header information

The metadata fields carried in the structured prefix of a network packet or protocol message — such as source and destination addresses, port numbers, protocol identifiers, and routing directives — as distinct from the payload or body of that message. A packet consists of a header and a payload; the header contains parameters including source and destination addresses, port numbers, and protocol identifiers. Security and compliance controls use these fields as the primary decision surface for traffic enforcement: firewalls filter packets by examining header information separately from content information, while deep packet inspection extends analysis beyond header information into the contents of network traffic. The term applies across protocol layers and data formats — network packets, email messages, HTTP responses, and other structured data units all carry analogous header fields — and authority documents such as NIST SP 800-53 use it to distinguish the lighter-weight, faster class of filtering decisions (made on routing and addressing metadata alone) from content-based inspection. In the context of electronic mail, for example, header information means "the source, destination, and routing information attached to an electronic mail message, including the originating domain name and originating electronic mail address."

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