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 is sometimes called the payload or body.
It is vital that header composition follows a clear and unambiguous specification or format, to allow for parsing.
Examples
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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 agents, and much more.
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Similar headers are used in Usenet (NNTP) messages, and .
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In a data packet sent via the Internet, the data (payload) are preceded by header information such as the sender's and the recipient's , the protocol governing the format of the payload and several other formats. The header's format is specified in the Internet Protocol.
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In data packets sent by wireless communication, and in Disk sector of data stored on magnetic media, typically the header begins with a syncword to allow the receiver to adapt to analog amplitude and speed variations and for frame synchronization.
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In graphics file formats, the header might give information about an image's size, resolution, number of colors, and the like.
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In archive file formats, the file header might serve as a fingerprint or signature to identify the specific file format and corresponding software utility.
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In some programming languages (for example C and C++) the subroutine are declared in .
See also
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Page footer
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Protocol overhead
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Trailer (computing), used in computer networking
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Field (computer science)
