What is the name of the protocol used to transfer the HTML documents of the web when the users use a web browser to access the webpages?
Suggest a new Definition Show
Proposed definitions will be considered for inclusion in the Economictimes.com Definition: A web server is a computer that runs websites. It's a computer program that distributes web pages as they are requisitioned. The basic objective of the web server is to store, process and deliver web pages to the users. This intercommunication is done using Hypertext Transfer Protocol (HTTP). These web pages are mostly static content that includes HTML documents, images, style sheets, test etc. Apart from HTTP, a web server also supports SMTP (Simple Mail transfer Protocol) and FTP (File Transfer Protocol) protocol for emailing and for file transfer and storage. Description: The main job of a web server is to display the website content. If a web server is not exposed to the public and is used internally, then it is called Intranet Server. When anyone requests for a website by adding the URL or web address on a web browser’s (like Chrome or Firefox) address bar (like www.economictimes.com), the browser sends a request to the Internet for viewing the corresponding web page for that address. A Domain Name Server (DNS) converts this URL to an IP Address (For example 192.168.216.345), which in turn points to a Web Server. The Web Server is requested to present the content website to the user’s browser. All websites on the Internet have a unique identifier in terms of an IP address. This Internet Protocol address is used to communicate between different servers across the Internet. These days, Apache server is the most common web server available in the market. Apache is an open source software that handles almost 70 percent of all websites available today. Most of the web-based applications use Apache as their default Web Server environment. Another web server that is generally available is Internet Information Service (IIS). IIS is owned by Microsoft.
Related NewsHTTP (HyperText Transfer Protocol)IntroductionThe WEBInternet (or The Web) is a massive distributed client/server information system as depicted in the following diagram. Many applications are running concurrently over the Web, such as web browsing/surfing, e-mail, file transfer, audio & video streaming, and so on. In order for proper communication to take place between the client and the server, these applications must agree on a specific application-level protocol such as HTTP, FTP, SMTP, POP, and etc. HyperText Transfer Protocol (HTTP)HTTP (Hypertext Transfer Protocol) is perhaps the most popular application protocol used in the Internet (or The WEB).
BrowserWhenever you issue a URL from your browser to get a web resource using HTTP, e.g. http://www.nowhere123.com/index.html, the browser turns the URL into a request message and sends it to the HTTP server. The HTTP server interprets the request message, and returns you an appropriate response message, which is either the resource you requested or an error message. This process is illustrated below: Uniform Resource Locator (URL)A URL (Uniform Resource Locator) is used to uniquely identify a resource over the web. URL has the following syntax: protocol://hostname:port/path-and-file-nameThere are 4 parts in a URL:
For example, in the URL http://www.nowhere123.com/docs/index.html, the communication protocol is HTTP; the hostname is www.nowhere123.com. The port number was not specified in the URL, and takes on the default number, which is TCP port 80 for HTTP. The path and file name for the resource to be located is "/docs/index.html". Other examples of URL are: ftp://www.ftp.org/docs/test.txt mailto: news:soc.culture.Singapore telnet://www.nowhere123.com/HTTP ProtocolAs mentioned, whenever you enter a URL in the address box of the browser, the browser translates the URL into a request message according to the specified protocol; and sends the request message to the server. For example, the browser translated the URL http://www.nowhere123.com/doc/index.html into the following request message: GET /docs/index.html HTTP/1.1 Host: www.nowhere123.com Accept: image/gif, image/jpeg, */* Accept-Language: en-us Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1) (blank line)When this request message reaches the server, the server can take either one of these actions:
An example of the HTTP response message is as shown: HTTP/1.1 200 OK Date: Sun, 18 Oct 2009 08:56:53 GMT Server: Apache/2.2.14 (Win32) Last-Modified: Sat, 20 Nov 2004 07:16:26 GMT ETag: "10000000565a5-2c-3e94b66c2e680" Accept-Ranges: bytes Content-Length: 44 Connection: close Content-Type: text/html X-Pad: avoid browser bugIt works!The browser receives the response message, interprets the message and displays the contents of the message on the browser's window according to the media type of the response (as in the Content-Type response header). Common media type include "text/plain", "text/html", "image/gif", "image/jpeg", "audio/mpeg", "video/mpeg", "application/msword", and "application/pdf". In its idling state, an HTTP server does nothing but listening to the IP address(es) and port(s) specified in the configuration for incoming request. When a request arrives, the server analyzes the message header, applies rules specified in the configuration, and takes the appropriate action. The webmaster's main control over the action of web server is via the configuration, which will be dealt with in greater details in the later sections. HTTP over TCP/IPHTTP is a client-server application-level protocol. It typically runs over a TCP/IP connection, as illustrated. (HTTP needs not run on TCP/IP. It only presumes a reliable transport. Any transport protocols that provide such guarantees can be used.) TCP/IP (Transmission Control Protocol/Internet Protocol) is a set of transport and network-layer protocols for machines to communicate with each other over the network. IP (Internet Protocol) is a network-layer protocol, deals with network addressing and routing. In an IP network, each machine is assigned an unique IP address (e.g., 165.1.2.3), and the IP software is responsible for routing a message from the source IP to the destination IP. In IPv4 (IP version 4), the IP address consists of 4 bytes, each ranges from 0 to 255, separated by dots, which is called a quad-dotted form. This numbering scheme supports up to 4G addresses on the network. The latest IPv6 (IP version 6) supports more addresses. Since memorizing number is difficult for most of the people, an english-like domain name, such as www.nowhere123.com is used instead. The DNS (Domain Name Service) translates the domain name into the IP address (via distributed lookup tables). A special IP address 127.0.0.1 always refers to your own machine. It's domian name is "localhost" and can be used for local loopback testing. TCP (Transmission Control Protocol) is a transport-layer protocol, responsible for establish a connection between two machines. TCP consists of 2 protocols: TCP and UDP (User Datagram Package). TCP is reliable, each packet has a sequence number, and an acknowledgement is expected. A packet will be re-transmitted if it is not received by the receiver. Packet delivery is guaranteed in TCP. UDP does not guarantee packet delivery, and is therefore not reliable. However, UDP has less network overhead and can be used for applications such as video and audio streaming, where reliability is not critical. TCP multiplexes applications within an IP machine. For each IP machine, TCP supports (multiplexes) up to 65536 ports (or sockets), from port number 0 to 65535. An application, such as HTTP or FTP, runs (or listens) at a particular port number for incoming requests. Port 0 to 1023 are pre-assigned to popular protocols, e.g., HTTP at 80, FTP at 21, Telnet at 23, SMTP at 25, NNTP at 119, and DNS at 53. Port 1024 and above are available to the users. Although TCP port 80 is pre-assigned to HTTP, as the default HTTP port number, this does not prohibit you from running an HTTP server at other user-assigned port number (1024-65535) such as 8000, 8080, especially for test server. You could also run multiple HTTP servers in the same machine on different port numbers. When a client issues a URL without explicitly stating the port number, e.g., http://www.nowhere123.com/docs/index.html, the browser will connect to the default port number 80 of the host www.nowhere123.com. You need to explicitly specify the port number in the URL, e.g. http://www.nowhere123.com:8000/docs/index.html if the server is listening at port 8000 and not the default port 80. In brief, to communicate over TCP/IP, you need to know (a) IP address or hostname, (b) Port number. HTTP SpecificationsThe HTTP specification is maintained by W3C (World-wide Web Consortium) and available at http://www.w3.org/standards/techs/http. There are currently two versions of HTTP, namely, HTTP/1.0 and HTTP/1.1. The original version, HTTP/0.9 (1991), written by Tim Berners-Lee, is a simple protocol for transferring raw data across the Internet. HTTP/1.0 (1996) (defined in RFC 1945), improved the protocol by allowing MIME-like messages. HTTP/1.0 does not address the issues of proxies, caching, persistent connection, virtual hosts, and range download. These features were provided in HTTP/1.1 (1999) (defined in RFC 2616). Apache HTTP Server or Apache Tomcat ServerA HTTP server (such as Apache HTTP Server or Apache Tomcat Server) is needed to study the HTTP protocol. Apache HTTP server is a popular industrial-strength production server, produced by Apache Software Foundation (ASF) @ www.apache.org. ASF is an open-source software foundation. That is to say, Apache HTTP server is free, with source code. The first HTTP server is written by Tim Berners Lee at CERN (European Center for Nuclear Research) at Geneva, Switzerland, who also invented HTML. Apache was built on NCSA (National Center for Supercomputing Applications, USA) "httpd 1.3" server, in early 1995. Apache probably gets its name from the fact that it consists of some original code (from an earlier NCSA httpd web server) plus some patches; or from the name of an American Indian tribe. Read "Apache How-to" on how to install and configuare Apache HTTP server; or "Tomcat How-to" to install and get started with Apache Tomcat Server. HTTP Request and Response MessagesHTTP client and server communicate by sending text messages. The client sends a request message to the server. The server, in turn, returns a response message. An HTTP message consists of a message header and an optional message body, separated by a blank line, as illustrated below: HTTP Request MessageThe format of an HTTP request message is as follow: Request LineThe first line of the header is called the request line, followed by optional request headers. The request line has the following syntax:
Examples of request line are: GET /test.html HTTP/1.1 HEAD /query.html HTTP/1.0 POST /index.html HTTP/1.1Request HeadersThe request headers are in the form of name:value pairs. Multiple values, separated by commas, can be specified. request-header-name: request-header-value1, request-header-value2, ...Examples of request headers are: Host: www.xyz.com Connection: Keep-Alive Accept: image/gif, image/jpeg, */* Accept-Language: us-en, fr, cnExampleThe following shows a sample HTTP request message: HTTP Response MessageThe format of the HTTP response message is as follows: Status LineThe first line is called the status line, followed by optional response header(s). The status line has the following syntax: HTTP-version status-code reason-phrase
Examples of status line are: HTTP/1.1 200 OK HTTP/1.0 404 Not Found HTTP/1.1 403 ForbiddenResponse HeadersThe response headers are in the form name:value pairs: response-header-name: response-header-value1, response-header-value2, ...Examples of response headers are: Content-Type: text/html Content-Length: 35 Connection: Keep-Alive Keep-Alive: timeout=15, max=100The response message body contains the resource data requested. ExampleThe following shows a sample response message: HTTP Request MethodsHTTP protocol defines a set of request methods. A client can use one of these request methods to send a request message to an HTTP server. The methods are:
"GET" Request MethodGET is the most common HTTP request method. A client can use the GET request method to request (or "get") for a piece of resource from an HTTP server. A GET request message takes the following syntax: GET request-URI HTTP-version (optional request headers) (optional request body)
Testing HTTP RequestsThere are many way to test out the HTTP requests. Your can use utility program such as "telnet" or "hyperterm" (search for "telnet.exe" or "hypertrm.exe" under c:\windows), or write you own network program to send raw request message to an HTTP server to test out the various HTTP requests. Telnet"Telnet" is a very useful networking utility. You can use telnet to establish a TCP connection with a server; and issue raw HTTP requests. For example, suppose that you have started your HTTP server in the localhost (IP address 127.0.0.1) at port 8000: > telnet telnet> help ... telnet help menu ... telnet> open 127.0.0.1 8000 Connecting To 127.0.0.1... GET /index.html HTTP/1.0 (Hit enter twice to send the terminating blank line ...) ... HTTP response message ...Telnet is a character-based protocol. Each character you enter on the telnet client will be sent to the server immediately. Therefore, you cannot make typo error in entering you raw command, as delete and backspace will be sent to the server. You may have to enable "local echo" option to see the characters you enter. Check the telnet manual (search Windows' help) for details on using telnet. Network ProgramYou could also write your own network program to issue raw HTTP request to an HTTP server. You network program shall first establish a TCP/IP connection with the server. Once the TCP connection is established, you can issue the raw request. An example of network program written in Java is as shown (assuming that the HTTP server is running on the localhost (IP address 127.0.0.1) at port 8000): import java.net.*; import java.io.*; public class HttpClient { public static void main(String[] args) throws IOException { String host = "127.0.0.1"; int port = 8000; Socket socket = new Socket(host, port); BufferedReader in = new BufferedReader( new InputStreamReader(socket.getInputStream())); PrintWriter out = new PrintWriter(socket.getOutputStream(), true); out.println("GET /index.html HTTP/1.0"); out.println(); out.flush(); String line; while((line = in.readLine()) != null) { System.out.println(line); } in.close(); out.close(); } }HTTP/1.0 GET RequestThe following shows the response of an HTTP/1.0 GET request (issue via telnet or your own network program - assuming that you have started your HTTP server): GET /index.html HTTP/1.0 (enter twice to create a blank line)HTTP/1.1 200 OK Date: Sun, 18 Oct 2009 08:56:53 GMT Server: Apache/2.2.14 (Win32) Last-Modified: Sat, 20 Nov 2004 07:16:26 GMT ETag: "10000000565a5-2c-3e94b66c2e680" Accept-Ranges: bytes Content-Length: 44 Connection: close Content-Type: text/html X-Pad: avoid browser bugIt works!Connection to host lost.In this example, the client issues a GET request to ask for a document named "/index.html"; and negotiates to use HTTP/1.0 protocol. A blank line is needed after the request header. This request message does not contain a body. The server receives the request message, interprets and maps the request-URI to a document under its document directory. If the requested document is available, the server returns the document with a response status code "200 OK". The response headers provide the necessary description of the document returned, such as the last-modified date (Last-Modified), the MIME type (Content-Type), and the length of the document (Content-Length). The response body contains the requested document. The browser will format and display the document according to its media type (e.g., Plain-text, HTML, JPEG, GIF, and etc.) and other information obtained from the response headers. Notes:
Response Status CodeThe first line of the response message (i.e., the status line) contains the response status code, which is generated by the server to indicate the outcome of the request. The status code is a 3-digit number:
Some commonly encountered status codes are:
More HTTP/1.0 GET Request ExamplesExample: Misspelt Request MethodIn the request, "GET" is misspelled as "get". The server returns an error "501 Method Not Implemented". The response header "Allow" tells the client the methods allowed. get /test.html HTTP/1.0 (enter twice to create a blank line)HTTP/1.1 501 Method Not Implemented Date: Sun, 18 Oct 2009 10:32:05 GMT Server: Apache/2.2.14 (Win32) Allow: GET,HEAD,POST,OPTIONS,TRACE Content-Length: 215 Connection: close Content-Type: text/html; charset=iso-8859-1Method Not Implementedget to /index.html not supported. Example: 404 File Not FoundIn this GET request, the request-URL "/t.html" cannot be found under the server’s document directory. The server returns an error "404 Not Found". GET /t.html HTTP/1.0 (enter twice to create a blank line)HTTP/1.1 404 Not Found Date: Sun, 18 Oct 2009 10:36:20 GMT Server: Apache/2.2.14 (Win32) Content-Length: 204 Connection: close Content-Type: text/html; charset=iso-8859-1Not FoundThe requested URL /t.html was not found on this server. Example: Wrong HTTP Version NumberIn this GET request, the HTTP-version was misspelled, resulted in bad syntax. The server returns an error "400 Bad Request". HTTP-version should be either HTTP/1.0 or HTTP/1.1. GET /index.html HTTTTTP/1.0 (enter twice to create a blank line)HTTP/1.1 400 Bad Request Date: Sun, 08 Feb 2004 01:29:40 GMT Server: Apache/1.3.29 (Win32) Connection: close Content-Type: text/html; charset=iso-8859-1Bad RequestYour browser sent a request that this server could not understand.The request line contained invalid characters following the protocol string.
Note: The latest Apache 2.2.14 ignores this error and returns the document with status code "200 OK". Example: Wrong Request-URIIn the following GET request, the request-URI did not begin from the root "/", resulted in a "bad request". GET test.html HTTP/1.0 (blank line)HTTP/1.1 400 Bad Request Date: Sun, 18 Oct 2009 10:42:27 GMT Server: Apache/2.2.14 (Win32) Content-Length: 226 Connection: close Content-Type: text/html; charset=iso-8859-1Bad RequestYour browser sent a request that this server could not understand. Example: Keep-Alive ConnectionBy fault, for HTTP/1.0 GET request, the server closes the TCP connection once the response is delivered. You could request for the TCP connection to be maintained, (so as to send another request using the same TCP connection, to improve on the network efficiency), via an optional request header "Connection: Keep-Alive". The server includes a "Connection: Keep-Alive" response header to inform the client that he can send another request using this connection, before the keep-alive timeout. Another response header "Keep-Alive: timeout=x, max=x" tells the client the timeout (in seconds) and the maximum number of requests that can be sent via this persistent connection. GET /test.html HTTP/1.0 Connection: Keep-Alive (blank line)HTTP/1.1 200 OK Date: Sun, 18 Oct 2009 10:47:06 GMT Server: Apache/2.2.14 (Win32) Last-Modified: Sat, 20 Nov 2004 07:16:26 GMT ETag: "10000000565a5-2c-3e94b66c2e680" Accept-Ranges: bytes Content-Length: 44 Keep-Alive: timeout=5, max=100 Connection: Keep-Alive Content-Type: text/htmlIt works!Notes:
Example: Accessing a Protected ResourceThe following GET request tried to access a protected resource. The server returns an error "403 Forbidden". In this example, the directory "htdocs\forbidden" is configured to deny all access in the Apache HTTP server configuration file "httpd.conf" as follows: ForbiddenYou don't have permission to access /forbidden/index.html on this server. HTTP/1.1 GET RequestHTTP/1.1 server supports so-called virtual hosts. That is, the same physical server could house several virtual hosts, with different hostnames (e.g., www.nowhere123.com and www.test909.com) and their own dedicated document root directories. Hence, in an HTTP/1.1 GET request, it is mandatory to include a request header called "Host", to select one of the virtual hosts. Example: HTTP/1.1 RequestHTTP/1.1 maintains persistent (or keep-alive) connection by default to improve the network efficiency. You can use a request header "Connection: Close" to ask the server to close the TCP connection once the response is delivered. GET /index.html HTTP/1.1 Host: 127.0.0.1 (blank line)HTTP/1.1 200 OK Date: Sun, 18 Oct 2009 12:10:12 GMT Server: Apache/2.2.14 (Win32) Last-Modified: Sat, 20 Nov 2004 07:16:26 GMT ETag: "10000000565a5-2c-3e94b66c2e680" Accept-Ranges: bytes Content-Length: 44 Content-Type: text/htmlIt works!Example: HTTP/1.1 Missing Host HeaderThe following example shows that "Host" header is mandatory in an HTTP/1.1 request. If "Host" header is missing, the server returns an error "400 Bad Request". GET /index.html HTTP/1.1 (blank line)HTTP/1.1 400 Bad Request Date: Sun, 18 Oct 2009 12:13:46 GMT Server: Apache/2.2.14 (Win32) Content-Length: 226 Connection: close Content-Type: text/html; charset=iso-8859-1Bad RequestYour browser sent a request that this server could not understand. Conditional GET RequestsIn all the previous examples, the server returns the entire document if the request can be fulfilled (i.e. unconditional). You may use additional request header to issue a "conditional request". For example, to ask for the document based on the last-modified date (so as to decide whether to use the local cache copy), or to ask for a portion of the document (or range) instead of the entire document (useful for downloading large documents). The conditional request headers include:
Request HeadersThis section describes some of the commonly-used request headers. Refer to HTTP Specification for more details. The syntax of header name is words with initial-cap joined using dash (-), e.g., Content-Length, If-Modified-Since. Host: domain-name - HTTP/1.1 supports virtual hosts. Multiple DNS names (e.g., www.nowhere123.com and www.nowhere456.com) can reside on the same physical server, with their own document root directories. Host header is mandatory in HTTP/1.1 to select one of the hosts. The following headers can be used for content negotiation by the client to ask the server to deliver the preferred type of the document (in terms of the media type, e.g. JPEG vs. GIF, or language used e.g. English vs. French) if the server maintain multiple versions for the same document. Accept: mime-type-1, mime-type-2, ... - The client can use the Accept header to tell the server the MIME types it can handle and it prefers. If the server has multiple versions of the document requested (e.g., an image in GIF and PNG, or a document in TXT and PDF), it can check this header to decide which version to deliver to the client. (E.g., PNG is more advanced more GIF, but not all browser supports PNG.) This process is called content-type negotiation. Accept-Language: language-1, language-2, ... - The client can use the Accept-Language header to tell the server what languages it can handle or it prefers. If the server has multiple versions of the requested document (e.g., in English, Chinese, French), it can check this header to decide which version to return. This process is called language negotiation. Accept-Charset: Charset-1, Charset-2, ... - For character set negotiation, the client can use this header to tell the server which character sets it can handle or it prefers. Examples of character sets are ISO-8859-1, ISO-8859-2, ISO-8859-5, BIG5, UCS2, UCS4, UTF8. Accept-Encoding: encoding-method-1, encoding-method-2, ... - The client can use this header to tell the server the type of encoding it supports. If the server has encoded (or compressed) version of the document requested, it can return an encoded version supported by the client. The server can also choose to encode the document before returning to the client to reduce the transmission time. The server must set the response header "Content-Encoding" to inform the client that the returned document is encoded. The common encoding methods are "x-gzip (.gz, .tgz)" and "x-compress (.Z)". Connection: Close|Keep-Alive - The client can use this header to tell the server whether to close the connection after this request, or to keep the connection alive for another request. HTTP/1.1 uses persistent (keep-alive) connection by default. HTTP/1.0 closes the connection by default. Referer: referer-URL - The client can use this header to indicate the referrer of this request. If you click a link from web page 1 to visit web page 2, web page 1 is the referrer for request to web page 2. All major browsers set this header, which can be used to track where the request comes from (for web advertising, or content customization). Nonetheless, this header is not reliable and can be easily spoofed. Note that Referrer is misspelled as "Referer" (unfortunately, you have to follow too). User-Agent: browser-type - Identify the type of browser used to make the request. Server can use this information to return different document depending on the type of browsers. Content-Length: number-of-bytes - Used by POST request, to inform the server the length of the request body. Content-Type: mime-type - Used by POST request, to inform the server the media type of the request body. Cache-Control: no-cache|... - The client can use this header to specify how the pages are to be cached by proxy server. "no-cache" requires proxy to obtain a fresh copy from the original server, even though a local cached copy is available. (HTTP/1.0 server does not recognize "Cache-Control: no-cache". Instead, it uses "Pragma: no-cache". Included both request headers if you are not sure about the server’s version.) Authorization: Used by the client to supply its credential (username/password) to access protected resources. (This header will be described in later chapter on authentication.) Cookie: cookie-name-1=cookie-value-1, cookie-name-2=cookie-value-2, ... - The client uses this header to return the cookie(s) back to the server, which was set by this server earlier for state management. (This header will be discussed in later chapter on state management.) If-Modified-Since: date - Tell the server to send the page only if it has been modified after the specific date. GET Request for DirectorySuppose that a directory called "testdir" is present in the document base directory "htdocs". If a client issues a GET request to "/testdir/" (i.e., at the directory).
It is interesting to take note that if a client issue a GET request to "/testdir" (without specifying the directory path "/"), the server returns a "301 Move Permanently" with a new "Location" of "/testdir/", as follows. GET /testdir HTTP/1.1 Host: 127.0.0.1 (blank line)HTTP/1.1 301 Moved Permanently Date: Sun, 18 Oct 2009 13:19:15 GMT Server: Apache/2.2.14 (Win32) Location: http://127.0.0.1:8000/testdir/ Content-Length: 238 Content-Type: text/html; charset=iso-8859-1Moved PermanentlyThe document has moved here. Most of the browser will follow up with another request to "/testdir/". For example, If you issue http://127.0.0.1:8000/testdir without the trailing "/" from a browser, you could notice that a trailing "/" was added to the address after the response was given. The morale of the story is: you should include the "/" for directory request to save you an additional GET request. Issue a GET Request through a Proxy ServerTo send a GET request through a proxy server, (a) establish a TCP connection to the proxy server; (b) use an absolute request-URI http://hostname:port/path/fileName to the target server. The following trace was captured using telnet. A connection is established with the proxy server, and a GET request issued. Absolute request-URI is used in the request line. GET http://www.amazon.com/index.html HTTP/1.1 Host: www.amazon.com Connection: Close (blank line)HTTP/1.1 302 Found Transfer-Encoding: chunked Date: Fri, 27 Feb 2004 09:27:35 GMT Content-Type: text/html; charset=iso-8859-1 Connection: close Server: Stronghold/2.4.2 Apache/1.3.6 C2NetEU/2412 (Unix) Set-Cookie: skin=; domain=.amazon.com; path=/; expires=Wed, 01-Aug-01 12:00:00 GMT Connection: close Location: http://www.amazon.com:80/exec/obidos/subst/home/home.html Via: 1.1 xproxy (NetCache NetApp/5.3.1R4D5) edFoundThe document has moved here.0 Take note that the response is returned in "chunks". "HEAD" Request MethodHEAD request is similar to GET request. However, the server returns only the response header without the response body, which contains the actual document. HEAD request is useful for checking the headers, such as Last-Modified, Content-Type, Content-Length, before sending a proper GET request to retrieve the document. The syntax of the HEAD request is as follows: HEAD request-URI HTTP-version (other optional request headers) (blank line) (optional request body)ExampleHEAD /index.html HTTP/1.0 (blank line) HTTP/1.1 200 OK Date: Sun, 18 Oct 2009 14:09:16 GMT Server: Apache/2.2.14 (Win32) Last-Modified: Sat, 20 Nov 2004 07:16:26 GMT ETag: "10000000565a5-2c-3e94b66c2e680" Accept-Ranges: bytes Content-Length: 44 Connection: close Content-Type: text/html X-Pad: avoid browser bugNotice that the response consists of the header only without the body, which contains the actual document. "OPTIONS" Request MethodA client can use an OPTIONS request method to query the server which request methods are supported. The syntax for OPTIONS request message is: OPTIONS request-URI|* HTTP-version (other optional headers) (blank line)"*" can be used in place of a request-URI to indicate that the request does not apply to any particular resource. ExampleFor example, the following OPTIONS request is sent through a proxy server: OPTIONS http://www.amazon.com/ HTTP/1.1 Host: www.amazon.com Connection: Close (blank line)HTTP/1.1 200 OK Date: Fri, 27 Feb 2004 09:42:46 GMT Content-Length: 0 Connection: close Server: Stronghold/2.4.2 Apache/1.3.6 C2NetEU/2412 (Unix) Allow: GET, HEAD, POST, OPTIONS, TRACE Connection: close Via: 1.1 xproxy (NetCache NetApp/5.3.1R4D5) (blank line)All servers that allow GET request will allow HEAD request. Sometimes, HEAD is not listed. "TRACE" Request MethodA client can send a TRACE request to ask the server to return a diagnostic trace. TRACE request takes the following syntax: TRACE / HTTP-version (blank line)ExampleThe following example shows a TRACE request issued through a proxy server. TRACE http://www.amazon.com/ HTTP/1.1 Host: www.amazon.com Connection: Close (blank line)HTTP/1.1 200 OK Transfer-Encoding: chunked Date: Fri, 27 Feb 2004 09:44:21 GMT Content-Type: message/http Connection: close Server: Stronghold/2.4.2 Apache/1.3.6 C2NetEU/2412 (Unix) Connection: close Via: 1.1 xproxy (NetCache NetApp/5.3.1R4D5) 9d TRACE / HTTP/1.1 Connection: keep-alive Host: www.amazon.com Via: 1.1 xproxy (NetCache NetApp/5.3.1R4D5) X-Forwarded-For: 155.69.185.59, 155.69.5.234 0(To compare the TRACE request with trace route) Submitting HTML Form Data and Query StringIn many Internet applications, such as e-commerce and search engine, the clients are required to submit additional information to the server (e.g., the name, address, the search keywords). Based on the data submitted, the server takes an appropriate action and produces a customized response. The clients are usually presented with a form (produced using HTML A form contains fields. The types of field include:
Each field has a name and can take on a specified value. Once the client fills in the fields and hits the submit button, the browser gathers each of the fields' name and value, packed them into "name=value" pairs, and concatenates all the fields together using "&" as the field separator. This is known as a query string. It will send the query string to the server as part of the request. name1=value1&name2=value2&name3=value3&...Special characters are not allowed inside the query string. They must be replaced by a "%" followed by the ASCII code in Hex. E.g., "~" is replaced by "%7E", "#" by "%23" and so on. Since blank is rather common, it can be replaced by either "%20" or "+" (the "+" character must be replaced by "%2B"). This replacement process is called URL-encoding, and the result is a URL-encoded query string. For example, suppose that there are 3 fields inside a form, with name/value of "name=Peter Lee", "address=#123 Happy Ave" and "language=C++", the URL-encoded query string is: name=Peter+Lee&address=%23123+Happy+Ave&Language=C%2B%2BThe query string can be sent to the server using either HTTP GET or POST request method, which is specified in the The HTTP GET request method is used to send the query string. Suppose the user enters "Peter Lee" as the username, "123456" as password; and clicks the submit button. The following GET request is: GET /bin/login?user=Peter+Lee&pw=123456&action=login HTTP/1.1 Accept: image/gif, image/jpeg, */* Referer: http://127.0.0.1:8000/login.html Accept-Language: en-us Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1) Host: 127.0.0.1:8000 Connection: Keep-AliveNote that although the password that you enter does not show on the screen, it is shown clearly in the address box of the browser. You should never use send your password without proper encryption. http://127.0.0.1:8000/bin/login?user=Peter+Lee&pw=123456&action=loginURL and URIURL (Uniform Resource Locator)A URL (Uniform Resource Locator), defined in RFC 2396, is used to uniquely identify a resource over the web. URL has the following syntax: protocol://hostname:port/path-and-file-nameThere are 4 parts in a URL:
For example, in the URL http://www.nowhere123.com/docs/index.html, the communication protocol is HTTP; the hostname is www.nowhere123.com. The port number was not specified in the URL, and takes on the default number, which is TCP port 80 for HTTP [STD 2]. The path and file name for the resource to be located is "/docs/index.html". Other examples of URL are: ftp://www.ftp.org/docs/test.txt mailto: news:soc.culture.Singapore telnet://www.nowhere123.com/Encoded URLURL cannot contain special characters, such as blank or '~'. Special characters are encoded, in the form of %xx, where xx is the ASCII hex code. For example, '~' is encoded as %7e; '+' is encoded as %2b. A blank can be encoded as %20 or '+'. The URL after encoding is called encoded URL. URI (Uniform Resource Identifier)URI (Uniform Resource Identifier), defined in RFC3986, is more general than URL, which can even locate a fragment within a resource. The URI syntax for HTTP protocol is: http://host:port/path?request-parameters#nameAnchor
"POST" Request MethodPOST request method is used to "post" additional data up to the server (e.g., submitting HTML form data or uploading a file). Issuing an HTTP URL from the browser always triggers a GET request. To trigger a POST request, you can use an HTML form with attribute method="post" or write your own network program. For submitting HTML form data, POST request is the same as the GET request except that the URL-encoded query string is sent in the request body, rather than appended behind the request-URI. The POST request takes the following syntax: POST request-URI HTTP-version Content-Type: mime-type Content-Length: number-of-bytes (other optional request headers) (URL-encoded query string)Request headers Content-Type and Content-Length is necessary in the POST request to inform the server the media type and the length of the request body. Example: Submitting Form Data using POST Request MethodWe use the same HTML script as above, but change the request method to POST. LOGINSuppose the user enters "Peter Lee" as username and "123456" as password, and clicks the submit button, the following POST request would be generated by the browser: POST /bin/login HTTP/1.1 Host: 127.0.0.1:8000 Accept: image/gif, image/jpeg, */* Referer: http://127.0.0.1:8000/login.html Accept-Language: en-us Content-Type: application/x-www-form-urlencoded Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1) Content-Length: 37 Connection: Keep-Alive Cache-Control: no-cache User=Peter+Lee&pw=123456&action=loginNote that the Content-Type header informs the server the data is URL-encoded (with a special MIME type application/x-www-form-urlencoded), and the Content-Length header tells the server how many bytes to read from the message body. POST vs GET for Submitting Form DataAs mentioned in the previous section, POST request has the following advantage compared with the GET request in sending the query string:
Note that although the password is not shown on the browser’s address box, it is transmitted to the server in clear text, and subjected to network sniffing. Hence, sending password using a POST request is absolutely not secure. File Upload using multipart/form-data POST Request"RFC 1867: Form-based File upload in HTML" specifies how a file can be uploaded to the server using a POST request from an HTML form. A new attribute type="file" was added to the tag of HTML When the browser encountered an tag with attribute type="file", it displays a text box and a "browse..." button, to allow user to choose the file to be uploaded. When the user clicks the submit button, the browser send the form data and the content of the selected file(s). The old encoding type "application/x-www-form-urlencoded" is inefficient for sending binary data and non-ASCII characters. A new media type "multipart/form-data" is used instead. Each part identifies the input name within the original HTML form, and the content type if the media is known, or as application/octet-stream otherwise. The original local file name could be supplied as a "filename" parameter, or in the "Content-Disposition: form-data" header. An example of the POST message for file upload is as follows: POST /bin/upload HTTP/1.1 Host: test101 Accept: image/gif, image/jpeg, */* Accept-Language: en-us Content-Type: multipart/form-data; boundary=---------------------------7d41b838504d8 Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1) Content-Length: 342 Connection: Keep-Alive Cache-Control: no-cache -----------------------------7d41b838504d8 Content-Disposition: form-data; name="username" Peter Lee -----------------------------7d41b838504d8 Content-Disposition: form-data; name="fileID"; filename="C:\temp.html" Content-Type: text/plainHome page on main server-----------------------------7d41b838504d8--Servlet 3.0 provides built-in support for processing file upload. Read "Uploading Files in Servlet 3.0". "CONNECT" Request MethodThe HTTP CONNECT request is used to ask a proxy to make a connection to anther host and simply relay the content, rather than attempting to parse or cache the message. This is often used to make a connection through a proxy. (Under Construction) Other Request MethodsPUT: Ask the server to store the data. DELETE: Ask the server to delete the data. For security consideration, PUT and DELETE are not supported by most of the production server. Extension methods (also error codes and headers) can be defined to extend the functionality of the HTTP protocol. (Under Construction) Content NegotiationAs mention earlier, HTTP support content negotiation between the client and the server. A client can use additional request headers (such as Accept, Accept-Language, Accept-Charset, Accept-Encoding) to tell the server what it can handle or which content it prefers. If the server possesses multiple versions of the same document in different format, it will return the format that the client prefers. This process is called content negotiation. Content-Type NegotiationThe server uses a MIME configuration file (called "conf\mime.types") to map the file extension to a media type, so that it can ascertain the media type of the file by looking at its file extension. For example, file extensions ".htm", ".html" are associated with MIME media type "text/html", file extension of ".jpg", ".jpeg" are associated with "image/jpeg". When a file is returned to the client, the server has to put up a Content-Type response header to inform the client the media type of the data. For content-type negotiation, suppose that the client requests for a file call "logo" without specifying its type, and sends an header "Accept: image/gif, image/jpeg,...". If the server has 2 formats of the "logo": "logo.gif" and "logo.jpg", and the MIME configuration file have the following entries: image/gif gif image/jpeg jpeg jpg jpeThe server will return "logo.gif" to the client, based on the client Accept header, and the MIME type/file mapping. The server will include a "Content-type: image/gif" header in its response. The message trace is shown: GET /logo HTTP/1.1 Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/x-shockwave-flash, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */* Accept-Language: en-us Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1) Host: test101:8080 Connection: Keep-Alive (blank line)HTTP/1.1 200 OK Date: Sun, 29 Feb 2004 01:42:22 GMT Server: Apache/1.3.29 (Win32) Content-Location: logo.gif Vary: negotiate,accept TCN: choice Last-Modified: Wed, 21 Feb 1996 19:45:52 GMT ETag: "0-916-312b7670;404142de" Accept-Ranges: bytes Content-Length: 2326 Keep-Alive: timeout=15, max=100 Connection: Keep-Alive Content-Type: image/gif (blank line) (body omitted)However, if the server has 3 "logo.*" files, "logo.gif", "logo.html", "logo.jpg", and "Accept: */*" was used: GET /logo HTTP/1.1 Accept: */* Accept-Language: en-us Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1) Host: test101:8080 Connection: Keep-Alive (blank line)HTTP/1.1 200 OK Date: Sun, 29 Feb 2004 01:48:16 GMT Server: Apache/1.3.29 (Win32) Content-Location: logo.html Vary: negotiate,accept TCN: choice Last-Modified: Fri, 20 Feb 2004 04:31:17 GMT ETag: "0-10-40358d95;404144c1" Accept-Ranges: bytes Content-Length: 16 Keep-Alive: timeout=15, max=100 Connection: Keep-Alive Content-Type: text/html (blank line) (body omitted)Accept: */*The following Apache’s configuration directives are relevant to content-type negotiation:
Language Negotiation and "Options MultiView"The "Options MultiView" directive is the simpler way to implement language negotiation. For Example: AddLanguage en .enSuppose that the client requests for "index.html" and send an "Accept-Language: en-us". If the server has "test.html", "test.html.en" and "test.html.cn", based on the client’s preference, "test.html.en" will be returned. ("en" includes "en-us".) A message trace is as follows: GET /index.html HTTP/1.1 Accept: */* Accept-Language: en-us Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1) Host: test101:8080 Connection: Keep-Alive (blank line)HTTP/1.1 200 OK Date: Sun, 29 Feb 2004 02:08:29 GMT Server: Apache/1.3.29 (Win32) Content-Location: index.html.en Vary: negotiate TCN: choice Last-Modified: Sun, 29 Feb 2004 02:07:45 GMT ETag: "0-13-40414971;40414964" Accept-Ranges: bytes Content-Length: 19 Keep-Alive: timeout=15, max=100 Connection: Keep-Alive Content-Type: text/html Content-Language: en (blank line) (body omitted)The AddLanguage directive is needed to associate a language code with a file extension, similar to MIME type/file mapping. Note that "Options All" directive does not include "MultiViews" option. That is, you have to explicitly turn on MultiViews. The directive LanguagePriority can be used to specify the language preference in case of a tie during content negotiation or if the client does not express a preference. For example: Character Set NegotiationA client can use the request header Accept-Charset to negotiate with the server for the character set it prefers. Accept-Charset: charset-1, charset-2, ...The commonly encountered character sets include: ISO-8859-1 (Latin-I), ISO-8859-2, ISO-8859-5, BIG5 (Chinese Traditional), GB2312 (Chinese Simplified), UCS2 (2-byte Unicode), UCS4 (4-byte Unicode), UTF8 (Encoded Unicode), and etc. Similarly, the AddCharset directive is used to associate the file extension with the character set. For example: AddCharset ISO-8859-8 .iso8859-8 AddCharset ISO-2022-JP .jis AddCharset Big5 .Big5 .big5 AddCharset WINDOWS-1251 .cp-1251 AddCharset CP866 .cp866 AddCharset ISO-8859-5 .iso-ru AddCharset KOI8-R .koi8-r AddCharset UCS-2 .ucs2 AddCharset UCS-4 .ucs4 AddCharset UTF-8 .utf8Encoding NegotiationA client can use the Accept-Encoding header to tell the server the type of encoding it supports. The common encoding schemes are: "x-gzip (.gz, .tgz)" and "x-compress (.Z)". Accept-Encoding: encoding-method-1, encoding-method-2, ...Similarly, the AddEncoding directive is used to associate the file extension with the an encoding scheme. For example: AddEncoding x-compress .Z AddEncoding x-gzip .gz .tgzPersistent (or Keep-alive) ConnectionsIn HTTP/1.0, the server closes the TCP connection after delivering the response by default (Connection: Close). That is, each TCP connection services only one request. This is not efficiency as many HTML pages contain hyperlinks (via tag) to other resources (such as images, scripts – either locally or from a remote server). If you download a page containing 5 inline images, the browser has to establish TCP connection 6 times to the same server. The client can negotiate with the server and ask the server not to close the connection after delivering the response, so that another request can be sent through the same connection. This is known as persistent connection (or keep-alive connection). Persistent connections greatly enhance the efficiency of the network. For HTTP/1.0, the default connection is non-persistent. To ask for persistent connection, the client must include a request header "Connection: Keep-alive" in the request message to negotiate with the server. For HTTP/1.1, the default connection is persistent. The client do not have to sent the "Connection: Keep-alive" header. Instead, the client may wish to send the header "Connection: Close" to ask the server to close the connection after delivering the response. Persistent connection is extremely useful for web pages with many small inline images and other associated data, as all these can be downloaded using the same connection. The benefits for persistent connection are:
In Apache HTTP server, several configuration directives are related to the persistent connections: The KeepAlive directive decides whether to support persistent connections. This takes value of either On or Off. KeepAlive On|OffThe MaxKeepAliveRequests directive sets the maximum number of requests that can be sent through a persistent connection. You can set to 0 to allow unlimited number of requests. It is recommended to set to a high number for better performance and network efficiency. MaxKeepAliveRequests 200The KeepAliveTimeOut directive set the time out in seconds for a persistent connection to wait for the next request. KeepAliveTimeout 10Range DownloadAccept-Ranges: bytes Transfer-Encoding: chunked(Under Construction) Cache ControlThe client can send a request header "Cache-control: no-cache" to tell the proxy to get a fresh copy from the original server, even thought there is a local cached copy. Unfortunately, HTTP/1.0 server does not understand this header, but uses an older request header "Pragma: no-cache". You could include both headers in your request. Pragma: no-cache Cache-Control: no-cache(More, Under Construction) REFERENCES & RESOURCES
Latest version tested: HTTP 1.1, Apache HTTP Server 2.2.14 Which protocol transfers HTML data from servers to browsers?HTTP (Hypertext Transfer Protocol) is the set of rules for transferring files -- such as text, images, sound, video and other multimedia files -- over the web. As soon as a user opens their web browser, they are indirectly using HTTP.
What is the HTTP protocol used for?Hypertext Transfer Protocol (HTTP) is an application-layer protocol for transmitting hypermedia documents, such as HTML. It was designed for communication between web browsers and web servers, but it can also be used for other purposes.
Which protocol is used for transferring a web page from a web server to a web browser?Hypertext Transfer Protocol (HTTP) is a method for encoding and transporting information between a client (such as a web browser) and a web server.
What do you mean by HTTP?HTTP, in full HyperText Transfer Protocol, standard application-level protocol used for exchanging files on the World Wide Web.
|