H3C Technologies H3C SecPath F1000-E - Manual (Page 35)

3
IP Address Lease Extension
The IP address dynamically allocated by a DHCP server to a client has a lease. When the lease expires,
the IP address is 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 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 broadcasts another DHCP-REQUEST message for lease extension after
7/8 lease duration elapses. The DHCP server handles 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: 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.