Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 320
Lease Duration

Timers between the server and client may be different. There is not a synchronization of timers between clients and servers using DHCP. This may lead to a problem in that the server assumes the lease is up before the client does (because of the possibility of the inaccuracy of the clocks). To compensate for this, the DHCP server may return a lease smaller than requested by the client. But this is only in response to the client. The server writes the original time into its bindery. This may or may not be accomplished; it depends on how the developer interpreted the RFC.

A client transmits a DHCPREQUEST for the use of an IP address for a certain period of time. The server guarantees not to hand out an allocated IP address that is in use by another client. Furthermore, the server will try to reallocate the same IP address to a requesting client each time it requests an IP address. The length of time that a client uses an IP address is known as the lease. If a client needs to extend the lease, it may submit these requests to the server. It is understood between the client and the server that if the server does not receive a message from the client indicating that the client would like to extend the lease, the server assumes that the client is no longer using the IP address, and the lease expires. However, a DHCP server does not automatically reuse expired leased IP addresses. The server will continue down the number of IP addresses and assign those that have not been assigned until it exhausts its list of IP addresses. When this occurs, the server will reallocate an IP address that was previously assigned but has expired. The server may probe the network to see if the address is still being used by simply sending an ICMP ECHO request and waiting for a reply. As another consistency check, the host with the newly assigned address may issue an ARP to see if the address has already been assigned to another host. Notice, however, the RFC places these checks as “SHOULD,” which means the implementer of the protocol should implement the feature, but it is not required. You may want to ask before implementing a specific vendor’s DHCP code.

DHCP implementations may vary, but the lease time is a 32-bit unsigned integer. This number is expressed in seconds, which allows for a lease time in the range of 1 second to (approximately) 136 years—longer than I plan to be in this business! According to RFC 2131, there is no minimum lease time requirement.