There are no session or presentation layers in the TCP/IP model. Some of the duties of the session layer, as defined by the OSI model, are included in the transport layer. For example, TCP creates a full-duplex connection between two application-layer protocols. Other duties of the OSI model's session and presentation layers are included in the application protocols if there is a need for them. In other words, it is left to each protocol in the application layer to decide whether to include some to these functionalities. For example, an application protocol may have format translation if needed.
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