This document, titled "Key Words for Use in RFCs to Indicate Requirement Levels," is a Best Current Practice (BCP) from the Internet Engineering Task Force (IETF). It specifies how certain words, such as "MUST," "SHOULD," and "OPTIONAL," should be interpreted in IETF documents. The document emphasizes that these terms are used to indicate the requirements and recommendations in specifications, with their force modified by the requirement level of the document. It also provides guidelines for authors to use these terms appropriately, particularly when specifying behaviors with security implications. The document is a collaborative effort, incorporating definitions from multiple RFCs and contributions from various individuals.This document, titled "Key Words for Use in RFCs to Indicate Requirement Levels," is a Best Current Practice (BCP) from the Internet Engineering Task Force (IETF). It specifies how certain words, such as "MUST," "SHOULD," and "OPTIONAL," should be interpreted in IETF documents. The document emphasizes that these terms are used to indicate the requirements and recommendations in specifications, with their force modified by the requirement level of the document. It also provides guidelines for authors to use these terms appropriately, particularly when specifying behaviors with security implications. The document is a collaborative effort, incorporating definitions from multiple RFCs and contributions from various individuals.