The TCP/IP stack

The OSI model was devised using a committee process wherein the standard was set up and then implemented. Some parts of the OSI standard are obscure, some parts cannot easily be implemented, some parts have not been implemented.

The TCP/IP protocol was devised through a long-running DARPA project. This worked by implementation followed by RFCs (Request For Comment). TCP/IP is the principal Unix networking protocol. TCP/IP = Transmission Control Protocol/Internet Protocol.

The TCP/IP stack is shorter than the OSI one:

TCP is a connection-oriented protocol, UDP (User Datagram Protocol) is a connectionless protocol.

IP datagrams

The IP layer provides a connectionless and unreliable delivery system. It considers each datagram independently of the others. Any association between datagrams must be supplied by the higher layers.

The IP layer supplies a checksum that includes its own header. The header includes the source and destination addresses.

The IP layer handles routing through an Internet. It is also responsible for breaking up large datagrams into smaller ones for transmission and reassembling them at the other end.

UDP

UDP is also connectionless and unreliable. What it adds to IP is a checksum for the contents of the datagram and port numbers. These are used to give a client/server model - see later.

TCP

TCP supplies logic to give a reliable connection-oriented protocol above IP. It provides a virtual circuit that two processes can use to communicate. It also uses port numbers to identify services on a host.