose Toheeb, with a web$-$based email account $($such as Yahoo or Gmail$),$ sends a message to his

friend Chioma, who accesses her mail from her mail server using POP. Discuss how the message gets

from Toheeb's host to Chioma's host. Be sure to list the series of application$-$layer protocols that are used

to move the message between the two hosts. Note: This is not about transfer across network layers. $(13$ marks$)$

b$)$ Explain any three reasons against having one DNS server. $(6$ marks$)$

c$)$ In our rdt protocols, what are the separate roles of checksums, sequence numbers, timers,

acknowledgments, negative acknowledgments, window$/$pipelining$?(6$ marks$)$