BOOTP Overview, History and Standards
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BOOTP Deals With the First Phase of Bootstrapping
It should be noted that even though the name of BOOTP implies that it defines everything needed for a storageless device to boot, this isn't really the case. As the BOOTP standard itself describes, bootstrapping generally requires two phases. In the first, the client is provided with an address and other parameters. In the second, the client downloads software, such as an operating system and drivers, that let it function on the network and perform whatever tasks it is charged with. BOOTP really only deals with the first of these phases: address assignment and configuration. The second is assumed to take place using a simple file transfer protocol like the Trivial File Transfer Protocol (TFTP).
One smart decision made when BOOTP was created was the inclusion of a vendor-specific area. This was intended to provide a place where hardware vendors could define parameters relevant to their own products. As the complexity of TCP/IP increased, it was realized that this field could be used to define a method of communicating certain parameters that were commonly needed by IP hosts, and were in fact vendor-independent. This was first proposed in RFC 1048, BOOTP Vendor Information Extensions, published February 1988.
The fact that BOOTP can be used to provide information to a client beyond just an IP address makes it useful even in cases where a device already knows its address. BOOTP can be used to send parameters that the administrator wants all hosts to have, to ensure that they use the network in a consistent manner. Also, in the case of devices that do have local storage (and therefore do not need BOOTP to get an IP address), BOOTP can still be used to let these devices get the name of a boot file for the phase two of bootstrapping described above.
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