Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 321
Efficiencies

Clients usually do not need all the configuration parameters that are available using DHCP. To build some efficiency into this protocol, the protocol assumes defaults. Host requirement RFCs (1122, 1123) name the default for the parameters. When the client receives the DHCPNAK packets that contain configuration information, a lot of the information will not be included. The host assumes that default value for anything not contained in the DHCPNAK message.

Another question should have come into your mind by now (if you were paying attention). If a host is using DHCP for its initial configuration, how does it know to accept TCP/IP packets when TCP/IP has not been fully initialized in the host? To work around this problem, DHCP uses the Flags field. The field is 16 bits in length, but only 1 bit is used. The other 15 must be set to 0. The bit that is used is called the Broadcast bit, or B bit. Any station that cannot receive unicast datagrams (usually sent by BOOTP Relay agents and servers on DHCPOFFER, DHCPACK, and DHCPNAK messages) must set this bit on DHCPDISCOVER and DHCPREQUEST messages. DHCP servers processing the request will mark this bit and transmit their responses as broadcast in both the IP header and the MAC header.

If the broadcast bit is set to 0, the responses of the server will be transmitted as unicast, with the IP address set to Yiaddr and the MAC destination address set to Chaddr.

Finally, a client may use the DHCPRELEASE message to gracefully shut down or to indicate to the DHCP server that the client no longer needs the IP address assigned by the server. The client does not have to use this message and may simply let the lease expire.