RFC 7232 Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests

PROPOSED STANDARD · June 2014 · R. Fielding, Ed., J. Reschke, Ed.

The Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating state changes, request header fields for making preconditions on such state, and rules for constructing the responses to a conditional request when one or more preconditions evaluate to false.

Not ingested in full here — read it on rfc-editor.org.