Boot Protocol BOOTP

■ Dynamic Host Configuration Protocol (DHCP)

RARP and BOOTP work using the same basic process. To use either protocol, a PC needs a LAN interface card. The computer sends a LAN broadcast frame announcing its own MAC address and requests that someone assign it an IP address. Figure 5-11 outlines the process for both RARP and BOOTP.

Jessie

Jessie

Figure 5-11 RARP and BOOTP

© RARP Broadcast 2 RARP Reply

RARP

Hannah

RARP Server

MAC: 0200.1111.1111

Hey Everybody! My MAC Address Is 0200.1111.1111. If You Are a RARP Server, Please Tell Me My IP Address!

RARP Server

0200.1111.1111 10.1.1.1 0200.1234.567810.1.1.2 0200.5432.1111 10.1.1.3

Configuration

0200.1111.1111 10.1.1.1 0200.1234.567810.1.1.2 0200.5432.1111 10.1.1.3

Your IP Address Is 10.1.1.2

Hannah

BOOTP

BOOTP Server

MAC: 0200.1111.1111

Configuration

Gateway

0200.1111.1111 10.1.1 . 1 10.1.1.20C 0200.1234.567810.1.1.2 10.1.1.200 0200.5432.1111 10.1.1.3 10.1.1.200

Hey Everybody! My MAC Address Is 0200.1111.1111. If You Are a BOOTP Server, Please Tell Me My IP Address!

10.1.1.200

Your IP Address Is 10.1.1.2 Your Default Gateway Is 10.1.1.200

RARP and BOOTP requests sent to the LAN broadcast address simply ask for an IP address assignment. Both protocols allow for IP address assignment, but that is all that RARP can ask for—it can't even ask for the subnet mask used on the LAN. RARP is defined in RFC 903, whereas BOOTP was defined later in RFC 1542, including several improvements over RARP. So, BOOTP allows many more tidbits of information to be announced to a BOOTP client—its IP address, its subnet mask, its default gateway IP addresses, its other server IP addresses, and the name of a file that the computer should download.

Both RARP and BOOTP were created with the motivation to allow a diskless workstation to come up and start operating. With RARP, the creators of the protocol just wanted to get the machine an IP address so that a knowledgeable user could type in commands and copy the correct files from a server onto the diskless computer's RAM memory so that they could be used. The creators of BOOTP, anticipating a less sophisticated user in the future, wanted to automate as much of the process as possible—including the dynamic assignment of a default gateway (router) IP address.

BOOTP's name really comes from the feature in which BOOTP supplies the name of a file to the BOOTP client. Typically, the diskless workstations had enough permanent memory to boot a very simple operating system, with the expectation that the computer would use a simple protocol, such as the Trivial File Transfer Protocol (TFTP), to transfer a file containing a more sophisticated operating system into RAM. So, with the ultimate goal being to let a diskless computer complete the processing of initializing, or booting, a full operating system, BOOTP was aptly named.

Neither RARP nor BOOTP is used much today. (They are possible topics for the INTRO exam, though.) One of the problems with both RARP and BOOTP is that they required a computer to act as a server, and the server was required to know the MAC address of every computer and the corresponding configuration parameters that each computer should be told. So, administration in a network of any size was painful.

DHCP, which is very popular in real networks today, solves some of the scaling and configuration issues with RARP and BOOTP, while supplying the same types of information. The main protocols for DHCP are defined in RFC 2131, but a couple of dozen additional RFCs define extensions and applications of DHCP for a variety of other useful purposes.

Like BOOTP, DHCP uses the concept of the client making a request and the server supplying the IP address to the client, plus other information such as the default gateway, subnet mask, DNS IP address, and other information. The biggest advantage of DHCP compared to BOOTP and RARP is that DHCP does not require that the DHCP server be configured with all MAC addresses of all clients. DHCP defines a process by which the server knows the IP subnet in which the DHCP client resides, and it can assign an IP address from a pool of valid IP addresses in that subnet. So, the DHCP server does not need to know the MAC address ahead of time. Also, most of the other information that DHCP might supply, such as the default router IP address, is the same for all hosts in the same subnet, so DHCP servers simply can configure information per subnet rather than per host and save a lot of administrative hassle compared to BOOTP.

The basic DHCP messages for acquiring an IP address are shown in Figure 5-12.

Figure 5-12

DHCP Client

DHCP Messages to Acquire an IP Address

© DHCP Discover Message (LAN Broadcast)

@ DHCP Offer Message Directed to Client

@ DHCP Request Message Directed to Server

0 DHCP Acknowledgement Directed to Client

Broadcast, in. O.rd.e.r. .to. _ _D_is.co.v.e.r. .Server ^

Offer to Provide DHCP Service Request Information_

DHCP Client

Acknowledgement, with the Information

DHCP Server

DHCP Server

(IP Address, Mask, Gateway, Etc)

DHCP has become a very prolific protocol, with most end-user hosts on LANs in corporate networks getting their IP addresses and other basic configuration via DHCP.

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