BOOTP and DHCP Objectives Know the types of information required by a system on boot-up Know how BOOTP operates Know how DHCP operates Understand the differences between BOOTP and DHCP Understand the DHCP transition state diagram

16.1 BOOTP The Bootstrap Protocol (BOOTP) is a client/server protocol that configures a diskless computer or a computer that is booted for the first time. BOOTP provides the IP address net mask the address of a default router the address of a name server. BOOTP is static. When a client workstation asks for the above info, it is retrieved from a fixed table. Every time the client asks for the info, it gets the same results.

The Bootstrap Protocol (BOOTP) is a client/server protocol designed to provide physical address to logical address mapping. BOOTP is an application layer protocol. The administrator may put the client and the server on the same network or on different network.

Figure 16.1 Client and server on the same network The BOOTP server can be on the same network as the BOOTP client or on different networks. BOOTP places its packet inside a UDP packet (note that BOOTP is an application layer program). The BOOTP server issues a passive open command on UDP port number 67 and waits for a client. A booted client issues an active open command on port number 68. The message is encapsulated in a UDP user datagram and then in an IP packet. In the IP packet the source address is all 0s and the destination address is all 1s. Server responds with a UDP datagram source port 67 and destination port 68. Can also bypass ARP since server also knows the MAC address of the client.

Figure 16.1 Client and server on the same network The BOOTP server issues a passive open command on UDP port number 67 and waits for a client. A booted client issues an active open command on port number 68. The message is encapsulated in a UDP user datagram and then in an IP packet. In the IP packet the source address is all 0s and the destination address is all 1s. Server responds with a UDP datagram source port 67 and destination port 68. Can also bypass ARP since server also knows the MAC address of the client.

One of the advantages of BOOTP over RARP is that the client and server are application-layer processes. As in other application-layer processes, a client can be in one network and the server in another, separated by several other networks. This problem must be solved. The BOOTP request is broadcast because the client does not know the IP address of the server. A broadcast IP datagram cannot pass through any router. To solve the problem, there is a need for an intermediary. One of the hosts (or a router) can be used as a relay. The relay agent knows the unicast address of a BOOTP server. When it receives this type of packet, it encapsulates the message in a unicast datagram and sends the request to the BOOTP server. The BOOTP server knows the message comes from a relay agent because one of the fields in the request message defines the IP address of the relay agent. The relay agent, after receiving the reply, sends it to the BOOTP client.

Figure 16.2 Client and server on two different networks When client and server are on different networks, we need a relay agent, because client does not know IP address of server, and a limited broadcast address gets dumped by the local router. Relay agent knows the IP addr of the server.

16.2 DHCP The Dynamic Host Configuration Protocol (DHCP) provides static and dynamic address allocation that can be manual or automatic. The topics discussed in this section include: Static Address Allocation Dynamic Address Allocation

BOOTP is not a dynamic configuration protocol. When a client requests its IP address, the BOOTP server consults a table that matches the physical address of the client with its IP address. This implies that the binding between the physical address and the IP address of the client already exists. What if a host moves from one physical network to another? What if a host wants a temporary IP address? BOOTP cannot handle these situations because the binding between the physical and IP addresses is static and fixed in a table until changed by the administrator. BOOTP is a static configuration protocol. The Dynamic Host Configuration Protocol (DHCP) has been devised to provide static and dynamic address allocation that can be manual or automatic.

Static Address Allocation: DHCP acts as BOOTP does Static Address Allocation: DHCP acts as BOOTP does. It is backward compatible with BOOTP, which means a host running the BOOTP client can request a static address from a DHCP server. A DHCP server has a database that statically binds physical addresses to IP addresses. Dynamic Address Allocation DHCP has a second database with a pool of available IP addresses. This second database makes DHCP dynamic. When a DHCP client requests a temporary IP address, the DHCP server goes to the pool of available (unused) IP addresses and assigns an IP address for a negotiable period of time. When a DHCP client sends a request to a DHCP server, the server first checks its static database. If an entry with the requested physical address exists in the static database, the permanent IP address of the client is returned. If the entry does not exist in the static database, the server selects an IP address from the available pool, assigns the address to the client, and adds the entry to the dynamic database.

DHCP provides temporary IP addresses for a limited time. The addresses assigned from the pool are temporary addresses. The DHCP server issues a lease for a specific time. When the lease expires, the client must either stop using the IP address or renew the lease. The server has the option to agree or disagree with the renewal. If the server disagrees, the client stops using the address. One major problem with the BOOTP protocol is that the table mapping the IP addresses to physical addresses needs to be manually configured. This means that every time there is a change in a physical or IP address, the administrator needs to manually enter the changes. DHCP, on the other hand, allows both manual and automatic configurations. Static addresses are created manually and dynamic addresses are created automatically.

DHCP basics Bootp is static, but DHCP is dynamic (but it can also be static). DHCP has a pool of available addresses. When a request arrives, DHCP pulls out the next available address and assigns it to the client for a negotiable time period. When a request comes in from a client, the DHCP server first consults the static table. DHCP is great when devices and IP addresses change. The DHCP packet format is almost identical to the BOOTP packet format (in order to be compatible with BOOTP). Only difference is 1-bit flag.