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The HyperText Transfer Protocol

In the early days, the Internet was mainly used for remote terminal access with telnet, email and file transfer. The default file transfer protocol, FTP, defined in RFC 959 was widely used. FTP clients and servers are still included in some operating systems.

Many FTP clients offered a user interface similar to a Unix shell and allowed clients to browse the file system on the server and to send and retrieve files. FTP servers can be configured in two modes :

• authenticated : in this mode, the ftp server only accepts users with a valid user name and password. Once authenticated, they can access the files and directories according to their permissions

• anonymous : in this mode, clients supply the anonymous user identifier and their email address as password. These clients are granted access to a special zone of the file system that only contains public files.

FTP was very popular in the 1990s and early 2000s, but today it has mostly been superseded by more recent protocols. Authenticated access to files is mainly done by using the Secure Shell (ssh) protocol defined in RFC 4251 and supported by clients such as scp or sftp. Nowadays, anonymous access is mainly provided by web protocols.

In the late 1980s, high energy physicists working at CERN had to efficiently exchange documents about their ongoing and planned experiments. Tim Berners-Lee evaluated several of the documents sharing techniques that were available at that time [B1989]. As none of the existing solutions met CERN’s requirements, they chose to develop a completely new document sharing system. This system was initially called the mesh. It was quickly renamed the world wide web. The starting point for the world wide web are hypertext documents. An hypertext document is a document that contains references (hyperlinks) to other documents that the reader can immediately access. Hypertext was not invented for the world wide web. The idea of hypertext documents was proposed in 1945 [Bush1945] and the first experiments were done during the 1960s [Nelson1965] [Myers1998] . Compared to the hypertext documents that were used in the late 1980s, the main innovation introduced by the world wide web was to allow hyperlinks to reference documents stored on different remote machines.

World-wide web clients and servers

A document sharing system such as the world wide web is composed of three important parts.

1. A standardized addressing scheme that unambiguously identifies documents

2. A standard document format : the HyperText Markup Language

3. A standardized protocol to efficiently retrieve the documents stored on a server

Note

Open standards and open implementations

Open standards play a key role in the success of the world wide web as we know it today. Without open standards, the world wide web would have never reached its current size. In addition to open standards, another important factor for the success of the web was the availability of open and efficient implementations of these standards. When CERN started to work on the web, their objective was to build a running system that could be used by physicists. They developed open-source implementations of the first web servers and web clients. These open-source implementations were powerful and could be used as is, by institutions willing to share information. They were also extended by other developers who contributed to new features. For example, the NCSA added support for images in their Mosaic browser that was eventually used to create Netscape Communications and the first commercial browsers and servers.

The first components of the world wide web are the Uniform Resource Identifiers (URI), defined in RFC 3986. A URI is a character string that unambiguously identifies a resource on the world wide web. Here is a subset of the BNF for URIs

URI           = scheme ":" "//" authority path [ "?" query ] [ "#" fragment ]
scheme        = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
authority     = [ userinfo "@" ] host [ ":" port ]
query         = *( pchar / "/" / "?" )
fragment      = *( pchar / "/" / "?" )
pchar         = unreserved / pct-encoded / sub-delims / ":" / "@"
query         = *( pchar / "/" / "?" )
fragment      = *( pchar / "/" / "?" )
pct-encoded   = "%" HEXDIG HEXDIG
unreserved    = ALPHA / DIGIT / "-" / "." / "_" / "~"
reserved      = gen-delims / sub-delims
gen-delims    = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims    = "!" / "$" / "&" / "'" / "(" / ")" / "*" / "+" / "," / ";" / "="

The first component of a URI is its scheme. A scheme can be seen as a selector, indicating the meaning of the fields after it. In practice, the scheme often identifies the application-layer protocol that must be used by the client to retrieve the document, but it is not always the case. Some schemes do not imply a protocol at all and some do not indicate a retrievable document 1. The most frequent schemes are http and https. We focus on http in this section. A URI scheme can be defined for almost any application layer protocol 2. The characters : and // follow the scheme of any URI.

The second part of the URI is the authority. With retrievable URIs, this includes the DNS name or the IP address of the server where the document can be retrieved using the protocol specified via the scheme. This name can be preceded by some information about the user (e.g. a user name) who is requesting the information. Earlier definitions of the URI allowed the specification of a user name and a password before the @ character (RFC 1738), but this is now deprecated as placing a password inside a URI is insecure. The host name can be followed by the semicolon character and a port number. A default port number is defined for some protocols and the port number should only be included in the URI if a non-default port number is used (for other protocols, techniques like service DNS records can used).

The third part of the URI is the path to the document. This path is structured as filenames on a Unix host (but it does not imply that the files are indeed stored this way on the server). If the path is not specified, the server will return a default document. The last two optional parts of the URI are used to provide a query parameter and indicate a specific part (e.g. a section in an article) of the requested document. Sample URIs are shown below.

http://tools.ietf.org/html/rfc3986.html
mailto:infobot@example.com?subject=current-issue
http://docs.python.org/library/basehttpserver.html?highlight=http#BaseHTTPServer.BaseHTTPRequestHandler
telnet://[2001:db8:3080:3::2]:2323/
ftp://cnn.example.com&story=breaking_news@10.0.0.1/top_story.htm

The first URI corresponds to a document named rfc3986.html that is stored on the server named tools.ietf.org and can be accessed by using the http protocol on its default port. The second URI corresponds to an email message, with subject current-issue, that will be sent to user infobot in domain example.com. The mailto: URI scheme is defined in RFC 2368. The third URI references the portion BaseHTTPServer.BaseHTTPRequestHandler of the document basehttpserver.html that is stored in the library directory on the docs.python.org server. This document can be retrieved by using the http protocol. The query parameter highlight=http is associated to this URI. The fourth example is a server that operates the telnet protocol, uses IPv6 address 2001:db8:3080:3::2 and is reachable on port 2323. The last URI is somewhat special. Most users will assume that it corresponds to a document stored on the cnn.example.com server. However, to parse this URI, it is important to remember that the @ character is used to separate the user name from the host name in the authorization part of a URI. This implies that the URI points to a document named top_story.htm on the host having IPv4 address 10.0.0.1. The document will be retrieved by using the ftp protocol with the user name set to cnn.example.com&story=breaking_news.

The second component of the word wide web is the HyperText Markup Language (HTML). HTML defines the format of the documents that are exchanged on the web. The first version of HTML was derived from the Standard Generalized Markup Language (SGML) that was standardized in 1986 by ISO. SGML was designed to support large documents maintained by government, law firms or aerospace companies that must be shared efficiently in a machine-readable manner. These industries require documents to remain readable and editable for tens of years and insisted on a standardized format supported by multiple vendors. Today, SGML is no longer widely used beyond specific applications, but its descendants including HTML and XML are now widespread.

A markup language is a structured way of adding annotations about the formatting of the document within the document itself. Example markup languages include troff, which is used to write the Unix man pages or Latex. HTML uses markers to annotate text and a document is composed of HTML elements. Each element is usually composed of three parts: a start tag that potentially includes some specific attributes, some text (often including other elements), and an end tag. A HTML tag is a keyword enclosed in angle brackets. The generic form of an HTML element is

<tag>Some text to be displayed</tag>

More complex HTML elements can also include optional attributes in the start tag

<tag attribute1="value1" attribute2="value2">some text to be displayed</tag>

The HTML document shown below is composed of two parts: a header, delineated by the <head> and </head> markers, and a body (between the <body> and </body> markers). In the example below, the header only contains a title, but other types of information can be included in the header. The body contains an image, some text and a list with three hyperlinks. The image is included in the web page by indicating its URI between brackets inside the <img src=”…”> marker. It is important to note that the image can reside on any server. The client will automatically download it when rendering the web page. The <h1>…</h1> marker is used to specify the first level of headings. The <ul> marker indicates an unnumbered list while the <li> marker indicates a list item. The <a href=”URI”>text</a> indicates a hyperlink. The text will be underlined in the rendered web page and the client will fetch the specified URI when the user clicks on the link.

A simple HTML page

Over the years, various extensions to HTML have been proposed and implemented. These include the specification of style sheets that adjust the layout of the document and the possibility of adding or referencing javascript code. Additional details about the various extensions to HTML may be found in the official specifications maintained by W3C.

The third component of the world wide web is the HyperText Transfer Protocol (HTTP). HTTP is a text-based protocol like SMTP. The client sends a request and the server returns a response. HTTP runs above the bytestream service and HTTP servers listen by default on port 80. The design of HTTP has largely been inspired by the Internet email protocols. Each HTTP request contains three parts :

• a method, that indicates the type of request, a URI, and the version of the HTTP protocol used by the client

• a header, that is used by the client to specify optional parameters for the request. An empty line is used to mark the end of the header

• an optional MIME document attached to the request

The response sent by the server also contains three parts :

• a status line , that indicates whether the request was successful or not

• a header, that contains additional information about the response. The response header ends with an empty line.

• a MIME document

  

  HTTP requests and responses

Several types of method can be used in HTTP requests. The three most important ones are :

• the GET method is the most popular one. It is used to retrieve a document from a server. The GET method is encoded as GET followed by the path of the URI of the requested document and the version of HTTP used by the client. For example, to retrieve the http://www.w3.org/MarkUp/ URI, a client must open a TCP connection on port 80 with host www.w3.org and send a HTTP request containing the following line:

GET /MarkUp/ HTTP/1.0

• the HEAD method is a variant of the GET method that allows the retrieval of the header lines for a given URI without retrieving the entire document. It can be used by a client to verify if a document exists, for instance.

• the POST method can be used by a client to send a document to a server. The document is attached to the HTTP request as a MIME document.

HTTP clients and servers can include different HTTP headers in HTTP requests and responses. Each HTTP header is encoded as a single ASCII-line terminated by CR and LF. Several of these headers are briefly described below. A detailed discussion of the standard headers may be found in RFC 1945. The MIME headers can appear in both HTTP requests and HTTP responses.

• the Content-Length: header is the MIME header that indicates the length of the MIME document in bytes.

• the Content-Type: header is the MIME header that indicates the type of the attached MIME document. HTML pages use the text/html type.

• the Content-Encoding: header indicates how the MIME document has been encoded. For example, this header would be set to x-gzip for a document compressed using the gzip software.

RFC 1945 and RFC 2616 define headers that are specific to HTTP responses. These server headers include:

• the Server: header indicates the version of the web server that has generated the HTTP response. Some servers provide information about their software release and optional modules that they use. For security reasons, some system administrators disable these headers to avoid revealing too much information about their server to potential attackers.

• the Date: header indicates when the HTTP response has been produced by the server.

• the Last-Modified: header indicates the date and time of the last modification of the document attached to the HTTP response.

Similarly, the following header lines can only appear inside HTTP requests sent by a client:

• the User-Agent: header provides information about the client that has generated the HTTP request. Some servers analyze this header line and return different headers and sometimes different documents for different user agents.

• the If-Modified-Since: header is followed by a date. It enables clients to cache in memory or on disk recent or most frequently used documents. When a client needs to request a URI from a server, it first checks whether the document is already in its cache. If it is, the client sends an HTTP request with the If-Modified-Since: header indicating the date of the cached document. The server will only return the document attached to the HTTP response if it is newer than the version stored in the client’s cache.

• the Referrer: header is followed by a URI. It indicates the URI of the document that the client visited before sending this HTTP request. Thanks to this header, the server can know the URI of the document containing the hyperlink followed by the client, if any. This information is very useful to measure the impact of advertisements containing hyperlinks placed on websites.

• the Host: header contains the fully qualified domain name of the URI being requested.

Note

The importance of the Host: header line

The first version of HTTP did not include the Host: header line. This was a severe limitation for web hosting companies. For example consider a web hosting company that wants to serve both web.example.com and www.example.net on the same physical server. Both web sites contain a /index.html document. When a client sends a request for either http://web.example.com/index.html or http://www.example.net/index.html, the HTTP 1.0 request contains the following line :

GET /index.html HTTP/1.0

By parsing this line, a server cannot determine which index.html file is requested. Thanks to the Host: header line, the server knows whether the request is for http://web.example.com/index.html or http://www.dummy.net/index.html. Without the Host: header, this is impossible. The Host: header line allowed web hosting companies to develop their business by supporting a large number of independent web servers on the same physical server.

The status line of the HTTP response begins with the version of HTTP used by the server (usually HTTP/1.0 defined in RFC 1945 or HTTP/1.1 defined in RFC 2616) followed by a three digit status code and additional information in English. HTTP status codes have a similar structure as the reply codes used by SMTP:

• All status codes starting with digit 2 indicate a valid response. 200 Ok indicates that the HTTP request was successfully processed by the server and that the response is valid.

• All status codes starting with digit 3 indicate that the requested document is no longer available on the server. 301 Moved Permanently indicates that the requested document is no longer available on this server. A Location: header containing the new URI of the requested document is inserted in the HTTP response. 304 Not Modified is used in response to an HTTP request containing the If-Modified-Since: header. This status line is used by the server if the document stored on the server is not more recent than the date indicated in the If-Modified-Since: header.

• All status codes starting with digit 4 indicate that the server has detected an error in the HTTP request sent by the client. 400 Bad Request indicates a syntax error in the HTTP request. 404 Not Found indicates that the requested document does not exist on the server.

• All status codes starting with digit 5 indicate an error on the server. 500 Internal Server Error indicates that the server could not process the request due to an error on the server itself.

In both HTTP requests and responses, the MIME document refers to a representation of the document with the MIME headers indicating the type of document and its size.

As an illustration of HTTP/1.0, the transcript below shows a HTTP request for http://www.ietf.org and the corresponding HTTP response. The HTTP request was sent using the curl command line tool. The User-Agent: header line contains more information about this client software. There is no MIME document attached to this HTTP request, and it ends with a blank line.

GET / HTTP/1.0
User-Agent: curl/7.19.4 (universal-apple-darwin10.0) libcurl/7.19.4 OpenSSL/0.9.8l zlib/1.2.3
Host: www.ietf.org

The HTTP response indicates the version of the server software used with the modules included. The Last-Modified: header indicates that the requested document was modified about one week before the request. A HTML document (not shown) is attached to the response. Note the blank line between the header of the HTTP response and the attached MIME document. The Server: header line has been truncated in this output.

HTTP/1.1 200 OK
Date: Mon, 15 Mar 2010 13:40:38 GMT
Server: Apache/2.2.4 (Linux/SUSE) mod_ssl/2.2.4 OpenSSL/0.9.8e (truncated)
Last-Modified: Tue, 09 Mar 2010 21:26:53 GMT
Content-Length: 17019
Content-Type: text/html

<!DOCTYPE HTML PUBLIC .../HTML>

HTTP was initially designed to share text documents. For this reason, and to ease the implementation of clients and servers, the designers of HTTP chose to open a TCP connection for each HTTP request. This implies that a client must open one TCP connection for each URI that it wants to retrieve from a server as illustrated on the figure below, showing HTTP 1.0 and the underlying TCP connection. For a web page containing only text documents this was a reasonable design choice as the client usually remains idle while the (human) user is reading the retrieved document.

However, as the web evolved to support richer documents containing images, opening a TCP connection for each URI became a performance problem [Mogul1995]. Indeed, besides its HTML part, a web page may include dozens of images or more. Forcing the client to open a TCP connection for each component of a web page has two important drawbacks. First, the client and the server must exchange packets to open and close a TCP connection as we will see later. This increases the network overhead and the total delay of completely retrieving all the components of a web page. Second, a large number of established TCP connections may be a performance bottleneck on servers.

This problem was solved by extending HTTP to support persistent TCP connections RFC 2616. A persistent connection is a TCP connection over which a client may send several HTTP requests. This is illustrated in the figure below showing the persistent connection of HTTP 1.1.

To allow the clients and servers to control the utilization of these persistent TCP connections, HTTP 1.1 RFC 2616 defines several new HTTP headers:

• The Connection: header is used with the Keep-Alive argument by the client to indicate that it expects the underlying TCP connection to be persistent. When this header is used with the Close argument, it indicates that the entity that sent it will close the underlying TCP connection at the end of the HTTP response.

• The Keep-Alive: header is used by the server to inform the client about how it agrees to use the persistent connection. A typical Keep-Alive: contains two parameters: the maximum number of requests that the server agrees to serve on the underlying TCP connection and the timeout (in seconds) after which the server will close an idle connection

The example below shows the operation of HTTP/1.1 over a persistent TCP connection to retrieve three URIs stored on the same server. Once the connection has been established, the client sends its first request with the Connection: Keep-Alive header to request a persistent connection.

GET / HTTP/1.1
Host: www.kame.net
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_2; en-us)
Connection: Keep-Alive

The server replies with the Connection: Keep-Alive header and indicates that it accepts a maximum of 100 HTTP requests over this connection and that it will close the connection if it remains idle for 15 seconds.

HTTP/1.1 200 OK
Date: Fri, 19 Mar 2010 09:23:37 GMT
Server: Apache/2.0.63 (FreeBSD) PHP/5.2.12 with Suhosin-Patch
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Content-Length: 3462
Content-Type: text/html

<html>...   </html>

The client sends a second request for the style sheet of the retrieved web page.

GET /style.css HTTP/1.1
Host: www.kame.net
Referer: http://www.kame.net/
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_2; en-us)
Connection: keep-alive

The server replies with the requested style sheet and maintains the persistent connection. Note that the server only accepts 99 remaining HTTP requests over this persistent connection.

HTTP/1.1 200 OK
Date: Fri, 19 Mar 2010 09:23:37 GMT
Server: Apache/2.0.63 (FreeBSD) PHP/5.2.12 with Suhosin-Patch
Last-Modified: Mon, 10 Apr 2006 05:06:39 GMT
Content-Length: 2235
Keep-Alive: timeout=15, max=99
Connection: Keep-Alive
Content-Type: text/css

...

Then the client requested the web server’s icon 3. This server does not contain such an icon and thus replies with a 404 HTTP status. However, the underlying TCP connection is not closed immediately.

GET /favicon.ico HTTP/1.1
Host: www.kame.net
Referer: http://www.kame.net/
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_2; en-us)
Connection: keep-alive

HTTP/1.1 404 Not Found
Date: Fri, 19 Mar 2010 09:23:40 GMT
Server: Apache/2.0.63 (FreeBSD) PHP/5.2.12 with Suhosin-Patch
Content-Length: 318
Keep-Alive: timeout=15, max=98
Connection: Keep-Alive
Content-Type: text/html; charset=iso-8859-1

<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> ...

As illustrated above, a client can send several HTTP requests over the same persistent TCP connection. However, it is important to note that all of these HTTP requests are considered to be independent by the server. Each HTTP request must be self-contained. This implies that each request must include all the header lines that are required by the server to understand the request. The independence of these requests is one of the key design choices of HTTP. As a consequence of this design choice, when a server processes a HTTP request, it does not use any other information than what is contained in the request itself. This explains why the client adds its User-Agent: header in all of the HTTP requests that it sends over the persistent TCP connection.

However, in practice, some servers want to provide content tuned for each user. For example, some servers can provide information in several languages. Other servers want to provide advertisements that are targeted to different types of users. To do this, servers need to maintain some information about the preferences of each user and use this information to produce content matching the user’s preferences. HTTP contains several mechanisms to solve this problem. We discuss three of them below.

A first solution is to force the users to be authenticated. This was the solution used by FTP to control the files that each user could access. Initially, user names and passwords could be included inside URIs RFC 1738. However, placing passwords in the clear in a potentially publicly visible URI is completely insecure and this usage has now been deprecated RFC 3986. HTTP supports several extension headers RFC 2617 that can be used by a server to request the authentication of the client by providing his/her credentials. However, user names and passwords have not been popular on web servers as they force human users to remember one user name and one password per server. Remembering a password is acceptable when a user needs to access protected content, but users will not accept to remember a unique user name and password for each web sites that they visit.

A second solution to allow servers to tune that content to the needs and capabilities of the user is to rely on the different types of Accept-* HTTP headers. For example, the Accept-Language: header can be used by the client to indicate its preferred languages. Unfortunately, in practice this header is usually set based on the default language of the browser and it is difficult for a user to indicate the language it prefers by selecting options for each visited web server.

The third and widely adopted solution are HTTP cookies. HTTP cookies were initially developed as a private extension by Netscape. They are now part of the standard RFC 6265. In a nutshell, a cookie is a short string that is chosen by a server to represent a given client. Two HTTP headers are used : Cookie: and Set-Cookie:. When a server receives an HTTP request from a new client (i.e. an HTTP request that does not contain the Cookie: header), it generates a cookie for the client and includes it in the Set-Cookie: header of the returned HTTP response. The Set-Cookie: header contains several additional parameters including the domain names for which the cookie is valid. The client stores all received cookies on disk and every time it sends an HTTP request, it verifies whether it already knows a cookie for this domain. If so, it attaches the Cookie: header to the HTTP request. This is illustrated in the figure below with HTTP 1.1, but cookies also work with HTTP 1.0.

Note

Privacy issues with HTTP cookies

The HTTP cookies introduced by Netscape are key for large e-commerce websites. However, they have also raised many discussions concerning their potential misuses. Consider ad.com, a company that delivers lots of advertisements on web sites. A web site that wishes to include ad.com’s advertisements next to its content will add links to ad.com inside its HTML pages. If ad.com is used by many web sites, ad.com could be able to track the interests of all the users that visit its client websites and use this information to provide targeted advertisements. Privacy advocates have even sued online advertisement companies to force them to comply with the privacy regulations. More recent related technologies also raise privacy concerns.

Footnotes

1

An example of a non-retrievable URI is urn:isbn:0-380-81593-1 which is an unique identifier for a book, through the urn scheme (see RFC 3187). Of course, any URI can be made retrievable via a dedicated server or a new protocol but this one has no explicit protocol. Same thing for the scheme tag (see RFC 4151), often used in Web syndication (see RFC 4287 about the Atom syndication format). Even when the scheme is retrievable (for instance with http), it is often used only as an identifier, not as a way to get a resource. See http://norman.walsh.name/2006/07/25/namesAndAddresses for a good explanation.

2

The list of standard URI schemes is maintained by IANA at http://www.iana.org/assignments/uri-schemes.html

3

Favorite icons are small icons that are used to represent web servers in the toolbar of Internet browsers. Microsoft added this feature in their browsers without taking into account the W3C standards. See http://www.w3.org/2005/10/howto-favicon for a discussion on how to cleanly support such favorite icons.

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