CSC 401 Data and Computer Communications Networks

Transcription

1 CSC 401 Data and Computer Communications Networks Application Layer: Cookies, Web Caching, SMTP Sec Prof. Lina Battestilli Fall 2017

2 Outline Application Layer (ch 2) 2.1 principles of network applications 2.2 Web and HTTP Cookies, Caching 2.3 electronic mail SMTP, POP3, IMAP 2.4 DNS 2.5 P2P applications 2.6 Video streaming and Content Distribution Networks 2.7 socket programming with UDP and TCP 10

3 User- state: cookies Many Web sites use cookies enabled by four components: 1) cookie header line of HTTP response message 2) cookie header line in next HTTP request message 3) cookie file kept on s host, managed by s browser 4) back-end database at Web site example: Susan always access Internet from PC visits specific e-commerce site for first time when initial HTTP requests arrives at site, site creates: unique ID entry in backend database for ID 11

4 Cookies: keeping state client ebay 8734 cookie file ebay 8734 amazon 1678 usual http request msg usual http response set-cookie: 1678 Amazon creates ID 1678 for create entry backend database usual http request msg cookie: 1678 usual http response msg cookiespecific action access one week later: ebay 8734 amazon 1678 usual http request msg cookie: 1678 usual http response msg cookiespecific action access 12

5 Cookies (continued) what cookies can be used for: authorization shopping carts recommendations session state (Web ) stateful protocol: protocol endpoints: maintain state at sender/receiver over multiple transactions cookies: http messages carry state aside cookies and privacy: cookies permit sites to learn a lot about you you may supply name and to sites privacy risk Cookies are not enough to buy staff. You still need to authenticate/authorization. Let s visit at Let s look at some cookies Incognito Mode 13

6 Monitor Cookies & Trackers Ghostery Cookie Inspector Check Cookies for your browser: 14

7 Web Caches (proxy ) goal: satisfy client request without involving origin sets browser: Web accesses via cache browser sends all HTTP requests to cache object in cache: cache returns object else cache requests object from origin, then returns object to client client client proxy origin origin 15

8 More about Web caching cache acts as both client and for original requesting client client to origin typically cache is installed by ISP (university, company, residential ISP) Why Web caching? reduce response time for client request reduce traffic on an institution s access link reduces origin web load 16

9 Caching Example: slow access Link assumptions: avg object size: 1 Mbits avg request rate from browsers to origin s:15/sec avg data rate to browsers: 15 Mbps public Internet origin s access link rate: 15 Mbps RTT from institutional router R1 to any origin : 2 sec consequences: problem! LAN utilization: 15% access link utilization = 100% institutional network R2 R1 15 Mbps access link 100 Mbps LAN total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + µsecs Way too long! 17

10 Caching example: fatter access link assumptions: avg object size: 1 Mbits avg request rate from browsers to origin s:15/sec avg data rate to browsers: 15 Mbps public Internet origin s access link rate: 15 Mbps 100 Mbps RTT from institutional router to any origin : 2 sec consequences: LAN utilization: 15% access link utilization = 100% 15% institutional network R2 R1 15 Mbps access link 100 Mbps 100 Mbps LAN total delay = Internet delay + access delay + LAN delay = 2 sec + min + µsecs msecs Cost: increased access link speed (not cheap!) 18

11 Caching example: install local cache assumptions: avg object size: 1 Mbits avg request rate from browsers to origin s:15/sec avg data rate to browsers: 15 Mbps access link rate: 15 Mbps RTT from institutional router to any origin : 2 sec consequences: LAN utilization: 15% access link utilization = total delay =?? How to compute link utilization, delay? institutional network public Internet R2 R1 15 Mbps access link origin s 100 Mbps LAN local web cache Cost: web cache (cheap!) 19

12 Caching example: install local cache Calculating access link utilization, delay with cache: suppose cache hit rate is % requests satisfied at cache 60% requests satisfied at origin access link utilization: 60% of requests use access link data rate to browsers over access link = 0.6*15 Mbps = 9 Mbps utilization = 9/15 => 47% total delay = 0.6 * (delay from origin s) +0.4 * (delay when satisfied at cache) = 0.6 (2.01sec) (~msecs) = ~ 1.2 secs institutional network public Internet 15 Mbps access link origin s 100 Mbps LAN local web cache less than with 100 Mbps link (and cheaper too!) 20

13 Conditional GET Goal: don t send object if cache has up-to-date cached version no object transmission delay lower link utilization cache: specify date of cached copy in HTTP request client HTTP request msg If-modified-since: <date> HTTP response HTTP/ Not Modified object not modified before <date> If-modified-since: <date> : response contains no object if cached copy is up-todate: HTTP/ Not Modified HTTP request msg If-modified-since: <date> HTTP response HTTP/ OK <data> object modified after <date> 21

14 Outline Application Layer (ch 2) 2.1 principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 electronic mail SMTP, IMAP, POP3 2.5 DNS 2.6 P2P applications 2.7 socket programming with UDP and TCP 23

15 Electronic mail Three major components: s mail s simple mail transfer protocol: SMTP User Agent a.k.a. mail reader composing, editing, reading mail messages e.g., Outlook, Thunderbird, iphone mail client outgoing, incoming messages stored on mail SMTP mail SMTP SMTP outgoing message queue mail mailbox 24

16 Electronic mail: mail s mail s: mailbox contains incoming messages for message queue of outgoing (to be sent) mail messages SMTP protocol between mail s to send messages client: sending mail : receiving mail mail SMTP mail SMTP SMTP mail 25

17 Electronic Mail: SMTP [RFC 2821] uses TCP to reliably transfer message from client to, port 25 direct transfer: sending to receiving three phases of transfer 1. handshaking (greeting) 2. transfer of messages 3. closure command/response interaction (like HTTP, FTP) commands: ASCII text response: status code and phrase messages must be in 7-bit ASCI

18 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 27

19 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 2-28

20 2-29 Try SMTP interaction for yourself: telnet name 25 see 220 reply from enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send without using client (reader)

21 Comparison HTTP Persistent or non-persistent connections Pull ASCI request/response + status codes each object encapsulated in its own response message SMTP Persistent connections Push 7-bit ASCII request/response + status codes multiple objects sent in same message. ASCII command/response, status codes CRLF.CRLF for end of message ASCII command/response, status codes CRLF or CRLFCRLF for end of

22 Mail message format SMTP: protocol for exchanging msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: different from SMTP handshaking protocol MAIL FROM:, RCPT TO: Body: the message ASCII characters only header body blank line 2-31

23 Mail access protocols SMTP SMTP sender s mail receiver s mail mail access protocol (e.g., POP3, IMAP, HTTP) PULL operation! SMTP: delivery/storage to receiver s mail access protocol: retrieval from POP3: 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. 2-32

24 POP3 and IMAP POP3 POP3 download and delete mode Bob cannot re-read if he changes client POP3 download-and-keep : copies of messages on different clients POP3 is stateless across sessions IMAP keeps all messages in one place: at allows to organize messages in folders keeps state across sessions: names of folders and mappings between message IDs and folder name 2-33

25 Web-based Introduced by Hotmail in mid 1990s User is a web browser Connection from User Agent to Mail Servers uses HTTP 34

26 References Some of the slides are identical or derived from 1. Slides for the 7 th edition of the book Kurose & Ross, Computer Networking: A Top-Down Approach, 2. Slides by Jim Kurose for his CSC453 course at Umass