http Stands for the hypertext transfer protocol. This is the most important application layer protocol on the Internet today, because it provides the foundation for the world wide web. defined in Fielding, Roy T., James Gettys, Jeffrey C. Mogul, Paul J. Leach, Tim Berners- Lee ``Hypertext Transfer Protocol -- HTTP/1.1'' (1999), RFC 2616
history 1990: version 0.9 allows for transfer of raw data. 1996: rfc1945 defines version 1.0. by adding attribute:value headers. 1999: rfc 2616 –adds support for hierarchical proxies caching, virtual hosts and some Support for persistent connections –is more stringent.
http resource identification identification of resources is assumed through Uniform Resource Identifiers (URI). As far as http is concerned, URIs are string. http can use ``absolute'' and ``relative'' URIs. A URL is a special case of a URI.
rfc about http An application-level protocol for distributed, collaborative, hypermedia information systems. … HTTP is also used as a generic protocol for communication between user agents and proxies/gateways to other Internet systems, including those supported by the SMTP, NNTP, FTP, Gopher, and WAIS protocols. In this way, HTTP allows basic hypermedia access to resources available from diverse applications.
overall operation: client side Client sends request, required items are –method –request URI –protocol version optional items are –request modifiers –client information
overall operation server side Server sends response, required items are –status line –protocol version –success or error code optional items are –server information –body
http assumes transport http assumes that there is a reliable way to transport data from one host on the Internet to another one. All http requests and responses are separate TCP connections. The default is TCP port 80, but other ports can be used.
Absolute http URL the absolute http URL is http://host[:port][[abs_path][?query]] If abs_path is empty, it is /. The scheme name "http" and the host name are case- insensitive. Characters other than those in the ``reserved'' and ``unsafe'' sets of RFC 2396 are equivalent to their ``%HEX HEX'' encoding. optional components are in [ ]
character sets A character set is a method used with one of more tables to convert a sequence of binary digits into a sequence of characters. http shares the same registry as the MIME multimedia email extensions. It is based at the IANA, at http://www.isi.edu/innotes/iana/ assignments/media-types/media-types The default character set is ISO-8859-1.
http messages There are two types of messages. –Requests are sent form the client to the server. –Responses are sent from the server to the client. The generic format is the same as for email messages: –start line –message headers –empty line –body Empty lines before the start line are ignored. The request's start line is called the request-line. The response start line is called the status-line.
The status line The status line is a set of lines that are of the form HTTP-Version Status-Code Reason-Phrase The status code is a 3-digit number used by the computer. The reason line is a friendly note for a human to read.
Status code classes 1 Informational: Request received, continuing process 2 Success: The action was successfully received, understood, and accepted 3Redirection: Further action must be taken in order to complete the request 4 Client Error: The request contains bad syntax or cannot be understood 5Server error: The request is valid but can not be executed by the server
Error codes II 300Multiple Choices 301Moved Permanently 302Found 303See Other 304Not Modified 305Use Proxy 307Temporary Redirect
Error codes III 400Bad Request 401Unauthorized 402Payment Required 403Forbidden 404Not Found 405Method Not Allowed 406Not Acceptable 407Proxy Authentication Required 408Request Time-out
Error codes IV 409Conflict 410Gone 411Length Required 412Precondition Failed 413Request Entity Too Large 414Request-URI Too Large 415Unsupported Media Type 416Requested range not satisfiable 417Expectation failed
Error codes V 500Internal Server Error 501Not Implemented 502Bad Gateway 503Service Unavailable 504Gateway Time-out 505HTTP Version not supported
Entity headers, common to response and request Allow: Content-Encoding: Content-Language: Content-Length: Content-Location: Content-MD5: Content-Range: Content-Type: Expires: Last-Modified
The body The entity-body (if any) sent with an HTTP request or response is in a format and encoding defined by the entity-header fields. When an entity-body is included with a message, the data type of that body is determined via the header fields Content-Type and Content- Encoding
GET and HEAD method The GET method means retrieve whatever information (in the form of an entity) is identified by the Request-URI. If the Request-URI refers to a data-producing process, it is the produced data which shall be returned as the entity in the response and not the source text of the process. The HEAD method is identical to GET except that the server MUST NOT return a message-body in the response.
Conditional & partial GET The semantics of the GET method change to a ``conditional GET'' if the request message includes an –If-Modified-Since –If-Unmodified-Since –If-Match –If-None-Match –If-Range header The semantics of the GET method change to a ``partial GET'' if the request message includes a Range header field. A partial GET requests that only part of the entity be transferred
The POST method The POST method is used to request that the origin server accept the entity enclosed in the request as a new subordinate of the resource identified by the Request-URI in the Request- Line. POST is designed to allow a uniform method to cover the following functions: –Annotation of existing resources; –Posting a message to a bulletin board, newsgroup, mailing list, or similar group of articles; –Providing a block of data, such as the result of submitting a form, to a data-handling process; – Extending a database through an append operation.
PUT and DELETE methods The PUT method requests that the enclosed entity be stored under the supplied Request-URI. If the Request-URI refers to an already existing resource, the enclosed entity should be considered as a modified version of the one residing on the origin server. The DELETE method requests that the origin server delete the resource identified by the Request-URI.
example status: redirect If you use Apache, you can create a file.htaccess (note the dot!) with a line redirect 301 old_url new_url old_url must be a relative path from the top of your site new_url can be any URL, even outside your site This works on wotan by virtue of configuration set for apache for your home directory. Examples –redirect 301 /~krichel http://openlib.org/home/krichel –redirect 301 Cantcook.jpg http://www.foodtv.com
The Semantic Web The W3C has been developing a new architecture that applies knowledge representation technology to the WWW. Using the Resource Description Framework (RDF), Statements are made using a Subject, Predicate and Object (very similar to Lisp and other predicate based languages). Each Subject, Predicate or Object are Resources in the URI sense and are identified by URIs within an RDF Statement using XML Namespaces.
The Semantic Web The combination of Web Services and the Semantic Web should give the Web the ability to turn any existing Web Resource into a full node in a purposefully built knowledge representation system with a functional component that allows that knowledge to be acted on. And both are based on the simple Uniform Resource Identifier.
example This statement says that the Resource identified by the URI http://openlib.org/home/krichel was created by the person Thomas Krichel: Thomas Krichel
Apache Is a free, open-source web server that is produced by the Apache Software Foundation, see http://www.apache.org It has over 50% of the market share. It runs best on UN*X systems but can run an a Mickeysoft OS as well. I will cover it here because it is freely available. I am covering version 1.3
Apache in debian /etc/apache/httpd.conf in set main configuration file. /etc/init.d/apache action, where action is one of –start –stop –restart is used to fire the daemon up or down. The daemon runs user www-data
Virtual host On a single installation of apache serveral web servers can be supported. That means the server can behave in a different way according to how it is being addressed. The easiest way to implement addressing a server in different was is through DNS host names.
Directives in httpd.conf The configuration directives are grouped into three basic sections: –Directives that control the operation of the Apache server process as a whole (the 'global environment'). –Directives that define the parameters of the 'main' or 'default' server, which responds to requests that aren't handled by a virtual host. These directives also provide default values for the settings of all virtual hosts. –Settings for virtual hosts, which allow Web requests to be sent to different IP addresses or hostnames and have them handled by the same Apac he server process.
Server type On a UN*X machine, the server can either be fired up on its own, or it can be run as part of the overall Internet daemon inetd. Usually standalone is used.
Server root Sets the directory where apache finds its own configuration files. If log files names are not given as absolute paths, they will be placen in the server root directory.
Timeout This set s the number of seconds that the server waits for the result of a request to be computed before sending a timeout. On wotan this is set to 300 seconds, this is rather a long time, the user will have gone for coffee by then.
Listen Tells the server which port and ip address to listen to. This can be used to have the server only to respond to requests to a certain IP address or to listen to a non-standard port, i.e. Not port 80
Loadmodule To extend apache, modules have written. They have to be loaded explicitly: LoadModule module file Where module is the name of the module and file is the name of the file that contains the module Looking at this gives you vital information about what the server can do.
Server directives User –Gives the user name apache runs under Group –Gives the group name the server runs under ServerAdmin –Email of a human who runs the default server ServerName –The name of the default server DocumentRoot –The top level directory of the default server
Directory options Many options for a directory can be set with instructions Name is the name of a directory. Instructions can be a whole lot of stuff
Directory instructions Options sets global options for the directory, it can be –None –All –Or any of Indexes (form directory indexes?) Includes (all server side includes?) FollowSymlinks (allow to follow server-side includes) ExecCGI (allow cgi-scripts?) MultiViews
Access control Can be part of to set directory level access control Example –Allow from friendly.com –Deny from evil.com Sometimes you have to set the order, example –Order allow, deny
Authentication This is used to enable password access. In that case the authentication is handled by a file.htaccess in the directory. The AllowOverride instruction is used to state what the user can do within the.htaccess file. Depending on its values, you can password protect a web site. We will not discuss this further here.
Userdir This sets the directory that is created by the user in her home directory to be accessed by requests to ~user. On wotan, we have UserDir public_html That is the default, actually.
Set up permission for user home directories AllowOverride FileInfo AuthConfig Limit Options +Includes Options MultiViews Indexes SymLinksIfOwnerMatch IncludesNoExec Order allow,deny Allow from all Order deny,allow Deny from all
Logs The web server logs every transaction. The are severeal types of logs that used to be kept separately, in early days. 188.8.131.52 - - [26/Jan/2003:09:19:51 -0500] "GET /~ramon/videos/ntsc175.html HTTP/1.1" 206 808 Additional information may be kept in the referer and user agent log. The referer log may have some interesting information on who links to your pages.
Virtual hosts Most apache directive can be wrapped in a grouping. This implies that the only hold for the virtual host. Example, from wotan ServerAdmin email@example.com DocumentRoot /home/connect/public_html ServerName connections2003.liu.edu ErrorLog /var/log/apache/connections2003-error.log CustomLog /var/log/apache/connectios2003-access.log common
http://openlib.org/home/krichel Thank you for your attention!