Requirements for Internet Hosts — Communication Layers
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R. Braden · 1989-10
Abstract
RFC 1122 documents the requirements that internet host implementations must meet at the link, internet, and transport layers. It clarifies and corrects ambiguities in earlier protocol specifications and establishes the 'robustness principle' as a guiding design philosophy for internet implementations.
Why This RFC Matters
RFC 1122 is often paired with RFC 1123 (application-layer requirements) to form the definitive interoperability standard for internet hosts. Its most quoted contribution is Postel's Robustness Principle: 'Be conservative in what you do, be liberal in what you accept from others.' While this principle enabled interoperability in the early internet, later researchers argued it also allowed protocol ambiguities to accumulate. Nevertheless, RFC 1122 remains a landmark — it was the first systematic attempt to document what a correct internet host implementation actually looks like, resolving years of accumulated ambiguity across TCP, IP, ICMP, and ARP.