Hello Hussein.
That’s a very good question. The different use of terms can become confusing. First of all, when using protocols such as TCP/IP, we are actually using the TCP/IP protocol stack which has four layers: Network Access, Internet, Transport and Application. However, when we speak about the layers in which protocols function, we always use the OSI model. For example, we say “layer 3” (of the OSI) for the IP protocol, even though it’s “layer 2” in the TCP/IP model. For TCP and UDP, we say they function at layer 4 of the OSI even though they are at layer 3 of the TCP/IP protocol stack.
What makes it more confusing is the fact that for the Transport layer, TCP creates what is called a TCP session. This is not to be confused with the Session layer 5 of the OSI model. The session begins with a 3-way handshake between the two hosts that are communicating to signify the beginning of the session. Data is exchanged during the session and it ends with a four way handshake that terminates the session.
The Session layer of the OSI model isn’t actually used when using TCP, because TCP/IP does not follow the OSI model. To find out more about the Session layer of the OSI model, check this out: http://www.cisco.com/cpress/cc/td/cpress/fund/ith/ith01gb.htm#xtocid1668415. Keep in mind however, that in the world of TCP/IP, this layer is non existent.
I hope this has been helpful!
Laz