Consider Bob uses an e-mail client (mail reader) such as Outlook to send an e-mail to Alice who uses a Web-based e-mail account (Gmail). The IP address of Alice’s mail server is initially unknown to Bob’s mail server. Which of the following is the correct sequence of activity for transport and application layer protocols that are involved from the time when Bob sends the e-mail to the time when Alice reads it?
s1 : DNS over UDP is used by Bob’s mail server to get the IP address for Alice’s mail server.
s2 : SMTP over TCP is used to transfer the mail from Bob’s mail server to Alice’s mail server.
s3: SMTP over TCP is used to transfer the mail from Bob’s e-mail client to his SMTP server.
s4: HTTP over TCP is used by Alice to read this e-mail.