The client may set DST.ADDR and DST.PORT to the destination host and port of interest, or may send an IPv4 address of 0.0.0.0 to indicate the query is not for any specific host. The server may use this information, along with the address of the client, to customize the reply. If the client has requested a persistent connection and the server chooses to grant that request, it returns a FLAG of X'01'. The address returned should be an IPv4 address of 0.0.0.0. After the reply to the command is sent, the server sends a structure called the Feature Description List described below.
Tags consist of a main tag and a subtag.
These are written as separated by a hyphen, with the value following parentheses. When encoded into the structure, the tag and subtag each map to one byte in the tag table. The length of the value is a single byte, followed by the value itself. It defines the scope of the FDL, and advises whether a client which made a different FDL request would have been advertised the same feature set. 22 July 1997 determine when the FDL must be re-fetched.