It has seven layers:
First four are usually very clear
Physical: This is bottom layer, it has all signalling, electrical or light. All physical things after all manipulation being done.
Data Link: This layer dictates us how devices in same network communicate. Here the identifier is Media Access Control(MAC) address. This address is exptected to be unique and it is 6byte(48 bits).
Network: IP address comes up here. If two devices are in different network, this comes to rescue.
Transport: This layer corresponds to delievery of information to specific port in machine i.e. multiplexing of networking communications to single host.
By transport we are able to communicate between two machines, now session layer manages how communication between these two hosts will be like, e.g. will it be full or half duplex.
It establishes, manages (including providing security) and terminates connections between applications at each end.
[Wikipedia] An example of a session-layer protocol is the OSI protocol suite session-layer protocol, also known as X.225 or ISO 8327. 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.
Examples of session layer protocols include DLC (data link control), PAP (printer access control), SMB (server message block), ASP (AppleTalk session protocol), NetBIOS (network basic input/output system) and ZIP (zone information protocol).
Session layer protocols are particularly useful for multimedia applications for which it is necessary to coordinate the timing of two or more types of data, such as voice and moving images, with a high degree of precision. Examples include video conferencing and streaming.
TCP/IP is sometimes referred as Department of Defence(DOD) model, because the DoD provided funding for the Arpanet, when TCP and IP were developed(Book Page: 18)
In TCP Protocol suite:
|Link Layer||Physical + data link|
|Application||Session + Transport+ Application|