1. 2: Application Layer1 04 - World Wide Web (WWW)

2. Introduction 1-2 Internet protocol stack (recap) r application: supporting network applications m FTP, SMTP r transport: process-process data transfer r network: routing of datagrams from source host to destination host m IP, routing protocols r link: data transfer between neighboring network elements m PPP, Ethernet r physical: bits “on the wire” application transport network link physical 5 4 3 2 1

3. 2: Application Layer3 Processes communicating across network r process sends/receives messages to/from its socket r socket analogous to door m sending process shoves message out door m sending process assumes transport infrastructure on other side of door will deliver message to socket at receiving process process TCP with buffers, variables socket host or server process TCP with buffers, variables socket host or server Internet controlled by OS controlled by app developer

4. 2: Application Layer 4 Addressing processes: r For a process to receive messages, it must have an identifier r Every host has a unique 32-bit IP address r Q: Does the IP address of the host on which the process runs suffice for identifying the process? r Answer: No, many processes can be running on same host r Identifier includes both the IP address and port numbers associated with the process on the host. r Example port numbers: m HTTP server: 80 m Mail server: 25 r More on this later

5. 2: Application Layer5 What transport service does an app need? Data loss r some apps (e.g., audio) can tolerate some loss r other apps (e.g., file transfer, telnet) require 100% reliable data transfer Timing r some apps (e.g., Internet telephony, interactive games) require low delay to be “effective” Bandwidth r some apps (e.g., multimedia) require minimum amount of bandwidth to be “effective” r other apps (“elastic apps”) make use of whatever bandwidth they get

6. 2: Application Layer6 Transport service requirements of common apps Application file transfer e-mail Web documents real-time audio/video stored audio/video interactive games Data loss no loss loss-tolerant Bandwidth elastic audio: 5kbps-1Mbps video:10kbps-5Mbps same as above few kbps up Time Sensitive no yes, 100’s msec yes, few secs yes, 100’s msec

7. 2: Application Layer7 Internet transport protocols services TCP service: r connection-oriented: setup required between client and server processes r reliable transport between sending and receiving process r flow control: sender won’t overwhelm receiver r congestion control: throttle sender when network overloaded r does not provide: timing, minimum bandwidth guarantees UDP service: r unreliable data transfer between sending and receiving process r does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee r No setup required.

8. 2: Application Layer8 Internet apps: application, transport protocols Application e-mail remote terminal access Web file transfer streaming multimedia Internet telephony Application layer protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] proprietary (e.g. Youtuble) proprietary (e.g., Skype) Underlying transport protocol TCP TCP or UDP typically UDP

9. 2: Application Layer9 Chapter 2 outline r 2.1 Principles of app layer protocols m clients and servers m app requirements r 2.2 Web and HTTP

10. 2: Application Layer10 Web and HTTP First some jargon r Web page consists of objects r Object can be HTML file, JPEG image, Java applet, audio file,… r Web page consists of base HTML-file which includes several referenced objects r Each object is addressable by a URL r Example URL: www.someschool.edu/someDept/pic.gif host name path name

11. 2: Application Layer11 HTTP overview HTTP: HyperText Transfer Protocol r Web’s application layer protocol r client/server model m client: browser that requests, receives, “displays” Web objects m server: Web server sends objects in response to requests r HTTP 1.0: RFC 1945 r HTTP 1.1: RFC 2068 *NetCraft's Web Server SurveyNetCraft's Web Server Survey PC running Chrome Server running Apache Web server* Mac running Safari HTTP request HTTP response

12. 2: Application Layer12 HTTP overview (continued) Uses TCP: r client initiates TCP connection (creates socket) to server, port 80 r server accepts TCP connection from client r HTTP messages (application- layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) r TCP connection closed HTTP is “stateless” r server maintains no information about past client requests Protocols that maintain “state” are complex! r past history (state) must be maintained r if server/client crashes, their views of “state” may be inconsistent, must be reconciled aside

13. 2: Application Layer13 HTTP connections Nonpersistent HTTP r At most one object is sent over a TCP connection. r HTTP/1.0 uses nonpersistent HTTP Persistent HTTP r Multiple objects can be sent over single TCP connection between client and server. r HTTP/1.1 uses persistent connections in default mode

14. 2: Application Layer14 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)

15. 2: Application Layer15 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

16. 2: Application Layer16 Response time modeling Definition of Round Trip Time (RTT): time to send a small packet to travel from client to server and back. Response time: r one RTT to initiate TCP connection r one RTT for HTTP request and first few bytes of HTTP response to return r file transmission time total = 2RTT+transmit time time to transmit file initiate TCP connection RTT request file RTT file received time

17. 2: Application Layer17 Persistent HTTP Nonpersistent HTTP issues: r requires 2 RTTs per object r browsers often open parallel TCP connections to fetch referenced objects r server must maintain a separate connection for each object Persistent HTTP r server leaves connection open after sending response r subsequent HTTP messages between same client/server are sent over connection Persistent without pipelining: r client issues new request only when previous response has been received r one RTT for each referenced object Persistent with pipelining: r default in HTTP/1.1 r client sends requests as soon as it encounters a referenced object r as little as one RTT for all the referenced objects

18. 2: Application Layer18 HTTP request message r two types of HTTP messages: request, response r HTTP request message: m ASCII (human-readable format) GET /somedir/page.html HTTP/1.1 Host: www.someschool.edu User-agent: Mozilla/4.0 Connection: close Accept-language:fr (extra carriage return, line feed) request line (GET, POST, HEAD commands) header lines Carriage return, line feed indicates end of message header

19. 2: Application Layer19 HTTP request message: general format

20. 2: Application Layer20 Uploading form input Post method: r Web page often includes form input r Input is uploaded to server in entity body URL method: r Uses GET method r Input is uploaded in URL field of request line: www.somesite.com/animalsearch?monkeys&banana

21. 2: Application Layer21 Method types HTTP/1.0 r GET r POST r HEAD m asks server to leave requested object out of response HTTP/1.1 r GET, POST, HEAD r PUT m uploads file in entity body to path specified in URL field r DELETE m deletes file specified in the URL field

22. 2: Application Layer22 HTTP response message HTTP/1.1 200 OK Connection: close Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Jun 1998 …... Content-Length: 6821 Content-Type: text/html data data data data data... status line (protocol status code status phrase) header lines data, e.g., requested HTML file

23. 2: Application Layer23 HTTP response status codes 200 OK m request succeeded, requested object later in this message 301 Moved Permanently m requested object moved, new location specified later in this message (Location:) 400 Bad Request m request message not understood by server 404 Not Found m requested document not found on this server 505 HTTP Version Not Supported In first line in server->client response message. A few sample codes:

24. 2: Application Layer24 Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: Opens TCP connection to port 80 (default HTTP server port) at www.rose-hulman.edu. Anything typed in sent to port 80 at www.rose-hulman.edu telnet www.rose-hulman.edu 80 2. Type in a GET HTTP request: GET /class/csse/csse432/201430/ By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server 3. Look at response message sent by HTTP server!

25. 2: Application Layer25 Cookies: keeping “state” Many major Web sites use cookies Four components: 1) cookie header line in the HTTP response message 2) cookie header line in HTTP request message 3) cookie file kept on user’s host and managed by user’s browser 4) back-end database at Web site Example: m Susan access Internet always from same PC m She visits a specific e- commerce site for first time m When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID

26. 2: Application Layer26 Cookies: keeping “state” (cont.) client server usual http request msg usual http response + Set-cookie: 1678 usual http request msg cookie: 1678 usual http response msg usual http request msg cookie: 1678 usual http response msg cookie- specific action cookie- spectific action server creates ID 1678 for user entry in backend database access Cookie file amazon: 1678 ebay: 8734 Cookie file ebay: 8734 Cookie file amazon: 1678 ebay: 8734 one week later:

27. 2: Application Layer27 Cookies (continued) What cookies can bring: r authorization r shopping carts r recommendations r user session state (Web e-mail) Cookies and privacy: r cookies permit sites to learn a lot about you r you may supply name and e-mail to sites r search engines use redirection & cookies to learn yet more r advertising companies obtain info across sites aside

28. 2: Application Layer28 Web caches (proxy server) r user sets browser: Web accesses via cache r browser sends all HTTP requests to cache m object in cache: cache returns object m 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 HTTP response origin server origin server

29. 2: Application Layer29 More about Web caching r Cache acts as both client and server r Typically cache is installed by ISP (university, company, residential ISP) Why Web caching? r Reduce response time for client request. r Reduce traffic on an institution’s access link. r Internet dense with caches enables “poor” content providers to effectively deliver content

30. 2: Application Layer30 Caching example Assumptions r average object size = 100,000 bits r avg. request rate from institution’s browsers to origin servers = 15/sec r delay from Router A to any origin server and back to router = 2 sec Consequences r traffic intensity on LAN =.15 r traffic intensity on access link = 1 r total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + milliseconds origin servers public Internet institutional network 10 Mbps LAN 1.5 Mbps access link institutional cache Router A

31. 2: Application Layer31 Caching example (cont) Possible solution r increase bandwidth of access link to, say, 10 Mbps Consequences r traffic intensity on LAN =.15 r traffic intensity on access link =.15 r Total delay = Internet delay + access delay + LAN delay = 2 sec + msecs + msecs r often a costly upgrade origin servers public Internet institutional network 10 Mbps LAN 10 Mbps access link institutional cache

32. 2: Application Layer32 Caching example (cont) Install cache r suppose hit rate is.4 Consequence r 40% requests will be satisfied almost immediately r 60% requests satisfied by origin server r traffic intensity of access link reduced to.60, resulting in negligible delays (say 10 msec) r total avg delay = Internet delay + access delay + LAN delay =.6*(2.01) secs + milliseconds origin servers public Internet institutional network 10 Mbps LAN 1.5 Mbps access link institutional cache The “host” header field in the HTTP request message specifies the origin server.

33. 2: Application Layer33 Conditional GET: client-side caching r Goal: don’t send object if client has up-to-date cached version r client: specify date of cached copy in HTTP request If-modified-since: r server: response contains no object if cached copy is up- to-date: HTTP/1.0 304 Not Modified client server HTTP request msg If-modified-since: HTTP response HTTP/1.0 304 Not Modified object not modified HTTP request msg If-modified-since: HTTP response HTTP/1.0 200 OK object modified