of multiplexing and sockets...

http://www.inetdaemon.com/tutorials/internet/tcp/multiplexing.shtml

transport layer:

Multiplexing is the process of combining two or more data streams into a single physical connection. TCP provides multiplexing facilities by using source and destination port numbers. These port numbers allow TCP to set up a number of virtual connections over a physical connnection and multiplex the data stream through that connection.

session layer:

 http://www.tcpipguide.com/free/t_SessionLayerLayer5.htm

The primary job of session layer protocols is to provide the means necessary to set up, manage, and end sessions. In fact, in some ways, session layer software products are more sets of tools than specific protocols. These session-layer tools are normally provided to higher layer protocols through command sets often called application program interfaces or APIs.

Common APIs include NetBIOS, TCP/IP Sockets and Remote Procedure Calls (RPCs). They allow an application to accomplish certain high-level communications over the network easily, by using a standardized set of services. Most of these session-layer tools are of primary interest to the developers of application software. The programmers use the APIs to write software that is able to communicate using TCP/IP without having to know the implementation details of how TCP/IP works.

For example, the Sockets interface lies conceptually at layer five and is used by TCP/IP application programmers to create sessions between software programs over the Internet on the UNIX operating system. Windows Sockets similarly lets programmers create Windows software that is Internet-capable and able to interact easily with other software that uses that interface. (Strictly speaking, Sockets is not a protocol, but rather a programming method.)

what is a socket?

http://en.wikipedia.org/wiki/Network_socket

An Internet socket is characterized by a unique combination of the following:

• Local socket address: Local IP address and port number
• Remote socket address: Only for established TCP sockets. As discussed in the client-server section below, this is necessary since a TCP server may serve several clients concurrently. The server creates one socket for each client, and these sockets share the same local socket address.
• Protocol: A transport protocol (e.g., TCP, UDP, raw IP, or others). TCP port 53 and UDP port 53 are consequently different, distinct sockets.