DHCP Client States in the Lease Process
DHCP clients cycle through six different states during the DHCP lease process, as illustrated in Figures 4.3 and 4.4. Figure 4.4 illustrates the DHCP lease process for clients that are renewing a lease.
Figure 4.3 DHCP Client States During the Lease Process
Figure 4.4 DHCP Client States During the Lease Renewal Process
When the DHCP client and DHCP server are on the same subnet, the DHCPDiscover, DHCPOffer, DHCPRequest, and DHCPAck messages are sent via media access control and IP-level broadcasts.
In order for DHCP clients to communicate with a DHCP server on a remote network, the connecting router or routers must support the forwarding of DHCP messages between the DHCP client and the DHCP server using a BOOTP/DHCP Relay Agent. For more information, see "Supporting BOOTP Clients" and "Managing Relay Agents" later in this chapter.
This state occurs the first time the TCP/IP protocol stack is initialized on the DHCP client computer. The client does not yet have an IP address to request from the DHCP servers. This state also occurs if the client is denied the IP address it is requesting or the IP address it previously had was released. Figure 4.5 shows the Initialization state.
Figure 4.5 The Initialization State
When the DHCP client is in this state, its IP address is 0.0.0.0. To obtain a valid address, the client broadcasts a DHCPDiscover message from UDP port 68 to UDP port 67, with a source address of 0.0.0.0 and a destination of 255.255.255.255 (the client does not yet know the address of any DHCP servers). The DHCPDiscover message contains the DHCP client's media access control address and computer name.
Next, the client moves into the Selecting state, where it chooses a DHCPOffer. All DHCP servers that receive a DHCPDiscover message and have a valid IP address to offer the DHCP client respond with a DHCPOffer message sent from UDP port 68 to UDP port 67. The DHCPOffer is sent via the media access control and IP broadcast because the DHCP client does not yet have a valid IP address that can be used as a destination. The DHCP server reserves the IP address to prevent it from being offered to another DHCP client.
The DHCPOffer message contains an IP address and matching subnet mask, a DHCP server identifier (the IP address of the offering DHCP server), and a lease duration. Figure 4.6 shows the Selecting state.
Figure 4.6 The Selecting State
The DHCP client waits for a DHCPOffer message. If a DHCP client does not receive a DHCPOffer message from a DHCP server on startup, it will retry four times (at intervals of 2, 4, 8, and 16 seconds, plus a random amount of time between 0 and 1,000 milliseconds). If a DHCP client does not receive a DHCPOffer after four attempts, it waits 5 minutes, then retries at 5-minute intervals.
After a DHCP client has received a DHCPOffer message from a DHCP server, the client moves into the Requesting state. The DHCP client knows the IP address it wants to lease, so it broadcasts a DHCPRequest message to all DHCP servers. The client must use a broadcast because it still does not have an assigned IP address. Figure 4.7 shows the Requesting state.
Figure 4.7 The Requesting State
If the IP address of the client was known (that is, the computer restarted and is trying to lease its previous address), the broadcast is looked at by all of the DHCP servers. The DHCP server that can lease the requested IP address responds with either a successful acknowledgment (DHCPAck) or an unsuccessful acknowledgment (DHCPNak). The DHCPNak message occurs when the IP address requested is not available or the client has been physically moved to a different subnet that requires a different IP address. After receiving a DHCPNak message, the client returns to the Initializing state and begins the lease process again.
If the IP address of the client was just obtained with a DHCPDiscover or DHCPOffer exchange with a DHCP server, the client puts the IP address of that DHCP server in the DHCPRequest. The specified DHCP server responds to the request, and any other DHCP servers retract their DHCPOffer. This ensures that the IP addresses that were offered by the other DHCP servers go back to an available state for another DHCP client.
The DHCP server responds to a DHCPRequest message with a DHCPAck message. This message contains a valid lease for the negotiated IP address, and any DHCP options configured by the DHCP administrator. Figure 4.8 shows the Binding state.
Figure 4.8 The Binding State
The DHCPAck message is sent by the DHCP server using an IP broadcast. When the DHCP client receives the DHCPAck message, it completes initialization of the TCP/IP stack. It is now considered a bound DHCP client that can use TCP/IP to communicate on the network.
The IP address remains allocated to the client until the client manually releases the address, or until the lease time expires and the DHCP server cancels the lease.
IP addressing information is leased to a client, and the client is responsible for renewing the lease. By default, DHCP clients try to renew their lease when 50 percent of the lease time has expired. To renew its lease, a DHCP client sends a DHCPRequest message to the DHCP server from which it originally obtained the lease.
The DHCP server automatically renews the lease by responding with a DHCPAck message. This DHCPAck message contains the new lease as well as any DHCP option parameters. This ensures that the DHCP client can update its TCP/IP settings in case the network administrator has updated any settings on the DHCP server. Figure 4.9 illustrates the Renewing state.
Figure 4.9 The Renewing State
Once the DHCP client has renewed its lease, it returns to the Bound state. Renewal messages (DHCPRequest and DHCPAck) are sent by media access control and IP-level unicast traffic.
If the DHCP client is unable to communicate with the DHCP server from which it obtained its lease, and 87.5 percent of its lease time has expired, it will attempt to contact any available DHCP server by broadcasting DHCPRequest messages. Any DHCP server can respond with a DHCPAck message, renewing the lease, or a DHCPNak message, forcing the DHCP client to initialize and restart the lease process. Figure 4.10 shows the Rebinding state.
Figure 4.10 The Rebinding State
If the lease expires or a DHCPNak message is received, the DHCP client must immediately discontinue using its current IP address. If this occurs, communication over TCP/IP stops until a new IP address is obtained by the client.