CS 3516: Computer Networks

Transcription

1 Welcome to CS 3516: Computer Networks Prof. Yanhua Li Time: 9:00am 9:50am M, T, R, and F Location: AK219 Fall 2018 A-term 1 Some slides are originally from the course materials of the textbook Computer Networking: A Top Down Approach, 7th edition, by Jim Kurose, Keith Ross, Addison-Wesley March Copyright J.F Kurose and K.W. Ross, All Rights Resered.

2 Chapter 2: outline 2.4 electronic, POP3, Application Layer 2-2

3 Electronic Three major components: s serers simple transfer protocol: User Agent a.k.a. reader composing, editing, reading messages e.g., Outlook, Thunderbird, iphone client outgoing, incoming messages stored on serer serer serer outgoing message queue serer box Application Layer 2-3

4 Electronic : serers outgoing message queue serers: box contains incoming messages for message queue of outgoing (to be sent) messages protocol between serers to send e messages client: sending serer serer : receiing serer serer serer serer box Application Layer 2-4

5 Scenario: Alice sends message to Bob 1) Alice uses UA to compose message to 2) Alice s UA sends message to her serer; message placed in message queue 3) client side of opens TCP connection with Bob s serer 4) client sends Alice s message oer the TCP connection 5) Bob s serer places the message in Bob s box 6) Bob inokes his to read message 1 serer serer 6 Alice s serer 5 Bob s serer Application Layer 2-5

6 Electronic Mail: [RFC 2821] uses TCP to reliably transfer e message from client to serer, port 25 direct transfer: sending serer to receiing serer three phases of transfer handshaking (greeting) transfer of messages closure command/response interaction (like HTTP) commands: ASCII text response: status code and phrase messages must be in 7-bit ASCII Application Layer 2-6

7 Sample interaction (messaging) telnet serername 25 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, end with "." on a line by itself C: Do you like ketchup? C: How about pickles? C:. S: 250 Message accepted for deliery C: QUIT S: 221 hamburger.edu closing connection Application Layer 2-7

8 : final words uses persistent connections serer uses CRLF.CRLF to determine end of message comparison with HTTP: Both TCP both hae ASCII command/response interaction, status codes HTTP: pull : push HTTP: each object encapsulated in its own response msg : multiple objects sent in a multipart msg Application Layer 2-8

9 Mail message format : protocol for exchanging e msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: Body: the message ASCII characters only header body blank line Application Layer 2-9

10 Mail access protocols access protocol (e.g., POP, IMAP) sender s serer receier s serer : deliery/storage to receier s serer access protocol: retrieal from serer POP: Post Office Protocol [RFC 1939]: authorization, download IMAP: Internet Mail Access Protocol [RFC 1730]: more features, including manipulation of stored msgs on serer HTTP: g, Hot, Yahoo! Mail, etc. Application Layer 2-10

11 POP3 protocol Port 110, ia TCP connections authorization phase client commands: : declare name pass: password serer responses +OK -ERR transaction phase, client: list: list message numbers retr: retriee message by number dele: delete quit S: +OK POP3 serer ready C: bob S: +OK C: pass hungry S: +OK 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 serer signing off Application Layer 2-11

12 Questions? Application Layer 2-12