3com 4210 PWR 9-Port 3CR17341-91-ME 用户手册

If the DHCP client fails to update its IP address lease when half of the lease time 
elapses, it will update its IP address lease by broadcasting a DHCP-REQUEST packet 
to the DHCP servers again when seven-eighths of the lease time elapses. The 
DHCP server performs the same operations as those described above.

DHCP has eight types of packets. They have the same format, but the values of 
some fields in the packets are different. The DHCP packet format is based on that 
of the BOOTP packets. The following figure describes the packet format (the 
number in the brackets indicates the field length, in bytes):

Figure 86   DHCP packet format

The fields are described as follows:

■

op: Operation types of DHCP packets, 1 for request packets and 2 for response 
packets.

■

htype, hlen: Hardware address type and length of the DHCP client.

■

hops: Number of DHCP relay agents which a DHCP packet passes. For each 
DHCP relay agent that the DHCP request packet passes, the field value 
increases by 1.

■

xid: Random number that the client selects when it initiates a request. The 
number 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 the 
DHCP response packet is a unicast (set to 0) or broadcast (set to 1). Other bits 
are 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 passes 
after it sent the request packet.

■

chaddr: Hardware address of the DHCP client.

op (1)

0

7

15

htype (1)

hlen (1)

hops (1)

xid (4)

23

31

secs (2)

flags (2)

ciaddr (4)

yiaddr (4)

siaddr (4)

giaddr (4)

chaddr (16)

sname (64)

file (128)

options (variable)