HTTP Protocol

Early versions of the WWW developed a reputation as a versatile and convenient tool for accessing mission-critical data at the European Laboratory for Particle Physics (CERN).

Paradoxically the Web tools Tim Berners-Lee developed were the most successful and were widely regarded as the best way to access the CERN phone directory. Please note, that the first successful WWW application was not distribution of published papers it was a gateway to an existing and important application. Of course, the versatility of WWW became clearer as the technology spread among high energy physics institutions and then to the outside world. But is it really an accident that the Web took off as a gateway to existing information system? I think this is not an accident and that's why WWW served as a launch pad for several scripting languages, including Perl, JavaScript, PHP and Python.

HTTP Headers

HTTP header fields are components of the message header of requests and responses in the Hypertext Transfer Protocol (HTTP). They define the operating parameters of an HTTP transaction.

The header fields are transmitted after the request or response line, the first line of a message. Header fields are colon-separated name-value pairs in clear-text string format, terminated by a carriage return (CR) and line feed (LF) character sequence. The end of the header fields is indicated by an empty field, resulting in the transmission of two consecutive CR-LF pairs. Long lines can be folded into multiple lines; continuation lines are indicated by presence of space (SP) or horizontal tab (HT) as first character on next line.[1] Few fields can also contain comments (i.e. in. User-Agent, Server, Via fields), which can be ignored by software.[2]

A core set of fields is standardized by the Internet Engineering Task Force (IETF) in RFC 2616 and other updates and extension documents (e.g., RFC 4229), and must be implemented by all HTTP-compliant protocol implementations. Additional field names and permissible values may be defined by each application.

The permanent registry of headers and repository of provisional registrations are maintained by the IANA.

Many field values may contain a quality (q) key-value pair, specifying a weight to use in content negotiation.[3]

There is no limits to size of each header field name or value, or number of headers in standard itself. However most servers, clients and proxy software impose some limits for practical and security reasons. For example Apache 2.3 server by default limits each header size to 8190 bytes, and there can be at most 100 headers in single request.[4]

Requests

Field name	Description	Example
Accept	Content-Types that are acceptable	`Accept: text/plain`
Accept-Charset	Character sets that are acceptable	`Accept-Charset: utf-8`
Accept-Encoding	Acceptable encodings. See HTTP compression.	`Accept-Encoding: gzip, deflate`
Accept-Language	Acceptable languages for response	`Accept-Language: en-US`
Accept-Datetime	Acceptable version in time	`Accept-Datetime: Thu, 31 May 2007 20:35:00 GMT`
Authorization	Authentication credentials for HTTP authentication	`Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==`
Cache-Control	Used to specify directives that MUST be obeyed by all caching mechanisms along the request/response chain	`Cache-Control: no-cache`
Connection	What type of connection the user-agent would prefer	`Connection: keep-alive`
Cookie	an HTTP cookie previously sent by the server with `Set-Cookie` (below)	`Cookie: $Version=1; Skin=new;`
Content-Length	The length of the request body in octets (8-bit bytes)	`Content-Length: 348`
Content-MD5	A Base64-encoded binary MD5 sum of the content of the request body	`Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==`
Content-Type	The MIME type of the body of the request (used with POST and PUT requests)	`Content-Type: application/x-www-form-urlencoded`
Date	The date and time that the message was sent	`Date: Tue, 15 Nov 1994 08:12:31 GMT`
Expect	Indicates that particular server behaviors are required by the client	`Expect: 100-continue`
From	The email address of the user making the request	`From: [email protected]`
Host	The domain name of the server (for virtual hosting), and the TCP port number on which the server is listening. The port number may be omitted if the port is the standard port for the service requested.^[5] Mandatory since HTTP/1.1. Although domain name are specified as case-insensitive,^[6]^[7] it is not specified whether the contents of the Host field should be interpreted in a case-insensitive manner^[8] and in practice some implementations of virtual hosting interpret the contents of the Host field in a case-sensitive manner.^{[citation needed]}	`Host: en.wikipedia.org:80` `Host: en.wikipedia.org`
If-Match	Only perform the action if the client supplied entity matches the same entity on the server. This is mainly for methods like PUT to only update a resource if it has not been modified since the user last updated it.	`If-Match: "737060cd8c284d8af7ad3082f209582d"`
If-Modified-Since	Allows a 304 Not Modified to be returned if content is unchanged	`If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT`
If-None-Match	Allows a 304 Not Modified to be returned if content is unchanged, see HTTP ETag	`If-None-Match: "737060cd8c284d8af7ad3082f209582d"`
If-Range	If the entity is unchanged, send me the part(s) that I am missing; otherwise, send me the entire new entity	`If-Range: "737060cd8c284d8af7ad3082f209582d"`
If-Unmodified-Since	Only send the response if the entity has not been modified since a specific time.	`If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT`
Max-Forwards	Limit the number of times the message can be forwarded through proxies or gateways.	`Max-Forwards: 10`
Pragma	Implementation-specific headers that may have various effects anywhere along the request-response chain.	`Pragma: no-cache`
Proxy-Authorization	Authorization credentials for connecting to a proxy.	`Proxy-Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==`
Range	Request only part of an entity. Bytes are numbered from 0.	`Range: bytes=500-999`
Referer^[sic]	This is the address of the previous web page from which a link to the currently requested page was followed. (The word "referrer" is misspelled in the RFC as well as in most implementations.)	`Referer: http://en.wikipedia.org/wiki/Main_Page`
TE	The transfer encodings the user agent is willing to accept: the same values as for the response header Transfer-Encoding can be used, plus the "trailers" value (related to the "chunked" transfer method) to notify the server it expects to receive additional headers (the trailers) after the last, zero-sized, chunk.	`TE: trailers, deflate`
Upgrade	Ask the server to upgrade to another protocol.	`Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11`
User-Agent	The user agent string of the user agent	`User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:12.0) Gecko/20100101 Firefox/12.0`
Via	Informs the server of proxies through which the request was sent.	`Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)`
Warning	A general warning about possible problems with the entity body.	`Warning: 199 Miscellaneous warning`

The Last but not Least Technology is dominated by two types of people: those who understand what they do not manage and those who manage what they do not understand ~Archibald Putt. Ph.D

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