This is mainly for methods like PUT to only update a resource if it has not been modified since the user last updated it.
The domain name of the server (for virtual hosting), and the TCP port number on which the server is listening.
The port number may be omitted if the port is the standard port for the service requested.
Only perform the action if the client supplied entity matches the same entity on the server.
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.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 end of the header section is indicated by an empty field, resulting in the transmission of two consecutive CR-LF pairs.Historically, long lines could be folded into multiple lines; continuation lines are indicated by the presence of a space (SP) or horizontal tab (HT) as the first character on the next line. A core set of fields is standardized by the Internet Engineering Task Force (IETF) in RFCs 7230, 7231, 7232, 7233, 7234, and 7235.