1.0 HYPERTEXT TRANSFER PROTOCOL

1.1 INTRODUCTION

The hypertext transfer protocol (http) is an application level protocol used mainly to access data on the world-wide web (www). The protocol transfer data in the form of plain text, hypertext, audio, video and so on. However, it is called the http because its efficiency allows its use in a hypertext environment where they are rapid jumps from document to another document. The http is the basic underlying application-level protocol used to facilitate the transmission of data to and from a web server. Http provide a simple, fast way to specify the interaction between client and server. The protocol actually defines how a client must ask for data from the server and how the server returns it. Http does not specify how the data is actually transferred. Since http is only the application level protocol, it only specifies the discussion and negotiation between the client and server. The actual data transfer is done by low level protocol, like transmission control protocol (TCP). The default port for http is 80, but other ports can be used.

    The http protocol is connectionless and stateless. This means, after the server has responded to the client's request, the connection between client and server is closed and forgotten. There is no "memory" between client and server. The pure http server treats every request as if it is brand-new.

    Another feature of http is that it is flexible in the formats that it can handle. When a client issues a request to a server, it may include a prioritized list of formats that it can handle, and the server replies with the appropriate format. For example, if a browser cannot handle images, so a web server need not transmit any images on web pages. This arrangement prevents the transmission of unnecessary information and provides the basis for extending the set of formats with new standardized and proprietary specifications.

    The http message are not intended to be read by humans, they are read and interpreted by the http server and http client (browser). Http uses Internet Media Types (formerly know as MIME Content-types) to provide opened and extensive data typing and type negotiation. When http server transmits back information to the client, it includes a MIME-like header to inform the client what kind of data to be follows by the header. 

    MIME (Multipurpose Internet Mail Extensions) is a protocol to transmit text files with headers that indicate binary data that will be returned to the client, in order to allow the communication of multimedia. Each MIME header consists of 2 parts; i.e. type and subtype. Some examples are:

Content-type: text/html or video/quicktime or image/gif

When a server delivers a file to the client, the MIME type is specified by the Content-type. The client (browser) receives this information and processes accordingly with the appropriate utility (image viewer, movie player etc). If the content-type is text/html, the browser will render it in the browser window. If the content-type is application/quicktime and regconised by the browser, the browser may look for the appropriate add-in application (movie player) to display or process it, or ask the user if he want to save it to the disk if the browser does not regconised the file type. 

    The idea of http is very simple. A client (the browser of the home PC user, for example) sends a request, which looks like mail, to the server in the WWW. The server send the response, which looks like a mail reply to the client. The http messages are delivered immediately. The command from the client to the server are embedded in a letter-like request message. The content of the response file or other information are embedded in a letter-like response message.

1.2 VERSION AND HISTORY

The first version of http being used is version 0.9 started on year 1990, which allows row data transfer across the www. The current version of http 1 is supported by most servers and clients. It allows transmission of hypermedia with meta-information (information of the information) about the information being transferred and modifiers on the request/response semantics. However, http 1 does has some problem e.g. does not properly handle the effects of hierarchical proxies and caching. Http 1.1 solves many of these problems and is supported by newer version of web browsers and server.

1.3 TERMINOLOGY

Protocol - A method in transferring data between client and server.

Hypertext - Text or page of text with certain specific text or "hot-spot" that in-line with other text in the browser, which once being clicked with cause the browser window to display another page. The term hypermedia should be used when the browser displays other object such as image, animation, video, audio etc.

Client - The receiver which originates the request of data or information. The hardware could be a home personal computer and the software could be the web browser. 

User Agent - The client that initiates a request. These are often browsers, editor, spiders or other end-user tools.

Server - The computer/program that accept connections in order to service requests by sending back response. Which usually stores data readily to be sent or receive data from client and process it before sending to the client upon request. In the WWW, one server serves to numerous clients.

Origin server - The server on which a given resource resides or is to be created.

Message - Basic unit of http communication, consisting of a structured sequence of octets transmitted via the connection.

Connection - A transport layer virtual circuit established between two application programs for the purposes of communication.

World-wide web (www) - The www is composed of millions of parts called web sites, all of which are interconnected to form the largest electronic database in the world. Each web site is made up of at least one web page, the basic element of www. The web pages are constructed using Hypertext Markup Language (HTML) as discussed in Section 7.

Browser - A program that that interprets, renders and display the HTML code in the web page, such as Internet Explorer or Nestcape Navigator.

Catche - A program's local store of response messages and the information that controls its message storage, retrieval and deletion. A Cacheable response can reduce the response time and required network bandwidth.

2.0 HTTP TRANSACTION

Figure below shows the http transaction between the client and server. Although http uses the services of TCP, http itself is a stateless protocol. The client initialise the transaction by sending a request message. The server reply by sending a response.

Figure 1: HTTP transaction

There are 3 types of http operation:

1. The simplest http operation is in which a user agent established a direct connection with and origin server, as illustrated in first part of Figure 1.1. The user agent is the client that initiates the request, such as a web browser being run on behalf of an end user. The origin server is the server on which a resource of interest of interest resides, an example is a web server at which a desire web home page resides. For this case, the client opens a TCP connection that is end-to-end between the client and the server. The client then issues an http request. When the server receives the request, it attempts to perform the requested action and then returns an http response. The TCP connection is then closed.

2. The middle part of Figure 1.1 shows a case in which there is not an end-to-end TCP connection between the user agent and the origin server. Instead, there are one or more intermediate systems with TCP connections between logically adjacent systems. Each intermediate system acts as a relay, so that a request initiated by the client is relayed through the intermediate systems to the server, and the response from the server is relayed back to the client.

3. The lowest part of Figure 1.1 shows an example of a cache. A cache is a facility that may store previous requests and response for handling new requests. If a new request arrives that is the same as a stored request, then the cache can supply the stored response rather than accessing the resource indicated in the URL. The cache can operate on a client or server, or on an intermediate system other than a tunnel (as explained later). In this example, intermediary B has cached a request/response transaction, so that a corresponding new request from the client need not travel the entire chain to the origin server, but is handled by B.

Figure 1.1 : Example of HTTP Operation

There are 3 forms of intermediate systems as illustrated in Figure 1.2:

Figure 1.2 : Intermediate HTTP systems

There are 2 types of http message in http operation, i.e. request message and response message as described in Section 3 and 4.

3.0 REQUEST MESSAGE

A request message consists of a request line, header (to be discussed in Section 5), a blank line and body (in certain message).

Figure 2 : Request message

    The request line defines the request type, resource (URL), and http version. The request line consists of a request type, a space, a URL, a space and http version.

Figure 3 : Request line

3.1 Request type

- In version 1.1 of http, several request types are defined. The request type categorises the request message into several method (see Section 3.8).

3.2 URL

- A client that wants to access to a web page needs an address. To facilitate the access of document distributed throughout the world, http uses the concept of location. The Uniform Resource Locater (URL) is a standard for specifying any kind of information on the internet. The URL defines 4 things: method, host, computer, port and path.

Figure 4 : URL

3.3 Method

    The method is the protocol used to retrieved the document. Several different protocols can retrieve a document, among them are Gopher, ftp, http, News and Telnet.

3.4 Host

    The host is the computer where the information is located although the name of the computer can be an alias. Web pages are usually stored in computers, and computers are given alias name that usually begin with the character "www". This is not mandatory, however, as the host can be any name given to the computer.

3.5 Port

    The URL optionally can obtain the port number of the server. If the port is included, it should be inserted between the host and the path, and it should be separated from the host by a colon. The default port for http is 80.

3.6 Path

    Path is the pathname of the file where the information is located. The slash separate the directory from the subdirectory and filename.

3.7 HTTP version

    The latest http version is 1.0, butl version 0.9 is still in use.

3.8 Request type or method

The request type field in request message defines several kinds of messages referred to as method. The request method is the actual command or request that a client issues to the server.

    The table below shows some of the method and their purposes:

Table 1: Method name and description

4.0 RESPONSE MESSAGE

A response message consists of a status line, a header (to be discussed in Section 5.0), and sometimes a body.

Figure 5 : Response message

Status line:- The status line define the status of the response message. It consists of the http version, a space, a status code, a space, a status phrase.

Figure 6 : Status line

Http version:- This field is same as the field in the request line.

Status code:- The server response to the client with information about itself and the data being returned. The information about itself is given in a form of status code. Certain status code is returned corresponding to the method of request.

The status code consists of 3 digits. The codes in the 100 range (1xx) are only informational and the process will be continued, the codes in 200 range (2xx) indicate that the request is successful, understood and accepted. The codes in 300 range (3xx) redirect the client to another URL and further action must be taken in order to complete the request, the codes in 400 range (3xx) indicate an error at the client site where the request contains bad syntax or cannot be fulfilled. Lastly, the codes in 500 range (5xx) indicate an error at the server site where the server failed to fulfill an apparently valid request.

Table 2 : Status code and status phrase

Reason phrase/status phrase:- this fields explains the status code in text form, as shown in above table.

5.0 HEADER

The header gives additional information between the client and the server. For example, the client can request that the document be sent on a specific format or the server can send extra information about the document.

    The header can be one or more header lines. Each header line is made of a header name, a colon, a space, and a header value.

Figure 7 : Header Format

    A header line belongs to one or 4 categories: general header, request header, response header and entity header. A request message can contain only general, request and entity header. A response message can contain only general, response and entity header. 

Figure 8 : Request header and response header

General header:- General header gives general information about the message and can be present in both a request and response message. It is included only when required to give additional or special information in the transaction..

Table 3 : General header

Request header:- Request header can only be present in request message. It specifies the client's configuration and the client's preferred document format. The request header fields allow the client to pass additional information about the request, and about the client itself, to the server.

Table 4 : Request header

Response header:- Can only be present in response message. It specifies a server's configuration and special information about the request. Some of the response headers are same as request headers, but with different interpretation. E.g. MIME-Version in request header indicates the required version should be returned by the server, while MIME-Version in response header indicates the actual version returned by the server.

Table 5 : Response header

Entity header:- It gives information about the body of document. Although it is mostly present in response message, some request message like POST or PUT methods, that contain a body also use this type of header.

Table 6 : Entity header

6.0 EXAMPLE OF HTTP TRANSACTION

6.1 Example 1

Figure 9 : Example illustrating http transaction using GET method

Assuming that in a web page, there is an image with file name "image" needed to be displayed. The browser hence initiates a command to ask for a image file named "image" located on URL /user/bin/image/. In the http transaction, this involves the use of GET method in the request message. The request line in this example is "GET /user/bin/image/ HTTP/1.1", meaning that the client sends a command to retrieve a file name "image" which is located on /user/bin/image/, by using the http version of 1.1. There are 2 headers in this example, i.e. "Accept: image.gif" and "Accept: image.jpeg", which gives additional information to the server that, the file could be with extension of either .gif or .jpeg. Assuming that the server could found the said file, the response message will be as shown in Figure 9 with the response status "http/1.1 200 OK", meaning that the used http version is 1.1, and status code of 200 indicates that the document requested was found and returned in this response to the client. There are 3 headers from server to give additional information to the client, i.e. "Server: Chanllenger", "MIME Version:1.0", "Content length: 3045". The body of the document is followed as per request. 

6.2 Example 2

Figure 10: Example illustrating http transaction using of HEAD method

In this example, the client just wants to retrieve the information about a document named "file1.html" which is located at "/user/bin/file/". In http transaction, this involves the use of HEAD method, with a request line "HEAD /user/bin/file/ HTTP/1.1". The header specifies that only file "file1.html" can be accepted. The server responses with response status "HTTP/1.1 200 OK" indicating that the file "file1.html" is found. 4 headers are returned which give information of the file as requested. No content body is returned since the HEAD method does not actually request for the file but only the information of the file.

7.0 HYPERTEXT MARKUP LANGUAGE (HTML)

Since the http is used mainly to transfer hypertext data, it is herein worth to study more on HTML language. This article was written in HTML format and was published to http://www.oocities.org/compchew/assignment/http/http.htm. To view the HTML source code, select menu "View -  Source" in Internet Explorer, or select "View -  Page source" in Nestcape Navigator.

    HTML is a structured markup language that is used to create web pages. A markup language such as HTML is simply a collection of codes, called elements, that are used to indicate the structure and format of a document. The web browser will interpret the meaning of these codes to figure how to structure or display a document. An HTML document is written in a plain text file with extension .html or .htm. The World Wide Web Consortium (W3C) is the organisation that standardize the formal structure and syntax of this language.

    The latest version of HTML 4 allows the web page to be embedded with image, animation, video and audio object which makes the web page more interactive and attractive. 

    The idea of HTML is so that the code can be interpreted regardless of the operating system resides on the platform (Windows, Unix etc) and the web browser being used (Internet Explorer, Nestcape etc). This allows data transfer between different clients and servers with different operating environment, both in hardware or software. However, different browsers will render the document differently and certain browsers may not support certain HTML tags because the rules set forth by W3C are not strictly adhered to by the browsers. However, this doesn't impair the purpose of HTML as a mean which initially was developed for efficient information transmission between different platform.

    The HTML code consists of numbers of element or nested elements. The syntax of the individual element is as follow:

<OpeningTagName OptionalAttributeName="AttributeValue"> AffectedContent </OptionalClosingTagName>

    Each element is started with a opening tag name and ended with an optional closing tag, with the same name but preceded with a "/". It can be included with one or more optional attributes that define the parameter that affect how the content will be displaced or formatted. The affected content is the actual object that will be displaced on the web page, could be some text, image or other hypermedia.

    There are some basic HTML tags that let the browser know important things about the document itself rather than the content and how to display the body. These tags normally do not contain attribute:

    The followings are some tags that used inside the <body> element:

Example :

<!doctype html public "-//W3C//dtd html 4.0 Transitional//EN">

<html>

  <head>

      <title>Example of a simple web page</title>

  </head>

  <body>

     <!--These text is only comment and will not be displayed-->

     <center>These text will be aligned at center</center>

     <p><a href="http://www.home.com"><img src="MyDirectory/home.gif" width="198" height="150" ></a></p>

     <p>Click the above picture will bring you to <a href="http://www.home.com">http://www.home.com</a></p>

   </body>

</html>

The rendering of the above HTML file in browser will be as below:

Figure 11: Rendering of a simple web page

8.0 ACCESS AUTHENTICATION

HTTP/1.1 defines a simple challenge-response technique for authentication. This definition does not restrict http clients and servers from using other forms of authentication, but the current standard only covers this simple form.

    Two authentication exchanges are defined : one between a client and a server, and one between a client and a proxy. Both types of exchange use a challenge-response mechanism. 

1. Client-server authentication - A user agent that wishes to authenticate itself with a server may do so by including an authorization field in the request header; an agent may do this when initially sending the request. An alternative, which may be more common, is that a client sends a request message without an authentication field and is then required to return an authorization by the server. Figure 12 illustrates this scenario which involves 3 steps: 

   The client sends a request, such as a GET request to the server, with no authorization field in the request header. The server returns a response with a status code in the status line of Unauthorised and a WWW-Authenticate field in the response header. The WWW-Authentication field consists of a challenge that indicates the type of authentication required and may include other parameters. No entity body is returned. The client repeats the request but includes an authentication field that contains the authorisation data needed by the server. If the authentication succeeds, the server  returns a response with some other status code and without a WWW-Authenticate field. If authentication fails, the server can initiate a new authentication sequence by returning a response with a status of Unauthorized and a WWW-Authentication field containing the (possibly new) challenge. The entity body shuld explain the reason for the refusal. Figure 12 : HTTP access-authentication scenario

2. Proxy Authentication - A proxy may be configured so that a client must first authenticate itself to the proxy before being granted access to the origin server. The sequence is similar to that described for client-server authentication. In this case, the authentication information is carried in the Proxy-Authentication field in the request header. A client may authenticate itself when first issuing a request message. Alternatively, a scenario similar to Figure 12 occurs:

   The client sends a request, such as GET request to the server, with no Proxy-Authorization field in the request header. The proxy does not forward the request, but returns a response with a status code in the status line of Proxy-Authentication Required and a Proxy-Authentication field in the response header. The client repeats the request but includes a Proxy-Authorization field that contains the authorization data needed by the proxy. If the request is authenticated, then the proxy may forward the request to a server, but will omit the Proxy-Authentication field. The proxy could also return a cached response.

9.0 FUTURE TREND OF HTTP

The next generation of HTTP, dubbed " HTTP-NG", will be a replacement for HTTP 1.0 with much higher performance and adding some extra features needed for use in commercial applications. It's designed to make it easy to implement the basic functionality needed by all browsers, whilst making the addition of more powerful features such as security and authentication much simpler.

The current HTTP 1.0 often causes performance problems on the server side, and on the network, since it sets up a new connection for every request. " HTTP-NG "divides up the connection (between client and server) into lots of different channels, each object is returned over its own channel." HTTP-NG " allows many different requests to be sent over a single connection. These requests are asynchronous - there's no need for the client to wait for a response before sending out a new request. The server can also respond to requests in any order it sees fit and allowing several images to be transferred in "parallel".

To make these multiple data streams easy to work with, HTTP-NG divides the connection  into lots of different channels. HTTP-NG sends all control messages (GET requests, meta-information etc) over a control channel. Each object is returned over in its own channel. This also makes redirection much more powerful - for example, if the object is a video the server can return the meta-information over the same connection, together with a URL pointing to a dedicated video transfer protocol that will fetch the data for the relevant object. This becomes very important when working with multimedia aware networking technologies. The HTTP-NG protocol will permit complex data types such as video to redirect the URL to a video transfer protocol and only then will the data be fetched for the client.

10.0 SUMMARY

The http is the main protocol used to access data in world-wide web, where the data is not only limited to hypertext but also to all type of hypermedia likes image, audio, video etc. Throughout its extensive use of request method, request header, error codes and response headers, this protocol allow the data being transferred between the client and server. However, http is a stateless and generic protocol that defines the method and gives the information required for data transfer. The actual data transfer is still done by low level protocol, i.e. transmission control protocol (TCP).

The http protocol is a request/response protocol. A client sends a request to the server in the form of a request method, URL and protocol version, followed by a MIME-like message containing request modifiers, client information, and possible body content over a connection with a server. The server responds with a status line, including message's protocol version and a success or error code, followed by a MIME-like message containing server information, entity meta-information, and possible entity-body content.

There are 2 types of http message, i.e. request message and response message. A request  message consists of a request line, headers, a blank line, and a body (for certain messages) as illustrated in Figure 2. The request line consists of a request type, a space, a URL, a space, and the http version (as illustrated in Figure 3). The URL consists of method, host, computer, port and path (Figure 4). The request type or method is the actual command or request that a client issues to the server. Some common methods are GET (to retrieve a document from server), HEAD (to retrieve information about a document in the server) and POST (to send a document to a server).

A response message consists of a status line, headers, a blank line, and a body (for certain messages) as illustrated in Figure 5. The status line consists of http version, a space, a status code, a space,  and a status phrase (Figure 6). The status code relays general information, information related to a successful request, redirection information or error information in the form of 3 digits code. The status phrase gives description about the code status. E.g. the status phrase for status code 404 is "Not Found".

The header relays additional information between the client and server. A header consists of a header name and a header value. A general header gives general information about the request or response. A request header specifies a client's configuration and preferred document format. A response header specifies a server's configuration and special information about the request. An entity header provide information about the body of a document.

Web page written in Hypertext Markup Language (HTML) constitutes most of the data being transfer in http. The idea of HTML is to allow data transfer between different platform with different operating environment, e.g. Windows NT, Windows 2000 or Unix. Therefore HTML does not specify exactly how the data will be displayed in the browser but instead the structure of the web page. It is a machine-independent markup language that let the browser to render the web page according to the structure and information set-forth in the HTML code. 

11.0 REFERENCE

1. Data and computer communication, Prentice Hall

2. TCP/IP Protocol Suit by Forouzon, McGraw Hill

3. The Complete Reference for HTML by Thomas A. Powell, McGraw Hill

4. http://www.w3.org/Protocols/rfc2616/rfc2616.html

5. http://www.ics.uci.edu/pub/ietf/http/rfc1945.html

This page was last updated on 05/19/2000