TCP (Transmission Control Protocol) is the software that breaks down computer data into tiny data packets that can be between 1 and 1500 characters long. These packets are provided with the address of the recipient and sent separately on the big data journey. At the receiving point, TCP reassembles the packets in the correct order. Only then can they be read by the recipient computer. In reality, however, TCP does a lot more. Packages are repeatedly lost on the way, for example due to faulty telephone lines or other disruptions during transport. When sending the data, TCP therefore creates a checksum. After receiving the data, this sum is checked. If the computer detects discrepancies, then TCP is able to close the gaps that occurred during the transmission and replace missing data. Of course, this error correction only works up to a certain point. If too much of the original data is missing, it has to be retransmitted. TCP can do that automatically as well. IP (Internet Protocol) ensures that a data packet arrives at the correct address. Each of the many millions of computers on the Internet has a unique identifier made up of four numbers or groups of numbers, all of which must be less than 256. These address elements are each separated by a dot or a point. An IP address looks something like this: 185.31.22.3 UNIX computers are easiest to integrate into the Internet. TCP / IP was originally developed on UNIX computers. UNIX comes with TCP / IP drivers including all basic software or offers a TCP / IP extension for a surcharge. UNIX computers are therefore the easiest to integrate to the Internet. Commercial UNIX operating systems such as ISC-UNIX or Solaris can cost up to several thousand dollars. An inexpensive alternative for PCs is the public version of UNIX called Linux. The TCP / IP reference model The TCP / IP reference model, also called the DoD layer model (Department of Defense layer model), in comparison to the to the OSI layer model, does not define seven, but only four layers. These four layers include the network layer, internet layer, transport layer and the application layer The protocols of the TCP / IP family are active in layers 2 and 3 (Internet layer and transport layer). Above these are the protocols of the application layer such as SMTP (Simple Mail Transfer Protocol), HTTP (Hypertext Transfer Protocol) or FTP (File Transfer Protocol). They are served by the protocols of layer 2 and 3 and are responsible for the exchange of information between the various applications in an IP-based network. Tasks and protocols of the Internet and transport layer The task of the Internet layer is to convey the data packets in the network and to forward them to the correct nodes. The Internet Protocol (IP) in versions IPv4 and / or IPv6 is largely responsible for these tasks. IP ensures the unique addressing of the participants and nodes of the network. Each data packet has a sender and destination IP address. Routing through the network and delivery of the data packet to the desired recipient takes place on the basis of the destination IP address. IP also defines the size of the datagrams but does not provide any special mechanisms for controlling data transmission. Protocols such as: ICMP (Internet Control Message Protocol) for exchanging and reporting status or error messages Routing protocols such as RIP (Routing Information Protocol), OSPF (Open Shortest Path First), BGP (Border Gateway Protocol) and others for the exchange of route information The end-to-end connections from the sender to the destination are implemented on the transport layer. The Transmission Control Protocol is one of the protocols for this task. It ensures the defined establishment and disconnection of connections and provides mechanisms for flow control. TCP checks whether a packet has actually reached the recipient and, if necessary, regulates the resending of a lost data packet. The applications of the higher layers can be addressed via so-called ports. Also, UDP operates at the transport layer, and provides ports for use before addressing. However, it works without a connection and does not check whether data packets actually arrive at their destination.