DHCP Message Format 793
■ If there are multiple DHCP servers, IP addresses offered by other DHCP servers
are assignable to other clients.
IP Address Lease
Extension
The IP address dynamically allocated by a DHCP server to a client has a lease. After
the lease duration elapses, the IP address will be reclaimed by the DHCP server. If
the 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 a
DHCP-REQUEST unicast message 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 the DHCP-NAK message, 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 Figure 241 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.
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 IP
address allocation.
■ secs: Filled in by the client, the number of seconds elapsed since the client
began address acquisition or renewal process. Currently 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
op (1)
0715
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)
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