Editing Session layer (section)

=== Dialogue control ===

The session layer may provide three different dialogue types - two way simultaneous (full-duplex), two way alternate (half-duplex), and one way (simplex). It also provides the mechanisms to negotiate the type of the dialogue, and controls which side has the "turn" or "token" to send data or to perform some control functions.

Dialogue control is not implemented in TCP/IP, and is left to the application layer to handle, if necessary. 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.