NetApp This document is part of the set of documents intended to update the description of NFSv4 Minor Version One as part of the rfc8881bis respecification effort for NFSv4.1. It describes the structure and function of NFSv4 Access Control Lists within NFSv4.0 and NFSv4.1. These minor versions and forthcoming ones define ACLs using an ACL structure derived from Windows ACLs. Support for other ACL approaches such as draft-POSIX ACLs remains an option that could be taken advantage of in later minor versions such as NFSv4.2. This document describes the structure of these Windows-derived NFSv4 ACLs and their role in the NFSv4 security architecture. While the focus of this document is on the role of these ACLs in providing a more flexible approach to file access authorization than is made available by the POSIX-derived authorization-related attributes, the potential provision of other security-related functionality based on ACLs is covered as well. Because of the failure of previous specifications to provide a satisfactory description of the authorization semantics of NFSv4 ACLs, this document takes a different approach to many matters while maintaining compatibility with implementations based on previous specifications. When the resulting document is eventually published as an RFC, it will supersede the descriptions of ACL structure and semantics appearing in existing minor version specification documents for NFSv4.0 and NFSv4.1, thereby updating RFC7530 and RFC8881.