Chapter 2 Application Layer

Transcription

1 Chapter 2 Application Layer A note on the use of these ppt slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: v If you use these slides (e.g., in a class) that you mention their source (after all, we d like people to use our book!) v If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR Computer Networking: A Top Down Approach 6 th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 All material copyright J.F Kurose and K.W. Ross, All Rights Reserved Application Layer 2-1

2 Chapter 2: outline 2.1 principles of network applications app architectures app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail SMTP, POP3, IMAP 2.5 DNS 2.6 P2P applications 2.7 socket programming with UDP and TCP Application Layer 2-2

3 FTP: the file transfer protocol at host FTP interface FTP client local file system file transfer FTP remote file system v transfer file to/from remote host v client/ model client: side that initiates transfer (either to/from remote) : remote host v ftp: RFC 959 v ftp : port 21 Application Layer 2-3

4 FTP: separate control, data connections v FTP client contacts FTP at port 21, using TCP v client authorized over control connection v client browses remote directory, sends commands over control connection v when receives file transfer command, opens 2 nd TCP data connection (for file) to client v after transferring one file, closes data connection FTP client TCP control connection, port 21 TCP data connection, port 20 FTP v opens another TCP data connection to transfer another file v control connection: out of band v FTP maintains state : current directory, earlier authentication Application Layer 2-4

5 FTP commands, responses sample commands: v sent as ASCII text over control channel v USER name v PASS password v LIST return list of file in current directory v RETR filename retrieves (gets) file v STOR filename stores (puts) file onto remote host sample return codes v status code and phrase (as in HTTP) v 331 Username OK, password required v 125 data connection already open; transfer starting v 425 Can t open data connection v 452 Error writing file Application Layer 2-5

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8 Chapter 2: outline 2.1 principles of network applications app architectures app requirements 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail SMTP, POP3, IMAP 2.5 DNS 2.6 P2P applications 2.7 socket programming with UDP and TCP Application Layer 2-8

9 Electronic mail Three major components: v s v mail s v simple mail transfer protocol: SMTP User Agent v a.k.a. mail reader v composing, editing, reading mail messages v e.g., Outlook, Thunderbird, iphone mail client v outgoing, incoming messages stored on mail SMTP mail SMTP SMTP outgoing message queue mail mailbox Application Layer 2-9

10 Electronic mail: mail s mail s: v mailbox contains incoming messages for v message queue of outgoing (to be sent) mail messages v SMTP protocol between mail s to send messages client: sending mail : receiving mail mail SMTP mail SMTP SMTP mail Application Layer 2-10

11 Electronic Mail: SMTP [RFC 2821] v uses TCP to reliably transfer message from client to, port 25 v direct transfer: sending to receiving v three phases of transfer handshaking (greeting) transfer of messages closure v command/response interaction (like HTTP, FTP) commands: ASCII text response: status code and phrase v messages must be in 7-bit ASCI Application Layer 2-11

12 Scenario: Alice sends message to Bob 1) Alice uses UA to compose message to 2) Alice s UA sends message to her mail ; message placed in message queue 3) client side of SMTP opens TCP connection with Bob s mail 4) SMTP client sends Alice s message over the TCP connection 5) Bob s mail places the message in Bob s mailbox 6) Bob invokes his to read message 1 mail mail 6 Alice s mail 5 Bob s mail Application Layer 2-12

13 Sample SMTP interaction 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 Application Layer 2-13

14 Try SMTP interaction for yourself: v telnet name 25 v see 220 reply from v enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send without using client (reader) Application Layer 2-14

15 SMTP: final words v SMTP uses persistent connections v SMTP requires message (header & body) to be in 7-bit ASCII v SMTP uses CRLF.CRLF to determine end of message comparison with HTTP: v HTTP: pull v SMTP: push v both have ASCII command/response interaction, status codes v HTTP: each object encapsulated in its own response msg v SMTP: multiple objects sent in multipart msg Application Layer 2-15

16 Mail message format SMTP: protocol for exchanging msgs RFC 822: standard for text message format: v header lines, e.g., To: From: Subject: different from SMTP MAIL FROM, RCPT TO: commands! v Body: the message ASCII characters only header body blank line Application Layer 2-16

17 Mail access protocols SMTP SMTP mail access protocol (e.g., POP, IMAP) sender s mail receiver s mail v SMTP: delivery/storage to receiver s v mail access protocol: retrieval from POP: Post Office Protocol [RFC 1939]: authorization, download IMAP: Internet Mail Access Protocol [RFC 1730]: more features, including manipulation of stored msgs on HTTP: gmail, Hotmail, Yahoo! Mail, etc. Application Layer 2-17

18 POP3 protocol authorization phase v v client commands: : declare name pass: password responses +OK -ERR transaction phase, client: v v v v list: list message numbers retr: retrieve message by number dele: delete quit S: +OK POP3 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 signing off Application Layer 2-18

19 POP3 (more) and IMAP more about POP3 v previous example uses POP3 download and delete mode Bob cannot re-read e- mail if he changes client v POP3 download-andkeep : copies of messages on different clients v POP3 is stateless across sessions IMAP v keeps all messages in one place: at v allows to organize messages in folders v keeps state across sessions: names of folders and mappings between message IDs and folder name Application Layer 2-19