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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)                 9  TCP/IP Dynamic Host Configuration Protocol (DHCP)                      9  DHCP Configuration and Operation

DHCP Lease Renewal and Rebinding Processes 1 DHCP Parameter Configuration Process For Clients With Non-DHCP Addresses

A TCP/IP host can't really do much without an IP address; it's a fundamental component of the Internet Protocol, upon which all TCP/IP protocols and applications run. When a host has either a manual IP address assignment or an “infinite” lease, it obviously never has to worry about losing its IP address. When a host has a finite DHCP lease, it will use the renewal/rebinding process to try to “hang on” to its existing IP address as long as possible.

So, under normal circumstances, a client will continue trying to extend its existing lease indefinitely. In certain cases, however, a host may decide to terminate its lease. This usually will not be something the client just decides to do spontaneously; it will occur in response to a specific request from the user to end the lease. A user may terminate a lease for a number of reasons, including the following:

• The client is being moved to a different network;
  
  
• The network is having its IP addresses renumbered;
  
  
• The user wants the host to negotiate a new lease with a different server;
  
  
• The user wants to reset the lease to fix some sort of a problem.

In any of these cases, the user can end the lease through a process called early lease termination or lease release. This is a very simple, unidirectional communication. The client sends a special DHCPRELEASE message unicast to the server that holds its current lease, to tell it that the lease is no longer required. The server then records the lease as having been ended. It does not need to reply back to the client.

The reason that the client can just assume that the lease termination has been successful is that this is not a mandatory part of the DHCP protocol. Having clients send DHCPRELEASE to end a lease is considered a courtesy, rather than a requirement. It is more efficient to have clients inform servers when they no longer need a lease, and this also allows the IP address in the terminated lease to be reused more quickly. However, DHCP servers are designed to handle the case where a client “disappears” without formally ending an existing lease.

DHCP Lease Renewal and Rebinding Processes 1 DHCP Parameter Configuration Process For Clients With Non-DHCP Addresses