Communication sessions consist of requests and responses that occur between applications.
Session-layer services are commonly used in application environments that make use of remote procedure calls (RPCs).
In the OSI model, the transport layer is not responsible for an orderly release of a connection.
However, in modern TCP/IP networks, TCP already provides orderly closing of connections at the transport layer.
The session layer may provide three different dialogue types - two way simultaneous (full-duplex), two way alternate (half-duplex), and one way (simplex).
In the widely-used HTTP/1.1 protocol, the client and the server typically work in a half-duplex way.
HTTP/1.1 also supports HTTP pipelining for full-duplex operation, but many servers/proxies couldn't handle it correctly, and there was no dialogue negotiation mechanism to check whether full-duplex is usable or not, so its support was eventually dropped by most browsers.