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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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3. The value of the hops field is incremented.<br />

4. The relay agent then forwards the BOOTPREQUEST to one or more BOOTP<br />

servers. The address of the BOOTP server or servers is preconfigured at the<br />

relay agent. The BOOTPREQUEST is normally forwarded as a unicast frame,<br />

although some implementations use broadcast forwarding.<br />

5. When the BOOTP server receives the BOOTPREQUEST with the non-zero<br />

router <strong>IP</strong> address field, it sends an <strong>IP</strong> unicast BOOTREPLY to the BOOTP<br />

relay agent at the address in this field on port 67.<br />

6. When the BOOTP relay agent receives the BOOTREPLY, the H/W type,<br />

length, <strong>and</strong> client hardware address fields in the message supply sufficient<br />

link layer information to return the reply to the client. The relay agent checks<br />

the broadcast flag. If this flag is set, the agent forwards the BOOTPREPLY to<br />

the client as a broadcast. If the broadcast flag is not set, the relay agent<br />

sends a reply as a unicast to the address specified in your <strong>IP</strong> address.<br />

When a router is configured as a BOOTP relay agent, the BOOTP forwarding<br />

task is considerably different from the task of switching datagrams between<br />

subnets normally carried out by a router. Forwarding of BOOTP messages can<br />

be considered to be receiving BOOTP messages as a final destination, <strong>and</strong> then<br />

generating new BOOTP messages to be forwarded to another destination.<br />

3.6.2 BOOTP considerations<br />

The use of BOOTP allows centralized configuration of multiple clients. However,<br />

it requires a static table to be maintained with an <strong>IP</strong> address preallocated for<br />

every client that is likely to attach to the BOOTP server, even if the client is<br />

seldom active. This means that there is no relief on the number of <strong>IP</strong> addresses<br />

required. There is a measure of security in an environment using BOOTP,<br />

because a client will only be allocated an <strong>IP</strong> address by the server if it has a valid<br />

MAC address.<br />

3.7 Dynamic Host Configuration Protocol (DHCP)<br />

DHCP is a draft st<strong>and</strong>ard protocol. Its status is elective. The current DHCP<br />

specifications are in RFC 2131 with updates in RFC 3396 <strong>and</strong> RFC 4361. The<br />

specifications are also in RFC 2132 with updates in RFC3442, RFC3942, <strong>and</strong><br />

RFC4361.<br />

The Dynamic Host Configuration Protocol (DHCP) provides a framework for<br />

passing configuration information to hosts on a <strong>TCP</strong>/<strong>IP</strong> network. DHCP is based<br />

on the BOOTP protocol, adding the capability of automatic allocation of reusable<br />

network addresses <strong>and</strong> additional configuration options. For information<br />

130 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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