Recommendation Y.FaaS-reqts provides an overview and functional requirements of Function as a Service (FaaS). It introduces the definition and advantages of FaaS, and describes logical components of FaaS and lifecycle of function in FaaS. It provides typical use cases of FaaS and specifies the functional requirements of FaaS from the perspective of four logical components, which are interactive action support, elastic resource support, operation and maintenance support, measuring and billing support. It also provides a relationship analysis among FaaS, IaaS, PaaS, SaaS, and serverless computing.