H3C Technologies H3C SecPath F1000-E - Manual (Page 230)

3
IP address lease extension
The IP address dynamically allocated by a DHCP server to a client has a lease. When the lease elapses,
the IP address will be reclaimed by the DHCP server. If the client wants to use the IP address longer, it
has to extend the lease duration.
When the half lease duration elapses, the DHCP client sends to 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 no reply, it will broadcast another DHCP-REQUEST message for lease extension
when 7/8 lease duration elapses. The DHCP server will handle the request as above mentioned.
DHCP Message Format
Figure 3 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 3 DHCP message format
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)
• 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 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.