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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
To clearly mark that this particular
meaning of the Vend field is being used, a special, universal
magic cookie value of 220.127.116.11 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 18.104.22.168, 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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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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