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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.

a) s1,s2,s3,s4

b) s1,s3,s2,s4

c) s3,s1,s2,s4

a) s3,s2,s1,s4
asked in Computer Networks by (125 points) | 86 views
I think it is B
I think same too BUT ans is given C.
Ooooooooooohhhh.. right.. Now since you've said so. Option C makes more sense. lol
No . I think option B should be correct bcz IP address of the destination should be known before sending any request.
Yes you're right. But first the sender's client will tell it's server which destination it wants to send the email to. Then the server will look for the address. How can a sender's server know which is the destination server until and unless it is told by the client?

Suppose instead of Alice Bob wanted to send an email to you, now this has to be informed to the Bob's server that he has to send it to you, then the server will be like "Okay, let me go search Avijit's server address".

Makes sense?
@vikas Yes now its making sense . Thanks

1 Answer

+1 vote

Option:- C is right answer S3, S1, S2, S4

As Bob want to send email to Alice :-

1.  (S3) Very first at Bob end, SMTP with help of TCP is used to transfer email from Bob outlook client to Bob's outlook server.

2.  (S1) DNS with help of UDP  at Bob's outlook server will get Alice mail server IP address.

3.  (S2) SMTP with help of TCP is used to transfer e-mail from Bob's outlook server to Alice's Gmail Server.

4.  (S4) HTTP with help of TCP is used to read e-mail from Alice's Gmail server to Alice Gmail client in Web Browser.


If you think logically,

1. When bob write email and click send, the email is not on the way to Alice. Because according to email architecture, the send email need to visit first  it's own email server.  The email client knows the IP address of it's own email server. so it simply make use of SMTP protocol to establish TCP connection and transfer it reliably to own server(Bob's server). In this step UDP is not used.

2. a. Now Bob's email server will  make use of UDP to get the destination(Alice mail server) IP address. 

    b. After getting IP address, with the help of SMTP protocol transfer the email packet  from Bob's email server to Alice email server reliably.

3.   Now Alice email server receives email packet. This email packet need to be send to Alice email client whenever Alice request it   . Alice email server know Alice email client IP address. So Alice client simply make use of HTTP / IMPA4 / POP3 protocol to read  the email from it own email server.

answered by (91 points)
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