The Transmission Control Protocol (TCP) or Internet Protocol is an industry-standard protocol suite for wide area networks (WANs) developed in the 1970s and 1980s by the U.S. Department of Defense (DoD). TCP/IP is a routable protocol that is suitable for connecting dissimilar systems (such as Microsoft Windows and UNIX) in heterogeneous networks.

It is the protocol of the worldwide network known as the Internet. the Internet began as a project funded by the United States Department of Defence in the 1970s in interconnect educational institutions and government installations. At that tie it was called ARPAnet (Advanced Research Projects Agency Network). over time it has evolved into the huge worldwide network known as the Internet.

The tools that make up the Internet protocol suite, the best-known being TCP (Transmission control protocol) and IP (Internet Protocol), have become de-facto standards because of the success of the Internet. the entire protocol suite is referred to as TCP/IP.

The Internet protocol suite is unique in that it is made up of non-proprietary protocols. this means that they do not belong to any one company and that the technology is available to anyone who wishes to use it. as a result, the internet protocol suite is supported by the widest variety of network vendors.

The Internet suite was developed about ten years before the OSI model was defined and can therefore be only roughly mapped to it. the internet protocol suite was defined according to its own model, known as the Internet or DOD model.

Transmission Control & Internet Protocol (TCP/IP)

Transmission Control & Internet Protocol (TCP/IP) OSI model

The four DOD model layers as you saw in the above image and the OSI model layers correspond as follows:
  1. The Network Access layer corresponds to the physical and data link layers of the OSI model.
  2. The Internet layer corresponds to the OSI network layer. protocols at this layer are concerned with transporting packets through the internetwork. the main internet layer protocol is IP (Internet Protocol).
  3. The host-to-host layer corresponds roughly to the OSI transport layer. protocols at this layer communicate with peer processes in other hosts or networked devices. an example of a host-to-host protocol is TCP.
  4. The process/application layer corresponds to the OSI session, presentation, and application layers. protocols at this layer provide applications services on the network. Examples of protocols at this layer are Telnet (a terminal emulator) and FTP (a file transfer protocol).
The Internet protocols do not cover the lower two layers of the OSI model. this is because the designers of TCP/IP used existing physical and data link standards, such as Ethernet and Token Ring, to make TCP/IP hardware independent. as a result, the protocols of the internet suite are widely used to connect heterogeneous systems.

As the Internet protocol suite does not include lower-layer protocols, we shall discuss the individual protocols at the middle layers. the OSI model's network and transport layer are concerned with transporting packets across the internetwork. TCP/IP and other internet protocols use three types of addresses for network addressing. These are as follows:
  1. Hardware or physical addresses are used by the data link and physical link layers. Physical addresses are usually hardcoded into the network cards with each device.
  2. IP addresses provide logical node IDs. IP addresses are unique addresses assigned by an administrator according to certain guidelines. they are expressed in four-part dotted decimal notation. For example, 123.456.789.012.
  3. Logical node names are easier to remember than an IP address. For example
Each layer of the TCP/Ip protocol suite has its associated component protocols, the most important of which are as follows:

Application layer protocols

Responsible for application-level access to TCP/IP networking services. these include Dynamic Host Configuration Protocol (DHCP), Domain Name System (DNS), Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Telnet, Simple Mail Transfer Protocol (SMTP), and Simple Network Management Protocol (SNMP). In the Microsoft implementation of TCP/IP, application layer protocols interact with transport layer protocols by using either windows sockets or NetBIOS over TCP/IP (NetBT).

Transport layer protocols 

Establish communication through connection-oriented sessions and connectionless broadcasts. protocols at this layer include transmission control protocol (TCP) and User Datagram Protocol (UDP).

Internet layer protocols

Responsible for routing and encapsulation into IP packets Protocols at this layer include Internet Protocol (IP), Address Resolution Protocol (ARP), Internet Control Message Protocol (ICMP), and Internet Group Management Protocol (IGMP).

Network layer protocols

It places frames on the network. the protocols include the various local area network (LAN) architectures (such as Ethernet and Token Ring) and WAN telecommunication service technologies such as Plain Old Telephone Service (POTS), Integrated Services Digital Network (ISDN), and Asynchronous Transfer Mode (ATM).

TCP/IP is a constantly evolving protocol suite whose development is steered by such bodies as the Internet Society (ISOC), the Internet Architecture Board (IAB), and the Internet Engineering Task Force (IETF). the current version of TCP/IP is called IPv4 (Internet Protocol version 4). A new version called IPv6 is under implementation.