Why do we need both session layer if we have transport layer(in the osi model)?

Question

If tcp in the transport layer creates for us a session , so why we need the session layer to create for us a session?

Answer 1

A quick look at the Wikipedia page seems to indicate that a session-layer session could span multiple connections (TCP sessions).

In case of a connection loss this protocol may try to recover the connection. If a connection is not used for a long period, the session-layer protocol may close it and re-open it.

I don't know much about that protocol in particular, but in general if they made a separate layer for sessions, that would indicate to me that a Session-layer session is different than a TCP session. A TCP session might fail or might get closed if it is open too long, and they wanted an abstract way to talk about a communication "session" that might actually take place over multiple connections.

Answer 2

There is no concept for OSI's session layer in the TCP/IP model.

Something similar is usually implemented on the application layer. Examples include cookies or URL parameters for HTTP, or control connections for FTP.