The means by which all I/O is done under STREAMS. Data on a stream is passed in the form of messages. Each stream head, STREAMS module, and driver has a read side and a write side. When messages go from one module's read side to the next module's read side they are said to be traveling upstream. Messages passing from one module's write side to the next module's write side are said to be traveling downstream. Kernel-level operation of messages is discussed in "Messages".