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Which of the following transport layer protocols is used to support electronic mail?

1. $\textsf{SMTP}$
2. $\textsf{IP}$
3. $\textsf{TCP}$
4. $\textsf{UDP}$

This may help

SOURCE: KUROSE ROSS

The answer is option C: TCP.

There are three primary TCP/IP protocols for E-Mail management:

• Post Office Protocol $\textsf{(POP)}$
• Simple Mail Transfer Protocol $\textsf{(SMTP)}$
• Internet Message Access Protocol $\textsf{(IMAP)}$

They all are Application Layer Protocols

Once a client connects to the E-mail Server, there may be $0$(zero) or more $\textsf{SMTP}$ transactions. If the client has no mail to send, then there are no $\textsf{SMTP}$ transactions. Every e-mail message sent is an $\textsf{SMTP}$ transfer.

$\textsf{SMTP}$ is only used to send (push) messages to the server. $\textsf{POP}$ and $\textsf{IMAP}$ are used to receive messages as well as manage the mailbox contents(which includes tasks such as deleting, moving messages etc.).

So does that mean none of  using UDP as transport layer protocol ?

• Post Office Protocol (POP)
• Simple Mail Transfer Protocol (SMTP)
• Internet Message Access Protocol (IMAP)

Yes, none  of these AL protocols uses UDP at TL, because e-mail is a highly reliable application/service or communication process and thus you always use TCP at TL when it comes to E-mails and yes, for chatting (like - Messenger at AL can use UDP at TL, because chatting is not a reliable service)

Why is there a delay of 30 sec between Time_wait state and connection Closed state?

Is it to accept delayed packets or because of some other reason ?

the Time_wait state allows the client to resend the ack in case it is lost. Closed state shows that connection is closed and all the resources are released.

ans c)

SMTP is used in Application Layer.

TCP is used in transport layer to carry out mail which is iniated by application layer protocol SMTP.

So correct option will be -TCP

E-mail uses SMTP, application layer protocol which intern uses TCP transport layer protocol.
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