Dhcp Message Format - HP V1910 User Manual

When the half lease duration elapses, the DHCP client sends to the DHCP server a DHCP-REQUEST unicast
to extend the lease duration. Upon availability of the IP address, the DHCP server returns a DHCP-ACK
unicast confirming that the client's lease duration has been extended, or a DHCP-NAK unicast denying the
request.
If the client receives no reply, it will broadcast another DHCP-REQUEST message for lease extension after
7/8 lease duration elapses. The DHCP server will handle the request as above mentioned.

DHCP message format

a gives the DHCP message format, which is based on the BOOTP message format and involves eight types.
These types of messages have the same format except that some fields have different values. The numbers
in parentheses indicate the size of each field in bytes.
DHCP message format
a.
0
op (1)
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 IP address allocation.

secs: Filled in by the client, the number of seconds elapsed since the client began address acquisition

or renewal process. This field is reserved and set to 0.
flags: The leftmost bit is defined as the BROADCAST (B) flag. If this flag is set to 0, the DHCP server sent

a reply back by unicast; if this flag is set to 1, the DHCP server sent a reply back by broadcast. The
remaining bits of the flags field are reserved for future use.
ciaddr: Client IP address.

yiaddr: 'your' (client) IP address, assigned by the server.

siaddr: Server IP address, from which the clients obtained configuration parameters.

giaddr: IP address of the first relay agent a request message traveled.

chaddr: Client hardware address.

sname: Server host name, from which the client obtained configuration parameters.

7 15
htype (1)
xid (4)
secs (2)
ciaddr (4)
yiaddr (4)
siaddr (4)
giaddr (4)
chaddr (16)
sname (64)
file (128)
options (variable)
23
hlen (1)
flags (2)
276
31
hops (1)