Introduction
About this tutorial
This is an attempt to give a basic understanding of how the web worksIt covers the HTTP protocol, which is used to transmit and receive web pages, as well as some server workings and scripting technologies. It is assumed that you already know how to make web pages and preferably some HTML as well.. It is also assumed that you have some basic knowledge of URLs. (A URL is the address of a document, what you need to be able to get hold of the document.)
I'm not entirely happy with this, so feedback would be very welcome. If you're not really a technical person and this tutorial leaves you puzzled or does not answer all your questions I'd very much like to hear about it. Corrections and opinions are also welcome.
Some background
When you browse the web the situation is basically this: you sit at your computer and want to see a document somewhere on the web, to which you have the URL.Since the document you want to read is somewhere else in the world and probably very far away from you some more details are needed to make it available to you. The first detail is your browser. You start it up and type the URL into it (at least you tell the browser somehow where you want to go, perhaps by clicking on a link).
However, the picture is still not complete, as the browser can't read the document directly from the disk where it's stored if that disk is on another continent. So for you to be able to read the document the computer that contains the document must run a web server. A web server is a just a computer program that listens for requests from browsers and then execute them.
So what happens next is that the browser contacts the server and requests that the server deliver the document to it. The server then gives a response which contains the document and the browser happily displays this to the user. The server also tells the browser what kind of document this is (HTML file, PDF file, ZIP file etc) and the browser then shows the document with the program it was configured to use for this kind of document.
The browser will display HTML documents directly, and if there are references to images, Java applets, sound clips etc in it and the browser has been set up to display these it will request these also from the servers on which they reside. (Usually the same server as the document, but not always.) It's worth noting that these will be separate requests, and add additional load to the server and network. When the user follows another link the whole sequence starts anew.
These requests and responses are issued in a special language called HTTP, which is short for HyperText Transfer Protocol. What this article basically does is describe how this works. Other common protocols that work in similar ways are FTP and Gopher, but there are also protocols that work in completely different ways. None of these are covered here, sorry. (There is a link to some more details about FTP in the references.)
It's worth noting that HTTP only defines what the browser and web server say to each other, not how they communicate. The actual work of moving bits and bytes back and forth across the network is done by TCP and IP, which are also used by FTP and Gopher (as well as most other internet protocols).
When you continue, note that any software program that does the same as a web browser (ie: retrieve documents from servers) is called a client in network terminology and a user agent in web terminology. Also note that the server is properly the server program, and not the computer on which the server is an application program. (Sometimes called the server machine.)
What happens when I follow a link?
Step 1: Parsing the URL
The first thing the browser has to do is to look at the URL of the new document to find out how to get hold of the new document. Most URLs have this basic form: "protocol://server/request-URI". The protocol part describes how to tell the server which document the you want and how to retrieve it. The server part tells the browser which server to contact, and the request-URI is the name used by the web server to identify the document. (I use the term request-URI since it's the one used by the HTTP standard, and I can't think of anything else that is general enough to not be misleading.)Step 2: Sending the request
Usually, the protocol is "http". To retrieve a document via HTTP the browser transmits the following request to the server: "GET /request-URI HTTP/version", where version tells the server which HTTP version is used. (Usually, the browser includes some more information as well. The details are covered later.)One important point here is that this request string is all the server ever sees. So the server doesn't care if the request came from a browser, a link checker, a validator, a search engine robot or if you typed it in manually. It just performs the request and returns the result.
Step 3: The server response
When the server receives the HTTP request it locates the appropriate document and returns it. However, an HTTP response is required to have a particular form. It must look like this:HTTP/[VER] [CODE] [TEXT] Field1: Value1 Field2: Value2 ...Document content here...The first line shows the HTTP version used, followed by a three-digit number (the HTTP status code) and a reason phrase meant for humans. Usually the code is 200 (which basically means that all is well) and the phrase "OK". The first line is followed by some lines called the header, which contains information about the document. The header ends with a blank line, followed by the document content. This is a typical header:
HTTP/1.0 200 OK Server: Netscape-Communications/1.1 Date: Tuesday, 25-Nov-97 01:22:04 GMT Last-modified: Thursday, 20-Nov-97 10:44:53 GMT Content-length: 6372 Content-type: text/html <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> <HTML> ...followed by document content...We see from the first line that the request was successful. The second line is optional and tells us that the server runs the Netscape Communications web server, version 1.1. We then get what the server thinks is the current date and when the document was modified last, followed by the size of the document in bytes and the most important field: "Content-type".
The content-type field is used by the browser to tell which format the document it receives is in. HTML is identified with "text/html", ordinary text with "text/plain", a GIF is "image/gif" and so on. The advantage of this is that the URL can have any ending and the browser will still get it right.
An important concept here is that to the browser, the server works as a black box. Ie: the browser requests a specific document and the document is either returned or an error message is returned. How the server produces the document remains unknown to the browser. This means that the server can read it from a file, run a program that generates it, compile it by parsing some kind of command file or (very unlikely, but in principle possible) have it dictated by the server administrator via speech recognition software. This gives the server administrator great freedom to experiment with different kinds of services as the users don't care (or even know) how pages are produced.
What the server does
When the server is set up it is usually configured to use a directory somewhere on disk as its root directory and that there be a default file name (say "index.html") for each directory. This means that if you ask the server for the file "/" (as in "http://www.domain.tld/") you'll get the file index.html in the server root directory. Usually, asking for "/foo/bar.html" will give you the bar.html file from the foo directory directly beneath the server root.Usually, that is. The server can be set up to map "/foo/" into some other directory elsewhere on disk or even to use server-side programs to answer all requests that ask for that directory. The server does not even have to map requests onto a directory structure at all, but can use some other scheme.
HTTP versions
So far there are three versions of HTTP. The first one was HTTP/0.9, which was truly primitive and never really specified in any standard. This was corrected by HTTP/1.0, which was issued as a standard in RFC 1945. (See references). HTTP/1.0 is the version of HTTP that is in common use today (usually with some 1.1 extensions), while HTTP/0.9 is rarely, if ever, used by browsers. (Some simpler HTTP clients still use it since they don't need the later extensions.)RFC 2068 describes HTTP/1.1, which extends and improves HTTP/1.0 in a number of areas. Very few browsers support it (MSIE 4.0 is the only one known to the author), but servers are beginning to do so.
The major differences are a some extensions in HTTP/1.1 for authoring documents online via HTTP and a feature that lets clients request that the connection be kept open after a request so that it does not have to be reestablished for the next request. This can save some waiting and server load if several requests have to be issued quickly.
This document describes HTTP/1.0, except some sections that cover the HTTP/1.1 extensions. Those will be explicitly labeled.
The request sent by the client
The shape of a request
Basically, all requests look like this:[METH] [REQUEST-URI] HTTP/[VER] [fieldname1]: [field-value1] [fieldname2]: [field-value2] [request body, if any]The METH (for request method) gives the request method used, of which there are several, and which all do different things. The above example used GET, but below some more are explained. The REQUEST-URI is the identifier of the document on the server, such as "/index.html" or whatever. VER is the HTTP version, like in the response. The header fields are also the same as in the server response.
The request body is only used for requests that transfer data to the server, such as POST and PUT. (Described below.)
GETting a document
There are several request types, with the most common one being GET. A GET request basically means "send me this document" and looks like this: "GET document_path HTTP/version". (Like it was described above.) For the URL "http://www.yahoo.com/" the document_path would be "/", and for "http://www.w3.org/Talks/General.html" it is "/Talks/General.html".However, this first line is not the only thing a user agent (UA) usually sends, although it's the only thing that's really necessary. The UA can include a number of header fields in the request to give the server more information. These fields have the form "fieldname: value" and are all put on separate lines after the first request line.
Some of the header fields that can be used with GET are:
- User-Agent
- This is a string identifying the user agent. An English version of Netscape 4.03 running under Windows NT would send "Mozilla/4.03 [en] (WinNT; I ;Nav)". (Mozilla is the old name for Netscape. See the references for more details.)
- Referer
- The referer field (yes, it's misspelled in the standard) tells the server where the user came from, which is very useful for logging and keeping track of who links to ones pages.
- If-Modified-Since
- If a browser already has a version of the document in its cache it can include this field and set it to the time it retrieved that version. The server can then check if the document has been modified since the browser last downloaded it and send it again if necessary. The whole point is of course that if the document hasn't changed, then the server can just say so and save some waiting and network traffic.
- From
- This header field is a spammers dream come true: it is supposed to contain the email address of whoever controls the user agent. Very few, if any, browsers use it, partly because of the threat from spammers. However, web robots should use it, so that webmasters can contact the people responsible for the robot should it misbehave.
- Authorization
- This field can hold username and password if the document in question requires authorization to be accessed.
GET / HTTP/1.0 User-Agent: Mozilla/3.0 (compatible; Opera/3.0; Windows 95/NT4) Accept: */* Host: birk105.studby.uio.no:81
HEAD: checking documents
One may somtimes want to see the headers returned by the server for a particular document, without actually downloading the document. This is exactly what the HEAD request method provides. HEAD looks and works exactly like GET, only with the difference that the server only returns the headers and not the document content.This is very useful for programs like link checkers, people who want to see the response headers (to see what server is used or to verify that they are correct) and many other kinds of uses.
Playing web browser
You can actually play web browser yourself and write HTTP requests directly to web servers. This can be done by telnetting to port 80, writing the request and hitting enter twice, like this:larsga - tyrfing>telnet www.w3.org 80 Trying 18.23.0.23... Connected to www.w3.org. Escape character is '^]'. HEAD / HTTP/1.0 HTTP/1.1 200 OK Date: Tue, 17 Feb 1998 22:24:53 GMT Server: Apache/1.2.5 Last-Modified: Wed, 11 Feb 1998 18:22:22 GMT ETag: "2c3136-23c1-34e1ec5e" Content-Length: 9153 Accept-Ranges: bytes Connection: close Content-Type: text/html; charset=ISO-8859-1 Connection closed by foreign host. larsga - tyrfing>However, this works best under Unix as the Windows telnet clients I've used are not very suitable for this (and hard to set up so that it works). Instead you can use HTTPTest, a CGI script I've linked to in the references.
The response returned by the server
Outline
What the server returns consists of a line with the status code, a list of header fields, a blank line and then the requested document, if it is returned at all. Sort of like this:HTTP/1.0 code text Field1: Value1 Field2: Value2 ...Document content here...
The status codes
The status codes are all three-digit numbers that are grouped by the first digit into 5 groups. The reason phrases given with the status codes below are just suggestions. Server can return any reason phrase they wish.1xx: Informational
No 1xx status codes are defined, and they are reserved for experimental purposes only.2xx: Successful
Means that the request was processed successfully.- 200 OK
- Means that the server did whatever the client wanted it to, and all is well.
- Others
- The rest of the 2xx status codes are mainly meant for script processing and are not often used.
3xx: Redirection
Means that the resource is somewhere else and that the client should try again at a new address.- 301 Moved permanently
- The resource the client requested is somewhere else, and the client should go there to get it. Any links or other references to this resource should be updated.
- 302 Moved temporarily
- This means the same as the 301 response, but links should now not be updated, since the resource may be moved again in the future.
- 304 Not modified
- This response can be returned if the client used the if-modified-since header field and the resource has not been modified since the given time. Simply means that the cached version should be displayed for the user.
4xx: Client error
Means that the client screwed up somehow, usually by asking for something it should not have asked for.- 400: Bad request
- The request sent by the client didn't have the correct syntax.
- 401: Unauthorized
- Means that the client is not allowed to access the resource. This may change if the client retries with an authorization header.
- 403: Forbidden
- The client is not allowed to access the resource and authorization will not help.
- 404: Not found
- Seen this one before? :) It means that the server has not heard of the resource and has no further clues as to what the client should do about it. In other words: dead link.
5xx: Server error
This means that the server screwed up or that it couldn't do as the client requested.- 500: Internal server error
- Something went wrong inside the server.
- 501: Not implemented
- The request method is not supported by the server.
- 503: Service unavailable
- This sometimes happens if the server is too heavily loaded and cannot service the request. Usually, the solution is for the client to wait a while and try again.
The response header fields
These are the header fields a server can return in response to a request.- Location
- This tells the user agent where the resource it requested can be found. The value is just the URL of the new resource.
- Server
- This tells the user agent which web server is used. Nearly all web servers return this header, although some leave it out.
- Content-length
- This gives the size of the resource, in bytes.
- Content-type
- This describes the file format of the resource.
- Content-encoding
- This means that the resource has been coded in some way and must be decoded before use.
- Expires
- This field can be set for data that are updated at a known time (for instance if they are generated by a script). It is used to prevent browsers from caching the resource beyond the given date.
- Last-modified
- This tells the browser when the resource was last modified. Can be useful for mirroring, update notification etc.
Caching: agents between the server and client
The browser cache
You may have noticed that when you go back to a page you've looked at not too long before the page loads much quicker. That's because the browser stored a local copy of it when it was first downloaded. These local copies are kept in what's called a cache. Usually one sets a maximum size for the cache and a maximum caching time for documents.This means that when a new page is visited it is stored in the cache, and if the cache is full (near the maximum size limit) some document that the browser considers unlikely to be visited again soon is deleted to make room. Also, if you go to a page that is stored in the cache the browser may find that you've set 7 days as a the maximum storage time and 8 days have now passed since the last visit, so the page needs to be reloaded.
Exactly how caches work differ between browsers, but this is the basic idea, and it's a good one because it saves both time for the user and network traffic. There are also some HTTP details involved, but they will be covered later.
Proxy caches
Browser caches are a nice feature, but when many users browse from the same site one usually ends up storing the same document in many different caches and refreshing it over and over for different uses. Clearly, this isn't optimal.The solution is to let the users share a cache, and this is exactly what proxy caches are all about. Browsers still have their local caches, but HTTP requests for documents not in the browser cache are not sent to the server any more, instead they are sent to the proxy cache. If the proxy has the document in its cache it will just return the document (like the browser cache would), and if it doesn't it will submit the request on behalf of the browser, store the result and relay it to the browser.
So the proxy is really a common cache for a number of users and can reduce network traffic rather dramatically. It can also skew log-based statistics badly. :)
A more advanced solution than a single proxy cache is a hierarchy of proxy caches. Imagine a large ISP may have one proxy cache for each part of the country and set up each of the regional proxies to use a national proxy cache instead of going directly to the source web servers. This solution can reduce network traffic even further. More detail on this is linked to in the references.
Server-side programming
What is it and why do it?
Server-side scripts or programs are simply programs that are run on the web server in response to requests from the client. These scripts produce normal HTML (and sometimes HTTP headers as well) as output which is then fed back to the client as if the client had requested an ordinary page. In fact, there is no way for the client software to tell whether scripting has been used or not.Technologies such as JavaScript, VBScript and Java applets all run in the client and so are not examples of this. There is a major difference between server-side and client-side scripting as the client and server are usually different computers. So if all the data the program needs are located on the server it may make sense to use server-side scripting instead of client-side. (There is also the problem that the client may not have a browser that supports the scripting technology or it may be turned off.) If the program and user need to interact often client-side scripting is probably best, to reduce the number of requests sent to the server.
So: in general, if the program needs a lot of data and infrequent interactions with the server server-side scripting is probably best. Applications that use less data and more interaction are best put in the client. Also, applications that gather data over time need to be on the server where the data file is kept.
An example of the first would be search engines like Altavista. Obviously it's not feasible to download all the documents Altavista has collected to search in them locally. An example of the last would be a simple board game. No data are needed and having to send a new request to the server for each move you make quickly gets tedious.
There is one kind of use that has been left out here: what do you do when you want a program to work on data with a lot of interaction with the user? There is no good solution right now, but there is one on the horizon called XML
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