SNA defines protocols for controlling the flow of data within a session (for example, specifying which end of the session can send data at a particular time). Data-flow-control requests and indicators are used in these protocols, some of which are summarized in Using SNA protocols. These requests and indicators can be specified with a SEND macroinstruction and are made available with a RECEIVE macroinstruction, or in an exit routine. For additional information, refer to the SEND and RECEIVE macroinstruction descriptions in Conventions and descriptions of VTAM macroinstructions.