CS 355. Computer Networking. Wei Lu, Ph.D., P.Eng.

Transcription

1 CS 355 Computer Networking Wei Lu, Ph.D., P.Eng.

2 Chapter 2: Application Layer Overview: Principles of network applications? Introduction to Wireshark Web and HTTP FTP Electronic Mail: SMTP, POP3, IMAP DNS P2P applications Socket programming with TCP and UDP

3 An Overview of System How SMTP (Simple Mail Transfer Protocol) Works Mail Access Protocols: POP, IMAP and HTTP

4 An System Overview

5 An overview of system user agent SMTP SMTP access protocol user agent sender s mail server receiver s mail server Four major components: user agents mail servers simple mail transfer protocol: SMTP, used to deliver/storage to/from receiver s server mail access protocol: retrieval from server, including POP3, IMAP, or HTTP

6 An overview of system user agent SMTP SMTP access protocol user agent sender s mail server receiver s mail server User Agent a.k.a. mail reader composing, editing, reading mail messages e.g., Eudora, Outlook, elm, Mozilla Thunderbird outgoing, incoming messages stored on server

7 An overview of system user agent SMTP SMTP access protocol user agent Mail Servers sender s mail server receiver s mail server mailbox contains incoming messages for user message queue of outgoing (to be sent) mail messages SMTP protocol between mail servers to send messages client: sending mail server server : receiving mail server outgoing message queue user mailbox

8 An overview of system user agent SMTP SMTP access protocol user agent sender s mail server receiver s mail server SMTP define a procedure by which data packets are transferred from one networked machine to another text-based protocol, defines the message format to be transferred uses TCP port 25 or 465; requires message (header & body) to be in 7-bit ASCII and uses CRLF.CRLF to determine end of message

9 An overview of system user agent SMTP SMTP access protocol user agent sender s mail server receiver s mail server Mail Access Protocols POP3: Post Office Protocol Version 3 authorization (agent <-->server) and download IMAP: Internet Mail Access Protocol more features (more complex) manipulation of stored messages on server HTTP: gmail, Hotmail, Yahoo! Mail, etc.

10 An overview of system: a scenario Judy uses user agent (e.g. outlook) to compose message; to: tom@keene.edu Judy s outlook sends message to her mail server; message placed in message queue Client side of SMTP opens TCP connection with Tom s mail server SMTP client sends Judy s message over the TCP connection Tom s mail server places the message in Tom s mailbox Tom invokes his user agent to read message

11 How SMTP Works

12 SMTP Overview Uses TCP as its transport service: client initiates TCP connection (creates socket) to server, port 25 server accepts TCP connection from client client and server acknowledge connection set up done! SMTP Login: SMTP request and response messages exchanged between SMTP UA and SMTP server to do the authentication SMTP request and response messages exchanged between client and server to set the mail message header and edit message body SMTP mail transfer and TCP connection closed

13 client initiates a TCP connection (creates socket) from host :25915 to server :25, port 25

14 server :25 accepts TCP connection from client :25915

15 Both server :25 and client :25915 acknowledge connection set up done!

16

17

18

19

20

21 Encrypted? What s original text?

22 ASCII TO BASE64?: Wake encryption method, easy to be de-encrypted

23

24 Encrypted? What s original text?

25 ASCII TO BASE64?: Wake encryption method, easy to be de-encrypted

26 Login successfully!

27 The next step after login successful is to set mail header and edit mail body that to be transferred in between mail server and UA

28 The next step after login successful is to set mail header and edit mail body that to be transferred in between mail server and UA

29 The next step after login successful is to set mail header and edit mail body that to be transferred in between mail server and UA

30 The next step after login successful is to set mail header and edit mail body that to be transferred in between mail server and UA

31 The next step after login successful is to set mail header and edit mail body that to be transferred in between mail server and UA

32 The next step after login successful is to set mail header and edit mail body that to be transferred in between mail server and UA

33 The next step after login successful is to set mail header and edit mail body that to be transferred in between mail server and UA

34 SMTP Overview: Internet Mail Format SMTP: protocol for exchanging msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: different from SMTP commands! body the message, ASCII characters only header body blank line

35 The next step after mail edit is to transfer the data in between mail server and UA

36 The final step after mail message transfer completed is to close the connection in between mail server and UA

37 Summary on SMTP uses TCP to reliably transfer message from client to server, port 25/465 (secure smtp) three phases of transfer handshaking (greeting) transfer of messages closure command/response interaction commands: ASCII text response: status code and phrase

38 Mail Access Protocol: POP3 user agent SMTP SMTP access protocol user agent sender s mail server receiver s mail server POP: Post Office Protocol [RFC 1939] authorization (agent <-->server) and download

39 Mail Access Protocol: POP3

40 POP3 Protocol authorization phase client commands: user: declare username pass: password server responses +OK -ERR transaction phase, client: list: list message numbers retr: retrieve message by number dele: delete quit S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on C: list S: S: S:. C: retr 1 S: <message 1 contents> S:. C: dele 1 C: retr 2 S: <message 1 contents> S:. C: dele 2 C: quit S: +OK POP3 server signing off

41 SMTP Analysis S: 220 hamburger.edu C: HELO crepes.fr S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: S: 250 Sender ok C: RCPT TO: S: 250 Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C:. S: 250 Message accepted for delivery C: QUIT S: 221 hamburger.edu closing connection

42 Mail Access Protocol: IMAP user agent SMTP SMTP access protocol user agent sender s mail server receiver s mail server IMAP: Internet Mail Access Protocol [RFC 1730] more features and more complex than POP3 manipulation of stored messages on server

43 Comparing IMAP with POP3 POP3 uses download and delete mode; or download and keep the copy cannot re-read if user changes client if deleting mail from server Download-and-keep : copies of messages on different clients IMAP Keep all messages in one place: the server Allows user to organize and manage messages in different folders

44