2. Outline r Introduction to App Layer Protocols r Brief History of WWW r Architecture r HTTP Connections r HTTP Format r Web Performance r Cookies

3. Sejarah Web r World Wide Web, “Web”, “WWW” r Ditemukan oleh Tim Berners-Lee di CERN (European Organization for Nuclear Research) tahun 1991 m Didemonstasikan prototype dikonferensi tahun ’91 m Text-based r Marc Andreessen mengembangkan Web Browser graphical pertama dithn 1993: Mosaic r Andreessen membangun Netscape Communications r Pemakaian Browser dimulai1995-96 r America Online membeli Netscape in 1998

4. Beberapa Istilah “Web” r Web page berisi links ke halaman yang lain (disebut juga Web Objects) r Object dapat berupa HTML file, JPEG image, Java applet, audio file,… r Web pages adalah “Hypertexts” m One page points to another m Proposed by Prof. Vannevar Bush in 1945! r Setiap object menggunakan alamat URL: http://www.someschool.edu/someDept/pic.gif path name host name protocol

5. Sekilas HTTP 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 2616 Request for Comments (RFC) PC running Explorer Server running Apache Web server Mac running Navigator HTTP request HTTP response

6. HTTP overview (lanjutan) 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

7. HTTP connections Non-persistent HTTP r At most one object is sent over a TCP connection. r HTTP/1.0 uses non- persistent 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 m Pipelined m Non-pipelined

8. Response time modeling Definition of 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

9. Classical HTTP/1.0 http://www.somewhere.com/index.html index.html references: page1.jpg, page2.jpg, page3.jpg. time to transmit index.hml initiate TCP connection RTT GET index.html RTT file received GET page1.jpg time to transmit page1.jpg

10. Persistent HTTP Nonpersistent HTTP issues: r requires 2 RTTs per object r OS must work and allocate host resources for each TCP connection r but browsers often open parallel TCP connections to fetch referenced objects 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

11. HTTP request message 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

12. HTTP request message: general format

13. HTTP Methods r GET: retrieve a file (95% of requests) r HEAD: just get meta-data (e.g., mod time) r POST: submitting a form to a server r PUT: store enclosed document as URI r DELETE: removed named resource r LINK/UNLINK: in 1.0, gone in 1.1 r TRACE: http “echo” for debugging (added in 1.1) r CONNECT: used by proxies for tunneling (1.1) r OPTIONS: request for server/proxy options (1.1)

14. 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

15. HTTP Response Status Codes r 1XX: Informational (def’d in 1.0, used in 1.1) 100 Continue, 101 Switching Protocols r 2XX: Success 200 OK, 206 Partial Content r 3XX: Redirection 301 Moved Permanently, 304 Not Modified r 4XX: Client error 400 Bad Request, 403 Forbidden, 404 Not Found r 5XX: Server error 500 Internal Server Error, 503 Service Unavailable, 505 HTTP Version Not Supported

16. 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.eurecom.fr. Anything typed in sent to port 80 at www.eurecom.fr telnet www.eurecom.fr 80 2. Type in a GET HTTP request: GET /~ross/index.html HTTP/1.0 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!

17. Web Proxy Caching r user sets browser: Web accesses via cache r browser sends all HTTP requests to cache m object in cache => cache hit: returns object m else cache requests object from origin server, then returns object to client Objective: satisfy client request without involving origin server resulting in reduced server & network load, low latency to response client Proxy server client HTTP request HTTP response HTTP request HTTP response origin server origin server Cache acts as both client and server

18. Web Caching Hierarchy client local proxy cache (e.g., local ISP, University) regional proxy cache national/international proxy cache

19. Why Cache? 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

20. Weak Consistency r Each cached copy has a TTL beyond which it must be validated with the origin server r TTL = freshness life time – age m freshness life time: often heuristically calculated; sometimes based on MAX_AGE or EXPIRES headers m age = current time (at client) – timestamp on object (time at which server generated response) r Age Penalty?

21. 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

22. Content distribution networks (CDNs) r The content providers are the CDN customers. Content replication r CDN company installs hundreds of CDN servers throughout Internet m in lower-tier ISPs, close to users r CDN replicates its customers’ content in CDN servers. When provider updates content, CDN updates servers origin server in North America CDN distribution node CDN server in S. America CDN server in Europe CDN server in Asia

23. 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

24. 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:

25. 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