In SNA, the layer within a half-session that (a) controls whether the half-session can send or receive, or both send and receive request units (RUs) at the same time, (b) combines related RUs into RU chains, (c) defines the limits of transactions by using the bracket protocol, (d) controls the connection of requests and responses in accordance with control modes specified when the session is started, (e) creates sequence numbers, and (f) associates requests with responses.