The path control network of Systems Network Architecture (SNA) supports communication between pairs of network addressable units called logical units, usually referred to as LU's. When communicating, information flows between LU's over a logical connection called a session. SNA defines three categories of messages which flow on LU-to-LU sessions-- session control, data flow control, and function management data. Thispaper discusses the structui'e and semantics of messages and protocols used by one LU to manage functions performed by a remote LU. The relatio.nship between distribution of function, the nature of the information, the technology, and the architecture of the data stream will be discussed. The data Streams used by SNA printers and 3270 displays will be used as examples. The paper concludes with a discussion of the possible impact of advancing electronic technology on data stream architecture.