3com 2928 User Guide
1-3
IP Address Lease Extension
The IP address dynamically allocated by a DHCP server to a client has a lease. When the lease expires,
the DHCP server will reclaim the IP address. 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 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.
the DHCP server will reclaim the IP address. 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 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
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.
different values. The numbers in parentheses indicate the size of each field in bytes.
Figure 1-3
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)
z
op: Message type defined in option field. 1 = REQUEST, 2 = REPLY
z
htype, hlen: Hardware address type and length of a DHCP client.
z
hops: Number of relay agents a request message traveled.
z
xid: Transaction ID, a random number chosen by the client to identify an IP address allocation.
z
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.
acquisition or renewal process. Currently this field is reserved and set to 0.
z
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.
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.
z
ciaddr: Client IP address.
z
yiaddr: 'your' (client) IP address, assigned by the server.
z
siaddr: Server IP address, from which the clients obtained configuration parameters.
z
giaddr: IP address of the first relay agent a request message traveled.
z
chaddr: Client hardware address.
z
sname: Server host name, from which the client obtained configuration parameters.