In the beginning of networking, when ARPANET was still in its infancy, the collaboration that was necessary to make communication work smoothly between computers was built upon the RFC (Request for Comment) model. In building ARPANET, many of the most talented computer scientists of the time (circa 1969) started creating informal documents to share ideas on how networking should function and how connections should be managed.
To make networking universal and non-proprietary, the OSI model was established. OSI (Open System Interconnection) is considered to be the basis of TCP/IP and networking as it is deployed today. The OSI model consists of seven layers describing the functions and elements of a network and how they should all interact. The seven layers are Physical, Data Link, Network, Transport, Session, Presentation and Application. Each layer plays an important part in how data is sent and received on our networks of today.
The first layer of the OSI model, Physical, describes the aspects of the physical connection, be it voltages over copper to wavelength of light emitted in fiber-optic connections. It is responsible for establishing connections and terminating them too.
Layer 2, the Data Link layer, is responsible for the transmission between network nodes. The Data Link layer of the OSI model has two sub-layers, LLC and MAC. The Media Access Control, or MAC sub-layer, makes sure that the received frame was meant for a specific machine by verifying that the MAC was encoded within the frame. The LLC (Logical Link Control) sub-layer provides the tolerance for running several different protocols on one network medium. It also helps provide flow control as well as error management.
The Network Layer is responsible for the routing of data, logical addressing, datagram encapsulation and it too helps in error handling. Logical addressing in the network layer is based on the IP protocol from the TCP/IP suite. The addressing provided by IP (Internet Protocol) is also used in the routing at this layer.
Layer 4, Transport, makes sure data can be sent reliably from the sending node to the destination. It is at this layer that acknowledgement, or ACK, becomes part of the communication. An ACK is simply the step of the receiving node sending an acknowledgement that it has received the complete message. If no ACK is sent, depending on other factors, the sending node may choose to retransmit the message.
The Session Layer is the 5th layer of the OSI model. The Session Layer controls the timing of transmission and who will send versus who will receive at any given time. It is responsible for the integrity of the connection between the nodes. The Session Layer determines speed of data transmission based on other attributes from lower layers. Depending on the physical connection, it is the session layer that will decide if transmission will be simplex, full duplex or somewhere in between.
Layer 6, Presentation, is responsible for the how the data is encoded. Presentation takes the information from the Application Layer (layer 7) and breaks the data down into ASCII or EBCDIC language. It is the layer responsible for the syntax of the communication, as well as the encryption and compression of the data to be sent over the network.
Lastly, the Application Layer is the function of what you want the network to do. The protocols of HTTP, FTP, SMTP, POP and many others determine what data will be assembled by the lower levels and sent across the network to provide the outcome you plan for.
While this is by no means a comprehensive list of what each layer does, it is because of the RFC and OSI model that these defined layers were developed to regulate and standardize communications over networks.