1. Application Layer  We will learn about protocols by examining popular application-level protocols  HTTP  FTP  SMTP / POP3 / IMAP  Focus on client-server model  Web  HTML application transport network data link physical application transport network data link physical application transport network data link physical 1-1 Lecture 4

2. Client-server architecture server:  always-on host  permanent IP address clients:  communicate with server  may be intermittently connected  may have dynamic IP addresses  do not communicate directly with each other client/server 1-2 Lecture 4

3. Web and HTTP First some jargon  What is WWW?  Web page consists of objects  Objects can be HTML file, JPEG image, JavaScript, video file,…  Web page consists of base HTML-file which includes several referenced objects  Each object is addressable by a URL  Example URL: www.someschool.edu/someDept/pic.jpg host name path name 1-3 Lecture 4

4. HTTP overview HTTP:  hypertext transfer protocol  Web’s application layer protocol  Resources to be accessed by HTTP are identified using Uniform Resource Identifiers (URIs)—or, more specifically, Uniform Resource Locators (URLs) PC running Explorer Server running Apache Web server Mac running Navigator HTTP request HTTP response 1-4 Lecture 4

5. HTTP overview (continued) HTTP requires guarantee:  HTTP (app.) runs over TCP (transport)  client initiates TCP connection (creates socket) to server, port 80  server accepts TCP connection from client  HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)  TCP connection closed 1-5 Lecture 4

6. HTTP connections Nonpersistent HTTP  At most one object is sent over a TCP connection. Persistent HTTP  Multiple objects can be sent over single TCP connection between client and server. 1-6 Lecture 4

7. Nonpersistent HTTP Suppose user enters URL www.someSchool.edu/someDepartment/home.index 1a. HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80 2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object someDepartment/home.index 1b. HTTP server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket time (contains text, references to 10 jpeg images) 1-7 Lecture 4

8. Nonpersistent HTTP (cont.) 5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects 6. Steps 1-5 repeated for each of 10 jpeg objects 4. HTTP server closes TCP connection. time 1-8 Lecture 4

9. Non-Persistent HTTP: Response time Definition of RTT: time for a small packet to travel from client to server and back. Response time:  one RTT to initiate TCP connection  one RTT for HTTP request and first few bytes of HTTP response to return  file transmission time total = 2RTT+transmit time time to transmit file initiate TCP connection RTT request file RTT file received time 1-9 Lecture 4

10. Persistent HTTP Nonpersistent HTTP issues:  requires 2 RTTs per object  OS overhead for each TCP connection  Improved version:  browsers often open parallel TCP connections to fetch referenced objects Persistent HTTP  server leaves connection open after sending response  subsequent HTTP messages between same client/server sent over open connection  client sends requests as soon as it encounters a referenced object  as little as one RTT for all the referenced objects 1-10 Lecture 4

11. Numerical Example 1-11 Lecture 4

12. How to speed up Internet access? An example:  average object size = 100,000 bits  avg. request rate from institution’s browsers to origin servers = 15/sec  Internet delay from institutional router to any origin server and back to router = 2 sec Consequences  utilization on LAN = 15%  utilization on access link = 100%  total delay = Internet delay + access delay + LAN delay = 2 sec + seconds + milliseconds  Huge delay origin servers public Internet institutional network 10 Mbps LAN 1.5 Mbps access link 1-12 Lecture 4

13. How to speed up Internet access? (cont) possible upgrade  increase bandwidth of access link to, say, 10 Mbps consequence  utilization on LAN = 15%  utilization on access link = 15%  Total delay = Internet delay + access delay + LAN delay = 2 sec + msecs + msecs  often a costly upgrade origin servers public Internet institutional network 10 Mbps LAN 10 Mbps access link 1-13 Lecture 4

14. A better solution: Web Cache Install web cache  suppose hit rate is 0.4 consequence  40% requests will be satisfied almost immediately  60% requests satisfied by origin server  utilization of access link reduced to 60%, resulting in negligible delays origin servers public Internet institutional network 10 Mbps LAN 1.5 Mbps access link institutional cache 1-14 Lecture 4

15. Web caches (proxy server)  user 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 server, then returns object to client Goal: satisfy client request without involving origin server client Proxy server client HTTP request HTTP response HTTP request origin server origin server HTTP response 1-15 Lecture 4

16. More about Web caching Why Web caching?  reduce response time for client request  reduce traffic on an institution’s access link.  Internet dense with caches: enables “poor” content providers to effectively deliver content client Proxy server client HTTP request HTTP response HTTP request origin server origin server HTTP response 1-16 Lecture 4

17. HTTP overview (continued)  If HTTP is “stateless”, how does it remember our session information? HTTP Cookies 1-17 Lecture 4 HTTP is “stateless”  server maintains no information about past client requests

18. User-server state: cookies  Many major Web sites use cookies 1) cookie file kept on user’s host, managed by user’s browser 2) back-end database at Web site  Cookies are not cache! Example:  When initial HTTP requests arrives at site, site creates:  unique ID  entry in backend database for ID 1-18 Lecture 4

19. Cookies: keeping “state” (cont.) client server usual http response msg cookie file one week later: usual http request msg cookie: 1678 cookie- specific action access usual http request msg Amazon server creates ID 1678 for user create entry usual http response Set-cookie: 1678 amazon 1678 usual http request msg cookie: 1678 cookie- spectific action access amazon 1678 backend database 1-19 Lecture 4

20. Cookies (continued) What cookies can bring:  Authorization  Previous session (e.g., shopping carts) Cookies and privacy: r cookies permit sites to learn a lot about you!!! r Security??? aside Remember HTTP is “stateless” 1-20 Lecture 4

21. Category of cookies  First party cookies  Same domain  Third party cookies  Different domain than what is being shown  Mostly used for tracking users browsing websites (spyware)  Cookies are not malwares but they can violate privacy rules 1-21 Lecture 4

22. Application layer  2.1 Web browsing and HTTP  2.2 File transferring and FTP (File Transfer Protocol)  2.3 Email and SMTP Lecture 4 22

23. FTP: the file transfer protocol  transfer file to/from remote host  client/server model  client: side that initiates transfer (either to/from remote)  server: remote host  ftp server: port 21 file transfer FTP server FTP user interface FTP client local file system remote file system user at host Lecture 4 23

24. FTP: separate control, data connections 1. FTP client contacts FTP server at port 21 2. client authorized over control connection 3. client browses remote directory by sending commands over control connection. 4. when server receives file transfer command, server opens 2 nd TCP connection (for file) to client 5. after transferring one file, server closes data connection. FTP client FTP server TCP control connection port 21 TCP data connection port 20 Lecture 4 24

25. Application layer  2.1 Web browsing and HTTP  2.2 File transferring and FTP  2.3 Email and SMTP (Simple Mail Transfer Protocol) Lecture 4 25

26. Electronic Mail Three major components:  user agents  mail servers  simple mail transfer protocol: SMTP User Agent  a.k.a. “mail reader”  composing, editing, reading mail messages  e.g., Outlook, Mozilla Thunderbird  outgoing, incoming messages stored on server user mailbox outgoing message queue mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP Lecture 4 26

27. Electronic Mail: mail servers Mail Servers  mailbox contains incoming messages for user  message queue of outgoing (to be sent) mail messages  SMTP protocol between mail servers to send email messages  client: sending mail server  “server”: receiving mail server mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP Lecture 4 27

28. Scenario: Alice sends message to Bob 1) Alice uses UA to compose message and “to” bob@someschool.edu 2) Alice’s UA sends message to her mail server; 3) Message placed in message queue 4) Client side of SMTP opens TCP connection with Bob’s mail server and sends Alice’s message 5) Bob’s mail server places the message in Bob’s mailbox 6)Bob invokes his user agent to read message user agent mail server mail server user agent 1 2 3 4 5 6 Lecture 4 28

29. SMTP: comparison with HTTP  SMTP uses persistent connections Comparison with HTTP:  HTTP: pull  SMTP: push  HTTP: port 80  SMTP: port 25  Both require guarantee  Both use TCP connection Lecture 4 29

30. Mail access protocols user agent sender’s mail server user agent SMTP access protocol receiver’s mail server Post Office Protocol (POP3) Internet Message Access Protocol (IMAP) Lecture 4 30

31. POP3 and IMAP: Comparison POP3  Uses “download and delete” mode.  Bob cannot re-read e- mail if he changes client  POP3 is “stateless” across sessions IMAP  Keep all messages in one place: the server  Allows user to organize messages in folders  IMAP keeps user state across sessions:  names of folders and mappings between message IDs and folder name Lecture 4 31

32. Reading : comparison among the protocols  HTTP  FTP 1-32 Lecture 4  SMTP