DHCP and BOOTP Interoperability
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I've talked extensively about how DHCP was designed based on the Boot Protocol, and how they use the same basic communication method and message format. This was done for several reasons, one of the most important of which was ensuring interoperability of the two protocols. Given this, you might expect that we could simply say that BOOTP and DHCP are compatible with each other, and that's that. Then again, given that you can see I have a whole topic on the subject, maybe you wouldn't think that at all
It is in fact true that DHCP was intended to be compatible with BOOTP. RFC 2131 lists the following as one of DHCP's design goals: DHCP must provide service to existing BOOTP clients.. This seems pretty clear. The reuse of the BOOTP message format is one of the keys to DHCP and BOOTP compatibility. DHCP functionality is implemented not through new fields but rather through DHCP-specific options, such as the DHCP Message Type option that specifies the all important type of DHCP messages. DHCP devices can look for this extra information, while BOOTP devices can ignore it.
However, while DHCP and BOOTP are similar, they are not the same, and so there are some interoperability concerns that crop up when they are used together. The DHCP message format is structurally the same as the BOOTP format, but the interpretation of certain fields is slightly different. BOOTP clients don't understand DHCP, so when BOOTP and DHCP are used together, the DHCP client or server must sometimes behave slightly differently to compensate. Further complicating matters is the fact that not all implementations of DHCP and BOOTP are necessarily exactly the same, and the fact that certain specifications in the DHCP standard are not mandatory.
For these reasons, we cannot just assume that DHCP and BOOTP will work together. To address some of these issues, the IETF published RFC 1534, Interoperation Between DHCP and BOOTP, at the same time that DHCP was originally created. This document looks at how the protocols work together, focusing on the two distinct client/server interoperating combinations: a BOOTP client connecting to a DHCP server, and a DHCP client connecting to a BOOTP server. Let's consider each case one at a time.
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