The header fields are transmitted after the request or response line, which is 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 permanent registry of header fields and repository of provisional registrations are maintained by the IANA.
Additional field names and permissible values may be defined by each application.
Non-standard header fields were conventionally marked by prefixing the field name with The standard imposes no limits to the size of each header field name or value, or to the number of fields.
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 the size of each field to 8190 bytes, and there can be at most 100 header fields in a single request.