This document is the main high-level architecture specification of the SUNFISH cloud federation solution. Its main objective is to introduce the concept of Federation-as-a-Service (FaaS) and the SUNFISH platform. FaaS is the new and innovative cloud federation service proposed by the SUNFISH project. The document defines the functionalities of FaaS, its governance and precise objectives. With respect to these objectives, the document proposes the high-level architecture of the SUNFISH platform: the software architecture that permits realising a FaaS federation. More specifically, the document describes all the components forming the platform, the offered functionalities and their high-level interactions underlying the main FaaS functionalities. The document concludes by outlining the main implementation strategies towards the actual implementation of the proposed cloud federation solution.