Not all clients require initialization of all parameters listed in Appendix A. Two techniques are used to reduce the number of parameters transmitted from the server to the client. First, most of the parameters have defaults defined in the Host Requirements RFCs; if the client receives no parameters from the server that override the defaults, a client uses those default values. Second, in its initial DHCPDISCOVER or DHCPREQUEST message, a client may provide the server with a list of specific parameters the client is interested in. If the client includes a list of parameters in a DHCPDISCOVER message, it MUST include that list in any subsequent DHCPREQUEST messages.
The client SHOULD include the 'maximum DHCP message size' option to let the server know how large the server may make its DHCP messages. The parameters returned to a client may still exceed the space allocated to options in a DHCP message. In this case, two additional options flags (which must appear in the 'options' field of the message) indicate that the 'file' and 'sname' fields are to be used for options.
The client can inform the server which configuration parameters the client is interested in by including the 'parameter request list' option. The data portion of this option explicitly lists the options requested by tag number.
In addition, the client may suggest values for the network address and lease time in the DHCPDISCOVER message. The client may include the 'requested IP address' option to suggest that a particular IP address be assigned, and may include the 'IP address lease time' option to suggest the lease time it would like. Other options representing "hints" at configuration parameters are allowed in a DHCPDISCOVER or DHCPREQUEST message. However, additional options may be ignored by servers, and multiple servers may, therefore, not return identical values for some options. The 'requested IP address' option is to be filled in only in a DHCPREQUEST message when the client is verifying network parameters obtained previously. The client fills in the 'ciaddr' field only when correctly configured with an IP address in BOUND, RENEWING or REBINDING state.
If a server receives a DHCPREQUEST message with an invalid 'requested IP address', the server SHOULD respond to the client with a DHCPNAK message and may choose to report the problem to the system administrator. The server may include an error message in the 'message' option.