DHCP Message Format 793■ If there are multiple DHCP servers, IP addresses offered by other DHCP serversare assignable to other clients.IP Address LeaseExtensionThe IP address dynamically allocated by a DHCP server to a client has a lease. Afterthe lease duration elapses, the IP address will be reclaimed by the DHCP server. Ifthe client wants to use the IP address again, it has to extend the lease duration.After the half lease duration elapses, the DHCP client will send the DHCP server aDHCP-REQUEST unicast message to extend the lease duration. Upon availability ofthe IP address, the DHCP server returns a DHCP-ACK unicast confirming that theclient’s lease duration has been extended, or a DHCP-NAK unicast denying therequest.If the client receives the DHCP-NAK message, it will broadcast anotherDHCP-REQUEST message for lease extension after 7/8 lease duration elapses. TheDHCP server will handle the request as above mentioned.DHCP Message Format Figure 241 gives the DHCP message format, which is based on the BOOTPmessage format and involves eight types. These types of messages have the sameformat except that some fields have different values. The numbers in parenthesesindicate the size of each field in bytes.Figure 241 DHCP message format■ op: Message type defined in option field. 1 = REQUEST, 2 = REPLY■ htype,hlen: Hardware address type and length of a DHCP client.■ hops: Number of relay agents a request message traveled.■ xid: Transaction ID, a random number chosen by the client to identify an IPaddress allocation.■ secs: Filled in by the client, the number of seconds elapsed since the clientbegan address acquisition or renewal process. Currently this field is reservedand set to 0.■ flags: The leftmost bit is defined as the BROADCAST (B) flag. If this flag is set to0, the DHCP server sent a reply back by unicast; if this flag is set to 1, the DHCPop (1)0 7 15htype (1) hlen (1) hops (1)xid (4)23 31secs (2) flags (2)ciaddr (4)yiaddr (4)siaddr (4)giaddr (4)chaddr (16)sname (64)file (128)options (variable)
