Wasiq Ansari’s World

TCP/IP (Transmission Control Protocol/Internet Protocol) is the basic communication language or protocol of the Internet. It can also be used as a communications protocol in a private network (either an intranet or an extranet).

TCP/IP Model Layers
The TCP/IP model uses four layers that logically span the equivalent of the top six layers of the OSI reference model; The physical layer is not covered by the TCP/IP model because the data link layer is considered the point at which the interface occurs between the TCP/IP stack and the underlying networking hardware. The following are the TCP/IP model layers, starting from the bottom.

1. Application Layer
   In TCP/IP the Application Layer also includes the OSI Presentation Layer and Session Layer. In this document an application is any process that occurs above the Transport Layer. This includes all of the processes that involve user interaction. The application determines the presentation of the data and controls the session. In TCP/IP the terms socket and port are used to describe the path over which applications communicate. There are numerous application level protocols in TCP/IP, including Simple Mail Transfer Protocol (SMTP) and Post Office Protocol (POP) used for e-mail, Hyper Text Transfer Protocol (HTTP) used for the World-Wide-Web, and File Transfer Protocol (FTP). Most application level protocols are associated with one or more port number.
2. Transport Layer
   In TCP/IP there are two Transport Layer protocols. The Transmission Control Protocol (TCP) guarantees that information is received as it was sent. The User Datagram Protocol (UDP) performs no end-to-end reliability checks.
3. Internet Layer
   In the OSI Reference Model the Network Layer isolates the upper layer protocols from the details of the underlying network and manages the connections across the network. The Internet Protocol (IP) is normally described as the TCP/IP Network Layer. Because of the Inter-Networking emphasis of TCP/IP this is commonly referred to as the Internet Layer. All upper and lower layer communications travel through IP as they are passed through the TCP/IP protocol stack.
4. Network Access Layer
   In TCP/IP the Data Link Layer and Physical Layer are normally grouped together. TCP/IP makes use of existing Data Link and Physical Layer standards rather than defining its own. Most RFCs that refer to the Data Link Layer describe how IP utilizes existing data link protocols such as Ethernet, Token Ring, FDDI, HSSI, and ATM. The characteristics of the hardware that carries the communication signal are typically defined by the Physical Layer.

The four layer structure of TCP/IP is built as information is passed down from applications to the physical network layer. When data is sent, each layer treats all of the information it receives from the layer above as data and adds control information to the front of that data. This control information is called a header, and the addition of a header is called encapsulation. When data is received, the opposite procedure takes place as each layer removes its header before passing the data to the layer above.