DHCP Packet Format 283If the DHCP client fails to update its IP address lease when half of the lease timeelapses, it will update its IP address lease by broadcasting a DHCP-REQUEST packetto the DHCP servers again when seven-eighths of the lease time elapses. TheDHCP server performs the same operations as those described above.DHCP Packet Format DHCP has eight types of packets. They have the same format, but the values ofsome fields in the packets are different. The DHCP packet format is based on thatof the BOOTP packets. The following figure describes the packet format (thenumber in the brackets indicates the field length, in bytes):Figure 86 DHCP packet formatThe fields are described as follows:■ op: Operation types of DHCP packets, 1 for request packets and 2 for responsepackets.■ htype, hlen: Hardware address type and length of the DHCP client.■ hops: Number of DHCP relay agents which a DHCP packet passes. For eachDHCP relay agent that the DHCP request packet passes, the field valueincreases by 1.■ xid: Random number that the client selects when it initiates a request. Thenumber is used to identify an address-requesting process.■ secs: Elapsed time after the DHCP client initiates a DHCP request.■ flags: The first bit is the broadcast response flag bit, used to identify that theDHCP response packet is a unicast (set to 0) or broadcast (set to 1). Other bitsare reserved.■ ciaddr: IP address of a DHCP client.■ yiaddr: IP address that the DHCP server assigns to a client.■ siaddr: IP address of the DHCP server.■ giaddr: IP address of the first DHCP relay agent that the DHCP client passesafter it sent the request packet.■ chaddr: Hardware address of the DHCP client.op (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)
