Transcript module_21
Module 2.1: Application Layer • Application-level protocols provide high-level services – – – – – – – – – K. Salah Web and HTTP DNS Electronic mail Remote login FTP telnet SNMP Multimedia - VoIP All of these applications use client-server architecture 1 World Wide Web • Hypertext model (HTML) • Use of hypertext in World Wide Web (WWW) • WWW client-server model • Use of TCP/IP protocols in WWW • Nice tutorials on HTML and HTTP can be found on course web source in resources section. K. Salah 2 Identifying a page • • Page identified by: – Protocol used to access page – Computer on which page is stored – TCP port to access page – Pathname of file on server Specific syntax for Uniform Resource Locator (URL): protocol://computer_name:port/document_name – Protocol can be http, ftp, file, mailto – Computer name is DNS name – (Optional) port is TCP port – document_name is path on computer to page K. Salah 3 Client Architecture • Browser has the following components: Display driver for painting screen HTML interpreter for HTML-formatted documents Other interpreters (e.g., Shockwave, JVM, VB runtime environment) for other items HTTP client to fetch HTML documents from WWW server Other clients for other protocols (e.g., ftp, telnet) Controller to accept input from user – Must be multi-threaded K. Salah 4 HTTP overview HTTP: hypertext transfer protocol • • • • Web’s application layer protocol PC running Explorer client/server model – client: browser that requests, receives, “displays” Web objects – server: Web server sends objects in response to requests Server running Apache Web server HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068 K. Salah Mac running Navigator 5 HTTP overview (continued) Uses TCP (socket programming): • • • • • • • Server listens on port 80 or 8080 for accepting connections from client HTTP is “stateless” client initiates TCP connection (creates socket) to server, port 80 • server accepts TCP connection from client, and creates thread to handle accepted connection Connection is established now between client and server thread HTTP messages between Client and Server are exchanged • TCP connection closed • Meanwhile server keeps listening for new connections • We will illustrate this aside further in TCP layer. K. Salah 6 server maintains no information about past client requests, as each request is process independently, without knowledge of the old requests. Server does not know who the client is? Or is it? Sample HTTP1.0 Exchange To retrieve the file at the URL http://www.somehost.com/path/file.html first open a socket to the host www.somehost.com, port 80 (use the default port of 80 because none is specified in the URL). Then, send something like the following through the socket: GET /path/file.html HTTP/1.0 From: [email protected] User-Agent: HTTPTool/1.0 [blank line here] The server should respond with something like the following, sent back through the same socket: HTTP/1.0 200 OK Date: Fri, 31 Dec 1999 23:59:59 GMT Content-Type: text/html Content-Length: 1354 <html> <body> <h1>Happy New Millennium!</h1> (more file contents) . . </body> </html> After sending the response, the server closes the socket servicing only one request per connection. K. Salah 7 HTTP request message: general format K. Salah 8 HTTP request message • two types of HTTP messages: request, response • HTTP request message: – ASCII (human-readable format) request line (GET, POST, HEAD commands) GET /somedir/page.html HTTP/1.1 Host: www.someschool.edu User-agent: Mozilla/4.0 header Connection: close lines Accept-language:fr Carriage return, line feed indicates end of message K. Salah (extra carriage return, line feed) 9 HTTP response message status line (protocol status code status phrase) header lines data, e.g., requested HTML file K. Salah 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 ... 10 HTTP connections Nonpersistent HTTP Persistent HTTP • At most one object is • Multiple objects can be sent sent over a TCP connection. over single TCP connection between client and server. • HTTP/1.0 uses • HTTP/1.1 uses persistent nonpersistent HTTP connections in default mode with pipelining. • Without pipelining client has to • • issue one request at a time. It can not issue a new request until it receives the response of the previous one. What is? • With pipelining client doesn’t HTTP Transaction – HTTP request and response have to wait for first response. Client can request multiple objects. HTTP Hit – For server statistics – Server sends one object to client K. Salah 11 HTTP 1.1 and higher • • As of early 1997, the Web is moving from HTTP 1.0 to HTTP 1.1. Whenever practical, use HTTP 1.1. It's more efficient overall. WHY? HTTP1.0 Client Request telnet www.ccse.kfupm.edu.sa 80 GET http://www.ccse.kfupm.edu.sa/~salah/ics432/index.html • HTTP1.1 Client Request telnet www.ccse.kfupm.edu.sa 80 GET http://www.ccse.kfupm.edu.sa/~salah/ics432/index.html HTTP/1.1 Host: www.ccse.kfupm.edu.sa:80 • HTTP1.2 Client Request telnet www.ccse.kfupm.edu.sa 80 GET http://www.ccse.kfupm.edu.sa/~salah/ics432/index.html HTTP/1.2 No need to have Host: field for every request. K. Salah 12 Uploading form input Post method: • Web page often includes form input • Input is uploaded to server in entity body URL method: • Uses GET method • Input is uploaded in URL field of request line: www.somesite.com/animalsearch?monkeys&banana K. Salah 13 Method types HTTP/1.0 HTTP/1.1 • GET • POST • HEAD • GET, POST, HEAD • PUT – asks server to leave requested object out of the response. Headers is only sent. Used for debugging. K. Salah 14 – uploads file in entity body to path specified in URL field • DELETE – deletes file specified in the URL field Cookies: keeping “state” Many major Web sites use cookies Example: – Susan access Internet always from same PC – She visits a specific ecommerce site for first time – When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID 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 K. Salah 15 Cookies: keeping “state” (cont.) client Cookie file ebay: 8734 Cookie file amazon: 1678 ebay: 8734 one week later: Cookie file amazon: 1678 ebay: 8734 K. Salah 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 16 Amazon server creates ID 1678 for user cookiespecific action cookiespectific action Cookies (continued) Cookies and privacy: What cookies can bring: aside • cookies permit sites to learn • authorization • shopping carts • recommendations • user session state (Web e- a lot about you • you may supply name and email to sites • search engines use mail) redirection & cookies to learn yet more • advertising companies obtain info across sites K. Salah 17 Web caches (proxy server) Goal: satisfy client request without involving origin 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 Web proxies can be multipurpose: • • • • client client cache, Firewall Monitor and log activites NAT K. Salah 18 origin server Proxy server origin server More about Web caching • • • • • Local cache can be used to hold copies of visited pages Why Web caching? • Cache acts as both client and server • Cache can do up-to-date check using If-modifiedsince HTTP header HTTP response messages has Age header. – Specifies how long the info is valid for. • Typically cache is installed by ISP (university, company, residential ISP) K. Salah 19 Reduce response time for client request. Downloading HTML documents from servers may be slow – Internet congested – Dialup connection – Server busy Reduce traffic on an institution’s access link. Conditional GET: client-side caching • • • Goal: don’t send object if client has up-to-date cached version server client HTTP request msg client: specify date of cached copy in HTTP request If-modified-since: <date> If-modified-since: <date> server: response contains no object if cached copy is up-todate: HTTP/1.0 304 Not Modified HTTP/1.0 304 Not Modified HTTP response HTTP request msg If-modified-since: <date> This is basically what happens when you hit “Refresh” or “update” button on the browser, I.e. GET with Ifmodified-since of current date. K. Salah object not modified HTTP response HTTP/1.0 200 OK <data> 20 object modified