List Of HTTP Header Fields
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. Few fields can also contain comments (i.e. in. User-Agent, Server, Via fields), which can be ignored by software.
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.
There are no limits to the size of each header field name or value, or the number of headers, in the standard itself. However, most servers, clients and proxy software impose some limits for practical and security reasons. For example, the Apache 2.3 server by default limits each header size to 8190 bytes, and there can be at most 100 headers in single request.
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