Problem: Not All Networks are Directly Connected
3.3 Internet (IP)
3.3.7 Host Configuration (DHCP)
Ethernet addresses are configured into the network adaptor by the manufacturer, and this process is managed in such a way to ensure that these addresses are globally unique. This is clearly a sufficient condition to ensure that any collection of hosts connected to a single Ethernet (including an extended LAN) will have unique addresses. Furthermore, uniqueness is all we ask of Ethernet addresses.
IP addresses, by contrast, not only must be unique on a given internetwork but also must reflect the structure of the internetwork. As noted above, they contain a network part and a host part, and the network part must be the same for all hosts on the same network. Thus, it is not possible for the IP address to be configured once into a host when it is manufactured, since that would imply that the manufacturer knew which hosts were going to end up on which networks, and it would mean that a host, once connected to one network, could never move to another. For this reason, IP addresses need to be reconfigurable.
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In addition to an IP address, there are some other pieces of information a host needs to have before it can start sending packets. The most notable of these is the address of a default router—the place to which it can send packets whose destination address is not on the same network as the sending host.
Most host operating systems provide a way for a system administrator, or even a user, to manually configure the IP information needed by a host; however, there are some obvious drawbacks to such manual configu- ration. One is that it is simply a lot of work to configure all the hosts in a large network directly, especially when you consider that such hosts are not reachable over a network until they are configured. Even more importantly, the configuration process is very error prone, since it is necessary to ensure that every host gets the correct network number and that no two hosts receive the same IP address. For these reasons, auto- mated configuration methods are required. The primary method uses a protocol known as theDynamic Host Configuration Protocol(DHCP).
DHCP relies on the existence of a DHCP server that is responsible for providing configuration information to hosts. There is at least one DHCP server for an administrative domain. At the simplest level, the DHCP server can function just as a centralized repository for host configuration information. Consider, for example, the problem of administering addresses in the internetwork of a large company. DHCP saves the network administrators from having to walk around to every host in the company with a list of addresses and network map in hand and configuring each host manually. Instead, the configuration information for each host could be stored in the DHCP server and automatically retrieved by each host when it is booted or connected to the network. However, the administrator would still pick the address that each host is to receive; he would just store that in the server. In this model, the configuration information for each host is stored in a table that is indexed by some form of unique client identifier, typically the hardware address (e.g., the Ethernet address of its network adaptor).
A more sophisticated use of DHCP saves the network administrator from even having to assign addresses to individual hosts. In this model, the DHCP server maintains a pool of available addresses that it hands out to hosts on demand. This considerably reduces the amount of configuration an administrator must do, since now it is only necessary to allocate a range of IP addresses (all with the same network number) to each network.
Since the goal of DHCP is to minimize the amount of manual configuration required for a host to function, it would rather defeat the purpose if each host had to be configured with the address of a DHCP server. Thus, the first problem faced by DHCP is that of server discovery.
To contact a DHCP server, a newly booted or attached host sends aDHCPDISCOVERmessage to a special IP address (255.255.255.255) that is an IP broadcast address. This means it will be received by all hosts and routers on that network. (Routers do not forward such packets onto other networks, preventing broadcast to the entire Internet.) In the simplest case, one of these nodes is the DHCP server for the network. The server would then reply to the host that generated the discovery message (all the other nodes would ignore it).
However, it is not really desirable to require one DHCP server on every network, because this still creates a potentially large number of servers that need to be correctly and consistently configured. Thus, DHCP uses the concept of arelay agent. There is at least one relay agent on each network, and it is configured with just one piece of information: the IP address of the DHCP server. When a relay agent receives a DHCPDISCOVERmessage, it unicasts it to the DHCP server and awaits the response, which it will then send back to the requesting client. The process of relaying a message from a host to a remote DHCP server is shown inFigure 3.25.
Figure 3.26below shows the format of a DHCP message. The message is actually sent using a protocol called theUser Datagram Protocol(UDP) that runs over IP. UDP is discussed in detail in the next chapter, but the only interesting thing it does in this context is to provide a demultiplexing key that says, “This is a
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Figure 3.25.: A DHCP relay agent receives a broadcast DHCPDISCOVER message from a host and sends a unicast DHCPDISCOVER to the DHCP server.
DHCP packet.”
DHCP is derived from an earlier protocol called BOOTP, and some of the packet fields are thus not strictly relevant to host configuration. When trying to obtain configuration information, the client puts its hardware address (e.g., its Ethernet address) in thechaddrfield. The DHCP server replies by filling in theyiaddr (“your” IP address) field and sending it to the client. Other information such as the default router to be used by this client can be included in theoptionsfield.
In the case where DHCP dynamically assigns IP addresses to hosts, it is clear that hosts cannot keep ad- dresses indefinitely, as this would eventually cause the server to exhaust its address pool. At the same time, a host cannot be depended upon to give back its address, since it might have crashed, been unplugged from the network, or been turned off. Thus, DHCP allows addresses to be leased for some period of time. Once the lease expires, the server is free to return that address to its pool. A host with a leased address clearly needs to renew the lease periodically if in fact it is still connected to the network and functioning correctly.
Key Takeaway
DHCP illustrates an important aspect of scaling: the scaling of network management. While discussions of scaling often focus on keeping the state in network devices from growing too fast, it is important to pay attention to the growth of network management complexity. By allowing network managers to configure a range of IP addresses per network rather than one IP address per host, DHCP improves the manageability of a network.[Next]
Note that DHCP may also introduce some more complexity into network management, since it makes the binding between physical hosts and IP addresses much more dynamic. This may make the network man- ager’s job more difficult if, for example, it becomes necessary to locate a malfunctioning host.
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Figure 3.26.: DHCP packet format.