Motivation
The Domain Name System was first developed in the early 1980s, since which time it has been progressively enhanced with new features, while maintaining compatibility with earlier versions of the protocol.
The restrictions in size of several flags fields, return codes and label types available in the basic DNS protocol were not sufficient to support some desirable features. Moreover, DNS messages carried by UDP were restricted to 512 bytes, not considering the Internet Protocol (IP) and Transport Layer headers. Resorting to a virtual circuit transport, using the Transmission Control Protocol (TCP), would greatly increase overhead. This presented a major obstacle to adding new features to DNS. In 1999, Paul Vixie proposed extending DNS to allow for new flags and response codes, and to provide support for longer responses in a framework that is backwards compatible with previous implementations.
Read more about this topic: Extension Mechanisms For DNS
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