TCP/IP (Transmission Control Protocol/Internet Protocol) is a network model and also the protocol system that makes up the Internet. It takes its name from the two main protocols, TCP and IP. These were the first network protocols to become a standard. From the beginning of their design, the main goal was to connect as many networks as possible without obstacles. TCP/IP was divided into four stacked layers based on its functions. These are, from bottom to top, the link layer, the Internet layer, the transport layer and the application layer. The protocol system is maintained by the IETF (Internet Engineering Task Force), which is responsible for developing Internet standards.
- Application Layer
- The application layer contains the highest level protocols. Most applications use these protocols to communicate over the network. Initially it only included the TELNET protocols for logging on to a remote computer, FTP for file transfer, and SMTP for sending emails, but over time many other protocols have been added to this layer. When an application wants to send data over the network, the application layer sends it to the transport layer.
- Transport Layer
- The transport layer allows data to be transferred between two IP addresses. The most commonly used transport layer protocols are TCP and UDP. TCP is a reliable, connection-based protocol that provides error-free byte transfers between two computers. It splits data from the application layer into smaller packets and then forwards them one at a time to the Internet layer. TCP also performs traffic control so that the faster source only sends as much data to the destination as it can receive. UDP is an unreliable, connectionless protocol. It is used when message fragmentation and reassembling, as well as traffic control, are not required. It is used in applications where a fast response is the most important factor, such as voice or video transmission.
- Internet Layer
- The Internet layer is responsible for the ability of a host to send packets to any network that have been fragmented by the transport layer, and to forward them regardless of destination. It is also okay for packets to arrive out of order, as the transport layer will reorder them into their original order. The Internet layer is associated with the IP protocol, whose main job is to route packets and avoid congestion.
- Link Layer
- The TCP/IP model does not say what should be in the link model. The only requirement is that the host must be connected to a network that has a protocol capable of transmitting packets.