The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

The Application Layer (Layer 7) refers to communication services to applications and is the interface between the network and the application. Most application layer protocols offer services that programs use to access the network, such as the Simple Mail Transfer Protocol (SMTP). Application layer protocols often include the session and presentation layer functions. A typical protocol stack consists of four separate protocols, which run at the application, transport, network, and data-link layers.Examples include: Telnet, HTTP, FTP, Internet browsers, NFS, SMTP gateways, SNMP, X.400 mail, and FTAM.

The Presentation Layer (Layer 6) defining data formats, such as ASCII text, EBCDIC text, binary, BCD, and JPEG and is responsible for formatting data exchange.. 
Encryption also is defined as a presentation layer service. Examples include: JPEG, ASCII, EBCDIC, TIFF, GIF, PICT, encryption, MPEG, and MIDI.

Session layer:
The session layer establishes, manages, and terminates communication sessions. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. Some examples of session-layer implementations include AppleTalk’s Zone Information Protocol (ZIP), and Decent Phase Session Control Protocol (SCP).

Transport layer:
Transport layer is responsible for providing reliable service between the hosts. Upper layer datagrams are broken down into manageable datagrams and then appropriate header information (such as sequence number, port number, etc.) is added to the datagram before passing it on to the Network layer. Two frequently used transport protocols are the TCP (Transmission Control Protocol) and the UDP (User Datagram Protocol). 

Important features of Transport layer:

Transport layer ensures reliable service.

-Breaks the message (from sessions layer) into smaller datagrams, and appends appropriate unit header information.

-Responsible for communicating with the Session layer

-Important features of TCP/UDP:

-TCP/IP widely used protocol for Transport/Network layers

-TCP: (Transport Control Protocol) TCP ensures that a packet has reached its intended destination by using an acknowledgement. If not, it retransmits the lost messages. Hence, TCP is called a connection oriented protocol.

-UDP (Universal Data gram Protocol): UDP simply transmits packets over the internet. It does not wait for an acknowledgement. It is the responsibility of upper layer protocols to ensure that the information had reached the intended partner(s). Hence, UDP is often called connectionless protocol.

-Application programs that do not need connection-oriented protocol generally use UDP.

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