Portals is an interconnect API intended to allow scalable, high-performance network communication between nodes of a large-scale parallel computing system. Portals is based on a building blocks approach that enables multiple upper-level protocols, such as MPI and SHMEM, to be used simultaneously within a process. This approach also encapsulates important semantics that can be offloaded to a network interface controller (NIC) to optimize performance-critical functionality. Previous generations of Portals have been deployed on large-scale production systems, including the Intel ASCI Red machine and Cray’s SeaStar interconnect for their XT product line. The current generation API is being used to enable advanced NIC architecture research for future extreme-scale systems.