General
-- Communicators combine context and group for message security
-- Thread safety
Point-to-point communication
-- Structured buffers and derived datatypes, heterogeneity
-- Modes: normal (blocking and non-blocking), synchronous,
ready (to allow access to fast protocols), buffered
Collective
-- Both built-in and user-defined collective operations
-- Large number of data movement routines
-- Subgroups defined directly or by topology