Header (information technology)
Encyclopedia
In information technology
, header refers to supplemental data
placed at the beginning of a block of data being stored or transmitted. In data transmission
, the data following the header are sometimes called the payload or body
.
It is vital that header composition follow a clear and unambiguous specification or format, to allow for parsing
.
Information technology
Information technology is the acquisition, processing, storage and dissemination of vocal, pictorial, textual and numerical information by a microelectronics-based combination of computing and telecommunications...
, header refers to supplemental data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
placed at the beginning of a block of data being stored or transmitted. In data transmission
Data transmission
Data transmission, digital transmission, or digital communications is the physical transfer of data over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibres, wireless communication channels, and storage media...
, the data following the header are sometimes called the payload or body
Body
With regard to living things, a body is the physical body of an individual. "Body" often is used in connection with appearance, health issues and death...
.
It is vital that header composition follow a clear and unambiguous specification or format, to allow for parsing
Parsing
In computer science and linguistics, parsing, or, more formally, syntactic analysis, is the process of analyzing a text, made of a sequence of tokens , to determine its grammatical structure with respect to a given formal grammar...
.
Examples
- E-mail header: The text (body) is preceded by header lines indicating sender, recipient, subject, sending time stamp, receiving time stamps of all intermediate and the final mail transfer agentMail transfer agentWithin Internet message handling services , a message transfer agent or mail transfer agent or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture...
s, and much more. See RFC 5322 for details. Similar headers are used in UsenetUsenetUsenet is a worldwide distributed Internet discussion system. It developed from the general purpose UUCP architecture of the same name.Duke University graduate students Tom Truscott and Jim Ellis conceived the idea in 1979 and it was established in 1980...
(NNTP) messages, and HTTP headers. - In a data packet sent via the InternetInternetThe Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
, the data (payload) are preceded by header information such as the sender's and the recipient's IP addressIP addressAn Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
es, the protocolCommunications protocolA communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
governing the format of the payload and several other formats. The header's format is specified in the Internet ProtocolInternet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
. - In data packets sent by wireless communication, and in sectorDisk sectorIn computer disk storage, a sector is a subdivision of a track on a magnetic disk or optical disc. Each sector stores a fixed amount of user data. Traditional formatting of these storage media provides space for 512 bytes or 2048 bytes of user-accessible data per sector...
s of data stored on magnetic media, typically the header begins with a syncwordSyncwordIn computer networks, a syncword, sync character or preamble is used to synchronize a transmission by indicating the end of header information and the start of data.-Examples:For example an audio receiver is receiving a bit stream of data...
to allow the receiver to adapt to analog amplitude and speed variations and for frame synchronizationFrame synchronizationWhile receiving a stream of framed data, frame synchronization is the process by which incoming frame alignment signals, i.e., distinctive bit sequences , are identified, i.e., distinguished from data bits, permitting the data bits within the frame to be extracted for decoding or retransmission...
. - In graphics file formats, the header might give information about an image's size, resolution, number of colours, and the like.
- In Archive file formats, the file header might serve as a fingerprintFingerprintA fingerprint in its narrow sense is an impression left by the friction ridges of a human finger. In a wider use of the term, fingerprints are the traces of an impression from the friction ridges of any part of a human hand. A print from the foot can also leave an impression of friction ridges...
or signatureSignatureA signature is a handwritten depiction of someone's name, nickname, or even a simple "X" that a person writes on documents as a proof of identity and intent. The writer of a signature is a signatory. Similar to a handwritten signature, a signature work describes the work as readily identifying...
to identify the specific file formatFile formatA file format is a particular way that information is encoded for storage in a computer file.Since a disk drive, or indeed any computer storage, can store only bits, the computer must have some way of converting information to 0s and 1s and vice-versa. There are different kinds of formats for...
and corresponding software utility. - In some programming languageProgramming languageA programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
s (for example CC (programming language)C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
and C++C++C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
) the functionsSubroutineIn computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
are declared in header fileHeader fileSome programming languages use header files. These files allow programmers to separate certain elements of a program's source code into reusable files. Header files commonly contain forward declarations of classes, subroutines, variables, and other identifiers...
s.