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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)
           9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)
                9  TCP/IP Bootstrap Protocol (BOOTP)

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BOOTP Message Format
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BOOTP Relay Agents (Forwarding Agents)
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BOOTP Vendor-Specific Area and Vendor Information Extensions
(Page 2 of 3)

BOOTP Vendor Information Extensions

Including the Vend field in BOOTP gives the protocol extensibility for vendor-specific information. Unfortunately, the original field format didn't include any way of extending the information sent from a server to a client for generic, non-vendor-specific TCP/IP information.

This was a significant oversight in the creation of the protocol, because there are many types of information that a TCP/IP host needs when it starts up that really have nothing to do with its vendor. For example, when a host boots, we probably want it to be told the address of a default router; the subnet mask for its local subnet; the address of a local DNS server; the MTU of the local network; and much more. None of these things are vendor-specific, but there is no place to put them in the BOOTP reply message.

Since there was no “non-vendor-specific area” field in BOOTP, the decision was made to define a way of using the Vendor-Specific Area for communicating this additional generic information. This was first standardized in RFC 1048, and then refined in later RFCs as I explained in the BOOTP overview. This scheme basically represents one particular way of using the Vend field that most TCP/IP BOOTP implementations have chosen to adopt, regardless of their vendor. This enhancement is formally referred to as BOOTP vendor information extensions.

To clearly mark that this particular meaning of the Vend field is being used, a special, universal “magic cookie” value of “99.130.83.99” is inserted into the first four bytes of the field. Then, the remaining 60 bytes can contain a sequence of one or more vendor information fields. The overall structure of the Vendor-Specific Area when vendor information extensions are used is shown in Figure 257.


Figure 257: BOOTP Vendor-Specific Area Format Showing Vendor Information Fields

The BOOTP Vendor-Specific Area begins with the four-byte “magic cookie” and then contains a number of variable-length vendor information fields, each of which has the format shown above and in Table 187.

 


Note: Despite the use of dotted decimal notation to represent the value “99.130.83.99”, this is not an IP address. It's just a marker, a “magic number” that is universally recognized.


Key Concept: The BOOTP message format includes a Vend field that was originally intended for vendor-specific customized fields. It was later changed to a place where additional generic information could be sent from a BOOTP server to a BOOTP client. Each such parameter is carried in a BOOTP vendor information field.



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