Information Technology Reference
In-Depth Information
The contents of this message depend on how the DHCP server is configured—there are
numerous options aside from an IP address that you can specify to pass to the client on
a Windows Server DHCP server.
3.
Request
: The client receives one or more DHCPOFFERs (depending on how many
DHCP servers exist on the local subnet), chooses an address from one of the offers, and
sends a DHCPREQUEST message to the server to signal acceptance of the DHCPOF-
FER.
This message might also request additional configuration parameters.
Other DHCP servers that sent offers take the request message as an acknowledgment
that the client didn't accept their offer.
4.
Acknowledge
: When the DHCP server receives the DHCPREQUEST, it marks the IP
address as being in use (that is, usually, though it's not required). Then it sends a DHC-
PACK to the client.
The acknowledgment message might contain requested configuration parameters.
If the server is unable to accept the DHCPREQUEST for any reason, it sends a
DHCPNAK message. If a client receives a DHCPNAK, it begins the configuration pro-
cess over again.
5.
When the client accepts the IP offer, the address is assigned to the client for a
specified period of time, called a
lease
. After receiving the DHCPACK message, the
client performs a final check on the parameters (sometimes it sends an ARP request
for the offered IP address) and makes note of the duration of the lease. The client is
now configured. If the client detects that the address is already in use, it sends
a DHCPDECLINE.
If the DHCP server has given out all of the IP addresses in its pool, it won't make an
offer. If no other servers make an offer, the client's IP network initialization will fail, and
the client will use Automatic Private IP Addressing (APIPA).
DHCP Lease Renewal
No matter how long the lease period, the client sends a new lease request message directly
to the DHCP server when the lease period is half over (give or take some randomness
required by RFC 2131). This period goes by the name
T1
(not to be confused with the T1
type of network connection). If the server hears the request message and there's no reason
to reject it, it sends a DHCPACK to the client. This resets the lease period.
If the DHCP server isn't available, the client realizes that the lease can't be renewed. The
client continues to use the address, and once 87.5 percent of the lease period has elapsed
(again, give or take some randomness), the client sends out another renewal request.
This interval is known as
T2
. At that point, any DHCP server that hears the renewal can
respond to this
DHCP request message
(which is a request for a lease renewal) with a
DHCPACK and renew the lease. If at any time during this process the client gets a negative
DHCPNACK message, it must stop using its IP address immediately and start the leasing
process over from the beginning by requesting a new lease.
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