The client maintains two times, T1 and T2, that specify the times at
which the client tries to extend its lease on its network address.
T1 is the time at which the client enters the RENEWING state and
attempts to contact the server that originally issued the client's
network address. T2 is the time at which the client enters the
REBINDING state and attempts to contact any server. T1 MUST be
earlier than T2, which, in turn, MUST be earlier than the time at
which the client's lease will expire.
To avoid the need for synchronized clocks, T1 and T2 are expressed in
options as relative times [2].
At time T1 the client moves to RENEWING state and sends (via unicast)
a DHCPREQUEST message to the server to extend its lease. The client
sets the 'ciaddr' field in the DHCPREQUEST to its current network
address. The client records the local time at which the DHCPREQUEST
message is sent for computation of the lease expiration time. The
client MUST NOT include a 'server identifier' in the DHCPREQUEST
message.
Any DHCPACK messages that arrive with an 'xid' that does not match
the 'xid' of the client's DHCPREQUEST message are silently discarded.
When the client receives a DHCPACK from the server, the client
computes the lease expiration time as the sum of the time at which
the client sent the DHCPREQUEST message and the duration of the lease
in the DHCPACK message. The client has successfully reacquired its
network address, returns to BOUND state and may continue network
processing.
If no DHCPACK arrives before time T2, the client moves to REBINDING
state and sends (via broadcast) a DHCPREQUEST message to extend its
lease. The client sets the 'ciaddr' field in the DHCPREQUEST to its
current network address. The client MUST NOT include a 'server
identifier' in the DHCPREQUEST message.
Times T1 and T2 are configurable by the server through options. T1
defaults to (0.5 * duration_of_lease). T2 defaults to (0.875 *
duration_of_lease). Times T1 and T2 SHOULD be chosen with some
random "fuzz" around a fixed value, to avoid synchronization of
client reacquisition.
A client MAY choose to renew or extend its lease prior to T1. The
server MAY choose to extend the client's lease according to policy
set by the network administrator. The server SHOULD return T1 and
T2, and their values SHOULD be adjusted from their original values to
take account of the time remaining on the lease.
In both RENEWING and REBINDING states, if the client receives no
response to its DHCPREQUEST message, the client SHOULD wait one-half
of the remaining time until T2 (in RENEWING state) and one-half of
the remaining lease time (in REBINDING state), down to a minimum of
60 seconds, before retransmitting the DHCPREQUEST message.
If the lease expires before the client receives a DHCPACK, the client
moves to INIT state, MUST immediately stop any other network
processing and requests network initialization parameters as if the
client were uninitialized. If the client then receives a DHCPACK
allocating that client its previous network address, the client
SHOULD continue network processing. If the client is given a new
network address, it MUST NOT continue using the previous network
address and SHOULD notify the local users of the problem.
