H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 26)

15
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. To continue using the IP address, the client must extend
the lease duration.
After half the lease duration, the DHCP client sends a DHCP-REQUEST unicast to the DHCP server to
extend the lease. Depending on 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.
DHCP message format
Figure 8 shows the DHCP message format, which is based on the BOOTP message format although
DHCP uses some of the fields in significantly different ways. The numbers in parentheses indicate the size
of each field in bytes.
Figure 8 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.