The course that gives CMU its Zip! Web Services Nov 26, Topics HTTP Serving static content Serving dynamic content

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1 The course that gives CMU its Zip! Web Services Nov 26, 2002 Topics HTTP Serving static content Serving dynamic content

2 Web History 1945: 1989: 1990: Vannevar Bush, As we may think, Atlantic Monthly, July, Describes the idea of a distributed hypertext system. A memex that mimics the web of trails in our minds. Tim Berners-Lee (CERN) writes internal proposal to develop a distributed hypertext system. Connects a web of notes with links. Intended to help CERN physicists in large projects share and manage information Tim BL writes a graphical browser for Next machines , F 02

3 Web History (cont) NCSA server released 26 WWW servers worldwide Marc Andreessen releases first version of NCSA Mosaic browser Mosaic version released for (Windows, Mac, Unix). Web (port 80) traffic at 1% of NSFNET backbone traffic. Over 200 WWW servers worldwide. Andreessen and colleagues leave NCSA to form "Mosaic Communications Corp" (now Netscape) , F 02

4 Internet Hosts , F 02

5 # # # # Web Servers Clients and servers communicate using the HyperText Transfer Protocol (HTTP) Client and server establish TCP connection Client requests content Server responds with requested content Client and server close connection (usually) Current version is HTTP/1.1 # RFC 2616, June, Web client (browser)! " Web server , F 02

6 Web Content Web servers return content to clients content: a sequence of bytes with an associated MIME (Multipurpose Internet Mail Extensions) type Example MIME types $&%(')$+*-,)$/.)0 $&%(')$+* &5 3& $&4+785)* $+9&6+:+4&1;$ 4-.<3+=+%2*&=+42> 4-.<3+=+%2*+?-12%+= HTML document Unformatted text Postcript document Binary image encoded in GIF format Binary image encoded in JPEG format , F 02

7 Static and Dynamic Content The content returned in HTTP responses can be either static or dynamic. Static content: content stored in files and retrieved in response to an HTTP request Examples: HTML files, images, audio clips. Dynamic content: content produced on-the-fly in response to an HTTP request Example: content produced by a program executed by the server on behalf of the client. Bottom line: All Web content is associated with a file that is managed by the server , F 02

8 URLs Each file managed by a server has a unique name called a URL (Universal Resource Locator) URLs for static CD629ECD6-.2F CD629ECD6-.2F CD%+G&F)*&4&52G+%&' CD629ECD6-.2F CD%+G&F MNPO!Q RSUT Identifies a file called L managed by a Web server at VVV O WO RXO!M that is listening on port 80. URLs for dynamic CD629ECD6-.2F MM Identifies an executable file called a, managed by a Web server at VVV O <O RbO!M that is listening on port 8000, that should be called with two argument strings: cdeee and fcg , F 02

9 How Clients and Servers Use URLs Example URL: Clients use prefix ( What kind of server to contact (Web server) Where the server is ( B+B+B CD3+7+0 CD6+7-. ) What port it is listening on (80) Servers use suffix ( ) to: Determine if request is for static or dynamic content. No hard and fast rules for this. Convention: executables reside in h Lji kl directory ) to infer: Find file on file system. Initial l in suffix denotes home directory for requested content. Minimal suffix is l, which all servers expand to some default MNPO!Q RS home page (e.g., L ) , F 02

10 v Anatomy of an HTTP Transaction m/n-odprq sdt/udv-t-s w/w/w+xzy/{/u x~}/{j / -ƒd odn/ / /ˆ+ ~Š/ / + ~Š/Œ- & ~ /Ž+ r / / n/n/ - / D r / - ~ D - Dš D ƒ / D ƒ o œ~ -ž&œ Ÿ / / / - / Š+ ~Š + / / ) / - ~ D / / - / Š+ ~ / / / & J - - Jª ƒ/ o n2 Š+ ~ «D 2 n2 / n / / /Š /Œ+ ~ˆ- & ~Œ/ Ÿ j ƒd ƒ ± o ƒd ƒ² + ~ ³- / ƒd ƒ² + ~Ž+ ~Ž / n- D /n- D / š 2 D /p- - 8 / n- D /n- / /n/ - D 2 Œ/ / - D - & q / / - & q / n/n/ - / /o n D / ¹ ƒ -od /n + m/n-odp/q Client: open connection to server Telnet prints 3 lines to the terminal Client: request line Client: required HTTP/1.1 HOST header Client: empty line terminates headers Server: followed by five response headers Server: expect HTML in the response body Server: expect 42,092 bytes in the resp body Server: empty line ( µ~ +µ ) terminates hdrs Server: first HTML line in response body Server: 766 lines of HTML not shown. Server: last HTML line in response body Server: closes connection Client: closes connection and terminates , F 02

11 HTTP Requests HTTP request is a request line,, followed by zero or more request headers Request line: º2»2%+:¼9&4+7&5¼½ *+\EC/\ ¾Á + +À º2F2: 4+½ ) is HTTP version of request (¾2 + +À * \EC/I is typically URL for proxies, URL suffix for servers. º+.)%¼$-,27 G+½ is either Â8Ã2 Ä À&żÆ+ Ä Å2À 2Ç-Å+È¼Æ Ä ¾+Ã&É¼Ê Ä À&Ë2 Ä Ê+Ã2Ì&ü &Ã Ä or 8Í&É)Î8à C or , F 02

12 HTTP Requests (cont) HTTP methods: Â&Ã2 À&Å2Æ+ : Retrieve static or dynamic content Arguments for dynamic content are in URI Workhorse method (99% of requests) : Retrieve dynamic content Arguments for dynamic content are in the request body Å2À+ 2ÇÏÅ+È2Æ : Get server or file attributes ¾+Ã&É2Ê : Like Â&ü but no data in response body À&Ë2 Ê+Ã2Ì8Ã2 &à &Í&É)Î8à : Write a file to the server! : Delete a file on the server! : Echo request in response body Useful for debugging , F 02

13 Õ L L a S HTTP Requests (cont) Request headers: > Provide additional information to the server. Major differences between HTTP/1.1 and HTTP/1.0 HTTP/1.0 uses a new connection for each transaction. HTTP/1.1 also supports persistent connections multiple transactions over the same connection Ð PÑ Ò HTTP/1.1 requires ¾+żÆ+ header WÑ ÖQ izó LÔ V Õ O R SØO <O RbO!M HTTP/1.1 adds additional support for caching , F 02

14 Î Î HTTP Responses HTTP response is a response line followed by zero or more response headers. Response line: <version> is HTTP version of the response. <status code> is numeric status. <status msg> is corresponding English text. 200 OK Request was handled without error 403 Forbidden Server lacks permission to access file 404 Not found Server couldn t find the file. Response headers: Provide additional information about response 785)$&%85)$+Y &Ù MIME type of content in response body. 785)$&%&5)$ Y %&5¼=2$-, Length of content in response body , F 02

15 Request to Apache Server From IE Browser Ÿ / Ú + Û - & / / - / Š+ ~Š ³& / Ú š + Ü Ü ³& / D š n/ /m / 2 /n m ³& / D š D n- -odn/ 2 -Ý/o š / ¹ D D Þ- ƒ/ ³- / /n- + Ý/o / D Œ+ ~ ßÛ D /š- /oj +à D Œ+ ~ /Š&à á&odn/ D -â + /m ƒ ã - D ~ m2 Û / /m / n/n/ - r /o n2 / / š ³ o /ä ŕå ßÛæ ƒ ædn&â , F 02

16 Response From Apache Server «Ÿ ƒ ³ ß Þ ¹ Ÿ Œ š š ³ o š ³ o š q q q / / - r Š+ ~Š / / / D 2 /m¼ / Dm Š- / / /Œ+ ~ / + ~Š/ˆ j ƒd š- D / Š+ ~Ž+ ~Ž ç/ /n / Š+ ~ / n-odp&â J -o od / 2 /m2 / Dm Š- / / /Žè ~Ž/Ž+ ~ /Š - / 2 éúœ/ J / & ¹/ Ž/ê- / / DŠ/ /Š-é ³& / D ä/ n/ / D / n- D rn- / /n/ - D 2 ê- / / 2 /o m- Dë-Š/ˆ+ - p/ë-š/ / / n/n/ - r /o n2 / / / n- D /nï D / 2 D /p- - & rä /å Ï & / /q /or /q š- / /o/ /q / /q -ŠDq š- / -ŠDq - & , F 02

17 l l Ô O Serving Dynamic Content Client sends request to server. ìí h L(i kl OîS lc c If request URI contains the string, then the server assumes that the request is for dynamic content. Client Server , F 02

18 Ô l Serving Dynamic Content (cont) The server creates a child process and runs the program identified by the URI in that process Client Server ï Õ N OîS , F 02

19 Ô Serving Dynamic Content (cont) The child runs and generates the dynamic content. The server captures the content of the child and forwards it without modification to the client Client Content Server Content OîS , F 02

20 Ô Issues in Serving Dynamic Content How does the client pass program arguments to the server? How does the server pass these arguments to the child? How does the server pass other info relevant to the request to the child? How does the server capture the content produced by the child? These issues are addressed by the Common Gateway Interface (CGI) specification. Client Request Content Server Content Create OîS , F 02

21 CGI Because the children are written according to the CGI spec, they are often called CGI programs. Because many CGI programs are written in Perl,, they are often called CGI scripts. However, CGI really defines a simple standard for transferring information between the client (browser), the server, and the child process , F 02

22 add.com: THE Internet addition portal! Ever need to add two numbers together and you just can t find your calculator? Try Dr. Dave s addition service at add.com: THE Internet addition portal! Takes as input the two numbers you want to add together. Returns their sum in a tasteful personalized message. After the IPO we ll expand to multiplication! , F 02

23 The add.com Experience input URL host port CGI program args Output page , F 02

24 Serving Dynamic Content With GET Question: How does the client pass arguments to the server? Answer: The arguments are appended to the URI Can be encoded directly in a URL typed to a browser or a URL in an HTML CD6+7-.E*&6+=+42Y/Z)4&5)*&3+G+G+%+:2[+\+^&_ 3+G+G+% : is the CGI program on the server that will do the addition. argument list starts with ð arguments separated by ð spaces represented by ð&ò ¹ ƒ š ƒ - D /ë/ / - / /o n/ë&éj - / / ô 2 ~ D D -o &odn š- r / éjq , F 02 [+ñ ^+ñ ñ 7 : ð ó _2I ñ Can also be generated by an HTML form

25 Serving Dynamic Content With GET CD6+7-.E*&6+=+42Y/Z)4&5)*&3+G+G+%+:2[+\+^&_ Result displayed on browser: Welcome to add.com: THE Internet addition portal. The answer is: = 3 Thanks for visiting! Tell your friends , F 02

26 Serving Dynamic Content With GET Question: : How does the server pass these arguments to the child? Answer: In environment variable QUERY_STRING A single string containing everything after the? For add.com: õ+ë+ã+í ö& )Æ+ &Í)Ç-È2Â = \+^&_ Ü D -o / D / D / D / ƒ /m /n- o Ü o ¹ ß/ßø -m ¹ ë / - D /n ß~éJù Þ/ ä/údû& ä ± Ÿ é/â/â ë/ë ± Þ / &â ü /p-o/ 2ßÛŠ-â-à ý Ü /p- ƒ / ƒ -Š n/ ƒ - ¹/ƒ D -m ¹ n/ n ƒ Ü / / n-š ë o2ß ƒ -Š-â-à n- ë o2ß ƒ - -â-à , F 02

27 Serving Dynamic Content With GET Question: How does the server pass other info relevant to the request to the child? Answer: In a collection of environment variables defined by the CGI spec , F 02

28 Some CGI Environment Variables General Æ&Ã+Í&þ2Ã+Í& )Æ&Å2ÿ+ +É2Í+à Æ&Ã+Í&þ2Ã+Í& 2È&É 2à Â-É) &à +É2ö& EÇ-È2 &Ã+Í2ÿÏÉ)Î&à (CGI version) Request-specific Æ&Ã+Í&þ2Ã+Í& )À&Å+Í2 Í+Ã+õ+Ë+Ã2Æ+ - 2Ã2 &¾+Å+Ê (Â&Ã2, À&Å2Æ+, etc) õ+ë+ã Í+ö& )Æ+ &Í)ÇÏÈ2 (contains Â&Ã2 args) Í+à 2ż &Ã& 2¾+Å2Æ+ (domain name of client) Í+à 2Å2 &Ã& É2Ê+Ê+Í (IP address of client) Î&Å+È2 &Ã+È2 - ) &ö2à&ã (for À&Å2Æ+, type of data in message body, e.g., $&%&')$+*-,)$/.)0 ) Î&Å+È2 &Ã+È2 - )Ì&à È2Â+ &¾ (length in bytes) , F 02

29 Ó Ó Some CGI Environment Variables In addition, the value of each header of type type received from the client is placed in environment variable type Examples: ÐÐí í ìí (any - is changed to _ ) , F 02

30 # Serving Dynamic Content With GET Question: How does the server capture the content produced by the child? 9+$&G+7&F)$ Answer: The child generates its output on 9+$&G+7&F)$ to redirect 9+$&G+7&F)$ 9+$&GA7&F)$ to its connected socket.. Server uses G(F+12_ G&F+12_ Notice that only the child knows the type and size of the content. Thus the child (not the server) must generate the corresponding headers. Ü D -o / /n/ ƒ D D / ƒ m ƒ odn/ Ü š ƒ odn- ¹ ßÛ n- D /n- + é~á- D / 2 ~ D / / n- D ƒ n/ - / -o/ /o n š- ƒ - æ š q / n - ƒ o D ë D -æ š q n ¹ ƒ o o/ /odn/ ~æ ƒ ædn&é8 n-š+ n- + n-š/n- -â-à Ü D -o / /n/ ƒ D D / / / ƒ/ n/ n D o/ nï D /n- Ü š ƒ odn- ¹ ß~é / n- D rn- /n/ - D 2 D -æ ƒ ædn&é& ƒ /n)ßû n- Ú /n- -â/â-à š ƒ odn- ¹ ß~é / n- D /n- š 2 D /p- - & æ ƒ ædn&é/â-à š ƒ odn- ¹ ß~éDæ ƒ ædn&é/â-à š ƒ odn- ¹ ß~é é& n- D /n- -â-à , F 02

31 Serving Dynamic Content With GET q Ÿ / /odn / / / / D -o &odn / / ƒ Š D / / - / Š+ ~Š + / z ~ ~ m2 Û / /m ~ / / / /ä ŕå/q HTTP request received by Tiny Web server ƒ ƒ o Ÿ ƒ ƒ š ƒ n ƒ š q n ƒ o Š ë Ž š q ¹ ƒ o n ¹ ƒ -o/ / ²q D n/ - / / / / -ƒd odn/ Š/ / + ~ + ~ r / + ~ /ˆ+ / / / n/n/ - / D / çd³ / ¼ - -Þ /«/Þ - Dš D / D œ~ -ž&œ / D -o &odn / / ƒ Š D / / - / Š+ ~Š + / z ~ ~ m2 Û / /m ~ / / / /ä ŕå/q / / - / Š+ ~Š / / / ƒd odn á- Ú ƒd / n- D /n- /n/ Ï D 2 Š/ / / n- D /n- 2 Ú /p- - & /ä ŕå/q á- D / 2 ~ D / r n- D n/ - / -o/ /o š- - / - n o/ /odn/ / n/n/ - / /o D / -od /n + -o/ / ²q HTTP request sent by client HTTP response generated by the server HTTP response generated by the CGI program , F 02

32 Proxies A proxy is an intermediary between a client and an origin server. To the client, the proxy acts like a server. To the server, the proxy acts like a client. HTTP request HTTP request Client HTTP response Proxy HTTP response Origin Server , F 02

33 Why Proxies? Can perform useful functions as requests and responses pass by Examples: Caching, logging, anonymization Client A Request ¹ / Client B Request ¹ / ¹ / Û - & Û Ï & ¹ / Û - & Û - & Proxy cache Request ¹ / ¹ / Û - & Û - & Slower more expensive global network Origin Server Fast inexpensive local network , F 02

34 For More Information Study the Tiny Web server described in your text Tiny is a sequential Web server. Serves static and dynamic content to real browsers. text files, HTML files, GIF and JPEG images. 220 lines of commented C code. Also comes with an implementation of the CGI script for the add.com addition portal , F 02