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.The client is now in the RENEWING state.The server wouldusually respond with a DHCPACK message indicating the new lease time, andtimers T1 and T2 are reset at the client accordingly.The server also resets itsrecord of the lease time.In normal circumstances, an active client wouldcontinually renew its lease in this way indefinitely, without the lease everexpiring.3.If no DHCPACK is received until timer T2 expires, the client enters theREBINDING state.It now broadcasts a DHCPREQUEST message to extendits lease.This request can be confirmed by a DHCPACK message from anyDHCP server on the network.4.If the client does not receive a DHCPACK message after its lease has expired,it has to stop using its current TCP/IP configuration.The client may then returnto the INIT state, issuing a DHCPDISCOVER broadcast to try and obtain anyvalid address.Figure 239 on page 412 shows the DHCP process and changing client stateduring that process.Chapter 7.Dynamic IP, Mobile IP and Network Computers 411 ExternalINITINIT-REBOOTConfigurationDHCPDISCOVERDHCPREQUESTDHCPACKDHCPINFORMDHCPNACKnot acceptablenotify server,DHCPNACK,restartDHCPDECLINEget parametersLease expiredhalt networkSELECTINGREBOOTINGDHCPNACKDHCPACKdiscard offersupplyDHCPOFFER parametersDHCPACK collect repliesselect offerrecord lease,REQUESTINGDHCPREQUESTset T1, T2AdditionalConfigurationDHCPACKDHCPOFFERrecord lease,DHCPACKdiscard REBINDINGset T1, T2record lease,set T1, T2DHCPACKBOUNDT2 expiresrecord lease,DHCPREQUESTDHCPOFFER, set T1, T2DHCPACK,T1 expiresDHCPNACK,DHCPNACKDHCPREQUESTlease expiresdiscardRENEWINGhalt networkDHCPRELEASEINITBOUNDhalt networkFigure 239.DHCP Client State and DHCP Process7.2.5 Reusing a Previously Allocated Network AddressIf the client remembers and wishes to reuse a previously allocated network addressthen the following steps are carried out:1.The client broadcasts a DHCPREQUEST message on its local subnet.TheDHCPREQUEST message includes the client's network address.2.A server with knowledge of the client's configuration parameters responds witha DHCPACK message to the client (provided the lease is still current),renewing the lease at the same time.If the client's lease has expired, the server with knowledge of the clientresponds with DHCPNACK.3.The client receives the DHCPACK message with configuration parameters.The client performs a final check on the parameters and notes the duration ofthe lease and the lease identification cookie specified in the DHCPACKmessage.At this point, the client is configured and its T1 and T2 timers arereset.If the client detects a problem with the parameters in the DHCPACK message,the client sends a DHCPDECLINE message to the server and restarts theconfiguration process by requesting a new network address.If the clientreceives a DHCPNAK message, it cannot reuse its remembered networkaddress.It must instead request a new address by restarting the configurationprocess as described in 7.2.3, Allocating a New Network Address onpage 409.412 TCP/IP Tutorial and Technical Overview Note: A host should use DHCP to reacquire or verify its IP address and networkparameters whenever the local network parameters have changed, for example atsystem boot time or after a disconnection from the local network, as the localnetwork configuration may change without the host's or user's knowledge.If a clienthas multiple IP interfaces, each of them must be configured by DHCP separately.For further information please refer to the above-mentioned RFCs.7.2.6 Configuration Parameters RepositoryDHCP provides persistent storage of network parameters for network clients.ADHCP server stores a key-value entry for each client, the key being some uniqueidentifier, for example an IP subnet number and a unique identifier within thesubnet (normally a hardware address), and the value contains the configurationparameters last allocated to this particular client.One effect of this is that a DHCP client will tend to always be allocated the same IPaddress by the server, provided the pool of addresses is not over-subscribed andthe previous address has not already been allocated to another client.7.2.7 DHCP ConsiderationsDHCP dynamic allocation of IP addresses and configuration parameters relievesthe network administrator of great deal of manual configuration work
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